OVERVIEW OF CONTROL SYSTEM DESIGN

LECTURE 02

1

Objectives Students should be able to

◦ask necessary questions for developing a control system.

◦determine possible control objectives, input and output variables, and constraints.

◦assess the importance of process control from safety, environmental and economics point of view

2

Although the specific control objectives vary from plant to plant, there are a number of general requirements:

◦ Safety ◦ Stable plant operation ◦ Product specifications and production rate

◦ Environmental regulations

◦ Economic plant operation

3

Steps of Control System Design

◦ The most widely used process control strategy is multi-loop control.

◦ A multi-loop control system consists of a set of PI, PID controllers, one for each controlled variable.

4

Steps of Control System Design The key design decision for multi-loop control is to determine an ………………….., that is, to find a ………………… of controlled and manipulated variables.

5

The first step of developing a control strategy is to

Formulate control objective (s)

What are the control objectives ?

…………………..

…………………..

………………….

Typically they are dependent on the process boundary.

6

Coal-fired power plant

7

Steps of Control System Design

After the control objectives have been formulated, the control system can be designed. The first step is to identify:

Select input variables: …………………. and …………………… variables

Select output variables: ……………….. and ………………….. variables

8

Classification of the Variables in Chemical Process Control System

Input Variables :- effect of the surroundings on the chemical process

Output Variables:- effect of the process on the surroundings

9

Input Variables :- Effect of the surroundings on the chemical process

Manipulated (or adjustable) variables • they can be adjusted freely by the human operator or a control mechanism.

Disturbances • their values are not the result of adjustment by an operator or a control system.

10

Output Variables :- Effect of the process on the surrounding

Measured output variables • their values are known by directly measuring them.

Unmeasured output variables • they are not or cannot be measured directly.

11

Process: A tank with level controller

12

LT LC

Fi

Fo

SP

h

Process variables are:

h, Fi, F0, A, r

0FFdt

dhA i rrr

What are the input and output variables?

Process: A tank with level controller

13

LT LC

Fi

F0

SP

h

Process variables are: h, F

i, F0, A, r

0FFdt

dhA i rrr

Input variables are: ………….

Process: A tank with level controller

14

LT LC

Fi

F0

SP

h

Process parameters are:

A, r

0FFdt

dhA i rrr

Output variables are: ……

Steps of Control System Design

After that: we have to realize

operating constraints such as: ◦ hard constraints: max or min flow rate ◦ soft constraints: product composition

15

Steps of Control System Design

What are the operating characteristics of the considered process?

◦ Continuous

◦ Batch ◦ Semibatch

16

Steps of Control System Design

Safety, environmental, and economic considerations are all important.

◦Economics is the driving force

◦Unsafe and environmentally hazardous process will cost more to operate

17

Steps of Control System Design

Then, control structure or control configuration will have to select and implement to control the process.

18

Control configurations

A control configuration is the ……………………………………….. that is used to connect the available ……………………. to the available …………………… variables.

19

Control configurations

Feedback control

Feedforward control

Feedback-Feedforward control

Cascade control

20

Feedback control

CONFIGURATION

21

Feedback control

22

We want to keep the

liquid level at a certain

value.

Fi

Fo

What measurement should be performed?

Feedback control

23

Fi

Fo

24

0FFdt

dhA i rrr

LC SP

LT

Fi

Fo

),(0 SPhhfF

SPc hhKF 0

25

SP

Fi

Fo

Controller provides

linkage between flow

out and liquid level.

Feedback control System

26

Which variable do we want to control?

Which variable can we monitor?

Which variable can we adjust?

Feedback control

27

LC SP

LT

Fi

Fo

Another alternative, if Fi is free from upstream process.

Feedback control

28

Another

alternative, if Fi is

free from upstream process.

LC SP

LT

Fi

Fo

0FhhKdt

dhA SPc rrr

Feedforward control

CONFIGURATION

29

Feedforward control

30

We also want to

keep the liquid level at a certain value.

Fi

F

Feedforward control

31

We also want to

keep the liquid

level at a certain value.

What is feed-forward control configuration?

Fi

F

Feedforward control

32

What question should be asked?

What make the level change?

liquid feed

Fi

F

Feedforward control

33

What make the level changed?

Fi

F

Feedforward control

34

How to take the action?

Which flow should be changed?

There are two alternatives.

Fi

F

Feedforward control

35

First alternative

Fi

F

Feedforward control

36

Second alternative

Fi

F

Feedforward control

37

Water bath Heater

We want to keep T inside the heater constant.

Fi, Ti

Steam

condensate

F, T

Fs, Ts

Feedforward control

38

Water bath Heater

Fi, Ti

Steam

condensate

F, T

Revisit: A Blending Process

39

m1, x1

m, x

Mt m2, x2=1

x1 is varied

One solution

CT

CC

M

What control configuration is in this set-up?

Revisit: A Blending Process

40

m1, x1

m, x

Mt m2, x2=1

x1 is varied

Another solution

CT CC

M

What control configuration is in this set-up?

Consider both configurations

41

What are the advantages of feedback control?

What are the advantages of feedforward control?

How about disadvantages?

Consider both configurations

42

Apply a new configuration

Use both feedback and feedforward control

Feedback-Feedforward control

CONFIGURATION

43

Feedforward control

44

Feedback-

FT

FFW

+

LT LC

SP

+

Two control signals

combine together.

What are major differences between FB and FF control?

Monitor ……………….. variable (FB)

Monitor …………………. (FFW)

Adjust the ……………… input variable to compensate the change

45

Cascade control

CONFIGURATION

46

Cascade control Cascade control is a multi-loop feedback control configuration.

Typical there is one manipulated variable, but several controlled variables, (SIMO).

47

Cascade control

48

FT

LT

LC SP

FC SP

Develop a model for an isothermal CSTR

49

AAi CCF 0 VCek A

RT

Eq

0dt

dCV A

For first-order elementary reaction

with Arrhenius eqn.

F CA

Fi CA0

A --> B CA

RT

EkTk aexp)( 0

What are the input and output variables?

50

51

Select the control configuration How do we decide on the control configuration?

◦Accuracy

◦Speed of response

◦Fluctuation

◦Budget

52

Steps of Control System Design After selecting the control structure, control algorithm or controller type has to choose for taking action during the controller correction.

The most widely used process control strategy is multi-loop control.

A multi-loop control system consists of a set of PI, PID controllers, one for each controlled variable.

53

Steps of Control System Design Typical types of control algorithm or controller are

◦ ON/OFF controller

◦ PID controller

◦ Fuzzy logic controller

◦ Model based controller

etc.

54

Revisit

If the unit of (1.0) is m3, what is the unit of (0.7)?

55

FCFCdt

dCii 7.05.20.15.00.1

Steps of Control System Design

The most popular control algorithm or controller in industries are:

ON/OFF controller

PID controller ◦Proportional controller

◦Proportional integral controller

◦Proportional integral derivative controller

56

Controller algorithm / PID controller

57

dtde

D

t

c edteKCOi

0

1

msp yye

CO = Controller output

58

Controller algorithm / PID controller

dtde

D

t

c edteKCOi

0

1

msp yye

Proportional Controller

59

Time

Controlled Variable

Setpoint

0 t

{ysp

(t)-y(t)}

Proportional action

60

PID controller

dtde

D

t

c edteKCOi

0

1

msp yye

Integral Controller

61

Time

Controlled Variable

Setpoint

0 t

t

dtte0

)( = shade area

Integral action

62

dtde

D

t

c edteKCOi

0

1

msp yye

Derivative Controller

PID controller

63

Time

Controlled Variable

Setpoint

0 t

{ysp

(t)-ym

(t)}

Magnify

64

t tdt

dy

Derivative action

Data for each mode Proportional controller uses current data for decision making

Integral controller uses past til present data for decision making

Derivative controller uses future data for decision making

65

Design control objective Which variable will be kept constant?

Level

Temperature

Pressure

Concentration

Flow

66

Selection of Controlled Variables

Guideline 1

All variables that are not self regulating must be controlled.

67

Which one is self regulating system?

F is the function of the liquid height

68

Fi

F

Which one is self regulating system?

F has constant flow rate

69

Fi

F

Selection of Controlled Variables

Guideline 1

Choose output variables that must be kept within equipment and operating constraints

Select output variables that represent a direct measure of product quality

70

Selection of Controlled Variables

Guideline 2

Choose output variables that seriously interact with other controlled variables such as steam header pressure is affected the pressure of downstream units

71

Selection of Controlled Variables

Guideline 2

Choose output variables that have favorable dynamic and static characteristics.

Variable that has large time delay or insensitive to manipulated variables are poor choices.

72

Selection of Manipulated Variables

Guideline 3

Choose input variables that have the large impact on controlled variables.

Choose input variables that rapidly affect the controlled variables.

73

Selection of Manipulated Variables

Guideline 3

The manipulated variables should affect the controlled variables directly rather than indirectly.

Avoid recycling of disturbances

74

Selection of Measured Variables

Guideline 4

Reliable, accurate measurements are essential for good control

Select measurement points that have an adequate degree of sensitivity

Select measurement points that minimize time delays and time constants.

75

Select the measurements Besides the guideline 4, the following issue has to be considered as well:

The availability of the sensor

Its operating range

Its response time

Its cost

76

77

Type of Pressure Sensors

1. Gage pressure sensors

2. Vacuum pressure sensors

3. Differential pressure sensors

4. Absolute pressure sensors

5. Barometric pressure sensors

5 2 3

1

Pressure sensor

78

Temperature sensor

79

Resistance Temperature Detector, RTD

Thermocouple

Level sensor

80

Float-type level sensors

Flow sensor

81

Concentration sensor

82

Steps of Control System Design

The key design decision for multi-loop control is to determine an appropriate control structure, that is, to find a suitable pairing of controlled and manipulated variables.

83

Select control configuration Feedback control system

Feedforward control system

Feedback- Feedforward control system

Cascade control system

84

Select a controller Finally, select a controller

◦ON/OFF controller

◦PID controller

◦Fuzzy logic controller

◦Model based controller

◦etc.

85

Process: A tank with level controller

86

LT LC

Fi

F0

SP

h

0FFdt

dhA i rrr

At the controller:

hm is sent by a sensor.

hm is compared with hsp.

e = (hm – hsp)

F0 = F0s+ Kce

F0 = F0s + Kc(hm – hsp)

Process: A tank with level controller

)(0 spmcsi hhKFFdt

dhA rrr

)(0 spmcsi hhKFFdt

dhA

87

LT LC

Fi

Fo

SP

h

0FFdt

dhA i rrr

For liquid

Control systems present in the form of Block diagram

88

The Feedback control loop can be simplified in a block diagram.

89

Converter

Controller Final

Control Element

Process

Measuring devices

+ -

Set

Point

y

ym

e

The Feedforward control loop can be simplified in a block diagram.

90

Converter

Controller Final

Control Element

Process

Measuring devices

+ -

Set

Point

y

ym

e

91

LC

LT

LC

LT CT CC

TT TC

FT FC

Distillation column and its control system

Hierarchy of process control activities

92

Process

1. Measurement & actuation

2. Safety and Environmental Equipment Protection

3. Regulatory control

4. Multivariable and constraint Control

5. Real-time Optimization

6. Planning and Scheduling

< 1 sec.

Days-months

Hrs-Days

Minutes-Hrs

Sec.-Minutes

< 1 sec.

Advantages of Automatic Control Systems Less human manual effort

Lower labor costs

Better uniformity and possibility of products

Increased production

Saving in Raw Material

Saving in Energy

Saving in Plant Equipment

93

The development of a control strategy consists of formulating or identifying the following.

Control objectives

Input variables

Output variables

Constraints

Operating characteristics: batch, continuous, semibatch,

Safety, environmental, and economic considerations

Control structure / controller

94

Tutorial 2.2 Design a control system in order to

obtain the desired concentration of the solution at 20% wt

produce the solution at a specified flow rate

maintain the level in the dilution tank

reduce the temperature of the solution before sending it to the dilution tank

95

Three general classes of needs that a control system is called on to satisfy:

Suppressing the influence of external disturbances

Ensuring the stability of a chemical process

Optimizing the performance of a chemical process

96

A Process to be controlled

97

End of Slide set 02

98

Read textbook