Proximity Switch with Nematic Press

I am working on a plc system that works with conveyor belt and nematic press as outputs and 2 reed switches on the air cylinder and a proximity sensor on the side of the press. The machine is suppose to run food on the convoyer belt, sense it when its under the press, press it and then keep the conveyor running to the next food pressing that one and so on.
The only problem I got solving is the algorithm for the proximity sensor. Is there a way I can keep the input on for a few seconds only then turn off? Because at the moment the proximity sensor stays on and the press keeps going up and down, up and down :S
I tried timers but they only seem to work when the input is impulsed (the sensor input is constant).
How can I achieve a constant input being turned into an impulsed input is the question?

Since you didn't specify which brand and model of PLC you have, it's difficult to give you a definitive answer, but look into using a one-shot and an off-delay timer.

Has the system ever worked?
What PLC are you using?
Are your valves sprung return?
Have you programmed the PLC?
Check your logic it seems dodgy.

Its a siemen s7-200 and I am using step 7 micro/win 4.0

Steve Bailey said:

Since you didn't specify which brand and model of PLC you have, it's difficult to give you a definitive answer, but look into using a one-shot and an off-delay timer.

Click to expand...

Badger said:

Has the system ever worked?
What PLC are you using?
Are your valves sprung return?
Have you programmed the PLC?
Check your logic it seems dodgy.

Click to expand...

I am using Siemens s7-200, yes the valves are returned and the PLC is programmed. I might have wrote this in a rush but all is working except the proximity sensor.

1. conveyor belt runs
2. product hits sensor
3. conveyor belt stops
4. press presses down on product and comes back up (works off the two reed switches, which i've programmed accordingly)
5. once its all the way up the conveyor belt starts again and does the same with the next product

I have the problem at step 5 where because the proximity is still on the press keeps going up and down. I need to sort of either impulse the sensors input or cause the circuit to reset once the sensor goes off.

Thanks in advance

Steve Bailey said:

Since you didn't specify which brand and model of PLC you have, it's difficult to give you a definitive answer, but look into using a one-shot and an off-delay timer.

Click to expand...

Steve indicated you should use a rising edge one-shot (to 'impulse') your prox switch - feed this into a delay-off timer.

This will provide a single timed-period signal which should do what you describe.

I have the problem at step 5 where because the proximity is still on the press keeps going up and down. I need to sort of either impulse the sensors input or cause the circuit to reset once the sensor goes off.

Cancel the fwd command when the rev command is set.

I have the problem at step 5 where because the proximity is still on the press keeps going up and down.

Click to expand...

This may be a case where you need some Step Relays - internal PLC relays that are latched for each process Step, so the PLC knows where things are and does not try to repeat a step that has already been done. Often, the only way you can know if a step has been done (because the limit switches and prox switches keep repeating for the same actions) is to keep up with past history, and the way to do that is with Step Relays. At the end of a step, the next Step Relays is Set, and the previous one is Reset.

In this case, if the other steps are working, then you may only need one Step 5 Relay.

(PS: Is a "Nematic" press a new improved type of pneumatic press, or is that a brand name?