Introduction: Control System | GATE Notes & Videos for Electrical Engineering - Electrical Engineering (EE) PDF Download

 Page 1


                              
 
CONTROL SYSTEMS 
 
 
 
INTRODUCTION 
A control system manages commands, directs or regulates the behavior of other devices or 
systems using control loops. It can range from a single home heating controller using 
a thermostat controlling a domestic boiler to large Industrial control systems which are used 
for controlling processes or machines. 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. 
 
Examples - Traffic lights control system, washing machine 
Traffic lights control system is an example of control system. Here, a sequence of input 
signal is applied to this control system and the output is one of the three lights that will be 
on for some duration of time. During this time, the other two lights will be off. Based on the 
traffic study at a particular junction, the on and off times of the lights can be determined. 
Accordingly, the input signal controls the output. So, the traffic lights control system 
operates on time basis. 
Classification of Control Systems 
Based on some parameters, we can classify the control systems into the following ways. 
Continuous time and Discrete-time Control Systems 
? Control Systems can be classified as continuous time control systems and discrete 
time control systems based on the type of the signal used. 
? In continuous time control systems, all the signals are continuous in time. But, 
in discrete time control systems, there exists one or more discrete time signals. 
SISO and MIMO Control Systems 
? Control Systems can be classified as SISO control systems and MIMO control systems 
based on the number of inputs and outputs present. 
? SISO (Single Input and Single Output) control systems have one input and one output. 
Whereas, MIMO (Multiple Inputs and Multiple Outputs) control systems have more 
than one input and more than one output. 
Page 2


                              
 
CONTROL SYSTEMS 
 
 
 
INTRODUCTION 
A control system manages commands, directs or regulates the behavior of other devices or 
systems using control loops. It can range from a single home heating controller using 
a thermostat controlling a domestic boiler to large Industrial control systems which are used 
for controlling processes or machines. 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. 
 
Examples - Traffic lights control system, washing machine 
Traffic lights control system is an example of control system. Here, a sequence of input 
signal is applied to this control system and the output is one of the three lights that will be 
on for some duration of time. During this time, the other two lights will be off. Based on the 
traffic study at a particular junction, the on and off times of the lights can be determined. 
Accordingly, the input signal controls the output. So, the traffic lights control system 
operates on time basis. 
Classification of Control Systems 
Based on some parameters, we can classify the control systems into the following ways. 
Continuous time and Discrete-time Control Systems 
? Control Systems can be classified as continuous time control systems and discrete 
time control systems based on the type of the signal used. 
? In continuous time control systems, all the signals are continuous in time. But, 
in discrete time control systems, there exists one or more discrete time signals. 
SISO and MIMO Control Systems 
? Control Systems can be classified as SISO control systems and MIMO control systems 
based on the number of inputs and outputs present. 
? SISO (Single Input and Single Output) control systems have one input and one output. 
Whereas, MIMO (Multiple Inputs and Multiple Outputs) control systems have more 
than one input and more than one output. 
                              
 
CONTROL SYSTEMS 
 
 
Open Loop and Closed Loop Control Systems 
Control Systems can be classified as open loop control systems and closed loop control 
systems based on the feedback path. 
In open loop control systems, output is not fed-back to the input. So, the control action is 
independent of the desired output. 
The following figure shows the block diagram of the 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. The traffic lights control system which we 
discussed earlier is an example of an open loop control system. 
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. 
 
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. 
Page 3


                              
 
CONTROL SYSTEMS 
 
 
 
INTRODUCTION 
A control system manages commands, directs or regulates the behavior of other devices or 
systems using control loops. It can range from a single home heating controller using 
a thermostat controlling a domestic boiler to large Industrial control systems which are used 
for controlling processes or machines. 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. 
 
Examples - Traffic lights control system, washing machine 
Traffic lights control system is an example of control system. Here, a sequence of input 
signal is applied to this control system and the output is one of the three lights that will be 
on for some duration of time. During this time, the other two lights will be off. Based on the 
traffic study at a particular junction, the on and off times of the lights can be determined. 
Accordingly, the input signal controls the output. So, the traffic lights control system 
operates on time basis. 
Classification of Control Systems 
Based on some parameters, we can classify the control systems into the following ways. 
Continuous time and Discrete-time Control Systems 
? Control Systems can be classified as continuous time control systems and discrete 
time control systems based on the type of the signal used. 
? In continuous time control systems, all the signals are continuous in time. But, 
in discrete time control systems, there exists one or more discrete time signals. 
SISO and MIMO Control Systems 
? Control Systems can be classified as SISO control systems and MIMO control systems 
based on the number of inputs and outputs present. 
? SISO (Single Input and Single Output) control systems have one input and one output. 
Whereas, MIMO (Multiple Inputs and Multiple Outputs) control systems have more 
than one input and more than one output. 
                              
 
CONTROL SYSTEMS 
 
 
Open Loop and Closed Loop Control Systems 
Control Systems can be classified as open loop control systems and closed loop control 
systems based on the feedback path. 
In open loop control systems, output is not fed-back to the input. So, the control action is 
independent of the desired output. 
The following figure shows the block diagram of the 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. The traffic lights control system which we 
discussed earlier is an example of an open loop control system. 
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. 
 
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. 
                              
 
CONTROL SYSTEMS 
 
 
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. Traffic 
lights control system having sensor at the input is an example of a closed loop control system. 
The differences between the open loop and the closed loop control systems are mentioned 
in the following table. 
 
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. In this chapter, let us discuss the 
types of feedback & effects of feedback. 
Types of Feedback 
There are two types of feedback - 
? Positive feedback 
? Negative feedback 
Positive Feedback 
The positive feedback adds the reference input, R(s)R(s) and feedback output. The following 
figure shows the block diagram of positive feedback control system 
Page 4


                              
 
CONTROL SYSTEMS 
 
 
 
INTRODUCTION 
A control system manages commands, directs or regulates the behavior of other devices or 
systems using control loops. It can range from a single home heating controller using 
a thermostat controlling a domestic boiler to large Industrial control systems which are used 
for controlling processes or machines. 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. 
 
Examples - Traffic lights control system, washing machine 
Traffic lights control system is an example of control system. Here, a sequence of input 
signal is applied to this control system and the output is one of the three lights that will be 
on for some duration of time. During this time, the other two lights will be off. Based on the 
traffic study at a particular junction, the on and off times of the lights can be determined. 
Accordingly, the input signal controls the output. So, the traffic lights control system 
operates on time basis. 
Classification of Control Systems 
Based on some parameters, we can classify the control systems into the following ways. 
Continuous time and Discrete-time Control Systems 
? Control Systems can be classified as continuous time control systems and discrete 
time control systems based on the type of the signal used. 
? In continuous time control systems, all the signals are continuous in time. But, 
in discrete time control systems, there exists one or more discrete time signals. 
SISO and MIMO Control Systems 
? Control Systems can be classified as SISO control systems and MIMO control systems 
based on the number of inputs and outputs present. 
? SISO (Single Input and Single Output) control systems have one input and one output. 
Whereas, MIMO (Multiple Inputs and Multiple Outputs) control systems have more 
than one input and more than one output. 
                              
 
CONTROL SYSTEMS 
 
 
Open Loop and Closed Loop Control Systems 
Control Systems can be classified as open loop control systems and closed loop control 
systems based on the feedback path. 
In open loop control systems, output is not fed-back to the input. So, the control action is 
independent of the desired output. 
The following figure shows the block diagram of the 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. The traffic lights control system which we 
discussed earlier is an example of an open loop control system. 
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. 
 
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. 
                              
 
CONTROL SYSTEMS 
 
 
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. Traffic 
lights control system having sensor at the input is an example of a closed loop control system. 
The differences between the open loop and the closed loop control systems are mentioned 
in the following table. 
 
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. In this chapter, let us discuss the 
types of feedback & effects of feedback. 
Types of Feedback 
There are two types of feedback - 
? Positive feedback 
? Negative feedback 
Positive Feedback 
The positive feedback adds the reference input, R(s)R(s) and feedback output. The following 
figure shows the block diagram of positive feedback control system 
                              
 
CONTROL SYSTEMS 
 
 
 
he concept of transfer function will be discussed in later chapters. For the time being, 
consider the transfer function of positive feedback control system is, 
 
Where, 
? T is the transfer function or overall gain of positive feedback control system. 
? G is the open loop gain, which is function of frequency. 
? H is the gain of feedback path, which is function of frequency. 
 
Negative Feedback 
Negative feedback reduces the error between the reference input, R(s)R(s) and system 
output. The following figure shows the block diagram of the negative feedback control 
system. 
 
Transfer function of negative feedback control system is, 
 
 
Page 5


                              
 
CONTROL SYSTEMS 
 
 
 
INTRODUCTION 
A control system manages commands, directs or regulates the behavior of other devices or 
systems using control loops. It can range from a single home heating controller using 
a thermostat controlling a domestic boiler to large Industrial control systems which are used 
for controlling processes or machines. 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. 
 
Examples - Traffic lights control system, washing machine 
Traffic lights control system is an example of control system. Here, a sequence of input 
signal is applied to this control system and the output is one of the three lights that will be 
on for some duration of time. During this time, the other two lights will be off. Based on the 
traffic study at a particular junction, the on and off times of the lights can be determined. 
Accordingly, the input signal controls the output. So, the traffic lights control system 
operates on time basis. 
Classification of Control Systems 
Based on some parameters, we can classify the control systems into the following ways. 
Continuous time and Discrete-time Control Systems 
? Control Systems can be classified as continuous time control systems and discrete 
time control systems based on the type of the signal used. 
? In continuous time control systems, all the signals are continuous in time. But, 
in discrete time control systems, there exists one or more discrete time signals. 
SISO and MIMO Control Systems 
? Control Systems can be classified as SISO control systems and MIMO control systems 
based on the number of inputs and outputs present. 
? SISO (Single Input and Single Output) control systems have one input and one output. 
Whereas, MIMO (Multiple Inputs and Multiple Outputs) control systems have more 
than one input and more than one output. 
                              
 
CONTROL SYSTEMS 
 
 
Open Loop and Closed Loop Control Systems 
Control Systems can be classified as open loop control systems and closed loop control 
systems based on the feedback path. 
In open loop control systems, output is not fed-back to the input. So, the control action is 
independent of the desired output. 
The following figure shows the block diagram of the 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. The traffic lights control system which we 
discussed earlier is an example of an open loop control system. 
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. 
 
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. 
                              
 
CONTROL SYSTEMS 
 
 
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. Traffic 
lights control system having sensor at the input is an example of a closed loop control system. 
The differences between the open loop and the closed loop control systems are mentioned 
in the following table. 
 
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. In this chapter, let us discuss the 
types of feedback & effects of feedback. 
Types of Feedback 
There are two types of feedback - 
? Positive feedback 
? Negative feedback 
Positive Feedback 
The positive feedback adds the reference input, R(s)R(s) and feedback output. The following 
figure shows the block diagram of positive feedback control system 
                              
 
CONTROL SYSTEMS 
 
 
 
he concept of transfer function will be discussed in later chapters. For the time being, 
consider the transfer function of positive feedback control system is, 
 
Where, 
? T is the transfer function or overall gain of positive feedback control system. 
? G is the open loop gain, which is function of frequency. 
? H is the gain of feedback path, which is function of frequency. 
 
Negative Feedback 
Negative feedback reduces the error between the reference input, R(s)R(s) and system 
output. The following figure shows the block diagram of the negative feedback control 
system. 
 
Transfer function of negative feedback control system is, 
 
 
                              
 
CONTROL SYSTEMS 
 
 
 
Where, 
? T is the transfer function or overall gain of negative feedback control system. 
? G is the open loop gain, which is function of frequency. 
? H is the gain of feedback path, which is function of frequency. 
The derivation of the above transfer function is present in later chapters. 
Effects of Feedback 
Let us now understand the effects of feedback. 
Effect of Feedback on Overall Gain 
? From Equation 2, we can say that the overall gain of negative feedback closed loop 
control system is the ratio of 'G' and (1+GH). So, the overall gain may increase or 
decrease depending on the value of (1+GH). 
? If the value of (1+GH) is less than 1, then the overall gain increases. In this case, 'GH' 
value is negative because the gain of the feedback path is negative. 
? If the value of (1+GH) is greater than 1, then the overall gain decreases. In this case, 
'GH' value is positive because the gain of the feedback path is positive. 
In general, 'G' and 'H' are functions of frequency. So, the feedback will increase the overall 
gain of the system in one frequency range and decrease in the other frequency range. 
Effect of Feedback on Sensitivity 
Sensitivity of the overall gain of negative feedback closed loop control system (T) to the 
variation in open loop gain (G) is defined as