Chapter 1

Introduction to Process

Control

Control Components

Dr. Mohamed Sobh

Reference: Chapter 1, Process Control Instrumentation Technology, 8th, Johnson

Contents

• Automatic Control

• Data Representation

• Control Definitions

2

Why Control

System

without

control

The Process

Qout > Qin h Decrease

Qout < Qin h Increase

Manual Control

Error Evaluation

Control Decision

Measurement

Control Action

Automatic Control

Measurement Sensor

Error Evaluation Difference Amp.

Control Decision Analog Circuit,

Computer, …

Control Action Actuator

More Control Examples

Servomechanism

Servo Control Systems

Reasons of Using Automatic

Control

Tracking Regulation

Maintain

Set Point

Reasons

?

Process Control Block Diagram

Set point

Error signal

Controlled

Variable

Manipulated

signal

Measured signal

Control

Signal

10

ON/OFF Control

+

-

11

Analog Control • All variables in the system are analog representations of another

variable.

• VT is an analog of T

Control System Objectives

1. System stability

2. Steady-state regulation

3. Transient regulation.


Output

Time

Unstable (un-damped) Response

Under-damped-response

Over-damped-response

Critical-damped-response

Transient Stability


Output

Time

Disturbance

Steady State Error +/- 5%

Steady State Stability

Unstable Response

Stable Response

Evaluation Criteria

1. System must be stable

2. Best Response Parameters

1. Fast response

2. Minimum overshot

3. Minimum steady state error

3. Minimum Integral Absolute Error

4. Maximum quarter amplitude damping

Response Parameters

Output

Time

Steady State Error +/- 5%

First Overshot

Second Overshot

Period of Oscillation

0.1

0.9

Rise Time

Desired Value

Integral Absolute Error

Error

Time

dtEIAE

Quarter Amplitude Damping

Output

Time

A1 = First Overshot

A2 = Second Overshot

Damping Ratio = A2/A1 < (1/4)

Controller Tuning

Adjust controller parameter to get the best

system response

Output Steady State Error

Time

Controller Implementation

Analog

Control

Unit

Digital

Control

Unit

D/A A/D

Analog

Digital

Data Representation

Digital Representation Analog Representation

Data Conversions

ADC (Analog-to-digital converter) DAC (Digital-to-Analog converter)

Data Units

• It is essential to use a well-defined set of

units of measurement

• Two system of units:

– The metric system

– The English system

• In process control, a particular set of

metric units is used called the International

System (SI)

International System of Units

Quantity Unit Symbol

Length Meter m

BASE

Mass Kilogram kg

Time Second s

Electric Current Ampere A

Temperature Kelvin K

Amount of substance Mole mol

Luminous intensity Candela cd

Plane Angle Radian rad

SUPPLEMENTARY Solid angle Steradian sr

Data Transmission

Data Transmission

Process Control Definitions

• Control: Force parameters in the environment to

have specific values.

• Sensor: Convert process output signal to

measurable control signal.

• Actuator: Convert control signal to process

input signal.

• Controller: Responsible on evaluating the error,

taking the control decision and producing the

control signal.


• Error:

• Difference between the measured value of

the variable and the desired one.

• Transfer Function

• Relationship between the input and output

for the block


• Transfer Function

• Relationship between the input and output

for the block, consists of two parts: static

and dynamic

– Static: Output depends only on input values

– Dynamic: Output depends on input and on

previous system status


Accuracy

• Maximum overall error to be expected from a device

• Accuracy is usually expressed as:

– Measured value

• EX (± 0.3 ma)

– Percentage of the instrument full-scale (FS) reading

• EX (Full Scale 20 ma and accuracy ± 1% ± 0.01*20 ma)

– Percentage of instrument span

• EX (Span 4 to 20 ma and accuracy ± 2% ± 0.02*(20-4)ma)

– Percentage of the actual reading

• EX (Read value is 15 ma and accuracy ± 0.5% ± 0.005*15 ma)


Sensitivity • Measure of the change in output of an

instrument for a change in input

• High sensitivity is desirable in an

instrument: a large change in output for a

small change in input

• Ex: A temperature transducer outputs 10

mV per degree Celsius; sensitivity = 10

mV/ºC


Hysteresis and Reproducibility • An instrument will not have the same output

value for a given input in many trials

• This is reproducibility of the device

• This variation is random and unpredictable

• Hysteresis: Different readings results for a

specific input, depending on whether the

input value is approached from higher or

lower values.


Hysteresis and Reproducibility


Resolution • Minimum measurable value of the input

variable.

• Can be changed only by redesign.

• Analog systems: smallest measurable change in the analog output signal of the measurement device.

• Digital systems: change in dynamic variable represented by a 1-bit change in the binary word output.


Linearity • In sensor and signal conditioning, output is

represented in some functional

relationship to the input

• This relationship must be unique: for each

value of the input variable there exists one

unique value of the output variable.

• A linear relationship between input and

output is highly desirable.


Linearity


Sensor Time Response • A process-control loop element specifies

how the output is related to the input if the

input is constant

• An element also has a time dependence

that specifies how the output changes in

time when the input is changing in time

• This dynamic transfer function is called the

time response.


Sensor Time Response

b(t) = bi+(bf – bi)[1 – e-t/]

First Order

Response

• a: output damping

constant

• fn: natural

frequency of the

oscillation


Sensor Time Response

Second Order

Response


Significance and Statistics

• Significant figures:

– Do not attach more significance to a variable

value than the instrument can support.

• Statistics:

– Arithmetic mean and

– Standard deviation

Education

Chapter 1