過程控制儀表與裝置Chap01 Introduction to Process Control

Chapter

1Introduction

to

Process

ControlIntroductionControl

SystemsProcess-Control

Block

DiagramControl

System

EvaluationAnalog

and

Digital

ProcessingUnits,

Standards,

And

DefinitionsSensor

Time

ResponseChapter

1

Introduction

to

Process

ControlControlControl

systemProcess

control

system1.1

IntroductionProcess

control

systemsServomechanismsDiscrete-state

control

systems1.2

Control

systems1.2

Control

systems1.2.1

Process

control

principles(1)

The

ProcessProperty:

self-regulationFor

some

input

flow

rate,

theliquid

height

will

rise

until

it

reacheheight

for

which

the

output

flow

ratematches

the

input

flow

rate.A

self-regulation

system

does

not

provide

a

regulation

of

a

variable

to

any

particular

reference

value.Non-self-regulation(1)

The

Process1.2.1

Process

control

principles1.2

Control

systemsTo

maintain

a

value,

a

sensor

is

necessary

to

provide

a

sight

tube.

Theliquid

level

is

called

controlled

variable.A

valve

has

been

added

to

the

output

flow

rate

can

be

changed

by

the

humaThe

output

flow

is

called

manipulated

variable

or

Controlling

variable.Disadvantages:

tired,

human

cost,

etc.1.2.1

Process

control

principles(2)

Human-aided

control,

artificial

regulation1.2

Control

systemsSensor:

a

floating

ball

mechanism,

human

eyesController:

a

machines

,

electronic

circuits

or

computer

,

huFinal

control

operation:

actuator

and

valve,

human

hands1.2.1

Process

control

principles(3)

Automatic

control1.2

Control

systems(3)

Error

detector

–the

difference

between

the

actual

provalue

and

setpoint.

Controller

-

to

examine

the

error

and

determine

what

acshould

be

taken.

Control

Element

(final

control

element):

refers

to

ththat

exerts

a

direct

influence

on

the

process.

e.g.

valvactuator

etc.Identification

of

elements

Process:

single-variable

process,

multi-variable

pro(perhaps

interrelated)Measurement:

sensor,

transducer.

All

sensors

are

tran but

not

all

transducers

are

sensors.1.3

Process-control

block

diagram1.3.2

Block

Diagram1.3

Process-control

block

diagramChap.

2,

3:

transducersChap

4,

5,

6:

sensorsChap.

7Chap.

8,

9,

10,

11Chap.

1,

121.3.2

Block

Diagram1.3

Process-control

block

diagramPhysical

diagram

of

a

process-control

loop1.3.2

Block

Diagram1.3

Process-control

block

diagramPhysical

diagram

of

aprocess-control

loopBlock

diagram

of

theprocess-control

loop1.4

Control

System

EvaluationControl

system

objective:The

system

should

be

stable.The

system

should

provide

the

best

possible

steady-state

regulatiThe

system

should

provide

the

best

possible

transient

regulation.1.4

Control

System

EvaluationMinimium

areaQuarter

amplitude1.4

Control

System

Evaluation1.5.1

Data

Representation1.5Analog

and

Digital

Processing1.5.2

ON/OFF

Control1.5Analog

and

Digital

Processing1.5.2

ON/OFF

ControlDeadband

-

a

range,

of

temperature

in

this

case,

whereinno

action

will

occur.Hysteresis

-

the

behavior

of

the

system

is

difference

at

thsame

value

depending

on

whether

the

temperature

isincreasing

or

decreasing.1.5

Analog

and

Digital

Processing1.5.3

Analog

Control1.5Analog

and

Digital

ProcessingSupervisory

control1.5.4

Digital

Control1.5

Analog

and

Digital

ProcessingDirect

Digital

Control

(DDC)1.5.4

Digital

Control1.5Analog

and

Digital

ProcessingSmart

Sensor1.5.4

Digital

Control1.5

Analog

and

Digital

ProcessingNetworked

Control

System1.5.4

Digital

Control1.5

Analog

and

Digital

Processing1.6.1

Units1.6

Units,

Standards,

and

Definitions1.6

Units,

Standards,

andDefinitions1.6.1

Units·

English

system,

Appendix

p6001.6

Units,

Standards,

andDefinitions1.6.1

Units·

CGS---centimeter-gram-second

system,

Appendix

p6013-15psi(pound/inch2)

,

20~100kPa4~20mA1.6.2

Analog

Data

Representation1.6

Units,

Standards,

andDefinitions1.6

Units,

Standards,

and

Definitions1.6.2

Analog

Data

Representation1.6.2

Analog

Data

RepresentationExample

1.7

Suppose

the

temperature

range 20℃

to

120℃

islinearly

converted

to

the

standard

current

range4to

20mWhat

current

will

result

from

66℃?

What

temperature

doe6.5mA

represent?1.6

Units,

Standards,

andDefinitions1.6.3

DefinitionsErrorBlock

definitionsTransfer

function

(T.

F.)StaticDynamic1.6

Units,

Standards,

and

Definitions1.6

Units,

Standards,

and

Definitions1.6.3

Definitions4)

Accuracy

-

specify

the

maximum

overall

ever

of

a

device,

usuallyexpressed

as

inaccuracy.

Several

forms:Measured

variable.

e.g.

±2℃

-uncertainty

of

±2℃in

any

value

o temperature

measuredPercentage

of

the

instrument

full-scale

(FS)

reading.

e.g.

±5%

F

5-V

full-scale

range

meter,

the

accuracy

would

be

±5%FS×5V=

±

0.025VPercentage

of

instrument

span,

i.e.

percentage

of

the

range.

e.g. span

for

a

20

to

50

psi

range,

the

accuracy

would

be

(±0.03

)(50-±0.9psiPercentage

of

the

actual

reading.

e.g.

±2%

of

reading

2V,

the

acc would

be

±0.04V1.6

Units,

Standards,

and

Definitions1.6.3

Definitions5)

System

accuracy

–

the

overall

accuracy

of

many

elementsprocess

control

loop

to

represent

a

process

variable.The

best

way

is

in

the

terms

of

the

T.

F.6)

Sensitivity

-

a

measurement

of

the

change

in

output

of

aninstrument

for

a

change

in

input.High

sensitivity

is

desirable,

e.g.

10mV/℃

is

higher

than

5mV/℃evaluated

together

with

other

parameters

(linearity,

range,

accuracGenerally

indicated

by

the

T.

F.e.g.

5mV/℃

means:T.

F.,

i.e.

when

input

is

1

℃,

output

is

5mV.sensitivity,

i.e.

if

input

change

by

1℃,

output

change

5mV.1.6

Units,

Standards,

and

Definitions1.6.3

Definitions1.6

Units,

Standards,

and

Definitions1.6.3

Definitions7)

Hysteresis

and

Reproducibility8)

ResolutionComparing

Sensitivity

and

ResolutionSensitivity:

refers

to

the

output

when

input

change

by

oResolution:

refers

to

the

input

which

causes

the

minimumrecognized

output.1.6

Units,

Standards,

and

Definitions1.6.3

Definitions1.6

Units,

Standards,

and

Definitions1.6.3

Definitions9)

LinearityEXAMPLE

1.14

A

sensor

resistance

changes

linearly

from

100180

Ohm

as

temperature

changes

from20to

120℃.

Find

alinear

equation

relating

resistance

and

temperature.Solution:Thedesired

equationwouldbe

of

the

form1.6.4

Process-Control

DrawingsP&ID:

Piping

and

Instrumentation

Diagram

Page

6221.6

Units,

Standards,

and

Definitions1.6.4

Process-Control

DrawingsP&ID:

Piping

and

Instrumentation

Diagram

Page

6221.6

Units,

Standards,

and

Definitions1.7

Sensor

Time

ResponsePerfect

sensor:

b(t)

reaches

the

finalvalue

with

nolag.However,

all

sensors

will

exhibitsome

lag

between

the

output

and

the

inpu