Re: Inductance meter for BIST

john larkin <jl@650pot.com> 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.
 
 
 

 
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