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

A controller is being tuned for a fast control loop. It has proportional action and the gain has been adjusted for a value by the technician. As reset action is added to the loop, which of the following will need to take place?  

The technician will need to:
A. add derivative
B. increase the gain
C. decrease the gain
D. increase the dead time

 

CCST Answer

The correct answer is C, decrease the gain. Both gain and reset (integral action) act upon the error in the same direction, per the simplified proportional, integral, derivative (PID) control algorithm:

Output 
=  Gain Term + Integral Term
=  Kc (E)  +   Kc · E(t) / Ti

If integral action is added, it is often necessary to reduce Kc, the gain, in order to keep the resulting tuning from being too aggressive.

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