Quote:
Originally Posted by cts_casemod
Not quite. Yes the wheel will accelerate, but by the time you gain traction, if nothing is done, it will kick all the mechanical inertia of the motor to the pavement. Dangerous with icy/rainy conditions.
Most new controllers can drive two motors from the same CPU with little overhead. ST has a few. Then you need to implement a differential in software. The torque is only the same if both wheels are in sync, otherwise kill torque to allow the wheel to adhere to the road again. Once they are synced apply torque again.
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Good point, except that you may not want them to be synced. For example, let's imagine that you are in a corner ( say, a skidpad ) and the inside wheel lifts up and looses traction. You see this all the time with FWD racecars - usually the inside rear corner is in the air. If it were an AWD car, then that wheel would be having the same issues we're discussing.
So, the point that wheel/motor/drivetrain inertia adds to the torque output of the motor in this case is definitely an issue. However, I wouldn't want to wait for the motors to be synced before applying torque. This won't happen until the car is going straight and it would require tight speed control, which I'm trying to avoid.
Perhaps looking at the relative velocities would be a good thing. Even in an extreme corner they really won't be very different, like they would be if one were on ice or in the air. If the relative speeds are > than XX% different, reduce the torque of the faster motor until the relative speeds are close enough. I wouldn't completely cut the torque, just reduce it so a controlled increase is possible.
Edit: Oh, and I forgot - there's a Majik thing that motors do to help combat this. Look at the torque-speed curves for the MGR - or the Prius - or any motor for that matter. Beyond the base speed, torque drops off fairly dramatically as speed increases. So, let's say you're @ highway speed with a gear ratio that puts 60mph at about 5000 rpm. The torque LIMIT of the motor is about 100 N-m. However, if the other motor spun out and met the max rpm, it's torque LIMIT would be less than 20 N-m.