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    Development of Brake System and RegenerativeBraking Cooperative Control Algorithm

    for Automatic-Transmission-BasedHybrid Electric Vehicles

    Jiweon Ko, Sungyeon Ko, Hanho Son, Byoungsoo Yoo, Jaeseung Cheon, and Hyunsoo Kim

    AbstractIn this paper, a brake system for an automatic trans-mission(AT)-based hybrid electric vehicle (HEV) is developed, anda regenerative braking cooperative control algorithm is proposed,with consideration of the characteristics of the brake system. Thebrake system does not require a pedal simulator or a fail-safedevice, because a hydraulic brake is equipped on the rear wheels,and an electronic wedge brake (EWB) is equipped on the frontwheels of the vehicle. Dynamic models of the HEV equipped withthe brake system developed in this study are obtained, and aperformance simulator is developed. Furthermore, a regenerativebraking cooperative control algorithm, which can increase theregenerative braking energy recovery, is suggested by consideringthe characteristics of the proposed hydraulic brake system. Asimulation and a vehicle test show that the brake system andthe regenerative braking cooperative control algorithm satisfythe demanded braking force by performing cooperative controlbetween regenerative braking and friction braking. The regener-ative braking cooperative control algorithm can increase energyrecovery of the regenerative braking by increasing the gradient ofthe demanded braking force against the pedal stroke. The gradientof the demanded braking force needs to be determined withconsideration of the drivers braking characteristics, regenerativebraking energy, and the driving comfort.

    Index TermsCooperative control, electronic wedge brake(EWB), hybrid electric vehicle (HEV), hydraulic brake, regener-ative braking.


    R EGENERATIVE braking is a core technology for in-creasing the fuel efficiency of the electric vehicle(x-EV) equipped with energy storage units, such as a batteryand an ultracapacitor [1]. Toyota reported that the greatestfactor for an improved fuel efficiency hybrid electric vehicle

    Manuscript received August 8, 2013; revised December 24, 2013 andMarch 18, 2014; accepted May 9, 2014. Date of publication May 21, 2014;date of current version February 9, 2015. This work was supported in part bythe Ministry of Trade Industry and Energy and in part by the Korea Institute forAdvancement of Technology.

    J. Ko, S. Ko, H. Son, and H. Kim are with the School of Mechanical Engi-neering, Sungkyunkwan University, Suwon 440-746, Korea (e-mail:;;;

    B. Yoo is with the Hyundai-Kia R&D Center, Hwasung 445-706, Korea(e-mail:

    J. Cheon is with the Hyundai Mobis R&D Center, Youngin 446-912, Korea(e-mail:

    Color versions of one or more of the figures in this paper are available onlineat

    Digital Object Identifier 10.1109/TVT.2014.2325056

    (HEV) is regenerative braking, which accounts for about 35%of the total energy efficiency improvement, as evident in theToyota Prius. Studies showed that HEVs have remarkably im-proved their fuel efficiencies by 30%40% through regenerativebraking [2][4].

    However, the braking force required by a driver cannot beguaranteed through regenerative braking alone, due to variouslimitations, such as battery state of charge, and vehicle speed.Therefore, separate friction braking, which enables active coop-erative control in response to regenerative braking, accordingto the braking demand of the driver and the driving state, isneeded [5].

    A brake-by-wire system, such as the electronic hydraulicbrake (EHB) or an electric actuator, has been used in thefriction brake system for regenerative braking cooperative con-trol. For an EHB, an electronically controlled brake system,and electronic stability control, the use of an oil pump and ahydraulic control unit has been developed [6], [7]. Furthermore,a system using an electronic hydraulic servo, as well as a smartbooster using an electrical booster, has been proposed [8], [9].Electronic brakes for cooperative control, which use the x-by-wire technology, include the screw-type electronic mechanicalbrake (EMB) and the electronic wedge-type brake (EWB). TheEMB pushes the brake pad against the brake disk using a piston,which is connected to the screw, to produce a clamping force.The EWB uses a self-reinforcing effect using a wedge, whichis connected to the screw, to produce a clamping force.

    The EMB and EWB are new brake types that use a motor,instead of hydraulic pressure, to generate the braking force.Thus, they respond faster and can be operated under activeelectronic control, which can be used to independently controlthe braking force of each wheel [10][14]. In addition, a pedalsimulator is required to provide the same braking feeling as thatin conventional vehicles [15], and a separate fail-safe device isrequired, because there is no physical connection between thedriver and the brake system.

    The regenerative braking cooperative control algorithm is de-termined by the structure of the friction brake system. Researchdone on the regenerative braking cooperative control algorithminclude a study based on the method of increasing the energyrecovery by considering the efficiencies of both the front- andrear-wheel motors for four-wheel drive (4WD) HEVs [6]; astudy based on an algorithm that focuses the braking force on

    0018-9545 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission.See for more information.


    Fig. 1. Structure of the brake system.

    the front wheels to increase the energy recovery of regenerativebraking of a front-wheel drive fuel cell electric vehicle [16];and a study based on regenerative braking cooperative con-trol, using a genetic algorithm and fuzzy control to improvevehicle stability and energy recovery [17], [18]. Another studydeveloped an electric regenerative power-assisted brake for thecooperative control of a 4WD hybrid electric van and carriedout a simulation to verify its performance [19]. Mi et al.proposed an electronic control brake system that integratedregenerative braking and an antilock braking system to preventwheel lock [20].

    This study proposes an electronic friction brake system forregenerative braking cooperative control that does not requirea separate pedal simulator, or a fail-safe device with EWBin the front wheels, as well as a hydraulic brake in the rearwheels of an automatic transmission (AT)-based parallel-typeHEV. Furthermore, a regenerative braking cooperative controlalgorithm, which can increase the regenerative braking energyrecovery, is suggested by considering the characteristics of theproposed brake system. A dynamic model of the HEV equippedwith the regenerative braking cooperative control system wasobtained, and a performance simulator was developed. Theperformance of the regenerative braking cooperative controlsystem was evaluated, by simulation and vehicle test.


    A. Brake System

    Fig. 1 shows the structure of the brake system that is pro-posed in this study. The target vehicle is a front-wheel-drive-type six-speed AT-based HEV. During braking, the front wheelsperform both regenerative and friction braking, and the rearwheels perform only friction braking. The front wheels havetwo EWBs, whereas the rear wheels have two hydraulic brakes.

    As in conventional hydraulic friction braking, the rear-wheelhydraulic brake generates a master cylinder pressure when thedriver steps on the brake pedal, and this pressure is transmittedto the rear-wheel caliper through the hydraulic line, to generatea clamping force. The hybrid control unit (HCU) determinesthe amount of regenerative braking, according to the requiredregenerative braking force, determined by the brake control unit(BCU) and the gear ratio. The difference between the conven-tional regenerative braking cooperative control system and thenewly proposed control system can be explained as follows.

    The regenerative braking cooperative control algorithmneeds to be determined based on the structure of the fric-tion brake system. In the electrohydraulic four friction brakeswith regenerative braking, which have been applied to existingHEVs, the friction braking force is determined from the totalbraking force minus the regenerative braking force at first and isthen distributed to the front and rear wheels. In this system, thesame braking pressure is supplied to the front and rear wheels.Then, the friction braking force of the front and rear wheels isdetermined by the brake design parameters, such as the caliperpiston area and the effective radius of the disk. This brake sys-tem has the advantage of using conventional hydraulic brake de-vices, such as a master cylinder and a caliper. However, the dragforce by the residual pressure in the caliper has been pointed outas a disadvantage, which decreases fuel efficiency [7].

    In current technology, four EWBs for all four wheels with re-generative braking have yet to be applied to any HEV, because,in this system, the brake pedal is not physically connected tothe brake, which does not meet the vehicle safety regulations.In addition, there are some problems that need to be solved,such as wedge jamming and high cost [10], [15].

    The new brake system proposed in this study consists oftwo EWBs at the front wheel and two hydraulic brakes atthe rear wheel. In the proposed new brake system, first, therear friction braking force