Recently, according to media reports, like most large retailers, Wal-Mart Stores in the United States is planning to replace paper labels on shelves with small electronic screens so that prices can be changed and promoted quickly and cost-effectively. But the conversion from paper to screen faces another problem-the screen needs electricity. At present, Wal-Mart is cooperating with Osia to adopt “remote charging” technology, also known as “Cota” to solve this problem.
Charging in the air sounds magical. What is the principle and how does it work?
“Collect” electromagnetic energy in the air to achieve charging
“The principle of Wal-Mart’s” isolated charging “is not esoteric. It is a specific application of ‘wireless charging’ technology.” Recently, Yan Huaizhi, director of the Institute of Computer Networks and Confrontation at Beijing Institute of Technology, was interviewed by Science and Technology Daily reporter Say.
Yan Huaizhi introduced that the so-called wireless charging refers to the wireless power transmission technology realized by non-physical contact.
The idea of wireless energy transmission has a long history. Tesla proposed the idea of achieving global wireless power supply through the atmospheric ionosphere more than 100 years ago.
The laws of energy conservation and transformation are generally applicable in nature, and electromagnetic space is no exception. This 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 the magnetic flux that causes the induced current. When the magnetic flux in the coil increases, the direction of the current generated by induction is opposite to the direction of the magnetic field generated by it. When the magnetic flux decreases, the direction of the current generated by induction is the same as the direction of the magnetic field generated by it. Because the phenomenon of electromagnetic induction also follows the law of conservation of energy and conversion, people can achieve the conversion and propagation of energy through electromagnetic induction.
“Specifically, the air-conditioning charging method adopted by Wal-Mart is an electric energy transmitter that provides an energy source to emit a certain frequency of radio frequency spectrum in a specific direction range, thereby creating a three-dimensional electromagnetic energy space.”
Yan Huaizhi Explained that if the electronic equipment in the space covered by the radio frequency signal is equipped with a signal receiving device of the corresponding spectrum, the signal can be received, and electromagnetic induction with the transmitting end can be achieved, so that the transmitting end can be “collected” through the air through the air.
The transmitted electromagnetic energy realizes wireless charging of the electronic device.
Wei Zhiqing, an associate professor at the School of Information and Communication Engineering, Beijing University of Posts and Telecommunications, further explained that the working principle of Wal-Mart air-conditioning charging devices is similar to WiFi and Bluetooth, and consists of a transmitter and multiple receivers.
Hundreds of miniature antennas are integrated into the transmitter, while the receiver is only the size of a coin and is mounted on the device that needs to be charged. The receiver sends a charging request signal to the transmitter when the power is low.
After receiving the request signal, the transmitter charges the device at a frequency of 100 Hz per second. In the APP that controls charging, you can specify charging devices and charging strategies (such as charging devices with low battery priority), and can charge up to 32 devices at the same time.
People are concerned about the distance and charging time that can be achieved by this air-space charging technology. Yan Huaizhi said that it 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, Wal-Mart ’s laboratory prototype system for this air-conditioning charging device is CotaTile, with a transmitting power of 20 watts, a device that is 1 meter away receives approximately 6 watts, and a device that is 2 meters away receives 2 to 3 Watts for charging devices up to 10 meters away.
You can also see these wireless charging technologies
In addition to this method of charging in the air, there are many current wireless charging methods.
Yan Huaizhi introduced that the first is wireless charging with electromagnetic induction technology, also known as Qi standard wireless charging. At present, the wireless charging of the latest models of Xiaomi, Huawei, and Apple on the market mostly adopts this method. Users can place the phone on the charging base to charge without a 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, and use the electromagnetic induction phenomenon. When the two are close to each other, the transmitting coil in the charging base is connected to alternating current to generate an alternating magnetic field.
A current is generated in the receiving coil, thereby 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 the smartphone can reach 80%. The disadvantage is that the charging distance is short. Usually, the mobile phone needs to be held close to the charging base.
The second is electromagnetic resonance wireless charging. This charging method uses a resonant device to make the transmitting end and the receiving end reach the same frequency, and the same resonant frequency is a necessary condition for energy transfer. The electromagnetic energy exchange can be achieved through magnetic field resonance. Compared with electromagnetic induction, the advantage of this method is that there is no need to “close”, the charging distance of the smartphone 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 a space electric field as an energy transmission medium and generates an alternating current under the action of an alternating electromagnetic field, thereby realizing the long-range wireless transmission of electrical energy. The advantage of this method is that the charging distance is long, and the disadvantages are strong electromagnetic radiation and low conversion efficiency.
“Wireless” is not a small challenge
But he believes that these problems will be effectively solved with the further development of technology.
Wei Zhiqing added that the biggest problem with wireless charging is the low charging efficiency. Generally, it only supports 5W charging power, and slightly faster charging only has 7.5W power. In addition, the public’s concerns about the possible electromagnetic radiation caused by wireless charging require rigorous tests and extensive scientific popularization by companies and evaluation agencies in order to dispel public concerns. Due to the large charging range of wireless charging, it will have broad application prospects in the field of Internet of Things in the future.