Motor Speed Control

ewsgroups: sci.electronics.design
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pullys approach? Would simple PWM be enough or
would there be some additional trickery needed?

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.

Were you thinking of AC or DC motors ?

The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.

They have probably gotten better in the last 10 years.

Cursitor Doom

2024-02-18 20:50:19 UTC

On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

DC. This will be for an old reel-to-reel tape recorder that's been in
storage for decades. All the rubber drive belts have perished and
replacements are unobtainable. It has 3 speeds: 3 inches per second,
7.5 IPS and 15. I believe it's a 24V motor but will have to check. If
the idea is feasible, I have a couple of other R-Rs I'd like to get
working again as well.

John Larkin

2024-02-18 21:23:04 UTC

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

I saw an article somewhere that suggested one can make replacement
drive belts from strips of back-to-back Scotch tape.

The old Ampex and such tape recorders usually used AC motors.

Cursitor Doom

2024-02-18 22:10:09 UTC

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

I saw an article somewhere that suggested one can make replacement
drive belts from strips of back-to-back Scotch tape.
The old Ampex and such tape recorders usually used AC motors.

One of the other tape recorders I have used a 240V motor. Not sure
whether AC or DC motors are more suited to PWM control.

John Larkin

2024-02-18 23:00:19 UTC

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

I saw an article somewhere that suggested one can make replacement
drive belts from strips of back-to-back Scotch tape.
The old Ampex and such tape recorders usually used AC motors.

One of the other tape recorders I have used a 240V motor. Not sure
whether AC or DC motors are more suited to PWM control.

The Ampex r-r recorders, and others, used a synchronous AC motor for
the capstain drive and shaded-pole AC motors for the reels.

The takeup reel motor ran stalled, and the feed reel motor had DC
applied to make a mild drag. Cheaper recorders had a single motor and
a mess of belts and clutches and such.

My first job was as a tech in a language lab, so I learned a lot about
tape recorders. I recall that the pay was about 70 cents per hour.

AC motors don't PWM well. DC motors don't speed control well, without
some sort of feedback loop. A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

Cursitor Doom

2024-02-18 23:18:44 UTC

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

I saw an article somewhere that suggested one can make replacement
drive belts from strips of back-to-back Scotch tape.
The old Ampex and such tape recorders usually used AC motors.

One of the other tape recorders I have used a 240V motor. Not sure
whether AC or DC motors are more suited to PWM control.

The Ampex r-r recorders, and others, used a synchronous AC motor for
the capstain drive and shaded-pole AC motors for the reels.
The takeup reel motor ran stalled, and the feed reel motor had DC
applied to make a mild drag. Cheaper recorders had a single motor and
a mess of belts and clutches and such.
My first job was as a tech in a language lab, so I learned a lot about
tape recorders. I recall that the pay was about 70 cents per hour.
AC motors don't PWM well. DC motors don't speed control well, without
some sort of feedback loop. A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

Thanks, John; very interesting. What do you mean by takeup reel motor
ran stalled? And what is it that controls the speed of the tape - the
capstan/pinch-wheel motor or the relevant reel motor?

Bertrand Sindri

2024-02-18 23:47:04 UTC

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

Post by Cursitor Doom
Can motor speed control ever approach the effectiveness of the old
style drive belts and pullys approach? Would simple PWM be enough
or would there be some additional trickery needed?

When I retired about 10 years ago we had many motor speed controls
that worked very well ranging from less than 1 HP to 300 HP. Some
were for DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes
the motors would produce a sound of say 1000 Hz in frequency. The
speed controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

[snip]
The old Ampex and such tape recorders usually used AC motors.

One of the other tape recorders I have used a 240V motor. Not sure
whether AC or DC motors are more suited to PWM control.

The Ampex r-r recorders, and others, used a synchronous AC motor for the
capstain drive and shaded-pole AC motors for the reels.
The takeup reel motor ran stalled, and the feed reel motor had DC applied
to make a mild drag. Cheaper recorders had a single motor and a mess of
belts and clutches and such. [snip] AC motors don't PWM well. DC motors
don't speed control well, without some sort of feedback loop. A BLDC
motor with tach could make a very nice capstain drive, with a lot of
electronics.

Thanks, John; very interesting. What do you mean by takeup reel motor ran
stalled?

The take-up motor is set to a speed (RPM) such that it maintains a tension
on the tape on the capstan/pinch-wheel. This is to assure that the tape
spools onto the take-up reel instead of unspooling on the floor. The result
is that the motor is always being held back from its desired speed (i.e.,
stalled) by the tape.

And what is it that controls the speed of the tape - the
capstan/pinch-wheel motor or the relevant reel motor?

The capstan and pinch-wheel controls the linear tape speed across the heads.
The take-up motor maintains tension to keep the tape spooling onto the
take-up reel, and the feed motor maintains a small back tension to prevent
the feed reel from unspooling onto the floor.

Cursitor Doom

2024-02-19 00:36:53 UTC

On Sun, 18 Feb 2024 23:47:04 GMT, Bertrand Sindri

Post by Bertrand Sindri

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls
that worked very well ranging from less than 1 HP to 300 HP. Some
were for DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes
the motors would produce a sound of say 1000 Hz in frequency. The
speed controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

[snip]
The old Ampex and such tape recorders usually used AC motors.

One of the other tape recorders I have used a 240V motor. Not sure
whether AC or DC motors are more suited to PWM control.

The Ampex r-r recorders, and others, used a synchronous AC motor for the
capstain drive and shaded-pole AC motors for the reels.
The takeup reel motor ran stalled, and the feed reel motor had DC applied
to make a mild drag. Cheaper recorders had a single motor and a mess of
belts and clutches and such. [snip] AC motors don't PWM well. DC motors
don't speed control well, without some sort of feedback loop. A BLDC
motor with tach could make a very nice capstain drive, with a lot of
electronics.

Thanks, John; very interesting. What do you mean by takeup reel motor ran
stalled?

And what is it that controls the speed of the tape - the
capstan/pinch-wheel motor or the relevant reel motor?

Thanks, Betrand. That pretty much ties in with what I'm seeing when
it's powered up. Looks like all of them will need a pretty thorough
service to get them back into spec. A lot can go wrong in 40 years!

Cursitor Doom

2024-02-19 00:34:11 UTC

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

I saw an article somewhere that suggested one can make replacement
drive belts from strips of back-to-back Scotch tape.
The old Ampex and such tape recorders usually used AC motors.

One of the other tape recorders I have used a 240V motor. Not sure
whether AC or DC motors are more suited to PWM control.

The Ampex r-r recorders, and others, used a synchronous AC motor for
the capstain drive and shaded-pole AC motors for the reels.
The takeup reel motor ran stalled, and the feed reel motor had DC
applied to make a mild drag. Cheaper recorders had a single motor and
a mess of belts and clutches and such.
My first job was as a tech in a language lab, so I learned a lot about
tape recorders. I recall that the pay was about 70 cents per hour.
AC motors don't PWM well. DC motors don't speed control well, without
some sort of feedback loop. A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

Thanks, John; very interesting. What do you mean by takeup reel motor
ran stalled? And what is it that controls the speed of the tape - the
capstan/pinch-wheel motor or the relevant reel motor?

The right reel was usually the takeup. It had 120 vac applied through
a power resistor, to give a gentle takeup torque. The left reel motor,
the feed side, had a little DC applied to make some viscous drag. In
rewind, the roles were reversed, full AC on the left and no drag on
the right.
Stopping, specially during rewind, was tricky. A really good deck
would seldom dump a reel of tape on the floor.
Tape speed was controlled by the capstain, with its synchronous motor.
The takeup reel motor was then forced to run at a tiny fraction is its
native speed, basically stalled.
This was a studio-grade recorder. Cheaper decks had one
non-synchronous motor that did everything.

Very clear explanation, John; many thanks indeed for that.
The two main decks I'd like to get working again are both Ferrograph
ones, so basically top the range of non-studio decks. Built like
tanks. I'm guessing each must weigh 120lbs! They had issues at the
time with rubber components disintegrating. Kind of damaged their
reputation by the end of the 1970s. A real shame, as they were
exceptionally high quality in every other respect.

Jeff Layman

2024-02-19 07:57:50 UTC

Post by Cursitor Doom
The two main decks I'd like to get working again are both Ferrograph
ones, so basically top the range of non-studio decks. Built like
tanks. I'm guessing each must weigh 120lbs! They had issues at the
time with rubber components disintegrating. Kind of damaged their
reputation by the end of the 1970s. A real shame, as they were
exceptionally high quality in every other respect.

Nothing to do with what you're after, but you might find something of
eventual use here:
<http://ukhhsoc.torrens.org/makers/Ferrograph/TapeRecorders/index.html>

--
Jeff

John Larkin

2024-02-19 00:34:47 UTC

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

I saw an article somewhere that suggested one can make replacement
drive belts from strips of back-to-back Scotch tape.
The old Ampex and such tape recorders usually used AC motors.

One of the other tape recorders I have used a 240V motor. Not sure
whether AC or DC motors are more suited to PWM control.

Actually, my first job was a summer thing, in a physics lab
researching Stark-effect microwave spectroscopy. I designed and built
a couple of high-voltage square wave generators. That was the first
time I saw a Tektronix scope, which was the most beautiful thing I'd
ever seen, until I discovered girls.

Post by John Larkin
AC motors don't PWM well. DC motors don't speed control well, without
some sort of feedback loop. A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

Thanks, John; very interesting. What do you mean by takeup reel motor
ran stalled? And what is it that controls the speed of the tape - the
capstan/pinch-wheel motor or the relevant reel motor?

The right reel was usually the takeup. It had 120 vac applied through
a power resistor, to give a gentle takeup torque. The left reel motor,
the feed side, had a little DC applied to make some viscous drag. In
rewind, the roles were reversed, full AC on the left and no drag on
the right.

Stopping, specially during rewind, was tricky. A really good deck
would seldom dump a reel of tape on the floor.

Tape speed was controlled by the capstain, with its synchronous motor.
The takeup reel motor was then forced to run at a tiny fraction is its
native speed, basically stalled.

This was a studio-grade recorder. Cheaper decks had one
non-synchronous motor that did everything.

Robert Roland

2024-02-19 13:41:50 UTC

Post by John Larkin
A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

A BLDC motor does not really need a tach feedback. The speed
controller performs the commutation, so it already knows how fast the
motor is spinning.

Some R/C hobby BLDC controllers have a governor mode, where they keep
the motor speed constant regardless of torque.

The electronics is not complicated at all. It is essentially a
microcontroller and six MOSFETs.

--
RoRo

Anthony William Sloman

2024-02-19 15:51:18 UTC

Post by John Larkin
A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

A BLDC motor does not really need a tach feedback. The speed
controller performs the commutation, so it already knows how fast the
motor is spinning.

A brushless DC motor has already got built-in shaft position sensing, so a tachometer is redundant.

If you want to control. torque as well as rotational speed, you do need to know where the rotating magnets are with respect to the static coils, and a brushless DC motor won't necessarily let you get at that information.

A tachometer just tells you how faster the motor shaft is spinning, which may not be all that you need to know.

Post by Robert Roland
Some R/C hobby BLDC controllers have a governor mode, where they keep
the motor speed constant regardless of torque.
The electronics is not complicated at all. It is essentially a microcontroller and six MOSFETs.

And the hard-ware that senses where the rotating magnets are with respect to the frame, which can be pretty simple too.

--
Bill Sloman, Sydney

Anthony William Sloman

2024-02-20 03:06:10 UTC

Post by John Larkin
A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

A BLDC motor does not really need a tach feedback. The speed
controller performs the commutation, so it already knows how fast the
motor is spinning.

Most have speed that's about linearly dependent on supply voltage.
Speed droops with load. Ears are pretty sensitive to wow and flutter.

Brushless DC motors are AC motors with build-in sensors and electronics that replace the commutator in a DC motor.

The electronics can be as complicated as you are prepared to pay for, and if you throw in a stable crystal oscillator that can be used to determine the motor speed, independent of the supply voltage (if the supply voltage is high enough).

Yeah, microcontrollers are simple.

From a printed circuit layout or a bill of materials point of view.

--
Bill Sloman, Sydney

John Larkin

2024-02-20 15:39:31 UTC

Post by John Larkin
A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

A BLDC motor does not really need a tach feedback. The speed
controller performs the commutation, so it already knows how fast the
motor is spinning.

The usual BLDC doesn't speed regulate. Speed depends on the DC supply
voltage and the loading. Basically, it tries as hard as it can.

Anthony William Sloman

2024-02-20 16:06:54 UTC

Post by John Larkin
A BLDC motor with tach could make a very
nice capstain drive, with a lot of electronics.

A BLDC motor does not really need a tach feedback. The speed
controller performs the commutation, so it already knows how fast the
motor is spinning.

The usual BLDC doesn't speed regulate. Speed depends on the DC supply
voltage and the loading. Basically, it tries as hard as it can.

The "usual" brushless DC motor might not regulate speed, but it has got all the hardware built-in that would let it do that. If the application doesn't call for speed regulation the customer won't be offered the option, but that's just a commercial choice.

--
Bill Sloman, Sydney

Robert Roland

2024-02-20 18:47:42 UTC

Post by John Larkin
The usual BLDC doesn't speed regulate. Speed depends on the DC supply
voltage and the loading. Basically, it tries as hard as it can.

That's correct. But some speed controllers also support a constant
speed mode, also called governor mode. Some modellers prefer this with
helicopters, as it helps keep the rotor speed constant.

Here's one example:

https://hobbyking.com/en_us/yep-120a-hv-4-14s-brushless-speed-controller-opto.html

--
RoRo

DJ Delorie

2024-02-20 20:55:42 UTC

Post by John Larkin
The usual BLDC doesn't speed regulate. Speed depends on the DC supply
voltage and the loading. Basically, it tries as hard as it can.

That's correct. But some speed controllers also support a constant
speed mode,

And when you get to industrial BLDC motors, you add smarter controllers
and position feedback, and you can control pretty much anything wrt that
motor - speed, torque, position, acceleration, etc. Yup, model
helicopters and big CNC machines use the same type of motors :-)

Cursitor Doom

2024-02-20 23:09:10 UTC

Post by John Larkin
The usual BLDC doesn't speed regulate. Speed depends on the DC supply
voltage and the loading. Basically, it tries as hard as it can.

That's correct. But some speed controllers also support a constant
speed mode,

Small fan-type BLDC motors often have 4 pins: V+, ground, PWM in, and
tach out. The PWM input controls speed, not very accurately, from zero
to max.

Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

john larkin

2024-02-20 23:50:28 UTC

Post by John Larkin
The usual BLDC doesn't speed regulate. Speed depends on the DC supply
voltage and the loading. Basically, it tries as hard as it can.

That's correct. But some speed controllers also support a constant
speed mode,

Small fan-type BLDC motors often have 4 pins: V+, ground, PWM in, and
tach out. The PWM input controls speed, not very accurately, from zero
to max.

Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

AC synchronous. Old clocks use them.

Or a stepper. Or a "torque motor"

Dave Platt

2024-02-21 00:16:02 UTC

Small fan-type BLDC motors often have 4 pins: V+, ground, PWM in, and
tach out. The PWM input controls speed, not very accurately, from zero
to max.

Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

Yes. Some brushless DC motors have integral controllers which are
designed with this in mind. Since the controller for a BLDC motor has
to be aware of the rotor position (in order to commutate the phases at
the right times) it has the information it needs to control the speed.

There's one such in my LP turntable, for example... it has a couple of
speed-adjust pots. Once set properly, it keeps the platter rotating
at a stable 33 1/3 RPM, despite variations in the torque required to
overcome drag (from the stylus, record brush, etc.).

The simpler BLDC motor controllers simply hard-switch the supplied DC
voltage to the coils at the proper times - for these motors, the speed
depends on the supplied DC voltage.

The more sophisticated controllers will PWM the supplied DC to the
coils. As I understand it, the timing of the switching between coils
depends on the rotor position, while the PWM duty cycle (and thus the
average voltage applied) is altered to control the speed.

Three Jeeps

2024-02-21 01:01:45 UTC

Post by John Larkin
The usual BLDC doesn't speed regulate. Speed depends on the DC supply
voltage and the loading. Basically, it tries as hard as it can.

That's correct. But some speed controllers also support a constant
speed mode,

Small fan-type BLDC motors often have 4 pins: V+, ground, PWM in, and
tach out. The PWM input controls speed, not very accurately, from zero
to max.

Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

The AC synchronous motor is probably the typical example. However there are some situations that make it hard to control, such as starting and exceeding slip spec. (Not the best choice for use in a R-R IMHO

A stepper motor can do this but there are challenges in other areas as well, they can miss steps, and starting speed vs torque control.
Any book on electric machinery will provide a good understanding of motor types, classifications, and performance curves.
In general, If you want good controllability (speed, position) a DC motor driven by power amplifier will do well....dig out your books on classical control theory and modern (state space) control for understanding on how to design various position, speed controllers/regulators. Central to this is understanding static and dynamic loads the system will experience along with motor mechanics (e.g. inertia, etc). IF you really want to redesign the R-R motor system, you should get matlab and simulink and the control theory tool box and have at it. Sure beats the bad old days of hand drawing bode plots and gain phase plots and overlaying the effects of compensation networks....

If you have never implemented discretized PID, filters, and other control approaches, you will have fun discovering things like integral windup, digital limiting, and correctly implementing digital filters and precise periodic sampling. These issues and solutions have been well documented, but if you never have done this before, or know where to look, you are in for a wonderful learning experience.
You just might want to get new belts and clean up the motors for the R-R deck....

Robert Roland

2024-02-21 13:52:19 UTC

Post by Cursitor Doom
Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

That would be called a synchronous motor. A BLDC motor is actually a
synchronous motor. If it gets blindly commutated at a certain speed,
it will rotate at that speed (but it will be inefficient). It is
possible to abuse a BLDC motor as a stepper motor. If you apply
current to one of its windings, the rotor will snap into one position
and hold that position.

In order to optimize efficiency, the controller needs to know when to
commutate. Hobby controllers are available in two types, sensorless
and sensored. Sensorless systems need almost no additional hardware
for the feedback. They simply measure the EMF produced by the rotating
magnets.

--
RoRo

Lasse Langwadt Christensen

2024-02-21 14:30:12 UTC

Post by Cursitor Doom
Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

That would be called a synchronous motor. A BLDC motor is actually a
synchronous motor. If it gets blindly commutated at a certain speed,
it will rotate at that speed (but it will be inefficient).

only at very slow speeds, else it'll just vibrate

Anthony William Sloman

2024-02-21 16:30:24 UTC

Post by Cursitor Doom
Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

That would be called a synchronous motor. A BLDC motor is actually a
synchronous motor. If it gets blindly commutated at a certain speed,
it will rotate at that speed (but it will be inefficient).

only at very slow speeds, else it'll just vibrate.

You do need to have some kind of sensor for rotation. If you have got that - no matter how crude - you can feed an acceleration sequence in to the motor coils and spin it up gradually enough that you can be pretty confident is will end up rotating at the synchronous speed.

At EMI Central Research we put together a linear scan driven by a pin on a loop of chain, and a stepper motor drove the gear that moved the chain around its loop.

If the magnet at the pin didn't got past the Hall effect sensor when it should, the motor had been stalled, and the state machine read the 16 step acceleration sequence out of PROM to get the system going again. It was quick and reliable - we did four scans a second - and people would lean on the system to stop it to watch it recover.

--
Bill Sloman, Sydney

Cursitor Doom

2024-02-22 00:40:01 UTC

Post by Cursitor Doom
Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

That would be called a synchronous motor. A BLDC motor is actually a
synchronous motor. If it gets blindly commutated at a certain speed,
it will rotate at that speed (but it will be inefficient). It is
possible to abuse a BLDC motor as a stepper motor. If you apply
current to one of its windings, the rotor will snap into one position
and hold that position.
In order to optimize efficiency, the controller needs to know when to
commutate. Hobby controllers are available in two types, sensorless
and sensored. Sensorless systems need almost no additional hardware
for the feedback. They simply measure the EMF produced by the rotating
magnets.

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

Cursitor Doom

2024-02-22 09:31:27 UTC

Post by Cursitor Doom
Hence the need for feedback. I wonder if there's a motor that can spin
at a given speed accurately without f/back?

That would be called a synchronous motor. A BLDC motor is actually a
synchronous motor. If it gets blindly commutated at a certain speed,
it will rotate at that speed (but it will be inefficient). It is
possible to abuse a BLDC motor as a stepper motor. If you apply
current to one of its windings, the rotor will snap into one position
and hold that position.
In order to optimize efficiency, the controller needs to know when to
commutate. Hobby controllers are available in two types, sensorless
and sensored. Sensorless systems need almost no additional hardware
for the feedback. They simply measure the EMF produced by the rotating
magnets.

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

The obvious answer is a stepper motor (synchronous motor) with a
crystal-controlled frequency drive.
It will work better if the controller can generate acceleration and
deceleration sequences to make slow and smooth changes in rotational
speed - the spools of tape have rotational intertia and you can't change
their speed of rotation all that quickly.

That *is* something I'm concerned could spoil the party with the
simpler solutions proposed here. Not sure if it'll make much
difference in practice, but we'll find out empirically I guess.

Once you have got it up to speed, the rotational frequency will be as
stable as your crystal clock. There will some phase lag between the
drive waveform and the position of the rotor - it creates the torque
that counteracts the friction losses, but that should be pretty stable.
You do need some kind of stall detector to accelerate the motor up to
speed again after some ham-fisted user has stopped it's rotation.

Robert Roland

2024-02-25 13:34:42 UTC

Post by Robert Roland
Hobby controllers are available in two types, sensorless
and sensored.

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

The most important difference between the two systems, is starting
torque.

Since the sensorless systems use the moving magnets to determine
commutation timing, the motor must be spinning in order to commutate.
Of course, it needs commutation in order to spin, so you essentially
have a catch-22 situation.

There are different strategies to overcome the startup problem. The
simplest one is to simply commutate "blindly" at low current and see
if any timing signals show up. There are more sophisticated methods,
but common to them all is that they provide very low torque at zero
speed. For propellers or helicopter rotors, this is not a problem, so
sensorless systems are used. For cars, however, starting torque is
important, so sensored systems are used.

The sensors are simply a few Hall effect sensors. There is no need for
any shaft encoders. In hobby products, the sensors are built in to the
motor at the factory, so the end user simply sees a few extra wires
that need to be connected to the controller.

--
RoRo

Bill Sloman

2024-02-25 14:57:01 UTC

Post by Robert Roland
Hobby controllers are available in two types, sensorless
and sensored.

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

The most important difference between the two systems, is starting
torque.
Since the sensorless systems use the moving magnets to determine
commutation timing, the motor must be spinning in order to commutate.
Of course, it needs commutation in order to spin, so you essentially
have a catch-22 situation.
There are different strategies to overcome the startup problem. The
simplest one is to simply commutate "blindly" at low current and see
if any timing signals show up. There are more sophisticated methods,
but common to them all is that they provide very low torque at zero
speed. For propellers or helicopter rotors, this is not a problem, so
sensorless systems are used. For cars, however, starting torque is
important, so sensored systems are used.

Stepper motors always provide the same torque when they step slowly at
any speed - as long as the current through coil can get up to the
tolerable peak, you will get the same torque.

If the magnetic field lines up with position of the rotor, you won't get
any torque, so the strategy is to start by stepping the magnetic field
slowly enough that rotor can follow the rotating magnetic field, which
gets rid of any initial stiction. At low step rates the rotor can
oscillate around the zero torque position, and you have to avoid steps
rates that match that oscillation frequency. Once you have got the rotor
moving slowly, you know where it is and you can start your acceleration
sequence.

Post by Robert Roland
The sensors are simply a few Hall effect sensors. There is no need for
any shaft encoders. In hobby products, the sensors are built in to the
motor at the factory, so the end user simply sees a few extra wires
that need to be connected to the controller.

The Hall sensors are shaft encoders - the rotor is bonded to the shaft,
and magnets in the rotor are what you are detecting.

The end user may see them as a few extra wires, but sophisticated users
will see them for what they are.

Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

--
Bill Sloman, Sydney

Cursitor Doom

2024-02-25 17:46:41 UTC

Post by Robert Roland
Hobby controllers are available in two types, sensorless
and sensored.

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

The most important difference between the two systems, is starting
torque.
Since the sensorless systems use the moving magnets to determine
commutation timing, the motor must be spinning in order to commutate.
Of course, it needs commutation in order to spin, so you essentially
have a catch-22 situation.
There are different strategies to overcome the startup problem. The
simplest one is to simply commutate "blindly" at low current and see
if any timing signals show up. There are more sophisticated methods,
but common to them all is that they provide very low torque at zero
speed. For propellers or helicopter rotors, this is not a problem, so
sensorless systems are used. For cars, however, starting torque is
important, so sensored systems are used.

Stepper motors always provide the same torque when they step slowly at
any speed - as long as the current through coil can get up to the
tolerable peak, you will get the same torque.
If the magnetic field lines up with position of the rotor, you won't get
any torque, so the strategy is to start by stepping the magnetic field
slowly enough that rotor can follow the rotating magnetic field, which
gets rid of any initial stiction. At low step rates the rotor can
oscillate around the zero torque position, and you have to avoid steps
rates that match that oscillation frequency. Once you have got the rotor
moving slowly, you know where it is and you can start your acceleration
sequence.

The Hall sensors are shaft encoders - the rotor is bonded to the shaft,
and magnets in the rotor are what you are detecting.
The end user may see them as a few extra wires, but sophisticated users
will see them for what they are.
Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

Up until that last paragraph I was just about to commend you on being
more like the old Bill Sloman who posted helpful advice here back in
the day. You just can't resist throwing barbs, can you? Sigh...

John Larkin

2024-02-25 18:13:45 UTC

Post by Robert Roland
Hobby controllers are available in two types, sensorless
and sensored.

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

The most important difference between the two systems, is starting
torque.
Since the sensorless systems use the moving magnets to determine
commutation timing, the motor must be spinning in order to commutate.
Of course, it needs commutation in order to spin, so you essentially
have a catch-22 situation.
There are different strategies to overcome the startup problem. The
simplest one is to simply commutate "blindly" at low current and see
if any timing signals show up. There are more sophisticated methods,
but common to them all is that they provide very low torque at zero
speed. For propellers or helicopter rotors, this is not a problem, so
sensorless systems are used. For cars, however, starting torque is
important, so sensored systems are used.

Stepper motors always provide the same torque when they step slowly at
any speed - as long as the current through coil can get up to the
tolerable peak, you will get the same torque.
If the magnetic field lines up with position of the rotor, you won't get
any torque, so the strategy is to start by stepping the magnetic field
slowly enough that rotor can follow the rotating magnetic field, which
gets rid of any initial stiction. At low step rates the rotor can
oscillate around the zero torque position, and you have to avoid steps
rates that match that oscillation frequency. Once you have got the rotor
moving slowly, you know where it is and you can start your acceleration
sequence.

The Hall sensors are shaft encoders - the rotor is bonded to the shaft,
and magnets in the rotor are what you are detecting.
The end user may see them as a few extra wires, but sophisticated users
will see them for what they are.
Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

Sloman's real enemy is Sloman.

Cursitor Doom

2024-02-25 18:52:41 UTC

Post by Robert Roland
Hobby controllers are available in two types, sensorless
and sensored.

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

The most important difference between the two systems, is starting
torque.
Since the sensorless systems use the moving magnets to determine
commutation timing, the motor must be spinning in order to commutate.
Of course, it needs commutation in order to spin, so you essentially
have a catch-22 situation.
There are different strategies to overcome the startup problem. The
simplest one is to simply commutate "blindly" at low current and see
if any timing signals show up. There are more sophisticated methods,
but common to them all is that they provide very low torque at zero
speed. For propellers or helicopter rotors, this is not a problem, so
sensorless systems are used. For cars, however, starting torque is
important, so sensored systems are used.

Stepper motors always provide the same torque when they step slowly at
any speed - as long as the current through coil can get up to the
tolerable peak, you will get the same torque.
If the magnetic field lines up with position of the rotor, you won't get
any torque, so the strategy is to start by stepping the magnetic field
slowly enough that rotor can follow the rotating magnetic field, which
gets rid of any initial stiction. At low step rates the rotor can
oscillate around the zero torque position, and you have to avoid steps
rates that match that oscillation frequency. Once you have got the rotor
moving slowly, you know where it is and you can start your acceleration
sequence.

The Hall sensors are shaft encoders - the rotor is bonded to the shaft,
and magnets in the rotor are what you are detecting.
The end user may see them as a few extra wires, but sophisticated users
will see them for what they are.
Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

Sloman's real enemy is Sloman.

His only supporter here was 3rdWit, who it transpired was just a
sock-puppet. I'm afraid Bill's become something of a sad and rather
tragic figure here in recent years.

John Larkin

2024-02-25 19:09:26 UTC

Post by Robert Roland
Hobby controllers are available in two types, sensorless
and sensored.

Thanks. I'm just trying to work out which type would be most suited to
the role of a capstan roller motor to use at 3 fixed speeds (after
gearing down if necessary).

The most important difference between the two systems, is starting
torque.
Since the sensorless systems use the moving magnets to determine
commutation timing, the motor must be spinning in order to commutate.
Of course, it needs commutation in order to spin, so you essentially
have a catch-22 situation.
There are different strategies to overcome the startup problem. The
simplest one is to simply commutate "blindly" at low current and see
if any timing signals show up. There are more sophisticated methods,
but common to them all is that they provide very low torque at zero
speed. For propellers or helicopter rotors, this is not a problem, so
sensorless systems are used. For cars, however, starting torque is
important, so sensored systems are used.

Stepper motors always provide the same torque when they step slowly at
any speed - as long as the current through coil can get up to the
tolerable peak, you will get the same torque.
If the magnetic field lines up with position of the rotor, you won't get
any torque, so the strategy is to start by stepping the magnetic field
slowly enough that rotor can follow the rotating magnetic field, which
gets rid of any initial stiction. At low step rates the rotor can
oscillate around the zero torque position, and you have to avoid steps
rates that match that oscillation frequency. Once you have got the rotor
moving slowly, you know where it is and you can start your acceleration
sequence.

The Hall sensors are shaft encoders - the rotor is bonded to the shaft,
and magnets in the rotor are what you are detecting.
The end user may see them as a few extra wires, but sophisticated users
will see them for what they are.
Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

Sloman's real enemy is Sloman.

His only supporter here was 3rdWit, who it transpired was just a
sock-puppet. I'm afraid Bill's become something of a sad and rather
tragic figure here in recent years.

Neither supporters nor enemies make sense in an electronic design
forum. This ain't social media.

Bill Sloman

2024-02-26 05:04:37 UTC

<snip>

Post by Bill Sloman
Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

Sloman's real enemy is Sloman.

His only supporter here was 3rdWit, who it transpired was just a
sock-puppet.

Only in Cursitor Doom's imaginary universe.

Post by Cursitor Doom
I'm afraid Bill's become something of a sad and rather
tragic figure here in recent years.

Neither supporters nor enemies make sense in an electronic design
forum. This ain't social media.

It's individuals interacting, which makes it a social medium.

When the individual are as flawed as Cursitor Doom and John Larkin, some
of the interactions can get downright ugly, and they resent being called
to account.

--
Bill Sloman, Sydney (not perfect, but closer to it than some I could name).

Bill Sloman

2024-02-26 04:57:01 UTC

Post by Bill Sloman
Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

Sloman's real enemy is Sloman.

His only supporter here was 3rdWit, who it transpired was just a
sock-puppet.

Only in Cursitor Doom's imaginary universe.

Post by Cursitor Doom
I'm afraid Bill's become something of a sad and rather tragic figure here in recent years.

Only in Cursitor Doom's imaginary universe. Cursitor Doom is fond of his
fatuous and implausible delusions. John Larkin's delusion that he
actually designs his electronic circuits is the same kind of
self-serving error.

--
Bill Sloman, Sydney

Bill Sloman

2024-02-26 04:51:09 UTC

<snip>

The Hall sensors are shaft encoders - the rotor is bonded to the shaft,
and magnets in the rotor are what you are detecting.
The end user may see them as a few extra wires, but sophisticated users
will see them for what they are.
Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

Sloman's real enemy is Sloman.

John Larkin divides the world into people who praise him as he feels he
deserves to be praised, and the rest, who are his enemies.

He'd be a lot more successful if he had a more realistic idea of the
limits of his capabilities, so we aren't actually his enemies, even if
he likes to think we are.

An enemy is somebody who tries to damage you. Vanity is a vice, and
feeding somebody's vanity is what an enemy would do.

--
Bill Sloman, Sydney

Bill Sloman

2024-02-26 04:42:50 UTC

<snip>

The Hall sensors are shaft encoders - the rotor is bonded to the shaft,
and magnets in the rotor are what you are detecting.
The end user may see them as a few extra wires, but sophisticated users
will see them for what they are.
Cursitor Doom isn't a sophisticated user, but if he is posting here we
need to treat him as if he could acquire some sophistication.

It's a moral obligation, in your case. Your undiscriminating enthusiasm
for pro-Putin, pro-Trump and pro-climate change denial propaganda means
that I do need to remind people that you shouldn't be taken seriously.

I do try to educate you, but you don't seem to want to learn.

--
Bill Sloman, Sydney

Three Jeeps

2024-02-19 20:06:16 UTC

Post by Cursitor Doom
On Sun, 18 Feb 2024 15:34:52 -0500, Ralph Mowery

When I retired about 10 years ago we had many motor speed controls that
worked very well ranging from less than 1 HP to 300 HP. Some were for
DC motors and some were for 3 phase AC motors.
Were you thinking of AC or DC motors ?
The AC motor speed controlers used a very odd waveform and sometimes the
motors would produce a sound of say 1000 Hz in frequency. The speed
controlers were microprocessed based and we had to set several
parameters depending on the motor and type of service.
They have probably gotten better in the last 10 years.

I suggest you try these websites for replacement belts:

fixyouraudio.com
webspareparts.com
https://www.thevoiceofmusic.com/
https://www.vintage-electronics.net/
thakker.eu

On occasion, I got a belt size that was close but slightly oversized and cut it to correct size and used superglue to join the two ends
I've also got small rubber belt material from (I can't remember the place), cut it to size and joined with superglue.

Using the above resources or making my own belts, I've repaired some vintage late 1970s JVC cassette decks (KD85, KD-A7), and two prize R2R's Akai GX77 and Pioneer RT-707.

Still looking for that 'affordable' Nak Dragon....direct drive.....

good luck

Anthony William Sloman

2024-02-19 00:44:33 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pulleys approach? Would simple PWM be enough or
would there be some additional trickery needed?

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.

It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

Brushless motors work that way, but hide the details from the customer, Classical DC motors rely on the commutator for phase control. and commutators eventually wear out.

--
Bill Sloman, Sydney

Carlos E.R.

2024-02-19 12:21:18 UTC

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

Post by Anthony William Sloman
Brushless motors work that way, but hide the details from the customer, Classical DC motors rely on the commutator for phase control. and commutators eventually wear out.

--
Cheers, Carlos.

Anthony William Sloman

2024-02-19 13:10:54 UTC

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

You do have to keep pulse width modulation fast, and filter the waveforms to keep the higher frequency components out of the motor (where they can warm up the
magnetic elements).

https://iopscience.iop.org/article/10.1088/0957-0233/7/11/015/meta

does go into that - I was driving a Peltier element rather than a motor, but was really intent on minimising high frequency interference.

At the time 200kHz was a nice chopping frequency to aim for. There are now faster MOSFets around and a couple of MHz is possible without excessive switching losses. At the time we used the same technique on motors for shifting samples around and a cheap transputer (which was fashionable at the time) produced all the fast waveforms needed for that. There are now some very fast simpler processors around

<snip>

--
Bill Sloman, Sydney

Cursitor Doom

2024-02-19 13:35:58 UTC

On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

John Larkin

2024-02-19 17:10:13 UTC

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.

A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

That could be a product, if there's enough market for fixing up old
tape decks. A Pi Pico could be the compute engine. Micro Python would
be fast enough.

The dynamics of handling tape are non-trivial. Transitioning between
play or rewind or fast foreward, to stop, is tricky and involves state
memory.

Cursitor Doom

2024-02-19 23:18:01 UTC

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.

Very true.

Post by John Larkin
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

It's another option - albeit perhaps a last one on grounds of
complexity.

Post by John Larkin
That could be a product, if there's enough market for fixing up old
tape decks. A Pi Pico could be the compute engine. Micro Python would
be fast enough.

Or maybe an Arduino.

Post by John Larkin
The dynamics of handling tape are non-trivial. Transitioning between
play or rewind or fast foreward, to stop, is tricky and involves state
memory.

I dunno what "state memory" is, but the rest of that paragraph had
already unhappily occurred to me.

That

John Larkin

2024-02-19 23:38:12 UTC

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.

Very true.

Post by John Larkin
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

It's another option - albeit perhaps a last one on grounds of
complexity.

Post by John Larkin
That could be a product, if there's enough market for fixing up old
tape decks. A Pi Pico could be the compute engine. Micro Python would
be fast enough.

Or maybe an Arduino.

Post by John Larkin
The dynamics of handling tape are non-trivial. Transitioning between
play or rewind or fast foreward, to stop, is tricky and involves state
memory.

I dunno what "state memory" is, but the rest of that paragraph had
already unhappily occurred to me.
That

It means that something has to remember what was going on before it
was told to stop. In old tape decks, the memory was something
mechanical, or part of the play/stop/foreward/rewind switch.

Pulling the line cord during rewind *would* throw tape all over the
place.

The student decks in the language lab got used all day, and abused, so
the switches failed a lot. Funny that I hadn't thought about that for
maybe 50 years but I remember it pretty well now.

A uP could do the motion control really well, better than mechanical
logic.

Anthony William Sloman

2024-02-20 03:12:38 UTC

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.

Very true.

Post by John Larkin
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

It's another option - albeit perhaps a last one on grounds of
complexity.

Post by John Larkin
That could be a product, if there's enough market for fixing up old
tape decks. A Pi Pico could be the compute engine. Micro Python would
be fast enough.

Or maybe an Arduino.

Post by John Larkin
The dynamics of handling tape are non-trivial. Transitioning between
play or rewind or fast forward, to stop, is tricky and involves state
memory.

I dunno what "state memory" is, but the rest of that paragraph had
already unhappily occurred to me.

What John Larkin presumably meant was that tap reels have intertia, and the software needs to keep track of the rotational energy stored in the reel and drive the motor in a way that removes that stored energy in a way that minimises the length of tape that might form a loose loop while it is going on.

--
Bill Sloman, Sydney

Lasse Langwadt Christensen

2024-02-19 23:31:47 UTC

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

I doubt it, a regular stepper will be noisy

John Larkin

2024-02-20 00:13:51 UTC

On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

I doubt it, a regular stepper will be noisy

Most any stepper motor can be microstepped, and make smooth quiet
motion. Drive the windings with sine/cosine waves instead of brutal
steps. You can do that in uP code: software DDS and a sin/cos lookup
table, into a couple of PWM blocks.

I did that ages ago with a 68332, with maybe 1% of the compute power
of a Raspberry Pi Pico.

John Larkin

2024-02-20 00:19:13 UTC

Post by John Larkin
On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

I doubt it, a regular stepper will be noisy

Most any stepper motor can be microstepped, and make smooth quiet
motion. Drive the windings with sine/cosine waves instead of brutal
steps. You can do that in uP code: software DDS and a sin/cos lookup
table, into a couple of PWM blocks.
I did that ages ago with a 68332, with maybe 1% of the compute power
of a Raspberry Pi Pico.

No, even worse, my quad microstepper used a 6803 cpu!

Lasse Langwadt Christensen

2024-02-20 00:32:22 UTC

Post by John Larkin
On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

I doubt it, a regular stepper will be noisy

sure, and almost all steppers are now driven like that and there a plenty of
cheap integrated IC that does it all, but unless you run very slow they still make noise

Anthony William Sloman

2024-02-20 03:21:21 UTC

<snip>

Post by John Larkin
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

I doubt it, a regular stepper will be noisy.

sure, and almost all steppers are now driven like that and there a plenty of
cheap integrated IC that does it all, but unless you run very slow they still make noise.

How? Magnetostriction in the magnetic path? The static coils in the frame are applying force to the moving magnets in the rotor, so they will move around a bit - more if the mechanical design didn't spend enough money on keeping them still and keeping any structural resonances well damped.

--
Bill Sloman, Sydney

John Larkin

2024-02-20 15:51:07 UTC

On Mon, 19 Feb 2024 16:32:22 -0800 (PST), Lasse Langwadt Christensen

Post by John Larkin
On Mon, 19 Feb 2024 15:31:47 -0800 (PST), Lasse Langwadt Christensen

Post by Cursitor Doom
On Mon, 19 Feb 2024 13:21:18 +0100, "Carlos E.R."

If you want to control the speed and torque of a motor, chose a three phase or five phase synchronous motor, monitor where the rotor is with respect to where you want it to be, and control the phase and current through each winding to generate the torque you want. Fast pulse width modulation - quite a lot faster than the AC frequencies s being fed into the motor - will let you do that pretty precisely.
It's complicated but not all that expensive, unless the motors are big - and tape recorders don't use big motors.

There might be a problem if this causes mechanical vibration in the
motor (maybe be audible). This vibration might affect the tape speed and
be audible in the result.

In these machines they use a heavy flywheel on the end of the capstan
roller, so that shouldn't be an issue.

If there's a belt, that will further lowpass filter angular vibration.
A microstepper would be a great capstain driver, but needs drive
logic, a uP with PWM blocks maybe. And a bunch of code.

I doubt it, a regular stepper will be noisy

sure, and almost all steppers are now driven like that and there a plenty of
cheap integrated IC that does it all, but unless you run very slow they still make noise

Less than an AC motor and generally inaudible.

With more effort, a non-sinusoidal lookup table can tune the drive
waveforms for even higher angular precision and less noise,
compensating for imperfect tooth profiles. But that's over the top.

Fred Bloggs

2024-02-19 19:03:07 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pullys approach? Would simple PWM be enough or
would there be some additional trickery needed?

The only kind of motor control of any importance:

https://www.powerelectronicsnews.com/overview-of-motor-control-in-electric-vehicles/

Post by Cursitor Doom
CD.

Cursitor Doom

2024-02-20 12:27:10 UTC

Can I just get some clarification on one point here. The two spools
are not speed controlled as such and just spin or drag (as the case
may be) at the same speed regardless of the tape speed selected? So
it's only the capstan motor that needs precise control speed? That
seems to be implication of what's been posted here so far and it would
make things much simpler if there was only one motor's speed to worry
about.

Cursitor Doom

2024-02-20 12:39:34 UTC

Post by Cursitor Doom
Can I just get some clarification on one point here. The two spools
are not speed controlled as such and just spin or drag (as the case
may be) at the same speed regardless of the tape speed selected? So
it's only the capstan motor that needs precise control speed? That
seems to be implication of what's been posted here so far and it would
make things much simpler if there was only one motor's speed to worry
about.

When the machine is in "play" or "record" mode, I mean; not during FF
or rewind.

Jan Panteltje

2024-02-20 13:34:11 UTC

On a sunny day (Tue, 20 Feb 2024 12:39:34 +0000) it happened Cursitor Doom

Indeed

Post by Cursitor Doom
When the machine is in "play" or "record" mode, I mean; not during FF
or rewind.

Yes

DJ Delorie

2024-02-20 14:52:13 UTC

The speed of the tape depends not only on the spool RPM but also how
much tape is present, since that changes the effective diameter. If the
spools have different amounts of tape on them (normal) they'll have to
move at different RPMs to have the same linear tape speed.

Cursitor Doom

2024-02-20 15:41:15 UTC

If I understand this correctly, one doesn't need to worry about that
aspect, because it will 'just happen automatically' on play and
record.

DJ Delorie

2024-02-20 16:56:48 UTC

Post by DJ Delorie
The speed of the tape depends not only on the spool RPM but also how
much tape is present, since that changes the effective diameter. If the
spools have different amounts of tape on them (normal) they'll have to
move at different RPMs to have the same linear tape speed.

If I understand this correctly, one doesn't need to worry about that
aspect, because it will 'just happen automatically' on play and
record.

Ah, I misunderstood the question. The reels can be run at a "constant
speed" which is known to be slower[*]/faster enough than the capstain, but
with sufficiently low torque that the capstain can override them.

[*] or unpowered, using only drag

I would consider this to be run at constant *torque* mode, not constant
*speed* mode, since you don't care how fast the spools are moving, just
how much drag or tension they're creating for the tape.

If you *forced* the reels to run at a set speed, the tape would break.

Post by Cursitor Doom
The capstain determines the tape speed. The takeup reel motor just
applies a gentle torque and the feed reel motor a bit of drag. The
reel speeds are controlled by the tape speed, namely the capstain.

Well yeah, I knew that. I was oversimplifying.

John Larkin

2024-02-20 15:56:35 UTC

The capstain determines the tape speed. The takeup reel motor just
applies a gentle torque and the feed reel motor a bit of drag. The
reel speeds are controlled by the tape speed, namely the capstain.

The tape could come out of a box on the floor and output to another
heap and it would work about the same.

Anthony William Sloman

2024-02-20 16:16:50 UTC

The capstain determines the tape speed. The takeup reel motor just
applies a gentle torque and the feed reel motor a bit of drag. The
reel speeds are controlled by the tape speed, namely the capstain.
The tape could come out of a box on the floor and output to another
heap and it would work about the same.

The IBM 7044 that I used (and operated from time to time) when I was a graduate student relied on magnetic tape for mass storage.

The tape drives fed the tape into a deep trough on one side of the reader before it went into the reading head, and had a second trough on the other side to take up the tape after it had been read. Air was pumped into both troughs to keep two fairly long loops under minimal tension. If you were operating the machine at four in the morning there wasn't a lot other stuff going on to attract your attention.

--
Bill Sloman, Sydney

DJ Delorie

2024-02-20 17:06:45 UTC

Post by Anthony William Sloman
The tape drives fed the tape into a deep trough on one side of the
reader before it went into the reading head, and had a second trough
on the other side to take up the tape after it had been read. Air was
pumped into both troughs to keep two fairly long loops under minimal
tension. If you were operating the machine at four in the morning
there wasn't a lot other stuff going on to attract your attention.

I remember the Data General tape drives like that. Those were vacuum
driven instead - the tape was sucked into the trough, past a column of
tiny holes that measured the vacuum - and thus the tape position - so
that the reels knew when to spin. Or so the operator explained to me.
From my point of view it was just magic.

John Larkin

2024-02-20 16:11:44 UTC

I don't recall any decks that changed the reel motor drives at
different capstain speeds. Certainly none that were aware of the
amount of tape on each reel.

Big sci-fi movie type computer tape drives used air columns to buffer
the reels. That reduced the effective inertia of the reels to about
zero. The capstains could start/stop every record, ballpark an inch of
tape, as needed.

I designed a tape controller, Ampex 9-track to PDP-11. I'd forgotten
all about that until this thread.

Liz Tuddenham

2024-02-20 17:32:51 UTC

I don't recall any decks that changed the reel motor drives at
different capstain speeds. Certainly none that were aware of the
amount of tape on each reel.

I have a vague recollection that there was a machine that boosted the
takeup motor temporarily at high tape speeds to reduce 'billowing'
during startup -- but I can't remember which machine it was (EMI BTR2 or
Marconi-Stille ??). The Ferrograph Series 7 had two torque settings to
deal with small and large-hub reels - but it didn't seem to make much
difference.

The Collaro 'pushmi-pullyu' deck had a constant speed spool motor with
variable friction drive to the spool hubs controlled by tension arms.
The spool motor and the capstan motors were identical (apart from
diection of rotations) and swapped functions when the deck went into
reverse.

Grundig 'Stenorette' dictating machines had a constant rotational speed
spool permanently built into the machine, and no capstan. The tape
cassette had a loose end hanging out with a loop which you hooked around
a pillar on the drive spool and the tape gradually sped up as it built
up on the spool hub. As the recordings had also been made on the same
type of machine, the pitch didn't vary on playback.

Wire recorders almost all used spool drive, but there was one which used
a capstan with the wire wrapped around it in a single turn with no
pressure roller.

--
~ Liz Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

Jan Panteltje

2024-02-21 06:38:15 UTC

On a sunny day (Tue, 20 Feb 2024 08:11:44 -0800) it happened John Larkin

I don't recall any decks that changed the reel motor drives at
different capstain speeds. Certainly none that were aware of the
amount of tape on each reel.
Big sci-fi movie type computer tape drives used air columns to buffer
the reels. That reduced the effective inertia of the reels to about
zero. The capstains could start/stop every record, ballpark an inch of
tape, as needed.
I designed a tape controller, Ampex 9-track to PDP-11. I'd forgotten
all about that until this thread.

The video quadruplex AVR1 from Ampex had the air-column buffers
it used photocells to see were the tape was in the air compartment
and a servo on the suppply reel motor to keep just enough tape in that air space
https://www.flickr.com/photos/watvhistory/3365534784/in/photostream/
note the holes in the capstan drive, no rubber wheel! it sucked the tape fixed...
The white thing top left is motor with a wheel with 4 rotating heads....
more pictures in that link, look a the electronics
https://www.flickr.com/photos/watvhistory/3365533536/in/photostream/
we had several of those, often not working when powered on in the morning
finaly it was decided to leave it on all the time,...
Used for video editing, a reel with tape was really heavy...
Had to keep it running, some factory mods were applied over time too IIRC.
vacuum system?
https://www.flickr.com/photos/watvhistory/3365534384/in/photostream/
Expensive machine...

Jasen Betts

2024-03-04 07:45:43 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pullys approach?

you mean like a centrifugal governor?

Post by Cursitor Doom
Would simple PWM be enough or would there be some additional trickery
needed?

PWM could. if you sample the back EMF during the off time of the PWM and feed
that back to the regulator... (or read the motor speed some other way,
you could have an interruptor typse sensor and control speed using a
PLL)

Oldschool when they weren't using centrifugal governors they would put
a compensating negative resistance in series with the motor and feed
the combination from a fixed DC voltage or fake that result.

--
Jasen.
🇺🇦 Слава Україні

Cursitor Doom

2024-03-04 23:06:05 UTC

On Mon, 4 Mar 2024 07:45:43 -0000 (UTC), Jasen Betts

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pullys approach?

you mean like a centrifugal governor?

Post by Cursitor Doom
Would simple PWM be enough or would there be some additional trickery
needed?

PWM could. if you sample the back EMF during the off time of the PWM and feed
that back to the regulator... (or read the motor speed some other way,
you could have an interruptor typse sensor and control speed using a
PLL)
Oldschool when they weren't using centrifugal governors they would put
a compensating negative resistance in series with the motor and feed
the combination from a fixed DC voltage or fake that result.

Thanks for your input, but this has been thrashed out by now. This
thread is stale.

Bill Sloman

2024-03-05 02:19:55 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pulleys approach?

you mean like a centrifugal governor?

Post by Cursitor Doom
Would simple PWM be enough or would there be some additional trickery
needed?

PWM could. if you sample the back EMF during the off time of the PWM and feed
that back to the regulator... (or read the motor speed some other way,
you could have an interruptor typse sensor and control speed using a
PLL)
Old school when they weren't using centrifugal governors they would put
a compensating negative resistance in series with the motor and feed
the combination from a fixed DC voltage or fake that result.

That's not all that "old school" - Philips got a patent on it around the
1970's. It wasn't remotely good enough for audio work, and neither were
centrifugal governors. Synchronous motors with stable frequency drives
was what the old school relied on

--
Bill Sloman, Sydney

KevinJ93

2024-03-05 18:57:18 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pulleys approach?

you mean like a centrifugal governor?

Post by Cursitor Doom
Would simple PWM be enough or would there be some additional trickery
needed?

PWM could. if you sample the back EMF during the off time of the PWM and feed
that back to the regulator... (or read the motor speed some other way,
you could have an interruptor typse sensor and control speed using a
PLL)
Old school when they weren't using centrifugal governors they would put
a compensating negative resistance in series with the motor and feed
the combination from a fixed DC voltage or fake that result.

Philips used the negative resistance approach for speed control in their
portable cassette players - so it wasn't too bad. Synchronous AC motors
weren't an option in a portable unit.

Other manufacturers did use centrifugal governors.

kw

Cursitor Doom

2024-03-05 23:11:49 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pulleys approach?

you mean like a centrifugal governor?

Post by Cursitor Doom
Would simple PWM be enough or would there be some additional trickery
needed?

PWM could. if you sample the back EMF during the off time of the PWM and feed
that back to the regulator... (or read the motor speed some other way,
you could have an interruptor typse sensor and control speed using a
PLL)
Old school when they weren't using centrifugal governors they would put
a compensating negative resistance in series with the motor and feed
the combination from a fixed DC voltage or fake that result.

So did steam engines.

Bill Sloman

2024-03-06 01:59:27 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pulleys approach?

you mean like a centrifugal governor?

Post by Cursitor Doom
Would simple PWM be enough or would there be some additional trickery
needed?

PWM could. if you sample the back EMF during the off time of the PWM and feed
that back to the regulator... (or read the motor speed some other way,
you could have an interruptor typse sensor and control speed using a
PLL)
Old school when they weren't using centrifugal governors they would put
a compensating negative resistance in series with the motor and feed
the combination from a fixed DC voltage or fake that result.

Philips used the negative resistance approach for speed control in their
portable cassette players - so it wasn't too bad. Synchronous AC motors
weren't an option in a portable unit.

They were. Electronic watches used them, with 32,768Hz watch crystal as
the frequency reference.

Post by KevinJ93
Other manufacturers did use centrifugal governors.

So did steam engines.

Which weren't sold on the basis of their frequency accuracy or the
absence of wow and flutter.

--
Bill Sloman, Sydney

Bill Sloman

2024-03-06 01:51:34 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pulleys approach?

you mean like a centrifugal governor?

Post by Cursitor Doom
Would simple PWM be enough or would there be some additional trickery
needed?

PWM could. if you sample the back EMF during the off time of the PWM and feed
that back to the regulator... (or read the motor speed some other way,
you could have an interruptor typse sensor and control speed using a
PLL)
Old school when they weren't using centrifugal governors they would put
a compensating negative resistance in series with the motor and feed
the combination from a fixed DC voltage or fake that result.

That's not all that "old school" - Philips got a patent on it around
the 1970's. It wasn't remotely good enough for audio work, and neither
were centrifugal governors. Synchronous motors with stable frequency
drives was what the old school relied on

Philips used the negative resistance approach for speed control in their
portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the permanent
magnets in the motor being regulated, and that is temperature dependent.
Philips may have relied on it, but it was still ghastly.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz watch
crystal as the frequency reference. Some of them included stepper motors
to drive a mechanical display.

Synchronous motors obviously are a practical option in a portable unit,
though perhaps not in a really cheap one.

Post by KevinJ93
Other manufacturers did use centrifugal governors.

Only at the very cheap and nasty end of the market.

--
Bill Sloman, Sydney

KJW93

2024-03-06 18:36:59 UTC

...

Post by Bill Sloman
That's not all that "old school" - Philips got a patent on it around
the 1970's. It wasn't remotely good enough for audio work, and
neither were centrifugal governors. Synchronous motors with stable
frequency drives was what the old school relied on

Philips used the negative resistance approach for speed control in
their portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the permanent
magnets in the motor being regulated, and that is temperature dependent.
Philips may have relied on it, but it was still ghastly.

Obviously Philips didn't agree with you. For a consumer product used
over a benign temperature range it was fine.

The temperature coefficient was low enough to keep the tape speed within
1% or so.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz watch
crystal as the frequency reference. Some of them included stepper motors
to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable unit,
though perhaps not in a really cheap one.

At the time these devices were first designed (mid-late 60's)
synchronous motors weren't a practical option for a consumer item.

...

kw

Bill Sloman

2024-03-07 04:05:23 UTC

...

Philips used the negative resistance approach for speed control in
their portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the permanent
magnets in the motor being regulated, and that is temperature
dependent. Philips may have relied on it, but it was still ghastly.

Obviously Philips didn't agree with you. For a consumer product used
over a benign temperature range it was fine.
The temperature coefficient was low enough to keep the tape speed within
1% or so.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz watch
crystal as the frequency reference. Some of them included stepper
motors to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable
unit, though perhaps not in a really cheap one.

At the time these devices were first designed (mid-late 60's)
synchronous motors weren't a practical option for a consumer item.

Back then they were called "stepper motors" and would have been entirely
practical. Admittedly, I didn't get to design one into what would have
been a cheap product until 1978 (and at EMI Central Research) but they
were pretty cheap.

--
Bill Sloman, Sydney

KevinJ93

2024-03-07 10:14:49 UTC

...

Post by Bill Sloman
That's not all that "old school" - Philips got a patent on it
around the 1970's. It wasn't remotely good enough for audio work,
and neither were centrifugal governors. Synchronous motors with
stable frequency drives was what the old school relied on

Philips used the negative resistance approach for speed control in
their portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the permanent
magnets in the motor being regulated, and that is temperature
dependent. Philips may have relied on it, but it was still ghastly.

Obviously Philips didn't agree with you. For a consumer product used
over a benign temperature range it was fine.
The temperature coefficient was low enough to keep the tape speed
within 1% or so.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz
watch crystal as the frequency reference. Some of them included
stepper motors to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable
unit, though perhaps not in a really cheap one.

At the time these devices were first designed (mid-late 60's)
synchronous motors weren't a practical option for a consumer item.

Stepper motors are much too inefficient and have too much torque ripple
for capstan drive - not at all suitable for a battery powered device,
they also tend to be noisy.

Even implementing the discrete drive electronics would be more costly
than necessary at a time where individual transistors were a significant
cost; Philips' solution used two transistors - creating a divide by 4
plus driver transistors plus an oscillator would probably require about
ten transistors plus numerous other components.

If stepper motors would be such a great solution how come nobody has had
your insight and used them in the past sixty years for tape drives?

The permanent magnet DC motor with negative resistance driver worked
perfectly well. It was low cost, used available technology, low power,
was quiet and met the design requirements.

kw

Bill Sloman

2024-03-07 14:07:41 UTC

...

Post by Bill Sloman
That's not all that "old school" - Philips got a patent on it
around the 1970's. It wasn't remotely good enough for audio work,
and neither were centrifugal governors. Synchronous motors with
stable frequency drives was what the old school relied on

Philips used the negative resistance approach for speed control in
their portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the
permanent magnets in the motor being regulated, and that is
temperature dependent. Philips may have relied on it, but it was
still ghastly.

Obviously Philips didn't agree with you. For a consumer product used
over a benign temperature range it was fine.
The temperature coefficient was low enough to keep the tape speed
within 1% or so.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz
watch crystal as the frequency reference. Some of them included
stepper motors to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable
unit, though perhaps not in a really cheap one.

At the time these devices were first designed (mid-late 60's)
synchronous motors weren't a practical option for a consumer item.

Back then they were called "stepper motors" and would have been
entirely practical. Admittedly, I didn't get to design one into what
would have been a cheap product until 1978 (and at EMI Central
Research) but they were pretty cheap.

Stepper motors are much too inefficient and have too much torque ripple
for capstan drive - not at all suitable for a battery powered device,
they also tend to be noisy.

Twaddle. A stepper motor is a synchronous motor, and if you are careful
how you drive it, it doesn't have any torque ripple, and it isn't any
less efficient than any other synchronous motor.

ESCAP did do a range of small stepper motors where a sine wave drive did
give a uniform rate of rotation - with others you had to massage the
waveform a bit to get uniform rotation.

Post by KevinJ93
Even implementing the discrete drive electronics would be more costly
than necessary at a time where individual transistors were a significant
cost; Philips' solution used two transistors - creating a divide by 4
plus driver transistors plus an oscillator would probably require about
ten transistors plus numerous other components.

Which you could could buy in an integrated circuit. Most of mine were in
a chunk of PROM.

Post by KevinJ93
If stepper motors would be such a great solution how come nobody has had
your insight and used them in the past sixty years for tape drives?

Beats me.

Post by KevinJ93
The permanent magnet DC motor with negative resistance driver worked
perfectly well. It was low cost, used available technology, low power,
was quiet and met the design requirements.

The strength of the permanent magnet depends on the it's temperature, so
the velocity feedback you get out of the motor coils does too.

It might have been "adequate" but it wasn't all that good.

--
Bill Sloman, Sydney

Bert Hickman

2024-03-07 16:06:02 UTC

...

Philips used the negative resistance approach for speed control in
their portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the permanent
magnets in the motor being regulated, and that is temperature
dependent. Philips may have relied on it, but it was still ghastly.

Obviously Philips didn't agree with you. For a consumer product used
over a benign temperature range it was fine.
The temperature coefficient was low enough to keep the tape speed within
1% or so.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz watch
crystal as the frequency reference. Some of them included stepper
motors to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable unit,
though perhaps not in a really cheap one.

At the time these devices were first designed (mid-late 60's)
synchronous motors weren't a practical option for a consumer item.

Stepper motors are much too inefficient and have too much torque ripple
for capstan drive - not at all suitable for a battery powered device, they
also tend to be noisy.

Twaddle. A stepper motor is a synchronous motor, and if you are careful how
you drive it, it doesn't have any torque ripple, and it isn't any less
efficient than any other synchronous motor.
ESCAP did do a range of small stepper motors where a sine wave drive did
give a uniform rate of rotation - with others you had to massage the
waveform a bit to get uniform rotation.

Post by KevinJ93
Even implementing the discrete drive electronics would be more costly than
necessary at a time where individual transistors were a significant cost;
Philips' solution used two transistors - creating a divide by 4 plus
driver transistors plus an oscillator would probably require about ten
transistors plus numerous other components.

Which you could could buy in an integrated circuit. Most of mine were in a
chunk of PROM.

Post by KevinJ93
If stepper motors would be such a great solution how come nobody has had
your insight and used them in the past sixty years for tape drives?

Beats me.

The strength of the permanent magnet depends on the it's temperature, so the
velocity feedback you get out of the motor coils does too.
It might have been "adequate" but it wasn't all that good.

A capstan motor sounds more could use a small AC Slosyn synchronous motor
rather than a DC Slosyn stepper. They do look quite similar and both are
made by the same vendors but they're not the same.

KevinJ93

2024-03-07 20:13:59 UTC

...

Post by Bill Sloman
Back then they were called "stepper motors" and would have been
entirely practical. Admittedly, I didn't get to design one into what
would have been a cheap product until 1978 (and at EMI Central
Research) but they were pretty cheap.

Stepper motors are much too inefficient and have too much torque
ripple for capstan drive - not at all suitable for a battery powered
device, they also tend to be noisy.

Twaddle. A stepper motor is a synchronous motor, and if you are careful
how you drive it, it doesn't have any torque ripple, and it isn't any
less efficient than any other synchronous motor.

Stepper motors are invariably of the reluctance type. With simple
drivers they have a great deal of cogging, which is undesirable in a
capstan drive motor.

Post by Bill Sloman
ESCAP did do a range of small stepper motors where a sine wave drive did
give a uniform rate of rotation - with others you had to massage the
waveform a bit to get uniform rotation.

Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.

Even for AC powered units where power was not an issue stepper motors
were never used. Synchronous motors with synthesized drive were
occasionally a feature but many/most used back-emf stabilization with DC
motors.

ICs were available to integrate that circuitry:

eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more costly
than necessary at a time where individual transistors were a
significant cost; Philips' solution used two transistors - creating a
divide by 4 plus driver transistors plus an oscillator would probably
require about ten transistors plus numerous other components.

Which you could could buy in an integrated circuit. Most of mine were in
a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

Post by KevinJ93
If stepper motors would be such a great solution how come nobody has
had your insight and used them in the past sixty years for tape drives?

Beats me

The strength of the permanent magnet depends on the it's temperature, so
the velocity feedback you get out of the motor coils does too.
It might have been "adequate" but it wasn't all that good.

There is little benefit to being more than adequate if it costs more and
will not be perceived by the customer as being better.

I'm afraid history is against you and regardless of your remonstrations
stepper motors were never used significantly or at all for capstan motors.

kw

John Larkin

2024-03-08 03:18:51 UTC

Post by KevinJ93
...

Stepper motors are much too inefficient and have too much torque
ripple for capstan drive - not at all suitable for a battery powered
device, they also tend to be noisy.

Twaddle. A stepper motor is a synchronous motor, and if you are careful
how you drive it, it doesn't have any torque ripple, and it isn't any
less efficient than any other synchronous motor.

Stepper motors are invariably of the reluctance type. With simple
drivers they have a great deal of cogging, which is undesirable in a
capstan drive motor.

There are two types, PM and VR. PM steppers use bipolar coil drive and
have a strong unpowered detent. And can act as generators.

Both can microstep nicely, for smooth motion.

Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.
Even for AC powered units where power was not an issue stepper motors
were never used. Synchronous motors with synthesized drive were
occasionally a feature but many/most used back-emf stabilization with DC
motors.

I designed a tape drive system for data storage, using the 3M tape
cartriges. The capstain driver was a stepper motor driven from 60 Hz
AC, with a cap in one leg to get a 90 degree phase shift. Motion was
very smooth.

Post by KevinJ93
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more costly
than necessary at a time where individual transistors were a
significant cost; Philips' solution used two transistors - creating a
divide by 4 plus driver transistors plus an oscillator would probably
require about ten transistors plus numerous other components.

Which you could could buy in an integrated circuit. Most of mine were in
a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

Post by KevinJ93
If stepper motors would be such a great solution how come nobody has
had your insight and used them in the past sixty years for tape drives?

Beats me

The strength of the permanent magnet depends on the it's temperature, so
the velocity feedback you get out of the motor coils does too.
It might have been "adequate" but it wasn't all that good.

There is little benefit to being more than adequate if it costs more and
will not be perceived by the customer as being better.
I'm afraid history is against you and regardless of your remonstrations
stepper motors were never used significantly or at all for capstan motors.

I did it.

Post by KevinJ93
kw

KevinJ93

2024-03-08 18:30:08 UTC

...

Post by KevinJ93
Stepper motors are invariably of the reluctance type. With simple
drivers they have a great deal of cogging, which is undesirable in a
capstan drive motor.

There are two types, PM and VR. PM steppers use bipolar coil drive and
have a strong unpowered detent. And can act as generators.

Yes, I was wrong.

Post by John Larkin
Both can microstep nicely, for smooth motion.

Given appropriate driving circuitry that would have been expensive and
power consuming in 1970.

...

kw

john larkin

2024-03-08 18:53:00 UTC

Post by KevinJ93
...

Post by KevinJ93
Stepper motors are invariably of the reluctance type. With simple
drivers they have a great deal of cogging, which is undesirable in a
capstan drive motor.

There are two types, PM and VR. PM steppers use bipolar coil drive and
have a strong unpowered detent. And can act as generators.

Yes, I was wrong.

Post by John Larkin
Both can microstep nicely, for smooth motion.

Given appropriate driving circuitry that would have been expensive and
power consuming in 1970.

My phase-shifted 60 Hz tape drive thing worked pretty well, for the
time. I used a few triacs to get stop/fwd/reverse. One phase of the
stepper was raw 60 Hz from a transformer, and the other phase had a
series R+C to get a 90 degree phase shift. We tweaked the RC to get
the smoothest rotation.

I've done a fair number of stepper drivers, like the ones to tune the
superconductive cavities at CEBAF, but I've never driven, or seen, a
VR type.

Aerospace people like to use "torque motors", which are basically big
VR steppers.

Bill Sloman

2024-03-09 04:31:49 UTC

Post by KevinJ93
...

Post by KevinJ93
Stepper motors are invariably of the reluctance type. With simple
drivers they have a great deal of cogging, which is undesirable in a
capstan drive motor.

There are two types, PM and VR. PM steppers use bipolar coil drive and
have a strong unpowered detent. And can act as generators.

Yes, I was wrong.

Post by John Larkin
Both can microstep nicely, for smooth motion.

Given appropriate driving circuitry that would have been expensive and
power consuming in 1970.

Nonsense. The cheap way of making an approximation to a sine wave is
pulse width modulation.

https://www.tinaja.com/glib/sinquest.pdf

That document is from 1997, but the idea has been around for a lot
longer. I used it in 1975 - if not to make sine waves - and it is cheap
and efficient. The "modified square wave" - which has no third harmonic
content - is equally old.

<snip>

--
Bill Sloman, Sydney

John Larkin

2024-03-09 05:08:10 UTC

Post by KevinJ93
...

Stepper motors are much too inefficient and have too much torque
ripple for capstan drive - not at all suitable for a battery powered
device, they also tend to be noisy.

Twaddle. A stepper motor is a synchronous motor, and if you are careful
how you drive it, it doesn't have any torque ripple, and it isn't any
less efficient than any other synchronous motor.

Stepper motors are invariably of the reluctance type. With simple
drivers they have a great deal of cogging, which is undesirable in a
capstan drive motor.

There are two types, PM and VR. PM steppers use bipolar coil drive and
have a strong unpowered detent. And can act as generators.
Both can microstep nicely, for smooth motion.

Could a VR stepper be used as a generator? I think so.

Bill Sloman

2024-03-08 04:48:15 UTC

Post by KevinJ93
...

Stepper motors are much too inefficient and have too much torque
ripple for capstan drive - not at all suitable for a battery powered
device, they also tend to be noisy.

Twaddle. A stepper motor is a synchronous motor, and if you are
careful how you drive it, it doesn't have any torque ripple, and it
isn't any less efficient than any other synchronous motor.

Stepper motors are invariably of the reluctance type. With simple
drivers they have a great deal of cogging, which is undesirable in a
capstan drive motor.

Post by Bill Sloman
ESCAP did do a range of small stepper motors where a sine wave drive
did give a uniform rate of rotation - with others you had to massage
the waveform a bit to get uniform rotation.

Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.

Don't be silly. Back-emf depends on the strenght of the magnetic field
generating the basck-emf, and that is temperature dependent.

Synchronous motors rotate at a rate that reflects the stability of the
frequency source that determines the drive frequency, and reasonably
stable frequency source - watch crystals have been around for ages.

Post by KevinJ93
Even for AC powered units where power was not an issue stepper motors
were never used. Synchronous motors with synthesized drive were
occasionally a feature but many/most used back-emf stabilization with DC
motors.
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more costly
than necessary at a time where individual transistors were a
significant cost; Philips' solution used two transistors - creating a
divide by 4 plus driver transistors plus an oscillator would probably
require about ten transistors plus numerous other components.

Which you could could buy in an integrated circuit. Most of mine were
in a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

It didn't - and it wasn't bipolar.

Post by KevinJ93
If stepper motors would be such a great solution how come nobody has
had your insight and used them in the past sixty years for tape drives?

Beats me

The strength of the permanent magnet depends on the it's temperature,
so the velocity feedback you get out of the motor coils does too.
It might have been "adequate" but it wasn't all that good.

There is little benefit to being more than adequate if it costs more and
will not be perceived by the customer as being better.

Tape recorder that didn't play back the recorded frequency weren't
perceived to be "good" by their customers. That didn't worry the bottom
end of the market.

Post by KevinJ93
I'm afraid history is against you and regardless of your remonstrations
stepper motors were never used significantly or at all for capstan motors.

History doesn't make a cheap and nasty solution anything other than
cheap and nasty. The thread is about what Cursitor Doom should do to get
his antique tape recorder working again, and getting hold of the
original motors used to drive it doesn't seem to be an option.

--
Bill Sloman, Sydney

KevinJ93

2024-03-08 18:49:21 UTC

...

Post by KevinJ93
Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.

Don't be silly. Back-emf depends on the strenght of the magnetic field
generating the basck-emf, and that is temperature dependent.

At about 0.2% per deg the magnetic field strength stability was adequate
for the speed accuracy required under the required environmental conditions.

Post by Bill Sloman
Synchronous motors rotate at a rate that reflects the stability of the
frequency source that determines the drive frequency, and reasonably
stable frequency source - watch crystals have been around for ages.

Post by KevinJ93
Even for AC powered units where power was not an issue stepper motors
were never used. Synchronous motors with synthesized drive were
occasionally a feature but many/most used back-emf stabilization with
DC motors.
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more
costly than necessary at a time where individual transistors were a
significant cost; Philips' solution used two transistors - creating
a divide by 4 plus driver transistors plus an oscillator would
probably require about ten transistors plus numerous other components.

Which you could could buy in an integrated circuit. Most of mine were
in a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple
supplies required. The logic to drive them would have been TTL consuming
significant amounts of power as well as expensive.

The first EPROMS that were easy to use, such as the 2708 weren't widely
available till the late 70's.

Post by KevinJ93
If stepper motors would be such a great solution how come nobody has
had your insight and used them in the past sixty years for tape drives?

Beats me

The strength of the permanent magnet depends on the it's temperature,
so the velocity feedback you get out of the motor coils does too.
It might have been "adequate" but it wasn't all that good.

There is little benefit to being more than adequate if it costs more
and will not be perceived by the customer as being better.

Tape recorder that didn't play back the recorded frequency weren't
perceived to be "good" by their customers. That didn't worry the bottom
end of the market.

Few customers had perfect pitch, an error of 1% was much preferable to
high cost.

You may call them 'cheap and nasty' but the major portion of the market
found this solution acceptable, only the high end went for more exotic
approaches.

Wow and flutter performance was much more important and using a DC motor
and belt drive with small capstans and a flywheel gave acceptable
performance.

I see that tape decks available at Crutchfield currently have a pitch
control so the speed can be varied anyway.

Post by KevinJ93
I'm afraid history is against you and regardless of your
remonstrations stepper motors were never used significantly or at all
for capstan motors.

Bill Sloman

2024-03-09 04:42:32 UTC

Post by KevinJ93
...

Post by KevinJ93
Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.

Don't be silly. Back-emf depends on the strenght of the magnetic field
generating the basck-emf, and that is temperature dependent.

At about 0.2% per deg the magnetic field strength stability was adequate
for the speed accuracy required under the required environmental conditions.

Motors run hotter than their environment

Post by KevinJ93
Even for AC powered units where power was not an issue stepper motors
were never used. Synchronous motors with synthesized drive were
occasionally a feature but many/most used back-emf stabilization with
DC motors.
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more
costly than necessary at a time where individual transistors were a
significant cost; Philips' solution used two transistors - creating
a divide by 4 plus driver transistors plus an oscillator would
probably require about ten transistors plus numerous other components.

Which you could could buy in an integrated circuit. Most of mine
were in a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple
supplies required.

It was one-time programmable, not an EPROM.

Post by KevinJ93
The logic to drive them would have been TTL consuming
significant amounts of power as well as expensive.

CMOS was around and cheap. I'd first used it around 1975, and the price
fell by a factor of three as I was developing the 1975 circuit.

Post by KevinJ93
The first EPROMS that were easy to use, such as the 2708 weren't widely
available till the late 70's.

The stepper motor circuit that I worked on was developed in 1978.

<snip>

--
Bill Sloman, Sydney

KevinJ93

2024-03-09 22:56:43 UTC

Post by KevinJ93
...

Post by KevinJ93
Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.

Don't be silly. Back-emf depends on the strenght of the magnetic
field generating the basck-emf, and that is temperature dependent.

At about 0.2% per deg the magnetic field strength stability was
adequate for the speed accuracy required under the required
environmental conditions.

Motors run hotter than their environment

With only 50-100mW being consumed by the motor (10's of mA at 3-6V) the
temperature differential was small.

Post by Bill Sloman
Synchronous motors rotate at a rate that reflects the stability of
the frequency source that determines the drive frequency, and
reasonably stable frequency source - watch crystals have been around
for ages.

Post by KevinJ93
Even for AC powered units where power was not an issue stepper
motors were never used. Synchronous motors with synthesized drive
were occasionally a feature but many/most used back-emf
stabilization with DC motors.
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more
costly than necessary at a time where individual transistors were
a significant cost; Philips' solution used two transistors -
creating a divide by 4 plus driver transistors plus an oscillator
would probably require about ten transistors plus numerous other
components.

Which you could could buy in an integrated circuit. Most of mine
were in a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple
supplies required.

It was one-time programmable, not an EPROM.

If it was NMOS it was almost certainly an EPROM in a cheaper package
without the quartz window.

Post by KevinJ93
The logic to drive them would have been TTL consuming significant
amounts of power as well as expensive.

CMOS was around and cheap. I'd first used it around 1975, and the price
fell by a factor of three as I was developing the 1975 circuit.

Post by KevinJ93
The first EPROMS that were easy to use, such as the 2708 weren't
widely available till the late 70's.

The stepper motor circuit that I worked on was developed in 1978.
<snip>

John Larkin

2024-03-09 23:13:20 UTC

Post by KevinJ93
...

Post by KevinJ93
Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.

Don't be silly. Back-emf depends on the strenght of the magnetic
field generating the basck-emf, and that is temperature dependent.

At about 0.2% per deg the magnetic field strength stability was
adequate for the speed accuracy required under the required
environmental conditions.

Motors run hotter than their environment

With only 50-100mW being consumed by the motor (10's of mA at 3-6V) the
temperature differential was small.

Post by Bill Sloman
Synchronous motors rotate at a rate that reflects the stability of
the frequency source that determines the drive frequency, and
reasonably stable frequency source - watch crystals have been around
for ages.

Post by KevinJ93
Even for AC powered units where power was not an issue stepper
motors were never used. Synchronous motors with synthesized drive
were occasionally a feature but many/most used back-emf
stabilization with DC motors.
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more
costly than necessary at a time where individual transistors were
a significant cost; Philips' solution used two transistors -
creating a divide by 4 plus driver transistors plus an oscillator
would probably require about ten transistors plus numerous other
components.

Which you could could buy in an integrated circuit. Most of mine
were in a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple
supplies required.

It was one-time programmable, not an EPROM.

If it was NMOS it was almost certainly an EPROM in a cheaper package
without the quartz window.

1702 was a p-mos UV-erase part. It was called an eprom.

Cursitor Doom

2024-03-10 08:59:30 UTC

Post by KevinJ93
...

Post by KevinJ93
Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.

Don't be silly. Back-emf depends on the strenght of the magnetic
field generating the basck-emf, and that is temperature dependent.

At about 0.2% per deg the magnetic field strength stability was
adequate for the speed accuracy required under the required
environmental conditions.

Motors run hotter than their environment

With only 50-100mW being consumed by the motor (10's of mA at 3-6V) the
temperature differential was small.

Post by Bill Sloman
Synchronous motors rotate at a rate that reflects the stability of
the frequency source that determines the drive frequency, and
reasonably stable frequency source - watch crystals have been around
for ages.

Post by KevinJ93
Even for AC powered units where power was not an issue stepper
motors were never used. Synchronous motors with synthesized drive
were occasionally a feature but many/most used back-emf
stabilization with DC motors.
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more
costly than necessary at a time where individual transistors were
a significant cost; Philips' solution used two transistors -
creating a divide by 4 plus driver transistors plus an oscillator
would probably require about ten transistors plus numerous other
components.

Which you could could buy in an integrated circuit. Most of mine
were in a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple
supplies required.

It was one-time programmable, not an EPROM.

If it was NMOS it was almost certainly an EPROM in a cheaper package
without the quartz window.

1702 was a p-mos UV-erase part. It was called an eprom.

Are EPROMs obsolete now? I assume they must be or we wouldn't have USB
drives and SD cards etc.

John Larkin

2024-03-10 09:41:56 UTC

Post by KevinJ93
...

Post by KevinJ93
Not in 1970. Even after that time they did not possess any advantage
over DC motor drive with speed stabilization based on back-emf.

Don't be silly. Back-emf depends on the strenght of the magnetic
field generating the basck-emf, and that is temperature dependent.

At about 0.2% per deg the magnetic field strength stability was
adequate for the speed accuracy required under the required
environmental conditions.

Motors run hotter than their environment

With only 50-100mW being consumed by the motor (10's of mA at 3-6V) the
temperature differential was small.

Post by Bill Sloman
Synchronous motors rotate at a rate that reflects the stability of
the frequency source that determines the drive frequency, and
reasonably stable frequency source - watch crystals have been around
for ages.

Post by KevinJ93
Even for AC powered units where power was not an issue stepper
motors were never used. Synchronous motors with synthesized drive
were occasionally a feature but many/most used back-emf
stabilization with DC motors.
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more
costly than necessary at a time where individual transistors were
a significant cost; Philips' solution used two transistors -
creating a divide by 4 plus driver transistors plus an oscillator
would probably require about ten transistors plus numerous other
components.

Which you could could buy in an integrated circuit. Most of mine
were in a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up all
your power budget.

It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple
supplies required.

It was one-time programmable, not an EPROM.

If it was NMOS it was almost certainly an EPROM in a cheaper package
without the quartz window.

1702 was a p-mos UV-erase part. It was called an eprom.

Are EPROMs obsolete now? I assume they must be or we wouldn't have USB
drives and SD cards etc.

Some of our older VME modules use DIP eproms in sockets, and 68332
CPUs. We seem to be able to get both. We don't buy the UV window parts
any more.

Bill Sloman

2024-03-10 10:02:17 UTC

<snip>

Post by John Larkin
1702 was a p-mos UV-erase part. It was called an eprom.

Are EPROMs obsolete now? I assume they must be or we wouldn't have USB
drives and SD cards etc.

EPROMs are obsolete, but they were replaced by electrically erasable PROM.

USB cards and SD cards do the same job, but they cost more and take up
more space on the board - if you don't need much programmable memory an
EEPROM can be big enough.

https://au.element14.com/w/c/semiconductors-ics/memory/eeprom/prl/results?ost=eeprom&sort=P_PRICE

--
Bill Sloman, Sydney

KJW93

2024-03-10 22:13:39 UTC

...

Post by Bill Sloman
It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple
supplies required.

It was one-time programmable, not an EPROM.

If it was NMOS it was almost certainly an EPROM in a cheaper package
without the quartz window.

1702 was a p-mos UV-erase part. It was called an eprom.

I know, that's why I said that the 2708 series were the first convenient
to use ones with a single power supply.

Although an EPROM, some equivalent parts were available in a cheaper
package without a window - they were one-time programmable.

kw

Bill Sloman

2024-03-10 02:40:18 UTC

Post by KevinJ93
...

Post by KevinJ93
Not in 1970. Even after that time they did not possess any
advantage over DC motor drive with speed stabilization based on
back-emf.

Don't be silly. Back-emf depends on the strenght of the magnetic
field generating the basck-emf, and that is temperature dependent.

At about 0.2% per deg the magnetic field strength stability was
adequate for the speed accuracy required under the required
environmental conditions.

Motors run hotter than their environment

With only 50-100mW being consumed by the motor (10's of mA at 3-6V) the
temperature differential was small.

But it was lot bigger inside the motor than you could detect from
outside it.

Post by Bill Sloman
Synchronous motors rotate at a rate that reflects the stability of
the frequency source that determines the drive frequency, and
reasonably stable frequency source - watch crystals have been around
for ages.

Post by KevinJ93
Even for AC powered units where power was not an issue stepper
motors were never used. Synchronous motors with synthesized drive
were occasionally a feature but many/most used back-emf
stabilization with DC motors.
eg https://www.precisionmicrodrives.com/ab-026

Post by KevinJ93
Even implementing the discrete drive electronics would be more
costly than necessary at a time where individual transistors were
a significant cost; Philips' solution used two transistors -
creating a divide by 4 plus driver transistors plus an oscillator
would probably require about ten transistors plus numerous other
components.

Which you could could buy in an integrated circuit. Most of mine
were in a chunk of PROM.

Not in 1970. Even by the late 70's a bipolar (P)ROM would use up
all your power budget.

It didn't - and it wasn't bipolar.

MOS EPROMS such as the 1702 were cumbersome to use with multiple
supplies required.

It was one-time programmable, not an EPROM.

If it was NMOS it was almost certainly an EPROM in a cheaper package
without the quartz window.

Perhaps. It was 46 years ago.

<snip>

--
Bill Sloman, Sydney

John Larkin

2024-03-07 15:26:08 UTC

...

Post by Bill Sloman
That's not all that "old school" - Philips got a patent on it
around the 1970's. It wasn't remotely good enough for audio work,
and neither were centrifugal governors. Synchronous motors with
stable frequency drives was what the old school relied on

Philips used the negative resistance approach for speed control in
their portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the permanent
magnets in the motor being regulated, and that is temperature
dependent. Philips may have relied on it, but it was still ghastly.

Obviously Philips didn't agree with you. For a consumer product used
over a benign temperature range it was fine.
The temperature coefficient was low enough to keep the tape speed
within 1% or so.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz
watch crystal as the frequency reference. Some of them included
stepper motors to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable
unit, though perhaps not in a really cheap one.

At the time these devices were first designed (mid-late 60's)
synchronous motors weren't a practical option for a consumer item.

Stepper motors are much too inefficient and have too much torque ripple
for capstan drive - not at all suitable for a battery powered device,
they also tend to be noisy.

Efficiency wouldn't matter for a capstain motor (they may well absorb
power!) and microstepping is easy and smooth.

Does anybody still make audio tape drives?

Cursitor Doom

2024-03-07 19:17:03 UTC

...

Post by Bill Sloman
That's not all that "old school" - Philips got a patent on it
around the 1970's. It wasn't remotely good enough for audio work,
and neither were centrifugal governors. Synchronous motors with
stable frequency drives was what the old school relied on

Philips used the negative resistance approach for speed control in
their portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the permanent
magnets in the motor being regulated, and that is temperature
dependent. Philips may have relied on it, but it was still ghastly.

Obviously Philips didn't agree with you. For a consumer product used
over a benign temperature range it was fine.
The temperature coefficient was low enough to keep the tape speed
within 1% or so.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz
watch crystal as the frequency reference. Some of them included
stepper motors to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable
unit, though perhaps not in a really cheap one.

At the time these devices were first designed (mid-late 60's)
synchronous motors weren't a practical option for a consumer item.

Stepper motors are much too inefficient and have too much torque ripple
for capstan drive - not at all suitable for a battery powered device,
they also tend to be noisy.

Efficiency wouldn't matter for a capstain motor (they may well absorb
power!) and microstepping is easy and smooth.

Does anybody still make audio tape drives?

Prepare to be shocked!

John Larkin

2024-03-08 03:25:13 UTC

...

Post by Bill Sloman
That's not all that "old school" - Philips got a patent on it
around the 1970's. It wasn't remotely good enough for audio work,
and neither were centrifugal governors. Synchronous motors with
stable frequency drives was what the old school relied on

Philips used the negative resistance approach for speed control in
their portable cassette players - so it wasn't too bad.

The feedback from a DC motor depends on the strength of the permanent
magnets in the motor being regulated, and that is temperature
dependent. Philips may have relied on it, but it was still ghastly.

Obviously Philips didn't agree with you. For a consumer product used
over a benign temperature range it was fine.
The temperature coefficient was low enough to keep the tape speed
within 1% or so.

Post by KevinJ93
Synchronous AC motors weren't an option in a portable unit.

Watches are portable, and electronic watches rely on a 32,768 Hz
watch crystal as the frequency reference. Some of them included
stepper motors to drive a mechanical display.
Synchronous motors obviously are a practical option in a portable
unit, though perhaps not in a really cheap one.

At the time these devices were first designed (mid-late 60's)
synchronous motors weren't a practical option for a consumer item.

Stepper motors are much too inefficient and have too much torque ripple
for capstan drive - not at all suitable for a battery powered device,
they also tend to be noisy.

Efficiency wouldn't matter for a capstain motor (they may well absorb
power!) and microstepping is easy and smooth.

Does anybody still make audio tape drives?

Prepare to be shocked!
http://youtu.be/38_SVIa8BDQ

It's shocking how annoying that guy is.

Tape is awful. Noisy, wobbly, hard to handle. But if you charge enough
for goofy high-end audio, some people will buy it.

KevinJ93

2024-03-07 20:30:48 UTC

...

Post by KevinJ93
Stepper motors are much too inefficient and have too much torque ripple
for capstan drive - not at all suitable for a battery powered device,
they also tend to be noisy.

Efficiency wouldn't matter for a capstain motor (they may well absorb
power!) and microstepping is easy and smooth.

Most(all?) portable cassette players used a single motor for capstan and
take-up reel; it would definitely consume power and would probably be
the largest item in the power budget - probably only 50-100mW allowable
determined by battery life from a few C-cells or even two AA cells in
later units.

Microstepping is easy now - not so much even at the end of the cassette
tape era 30-40 years ago when CDs started to take over

Does anybody still make audio tape drives?

Crutchfield still has a couple of tape decks being sold. I'm sure the
market is very small.

The only reason I've used a cassette player in the last 20-30 years is
to transcribe tapes I already have into a digital format or to be able
to play things in a car that has a cassette player installed.

I wouldn't expect there is any significant new development being done.

kw

Jasen Betts

2024-03-06 11:43:33 UTC

Post by Cursitor Doom
Gentlemen,
Can motor speed control ever approach the effectiveness of the old
style drive belts and pulleys approach?

you mean like a centrifugal governor?

Post by Cursitor Doom
Would simple PWM be enough or would there be some additional trickery
needed?

PWM could. if you sample the back EMF during the off time of the PWM and feed
that back to the regulator... (or read the motor speed some other way,
you could have an interruptor typse sensor and control speed using a
PLL)
Old school when they weren't using centrifugal governors they would put
a compensating negative resistance in series with the motor and feed
the combination from a fixed DC voltage or fake that result.

All the old the tape recorders I pulled apart to see how they worked
used centrifugal governors (little leaf switches on the rotor),
except the one with high speed dubbing.

The Philips patent was used on record players, somehow that's not audio?
they didn't even use an op-amp just 2 transistors and used V_BE as a
voltage reference.

To me old-school is analogue speed control.

--
Jasen.
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Jan Panteltje

2024-03-05 06:05:12 UTC

On a sunny day (Mon, 4 Mar 2024 07:45:43 -0000 (UTC)) it happened Jasen Betts