ISA CAP Advanced Techniques & Methods 3 — Questions and Answers
Question 1: In multivariable control, the Relative Gain Array (RGA) is used to:
- Calculate PID tuning constants directly
- Assess loop interaction and select input-output pairings (Correct answer)
- Determine the minimum number of sensors required
- Evaluate sensor accuracy and calibration drift
Correct answer: Assess loop interaction and select input-output pairings
The RGA quantifies steady-state interaction between manipulated and controlled variables, guiding optimal pairing to minimize loop coupling.
Question 2: A decoupler in a multivariable control system is designed to:
- Increase the speed of response of the slowest loop
- Cancel the cross-coupling effects between interacting loops (Correct answer)
- Convert a MIMO system into a single SISO loop
- Add integral action to eliminate offset in all loops
Correct answer: Cancel the cross-coupling effects between interacting loops
A decoupler adds compensating signals so that a change in one manipulated variable does not affect the other controlled variables.
Question 3: What does the concept of 'inferential control' (soft sensing) involve?
- Using multiple redundant sensors for the same variable
- Estimating an unmeasured controlled variable from related measured variables (Correct answer)
- Controlling a process using only binary (on/off) signals
- Inferring fault conditions from alarm history
Correct answer: Estimating an unmeasured controlled variable from related measured variables
Inferential control estimates a key quality variable (e.g., composition) that cannot be measured directly or in real time, using a mathematical model and available sensor readings.
Question 4: In statistical process control (SPC), a Western Electric Rule violation occurs when:
- A single point exceeds the 1-sigma limit
- A specific non-random pattern is detected on a control chart (Correct answer)
- The process capability index Cp drops below 2.0
- Two consecutive points fall exactly on the centerline
Correct answer: A specific non-random pattern is detected on a control chart
Western Electric Rules define patterns—such as runs, trends, or clusters near control limits—that indicate a non-random (assignable) cause is present.
Question 5: Process capability index Cpk differs from Cp in that Cpk:
- Uses the range instead of standard deviation
- Accounts for the centering of the process relative to specifications (Correct answer)
- Applies only to attribute (pass/fail) data
- Is calculated from only the upper specification limit
Correct answer: Accounts for the centering of the process relative to specifications
Cpk incorporates the process mean's distance from the nearest specification limit, penalizing processes that are not centered within the spec window.
Question 6: When tuning a PID controller using the Cohen-Coon method, the primary inputs to the tuning equations are derived from:
- Frequency response data from a relay test
- Process reaction curve parameters: dead time, time constant, and steady-state gain (Correct answer)
- Closed-loop step response amplitude ratio
- Ziegler-Nichols ultimate gain and period
Correct answer: Process reaction curve parameters: dead time, time constant, and steady-state gain
Cohen-Coon uses open-loop step test data—specifically dead time (td), time constant (τ), and process gain (K)—to compute PID settings with good disturbance rejection.
Question 7: In the context of ISA-88 batch control, a 'phase' is best described as:
- A time interval between two equipment transitions
- The lowest-level procedural element that directs process-oriented tasks (Correct answer)
- A collection of unit procedures across multiple trains
- A network segment connecting field devices to the DCS
Correct answer: The lowest-level procedural element that directs process-oriented tasks
According to ISA-88, a phase is the lowest-level procedural element that groups process-oriented steps executed sequentially or in parallel within a unit.
ISA CAP Advanced Techniques & Methods 3
In multivariable control, the Relative Gain Array (RGA) is used to: