In an open loop system, the input is directly applied to the system through a controller and output known as controlled output is obtained.
In this case, the input to the controller or the control action is no way affected by the value of controlled output i.e. the output is neither measured nor feedback for comparison with the input.
Open Loop Control System Definition
A system in which control action is totally independent of the output of the system is called as open loop control system.
Fig. shows the block diagram of open loop system.
As shown in Fig.
• Reference input r(t) is applied to controller which generates the actuating signal u(t).
• Actuating signal u(t) actuates the process to give controlled output c(t).
• The control action has nothing to do with output c(t) i.e. there is no any relation between input and output so that can form a loop.
• Hence, system is open loop.
• Also it is known as non-feedback system.
Examples for Open Loop Control System
Example for Open Loop Control System : Automatic Washing Machine
In washing machine, input is dirty cloths where output is clean cloths. The water, soap etc. can be taken as inputs.
The control action, is determined by the timer (time set). But at the particular time setted we can't get the 100% clean cloths i.e. control action is not responsible for desired output. Hence washing machine action is open loop.
Control of Traffic by Road Traffic Lamps
The glowing of red and green lamps is the input.
When red lamp glows that direction traffic stops.
When green lamp glows, that direction traffic starts to flow.
So the traffic action (flow or stop) is the output.
The control action, is determined by time set to glow the red or green lamp.
But the control action nothing to do with the actual status of the output i.e. Suppose suddenly an accident happens on road, the traffic will stop their, but red and green lamp will glow alternately at time set. Hence it is open loop control system.
Bread Toaster
A toaster simply heats the bread for sometime. This time determines the quality of bread i.e. either soft or hard.
Therefore, bread is input.
Quality of bread soft or hard is output.
The time determining the quality of bread is the control action.
This control action, has nothing to do with the status of output.
Hence, the open loop system.
Advantages of open loop control system
1. Simple design and construction.
2. Economic.
3. Easy maintenance.
4. Stable.
Disadvantages of open loop control system
1. Inaccurate and unreliable.
2. To maintain quality and accuracy. Recalibration is necessary every time.