Study on Energy System Conﬁguration of Wireless In-Wheel Motor with Supercapacitor

In-Wheel Motor (IWM) have been proposed to increase an efﬁciency and controllability of electric vehicle (EV). However, a risk of disconnection of power lines have been considered as a fundamental problem of IWM. Therefore, we proposed Wireless In-Wheel Motor (W-IWM) to solve this problem radically by using Wireless Power Transfer (WPT). In this paper, an advanced system of W-IWM which has multiple power sources, supercapacitor and dynamic charging circuit on its wheel side is proposed for a more effective operation. Additionally, a power-ﬂow control of the advanced system is proposed and is veriﬁed by simulations and experiments. Furthermore, efﬁciency of the proposed system is investigated.


Introduction
Electric vehicle (EV) has been gathered a great deal of public attention from the perspective of environmental performance. However, due to a limited battery capacity, EV has been only able for a short distance. To deal with this issue, a number of studies have been done on range extension of EV [1] [2].
Especially vehicle motion control using In-Wheel Motor (IWM) can increase not only a driving efficiency but also a driving safety by its high controllability [3] [4]. In-Wheel Motor is one of the propagation system for EV realizing independent torque control of each wheels. Nevertheless, IWM has not been put into practical use because of the risk of power lines disconnection. Therefore we have proposed Wireless In-Wheel Motor (W-IWM) to solve this problem radically and make IWM more practical by using Wireless Power Transfer (WPT) via magnetic resonance coupling [5].
In the purpose of a more efficient operation of W-IWM, we have proposed an advanced energy system of W-IWM having supercapacitor on its wheel side for a more effective regeneration and a circuit for dynamic charging. By applying Hybrid Energy Storage System [8] [9], we propose a novel powerflow control for W-IWM system with multiple power sources on the wheel side. The proposed powerflow control is verified by simulation and experiment. Furthermore, we measured the efficiency of the proposed system and verified the effectiveness of the proposed system. Figure 1 shows the first trial unit of W-IWM which was successfully drove the experimental vehicle shown in figure 2. The maximum power of the unit is 3.3 kW/wheel and DC to DC efficiency is about 89 %. The gap between a chassis side transmitter coil to a wheel side receiver coil is 100mm. Figure   EVS29 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium 3 shows the previous system of W-IWM which has SS topology for wireless power transfer. This system is configured symmetrically at the chassis side and the wheel side for bidirectional power transfer. Therefore it can regenerate power from the wheel side to the chassis side.

Wireless In-Wheel Motor (W-IWM)
Due to an instability of a motor driving system with WPT using SS topology, a wheel side DC-link voltage control is necessary [6]. The wheel side voltage control of W-IWM is achieved by using a hysteresis control [7]. Hysteresis control accomplishes the voltage control and a power control at the same time.
The maximum power of MITSUBISHI i-MiEV (the base vehicle of the experimental vehicle) is 47 kW. On the other hand, the experimental vehicle is expected to generate 13.2 kW with W-IWM on its all wheels. Improving the output power and efficiency are required for furthermore practicability.

Second trial unit (W-IWM2)
Considering a requirement for range extension of EV, more effective system is desired. In this paper, to achieve these goal, an advanced system of W-IWM is proposed. Figure4 shows circuit structure of the proposed system of second trial unit of W-IWM (W-IWM2). This system has Lithium ion Capacitor (LiC) connected to wheel side DC-link through DC/DC converter and dynamic charging circuit connected through AC/DC converter. Therefore wheel power can be regenerated to LiC or get power from road side transmitter.
In proposed system, because of regenerative power go through less converters compared to the previous system, DC to DC regenerative efficiency is expected to improve 89 % to 96 %. Additionally, since dynamic charging can be applied on this system, a more effective operation and range extension are expected.

Proposed power-flow control
W-IWM2 has multiple power sources on the wheel side different from W-IWM. Therefore a power-flow control on these power sources is required for a stability of motor drive.

Wheel side DC/DC converter
W-IWM stabilizes the wheel side DC-link voltage V DC using hysteresis control on the wheel side AC/DC converter. On the other hand, W-IWM2 controls V DC using a feedback control on a wheel side DC/DC converter for chassis. Herewith, the wheel side DC/DC converter restrains change of V DC and LiC compensates the power balance by changing a power of LiC P LiC automatically. Accordingly, a wheel side power-flow control can be achieved just using the feedback control of V DC on the wheel side DC/DC converter.

wheel side AC/DC converter
W-IWM2 controls a transmitting power from the chassis side to the wheel side P WPT using the wheel side AC/DC converter. Hence, P LiC can be controlled indirectly. By means of these two controls, a relational expression on the power-flow of the wheel side is completed with the voltage feedback control on the wheel side DC/DC converter.
where P L is load power of PMSM and 3 phase PWM inverter.
In proposed system, V DC is controlled by the wheel side DC/DC converter and 2 mode control [6] is utilized for controlling P WPT on the wheel side AC/DC converter. Thus, P WPT can be controlled by the wheel side only. Moreover, LiC powers the motor or be charged automatically by changing the powerflow on the wheel side. Therefore, this system can operate with multiple power sources. Furthermore, controlling P WPT indicates that power of SC P LiC can be controlled indirectly. Applying proposed control method for the system, the power-flow control is practicable. Figure 5 shows the block diagram of the power-flow control. To analysis this circuit, the state-space averaging method is applied. In this paper, because of switching of this half bridge is reciprocal, this model works in continuous current mode. Since the system includes nonlinearity, linearizion is conducted on its equilibrium point as where d(t) indicates the time of lower arm switch. By assuming the fluctuation of equilibrium point is slow compare with carrier period, each equilibrium where From formula (6), (7), the transfer function ∆P v from ∆d ′ (s) to ∆v DC (s) is, On this transfer function, we design the quadrupole of PID controller on -1000 rad/s and discretized it using tustin transfer with sampling period T s .

Power control on wheel side AC/DC converter
The wheel side AC/DC converter controls the transmitting power from the chassis side to the wheel side by 2 mode control [6]. Figure 7 shows operation modes of the wheel side AC/DC converter with 2 mode control.

Short mode
The under side switches of the wheel side AC/DC converter is turn on. Then the wheel side receiver coil shorts from the wheel side circuit as showed figure 7(a) and does not supply the transmitting power from the chassis side to the wheel side.

Rectification mode
The wheel side AC/DC converter operates as a rectifier. The wheel side receives the transmitting power from chassis to wheel.
By converting two modes mentioned above periodically, we are able to controlĪ WPT . Assuming that the wheel side coil current is a sinusoidal current with the resonant frequency, an input voltage of the wheel side AC/DC converter can be approached to its fundamental harmonic. Moreover, an approximate value of an effective current of the wheel side coil I 21 is determined as below.
where, ω 0 is resonance angular frequency, L m is mutual inductance between the chassis side and the wheel side coils and R 1 , R 2 are the chassis side, the wheel side coils resistance. An output current on each mode I dc is expressed as below. Therefore,Ī where, α means time ratio of rectification mode. Consequently, we can control an average output current of the wheel side AC/DC converterĪ WPT and the transmitting power from the chassis side via WPT P WPT by changing α. The experimental results are sampled at 50 kHz. We filtered a result of power by moving average with a window size 100 to reduce an effect of V DC ripple which causes by 2 mode control of the wheel side AC/DC converter. We applied 1 kHz primary low pass filter on the load current I L and current on supercapacitor I LiC . Furthermore, we applied 500 Hz primary low pass filter on the output averaging current of the wheel side AC/DC converterĪ WPT . (

Simulation
We performed simulations on the proposed power-flow control using MATLAB Simulink Simpower Systems. Simulation conditions are determined, considering the value of W-IWM2 which values are given in table 1. Now, the maximum output 12 kW is determined by assuming that the experimental vehicle with W-IWM2 on its all wheels achieves the equivalent output of MITSUBISHI i-MiEV 47 kW. Furthermore, the operation voltage of LiC is assumed 38.4-57.6 V and 48 V (SOC = 0.5) was used as nominal voltage on the simulation. To simplify the simulation, we replaced LiC by voltage source. The wheel side DC-link voltage V DC and a fundamental value of effective voltage of the chassis side inverter output V 11 are determined by formula (14),(15) to make transmitting efficiency of WPT between the chassis side and the wheel side maximum. Moreover, we took 8 kW for P WPT .
Where R ac is equivalent AC resistance of the wheel side and R ηopt is equivalent AC resistance of the wheel side at the maximum efficiency of WPT (α = 1). Simulation results are sampled at 20 kHz. Moving average with a window size 40 is applied to reduce an effect of V DC ripple which causes by 2 mode control of the wheel side AC/DC converter. EVS29 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium  Figure 9 shows a power-flow transition with the stepwise load change. α decided 0.75 to make P WPT 6 kW. Load changed stepwise from 12 kW powering to -6 kW regenerate. Figure 10 shows simulation results. Figure 10(a) shows power of each power sources and P LiC is controlled automatically by only applying the voltage feedback control on the wheel side DC/DC converter. Figure 10(b) shows that the voltage feedback control of the wheel side DC/DC converter stabilizes V DC before and after the load change. Note that, ripple of V DC is caused by 2 mode control. Accordingly, the power-flow control effects solely by applying the voltage feedback control on the wheel side DC/DC converter. Figure 11 shows a power-flow with P WPT change. α is changed stepwise to make P WPT from 4.0 kW to 7.0 kW. Figure 12 shows the simulation result. Figure 12(a) shows power of each power sources and indicates that P LiC is controlled indirectly by changing P WPT with 2 mode control on wheel side AC/DC converter. Figure 12 (b) shows that the voltage feedback control of wheel side DC/DC converter stabilizes V DC before and after P WPT change. Note that, ripple of V DC is caused by 2 mode control.

Power-flow transition with P WPT change
Accordingly, the power-flow control is achieved only by the voltage feedback control on the wheel side DC/DC converter and P LiC is determined automatically. Moreover, P LiC can be controlled indirectly with 2 mode control using the wheel side AC/DC converter.

Experiment
We conducted congenial experiments as simulations using the small power experimental setup. Additionally, we investigate the efficiency of the wheel side DC/DC converter. Figure 13 shows an experimental result of a power-flow transition with load change. During transmitting a constant power via WPT from chassis to wheel, load changed 1.2 W powering to -0.6 kW regeneration. Because of the wheel side DC/DC converter controls V DC , P LiC compensates the power-flow change. Figure 13(a) shows power of each wheel side power sources. By applying the voltage feedback control on the wheel side DC/DC converter, the wheel side power-flow control is achieved automatically. Figure  13 (b) shows V DC . Before and after the load change, V DC is stabilized by the voltage feedback control of the wheel side DC/DC converter. Note that, ripple of V DC is caused by 2 mode control. Figure 14 the shows an experimental result of power-flow transition with P WPT change. While the load is powering 0.6 kW, P WPT is changed from 0.3 kW to 0.6 kW stepwise using the wheel side AC/DC converter with 2 mode control. Figure 14 (b) shows that change of V DC accompanied by P WPT change. Before and after the P WPT change, V DC is stabilized by the voltage feedback control of the wheel side DC/DC converter. Note that, ripple of V DC is caused by 2mode control. Figure14 (a) shows that power of supercapacitor P LiC is controlled by P WPT indirectly.

Power-flow transition with P WPT change
Therefore, proposed power-flow control, synthesis of V DC control of DC/DC converter and 2 mode control on wheel side AC/DC converter, is verified by experiments.

Efficiency measurement on wheel side DC/DC converter
We measured DC to DC efficiency of the wheel side DC/DC converter from supercapacitor to regenerative power supply powering at 1 kW using power analyzer (PPA5530 : IWATSU). To reduce the size of the wheel side DC/DC converter, we use a small inductor and the inductance of this small inductor is 60.8 µH. Therefore we operates the wheel side DC/DC converter at a switching frequency 40 kHz to reduce a current ripple. Table2 shows experimental condition and result of the experiment. The efficiency of the wheel side DC/DC converter powering 1.0 kW was 95.2 %. Consequently, the regenerative efficiency of the proposed system is expected to improve 89 % to 95.2 %.

Conclusion
In this research, we proposed advanced energy system configuration of W-IWM. This system has supercapasitor and circuit for dynamic charging on the wheel side. The power-flow of the system is controlled by the voltage feedback control on wheel side DC/DC converter only. Moreover, output/input power of supercapacitor is controlled indirectly by 2 mode control on the wheel side AC/DC converter. Therefore, the proposed power-flow control, synthesis of the voltage feedback control on the wheel side EVS29 International Battery, Hybrid and Fuel Cell Electric Vehicle Symposium DC/DC converter and 2 mode control on the wheel side AC/DC converter are verified by simulation and experiment. Moreover we investigated the efficiency of the wheel side DC/DC converter and the efficiency was 95.2 %. Comparing that the DC to DC efficiency of regeneration using WPT is about 89 % to the efficiency using the wheel side DC/DC converter 95.2 %, the effectiveness of the advanced system was verified.