1.Introduction to Control System Engineering.pdf

1
EEE 2221
Introduction to Control System Engineering
Nayeema Hasan
Lecturer
Dept. of EEE, KUET.
Reference Books:
‘Modern Control Engineering’- by Katsuhiko Ogata.

System
 A system is a collection of components which interact with each other
and with the environment (by information or energy links) from which
the system is separated by a notational boundary.

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Systems include physical,
biological, organizational,
and other entities, and
combinations thereof, which
can be represented through
a common mathematical
symbolism. The study of
feedback control systems is
essentially a study of an
important aspect of systems
engineering and its
application.

System
 Input: The input is the stimulus, excitation or command applied to a
control system. Typically from external energy source, usually in order
to produce a specified response from the control system.
 Output: The output is the actual response obtained from a control
system. It may or may not be equal to specified response implied by the
input.
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Control Engineering
 Control engineering or Control systems engineering is based on the
foundations of feedback theory and linear system analysis, and it
integrates the concepts of network theory and communication theory.
 It is the engineering discipline that applies control theory to design
systems with predictable behaviors. The practice uses sensors to measure
the output performance of the device being controlled (often a vehicle)
and those measurements can be used to give feedback to the input
actuators that can make corrections toward desired performance. When
a device is designed to perform without the need of human inputs for
correction it is called automatic control (such as cruise control for
regulating a car's speed). Multi-disciplinary in nature, control systems
engineering activities focus on implementation of control systems mainly
derived by mathematical modeling of systems of a diverse range.
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Why Control is important? [for production process/ in
plant]
 (1) Safety: Prevent injury to plant personnel, protect the environment by
preventing emission and minimizing waste and prevent damage to the
process equipment.
 (2) Maintain product quality (composition, purity, color, etc.) on a
continuous basis and with minimum cost.
 (3) Maintain plant production rate at minimum cost.
 So, we can say that the reasons for automation of process plants are to
provide safety and at same time maintain desired product quality, high
plant throughput, and reduce demand on human labor.
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Control System
 A control system consists of subsystems and processes (or plants)
assembled for the purpose of controlling the outputs of the process. A
control system is a system capable of monitoring and regulating the
operation of a process or a plant. For example, a furnace produces heat
as a result of the flow of fuel. In this process, flow of fuel in the input,
and heat to be controlled is the output.
 There are two common classes of control systems, with many variations
and combinations: logic or sequential controls, and feedback or linear
controls. There is also fuzzy logic, which attempts to combine some of the
design simplicity of logic with the utility of linear control. Some devices
or systems are inherently not controllable
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Advantages of Control System
We build control systems for four primary reasons:
 Power amplification
 Remote control
 Convenience of input form
 Compensation of the disturbances
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Analysis and Design Objectives of Control
System
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Analysis and Design Objectives of Control
System
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Classification of Control System
 Controls are classified with respect to:
 Technique involved to perform control (i.e. human/machines):
i) manual control
ii) automatic control
 Time dependence of output variable (i.e. constant/changing):
i) regulator / regulating control
ii) servo / tracking control)
 Fundamental structure of the control (i.e. the information used for
computing the control):
i) Open loop control
ii) Feedback control / Closed-loop control
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 Manual and Automatic Controls - Examples
A system that involves:
 a person controlling a machine is called manual control. Ex: Driving a
car.
 machines only is called a automatic control. Ex: Central AC
 Servo/Regulator Controls – Examples
An automatic control system designed to:
 follow a changing reference is called tracking control or a servo. Ex:
Remote control car.
 maintain an output fixed (regardless of the disturbances present) is
called a regulating control or a regulator. Ex: Cruise control.
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 Open-Loop Control and Feedback control:
 The structures are fundamentally different:
 In an open-loop control, the system does NOT measure the actual
output and there is no correction to make that output conform to the
desired output. An Electric toaster is an open-loop control, Since:
 The controller is based on the knowledge.
 The output is not used in control computation.
 In a closed loop control, the system includes a sensor to measure the
output and uses feedback of the sensed value to influence the control
input variable. A water tank of an ordinary flush toilet is a (basic)
feedback control, since the output is fed back for control computation.
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Block Diagram
 It represents the structure of a control system.
 It helps to organize the variables and equations representing the control
system.
 It is composed of:
 – boxes, that represents the components of the system including their
causality;
 – Lines with arrows that represents the actual dynamic variables, such
as speed, pressure, velocity, etc.
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A simple open-loop Block Diagram example
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Spring/mass system:
System = mass + spring:
• Control Input: force u
• Output: displacement x(t)
• Block diagram; derived using
Laplace transforms.

Types of Block Diagram
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Block diagrams are of two types:
i) Specific Component block diagram
ii) Generic Component Block Diagram
For the Spring/mass system:
Specific Component block diagram
Generic Component block diagram

Definitions of Significant Terms
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• Process : Component whose the output is to be controlled, Ex: Mass.
• Actuator : Device that can influence the control input variable of the
process, Ex: Spring.
• Plant = actuator + process, Ex: Spring/mass system.
• Control input : external variable (signal/action) applied to the plant.
• Controller : computes the desired control input variable
• Sensor : measures the actual output variable and converts the
measured output into an electric signal for use by the controller.
• Comparator (or Σ) = computes the difference between the desired
and actual output variables to give the controller a measure of the
system error.
• Input filter = converts the desired output variable to electric form for
later manipulation by the controller.

Open-loop Control System
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• Open-loop control: An open-loop control is applied to achieve desired
system response using a controller or an actuator without feedback.
Figure below is the block diagram of it:
Features of Open-loop Control:
Two outstanding features of open-loop control systems are:
1. Their ability to perform accurately is determined by their calibration.
To calibrate means to establish or reestablish the input-output relation
to obtain desired system accuracy.
2. They are not usually troubled with problems of instability.

Example of an Open-loop Control System
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• An open-loop control system utilizes an actuating device to control
the process directly without using feedback. Below are the schematic
diagram (a) and block diagram (b) of Open loop speed-control of a
rotating disk.

Advantages of Open-loop Control System
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The main advantages of an open Loop Production Control System are:
• Simplicity: It is simple in construction and design.
• Low-cost: It is inexpensive compared to closed loop control systems.
• Easy Maintenance: It costs less time and money to maintain.
• No measurement: Useful when the production output is impossible or
hard to measure.
To put it simply, an open loop control system is best for manufacturing
processes that do not require output changes, run cheap, and come with
little to no ways to quantify the measurement. It should also have a
meager chance of disruptions during the production process.

Disadvantages of Open-loop Control System
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The main disadvantages of an open Loop Production Control System
are:
• Can’t handle disruptions: It can hardly handle any major
disruptions.
• Poor Reliability: It is unreliable as it cannot adapt to uncertainties.
• Poor Flexibility: It lacks the flexibility to self-correct and recover
from accidental stoppage.
• Poor Accuracy: With no feedback system, it is very inaccurate
when it needs to reflect the actual manufacturing output.

Examples of Open-loop Control System
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Closed Loop Control System
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• Closed-loop control: A closed-loop control is used to achieve desired
system response using a controller with the output measurement as a
feedback signal.
• The use of feedback enables us to improve system performance at the
cost of introducing the measurement noise and stability problem.
• Figure below is the block diagram of closed-lop control system:

Example of an Closed-loop Control System
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A closed-loop control system uses a measurement of the output and
feedback of this signal to compare it with the desired output. Below are the
schematic diagram (a) and block diagram (b) of Closed loop speed-control
of a rotating disk. The system is a negative feedback control system.

Advantages of Closed-loop Control System
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• Reliable and Accurate:
• It can reflect the actual production state by using feedback as input
data.
• Flexible and Faster Recovery:
• It could recover quicker from disruptions and avoid further damage
to production efficiency.
• Continuous Improvement:
• Collected feedback can be analyzed to streamline the manufacturing
process further, saving time and operation costs.
• Combat Risk Factors:
• System with well designed feedback control can respond to
unforeseen events.
• Eliminates need for human adjustment: of control variable and
reduces human workload.
• Gives much better performance than : it is possible with open-loop.

Disadvantages of Closed-loop Control System
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• More complex than open-loop control.
• May have steady state error.
• Depends on accuracy with which you can measure the output.
• May cause stability problems.
• Required heavy documentation or spreadsheets management.
• Software solutions were expensive and exclusive.
• Required costly and intensive training for employees.

Examples of Closed-loop Control System
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What is Feedback?
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• Feedback is that property of a closed-loop system which permits the
output (or some other controlled variable) to be compared with the
input to the system (or an input to some other internally situated
component or subsystem) so that the appropriate control action may
be formed as some function of the output and input.
• More generally, feedback is said to exist in a system when a closed
sequence of cause-and-effect relations exists between system
variables.

Characteristics of Feedback
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• CHARACTERISTICS OF FEEDBACK:
• 1. Increased accuracy: For example, the ability to faithfully
reproduce the input.
• 2. Reduced sensitivity: of the ratio of output to input to variations in
system parameters.
• 3. Reduced effects of nonlinearities.
• 4. Reduced effects of external disturbances or noise
• 5. Increased bandwidth: The bandwidth of a system is a frequency
response measure of how well the system responds to (or filters)
variations (or frequencies) in the input signal

Example of a Closed-loop Control System
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An example of a closed-loop control system is a person steering an
automobile (assuming his eyes are open) by looking at the auto’s location on
the road and making the appropriate adjustments.
A person steering an automobile:

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Regulate the level of fluid by adjusting the output valve : The input is a
reference level of fluid and is memorized by operator. The power amplifier is
the operator. The sensor is visual. Operator compares the actual level with
the desired level and opens or closes the valve ( actuator) adjusting the fluid
flow out, to maintain the desired level.
.

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Temperature Control System:
The temperature in the electric furnace is measured by a thermometer.
The analog temperature is converted to a digital temperature by an A/D
converter. This digital temperature is compared with the programmed
input temperature, and if there is any discrepancy (error), the controller
sends out a signal to the heater, through an interface, amplifier, and relay,
to bring the furnace temperature to a desired value..

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Temperature Control of passenger compartment of a car:
Figure above is a functional block diagram of temperature control of the
passenger compartment of a car. Note that the ambient temperature and
radiation heat transfer from the sun, which are not constant while the car
is driven, act as disturbances.

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Control System of the National Income:
It has become interesting and valuable to attempt to model the feedback
processes prevalent in the social, economic, and political spheres.

Multi-variable Control System
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Open-loop vs. Closed Loop Control System
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1. Feedback Element:
An advantage of the closed loop control system is the fact that the use of
feedback makes the system response relatively insensitive to external
disturbances and internal variations in system parameters. It is thus
possible to use relatively inaccurate and inexpensive components to obtain
the accurate control of a given plant, whereas doing so is impossible in the
open-loop case.
2. Stability:
From the point of view of stability, the open-loop control system is easier
to build because system stability is not a major problem. On the other
hand, stability is a major problem in the closed-loop control system, which
may tend to overcorrect errors and thereby can cause oscillations of
constant or changing amplitude.

Open-loop vs. Closed Loop Control System
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3. Cost and Required Power:
Note that the output power rating partially determines the cost, weight,
and size of a control system. The number of components used in a closed-
loop control system is more than that for a corresponding open-loop
control system. Thus, the closed-loop control system is generally higher in
cost and power. To decrease the required power of a system, open-loop
control may be used where applicable.
When to use open-loop: It should be emphasized that for systems in which
the inputs are known ahead of time and in which there are no
disturbances it is advisable to use open-loop control.
When to use closed-loop: Closed loop control systems have advantages
only when unpredictable disturbances and/or unpredictable variations in
system components are present.
A proper combination of open-loop and closed-loop controls is usually less
expensive and will give satisfactory overall system performance.

Definitions
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• Servomechanism (often abbreviated as servo). The term is often used
to refer to a mechanical system in which the steady-state error is zero
for a constant input signal. Sometimes, by generalization, it is used to
refer to any feedback control system.
• Regulator. This term is used to refer to systems in which there is a
constant steady-state output for a constant signal. The name is
derived from the early speed and voltage controls, called speed and
voltage regulators
• Command input: The motivating input signal to the system, which is
independent of the output of the system and exercises complete
control over it (if the system is completely controllable).
• Reference selector (reference input element). The unit that establishes
the value of the reference input. The reference selector is calibrated in
terms of the desired value of the system output.

Definitions
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• Reference input. The reference signal produced by the reference
selector, i.e., the command expressed in a form directly usable by the
system. It is the actual signal input to the control system.
• Disturbance input. An external disturbance input signal to the system
that has an unwanted effect on the system output.
• Forward element (system dynamics): The unit that reacts to an
actuating signal to produce a desired output. This unit does the work
of controlling the output and thus may be a power amplifier.
Feedback element: The unit that provides the means for feeding back
the output quantity, or a function of the output, in order to compare
it with the reference input.

Definitions
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• Output (controlled variable). The quantity that must be maintained
at a prescribed value, i.e., following the command input without
responding the disturbance inputs.
• Actuating signal. = Reference input – feedback signal. It is the input
to the control unit that causes the output to have the desired value.
• The fundamental difference between the open-and closed-loop
systems is the feedback action, which may be continuous or
discontinuous. In one form of discontinuous control the input and
output quantities are periodically sampled and discontinuous.
Continuous control implies that the output is continuously fed back
and compared with the reference input compared; i.e., the control
action is discontinuous in time. This is commonly called a digital,
discrete-data or sampled-data feedback control system.

1.Introduction to Control System Engineering.pdf

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1.Introduction to Control System Engineering.pdf