Integrated Slip-Based Torque Control of Antilock Braking System for In-Wheel Motor Electric Vehicle

IEEJ Journal of Industry Applications Vol.3 No.4 pp.318–327 DOI: 10.1541/ieejjia.3.318

Paper

∗a) ∗∗ Wen-Po Chiang Non-member, Dejun Yin Non-member ∗ ∗ Manabu Omae Non-member, Hiroshi Shimizu Non-member

(Manuscript received March 31, 2013, revised Jan. 17, 2014)

In electric vehicles (EVs), cooperative control between a regenerative brake system (RBS) and the conventional hydraulic brake system (HBS) enhances the brake performance and energy regeneration. This paper presents an integrated antilock braking control system based on estimating the wheel slip to improve the anti-slip performance of a four wheel drive in-wheel motor EV. A novel anti-slip control method for regenerative braking was developed and integrated with a hydraulic antilock braking system (ABS) according to the logic threshold concept. When combined with the features of driving motors, the proposed method can improve the brake performance under slip conditions. Com- parative simulations showed the proposed control approach provided a more eﬀective antilock braking performance than the conventional ABS.

Keywords: anti-slip control, antilock braking system, brake force distribution, in-wheel motor electric vehicle, regenerative brake system

The productions of HEV and EV in current automotive 1. Introduction market obviously have also taken advantage of regenerative The electric vehicle (EV) as a zero carbon dioxide emis- technology to enhance the energy efficiency. However, dur- sion automobile having superior efficiency is currently be- ing the emergency braking i.e. the slip condition, commer- ing developed due to deterioration of environment issues (1) (2). cial EVs or HEVs do not reply on the regenerative braking Electric motor drive EV has not only the character of high ef- but the antilock braking system (ABS) of HBS. The ABS ficiency, but the outstanding features of rapid response, easy control algorithm makes braking force and cornering force measurement and accurate controllability by comparing to in- to be well used as possible by monitoring wheel accelera- ternal combustion engine vehicle (ICEV) (3) (4). The regeneration and slip ratio based on the logic threshold control (LTC) tive brake system (RBS) that drives motor as a generator to method. When control unit detects the potential sliding, con- convert moving kinetic energy into the electric energy storing trol system reduces regenerative braking force and reverts to in the battery pack has been researched widely to improve en- the hydraulic braking only so that the hydraulic ABS can take ergy efficiency. full responsible during the skidding. The main purpose is In the early research works, the RBS and hydraulic brake that the brake performance is prior to the energy regenera- system (HBS) are coordinated to concentrate on either energy tion. However, conventional hydraulic ABS leads to the os- regeneration in normal braking or vehicle stability perfor- cillatory deceleration causing unfavorable ride comfort and mance in emergency braking. During normal braking, how the advantages of the motor are not well utilized. Neverthe- to obtain the maximum energy regeneration is an interesting less, because the hydraulic braking is not able to compensate issue and had been respectfully studied (5)–(7). Nevertheless, the loss of reduced regenerative braking rapidly, the decel- some studies are devoted to anti-slip control for vehicle sta- eration decreases suddenly causing incongruous deceleration bility regardless of energy efficiency (8)–(11). Recently, due to feeling (15). the benefits of hybrid brakes, the brake control strategies have In order to cope with problems, some research works attracted attention on coordination between energy recovery proposed the independently regenerative anti-slip control and vehicle stability for hybrid electric vehicle (HEV) (12) (13) method (9) (16) or the cooperative control strategy with hydraulic and EV (14) applications. These studies have presented that ABS to maintain the vehicle stability and brake performance the developed control strategies can carry out sufficient en- during the emergency braking (8) (13) (14) (17)–(21). In these studies, ergy recovery while maintaining vehicle stability. the cooperative ABS control methods are presented in sev- eral control scenarios. The first type is to propose an ABS a) Correspondence to: Wen-Po Chiang. E-mail: bradbogy@sfc. reinforcement system to improve the traditional hydraulic keio.ac.jp ∗ ABS performance by adjusting the proper regenerative brak- Graduate School of Media and Governance, Keio University ing force. The regenerative braking is taken as auxiliary 5322, Endo, Fujisawa-shi, Kanagawa 252-0882, Japan ∗∗ School of Mechanical Engineering, Nanjing University of Sci- brake system to support hydraulic ABS. However, these sys- ence and Technology tems do not contain the individual regenerative anti-slip con- No. 200, Xiaolingwei, Nanjing 210094, China trol especially on the low friction road in which there is no

c 2014 The Institute of Electrical Engineers of Japan. 318 Integrated Slip-Based Torque Control of ABS for EV（Wen-Po Chiang et al.） need of hydraulic braking. The second kind is to develop a leveled ABS control strategy to use either regenerative ABS or hydraulic ABS under certain brake strength for better energy regeneration. However, these strategies cannot make advantage of motor to improve the anti-slip ability comparing to the conventional ABS method. The third type is to con- struct a simultaneously cooperative control method between two distinct brake systems. Although the ABS performance is upgraded, but most of them use the same anti-slip control Fig. 1. Dynamic longitudinal model of vehicle method for both two brake systems, the merits of electric motor are not well utilized. This study aims to make full use of the advantages of motors to achieve ABS performance improvement. We proposed an integrated antilock braking control strategy on a series braking system. The control strategy includes two individual ABS control systems and an integrated control method. Dur- ing slight braking (i.e., only regenerative braking is used), the developed regenerative ABS works individually if wheel slip occurs. If slip occurs during the composite braking (i.e., two brake systems are needed), both ABS control systems work simultaneously. Meanwhile, in order to realize better brake performance during the normal braking, brake force distribution between the front and rear wheels follows the ideal brake force distribution curve (I-curve) to ensure good usage of road friction. A suitable distribution between RBS and HBS is determined according to the motor’s torque-speed curve and state of charge (SOC) of battery for maximum energy recovery purpose. 2. System Modeling 2.1 Vehicle Dynamics A half-car vehicle model for Fig. 2. Magic formula in half-car vehicle model longitudinal motion of a four-wheel drive in-wheel motor EV is considered on flat road surface without incline shown in Fig. 1. The dynamic differential equations of the vehicle are where Vw is the wheel speed. During longitudinal braking, described as (1)–(5) on the basis of the assumption that the load transference occurs within front and rear axles. The fol- driving resistance is considered to be small and ignorable. lowing two equations describe the dynamic load transference However, driving resistance is a variable that is related to h · aerodynamics, which can be synthesized in real time if higher N = M g − M V ························· (8) f f L c anti-slip performance is required or if the vehicle runs at high speed (4). h · Nr = Mrg + M Vc ·························· (9) . L Jω f ω f = Tmf + Thf − rFdf ····················· (1) . herein g is the acceleration of gravity, h is the center of grav- Jωr ωr = Tmr + Thr − rFdr ······················· (2) . ity height, L is the wheel base. Fig. 2 shows a two-wheel (8) M Vc = Fd = Fdf + Fdr ························· (3) vehicle model with Magic Formula for deceleration . 2.2 Hydraulic Brake Torque Model The actual hy- Fdf = μ f (λ f )N f ································ (4) draulic brake torque can be measured directly with a sensor F = μ (λ )N ································· (5) dr r r r or estimated from hydraulic pressure of the wheel cylinder (22). The latter method is utilized in this study as where, Jω is the wheel inertia, ω is the wheel angular speed. T is the regenerative brake torque, T is the hydraulic fric- m h Th = BEF · Aw · re · Pw ·························(10) tion torque, Fd is the friction force between tire and road sureface, M is the vehicle mass, Vc is the vehicle speed, r where BEF is the brake effective factor of brake lining. Aw is the wheel radius, N is the vehicle normal load, μ is the is the wheel cylinder area. re is the brake effective radius. friction coefficient, λ is the slip ratio. The subscript r and f Pw is the wheel hydraulic pressure. Here, we assume that the indicate front wheel and rear wheel, respectively. Here, the variation of BEF due to the brake lining wearing or ambient slip ratio is defined as conditions is negligible. Although the wheel hydraulic pressure can be estimated V − V λ = w c ······························· (6) according to hydraulic control unit (HCU) dynamic property max(Vc, Vw) and master cylinder pressure sensor which saves the cost of (23) Vw = r · ω ······································ (7) manufacturing , the usage of the estimated pressure must

319 IEEJ Journal IA, Vol.3, No.4, 2014 Integrated Slip-Based Torque Control of ABS for EV（Wen-Po Chiang et al.） be calibrated wisely, otherwise a signiﬁcantly longer stopping distance can result. Moreover, because the applied brake forces are critical to ABS, it obviously would be an advantage to know cylinder pressure directly. Therefore, in this study the wheel cylinder pressure sensor is utilized to mea- sure pressure directly. 2.3 Regenerative Brake Torque Model The transfer function for regenerative braking between target toque and executed torque is described as ﬁrst order delay system with dead time (4) as

−τ1 s = Tm = e ························· Fig. 3. Motor/Generator map Gm(s) ∗ τ + (11) Tm 2 s 1 where τ1 represents the dead time of CAN communication or mechanical response lag, τ2 represents the response time of motor with first order lag. 3. Brake Torque Distribution Control 3.1 Brake Torque Distribution between RBS and HBS Despite of the merits of the electric motor, the hydraulic brake unit is reserved on considering the possible failure of the electric system. To employ both two brake systems, the architecture of the brake system needs to be changed instead of conventional construction. There are two techniques to Fig. 4. Weighting factor of the battery SOC coordinate the regenerative torque of the motor and friction torque of the hydraulic unit: parallel and series (7).Inorder in this paper. Because the regenerative torque is still avail- to recover kinetic energy as much as possible during braking, able, even the regenerative efficiency is low at slow speed the hydraulic brake force prefers not to operate until regen- due to the lesser electric voltage generated (24). Accordingly, erative brake force reaches its maximum availability. Herein, the available motor torque is obtained as (14) with a weight- the electro-hydraulic series brake system which allows inde- ing factor WSOC. Figure 4 shows the weighting factor for pendent control of the hydraulic braking according to regen- battery’s SOC. The equation of it is described as (15). erative braking is introduced to recover more kinetic energy, T = T W ························ (14) Hence, the distribution between regenerative torque and fric- m avail ⎧ m max SOC ⎪ < . tion torque is described as below ⎨⎪ 1 SOC 0 8 W = ⎪ 10(0.9 − SOC)0.8 ≤ SOC < 0.9 ∗ ≤ = ∗, = SOC ⎩⎪ if T Tm avail,thenTm T Th 0 0 SOC ≥ 0.9 ∗ > = , = ∗ − if T Tm avail,thenTm Tm avail Th T Tm ··················· (15) ··················· (12) 3.2 Brake Torque Distribution between Front and ∗ where T is the target brake torque, Tm avail is the available Rear Wheels Dynamic load transference causes rear motor torque. During braking, the control unit calculates the wheels lock become easier than front wheels if brake forces target deceleration as well as the target torque according to of both axles are set to be the same. The higher deceleration, the brake pedal stroke θ commanded by the driver. The avail- the more weight transference increases. Hence, appropriate able motor brake torque is determined by the vehicle status brake force design on the front and rear wheels is essential which includes the motor maximum power, the vehicle speed and required for the consideration of vehicle stability. By and the battery’s SOC (14). Eq. (13) reveals the calculation of substituting (8) and (9) to (4) and (5) respectively, a distribu- the maximum motor torque. The motor characteristic curve tion ratio α can be calculated as (16) which is a function of of application motor for simulation in this study is shown in vehicle acceleration measured by accelerometer. Therefore, Fig. 3 T ∗ with known demand of target brake torque , the appropri- ω ≤ ω ate brake torques on the front and rear wheels can be calcu- = TmN m mN ········ Tm max (13) lated by (17) and (18). Front/rear brake torques distribution 9550PmN/ωm ωm >ωmN that follows the distribution ratio α achieves the ideal brake where Tm max is the maximum motor torque. TmN is the mo- force distribution and gives the optimal braking which makes ω ω tor rated torque, m is the motor speed, mN is the motor base the front and rear wheels lock simultaneously for any road speed, PmN is the motor rated power. condition, i.e. the maximum brake effectiveness. However, it is not available to utilize the regenerative braking due to the electric recovery causing battery overcharging Fdri h h α = = Mrg + Ma M f g − Ma if SOC of battery is too high. Thus, a weighting factor WSOC Fdfi L L is employed to protect the battery from overcharging that may ··················· (16) deteriorate the battery’s life. The weighting factor for vehi- T ∗ T = ··································· (17) cle speed which is utilized in some studies is not considered fi 1 + α

320 IEEJ Journal IA, Vol.3, No.4, 2014 Integrated Slip-Based Torque Control of ABS for EV（Wen-Po Chiang et al.）

large consequently. . Vw · = (λopt + 1) ································(22) Vc 4.1 Vehicle Speed Calculation for Slip Ratio Estima- tion For calculation of slip ratio the vehicle speed is needed according to (6). Considering that the driving resistance exits, this paper utilizes the acceleration sensor to cal- culate vehicle speed by integration based on an assumption (21) of flat road deceleration in short time interval as below ∧ Vc = a · dt + Vc0 ···························· (23) Fig. 5. Typical μ-λ curves Vc0 = max(Vω f , Vωr)··························· (24) α · ∗ T where Vc0 is the initial value for integral calculation. When Tri = ··································· (18) 1 + α brake pedal is steeped on, Vc0 is set to be wheel speed value which is a larger value between the front and rear wheels on 4. Proposed Anti-Slip Control Strategy the instant of nonzero pedal stroke. In order to avoid accumu- The wheel slip will increase to result in wheel lock and lated error causing inaccurate calculation in integration, the cause vehicle instability, once brake force is greater than fric- initial value Vc0 is acquired every single brake pedal operation force that road surface can supply. According to the basic tion. Here, we assume the ineffective stroke of brake pedal to principle of conventional hydraulic ABS system, the wheel be zero. With calculated vehicle speed, the wheel slip ratio dynamics that remains at the stable region or approaches to can be calculated as the unstable region can be judged by monitoring the wheel ∧ ∧ V − V slip. Figure 5 shows four typical μ-λ curves which are in re- λ = w c ···································(25) ffi ∧ lation to the slip ratio and longitudinal friction coe cients Vc ff on di erent types of road surfaces. It can be found that the 4.2 Hydraulic Antilock Braking Control The con- vehicle is able to stay stable within a certain slip ratio value trol method of conventional hydraulic ABS is so called logic λ ffi opt which has the maximum friction coe cient i.e. the max- threshold control. The ABS control tries to avoid high slip imum braking force. On the contrary, when wheel slip is occurrence by adjusting the hydraulic pressure in the brake λ greater than opt, the vehicle turns to unstable. calipers and consequently the brake torque clamped. The λ The ideal anti-slip control is to keep slip ratio as opt all most common ABS algorithm is based on vehicle decelera- the time until vehicle stops except for the timing of slip ratio tion and slip ratio threshold. By means of designed threshold λ increasing from 0 to opt. In other words, once the wheel slip values, HUC of ABS controls increase, hold or decrease of is controlled precisely, the brake performance reaches opti- the existing wheel hydraulic pressure (25). Accordingly, in this mum and the stopping distance is the shortest. In this study study we designed an ABS control algorithm with two slip we utilize wheel slip as a reference parameter in the control ratio threshold values to achieve the equivalent operation as method as in the case of the conventional hydraulic ABS. The (26). The wheel hydraulic pressure is calculated as (27) and identical control object aids in integration of the two distinct the final friction torque is derived by substituting (27) into ABS control systems. Here, considering the definition of slip (10). ratio, we can rewrite (6) as (19). By taking differential of it, ⎧ ⎪ √ ∧ we can get (20) ⎪ k (P − P )0≤ |λ| < λ∗ ⎪ 1 m w L ⎨ ∧ = λ + · ······························ Q = ⎪ λ∗ ≤ |λ| < λ∗ ····· (26) Vw ( 1) Vc (19) ⎪ 0 L H λ ⎪ √ ∧ dVw dVc d ⎩ ∗ = (λ + 1) + V ···················· (20) −k2 Pw − Pr λ ≤ |λ| ≤ 1 dt dt c dt H = · = · ······················ When slip occurs, if slip ratio can be maintained at value of Pw k3 U k3 Qdt (27) λ during braking as mentioned ideal control, i.e. dλ/dt = opt Q P 0, (20) can be rewritten as (21). where is the hydraulic flow in brake caliper. m is the master hydraulic pressure. Pr is the reservoir hydraulic pressure. . · λ∗ and λ∗ are lower slip ratio threshold and upper slip ratio Vw = (λopt + 1) · Vc ····························(21) L H threshold, respectively. These two thresholds are designated (4) AccordingtoYinet al. , “if wheel and chassis accelera- nearby the λopt in Fig. 5. k1 and k2 are coefficients determined tions are well controlled, then the difference between wheel by orifice size and oil viscosity. U is the fluid capacity. k3 is and chassis velocities, i.e. the slip is also well controlled”. acoefficient decided by characteristic of brake parts. Here, by rewriting (21) into the form of (22) to describe the Furthermore, in order to increase the pressure unhurriedly accelerations of the vehicle and the wheel as a ratio. It is to avoid quick recovery of slip once ABS control is operated. truth that if braking torque is controlled based on this ratio, The control technique of pulse step increasing is introduced. i.e. maintain the wheel and vehicle acceleration constantly This control method is also wildly used in current ABS con- (24) with respect to value of λopt, the wheel slip does not become trol technology .

321 IEEJ Journal IA, Vol.3, No.4, 2014 Integrated Slip-Based Torque Control of ABS for EV（Wen-Po Chiang et al.）

4.3 Regenerative Antilock Braking Control The target of this study is to maintain acceleration ratio shown in (22) to be a constant to achieve anti-slip control. Based on it, Yin et al. (4) had presented a control method for slip preven- tion which provides a higher anti-slip performance for accel- erating traction control. However, in the braking behavior the acceleration ratio of wheel to vehicle will be limited all the time so that the brake performance might be limited, because the brake forces will be constrained early i.e. the wheel slip is not sever enough. Moreover, the presented control method did not consider the effect of inaccuracy of hydraulic brake torque. Fig. 6. Regenerative antilock braking control system In this study, a designated value is defined for anti-slip control usage and we conduct control conditions as (28) by comparing the designated slip ratio to estimated one. This paper adopted the control method using the logic thresh- ⎧ ff ⎪ ∧ old concept with di erent designed threshold values. The ⎪ ∗ ⎨ Tm 0 ≤ |λ| < |λ | regenerative ABS works at appropriate designated slip ratio Tm = ⎪ ∧ ···················(28) ⎩⎪ ∗ ∗ and hydraulic ABS works around this slip ratio by increas- T |λ | ≤ |λ| ≤ 1 mt ing, holding, decreasing of hydraulic pressure. For example, ∗ λ∗ if the reference slip ratio for regenerative ABS is designated where Tmt is the target torque for anti-slip purpose. is the to be 0.2, the slip ratio threshold values of hydraulic ABS are reference slip ratio, which is equal to the general value λopt for most road surfaces showing in Fig. 5. During the nor- between 0.15 and 0.25 which assists to avoid the occurrence mal braking, i.e. a braking period without slip occurrence, of slip. the output brake torques follows the I-curve to make full use When controller detects that the vehicle tends to slip, i.e. of road friction efficiently and assure that the front and rear the potential lock on wheel, the integrated anti-slip control wheels behave the same. If brake force is greater than friction strategy holds the hydraulic bake pressure in advance and force that road surface can supply i.e. the estimated slip ratio then constrains regenerative torques by derived ideal allow- is greater than λ∗, the vehicle slips and wheel tends to lock able torques. If regenerative ABS can suppress the wheel slip and cause vehicle instable. alone, the hydraulic ABS is not used to reduce the hydraulic This study proposed a novel control approach to limit the pressure during the braking operation. On the other hand, if ∗ the wheel slip is not sufficiently suppressed and continues to brake torques by derived a target motor torque Tmt, called ideal allowable torque. By substituting the reference slip ra- increase, the hydraulic ABS is used to reduce the hydraulic tio λ∗ into (22), the target wheel angular acceleration can be pressure to ensure that wheel slip stays within the desired calculated as (29). Although the implement of acceleration range. estimation on the EV is possible, but the anti-slip perfor- 5. Simulation Results mance will be affected severely if vehicle acceleration is not sensed directly, especially braking on slope or the hydraulic This section evaluates the effectiveness of proposed con- friction torque calculation exits inaccuracy. Therefore, the trol strategy. The simulation environment showing in Fig. 7 accelerometer is utilized. is constructed in MATLAB/Simulink. The vehicle model is · λ∗ + built with Magic Formula based on Fig. 2 containing the lon- ω∗ = a( 1) ································ t (29) gitudinal weight transference. The major specification of ap- r plication vehicle is shown in Table 1. Due to the maximum With target wheel angular acceleration, calculated friction torque ability of motor, the regenerative braking is able to be torque and estimated friction force, the ideal allowable torque utilized up to 0.4g of deceleration i.e. motors can generate can be derived by (30). The estimated friction force of wheel maximum total torque about 2000 Nm. The distribution ra- between tire and road surface is derived as (31) by observer tio of front/rear hydraulic friction torques is set to be a fixed according to (1) and (2). value of 1.5 which is widely used for passenger car. The conditions of road surface are specified by the pick · ∧ ∗ = ω∗ + − ························· value of friction coefficient of μ-λ curves. The reference slip Tmt Jω t rFd Th (30) . λ∗ ∧ ratio for regenerative ABS and two slip ratio thresholds for T + T − Jω ω ∗ ∗ F = m h ·························· (31) hydraulic ABS, λ and λ are −0.2, −0.18 and −0.22, respec- d r L H tively. The time constants are set as following, τ1 = 0.02, With the derived ideal allowable torque, the wheel angu- τ2 = 0.01. The speed limit of hydraulic ABS is 3.6 Km/h lar acceleration can be controlled to achieve desired anti-slip (1 m/s). The SOC of battery is assumed to be around 50% performance by restraining the surplus regenerative torque. during the braking. Figure 6 shows the block diagram of proposed regenerative 5.1 Slight Braking on Low µ Road Surface This antilock braking control system for a single wheel. simulation performs the vehicle to decelerate with a decel- 4.4 Integrated Antilock Braking System By in- eration close to −0.3g (−2.94 m/s2)onjumpμ road surface specting two distinct ABS systems, the regenerative ABS and which begins with dry concrete (μpeak = 0.8) and changes to the hydraulic ABS, the same referring variable is slip ratio. compacted snow road (μpeak = 0.3) at t = 2 s. Brake operation

322 IEEJ Journal IA, Vol.3, No.4, 2014 Integrated Slip-Based Torque Control of ABS for EV（Wen-Po Chiang et al.）

Fig. 7. Block diagram of entire system in MATLAB/Simulink

Table 1. Speciﬁcation of application vehicle

(a) Conventional ABS (b) Integrated ABS

limit of road surface. Because the brake torques of proposed control are distributed effectively, the stopping distance started at t = 1 s with initial speed of 60 Km/h and target brake is shortened about 3.03 m. In other words, almost 7.4% of torque is about 1645 Nm according to brake pedal input. Be- needed stopping distance is reduced. cause the deceleration of this simulation case is below 0.4g, 5.2 Heavy Braking on Low µ Road Surface In the regenerative braking is responsible for the decelerating. this simulation, the vehicle is performed on dry concrete- Figure 8 shows the comparison results between conven- compacted snow jump μ road surface as well but with heavy tional hydraulic ABS method and proposed integrated ABS brake operation of −0.7g (−6.86 m/s2). The target brake method. Figure 8(a) gives the brake performance results of torque is about 4090 Nm. Because the deceleration is higher conventional hydraulic ABS. Figure 8(b) represents the sim- than 0.4g, both the regenerative braking and hydraulic brak- ulation results of proposed one. Figure 8(c) and Fig. 8(d) ing are needed during the normal brake at initial two seconds. compare the vehicle speeds and decelerations between two As simulation results in Fig. 9(a) and Fig. 9(b), the brake ABS control methods, respectively. system with conventional ABS relying on conventional ABS As simulation results in Fig. 8(a) and Fig. 8(b), once the stops the regenerative torques and suppresses the front/rear vehicle runs into the slippery road surface at 2 s, two brak- friction torques after entering the snowy road surface. On ing systems with different ABS control method have dis- the contrary, the brake system with integrated ABS decreases tinct behavior of brake performance. For the brake system the hydraulic braking torque to be zero and suppresses the with conventional ABS, front slip ratio increases greater then regenerative torque according to anti-slip control when the λ∗ lower slip ratio threshold value L so that the regenerative vehicle runs into the compacted snow road. Because only re- torques are turned off and hydraulic braking joins in. Then, generative braking is enough for slippery road, the hydraulic hydraulic braking follows the hydraulic ABS control method braking is thought as redundancy. to avoid the front wheel lock and maintain its wheel speed As comparison results in Fig. 9(c) and Fig. 9(d), the brake oscillationally until vehicle stops. On the other hand, for the system with integrated ABS achieves shorter stopping time brake system with integrated ABS it is apparent that at the and shortens about 3.32 m of stopping distance which is proposed distribution control method distributes appropriate about 13.2% reduction. The deceleration also approaches to brake torques on front and rear wheels so that the slip rations 2.9 m/s2 stably to state that the road surface is utilized effec- are about the same during the normal braking. Because the tively better than conventional one. friction of road surface is well utilized, the wheel slips do not 5.3 Strong Braking on Medium µ Road Surface exceed the reference slip ratio λ∗. Unlike conventional ABS, Above simulations give an example to inspect when vehicle the proposed anti-slip control is not activated. encounters an extreme condition, i.e. the low μ condition. In By comparing the stopping time and deceleration between fact, the slip happens also under a normal adhesive road sur- two ABSs as shown in Fig. 8(c) and Fig. 8(d), the brake sys- face in common use, such as wet asphalt (μpeak = 0.5). This tem with integrated ABS achieves shorter stopping time and simulation performs with −0.6g (−5.88 m/s2) of deceleration higher stable deceleration of 2.9 m/s2 which is close to the on dry concrete-wet asphalt jump μ road. The target brake

323 IEEJ Journal IA, Vol.3, No.4, 2014 Integrated Slip-Based Torque Control of ABS for EV（Wen-Po Chiang et al.）

(a) Conventional ABS (b) Integrated ABS (a) Conventional ABS (b) Integrated ABS

(c) Vehicle speeds (d) Accelerations (c) Vehicle speeds (d) Accelerations μ Fig. 9. Brake performance comparison on low μ road Fig. 10. Brake performance comparison on medium surface with heavy braking road surface with strong braking torque is about 3540 Nm. In this condition, the brake system with conventional ABS has the same operation of relying on regenerative ABS without regenerative braking usage. On the contrary, the system with integrated ABS needs to coordinate the regenerative ABS with hydraulic ABS to achieve anti-slip performance because the deceleration is higher than 0.4g. (a) Heavy braking on low μ road surface As simulation results in Fig. 10(a) and Fig. 10(b), the brake system with conventional ABS operates as same as previous simulation. On the other hand, the brake system with integrated ABS has coordinated the hydraulic torques and regenerative torques according to integrated ABS control method. As comparing results showed in Fig. 10(c) and Fig. 10(d), the brake system with proposed integrated ABS can achieve (b) Strong braking on medium μ road surface shorter stopping time and shorten the stopping distance about Fig. 11. Brake performance comparison with friction 2.33 m which is about 13.7% reduction of needed stopping torque inaccuracy distance. The deceleration reaches to 4.8 m/s2 most of time stably to verify that the road surface is utilized effectively better than conventional ABS with oscillatory deceleration. still works effectively even with friction torques inaccuracy. 5.4 Performance with Friction Torque Inaccuracy The shortened stopping distance comparison is only effect of In Sect. 2, we have assumed that the friction torque of 0.5% which is neglectablely small. hydraulic braking can be calculated correctly from the mea- 5.5 Performance with Vehicle Mass Variation surement of wheel hydraulic pressure. However, the uncer- Figure 12 shows the comparative anti-slip performance tain variation of brake lining due to the wearing or ambient results with perturbations in vehicle mass. In a real driv- conditions is unpredictable especially during the long time ing environment, the vehicle mass can vary significantly. In braking or urgent braking. In order to verify the adaptabil- order to cover the variational possibility in vehicle mass, ity of proposed integrated ABS control method, this simu- two conditions of vehicle loading are performed to evalu- lation performs same scenarios in Sect. 5.2 and Sect. 5.3 but ate the robustness of proposed controller. One condition is adding 30% inaccuracy d to calculated friction torque, i.e. the so called Light Vehicle Weight (LVW) which is mass con- calculated friction torque is 30% larger than actual one. As tains net weight of a vehicle and driver standard mass. An- vehicle speed comparison results showed in Fig. 11(a) and other condition is so called Gross vehicle weight (GVW) is Fig. 11(b), the proposed integrated anti-slip control method the maximum operating weight of a vehicle as specified by

324 IEEJ Journal IA, Vol.3, No.4, 2014 Integrated Slip-Based Torque Control of ABS for EV（Wen-Po Chiang et al.）

(a) Heavy braking on low μ road surface

(b) Strong braking on medium μ road surface Fig. 12. Brake performance comparison with variation in vehicle mass (a) Hydraulic ABS (b) Integrated ABS the manufacturer. In this case, the GVW of vehicle is set to be 2025 kg. Same scenarios in Sect. 5.2 and Sect. 5.3 are also performed in this simulation to show the comparisons. As results showed in Fig. 12(a) and Fig. 12(b), the proposed control method is able to perform effectively in both cases without apparent changes. (c) Vehicle speeds (d) Accelerations Fig. 13. Brake performance comparison results on split 6. Extended Simulation Study μ road surface from CarSim 6.1 Verification by using CarSim In order to verify the proposed method on a detailed full-order model that con- the appropriate distribution and control between regenera- tains the modeling uncertainties and non-linearity, a vehicle tive braking and hydraulic braking, and showed the better model and road model in simulation software CarSim 8.11 brake performance and anti-slip performance than conven- were utilized. The built-in vehicle model of B-Class, Hatch- tional ABS control. During normal braking, the brake torques back (with ABS), is employed and re-built with powertrain are distributed appropriately to make full usage of friction of four in-wheel-motor drive. The road surface is with split force of road surface by following I-curve. During slip con- friction coefficient (0.5L/0.8R). For fair comparison, the ra- dition, the oscillatory behavior of slip ratio is improved and tio of regenerative torque output between front and the rear the stopping distance is shortened average about 13% com- wheels is 8 : 3 which is default setting of brake system on the paring to conventional ABS control. Moreover, an extened simulation study using CarSim in vehicle model of B-Class, Hatchback. Because the default ff threshold values λ∗ /λ∗ of hydraulic ABS control of B-Class, section 6 is performed to verify the e ectiveness of proposed L H method on a detailed full-order vehicle model. The compar- Hatchback are 0.2/0.1 for front wheel and 0.1/0.06 for rear ∗ ison results of shorter stopping distance and stable decelera- wheel, the reference slip ratios λ for regenerative ABS are tion are in agreement on the simulation results in Sect. 5. set to be 0.15 and 0.08 for front and rear wheels, respectively. By inspecting the behavior of vehicle deceleration, the con- In the following simulation, the wheels on the left side are tinuously stable deceleration can achieve better ride comfort used to evaluate. The vehicle decelerates at initial speed of (26) / i.e., no jerk (defined as differentiation of deceleration) . 80 Km h. the regenerative braking is able to be utilized up to ffi 0.4g of deceleration. Furthermore, the regenerative e ciency is expected to be en- hanced because of continuous usage of regenerative braking. 6.2 Comparison Results Figure 13 shows the brake In Sect. 5.4, the inaccuracy of friction torque is added to comparison results for the pure hydraulic ABS and the pro- verify the robustness of proposed control strategy. By substi- posed integrated ABS. As results showing in Fig. 13(a) and tuted (31) into (30), we can know that ideal allowable torque Fig. 13(b), each case can retrieve the increasing slip ratios by ∗ Tmt which restrains the surplus regenerative torque if wheel reducing the brake torques. However, the proposed method λ∗ can achieve less quantity of oscillation, and smaller ampli- slip exceed reference slip ratio is independent of calculated tude as well, which benefits the anti-slip performance better friction torque as below · than the hydraulic one. From Fig. 13(c) and Fig. 13(d), the ∗ ∗ · T = Jω(ω − ω) + T ·························(32) stopping distance is shortened about 1.6 m, and the deceler- mt t m ation achieves to 6 m/s2 stably which is utilized effectively The simulation results demonstrate the braking perfor- better than conventional one (oscillates within 5 to 6 m/s2). mance is nearly the same even the inaccuracy of friction torques is introduced. That is, the adaptability of proposed 7. Discussion control approach has proved. First three simulations in section 5 are performed on the Detailed analysis of control characteristics can be per- designated road surface to verify the effectiveness of pro- formed with a partially linearized one wheel vehicle model. posed control strategy. The simulation results demonstrated When the control system falls into an anti-slip state, the entire

325 IEEJ Journal IA, Vol.3, No.4, 2014 Integrated Slip-Based Torque Control of ABS for EV（Wen-Po Chiang et al.）

motor EVs. In the braking system, brake torque distribution controls regarding to available motor torque and maximum brake effectiveness are considered. A novel control strategy based on ideal anti-slip control is proposed. We adopted the logic threshold concept as control conditions with different designed thresholds for slip-based torque control. The integrated system coordinates regenerative ABS and hydraulic ABS to improve the anti-slip performance and realize a good usage of road friction when vehicle encounters slippery roads. The comparative simulation results and discussion demonstrated its effectiveness and adaptability with regard to the antilock braking performance. The proposed anti-slip control stably constrains the skidding superior to the conventional hydraulic ABS, which can shorten the stopping Fig. 14. Partially linearized control system distance and provide better ride comfort on different road surfaces. Moreover, the control strategy makes full use of regenerative braking so that the better energy efficiency can be control system can be simplified into a closed-loop feedback expected. system shown as Fig. 14. Here, D is speed difference which represents the extent of the slip, and Ku represents the relation between D and friction force. Ku isassumedtobeposi- tive because the discussion is limited to conditions where the References slip ratio is in the unstable region. Fdo is the friction force between tire and road surface when anti-slip control starts. ( 1 ) H. Shimizu, J. Harada, C. Bland, K. Kawakami, and L. Chan: “Advanced concepts in electric vehicle design”, IEEE Trans. Ind. Electron., Vol.44, No.1, The delay of the electromechanical system is simplified as an pp.14–18 (1997-2) LPF with a time constant of τ for easy analysis. ( 2 ) K.T. Chau, C.C. Chan, and C. Liu: “Overview of permanent-magnet brush- less drives for electric and hybrid electric vehicles”, IEEE Trans. Ind. Elec- The transfer functions from Fdo to D, from Th to D and from d to D are defined as T , T and T respectively. tron., Vol.55, No.6, pp.2246–2257 (2008-6) DF DT Dd ( 3 ) S. Sakai, H. Sado, and Y. Hori: “Motion control in an electric vehicle with ∗ four independently driven in-wheel motors”, IEEE/ASME Trans Mechatron- −J τs + Jωλ T = n ics, Vol.4, No.1, pp.9–16 (1999-3) DF 2 ∗ Jω Mτs +(Jω M−JnKuτ)s+JωKuλ ( 4 ) D.J. Yin, S. Oh, and Y. Hori: “A novel traction control for EV based on Max- Mrτs imum Transmissible Torque Estimation”, IEEE Trans Ind. Electron., Vol.56, T = ····· No.6, pp.2086–2094 (2009-6) DT 2 ∗ (33) Jω Mτs +(Jω M−JnKuτ)s+JωKuλ ( 5 ) Z.C. Sun, Q.H. Liu, and X.D. Liu: “Research on electro-hydraulic parallel brake system for electric vehicle”, International Conference on Vehicular TDd = 0 Electronic and Safety, pp.376–379 (2006) where J is defined as the equivalent system inertia as below ( 6 ) H. Yeo, S. Hwang, and H. Kim: “Regenerative braking algorithm for a hybrid n electric vehicle with CVT ratio control”, J. Autom. Eng., Vol.220, pp.1589– 2 1600 (2006-8) Jn = Jω + Mr ·································(34) ( 7 ) G. Xu, W. Li, K. Xu, and Z. Song: “An intelligent regenerative braking strategy for electric vehicles”, Energies, Vol.4, pp.1461–1477 (2011-9) Therefore, the steady state response of the system to inputs ( 8 ) S.I. Sakai and Y. Hori: “Advanced vehicle motion control of electric vehicle can be described as below: based on the fast motor torque response”, 5th International Symposium on Advanced Vehicle Control, pp.729–536 (2000) ( 9 ) T. Suzuki and H. Fujimoto: “Slip ratio estimation and regenerative brake con- lim D(t) = lim sTDF(s)Fdo(s) + lim sTDT (s)Th(s) t→∞ s→0 s→0 trol for decelerating electric vehicles without detection of vehicle velocity and ··················· acceleration”, IEEJ Trans. on IA, Vol.130, No.4, pp.512–517 (2010) (35) (10) N. Mutoh, Y. Hayano, H. Yahagi, and K. Takita: “Electric braking control methods for electric vehicles with independently driven front and rear Here, assume Fdo and Th to be a step reference, (35) can wheels”, IEEE Trans Ind. Electron., Vol.54, No.2, pp.1168–1176 (2007-4) be used to examine the relation between the friction force and (11) D.H. Kim, J.M. Kim, S.H. Hwang, and H.S. Kim: “Optimal brake torque the speed difference, and the relation between friction torque distribution for a four-wheel drive hybrid electric vehicle stability enhance- ff ment”, Proceedings of the Institution of Mechanical Engineers, Part D: J. and speed di erence, separately. Autom. Eng., Vol.221, No.11, pp.1357–1366 (2007-11) In the results, the speed difference is positively propor- (12) E. Cacciatori, B. Bonnet, N.D. Vaughan, M. Burke, D. Price, and K. tional to the friction force, and is not affected by friction Wejizanowski: “Regenerative braking strategies for a parallel hybrid powertrain with torque controlled IVT”, SAE Technical Paper, 2005-01-3826 torque in the steady state. Thus, the proposed control system (2005) can be verified that the worse slip condition has the better (13) J. Zhang, X. Lu, J. Xue, and B. Li: “Regenerative braking system for se- anti-slip performance. Moreover, because positively propor- ries hybrid electric city bus”, World Elect. Veh. J., Vol.2, No.4, pp.128–134 (2008-12) tional relation is only related to Ku,theeffect of vehicle mass (14) L. Zhou, Y. Luo, K. Li, and X. Lian: “A novel brake control strategy for variation is negligible. The analysis results have agreed with electric vehicle based on slip trial method”, IEEE International Conference simulation results. on Vehicular Electronic and Safety, pp.1–6 (2007) (15) G. Zhuo and H. Li: “Research on Electro-hydraulic parallel brake system 8. Conclusion based on ABS”, International Conference on Electrical and Control Engi- neering, pp.782–787 (2011) This paper presented an integrated antilock braking system (16) Y. Zhou, S. Li, Z. Fang, and Q. Zhou: “Control strategy for ABS of EV based on a series configuration for four wheel drive in-wheel with independently controlled four in-wheel motors”, 4th IEEE Conference

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on Industrial Electronics and Applications, pp.2471–2476 (2009) Dejun Yin (Non-member) received his B.S. and M.S. degrees in Elec- (17) C. Song, J. Wang, and L. Jin: “Study on the composite ABS control of vehi- trical Engineering from the Harbin Institute of Tech- cles with four electric wheels”, J. Comput, Vol.6, No.3, pp.618–626 (2011-3) nology, China in 1999 and 2001, respectively. He re- (18) C. Mi, H. Lin, and Y. Zhang: “Iterative learning control of antilock braking ceived another M.S. degree in Electronics Engineer- of electric and hybrid vehicles”, IEEE Trans. Veh. Technol., Vol.54, No.2, ing from Chiba Institute of Technology, Japan, in pp.486–494 (2005-5) 2002. Then, he worked on embedded control sys- (19) D. Peng, Y. Zhang, C.-L. Yin, and J.-W. Zhang: “Combined control of a re- tem design in Mechanical-Electronics Engineering generative braking and anti-lock braking system for hybrid electric vehicles”, for nearly 5 years. In 2006, he entered the University International J. Autom. Technol., Vol.9, No.6, pp.749–757 (2008-5) (20) H. Zhang and X. Chen: “Study on the Electronic-hydraulic Compound Anti- of Tokyo and received his Ph.D. degree in Electrical lock Braking System for Four In-Wheel Motor Driving Vehicle”, Interna- Engineering in 2009. He is currently a professor of tional Conference on Electric Information and Control Engineering (ICE- Nanjing University of Science and Technology in China performing research ICE), pp.6176–6183 (2011) and development on vehicle control and drive system design for next gener- (21) T. Suzuki and Fujimoto: “Regenerative braking control based on slip ratio ation electric vehicles. with integrated accelerometer for short time interval for electric vehicle”, IEE of Japan Industry Applications Society Conference, pp.1–6 (2008) (22) T. Ishige, H. Furusho, Y. Aoki, and K. Kawagoe: “Adaptive slip control using Manabu Omae (Non-member) is a professor of the Graduate School a brake torque sensor”, 9th International Symposium on Advanced Vehicle of Media and Governance, Keio University. He re- Control, Vol.1, pp.417–422 (2008) ceived his Ph.D. in Mechanical Engineering from the (23) O’Dea and Kevin: “Anti-Lock Braking Performance and Hydraulic Brake University of Tokyo in 2000. His major research in- Pressure Estimation”, SAE Technical Paper, 2005-01-1061 (2005) terests are advanced vehicle control and safety sys- (24) J. Guo, J. Wang, and B. Cao: “Regenerative braking strategy for electric ve- tems, and automatic driving. hicles”, Intelligent Vehicles Symposium, pp.864–868 (2009) (25) Japan. ABS Co., Ltd.: “ABS Research for Automobile”, Sankaido, pp.59–70 (1993) (26) F. Wang, K. Sagawa, and H. Inooka: “A study of the relationship between the longitudinal acceleration/deceleration of automobiles and ride comfort”, Jpn. J. Ergon, Vol.36, No.4, pp.191–200 (2000)

Hiroshi Shimizu (Non-member) is a professor emeritus of Keio Uni- veisity. He earned his Ph.D. from the Graduate Wen-Po Chiang (Non-member) received a M.S. degree in Mechan- School of Engineering of Tohoku University in 1975. ical Engineering from the Michigan Technological In 1976 he began his work as a researcher at the Na- University, MI, United States in 2004. He was a tional Institute for Environmental Studies. He be- senior engineer and worked on brake system design came a professor in the Faculty of Environment and and development at China Motor Corp., Taiwan for Information Studies at Keio University in 1997. He 6 years. He studies his Ph.D. degree at Keio Univer- has been a leading ﬁgure in Japan for electric vehicle sity since 2011, performing research on vehicle mo- development, working on development of 8 concept tion control, especially in the ﬁeld of brake system for cars in 30 years. He was a technology leader in the next generation electric vehicles. Eliica project. The Eliica concept car which utilizes next generation “in- wheel motor” technology reached a speed of 370 km/h in 2004 tests. Author Name (Membership Category of IEEJ) the quick brown fox jump over the lazy dog. The quick brown fox jump over the lazy dog. The quick brown fox jump over the lazy dog. The quick brown fox jump over the lazy dog. The quick brown fox jump over the lazy dog. The quick brown fox jump over the lazy dog. The quick brown fox jump over the lazy dog. The quick brown fox jump over the lazy dog. The quick brown fox jump over the lazy dog

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