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

Measurement Error Evaluation

Control Action Control Decision

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
Maintain
Set Point

Reasons

Tracking Regulation

?

Process Control Block Diagram
Manipulated
signal

Set point
Control
Error signal Signal

Controlled
Variable

Measured signal

ON/OFF Control

+
-

10

Analog Control
• All variables in the system are analog representations of another
variable.
• VT is an analog of T

11

Control System Objectives
1. System stability
2. Steady-state regulation
3. Transient regulation.


Unstable (un-damped) Response

Output
Under-damped-response

Critical-damped-response

Over-damped-response
Time

Transient Stability

Unstable Response

Disturbance

Output

Steady State Error +/- 5% Time

Stable Response
Steady State Stability

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

First Overshot

Second Overshot Desired Value
Output

0.9

Steady State Error +/- 5%
Period of Oscillation
0.1
Time

Rise Time

Integral Absolute Error

Error

Time

IAE   E  dt

Quarter Amplitude Damping

A1 = First Overshot

A2 = Second Overshot
Output

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

Time

Controller Tuning
Adjust controller parameter to get the best
system response

Steady State Error
Output

Time

Controller Implementation

Analog
Analog Control
Unit

Digital
Digital A/D Control D/A
Unit

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
Mass Kilogram kg
Time Second s
BASE
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

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.


b(t) = bi+(bf – bi)[1 – e-t/]

First Order
Response

Second Order
Response

• a: output damping
constant
• fn: natural
frequency of the
oscillation

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

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