Definition : A system in which output is dependent on input but controlling action or input is totally independent of the output or changes in output of the system, is called an Open Loop System.
In a broad manner it can be represented as in Fig.1.
Reference input {r(t)} is applied to the controller which generates the actuating signal (u) required to control the process which is to be controlled. process is giving out the necessary desired controlled output c(t).
1. Advantages
The advantages of open loop control system are,
1) Such systems are simple in construction.
2) Very much convenient when output is difficult to measure.
3) Such systems are easy from maintenance point of view.
4) Generally these are not troubled with the problems of stability.
5) Such systems are simple to design and hence economical
2. Disadvantages
The disadvantages of open loop control system are,
1) Such system are inaccurate and unreliable because accuracy of such systems are totally dependent on the accurate precalibration of the controller.
2) Such systems give inaccurate results if there are variations in the external environment i.e. such systems cannot sense environmental changes.
3) Similarly they cannot sense internal disturbances in the system, after the controller stage.
4) To maintain the quality and accuracy, recalibration of the controller is necessary from time to time.
To overcome all the above disadvantages, generally in practice closed loop systems are used.
The good example of an open loop system is an electric switch. This is open loop because output is light and switch is controller of lamp. Any change in light has no effect on the the ON-OFF position of the switch, i.e. its controlling action.
Similarly automatic washing machine. Here output is degree of cleanliness of clothes. But any change in this output will not affect the controlling action or will not decide the operation time or will not decide the amount of detergent which is to be used. Some other examples are traffic signal, automatic toaster system etc.
1. Real Time Applications of an Open Loop SystemThe various illustrations of an open loop system are discussed below,
1.1 Sprinkier Used to Water Alawn
The syetem is adjusted to water a given area by opening the water valve and observing the resulting pattern. When the pattern is considered satisfactory, the system is ''calibrated'' and no further valve adjustment is made.
1.2 Stepper Motor Positioning System
The actual position in such system is usually not monitored. The motor controller command a certain number of step by the motor to drive the output to a previously determined location.
1.3 Automatic Toaster System
In this system, the quality of toast depends up on the time for which the toast is heated. Depending on the time setting, bread is simply heated in this system. The toast quality is to be judged by the user and has no effect on the inputs.
A traffic flow control system used on roads is time dependent. The traffic on the road becomes mobile or stationary dependent on the duration and sequence of lamp glow. The sequence and duration are controlled by relay which are predetermined and not dependent on the rush on the road.
In this system, photo sensitive devices are used. When a person interrupts a light, photo device generates actuating signal which opens the door for specific time. When person passes through the door, light becomes continuous closing the door. The opening and closing of the door is the output which has nothing to do with the inputs, hence an open loop system.
The room heater, fan regulator, automatic coffee server, electric lift, theatre lamp dimmer, automatic dryer are examples of open loop system.