Re: Inductance meter for BIST

On 4/9/24 19:17, john larkin wrote:

On Tue, 9 Apr 2024 17:36:28 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:
 

On 4/9/24 16:19, John Larkin wrote:

On Mon, 08 Apr 2024 17:17:15 -0700, john larkin <jl@650pot.com> wrote:
>

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:
>
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.
 

>
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