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Unread 23-06-2002, 22:34
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Re: Drill Motor Inconsistency

Posted by Rick Berube, Engineer on team #121, Rhode Warriors, from Middletown H.S..

Posted on 4/17/99 4:35 PM MST


In Reply to: Drill Motor Inconsistency posted by Raul on 4/15/99 10:14 AM MST:



: We have noticed that the drill motors on our practice robot do not run at the same speed at full throttle. One of the motors is running as much as 20% faster in one direction and 5% in the other. The obvious affect is that the robot travels on an arc instead of in a straight line.

: Has anyone else dealt with this problem? We obviously cannot try to fix this with HW because then spare drill motors will have different characteristics. Did anyone have an efficient SW fix for this?

: Raul

Yes. We saw this problem both last year and this year. our motor's this year are much more closely matched however. The biggest problem is due to the topology. Mounting the motors in a symetric fashion typically forces one motor to always run in the reverse direction of the other.
We got around this in software. basically we generated PWM to RPM curves for the motors. Since motor's are pretty much linear creatures at heart, (within most of their operating range), we limited the operating range of the faster of the two motors to that of the slower. We then overlaid the two PWM/RPM profiles using y= mx + b. By tweaking m & b coefficients accordingly, you should be able to align any two motors. We felt being able to control the robot worth the trade off in speed/power.
this year, since the mismatch is not as nearly pronounced, our kids simply make the adjustment when driving. They drive tank-style so this has not been a problem for them. we do not use the X-axis of the sticks. If we did, we would have chacterized and made the same adjusments we made last year.

PS. A word of warning. While it is tempting to use the look up features of the PBASIC language, this can be terribly slow when the number of terms get large. the lookup function on the PIC is reading EEPROM for stored coeffecients to compare against. If you plan to use a lookup statement, keep the number of terms down to a minimum. Otherwise, while you may find the control very linear, but the response very sluggish. Also because the lookup function may traverses the list differently base on he value of the stick position, the delay imposed on your control loop will vary accordingly. Scaling and adding an offset imposes a small delay which is always fixed in duration.

Hope this helps,

Rick


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