Drivetrain PID tuning

[AustinSchuh]

Originally Posted by SamcFuchs

This is a good point, and I feel it would probably solve a lot of out problems. However, I haven't found a good way to get real time plots from the robot. Do you just use the smartdashboard?

This stuff happens so fast that it's not worth doing it in realtime. Each test is like 2 seconds, so we end up writing scripts to copy the data back and plot it.

This is almost a whole new thread, but it's actually hard to write code that is hard real-time on the roboRIO. Hard real-time means that 100% of the time, your code will finish in X us. That means all your algorithms need to run in constant bounded time, and they can't use any system calls which don't run in constant bounded time either.

This means you need to avoid
- Allocating memory (new, malloc, etc).
- file IO. (It can take an unbounded amount of time for your write system call to complete).
- algorithms which don't take constant execution time.
- Any other operations which aren't constant time.

https://rt.wiki.kernel.org/index.php/RT_PREEMPT_HOWTO has some good info on what real-time means. If you are interested in debugging real-time issues, I'm happy to post some more detailed information. I should really do a CD post some time on one of the ones I've found.

For us for logging, this means that we don't log data from our controls thread. We queue it up with a real-time bounded length queue, and write it to a USB stick mounted on the roboRIO from another process. This is a pain, but well worth it.

Another hack I'll use for debugging is to monitor the execution time of the syscalls I care about, (for example, the control loop execution time including logging), and re-run the test if there was a timing violation. This won't be real-time for running during a match where you can't replay if there was a timing violation, but lets you debug something quickly and know when you've affected your test results.