Quiz on Level Control Case Study

1. A doublet test on the level control model shows some nonlinear behavior. The First-Order Plus Dead Time (FOPDT) model is linear. What does this mean for the system?

A. The derivative of the model changes over time
Incorrect. Nonlinear behavior means that the FOPDT parameters are different in some regions. Derivatives for a linear system can change over time.
B. The process response is not straight (has a curve)
Incorrect. Nonlinear behavior means that the FOPDT parameters are different in some regions. A linear system can have a curved response.
C. The linear FOPDT model does not fully describe the system in some regions
Correct. Nonlinear behavior means that the FOPDT parameters are different in some regions.
D. The linear FOPDT model cannot be used
Incorrect. The FOPDT model may not be a perfect fit, but the model is often useful to obtain initial PID tuning parameters.

2. What is not a common way to quantify PID tuning performance?

A. Rise time / Peak time
Incorrect. Rise time and peak time are valid metrics for quantifying controller performance
B. Overshoot ratio
Incorrect. Overshoot ratio is a valid metric for quantifying controller performance
C. Controller cycle time
Correct. How often a controller recomputes a new output is not a common measure of performance.
D. Settling time
Incorrect. Settling time is a valid metric for quantifying controller performance

3. For a particular implementation of the two-tank level control the objective is to maintain a given level in the second tank, adapting to disturbances. Which tuning correlations would you suggest when there are no expected set points?

A. Set point tracking
Incorrect. Set point tracking or servo control is best for a frequently changing set point
B. Disturbance rejection
Incorrect. The answer is both B and C
C. Regulatory control
Incorrect. The answer is both B and C
4. What controller tuning type is more aggressive with a higher controller gain K_c?
Correct. Regulatory control for disturbance rejection is used when the set point is not expected to change. The tuning correlations recommend a higher controller gain K_c compared to servo control. This is because a set point change is a more difficult transition from one operating region to another and an aggressive controller is not desirable for this action.
Incorrect. Servo control correlations are used when the set point is expected to change. The tuning correlations recommend a lower controller gain K_c compared to regulatory control for disturbance rejection. This is because a set point change is a more difficult transition from one operating region to another and an aggressive controller is not desirable for this action.