Boe-bot servo tuning...

OSOK

Hello all...

Calibrating my servos, the boe-bot will go straight with the settings 650 and 810. The book mentions that if the servos are 'severely' mismatched, then it'd be that kind of adjustment. Before I continue, should I go back to hardware tuning or continue with that much of a difference?

Thanks!
OSOK

D Faust

Do they both stop when set to 750?· If so I can't think of anything else that you can do.· If not then the screw that changes the the middle setting must be adjusted.· Hope this helps.· (Mine runs straight at 850 and 682)

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
D Faust

OSOK

Yea... they both stop at 750. But, isn't that screw also responsible for motions at other pulse widths? What if even though they both stop at 750, I adjusted one a tiny bit more than the other? my settings now are 807/650 for fwd and 679/850 for backwards.

Franklin

Servos are not exact devices as they were designed to be adjusted continously as they worked so repeatability was not a concern. Since they stop at 750 that's what the screw was for. The rest needs to be addressed in software. You might get by with just specifing an offset value or you might compute the offset on the fly.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
- Stephen

D Faust

First, Continuous rotation servos are regular servos with te potentiometer removed/disabled.· The screw tells the pot what is the center of movement.· The pot (if it is just disabled) will give a consistent reading. The screw is used to make the "center" where the pot is so it has achieved its goal and thus stops.· The servo changes speeds at different·pulses because it is trying to reach different positions and the speed is proportional to the distance from the center.· In your case one of the servos just goes faster that the other.· If you move the screw the servos will never meet the center requirement and thus will never stop.· I hope that I explained all of that correctly.· Hope this helps, post any questions.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
D Faust

OSOK

Thanks for the info guys... I was just wondering if the difference I have was too large... but I guess not, so I'll continue on.

Thanks again!

Andy Lindsay (Parallax)

Hi OSOK,

Your servos are fine, no need to worry. The 850 and 650 values are outside the the range of linear speed control. So, for a large change in pulse duration, like 850 to 810, there won't be that much of a change in speed. So your speed adjustment was actually pretty small.

It's only when you get in the 700 to 800 PULSOUT Duratoin range that the pulse duration starts looking proportional to speed control. Check out the graph on page 115 of Robotics with the Boe-Bot. Speed control vs. pulse duration is mostly between 1.4 and 1.6 ms pulses, which translates to PULSOUT pin, 700 to PULSOUT pin, 800.

Given that 810 straigtened out the Boe-Bot's forward motion, you'll probably see very similar graphs for the left and right servos when you try Chapter 3, Activity #4.

Andy

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Andy Lindsay

Education Department
Parallax, Inc.

OSOK

Yes Andy... I did that portion, filled out the table, and my graph looks identical to the one in the book. I was just worried because the book mentioned it as a "severely mismatched" if it was a diff of 40 or more... but it's ok now.
Thanks again!

Andy Lindsay (Parallax)

Ah, okay. That's a mistake in the book. Sorry. I'll send a link to this thread to editor@parallax.com, and we'll make sure to fix that in the next revision.

Thanks OSOK!

Andy

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Andy Lindsay

Education Department
Parallax, Inc.

OSOK

Do I get a mention in the back of the book?