**1. Introduction**

Faraday's experiment on electromagnetic induction and energy transfer was the first experiment to transfer electrical energy wirelessly [1]. Since then, researchers have been interested in wireless power transfer and radio-frequency communication technologies. The concept of wireless power transfer (WPT) was demonstrated by Nikola Tesla in the early 1900 s [2]. However, engineers have experienced di fficulties in commercialization due to specific problems with WPT, viz., low e fficiency and di fficulty in long-distance transmission when compared with conductive power transfer. Therefore, researchers have focused on contact wireless power transfer, which has been commercialized and used in many electronic and electrical devices [3–5]. Long-range WPT technology regained attention in 1964 owing to William C. Brown [6], who successfully supplied power to fuel-free helicopters using 2.45 GHz microwaves. In 2007, owing to the continued research and development by engineers, Professor Marin Soljacic at Massachusetts Institute of Technology (MIT) succeeded in verifying 40% e fficiency of WPT technology at a distance of two meters using a coil with a diameter of 60 cm [7].

The application of WPT technology to electric vehicles (EVs) is widely studied. In 2010, the United States (U.S.) Department of Energy (DOE) and the Society of Automotive Engineers (SAE) led the wireless power transfer and alignment task force and began research and standardization of WPT technology [8]. Researchers from major automotive manufacturers and charging infrastructure suppliers [9] wished to develop WPT technology suitable for EVs while participating in the EV WPT standardization. The primary goal of the development of WPT for the EVs in its early stages was to ensure the safety of users during the implementation of WPT and maximizing the charging e fficiency of wireless charging. Therefore, the coil shape or electric power circuit was studied [10].

Further, engineers from automotive manufacturers began to study fine and precise positioning methods suitable forWPT in EVs and various indoor and outdoor positioning technologies. The contents of the discussion are as follows [11]: (i) As a mechanical method, stopping a vehicle in the center of a primary device using a curbside block or parking block was considered. Further, a method of using a robotic arm to move the primary device to the center of the secondary device after the vehicle is parked was considered. (ii) As a communication-based method [12–16], technologies such as global positioning system (GPS), Bluetooth low energy (BLE), radio frequency identification (RFID), Wi-Fi and ultra-wideband (UWB) were discussed. (iii) As a video-based method [17,18], a parking assistant system (PAS) applied to a vehicle, 2D/3D marker notified to a user by a camera installed in a parking lot and optical character recognition (OCR) was mentioned. However, the mechanical methods were excluded from the discussion owing to an increase in the production cost of the WPT manufacturer, and the communication-based methods were excluded because it is di fficult to satisfy the vehicle electromagnetic compatibility (EMC) standards regulated by the International Telecommunication Union (ITU) [19]. The video-based method was found to be di fficult to apply to external public parking lots due to weather or environmental factors. Therefore, the experts of the SAE J2954 task force is focused on technologies that could easily be mounted on an EV, was inexpensive, did not interfere with an electronic component in the vehicle, and, satisfied the conditions for positioning within the alignment tolerance range for the WPT [20]. It also focuses on technology that satisfies the fine positioning condition, where the central alignment distance between the primary and secondary devices is approximately 1.5 m or more and the precise positioning condition, where the primary and secondary devices begin to overlap. Among them, low power excitation (LPE) is a technology whose primary device, i.e., the power transfer device, performs fine and precise positioning by transmitting a minute quantity of power to the secondary device. Another method is to mount a ferrite antenna using a low frequency (LF) in primary device or a secondary device and perform fine and precise positioning using the magnetic field change value of the ferrite antenna. Therefore, LPE and ferrite antennas were applied to the SAE J2954 standard as a method for fine and precise positioning in an EV WPT system [20]. In addition, the International Electrotechnical Commission (IEC), an international standards and conformity assessment body, also addresses LPE and ferrite antennas as a method to fine and precise positioning in the 61980-2 document [21].

This article describes how to find the central alignment point between the primary device and secondary device within the alignment tolerance area that requires the minimum power transfer efficiency of the EV WPT system using the ferrite antenna. This method suggests that it is necessary to calculate all induced loop voltages in the relationship between the incident magnetic field signal strength and the induced loop voltage because of the distance between the transmitter and receiver of the ferrite antenna in EV WPT precise positioning is short—to within 250 mm. It also suggests a sequence to find the fitting location of the ferrite antenna, the number of antennas used and the center alignment point. After the simulation is performed on the suggestions, unit-level, component-level and vehicle-level tests are performed to validate the simulation results. Therefore, we propose that the ferrite antenna was suitable for the precise positioning of EV WPT.

The content is organized as follows: In Section 2, we review the SAE J2954 document, the magnetic flux density of ferrite antenna and open-circuit voltage of a ferrite antenna. In Section 3, we verify that ferrite antennas already applied in the automotive field are suitable for use in the precise positioning of WPT systems for EVs. Then, we simulate and validate the performance of the ferrite antenna as a transmitter and receiver as a method of finding the central alignment point within the alignment tolerance area of the EV WPT system. In Section 4, we extend the magnetic flux density and open-circuit voltage models of a ferrite antenna to the EV WPT system and conducts simulations. In Section 5, we present the results of performed experiments at the component-level and vehicle-level to verify the validity of modeling and simulation results and compare them with simulation results. We present conclusions in Section 6. In Section 7, we describe registered patents. Finally, in Appendix A, we describe the magnetic flux density, the open-circuit voltage applied to the geometric dimensions of the WPT system for EV and the power received by the ferrite antenna of the primary device.
