CPAT Process Control Systems & Automation 3

Question 1

What is feedforward control, and how does it differ from feedback control?

Accepted Answer

Feedforward measures a disturbance and applies corrective action before it affects the process variable; feedback corrects after the process variable has deviated

Answer

Feedforward corrects errors after they occur; feedback predicts errors before they affect the process variable

Answer

Feedforward control only applies to temperature loops; feedback applies to pressure and flow

Answer

Feedforward uses derivative action; feedback uses only proportional action

Question 2

What is the Ziegler-Nichols closed-loop method used for in process control?

Accepted Answer

Determining initial PID tuning parameters by finding the ultimate gain and ultimate period of the loop

Answer

Sizing control valves based on flow coefficient calculations

Answer

Diagnosing loop oscillations caused by valve stiction

Answer

Calculating the required sample conditioning flow for a process analyzer

Question 3

What is 'integral windup' in a PID controller, and when is it most likely to occur?

Accepted Answer

Accumulation of integral error during a period when the controller output is saturated (at its maximum or minimum limit), leading to overshoot when the constraint is removed

Answer

Excessive derivative action causing high-frequency noise amplification during steady-state operation

Answer

A sudden setpoint change causing the proportional term to spike and create oscillations

Answer

Slow controller response due to an incorrectly set integral time constant that is too small

Question 4

In a split-range control strategy, what is the typical application?

Accepted Answer

Dividing a single controller output signal (0–100%) to operate two final control elements in sequence — often one for heating and one for cooling

Answer

Using two separate controllers to control the same process variable independently

Answer

Splitting a high-pressure stream into two equal flows using separate valves

Answer

Assigning half the PID tuning to each of two redundant controllers

Question 5

What is 'dead band' in a process control valve or controller, and what is its primary negative effect?

Accepted Answer

A range of input change to which the final control element does not respond, causing offset and limit cycling around the setpoint

Answer

A safety zone around the setpoint where the controller ramps output slowly to avoid valve wear, improving stability

Answer

The minimum travel speed of a pneumatic control valve when supply pressure is low

Answer

The region between the upper and lower alarm limits where no operator action is required

Question 6

What is the purpose of a ratio controller in process automation?

Accepted Answer

Maintain the ratio of two process streams at a desired value by adjusting one stream proportionally to the other

Answer

Control the ratio of P to I gain in a PID controller for optimal tuning

Answer

Set the ratio of redundant transmitter readings to select the best measurement

Answer

Divide the controller output equally between two final control elements in a split-range scheme

Question 7

Which ISA (Instrumentation, Systems, and Automation Society) standard defines the symbols and identification used on Process & Instrumentation Diagrams (P&IDs) for control loops?

Accepted Answer

ISA-5.1

Answer

ISA-18.2

Answer

ISA-84.00.01

Answer

ISA-51.1