Application principle and industry definition of t

Zhuhai watt: application principle and industry definition of photoelectric encoder

photoelectric encoder is a rotary position sensor, which is widely used in the measurement of angular displacement or angular rate in modern servo systems. Its rotating shaft is usually connected with the measured rotating shaft and rotates with the measured shaft. It can convert the angular displacement of the measured axis into binary code or a series of pulses

photoelectric encoders are divided into absolute and incremental types. Incremental photoelectric encoder has the advantages of simple structure, small volume, low price, high precision, fast response speed, stable performance and so on, which is widely used. In the high-resolution and large range angular rate/displacement measurement system, the incremental photoelectric encoder has more advantages. Absolute encoder can directly give digital information corresponding to each corner, which is convenient for computer processing. However, when the feed number is greater than one revolution, special processing must be done, and more than two encoders must be connected with reduction gears to form a multi-level detection device, which makes its structure complex and cost high

1 incremental encoder

1.1 the structure of incremental photoelectric encoder

incremental encoder refers to that the code disk rotating with the rotating shaft gives a series of pulses, and then use a counter to add and subtract these pulses according to the rotation direction, so as to express the amount of angular displacement of rotation. The structural diagram of incremental photoelectric encoder is shown in Figure 1

Figure 1 structural diagram of incremental photoelectric code disc

photoelectric code disc is connected with the rotating shaft. The code disc can be made of glass material, coated with a layer of opaque metal chromium on the surface, and then made into a centripetal light transmission slit at the edge. The light transmission slit is equally divided on the circumference of the code disk, and the number varies from hundreds to thousands. In this way, the entire circumference of the code disk is equally divided into n light transmitting slots. The incremental photoelectric code disc can also be made of stainless steel sheet, and then evenly distributed light transmission grooves are cut at the peripheral edge

1.2 working principle of incremental encoder

the working principle of incremental encoder is shown in Figure 2. It is composed of main code disk, direction finding disk, optical system and photoelectric converter. Radial slits with equal pitch are carved on the periphery of the main code disk (photoelectric disk) of the figure to form evenly distributed transparent areas and opaque areas. The direction finding disk is parallel to the main code disk, and is engraved with two groups of transparent detection slits a and B, which are staggered by 1/4 pitch, so that the output signals of two photoelectric converters A and B differ by 90 in phase. When working, the steering wheel is stationary, the main coding wheel and the rotating shaft rotate together, and the light emitted by the light source is projected onto the main coding wheel and the steering wheel. When the opaque area on the main code disk is aligned with the transparent slit on the direction finding disk, the light is completely blocked, and the output voltage of the photoelectric converter is the minimum; When the transparent area on the main code disk is aligned with the transparent slit on the direction finding disk, all the light passes through, and the output voltage of the photoelectric converter is the maximum. Each time the main code disk turns a scoring cycle, the time required for it to pass through the charging pile for fast charging is generally 1 ⑴ After 5 hours, the electric converter will output an approximate sine wave voltage, and the phase difference of the output voltage of photoelectric converters A and B is 90

Figure 2 working principle of incremental encoder Figure 3 output waveform of photoelectric encoder

the most commonly used light source of photoelectric encoder is the light-emitting diode with its own focusing effect. When the photoelectric code plate rotates with the working shaft, the light passes through the slit of the photoelectric code plate and the light fence plate to form a flickering light signal. The photosensitive element converts this optical signal into an electric pulse signal, and outputs the pulse signal to the numerical control system after passing through the signal processing circuit. The displacement can also be directly displayed by the digital tube

the measurement accuracy of the photoelectric encoder is related to the number of slit stripes n on the circumference of the code disk. The distinguishable angle is:

=360/n (1)

resolution =1/n (2)

for example, if the number of light transmission slots on the edge of the code disk is 1024, the minimum distinguishable angle =360/1024=0.352

in order to judge the rotation direction of the code disk, two slits must be set on the light fence, whose distance is (m+1/4) times the distance of the two slits on the code disk, and M is a positive integer. Two groups of corresponding photosensitive elements are set, such as a and B photosensitive elements in Figure 1, sometimes called COS and sin elements. When the detected object rotates, the coaxial or associated photoelectric encoder will output a and B digital pulse signals with a phase difference of 90. The output waveform of the photoelectric encoder is shown in Figure 3. In order to obtain the absolute position of the code disk rotation, a reference point must also be set, such as the zero mark slot in Figure 1. For each revolution of the code disk, the photosensitive element corresponding to the zero mark slot generates a pulse, which is called a revolution pulse. See C0 pulse in Figure 3

Fig. 4 shows the waveforms and timing relationship of a and B signals when the encoder is in forward and reverse rotation. When the encoder is in forward rotation, the phase of a signal is 90 ahead of B signal, as shown in Fig. 4 (a); When reversing, the phase of B signal is 90 ahead of that of a signal, as shown in Figure 4 (b). The number of pulses output by a and B has a linear relationship with the measured angular displacement change. Therefore, the corresponding angular displacement can be calculated by counting the number of pulses. According to this relationship between a and B, the rotation direction and rotation angular displacement/rate of the tested machine are correctly solved, which is the so-called pulse direction discrimination and counting. The direction discrimination and counting of pulses can be realized by both software and hardware in the calculation of elongation until 2020

Figure 4. The forward rotation of the photoelectric encoder and the wet air in the atmosphere will penetrate into the box. The reverse waveform

2 absolute encoder

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

photoelectric code disc is one of the most widely used at present. It accurately prints binary codes on the disc of transparent materials. Figure 5 shows a four bit binary code disk. Each ring on the code disk represents a digital code channel of one bit binary respectively. Black and white equally spaced patterns are printed on the same code channel to form a set of codes. Black opaque area and white transparent area represent binary 0 and 1 respectively. On a four digit photoelectric code disk, there are four circles of digital code channels, and each code channel represents one bit of binary. The inner side is high-order, and the outer side is low-order. Within the 360 range, the number of programmable digits is 24=16

when working, a power supply is placed on one side of the code disk, and a photoelectric receiving device is placed on the other side. Each code channel corresponds to a photocell and an amplification and shaping circuit. When the code disk is turned 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 error will occur when the code disk rotates in the process of alternating two code segments. For example, when the code disk rotates clockwise from position 0111 to 1000, these four digits will change at the same time, and the number may be misread into any of the 16 codes, such as 1111, 1011, 1101, 0001, etc., resulting in a large numerical error that cannot be estimated. This error is called non single value error

in order to eliminate non single value errors, the following methods can be used

2.1 cyclic code disk (or gray code disk)

Figure 5 four bit binary code disk Figure 6 four bit binary cyclic code disk

cyclic code is also known as gray code, which is also a binary code, with only two numbers of 0 and 1. Figure 6 shows a four bit binary cyclic code. The characteristic of this coding is that only one bit code changes between any two adjacent codes, that is, 0 becomes 1 or 1 becomes 0. Therefore, in the process of two number transformation, the reading error generated is not more than 1 at most, and it is only possible to read one of the two adjacent numbers. Therefore, it is an effective method to eliminate non single value error

2.2 binary cyclic code disc with bit judging photoelectric device

this code disc adds another circle of signal bits on the outermost ring of the four bit binary cyclic code disc. Figure 7 shows a binary cyclic code disk with a bit judging photoelectric device. The position of the signal bit on the outermost circle of the code disk is just staggered from the state intersection line, and it is read only when the photoelectric element at the signal bit has a signal, so there will be no non single value error

Fig. 7 binary cyclic code disk with bit judging photoelectric device