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Berna

PLC-5 PID Convertion to CLX

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What is the best way to convert a PLC5 (RSlogix5) PID control to a CLX(RSLogix5000). We just did the same programming that was in the PLC5 and put it in the Controllogix. Put the same Gains same parameters etc. Didn't worked. We made some adjustments like, performing the PID via a Periodic Task and read at least 2 times the analog data on each loop (20ms). changed the gains . But the control is not stable , very noisy. We have changed the whole PID that was working on the PLC-5 program , so wanted to know if anyone has already done this before. Or maybe some recomendations , I dont know if it is high recommendable to buy the RSTune licence or so?. Thanks in advance.

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I did a PLC-5 to ControlLogix conversion last year in which I encountered timing problems because of the way the original PLC-5 programmer had used the PID instruction execution. I used the ordinary "PLC like" PID instruction in ladder logic rather than converting to the PIDE form in the Function Block Diagram language. In the PLC-5 you must execute the PID instruction at the same rate as the Loop Update Time in order for it to work properly. In my case, most of the PID instructions were configured for a 100 ms Loop Update Time but were programmed to execute ever scan (no pre-conditions, no Selectable Timed Interrupt). Because the PLC-5/80 controller was very full, the scan time was around 110 milliseconds, so most of the PID loops worked, but only by coincidence. When the same code was run in the ControlLogix, with a 6 millisecond scan time for the same logic, of course none of the tuning worked. There were also loops that were configured for 1000 ms Loop Update Times that executed every scan, so we just executed those with the same gains based on 1000 ms timers. Examine the timing and execution of your PID instructions in the PLC-5 carefully.

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Thank you Ken, in fact, we did the same as you, we worked the PID just like in the PLC5 program timing with periodic task in ladder, we are controlling level here, but it is very variable (its continuos casting of steel). The PLC-5 had the level oscillating the set point around +-5 and with the CLX we have it oscillating around +- 8. Im acknowledge that is very probable that we buy the RS Tune so we would expect to reduce that noice from the PID control. I think this is the way to do it, but if someone has done it better I would like to know, Thanks.

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If you are using the same gains and the same I/O systems, and executing the PID instructions using equivalent logic, the performance should be exactly the same. It's math. If something else has changed, like the I/O system (are you using 1756 I/O instead of older 1771 ?) or the timing logic is not really the same, or you changed something about the gains, you could get different performance. You have a lot more processing power in the ControlLogix so it certainly can't hurt to re-tune some of your control loops. Whether or not RSTune is appropriate is an engineering decision.

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