Detailed explanation of the working principle of photoelectric encoder

Photoelectric encoder is a kind of sensor that converts the mechanical displacement on the output shaft into pulse or digital quantity through photoelectric conversion. It is the most used sensor at the beginning. The normal photoelectric encoder is mainly caused by the grating disc and photoelectric detection device. In the servo system, since the photoelectric encoder is coaxial with the motor, when the motor rotates, the grating disk rotates at the same speed as the motor. How many pulse signals are detected and output by the detection device caused by electronic components such as light-emitting diodes, the reason is shown in the figure. By calculating the number of output pulses per second of the photoelectric encoder can reflect the motor speed in the future. In addition, in order to determine the rotation direction, the code wheel can also provide two-channel optical code output with a phase difference of 90°, and change the rotation of the motor according to the state of the dual-channel optical code. According to the detection principle, the encoder can be divided into optical, magnetic, sensor and capacitive. According to its scale method and signal output state, it can be divided into three types: incremental, absolute and mixed.

Classification of photoelectric encoder

The photoelectric encoder is a sensor that converts the mechanical displacement on the output shaft into pulses or digital quantities through photoelectric conversion. This is the most used sensor at the beginning. The photoelectric encoder is caused by the grating disc and photoelectric detection device. The grating disc is a circular plate with a certain diameter that is divided into several rectangular holes. Since the photoelectric encoder is coaxial with the motor, when the motor rotates, the grating disk rotates at the same speed as the motor, and how many pulse signals are detected by the detection device caused by electronic components such as light-emitting diodes. The schematic diagram of the principle is shown in Figure 1. The process calculates the number of output pulses of the photoelectric encoder per second to reflect the motor speed in the future. In addition, in order to determine the rotation direction, the encoder can also provide two pulse signals with a phase difference of 90°.

The obligation of encoder

The photoelectric encoder is a kind of angle (angular velocity) detection device. It uses the principle of photoelectric conversion to convert the angular quantity input to the shaft into a response electric pulse or digital quantity. It occupies small size, high precision, reliable obligation, 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 purposes.

It consists of a photoelectric code disc with an axis in the center, on which there are ring-shaped and dark engraved lines, which are read by photoelectric transmitting and receiving devices. The four groups of sine wave signals are combined into a, b, c, d, each The sine wave has a phase difference of 90 degrees (absolutely 360 degrees in a cycle). The c and d signals are reversed and superimposed on the a and b phases to strengthen and stabilize the signal; in addition, output a z-phase per revolution Pulse to represent the zero reference position. Since the phase a and b are different by 90 degrees, you can compare whether phase a is first or phase b first to determine the forward and reverse rotation of the encoder. Through the zero pulse, the zero reference position of the encoder can be lost . The materials of the encoder code disc are glass, metal, plastic. The glass code disc is a thin line on the glass, which has good thermal stability and high accuracy. The metal code disc is directly engraved with through and impassable lines and is not easily broken. However, because metal has a certain thickness, accuracy is restricted, and its thermal stability is an order of magnitude worse than that of glass. Plastic code discs are economical, and their cost is low, but accuracy, thermal stability, and life are worse. . Resolution-How many pass or dark engraved lines are supplied by the encoder per 360 degrees of rotation is called resolution, also called analytical indexing, or directly called how many lines, normally 5 to 10,000 lines per revolution.

★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 completely the same as the output information of the incremental encoder.

2. Optoelectronics

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

★Absolute encoder

Absolute encoder is a sensor that directly outputs digital quantities. There are multiple concentric code tracks along the radial direction on its circular code disc. Each track is caused by light-transmitting and opaque fan-shaped areas. The number of sectors is double-associated. The number of code channels on the code disc is the number of binary digits. On one side of the code disc is the light source, and the other side has a photosensitive element corresponding to each code track; when the code disc is in a different position When in position, each photosensitive element converts a corresponding level signal according to whether it receives light or not, resulting in a binary number. The feature of this kind of encoder is that it does not need a counter, and a stable digital code corresponding to the position can be read 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 the beginning, there were 16-bit absolute encoder products in China.

Absolute encoders use natural binary or recurrent binary (Grey code) method to stop photoelectric conversion. The difference between an absolute encoder and an incremental encoder is the light-transmissive and opaque line patterns on the disc. The absolute encoder can have multiple codes. The absolute position can be detected according to the code on the read-out code disc. The coding design can adopt binary code, recurrent code, two's complement code, etc. Its features are:

1. Ability to directly read the absolute value of angle coordinates;

2. Does not accumulate deviation;

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 were 10 bits, 14 bits and so on.

★Incremental encoder

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

Signal output:

The signal output has sine wave (current or voltage), square wave (ttl, htl), open collector (pnp, npn), push-pull multiple modes, among which ttl is a long-line differential drive (symmetric a, a-; b,b-;z,z-), htl is also called push-pull or push-pull output. The signal receiving device interface of the encoder should correspond to the encoder. Signal connection—The pulse signal of the encoder is normally connected to the counter, plc, and computer. The modules connected to the plc and the computer are divided into low-speed modules and high-speed modules, and the switching frequency is low and high. Such as single-phase connection, used for bidirectional counting and bidirectional speed measurement. a.b Two-phase connection, used for forward and reverse counting, judgment of forward and reverse and speed measurement. A, b, z three-phase connection, used for position measurement with reference position modification. a, a-, b, b-, z, z-connections, due to the connection with symmetrical negative signals, the electromagnetic field contributed by the current to the cable is 0, the attenuation is the smallest, the anti-interference is the best, and the transmission distance is longer . For ttl encoders with symmetrical negative signal output, the signal transmission interval can reach 150 meters. For the encoder with symmetrical negative signal output of htl, the signal transmission interval can reach 300 meters.