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[DPRG] Continuous rotation servo question

Subject: [DPRG] Continuous rotation servo question
From: Dick Swan dickswan at sbcglobal.net
Date: Mon Feb 7 11:31:06 CST 2011

Some related information on continuous rotation servos.

 

1.       Parallax makes a good quality CR servo for their robotic kits. It
has access to a adjustable pot to trim the center point. Adjustment is with
a small slotted screwdriver. They're the only vendor I've seen that has this
adjustment.

 

2.       The center point adjustment is necessary. Here's why. HiTec came
out with a productized version of CR servo in the last year or so
specifically designed for use with the FIRST FTC tournament. The first batch
of prototypes had many servos that were way outside the nominal 1.5
milliseconds for stopped; I recall more than one third had stopped points
that were more than 0.15 msec away from center so that when you sent 1.5
msec pulse they slowly moved forward or reverse. The prototypes used HiTec's
standard servo electronics board likely with the same sort of modification
that you can find on the web. I think the resolution was that they modified
a few of the components to use higher precision components.

 

3.       If you can't adjust the stopped pulse duration via hardware and try
to compensate for it in firmware then you'll need a custom value for each
servo.

 

4.       The "dead zone" in servos is quite narrow - +/- 5 microseconds or
so. So the nominal 1.5 msec stopped pulse generated by your firmware and the
actual stopped point in the CR servo need to be within this tolerance. The
electronics within the VEX CR servos (i.e. their motors) has a much wider
dead zone specifically for this problem.

 

If you don't want to avoid the above hassle, use Parallax CR servos.

 

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