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homing using Find Reference giving me Modal errors

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Hello,

We have an NI PXI-7354 Stepper/Servo controller controlling a Primatics PN 0-7205-0012 2-axis drive, running 2 Micos DT-120 rotary stages.  Recently we have been experiencing modal errors during homing "-70119 (NIMC_limitSwitchActiveError) The desired move cannot be completed because the limit input is active in the direction of travel."

 

This system has been running for years, an dthi sjust started popping up on homing which occurs once a day.  This happens when the Find Reference move goes to the reverse limit, as it is programmed and configured to do so.  It then performs a small move in the forward direction, and then this error occurs.  Now, i do understand that this is part of the Find Rreference command.  It seems as if through wear on the switch or perhaps in the stage itself that when it moves in the forward direction to go off the switch in the atempt to approach the revers switch slowly for an accurate home, that it is not actually off the switch yet, and the move fails.  It is not easy for me to change the switch on the stage, but it can be done.  I would like to be able to tell Find Reference to move farther in the forward direction so it actually gets off the switch before it approaches it again.

 

So, this is what I see is supposed to happen

1: Approach the reverse limit switch during homing

2: Hits switch

3: small move in the forward direction

4: move in reverse direction

5: hit switch - successful home

 

Does step 3 move a preprogrammed amount?  If so how do I change it?

Or, does step 3 move until the limit becomes inactive, in which case I am having bouncing or noise on my switch input.

 

I tried to adjust the home switch position, there is a small amount of adjustment possibe.  I also changed the speed of the Find Reference move from 50000 c/s to 25000 c/s and I have not seen another failure yet, but I am not confident that this is the best and most permanent course of action.

 

Any help would be greatly appreciated.

 

 

 

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Accepted by topic author Palickar

Have you had a chance to look at http://www.ni.com/product-documentation/4190/en/ ?  Your thoughts sound correct that the limit switch is still being activated even after step 3, which should deactivate the switch.  It sounds like ways to get around this issues may be to either change the final approach direction or the speed of the move.  From this page about find limits:  http://zone.ni.com/reference/en-XX/help/372134F-01/ni-motion_help/findforwardandreverselimits/ it seems like either final approach direction should work, because in step 3, it should see the switch deactivate before approaching again, but maybe give changing the final approach direciton a try.

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Accepted by topic author Palickar

Thanks!  Yup, I read the help I have with my installation of NI-Motion, but the links you referenced had some more detail and were very helpful.  I did reduce the speed of the initial approach, and for now, I am OK.  I think eventually I will need to replace the switch.  I might try approaching the the limit from the other direction, but am a little apprehensive on what that would do in this application.  Still it's easy to try.

 

Thanks for yuor response!  I am used to doing servo programming on a very low level, and not having every single action totally under control is a bit tough getting used to.  But then again, this whole system is pretty easy to deal with.

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