Abstract: Electromagnetic compatibility (EMC) is crucial in computer control systems composed of multiple photoelectric sensors. Through EMC design, the electromagnetic radiation intensity of photoelectric sensors is reduced, and their anti-electromagnetic interference capability is significantly improved. This paper introduces the main components of photoelectric sensors and the method for EMC design as a whole, and provides EMC testing methods. Keywords: photoelectric sensor; electromagnetic compatibility; design; test 0 Introduction Photoelectric sensors, widely used in industrial automation, are generally switch-type and called photoelectric switches. A photoelectric sensor consists of a projector, receiver, integrated circuit, output circuit, etc., and belongs to the category of weak current detection sensors. It is easily affected by electromagnetic interference from strong current equipment. During operation, it causes electromagnetic radiation, forming electromagnetic interference. When the operating frequency is high and multiple sensors are close together, weak current equipment such as instruments, bus systems, and computers will be strongly interfered with, and the sensor itself will also be strongly interfered with. At this time, the EMC problem becomes more prominent. The purpose of EMC design is twofold: first, to ensure that the sensor itself can work normally in the electromagnetic environment and avoid malfunctions; and second, to prevent the sensor from becoming a source of electromagnetic interference. 1 Suppression of Light Interference Photoelectric sensors use light as a medium for non-contact detection. Light is a high-frequency electromagnetic wave [1]. Light interference is also a kind of electromagnetic interference, and light interference is one of the main factors causing sensor malfunction. Ambient light, background light and light emitted by other photoelectric sensors in the surrounding area are sources of light interference. Using infrared light as a medium for detection can reduce the influence of visible light, and infrared light does not affect visible light. Infrared photoelectric sensors can use filters to filter out visible light. For light interference from other photoelectric sensors in the surrounding environment, shells, sleeves and gaps can be used to suppress it [2]. Figure 1 is a structure diagram of a light receiver. If the structure of the projector shell is properly designed, the emitted light can become a regular beam rather than a scattered beam. If it is properly installed during use, the projector is unlikely to become a source of light interference. When designing the sensor, using polarized light and high-frequency modulated pulsed light, and using synchronous detection, are all beneficial to suppressing light interference. 2 Electromagnetic compatibility of circuit board The photoelectric sensor generates a high-frequency electrical signal through high-frequency modulation. At high frequencies, a wire is equivalent to an inductor. Therefore, the length of the high-frequency line inside the sensor must be shortened as much as possible. When designing printed circuit board traces, it is necessary to consider minimizing electromagnetic radiation as much as possible. Trace radiation is stronger than integrated circuit radiation. If the traces form current loops of equal size and opposite direction as shown in Figure 2, they will radiate magnetic fields into space and also receive magnetic fields from space. In the figure, i is the loop current. Let the loop area be S, the electrical signal frequency be f, the distance from the measuring antenna to the radiation plane be d, and the angle between the measuring antenna and the printed circuit board plane be θ, then the measured electric field strength is [3]. This is differential mode radiation. Differential mode radiation can be reduced by reducing the loop area S and the loop current i. Common mode radiation can be simulated by a short monopole antenna excited to ground voltage with a length less than 1/4 wavelength. The electric field strength of common mode radiation is [3]. In the formula, l is the length of the short monopole antenna, and the common mode voltage is converted into differential mode current i. Reducing the ground voltage and bypassing the differential mode current to ground can reduce common mode radiation. In general, the length of the printed circuit board trace should be short and the width should be increased, but it should not be suddenly widened or suddenly turned. The ground area should be increased as much as possible to reduce the ground impedance. High-frequency signal lines and power lines should be as parallel as possible to the ground. Low-power devices, such as CMOS integrated circuits, should be selected for the sensor. CMOS devices have strong anti-interference ability, and the anti-interference ability of CMOS logic circuits is as high as 20%. Low-power devices generate less heat, which is conducive to the sensor design being more compact and conducive to the stable operation of the sensor. 3 Electromagnetic compatibility of output circuit The output circuit is the last stage circuit of the sensor, which is generally a contactless switch circuit. Commonly used output components are transistors and thyristors. When the state of the transistor changes, it causes electromagnetic radiation. An RC absorption circuit is connected in parallel between the collector and emitter of the transistor, and an inductor L is used to suppress di/dt, which can reduce the energy of electromagnetic radiation [4], as shown in Figure 3. Another method of electromagnetic compatibility design for the output circuit is shielding. In Figure 4, the thyristor is shielded, and the LC filter network is used to suppress the surge generated when the thyristor operates. Both the filter network and the shielding cover are grounded. 4. Anti-interference Coding Currently, the development of communication, interface, and bus technologies has also promoted the intelligentization of binary sensors. AS-Interface is an actuator-sensor-interface bus system. The AS-Interface bus system can install up to 248 binary sensors. The AS-Interface chip, in conjunction with a photoelectric sensor, allows the sensor to connect to the AS-Interface bus. On the same bus, the problem of mutual interference between multiple sensors is quite serious. This can be addressed by adding a check code (redundant code) to the information during sensor communication to prevent interference. For example, 00000 and 11111 can be used to replace 0 and 1. When 10111 is received, it can be considered that one bit is wrong, and the second bit will be automatically corrected. 5. Electromagnetic Compatibility Testing Electromagnetic compatibility design relies on electromagnetic compatibility testing for verification and improvement. Currently, there are relatively complete electromagnetic compatibility testing methods and specialized testing equipment. When switching electrical appliances operate, they generate strong interference and transient pulses. The IEC 61000-4-4 standard specifies electrical fast transient pulse interference, categorizing it into single pulses, pulse groups, and pulse clusters. The pulse cluster waveform is shown in Figure 5, with a period of 300 ms and a pulse beam width of 15 ms. The width of a single pulse or pulse group must be less than 15 ms. The width of the lightning simulation pulse specified in IEC 1024 must also be less than 15 ms. A small capacitor internally used in the photoelectric sensor is well-suited for transient pulse filtering. If the sensor is controlled by a host computer, digital filtering and delay instructions can be used in the software to suppress transient interference. The radiation sensitivity test results of the photoelectric sensor are shown in Table 1. 6. Conclusion After adopting the above design, the photoelectric sensor does not interfere with either the 80C51 microcontroller experimental system or the F-40M PLC. It can operate stably in the harsh environment of a circuit breaker. Through electromagnetic compatibility design, the reliability and quality of the photoelectric sensor are significantly improved. References: [1] Huang Jichang, Xu Qiaoyu, Zhang Haigui, et al. Working principle and application examples of sensors [M]. Beijing: People's Posts and Telecommunications Press, 1998. 160. [2] Lei Yutang, Wang Qingyou. Photoelectric detection technology [M]. Beijing: China Metrology Press, 1997. 236-238. [3] Bai Tongyun, Lü Xiaode. Electromagnetic compatibility design [M]. Beijing: Beijing University of Posts and Telecommunications Press, 2001. 55-60. [4] Zhang Guozhong, Zhao Jiagui. Detection technology [M]. Beijing: China Metrology Press, 1998. 378-382. Source: www.tede.cn