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WhatistheWorkingPrincipleofabsoluteencoder

WhatistheWorkingPrincipleofabsoluteencoder

2021-09-15

Absolute encoders are feedback devices that provide speed, position information by outputting a digital word or bit in relation to motion. Different from incremental encoders that output a continuous stream of ubiquitous pulses, absolute encoders output unique words or bits for each position.

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By outputting a digital word or bit instead of a stream of pulses, absolute rotary encoders offer several advantages:

1.Higher overall resolution compared to incremental encoders

2.Better starting performance due to low initial position time

3.More accurate motion detection along multiple axes

4.Multiple output protocols for better electronics integration

5.Offer a better recovery from system or power failure

 

Absolute encoders can be categorized based on their sensing technology type  and  their output over multiple turns of the motor shaft (single-turn or multi-turn, both are available).

Optical absolute encoders use a code disc with markings and an LED that shines light through the encoder disc. When the encoder disc turns with the motor shaft, changes in positions can be detected easily.

While all absolute rotary encoders provide feedback based on the rotation of shaft (position of the encoder within 360 degrees or single turn), applications differ based on the requirement to know how many times the encoder has made a complete rotation or multiple turns. Multi-turn absolute encoders offer additional feedback for the number of 360 degree turns.

An absolute rotary encoder determines its position by using a static reference point. Absolute encoders work by outputting a digital word of bit as the shaft rotates. There are two discs, both with concentric rings with offset markers. One disc is fixed to the central shaft; the other moves freely. As the disc turns, the markers along the track of absolute encoders change position on the fixed disc. Each configuration along the disc of an absolute rotary encoder represents a unique binary code. Looking at the binary code within the absolute rotary encoder determines the absolute position of the object.

Absolute encoders offer unique advantages over incremental encoders. They have a unique code for each shaft position, meaning that they can provide very special position information as no two positions on a track are identical. They measured actual position by generating a stream of unique digital codes that represent the encoder’s actual position and therefore do not require an index or reference point. This also provides absolute encoders an advantage in applications returning to a home position may present issues in the event of a power loss.

Absolute encoders also offer a higher resolution option compared to incremental encoders. While incremental encoders must add more increments to a single track on a code disc and are thereby limited to the physical size of the disc and the number of pulses that can be decoded vs the rotating speed of the encoder (frequency response), absolute encoder just adds additional tracks to achieve higher resolutions and do not continuously output a stream of pulses. Instead they are limited by the number times the encoders is interrogated within a given sample period of baud rate.

The absolute rotary encoder itself understands the positioning information – it doesn’t need to rely on outside electronics to provide a baseline index for the encoder position. Absolute encoders enable applications which rely on non-linear positioning to work without additional external components.

In actual applications, absolute encoders allow more precision than incremental encoders.

Determining multi-axis orientation for CNC machines used in manufacturing

Automatically determining the height of scissor beds used in armariums. 

Accurately positioning multiple stabilizers for large vehicles.

Moving automatic doors or bays without a limiting switch

Continuing robotic movement cleanly even after a power failure

 


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