View Full Version : Switching duty cycles of machines

March 8th, 2006, 10:06 PM
I am currently designing a control system to control 2 compressors. The current design uses an Allen Bradley PLC (1747-L542B 5/04 CPU - 32K mem). I am having a little difficulty with the implementation of the ladder logic.
The way the compressors work is that 1 compressor will pump up to the required pressure then switch off. When the pressure drops to a certain low level the other compressor will switch on and pump the pressure back up then switch off. The first compressor will then kick back in once again when the pressure drops below a certain pressure (and so on). To add complexity, if the required pressure is not achieved in 20mins, the next compressor switches on at the same time to pump up the pressure faster. Regardless of pressure they will both switch off after 50minutes of continuous run time for a period of 10minutes.
The Alan Bradley PLC that I am using is prgrammed using RSLogix 500 and this is where the problem occurs. I am not very familiar with this software. Would anyone be able to suggest a ladder logic (even prmitive logic which I can convert to ladder would be fine) which would ******* the compressors in the afore mentioned way?


March 8th, 2006, 10:58 PM
Do a forum search for JK flip flops.
You can find an AB solution on the forum.
Divide up your application as follows:

If air pressure < setpoint
then Compressor (FlipFlop) on

If air pressure < setpoint for 20 minutes
then Compressor (FlipFlop & notFlipFlop) on

If Compressor On < 50 minutes
then Compressor off

Myself or someone else can help you with the ladder specifics if you'll work at it.

Welcome to the forum.

March 9th, 2006, 01:27 AM
I would suggest to draw a flow chart specifying exactly how you want this to run, including all branches, time limits etc.
This will then not be too hard to implement as a ladder logic sequence.

March 9th, 2006, 03:42 PM
Thanks for the help guys. I think I know how I will go about this now.