Limit Switches interfacing with PCI-7344

[Perryg114]

I am trying to figure out the best way to calibrate encoders with the PCI-7344 interface card. Should I setup my limit switches to stop the axis at the limit and then reset encoders there or what? Or should I have a home switch somewhere in the center of travel and let Labview reset the encoders at that point? The 7344 card has digital I/O but they are not configured as anything at the moment. I assume there is a vi to do this? What is the accepted way to do this?

Perry

[Matt-J]

Hi Perry,

What sort of behavior is your motion system going to have? Typically, you would want for one end (if linear) to be the home. So, I would probably approach it that way. But, what is your system going to be like? As for the digital I/O, there is a palette of VIs to handle this within the NI 73XX functions palette. See the pic.

[Perryg114]

It will be a linear axis with a limit switch at both ends. I have not researched it but some motor drives have a setting to automatically home and I would assume they would reset the encoder to 0 or a specified value at that point. I would prefer the drive controller handle limit switches directly for safety purposes. When Labview is not running, the motor is free spinning so something to safe the system when the computer system is not on would be a good idea. In the past, I have defeated the axis with a normally open relay. If limit switches are run through Labview they don't do anything to limit axis movement when the computer is not running and Labview is not in control.

Perry

[Buechsenschuetz]

I think there should be no problem connecting the limit switches to both the NI controller AND your driver system. You should check the input specifications (and, if possible, input circuitry) of both to make sure that there are no 'side effects', i.e. the limit switch inputs of both devices should be properly decoupled. 

We are using NI motion controllers for controlling linear axis, too. Since our systems require a well-defined home position we use a home switch somewhere between the limit switches (not necessarily the center. We had some problems with using a mechanical switch as home position until we added some circutry to debounce the switch. For the limit switches you do not need any debouncing.