Talking about the classification and principle of encoder

How do the elevators that are often used in life accurately send people to the designated floors? How does the machine tool accurately cut materials? How does the servo motor ensure the accuracy of the rotation position? All this is due to an artifact— —Encoder, but what is an encoder? How does he accurately measure the position of the motor? Let’s talk about the encoder today.

1. What is an encoder

　　 Encoder is a device that compiles and converts signals or data into signal forms that can be used for communication, transmission and storage. The encoder converts angular displacement or linear displacement into electrical signals. The former is called a code wheel and the latter is called a code ruler. It is a commonly used motor positioning device in the industry, which can accurately test the angular displacement and rotation position of the motor.

Figure 1 Encoder

　　 2. Encoder classification

　　According to the working principle, encoders can be divided into two types: incremental and absolute. The incremental encoder converts the displacement into a periodic electric signal, and then converts this electric signal into a counting pulse, and the number of pulses is used to indicate the magnitude of the displacement. Each position of the absolute encoder corresponds to a certain digital code, so its indication is only related to the start and end positions of the measurement, and has nothing to do with the middle process of the measurement.

　　Incremental

Incremental encoders usually have 3 output ports, which are A-phase, B-phase, and Z-phase output. The pulse output between A-phase and B-phase is delayed by 1/4 cycle. According to the delay relationship, it can distinguish between positive and negative And by taking the rising and falling edges of phase A and phase B, the frequency can be doubled or quadrupled; phase Z is a single-turn pulse, that is, one pulse is sent out every turn.

The grating of the incremental measurement method is composed of periodic gratings. The position information is obtained by calculating the number of increments (measurement steps) from a certain point. Since the absolute reference point must be used to determine the position value, there is also a reference point track on the rotary encoder.

　　Absolute

　　Absolute encoder corresponds to one circle, and each reference angle emits a unique binary value corresponding to the angle. It can record and measure multiple positions through an external circle device.

　　 When the encoder is powered on, the position value can be obtained immediately and read by the subsequent signal processing electronic circuit at any time. There is no need to move the axis to perform the reference point return operation. The absolute position information comes from a circular grating code disc, which consists of a series of absolute codes. A separate incremental track signal generates position values ​​through subdivision, and can also generate optional incremental signals.

　　 The absolute position value information of the single-turn encoder is repeated once every revolution. Multi-turn encoders can also distinguish the position value of each turn.

Figure 2 Disc grating of absolute rotary encoder

　　 They have the biggest difference: In the case of an incremental encoder, the position is determined by the number of pulses counted from the zero mark, while the position of an absolute encoder is determined by the reading of the output code. In a circle, the reading of the output code of each position is unique, so when the power is disconnected, the absolute encoder is not separated from the actual position. If the power is turned on again, the position reading is still current and valid, unlike the incremental encoder, which must search for the zero mark.

　　 Three, encoder working principle

A photoelectric code disc with a shaft in the center, on which there are ring-shaped and dark engraved lines, read by photoelectric transmitting and receiving devices, to obtain four groups of sine wave signals combined into A, B, C, D, each sine wave Phase difference of 90 degrees (relative to a cycle of 360 degrees), the C and D signals are reversed and superimposed on the A and B phases to enhance the stable signal; in addition, a Z-phase pulse is output every revolution to represent the zero reference Bit.

　　 Since the phase A and B are 90 degrees apart, the encoder's forward and reverse rotation can be judged by comparing the phase A or the B phase. The zero reference position of the encoder can be obtained through the zero pulse. The materials of the encoder code disc are glass, metal, plastic. The glass code disc is deposited on the glass with very thin scribe lines, which has good thermal stability and high precision. The metal code disc is directly engraved with through and impassable lines and is not fragile. However, because metal has a certain thickness, the accuracy is limited, and its thermal stability is one order of magnitude worse than that of glass. The plastic code disc is economical and its cost is low, but the accuracy, thermal stability, and life are worse.

　　 Resolution—The number of open or dark engraved lines provided by the encoder per 360 degree rotation is called resolution, which is also called resolution division, or directly called the number of lines, generally 5 to 10,000 lines per revolution.

Figure 3 Encoder

　　 Fourth, position measurement and feedback control principle

　　In elevators, machine tools, material processing, motor feedback systems, and measurement and control equipment, encoders occupy an extremely important position. The encoder uses the grating and infrared light source to convert the optical signal into a TTL (HTL) electrical signal through the receiver. Through the analysis of the TTL level frequency and the number of high levels, it intuitively reflects the rotation angle and rotation position of the motor.

　　 Since the angle and position can be accurately measured, the encoder and frequency converter can be combined into a closed-loop control system to make the control more precise. This is why elevators, machine tools, etc. can be used so accurately.

　　 In summary, we understand that encoders are divided into two types: incremental and absolute according to their structure. They also convert other signals, such as optical signals, into electrical signals that can be analyzed and controlled. The common elevators and machine tools in our lives are precisely based on the precise adjustment of the motor. Through the feedback closed-loop control of the electric signal, the encoder and the frequency converter can naturally realize the precise control.