November/December 2012
Certification Review

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

Using forward decoupling, the objective is to:

A.     accumulate the interaction between two process variables and their outputs

B.     cancel out the interaction between two process variables and their outputs

C.     use feedback to eliminate the gain of one process variable and its output

D.     use a decoupling algorithm to eliminate all gain between outputs

CAP answer

The correct answer is B, "Cancel out the interaction between two process variables and their outputs." In MIMO (multiple input, multiple output) systems, often the process variables and outputs interact with one another, which makes control of the independent variables difficult. A forward decoupling algorithm can be used to cancel out these interactions, making more traditional control methods applicable to these complex systems.

Answer A is not correct; accumulation of the interactions would be counterproductive to the loop performance, and would simply magnify the coupling between inputs and outputs.

Answer C is not correct. Simple feedback can be used to make adjustments to the output based on a measured or calculated quantity, but simple feedback cannot be used to eliminate the process gain between a process variable and its own output.

Answer D is not correct. Similar to Answer C, a decoupling algorithm does not eliminate "all gains between outputs," but rather, when used in a forward decoupling method, can be used to cancel the interactions between multiple inputs and their outputs.

Reference: McMillan, Gregory, et al., "Models Unleashed-Virtual Plant and Model Predictive Control Applications-A Pocket Guide," ISA, 2004.