Article Auxiliary Power Supply System for Electric Power Steering (EPS) and High-Heat-Resistant Lithium-Ion † Capacitor

Takumi Mio 1,* , Yukihiro Komatsubara 1, Naoki Ohmi 1, Yusuke Kimoto 1, Kentaro Iizuka 1, Tomoki Suganuma 1, Shun Maruyama 1, Toyoki Sugiyama 2, Fumihiko Sato 2,*, Satoshi Shinoda 2, Tokuaki Hibino 2 and Koji Nishi 1 1 Power Storage Device Dev. Group, BR Electrical Power Storage Device Business Dept., JTEKT Corporation 1-7 Kitajizouyama, Noda-cho, Kariya, Aichi 448-0803, Japan; [email protected] (Y.K.); [email protected] (N.O.); [email protected] (Y.K.); [email protected] (K.I.); [email protected] (T.S.); [email protected] (S.M.); [email protected] (K.N.) 2 Power Supply System Dev. Group, BR Electrical Power Storage Device Business Dept., JTEKT Corporation 1-7 Kitajizouyama, Noda-cho, Kariya, Aichi 448-0803, Japan; [email protected] (T.S.); [email protected] (S.S.); [email protected] (T.H.) * Correspondence: [email protected] (T.M.); [email protected] (F.S.) Presented at EVS 31 & EVTeC 2018, Kobe, Japan, 1–3 October 2018 †


Received: 19 April 2019; Accepted: 18 May 2019; Published: 22 May 2019


Abstract: Advanced Driver Assistance System (ADAS) and Automated Driving (AD) are the two major topics for the current and next generations of vehicles. To realize them in full-size vehicles equipped with a 12 V power supply, the need for electric power steering (EPS) is increasing. Currently, the steering system of full-size vehicles is equipped with hydraulic power steering due to electric power shortage. An auxiliary power supply system using a lithium-ion capacitor was developed in order to solve the concern. In addition, to mount the system in the vehicle with no cooling–heating equipment, development of expanding the operating temperature range of the lithium-ion capacitor was conducted. Several improvements have made the capacitor operate stably in high-temperature environments above 100 ◦C.

Keywords: electric power steering; temporary power boost; redundant power supply; lithium-ion capacitor

1. Introduction Advanced Driver Assistance System (ADAS) and Automated Driving (AD) are the two major topics for the current and next generations of vehicles. According to the roadmap issued by the Prime Minister’s Office in Japan, Level 4 of the AD level defined by the SAE International in J30162 (September 2016) will be realized around 2025 in limited scenes such as expressways and highways [1,2]. AD functions take over the longitudinal and lateral guidance of the vehicle and must be fail-operational in case of any failure [3–5]. Therefore, it is also essential to improve functional safety in electric power steering (EPS) to protect SAE Level 3 or higher from power failure during driving [6,7]. To realize ADAS and AD functions in full-size vehicles equipped with a 12 V power supply, the need for EPS is increasing. Currently, the steering system of full-size vehicles is equipped with hydraulic power steering due to electric power shortage. In order to steer a 3.5-ton class full-size vehicle, up to about 18 kN of steering rack force is required. Assuming that the overall efficiency of the EPS, including the ECU, motor, and gears, is approximately 50%, the electric power required to output the steering force is about 2000 W. If the necessary power is supplied from a 12 V power supply,

World Electric Vehicle Journal 2019, 10, 27; doi:10.3390/wevj10020027 www.mdpi.com/journal/wevj World Electric Vehicle Journal 2019, 10, 27 2 of 13 about 170 A is required, but the current supplied to the EPS is up to 120 A, so the power demand of EPS cannot be satisfied. 12 V power supply can only drive EPS for vehicles weighing less than 3 tons

(aboutWorld 12 Electric kN rack Vehicle force). Journal 2019, 10, x FOR PEER REVIEW 2 of 13 The shortage for the electric power required by the EPS must be compensated by the driver’s steeringsupply, force. about This 170 is manifestedA is required, as but an the “EPS current torque-assist supplied delayto the phenomenon”EPS is up to 120 in A, which so the thepower steering wheeldemand suddenly of EPS becomes cannot be heavy satisfied. during 12 V abrupt power steeringsupply can or only stationary drive EPS steering. for vehicles So, inweighing order toless install EPS tothan full-size 3 tons (about vehicles 12 kN equipped rack force). with a 12 V power supply, torque-assist delay phenomenon must The shortage for the electric power required by the EPS must be compensated by the driver’s be solved. steering force. This is manifested as an “EPS torque-assist delay phenomenon” in which the steering In recent years, 48 V mild-hybrid vehicles have been launched, mainly in Europe and the United wheel suddenly becomes heavy during abrupt steering or stationary steering. So, in order to install States,EPS but to 48full V-size is avehicle drives systemequipped power with a source 12 V power only, supply and auxiliary, torque-assist electronics delay phenomenon components must are still conventionalbe solved.12 V [8]. Since EPS belongs to auxiliary electronics components, the electric power shortage cannotIn recent be years, resolved. 48 V It mild will-hybrid take a vehicleslong time have for beenthe overall launched, voltage mainly of the in Europe vehicle and to rise. the InUnited addition, States, when but 48 installing V is a drive EPS system in a full-size power source vehicle, only, it is and necessary auxiliary to electronics consider thecomponents steering force changeare atstill the conventional time of temporary 12 V [8] power. Since EPS loss. belongs There isto aauxiliary possibility electronics that driving components, cannot the be continuedelectric if EPS assistpower is shortage lost. Even cannot while be resolved. the torque-assist It will take delay a long phenomenon time for the overall occurs, voltage a part of the v steeringehicle to force necessaryrise. for steering a full-size vehicle is output from the EPS, but in the situation of a power supply In addition, when installing EPS in a full-size vehicle, it is necessary to consider the steering failure, all the torque-assist force is lost. It is impossible to satisfy the steering force only with the force change at the time of temporary power loss. There is a possibility that driving cannot be power of the driver. continued if EPS assist is lost. Even while the torque-assist delay phenomenon occurs, a part of the Asidesteering from force the necessary realization for steering of ADAS a full and-size AD,vehicle there is output are benefits from the to EPS, introducing but in the EPSsituation to full-size of vehicles.a power A typicalsupply failure point,is all that the torque it is environmentally-assist force is lost. friendly. It is impossible The conventional to satisfy the steering hydraulic force power steeringonly useswith the power driving of the force driver. of the engine to constantly power the hydraulic pump in order to generate hydraulicAside from power. the realization This creates of ADAS excess and burden AD, there on are the benefits engine andto introducing is one of theEPS reasons to full-size for high fuel consumption.vehicles. A typical The point EPS systemis that it operatesis environmentally the motor friendly. only during The conventional steering, therefore hydraulic reducing power the load onsteering the engine uses the and driving improving force of fuel the consumption engine to constantly by approximately power the hydraulic 2.5% [9]. pump in order to Consequently,generate hydraulic JTEKT power. worked This oncreates development excess burden of the on auxiliary the engine power and is supply one of systemthe reasons for EPS for with high fuel consumption. The EPS system operates the motor only during steering, therefore reducing the following features [10,11]: the load on the engine and improving fuel consumption by approximately 2.5% [9]. 1. EliminationConsequently, of the JTEKT torque-assist worked delayon development during abrupt of the steering auxiliary and power stationary supply steeringsystem for EPS 2. withPeak the cut following of the electric features power [10,11 consumption]: during abrupt steering and stationary steering 1. Elimination of the torque-assist delay during abrupt steering and stationary steering 3. Redundant power supply in case of 12 V power supply failure 2. Peak cut of the electric power consumption during abrupt steering and stationary steering 3. Redundant power supply in case of 12 V power supply failure 2. EPS Electric Power Consumption First,2. EPS FigureElectric1 Pshowsower C theonsumption result of maximum electric power consumption of various electrical componentsFirst, of Figure a vehicle. 1 shows The the electric result powerof maximum consumption electric power of the consumption EPS system of during various abrupt electrical steering is largercomponents than that of a of vehicle. the air The conditioner electric power and consumption headlights. of the On EPS the system other during hand, abrupt the electric steering power consumptionis larger thanof the that EPS of during the air straight-line conditioner and driving headlight is onlys. On 3% the of stationaryother hand, steering. the electric power consumption of the EPS during straight-line driving is only 3% of stationary steering. 1.0

0.8

0.6

0.4 (Normalization)

Power consumption 0.2 0.03 0.0

FigureFigure 1. Measuring 1. Measuring result result of maximum of maximum electric electric power consumption power consumption of various of electricalvarious electrical components of vehicle.components of vehicle. World Electric Vehicle Journal 2019, 10, 27 3 of 13 World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 3 of 13 World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 3 of 13 Next,Next, FigureFigure2 2 shows shows the the power power consumption consumption during during abrupt abrupt steering steering of of a a full-size full-size vehicle. vehicle. The The Next, Figure 2 shows the power consumption during abrupt steering of a full-size vehicle. The 1212 VV powerpower supplysupply temporarilytemporarily cannotcannot satisfysatisfy thethe EPSEPS electricelectric powerpower demanddemand (A,(A, B).B). InIn other words, 12 V power supply temporarily cannot satisfy the EPS electric power demand (A, B). In other words, toto installinstall EPSEPS inin aa full-sizefull-size vehicle,vehicle, itit isis onlyonly necessarynecessary toto eliminateeliminate thethe severalseveral secondsseconds ofof powerpower to install EPS in a full-size vehicle, it is only necessary to eliminate the several seconds of power shortageshortage thatthat occursoccurs duringduring abruptabrupt steering.steering. The reinforcementreinforcement of thethe wholewhole 1212 VV powerpower supplysupply isis shortage that occurs during abrupt steering. The reinforcement of the whole 12 V power supply is notnot anan optimaloptimal solution.solution. ForFor thethe aboveabove reasons,reasons, developmentdevelopment ofof anan auxiliaryauxiliary powerpower supplysupply systemsystem not an optimal solution. For the above reasons, development of an auxiliary power supply system hashas started.started. has started. 1000 1000 Power shortage Power shortage

during 12V power supply limit

during 12V power supply limit A

A BB

abrupt steering abrupt

abrupt steering abrupt

EPS power demand power EPS EPS power demand power EPS 0 0 0 Time 1000 0 Time 1000 FigureFigure 2.2. ElectricElectric powerpower consumptionconsumption ofof EPSEPS duringduring abruptabrupt steering.steering. Figure 2. Electric power consumption of EPS during abrupt steering. 3. The Configuration of Auxiliary Power Supply System for EPS 3. The Configuration of Auxiliary Power Supply System for EPS 3. The Configuration of Auxiliary Power Supply System for EPS 3.1. System Summary 3.1. System Summary 3.1. FigureSystem3 Summary shows the configuration of the auxiliary power supply system for EPS. The system Figure 3 shows the configuration of the auxiliary power supply system for EPS. The system consistsFigure of a charge–discharge3 shows the configuration controller of and the electrical auxiliary power power storage supply device. system It isfor installed EPS. The between system consists of a charge–discharge controller and electrical power storage device. It is installed between theconsist 12 Vs power of a charge supply–discharge and the EPS. controller The lithium-ion and electrical capacitor power was storage adopted device as a. powerIt is installed storage between device the 12 V power supply and the EPS. The lithium-ion capacitor was adopted as a power storage giventhe 12 its V rapid power charge–discharge supply and the characteristics, EPS. The lithium cycle-ion life, capacitor safeness, was operating adopted voltage, as a power and volume storage device given its rapid charge–discharge characteristics, cycle life, safeness, operating voltage, and energydevice density. given its rapid charge–discharge characteristics, cycle life, safeness, operating voltage, and volume energy density. volume energy density. Vehicle Developed system EPS Vehicle Developed system EPS Alternator Capacitor module Alternator Capacitor module

12V Charge-Discharge EPS 12V Charge-Discharge EPS Motor Battery controller controller Motor Battery controller controller

Figure 3. Configuration of auxiliary power supply system for EPS. Figure 3. Configuration of auxiliary power supply system for EPS. 3.2. Block Diagram andFigure Control 3. Method,Configuration Design of ofauxiliary Capacitance power of supply Capacitor system for EPS. 3.2. Block Diagram and Control Method, Design of Capacitance of Capacitor 3.2. FigureBlock D4 iagramshows and a block Control diagram Method of, theDesign charge–discharge of Capacitance of system. Capacitor The system possesses a charge circuitFigure [D] and 4 shows a discharge a block circuitdiagram [E]. of Thethe charge central–discharge processing system. unit (CPU) The system of the possesses charge–discharge a charge Figure 4 shows a block diagram of the charge–discharge system. The system possesses a charge controllercircuit [D] monitors and a discharge the 12 V powercircuit supply[E]. The and central EPS. processing The CPU controls unit (CPU) the discharge of the charge circuit–discharge to boost circuit [D] and a discharge circuit [E]. The central processing unit (CPU) of the charge–discharge thecontroller power supplymonitors voltage the 12 by V connectingpower supply the lithium-ionand EPS. The capacitor CPU controls in series the with discharge the 12 V circuit power to supply boost controller monitors the 12 V power supply and EPS. The CPU controls the discharge circuit to boost (x)the when power the supply EPS requires voltage energy by connecting that is beyond the lithium the upper-ion limit capacitor of the in 12 series V power with supply. the 12 When V power the the power supply voltage by connecting the lithium-ion capacitor in series with the 12 V power lithium-ionsupply (x) when capacitor the EP dischargesS requires and energy the cellthat voltage is beyond drops, the theupper CPU limit controls of the the 12 chargeV power circuit supply to. supply (x) when the EPS requires energy that is beyond the upper limit of the 12 V power supply. rechargeWhen the the lithium lithium-ion-ion capacitor capacitor discharges from the12 and V powerthe cell supply voltage (y). drops, the CPU controls the charge When the lithium-ion capacitor discharges and the cell voltage drops, the CPU controls the charge circuitAs to the recharge number the of lithium lithium-ion-ion capacitor capacitors from connected the 12 V inpower series supply is increased, (y). the system output circuit to recharge the lithium-ion capacitor from the 12 V power supply (y). increases.As the Generally, number a of 12 l Vithium electronic-ion capacitor circuit boards connected has a performance in series is of increased, 24 V withstand the system voltage. output Even As the number of lithium-ion capacitors connected in series is increased, the system output ifincreases. the capacitor Generally, is connected a 12 V in electronic series to thecircuit 12 V board power has supply a performance and boosted, of no24 problemV withstand occurs voltage. if the increases. Generally, a 12 V electronic circuit board has a performance of 24 V withstand voltage. voltageEven if is the up capacitor to about 20 is V. connected in series to the 12 V power supply and boosted, no problem Even if the capacitor is connected in series to the 12 V power supply and boosted, no problem occurs if the voltage is up to about 20 V. occurs if the voltage is up to about 20 V. World Electric Vehicle Journal 2019, 10, 27 4 of 13 World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 4 of 13 World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 4 of 13

FurthermoreFurthermore,, because the l lithium-ionithium-ion capacitor has an average voltage of 3 V/cell, V/cell, a 12V power Furthermore, because the lithium-ion capacitor has an average voltage of 3 V/cell, a 12V power supply can be obtained by connecting four capacitors in series.series. In this configuration, configuration, it is suitable as supply can be obtained by connecting four capacitors in series. In this configuration, it is suitable as a backup power supply during a 12 V powerpower supplysupply failure. It contributes to the realization of AD a backup power supply during a 12 V power supply failure. It contributes to the realization of AD Level 4 that requires complete failure operationoperation ofof EPSEPS includingincluding powerpower supplysupply failurefailure [[12,1312,13].]. Level 4 that requires complete failure operation of EPS including power supply failure [12,13]. Lithium-ion [C] Lithiumcapacitor-ion [C] capacitor Charge Discharge [D]Charge [F] Discharge[E] [D]Charge [F] Discharge[E] [A] Charge y CPU x Discharge [B] [A] circuit y CPU x circuit [B] 12V circuit circuit EPS 12V EPS power supply Charge-discharge ECU system power supply Charge-discharge ECU system

Figure 4. ChargeCharge–discharge–discharge system for EPS block diagram diagram.. Figure 4. Charge–discharge system for EPS block diagram. Detailed circuitcircuit operationoperation is is shown shown in in Figure Figure5. If5. the If the power power consumption consumption of the of EPS the isEPS low, is suchlow, assuch forDetailed as straight-line for straight circuit driving,-line operation driving, the FETsis the shown 1FETs and in 1 2 and areFigureclosed 2 are 5. closedIf and the the powerand EPS theis consumption EPS operated is operated only of by theonly the EPS 12Vby theis power low, 12V suchsupply.power as for supply. When straight the When- EPSline the driving, requires EPS requiresthe a large FETs amount a1 and large 2 ofare amount power, closed of suchand power, the as forEPS such abrupt is operated as for steering abrupt only or by steering stationary the 12V or powersteering,stationary supply. FETs steering, 1 When and FETs 3 the are 1 EPS closed,and requires3 are and closed, the a lithium-ionlarge and amountthe lithium capacitor of - power,ion capacitor is connected such asis confor innected seriesabrupt within steering series the with 12 or V stationarypowerthe 12 V supply. power steering, After supply. FETs the systemAfter1 and the3 operatesare system closed, and operate and the the capacitors andlithium the voltage- capacitorion capacitor drops, voltage is the con lithium-iondrops,nected the in serieslithium capacitor with-ion is therechargedcapacitor 12 V power is using recharged supply. FETs 4 using andAfter 5 FETs andthe system the4 and coil. operate5 Rechargingand thes and coil. startsthe Recharging capacitor when the voltagestarts electric when drops, power the the demandelectric lithium power of-ion the capacitorEPSdemand falls of belowis therecharged EPS the 12falls Vusing below power FETs the supply 124 and V limit. power 5 and If supply a the power coil. limit. supply Recharging If a failure power starts occurs, supply when the failure EPSthe isoccurs,electric driven thepower by EPS the demandelectricis driven powerof bythe theEPS of the electricfalls lithium-ion below power the capacitor12 of V the power lithium by supply closing-ion limit. capacitor FETs If 3 a and power by 5. closing Also, supply if FETs somethingfailure 3 occurs, andgoes 5. theAlso wrong EPS, if iswithsomething driven the circuit, by goes the FETwrong electric 6 is with closed power the andcircuit, of the the EPSFET lithium is6 is driven closed-ion with capacitor and the the bypass EPS by closingis circuit.driven FETs with 3the and bypass 5. Also circuit., if something goes wrong with the circuit, FET 6 is closed and the EPS is driven with the bypass circuit. [Developed system] [Developed system] Charging Discharging Chargingcircuit (+6V) Dischargingcircuit

circuit (+6V) circuit

4 High High load

4 +

FET3 FET

High High load + FET3 FET Lithium- Lithiumion -

5 Capacitorion

5 Capacitor FET2

FET1 FET FET2 FET1 FET 12V 12V 12V Backup FET6 12V Backup Normal power FET6 Normal EPS powersupply EPS supply System failure System failure

Figure 5. Circuit operation of charge/discharge controller. Figure 5. Circuit operation of charge/discharge controller. Figure 5. Circuit operation of charge/discharge controller. The energy amount design of the lithium-ion capacitor required by the system is explained using the powerThe energy waveform amount when design performing of the a l continuousithium-ion stationary capacitor steering required test by on the a 3.5-tonsystem class is explained full-size The energy amount design of the lithium-ion capacitor required by the system is explained vehicleusing the shown power in waveform Figure6. when performing a continuous stationary steering test on a 3.5-ton class usingfull-size the vehicle power shownwaveform in Figure when 6performing. a continuous stationary steering test on a 3.5-ton class full-size vehicle shown in Figure 6. World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 5 of 13 WorldWorld ElectricElectric VehicleVehicle JournalJournal2019 2019,,10 10,, 27x FOR PEER REVIEW 55 ofof 1313

Required electric power Required electric power Capacitor (Approximately 2,000W) (Approximately 2,000W) Capacitordischarging Capacitor discharging Capacitorrecharging recharging 12V power supply limit 12V(Approximately power supply 1,400W) limit

(Approximately 1,400W)

consumption of EPS, W EPS, of consumption consumption of EPS, W EPS, of consumption Approximate electric power power electric Approximate 0 5 10 Approximate electric power power electric Approximate 0 5 10 Stationary steering time, sec Stationary steering time, sec

FigureFigure 6. Electric 6. Electric power power waveform waveform during during continuous continuous stationary stationary steering steering test of 3.5-ton test of class 3.5- full-sizeton class Figure 6. Electric power waveform during continuous stationary steering test of 3.5-ton class vehiclefull-size (the vehicle steering (the speed steering is 600 speed◦/s). is 600 °/s). full-size vehicle (the steering speed is 600 °/s). AsAs described described in in the the introduction, introduction, the the 12 V12 power V power supply supply limit limit is approximately is approximately 1400 1400 W, and W, theand As described in the introduction, the 12 V power supply limit is approximately 1400 W, and powerthe power required required for 3.5-ton for 3.5 class-ton stationary class stationary is approximately is approximately 2000 W. As 2000 the W. steering As the angle steering increases, angle the power required for 3.5-ton class stationary is approximately 2000 W. As the steering angle theincreases, electric powerthe electric demand power of EPS demand increases. of EPS In increases. the power In waveform, the power the waveform, integrated the value integrated of a region value increases, the electric power demand of EPS increases. In the power waveform, the integrated value (redof a area) region in which(red area) the electricin which power the electric demand power of EPS demand exceeds of theEPS 12 exceeds V power the supply12 V power limit issupply the of a region (red area) in which the electric power demand of EPS exceeds the 12 V power supply requiredlimit is energythe required amount energy of the amount lithium-ion of the capacitor lithium-ion per capacitor 1 time of per stationary 1 time of steering. stationary Assuming steering. limit is the required energy amount of the lithium-ion capacitor per 1 time of stationary steering. thatAssuming the power that shortage the power region shortage (red area)region is (red approximated area) is approximated as a triangle as and a triangle the power and assist the power time Assuming that the power shortage region (red area) is approximated as a triangle and the power isassist approximately time is approximately 1 s, the required 1 s, the energy required amount energy of the amount lithium-ion of the capacitorlithium-ion is aboutcapacitor 300 is J. about The assist time is approximately 1 s, the required energy amount of the lithium-ion capacitor is about required300 J. The energy required amount energy of the amount system of isthe obtained system byis obtained multiplying by multiplying the number the of times number to guarantee of times to 300 J. The required energy amount of the system is obtained by multiplying the number of times to theguarantee stationary the steering, stationary and steering as shown, and in theas shown blue area in ofthe Figure blue 6area, it can of beFigure reduced 6, it bycan controlling be reduced to by guarantee the stationary steering, and as shown in the blue area of Figure 6, it can be reduced by rechargecontrolling as soon to recharge as the power as soon demand as the power of EPS demand falls below of EPS the 12falls V powerbelow the supply 12 V limit. power supply limit. controlling to recharge as soon as the power demand of EPS falls below the 12 V power supply limit. TheThe system system can can be be built built with with a a very very small small amount amount of of energy energy and and number number of of cells, cells, but but since since it it is is The system can be built with a very small amount of energy and number of cells, but since it is necessarynecessary to to supply supply a a large large current current of of up up to to 120 120 A, A, the the system system can can only only be be designed designed with with capacitors. capacitors. necessary to supply a large current of up to 120 A, the system can only be designed with capacitors. WhenWhen building building the the system system with with a battery, a battery, it is necessary it is necessary to have to a very have large a very capacity, large mostcapacity, of which most is of When building the system with a battery, it is necessary to have a very large capacity, most of notwhich used. is not used. which is not used. 4. Mechanism of High Power Output 4. Mechanism of High Power Output 4. Mechanism of High Power Output The torque-assist delay phenomenon that occurs at the time of abrupt steering in EPS using The torque-assist delay phenomenon that occurs at the time of abrupt steering in EPS using brushlessThe torque DC motor-assist is delay described phenomenon below with that referenceoccurs at tothe the time motor of abrupt rotation steering speed in and EPS torque using brushless DC motor is described below with reference to the motor rotation speed and torque characteristicbrushless DC diagram motor is (N–T described characteristic below diagram)with reference shown to in theFigure motor7. rotation speed and torque characteristic diagram (N–T characteristic diagram) shown in Figure 7. characteristic diagram (N–T characteristic diagram) shown in Figure 7. 12V power supply limit 12V power supply limit

Full-size vehicle Full-size vehicle Voltage boosting X B C Voltage= Motor boosting rotation speed increase X B C = Motor rotation speed increase Medium-size vehicle

Medium-size vehicle AA

Motor torque Motor Motor torque Motor (Steering rack force) rack (Steering Y (Steering rack force) rack (Steering Y Motor rotation speed (TorqueMotor assist rotation responsiveness) speed (Torque assist responsiveness)

Figure 7. Motor rotation speed and torque characteristic diagram (N–T characteristic diagram of brushlessFigure 7. DC Motor motor). rotation speed and torque characteristic diagram (N–T characteristic diagram of Figure 7. Motor rotation speed and torque characteristic diagram (N–T characteristic diagram of brushless DC motor). brushless DC motor). World Electric Vehicle Journal 2019, 10, 27 6 of 13

WorldThe Electric N–T Vehicle characteristic Journal 2019, when 10, x FOR the PEER torque REVIEW command value is constant is indicated by the solid6 line of 13 in Figure7. The dashed line is the upper limit of the electric power that can be supplied from the 12 V powerThe supply. N–T characteristic when the torque command value is constant is indicated by the solid line in FigureWhen 7 the. The motor dashed is rotated line is the with upper thetorque limit of command the electric value power set that to a can certain be supplied value, the from rotation the 12 speedV power at the supply point. (rated point A) intersecting the dashed line of electric power is the maximum rotation speedWhen at which the themotor commanded is rotated torque with the can torque be outputted. command value set to a certain value, the rotation speedIf the at commandthe point torque(rated point is increased A) inter to outputsecting thethe steering dashed rackline forceof electric required power for a is full-size the maximum vehicle, therotation rated pointspeed moves at which on thethe brokencommanded line of torque electric can power be outputted. from rated point A to rated point B as shown in FigureIf the7 “X”. command The maximum torque is value increased of motor to output rotation the speed steering that can rack output force required the command for a torquefull-size decreasesvehicle, the as shownrated point in Figure moves7 “Y”. on the broken line of electric power from rated point A to rated point B asThe shown output in torque Figure decreases 7 “X”. The in the maximum region above value this of rotation motor speed. rotation Since speed the steeringthat can force output of the the drivercommand increases torque as decreases EPS torque as assist shown decreases, in Figure abrupt 7 “Y”. steering causes the steering wheel to become heavy,The resulting output in torque “torque-assist decreases delay”. in the region above this rotation speed. Since the steering force of the When driver the increases capacitor as EPS voltage torque is added assist decreases, to the 12 V abrupt power steering supply causes using the steeringauxiliary wheel power to supplybecome system, heavy, theresulting rated pointin “torque moves-assist from delay B to C,”. and motor rotation speed can be increased while maintainingWhen the commandcapacitor voltage torque. is This added is the to same the 12 as V EPS power assist supply torque using reinforcement the auxiliary under power the abrupt supply steeringsystem, situation. the rated point moves from B to C, and motor rotation speed can be increased while maintaining the command torque. This is the same as EPS assist torque reinforcement under the 5.abrupt Actual steering Vehicle situation. Evaluation

Testing5. Actual Method Vehicle Evaluation The steering system of a full-size SUV (car weight 3.5-ton class) equipped with hydraulic power steering5.1 Testing was M replacedethod with rack assist type EPS equipped with a 600 W brushless DC motor. It could outputThe the steering steering system rack force of a upfull to-size 18 kN.SUV The (car torque weight assist 3.5-ton responsiveness class) equipped of EPS with to hydraulic steering speedpower wassteering evaluated was replaced by following with tworack patterns. assist type The EPS results equipped are shown with a in 600 Figure W brushless8. DC motor. It could output the steering rack force up to 18 kN. The torque assist responsiveness of EPS to steering speed 1. Steer with only 12 V power supply. was evaluated by following two patterns. The results are shown in Figure 8. 2. Steer with 12 V power supply + developed system 1. Steer with only 12 V power supply. (lithium-ion capacitor 2 cells; voltage boost + 6 V). 2. Steer with 12 V power supply + developed system (lithium-ion capacitor 2 cells; voltage boost + 6 V).

1 2V power supply only 1 2V power supply

+Developed system

Heavy

Steering wheel Steering Light Slow Fast Steering speed

FigureFigure 8. 8.Comparison Comparison of of torque torque assist assist responsiveness. responsiveness.

TheThe 12 12 V powerV power supply supply alone couldalone not could meet not the meetEPS power the EPS demand. power This demand caused. a This “torque-assist caused a delay”“torque that-assist made delay” the steering that made wheel the heavysteering during wheel high-speed heavy during steering. high- Onspeed the steering. other hand, On thethe 12other V powerhand, supplythe 12 V with power the developedsupply with system the developed experienced system no torque-assistexperienced no delay; torque the-assist electric delay power; the shortageelectric power of the shortage 12 V power of the supply 12 V power was compensated supply was compensated by the electric by powerthe electric supplied power from supplied the lithium-ionfrom the lithium capacitor.-ion capacitor. Lowering the power-assist threshold from the lithium-ion capacitor reduces the load on the 12 V power supply required to operate the EPS. This “peak cut function” is useful to prevent battery overload degradation in situations where some electrical components require large power demands. World Electric Vehicle Journal 2019, 10, 27 7 of 13

Lowering the power-assist threshold from the lithium-ion capacitor reduces the load on the 12 V power supply required to operate the EPS. This “peak cut function” is useful to prevent battery overloadWorld Electric degradation Vehicle Journal in 2019 situations, 10, x FOR where PEER REVIEW some electrical components require large power demands.7 of 13 Next, we compared the steering force change with the 12 V power supply shut down while driving Next, we compared the steering force change with the 12 V power supply shut down while with and without the development system equipped with four series-connected 500 F lithium-ion driving with and without the development system equipped with four series-connected 500 F capacitors (total energy amount was 9600 J). The results are shown in Figure9. lithium-ion capacitors (total energy amount was 9600 J). The results are shown in Figure 9. 20 20

▼ Power shut down ▼ Power shut down

[V]

[V] System System voltage 0 System voltage 0

20 20

torque

torque

[Nm] [Nm]

0 0

Steering Steering

160 160

angle

angle

[deg]

[deg] Steering Steering 100 100 0 Time [sec] 30 0 Time [sec] 30 1 2V power supply only 1 2V power supply + developed system

Figure 9. Comparison of steering force change when the 12 V power supply is shut down. Figure 9. Comparison of steering force change when the 12 V power supply is shut down. In a situation where the EPS was operated by a 12 V power supply alone, shutting down the 12 VIn vehicle a situation power where significantly the EPS was changed operated the steeringby a 12 V force power and supply driving alone, could shutting not be down continued. the 12 InV avehicle situation power where significantly the EPS was change drivend the by steering a 12 V power force and supply driving and thecould developed not be continued. system, even In a withsituation the 12 where V power the supplyEPS was shut driven down, by the a 12 steering V power force supply was stable and the enough developed that the system, driver even could with not detectthe 12it. V power supply shut down, the steering force was stable enough that the driver could not detectThe it. amount of time the developed system can sustain EPS power varies depending on the steering situation.The amount In a steady of time circular the turning developed with system a radius can of sustain 10 m, it EPS was power driven varies for 3 min depending or more on with the foursteering lithium-ion situation capacitors. In a steady having circular a capacitance turning with of a 500 radius F. The of reason10 m, it is was that, driven as shown for 3 inmin Figure or more 10, thewith backup four lithium power-ion supply capacitors voltage havin cang maintain a capacitance 12 V of or 500 more. F. The This reason test resultis that, is as considered shown in Figure to be enough10, the backup time for power the vehicle supply to stopvoltage safely can on maintain the roadside 12 V or after more. the This driver test detects result a is power considered failure. to be enoughIn addition, time for inthe order vehicle to make to stop it easier safely for on the the driver roadside to notice after athe shutdown driver detects of the a 12 power V power failure. supply, an EPSIn controladdition, was in developed order to make to gradually it easier reduce for the the driver steering to notice assist force.a shutdown This system of the is 12 suitable V power for autonomoussupply, an EPS vehicle control required was developed redundancy to ofgradually a power reduce supply the system steering and isassist a highly force. safe This system. system is suitableTo mount for autonomous the developed vehicle system required with noredundancy cooling–heating of a power device supply in a vehicle, system the and improvement is a highly safe of thesystem. operating temperature range of the lithium-ion capacitor was essential. Conventional capacitors have only a temperature16 range of about 20 to 60 C and cannot withstand the temperature240 requirement − ◦ of the passenger compartment of a vehicle ( 40 to 85 ◦C). Since the addition of a cooling–heating 15 − 200 device makes the system larger, heavier, less efficient, and expensive, the developed system could hardly be installed to14 a vehicle. As a result, JTEKT improved the operating temperature160 range of the lithium-ion capacitor. 13 120

12 80 Steering Steering angle[deg]

Capacitor Capacitor voltage[V] Capacitor voltage 11 40 Steering angle

10 0 0 1 2 3 time[min]

World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 7 of 13

Next, we compared the steering force change with the 12 V power supply shut down while driving with and without the development system equipped with four series-connected 500 F lithium-ion capacitors (total energy amount was 9600 J). The results are shown in Figure 9. 20 20

▼ Power shut down ▼ Power shut down

[V]

[V] System System voltage 0 System voltage 0

20 20

torque

torque

[Nm] [Nm]

0 0

Steering Steering

160 160

angle

angle

[deg]

[deg] Steering Steering 100 100 0 Time [sec] 30 0 Time [sec] 30 1 2V power supply only 1 2V power supply + developed system

Figure 9. Comparison of steering force change when the 12 V power supply is shut down.

In a situation where the EPS was operated by a 12 V power supply alone, shutting down the 12 V vehicle power significantly changed the steering force and driving could not be continued. In a situation where the EPS was driven by a 12 V power supply and the developed system, even with the 12 V power supply shut down, the steering force was stable enough that the driver could not detect it. The amount of time the developed system can sustain EPS power varies depending on the steering situation. In a steady circular turning with a radius of 10 m, it was driven for 3 min or more with four lithium-ion capacitors having a capacitance of 500 F. The reason is that, as shown in Figure 10, the backup power supply voltage can maintain 12 V or more. This test result is considered to be enough time for the vehicle to stop safely on the roadside after the driver detects a power failure. In addition, in order to make it easier for the driver to notice a shutdown of the 12 V power supply, an EPS control was developed to gradually reduce the steering assist force. This system is Worldsuitable Electric for Vehicle autonomous Journal 2019 vehicle, 10, 27 required redundancy of a power supply system and is a highly8 ofsafe 13 system.

16 240

15 200

14 160

13 120

12 80 Steering Steering angle[deg]

Capacitor Capacitor voltage[V] Capacitor voltage 11 40 Steering angle

10 0 0 1 2 3 time[min]

Figure 10. Capacitor voltage transition during backup power supply operation with steady circular turning (radius of 10 m).

6. Adaptation of Electrical Power Storage Device in Vehicle Environment

6.1. Lithium-Ion Capacitor The structure combines the positive electrode of an electric double layer capacitor, and the negative electrode and electrolyte solution of a lithium-ion secondary battery [14,15]. It is an electricity storage device with improved volumetric energy density that maintains the advantages of an electric double layer capacitor capable of instantaneously supplying and regenerating large amounts of power. It is expected to be used in various industries as well as automotive [16].

6.2. Development of High-Heat-Resistant Lithium-Ion Capacitor Through research and testing, the phenomenon that occurs at temperatures higher and lower than the operating temperature range of conventional products was identified, and how that contributes to the deterioration of the lithium-ion capacitor performance. With high temperature, there was an increase of internal resistance and capacitance reduction proceeded irreversibly with the thermal decomposition of the electrolyte solution. The generation of decomposition gas causes cell expansion and eventually the cell ruptures. It has been found that the decomposition of electrolyte solution is due to the thermal decomposition of lithium hexafluorophosphate (LiPF6), which is a popular electrolyte used in lithium-ion capacitors and lithium-ion batteries. With low temperature, output reduction with increase of internal resistance occurs reversibly. When the temperature was lowered to 20 C or less, the electrolyte solution was frozen and the − ◦ capacitor could not be charged/discharged. The negative electrode of a lithium-ion capacitor utilizes a chemical reaction during charge/discharge. The chemical reaction rate depends on temperature so an output reduction occurs on the low-temperature side. From the above results, it was found that primarily an improvement of the electrolyte solution was necessary in order to extend the operating temperature range of the lithium-ion capacitor.

7. Implementation Items

Table1 shows implementation items. The electrolyte salt LiPF 6, which caused thermal decomposition of the electrolyte at high temperatures, was changed to high-heat-resistant lithium compound salt (imide-based lithium-compound salt). Next, the composition ratio of the electrolyte solvent was reviewed and changed to a unique carbonate-based composition that does not freeze at 40 C or boil at 100 C. Furthermore, in order to make the material perform properly, we adopted our − ◦ ◦ proprietary method of selecting a capacitor material based on certain chemical rules. World Electric Vehicle Journal 2019, 10, 27 9 of 13

Table 1. Technical challenge and implementation items.

Technical Challenge Implementation Items Prevention of electrolyte solution Adoption of high-heat resistance decomposition electrolyte salt Heat-resistance Prevention of electrolyte solution Adoption of high boiling-point Improvement boiling organic solvent JTEKT proprietary method Adoption of low freezing point Prevention of electrolyte solution organic solvent freezing Optimization of organic solvent Improvement of low mixing ratio temperature output Prevention of internal resistance Affecter identification increase Changing of positive and negative materials

8. Test Specimen and Testing Method

8.1. Test Specimen A sheet of activated carbon coated on aluminum foil was used as a positive electrode. A sheet of graphite coated on copper foil was used as a negative electrode. The positive electrode and the negative electrode were alternately stacked via a separator. Three types of lithium-ion capacitors (capacitance 500 F, operating voltage 2.2 to 3.8 V) shown below were prepared.

1. Conventional capacitor using electrolyte solution in which 1.0 mol/L of LiPF6 was dissolved (ethylene carbonate (EC):ethyl methyl carbonate (EMC):dimethyl carbonate (DMC) = 3:4:3) 2. Change only electrolyte solution 3. Change electrolyte solution and JTEKT proprietary method

8.2. Heat Resistance Evaluation The high-temperature endurance test of lithium-ion capacitors was based on “High temperature continuous rated voltage application test” in IEC 62813-2015: Test method for lithium ion capacitors for electrical and electronic devices—Electrical characteristics. A 3.8 V float charge test was conducted with an ambient temperature of 85 ◦C. The standard requires that the internal resistance increase rate after the float-charging test 1000 h is 50% or less, and the capacitance reduction ratio is required to be 20% or less.

8.3. Evaluation of Large Current Charge–Discharge Properties The resistance to high current charge–discharge of the developed capacitor was investigated. The performance deterioration of the capacitor was confirmed by charge–discharge cycle test in which the maximum temperature was changed from 85 to 110 ◦C in steps of 5 ◦C. The upper voltage of the capacitor was changed stepwise from 3.6 to 3.8 V. The charge–discharge rate was 900 C, CC/CV mode, and 10,000 cycles were performed. After the cycle test, internal resistance and discharge capacity were measured according to IEC 62813-2015, and test threshold was set to performance degradation of approximately 10% or more.

8.4. Low-Temperature Properties Evaluation The internal resistance rise multiple in a low-temperature environment was investigated based on internal resistance at 25 ◦C. The measurement method conforms to IEC 62813-2015. World Electric Vehicle Journal 2019, 10, 27 10 of 13

The charge–discharge cycle performance at 40 ◦C of the developed capacitor was also investigated. World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW− 10 of 13 Capacitor voltage range was 2.2–3.8 V, CC/CV charge–discharge mode, and charge–discharge rate was 85returned C. Before to 25checking °C . Internal the capacitor resistance performance, and discharge the capacitorcapacity were temperature confirmed was in returned accordance to 25 with◦C. InternalIEC 6281 resistance3-2015. and discharge capacity were confirmed in accordance with IEC 62813-2015. 9. Results 9. Results 9.1. Heat Resistance Evaluation 9.1. Heat Resistance Evaluation The results are shown in Figure 11. In the conventional capacitors, deterioration of both the capacityThe retention results are rate shown and internal in Figure resistance 11. In the increase conventional rate were capacitors, remarkable. deterioration The required of both value the wascapacity lowered retention about rate 100 and h after internal the startresistance of the increase test. The rate test were was remarkable interrupted. The due required to remarkable value was cell expansionlowered about from the100 decompositionh after the start of the of the electrolyte test. The into test gas was triggered interrupted by the due thermal to remarkable decomposition cell expansion from the decomposition of the electrolyte into gas triggered by the thermal of LiPF6 [17]. decompositionThe capacitor of LiPF with6 [17]. only the electrolyte liquid was improved in capacity retention rate and eliminatedThe capacitor the cell expansion. with only However, the electrolyte the internal liquid resistance was improved increase in rate capacity was not retention improved. rate and eliminatedThe capacitor the cell expansion with electrolyte. However, change the andinternal JTEKT resistance proprietary increase method rate was continued not improved. to meet the requiredThe values capacitor of capacity with electrolyte retention change rate and and internal JTEKT resistanceproprietary increase method rate continue even afterd to meetthe float the chargerequired test values was over of capacity the 1000 retention h specified rate in and the standard.internal resistance [18] increase rate even after the float charge test was over the 1000 h specified in the standard. [18]

120% 2. Change only electrolyte solution 100%

100% GOOD 80%

3. Electrolyte solution change GOOD + JTEKT proprietary method 80% 60% Threshold of IEC 62813-2015

Threshold of IEC 62813-2015 60% 40%

1. Conventional capacitor capacity capacity retention rate

40% 20% Internal Internal resistance increase rate

20% 0% 0 200 400 600 800 1,000 1,200 1,400 1,600 0 200 400 600 800 1,000 1,200 1,400 1,600 85 degree C, 3.8V float charging time [h] 85 degree C, 3.8V float charging time [h] Figure 11. Test result of floatfloat charging atat 8585 ◦°CC (according(according toto IEC-62813-2015).IEC-62813-2015).

FromFrom thethe aboveabove results, in order to improve the heat resistance of the lithium-ionlithium-ion capacitor, it is necessarynecessary to devise not only the improvement of heat resistance of constituent materialsmaterials butbut alsoalso thethe selectionselection ofof materialmaterial forfor susufficientlyfficiently exhibitingexhibiting thethe performanceperformance ofof eacheach material.material. 9.2. Evaluation of Large Current Charge–Discharge Properties 9.2. Evaluation of Large Current Charge–Discharge Properties The results are shown in Tables2 and3. By limiting upper voltage of the capacitor from 3.8 V to The results are shown in Tables 2 and 3. By limiting upper voltage of the capacitor from 3.8 V to 3.6 V,the capacitor performance hardly changed even at 100 C. It should be noted that the improvement 3.6 V, the capacitor performance hardly changed even◦ at 100 °C . It should be noted that the of heat resistance of the capacitor also contributes to suppression of deterioration due to self-heating improvement of heat resistance of the capacitor also contributes to suppression of deterioration due when large current charge–discharge is repeated. to self-heating when large current charge–discharge is repeated.

World Electric Vehicle Journal 2019, 10, 27 11 of 13 World Electric Vehicle Journal 2019, 10, x FOR PEER REVIEW 11 of 13

Table 2. Internal resistance increase behavior. Upper Limit Voltage of Capacitor, V Table 2. Internal resistance increase behavior. 3.60 3.65 3.70 3.75 3.80 Upper Limit Voltage of Capacitor, 110 24.6% - - - - V Test atmospheric 105 7.1% 5.7% - - - temperature, 3.60 3.65 3.70 3.75 3.80 100 3.1% 1.8% 6.8% - - degree C 110 24.6% - - - - 95105 0.3% 7.1% 2.2%5.7% 3.7%- - 5.1%- - Test atmospheric 90100 0.7% 3.1% 0.2%1.8% 3.0%6.8% - 2.1%- - temperature, − 95 0.3% 2.2% 3.7% 5.1% - degree C85 0% 2.1% 3.5% 3.5% 7.7% Measuring90 method:0.7% IEC− 62813-2015.0.2% 3.0% 2.1% - 85 0% 2.1% 3.5% 3.5% 7.7% Measuring method: IEC 62813-2015. Table 3. Capacitance reduction behavior.

Table 3. CapacitanceUpper reduction Limit Voltage behavior of. Capacitor, V 3.60Upper 3.65 Limit Voltage 3.70 of Capacitor, 3.75 3.80 110 3% -V - - - Test atmospheric 105 2.3% 3.60 0.3%3.65 3.70 - 3.75 -3.80 - − temperature, 100 1.6%110 3% 0.2%- 0.7%- - -- - degree C 105 2.3%− −0.3% - - - Test atmospheric95 0.8% 0.5% 0.2% 0% - 100 1.6% −0.2% 0.7% - - temperature,90 2.0% 0.9% 1.4% 0.6% - 95 0.8% 0.5% 0.2% 0% - degree C 85 0%90 2.0% 0.8% 0.9% 0.5%1.4% 0.6% 1.4% - 0.2% Measuring85 method:0% IEC0.8% 62813-2015. 0.5% 1.4% 0.2% Measuring method: IEC 62813-2015. 9.3. Low-Temperature Properties Evaluation 9.3. Low-Temperature Properties Evaluation The measurement result of internal resistance is shown in Figure 12. The conventional capacitor The measurement result of internal resistance is shown in Figure 12. The conventional capacitor could not be charged and discharged at 30 C or less due to freezing of the electrolyte solution. could not be charged and discharged at −−30 °C◦ or less due to freezing of the electrolyte solution. In In contrast, the developed capacitor could be charged and discharged even at 40 C. The internal contrast, the developed capacitor could be charged and discharged even at −−40 ◦°C . The internal resistance was almost the same as the conventional one at 20 C. resistance was almost the same as the conventional one at− −20 ◦°C .

x40 )

x30 Developed Conventional

x20

value: 25 degree 25C degree value: resistance rise multiple rise resistance

x1 0

(Reference Internal x0 -40 -20 0 20 40 Temperature [degree C] Figure 12. Measurement result of internal resistance at low temperature. Figure 12. Measurement result of internal resistance at low temperature.

The result of the charge–discharge cycle test at −40 °C is shown in Figure 13. The developed capacitor showed a slight deterioration in performance even after 200,000 cycles of testing. Not only World Electric Vehicle Journal 2019, 10, 27 12 of 13

The result of the charge–discharge cycle test at 40 C is shown in Figure 13. The developed − ◦ capacitorWorld Electric showed Vehicle aJournal slight 2019 deterioration, 10, x FOR PEER in performanceREVIEW even after 200,000 cycles of testing. Not only12 of at 13 high temperature but also at low temperature, it has been confirmed that a large current supply, which isat an high advantage temperature of the but capacitor, also at islow possible. temperature, it has been confirmed that a large current supply, which is an advantage of the capacitor, is possible. 1 00 1 00

80 80

60 60

40 40

20 20 Capacitance reduction rate [%] rate reduction Capacitance

0 [%] rate increase resistance Internal 0 0 50,000 1 00,000 1 50,000 200,000 0 50,000 1 00,000 1 50,000 200,000 Charge/discharge cycle [times] Charge/discharge cycle [times]

FigureFigure 13. 13.Test Test result result of of charge–discharge charge–discharge test test at at low low temperature. temperature. 10. Conclusions 10. Conclusions The newly developed auxiliary power supply system for EPS can eliminate the power shortage The newly developed auxiliary power supply system for EPS can eliminate the power shortage when mounting the EPS on a large vehicle without reinforcing the entire vehicle power supply. when mounting the EPS on a large vehicle without reinforcing the entire vehicle power supply. In In addition, the developed system has excellent functional expandability, and can be operated as addition, the developed system has excellent functional expandability, and can be operated as a a redundant power supply in the event of a vehicle power failure or temporary blackout. It will redundant power supply in the event of a vehicle power failure or temporary blackout. It will support the realization of autonomous vehicles that require higher levels of safety. support the realization of autonomous vehicles that require higher levels of safety. In addition, the lithium-ion capacitor whose operating temperature range has been expanded In addition, the lithium-ion capacitor whose operating temperature range has been expanded helps to realize a system that can be mounted on a vehicle without a cooling–heating device. It expands helps to realize a system that can be mounted on a vehicle without a cooling–heating device. It the field of utilization not only in automobiles but also in various industrial fields. expands the field of utilization not only in automobiles but also in various industrial fields.

AuthorAuthor Contributions: Contributions:Data Data curation, curation, K.I. K.I. and and T.H.; T.H. Investigation,; Investigatio N.O.,n, N.O. Y.K.,, Y.K. K.I.,, F.S.,K.I., andF.S. S.S.;, and Methodology, S.S.; Methodology, T.M., Y.K., Y.K., F.S., and T.H.; Project administration, T.M., T.S. (Toyoki Sugiyama), and K.N.; Software, F.S.; Supervision, K.N.;T.M. Validation,, Y.K., Y.K. Y.K.,, F.S. N.O.,, and T.S. T.H. (Tomoki; Project Suganuma), administration, S.M.(Shun T.M., Maruyama), Toyoki Sugiyama and S.S.;, and Writing—original K.N.; Software, draft, F.S.; T.M.;Supervision, Writing—review K.N.; Validation, & editing, Y.K. K.N., N.O., Tomoyuki Suganuma, Shun Maruyama, and S.S.; Writing—original draft, T.M.; Writing—review & editing, K.N. Funding: This research received no external funding. ConflictsFunding: of This Interest: researchThe received authors declareno external no conflict funding. of interest. Conflicts of Interest: The authors declare no conflict of interest. References References 1. Japan Prime Minister’s Office; Advanced Information and Telecommunications Network Society Promotion 1. StrategyJapan PrimeHeadquarters Minister’s (IT Comprehensive Office; Advanced Strategy, Information Headquarters). and Telecommunications Public-Private ITS Concept Network Road Society Map. 2018.Promotion Available Strategy online: Headquartershttp://www.kantei.go.jp (IT Comprehensive/jp/singi/it2 /kettei Strategy,/pdf /20180615 Headquarters)/siryou9.pdf. Public (accessed-Private on ITS 10Concept May 2019). Road Map. 2018. Available online: 2. SAE.http://www.kantei.go.jp/jp/singi/it2/kettei/pdf/20180615/siryou9.pdf Automated Driving Levels of Driving Automation are Defined in(Access New SAEon 10 International May 2019). Standard 2. J3016.SAE. AvailableAutomated online: Driving http: Levels//www.sae.org of Driving/misc Automation/pdfs/automated_driving.pdf are Defined in New (accessed SAE International on 10 May Standard 2019). 3. ISOJ3016 26262.. AvailableRoad Vehicles online: Functional http://www.sae.org/misc/pdfs/automated_driving.pd Safety; ISO: Geneva, Switzerland, 2011. f (Access on 10 May 2019). 4.3. IECISO 61508-6. 26262. RoadFunctional Vehicles Safety Functional of Electrical Safety/Electronic; ISO: Geneva Programmable, Switzerland, Electronic 2011. Safety-Related Systems, Annex B; 4. IEC:IEC Geneva, 61508-6. Switzerland, Functional Safety 2010. of Electrical/Electronic Programmable Electronic Safety-Related Systems, Annex 5. Kaneko,B; IEC: Geneva, T.; Nakamura, Switzerland, H.; Fukasawa, 2010. R. Safety design for Automated Driving Systems. JARI Res. J. 2016, 5. 2016Kaneko,, 20161003. T.; Nakamura, H.; Fukasawa, R. Safety design for Automated Driving Systems. JARI Res. J. 2016, 6. Murata,2016, 20161003 M. Jtekt’s. Perspective On Steering Systems for Autonomous Driving. Automotive Mirai Summit 6. Spring,Murata, Tokyo, M. Jtekt’s Japan, Perspective 11 April 2019. On Steering Systems for Autonomous Driving. Automotive Mirai Summit Spring. Tokyo, Japan, 11 April 2019. 7. Ohashi, M. 2-Drive motor control unit for electric power steering. Denso Tech. Rev. 2016, 21, 48–53. 8. Fuji Keizai Management Co., Ltd. Business Technology Office. Latest Trend of 48V Conversion for Vehicle Power Supply; Fuji Marketing Report; Fuji Keizai Management Co., Ltd. Business Technology Office: Tokyo, Japan, 2014; Volume 76. World Electric Vehicle Journal 2019, 10, 27 13 of 13

7. Ohashi, M. 2-Drive motor control unit for electric power steering. Denso Tech. Rev. 2016, 21, 48–53. 8. Fuji Keizai Management Co., Ltd. Business Technology Office. Latest Trend of 48V Conversion for Vehicle Power Supply; Fuji Marketing Report; Fuji Keizai Management Co., Ltd. Business Technology Office: Tokyo, Japan, 2014; Volume 76. 9. Miyazaki, H. Future views on EPS system technological development. JTEKT Eng. J. 2012, 1009, 19–22. 10. Shinoda, S.; Higashi, M.; Sugiyama, T. Development of Auxiliary Power Supply System for Electric Power Supply Steering. In Proceedings of the JSAE Annual Congress (Spring) Proceedings, Yokohama, Japan, 21–23 May 2014; p. 20145065. 11. Sato, F.; Higashi, M.; Sugiyama, T. Development of Auxiliary Power Supply System for Electric Power Supply Steering. In Proceedings of the JSAE Annual Congress (Spring) Proceedings, Yokohama, Japan, 20–22 May 2015; p. 20155307. 12. Hibino, T.; Sato, F.; Shinoda, S.; Sugiyama, T. Auxiliary Power Supply System for Electric Power Supply Steering. In Proceedings of the JSAE Annual Congress (Spring) Proceedings, Yokohama, Japan, 23–25 May 2018; p. 20180195. 13. Kaji, R. ISO 26262 Compliant and Redundant Design of Electric Power Steering (EPS), MONOist Automotive Forum; ISO: Geneva, Switzerland, 2015. 14. Tanizaki, H.; Matsui, K.; Tasaki, S.; Ando, N.; Hato, Y.; Shibutani, H. Development of Lithium-ion Capacitor (3). In The 47th Battery Symposium in Japan Proceedings; ESJ: Tokyo, Japan, 2006; p. 3E04. 15. Tasaki, S.; Tanisaki, H.; Miyakawa, R.; Shirakami, A.; Taguchi, H.; Matsui, K.; Ando, N.; Hatozaki, O.; Hato, Y.; Shibuya, H. Development of Lithium-ion Capacitor (4). In The 47th Battery Symposium in Japan Proceedings; ESJ: Edogawa-ku, Tokyo, Japan, 2006; p. 3E05. 16. Miyakawa, R.; Hato, Y.; Inagawa, M.; Inoue, K.; Seki, D. Development of High-Power Lithium-Ion Capacitor. NEC Tech. Rep. 2010, 63, 70–74. 17. Aurbach, D.; Zaban, A.; Ein-Eli, Y.; Weissman, I.; Chusid, O.; Markovsky, B.; Levi, M.; Levi, E.; Schechter, A.; Granot, E. Recent studies on the correlation between surface chemistry, morphology, three-dimensional structures and performance of Li and Li-C intercalation anodes in several important electrolyte systems. J. Power Sources 1997, 68, 91–98. [CrossRef] 18. Takumi, M.; Komatsubara, Y.; Naoki, O.; Kimoto, Y.; Kentaro, I.; Nishi, K. Development of high heat-resistant Lithium-ion Capacitor. In Proceedings of the JSAE Annual Congress (Spring) Proceedings, Yokohama, Japan, 23–25 May 2018; p. 20180194.

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