What is Control System?
Let us study about a new type of engineering study which is called as Control Systems Engineering.It’s very interesting subject and has a lot of calculation part.Control system theory evolved as an engineering discipline and due to the universality of the principles involved, it is extended to various fields like economy, sociology, biology, medicine etc.In this, you will learn about open and closed loop control system and also their differences.
Control System theory has played a vital role in the advance of engineering and science.The automatic control has become an integral part of modern manufacturing and industrial processes.For example, numerical control of machine tools in manufacturing industries, controlling pressure, temperature, humidity, viscosity and flow in the process industry.
When a number of elements or components are connected in a sequence to perform a specific function, the group thus formed is called a system.In a system when the output quantity is controlled by varying the input quantity, the system is called control system.The output quantity is called controlled variable or response and input quantity is called command signal or excitation.
Types of Control systems:
Control systems are of two types.They are
1)Open Loop System
2)Closed Loop System
Any physical system which does not automatically correct the variation in its output is called an open loop system or control system in which the output quantity has no effect upon the input quantity are called open loop control system.This means that the output is not feedback to the input for correction.
In open loop control system, the output can be varied by varying the input.But due to external disturbances, the system output may change.When the output changes due to disturbances, it is not followed by changes in input to correct the output.In open loop systems, the changes in output are corrected by changing the input manually.
2)Closed loop control system:
Control systems in which the output has an effect upon the input quantity in order to maintain the desired output value are called closed loop systems.
The open loop system can be modified as closed loop system by providing a feedback.The provision of feedback automatically corrects the changes in output due to disturbances.Hence the closed loop system is also called automatic control system.The general block diagram of an automatic control system is shown in the figure below.It consists of an error detector, a controller, plant (open loop system) and feedback path elements.
The reference signal (or input signal) corresponds to the desired output.The feedback path elements sample the output and convert it to the same type as that of the reference signal.The feedback signal is proportional to the output signal and it is fed to the error detector.The error signal generated by the error detector is the difference between the reference signal and the feedback signal.The controller modifies and amplifies the error signal to produce better control action.The modified error signal is fed to the plant to correct its output.
Advantages of Open loop control system:
1.The open loop systems are simple and economical.
2.The open loop systems are easier to construct.
3.Generally, the open loop systems are stable.
Disadvantages of open loop systems:
1.The open loop systems are inaccurate and unreliable.
2..The changes in the output due to external disturbances are not corrected automatically.
Advantages of closed loop systems:
1.The closed loop systems are accurate.
2.The closed loop systems are accurate even in the presence of non-linearities.
3.The sensitivity of the systems may be made small to make the system more stable.
4.The closed loop systems are less affected by noise.
Disadvantages of closed loop systems:
1.The closed loop systems are complex and costly.
2.The feedback in closed loop system may lead to an oscillatory response.
3.The feedback reduces the overall gain of the system.
4.Stability is a major problem in closed loop system and more care is needed to design a stable closed loop 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.
Here, the control system is represented by a single block. Since, the output is controlled by varying input, the control system got this name. We will vary this input with some mechanism. In the next section on open loop and closed loop control systems, we will study in detail about the blocks inside the control system and how to vary this input in order to get the desired response.
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.
Based on some parameters, we can classify the control systems into the following ways.
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.
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 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.
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.
Open Loop Control Systems Closed Loop Control Systems Control action is independent of the desired output. Control action is dependent of the desired output. Feedback path is not present. Feedback path is present. These are also called as non-feedback control systems. These are also called as feedback control systems. Easy to design. Difficult to design. These are economical. These are costlier. Inaccurate. Accurate.Career
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