Connecting a 10-30V 3-wire proximity switch to the UMI-7764

[TedAnderson]

Hello all,

We're getting started in motion control with a fairly simple stepper motor/actuator project.  We are using the PXI-7344, the UMI-7764, P70530 (NI & Danaher) stepper drive, and an N31HRHJ motor.   The actuator and limit switches are from Festo.  I'm attaching a pdf of the Festo connection diagram.  The switch we selected is of the PNP variety.

So far I've not been able to get the Festo switch to trigger the limit input on the UMI-7764.  The switch has an LED on it which indicates presence of a magnetic field or not.  I connected things in various configurations to no avail.  The UMI uses a 5V supply, so I figured that it wouldn't be enough for the switch, which is specified as 10-30V range.  I've tried connecting 24V power across the switch but must have some other connection to the UMI wrong....  I'd like to configure this so that the switches are Normally Closed. Are any resistors or other connections required to get this to work?

I've tested the UMI with a SPST switch and "MAX interactive" responds appropriately showing when the limit switch is active or not.

If you've been through this before, your guidance would be much appreciated...

Thanks much,
Ted



Message Edited by Bairoa on 07-29-2008 02:39 PM

[Jochen · Accepted solution]

Ted,

there are several potential reasons for this behavior. Here is a link that could help:
Limit Switches Do Not Work Correctly With NI-Motion Hardware

If the problem is not caused by a wrong cable as described in the link above, there is a chance, that the output impedance of your limit switch is too high. The input circuitry for limit switches on 73xx boards looks like this (for limit switches the UMI is just a passive connector block):



The 73xx boards in combination with a UMI-7764 are designed for direct connectivity to mechanical limit switches. Proximity sensors typically have a significant output impedance, which results in a voltage divider and there is a chance that the voltage at the input never drops below a value that could be detected as a low level. Please note, that the output voltage of your sensor should be between 0 and 5 V.

For proximity sensors it's not recommended to use the UMI-7764 but the UMI-7774, which has opto-coupled inputs which are compatible to typical proximity sensors.
If exchanging the UMI is not an option for you, you could try to put a resistor in parallel to your sensor to decrease the voltage level in the low state. The resistor shouldn't be too low, as this would reverse the effect (high level can't be detected anymore). You could try to find the correct value by using a potentiometer.
Still I recommend the UMI-7774 as a clean solution.

Kind regards,

Jochen Klier
National Instruments

Message Edited by Jochen on 07-30-2008 09:29 AM