PLC assignment Help needed

I have been asked to design a PLC program that will contain a start push-button that will turn on a heater immediately after being pressed. Then 3 seconds later the injection filler will be turned on and after 3seconds the injection filler has to turn off and 3 seconds later a vacuum pump has to be switched on.

The stopping process has to contain a stop push-button that once pressed will immediately turn the heater off and after 3 seconds will turn the vacuum pump off.

The start push-button and stop push button must be interlocked so that the start process cannot be activated whilst the stop process is ongoing.

I’m using the Siemens STEP 7Micro/Win plc at college.

I’m having 2 big problems. Firstly the T38 timer keeps timing up to its preset value then it opens the input T38(rung 3) very quickly then it becomes closed again this means that Q0.1 doesn’t go off this also prevents the input T38(rung 5) from closing and turning the T39 timer on.

Secondly when I press my I0.1 stop button the whole process goes off at once.

Can anyone tell me where I’m going wrong? I really could use the help.

Network 3 and 4 are acting like a cycle timer. When T38 reaches the PV, it turns off Q0.1 for one PLC scan, which disables T38. On the next PLC scan, since the T38 contact is closed again, it will begin counting up T38 until the cycle repeat itself. It will happen so fast you don't see it. T38 probably counts 0 to 300 over and over...

In network 4, try changing Q0.1 to T37.



-Eric

Ive changed the start process but not sure if it will work as I've not got a plc at the house.

Still not sure about the stop process.

Is it possible to put a normally open M0.0 latch under input T39 so that it would continue to allow Q0.2 to be ON even after Q0.0 has been switched off?