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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: Inductance meter for BIST
Date: Wed, 10 Apr 2024 17:38:00 +1000
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On 10/04/2024 4:29 pm, piglet wrote:
> 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.

Why would you need brushes in a  torque motor?

> 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