Question:
What is the effect of valve stiction on PID performance?
Options:
A) Stable response
B) Zero overshoot
C) Hunting behavior
D) Zero error
View Answer
Correct Answer: C) Hunting behavior
Detailed Explanation
Valve stiction (static friction) occurs when a control valve sticks and requires extra force to start moving. Instead of smooth motion, the valve moves in a stick–slip pattern.
In a PID loop, this causes:
- Delayed valve response
- Sudden jumps in valve position
- Oscillations around setpoint
- Limit cycling (hunting behavior)
The controller keeps correcting because the valve does not move smoothly, leading to continuous cycling.
Practical Symptoms
Valve stiction often results in:
- Saw-tooth PV trend
- Repeated oscillations
- Actuator chattering
- Increased mechanical wear
Why Other Options Are Incorrect
A) Stable response → Stiction reduces stability.
B) Zero overshoot → Stiction typically increases oscillations.
D) Zero error → Stiction can actually increase steady-state deviation and instability.