CCST CCST Process Optimization & Performance Tuning 2 — Questions and Answers
Question 1: In a cascade control configuration, which controller directly manipulates the final control element (e.g., control valve)?
- Primary (outer) controller
- Secondary (inner) controller (Correct answer)
- Feedforward controller
- Ratio controller
Correct answer: Secondary (inner) controller
In cascade control, the secondary (inner) controller receives its setpoint from the primary controller and directly positions the final control element.
Question 2: What is the main advantage of feedforward control compared to feedback control in a process loop?
- Feedforward eliminates the need for a process model
- Feedforward acts on measured disturbances before they affect the process variable, reducing disturbance response time (Correct answer)
- Feedforward automatically rejects all unmeasured disturbances
- Feedforward replaces the need for integral action
Correct answer: Feedforward acts on measured disturbances before they affect the process variable, reducing disturbance response time
Feedforward control measures a known disturbance and adjusts the controller output proactively, compensating for the disturbance before it can affect the controlled variable.
Question 3: A CCST technician is analyzing a process trend and observes a sinusoidal oscillation with a period equal to the loop scan time. What is the most likely cause?
- Excessive integral time (reset rate too slow)
- Aliasing due to the scan rate being too slow relative to process dynamics (Correct answer)
- Incorrect proportional band
- Setpoint step change
Correct answer: Aliasing due to the scan rate being too slow relative to process dynamics
Aliasing occurs when the control system scan rate is slower than twice the frequency of a process oscillation (violating Nyquist), causing the oscillation to appear at the wrong frequency in trends.
Question 4: In process optimization, what does the term 'process capability index (Cpk)' quantify?
- The average production rate of a process per shift
- How well a process performs relative to its specification limits, accounting for both spread and centering (Correct answer)
- The ratio of setpoint to process variable
- The number of control valves in a loop
Correct answer: How well a process performs relative to its specification limits, accounting for both spread and centering
Cpk measures how centered and how tightly controlled a process is relative to its upper and lower specification limits — higher Cpk means fewer defects.
Question 5: What is the purpose of a 'bump test' (step test) performed on a control loop that is switched to manual mode?
- To calibrate the transmitter zero and span
- To identify process dynamics such as gain, dead time, and time constant for tuning calculations (Correct answer)
- To verify alarm setpoints are correct
- To test the control valve actuator stroke
Correct answer: To identify process dynamics such as gain, dead time, and time constant for tuning calculations
A bump test applies a step change to the controller output in manual mode and records the process variable response to identify the process model parameters needed for tuning.
Question 6: Which statistical process control tool is used to determine whether process variability is due to common cause or special cause variation?
- Pareto chart
- Control chart (Shewhart chart) (Correct answer)
- Fishbone (Ishikawa) diagram
- Scatter plot
Correct answer: Control chart (Shewhart chart)
Control charts plot process data over time with statistical control limits, allowing technicians to distinguish natural common-cause variation from abnormal special-cause events requiring investigation.
CCST CCST Process Optimization & Performance Tuning 2
In a cascade control configuration, which controller directly manipulates the final control element (e.g., control valve)?