System Optimization & Troubleshooting Flashcards

A control loop is oscillating with increasing amplitude after a setpoint change. What is the most likely cause?

Answer: Proportional gain too high

Excessively high proportional gain causes the controller to over-correct, leading to growing oscillations and instability.

Which method is best suited for tuning a controller on a process that cannot be taken offline for open-loop step testing?

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.

A flow transmitter consistently reads 5% higher than a reference meter under all conditions. This is best described as:

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.

What does a negative dead time estimate in a process model indicate during system identification?

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.

During troubleshooting, a technician finds that a control valve's stem position changes but flow does not. What should be checked first?

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.

Which performance index penalizes large initial errors more heavily and is commonly used to compare controller tuning?

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.

A batch reactor temperature control loop performs well at low temperatures but becomes unstable at high temperatures. The most probable cause is:

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.