Quote:
Originally Posted by Oblarg
This is likely a static friction problem, and the easiest solution I've found to deal with it is to implement a "minimum output" parameter that roughly corresponds to the smallest motor voltage which will result in actual movement. Then, you change your PID response so that the magnitude of the output is never less than that value, e.g. you can just add the value (with the proper sign) to any output, or you can simply set any output that is too small to the minimum value (again, with the proper sign). I think the former behaves somewhat better, but both work.
You can also implement cascading control, where the output of your angle PID is fed to a velocity PID for the wheels instead of directly to the motors. Thus, the internal PID loop will account for the problem automatically (provided it is tuned correctly).
You also really should not need an I or D term for a turn-to-angle loop.
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I've found the opposite. I and D are quite important for pneumatic tires on carpet. We added the equivalent of I this year, and have found that D has been crucial for years to get smooth fast performance.
I don't think you need to go to cascading control here. That sounds more complicated than needed. We run statespace controllers, but the only real difference between a SS controller and a PID loop is the improved filtering on the velocity term. Our heading controller works very well.
The first thing I tell people who ask me about tuning controllers like this is to start plotting things. With time as the X axis, plot left power, right power, right - left position, and gyro heading. You'll learn a lot from those plots. Feel free to post them as well if you want some more feedback.
Backlash in your drivetrain is also important. If you have too much backlash, you will hunt around the goal. A small move will stabilize because you don't get moving fast enough to need to flip to the other side of the backlash in order to slow down.
What frequency are you running your control loops at? Are you using the WPILib PID loop or your own? Too slow a loop time, or running the controller in the joystick code can cause a lot of problems.
Consider also adding in a motion profile. You can do a constant velocity profile, or try a trapezoidal profile. This will give your controller a bunch of small moves in a (more) feasible path which will be a lot easier to follow. We do trapezoidal motion profiles for all of our movements.
OP: From your profile, it looks like you are in the Bay Area. If the timing works out, 971 would be happy to chat in person with you guys, and take a look.