The concept of control systems has received an important position in the advancement of modern science. They have played an indispensable role in improving the functionality and efficiency of various electronic devices. They play an essential role in automating processes.
A system is a combination of various physical components that act together as an entire unit to achieve certain goals and also to control means to regulate or direct. Therefore, a control system is an arrangement of various physical elements connected in such a way as to command or direct its system or some other system.
Let's look at an example to understand the concept of a control system better. Every physical object is a system, for example, a lamp. A lamp made of filament, and glass is a physical system and if it is supplied with power and connected to an on or off switch, then we can control its function by switching on and off the switch. So, this entire system can be called a control system. This concept of a physical system and a control system is shown in the figure below.
Hence, we can say that a control system is an interconnection of the physical components to provide a desired function with some controlling action.
Components of a Control System
The main components of a control system are,
- Plant: This portion of a system that is to be controlled or regulated is called the plant or the process.
- Controller: The element of the system itself or external to the system that controls the process or the plant is called a controller. It accepts the error and produces the manipulated signal that controls the plant or the process.
- Input: An input is an applied signal or an excitation signal applied to a control system from an external energy source in order to produce a specific output. A control system may have more than one input.
- Output: It is a particular signal or the actual response obtained from a control system when input is applied. A control system may have more than one output.
- Disturbance: It is a signal which tends to adversely affect the value of the output of a system. If a disturbance is generated within the control system, then it is called an internal disturbance, or if, the disturbance is generated outside the control system acting as an extra input to the system in addition to its normal input and affecting the output adversely is called an external disturbance.
- Error Detector: It compares the actual output with the desired output to produce the error, which is the difference between the actual output and the desired output.
- Feedback: It senses the output and feeds it back to the input for comparison purposes.
Classification of Control Systems
Below are some of some of the broadly classified control systems,
Time-varying and Time-invariant systems
Time-varying control systems are those in which the parameters is the system vary with time. It is not dependent on whether input and output are functions of time or not.
A control system is said to be a time-invariant system if, even though its inputs and outputs are functions of time, the parameters of the system are constant and independent of time.
Linear and Nonlinear Systems
A system is said to be a linear system if it satisfies the following properties,
1. The principle of superposition applies to the system. This means the response to multiple inputs can be obtained by considering one input at a time and then adding the individual result algebraically.
Mathematically, the principle of superposition is expressed by two properties,
- Additive property: it says that if x and y belong to the domain of the function f then we have,
f(x + y) = f(x) + f(y) - Homogeneous property: it says that if a belongs to the domain of the function f and for any scalar constant α we have,
f(α x) = α f(x)2. The differential equation describing the system should be linear with constant coefficients.
3. Practically, the output of the system varies linearly with the input.
A system is said to be nonlinear, if
1. It does not satisfy the principle of superposition.
2. The equations that describe the system are nonlinear.
The function f(x) = x2 is nonlinear because.
f(x1 + x2) = (x1 + x2)2 which is not equal to (x1)2 + (x2)2,
and f(αx) = (αx)2 which is not equal to αx2 where α is a constant
3. The equations of nonlinear systems involve such nonlinear functions.
4. The output of the system doesn't vary linearly for a nonlinear system.
Continuous time and Discrete time Control System:
In a continuous time control system, all system variables are the functions of a continuous time variable 't'. However, in discrete time systems one or more system variables are known only at certain discrete intervals of time. They do not continuously depend on time.
Deterministic and Stochastic Control System
A deterministic control system is one in which the response to the input and behaviour to external disturbances is predictable and repeatable. However, in a control system if the response to the input and behaviour to the external disturbance is unpredictable, then the system is said to be stochastic.
Open loop and Closed loop Systems
A system is said to be an open-loop system, if its output is dependent on input but controlling action or input is independent of the output or changes in the output of the system.
A representation of an open-loop system is shown below,
The reference input is applied to the controller that generates the actuating signal required to control the plant or process that is to be controlled. The process is giving out the necessary desired controlled output.
Advantages of the Open-loop Control System
- They are simple in construction
- They are very convenient when output is difficult to measure.
- Such systems are easy to maintain.
- In general, there is no problem of stability.
- Also, they are less expensive.
Disadvantages of the Open-loop Control System
- Optimization in such systems is not possible.
- They are unreliable because the accuracy of such systems is dependent on the accurate pre-calibration of the controller.
- They give inaccurate results if there are variations in the external environment, these systems can not sense environmental changes.
- They can not sense internal disturbance in the system, after the controller stage.
- To maintain the quality and accuracy of such a system, recalibration of the controller is necessary from time to time.
- Generally, to overcome these drawbacks closed-loop systems are used in practice.
A closed-loop control system is a system in which the controlling action or input is somehow dependent on the output or changes in the output. Such a system uses feedback property to have a dependence of input on the output. In such systems, the output or part of the output is feedback to the input for comparison with the reference input applied.
A representation of a closed-loop system is shown below,
The main excitation to the system is called the command input which is applied to the reference transducer to generate reference input. The reference input is the changed input as per requirement. The part of output, which is to be decided by the feedback element is fed back to the reference input. The signal that is the output of the feedback element is called the feedback signal. It is then compared to the reference input to give the error signal. This error signal is then modified by the controller.
This manipulation is such that the error will approach zero. This signal then actuates the actual system and produces an output, as the output is a controlled one, hence called controlled output.
Advantages of the Closed-loop Control System
- The accuracy of such a system is always high, as the controller modifies and manipulates the actuating signal such that the error in the system will be zero.
- These systems sense environmental change, and internal disturbance and accordingly modifies the error.
- These systems have reduced the effects of nonlinearities and distortions.
- The bandwidth of such a system is very high.
Disadvantages of the Closed-loop Control System
- Such systems are complex and time-consuming to design.
- Such systems are expensive.
- The stability problem is severe in such systems and must be considered while designing the system.
Applications of Control System
Some popular applications of control systems are as follows,
- Automatic toaster system.
- Traffic light controller.
- Automatic door opening and closing system.
- Home heating system.
- Manual speed control system.
- Temperature control system.
- D.C. motor speed control system.
- Ship stabilization system.
- Flight control system.
- Voltage Stabilizer.
- Missile launching system.