Thread scheduler for PID period control ?

[Greg McKaskle]

What is your CPU usage when doing the 15ms PID and the rest of your robot code? It is on the charts tab.

If you make it easy to change the period of the PID, you can decrease it and watch the CPU. This will also help understand whether the PID loop is waiting on I/O or CPU bound. Again, it may be useful to know the time spent within the PID routine, so if you add your getFPGA timing to the top and bottom of the function and display that delta as well.

Greg McKaskle

[gpetilli]

Originally Posted by Greg McKaskle

What is your CPU usage when doing the 15ms PID and the rest of your robot code? It is on the charts tab.

If you make it easy to change the period of the PID, you can decrease it and watch the CPU. This will also help understand whether the PID loop is waiting on I/O or CPU bound. Again, it may be useful to know the time spent within the PID routine, so if you add your getFPGA timing to the top and bottom of the function and display that delta as well.

Greg McKaskle

Agreed. We have a variable dt that sets the period for the timed loop as well as changes the PID values. We just need time on the robot to finish our tuning.

[TravSatEE]

I'd like the OP or any others that have done PID loop timing changes to share what inputs/outputs they are using. What do these custom PID loops do on the robot (drive, interact with game/field items)? I am curious as to what needs a custom timing rate, as I haven't done anything myself that needed this for a PID controller. What was the behavior that led you to believe you needed a faster timing rate? If you post your code repositories publicly, please share the URL so I can try to follow along with what you did. One of the posts mention a change of "PID values" (I assume the coefficients), why did you do this? Thanks!

[gpetilli]

Originally Posted by TravSatEE

I'd like the OP or any others that have done PID loop timing changes to share what inputs/outputs they are using. What do these custom PID loops do on the robot (drive, interact with game/field items)? I am curious as to what needs a custom timing rate, as I haven't done anything myself that needed this for a PID controller. What was the behavior that led you to believe you needed a faster timing rate? If you post your code repositories publicly, please share the URL so I can try to follow along with what you did. One of the posts mention a change of "PID values" (I assume the coefficients), why did you do this? Thanks!

We are controlling a Killough style holonomic drive (which depends on wheel "slippage") by using two orthogonal instrumented follower wheels (no encoders on gearbox) plus a 9DOF IMU sensor ($16 GY-85). For the X&Y translation velocity PD we use the velocity from followers plus the accellerometer on the IMU. For rotation velocity PI we use the gyro and compass on the IMU.

We found that the 20ms "periodic" loop rate varied too much for repeatable control - it was difficult to tune the overshoot vs. accuracy since we are controlling velocity but observing the integrated distance. We evaluated timed loops and found them to be significantly more consistent. The standard PID loop uses timed loops but defaults to 50ms, which is way too long for good control. There was a visible difference in the overshoot going from a timed loop of 20ms to 10ms, however measurements showed we were really only achieving 14ms. Our competition bot has the loop set for 15ms, is extremely responsive and exhibits very little overshoot.