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Old 01-31-2011, 11:50 AM   #3 (permalink)
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Hmmm, is there a wiki page for this yet? I've read the Cougar code pretty carefully and could likely add some useful info.

The comment about "a weird pot that doesn't close or open all the way" is a little off the mark.

The throttle input is deliberately set up to not span the full 0-5V range. This is so that the throttle "fails safe". With a good physical design, there is no wire or trace break that could result in a valid, non-zero throttle input. And no persistent short should result in a valid input. Part of the reason for HPL / High Pedal Lockout is that a internally shorted throttle position sensor (TPS) shouldn't result in half-throttle operation.

The controller typically has resistors to a 0-5V supply, so that a connected variable resistor (TPS or "pot box") ends up with a 1V-4V reference range. That way most breaks or shorts will report 0V or 5V (or even 12V) -- clearly outside the valid range. Further, the TPS is often set up with physical limits to never use the full range, perhaps only 1.25V to 3.75V. That way the controller can localize a failure -- a 1V input suggests the failure is internal to the TPS or wiring rather than on the controller or an external short.

Yes, reducing the input voltage range like this reduces the resolution of the throttle input. But the ability to detect a very slight movement of the gas pedal isn't nearly as important as being able to detect a failure.
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