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ChE / MET 433

ChE / MET 433. Advanced control schemes. 18 Apr 12 Cascade Control: Ch 09 Ratio Control: Ch 10. Tuning a Cascade System. Both controllers in manual Secondary controller set as P-only (could be PI, but this might slow sys) Tune secondary controller for set point tracking

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ChE / MET 433

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  1. ChE / MET 433 Advanced control schemes 18 Apr 12 Cascade Control: Ch 09Ratio Control: Ch 10

  2. Tuning a Cascade System • Both controllers in manual • Secondary controller set as P-only (could be PI, but this might slow sys) • Tune secondary controller for set point tracking • Checksecondaryloop for satisfactory set point tracking performance • Leave secondary controller in Auto • Tune primary controller for disturbance rejection (PI or PID) • Both controllers in Auto now • Verify acceptable performance

  3. In-Class Exercise: Tuning Cascade Controllers • Select Jacketed Reactor • Set T cooling inlet at 46 oC(normal operation temperature; sometimes it drops to 40 oC) • Set output of controller at 50%. • Desired Tout set point is 86 oC(this is steady state temperature) • Tune the single loop PI control • Criteria: IMC aggressive tuning • Use doublet test with +/- 5 %CO • Test your tuning with disturbance from 46 oC to 40 oC

  4. In-Class Exercise: Tuning Cascade Controllers • Select Cascade Jacketed Reactor • Set T cooling inlet at 46 oC (again) • Set output of controller (secondary) at 50%. • Desired Tout set point is 86 oC (as before) • Note the secondary outlet temperature (69 oC) is the SP of the secondary controller • Tune the secondary loop; use 5 %CO doublet open loop • Criteria: ITAE for set point tracking (P only) • Use doublet test with +/- 5 %CO • Test your tuning with 3 oCsetpoint changes • Tune the primary loop for PI control; make 3 oC set point changes (2nd-dary controller) • Note: MV = sp signal; and PV = T out of reactor • Criteria: IAE for aggressive tuning (PI) • Implement and with both controllers in Auto… change disturbance from 46 to 40 oC. • How does response compare to single PI feedback loop?

  5. Ratio Control • Special type of feed forward control A B • Blending/Reaction/Flocculation • A and B must be in certain ratio to each other

  6. Ratio Control Possible control system: FY FY FC FC FT FT B A • What if one stream could not be controlled? • i.e., suppose stream A was “wild”; or it came from an upstream process and couldn’t be controlled.

  7. Ratio Control Possible cascade control systems: “wild” stream A FT Desired Ratio FC FY FT B “wild” stream A FT Desired Ratio This unit multiplies A by the desired ratio; so output = FY FC FT B

  8. Ratio Control Uses: • Constant ratio between feed flowrate and steam in reboiler of distillation column • Constant reflux ratio • Ratio of reactants entering reactor • Ratio for blending two streams • Flocculent addition dependent on feed stream • Purge stream ratio • Fuel/air ratio in burner • Neutralization/pH

  9. In-Class Exercise: Furnace Air/Fuel Ratio • Furnace Air/Fuel Ratio model • disturbance: liquid flowrate • “wild” stream: air flowrate • ratioed stream: fuel flowrate • Minimum Air/Fuel Ratio 10/1 • Fuel-rich undesired (enviro, econ, safety) • If air fails; fuel is shut down Check TC tuning to disturbance & SP changes. PV Desired 2 – 5% excess O2 Disturbance var. TC Dependent MV TC output Ratio set point Independent MV

  10. ChE / MET 433 Advanced control schemes 18 Apr 12Feed Forward Control: Ch 11

  11. Feed Forward Control steam Suppose qiis primary disturbance TC TT Heat Exchanger ? What is a drawback to this feedback control loop? ? Is there a potentially better way? steam What if Ti changes? FF Heat Exchanger FT TT FF must be done with FB control!

  12. Feed Forward and Feedback Control TC steam TY FF TY FT TT Heat Exchanger Block diagram: + + + + + -

  13. Feed Forward Control + + + + + - Response to MFF No change; perfect compensation!

  14. Feed ForwardControl + + + + + - Examine FFC T.F. For “perfect” FF control: + +

  15. Feed ForwardControl: FFC Identification Set by traditional means: Model fit to FOPDT equation: Eqn: 11-2.5 p 379 Dead time compensator FF Gain Lead/lagunit Often ignored; if set term to 1 Accounts for time differences in 2 legs { FFC ss } steady state FF control { FFC dyn } dynamic FF control

  16. Feed ForwardControl: FFC Identification How to determine FOPDT models : • With Gc disconnected: • Step change COFB, say 5% • Fit C(s) response to FOPDT + + • Still in open loop: • Step change Q, say 5 gpm • Fit C(s) response to FOPDT lead time lag time

  17. Lead/Lag or Dynamic Compensator Look at effect of these two to step change in input Output or response • Final Change from: • Magnitude of step change, • Initial response by the lead/lag, • Exponential decay from lag,

  18. Feed Forward Control Rule of Thumb: if lead-lag won’t help much; use FFCss (p 389) In text: pp 393-395, useful comments if implementing FFC No improvement using FFC with set point changes.

  19. In-Class PS Exercise: Feed Forward Control What is the Gm, and what is the GD? Determine FCC Tune PI controller to aggressive IMC For disturbance:Tjacket in 50oC – 60oC – 50oC • Test PI Controller • Test PI + FFCss only • Test PI + FFC full

  20. In-Class PS Exercise: Feed Forward Control PI only PI + FFCss only PI + full FFC

  21. ChE / MET 433

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