Motion Profiling

[AustinSchuh]

Good point Jared. Once you have a profile, you need to follow it.

My favorite controller for following trajectories is as follows (I think this is a modification of 3). I used it before I got tired of hand-tuning the PD for the drivetrain and switched to state feedback.

pwm = P * error + D * ((error - prev_error) / dt - goal_velocity) + Kv * goal_velocity + Ka * goal_acceleration

This is close to a PD controller, but with some feed forward terms. The idea behind feed forwards is that if you have a pretty good idea about how much power it will take to do something, go ahead and add it in. The control loop will take up the slop from there when you are wrong, or someone bumped the bot.

The D term is special in that you subtract off the goal velocity. This falls out from the state feedback controllers. Think about it this way. If you are at the goal, and moving at the right speed, you don't want to apply corrective power to decelerate the robot (This is what D would do if you weren't trying to move but were moving and at the goal.)

The Kv and Ka feed forwards terms make a lot of sense. It takes power to move at a velocity, so add that power on... Same for acceleration. It takes power to accelerate, so just add the power right on. You can get the goal velocity and acceleration from your profile.

A properly tuned controller with motion profiles will produce some very nice and smooth motion.