Re: [WheelWatcher] Help with tuning

[Pete Skeggs <plskeggs@xxxxxxxxxxxxxxxx>]

Rob,

Sorry you are having trouble.  I do need to update that example to fix 
some problems in it -- it's not your fault that it is unstable at low 
speeds.  I've been buried with putting finishing touches on the new 
WheelCommander WC-132, which includes many of the same ideas as the 
example you refer to, but has a lot more in it, and is a "canned" 
solution.

One big difference internally is that I gave up on having nested 
velocity and position loops, and made the position loop such that it 
produces motor commands (PWM values and directions) directly, bypassing 
the velocity loop.  I also found that I had to add an offset to the 
calculated PWM value to overcome friction and deadbands (in servos at 
least) at the low speed end.  Finally, it helps to have a low loop 
bandwidth (6 Hz) when slow motions are required.

Making low speeds stable was not trivial, but is doable.  I'm really 
busy tomorrow, so it might be a day or two before I get back to you.

Thanks.

Sincerely,
Pete Skeggs
WheelWatcher Tech Support

rob.stehlik@xxxxxxxxxxxx wrote:
> Hello,
>
> I have just installed a couple WW02's on my robot which has custom 
> wheels and motors. I am currently trying to get the advanced velocity 
> and position control code working (I am using a PIC 16F877A with CCS), 
> but am quite confused about the way the feedback controller is tuned. I 
> am used to adjusting gains for P, I and D terms, but this code seems 
> different. Currently I have it so the code functions properly, but the 
> robot's movements are not well controlled. IE when driving in a square, 
> the robot always overshoots, especially in the turns, and sometimes does 
> not recover. Many times the robot will get stuck oscillating around the 
> target really slowly. The positioning accuracy is terrible.  I have 
> tried adjusting the divisor in the integral calculation, as well as the 
> acceleration value. And I played with the deceleration term (was 
> originally 150). After running tests for several hours, the robot 
> performs a little better, but it really leaves a lot to be desired.
> So, I guess my question is what approach should I take when tuning the 
> controller? Which parameters should I vary? And considering my robot is 
> quite heavy and fast, is there anything I should keep in mind when 
> adjusting the parameters? Is there a way to set a minimum PWM duty 
> cycle? Because of my robots weight, it does not move well at slow speeds.
>
> I appreciate your help,
>
> Rob