March/April 2012

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

Which choice is NOT a major advantage of using cascade control?

A.    Any disturbances that affect the slave variable are detected and compensated for by the slave controller before they have time to affect the primary control variable.

B.    The controllability of the outside loop is improved because the inside loop speeds up the response of the process dynamic elements between the control variable and slave variable.

C.    Nonlinearities of the process in the inner loop are handled by that loop and removed from the more important outer loop.

D.    It can further improve the performance of loops that are already very controllable.

CAP answer

The correct answer is D. Cascade control does very little to improve upon control loops that were previously stable. Examples are liquid level and gas pressure control loops.

Reference: Corripio, A.B, Tuning of Industrial Control Systems, 2nd Edition, pages 127-129.