Drivetrain PID tuning

[sspoldi]

Originally Posted by ollien

Today, I was encouraging my rookie programmers to utilize the NavX to experiment with PID Control in order to make our robot drive straight. Given that we've never done this in a season, one of the things that crossed my mind is how we should deal with the differences in CoFs between our lab's carpet and that of a true to life field. What have other teams done to account for this? Have you guys just purchased carpet, and just calibrated it on that? What would you use to re-tune your loop at competition?

Thanks so much!

We have never needed to re-tune for a change in carpet. Our typical heading stabilization is a PI controller for angular rate via gyro to stabilize heading. We tune via the following:

1.We do a quick system id on the yaw axis
a. Apply a moderate amplitude yaw step
(usually 1/2 max input on whatever yaw command input you have)
b. Model the angular rate output with the equation
rate = input*k*(1 - exp(-t/tau))

2. design a controller using the following architecture;
error = (yaw_cmd - yaw_rate)
motor_cmd = PID(error, Kp, Ki, Kd)
where:
Kp = gain/K*tau
Ki = gain/K
Kd = 0
gain = 5 to 10 (arbitrary, desired bandwith in rad/sec);

3. Vary gain to get the command response you want. We start at 10, and if it's too sluggish bump it up, if it oscillates or limit cycles (bounces back and forth due to backlash) we back it off.

Sorry for the terse explanation, but that's pretty much it. We do all our own code, so the PI controller is just written into the yaw service function. Sorry I don't have the code right now, but I can find it.

Cheers,
Steve.