I. Principle of Photoelectric Encoder
The grating disk consists of a circular plate of a certain diameter with several rectangular holes evenly spaced. Since the photoelectric encoder disk is coaxial with the motor, it rotates at the same speed as the motor. A detection device composed of light-emitting diodes and other electronic components detects and outputs several pulse signals. By calculating the number of pulses output per second by the photoelectric encoder, the current motor speed can be reflected. Furthermore, to determine the direction of rotation, the encoder disk can also provide two pulse signals with a 90° phase difference.
An optical encoder is a sensor that converts the mechanical geometric displacement of an output shaft into pulses or digital signals through photoelectric conversion. It is currently the most widely used sensor. An optical encoder consists of a grating disk and a photoelectric detection device. The grating disk has several rectangular holes evenly spaced on a circular plate of a certain diameter. Because the optical encoder disk is coaxial with the motor, it rotates at the same speed as the motor. The detection device, composed of light-emitting diodes and other electronic components, detects and outputs several pulse signals. By calculating the number of pulses output per second, the current motor speed can be reflected. Furthermore, to determine the direction of rotation, the encoder disk can also provide two pulse signals with a 90° phase difference.
II. How to classify photoelectric encoders according to their working principle
Based on their detection principles, encoders can be classified into optical, magnetic, inductive, and capacitive types. Based on their calibration methods and signal output formats, they can be classified into incremental, absolute, and hybrid types.
(I) Incremental Encoder
Incremental encoders directly utilize photoelectric conversion to output three sets of square wave pulses: A, B, and Z phases. The A and B pulses have a 90° phase difference, allowing for easy determination of the rotation direction. The Z phase consists of one pulse per revolution, used for reference point positioning. Its advantages include simple principle and construction, an average mechanical lifespan exceeding tens of thousands of hours, strong anti-interference capability, high reliability, and suitability for long-distance transmission. Its disadvantage is that it cannot output the absolute position information of the shaft rotation.
(II) Absolute Encoder
An absolute encoder is a sensor that directly outputs digital values. Its circular code disk has several concentric tracks radially, each track consisting of alternating transparent and opaque sectors. The number of sectors in adjacent tracks is double the number of bits in its binary code. A light source is located on one side of the code disk, and a photosensitive element corresponds to each track on the other side. When the code disk is in different positions, each photosensitive element converts the light source into a corresponding voltage level signal, forming a binary number. The key feature of this type of encoder is that it does not require a counter; a fixed digital code corresponding to the position can be read from any position of the shaft. Obviously, the more tracks, the higher the resolution. For an encoder with N-bit binary resolution, its code disk must have N tracks. Currently, 16-bit absolute encoders are available in China.
Absolute encoders utilize natural binary or cyclic binary (Gray code) methods for photoelectric conversion. The difference between absolute and incremental encoders lies in the translucent and opaque lines on the code disk. Absolute encoders can have multiple codes, and the absolute position is detected by reading the codes on the code disk. The code design can employ binary code, cyclic code, binary complement code, etc. Its characteristics are:
1. The absolute value of the angle coordinate can be read directly;
2. No cumulative error;
3. Location information is not lost after power is cut off. However, resolution is determined by the number of bits in the binary representation, meaning precision depends on the number of bits; currently, there are various types, such as 10-bit and 14-bit.
(III) Hybrid Absolute Encoder
The hybrid absolute encoder outputs two sets of information: one set is used to detect the magnetic pole position and has absolute information function; the other set is exactly the same as the output information of the incremental encoder.
An optical encoder is an angle (angular velocity) detection device that converts the angle input to a shaft into corresponding electrical pulses or digital signals using the photoelectric conversion principle. It features small size, high precision, reliable operation, and a digital interface. It is widely used in CNC machine tools, rotary tables, servo drives, robots, radar, military target measurement, and other devices and equipment requiring angle detection.
