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

In an analog electronic single loop controller, a _________________ op-amp is used to compare the process variable to the set point, which creates the error signal in the controller.

A. integrating
B. differential
C. summing
D. inverting

CCST Answer

The correct answer is B: "differential".

In a single loop controller, the error is "calculated" by subtracting the process variable (PV) from the setpoint (SP).  Electrically, in an analog controller, this is done by taking the difference of an analog voltage signal that represents the SP and an analog voltage signal that represents the PV.

This subtraction is carried out using a differential op amp circuit, which can be shown diagrammatically as:

CCST Intech Plus Question img 

If all resistor values are equal, Vout = V2 - V1.  If V1 = the PV signal and V2 = SP signal, then the difference is the Error = V2 - V1.

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