Definition of the Open-Loop Control System
The control system in which the output of the system does not have any effect on its input is known as the open-loop control system. This means that the system does not have any feedback structure.
Based on the required output we set input to the controller in the system, depending on the input the controller generates the control signal, and this control signal will be fed to the processing unit. So, according to the control signal, proper processing will be done, and the output is achieved, it could not judge whether the achieved output is desired or not. If the system has the capability of judging whether the achieved output from the control signal is desired or not, such systems are known as closed-loop systems, they can accomplish this judgment with a feedback structure.
We can understand this better with the help of some practical examples of open-loop control systems. Block-diagram representation of the open-loop control system is shown in the below figure.
Gain of Open-Loop Control System
Let G1(s) be the gain of the controller block and G2(s) be the gain of the processing unit then the block diagram representation of the open-loop control system is given below.
If the gain of the system,
Here G(s) is the system gain, From the block diagram
⇒x0(s)/xi(s)= G1(s)G2(s)= G(s)
Hence the block diagram can be redrawn as shown below.
There may be any number of blocks in the forward path, the system gain is the product of individual gains of each element in the forward path.
Examples of an Open-Loop Control System
Let us consider an example of an induction cooker, let us assume that it is equipped with a timer that controls the on and off durations of the cooker. The operator will set the timer or cooking mode of the cooker. When the pre-set time is up, then the cooker will turn off. However, it is quite likely that the cooker temperature is above or below the desired level, due to the inaccuracy in the estimation made by the user. Here the time set by the user is the control signal, and the timer will execute the control process.
The timer-based applications are the best examples of open-loop control systems such as traffic signal controllers, the signals produced by the controller are time-dependent. When the controller is installed at any crossing, each signal will be displayed as per the pre-set timings, irrespective of the amount of traffic present. Like, the other examples of open-loop control systems are the washing machine, fan regulator, room heater, etc.
Advantages of Open-Loop Control System
The open-loop control systems are easier to construct. They are economical as they comprise a smaller number of components as compared to closed-loop control systems. Generally, the open-loop control systems are not sensitive to external disturbances hence these are more stable when compared with the closed-loop control systems.
Disadvantages of Open-Loop Control System
The open-loop control systems lack accuracy and versatility, they cannot adapt to the variations in the environmental conditions or the external disturbances. Hence these can be used in simple applications only.
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