The Complete Guide to Open Loop And Closed Loop Control System for 2020

What is open loop control system 

In open-loop control systems, the output signal has no effect upon the controlled variable(s). In other words, in an openloop control system the output is neither measured nor “fed back” for comparison with the input. The control is executed.

Example of open loop control system  :-

Operation of a washing machine, where the duration of soaking, washing, and rinsing are set in advance and on the basis of experience with how these settings affect the output signal, which is the cleanness of the clothes. In such an open-loop system, the output signal is not automatically measured.

  • Clothes drier
  • Electric bulb
  • Automatic washing machine
  • Bread toaster (time-based)
  • Electric hand drier
  • Door lock systems
  • Servo motors
  • Remote control of TV
  • Inkjet printers
  • Word processor

Advantage and disadvantage of open loop control system :-

Advantage :-

  • Simplicity and stability: they are simpler in their layout and hence are economical and stable too due to their simplicity.
  •  Construction: Since these are having a simple layout so are easier to construct.

Disadvantage :- 

    • Accuracy and Reliability: since these systems do not have a feedback mechanism, so they are very inaccurate in terms of result output and hence they are unreliable too.
    •  Due to the absence of a feedback mechanism, they are unable to remove the disturbances occurring from external sources.

What is Close loop control system 

The main tool utilized by closed-loop controllers is the negative feedback. Here, a detector measures the output signal of the controlled process and its value is compared to a reference value (set point). The difference between the reference signal and the output signal is the error signal, which is driving a controller. The output of the controller generates the manipulated variable, which throttles the final control element that influences the input of the process. This is how the control loop is closed.

Example of close loop control system  :-

We use closed loop control systems in many applications of our day to day life. five very common examples to understand the close loop system as given given below.

  • Automatic Electric Iron – Depending on the temperature of the iron heating elements were controlled automatically up and down depend upon the settings.
  • Servo Voltage Stabilizer – Stabilization in voltage is achieved by the feeding the output voltage back to the system.
  • Water Level Controller– Water level in the reservoirs decides the input water into it on particular level after the output feedback signal close the pump.
  • Air Conditioner –Air conditioner automatically adjusts its temperature depending on its room temperature.
  • In motor speed regulator using tachometer and/or current sensor , sensor senses the speed and sends a feedback to the system to regulate its speed.

Advantage and disadvantage of close loop control system :-

Advantage : –

  •  Accuracy: They are more accurate than open loop system due to their complex
    construction. They are equally accurate and are not disturbed in the presence of nonlinearities.
  •  Noise reduction ability: Since they are composed of a feedback mechanism, so they clear out the errors between input and output signals, and hence remain unaffected to the external noise sources.

Disadvantage :- 

    • Construction: They are relatively more complex in construction and hence it adds up to the cost making it costlier than open loop system.
    •  Since it consists of feedback loop, it may create oscillatory response of the system and it also reduces the overall gain of the system.
    •  Stability: It is less stable than open loop system but this disadvantage can be striked off since we can make the sensitivity of the system very small so as to make the system as stable as possible.

Difference Between open loop and closed loop system 

Open loop system 

Close loop system 

  • Open-loop control is simple and cheap because it does not require any sensors. The characteristics of the operating elements, the relationships between the input and output signals, and the expected disturbances have to be known in advance, as this knowledge is needed to set up the rules that govern the system operation.
  • Open loop control cannot tolerate changes in the loop parameters or in the disturbances, thus, errors in the assumed data will lead to improper operation. However, designing an open-loop control system is simple and does not require stability analysis.
  • Simple construction 
  • Response time fast
  • No feedback
  •  less accurate
  • Open loop consist of Controller + controller process with no feedback
  • Open loop system Example :- Water sprinkler, Traffic light controller.
  • Closed-loop control is more expensive, but in addition to being able to keep the process variable on set point, it is also capable of correcting for process disturbances and compensating for the errors in the model, which is implemented by the controller.
  • Complex construction.
  • Response time slow as compared open loop.
  • Feedback is there
  • Close loop consist of Controller + controller process or plant with feedback element. 
  • Very accurate as compared open loop
  • Close loop system Example :-  Missile launching system, Voltage stabilizer

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