Feed-forward coefficient for motion-constrained system

[AustinSchuh]

Originally Posted by Jared Russell

While a constant velocity test is the simplest way to calculate this gain, it isn't the only way. The first step is an initial guess. This is usually pretty easy...All you need to do is figure out the free speed of your mechanism by looking at your motor specs and effective gear ratio (and compensating for efficiency losses if you want). Kf = Full power / free speed. Once you have this guess, come up with a couple reference motion profiles to test and use a controller to follow them using *only* feedforward gain. This won't be precise or robust of course, but it should give you a way to figure out if you require more or less. Iteratively tune your feedforward gain until you are tracking various profiles closely. Remember, it doesn't have to be perfect...That's why we use feedback.

When you are doing this tuning with the feedback off, make sure to do some short and long moves, especially on systems (like drivetrains) with lots of friction or other nonlinearities. You want to make sure you aren't over-fitting some specific motion. I worry more about making sure the shape of the acceleration is correct than making sure it matches perfectly.

It is possible to do the entire feed-forwards tuning and calculation from models, but that takes more math. That's how we did all the feed-forwards this year, and it worked really well.