Wireless charging of mobile phones is not new. ＂Charging in the air＂ Learn about it!
Photo courtesy of vision china
Our reporter Fu Lili
Recently, according to media reports, like most large retailers, Wal-Mart stores in the United States are planning to replace paper labels on shelves with small electronic screens, so that prices can be changed quickly and at low cost. But the conversion from paper to screen faces another problem — — The screen needs power. At present, Wal-Mart is cooperating with Osiya Company to adopt the technology of "long-distance charging", also known as "charging in the air" (Cota) to solve this problem.
Charging from a distance sounds amazing. What is its principle and how is it realized?
"Collecting" electromagnetic energy from space to realize charging
"Wal-Mart this ‘ Charging in the air ’ The principle of is not profound, it is ‘ Wireless charging ’ A specific application of technology. " Recently, Yan Huaizhi, director of the Institute of Computer Network and Countermeasure of Beijing Institute of Technology, said in an interview with a reporter from Science and Technology Daily.
Yan Huaizhi introduced that the so-called wireless charging refers to the wireless transmission technology of electric energy realized by non-physical contact.
The idea of wireless energy transmission has a long history. As early as more than 100 years ago, Tesla proposed the idea of realizing global wireless power supply through the atmospheric ionosphere.
The law of conservation and transformation of energy is universally applicable in nature, and electromagnetic space is no exception, which can be proved by Lenz’s law. The essence of Lenz’s law is that "the magnetic field of induced current always hinders the change of magnetic flux that causes induced current." When the magnetic flux in the coil increases, the direction of the induced current is opposite to that of the magnetic field generated by it, while when the magnetic flux in the coil decreases, the direction of the induced current is the same as that of the magnetic field generated by it. Because electromagnetic induction also follows the law of energy conservation and transformation, people can realize energy transformation and propagation through electromagnetic induction.
"Specifically, Wal-Mart’s charging method from a distance is that an electric energy transmitter provides an energy source and emits a certain frequency spectrum in a specific direction range, thus creating a three-dimensional electromagnetic energy space." Yan Huaizhi explained that electronic equipment in the space covered by the RF signal can receive the signal if it is equipped with a signal receiving device with the corresponding frequency spectrum, so as to realize electromagnetic induction with the transmitter, thus "collecting" the electromagnetic energy transmitted by the transmitter through the air from a distance and realizing wireless charging of the receiving electronic device.
Wei Zhiqing, an associate professor at the School of Information and Communication Engineering of Beijing University of Posts and Telecommunications, further explained that the working principle of Wal-Mart’s air charging equipment is similar to that of WiFi and Bluetooth, and it consists of a transmitter and multiple receivers. Hundreds of micro antennas are integrated in the transmitter, while the receiver is only the size of a coin and is installed on the equipment that needs charging. The receiver will send a request signal for charging to the transmitter when the power is low. After receiving the request signal, the transmitter will charge the equipment at a frequency of 100Hz per second. In the APP that controls charging, you can specify charging devices and charging strategies (for example, give priority to charging devices with low power), and you can charge up to 32 devices at the same time.
Yan Huaizhi said that the distance and charging time that people care about this kind of charging technology can reach mainly depends on two factors, one is the transmitting power of the wireless spectrum of the transmitting device, and the other is the transmitting frequency of the wireless spectrum.
According to reports, the laboratory prototype system of Wal-Mart’s air charging device is CotaTile, with the transmitting power of 20 watts, the receiving power of equipment at a distance of 1 meter is about 6 watts, and the receiving power of equipment at a distance of 2 meters is 2 to 3 watts, which can charge equipment at a distance of 10 meters at the farthest.
You can also see these wireless charging technologies.
In addition to this way of charging from a distance, there are many commonly used wireless charging methods.
Yan Huaizhi introduced that the first is electromagnetic induction technology wireless charging, also known as Qi standard wireless charging. At present, this method is mostly used for wireless charging of the latest models of Xiaomi, Huawei and Apple on the market. The user can put the mobile phone on the charging base to charge without charging cable.
Wei Zhiqing said that its working principle is that the wireless charging base and the mobile phone are respectively equipped with a transmitting coil and a receiving coil. By using electromagnetic induction, when they are close, the transmitting coil in the charging base is connected with alternating current to generate an alternating magnetic field, and the change of the magnetic field makes the receiving coil generate current, thus transferring energy from the transmitting end to the receiving end, that is, supplying power to the mobile phone from the charging base. The advantage of this method is that the charging efficiency is high, and the charging efficiency of smart phone can reach 80%. The disadvantage is that the charging distance is short, so it is usually necessary to put the phone close to the charging base.
Secondly, electromagnetic resonance wireless charging, which uses resonant devices to make the transmitter and receiver reach the same frequency, and the same resonant frequency is a necessary condition for energy transfer, and electromagnetic energy can be exchanged through magnetic resonance. Compared with electromagnetic induction, the advantage of this method is that there is no need to "cling". The charging distance of smart phones can reach 10 cm, but its charging efficiency is slightly lower, usually less than 70%.
In addition, there is wireless radio frequency technology, which uses space electric field as energy transmission medium and generates alternating current under the action of alternating electromagnetic field, thus realizing long-distance wireless transmission of electric energy. The advantage of this method is long charging distance, but the disadvantage is strong electromagnetic radiation and low conversion efficiency.
Although "wireless" is good, it faces many challenges.
"Compared with wired charging, the advantages of wireless charging are very unique, including the convenience of use, the security of transmission, the compactness of the device and the integration of the environment, but there are also many challenges, which are embodied in both technical and non-technical aspects." Yan Huaizhi said that technically, wireless charging involves technical problems such as wireless transmission of power signals, high-frequency conversion, coupling coil design, system collaborative control, electromagnetic shielding and electromagnetic interference. For example, although close-range RFID devices, smart phones, tablets, smart watches, hearing AIDS, etc., to long-distance drones and even spacecraft in space, they can be charged in the air. However, due to the principle of electromagnetic induction, any factors that damage or weaken the propagation of electromagnetic spectrum signals, such as electromagnetic shielding, insufficient power and unstable frequency, will have a fatal impact on it. On the non-technical level, there are problems such as high cost of wireless charging equipment, difficulty in unifying technical standards, and public doubts about electromagnetic radiation.
But he believes that these problems will be effectively solved with the further development of technology.
Wei Zhiqing added that the biggest problem of wireless charging is the low charging efficiency, which generally only supports 5W charging power, and the slightly faster charging power is only 7.5W. In addition, regarding the public’s concerns about the electromagnetic radiation that may be brought by wireless charging, it is necessary for enterprises and evaluation institutions to conduct rigorous experiments and extensive popularization of science, so as to dispel public concerns. Due to the wide charging range of wireless charging, it will have broad application prospects in the field of Internet of Things in the future.