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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 openloop system Tune the controller parameters for a closedloop system with feedback control 26 October 2009 28150 Outline Performance criteria for closedloop systems Modelbased design Controller tuning Controllers with two degrees of freedom Online controller tuning 26 October 2009 28150 Outline Performance criteria for closedloop systems Modelbased design Controller tuning Controllers with two degrees of freedom Online controller tuning 26 October 2009 28150 Desirable controller features The closedloop system is stable Quick response to set point changes (good set point tracking ) Effects of disturbances are minimal (adequate disturbance rejection ) No steadystate 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 closedloop response Controller design = procedure where the feedback control law (= equation) is derived for a given openloop system 26 October 2009 28150 Outline Performance criteria for closedloop systems Modelbased design Controller tuning Controllers with two degrees of freedom Online 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 ( online 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 closedloop TF 1. Specify desired closedloop response (transfer function) Key design decision!!!...
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 Fall '09
 RafiqulGani

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