CPT Process Control & Automation 2 — Questions and Answers
Question 1: In a cascade control system, which controller receives the setpoint from the operator?
- Primary (master) controller (Correct answer)
- Secondary (slave) controller
- Feed-forward controller
- Ratio controller
Correct answer: Primary (master) controller
In cascade control, the operator sets the setpoint on the primary controller, which then outputs a setpoint to the secondary controller.
Question 2: What does the term 'offset' refer to in a proportional-only control loop?
- The time delay between input and output
- A sustained steady-state error between setpoint and process variable (Correct answer)
- The overshoot following a setpoint change
- The rate of change of the error signal
Correct answer: A sustained steady-state error between setpoint and process variable
Proportional-only control produces a permanent offset (steady-state error) because it cannot eliminate error without integral action.
Question 3: Which valve characteristic provides an equal percentage change in flow for equal increments of valve travel?
- Linear
- Quick-opening
- Equal percentage (Correct answer)
- Parabolic
Correct answer: Equal percentage
Equal percentage valves produce a constant percentage change in flow per unit of stem travel, making them suitable for processes with variable pressure drops.
Question 4: A control valve with an air-to-open (ATO) actuator will fail in which position on loss of instrument air?
- Open
- Closed (Correct answer)
- Last position
- Mid-stroke
Correct answer: Closed
An air-to-open valve requires air pressure to open; when air is lost, the spring returns the valve to the closed (fail-closed) position.
Question 5: What is the function of a positioner on a control valve?
- To convert a 4–20 mA signal to a pneumatic signal only
- To ensure the valve stem reaches and maintains the position commanded by the controller output (Correct answer)
- To prevent the valve from exceeding its pressure rating
- To measure process flow through the valve
Correct answer: To ensure the valve stem reaches and maintains the position commanded by the controller output
A valve positioner uses feedback from the stem position to ensure the valve accurately follows the controller output signal, overcoming friction and hysteresis.
Question 6: In process control, what is 'integral windup' and when does it occur?
- Rapid oscillation caused by excessive derivative gain
- Accumulation of integral action while the controller output is saturated (Correct answer)
- An increase in process gain at high throughput
- Noise amplification in the derivative term
Correct answer: Accumulation of integral action while the controller output is saturated
Integral windup occurs when the controller output saturates (hits a limit) and the integral term continues to accumulate error, causing delayed response when the process returns to normal.
Question 7: Which control strategy is best suited for compensating for measurable disturbances before they affect the process variable?
- Feedback control
- Feed-forward control (Correct answer)
- On/off control
- Derivative-only control
Correct answer: Feed-forward control
Feed-forward control measures a disturbance and applies a corrective action before the disturbance affects the controlled variable, unlike feedback which reacts after the fact.
CPT Process Control & Automation 2
In a cascade control system, which controller receives the setpoint from the operator?