How to monitor basic vital signs such as heartbeat and breathing without contact? The answer lies in millimeter wave radar
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Millimeter-wave radar sensors are constantly "going viral" and have promising market prospects
How to monitor basic vital signs such as heartbeat and breathing? For this demand, I believe most people's first reaction is to use professional medical monitoring equipment such as electrocardiographs in hospitals. With the rise of wearable devices in recent years, many smart watches and bracelets can also obtain users' pulse, blood oxygen and other vital sign parameters in real time through photoelectric methods, which has brought great convenience to people.
However, these methods have one thing in common: they are all "contact" measurement solutions, which means that the sensor must be directly connected to the body of the object being measured. If you want to find a "non-contact" measurement method to complete real-time monitoring of heartbeat and breathing in the air, is it feasible?
Such a measurement solution already exists! It uses millimeter wave radar technology.
A latecomer with good performance
As we all know, the earliest application of radar was in the military. Its principle is to generate and send a signal with a frequency that gradually increases over time through a transmitter. When this signal encounters an object, it will be reflected back, and its time delay is twice the distance/speed of light value. There is a frequency difference between the returned waveform and the emitted waveform. This frequency difference is linearly related to the time delay. By subtracting these two frequencies, we can get the difference frequency (beat frequency) of the two frequencies, and thus calculate the distance between the object and the radar. On this basis, we can not only know whether the target exists, but also calculate the precise information of the target such as position, direction, distance and speed.
Later, the application of radar gradually expanded to other specialized fields, and finally entered the civilian market with millimeter-wave radar. Automobiles have become an important entry point for millimeter-wave radar in the civilian field. With the rise of autonomous driving in recent years, the development momentum is even stronger. At present, millimeter-wave radar has basically become a standard feature in autonomous driving systems above L2.5.
Millimeter-wave radar essentially senses the position and movement of objects. In the broader civilian field, there were already some mature sensing technologies (such as PIR and ultrasound) before the arrival of radar. If you want to break out of your original "comfort zone" and enter this new field, millimeter-wave radar must face the challenges of these technologies.
Table 1: Comparison of mmWave radar with other sensor technologies
However, in terms of technical characteristics, millimeter-wave radar is still very confident in its ability to conquer the civilian market. Specifically, millimeter-wave radar is not easily affected by the environment (temperature, light, etc.), and can penetrate non-metallic materials such as plastics, wall panels and clothes, which determines that it can occupy the C position in some specific application scenarios; on the other hand, compared with other sensors, millimeter-wave radar can not only detect whether an object exists, but also determine the direction, speed, and distance of a moving object, and even determine the exact position of a moving object based on the antenna configuration. These high-precision data and rich information will undoubtedly win new development space for millimeter-wave radar in the industrial and consumer markets, especially in areas related to the Internet of Things.
Millimeter-wave radar "three brothers"
According to the working frequency band, millimeter wave radars for the civilian market can be divided into three categories:
24GHz millimeter-wave radar. This is also the first millimeter-wave radar put into civilian use, with a wavelength of 1.25cm (barely considered a millimeter wave). Due to the relatively low frequency and narrow bandwidth (only 250MHz), the 24GHz millimeter-wave radar is limited in measurement accuracy, which also limits the expansion of its application range to a certain extent. However, due to its mature technology and low cost, there is still a lot of room for development.
77GHz millimeter-wave radar. The wavelength of millimeter-wave radar in this frequency band is only 3.9mm, the frequency is relatively high, and the bandwidth can reach 4GHz. Generally speaking, the shorter the wavelength of the radar, the higher the resolution/accuracy (of course, the cost will also be higher), and the size of the entire system will be smaller. Therefore, 77GHz millimeter-wave radar has gradually taken over 24GHz products and become the mainstream in the automotive field. In addition, many countries and regions have also allocated this frequency band as a dedicated frequency band for automobiles.
With the maturity of 77GHz millimeter-wave radar technology, the cost-effectiveness advantage of the 24GHz solution in automotive applications is not so significant. Therefore, in recent years, 24GHz millimeter-wave radar has gradually shifted the focus of subsequent development to the industrial and consumer markets.
60GHz millimeter-wave radar. The radar wavelength in this frequency band is 5mm, and it has a license-free bandwidth of up to 7GHz that can be used for short-range applications, so it can provide better resolution. It was mainly created to address the problems of limited bandwidth, insufficient accuracy, and limited sensing of objects in 24GHz radar. Because of this, the competition among major technology manufacturers around millimeter-wave radar in the non-automotive field has gradually shifted from 24GHz to the 60GHz band.
Obviously, 60GHz millimeter-wave radar is better at detecting subtle movements. The wavelength of 60GHz radar is 5mm, and a micro-movement of 0.5mm is equivalent to 10% of the wavelength, which is already an obvious displacement in the radar's view.
“New track” in non-automotive fields
It can be said that with the 24GHz radar leading the way, the 77GHz radar making a breakthrough in the automotive field, and the 60GHz radar making a reinforcement, the "three brothers" of millimeter-wave radar have already opened up a new world in the civilian market. Especially in the non-automotive field, the performance of millimeter-wave radar in four fields is worth looking forward to.
The first is the field of building automation and smart home. As a human body and motion sensor, millimeter wave radar can accurately sense the number and location of people in the area, and use this as a basis to control lighting, air conditioning and other systems to achieve more efficient energy saving. Some developers are also exploring the combination of millimeter wave radar and elevator control, and accurately calculate the number of passengers waiting in the elevator room based on the algorithm - and even accurately judge irrelevant passers-by and real passengers - and then reasonably dispatch and improve the operating efficiency of the elevator.
The second is the field of health monitoring. The heartbeat and breathing monitoring mentioned above is a typical use case. Moreover, with the acceleration of the aging process, the combination of millimeter-wave radar and smart elderly care can also evolve into more new use cases such as fall alarm and sleep monitoring. The future prospects of this field are also worthy of attention. For example, Avnet has developed a breathing and heartbeat detection solution based on Infineon's BGT60TR1X series millimeter-wave radar chip. The solution works with two antennas, one for transmission and one for reception. The collected data is processed on a low-cost MCU based on Arm Cortex-M7, which can automatically detect and capture subtle movements caused by breathing and heartbeat in a large range.
Figure 1: Schematic diagram of Avnet's millimeter wave radar function
Figure 2: Avnet millimeter wave radar breathing and heartbeat detection solution actual measurement diagram,
The upper left and upper right images show the changes in chest displacement caused by breathing and heartbeat
The third is the field of security monitoring. Although cameras are currently the mainstream solution in this field, personal privacy has always been a sensitive topic accompanying visual solutions. Therefore, in certain occasions, millimeter-wave radar can be used as an alternative solution to replace cameras for traffic analysis, intrusion detection and other tasks. In addition, the use of a "camera + millimeter-wave radar" solution that is interconnected can also enhance the performance and reliability of the original solution, which is conducive to achieving higher recognition rates, lower false alarm rates, and lower labor costs.
Finally, in the field of human-machine interaction (HMI), millimeter-wave radar can achieve more sophisticated gesture and body recognition with precise motion detection, providing users with a more natural HMI experience. This will bring a new user experience in non-contact gesture control of small or short-range screens, real-time game interaction, home appliance control, etc.
In short , from military to special, to automobile, to industrial and consumer IoT applications, millimeter-wave radar has continuously expanded its application map by constantly "going out of the circle". According to data from MarketsandMarkets, the total market size of radar sensors will reach US$20.6 billion by 2023, with a compound annual growth rate of 19.5% from 2017 to 2023. Among them, automotive radar is the biggest driving force for this wave of growth. It can be foreseen that the IoT market will become another wheel driving the millimeter-wave radar market, pushing it to run forward at a higher acceleration.
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