CHAPTER 15: FEEDFORWARD CONTROL

Transcription

1 CHAPER 5: EEDORWARD CONROL When I complete this chapter, I want to be able to do the following. Identify situations for which feedforward is a good control enhancement Design feedforward control using the five design rules Apply the feedforward principle to other challenges in life

2 CHAPER 5: EEDORWARD CONROL Outline of the lesson. A process challenge - improve performance eedforward design rules Good features and application guidelines Several process examples Analogy to management principle

3 CHAPER 5: EEDORWARD CONROL feed L product Discuss this stirred tank heat exchanger. C PID controller 3 heating stream

4 CHAPER 5: EEDORWARD CONROL Class exercise: What do we do? 76 C IAE = ISE = feed Disturbance = feed temperature Control performance not acceptable! L temperature C minimum Let s use cascade 3 heating stream

5 CHAPER 5: EEDORWARD CONROL CASCADE DESIGN CRIERIA OR Cascade is desired when. Single-loop performance unacceptable. A measured variable is available A secondary variable must 3. Indicate the occurrence of an important disturbance 4. Have a causal relationship from valve to secondary 5. Have a faster response than the primary

6 CHAPER 5: EEDORWARD CONROL OK CASCADE DESIGN CRIERIA OR Cascade is desired when. Single-loop performance unacceptable. A measured variable is available A secondary variable must 3. Indicate the occurrence of an important disturbance 4. Have a causal relationship from valve to secondary 5. Have a faster response than the primary

7 CHAPER 5: EEDORWARD CONROL OK OK CASCADE DESIGN CRIERIA OR Cascade is desired when. Single-loop performance unacceptable. A measured variable is available A secondary variable must 3. Indicate the occurrence of an important disturbance 4. Have a causal relationship from valve to secondary 5. Have a faster response than the primary

8 CHAPER 5: EEDORWARD CONROL OK OK CASCADE DESIGN CRIERIA OR Cascade is desired when. Single-loop performance unacceptable. A measured variable is available OK A secondary variable must 3. Indicate the occurrence of an important disturbance 4. Have a causal relationship from valve to secondary 5. Have a faster response than the primary

9 CHAPER 5: EEDORWARD CONROL OK OK CASCADE DESIGN CRIERIA OR Cascade is desired when. Single-loop performance unacceptable. A measured variable is available OK A secondary variable must 3. Indicate the occurrence of an important disturbance 4. Have a causal relationship from valve to secondary 5. Have a faster response than the primary NO! Cascade not possible. We need another enhancement!

10 CHAPER 5: EEDORWARD CONROL Let s think about the process behavior. Causal relationship from disturbance to (without control) feed L C product How can we manipulate valve to compensate? 3 heating stream v (valve) Q C 0 (eed temperature)

11 CHAPER 5: EEDORWARD CONROL 76 We want to adjust the valve to cancel the effect of the disturbance ime 0 D m (t) = ime

12 CHAPER 5: EEDORWARD CONROL 76 We want to adjust the valve to cancel the effect of the disturbance CV A (t) = disturbance effect ime 0 D m (t) = ime

13 CHAPER 5: EEDORWARD CONROL CV B (t) = compensation effect 76 We want to adjust the valve to cancel the effect of the disturbance CV A (t) = disturbance effect ime 0 D m (t) = ime

14 CHAPER 5: EEDORWARD CONROL CV B (t) = compensation effect We want to adjust the valve to cancel the effect of the disturbance CV A + CV B = no deviation CV A (t) = disturbance effect ime 0 D m (t) = ime

15 CHAPER 5: EEDORWARD CONROL CV B (t) = compensation effect We want to adjust the valve to cancel the effect of the disturbance. v CV A + CV B = no deviation CV A (t) = disturbance effect MV(t) = v ime 0 D m (t) = ime

16 CHAPER 5: EEDORWARD CONROL We use block diagram algebra to determine the form of the calculation [G ff (s)] to achieve the desired performance. Measured disturbance, 0 D m (s) G d (s) CV A (s) Controlled variable, eedforward controller Manipulated variable G ff (s) + MV (s) G p (s) CV B (s) CV (s) How do we measure CV A?

17 CHAPER 5: EEDORWARD CONROL [ ] 0 0 = + = = + = ) ( ) ( ) ( ) ( ) ( ) ( ) ( s D s G s G s G s CV s CV s CV m p ff d B A ) ( ) ( ) ( ) ( ) ( s G s G s D s MV s G p d m ff = = Not a PID algorithm! Why??? his is general!

18 CHAPER 5: EEDORWARD CONROL G ff ( s) = MV ( s) D m ( s) = G G d p ( s) ( s) Special case of G p (s) and G d (s) being first order with dead time Please verify. G ff (s) = MV(s) D m (s) = K ff ld lg s s + + e θ ff s Gain Lead-lag Dead time

19 CHAPER 5: EEDORWARD CONROL G ff ( s) = K ff ld s + lgs + e θ ff s Lead-lag controller gain = ( ld s+)/ lg s+) = K ff = - K d /K p controller dead time = θ ff = θ d - θ p 0 Lead time Lag time = ld = τ p = lg = τ d How do we get values for these parameters?

20 CHAPER 5: EEDORWARD CONROL s ld ff ff ff e s s K s G θ + + = ) ( lg Digital implementation is straightforward. Its derived in textbook. Γ Γ Γ Γ + + = = N m N m N ff N ff N m ld ff N m ld ff N ff N ff D c D b MV a MV D t t K D t t K MV t t MV ) ( ) ( ) ( ) ( ) ( / / ) ( / / ) ( / / ) ( lg lg lg lg

21 CHAPER 5: EEDORWARD CONROL ypical dynamic responses from the lead-lag element in the feedforward controller. It synchronizes the compensation and disturbance effects. Results for several cases of lead/lag : a. 0.0 b. 0.5 c..0 d..5 e..0

22 CHAPER 5: EEDORWARD CONROL feed L How do we combine feedback with feedforward? highlighted in red Y MV fb C Y + MV ff 3 heating stream

23 CHAPER 5: EEDORWARD CONROL Control Performance Comparison for CS Heater Single-Loop eedforward with feedback 76 IAE = ISE = IAE = 7.77 ISE = temperature 74 7 temperature Much better performance! WHY?

24 CHAPER 5: EEDORWARD CONROL temperature C IAE = 7.77 ISE = he MV changed before deviated from its set point! heating valve (% open) SAM =.4394 SSM = Valve adjustment not too aggressive ime Disturbance occurred at this time Why wait after disturbance?

25 CHAPER 5: EEDORWARD CONROL What have we gained and lost using feedforward and feedback? feed Y L C or each case, is with B better, same, worse than single-loop feedback (C v)?? 3 Y + heating stream A disturbance in feed inlet temperature A disturbance in heating medium inlet pressure A disturbance in feed flow rate A change to the C set point

26 CHAPER 5: EEDORWARD CONROL What have we gained and lost using feedforward and feedback? feed Y L C or each case, is and B better, same, worse than single-loop feedback (C v)?? 3 Y + heating stream A disturbance in feed inlet temperature A disturbance in heating medium inlet pressure A disturbance in feed flow rate A change to the C set point /B better Both the same Both the same Both the same

27 CHAPER 5: EEDORWARD CONROL EEDORWARD DESIGN CRIERIA eedforward is desired when. Single-loop performance unacceptable. A measured variable is available A measured disturbance variable must 3. Indicate the occurrence of an important disturbance 4. NO have a causal relationship from valve to measured disturbance sensor 5. Not have a much faster affect on the CV than the MV (when combined with feedback)

28 CHAPER 5: EEDORWARD CONROL eedforward and eedback are complementary Advantages Disadvantages eedforward Compensates for disturbance before CV is affected Does not affect the stability of the control sysytem (if G ff (s) stable) Cannot eliminate steadystate offset Requires a sensor and model for each disturbance eedback Provides zero steadystate offset Effective for all disturbances Does not take control action until the CV deviates from its set point Affects the stability of the control system

29 CHAPER 5: EEDORWARD CONROL CLASS EXERCISE: SOME QUESIONS ABOU EEDORWARD CONROL Why do we retain the feedback controller? When would feedforward give zero steady-state offset? Why does the feedforward controller sometimes delay its compensation? Don t we always want fast control? What is the additional cost for feedforward control? How can we design a strategy that has two controllers both adjusting the same valve? What procedure is used for tuning feedforward control?

30 CHAPER 5: EEDORWARD CONROL heating stream Discuss this packed bed reactor. feed A 3 packed bed reactor Notes:. A measures reactant concentration. Circle is shell & tube heat exchanger 3. eed valve is adjusted by upstream process 4. Increasing temperature increases reaction rate A product

31 CHAPER 5: EEDORWARD CONROL Class exercise: Design feedforward control to improve the performance. Performance not acceptable for feed composition disturbance heating stream What about cascade? 0. AC feed IAE =.9349 ISE = A packed bed reactor 3 CV maximum Disturbance in feed composition AC product

32 CHAPER 5: EEDORWARD CONROL Class exercise: Design feedforward control to improve the performance. eedforward design criteria A 3. Single-loop not acceptable. Disturbance variable is measured 3. Indicates a key disturbance 4. No Causal relationship, valve D m 5. Disturbance dynamics not much faster than compensation Let s use the feedforward design rules! Remember: he disturbance is the feed composition.

33 CHAPER 5: EEDORWARD CONROL Class exercise: Design feedforward control to improve the performance. eedforward design criteria A 3. Single-loop not acceptable Y Y Y Y Y Y. Disturbance variable is Y Y Y Y Y Y measured 3. Indicates a key disturbance Y N N N N N 4. No Causal relationship, valve Y Y N Y Y N D m 5. Disturbance dynamics not much faster than compensation Y N/A N/A N/A N/A N/A A satisfies all of the rules and can be used as a feedforward variable.

34 CHAPER 5: EEDORWARD CONROL heating stream MV AC MV ff + feed A 3 AY D m packed bed reactor MV fb CV AC product SP from person

35 CHAPER 5: EEDORWARD CONROL Control Performance Comparison for Packed Bed Reactor Single-Loop eedforward and feedback 0. IAE =.9349 ISE = IAE =.794 ISE = AC AC CV Much better performance! WHY? Little model error, most experimental feedforward not this good!

36 CHAPER 5: EEDORWARD CONROL What have we gained and lost using feedforward and feedback? How does the system respond to the following? A disturbance in A disturbance in heating medium inlet pressure A disturbance in A disturbance to feed composition, A feed A change to the AC- set point AY heating stream A D m Both the same packed bed reactor MV Both the same 3 CV Both the same AC AC /B better MV ff + MV fb product SP from person Both the same

37 CHAPER 5: EEDORWARD CONROL We can combine cascade and feedforward to gain the advantages of both. heating stream MV MV ff feed A CV C 3 secondary Y 3 AY packed bed reactor MV primary CV AC SP from person

38 CHAPER 5: EEDORWARD CONROL Ratio control is a simple and frequently used feedforward application. In ratio control, the dynamics are negligible. Uncontrolled (wild) flow Desired / = R Y x Blended flow SP = *R C Goal is to keep / constant. Manipulated flow

39 CHAPER 5: EEDORWARD CONROL CLASS EXERCISE: Use analyzer in automatic control while retaining the good aspects of ratio control. Uncontrolled (wild) flow Desired / = R Y x A Blended flow SP = *R C Goal is to keep A constant. Manipulated flow

40 CHAPER 5: EEDORWARD CONROL CLASS EXERCISE: Use analyzer in automatic control while retaining the good aspects of ratio control. Uncontrolled (wild) flow R Y x AC eedback PID Blended flow SP = *R C Goal is to keep A constant. Manipulated flow

41 CHAPER 5: EEDORWARD CONROL In many organizations, we take actions on inputs to prevent large disturbances to outputs. Sometimes, these are called pre-actions. After you have measured the change, you have some time to react before it hits you What would you do if? Number of births per year increases by 0% in your country A drought occurs in in the most fertile area of your country New legislation will impose stricter emissions regulations in three years Do we need feedback? What is your algorithm? What would you do if the measurement were noisy?

42 CHAPER 5: EEDORWARD WORKSHOP Evaluate feedforward control for a disturbance in the heating medium inlet temperature. You may add a sensor but make no other changes to the equipment. feed L product C 3 heating stream

43 CHAPER 5: EEDORWARD WORKSHOP Prepare a flowchart for the calculations performed by the packed bed feedforward controller. Show every calculation and use process variable symbols (e.g., A), not generic symbols (CV ). Report the equations for digital control. heating stream MV AC MV ff + feed A 3 AY D m packed bed reactor MV fb CV AC SP from person

44 CHAPER 5: EEDORWARD WORKSHOP 3 Answer each of the following questions true or false. he feedback controller tuning does not change when combined with feedforward compensation.. he feedforward controller has no tuning parameter. 3. he feedforward controller should react immediately when the measured disturbance is measured. 4. eedforward could be applied for a set point change.

45 CHAPER 5: EEDORWARD WORKSHOP 4 Identify a process that would benefit from ratio control. You may select from examples in your summer/co-op jobs, engineering laboratories, and course projects. Draw a sketch of the process with ratio control. Explain the advantages and any disadvantages of the design.

46 CHAPER 5: EEDORWARD When I complete this chapter, I want to be able to do the following. Identify situations for which feedforward is a good control enhancement Design feedforward control using the five design rules Apply the feedforward principle to other challenges in life Lot s of improvement, but we need some more study! Read the textbook Review the notes, especially learning goals and workshop ry out the self-study suggestions Naturally, we ll have an assignment!

47 CHAPER 5: LEARNING RESOURCES SIE PC-EDUCAION WEB - Instrumentation Notes - Interactive Learning Module (Chapter 5) - utorials (Chapter 5) he extbook, naturally, for many more examples

48 CHAPER 5: SUGGESIONS OR SEL-SUDY. Suggest some methods for fine-tuning a feedforward controller.. Program a feedforward controller for one of the processes modelled in Chapters Explain why the feedforward compensation should not be much slower than the disturbance. Why doesn t this guideline apply when no feedback is implemented? 4. Discuss whether you would recommend more than one feedforward controller on the same process. 5. Write a memorandum explaining feedforward compensation for a company with non-technical employees

49 CHAPER 5: SUGGESIONS OR SEL-SUDY 6. A friend asks whether the general sketch for feedback, textbook igure.4, applies to feedforward. Answer completely, including any changes to the sketch. 7. Discuss why the feedforward controller dead time must be positive.