Process Control and Controller Tuning Notes

Process Control and Controller Tuning

Overview of Control Systems
  • Feedback Control: Utilizing system output to adjust input for desired performance.
  • Closed-Loop Responses: Different feedback control systems exhibit various responses; for instance, controller performance may vary in settling time and error minimization.
Performance Criteria for Controller Selection
  • Common Criteria:
    • Maximum Deviation: Keep deviation from the desired set point as small as possible.
    • Settling Time: Aim for minimal time to reach steady state.
    • Integral of Errors: Minimize the cumulative error over time leading to the set point.
  • Types of Performance Criteria:
    1. Steady-State Performance: Primarily focuses on achieving zero error at steady-state.
    2. Dynamic Response Performance: Evaluated based on speed and accuracy of reaching the set point.
Simple and Time Integral Performance Criteria

  • Simple Performance Criteria:

    • Simplifies characteristics of closed-loop response like overshoot, rise time, settling time.
    • Decay Ratio: A popular measure where each successive peak decreases by a quarter, aiding in balancing rise time and settling time.

  • Time Integral Performance Criteria:

    • Utilize the entire response curve from start to steady-state for more precision.
    • Three Key Integral Criteria:
    1. Integral of Square Error (ISE): Weighs larger deviations more heavily.
    2. Integral of Absolute Error (IAE): Penalizes deviations linearly, advantageous for small errors.
    3. Integral of Time-Weighted Absolute Error (ITAE): Further emphasizes deviations over longer times.
Optimization Characteristics
  • Cyclic Response: In underdamped responses, focus on minimizing both deviation magnitude and duration.
  • Minimum Area Criterion: Adjusts system response to reduce the net area of deviation, maximizing performance.
Guidelines for Selecting the Appropriate Performance Criteria
  • Choosing between ISE, IAE, ITAE based on specific objectives:
    • To suppress large errors, prefer ISE.
    • For small errors, IAE is more suitable.
    • For persistent errors, ITAE is the optimal choice.
Controller Selection Process
  1. Define Performance Criteria: Choose between ISE, IAE, or ITAE.
  2. Compute Performance Values: Use selected controllers (P, PI, PID) and determine optimal settings for parameters Kp, Ki, and Kd.
  3. Select Best Controller: Choose the controller that optimizes the established performance criterion.
Key Considerations
  • Different tuning criteria can lead to distinct controller designs, highlighting the importance of correct criteria selection based on process requirements.