Quote:
Originally Posted by thatprogrammer
This post is very useful, but I had a few questions based on it.
1. Assuming that you use your gyro to calculate your angular offset rather than "angle = (right_distance - left_distance) / width." How would you calculate the turn voltage part of the bottom equation?
Would you just use a full PID loop to calculate a voltage (and tune it) and then just add the result of that to your distance PID loop?
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Pick your favorite controller. P, PD, PID, LQR, Lead/Lag, MPC, YMMV
I try to start with what boils down to a PD loop before I pull out I or any other terms. The input to your controller is the angle, and the output is the voltage.
Quote:
Originally Posted by thatprogrammer
2. Do you have any tips for tuning PID loops in cases with lots of friction like turning PID loops? Jared (very kindly!) sent me some tips on tuning PID loops before, but I'm curious to see if there are other viewpoints on this in situations with lots of friction.
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I've got a bit of a random pile of advice, but hopefully some of it is useful when the robot isn't working right. Sometimes you just need to try different things to figure out what's going wrong.
If you've got a bunch of friction, you end up needing to slow things down a bit. Friction is a pain to deal with. You also need to ask yourself really how accurate you need to be. If you can tolerate moderate steady state error, leave it as a PD loop. As the robot goes faster, it takes less energy to get it to steer, which works in your favor. If you really need good tracking, you are going to have to work at tuning I correctly.
Consider running in low gear if you have a transmission so friction is a smaller portion of your overall torque. The most annoying steering loop I tuned was 254's 2011 robot, geared for crazy speeds. We needed to run auto in high gear too, which meant we were close to saturation all the time.
The most annoying issue I've had tuning heading PID loops was where there was lag in the gyro angle reading. That phase lag meant I couldn't push the bandwidth of the loop up to anything useful. I had to fix that before I could get it to stabilize well. I debugged that by plotting the gyro and encoder headings.
Run your loops at 100 - 200 hz. You want to run your loops at 10x the frequency of the highest frequency you want to control. So, if you want to control at 10 hz, you need a 100 hz loop. The more reliable you can get your loop timing, the better. We go to great lengths to hit a 5 ms +- 5% loop timing, and it helps a lot.
The most important part here is to plot everything. Plot your error vs time, and watch it evolve. Plot the power due to P, I, D, FF. Plot the encoder based heading and the gyro based heading. It's possible but hard to tune these things by eye. Honestly, sometimes I think it's easier to rough tune them by listening to them and listening for the overshoot, and then reaching for the plots when I'm stuck. I also like to grab it and feel the loop, though you have to be very careful since robots can cause lots of damage fast.
We use more complicated controllers, which means I have less recent relevant robot experience than I'd like here. I should probably go grab a robot and tune a PID heading loop again just to have some more guidance.