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PID

(1) (Process IDentifier) A temporary number assigned by the operating system to a process or service.

(2) (Proportional-Integral-Derivative) The most common control methodology in process control. It is a continuous feedback loop that keeps the process flowing normally by taking corrective action whenever there is any deviation from the desired value ("setpoint") of the process variable (rate of flow, temperature, voltage, etc.). An "error" occurs when an operator manually changes the setpoint or when an event (valve opened, closed, etc.) or a disturbance changes the load, thus causing a change in the process variable.

The PID controller receives signals from sensors and computes corrective action to the actuators from a computation based on the error (proportional), the sum of all previous errors (integral) and the rate of change of the error (derivative). See PAC.




Inputs from a PID
The large white boxes are Opto 22 SNAP PACs, which are programmable automation controllers that are processing the PID loops in Chevron's research facility in Richmond, Virginia. The wires come from thermocouples that sense pipe temperature in a laboratory test that analyzes the best way to break down crude oil. The PACs determine when to raise and lower the temperature, and the three modules with the black sockets send digital signals to the heaters (cables not connected in this image). (Image courtesy of Opto 22, www.opto22.com)






PAC

(1) See perceptual audio coding.

(2) (Programmable Automation Controller) A programmable microprocessor-based device that is used for discrete manufacturing, process control and remote monitoring applications. Designed for use in rugged, industrial environments, PACs combine the functions of a programmable logic controller (PLC) with the greater flexibility of a PC. They are also more easily set up for data collection and integration with the company's business applications than PLCs.

Although each PAC vendor uses its own development environment (IDE) and programming language, PAC networking is typically based on IP and Ethernet. Taking advantage of off-the-shelf microprocessors, PACs were developed in the 1990s to provide a single industrial controller that would provide the functions of a DCS and PLC. The term was coined in 2001 by the ARC Advisory Group (www.arcweb.com). Contrast with PLC and DCS.




PACs in Operation
The large white boxes are Opto 22 SNAP PACs in Chevron's Richmond, Virginia research facility. Processing the PID loops, the wires come from thermocouples that sense pipe temperature in a laboratory test that analyzes the best way to break down crude oil. The PACs determine when to raise and lower temperatures, and the three modules with the black sockets send digital signals to the heaters (cables not connected in this image). (Image courtesy of Opto 22, www.opto22.com)






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