CONTROL ENGINEERING I (EEC323)
CHAPTER ONE
INTRODUCTION
Every activity in our lives is influenced by sort of control systems. The concept of control
system plays an important role in working of space vehicle, satellites, guided missiles,
automobile vehicle etc
Control systems are found in various practical applications like computerised control systems,
transportation systems, power systems, temperature limiting systems, robotics etc
BASIC CONCEPT OF CONTROL SYSTEMS
System is the interconnection of elements and devices for a desired purpose. For example, a
classroom is a system consisting of benches, board, fans, students etc. another example of a
system is a lamp consisting of glasses, filament, gasses for the purpose of providing light.
Control means to regulate, to direct or to command to get desired output.
A control system consists of subsystems and processes (or plants) assembled for the purpose
of obtaining a desired output with desired performance, given a specified input.
Control systems are concerned with finding technically the possible ways of acting on a
technological system to control its output to desired values while ensuring a desired level of
performance.
Considering the examples of systems given above.
▪ For the classroom, if a teacher is delivering lecture, the combination becomes a control
system.
▪ For the lamp, if it is switched ON and OFF using switch, the combination becomes a
control system.
A control system is a system, which provides the desired response by controlling the output.
The following figure shows the simple block diagram of a control system.
Figure 1: CONTROL SYSTEM
The figure above shows a control system in its simplest form, where the input represents a
desired output. Example of this is an elevator. When the fourth-floor button is pressed on the
first floor, the elevator rises to the fourth floor with a speed and floor-leveling accuracy
designed for passenger comfort. The push of the fourth-floor button is an input that represents
our desired output.
EXAMPLES OF CONTROL SYSTEMS: Control systems are an integral part of our
society. NUMEROUS applications of control systems influence all facets of modern life.
Engineering Examples of Control Systems
▪
Manual Control Systems: A switch for turning on/off a simple filament lamp, the
accelerator or throttle pedal of a motor car, the oven control knob on a gas cooker etc.,
▪
Automatic Control Systems: Automatic switches, automatic car, automatic washers and
dryers, automatic microwave ovens, chemical process plants, navigation and guidance
systems, space satellites, spaceships control panels. These are just a few examples of the
automatically controlled systems.
Biological Examples of Control Systems
Control Systems also exist in nature. Within our own bodies are numerous control systems;
the pancreas, which regulates our blood sugar, the precipitation system which secretes sweat
in controlling body temperature.
PURPOSE OF CONTROL SYSTEMS
With control systems we can move large equipment with precision that would otherwise be
impossible. We can point huge antennas toward the farthest reaches of the universe to pick up
faint radio signals; controlling these antennas by hand would be impossible. Because of control
systems, elevators carry us quickly to our destination, automatically
stopping at the right floor. We alone could not provide the power required for the load and the
speed; motors provide the power, and control systems regulate the position and speed.
We build control systems for four primary reasons:
1. Power Amplification (Gain)
For example, a radar antenna, positioned by the low-power rotation of a knob at the input,
requires a large amount of power for its output rotation. A control system can produce the
needed power amplification, or power gain.
2. Remote Control
Robots designed by control system principles can compensate for human disabilities. Control
systems are also useful in remote or dangerous locations. For example, a remote-controlled
robot arm can be used to pick up material in a radioactive environment.
3. Convenience of Input Form
Control systems can also be used to provide convenience by changing the form of the input.
For example, in a temperature control system, the input is a position on a thermostat. The
output is heat. Thus, a convenient position input yields a desired thermal output.
4. Compensation for Disturbances
Another advantage of a control system is the ability to compensate for disturbances. Typically,
we control such variables as temperature in thermal systems, position and velocity in
mechanical systems, and voltage, current, or frequency in electrical systems. The system must
be able to yield the correct output even with a disturbance. For example, consider an antenna
system that points in a commanded direction. If wind forces the antenna from its commanded
position, or if noise enters internally, the system must be able to detect the disturbance and
correct the antenna's position. Obviously, the system's input will not change to make the
correction. Consequently, the system itself must measure the amount that the disturbance has
repositioned the antenna and then return the antenna to the position commanded by the input.
CLASSIFICATION OF CONTROL SYSTEMS
Based on some parameters, we can classify the control systems into the following ways:
1. Based on The Type of Signal
Control Systems can be classified as continuous time control systems and discrete time
control systems based on the type of the signal used.
(I)
In continuous time control systems, all the signals are continuous in time. For
example, speed control of a d.c motor using a tachogenerator feedback.
(II) In discrete time control systems, there exists one or more discrete time signals.
Example of this is a microprocessor or a computerized control system.
2. Based on Number of Inputs and Outputs
Control Systems can be classified as SISO control systems and MIMO control systems
based on the number of inputs and outputs present.
(I)
SISO (Single Input and Single Output) control systems have one input and one
output. Examples are position control system and fan speed control.
(II) MIMO (Multiple Inputs and Multiple Outputs) control systems have more than one
input and more than one output. Example of this include radio system temperature
and humidity control of a room. The input of this system is the water sprayer to
control the humidity and the heating element to control the temperature of the room.
3. Based on Response to Time
Control Systems can be classified as open loop control systems and closed loop control
systems based on response to time.
I. Time Variant: If parameters vary with respect to time. Example: space vehicle in
which the mass of the vehicle decreases with time as it leaves the earth.
II.
Time Invariant: If the parameters do not vary with respect to time. Example: an
electrical circuit with elements R, L and C.
4. Based on System Configuration
Control Systems can be classified as open loop control systems and closed loop control
systems based on the system configuration.
(I)
In open loop control systems, output is not fed-back to the input. So, the control action
is independent of the desired output.
The figure below shows the block diagram of an open loop control system.
Figure 2: OPEN LOOP CONTROL SYSTEM
Here, an input is applied to a controller and it produces an actuating signal or controlling signal.
This signal is given as an input to a plant or process which is to be controlled. So, the plant
produces an output, which is controlled. Examples of open loop control systems include
washing machine, immersion rod, air conditioning system, bread toasters and control of the
traffic light, hand drier. To have a clear understanding of open loop systems, assume that you
calculate the amount of time you need to study for an examination that covers three chapters
in order to get an A. If the professor adds a fourth chapter—a disturbance—you are an openloop system if you do not detect the disturbance and add study time to that previously
calculated. The result of this oversight would be a lower grade than you expected.
Advantages of Open Loop Control System
i.
Open loop systems are simple.
ii.
They are economical.
iii. Less maintenance is required.
iv.
Proper calibration is simple.
Disadvantages of Open Loop Control System
i.
Open loop systems are inaccurate.
ii.
They are not reliable.
iii. They are slow.
iv.
Optimization is not possible.
(II)
In closed loop control systems, output is fed back to the input. So, the control action
is dependent on the desired output.
The following figure shows the block diagram of negative feedback closed loop control
system.
Figure 3: CLOSED LOOP CONTROLSYSTEMS
The error detector produces an error signal, which is the difference between the input and the
feedback signal. This feedback signal is obtained from the block (feedback elements) by
considering the output of the overall system as an input to this block. Instead of the direct
input, the error signal is applied as an input to a controller.
So, the controller produces an actuating signal which controls the plant. In this combination,
the output of the control system is adjusted automatically till we get the desired response.
Hence, the closed loop control systems are also called the automatic control systems.
Examples of closed loop systems include automatic electric iron, cooling system in car, air
conditioner and washing machine with fuzzy logic system.
Advantages of Closed Loop Control Systems
(i)
Closed loop systems are more reliable.
(ii) They are faster.
(iii) Optimization is possible.
(iv) A number of variables can be handled simultaneously.
Disadvantages of Closed Loop Control Systems
(i)
Closed loop systems are expensive.
(ii) Maintenance is difficult.
(iii) Complicated installation
COMPARISONS BETWEEN OPEN LOOP AND CLOSED LOOP CONTROL SYSTEMS
Open Loop Control Systems
Closed Loop Control Systems
1. Control action is independent of the desired Control action is dependent of the desired
output.
output.
2. Feedback path and error detector are not Feedback path and error detector are
present.
present.
3. They are not reliable.
These are reliable
4. More stable
Less stable
5. Optimization is not possible
Optimization is possible
6. Highly sensitive to disturbance
Less sensitive to disturbance
4. Easy to design.
Difficult to design.
5. These are economical.
These are costlier.
6. Inaccurate.
Accurate.
If either the output or some part of the output is returned to the input side and utilized as part
of the system input, then it is known as feedback. Feedback plays an important role in order
to improve the performance of the control systems.
COMPONENTS OF CONTROL SYSTEMS
The diagram below shows some basic components of automatic control system.
Figure 4: COMPONENTS OF CONTROL SYSTEM
▪ Set-point or Reference input: - This is the externally produced input to the system. It
represents the desired output
▪ Input Transducer: - This converts the form of the input to that used by the controller.
▪ Error Detector or Summing Junction: - this receives the measured signal and
compared it with the reference input. It is also called a comparator or summing junction.
The difference of the two signals, produces the error signal, hence the name error
detector.
▪ Controller: - The controller drives a process or a plant. The output of the controller is
called the controlled signal.
▪ Disturbances: - These are the unwanted signals which adversely affect the output of
the system. Disturbance may be internal or external.
▪ System, Plant or Process: -This is the plant or system to be controlled.
▪ Output Transducer, Feedback Element or Sensor: - Provides measurement of the
system output response and converts it into the form used by the controller. The output
signal is also called control variable.
TUTORIAL QUESTIONS
1. Define the term Control System with at least three examples from various aspect of life.
2. Draw a block diagram of a feedback control system indicating the suming point, controller, plant, and
feedback path (sensor).
3. What are the four basic purposes of control system; explain two.
4. List and explain the types of control system based on:
(a) System configuration
(b) Number of input and output
(c) Type of signal
(d) Response to time
5. In tabular form, compare between Open Loop and Closed Loop Control.
6. State the advantages and disadvantages of both open loop and closed loop control system.
7. Explain the following terms with respect to control system.
(a) Set point
(b) Input Transducer
(c) Error Detector
(d) Controller
(e) Disturbances
(f) Plant
(g) Feedback Element