Re: Erratic behavior, possibly contact related


M. Collins
 

On Thu, Sep 16, 2021 at 09:43 AM, Roland Christen wrote:
You probably had corrosion on the pins of the Y cable and possibly on the motor box as well.
  This seems to be the most plausible explanation. It's odd that everything worked during bench testing and the first night at the site, then the problem surfaced and persisted after cycling the connectors, replacing the mount controller, etc., and finally went away without any actions which involved the Y cable connections. But I've seen enough strange failure modes involving old equipment and poor contacts to know that almost anything is possible.

  Are there any common connections through the Y cable, e.g., a signal ground pin which is connected to both motor assemblies? Poor contact at one pin which can affect both motor drives would fully explain the symptoms observed last week. (The mount is at one of our remote sites, so I can't readily pull the cable to see how it's wired.)
Runaways occur when the encoder signals don't reach the servo controller, so the controller thinks that the motor is not turning and supplies more and more voltage and current to turn the motor shaft. Of course, the motor shaft is turning, but without the encoder feedback, the controller runs open loop.
  A question about this. On more than one occasion when one or both axes were running away, pressing the stop button on the hand controller had no effect and it was only possible to stop the motors by pulling the power connection. Does the fact that the motors continued to be powered despite the stop operation point to another possible fault?

  Thanks for the details regarding the servo loops and hand controller interface.

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