Research and development of electric vehicles for
clean transportation
Abetract
This article presents the research and development of all electric vehicle (EV)in Department of Human·Robotics Saitama Institute of Technology,Japan．Electric mobile systems developed in our laboratory include aconverted electric automobile，electric wheelchair and personal mobile robot．These mobile systems contribute to realize clean transportation since energy sources and devices from all vehicles，i．e．，batteries and electric motors，does not deteriorate the environment．To drive motors for vehicle traveling，robotic technologies were applied．
Key words：electric vehicle；robot technology；converted electric vehicle；clean transportation
Introduction
The influences of internal combusti on sy?stems such as cars with gas engines become a serious social problem because of the environment pollution．To alleviate the problems automobile manufacturers forced to shift their part of productions from pure internal combustion systems to hybrid systems or electric systems．An electric system provides the best solution to the problem，however'weight and energy capacity ofelectric batteries ale still notenough for the substitution of the internal combustion system witIl gasoline．Therefore，a hybrid system might be a reasonable solution for the substitute of pure internal combustion syrstem so far．Toyota Motor Corp．provided their first hybrid car“P刚US”(Toyota Motor Corporation，1997)，and the hybdd technology has been extended to large class of cars．such as…'Estima hybrid'?and“Hartier hybrid”．n is estimated that electric systems will be widely used in the near future．To get initiative in this direction ofdevelopment,Mitsubishi Motors developed a pure electric vehicle．“i—MiEV”and Fuji Heavy Industry also develo ped……Rl e”．both of which will be commercially available soon．Electric drive systems ale relatively simple compared with an engine．It needs no cooling system，lubricant system．timing control between electric and gas responses．In our electric vehicle(EV)project,the first stage of the research has been started in which we study the fundamen—tal capabilities of Ev．For the objective．we developed a converted EV(eleclric mini．E-MINI)in our laboratary．In this article，the procedure ofconversion and results ofbasicteSts including road tests am reported．
1 Electric vehicle development
1．1 Developments of electric vehicle in laboratory or as personal project Good features of EVs ale clean and easy to handie not only in driving but also in the development in university laboratory or as personal project．To develop a gas engine，a large space witIl expensive facilities is needed for testing engine．such as an isolated room with a noise reduction system，all air ventilations，a safety system for eliminating the damages of engine explosion，ete．
Unlike the development of engine．a drive system winl an electric motor is quite simple and easy to handle．No special facility for noise reduction is needed，because electric motor is very quiet compared with engine．No air ventilation system is needed since electric system does not contaminate the environments，and smeHy or volatile chemistry such as gasoline is not used．1．2 Basecar
As a base car for converted EV,a class of light—car(1ess than 660 cc of gas engine)or a small 一car around 1000-1 300 cc might be suitable．Since the light—car shows a better performance in traveling range and acceleration．After the conversion，it would gain 100 kg and sometimes additional weight could reach 300 kg．
W色chose a Rover Mini(known as a Mini Cooper)as a base car to develope the converted EV because of its relatively small size，rigid monocoque chassis construc—tion，and easily obtainable of parts．The car has been sold for about 40 years without major design change．Additionally，its electric systems are quite simple which has no computers，nor brake—servo system．
Figure 1 shows a snap shot of a base car before conversion to EV，11他base car used was manufactured in 1986 and its speeifications ale shown in亿Iblc 1．
1．3 Motortype
Some types of motors which can be used for electric vehicle propulsions are now cormnercially available such as direct current(DC)brash motors，DC brash—less motors,induction motors．etc．However,if a base cal"with a back gear was chosen．a DC brash motor is a solution with the highest pri嘶ty when consider the cost,weight，and performances．
Currently，digital control technology is applied to drive a such high power motor in which power switching devices such as FETs．IGBTs are used．The full voltage power line is switched on and ofr in high frequency higher than 10 kI-Iz by the switching devices and the pulse shaped voltage is applied to motor windings．The switching frequency is constant while a ratio between on．time and off-time in one cycle is varied based on the operation valRe．This drive method is called as a pulse width modulation(PWM)．For rotate a DC brash motor in specific direction．It
needs iust one switching device which chops a power line connected to a motor．However,if rotating a motor to drive in multiple directions is needed，namely clockwise(C、聊and counterclockwise(CCW)，four switching devices are needed to be installed．Compared to DC brash motor,a DC brash—legs motor or an alternate current(AC)motor need at least six switching devices which have to be controlled independently．AIso，a sensor for detecting rotor angle is essential and Call make a motor and a driver to be complicated，heavy and bulky．From the discussion．it is clarified that a DC brash motor rotating only in one direction is the simplest configuration compared with others．Tt圯configuration can be available by installing or remaining a back gear on the vehicle．Tllis is not a difficult requirement because a normal gas engine Call rotate in ofle direction as well．It could be an another solution to alter the connection between batteries and a motor using circuit contactors to provide a current flow in negative direction．In this case．the moving direction，forward or back，is controlled by an electric switch，unlike a usual automobile which moving direction is controlled by a mechanical gear transmission．However,this is a unique solution only applicable for EVs．
1．4 Battery type
Wb have to consider the type of drive baaeries for propelling EV A Li—ion battery may present the best performance among the existing batteries in its energy／weight ratio．However,the strict current control in charging and discharging situations is required and a special ordered baUery chargot has to be used in many cases．If the baRery configuration has been fixed．it would be di伍cult to change it for a new specification。such雒changing voltage，increasing the number of batteries to enlarge maximum current,etc．Beside the inconveniences，Li—ion battery systems are extremely expensive which are lo_100 times of lead type battery in cost．In addition，it has not been accepted by the public that the safety ofthe Li—ion batteries has been established.
Due to above reasons，lead based batteries was chosen for drive EV．Optima yellow top，the deep cycle battery from Optima batteries，Inc．，provides quite high pefformances compared with standard lead batteries．
2 Electric vehicle design
2．1 Components for electric vehicles
Most important components for EV ale a drive motor and a motor controller as discussed in the previous section．A motor used for converted EV is a series．wound DC brash motor which is available on 72-1 20 VDC and 400 A current at most．A motor controller with power switching devices Call turn on and ofr 120 VDC power at most in 15 kHz frequency．Some of the important
components used in EV are shown in T．able 2 together with photos of a motor and a controller in Fig．2．
2．2 Design of drive unit
The major subjects of the conversion are a design and assembling of a drive unit which consists of a drive motor,a clutch，a fly wheel and a transmission．A transmission is not always needed tO all EV however,we utilize the original transmission tO extend the limited motor torque and velocity tO adapt various drive situations together witll for remaining a back gear tO realize back drive using DC motor rotating onlY in one direction．Figure 3 shows a snap shot of removing the gas engine from the vehicle．T11e case of the transmission Was partly removed by machining process to locate the motor shaft to the exact 10cation of a crank shaft of the original engine．Figures 4 and 5 show all original transmission，a machined transmission case,and the drive unit assembly with a drive motor mounted on the top of the transmission，respectively．The weight of the original engine Was 141 kg including small peripheral devices such as a generator,a starter motor,etc．，while the drive unit with a motor weights 87 kg．The maximum torque ofdeveloped drive unit is 6．91 kgf．m and maximum power is 22．70 kW(30．45 liP)．
2．3 Battery configuration
Various available combinations exist for connecting a multiple batteries to a drive motor for an EV．MoSt of motors and motor drivers are available for the power range DC 72_144 v．TIle appropriate battery configuration is oue of tlle most important subiectS for developing EV n depends on the size and weight of the original car,the weight of batteries。and motor performances．
For generating 72 V at 1east 6 batteries(12 V each)are needed to connect them in series．However,the batteries providing 4．32 k聃，ll power under the assumption require one battery pack has 60 Ah capacity．Normally,an EV cai!travel 7 to 10 km per l kWh energy,it is obvious tllat the energy is not enough for a Car in normal use．If 72 V iS believed the best for system．the next solution for the system is tO install 12 batteries in a car,in which a pear of 6 batteries are connected in parallel．Table 3 shows me possible connections for EV drive with lead type 12 V batteries．
For driving E—MDⅡ．a DC power iS generated by 8 batteries connected in series which rated voltage is 96 V The capacity of a battery is 60 Ah，tOtal energy on a cal" becomes 5．76 kⅥm．The batteries Can be charged by all on—board battery charger with plugging it in all AC 100 V socket which is found at every home in Japan．Since the maximum current for battery charge is limited at 8 A，110 special wiring or large breaker is required．
2．4 Installation
Figure 6 shows the location of components and the wiring of E．MDⅡ．A motor,a motor controller and a throttle are located in an engine compartment．The throttle is connected to a gas pedal by wire．driver Can operate the car in an identical manner with a standard gas Car．Batteries are placed in separated locations including all engine compartment，a space under the back seats and a rear luggage room．A battery charger is installed also in 1uggage room which electric cord is pulled OUt from the Cap of gasoline tank for battery charging．Figures 6b and 6c show engine compartmentand the overview of E．MINI respectively．1nhe total weight of the car reaches 760 kg which is 80 kg gained from the Car before conversion．
2．5 Meters and indicators
To manage and get information of drive batteries，an Emater is installed on a cockpit of E—MDⅡwhich provides a total voltage，a current flOW,a reinained energy in kWll，and the estimated possible time for driving(Fig。71．Tachometer, and ampmeter are alSO installed around me cockpit for getting instantaneous information of drive motor．A speedo—meter works as a standard Car．
3 System test
3．1 Road test
E—M五阿has been anthorized tO drive on pubic roads with a license plate．B．M玳I COBld travel 35 tO 38 km distance by one charge where approx．3．5 to 4 kWh energy had been consumed．Therefore．efficiency is in the range of 8．75-10．85 km／l(Ⅵ，11．Figure 8 shows the energy consumption and the voltage drop as travel distance increased from 0(start)tO approximate 30 km．
3．2 Battery charge
Figum 9 shows the change of the stored energy in battery charging using on-board charger．E．MDⅡhas all on．board battery charger which is available for 100 V AC using home power lines，because the maximum current is restricted at 8 A．From Fig．9．50％charge is done in approximate 2．5 h at almost empty condition，thus full charge needs approximate 5 h．
4 Conclusions
In this atticle．the EV project has been reported in which all electric vehicle E—MINl was developed by conveaing Rover Mini for realizing clean transportation．E—MINI has been authorized tO drive on pubfic roads and achieved 38 km traveling by one charge．The efficiency is 10．85 km／kWh at most while it is varied depending Oll road conditions and drivers．To extend a traveling distance by one charge would be one of our future works．