ISA CAP Advanced Techniques & Methods 2 — Questions and Answers
Question 1: Which model predictive control (MPC) feature distinguishes it from conventional PID control?
- It uses only proportional action
- It optimizes over a future prediction horizon subject to constraints (Correct answer)
- It eliminates the need for process models
- It reacts only to past errors
Correct answer: It optimizes over a future prediction horizon subject to constraints
MPC explicitly optimizes manipulated variables over a receding prediction horizon while satisfying process and equipment constraints.
Question 2: In cascade control, which loop is tuned first and why?
- The primary (outer) loop, because it sets the overall setpoint
- The secondary (inner) loop, because it must respond faster than the primary (Correct answer)
- Both loops simultaneously to avoid interaction
- The loop with the larger time constant
Correct answer: The secondary (inner) loop, because it must respond faster than the primary
The secondary (inner) loop is tuned first because it must be sufficiently fast to reject disturbances before the primary loop responds.
Question 3: What is the primary purpose of a Smith Predictor in a control loop?
- To add derivative action for noisy signals
- To compensate for large dead time in the process (Correct answer)
- To implement gain scheduling
- To provide anti-reset windup protection
Correct answer: To compensate for large dead time in the process
The Smith Predictor uses a process model to remove dead time from the closed-loop characteristic equation, enabling better control of processes with large time delays.
Question 4: When applying the Internal Model Control (IMC) design method, the IMC filter time constant (λ) is adjusted primarily to trade off:
- Integral versus derivative action
- Closed-loop speed of response versus robustness to model error (Correct answer)
- Feedforward versus feedback gain
- Signal filtering versus setpoint tracking
Correct answer: Closed-loop speed of response versus robustness to model error
Increasing λ slows the closed-loop response but provides greater robustness when the process model is imperfect.
Question 5: In a ratio control scheme for blending two streams, the ratio controller output adjusts which variable?
- The setpoint of the wild stream flow controller
- The setpoint of the controlled stream flow controller (Correct answer)
- The pressure upstream of both streams
- The combined total flow setpoint
Correct answer: The setpoint of the controlled stream flow controller
The ratio controller multiplies the measured wild (uncontrolled) stream flow by the desired ratio to set the setpoint for the controlled stream flow controller.
Question 6: Gain scheduling in a nonlinear process controller changes the controller gains based on:
- The integral of the error signal over time
- A measured or computed operating-point variable (Correct answer)
- The derivative of the output signal
- Fixed time intervals regardless of process state
Correct answer: A measured or computed operating-point variable
Gain scheduling selects pre-computed controller parameters as a function of a scheduling variable that characterizes the current operating point of the nonlinear process.
Question 7: Which technique is specifically used to prevent integrator windup in a controller with output limits?
- Derivative filtering
- Back-calculation or conditional integration (Correct answer)
- Lead-lag compensation
- Feedforward decoupling
Correct answer: Back-calculation or conditional integration
Back-calculation feeds the difference between the actual and limited output back to the integrator, preventing it from accumulating beyond usable limits.
ISA CAP Advanced Techniques & Methods 2
Which model predictive control (MPC) feature distinguishes it from conventional PID control?