Discussion:
Inductance meter for BIST
(too old to reply)
john larkin
2024-04-09 00:17:15 UTC
Permalink
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.

If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.

Loading Image...

https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
John Larkin
2024-04-09 14:19:26 UTC
Permalink
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.

This is better:

Loading Image...

Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.

I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.

I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
Jeroen Belleman
2024-04-09 15:36:28 UTC
Permalink
Post by John Larkin
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.
https://www.dropbox.com/scl/fi/9tex1slpnaqz4n2cbxp3n/Ind_Meter_Apr_9.jpg?rlkey=52vbyylc5ax48w54qdxm9r9cq&raw=1
Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.
I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.
I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
What's R1 (1G to +5) for?

Are torque motors actually predominantly inductive? Just
because it has coils doesn't necessarily mean it is so.

Jeroen Belleman
john larkin
2024-04-09 17:17:26 UTC
Permalink
On Tue, 9 Apr 2024 17:36:28 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by John Larkin
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.
https://www.dropbox.com/scl/fi/9tex1slpnaqz4n2cbxp3n/Ind_Meter_Apr_9.jpg?rlkey=52vbyylc5ax48w54qdxm9r9cq&raw=1
Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.
I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.
I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
What's R1 (1G to +5) for?
It can be switched in, and R2 switched out, to make an ohmmeter. Then
the opamp is a DC amplifier into an ADC.
Post by Jeroen Belleman
Are torque motors actually predominantly inductive? Just
because it has coils doesn't necessarily mean it is so.
One case is used in some jet engines, for controlling fuel flow I
think. 750 mH and maybe 200 ohms..

Unlike steppers, they seem to have no detent torque and I suspect L
doesn't change much with angular position. Gotta machine a test setup
and verify that.

If the brushes short windings sometimes, it could get weird.
Post by Jeroen Belleman
Jeroen Belleman
Jeroen Belleman
2024-04-09 19:13:30 UTC
Permalink
Post by john larkin
On Tue, 9 Apr 2024 17:36:28 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by John Larkin
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.
https://www.dropbox.com/scl/fi/9tex1slpnaqz4n2cbxp3n/Ind_Meter_Apr_9.jpg?rlkey=52vbyylc5ax48w54qdxm9r9cq&raw=1
Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.
I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.
I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
What's R1 (1G to +5) for?
It can be switched in, and R2 switched out, to make an ohmmeter. Then
the opamp is a DC amplifier into an ADC.
Post by Jeroen Belleman
Are torque motors actually predominantly inductive? Just
because it has coils doesn't necessarily mean it is so.
One case is used in some jet engines, for controlling fuel flow I
think. 750 mH and maybe 200 ohms..
Unlike steppers, they seem to have no detent torque and I suspect L
doesn't change much with angular position. Gotta machine a test setup
and verify that.
If the brushes short windings sometimes, it could get weird.
At first sight, if they're like brushed DC motors, the equivalent
circuit would be a series combination of an inductor, a resistor
and a voltage source, function of rotational speed. There will be
some torque ripple and commutator noise, but there's no telling
how important that may be.

Jeroen Belleman
john larkin
2024-04-09 20:57:47 UTC
Permalink
On Tue, 9 Apr 2024 21:13:30 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by john larkin
On Tue, 9 Apr 2024 17:36:28 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by John Larkin
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.
https://www.dropbox.com/scl/fi/9tex1slpnaqz4n2cbxp3n/Ind_Meter_Apr_9.jpg?rlkey=52vbyylc5ax48w54qdxm9r9cq&raw=1
Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.
I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.
I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
What's R1 (1G to +5) for?
It can be switched in, and R2 switched out, to make an ohmmeter. Then
the opamp is a DC amplifier into an ADC.
Post by Jeroen Belleman
Are torque motors actually predominantly inductive? Just
because it has coils doesn't necessarily mean it is so.
One case is used in some jet engines, for controlling fuel flow I
think. 750 mH and maybe 200 ohms..
Unlike steppers, they seem to have no detent torque and I suspect L
doesn't change much with angular position. Gotta machine a test setup
and verify that.
If the brushes short windings sometimes, it could get weird.
At first sight, if they're like brushed DC motors, the equivalent
circuit would be a series combination of an inductor, a resistor
and a voltage source, function of rotational speed. There will be
some torque ripple and commutator noise, but there's no telling
how important that may be.
Jeroen Belleman
They behave about like 2-phase stepper motors. DC applied to the two
windings sets an angular position, not a speed.
John Larkin
2024-04-10 15:45:09 UTC
Permalink
On Tue, 9 Apr 2024 21:13:30 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by john larkin
On Tue, 9 Apr 2024 17:36:28 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by John Larkin
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.
https://www.dropbox.com/scl/fi/9tex1slpnaqz4n2cbxp3n/Ind_Meter_Apr_9.jpg?rlkey=52vbyylc5ax48w54qdxm9r9cq&raw=1
Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.
I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.
I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
What's R1 (1G to +5) for?
It can be switched in, and R2 switched out, to make an ohmmeter. Then
the opamp is a DC amplifier into an ADC.
Post by Jeroen Belleman
Are torque motors actually predominantly inductive? Just
because it has coils doesn't necessarily mean it is so.
One case is used in some jet engines, for controlling fuel flow I
think. 750 mH and maybe 200 ohms..
Unlike steppers, they seem to have no detent torque and I suspect L
doesn't change much with angular position. Gotta machine a test setup
and verify that.
If the brushes short windings sometimes, it could get weird.
At first sight, if they're like brushed DC motors, the equivalent
circuit would be a series combination of an inductor, a resistor
and a voltage source, function of rotational speed. There will be
some torque ripple and commutator noise, but there's no telling
how important that may be.
Jeroen Belleman
https://www.dropbox.com/scl/fo/d9qffkwqzikd9visdt0zi/h?rlkey=mdon4i9xblxkerk470umdbsw3&dl=0


This is charmingly bizarre. There are 41 (prime number!) of commutator
segments. If I pick one arbitrarily, and call it #1, and go clockwise
with an ohmmeter, resistance goes up to about 8 ohms in steps and goes
back down to about 1 ohm at commutator 17, 26, and 34.

This is an NOS motor from ebay. It's an older style with brushes and
alnico magnets.

Imagine designing this 50 years ago without a computer.
Klaus Vestergaard Kragelund
2024-04-09 23:01:55 UTC
Permalink
Post by john larkin
On Tue, 9 Apr 2024 17:36:28 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by John Larkin
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.
https://www.dropbox.com/scl/fi/9tex1slpnaqz4n2cbxp3n/Ind_Meter_Apr_9.jpg?rlkey=52vbyylc5ax48w54qdxm9r9cq&raw=1
Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.
I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.
I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
What's R1 (1G to +5) for?
It can be switched in, and R2 switched out, to make an ohmmeter. Then
the opamp is a DC amplifier into an ADC.
Post by Jeroen Belleman
Are torque motors actually predominantly inductive? Just
because it has coils doesn't necessarily mean it is so.
One case is used in some jet engines, for controlling fuel flow I
think. 750 mH and maybe 200 ohms..
Unlike steppers, they seem to have no detent torque and I suspect L
doesn't change much with angular position. Gotta machine a test setup
and verify that.
I did some tests on that maybe 10 years back. I can't remember the
details, but I am pretty sure there is a significant change in the
inductance vs position
piglet
2024-04-10 06:29:33 UTC
Permalink
Post by john larkin
On Tue, 9 Apr 2024 17:36:28 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by John Larkin
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.
https://www.dropbox.com/scl/fi/9tex1slpnaqz4n2cbxp3n/Ind_Meter_Apr_9.jpg?rlkey=52vbyylc5ax48w54qdxm9r9cq&raw=1
Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.
I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.
I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
What's R1 (1G to +5) for?
It can be switched in, and R2 switched out, to make an ohmmeter. Then
the opamp is a DC amplifier into an ADC.
Post by Jeroen Belleman
Are torque motors actually predominantly inductive? Just
because it has coils doesn't necessarily mean it is so.
One case is used in some jet engines, for controlling fuel flow I
think. 750 mH and maybe 200 ohms..
Unlike steppers, they seem to have no detent torque and I suspect L
doesn't change much with angular position. Gotta machine a test setup
and verify that.
If the brushes short windings sometimes, it could get weird.
Post by Jeroen Belleman
Jeroen Belleman
They won’t have nice linear ferrite cores but nasty lumps of iron so
measured inductance could depend on test frequency, AC amplitude, amount of
DC flowing, rotor position, temperature etc etc etc. Precision is futile?
--
piglet
Bill Sloman
2024-04-10 07:38:00 UTC
Permalink
Post by piglet
Post by john larkin
On Tue, 9 Apr 2024 17:36:28 +0200, Jeroen Belleman
Post by Jeroen Belleman
Post by John Larkin
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
Most L-meters, the AADE sort, lie or don't work with big Ls.
https://www.dropbox.com/scl/fi/9tex1slpnaqz4n2cbxp3n/Ind_Meter_Apr_9.jpg?rlkey=52vbyylc5ax48w54qdxm9r9cq&raw=1
Given a small uP, like the Rasp Pi thing, it could measure L and Rl
pretty easily. One could get compusive and math out the small
inductance measuring error from the inductor copper resistance.
I need repeatability more than accuracy, and don't have time to
explore this very deep, but it's interesting.
I've been buying shaftless torque motors off ebay, to understand them
electrically. We need to simulate a torque motor.
What's R1 (1G to +5) for?
It can be switched in, and R2 switched out, to make an ohmmeter. Then
the opamp is a DC amplifier into an ADC.
Post by Jeroen Belleman
Are torque motors actually predominantly inductive? Just
because it has coils doesn't necessarily mean it is so.
One case is used in some jet engines, for controlling fuel flow I
think. 750 mH and maybe 200 ohms..
Unlike steppers, they seem to have no detent torque and I suspect L
doesn't change much with angular position. Gotta machine a test setup
and verify that.
If the brushes short windings sometimes, it could get weird.
Why would you need brushes in a torque motor?
Post by piglet
They won’t have nice linear ferrite cores but nasty lumps of iron so
measured inductance could depend on test frequency, AC amplitude, amount of
DC flowing, rotor position, temperature etc etc etc. Precision is futile?
Not futile but probably difficult to achieve. Lots of variables to
measure and correct for, and non-linear correction equations.
--
Bill Sloman, Sydney
Lasse Langwadt
2024-04-09 20:58:30 UTC
Permalink
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
https://github.com/coreWeaver/LC-Meter/blob/main/LC%20Meter%20v1.0/1.%20Schematic%20and%20Gerber%20UPDATED%20Apr.08%2C%202021/LC%20Meter%20-%20Schematic.pdf
John Larkin
2024-04-10 03:18:01 UTC
Permalink
Post by Lasse Langwadt
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
https://github.com/coreWeaver/LC-Meter/blob/main/LC%20Meter%20v1.0/1.%20Schematic%20and%20Gerber%20UPDATED%20Apr.08%2C%202021/LC%20Meter%20-%20Schematic.pdf
Are there specs on that one? I didn't see anything like a manual.
Lasse Langwadt
2024-04-10 23:16:35 UTC
Permalink
Post by John Larkin
Post by Lasse Langwadt
Post by john larkin
I can build this into my new programmable inductor module, for
self-test. With a 1% cap, it should be plenty good. L range will be
750 mH down to maybe 10 mH, part of simulating solenoids and torque
motors and such.
If I add a switch to open R3 and run some current through R1, it can
measure series resistance too.
https://www.dropbox.com/scl/fi/tn8lpk38677ioylhdkget/P955_L-meter.jpg?rlkey=xynqyzfc2x020llr3a945c0td&raw=1
https://www.dropbox.com/scl/fi/gojlqxj23r4m57ke9jhui/Ind_Meter_1.asc?rlkey=if40kmtiz49gp62bacfvju3uv&dl=0
https://github.com/coreWeaver/LC-Meter/blob/main/LC%20Meter%20v1.0/1.%20Schematic%20and%20Gerber%20UPDATED%20Apr.08%2C%202021/LC%20Meter%20-%20Schematic.pdf
Are there specs on that one? I didn't see anything like a manual.
afaict there's a video explaining all the fucntions and equations
https://github.com/coreWeaver/LC-Meter

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