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Old 03-07-2011, 05:12 PM   #4443 (permalink)
jyanof
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Quote:
Originally Posted by vwdevotee View Post
Also, about 1000 posts ago, there was talk of using motor voltage and a motor constant to calculate the RPM in the MCU without actually needing any new hardware. Did that fizzle or would that still be an option? In order to use the MCU to drive one of the factory gauges, what would the additional hardware consist of?
It's been implemented in the form of overspeed protection. It doesn't calculate a specific RPM, but calculates a number; if that number is above the threshold you provide, power to the motor is cut.

The calculation could probably be modified to produce an actual rpm value or a proportional PWM signal. It will be very inaccurate at low speeds and will also vary in accuracy at different output currents. For example, I noticed that the overspeed would trip at lower RPM if the motor was lightly loaded. The difference was small though, maybe 400 RPM between an unloaded motor and a highly loaded motor.

Also, no calculations are performed when you're coasting (since motor voltage is zero) so the tach would also go to zero. Might be a little weird to get used to...

Lastly, this calculation only works with series wound motors, hence the modifications required for Jack and his compound motor (i think it's a compound, anyway).

If you try it though, let us know how it works! I've had a dead tach for 2 years now...

btw, you can feed a traditional tach with the signal produced by a couple of magnets on the motor tailshaft (or shaft adapter, i guess) and a home made sensor of a couple hundred turns of fine magnet wire around a screw. Put the screw/magnet wire close to where the magnets pass and they will produce a voltage as they pass by. Use a 12v zener to clamp the output to 12V and feed it to your tach signal input. That's the jist of it, let me know if you want more details...
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