Explain Closed loop and Open loop system with sketch.

Explain Closed loop and Open loop system with sketch. 
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1 Answer

Answer :

Closed Loop System:

[1] The name indicates that the closed loop control system has a loop that is closed as shown in figure.

[2] A feedback device is used for this purpose. This makes the design of closed loop a little complicated and expensive.

[3] But a very high degree of accuracy is achieved in the movement of slide.

[4] This system is similar to open loop control system. But it consists of two additional devices in the form of feedback transducer and a comparator.

[5] The transducer feedbacks the actual slide displacement to the comparator.

[6] The comparator compares the actually achieved slide movement with command signal. If there is any error then it is feedback to the Machine Control Unit (MCU).

[7] The MCU then sends the corrective commands to the drive unit and the cycle repeats until there is no error signal from the comparator.

[8] Nowadays, almost all CNC machines use this control system. 

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Open Loop System:

[1] The term open-loop means that there is no feedback, and in open loop systems the motion controller produces outputs depending only on its set points, without feedback information.

[2] Machine Tool Controls in which there is no provision to compare the actual position of the cutting tool or work piece with the input command value, are called Open loop systems.

[3] In this, the electric motor continues to run until the absence of power from input command signal, indicates that the programmed location has been attained.

[4] There is no monitoring of the actual displacement of the machine slide.

[5] In this system, the control may indicate a movement of 50 mm whereas actually the slide may have moved only 49.8 mm.

[6] Open loop system are less accurate compared to a closed loop system, but it is less expensive than closed loop system due to the absence of monitoring devices and their maintenance is not complicated.

[7] For these reasons, the open-loop system is generally used in point-to-point systems where the accuracy requirements are not critical. 

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