Quote:
Originally Posted by Jay Lundy
By the way, if you do use a PID loop to control the velocities I think you can improve stability (ie overshooting) and probably even response time by using both feedback and feed-forward control.
Instead of sending this to the motors:
Code:
pwm_out = PID(e) // PID is your PID function, e is the speed error
send this:
Code:
pwm_out = guess(target_velocity) + PID(e)
The guess function takes your PID setpoint, the target velocity, and guesses what PWM output you need to reach that velocity. It doesn't have to be exact, but as long as you're close you can improve the performance of your PID loop. The guess function ideally ends up providing most of the output and the PID loop just makes small corrections.
By the way your straight drive code uses the same idea (both sides are fed throttle and the PID loop just makes small adjustments to keep them at the same speed). |
The feed forward option is almost always going to have better transient response in velocity control in terms of overshoot and oscillation, although it needs to be well tuned to improve rise time.
I would caution, though, that you probably want to code it like this:
Code:
pwm_out = feed_forward_gain*guess(target_velocity) + (1-feed_forward_gain)*PID(e)
Where "feed_forward_gain" is between 0 and 1 (functionally it will probably be .7+). You will want to do this using integer math, of course.
This clamps your output, and lets you use the same gains as in the pure feedback case.