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From: Jeroen Belleman <jeroen@nospam.please>
Newsgroups: sci.electronics.design
Subject: Re: Inductance meter for BIST
Date: Tue, 9 Apr 2024 21:13:30 +0200
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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