Experimental Physics and
Industrial Control System

The problem of the motor moving away from a limit switch and then quickly stopping again is often caused by noise on the limit switch lines.  When one limit switch is open (the one you originally hit) it can make the second limit switch even more susceptible to noise.  When you move away from the first limit switch noise on the second limit line causes the motor to stop again.

We often fix this by putting large capacitors on the limit switch lines to increase their time constant and reduce the susceptibility to noise spikes.  You can put the limit switch signals on a scope, and look to see if there are noise spikes.

This problem can be difficult to track down, because a noise spike only a few microseconds long is enough to cause the motor to stop, but when you then look at the limit status it is OK because the spike is gone.

Mark

________________________________
From: [email protected] [[email protected]] on behalf of Mi Qingru [[email protected]]
Sent: Thursday, May 30, 2013 3:14 AM
To: tech-talk epics
Subject: PI C863's direction status can't change in time as leaving the limit switch

Hi all,

    After the motor drived by PI C863 hit the limit switch and stopped, it would be commanded to leave the limit switch and should move in reverse direction. But the motor record often stopped immedietely as the moving started, and the motor actually did move on the command. The motor record seemed to restart after the “STOP/GO” button is pressed. Sometimes the button didn’t work too.

    The debug information showed that the direction status and the limit switch status in C863 couldn’t change when the motor record queried C863 first time. So the motor record thought no motor in moving , then wait forever. Even if we reduced the scan rate, the problem still occurred. The engineer from PI told us the time in which the direction status changed was not sure, and it depended on the difference between the current position and the origin position from the encoder.

    Is there any ways to resolve the problem?


Mi Qingru
SSRF

References:

PI C863's direction status can't change in time as leaving the limit switch Mi Qingru

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