How do I change behavior after finding limit switch

[Richard_Jennings]

Hello,

I'm using LV8.2, PXI-7350, NI Motion 7.4.0.3002, and a UMI-7774 along with a 2-axis linear servo system from Parker. When I use the find reference VI in LabVIEW to find the forward or reverse limit the correct axis moves in the correct direction, hits the correct limit switch, and then slowly reverses direction until it hits the opposite limit switch, then reverses direction until it hits the correct limit switch, then it stops. Is there any way to force the system to halt the first time it hits a limit switch? I've tried setting the Set U32.flx attribute "Stop Type on Switch" to 1 "Halt", but this did not seem to help. I did not see a way to change this behavior in MAX either.

Thanks,
Richard

[Jochen]

Hi,

the stop type just defines the type of stop operation that is executed when a limit switch is hit. Some users prefer the axis not to do a halt stop but a decelerated stop when a limit switch is hit to avoid high torque.
The behavior that you describe sounds a bit strange to me. Is there a chance that you have connected the fwd limit switch to the rev input and vice versa?

Best regards,

Jochen Klier
National Instruments

[Richard_Jennings]

Hi Jochen,

The behavior seems strange to me also. If it was a matter of the wrong limit switch then I would expect the system to stop on the other switch. I've reset the board to factory defaults and it didn't help. The board/system worked correctly when I first started development 3 weeks ago. The first task with the system was to get the motors driving in the correct directions and get the limit switches operational. After this was working I went about working on the application. As it's close to shipping, I'm going back over the system and re-validating everything. That's when I noticed the issue with the limits.
I've verified all signals with a scope and meter. When the stage hits a limit the limit input goes to 5.7VDC and 8.8 mA of current flows into the limit input. Additionally the correct limit switch indicator flashes on the UMI7774.

Richard

[Jochen]

Hi Richard,

this is the normal sequence of moves when you search for the forward limit switch with find reference (initial search direction = fwd, final approach direction = fwd):

1. The axis moves forward until it hits the fwd limit switch.
2. The axis moves slowly backwards until the limit switch is not active anymore.
3. The axis moves slowly forward until the limit switch becomes active again.

It looks like that in your case step 2 doesn't work as expected. I could imagine that a badly bouncing limit switch could cause such a behavior, especially if the "Limit Filters" are disabled. Please check this setting in MAX (Here you can see where this setting is located in MAX).

Another thing you could try is reducing the velocity for for the initial search move.

I hope this helps,

Jochen