ISA Certified Automation Professional (CAP) program

Certified Automation Professionals (CAPs) are responsible for the direction, design, and deployment of systems and equipment for manufacturing and control systems.

CAP question

Processing of the PID algorithm in a digital controller presents which of the following alternatives not available in an analog or pneumatic controller system?

A. correction of integral windup, eliminating overshoots in the controlled response
B. setting of the control output to any reasonable arbitrary level
C. reduction of the error between set point and process variable
D. calculation of the increment by which the controller output should be changed

CAP Answer

The correct answer is D, "calculation of the increment by which the controller output should be changed." Execution of the proportional, integral, derivative (PID) algorithm in any type of controller (digital, pneumatic, or analog) has the following characteristics:

  • capable of switching between automatic and manual output modes
  • capable of directly setting and adjusting the control output (answer B) in manual mode
  • capable of reducing the error between the set point and process variable (answer C) by adjusting the control output via the PID algorithm in automatic mode
  • an option to correct or limit integral windup (answer A)

However, only digital controllers have the computational ability to calculate the actual discrete increment in which the controller output should be increased or decreased (answer D) at the end of each PID execution cycle. Analog and pneumatic controllers are able to increase or decrease the output to the final control element "by some amount" based on the mechanical (pneumatic) or electronic (analog) comparison of the set point to the process variable.

Reference: Trevathan, Vernon L., A Guide to the Automation Body of Knowledge, Second Edition, ISA, 2006.

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