automotive control devices and belong to the obligatory equipment to be installed ..... [1] Bosch Automotive Handbook, Wiley & Sons, 2007. [2] T. Yoshimura, H.
Electric Vehicles with Individually Controlled On-board Motors: Revisiting the ABS Design Valentin Ivanov, Dzmitry Savitski, Klaus Augsburg
Phil Barber
Department of Automotive Engineering Ilmenau University of Technology Ilmenau, Germany
Vehicle Capability Research Jaguar Land Rover Limited Abbey Road, Whitley, Coventry, UK
Abstract—The paper introduces the anti-lock braking system (ABS) designed for the all-wheel drive full electric vehicle equipped with four on-board motors. The main features of the ABS under consideration are (i) continuous wheel slip control with feedforward and feedback controller parts, and (ii) versatile actuation architecture realizing pure electric braking, braking with electro-hydraulic decoupled brake systems, and blended braking with operation both of friction brakes and electric motors in regenerative mode. Results of ABS validation demonstrate essential benefits of the developed control strategy for brake performance, precise wheel slip tracking, and drive comfort at braking. Adaptive properties of the proposed ABS are discussed for test cases including the transient road friction.
higher bandwidth of motors, compared to friction brakes) and energy efficiency of the vehicle (with the possibility of energy regeneration by the brake operation).
Keywords—electric vehicle; anti-lock braking system; wheel slip control; continuous control; on-board motors; vehicle test
I. INTRODUCTION Anti-lock braking systems are one of the most traditional automotive control devices and belong to the obligatory equipment to be installed on passenger cars and commercial vehicles in accordance with safety regulations. As of now, one can speak about established approaches to the ABS design that can be characterized by: (i) Individual control of friction brakes to avoid the wheel lock at braking; (ii) Use of wheel slip estimates and deceleration as the ABS control parameters; (iii) Dominating application of the rule-based control to the ABS strategy. These positions are valid for most commercial anti-lock braking systems and explained in more details in various related reference books, for example in [1]. The emergence of hybrid and full electric vehicles, which are receiving now more and more attention not only in research but also in the market, opens up possibilities for new approaches to the ABS design. The reason is that an electric powertrain can operate in a braking mode by producing negative torques to recuperate energy, which is normally dissipated in conventional brake systems. This kind of braking is especially effective in the case of installation of individual electric motors for each wheel due to better opportunity for integration of the electric and friction brake systems. The ABS architecture, which uses electric motors as the actuators, can bring additional benefits; for the vehicle safety (quicker system response and
The listed arguments have motivated intensive studies in the development of anti-lock braking systems for electric vehicles. It should be mentioned that most of related research works in this area are mainly concentrated on the strategies of regenerative braking in the case of ABS intervention. Problems of the ABS design based on pure actuation of electric motors are still rarely addressed. By contrast, the adoption of traction control (TC) functions to electric motors has well-established approaches. In particular, relevant TC system based on fuzzy control was introduced by Yoshimura et al. in [2] for an all-wheel drive electric vehicle. Model-based and model-following wheel slip control methods for electric vehicles are described in publications of Hori, Toyoda and Tsuruoka [3], Akiba et. al. [4], and Kato [5]. Of special interest are also the studies proposing advanced methods for wheel slip estimation in the case of cars with individually controlled electric motors. Relevant control systems are discussed, in particular, in studies of Fujimoto, Maeda, Hori, and Fujii [6-7]. Several research works have specifically discussed methodological approaches to electric motor actuation in ABS mode. In particular, fuzzy logic [8], linear quadratic control [9], combination of wavelet transforms and back EMF analysis [10], and iterative learning [11] are known in this context. Recent advancements of electric motor technologies have allowed the introduction of several variants of electric ABS installed on vehicle platforms. For instance, industrial implementation of the anti-lock braking system actuating inwheel motors is described in [12, 13] for Toyota electric vehicle and in [14] for Roading car with motors of Siemens AG. Other applicative examples can be found in [15]. In spite of many-sided research activities for anti-lock braking control in electric vehicles, certain topics are still insufficiently explored. Especially, it is the case for (i) influence of electric powertrain dynamics on the ABS operation, (ii) adaptation of the pure electric and blended ABS actuation to variation of the road and operational conditions, (iii) experimental procedures for validation of electric ABS functionality. Contributing to the mentioned range of problems, the presented paper will introduce several outcomes of the new ABS design applied for all-wheel drive sport utility vehicle
with individually controlled on-board electric motors. The main target of this study is to prove experimentally the feasibility of the continuous wheel slip control realized through electric motor torque modulation. The architecture of relevant continuous ABS differs essentially from conventional anti-lock braking systems using rule-based controllers. The comparison of rule based and continuous ABS control lies out of scope of the presented paper, however such an analysis is given in the following work of authors [16]. Next paper sections discuss the continuous ABS design in more details as well as experimental results of the ABS operation in various test conditions. II. ELECTRIC VEHICLE ARCHITECTURE The anti-lock braking control under discussion has been developed for an experimental electric vehicle based on the Range Rover Evoque platform. The powertrain architecture of the vehicle includes four on-board electric motors connected to the wheels through gearboxes and half-shafts. Fig. 1 shows a drawing illustrating the powertrain components. Technical specification of the vehicle:
Total weight: 2117 kg; Motors: Switched reluctance type; Peak torque / power (30 sec) – 200 Nm / 100 kW; Nominal torque / power – 125 Nm / 42 kW; Maximum speed – 15000 m-1; Transmission (motor-to-wheel): Twostage reducer with helical gears and half-shaft; Overall gear ratio – 1:10,5; Estimated torsional stiffness of half-shafts – 6500 Nm/rad; Battery packs: 400 VDC, 6 kWh; Peak / nominal power – 160 kW / 80 kW; Tyres: 235/55 R19.
The global vehicle controller implemented in a rapid prototyping system dSPACE, includes the following subfunctions: base brake control, regenerative brake control, ABS and TC functions, direct yaw moment control, torque distributor, generator of reference slip ratio, energy coordinator, vehicle state estimator, and error handler.
III. DESIGN FEATURES OF CONTINUOUS ABS The individual control of electric motors can be realized as the continuous regulation of the driving and braking torques on wheels that contributes to the constant and seamless control of the vehicle dynamics under all possible driving conditions. Involvement of the electric powertrain in continuous wheel slip control with in-wheel and on-board electric drives for ABS purposes allows the torque modulation frequency from 3-8 Hz (typical for conventional hydraulic ABS) to be expanded to a level of >8 Hz. The structure of the continuous anti-lock braking system implemented in the presented study has been firstly published in [18] and now is advanced with extra functions like adaptation to uneven road surfaces and surfaces with inhomogeneous friction. The main elements of the proposed ABS are depicted on Fig. 2. The ABS controller consists of predictive (feedforward) and reactive (feedback) parts. The predictive formulation of the brake torque demand Tdem for each wheel is realized by the base brake controller. The torque demand Tdem is computed from the driver control action Fdriver, estimated by the signal of the brake pedal sensor, and from the estimated maximum road friction coefficient µmax_est and the normal wheel load Fz:
min Tdem _ prim,Tpred if Tdem _ prim 0 Tdem max Tdem _ prim ,Tpred if Tdem _ prim 0 where Tdem_prim is the preliminary value of the torque demand derived as the function of the brake pedal travel. Tpred is the predictive torque calculated as Tpred µmax_ est Fz r k pred µmax_ est Fz r
where r is the tire rolling radius, kpred is the correction coefficient related to the type of the tyres.
The vehicle is also equipped with the decoupled electrohydraulic brake system actuating the friction brakes. Therefore, three modes are possible for the ABS operation: pure electric braking, pure friction braking, blended (combined) braking.
Battery pack
Front axle assembly
High voltage connection box MGU (dSpace Autobox)
Battery charger
Rear axle assembly
Fig. 1. Electric vehicle packaging (reproduced from [17]).
Fig. 2. Continuous ABS controller structure (adapted from [18]).
The control law for the reactive torque is
react PI driver _ dem where the correction factor driver_dem is used to control the saturation in order to prevent generation of inappropriate torques and is calculated depending on the driver demand. The proportional part of the controller is the function of the vehicle velocity Vx
P K P Vx min 0, e and the integral part is computed as:
I KI Vx , mode min 0, e Tdem _ wheel sat e I where the coefficient is a modifying factor to control the quickness of the integral part changing, Tdem_wheel is the torque demand for single wheel. In Eqs. (4) and (5), the variable e is the control error. The saturation of the control error is performed in the following way:
1, max(0, e) 0.02 sat e . e, max(0, e) 0.02
(6)
The sum of predictive and reactive torques is further blended and used to control the slip ration on individual wheels. The torque blending procedure block splits the total wheel torque demand into the torque demand for the electric motors Tem_dem and for the friction brakes Tbr_dem. Both torques are being realized by corresponding subsystems with the outputs Tem and Tbr. The reference slip ref for each wheel is generated from a lookup table based on the tyre model. During the ABS operation the value of ref is subjected to the procedure of slip target adaptation based on the assessment of the x- slope. The actual slip is computed utilizing the slip and velocity observers. The observers use the actual wheel velocity Vw, the longitudinal vehicle acceleration ax, and individual wheel torques Tw as the input information. More detailed description of the controller gain definition as well as torque blending procedure is given in [18]. The functional validation of the controller and corresponding experimental results are presented in the next section. IV. EXPERIMENTAL ABS VALIDATION A. Specification of Test Procedures Initial tuning of the controller gains has been done using hardware-in-the-loop test setup and then the controller has been implemented in the test vehicle. The vehicle, Fig. 3, has following test equipment: dSPACE Autobox controller platform; Corrsys Datron velocity sensor (0 - 250 km/h); accelerometer ( 5g); gyroscope ( 300 °/s); magnetometer ( 1.2 Gauss); Kistler 19” RoaDyn S635 wheel force sensor.
Fig. 3. Test vehicle demonstrator.
The testing procedures were performed at Ford Proving Ground in Lommel, Belgium. The baseline test program includes straight-line braking maneuvers for different system configurations: (i) without ABS; (ii) friction (hydraulic) ABS on both axles; (iii) only electric ABS on the front axle; (iv) blended electric and friction ABS on both axles. The initial braking velocity was set up at 40, 50 and 60 km/h. These tests were performed for regular road conditions corresponding to the low-friction surface constructed of basalt tiles, which are continuously wetted by a sprinkler system during the vehicle tests. This kind of surface has an inhomogeneous character with the average friction coefficient of about µx=0.2. Initial value of the reference slip ref was 0.05. The advanced test program covers (i) braking with variation of appointed operational parameters, in particular, tire pressure and (ii) braking on road surface with transition friction characteristics. Several test results of special interest are further discussed. B. Straight-Line Braking under Regular Road Conditions To give an example, Fig. 4 and Fig. 5 shows the diagrams of ABS braking from 50 km/h on the regular lowfriction surface for two system configurations: (i) electric ABS on the front axle only, (ii) blended electric and hydraulic ABS on both axles. Table 1 compares the brake distance and average deceleration for blended electric and hydraulic ABS on both axles in cases of braking without ABS and with conventional hydraulic ABS. The obtained experimental results allow a number of essential observations and conclusions to be drawn: 1) Braking with electric ABS on the front axle only. Even such a limited ABS configuration guarantees the required performance for the given road conditions. In particular, the average deceleration of the vehicle has reached the level > 1.25 m/s2 (as for service braking), which is considerably more than the case of braking without ABS. The initial drop of the wheel slip did not exceeded the level 0.2 with the subsequent stabilization characterized by minor oscillations around the reference value ref=5%. Regardless of initial braking velocity, the drivetrain torque demand oscillates around 40 Nm and reached short-time peak value about of 70 Nm to the end of maneuver.
Fig. 4. Diagrams of braking from 50 km/h: pure electric ABS on the front axle only. Wheel indices: FL – front left; FR – front right.
TABLE I.
BRAKE DISTANCE AND AVERAGE DECELERATION Initial braking velocity 40 km/h
50 km/h
60 km/h
Braking without ABS Brake distance, m 68.5 96.4 153.4 Average deceleration, m/s2 -1.10 -1.01 -1.03 Braking with conventional hydraulic ABS on both axles Brake distance, m 49.4 68.6 114.3 Average deceleration, m/s2 -1.52 -1.51 -1.50 Braking with blended electric / hydraulic ABS on both axles Brake distance, m 44.9 64.0 105.3 Average deceleration, m/s2 -1.81 -1.76 -1.76
Fig. 5. Diagrams of braking from 50 km/h: blended electric/friction ABS on both axles. Wheel indices: FL – front left; FR – front right; RL – rear left; RR – rear right.
2) Braking with blended electric/hydraulic ABS on both axles. This ABS configuration demonstrated evident reduction in the brake distance compared with both nonABS braking (>31 % depending on initial braking velocity) and braking with the conventional hydraulic ABS (>6.5% depending on initial braking velocity). The absolute values of the average deceleration are also higher than the hydraulic ABS (>16.5% depending on initial braking velocity). The deceleration profile of the continuous ABS deserves special attention: it was observed that this type of ABS operation does not produce characteristic oscillations of the deceleration. This can be considered as a significant enhancement of driving comfort. 3) Reference slip tracking. The most distinctive feature of the continuous ABS operation is accurate tracking of the reference slip ratio that was confirmed both for pure electric and blended configurations of the anti-lock braking system. As it can be seen from Fig. 4 and Fig. 5, only few initial slip deviations took place during the first seconds of the brake process similar to ordinary ABS operation. To give a quantitative example, Fig. 6 depicts the distribution of actual slip values during the whole duration of a single braking maneuver (blended ABS, low-friction road, braking from 50 km/h). This distribution in the form of hit percentage confirms clearly that most of the braking time all the wheels are working in the area close to the reference slip value of 5%. C. Straight-Line Braking with Variation of Test Conditions Within the framework of the presented paper, one specific test procedures is further introduced. Evaluation of the adaptive properties of the ABS controllers is usually subjected to several test profiles, where different operational parameters are changed in a quasi-static or dynamic manner. One of the relevant experiments is braking on a surface with transition from one to another type of the road surface. Fig. 7 introduces ABS diagrams for one corresponding maneuver: the vehicle starts to brake from 60 km/h on lowfriction surface and then the transition to the high-friction surface takes place after 3 seconds. The diagrams confirm that in this case the blended continuous ABS controller properly recognized the change of road conditions.
Fig. 7. Diagrams of transition braking from lo-friction to high-friction surface.
The proper adaptation is also confirmed for another variant of the transition braking from high-friction to low friction surface, Fig. 8. It should be mentioned for both tests that the transition time did not exceed 200 ms and even short-time locking of the wheels was avoided. Fig. 6. Distribution of actual slip values for each wheel during the braking.
REFERENCES [1] [2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13] Fig. 8. Diagrams of transition braking from high- to low-friction surface. [14]
V. CONCLUSIONS The presented study has confirmed by way of experimental investigations the feasibility of continuous ABS control with the use of individually controlled motors of AWD electric vehicle. Both pure electric and blended electric/hydraulic ABS configurations ensure required brake performance on low-friction surfaces and a smooth tracking of the reference slip. This is also beneficial for driving comfort. A series of transition braking tests from high to low friction surfaces, have also confirmed sufficient adaptive properties of the developed methods of the continuous wheel slip control.
[15]
[16]
[17]
[18]
ACKNOWLEDGEMENT The research leading to these results has received funding from the European Union Seventh Framework Program FP7/2007-2013 under grant agreement no. 284708.
Bosch Automotive Handbook, Wiley & Sons, 2007. T. Yoshimura, H. Uchida, H. Nasu, J. Hino and R. Ueno, "Traction force control of an electric vehicle in 2WS-4WD using fuzzy reasoning," Int. J. of Vehicle Design, vol. 18, no. 5, pp. 442-454, 1997. Y. Hori, Y. Toyoda and Y. Tsuruoka, "Traction control of electric vehicle: basic experimental results using the test EV “UOT electric march”," IEEE Trans. on Industry Applications, vol. 34, no. 5, pp. 1131-1138, 1998. T. Akiba, R. Shirato, T. Fujita and J. Tamura, "A study of novel traction control method for electric motor driven vehicle," in Proc. PCC’07 Power conversion conference, Nagoya, Japan, 2007. M. Kato, "Proposal of anti-slip control method for electric vehicles with adaptive torque limiter," in JSAE Annual Congress, Yokohama, 2013. K. Fujii and H. Fujimoto, "Traction control based on slip ratio estimation without detecting vehicle speed for electric vehicle," in Proc. PCC '07 Power Conversion Conference, Nagoya, Japan, 2007. K. Maeda, H. Fujimoto and Y. Hori, "Four-wheel driving-force distribution method based on driving stiffness and slip ratio estimation for electric vehicle with in-wheel motors," in Proc. IEEE Vehicle Power and Propulsion Conference (VPPC), Seoul, Korea, 2012. P. Khatun, C.M. Bingham, N. Schofield and P.H. Mellor, "Application of fuzzy control algorithms for electric vehicle antilock braking/traction control systems," IEEE Transactions on Vehicular Technology, vol. 52, no. 5, pp. 1356-1364, 2003. M. Ringdorfer and M. Horn, "Development of a wheel slip actuator controller for electric vehicles using energy recuperation and hydraulic brake control," in Proc. 2011 IEEE International Conference on Control Applications (CCA), Denver, CO, USA, 28-30 September 2011. A. Dadashnialehi, A. Bab-Hadiashar, C. Zhenwei and A. Kapoor, "Intelligent sensorless ABS for in-wheel electric vehicles," IEEE Trans. on Industrial Electronics, vol.61, no.4, pp.1957-1969, 2014. C. Mi, H. Lin and Y. Zhang, "Iterative learning control of antilock braking of electric and hybrid vehicles," IEEE Transactions on Vehicular Technology, vol. 54, no. 2, pp. 486-494, 2005. D. Akaho, M. Nakatsu, E. Katsuyama, K. Takakuwa and K. Yoshizue, "Development of vehicle dynamics control system for inwheel-motor vehicle," in JSAE Annual Congress (Spring), Yokohama, Japan, 2010. S. Murata, “Innovation by in-wheel-motor drive unit,” Vehicle System Dynamics: International Journal of Vehicle Mechanics and Mobility, vol. 50, no. 6, pp. 807-830, 2012. G. Freitag, M. Klopzig, K. Schleicher, M. Wilke and M. Schramm, "High-performance and highly efficient electric wheel hub drive in automotive design," in Proc. The 3rd International Electric Drives Production Conference (EDPC), Nuremberg, Germany, 2013. V. Ivanov, D. Savitski and B. Shyrokau, "A Survey of Traction Control and Anti-lock Braking Systems of Full Electric Vehicles with Individually-Controlled Electric Motors," IEEE Transactions on Vehicular Technology, DOI: 10.1109/TVT.2014.2361860. D. Savitski, V. Ivanov, B. Shyrokau, J. De Smet and J. Theunissen, "Experimental study on continuous ABS operation in pure regenerative mode for full electric vehicle," SAE International Journal of Passenger Cars - Mechanical Systems (accepted). J. Theunissen et al., "Powertrain architecture of electric vehicles with individually controlled switched reluctance motors," in Proc. FISITA World Automotive Congress, Maastricht, The Netherlands, 2014. V. Ivanov, B. Shyrokau, D. Savitski, J. Orus, R. Meneses, J.M. Rodriguez-Fortun, J. Theunissen and K. Janssen, "Design and testing of ABS for electric vehicles with individually controlled on-board motor drives," SAE International Journal of Passenger Cars Mechanical Systems, vol. 7, no. 2, pp. 902-913, 2014.
