The document discusses process control components, definitions, and concepts. It introduces automatic and manual control systems, control objectives like stability and regulation, and control fundamentals such as sensors, actuators, controllers, errors, and transfer functions. It also covers data representation, conversions between analog and digital signals, and important metrics for control elements like accuracy, sensitivity, resolution, and time response.

1. Introduction to Process
Control
Control Components
Dr. Mohamed Sobh
Reference: Chapter 1, Process Control Instrumentation Technology, 8th, Johnson

2. Contents
• Automatic Control
• Data Representation
• Control Definitions
2

3. Why Control
System
without
control

4. The Process
Qout > Qin  h Decrease
Qout < Qin  h Increase

5. Manual Control
Measurement Error Evaluation
Control Action Control Decision

6. Automatic Control
Measurement Sensor
Error Evaluation Difference Amp.
Control Decision Analog Circuit,
Computer, …
Control Action Actuator

7. More Control Examples
Servomechanism
Servo Control Systems

8. Reasons of Using Automatic
Control
Maintain
Set Point
Reasons
Tracking Regulation
?

9. Process Control Block Diagram
Manipulated
signal
Set point
Control
Error signal Signal
Controlled
Variable
Measured signal

10. ON/OFF Control
+
-
10

11. Analog Control
• All variables in the system are analog representations of another
variable.
• VT is an analog of T
11

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

13. Control System Objectives
Unstable (un-damped) Response
Output
Under-damped-response
Critical-damped-response
Over-damped-response
Time
Transient Stability

14. Control System Objectives
Unstable Response
Disturbance
Output
Steady State Error +/- 5% Time
Stable Response
Steady State Stability

15. 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

16. Response Parameters
First Overshot
Second Overshot Desired Value
Output
0.9
Steady State Error +/- 5%
Period of Oscillation
0.1
Time
Rise Time

17. Integral Absolute Error
Error
Time
IAE   E  dt

18. Quarter Amplitude Damping
A1 = First Overshot
A2 = Second Overshot
Output
Damping Ratio = A2/A1 < (1/4)
Time

19. Controller Tuning
Adjust controller parameter to get the best
system response
Steady State Error
Output
Time

20. Controller Implementation
Analog
Analog Control
Unit
Digital
Digital A/D Control D/A
Unit

21. Data Representation
Digital Representation Analog Representation

22. Data Conversions
ADC (Analog-to-digital converter) DAC (Digital-to-Analog converter)

23. 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)

24. 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

25. Data Transmission

26. Data Transmission

27. 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.

28. 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

29. 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

30. 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)

31. 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

32. 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.

33. Process Control Definitions
Hysteresis and Reproducibility

34. 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.

35. 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.

36. Process Control Definitions
Linearity

37. 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.

38. Process Control Definitions
Sensor Time Response
b(t) = bi+(bf – bi)[1 – e-t/]
First Order
Response

39. Process Control Definitions
Sensor Time Response
Second Order
Response
• a: output damping
constant
• fn: natural
frequency of the
oscillation

40. 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