What are the characteristics and working principle of the photoelectric encoder

What are the characteristics and working principle of the photoelectric encoder?

The photoelectric encoder is a sensor that converts the mechanical geometric displacement on the output shaft into pulses or digital quantities through photoelectric conversion. This is the most commonly used sensor at present. The photoelectric encoder is composed of a grating disc and photoelectric detection equipment. The grating disc is a circular plate with a certain diameter and a number of rectangular holes are equally opened. Because the photoelectric code disc is coaxial with the motor, when the motor rotates, the grating disc rotates at the same speed as the motor, and several pulse signals are detected by the detection device composed of electronic components such as light-emitting diodes. The photoelectric encoder converts the angle input to the shaft into the corresponding electric pulse or digital quantity by the principle of photoelectric conversion. It has the advantages of small size, high precision, reliable work, and digital interface. It is widely used in devices and equipment that need to detect angles such as CNC machine tools, rotary tables, servo drives, robots, radars, and military target determination.

According to the detection principle, encoders can be divided into optical, magnetic, inductive and capacitive. According to its scale method and signal output form, it can be divided into three types: incremental, absolute and mixed.

1.1 Incremental encoder

Incremental encoders directly use the principle of photoelectric conversion to output three sets of square wave pulses A, B, and Z; the phase difference of the two sets of pulses is 90 per revolution. The phase difference of the two sets of pulses is 90. Pulse, used for reference point positioning. Its advantages are simple structure, average mechanical life of more than tens of thousands of hours, strong anti-interference ability, high reliability, and suitable for long-distance transmission. The disadvantage is that the absolute position information of the shaft rotation cannot be output.

1.2 Absolute encoder

The absolute encoder is a sensor that directly outputs digital quantities. There are several concentric code tracks along the radial direction on its circular code disc. Each track is composed of light-transmissive and opaque sectors. The fans of adjacent code channels The number of zones is doubled. The number of code channels on the code wheel is the number of binary digits. On one side of the code wheel is the light source, and the other side corresponds to each code channel with a photosensitive element; When, each photosensitive element converts a corresponding level signal according to whether it receives light or not, and forms a binary number. The characteristic of this kind of encoder is that it does not need a counter, and a fixed digital code corresponding to the position can be read out at any position of the shaft. Obviously, the more code channels, the higher the resolution. For an encoder with N-bit binary resolution, the code disk must have N code channels. At present, there are 16-bit absolute encoder products in China.

The absolute encoder uses natural binary or cyclic binary (Grey code) method for photoelectric conversion. The difference between an absolute encoder and an incremental encoder is the transparent and opaque line pattern on the disc. The absolute encoder can have several codes, and the absolute position can be detected according to the code on the read code disc. The coding design can adopt binary code, cyclic code, two's complement code, etc. Its unique features are:

1.2.1 The absolute value of angle coordinates can be read directly;

1.2.2 There is no cumulative error;

1.2.3 The position information will not be lost after the power is cut off. However, the resolution is determined by the number of bits in the binary system, which means that the accuracy depends on the number of bits. There are currently 10 bits, 14 bits, and so on.

1.3 Hybrid absolute encoder

Hybrid absolute encoder, it outputs two sets of information: one set of information is used to detect the magnetic pole position, with absolute information function; the other set is exactly the same as the output information of the incremental encoder.

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