Change sequence of contactors each time they start up using plc program?

[Murrayrfc]

Four contactors k1, k2, k3 & k4. If temperature goes over set temp by, 1 degree bring one contactor in, two degrees bring two contactors in, three degrees bring three contactors in & four degrees bring four contactors in. But my problem is: how do I write the plc program so that it's not always the same contactor that comes in first. (different sequence everytime it goes overtemp)?

 

[rstech]

Try to think of how you would systematically approach this problem using plain English. Worry about the programming after you have a clear idea of how you will solve the problem.

For example. If you started with K1 you will need to remember that you started with K1 so you don't start with it next time. The plain English is "remember that you started with K1" where the PLC logic maybe to latch a bit called STARTED_WITH_K1. Next time you start up, the bit will be latched and your logic could have a condition to start K2 first if STARTED_WITH_K1 is latched. Continue this process with the other contactors and don't forget to unlatch your bits at the appropriate time.

 

[Murrayrfc]

Thanks. Ive heard that a shift register might be useful to tackle this problem? Any ideas?

 

[Jim Morton]

You could use a counter that is updated every time it overtemps. If the counter is at one, start this sequence. If the count is two, then start this sequence, etc. Once the count goes to four, start the last sequence and reset the counter.