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Abstract: With the rapid development of modern society and the widespread application and continuous upgrading of IoT products, people have put forward higher requirements for the privacy, security, efficiency, and concealment of intelligent products. Radar sensors utilize the echo characteristics of electromagnetic waves to obtain characteristic information of the detected target and its surrounding environment. Compared to other human motion detection sensors, they have strong anti-interference ability and higher concealment. This article provides a detailed introduction to the principle, functional structure, and application of radar sensors for detecting human motion, and prospects their future development trends and market prospects.

Keywords: radar sensor; Internet of Things; Human body induction

introduction：

In recent years, the development of modern wireless communication technology has accelerated the implementation and upgrading of Internet of Things (IOT) applications. The application of intelligent devices in the Internet of Things has greatly saved human resources and improved the quality of life in human society, while also bringing new troubles to people's lives. On the one hand, the application of the Internet of Things requires increasingly high reliability and concealment of sensors. On the other hand, the collection of information by visual sensors has raised concerns about privacy and security issues.

The main reason for the above troubles is that the widely used human motion detection sensors in the market are mainly PIR infrared sensors and camera sensors. Although the equipment cost of infrared sensing red is low, the detection error is large, and the installation environment requirements are also relatively strict. It cannot function normally when the environmental temperature is equivalent to human body temperature or when the environmental brightness is strong. Although the camera sensor does not have the above problems, its installation cost is too high and its confidentiality is poor, and it cannot be used in relatively hidden spaces such as bathrooms and bedrooms. Radar sensors obtain target information by analyzing the transmitted and received electromagnetic wave information. Determined by the characteristics of electromagnetic waves, their working conditions are not affected by temperature, brightness, and humidity, and have the ability to penetrate obstacles. At the same time, radar sensors do not collect image information, so there is no risk of privacy leakage, and are fully suitable for special places such as bathrooms and bathrooms.

In recent years, with the development of CMOS (Complex Metal Oxide Semiconductor) integrated circuit technology, radar integration has been higher, power consumption and size have been significantly reduced. Combined with modern advanced signal processing technology, the sensing ability of radar sensors has been enhanced. The radar sensor, which enables low-power and lightweight human motion, has a wider application range and is more suitable for the development of IoT close range detection and sensing.

This article will explore and research the human motion presence detection radar sensor, mainly including the principle, functional structure, application scenarios, and other aspects of the human motion presence detection radar sensor.

1The principle of radar sensors for human motion

Radar technology is a wireless sensing technology [2]. It mainly uses electromagnetic waves for detection and measurement, and obtains information such as the position, velocity, direction, and distance of the detected target through analysis of echo characteristics. It emits a beam of electromagnetic waves, which are reflected back and received by the receiver when they encounter an object. By measuring the time and intensity of the reflected electromagnetic waves, the radar system can determine the position, shape, size, and other information of an object. The radar sensors used for detecting human motion in the Internet of Things are generally Doppler radar or FM continuous wave radar.

Doppler radar mainly utilizes the Doppler effect, transmitting a section of electromagnetic wave through a transmitter, and then obtaining the electromagnetic wave reflected back by the detected object through the receiver. If relative motion occurs between the detected object and the radar, the frequency of the electromagnetic wave received by the radar will change, thereby determining the velocity information of the target. Due to the propagation loss of electromagnetic waves in the air, the farther the target is relative to the radar, the greater the loss, and the smaller the amplitude of electromagnetic waves received by the radar. Therefore, the direction of motion of the target is determined by receiving changes in the amplitude of electromagnetic waves.

FM continuous wave radar is different from Doppler radar in that its transmitted signal is no longer a fixed frequency continuous wave, but a continuously changing frequency continuous wave emitted at a certain period. The receiver calculates the distance and motion speed between the measured object and the radar by measuring the frequency difference between the reflected signal and the transmitted signal. For FM continuous wave radar, the farther the detection distance of the target, the longer the detection time and the higher the processed differential frequency. Conversely, the lower the difference frequency. The modulation methods of frequency modulated continuous wave radar are generally triangular wave, sawtooth wave, encoding modulation, or noise modulation.

Figure 1.1 is a schematic diagram of the ranging principle of FM continuous wave radar. The frequency of the radar's transmitted signal is proportional to time, and due to the propagation of electromagnetic waves in the air, there is a frequency difference between the transmitted and received signals Δ f. Calculate the propagation time of electromagnetic waves using the received frequency difference Δ t. Calculate the distance of the radar to detect the target. It should be noted that the propagation time here is the time when the radar emits to the target and then reflects back to the radar receiver from the target. Therefore, there is a double relationship between the propagation distance of electromagnetic waves and radar detection during calculation.

图1.1 调频连续波雷达测距原理示意图

Both Doppler radar and FM continuous wave radar have been designed with low power consumption and high integration, and can be used as IoT sensors for human motion detection. They are suitable for different operating frequency bands and different working scenarios. Doppler radar, because it only needs to emit fixed frequency continuous waves, has a simpler structure implementation, but can only detect moving objects and cannot detect fixed objects. And the frequency modulated continuous wave radar has no blind spots in detection, which can measure both speed and distance, and has obvious advantages in the field of short distance measurement. However, due to the need to generate FM signals within a fixed period, its structure is relatively complex. At the same time, there are errors in measuring distance and speed when measuring complex targets.

Both Doppler radar and FM continuous wave radar have the advantages of easy implementation, simple structure, low power consumption, small size, and low cost. As sensors for detecting human motion presence, they can be widely used in the field of the Internet of Things.

2The Function and Structure of Radar Sensors for Human Motion

For the convenience of understanding, before introducing the existence of radar sensors in human motion, the working process of radar sensors should be explained first. In order to obtain detection target information, radar sensors first transmit a specific frequency of radio frequency signal from the antenna, which is then propagated. The antenna then receives a specific frequency of radio frequency signal in nature and inputs it to the receiver. The receiver amplifies the required RF signal and inputs it to the mixer, which is multiplied by the transmission frequency to obtain an easily processed intermediate frequency signal; The filter then filters out the out of band interference signal and outputs it to the baseband amplifier to amplify the effective signal. The digital signal is obtained through ADC sampling, and after digital signal processing (usually in the frequency domain), relevant information of the detected target is obtained. Figure 2.1 shows the functional structure diagram of the radar sensor, and the functions of each structure are as follows.

A frequency synthesizer mainly generates a certain frequency signal as a transmission signal and an intrinsic signal to be transmitted to the transmitter and mixer, respectively. For Doppler radar, only a fixed frequency RF signal needs to be generated [3], while for FM continuous wave radar, a sweep signal needs to be generated within a certain period [4].

The transmitter is mainly composed of an RF power amplifier, which is used to amplify the signal power generated by the frequency synthesizer to a high enough level to achieve the detection distance required by the radar sensor.

The receiver is mainly composed of low noise amplifiers, and RF filters will be added to improve receiver performance according to actual situations. Mainly amplifies the effective signal received by the antenna, facilitating the processing and analysis of the system's subsequent circuits. This is mainly due to the complexity and variability of the natural environment, which leads to varying degrees of RF signal loss, resulting in significant fluctuations in the signal received by the antenna. When introducing sufficiently low noise, amplify the effective signal received by the antenna to improve the sensitivity of the radar sensor, i.e. increase the detection distance.

图 2.1 雷达传感器的功能结构图

The mixer multiplies the RF signal generated by the frequency synthesizer with the echo signal received by the receiver, converting the RF signal into a low-frequency signal that is easy to process.

A baseband amplifier/filter is used to filter and amplify the low-frequency signal converted by the mixer, and to achieve a certain amplitude of effective signal for ADC sampling. The baseband amplifier for radar sensing generally adopts low-pass or bandpass filtering, mainly because the signal output by the mixer will contain high-frequency interference signals. By increasing filtering and amplification, the effective signal can be separated, which can improve the detection distance of the radar sensor.

Finally, the amplified baseband signal is sampled by ADC and converted into a digital signal that is easy to process. After digital signal processing and MCU operation, the distance, speed, and other information of the detected target are obtained.

The principles of Doppler radar and frequency modulated continuous wave radar are widely used in radar sensors for detecting the presence of human motion. The circuit structure of the human motion detection radar sensor based on these two radar principles adopts a zero intermediate frequency down conversion transceiver architecture. The biggest difference in circuit modules is in the frequency synthesizer. The human motion presence detection sensor based on Doppler radar only needs to generate a fixed frequency RF signal using a simple voltage controlled oscillator (VCO) [2], while the human motion presence sensor based on frequency modulated continuous wave radar needs to generate a variable frequency signal within a fixed period. To ensure the linearity of the output RF signal, a voltage controlled oscillator with a phase-locked loop is needed to achieve it, and its circuit structure is also more complex.

3The Application of Radar Sensors in Human Motion

With the rapid development of integrated circuits, small-sized and low-power lightweight radar sensors have emerged and are gradually being applied in intelligent devices. Millimeter wave radar is widely used in automotive automation and assisted driving. Lower cost, lower power consumption, and smaller size human motion detection radar sensors are gradually being applied in the control applications of intelligent devices and home appliances. Compared to other human body detection sensors, radar sensors have strong anti-interference ability and certain penetration ability to materials such as plastic, walls, and wooden boards. Therefore, radar sensors can be installed inside household appliances (metal materials cannot be directly in front of the radar antenna), and are not affected by external installation components that cannot work properly, ensuring the appearance and consistency of the product.

In the field of intelligent lighting, radar sensors for detecting human motion presence are gradually being applied and promoted. This is thanks to its higher accuracy, longer detection distance, and lower power consumption, as well as its ability to accurately detect the position and movement of the human body. In contrast, infrared sensors are susceptible to environmental factors, such as being unable to function properly when the ambient temperature is close to human body temperature or when exposed to strong light [5] [6]. Radar sensors are not affected by these factors. Secondly, in large spaces or corridors, radar sensors can detect the position and movement of the human body over a long distance, thereby achieving control and management of personnel throughout the entire space. However, other sensors require distributed installation and ensure that they do not interfere with each other in order to be applied in large spaces, which greatly increases their installation difficulty and usage cost. At the same time, radar sensors can be set to different operating frequencies to achieve dense installation. According to the penetration of radar sensors, the integration of the lighting system is higher, without the need for a separate sensor power supply system, greatly reducing the threshold and installation cost of smart lights. Integrated radar bulb lights, LED ceiling lights, bedroom floor lights, etc. have all been applied in the market and have broad application prospects with the promotion of the market.

In the field of intelligent security [8], the combination of camera and human motion detection radar sensors can improve the detection accuracy and range of security systems, and achieve comprehensive monitoring of personnel. In the intelligent access control system, low-power radar sensors are used to wake up the screen, activate the camera function for facial recognition, and reduce the system's operating power consumption. Simultaneously combining radar sensors for multiple detections to improve the security of the access control system.

In the field of smart home appliances, human motion detection radar sensors also have broad application prospects. Integrating sensors into the interior of home appliances can achieve intelligent and convenient control. For example, integrating radar sensors into an intelligent air conditioning system can automatically adjust the temperature, wind speed, etc. of the air conditioning system based on the presence of personnel in the space detected by the radar sensors, optimize equipment power configuration, and reduce usage costs. At the same time, radar sensors are used to detect human movements and achieve diverse switching of the air conditioning system; Track the direction of personnel's movement to achieve wind direction tracking or direct blow prevention functions for the air conditioning system. In short, combining human motion detection radar sensors with smart home appliances and using other sensor devices in combination with intelligent technology can achieve the intelligent and convenient development of smart home appliances, reduce user usage costs, and improve user convenience and comfort [9] [10].

4Current Status and Prospects of Radar Sensors for Human Motion

With the continuous development of the Internet of Things technology, the application of radar sensors for human motion in the field of the Internet of Things is becoming increasingly widespread. Currently, high-end radar sensors have been widely used in the field of automotive autonomous driving. The consumer market for relatively low-cost human motion sensors with radar sensors has just started, and is currently mainly applied in fields such as intelligent lighting, intelligent security, and smart home appliances, with huge market potential in the future. The presence of radar sensors in human motion can provide information such as position, speed, and distance, which has significant advantages compared to other sensors such as infrared, ultrasonic, and acoustooptic. At present, low power consumption, miniaturization, and low cost are still the main directions for its technological breakthroughs.

With the further development of the Internet of Things technology and the continuous upgrading of the digital industry in the future, the application prospects of radar sensors for human motion will also be broader. On the one hand, with the continuous development of radar technology, the performance of radar sensors used for detecting human motion is constantly improving, and their smaller size and lower power consumption make their application areas more extensive. On the other hand, with the continuous development of artificial intelligence, big data and other technologies, the application scenarios of radar sensors for human motion will also continue to expand, and the application fields will also be more extensive.

5conclusion

Radar sensors for human movement have strong anti-interference and penetration capabilities, and can provide information on personnel movement direction, speed, and distance. Compared to other human motion detection sensors, their detection distance is longer, detection accuracy, and application scenarios are more diverse. With the continuous promotion of the radar sensor market for human motion, there are further breakthroughs in radar sensor technology for low-power, low-cost, and small-scale human motion, combined with the rapid development of artificial intelligence and big data technology, making it have strong market potential.

References

[1] Peng Nan, Lin Guoqiang, Lin Hehui et al. Research on Network Security Issues in Smart Home Systems [J]. Electrical Era, 2023 (07): 84-89

[2] Fan Zhengguang, Yang Tianhong, Zhang Jian, et al. Continuous human motion recognition method based on radar sensors [J]. Computer and Information Technology, 2022,30 (01) 1005-1228

[3] Pan Huichong, Luo Dingli, Xu Baoqing. Research on Radar Target Classification and Recognition Based on Doppler Spectrum Characteristics [J]. Fire Control Radar Technology, 2023,52 (02): 50-55

[4] Lin Jiahui, Yin Huibin, Xiong Rongpeng. Indoor personnel positioning method based on FMCW radar sensors [J]. Sensors and Microsystems, 2021,40 (11): 50-53

[5] Sangchuan Chuan, Tanya Feng. Wireless temperature sensor based on 5.8G microwave radar Bluetooth communication technology [J]. Electrical Switch, 2023,61 (03): 72-74+78

[6] Yang Di, Han Chunlei, Lu Yao, et al. Multi radar/infrared sensor data fusion algorithm [J]. Modern Navigation, 2023,14 (02): 135-140

[7] Xu Xinyin, Liu Mengjie, Fu Baochuan. Overview of Indoor Intelligent Lighting Control Systems [J]. Journal of Suzhou University of Science and Technology (Natural Science Edition), 2023,40 (02): 11-19

[8] Qu Jicheng. Application of Intelligent Security Based on Internet of Things Technology [J]. Intelligent Building and Smart City, 2023 (03): 157-159

[9] Shi Duanyang, Lin Qiang, Hu Bing, et al. Review of the Application of Deep Learning in Radar Target Detection [J]. Radar Science and Technology, 2022-20 (06): 589-605

[10] Research on the Development of Smart Home Technology in the 5G Era by Cha Weijin [J]. Smart City, 2023,9 (05): 10-12