BX servo lock: jerky motion, not returning to position?

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BX servo lock: jerky motion, not returning to position?

Postby om_tech_support_JT » Wed Jul 03, 2013 2:43 pm

Q: I'm doing some testing with your BX series brushless motor system and I found something strange. After discussing with your technical support group, I've been told that if I put my BX motor in "servo lock" mode, it should provide holding torque and hold the position still. This works. Whenever I grab the shaft, rotate it to some position, then let go before the overload alarm generates (5 seconds), it will return to its original position. This also works. The problem starts whenever I grab the motor shaft in servo lock and rotate it manually to more than two revolutions (at the motor not gearhead shaft), it doesn't return to its original position. What could be causing this issue? Another issue I have on the same motor is that at servo lock/standstill, with the load, the motor shaft seems to hunt back and forth. When I rotate my disc, the motion seems very jerky. I have a pretty large disc for my load, so the jerky motion is even worse on the outside. Can you help me understand my problems?

A: Sure we can help. Your jerky motion issue is typically because the BX motor is undersized. Every BX model has a specific maximum load inertia specification according to the motor's rotor inertia and gear ratio. If this spec is exceeded, then the jerky motion would occur during rotation and holding. What the BX is really doing is stalling. Since BX uses an encoder to close the loop, when it senses a stall, it would try to generate more torque and speed to try to get the motor shaft back into synchronism. The switching between open-loop and closed-loop operation is causing the jerky motion. To accomodate for larger load inertial loads, I would advise that you use a bigger motor or a higher gear ratio on your gearhead if you can sacrifice speed.

http://www.orientalmotor.com/products/p ... _bl_bx.pdf

Your strange issue of the shaft not returning to its original position during servo lock took a bit more research.

The BX controller is designed with an internal limitation on rotation speed of 3500rpm for safety reasons. When the GP Gain is set to 50, the return speed will reach 3500 rpm when the motor shaft is externally rotated 806.4 degrees and released back to the original position. If the motor shaft is rotated greater than this angle, it can not return to the original position successfully since the return speed exceeds 3500rpm before it reach the original position, and the motor stops. The more the shaft is rotated externally, the greater deviation the motor will have when the shaft is released. If the grain is set lower, the forced angle can be greater.

The following is a sample equation:
40320 (speed parameter for internal calculation) / 50 (GP Gain) = 806.4 degrees at motor shaft

If the shaft is rotated greater than this angle, it can not return original position.

If we set the GP gain setting to 10,
40320 / 10 = 4032 deg or 11 rev at motor shaft

Hopefully this helps. Feel free to contact our knowledgeable technical support group or post here if you have any more questions.

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