The Principle of Operation of Optical Encoders

An encoder is a device that provides a digital output as a result of an angular or linear displacement.

The position encoders can be grouped into two categories:

• Incremental encoders, which detect changes in displacement from some datum position.
• Absolute encoders, which give the actual position.

Incremental Encoders

Let’s consider the basic form of an incremental encoder for the measurement of angular displacement of shaft shown below:

The incremental encoder shown above consists of a disc which rotates along with the shaft. The rotatable disc has a number of windows through which a beam of light can pass and be detected by a suitable light sensor. When the shaft and disc rotates, a pulsed output is produced by the sensor with the number of pulses being proportional to the angle through which the disc rotates. The angular displacement of the disc and hence the shaft rotating it can therefore be determined by the number of pulses produced in the angular displacement from some datum position. Typically the number of windows on the disc varies from 60 to over a thousand with multi-tracks having slightly offset slots in each track.

For example, consider 60 slots occurring in 1 revolution, and since 1 revolution is a rotation of 360°, the minimum angular displacement, i.e. the resolution that can be detected is 360°/60 = 6°. The resolution of incremental encoders typically vary from about 6° to 0.3° or even better than this.

Related: Types of Sensors used in Measurement and Process Control

Absolute Encoder

While the incremental encoder detects changes in displacement from some datum position, the absolute encoders provides the actual position. The absolute encoder gives an output in the form of a binary number of several digits, each number representing a particular angular position. The figure below shows a basic form of an absolute encoder for the measurement of angular position.

The rotating disc of the absolute encoder shown above has four concentric circles of slots and four sensors to detect the light pulses. The slots are arranged in such a way that the, sequential output from the sensors is a number in the binary code, each such number corresponding to a particular angular position.

Typical encoders tend to have up to 10 or 12 tracks. The number of bits in the binary number will be equal to the number of tracks. Therefore with 10 tracks there will be 10 bits and hence the number of positions that can be detected is 2¹⁰ i.e. 1024 a resolution of 360°/1024 = 0.35.

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