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`Leave PID loops out of the PLC network`

01 November, 1999

`Leave PID loops out of the PLC network`

PID (proportional integral derivative) loops should be handled by dedicated hardware devices rather than being built into PLC networks, argues West Instruments.

The company, part of the Danaher Corporation, has just launched a multi-loop PID controller designed to remove the burden of processing PID parameters from PLC and SCADA networks.

Traditionally, PLCs have had limited analogue-handling capabilities and have not been particularly user-friendly. They are especially complex to program for loop control, West contends.

Also, the performance of most PLCs suffers when they have to handle high-density analogue inputs; most will only support simple floating-point PID terms. Furthermore, variable scanning speeds can affect their PID performance and prioritisation requires complex programming.

"This is the PLC dilemma - when loop meets logic," suggests Robert Jelski, managing director of West Instruments. "PLCs are the first choice for automation, but analogue loop control is not their strength, because logic deals with Boolean on/off states, whereas loop control deals with continuously variable analogue parameters."

"PID loop control demands direct connection of sensors, continuous PID calculations and single loop integrity," Jelski argues.

West Instruments` MLC 9000 controller allows up to eight DIN-rail-mounted PID loop modules to run off a single bus communications module. The modules can be "hot swapped" without powering down or interrupting the process. Up to 32 of the modules can be linked in a Modbus network.

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