The detection of angular and linear motion is a key function in controlling the machines in the electronics factory. The microcomputers in these machines often need information about the position, direction of rotation, and speed of rotation of a shaft or axle, which needs to be converted into digital form. Optical encoders are the electro-mechanical devices used to measure either angular or linear positions. The ones used for angular detection are commonly referred to as rotary or shaft encoders. These are increasingly used for a multitude of jobs in consumer and industrial equipment. Rotary encoders, or shaft encoders, can in principle, be absolute or incremental. An absolute encoder provides position information when the power is lost, whereas an incremental encoder is used where velocity and direction information is required. Both can be used with angular as well as linear displacements, but they operate differently. Let’s take a detailed look at how they differ from each other.

What is an Absolute Encoder?

An absolute encoder has a unique code for each shaft position which represents the absolute position of the encoder. It directly provides the digital output representing the absolute displacement. The value of the actual position is measured immediately the moment the system is switched on. Thus, an absolute encoder doesn’t need a counter as the measured value is derived directly from the graduation pattern. It provides the digital output corresponding to the position directly. Each bit position is separately encoded through a dedicated LED pair. Each code represents an absolute angular position of the shaft in its rotation. The disc of an absolute encoder uses a Gray code in which one bit changes at a time, which reduces encoder communication errors. They can be divided into single-turn and multi-turn encoders.

What is an Incremental Encoder?

An incremental encoder is an electro-mechanical device that transforms the angular position of the shaft into digital or pulse signals. It generates a certain number of pulses per revolution, providing a pulse for each increment corresponding to the revolution. It can measure the change in position, not the absolute position. Therefore, it cannot specify the position relative to a known reference. The number of pulses generated is proportional to the angular position of the shaft. Incremental encoders are used in applications where a velocity or a velocity and direction information is required. Every time the device is switched on or reset, it begins counting from zero and it generates an output signal each time the shaft moves. The types of an incremental encoder maybe further subdivided into quadrature encoders and tachometers.

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Difference Between Absolute and Incremental Encoders

Basics of Absolute vs. Incremental Encoders

– Both are electro-mechanical devices used to measure either angular or linear positions of the shaft and convert them into digital or pulse signals. An absolute encoder has a unique code for each shaft position which represents the absolute position of the encoder, while an incremental encoder generates an output signal each time the shaft rotates a certain angle and the number of generated pulses is proportional to the angular position of the shaft. An incremental encoder can measure the change in position, not the absolute position.

Operating Principle of Absolute vs. Incremental Encoders

– An absolute encoder consists of a binary coded disk mounted on the shaft such that it rotates with the shaft. Thanks to a number of output channels, every shaft angular position is described by its own unique code. The number of channels increases as the required resolution increases. Unlike an incremental encoder, it’s not a counting device which doesn’t lose the position information when the power is lost. An incremental encoder, on the other hand, provides an output signal for a given increment of angular position of the shaft which is determined by counting the output pulses relative to a reference point.

Cost Efficiency

– The code matrix of the encoder disk is more complex and because more light sensors are required, an absolute encoder typically costs twice as much as the incremental encoders. The resolution is limited by the number of tracks on the encoder disk, so it becomes more expensive to obtain finer resolutions without adding more tracks. Incremental encoders, on the contrary, are less complex than their absolute counterparts, thus typically less expensive.

Stability

– Absolute encoders can offer better performance, accurate results, and lower overall costs. Thanks to its ability to provide absolute angle readings, even if a reading is missed, it won’t affect the next reading. A particular reading is not dependent on accuracy of a previous reading. An incremental encoder, on the other hand, needs to be powered on throughout the operation of the device. Each time the power is lost, the reading must be reinitialized or the system shows an error. This slows down the system performance. Absolute encoders do not lose position information in case of power failure.

Absolute vs. Incremental Encoder: Comparison Chart

Summary of Absolute vs. Incremental Encoders

In a nutshell, an incremental encoder needs to be powered throughout the operation of the device. In case of power failure, the reading must be reinitialized or the system introduces an error. An absolute encoder, on the contrary, needs power only when a reading is taken and thanks to its ability to provide absolute angle readings, a particular reading is independent of the accuracy of a previous reading. However, the code matrix of the disk in an absolute encoder is more complex, thus typically costs twice as much as an incremental encoder, which on the other hand, is less complex, so costs less expensive.

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