DC servor motor

Friends, I'm using a DC servor motor with an quadrature optical encoder on the back end of the motor shaft. Ran into a problem as the shaft from the
motor that I have to work with has a tendency to settle into 6 positions in a revolution. The shaft will resist rotation initially when it is moved away from one of its 6 "comfort position" up until it is half way between the two "comfort positions". And then it accelerates the rotation to the second "comfort position". This is causing havoc when we try to run the motor at slow rate. Anybody has run into this problem? What's your solution? Do I have a solution other than replacing the motor? I'm not an expert on motors, so if anybody can explain to me what's causing this, it will be much appreciated. Thank you.
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Hi
It's called cogging, an ironless rotor would help, but I am wondering if maybe you are testing the drive circuit without connecting the motor up to whatever you plan to connect it to. You won't see the judder anything like as bad once you hang an inertial load on it.
just my .02p
Robin G Hewitt
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On Thu, 13 May 2004 10:35:37 -0700, motorguy wrote:

Looks like motor cogging or detent torque. This effect _should_ be hidden by the servo loop which should keep the motor withing a few encoder tics if well tuned...
Maybe your loop gain (KP) is too low.
Peter Wallace
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