Having difficulty with timer

Hello all. I am trying to create an instruction that will do the following function:

When the input from either a momentary pushbutton OR a differential pressure switch becomes TRUE, immediately energize a solenoid coil, and keep it energized for 15 seconds, REGARDLESS of state of input. (This is because the pushbutton will first turn input TRUE then FALSE when it is released, but the pressure switch will keep the input TRUE until the differential is eliminated).

I have made several attempts, but they all end up having the same problem, which is how to trigger an OFF timer by turning input TRUE, then instantly turning it FALSE to start the timed output. The diff press switch being maintained is my obstacle, I think.

Shane

I think I may have found a good place to start already, with a one-shot.

Shane

If I was doing this with an Allen Bradley PLC I would use the Timer Timing Bit. (.TT) as a holding circuit.

if push button OR Timer Timing then Timer Runs.

If Timer Timing OR Pressure Switch Then Solenoid.


Use a normal timer TON

Tof

This would probably work too, although the TON explained above may be better as this could fire the timer again immediately after the 15 seconds if the pressure switch or pushbutton is still made. Not sure if that's a good or bad problem.

sigshane said:

Hello all. I am trying to create an instruction that will do the following function:

When the input from either a momentary pushbutton OR a differential pressure switch becomes TRUE, immediately energize a solenoid coil, and keep it energized for 15 seconds, REGARDLESS of state of input. (This is because the pushbutton will first turn input TRUE then FALSE when it is released, but the pressure switch will keep the input TRUE until the differential is eliminated).

I have made several attempts, but they all end up having the same problem, which is how to trigger an OFF timer by turning input TRUE, then instantly turning it FALSE to start the timed output. The diff press switch being maintained is my obstacle, I think.

Shane

Click to expand...

I'm guessing there is an ambiguity here... on one hand you state that the solenoid should be turned on for 15 seconds by the pushbutton, but you also suggest that the solenoid should be operated until the differential pressure is removed.....

I'm guessing (again) that the purpose of energising the solenoid valve is to reduce the differential pressure....

The ambiguity is here "and keep it energized for 15 seconds, REGARDLESS of state of input. " - what input? pushbutton?, pressure switch?, or both?

Lots of people can "guess" what you really want, and come up with workable solutions.... but you need to clearly define the goalposts with a functional specification.

Reading between your lines, I'm guessing a reasonable functional specification might be....

1. If the differential pressure switch input is ON - energise the solenoid output until the differential pressure switch goes off.

2. If the operator presses the pushbutton, energise the solenoid for 15 seconds, even if the operator holds the pushbutton on.

That's my take from you put in your original post, but your requirements may be different - that's what's missing...

Once you have an unambiguous specification, the coding is relatively trivial.

Now I'm wondering why you have an operator input to do what the pressure switch can do.. are you expecting operators to be watching the process continuously ?

Okay, here's the description of the circuit. At the hydroelectric power plant where I work, we have dual chamber, self-shifting, self-cleaning water strainers, for filtering the water used for cooling.

Right now there are numerous electromechanical relays which, upon sensing a ten psi difference between inlet and outlet, will initiate a fifteen second strainer chamber shift by energizing a solenoid on an air valve, then a thirty second backflush on the clogged chamber. The cost of one of these timer relays is well over $300, and with eleven strainers, with 4 timer relays per one, the cost to repair just one of those could easily exceed that of just retrofitting the controllers with a PLC.

I want to do that with a small, standalone PLC. I'm seriously considering a Rockwell Micro810 for this purpose.

I think I may have solved my timer problem, though. I will post my workings when I get back to work on Monday.

Shane

daba said:

...
Now I'm wondering why you have an operator input to do what the pressure switch can do.. are you expecting operators to be watching the process continuously ?

Click to expand...

The purpose of the momentary switch is to allow for a manual shift/flush of the strainer during monthly inspections. Sorry I failed to explain that in my previous response.