IEEE International Symposium on Industrial Electronics (ISlE 2009)Seoul Olympic Parktel, Seoul, Korea July 5-8, 2009

Anti-slip Regulation of Electric Vehicle WithoutSpeed Sensor

Xu Pengl,2,Hou Zhe l, Guo Guifang', Zhang Liping', Cao Binggang', Long Hongyu" Member, IEEE, Chen Xi4

1. R&D Center of Electric Vehicle, Xi'an Jiaotong University, Xi'an, China2. The 71811 Unit ofPLA, xinyang, henan, China

3. Department of Electromechanical Engineering, Construction Machinery School, Chang'an University, Xi'an, China.4. school of electrical engineering, Xi'an Jiaotong University, Xi'an, China

xuipeng@126.com

Abstract-In order to research the anti-slip regulation(ASR)of electric vehicle(EV), this paper analyzes the operatingprinciple and control strategy of electric vehicle's ASR , buildsthe motion model of the EV and vehicle dynamics modelincluding the tyre, the wheel and the model between the tyre andthe road. The traditional ASR structure is com plex. In order tosolve the problem, a novel anti-slip controller for EV withoutspeed sensor is designed to prevent the slip between tyre and road.A back electromotive force (back-EMF) observer is constructedto acquire the information of speed as well as acceleration. Byselecting the time constant or observer gain properly, the currentcommand reduction characteristics can be adjusted freely insome range. The results of simulation and experiment verified theeffectiveness of the proposed controller, which can prevent thedriven wheels skidding, improve the traction performance,enhance the driving performance and the stability of EV, andavoid the occurrence of traffic accident. The anti-slip control forEV without speed sensor is an ideal anti-slip regulation method.

I. INTRODUCTION

The pure electric vehicle(PEV) is "nopolluting" vehicle.The term "nopolluting" is only relative. While the electricautomobiles do not pollute, the original source of energy­electricity does cause pollution during its generation. However,the centrally generated pollutions can be controlled andreduced much easier than the distributed pollution sources ofindividual vehicles. [1]

Recently, PEVs have achieved sufficient drivingperformance thanks to drastic improvements in motors andbatteries. On the other hand, hybrid EVs (HEVs), like theToyota Prius, will be widely used in the next ten years. Fuel­cell vehicles (FCVs) will be the major vehicles in the 21stcentury. Such development has the strong incentives of energyefficiency and global environmental protection. However, it isnot well recognized that the most distinct advantage of the EVis the quick and precise torque generation of the electric motor.If we do not utilize this merit, the EV will never be used in thefuture. For example, if a diesel HEV is developed, its energyconsumption will be extremely low. The EV cannot competeagainst such vehicles in terms of energy efficiency or CO2

emissions. On the contrary, if we recognize the advantage ofthe EV in control performance and succeed in the developmentof new concept vehicles, a bright future will be waiting for us.[2]

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978-1-4244-4349-9/09/$25.00 ©2009 IEEE

In the closed loop system comprising the driver, EV andenvironment, the primary connection of the EV andenvironment is the acting force between the tyre and the roadsurface, which includes longitudinal force, lateral force andnormal force, etc. The driving behavior of the EV is decidedby the acting force between the tyre and road surface. Thelongitudinal force decides the drive capability and brakecapability, and lateral force decides the turning capability andthe anti-interference capability of transverse interference. Sothe driver is controlling the force between tyre and the roadsurface in effect, and the force is limited by the adherencecharacter between the tyre and the road surface. When theacting force approaches the adherence limit and the attachmentcoefficient is small, the driving force of the EV alwaysexceeds the longitudinal adherence limit between the tyre androad surface, and create excessive wheelspin of the drivingwheel. It reduces the driving performance, exacerbates theabrasion of the tyre, enhances the bear of the loading weight ofthe transmitting mechanism and driver, and increases energyconsumption. The lateral force approaches the lateralattachment coefficient, thus the EV can't follow the correctway and it damages the maneuverability, stability and securityof the EV, easy to occur accident. [3][4] So, the control of thedriving wheel at the driving process is very important. TheEV's driving adopts motor with good electronic drivingsystem, which lays a certain foundation for the driving anti­skid control.

At the present time, due to the technology of EV at theresearch state, the reports of the EV's ASR are lack. From theinformation searched, professor HorLY in Tokyo University isthe represent in this area. It adopts the fast angular forceresponse of the motor to attain the goal of control the EV'sdrive wheel wheelspin rate through the observation of theopposing electromotive force or the motor's phase current.[5][6]

II. OPERATING PRINCIPLE OF EV'S ASR

When the EV hard braking on the wet-skidding or iceroad, the EV will revolute or tum around, and the EV will beout of control. When the EV runs, the driving force dependson the output torque of the motor delivered to the propellingwheel and the attachment limit. The output torque is decidedby the external character of the motor and the transfer

behavior of the drive unit (velocity ratio, transrmssronefficiency, radius of the propelling wheel). It puts up definitecontrol law according to the operation of the driver. Theattachment limit between the type and the road surface relatesto the structure of the tyre, the road surface condition, theweather condition, the speed of the EV and etc. It is anindeterminacy value with wide variation range. Vast scaleexperiments indicate that the relationship of the attachmentlimit and the wheelspin rate of the driving wheel are shown inFig.l. [7J

a -It' .J~

~

i,

~ tF' .1~

.,

Fig.I the relation of the attachment coeffic ient and wheelspin rate

From the Fig.l, when the drive wheelspin rate (A)increases from zero, the lengthwise direction factor ofadhesion increases rapidly along with it. When the wheelspinrate reaches a certain value (At) between 0.10 and 0.30, thelengthwise direction factor of adhesion reaches the maximumvalue. After this, the drive wheelspin rate (A) increases rightalong, the factor of adhesion dcereases on the contrary. WhenAreaches 100%, namely the wheels simple sliding on the road,the factor of adhesion is far less than the peak value of factorof adhesion. When the wheelspin rate is between 0 and At,namely the ascending branch of the lengthwise directionattachment coefficient, the EV is in the stable area. When thewheelspin rate is between At and I, namely the descendingbranch of the lengthwise direction attachment coefficient, theEV is in the unstable area. So considering from the haulingability, the lengthwise direction wheelspin rate of drivingwheel ought to be in a small area. On the other hand, when thewheelspin rate increases, the lateral direction attachmentcoefficient between the wheel and the road rapidly decreases.So considering from the stability performance of lateraldirection, the lengthwise direction wheelspin rate of the wheelshould be smaller. If the fore wheels loses the lateral directionattachment, the EV will lose the steering stability; if the backwheel loses the lateral direction attachment, the EV will lossthe directional stability, and occur tail-flick phenomenon.Therefore, the ideal lengthwise direction wheelspin rate shouldbe somewhat less than At, namely between 0.10 and 0.2.

III. ASR CONTROL OF THE EV

A. ASR CONTROL STRATEGY OF THE EV

Theoretically, the driving wheels spin when the drivingmoment exceeds the adherence moment between the tyre andthe road. So only decreasing the EV' s electric motor' smoment of torque or the delivery moment of torque can attain

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the goal of ASR. In the system consisting of driver, EV andexternal environment, the force and moment acting on thedriving wheels are as follows: the outputting angular force ofthe electric motor, the braking moment of the arresting gear,loading weight of the driving wheels, normal reaction of theroad and the moment of couple of resistance of rolling.Commonly used control strategies of ASR are as follows:adjusting the loading weight of the driving wheels, adjustingthe outputting angular force of electric motor, adjusting thedriving moment and applying brake moment on the drivingwheels, etc.

The different structural basis corresponds to differentcontrol method. There is no electronic control suspension onthe XJTUEV-! for the experimental use, and the EV uses thepermanent magnet DC motor. In order to reduce the rotationspeed of the propelling wheel, it can adopt the method ofelectric braking, and impose braking moment. The commonlyused methods of braking are energy consumption braking,reverse connection braking, and regenerative braking. Amongthe methods, the target of the reverse connection braking is tomake the motor stop, and the braking moment is big, which isnot accordance with the target of reducing the rotational speedof the propelling wheel. The energy consumption andregenerative braking are combined in a braking PWM periodin this paper.

When the propelling wheel wheelspining, it indicates thatthe driving moment is oversize. If reducing the output torqueof the motor by adopting self adapting method and reducingthe driving moment properly, it will control the wheelspin rateof the driving wheels, the expression of the permanent magnetDC motor are as follows:

(1)

Where, r.,m is the electromagnetic torque, C/J is the

excitation flux, 1a is the armature current, K m is the torque

constant.

The formula above indicates that, there are two mainpaths reducing the motor' s output torque: (1) Reduce themain flux of the motor ep. In this case, it can but proceed withflux-weakening control, and it can control the speed in smallscale yield above the rated speed. It belongs to constant output

power control method . (2) Modify the current of armature 1a ;

in this case, it can adopt PWM chopping control. This methodis adopted in this paper.

B.THE ASR CONTROL PRINCIPLE OF EV

When the EV runs at different speeds, the EV adoptsdifferent wheelspin control strategies to control the drivingwheel. In certain condition, making the emphasis performanceas the main control target, and considering the otherperformances properly, the control target is different when theEV runs at a different speed. So the path of putting the drivinganti-skid into effect is different'".

1) starting and acceleration: When the EV runs at thestarting and acceleration stage, the main target of anti-slipregulation is to enhance the starting acceleration performanceof EV, namely making full use of the adhesive force of everydriving wheel to obtain the maximum hauling power.

2) Mediate speed: When the EV runs at mediate speed, themain target of the anti-slip regulation is ensuring the stability ofdriving direction, and giving consideration to the accelerationperformance.

3) High speed driving: When the EV runs at high speed,the exclusive target of the anti-slip regulation is ensuring thestability of driving direction, and keeping the hauling power ofevery driving wheel consistent.

transformation coefficient. From the view of engineeringpractice, suppose the rolling resistance and wind resistance arerelatively small and can be ignored, so:

(3)

The adhesive characters between the tyre and road surface canbe expressed with the slip ratio, and its definitionis as follows:

(5)

(4)

0& 08D~ O ~o

,-. -0 " ------___

<> 0

i , -o~l§§~===~

v-vWhen the EV braking: A= W

V

v -VWhen the EV driving: A= --:;:W"--_

Vw

qJ=-1.05k[exp(-45,1) -exp(-0.45,1)] (6)

When the EV braking:

qJ = 1.05k[exp(35A) - exp(0.35A)] (7)

Where k is the state parameter of the road surface andthe value is the maximum of attachment coefficient,

namely k=qJrnax' When the k is different, the qJ- A curve is as

follows:

Where V is the velocity of the EV, the adhesioncoefficient of tyre is the function of slip ratio and therelationship is as follows:

When the EV driving:

"

IV. RESEARCH OF ASR CONTROL WITHOUT SPEEDSENSOR

The simulation of anti-slip regulation of EV without speedsensor is shown as follow. The driving structure of the EV isshown in Fig.iS

].

Where WI,W2 are the loading weight of the followerwheels and the driving wheels, mi -mi are the mass of thefollower wheels and the driving wheels , Fe, Fz2 are the normalreaction of the follower wheels and the driving wheels,

lWl,lwl are the moment of inertia, FpJ, Fp2 are the force that

the follower axis and the driving axis act on the followerwheels and driving wheels paralleling to the road, Fa. FX2 arethe tangential reaction from the road of the follower wheelsand driving wheels, Tn , Tn are the rolling resistance momentof couple, Tem' is the angular force of the half axis acting onthe driving wheels, V is the velocity of the EV, Jw is themoment of inertia of the wheel, Td , Tb are the driving momentand brake moment, (iJ(i, j) is the spin velocity of every wheel.

Rd is the radius of the wheel, Fd is the driving force, and 0,is the regenerative braking moment.

(2)

Fig.2 the force diagram when the EV driving

The dynamic equations of EV's single wheel model:

{

J dJ.=_T - FdRd-t. -r;«;MV -r.-r-».Vw = OJRd

Where F; is the air friction, T is the gross tractive effort,Ff is the rolling resistance, Vw is the linear velocity, M is themass of the EV, and H; is nondimensional windage

'''" IL''('l:-op in rule

Fig.3 qJ- A curve

The driving force Fd can be expressed:

(8)

Where Fz is the normal reaction, and the below function

is attainable from the function above:

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(j) 1= ~

T Js+MR/(1-A)SThe equation above can be denoted with the model of the

single wheel, as shown in FigA.

motor can't receive directly, so an observer must be set toevaluate the value of the EMF. In this paper, the EMF of theDC motor is disturbing signal. The EMF observer is shown in

Fig.7. The EV's model is expressed as }js'

FigA the model of the EV

In order to simplify the model, suppose

J' = J + MRd2(1- A) , so the relationship between the

torque and rotational speed can be expressed as Fig.5.

lIJ,(V

l iMsv

Fig.7. the block diagram of the EMF observer

The transfer function G(s) from the voltage instruction

to the motor's current is as follows:

G(s)=, " J'rs,'+J'sJ'Lts +J (L+ Rr)s' +(JR+ KEKTr)s+ KEKT(l- K)

(10)

ig is the drive ratio of the speed reducer; io is the drive

ratio of the main speed reducer; 1] is the general efficiency of

the driving device from the output axis to the driving wheels;K, is the moment coefficient of the motor; K; is the back EMFcoefficient.

When the EV driving steadily, the EV' s moment of inertia J' is

the constant value I n , K is the feedback gain, and the value of the

current regulator is:

G 1(s) J"Lrs3

+J,,(L+Rt)i+(Jfi+Ki(ri)s+~KrCI-K)J"d+J~

(II)From (10) and (1 I), the transfer function from the current

instruction i* to the motor's current i is as follows:

i(s) J"Lrs3

+J,,(L+Rt)I+(Jfi+~,~·i)s+~,~(1-K).f

t(s) .fLrs3

+.f(L+Rt)I+(.fR+~,Kri)s+~,~(l-K) J"(12)

From (12), when the EV is in the steady-state, namely

J' = I n , the value of the transfer function is I, temporality,

the motor's current follows the current instructionunconditionally, i.e. the driver can drive the EV freely.

From Fig.7, if the characteristic time t: is sufficientlysmall, the EMF can be completely compensated, and theinterference doesn' t affect the current of the motor.Contrarily, if the characteristic time is relatively large, thesystem response speed is very slow, and the interferenceaffects the motor's current greatly. So, by adjusting the

...

.---------< Ks

Fig.5 the relationship between the torque and rotational speed

When the EV runs from the road with high attachmentcoefficient into the road with low attachment coefficient, theadhesion between the tyre and the road decreases rapidly, andthe output torque of the EV's motors keeps the value ofprevious time. The driving force is greater than the adhesivepower of the road, thus the driving wheels enter the state ofwheelspin rapidly.

Because the back electromotive force (back EMF) isproportional to the rotational speed of the motor, namelyproportional to the rotational speed of the driving wheel.When the speed of the wheel increases, the EMF increases,and the wheel 's acceleration decreases, at the same time, themotor's torque reduces rapidly. So the anti-slip regulation canmake use of this principle. The method needn't complicatedcalculation. Its structure is shown in Fig.6.

Fig.6 the EV system with motor

When the EV wheelspins, A increases, from the equ.9,

J' reduces, equivalently the moment of inertia reduces. Sothe observer can be designed to adjust the EV's drivingmoment, and it doesn't change the character of the motor. Inthe system without speed sensor, the ASR control can berealized by using the EMF of the motor. The EMF of the

T

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characteristic time t: , it can control the effect of the EMF tothe motor's current.

V. SIMULATION AND EXPERIMENTAL RESEARCH ON ASR

OF THE REAR WHEEL DRIVE

The main parameters of XJTUEV-I shown inTABLE.l.

TABLE I TilE MAIN PARAMETERS OF TilE EV

2) Simulation results with EMF observer: observe theeffect by adjusting the characteristic time r and the feedbackgain K, and simulating. The simulation conditions are same asabove, at 5th second, the EV enters the wheelspin state.

At first, supposing the feedback gain K is l , and adjustingthe characteristic time t: , the simulation results are shown inFig.9.

Fig.8 the simulation results without EMF obs erv er

10

10

9

9

B

8

7

5 6tis

4 5 6tis

K=l

K=O

3

3

2

2

500

l lJC(1

500

2OCO

81:' 1500--c

2500

Fig.10 the rotational speed of the motor

As shown from the figures above, when the EV skids, ifthe feedback gain K is smaller, the decrease amplitude of therotational speed is larger. This character makes the rotationalspeed of motor can't increase rapidly; when the feedback gainK is big enough, the EMF of the motor can be compensated, atthis time, the current of the motor equals to the currentinstruction value. When the EV skids, the rotational speedincreases rapidly.

From Fig.9 and Fig.lO, when the EV skids, the drivingmoment is decided by the characteristic time t: of the EMFobserver and the gain K. By carefully adjusting these two

Fig.9 the rotational speed of the motor

As shown from the figure above, when the EV skids, ifthe characteristic time t: is bigger, the decrease of theamplitude of the rotational speed is larger. This charactermakes the rotational speed of motor can't increase rapidly;when the characteristic time r is sufficiently small, the EMFof motor can be compensated, at this time, the current of themotor equals to the current instruction value. When the EVskids, the rotational speed increases rapidly.

Supposing the characteristic time t: =0.00 I, adjusting thefeedback gain, the simulation results are shown in Fig.l O.

2000

8e- 15OO

<:l lJC(1

(a)wheelspin rate

(b) angular velocity of the wheel

::. ////~1~ 3: ~ Ib ,., '0

\,'~

M 1500kg Iz 1100kg",'

a 0.9m dJ 1.215m

b 1.06m d; l .2m

h. 0.6m R,( 0.287m

CD 0.5 A 2.43m2

Er 0. 1 K, 12.975

K.r 12.975 J... 0.5kgm 2

~: I- : ;<lQ

f ...,n.~ '00i .....0 .

°0

M is the mass of the EV; lz is the Z-moment of inertia ; ais the distance from the gravity center to the fore axle; b is thedistance from the gravity center to the back axle; df is the frontgauge ; d, is the rear track; hg is the height of the gravity center;Rd is the radius of the wheel; Cf) is the wind resistance factor;A is the front face area; Ef is the characteristic parameter oflongitudinal tyre force; K, is the initial nondimensionalsideslip stiffness of the tyre; K, is the nondimensional verticalslip stiffnessr.z, is the rotary inertia of the wheel.

I) Simulation without observer EMF: carrying out thesimulation of the EV without observer EMF as shown in Fig.6.Suppose the EV drives on the dry cement pavement, at 5thsecond, the EV enters the ice-snow road. The simulationresults are shown in Fig.8.

i ~ ~ 1 ::::[ : I0 0 1 2 3 4 5 6 7 8 9 10

V,

:~l/-~~~~-~~: 1o 1 2' :! .. !. B 'I BI { I ' 0

L"

(c) velocity of the car body

226

The simulation and experimental research results showthat when the EV runs on the low adherence road and

VI. CONCLUSION

In this paper, the operating principle, control strategy andcontrol method are discussed, and the mathematical model ofthe EV's ASR is built. The speed sensor-less ASR controller isdesigned, and the simulation and experimental research arecarried out in the refitted EV XJTUEV-1.

Fig.11 the ASR experimental results with speed sensor

As shown from the figure above, when the EV drives, thespeed sensor-less ASR controller with EMF observer canprevent the excessive wheelspin because of the oversizedriving force. It enhances the reliability and security of the EV,reduces the speed detector device, and lowers the cost. It is anideal EV ASR control method.

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IEEE TRANSACTIONS ON VEHICULAR TECHNOLO GY.VOL.42 .N0.4.NOVEMBER 1994.

[2] Yoichi I-Iori. Future Vehicle Driven by Electricity and Control­Research on Four Wheel Motored "UOT Electric March II ". IEEETRANSACTIONS IND USTRIAL ELECTRONICS,VOL.5I.5,OCTOBER 2004,pp.954-962.

[3] Keun-Ho Hyun.Design of a speed controller for permanent magnetsynchronous motor in pure electric vehicle applications . 2007Internat ional Conference on Control, Automation and Systems-IC CAS ,Seoul South Korea, 2007,pp.1623-8 .

[4] S. I-I. Kataoka . Optimal Drive Forum: Control of EV based on RoadStatus Observing [A]. MQSET dissertat ion of he University ofTobo .200\.

[5] Cern Unsal. Pushkin Kachroo . Sliding Mode Measurem ent FeedbackControl for Antilock Braking System[J]. In IEEE on Control SystemTechno logy, 1999,7(2) :271~28 1 .

[6] lIimshi Fujimoto . TakeOSaito. Toshihiko Noguch i. Motion Stabil izationControl of Electric Vehicle under Snowy Condit ions Based on Yaw­Moment Observer[ A]. The 8th IEEE International Workshop onAdvanced Motion Control. Kawasaki , 2004:35~40 .

[7] Shin-ichiro Sakai and Yoichi. Hori. Lateral motion stabil ization of 4wheel-motored EV based on wheel skid prevent ion[A]. The17th.Electr ic Vehicle Sympos ium EVSJ, Montreal , Canada , 2000:135~140.

[8] Binggang Cao; Zhifeng Bai; Wei Zhang ,Research on control forregenerative braking of electric vehicle.2005 IEEE InternationalConference on Vehicular Electronics and Safety Proeeedings,Shaan'xiChina.2005

dimidiate road, it is easy to cause the driving wheel to quick­speed wheelspin . The road lengthwise attachment coefficientcan be fully used, the side direction adherence capability ispoor, it cause the hauling ability isn't ideal, and threat to thesafety.

The speed sensor-less ASR controller with EMF observercan control the wheelspin rate around 0.15, and play the ASRrole. The controller reduces the speed detector device,decreases the cost, and enhances the drive performance andactive safety performance . It is an ideal EV ASR controlmethod .

74:./ s

]2

c

~ o."c,1,-;'

1.s:,.

0

parameters, it can play the role of ASR. In effect, it equals to aPI ASR controller .

For the refitted EV XJTUEV-1, the characteristic timet: =10 and the gain K=0.8, the experimental conditions are as

follows: the EV runs on the dry road initially, CfJx = 0.8 at 2nd

second, the EV runs onto the ice-snow road, CfJx =0.1 the

experimental results are shown in Fig.II .

\.( I,,--....---.----,,--,.---.----.---r---,

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