chapter 1
DESCRIPTION
Chapter 1, Introduction to ProcessControl, Control ComponentsTRANSCRIPT
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.
Control System Objectives
Output
Time
Unstable (un-damped) Response
Under-damped-response
Over-damped-response
Critical-damped-response
Transient Stability
Control System Objectives
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.
Process Control Definitions
• Error:
• Difference between the measured value of
the variable and the desired one.
• Transfer Function
• Relationship between the input and output
for the block
Process Control Definitions
• 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
Process Control Definitions
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)
Process Control Definitions
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
Process Control Definitions
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.
Process Control Definitions
Hysteresis and Reproducibility
Process Control Definitions
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.
Process Control Definitions
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.
Process Control Definitions
Linearity
Process Control Definitions
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.
Process Control Definitions
Sensor Time Response
b(t) = bi+(bf – bi)[1 – e-t/]
First Order
Response
• a: output damping
constant
• fn: natural
frequency of the
oscillation
Process Control Definitions
Sensor Time Response
Second Order
Response
Process Control Definitions
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