ISA CAP System Optimization & Troubleshooting 2

Question 1

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

Accepted Answer

Proportional gain too high

Answer

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?

Accepted Answer

Relay feedback (auto-tune) method

Answer

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:

Accepted Answer

Systematic bias (offset) error

Answer

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?

Accepted Answer

A modeling or data-timing error occurred

Answer

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?

Accepted Answer

Whether the valve is bypassed or blocked

Answer

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.

(ISA CAP) ISA Certified Automation Professional Practice Test