ISA Certified Control Systems Technician (CCST) program

CCST

Certified Control System Technicians (CCSTs) calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables.

CCST question

Which of the following terms describe a control strategy in which the output of one controller is used to manipulate the setpoint of another controller?

A. Ratio
B. Fail safe
C. Cascade
D. Feedforward

CCST Answer

The correct answer is C, "Cascade".

In cascade control, the output of the primary, or outer, control loop is used to set the setpoint of the secondary, or inner loop. This configuration is useful when there is a disturbance variable that can easily be controlled, thereby increasing the stability and effectiveness of the primary loop.

For example, in a heat exchanger using steam to heat a process stream, fluctuations in the steam pressure will cause the amount of heat that is transferred in the exchanger to fluctuate, causing the temperature to drift from setpoint. Instead of controlling the temperature directly with a single controller, the temperature controller output can be cascaded to a pressure controller’s setpoint, which will work to keep the steam pressure, and therefore, heat transfer constant for a given output.

Reference: Goettsche, L.D. (Editor), Maintenance of Instruments and Systems, 2nd Edition (2005), ISA Press