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What is the difference between Incremental rotary encoder and absolute rotary encoder?

What is the difference between Incremental rotary encoder and absolute rotary encoder?

2021-09-17

A rotary encoder, also called a shaft encoder, is an electro-mechanical device that converts the angular position or motion of a shaft or axle to analog or digital output signals.

There are two main types of rotary encoder: absolute and incremental. 

The output of an absolute encoder indicates the current shaft position, making it an angle transducer. The output of an incremental encoder provides information about the motion of the shaft, which typically is processed elsewhere into information such as position, speed and distance.

So whats the difference between Incremental rotary encoder and absolute rotary encoder? The incremental encoder will immediately report changes in position, which is an essential capability in some applications. However, it does not report or keep track of absolute position. As a result, the mechanical system monitored by an incremental encoder may have to be homed (moved to a fixed reference point) to initialize absolute position measurements. While an absolute encoder maintains position information when power is removed from the encoder. The position of the encoder is available immediately on applying power. The relationship between the encoder value and the physical position of the controlled machinery is set at assembly; the system does not need to return to a calibration point to maintain position accuracy. An absolute encoder has multiple code rings with various binary weightings which provide a data word representing the absolute position of the encoder within one revolution. This type of encoder is often referred to as a parallel absolute encoder. A multi-turn absolute rotary encoder includes additional code wheels and toothed wheels. A high-resolution wheel measures the fractional rotation, and lower-resolution geared code wheels record the number of whole revolutions of the shaft.

A rotary incremental encoder has two output signals, A and B, which issue a periodic digital waveform in quadrature when the encoder shaft rotates. This is similar to sine encoders, which output sinusoidal waveforms in quadrature (i.e., sine and cosine),[13] thus combining the characteristics of an encoder and a resolver. The waveform frequency indicates the speed of shaft rotation and the number of pulses indicates the distance moved, whereas the A-B phase relationship indicates the direction of rotation.

Some rotary incremental encoders have an additional "index" output (typically labeled Z), which emits a pulse when the shaft passes through a particular angle. Once every rotation, the Z signal is asserted, typically always at the same angle, until the next AB state change. This is commonly used in radar systems and other applications that require a registration signal when the encoder shaft is located at a particular reference angle.

Like our K series and P series. The K50 encoder and the PN58 encoder, both of them are the incremental one.

Optical absolute encoders like KJ50, SJ38.The optical encoder's disc is made of glass or plastic with transparent and opaque areas. A light source and photo detector array reads the optical pattern that results from the disc's position at any one time.[8] The Gray code is often used. This code can be read by a controlling device, such as a microprocessor or microcontroller to determine the angle of the shaft.

The absolute analog type produces a unique dual analog code that can be translated into an absolute angle of the shaft.


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