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Chapter 8

Chapter 8. Performance of P-only, PI and PID Controllers. Overall Course Objectives. Develop the skills necessary to function as an industrial process control engineer. Skills Tuning loops Control loop design Control loop troubleshooting Command of the terminology

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Chapter 8

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  1. Chapter 8 Performance of P-only, PI and PID Controllers

  2. Overall Course Objectives • Develop the skills necessary to function as an industrial process control engineer. • Skills • Tuning loops • Control loop design • Control loop troubleshooting • Command of the terminology • Fundamental understanding • Process dynamics • Feedback control

  3. P-only Control • For an open loop overdamped process as Kc is increased the process dynamics goes through the following sequence of behavior • overdamped • critically damped • oscillatory • ringing • sustained oscillations • unstable oscillations

  4. Dynamic Changes as Kc is Increased for a FOPDT Process

  5. Root Locus Diagram(Kc increases a to g)

  6. Effect of Kc on Closed-Loop z

  7. Effect of Kc on Closed-Loop tp

  8. P-only Controller Applied to First-Order Process without Deadtime • Without deadtime, the system will not become unstable regardless of how large Kc is. • First-order process model does not consider combined actuator/process/sensor system. • Therefore, first-order process model without deadtime is not a realistic model of a process under feedback control.

  9. PI Control • As Kc is increased or tI is decreased (i.e., more aggressive control), the closed loop dynamics goes through the same sequence of changes as the P-only controller: overdamped, critically damped, oscillatory, ringing, sustained oscillations, and unstable oscillations.

  10. Effect of Variations in Kc Effect of Variations in tI

  11. Analysis of the Effect of Kc and tI • When there is too little proportional action or too little integral action, it is easy to identify. • But it is difficult to differentiate between too much proportional action and too much integral action because both lead to ringing.

  12. Response of a Properly Tuned PI Controller

  13. Response of a PI Controller with Too Much Proportional Action

  14. Response of a PI Controller with Too Much Integral Action

  15. PID Control • Kc and tI have the same general effect as observed for PI control. • Derivative action tends to reduce the oscillatory nature of the response and results in faster settling for systems with larger deadtime to time constant ratios.

  16. Comparison between PI and PID for a Low qp/tp Ratio

  17. Comparison between PI and PID for a Higher qp/tp Ratio

  18. An Example of Too Much Derivative Action

  19. Effect of tD on Closed-Loop z

  20. Demonstration: Visual Basic Simulator Effect of Kc, tI, and tD

  21. Overview • As the controller aggressiveness is increased (i.e., Kc is increased or tI is decreased), the response goes from overdamped to critically damped to oscillatory to ringing to sustained oscillations to unstable. • Too little proportional or integral action are easy to identify while too much proportional or integral results in ringing. Differentiating between too much integral or proportional action requires comparing the lag between the controller output and the CV.

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