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CBE 491 / 433

CBE 491 / 433. 16 Oct 12 Deadtime in a Process. Dead Time in a Process. Show how dead time might show up How it affects block diagrams How it affects response. LT. LC. Level Loop (melt tank). +. +. +. -. Closed loop response: (no setpoint change). A. If:. t. 0.

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CBE 491 / 433

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  1. CBE 491 / 433 16 Oct 12 Deadtime in a Process

  2. Dead Time in a Process • Show how dead time might show up • How it affects block diagrams • How it affects response LT LC

  3. Level Loop (melt tank) + + + - Closed loop response: (no setpoint change) A If: t 0

  4. Level Loop (melt tank) + + + - Closed loop response: (setpoint change) If: A t 0

  5. Dead Time in a Process Suppose change manipulated variable LC LT FT FC

  6. Level Loop (melt tank) + + + - • D(s) not a polynomial; can’t do P.F. expansion, so different procedure needed. • Dead time effect is to reduce the ultimate loop gain (will oscillate at lower Kc values)

  7. Feedback Controller Tuning Tuning: adjust controller parameters to obtain specified closed loop response. • Values of parameters depend upon: • Desired response • Dynamic characteristics of other elements in the control loop. We’ll come back to general tuning approaches, but lets first explore the solid feeder example that has some dead time….

  8. Tuning Introduction Feeder with some dead time (to) aT= aV= aP= aC= +1 +1 +1 -1 WY WC WT + + + -

  9. Tuning Example (w/ P-only Controller)

  10. QAD or ¼ Decay Ratio ¼ Decay Ratio or Quarter Amplitude Damping: • Convenient • Relatively quick response • Relatively high overshoot on setpoint changes • Non-unique (theoretically infinite no. tuning parameters) If dead time in loop: • Makes closed loop closer to unstable • Reduce Kc … but then more sluggish response • Instead of pure feed back control … could implement Dead-Time Compensation (Smith Predictor)

  11. Ziegler Nichols Tuning Method (I) For our simple example with To achieve QAD we set WY WC WT Ziegler Nichols Tuning Method I • P-only control • Find • Set Empirical formula to get closed loop response close to QAD We’ll see another Ziegler Nichols: ZN II related to FOPDT fit

  12. Feedback Controller Tuning: (General Approaches) • Simple criteria; i.e QAD via ZN I, tr, etc • easy, simple, do on existing process • multiple solutions • Time integral performance criteria • ISE integral square error • IAE integral absolute value error • ITAE integral time weighted average error • Semi-empirical rules • FOPDT (ZN II) • Cohen-Coon • ATV, or Autotuning • Trial and error • Rules of thumb

  13. Questions ??

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