120v prox input to plc 1

[powersoff]

I am using a 3 wire 120vac prox switch as an input to a micrologix plc. The input is dropping out(indicating light flashes off then back on erratically). If I wire the prox directly into the plc bypassing all the field wiring the input responds the same way.
Is a resistor the answer?

 

[PLCMentor]

How about posting the part number of the prox and how you have it wired to the PLC.

Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com

 

[powersoff]

I will post later with the part #.
I added a 1/2 second off delay timer in the program. That has stopped the input from causing chaos but I would still like to know what the issue is.

 

[itsmoked]

Prox sensors always take some thought about their application. Almost never can you stick one on a machine and expect to use it immediately. They often have very little hysteresis. This means you command the machine to stop and vibration, or hydraulic leak-down, causes it to de-trigger which needs to be considered in the software.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[roydm]

There are very few 120V prox switches that will work with a PLC. Most have a leakage current that is higher than the PLC input card Turn Off current.
Post your Prox switch part No and your PLC input card. so we can recommend a solution.

Roy

 

[powersoff]

Banner S18RW3FF100 W/30 sensor

AB Micrologix1500 1764-24AWA

neutral -Blue(prox)-ACcom0(PLC)
120vac -Brown(prox)
Black(prox)-I/1(PLC)

 

[PLCMentor]

I looked at the particular prox switch you have. I did not realize that it is an IR prox switch, I am more familiar with the inductive type. The first thing I notice about this particular prox is that it is listed as a DO or Dark to Operate - ie the sensor conducts when it does not see the IR signal. With that said, you should see a signal when the prox is benched and has no target in front of it. Since it should be on, the problem with leakage and not being able to properly cutoff the input is possibly a separate issue, but shouldn't affect the on state - just turning off. The prox has a green led and a yellow led. Check them. The green is for power and should be on continuously. The yellow indicates light detected and should be off without a target. That light should be flashing similar to your input. Check that. If it is then the problem is not with the connection or the PLC input, but with the sensor. This type of prox may have problems with ambient lighting sources. Just a guess. Is there a gain pot on it? Maybe you can tweak it so that external sources are not a factor.

Russell White, P.E.
Automation Technologies, Inc.

http://www.AutomationNC.com

Automation Help

http://www.PLCMentor.com