View Full Version : Problem with speed control - smaller wheels or better PID

11-22-2009, 03:10 PM

it is the first time in my life I am programming a robot and going to a robotics competition and I have a problem with speed control.

Our robot has to find golf balls and kick them in to the goal. You can see it making it's first steps : YouTube- Robotex robot first goals

I want to have good speed control to be able to chase moving balls. We already have image processing giving fast enough feedback, but the motion control is not good.

I am suspecting that the choice of motors and wheels is the problem. We are using the wheels and motors from RD01 drive system (http://www.active-robots.com/products/motorsandwheels/rd01-drive.shtml)(the drive electronics and algorithm is our own).

To me it seems that the problem is the too high gear of this set-up. The wheel has too high diameter and the gearbox not high enough reduction ratio, meaning that it takes a lot of power to make the robot(~3kg!) move and when it moves, it wants to move quite fast.

With my controller, it takes too much time(0.5s) and distance for it to reach for the speed I want from stall. This is with the best P parameter I have, increasing the P would make it overshoot badly(full speed very quickly and then slowing down). And, already with the current P, decreasing the speed setpoint not too much will overshoot ie. the robot will stop for a moment and then regulate it to the right speed.

The controller is currently just P, but I think that in this case adding D or I won't help me much. The PID loop is running at 20Hz. May be running it faster could have a positive effect, but the encoders of this motor has bad resolution(20 clicks per period is max speed) so I would not get enough clicks between PID calculations.

I have the feeling that using wheels 2x smaller will achieve better control, but it will be a lot of work to change the current wheels and I am not doing the mechanics.

It would be good to hear other ideas and suggestions how to control the speed better and faster without changing the wheels before I ask the mechanics guy to take on this daunting task.

11-22-2009, 04:03 PM
Welcome to the forum!

Have you had a look at the motor calculator over at SOR? http://www.societyofrobots.com/RMF_calculator.shtml

12-02-2009, 06:46 AM
nice soccerbot btw!