using plc with magnetic proxy switch

I am currently programming a pnuematic oscilating machine to shake parts out of a sheet of sheetmetal. The problem that I am having is that when the machine is running the rams go right by the proxy switches so fast because of over travel of the ram that the proxy signal doesn't deactivate the air cylinder. The proxy's are 24vdc as are the inputs of the PLC. I am using an IDAC-24 PLC. Does the input need to be followed by a oneshot so the PLC will pick it up? Is this a timing Problem? I would appreciate any help as I am inexperienced at this. Thanks in advance for everyone's help
Danco

You may need to use a high speed interupt driven input so the PLC can react fast enough. PLC's execute program scans in milliseconds or longer, dpending onthe machine and length of code. I am not familiar with the PLc you are using so I cannot say if yours can do this.
The PLC may not have time during the normal scan to react to the proximity switch.
Not knowing the geometry makes it difficult to suggest a solution, however a couple possiblities come to mind. 1) make the target larger if possible. 2) change the proximity switch to a location at the end of travel, so the target is moving slower. 3) get an interupt driven PLc. Before I did that I would want to put a scope on output from the Prox so I can see how wide the pulse is an how often it is occuring.

If I understand you correctly, you're trying to reverse an air cylinder when it reaches a proximity switch that detects mid-stroke (by mid-stroke I mean somewhere other that the end of stroke).

If you've PROVEN that it's not working simply because the switch is not ON long enough for the PLC to read it, there are a few things you can do. Adding a one-shot is not one of those things...

You can INCREASE the length (time) of the signal by adding a longer actuator (or 'flag') to the switch. This will give the PLC time to read the signal.

OR, look at increasing the response time in the PLC. Can you lower the filter time on that specific input? You may have a 10ms or so delay as a default setting. Also look at using an 'interrupt' is the signal is really fast.

Do you really need the 'extra' stroke? IOW, could you add a physical stop to the cylinder so it can't go past the switch?

beerchug

-Eric

Beaten again!...

Hmmm, I see you and I think alike, APB...

You're just a faster typer!...

beerchug

-Eric

May want to check the speed of the cylinder and compare that with the scan time settings, if there are any. I assume you meant an IDEC plc but which one? I have worked with the Micro1 when Square D brand labeled it.

What you may want to do is "latch" a bit with the prox switch and use a timer to unlatch it after x period of time and reset the timer.

Make sure the prox is actually detecting the ram. A standard 12mm inductive prox detector should detect the ram at no more than 2 mm away from it. The larger the prox the farther away it can be. There are also extended range proxes that will double the distance detected. If that's not the problem you could try latching it in the program or use a hysteresis timer to ensure it stays on for whatever fraction of a second it needs to be.

Let us know how you make out.

Have you proven to yourself that the function will work at slow speeds? Put some restriction in the airline or run at a lower pressure so the cylinder moves at slower speed and see if the thing does what you expect. This will give you a better shot at evaluating the machine actions as well as your logic.

Keith