Technology | Highlight
Latest Wireless In-Wheel Motor Keeps All Components in Wheel
T

he research group led by Associate Professor Hiroshi Fujimoto of the Graduate School of Frontier Sciences at the University of Tokyo has developed a third-generation wireless in-wheel motor (WIWM-3) powered by electric energy from the road to WIWM, and has succeeded in a vehicle driving test with in-motion wireless charging (Fig. 1). The members of the research group include the University of Tokyo, Bridgestone Corporation, NSK Ltd., ROHM Co., Ltd., and TOYO ELECTRIC MFG. Co., Ltd.

The WIWM-3 is an evolution of the second-generation wireless in-wheel motor (WIWM-2) announced in March 2017, and features significantly improved wireless power transfer (WPT) performance, motor performance, and vehicle mountability for practical application. In order to further develop the technology, the group has also started research and development on tires and wheels that do not affect WPT performance.

Reducing Carbon Load
The realization of a low-carbon society that reduces greenhouse gas emissions in order to prevent global climate change is an international issue. In Japan, private vehicles are the cause of 17.9 percent of its carbon dioxide (CO2) emissions (Ministry of the Environment, FY2017), which are a leading cause of climate change. Therefore, the regulation for CO2 emissions from vehicles is becoming stricter every year worldwide.

Overview of IWM-3
Fig. 1: Overview of IWM-3
Electric vehicles have no internal combustion engines, therefore do not produce CO2 emissions and are more efficient, making them the best method for reducing vehicle emissions. On the contrary, EVs require inconvenient charging, and pose concerns on the amount of resources required to produce large quantities of batteries. For the sustainable spread of EVs, those that can run efficiently using a small battery are required. There is much research being carried out globally to realize WPT in-motion with the intent of reducing the necessary battery size for EVs as providing power supply while driving offers a lot of advantages: 1) Battery capacity can be small, the EV becomes lighter and can drive with less energy; 2) Battery capacity can be small, the EV becomes more reasonable price; and 3) Drivers can drive EVs without worrying about the remaining charge or charging time.

In addition, WPT in-motion is characterized by a positive relationship with renewable energy. As power generated by solar or wind fluctuates greatly depending on weather conditions, a system that absorbs the fluctuations is necessary for a reliable power grid. The idea of using an EV battery as a power storage facility to absorb the fluctuations has long been considered, but only an EV that is parked and connected to the power grid via the charger would be functional in this role. If electric power can be freely supplied to a driving EV by the WPT system, an EV battery can be connected to the grid at all times. Therefore, WPT in-motion is an important technology not only for reducing CO2 emissions in the transportation sector but also for supporting the flaws in renewable energy systems.

Comparison of vehicle mountability
Fig. 2: Comparison of vehicle mountability
Three Features of WIWM-3
All components in wheel
This research group have developed the WIWM-3 that has all its components, the motor/inverter which is the drive system for electric vehicles, and the power receiving circuit for WPT during driving, inside the wheel. The WIWM-2 is too large to fit the inside of a wheel, forcing some components to be mounted outside the wheel (Fig. 2). The WIWM-3 is not only compact but has a higher output than WIWM-2. Designed for private vehicles, the WIWM-3 can output 25kW/unit. This amount is twice that or greater than the WIWM-2, designed for small cars. One of the biggest reasons why both compact packaging and high power can be achieved is due to the ultrasmall SiC power module produced by ROHM.

One of the major features of the WIWM system is the receiving coil position. The receiving coil is set as unsprung coil. When the receiving coil is “onboard” like the bottom of the car body panel, the distance between the road coil and the receiving coil varies greatly depending on the evenness of the road surface and the weight of passengers in the car (Fig. 3). The performance of WPT varies greatly depending on the distance between the road coil and the receiving coil, then a large change in the distance between the coils makes it difficult to optimize the design of the WPT system.

Comparison of coil position
Fig. 3: Comparison of coil position
Two variations of receiving coil position for WIWM
Fig. 4: Two variations of receiving coil position for WIWM
In the case of the unsprung coil arrangement proposed by the research group, the change in distance between the road surface and the receiving coil is extremely small, facilitating the optimization of the WPT system design and improving the power supply capacity and efficiency.

In addition, the receiving coil position unsprung coil has another advantage. In the WPT system that transmits power using a magnetic field, if there is a metal foreign object between the transmitting coil and the receiving coil, the foreign object will be heated by the energy transfer. For this reason, it is desirable to have a structure that prevents foreign matters from entering between the coils.

The newer structural concept of the WIWM is proposed to prevent foreign matters from entering between the coils (Fig. 4). The coil position of the newer structure is inside of wheel. It can further reduce the possibility of foreign matter entering. However, it is necessary to change the tire and the wheel not to interfere WPT. Therefore, this research group is also doing R & D of materials and structures for tires and wheels with Bridgestone.

Infinity driving range
This research group studies the feasibility of infinity driving range with a small battery. In 2018, this group acquired vehicle driving data on urban roads in Kanagawa Prefecture, and quantified the percentage of time a car stays in front of intersection. As a result of the simulation using that data, if it is possible to supply power during driving in the section of 30m from the stop line of all intersections, the change in the state of charge of battery will be almost zero before and after driving. This result shows three points: 1) It is not necessary to install power supply on all the roads, and only during a limited section before the intersection; 2) Amount of vehicle battery can be significantly reduced; 3) Charging at home is not required.

WIWM-2 achieved 12kW WPT output. However, it is not enough for this simulation with passenger car. The research group proposed a new optimization methodology of coil design. As the result, 20kW WPT output with 92.5 percent efficiency was realized on test bench. WPT efficiency will be more improved by control optimization.

Future image of WPT in-motion
Fig. 5: Future image of WPT in-motion
If a smart city with a WPT system with this performance installed only in a limited area in front of the intersection is realized, the user of the EV can drive without self-charging, thus convenience of EV will greatly increase (Fig. 5)

Industry-academia collaboration open innovation
This project consists of main five members: University of Tokyo, Bridgestone, NSK, ROHM and Toyo Denki MFG. WIWM-3 is a collection of diverse technologies such as power electronics, control methods, mechanical parts, tire and wheel structures, electronic components, semiconductor power devices, and materials. This project is the result of maximizing the framework of joint research by industry-academia collaboration. For social implementation of WPT in-motion, it is necessary to build a large system that includes not only vehicles but also infrastructure, and collaboration beyond the industrial field is indispensable. Therefore, University of Tokyo, Bridgestone, NSK, and Toyo Denki MFG have agreed to open a basic patent related to this project, and the intellectual property mechanism that allows companies and organizations approved by the project’s steering committee to use the rights-free technology for free. This framework can promote research and development through open innovation.

Future Prospects
This research group continues to experiment and evaluate WIWM-3 and will proceed with the proposal and prototyping of next-generation WIWM incorporating new ideas and technologies. The research group will undergo trial demonstration of the WIMW of this project in 2025, incorporating the knowledge of a wide range of organizations and companies, on top the current participating members.