I apologize for continuing off on a tangent, but I feel we're on a roll with the tangent and it's pertinent to the original topic to an extent. The biggest advantage I see for traction control is the ability to climb rough terrain (ramps) without too much driver input.
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Originally Posted by sumadin
I also don't understand the idea of PID traction control. I've heard of PID velocity control using encoders, and I'm planning to implement that on our robot this year, but I don't understand the idea of PID traction control. How would you do that?
Jesse, there are some ways. One way is to have a passive wheel on an independent axis in the center of your robot, and encode it. It will only move if the robot is moving. You could also use mouse sensors or trackballs to accomplish similar things, I think.
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We too use the encoders for PID velocity control in order to keep the robot driving straight at high velocity.
Hmm, after a bit more thinking the mouse sensors seem easy enough to do if you have 1 mouse sensor on each side -- even though the PID control, for perfection and theory, would slightly change during a turn (higher I value) than in a straight (higher P value). I'll have to bring this up to the drive train design team tonight to see if we can focus a bit of time experimenting with it.
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Originally Posted by Kevin Sevcik
Wheels slip when applied force exceeds the static friction force. Applied force is proportional to applied torque which is (mostly) proportional to motor torque which is proportional to motor current. So your goal would be to PID control the current being supplied to (or sourced from) the motors. Current-mode motor drivers and amplifiers are awesome for this, but we don't have any, sooo the idea would be to use a solid state current sensor on your motor leads, and PID control this.
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This might be a good start for general traction control as I know exactly what you're talking about. We definitely need some data before we can come up with anything concrete for limiting values however, and be able to test many scenarios to make sure it dynamically understands turns vs straights. We'll also have to review the rules on custom circuits since this sensor would be inlined with the motor leads. Bah, such a great idea so little time!