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A photoelectric encoder is a rotary position sensor widely used in modern servo systems for measuring angular displacement or angular velocity. Its axis is usually connected to the measured rotation axis and rotates together with the measured axis. It can convert the angular displacement of the measured axis into binary code or a series of pulses.

Optical encoders are divided into two types: linear encoders and incremental encoders. Incremental photoelectric encoders have the advantages of simple structure, small size, low price, high accuracy, fast response speed, and stable performance, and are more widely used. In high-resolution and large-scale angular rate/displacement measurement systems, incremental photoelectric encoders are more powerful. The encoder can directly provide digital information corresponding to each corner, which is convenient for computer processing. However, when the feed rate is greater than one revolution, special processing is required, and two or more encoders must be connected by reduction gears to form a multi-stage detection device, making its structure complex and costly.

1 Incremental encoder

1.1 Structure of incremental photoelectric encoder

An incremental encoder refers to a series of pulses generated by a rotating encoder disk, which are then added or subtracted by a counter based on the direction of rotation to represent the angular displacement of the rotation. The schematic diagram of the incremental photoelectric encoder structure is shown in Figure 1.

Figure 1 Schematic diagram of incremental photoelectric encoder structure

The photoelectric encoder is connected to the rotating shaft. The code wheel can be made of glass material, coated with a layer of opaque metal chromium on the surface, and then made into concentric transparent slits at the edges. The transparent slits are evenly distributed on the circumference of the code wheel, with quantities ranging from several hundred to several thousand. In this way, the entire circumference of the code wheel is divided into n translucent grooves. The incremental photoelectric encoder can also be made of stainless steel sheet, and then evenly distributed transparent grooves can be cut at the circumferential edge.

1.2 Working principle of incremental encoder

The working principle of the incremental encoder is shown in Figure 2. It consists of a main code disk, a direction indicator disk, an optical system, and a photoelectric converter. On the periphery of the main code disk (photoelectric disk) of the graphic, there are radial narrow slits with equal pitch, forming uniformly distributed transparent and opaque areas. The reference disk is parallel to the main code disk and engraved with two sets of transparent detection slots, a and b. They are offset by 1/4 pitch from each other to make the output signals of the A and B photoelectric converters 90 ° out of phase. When working, the reference disk remains stationary, while the main code disk and the rotating shaft rotate together. The light emitted by the light source is projected onto the main code disk and the reference disk. When the opaque area on the main code disk aligns perfectly with the transparent narrow slit on the reference disk, all light is blocked and the output voltage of the photoelectric converter is low; When the transparent area on the main code disk aligns perfectly with the transparent narrow slit on the reference disk, all light passes through and the output voltage of the photoelectric converter is high. Every time the main code wheel rotates through one engraved period, the photoelectric converter will output an approximate sine wave voltage, and the phase difference between the output voltages of photoelectric converters A and B is 90 °.

Figure 2 Working principle of incremental encoder Figure 3 Output waveform of photoelectric encoder

The commonly used light source for photoelectric encoders is a light-emitting diode with its own focusing effect. When the photoelectric encoder rotates along with the working axis, light passes through the photoelectric encoder and the slit of the light barrier, forming a flickering light signal. The photosensitive element converts this light signal into an electrical pulse signal, which is then processed by a signal processing circuit and output to the numerical control system as a pulse signal. The displacement can also be directly displayed by a digital tube.

The measurement accuracy of the photoelectric encoder is related to the number of slit fringes n on the circumference of the encoder, and the angle α that can be resolved is:

α=360 °/n (1) Resolution=1/n (2)

For example, if there are 1024 transparent slots at the edge of the code wheel, the distinguishable small angle α=360 °/1024=0.352 °.

In order to determine the direction of rotation of the code wheel, two slits must be set on the light barrier, with a distance of (m+1/4) times the distance between the two slits on the code wheel, where m is a positive integer. Two corresponding photosensitive elements are also set, such as A and B photosensitive elements in Figure 1, sometimes also known as cos and sin elements. When the detection object rotates, the coaxial or associated photoelectric encoder will output digital pulse signals with a phase difference of 90 ° between A and B. The output waveform of the photoelectric encoder is shown in Figure 3. In order to obtain the position of the code wheel rotation, a reference point must also be set, such as the 'zero position mark slot' in Figure 1. For each revolution of the code wheel, the photosensitive element corresponding to the zero position mark slot generates a pulse called the 'one revolution pulse', as shown in C0 pulse in Figure 3.

Figure 4 shows the waveforms and timing relationship of signals A and B when the encoder rotates forward and backward. When the encoder rotates forward, signal A leads signal B by 90 ° in phase, as shown in Figure 4 (a); When reversed, the phase of signal B leads signal A by 90 °, as shown in Figure 4 (b). The number of pulses output by A and B is linearly related to the measured displacement change, therefore, the corresponding angular displacement can be calculated by counting the number of pulses. Correctly determining the rotation direction and rotation angle displacement/rate of the tested machine based on the relationship between A and B is known as pulse direction identification and counting. The identification and counting of pulses can be implemented both in software and hardware.

Figure 4: The forward and reverse waveforms of the photoelectric encoder

2. Type encoder

The encoder is a detection element that directly converts the measured angle into the corresponding code by reading the pattern information on the encoder disc. There are three types of encoding disks: photoelectric, contact, and electromagnetic.

The photoelectric encoder is currently one of the most widely used types, which prints binary codes on a transparent material disk. Figure 5 shows a four digit binary code disk, with each circular ring representing a binary digit code track. Black and white equidistant patterns are printed on the same code track to form a set of codes. The black opaque area and the white transparent area represent binary '0' and '1', respectively. On a four digit photoelectric encoder, there are four circles of digital code tracks, each representing one bit of binary. The inner side is the high bit and the outer side is the low bit. Within a 360 ° range, 24=16 digits can be encoded.

During operation, a power supply is placed on one side of the code wheel, and a photoelectric receiver is placed on the other side. Each code track corresponds to a photoelectric cell and amplification/shaping circuit. When the code wheel is rotated to different positions, the photoelectric element receives the optical signal and converts it into the corresponding electrical signal. After amplification and shaping, it becomes the corresponding digital electrical signal. However, due to the influence of manufacturing and installation accuracy, reading errors may occur when the code wheel rotates during the alternating process of two code segments. For example, when the code wheel rotates clockwise and changes from position '0111' to '1000', all four digits need to change simultaneously, which may misinterpret the digits as any of the 16 codes, such as 1111, 1011, 1101,... 0001, resulting in significant numerical errors that cannot be estimated. This type of error is called non singular error.

To eliminate non univariate errors, the following methods can be used.

2.1 Circular encoder (also known as Gray encoder)

Cyclic code, also known as Gray code, is a binary encoding with only two numbers, '0' and '1'. Figure 6 shows a four bit binary cyclic code. The characteristic of this encoding is that only one bit of code changes between any adjacent two codes, that is, '0' becomes' 1 'or' 1 'becomes' 0'. Therefore, in the process of two number transformation, the reading error generated is mostly not more than '1', and can only be read as one of the adjacent numbers. So, it is an effective method for eliminating non single valued errors.

2.2 Binary cyclic encoder with position determining photoelectric device

This type of code disk adds another signal bit to the outer circle of the four bit binary cyclic code disk. Figure 7 shows a binary cyclic encoder with a positioning photoelectric device. The position of the signal bit on the outer ring of the code wheel is exactly offset from the state intersection line, and it is only read when the photoelectric element at the signal bit has a signal, so as not to produce non single value errors.