CHAPTER 15: FEEDFORWARD CONTROL Outline of the lesson. A process - - PowerPoint PPT Presentation

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Jan 31, 2023 132 likes •245 views

CHAPTER 15: FEEDFORWARD CONTROL Outline of the lesson. A process challenge - improve performance Feedforward design rules Good features and application guidelines Several process examples Analogy to management

SLIDE 1

Outline of the lesson.

A process challenge - improve

performance

Feedforward design rules
Good features and application

guidelines

Several process examples
Analogy to management principle

CHAPTER 15: FEEDFORWARD CONTROL

SLIDE 2

TC 2 T 1 F 1 F 2 T 3 L 1

feed product heating stream

Discuss this stirred tank heat exchanger. PID controller

CHAPTER 15: FEEDFORWARD CONTROL

SLIDE 3

TC 2 T 1 F 1 F 2 T 3 L 1

feed heating stream

Disturbance = feed temperature Control performance not acceptable!

Class exercise: What do we do?

CHAPTER 15: FEEDFORWARD CONTROL

20 40 60 80 100 120 140 160 180 200 70 72 74 76 IAE = 237.6971 ISE = 758.425 temperature

minimum

TC

Let’s use cascade

SLIDE 4

CHAPTER 15: FEEDFORWARD CONTROL

CASCADE DESIGN CRITERIA FOR T1

Cascade is desired when 1. Single-loop performance unacceptable 2. 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

OK OK OK NO! Cascade not possible. We need another enhancement!

SLIDE 5

TC 2 T 1 F 1 F 2 T 3 L 1

feed product heating stream

Let’s think about the process behavior.

Causal relationship

from T1 disturbance to T2 (without control)

How can we

manipulate valve to compensate? v (valve) → Q → TC T0 (Feed temperature)

CHAPTER 15: FEEDFORWARD CONTROL

SLIDE 6

20 40 60 80 100 120 140 160 180 200 66 68 70 72 74 76

T

20 40 60 80 100 120 140 160 180 200 Time

T0

Time

Dm(t) = T0

CHAPTER 15: FEEDFORWARD CONTROL

We want to adjust the valve to cancel the effect of the disturbance.

CVA(t) = disturbance effect CVB(t) = compensation effect CVA + CVB = no deviation

20 40 60 80 100 120 140 160 180 200 50 52 54 56 58 60

v MV(t) = v

SLIDE 7

CHAPTER 15: FEEDFORWARD CONTROL

We use block diagram algebra to determine the form of the calculation [Gff(s)] to achieve the desired performance.

Gd(s) Gp(s) Gff(s)

+ Dm(s) CV A(s) CV B(s) CV (s) MV (s)

Measured disturbance, T0

Manipulated variable Controlled variable, T

How do we measure CVA?

Feedforward controller

SLIDE 8

CHAPTER 15: FEEDFORWARD CONTROL

) ( ) ( ) ( ) ( ) ( s G s G s D s MV s G

p d m ff

− = =

s lg ld ff m ff

e s T s T K ) s ( D ) s ( MV ) s ( G

θ −

+ + = = 1 1

Dead time Gain Lead-lag Special case of Gp(s) and Gd(s) being first order with dead time

Please verify.

SLIDE 9

CHAPTER 15: FEEDFORWARD CONTROL

s ld ff ff

e s T s T K s G

θ −

+ + = ) (

lg

1 1

Lead-lag = (Tlds+1)/Tlgs+1) FF controller gain = Kff = - Kd/Kp controller dead time = θff = θd - θp ≥ 0 Lead time = Tld = τp Lag time = Tlg = τd

How do we get values for these parameters?

SLIDE 10

CHAPTER 15: FEEDFORWARD CONTROL

TC 2 T 1 F 1 F 2 T 3 L 1

feed heating stream

TY 1 TY 2

+

FF

FF high- lighted in red How do we combine feedback with feedforward? MVff MVfb

SLIDE 11

Control Performance Comparison for CST Heater Single-Loop Feedforward with feedback

Much better performance! WHY?

CHAPTER 15: FEEDFORWARD CONTROL

20 40 60 80 100 120 140 160 180 200 70 72 74 76 IAE = 237.6971 ISE = 758.425 temperature 20 40 60 80 100 120 140 160 180 200 70 71 72 73 74 75 76 IAE = 27.772 ISE = 8.0059 temperature