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1 ChE / MET 433 11 Apr 12 Process Linearity, Integral Windup, PID Controllers Linearity, Windup, & PID.

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Presentation on theme: "1 ChE / MET 433 11 Apr 12 Process Linearity, Integral Windup, PID Controllers Linearity, Windup, & PID."— Presentation transcript:

1 1 ChE / MET Apr 12 Process Linearity, Integral Windup, PID Controllers Linearity, Windup, & PID

2 2 ChE / MET 433 Quiz Solutions

3 Process Linearity Test the Heat Exchanger process linearity by: Starting Loop Pro trainer Set %CO to 80% Make steps down (say 10% down) to the %CO Measure the response Calculate the process gain 3

4 4 K = K = K = K = K = K = Adaptive Control ?

5 Integral (Reset) Windup 5 “Windup” can occur if integral action present Most modern controllers have anti-windup protection If doesn’t have windup protection, set to manual when reach point of saturation, then switch back to auto, when drops below sat. level IE: LoopPro Trainer, select Heat Exchanger Set %CO to 90%; SP to 126; Kc to 1 %/deg C; Tau I to 1.0 min Set Integral with Anti-Reset Windup ON Change Set Point to 120 deg. C. (~10 min); then change back to 126 deg. C Repeat with controller at ON: (Integral with Windup)

6 Integral (Reset) Windup 6

7 In-Class PID Controller Exercise Tune the Heat Exchanger for a PID Controller: Use the built in IMC, and choose Moderately Aggressive Start Loop Pro trainer Tune at the initial %CO and exit temperature Compare PI with PID Compare PID with PID with filter 7

8 8 ChE / MET Apr 12 Cascade Control: Ch 9 Advanced control schemes

9 Improve Feedback Control Feedback control: Disturbance must be measured before action taken ~ 80% of control strategies are simple FB control Reacts to disturbances that were not expected 9 We’ll look at: Cascade Control (Master – Slave) Ratio Control Feed Forward

10 Cascade Control Control w/ multiple loops Used to better reject specific disturbances 10 Take slow process: - + Split into 2 “processes” that can measure intermediate variable? G p2 must be quicker responding than G P1. Inner (2 nd -dary) loop faster than primary loop Outer loop is primary loop

11 11 Material Dryer Example - + Heat Exchger T air blower MC MT steam % moisture

12 12 Separate G p into 2 blocks Heat Exchger T air blower MC MT steam % moisture TT TC

13 13

14 14 Problem Solving Exercise: Heat Exchanger Heat Exchger T Hot water TC TT steam Single feedback loop. Suppose known there will be steam pressure fluctuations… Design cascade system that measures (uses) the steam pressure in the HX shell. Heat Exchger T Hot water TT steam PT

15 Temperature Control of a Well-Mixed Reactor (CSTR) Responds quicker to T i changes than coolant temperature changes. TiTi 15

16 Temperature Control of a Well-Mixed Reactor (CSTR) TiTi If T out (jacket) changes it is sensed and controlled before “seen” by primary T sensor. Use Cascade Control to improve control. Secondary Loop Measures T out (jacket) Faster loop SP by output primary loop Primary Loop: Measures controlled var. SP by operator 16

17 Cascade Control Disturbances in secondary loop corrected by 2 nd ary loop controller Flowrate loops are frequently cascaded with another control loop Improves regulatory control, but doesn’t affect set point tracking Can address different disturbances, as long as they impact the secondary loop before it significantly impacts the primary (outer loop). Benefits: Secondary loop must be faster than primary loop Bit more complex to tune Requires additional sensor and controller Challenges: 17

18 18 Cascade Control Examples Objective: Regulate temperature (composition) at top and bottom of column Distillation Columns

19 19 Objective: Keep T 2 out at the set point T 2 out Objective: Keep T P out at the set point T P out Heat Exchanger Furnace

20 20 In-Class Exercise: Cascade Control System Design What affects flowrate? Valve position Height of liquid P (delta P across valve) Design a cascade system to control level (note overhead P can’t be controlled)

21 21 In-Class Exercise: Cascade Control System Design Does this design reject P changes in the overhead vapor space?

22 Tuning a Cascade System 22 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 Check secondary loop 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

23 23 In-Class Exercise: Tuning Cascade Controllers Select Jacketed Reactor Set T cooling inlet at 46 o C (normal operation temperature; sometimes it drops to 40 o C) Set output of controller at 50%. Desired T out set point is 86 o C (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 o C to 40 o C

24 24 In-Class Exercise: Tuning Cascade Controllers Select Cascade Jacketed Reactor Set T cooling inlet at 46 o C (again) Set output of controller (secondary) at 50%. Desired T out set point is 86 o C (as before) Note the secondary outlet temperature (69 o C) 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 o C setpoint changes Tune the primary loop for PI control; make 3 o C set point changes (2 nd -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 o C. How does response compare to single PI feedback loop?

25 25 ChE / MET Apr 12 Ratio Control: Ch 10 Advanced control schemes

26 Ratio Control Special type of feed forward control Blending/Reaction/Flocculation A and B must be in certain ratio to each other AB 26

27 Ratio Control Possible control system: 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. A B 27 FT FC FY FT FC FY

28 Ratio Control Possible cascade control systems: “wild” stream A B 28 FT FY FC Desired Ratio A B FT FY FC Desired Ratio This unit multiplies A by the desired ratio; so output = “wild” stream

29 Ratio Control Uses: 29 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

30 30 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 Independent MV PV Ratio set point Dependent MV Disturbance var. TC TC output Desired 2 – 5% excess O 2 Check TC tuning to disturbance & SP changes.

31 31 ChE / MET Apr 12 Feed Forward Control: Ch 11 Advanced control schemes

32 Feed Forward Control Suppose q i is primary disturbance 32 Heat Exchanger TC TT ? What is a drawback to this feedback control loop? ? Is there a potentially better way? Heat Exchanger TT FT FF What if Ti changes? FF must be done with FB control! steam

33 Feed Forward and Feedback Control 33 Heat Exchanger TT FT TY steam TC FF TY Block diagram:

34 Feed Forward Control No change; perfect compensation! Response to MFF

35 Feed Forward Control Examine FFC T.F For “perfect” FF control:

36 Feed Forward Control: FFC Identification Set by traditional means: 36 Model fit to FOPDT equation: FF GainLead/lag unit Dead time compensator { FFC ss } steady state FF control { FFC dyn } dynamic FF control Accounts for time differences in 2 legs Often ignored; if set term to 1

37 37 ChE / MET 433

38 38 Problem Solving Exercise: Heat Exchanger Draw the block diagram: what is the primary and what is the secondary loop? Heat Exchger T Hot water TC TT steam PT PC


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