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Applications controlled with a PLC that require accurate single- or multi-loop PID control and handling of discrete loops can add PID control modules supporting the PLC.
In recent years, it has become more common to integrate analog control loops and, more specifically, PID control, into a programmable logic controller (PLC) system. This practice is technically feasible but often not as functional or easy to support as off-the-shelf analog device technology in the long run. It often also is more costly than using off-the-shelf analog device technology.
The PLC normally is designed to work with digital I/O and sequence control. If a PID loop is added to the PLC application, the processing speeds of both the logic functions and the PID loop are decreased. To minimize speed lags, additional programming time, faster processors, extra system memory and more I/O cards are necessary. Further, if even a single loop must be added, multiple channel I/O cards must be purchased.
The increased cost of a more powerful PLC processor and analog input cards is in direct conflict with the desire for total-system-cost reductions and the larger desire tor single-loop integrity. More processors are using - or trying to use - subpanelmount devices that save panel space and reduce wiring...