08_PID_design_E2009 - 1 28150. Introduction to process...

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Unformatted text preview: 1 28150. Introduction to process control 8. PID controller design Krist V. Gernaey 26 October 2009 28150 Learning objectives At the end of this lesson you should be able to: Design a feedback controller for a given open-loop system Tune the controller parameters for a closed-loop system with feedback control 26 October 2009 28150 Outline Performance criteria for closed-loop systems Model-based design Controller tuning Controllers with two degrees of freedom On-line controller tuning 26 October 2009 28150 Outline Performance criteria for closed-loop systems Model-based design Controller tuning Controllers with two degrees of freedom On-line controller tuning 26 October 2009 28150 Desirable controller features The closed-loop system is stable Quick response to set point changes (good set point tracking ) Effects of disturbances are minimal (adequate disturbance rejection ) No steady-state error (no offset) Avoids excessive controller action Insensitive to model and measurement errors (robustness) Suitable over a wide range of operating conditions (robustness) 26 October 2009 28150 Stability regions in the complex plane Real part Imaginary part Stable region Unstable region 2 26 October 2009 28150 Practical control = trade off Performance versus robustness Robustness (satisfactory performance for a broad range of process conditions and allowing a reasonable degree of model accuracy) is achieved by selecting conservative controller settings (low K C , large I ) Disturbance rejection capability versus set point tracking capability 26 October 2009 28150 Example: FOPTD system in Simulink, response to a step disturbance G G c D 26 October 2009 28150 I = 5 I = 11 I = 20 K c = 5 K c = 3 K c = 1 26 October 2009 28150 Definitions Controller tuning = procedure where controller parameters are adjusted to improve the closed-loop response Controller design = procedure where the feedback control law (= equation) is derived for a given open-loop system 26 October 2009 28150 Outline Performance criteria for closed-loop systems Model-based design Controller tuning Controllers with two degrees of freedom On-line controller tuning 26 October 2009 28150 Introduction If a reasonably accurate dynamic model of the process is available, controller design based on the process model is advantageous Saves time for controller implementation ( on-line tuning ) when starting with a reasonable set of controller parameters 3 26 October 2009 28150 Direct synthesis method G G G G Y Y c c sp + = 1 d sp Y Y G c G Y sp D U E Y + + +- Controller design based on process model + a desired closed-loop TF 1. Specify desired closed-loop response (transfer function) Key design decision!!!...
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08_PID_design_E2009 - 1 28150. Introduction to process...

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