I. Introduction
Recently, wireless power transfer (WPT) system has attracted much researchers’ attention [1]-[12], [14]-[18]. WPT systems are applied to various applications such as electric vehicles [1], [2], medical equipment [3], and robots [10]-[12]. Construction of WPT systems for robots is an important research topic. By applying the magnetic connections at the joint parts, the risk of failure due to disconnection can be reduced. Because the robot arm has multiple joints in it, the WPT system has a multi-hop structure [10]-[12].