Motion Control and Motor Drives

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Forward motion stops when reverse limit transitions from high to low

Using MAX 1d interactive motion (just to recreate what is happening during normal stepper operation), I set up one axis of my motion system to cycle between two counts, with a 1 second pause between cycles. The stepper is attached to a butterfly valve, and the forward motion opens it about 45 degrees, and the reverse motion close it completely, actuating the reverse limit.

Most (99%) of the time, motion works as expected. Occasionally, when the reverse limit transitions from high to low during a forward move off of the limit, forward motion stops. I have attached an image that shows two screen caps of a VI that I used to monitor the limit switch and the voltage command. As can be seen in the image, when motion fails, the control voltage goes to zero at the same moment as the reverse limit goes low.

Servo drive: Kollmorgen MMC-SD-2.0-230
UMI: NI-7764
Motion Controller: PCI-7340
Software: NI-Motion 8.4, using MAX
OS: Windows XP

Thank You,

Rick Howard

 

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Hi Rick,

 

Just to confirm, in the axis configuration you have set Forward and Reverse software limits?  Also, do you have the Smart Enable configured for limits?

Cole R.
National Instruments
Software Engineer
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Software limits are disabled and smart enable is configured as well. To be clear, this is not happening during a find reference of any sort. It is happening during a normal move operation.

 

Regards,

Rick

 

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Hi Rick, 

 

Do you have a forward limit set up as well?  Is it possible that this limit is getting tripped at the same time as the reverse limit?

 

Cole R.
National Instruments
Software Engineer
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There is a forward limit, but I have verified (via datalogging) that it does not trip during the forward motion in question.

 

Regards,

Rick

 

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Do any errors or warnings occur using the motion error handler?  Also, does the code appear to just hang after the motion stops?    

Cole R.
National Instruments
Software Engineer
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Recall that I am using MAX in the 1-D Interactive mode to reproduce this error, and no errors are evident in the "Motion Function(s) Execution Status" detail. In fact, the "Current Trajectory Data" Position and Velocity values continue unabated, as if the motion were actually continuing.

 

Regards,

Rick

 

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Hi Rick,

 

Is there any bounce in your reverse limit switch? How did you get the plot from your first post? If you obtained it by using the analog input on the motion card, you probably wouldh ave missed the switch bouncing because the AI on the motion card is software timed. If you connect an oscilliscope to the reverse limit switch, you might see the limit go low, then high, then low again. This would cause the axis to stop and wouldn't throw an error beacuse the axis is supposed to stop when it hits any limit switch, even if it is moving in the direction opposite of the switch.

 

Thanks,

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The plot came from a separate data acquisition system in parallel with the limit switch and the voltage control lines, and was taken at 1000 Hz. If the switch were bouncing, I would expect to see it in the trace, but I do not.

 

If it turns out that bounce is an issue, is there a configuration setting to make the axis less sensitive?

 

Regards,

Rick

 

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