Question about all electronic auto instrument

I've got a junked Ford auto instrument panel. It has 6 actuators for the tach, speed, fuel, etc. Identical 1" X 1-1/8" X 1/2" white plastic modules with four wires. Inside is a little (for lack of a better word) geared down stepper motor.

It might be a bipolar stepper except the two identical 170 ohm coils have a common third pole (in the magnetic circuit). The coils are set in a 90 degree "V" configuration. Three double gears to turn the (1/16" X 1/2") output shaft.

It is very easy to take apart and reassemble. .

The rotor appears to be a permanent magnet type with at least four (probably more like 8) poles. Rotation is limited to 270 degrees with a mechanical stop on the output shaft's gear.

Anyone know how they work and what it takes to drive one?

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I have an acquaintance who used to work for a company that makes instruments like that for trucks. They work like stepper motors. I'd play around with a power supply and see if you can figure out what makes the motor step.

I expect that the motor is very wimpy (it doesn't need to be strong, because it's just moving a needle, after all) and that there's no position feedback -- when the controlling microprocessor comes out of reset it just drives the thing into the pin for 270 degrees worth of steps, then it just counts steps after that.

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Tim Wescott

The more I look at it, the more it looks like a stepper. The unusual pole arrangement may just be to accommodate the coils in a small package.

No telling with what passes for electronics in Detroit...

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I wonder if these things could be "selsyns", also known as "synchos"? Essentially, they are like steppers that get their drive pulses from a matching unit at the sensor. (As I recall, you can cross-wire certain conventional steppers to behave this way.) But the original selsyns were around long before steppers, used in aircraft and industrial instrumentation for example.

Best regards,

Bob Masta DAQARTA v6.02 Data AcQuisition And Real-Time Analysis

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Bob Masta

Selsyns are rotary transformers - the rotor is excited via slip rings and the stator is usually a three phase transformer (or motor). The phase of the rotor versus the field/stator determines position - 5 or

6 wires.
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Selsyns (self-syncronous servos) use an alternating field,

You can wire permanent magnet steppers together like selsyns, if you turn the master fast enough to generate voltage to turn the slave the slave will follow it, but at slow speed it won't follow.

selsyns work at any speed because the master acts as a variable transformer thus there's always voltage on the signal wires. the slave moves to match the master minimising the current flowing in the signal wires.

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Jasen Betts

Swisstech products are very common - Check:

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-- Roberto Waltman

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Roberto Waltman

Is domain waltman.com from you? No? then don't use that domain for your return address. Use waltman.invalid instead.

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Yes it is. - The "invalid" in my signature means that although the email address exists, I gave it a zero quota, so any message sent there will be rejected.

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Roberto Waltman

I fooled around with that - never could get any results with one stepper driving another directly, but did get reasonable results by adding some Darlington transistors - but still had to have enough speed to bias the Darlington on.

Adding op amps made a pretty good system. Open loop gain of 200K driving a Darlington with 1K gain. I couldn't turn the generator stepper so slow that the motor stepper wouldn't also turn. Downside was there was no holding torque except for any friction in the mechanical load or cogging in the motor stepper - doubtless some flip flops could be added to provide power to the motor coils when the generator was still - but I never carried it that far.

I used it for a remotely located antenna tuner.

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Thanks. Not the exact model I have but it looks like the same general idea - even including the snap apart case.

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I never used them, (except, unknowingly, in my car,) but I was looking to buy a few for a home-built flight simulator instrument panel. You may have older models - I remember different part numbers, they may have revamped the product line when bought by Juken. Also the documentation used to be free. Now some PDFs are marked "restricted" and require you to sign up to download them. There are many in eBay if you want to experiment without dissasembling what you have:

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-- Roberto Waltman

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Roberto Waltman

Like those. (1st clip: The green PCB in the center has the 4-channel driver chip controlling the others.)

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Roberto Waltman

4 wires will work. two for the rotor and one of the three for the stator can share one of the rotor wires as there's no other current path between the stator and the rotor.

R1 R2 R3 + S1 S2 but 5 wires will work better if the wiring has significant resistance.

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Reply to
Jasen Betts

Search for 'aircore motor'. Put sin on one coil and cos on another from a dac to drive it.

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BobG

I ruled that out already - the 90 degree coil arrangement suggested it and I remember a national IC designed just for the purpose. The beauty of sin/cos is absolute position information - no need for feedback to tell you where the pointer is. But the use of iron poles and magnetic rotor means it "cogs" (favors some angles and wouldn't be linear) then the >100:1 gear ratio would seem to render the absolute position advantage moot.

This is Detroit electronics - they evolve their current systems and adapt them to existing systems - so they aren't replacing their current stock of parts. Adaptive engineering, versus design engineering.

That is to say, it may not make a lot of sense, from an elegance of design perspective.

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