The growing number of publications concerning the dynamic compensation of inductive links has demonstrated the importance of the issue upon the WPT system design. There is not still a final and consolidated solution for the case when the coils are in relative movement with each other or misaligned. In this context, this chapter intends to give some contributions to that area, starting with the modeling of the inductive link in Section 2, where the authors show how the magnetic coupling coefficient and the load influence on the PDL. In addition, Section 2 also presents how the excitation frequency and the matching capacitances can improve the PDL. In Section 3, the variable excitation frequency method is explained as well as the hardware needed for the implementation. The voltage-controlled capacitance as a variable element in the matching network is shown in Section 4. Finally, in Section 5, optimization techniques are presented by using both frequency and capacitance to maximize the PDL.
TopIntroduction
The Wireless Power Transfer (WPT) has been studied since the early experiments of Heinrich Hertz in 19th century (Brown, 1984). However, it was from the Tesla’s work that WPT has been brought to the light and the theoretical basis has been developed, which have allowed constructing practical WPT systems. From the first Tesla’s work until the decade of 1930, many attempts have been made to transmit energy without wires in an efficiently way. A WPT system working at 100 MHz has been developed at Westinghouse’s labs, where hundreds of watts have been transmitted over a distance of eight meters (Tesla, 1914).
The technology available at the first half of 20th century did not allow fast and significant development of WPT systems. In fact, Brown (1984) states that a lack of interest in WPT during the first half of 20th century was related to the way that engineers and scientists believed to transmit energy without wires. They believed that for transmitting energy from one point to another, the energy in the magnetic field should be concentrated in a narrow beam. Thus, the implementation would require small wavelengths (radio frequencies) and suitable antennas. However, electronic devices at that time could operate with just a few miliwatts at those wavelengths. A greater step up has been made after the second Great War with the development of high power vacuum tubes (Brown, 1957). Thus, it was possible to transmit energy wirelessly at greater distances by using microwaves (Brown, 1974; Goubau & Schwering, 1961; Degenford, Sirkis, & Steier, 1964).
The WPT systems operating at low frequencies, that is, not involving electromagnetic wave propagation became popular from 1970 decade in the biomedical engineering area (Ko, Liang, & Fung, 1977; Hochmair, 1984). Such systems are considered safe in applications inside or in contact with the human body. Instead of using wave propagation theory, the transferred energy was based on the magnetic coupling between two or more coils and mathematical modeling can be done through the electric circuits theory (Hui, Zhong, & Lee, 2014). Other application fields have benefited from the WPT, such as the induction heating (Hurley & Kassakian, 1979), recharging of mobile devices (Choi et al., 2004; Jang & Jovanovic, 2003; Kim et al., 2001; Hui & Ho, 2005; Liu & Hui, 2007a), and other high power applications for electric vehicles (Green & Boys, 1994; Boys, Covic, & Green, 2000; Boys, Hu, & Covic, 2000; Elliot et al., 2006; Kissin, Boys, & Covic, 2009).
Since the 2000 decade, the consumer electronics and mobile devices have become an important field of application for WPT. After the remarkable results published by researches from Massachusetts Institute of Technology (MIT), WPT has gained much more interests from the scientific community (Kurs et al., 2007). In 2008, eight companies founded an open industrial group in order to standardize solutions for mobile phone recharging. In August 2010, the Qi interface was published, containing the specifications and test procedures for certifications of products with WPT embedded into them. In 2015, more than 200 companies have been already participating in the WPT consortium and more than 700 different products have been already certified with the Qi interface (Treffers, 2015).