1. UNIT-III FEEDBACK DEVICES
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2. Controller
A controller is a electronic regulating mechanism used on CNC machinery. For example the controller reads a CNC program and sends signals to the axis motors on the machine
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3. FEEDBACK DEVICES Feedback can be accomplished in one of two ways
Linear Scales 
Encoders
Note- SERVOMOTOR-it is an automatic device that uses error sensing negative feedback to correct the performance of a mechanism.
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4. It measures the absolute/exact position of the slide directly.
Linear scale-
It measures the absolute/exact position of the slide directly.
It is a graduated scale made of either glass or stainless steel along with a measuring surface and scanning unit.
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5. They convert rotary movement into digital signals.
Encoder-
An encoder is a sensor of mechanical motion that generates digital signals in response to motion. As an electro-mechanical device, an encoder is able to provide motion control system users with information concerning position, velocity and direction.
They convert rotary movement into digital signals.
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6. Types of encoders
There are two different types of encoders:
linear and 
Rotary.
A linear encoder responds to motion along a path, while a rotary encoder responds to rotational motion.
An encoder is generally categorized by the means of its output. An incremental encoder generates a train of pulses which can be used to determine position and speed. An absolute encoder generates unique bit configurations to track positions directly.

7. Difference between incremental and absolute encoder-
Incremental rotary encoder outputs the pulse corresponding to the rotation angle only while rotating, and is the counting measurement method that adds up the pulse from the measurement beginning point.
Absolute rotary encoder outputs the signal of position corresponding to the rotation angle by code. The Absolute position from the starting point is measured by the output code data. The position is memorized though the power supply is cut off.
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