Coexistence of Dual Inductive Wireless Power Transfer Systems

Abstract

Wireless power transfer is emerging into everyday life. The idea is to create a more convenient way of charging, or at least powering, devices. Ideally, all power transmitters and power-receiving devices should be interoperable so that the user does not need to investigate whether a specific power transmitter will work together with a specific device. Interoperability is what the standard Qi, invented by the Wireless Power Consortium, has achieved for the wireless charging of smartphones since its release in 2008, which also leads to its phenomenal success. However, when it comes to cordless kitchen appliances, the standard (called Ki) is still under development; the idea is to transform the kitchen into a cordless environment, for the user’s convenience. Unlike Qi, which standardizes power transfer in the order of tens of watts, Ki should safely handle power level of up to the kilowatt range. Hence, Ki is technically more challenging to develop. This thesis investigates how the dual wireless power systems proposed in Ki may coexist in space and time. One system is used to transfer high power levels in the kilowatt range for appliance operation, while the other utilizes low power of less than 1 watt for communication needs. To simply implementation, the current Ki approach is to separate the operation of two systems in time, to bypass the need for coexistence. In this investigation, the dual wireless power systems were first characterized. It was found that the transmitting and receiving units will act significantly differently depending on the material that surrounds the power transmitter and receiving units. With the system characterized, methods to decouple the two systems (i.e., a filter and a shielding barrier) were studied. The decoupling of the two systems is important as the signals from one system can otherwise interfere with the correct operation of the other system, and viceversa. Although coexistence has not been achieved in this work, the progress made towards that direction is presented. Adequate shielding of the combined of a low-frequency magnetic field and a high-frequency electric field will require further efforts. In addition, the low-power communication system is very susceptible to interference due to its lower signal power compared to the noise emitted by the high-power system. Increasing the power level in the low-power system may enable coexistence.

Popular Abstract

Wireless power transfer (WPT) is an emerging field with numerous exciting research opportunities. Today, the most common standardized WPT application is the wireless charging of small mobile devices, where the amount of power transferred is relatively small. When higher power levels are involved, such as the powering of kitchen appliances through WPT, the engineer must consider other technical aspects. For example, a high-power device that is not to be charged, but directly powered by WPT, may contain electronics within the device. Those electronics may need another power source available before or independent of the high power source. The Wireless Power Consortium, which came up with the Qi specification for wireless charging of smartphones, has been developing a specification for kitchen appliances, called Ki. As in the case of the Qi specification, this interface utilizes inductive coupling for both power transfer and communication. However, whereas the communication in Qi is realized by modulation of the power transfer signals (so called "in-band-communication"), the communication in Ki is performed based on the NFC standard. This means that in Ki, the WPT and communication is separated both physically and electrically; two separate coils are used and will operate at different frequencies. The receiver needs to communicate with the transmitter actively, even when the appliances are not operating with high power. Power-up or standby mode are two examples of such cases. Using the high-power coils to provide standby power is highly inefficient and adding a battery to the appliance is not the desired option. Thus, the NFC link will be utilized as a secondary low-power transfer link. One way of achieving coexistence is through time division duplex, meaning that the WPT and communication will each have defined slots in time where only one is allowed to be operational. The implications of such a solution are discussed later in this thesis. Neither WPT with inductive coupling nor NFC is a new research area. However, the coexistence of a dual WPT system with this specific setup is yet to be fully characterized. Therefore, this thesis will focus on the theoretical and practical integration of these coexisting systems. A complete setup comprising a control system and coils to it are used to perform this study. Custom coils are built and characterized merely for this purpose. A filter and shielding have been implemented to attenuate interference between the high-power and the communication coils. Shielding, in this case, implies enclosing sensitive parts of the communication system within a metallic cage. These two implementations were expected to ensure coexistence between the systems. However, the coexistence issue is still present with the mentioned implementations. Further investigations are needed to achieve coexistence between the high-power and communication systems.

• author: Hållsten, Eneas
• supervisor: Buon Kiong Lau; Laurens Swaans; Max Andersson
• organization: Department of Electrical and Information Technology
• course: EITM01 20221
• year: 2022
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• report number: LU/LTH-EIT 2022-890
• language: English
• id: 9099577
• date added to LUP: 2022-10-05 15:11:33
• date last changed: 2022-10-05 15:11:33