PLC programming problem

lilocomotiv · May 1, 2009

I got a problem here. I have to control Hydraulic Supply(Mot Starter) and 3 solenoid valves with a PLC. I have a feedback coming back from Hydraulic Supply. I dont want to turn the supply/valves ON or OFF if the feedback is above certain value. Look at the attached PDF. With that setup I can't turn ON the supply/valves if the feedback is greater than certain value. But it will turn OFF even if it is greater than certain value, which is not what I want. Can I do this in any way? When I use other programming languages(Labview) I just disable the pushbuttons. I dont know If I can do it with the PLC. Please help.

just_lionel · May 1, 2009

What do you want to happen to the valves when the feedback is above the setpoint? Hold current state?

WireGuy1950 · May 1, 2009

You are already disabling a pushbutton when you select to start the motor.
You can do the same thing with the valves also, but from you diagram I don't see how you turn off the valves. You inhibit their starting, but you can't turn them off.

lilocomotiv · May 1, 2009

Yes. Hold the current state. I should not be able to turn ON or OFF if the feedback is above setpoint.

lilocomotiv · May 1, 2009

Wireguy...thx for the reply. I know am disabling X1 is start motor Rung. I am talking about C30 contact. And it only works one way. If feedback is high, i still be able to turn OFF starter and valves. I dont want this to happen. In case of the valves, C0, C1 & C2 are toggle switches. So it controls the ON/OFF feature.

bernie_carlton · May 1, 2009

Try this

lilocomotiv · May 1, 2009

Thanks Bernie...It works!

Lancie1 · May 1, 2009

I know that Bernie's solution will work perfectly, but it bothers me to see a STOP button bypassed.

What would happen if your analog input failed high (sending the maximum signal) while the pump is running. How would you stop the pump? How would you close the valves? Maybe it is not a problem in this case, but when you make an exception, it leaves the door open for some idiot to do something really dangerous.

If you bypass the so-called Stop button, then it is no longer a stop, so you need to call it somethig else, but what? Maybe you could call it "Stop-Only If Things are OK", or "Stop-Maybe?"

Now you need another button that really will stop the pump if things get out of control. Perhaps a SHUTDOWN switch that overrides everything?

Another way would be to change your switches from the maintained-contact type to a momemtary-contact, so that the valves are not hard-locked on or off.

bernie_carlton · May 1, 2009

I had the same misgivings but, as he knows the system better than I do I figured to just implement his request. A software stop isn't an E-Stop anyway unless the system is REALLY BADLY designed.

I'm guessing that the valves he's turning on are designed to start but not stop a high flow (probably destroy them). Something else must affect the feedback but since the first output, by his own description, controls the Hydraulic Supply (Motor Starter) I kind of wonder what the setup looks like. (You can't turn it off until it's almost off anyway?)

I hope lilocomotiv get's back to us with more details just to satisfy our curiosity.

lilocomotiv · May 3, 2009

I understand your concerns. I do have a E-STOP button(Hardwired) which will kill everything if something like that happens. Only reason I asked for this change to not allow users accidently shut off valves/Hyd Power Supply while it is running at 3000 psi. Again thanks for your help!

gmferg · May 3, 2009

In this case, maintaining the valves open is safer than closing them before pressure is down to a safe level.

Reading the OP again, I see no mention of locking out the pump, you just want to lockout the valves, correct? If this is the case, remove the NOT C30 from the Mot Starter rung. This would make it safer since the stop button will still be active for the supply pump.

bernie_carlton · May 3, 2009

I have to control Hydraulic Supply(Mot Starter) and 3 solenoid valves with a PLC. I have a feedback coming back from Hydraulic Supply. I dont want to turn the supply/valves ON or OFF if the feedback is above certain value.

He refers to BOTH the supply and the valves in his description of the devices he wishes to control - specifically not to change state if the flow is greater than a certain level.

gmferg · May 3, 2009

Yes, you're right Bernie. I had to read it again. Forget my last post.

lilocomotiv · May 3, 2009

Initially I thought about doing it. After reading replies from you experts, I might just go with locking out just the valves. I will also have an E-STOP(Hardwired) for the worstcase. Thanks!

Lancie1 · May 3, 2009

Li locomotiv,

If you do lock out the Pump Stop PB, then you probably should re-label that button to something like "Conditional Shutdown" or "Low-Pressure Stop", meaning that you can only stop under certain conditions (if the pressure is not high).

PLC programming problem

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