ISA CAP System Optimization & Troubleshooting 2 — Questions and Answers
Question 1: A control loop is oscillating with increasing amplitude after a setpoint change. What is the most likely cause?
- Integral windup
- Excessive derivative gain
- Proportional gain too high (Correct answer)
- Insufficient integral action
Correct answer: Proportional gain too high
Excessively high proportional gain causes the controller to over-correct, leading to growing oscillations and instability.
Question 2: Which method is best suited for tuning a controller on a process that cannot be taken offline for open-loop step testing?
- Ziegler-Nichols open-loop method
- Cohen-Coon method
- Relay feedback (auto-tune) method (Correct answer)
- Manual bump test
Correct answer: Relay feedback (auto-tune) method
The relay feedback method introduces only a small, bounded oscillation and works while the process remains in closed-loop operation.
Question 3: A flow transmitter consistently reads 5% higher than a reference meter under all conditions. This is best described as:
- Random noise
- Hysteresis error
- Systematic bias (offset) error (Correct answer)
- Non-linearity
Correct answer: Systematic bias (offset) error
A consistent, constant deviation in one direction across all conditions is a systematic bias or offset error requiring calibration correction.
Question 4: What does a negative dead time estimate in a process model indicate during system identification?
- The process is overdamped
- A modeling or data-timing error occurred (Correct answer)
- The gain is negative
- Derivative action is needed
Correct answer: A modeling or data-timing error occurred
Dead time cannot be physically negative; a negative estimate signals a problem with data alignment, time-stamping, or the identification algorithm.
Question 5: During troubleshooting, a technician finds that a control valve's stem position changes but flow does not. What should be checked first?
- PID controller tuning parameters
- Signal wiring to the transmitter
- Whether the valve is bypassed or blocked (Correct answer)
- Setpoint ramp rate settings
Correct answer: Whether the valve is bypassed or blocked
If stem movement produces no flow change, a mechanical issue such as a closed block valve, bypass, or failed trim is the most likely culprit.
Question 6: Which performance index penalizes large initial errors more heavily and is commonly used to compare controller tuning?
- IAE (Integral of Absolute Error)
- ISE (Integral of Squared Error) (Correct answer)
- ITAE (Integral of Time-weighted Absolute Error)
- IE (Integral of Error)
Correct answer: ISE (Integral of Squared Error)
ISE squares the error, giving disproportionately large weight to large errors compared to small ones, making it sensitive to big initial deviations.
Question 7: A batch reactor temperature control loop performs well at low temperatures but becomes unstable at high temperatures. The most probable cause is:
- Integral windup during the batch
- Process gain variation with temperature (process non-linearity) (Correct answer)
- Noise on the thermocouple signal
- Incorrect engineering units in the HMI
Correct answer: Process gain variation with temperature (process non-linearity)
Non-linear processes have a gain that changes with operating point; fixed PID tuning optimized for one region will be poorly matched at another.
ISA CAP System Optimization & Troubleshooting 2
A control loop is oscillating with increasing amplitude after a setpoint change.
What is the most likely cause?