Development of a Cooperative Braking System for Front-Wheel Drive Electric Vehicles

energies

Article Development of a Cooperative Braking System for Front-Wheel Drive Electric Vehicles

Di Zhao ID , Liang Chu *, Nan Xu, Chengwei Sun and Yanwu Xu State Key Laboratory of Automotive Simulation and Control, Jilin University, Changchun 130022, China; [email protected] (D.Z.); [email protected] (N.X.); [email protected] (C.S.); [email protected] (Y.X.) * Correspondence: [email protected]; Tel.: +86-431-85095165

Received: 27 December 2017; Accepted: 3 February 2018; Published: 6 February 2018

Abstract: Most electric vehicles adopt cooperative braking systems that can blend friction braking torque with regenerative braking torque to achieve higher energy efﬁciency while maintaining a certain braking performance and driving safety. This paper presented a new cooperative regenerative braking system that contained a fully-decoupled hydraulic braking mechanism based on a modiﬁed electric stability control system. The pressure control algorithm and brake force distribution strategy were also discussed. Dynamic models of a front wheel drive electric car equipped with this system and a simulation platform with a driver model and driving cycles were established. Tests to evaluate the braking performance and energy regeneration were simulated and analyzed on this platform and the simulation results showed the feasibility and effectiveness of this system.

Keywords: electric vehicle; regenerative braking; friction braking; pressure control; brake controller

1. Introduction In recent years, the development of electric cars has been the leading direction in the automobile industry as they can improve the energy usage efficiency as well as reduce the emissions of pollutants [1]. In addition to having a more efficient electric powertrain, an electric car can also perform regenerative braking by using traction motors as generators when braking. Such a feature can convert kinetic energy into electric energy instead of turning it into heat, which will dissipate in the air. The recovered energy can be stored in the battery and used to power the vehicle in turn, thus improving the total energy efficiency of the car. Braking torque produced by the traction motor is affected by many factors, and in some occasions can even be zero; hence it is impossible to use electric motors as the only source of braking force. To ensure safety and maintain proper braking force when the electric motor is not able to perform regenerative braking, all the regenerative braking systems currently available need to work together with a friction braking system that can fill the difference between the total braking force and the motor braking force. This process is typically called “blending” and a system that can do “blending” is usually called “a cooperative braking system”. This kind of system is commonly used in railway applications [2,3], but it is not very common in electric cars. This cooperating friction braking system needs to have the ability to regulate the braking force it produced to avoid causing an uneven feeling due to the intervention and withdrawal of the motor braking force during a braking process. Research on both the hydraulic system layout and control method has been conducted using an electrohydraulic braking system as the cooperating friction braking system. In terms of system architecture, researchers in universities and top tier automotive part suppliers have proposed serval schemes: TRW Inc. (Cleveland, OH, USA) developed a braking system with a controllable hydraulic booster that could control the pressure in the front calipers. This system could be used on an electric vehicle to perform regenerative braking on the front axle [4]. Mando

Energies 2018, 11, 378; doi:10.3390/en11020378 www.mdpi.com/journal/energies Energies 2018, 11, 378 2 of 24 designed a system that consisted of a vacuum booster, an electric vacuum pump, and a modiﬁed Electronic Stability Control (ESC) hydraulic control unit that had two extra valves, but had no pedal travel simulation mechanism. This system is capable of performing regenerative braking while the pedal feel will be affected when the electric motor is engaged [5]. Toyota (Toyota, Aichi, Japan), cooperating with ADVICS (Kariya, Aichi, Japan), implemented an electro hydraulic braking system in the Prius. This system is characterized by a fully by-wire hydraulic control unit and a pedal feel simulation mechanism, making it able to adjust the pressure in each wheel cylinder precisely and independently [6]. Honda (Minato, Tokyo, Japan) mounted a braking system powered by an electric booster together with its i-MMD hybrid powertrain, which is the same as that used on the Accord Hybrid. The electric booster is a tandem master cylinder with its push rod connected to a ball screw structure driven by an electric motor. Identical to the layout used by Toyota, this system also has a pedal feel simulation device that has a similar design to that of Toyota. Such features made this system capable of controlling the brake pressure on each axle individually [7]. Yang et al. proposed a regenerative braking system based on an Anti-lock Braking System (ABS) hydraulic control unit as well as an electric vacuum booster assembly with pedal travel simulator and performed a number of simulations on it to evaluate its performance [8]. Ko et al. developed a regenerative braking system that combined an electric wedge brake with a conventional hydraulic braking system for a hybrid electric car and the control algorithm for it [9]. Wang et al. researched on a regenerative braking system based on an ABS hydraulic control unit and a robust wheel slip controller [10]. In terms of control strategy, Zhang et al. designed a cooperative control strategy for hydraulic braking and regenerative braking which was tested on an electric passenger car equipped with a conventional hydraulic braking system and an ESC hydraulic control unit [11]. Huang et al. undertook research on the application of model predictive control on the regenerative braking control of an electric car powered by in-wheel motors [12]. Guo et al. proposed a method for optimizing the brake force distribution [13]. Nian et al. applied fuzzy logic control on brake force distribution in a regenerative braking system [14]. However, as there is still no widely applied system layout approach and control method, the authors considered research on the cooperative braking system as an open area. In most of the currently available approaches, either a hydraulic booster or a vacuum booster has been used as a high-pressure hydraulic source to ensure that the braking system can produce enough braking force. Vacuum boosters are designed for gasoline powered cars and they rely on the vacuum generated by internal combustion engines. However, engines in hybrid electric vehicles do not work continuously and given that there are no internal combustion engines in pure electric vehicles, additional equipment is required to produce a stable source of vacuum. Therefore, to use a cooperative braking system with a vacuum booster, an electric vacuum pump must be used, making the system more complicated and cumbersome. Hydraulic boosters usually consist of a hydraulic pump, a high-pressure accumulator, and a hydraulic servo mechanism, thus, making the system very complicated and expensive. These kinds of boosters can only be found in very few models with most of them being luxury cars. However, it is possible to obtain a high-pressure source by using only hydraulic pumps and solenoid valves, which is what an ESC system does. ESC systems are widely used in conventional hydraulic braking systems and they can improve the stability of cars by controlling the traction and braking force on each wheel. Like ABS, an ESC system has many solenoid valves and a pump powered by an electric motor. The difference is that an ESC system has some extra valves to control the ﬂow direction of the pump, so it can both increase and decrease the braking force on each wheel individually, while an ABS can only decrease the braking force. Therefore, a regenerative braking system can be realized based on an electronic stability control system where there will be lower complexity and technical barriers. Energies 20182018,, 11,, x 378 3 of 24

2. Design of the Cooperative Braking System 2. Design of the Cooperative Braking System

2.1. Design of the Mechanical Structure In thisthis study, study, the the system system waswas deployeddeployed on on a a front-wheelfront-wheel drive drive electric car and the layout layout is shown in Figure1 1.. TheThe frontfront axleaxle waswas drivendriven byby aa permanentpermanent magnetmagnet synchronoussynchronous motormotor throughthrough aa transmissiontransmission withwith constantconstant geargear ratioratio andand aa differential. The motormotor was controlled by a motor control unit ( (MCU),MCU), which which ha hadd driving driving logic logic and and power power electronics. electronics. A high A high-voltage-voltage lithium lithium battery battery pack packwith withbattery battery management management system system (BMS) (BMS) was used was usedto provide to provide energy energy for traction for traction to propel to propel the car the and car andstore store the theelectric electric energy energy recovered. recovered. Each Each wheel wheel was was equipped equipped with with a adisc disc brake brake wit withh a hydraulic pressure sensor and the pressure in each brake cylinder was controlled by a hydraulic control unit, whichwhich waswas onlyonly hadhad an actuator withwith some solenoid valvesvalves andand electricelectric pumpspumps inside.inside. An additional PCU (pressure control control unit) unit) was was implemented implemented to to control control the the operation operation of the ofthe valves valves and and pumps. pumps. All Allthree three control control units units (MCU, (MCU, BMS, BMS, PCU) PCU) were were connected connected to to a avehicle vehicle control control unit unit ( (VCU),VCU), which can recognize the driver’sdriver’s intent byby acquiring and analyzing the signals from the accelerator and brake pedals andand controlcontrol allall thethe componentscomponents byby sendingsending requestsrequests toto otherother controlcontrol units.units.

FigureFigure 1.1. Layout of the powertrain and cooperative braking system.

A detailed layout of the hydraulic braking systemsystem isis shown inin Figure2 2.. This This consistedconsisted of of a a mastermaster cylinder assembly, aa pedalpedal traveltravel simulationsimulation mechanism,mechanism, a hydraulichydraulic control unit, and fourfour brakebrake calipers. The brake pedal was directly connected to the master cylinder instead of through a vacuum booster. A travel simulator was connected to the master cylinder through a normally closed solenoid valve calledcalled a a pedal pedal travel travel simulator simulator isolation isolation valve valve (PTSIV). (PTSIV). This This valve valve is designated is designated as a safety as a devicesafety thatdevice can that prevent can prevent the brake the ﬂuid brake from fluid ﬁlling from the filling travel the simulator travel simulator when the when system the is system not powered. is not Thepowered. travel The simulator travel simulator has a piston has and a piston two serially and two connected serially connected springs, which springs, can which be used can to be simulate used to thesimulate pressure-volume the pressure characteristic-volume characteristic of the wheel of cylinders the wheel by cylinders matching bythe matching stiffness and the preload stiffness of and the springs.preload of For the the springs. remaining For partsthe remaining of the hydraulic parts of brakethe hydraulic system, thebrake front system, and rear the axlesfront wereand rear identical axles inwere structure, identical so in only structure, the front so only axle the is discussed. front axle is Through discussed. a front Through isolation a front valve isolation (FIV), valve which (FIV), will decouplewhich will the decouple braking the actuator braking from actuator the brakingfrom the control braking mechanism, control mechanism, the outlet the port outlet of theport master of the cylindermaster cylinder is connected is connected to an enhanced to an enhanced hydraulic hydraulic control unit. control In addition,unit. In addition, a return linea return to the line reservoir to the isreservoir also connected is also connected to the output to the port output of theport master of the cylinder,master cylinder, and this and loop this is loop controlled is controlled by the by front the pressurefront pressure regulation regulation valve (FPRV). valve ( TheFPRV). hydraulic The hydraulic control unit, control marked unit, by marked the dash by line, the is dash structurally line, is similarstructurally to the similar hydraulic to the control hydraulic unit control in a conventional unit in a conventional electronic stability electronic control stability system. control However, system. However, they are not identical as the hydraulic control unit in the proposed system has enhanced electric pumps that can meet the demand of a higher flow rate and longer working life. Therefore, a

Energies 2018, 11, 378 4 of 24 theyEnergies are 2018 not, 11 identical, x as the hydraulic control unit in the proposed system has enhanced electric pumps4 of 24 that can meet the demand of a higher ﬂow rate and longer working life. Therefore, a braking system suchbraking as that system described such above as that can described control the above braking can force control on each the wheel braking independently, force on each making wheel it possibleindependently, to implement making torque it possible vectoring to implement control on torque the vehicle vectoring [15]. control on the vehicle [15].

Travel Simulator S U

PTSIV

FIV RIV

FPRV RPRV

FHSV FCV RCV RHSV

FL IV FR IV RL IV RR IV

FL OV FR OV RL OV RR OV

P P P P U U U U

FL FR RL RR

FigureFigure 2.2. Layout of the hydraulic braking system.

2.2. Design of the Cooperative Braking Controller The block diagram diagram of of the the controller controller for for the the cooperative cooperative braking braking system system is shown is shown in Figure in Figure 3. 3In. Inthis this paper, paper, the the controller controller was was embedded embedded as as a afunctional functional module module in in the the VCU. VCU. The The controller controller acquires the brakebrake pedalpedal traveltravel asas thethe inputinput andand recognizesrecognizes thethe driver’sdriver’s braking intention,intention, which is then sent to the the other other two two blocks. blocks. The The mode mode detection detection block block acquires acquires the status the status of the of motor the motor as well as as well the astarget the targetdeceleration deceleration to find to which ﬁnd which mode mode the systemthe system should should work work in. in. Finally, Finally, the the working working mode mode and and the demanded deceleration willwill bebe sent to thethe brake force distribution block to calculate the friction brake force onon each axle produced by the motor. The friction brake force is converted to hydraulic pressure and is then sent to the PCU for pressure modulation. Meanwhile, the controller sends a motormotor torque request to the MCUMCU toto trytry toto produceproduce thethe calculatedcalculated motormotor brakebrake torque.torque. Due to some limitations, however, thethe MCUMCU may may not not produce produce enough enough braking braking force force to to meet meet the the request, request, so itso will it will send send back back the actualthe actual brake brake torque torque produced produced to theto the controller controller as as feedback, feedback, which which is is then then used used to to correctcorrect thethe brake force distribution.distribution.

EnergiesEnergies 20182018,, 1111,, 378x 55of of 2424

Figure 3. Block diagram for cooperative braking controller. Figure 3. Block diagram for cooperative braking controller. 2.2.1. Operation Modes of the Cooperative Braking System 2.2.1. Operation Modes of the Cooperative Braking System The cooperative braking system has three operation modes: electric braking mode, cooperative brakingThe mode, cooperative and hydraulic braking brakingsystem mode.has three operation modes: electric braking mode, cooperative brakingWhen mode, the system and hydraulic is operating braking in the mode. electric braking mode, the PTSIV will be at an open position to enableWhen the the pedal system travel is operating simulator in whilethe electric the front braking isolation mode, valvethe PTSIV (FIV) will and be rear at an isolation open position valve (RIV)to enable will the be inpedal a closed travel position. simulator All while the the brake front fluid isolation from thevalve master (FIV) cylinder and rear flowsisolation into valve the pedal (RIV) travelwill be simulator. in a closed All position. other components All the brake of thefluid hydraulic from the braking master systemcylinder will flows be keptinto the in anpedal idle travel state. Thesimulator. electric All motor other in thecomponents traction system of the willhydraulic produce braking braking system torque will and be recover kept in electric an idle energy state. intoThe theelectric battery. motor in the traction system will produce braking torque and recover electric energy into the battery.For the cooperative braking mode, the electric motor and hydraulic braking system work together to applyFor braking the cooperative force on thebraking wheels. mode, Like the the electric electric motor braking and mode, hydraulic the electric braking motor system performs work regenerativetogether to apply braking braking and the force pedal on travel the wheels. simulator Like is the enabled. electric The braking difference mode, is that the electricthe VCU motor will calculateperforms the regenerative brake force braking distribution and the among pedal the travel motor, simulator front axle, is enabled. and rear The axle, difference and then is send that thethe pressureVCU will requests calculate to the the brake PCU toforce meet distribution the driver’s among demand the for motor, deceleration, front axle, simultaneously and rear axle, allowingand then thesend driver the pressure to obtain reque a similarsts to brake the PCU feel to meet that on the a driver conventional’s demand brake for system. deceleration, Meanwhile, simultaneously the PCU willallowing adjust the the driver pressure to obtain in each a brakesimilar cylinder brake feel by controlling to that on a the conventional electric pumps brake and system. solenoid Meanwhile, valves in thethe hydraulicPCU will adjust control the unit. pressure in each brake cylinder by controlling the electric pumps and solenoid valvesIf thein the electric hydraulic motor control cannot unit. recover energy, the system will work in only in the hydraulic braking mode.If Inthe this electric mode, motor the electric cannotmotor recover rotates energy, with the the system wheels will and work outputs in only no in braking the hydraulic torque andbraking the hydraulicmode. In this brake mode, torque the is electric distributed motor to rotates the front with and the rear wheels axles and at a outputs fixed ratio. no braking torque and the hydraulic brake torque is distributed to the front and rear axles at a fixed ratio. 2.2.2. Brake Force Distribution Strategy in Cooperative Braking Mode 2.2.2. Brake Force Distribution Strategy in Cooperative Braking Mode The braking force is distributed among the electric motor, the front brakes, and rear brakes to ensureThe that braking the total force braking is distributed force meets among the driver’s the electric demand motor, for deceleration.the front brakes, In previous and rear studies, brakes the to mostensure widely that the used total braking braking force force distribution meets the strategy driver was’s demand the “the for parallel deceleration. strategy”, In where previous the brakingstudies, forcethe most is distributed widely used at a fixedbraking ratio force between distribution the motor, strategy the front was axle, the and “the the parallel rear axle. strategy”, The advantage where the of thisbraking strategy force is is that distributed it can be easilyat a fixed implemented ratio between in a conventional the motor, the hydraulic front axle, braking and the system; rear however,axle. The itadvantage cannot maintain of this strategy the deceleration is that it ofcan the be vehicleeasily implemented when the brake in a pedal conventional is held at hydraulic a fixed position, braking norsystem; will however, it maximize it cannot energy maintain recovery the [16 deceleration]. In this paper, of the anvehicle improved when brakingthe brake force pedal distribution is held at a strategyfixed position, was proposed: nor will theit maximize motor braking energy force recovery was preferentially[16]. In this paper, used, an and improved the hydraulic braking braking force forcedistribution was adjusted strategy in real was time proposed during: the the braking motor process braking so forcethat the was braking preferentially rate always used, followed and the the driver’shydraulic intention. braking Theforce changes was adjusted of braking in real forces time with during the the braking braking rate process is shown so inthat Figure the braking4. rate always followed the driver’s intention. The changes of braking forces with the braking rate is shown in Figure 4. In this strategy, the distribution of the braking force is a range rather than a curve as the motor braking force will change within a certain range, and the front axle braking force and rear axle braking

Energies 2018, 11, x 6 of 24

force will also vary within a range. The electric motor will be used as the only source of brake force when the braking rate is below 0.1368 G (1.3406 ms−2), at which point the electric motor produces the maximum braking torque. When the braking rate extends beyond 0.1368 G, the controller will blend the motor braking force together with the hydraulic braking force to meet the driver’s braking

Energiesdemand.2018 The, 11 ,electric 378 motor will gradually withdraw from the braking process when the braking6 ofrate 24 increases from 0.65 G to 0.7 G and will completely detach when it exceeds 0.7 G.

FigureFigure 4.4. Brake force distribution vs. braking rate.

2.2.3. Pressure Control Method of the Hydraulic Braking System In this strategy, the distribution of the braking force is a range rather than a curve as the motor brakingMuch force research will change has been within conducted a certain on range, adjusting and the the front brake axle wheel braking cylinder force pressure and rear axlethrough braking the forcesolenoid will valves also vary and within pumps a and range. many The control electric methods motor will have be been used proposed. as the only A source widely of-used brake control force whenmetho thed is brakingthe PID rate(Pro isportional below 0.1368-Integral G- (D1.3406erivativems−)-2bang), at- whichbang control point theler, electricwhich is motor quite producessimple to theimplement. maximum However, braking this torque. method When cannot the braking achieve rate a sm extendsooth braking beyond process. 0.1368 G,This the is controller acceptable will in blendbrake thecontrol motor systems braking that force only together work in with emergency the hydraulic situations, braking but force this method to meet theis not driver’s quite suitable braking demand.for the system The electric proposed motor in this will paper, gradually especially withdraw for the from pressure the braking increasing process stage. when the braking rate increasesSome from researchers 0.65 G to have 0.7 G propo and willsed completelya control method detach whenthat uses it exceeds the linear 0.7 G.characteristics of inlet solenoid valves and have applied it in a traction control system [11,17]. In this method, the pressure 2.2.3.increase Pressure rate can Control be controlled Method by of adjusting the Hydraulic the opening Braking of System the inlet solenoid valves, thus the pressure in theMuch wheel research cylinder has can been be controlled conducted smoothly. on adjusting Obviously, the brake this wheel method cylinder can achieve pressure better through control the solenoidquality and valves can andgive pumps the driver and a many smoother control braking methods feel. have However, been proposed. according A to widely-used the data in controlKang’s methodstudy, a isfact the that PID can (Proportional-Integral-Derivative)-bang-bang be inferred is that the relationship between controller, opening the which valves is quite and simplepressure to implement.increase rate However, will change this when method the wheel cannot cylinder achieve pressure a smooth changes. braking Accordingly, process. This it iscan acceptable be inferred in brakethat this control method systems requires that quite only worka large in number emergency of calibrations. situations, but this method is not quite suitable for the systemAfter studying proposed the in thischaracteristics paper, especially of the forsolenoid the pressure valves in increasing the hydraulic stage. control unit (HCU) by usingSome the method researchers and havemodel proposed proposed a controlin Zhao method’s paper that[18], uses both the the linear simulation characteristics and experiment of inlet solenoidproved that valves the and front have control applied valve it in (FCV a traction) and controlrear control system valve [11,17 (].RCV In this) ha method,d proportional the pressure relief increasecharacteristics, rate can as be illustrated controlled in by Figure adjusting 5. This the means opening that of a the controllable inlet solenoid constant valves, pressure thus the source pressure can inbe therealized wheel by cylinder energizing can bethe controlled front/rear smoothly.high-pressure Obviously, switching this valve method (FHSV/ can achieveRHSV) and better adjusting control quality and can give the driver a smoother braking feel. However, according to the data in Kang’s study, a fact that can be inferred is that the relationship between opening the valves and pressure increase rate will change when the wheel cylinder pressure changes. Accordingly, it can be inferred that this method requires quite a large number of calibrations. Energies 2018, 11, 378 7 of 24

After studying the characteristics of the solenoid valves in the hydraulic control unit (HCU) by using the method and model proposed in Zhao’s paper [18], both the simulation and experiment proved that the front control valve (FCV) and rear control valve (RCV) had proportional relief characteristics,Energies 2018, as11, illustratedx in Figure5. This means that a controllable constant pressure source7 of can24 be realized by energizing the front/rear high-pressure switching valve (FHSV/RHSV) and adjusting the Energies 2018, 11, x 7 of 24 drivingthe current driving of current the FCV/RCV. of the FCV/RCV. The equivalent The equivalent structure structure of the of hydraulic the hydraulic braking braking circuit circuit is shown is in shown in Figure 6. In this circuit, the FCV/RCV will work like proportional relief valves, thus the Figure6the. In driving this circuit, current the of FCV/RCVthe FCV/RCV. will The work equivalent like proportional structure of the relief hydraulic valves, braking thus the circuit pressure is at pressure at the output port of the pump can be controlled by adjusting the set pressure of the the outputshown port in ofFigure the pump6. In this can circuit, be controlled the FCV/RCV by adjustingwill work like the proportional set pressure relief of thevalves, FCV/RCV. thus the Hence, FCV/RCV. Hence, the authors proposed the use of a method that combined the control of the pressure at the output port of the pump can be controlled by adjusting the set pressure of the the authorsFCV/RCV proposed and FIV/RIV the use to of control a method the pressure that combined in the wheel the control cylinders. of the This FCV/RCV method will and be FIV/RIV quite to FCV/RCV. Hence, the authors proposed the use of a method that combined the control of the controlsimple the pressureto implem inent the since wheel it only cylinders. needs a Thisfew calibrations, method will furthermore, be quite simple it is foreseeable to implement that the since it FCV/RCV and FIV/RIV to control the pressure in the wheel cylinders. This method will be quite only needs a few calibrations, furthermore, it is foreseeable that the pressure regulation process will be pressuresimple regulationto implem entprocess since will it only be smootherneeds a few by usingcalibrations, this method furthermore, and will it isbe foreseeable suitable for that a control the smootherprocesspressure by frequently using regulation this triggered method process during will and be will dailysmoother be driving. suitable by using for this a control method processand will be frequently suitable for triggered a control during daily driving.process frequently triggered during daily driving.

Figure 5. Experimental data of the FCV characteristics test. FigureFigure 5. 5Experimental. Experimental datadata of the FCV FCV characteristics characteristics test test..

IV IV CVCV HSVHSV

P P U U MM

OV OV

Figure 6. The equivalent structure of the hydraulic braking circuit. FigureFigure 6. The6. The equivalent equivalent structurestructure of the the hydraulic hydraulic braking braking circuit. circuit.

Energies 2018, 11, 378 8 of 24 Energies 2018, 11, x 8 of 24

3.3. Simulation PlatformPlatform andand SystemSystem ComponentsComponents ModelingModeling

3.1.3.1. Structure ofof thethe SimulationSimulation PlatformPlatform ToTo conductconduct simulationssimulations andand analyzeanalyze thethe characteristicscharacteristics ofof thethe system,system, aa simulationsimulation platformplatform waswas builtbuilt inin SimulinkSimulink asas shownshown inin FigureFigure7 .7. This This consisted consisted of of ﬁve five sub sub systems: systems: the the hydraulic hydraulic braking braking system,system, thethe tractiontraction system,system, thethe vehicle,vehicle, the controller,controller, andand aa simplesimple proportional–integralproportional–integral (PI)(PI) driverdriver modelmodel thatthat acceptsaccepts thethe speedspeed inputinput andand outputsoutputs thethe brakebrake pedalpedal andand accelerationacceleration pedalpedal traveltravel signal.signal.

Figure 7. Simulation platform layout. Figure 7. Simulation platform layout. 3.2. Vehicle Dynamic Modeling 3.2. Vehicle Dynamic Modeling Since only longitudinal motion and axle load transfer were considered, a simple longitudinal vehicleSince model only was longitudinal established. motion Aerodynamic and axle dragload forcetransfer is alsowere one considered, of the main a simple driving longitudinal resistances, especiallyvehicle model in high-speed was established. driving Aerodynamic conditions. To drag evaluate force the is effectalso one of energy of the recoverymain driving more resistances, accurately, theespecially aerodynamic in high drag-speed was not driving ignored. conditions. Force in the To longitudinal evaluate the direction effect of of the energy car can berecovery described more as:

accurately, the aerodynamic drag was not ignored.. Force in the longitudinal direction of the car can be described as: mv = F1 + F2 − Fa (1) 푚푣̇ = 퐹 + 퐹 − 퐹 (1) where F1 and F2 are the longitudinal tire forces of1 the2 front푎 and rear wheels, respectively; Fa is the aerodynamic drag force; m is the mass of the car; and v is the speed of sprung mass. Aerodynamic drag where 퐹1 and 퐹2 are the longitudinal tire forces of the front and rear wheels, respectively; 퐹푎 is the isaerodynamic one type of forcedrag thatforce needs; 푚 is to the be mass overcome of the by car the; and vehicle 푣 is while the speed moving of sprung and significantly mass. Aerodynamic affects the energydrag is efficiencyone type ofof theforce vehicle that needs [19]. Aerodynamic to be overcome drag by can the be vehicle calculated while by moving the following and significantly equation: affects the energy efficiency of the vehicle [19]. Aerodynamic drag can be calculated by the following 1 2 equation: Fa = C Aρv (2) 2 d 1 where C is the drag coefficient; A is the reference퐹 = area;퐶 퐴휌 and푣2 ρ is the mass density of air. (2) d 푎 2 푑

3.2.1.where Tire 퐶푑 Modelingis the drag coefficient; 퐴 is the reference area; and 휌 is the mass density of air. Almost all the external forces—with the exception of aerodynamic—exerted on the car are from 3.2.1. Tire Modeling the contact surface between the tires and ground. Since tire modeling is so important in vehicle dynamicAlmost modeling, all the external a lot of researchforces—with has beenthe except conductedion of inaerodynamic this area and—exerted many modelingon the car methodsare from havethe contact been discovered. surface between Among the these tires tire and models, ground. the “magic Since tireformula” modeling is the is most so important widely used in and vehicle has becomedynamic the modeling, de facto a industry lot of research standard. has More been tireconducted manufacturers in this area have and begun many to providemodeling customers methods withhave magicbeen discovered. formula parameters. Among these In this tir study,e models the, tiresthe “magic were modeled formula” using is the the m “magicost widely formula” used andand canhas be become presented the as: de facto industry standard. More tire manufacturers have begun to provide customers with magic formula parameters.F iIn= thisFxi − study,f Fzi the tires were modeled using the “magic(3) formula” and can be presented as:

퐹푖 = 퐹푥푖 − 푓퐹푧푖 (3)

Energies 2018, 11, 378 9 of 24

−1n h −1 io Fxi = D sin Ctan Bsi − E Bsi − tan (Bsi) Fzi (4) where Fzi and si are the normal load of the tire and slip rate, respectively. Fxi is the longitudinal force exerted on the tire. B, C, D, and E, which are constants, are the parameters of the magic formula, and the meanings are described in Pacejka [20]. The coefﬁcient of rolling resistance, which is presented as f in Equation (4), is regarded as speed independent, therefore it is a constant. From the tire model, the tire force is affected by the normal load. As the vehicle acceleration changes, the vertical load will be transferred between the two axles, and the load transfer can be expressed as:

. ! mg vhg F = L − (5) z1 L 2 g

. ! mg vhg F = L + (6) z2 L 1 g where g is the gravity constant; L is the wheel base, and it is assumed that the front axle and rear axle have the same wheel base. Li is the distance from the front or rear axle to the gravity center and hg is the height of the gravity center.

3.2.2. Wheel Modeling Since the car described herein is a front-wheel drive car, the wheels were divided into drive wheels and driven wheels. In addition to the force between the tire and the ground, the torque output from the axle as well as the braking torque generated by the brake calipers and motor were also applied to the drive wheel. Therefore, the force balance on the drive wheels can be described as:

J . 1 J + m1 ω = (T − RF − T ) (7) w1 2i2 1 2 m1 1 h1

Tm1 = iTm (8) where Jw1 is the moment of inertia of one front wheel; and Jm1 is the moment of inertia of the electric motor’s rotor. The front wheels were driven through a transmission and the gear ratio was i. Tm is the motor torque; R is the rolling radius of the wheel; ω1 is the angular speed of the wheel; Th1 is the hydraulic braking torque on the front axle; and Tm1 is the driving torque output by the motor through the transmission. The rolling radius was regarded as a constant and the moment of inertia of transmission ignored. For driven wheels, there is no driving torque exerted on them, so the force balance on them can be presented as: . 1 J ω = (−RF − T ) (9) w2 2 2 2 h2 where Jw2 is the moment of inertia of the rear wheel; and Th2 is the hydraulic braking torque of the rear wheel. Braking torque exerted on each wheel can be expressed as:

TBi = Pi AiCBF REi (10) where TBi is the brake torque applied on a wheel; Pi is the pressure of the brake ﬂuid in a wheel cylinder and Ai is the sectional area; CBF is the brake factor of a disc brake, usually double the friction coefﬁcient; and RE is the effective radius of the brake disc. Energies 2018, 11, 378 10 of 24

Energies 2018, 11, x 10 of 24 3.3. Traction System Modeling motor are regarded as a whole component, was established based on the experimental data. Since The traction system mainly consisted of three parts: the electric motor, the motor controller, only limited data points were acquired, to obtain a better simulation model, the data points were and the battery. Higher model complexity can achieve higher simulation accuracy, but it usually fitted as the following polynomial function and is shown in Figure 8: requires more parameters to be calibrated and is not easy to achieve. On the other hand, excessive 푚 푛 simplification of the simulation model may lead to misleading푖 results.푗 To simplify the model and 푇푚_푐푢푟푟푒푛푡(퐼푚, 휔푚) = ∑ ∑ 푃푖푗퐼푚 휔푚 (11) make the model sufficiently accurate, a steady-state black box model where the motor controller and 푖=0 푗=0 motor are regarded as a whole component, was established based on the experimental data. Since only limitedwhere 퐼 data푚 is pointsthe current were on acquired, the DC tobus; obtain 휔푚 ais betterthe motor simulation speed; model,and 푃푖푗 the is the data co pointsnstant werecoefficient. fitted as the followingThe motor polynomial controller functionreceives the and torque is shown command in Figure from8: the VCU during operation, after that it drives the motor to produce the requested torque. The regenerative braking torque is generated by m n the back EMF (electromotive force), so it will be limited at lowi speedj [21]. An experiment was Tm_current(Im , ωm) = ∑ ∑ Pij Im ωm (11) undertaken to find the maximum regenerative brakingi=0 j =torque0 when the motor is running at low speed, the result is shown in Figure 9. The dashed line marked as “A” shows the boundary of the whereregenerativeIm is the braking current torque. on the DC bus; ωm is the motor speed; and Pij is the constant coefficient.

FFigureigure 7 8.. SteadySteady-state-state torque response of the electric motor.motor.

TheThus, motor when controller the motor receives is running the at torque low speed, command the maximum from the VCU regenerative during operation, braking torque after thatcan itbe drives calculated the motor by using to producethe following the requested equation: torque. The regenerative braking torque is generated by the back EMF (electromotive force), so it will be limited at low speed [21]. An experiment was 푇푚_푠푝푒푒푑(휔푚) = 푎휔푚 + 푏 (12) undertaken to ﬁnd the maximum regenerative braking torque when the motor is running at low speed, thewhere result 푎 isand shown 푏 are in the Figure constant9. The coefficient dashed lines markedthat can as be “A” obtained shows through the boundary experiments. of the regenerative When the brakingelectric motortorque. is running at higher speed, the maximum torque it can output is limited by the maximum charging or discharging current of the battery, which can be calculated by using Equation (11).

Energies 2018, 11, 378 11 of 24 Energies 2018, 11, x 11 of 24

FFigureigure 8 9.. RegenerativeRegenerative braking braking torque torque is is limited limited when when motor motor is is running at low speed speed..

TheThus, motor when controller the motor accepts is running torque at lowrequests speed, from the maximumthe CAN (Controller regenerative Area braking Network torque) bus can and be drivecalculateds the electric by using motor the following to reach the equation: requested torque when it does not exceed the torque limitation. If the requested torque goes beyond the limitation at a certain working point, then the maximum torque will be output to the transmission.Tm _Inspeed summary,(ωm) = atheωm resp+ bonded motor torque at a certain motor(12) speed can be defined as: where a and b are the constant coefficients that can be obtained through experiments. When the electric motor is running at higher푚𝑖푛 [ speed,푇푚_푠푝푒푒푑 the(휔푚 maximum), 푇푚_푐푢푟푟푒푛푡 torque(퐼푏_푑푖푠푐 it_푚푎푥 can, 휔 output푚), 푇푟푒푞 is](푇 limited푟푒푞 ≥ 0) by the maximum 푇푚 = { (13) charging or discharging푚푎푥 current [푇푚_푠푝푒푒푑 of the(휔 battery,푚), 푇푚_푐푢푟푟푒푛푡 which(퐼 can푏_푐ℎ푎푟 be_푚푎푥 calculated, 휔푚), 푇푟푒푞 by]( using푇푟푒푞 < Equation0) (11). The motor controller accepts torque requests from the CAN (Controller Area Network) bus and where 퐼푏_푑𝑖푠푐_푚푎푥 is the maximum discharging current; 퐼푏_푑𝑖푠푐_푚푎푥 is the maximum charging current; drives the electric motor to reach the requested torque when it does not exceed the torque limitation. and 푇푟푒푞 is the requested motor torque sent from the VCU. If the requested torque goes beyond the limitation at a certain working point, then the maximum 3.4.torque Battery will Modeling be output to the transmission. In summary, the responded motor torque at a certain motor speed can be defined as: A simplified battery model was used in this research, and can be expressed as the following  equation: h i  min Tm_speed(ωm), Tm_current(Ib_disc_max, ωm), Treq Treq ≥ 0 Tm = h 휏 i (13) ( )  max Tm_speed(ωm), Tm_current∫0(I휂b_푏char퐼푚 _휏max푑휏, ωm), Treq Treq < 0 푆푂퐶 = 푆푂퐶0 − (14) 푄푐 where Ib_disc_max is the maximum discharging current; Ib_disc_max is the maximum charging current; where 푆푂퐶 and 푆푂퐶0 are the actual and initial state of charge of battery pack; 푄푐 is the total energy and Treq is the requested motor torque sent from the VCU. capacity of the battery pack; and 휂푏 is the charging or discharging efficiency of the battery. 3.4. Battery Modeling 3.5. Hydraulic Braking System Modeling A simplified battery model was used in this research, and can be expressed as the followingAs shown equation: in Figure 2, the hydraulic braking system consisted of the master cylinder assembly, travel simulator, hydraulic control unit, and brakes.R τ These components were modeled to conduct 0 ηb Im(τ)dτ simulations to evaluate the performanceSOC =andSOC characteristics0 − of the system. (14) Qc

Energies 2018, 11, 378 12 of 24

where SOC and SOC0 are the actual and initial state of charge of battery pack; Qc is the total energy capacity of the battery pack; and ηb is the charging or discharging efﬁciency of the battery.

3.5. Hydraulic Braking System Modeling As shown in Figure2, the hydraulic braking system consisted of the master cylinder assembly, travel simulator, hydraulic control unit, and brakes. These components were modeled to conduct simulations to evaluate the performance and characteristics of the system.

3.5.1. Hydraulic Control Unit Modeling The hydraulic control unit is a device characterized by twelve solenoid valves, two accumulators, and two pumps. This device can control the hydraulic pressure in each brake caliper independently through the controlling pumps and solenoid valves. Since the layout of the hydraulic braking system discussed in this paper was “axle by axle”, only the front braking circuit modeling is discussed, and the rear circuit can be modeled in the same way. As described above, the FCV can be treated as a solenoid proportional relief valve, so the relief pressure of FCV is a function of the input current on the solenoid expressed as: 3 2 PFCV = a0 IFCV + a1 IFCV + a2 IFCV + a3 (15) where PFCV is the relief pressure of the FCV, IFCV is the current in the solenoid of the FCV. a0, a1, a2 and a3 are constants that can be achieved through experimental tests. When the relief pressure changes, the pressure at output port of the pump will take some time to stabilize, and this process can be expressed as: ω ( ) = np ( ) Ps s 2 PFCV s (16) s + 2ξ pωnps + ωnp where Ps(s) is the pressure at the output port of the pump and PFCV is the relief pressure. When the HCU is regulating the pressure in the front left wheel cylinder, the brake fluid flowing out of the hydraulic pump flows into it through the FLIV (front-left inlet valve) and flows out of it through the FLOV (front-left outlet valve), before finally going into the front low-pressure accumulator. Therefore, the flow rate of the brake fluid flows into the front left wheel cylinder can be calculated using the following equation:

p p QFL = DFLIV CFLIV Ps − PFL − DFLOV CFOV PFL − PFA (17) where QFL is the flow rate; DFLIV and DFLOV are the control signals of FLIV and FLOV; and CFLIV and CFLOV are the flow coefficients of FLIV and FLOV. PFL and PFA represent the pressure of the front left wheel cylinder and front low-pressure accumulator, respectively. Furthermore, in a certain period, the volume that flows into or out of the wheel cylinder can be expressed as:

Z t VFL = QFL(τ)dτ (18) 0 where VFL is the volume of brake ﬂuid ﬂow into or out of the front left wheel cylinder.

3.5.2. Brake Caliper Modeling Modeling brake calipers based on mechanical structures is cumbersome since the deformation of the caliper bodies and brake pads are hard to measure and estimate. Therefore, the brake calipers were modeled based on pressure-volume characteristics, which can be measured easily through experiments. Energies 2018, 11, 378 13 of 24

The P-V characteristics of the front and rear calipers are shown in Figure 10. The measured data can be ﬁtted to a curve represented by Equation (19) [22]:

λi Pi = γiVi (19) where Pi is the pressure of the brake cylinders at the front or rear axle, and γi, λi are coefﬁcients of pressure-volumeEnergies 2018, 11, x characteristics that can be ﬁtted through the experimental data. 13 of 24

(a) (b)

Figure 10.10. PresPressure-volumesure-volume characteristics of front and and rearrear brake brake calipers. calipers. (a)) Pressure-volumePressure-volume characteristics of thethe frontfront brakebrake caliper;caliper; ((bb)) Pressure-volumePressure-volume characteristicscharacteristics ofof thethe rearrear brakebrake caliper.caliper.

3.5.3. Master Cylinder and Pedal Assembly Modeling 3.5.3. Master Cylinder and Pedal Assembly Modeling A tandem master cylinder was modeled in this paper. The input push-rod of the master cylinder A tandem master cylinder was modeled in this paper. The input push-rod of the master cylinder was connected directly to the pedal assembly instead of through a booster. The pressure in the master was connected directly to the pedal assembly instead of through a booster. The pressure in the master cylinder and the force applied on the brake pedal can be represented as: cylinder and the force applied on the brake pedal can be represented as: 1 퐹푃 = 1 (푃푀퐶퐴푀퐶 + 푆푀퐶푘푀퐶 + 퐿푀퐶) (20) FP = 푅푃(PMC AMC + SMCkMC + LMC) (20) RP where 푃푀퐶 is the pressure of brake fluid in the master cylinder; 퐴푀퐶 and 푆푀퐶 are the sectional area where PMC is the pressure of brake fluid in the master cylinder; AMC and SMC are the sectional area and piston stroke of master cylinder, respectively; and 푘푀퐶 and 퐿푀퐶 are the stiffness and preload of and piston stroke of master cylinder, respectively; and kMC and LMC are the stiffness and preload of the spring in it. 퐹푃 is the force applied on the brake pedal and 푅푃 is the leverage ratio of brake pedal. the spring in it. FP is the force applied on the brake pedal and RP is the leverage ratio of brake pedal. 3.5.4. Brake Pedal Travel Simulator Modeling 3.5.4. Brake Pedal Travel Simulator Modeling The brake pedal travel simulator is a component that can simulate the pressure-volume The brake pedal travel simulator is a component that can simulate the pressure-volume characteristics of the brake calipers. By deploying such a device in the hydraulic circuit, the brake characteristics of the brake calipers. By deploying such a device in the hydraulic circuit, the brake pedal maintains a smooth pedal feel when the HCU is regulating pressure in each brake. The pedal pedal maintains a smooth pedal feel when the HCU is regulating pressure in each brake. The pedal travel simulator is a spring cylinder with two springs of different stiffness, and the two springs are travel simulator is a spring cylinder with two springs of different stiffness, and the two springs are connected in series. Each spring is preloaded respectively, and the preload of the second spring connected in series. Each spring is preloaded respectively, and the preload of the second spring equals the force of the first spring at maximum compression. Therefore, when the first sprint is being equals the force of the first spring at maximum compression. Therefore, when the first sprint is being compressed, the pressure-volume characteristics of the travel simulator can be calculated by using compressed, the pressure-volume characteristics of the travel simulator can be calculated by using the the following equation: following equation: 11 V푉S푆 P푃S푆== (L퐿s푠11++K퐾s푠11 ) (21(21)) A퐴S푆 A퐴S푆 where 푃푆 is the pressure of the brake fluid in the brake pedal travel simulator; 퐴푆 is the sectional area of it; 퐿푠1 and 퐾푠1 are the preload and stiffness of the first spring, respectively; and 푉푆 is the volume of brake fluid flow into the simulator. When the first spring reaches maximum compression, the second spring will be compressed and the pressure-volume characteristics will be:

1 푉푆 퐿푠2 − 퐿푠1 푃푆 = (퐿푠2 + 퐾푠2 ( − )) (22) 퐴푆 퐴푆 퐾푠1 where 퐿푠2 and 퐾푠2 are the preload and stiffness of the second spring, respectively.

Energies 2018, 11, 378 14 of 24

where PS is the pressure of the brake fluid in the brake pedal travel simulator; AS is the sectional area of it; Ls1 and Ks1 are the preload and stiffness of the first spring, respectively; and VS is the volume of brake fluid flow into the simulator. When the first spring reaches maximum compression, the second spring will be compressed and the pressure-volume characteristics will be: 1 VS Ls2 − Ls1 PS = Ls2 + Ks2 − (22) AS AS Ks1 where Ls2 and Ks2 are the preload and stiffness of the second spring, respectively.

4. Veriﬁcation and Analysis of the System in Simulation Environment Several experiments were carried out in a simulation environment to verify the feasibility and effectiveness of the system. The main parameters used in the simulations are shown in Table1.

Table 1. Main parameters used in simulations.

Parameter Value Total weight of the car 1928 kg Wheelbase 2675 mm Distance from front axle to centroid 1263 mm Distance from rear axle to centroid 1412 mm Centroid height 530 mm Drag area 0.6789 m2 Moment of inertia of a wheel 1.12 kg·m2 Rolling radius of a wheel 0.308 m Transmission reduction ratio 8.28 Battery capacity 43, 200 C Initial SOC(state of charge) 80% Brake factor 0.7 Diameter of front wheel cylinder 57.15 mm Diameter of rear wheel cylinder 34.92 mm Diameter of master cylinder 18 mm Effective radius of front brake rotor 0.225 m Effective radius of rear brake rotor 0.230 m Brake pedal leverage ratio 3.5 Diameter of pedal travel simulator 15 mm Stiffness of pedal travel simulator spring (1st stage) 9.03 N/mm Stiffness of pedal travel simulator spring (2nd stage) 42.16 N/mm

4.1. Simulation and Analysis of Wheel Cylinder Pressure Modulation Algorithm To maintain a stable braking rate while the electric motor intervenes in the braking process, the hydraulic control unit will regulate the pressure in each wheel cylinder. The dynamic characteristics of the hydraulic control unit is a key factor of this pressure regulation process and can be obtained by observing the ramp response and sinusoidal response of the hydraulic braking system [8]. The simulation results are shown in Figures 11 and 12. In Figure 11, two curves, excited at two different frequencies, show that the hydraulic control unit could regulate the pressure in the wheel cylinders with relatively high precision when working in the pressure increasing stage. However, there will be a delay during the regulation process, which is mainly caused by the brake caliper gap and the ﬂow rate limitation of the valves. Unlike the delay shown in the pressure increasing stage, when working at the pressure decreasing stage and the pressure is higher than 7 MPa, there will be an overshot in the pressure decrease stage, which is caused by the excessive pressure difference and ﬁxed control cycles of the outlet valves. Energies 2018, 11, 378 15 of 24

For the ramp response test, the conclusion was almost the same as that in the sinusoidal response test where it was obvious that the delay of the low-pressure portion increased as the target pressure rate increased. Figure 13 is an enlarged view of the sinusoidal response curve for comparing the quality of the proposed pressure control method and PID-bang-bang control. Obviously, the proposed method had much lower noise than the PID-bang-bang control. Figure 14 shows the pressure of the front left brake and the control signals of valves associated with it. From the results, in the pressure rise stage, we can see that it is necessary to control FLIV simultaneously only at the high-pressure section together with the FCV, otherwise the pressure can be controlled by only controlling the duty cycle of theEnergies drive 2018 signal, 11, x of the FCV. 15 of 24 Energies 2018, 11, x 15 of 24

Figure 11. Sinusoidal response curve of front brake pressure. FigureF 11.igureSinusoidal 11. Sinusoidal response response curve of of front front brake brake pressure pressure..

Figure 12. Ramp response curve of front brake pressure. Figure 12. Ramp response curve of front brake pressure. Figure 12. Ramp response curve of front brake pressure.

Energies 2018, 11, 378 16 of 24 Energies 2018, 11, x 16 of 24 Energies 2018, 11, x 16 of 24

Figure 9. Partial enlarged view of the pressure response curves with different pressure control Figure 9. Partial enlarged view of the pressure response curves with different pressure control Figuremeth 13.odPartial. enlarged view of the pressure response curves with different pressure control method. method.

Figure 10. Pressure and valve control signals of the front left wheel. FigureFigure 14. 10Pressure. Pressure and and valve valve controlcontrol signals signals of of the the front front left left wheel wheel..

4.2. Simulation and Analysis of the System in Hydraulic Only Mode The electric motor may not be working in regenerative braking in many conditions such as if the SOC is too high as well as if the motor or the battery pack overheats [23]. In these cases, the braking system will work in hydraulic braking only mode and all the brake torque is generated by Energies 2018, 11, x 17 of 24 Energies 2018, 11, x 17 of 24 4.2. Simulation and Analysis of the System in Hydraulic Only Mode 4.2. Simulation and Analysis of the System in Hydraulic Only Mode The electric motor may not be working in regenerative braking in many conditions such as if the The electric motor may not be working in regenerative braking in many conditions such as if the EnergiesSOC2018 is too, 11, high 378 as well as if the motor or the battery pack overheats [23]. In these cases, the17 braking of 24 SOCsystem is too willhigh work as well in as hydraulic if the motor braking or the only battery mode pack and overheats all the brake[23]. In torque these cases,is generated the braking by the systemhydraulic will work control in unit. hydraulic A brake braking test with only a modeconstant and braking all the rate brake was torque simulated is generated and the by result the is thehydraulicshown hydraulic in control Figure control unit.s 15 unit. and A brake A 16 brake. test test with with a aconstant constant braking braking rate rate was was simulated simulated and and the result is is shownshown inin FiguresFigures 1515 and and 16 16..

Figure 11. Changes of vehicle speed and wheel speed in hydraulic braking only mode. FigureFigure 15.11. ChangesChanges ofof vehiclevehicle speedspeed andand wheelwheel speedspeed inin hydraulichydraulic brakingbraking onlyonly mode.mode.

FigureFigure 16. 16Measured. Measured brake brake force force distribution distribution in hydraulic in hydraulic braking braking only only mode. mode. Figure 16. Measured brake force distribution in hydraulic braking only mode.

In this simulation, the driver pressed the brake pedal to 51 mm at a constant rate of 51 mm/s and kept it at 51 mm until the vehicle stopped from an initial speed of 100 km/h. Figure 15 shows the Energiesspeed2018 changes, 11, 378 of the sprung mass and the wheels. During the whole braking process, the deceleration18 of 24 change was very smooth, and it can be inferred that the driver will not have significant discomfort. speedThe changesmaximum of the braking sprung rate mass was and 0.8 the g, wheels.meaning During that the the braking whole braking system process, could produce the deceleration sufficient changebraking was force very in smooth, the hydraulic and it canbraking be inferred only mode. that theFigure driver 16 shows will not the have measured significant braking discomfort. forces on Theeach maximum axle and braking the calculated rate was total 0.8 braking g, meaning force thaton the the vehicle, braking which system revealed could that produce the braking sufficient force brakingdistributio forcen in ratio the stayed hydraulic constant braking at different only mode. braking Figure rates 16 showsand was the the measured same as brakingdesigned. forces on each axle and the calculated total braking force on the vehicle, which revealed that the braking force distribution4.3. Simulation ratio and stayed Analysis constant of the at S differentystem in Regenerative braking rates Braking and was Mode the same as designed. With the regenerative braking function activated, the braking system will work in a more 4.3.complicated Simulation andmode. Analysis Figure ofsthe 17– System19 show in the Regenerative simulation Braking results Mode of a regenerative braking process with constantWith the braking regenerative rate. The braking measured function brake force activated, is shown the in braking Figure system17 and it will can work be concluded in a more that complicatedthe brake forcemode. distribution Figures 17 algorithm–19 show thewas simulation almost in line results with of expectations. a regenerative However, braking processthe motor withbrake constant torque braking did not rate. reduce The to measured zero at once brake when force the is braking shown rate in Figure was beyond 17 and 0.7 it can g, which be concluded may cause thatthe the ABS brake function force distributionto not work algorithmproperly. The was delay almost was in linecaused with by expectations. the torque response However, characteristics the motor brakeof the torque motor did and not the reduce brake to force zero atdistribution once when strategy, the braking which rate ha wasd a beyond sudden 0.7 change g, which in motor may cause torque theat ABS a high function braking to rate. not workIn this properly. simulation, The the delay motor was braking caused torque by the was torque much response less than characteristics its upper limit ofdue the motorto its working and the brakecondition. force Figure distribution 18 shows strategy, the change which of had brake a sudden force and change deceleration in motor in torque the first at1.5 a high s of brakingthe simulation, rate. In where this simulation, it can be seen the motorthat as brakingthe motor torque braking was force much gradually less than reduced, its upper the limithydraulic due to its braking working force condition. was compensated Figure 18 shows so that the the change deceleration of brake force could and change deceleration with the in target the firstdeceleration, 1.5 s of the simulation,however, there where was it still can a be small seen error thatas between the motor the brakingactual vehicle force gradually deceleration reduced, and the thetarget hydraulic deceleration, braking partly force was because compensated of the presence so that of the air deceleration resistance, and could also change because with the the brake target force deceleration,distribution however, strategy is there an open was- stillloop a strategy small error that betweendoes not thehave actual feedback vehicle correction deceleration on deceleration. and the targetRequested deceleration, and responded partly because motor of torques, the presence together of air with resistance, the current and are also shown because in theFigure brake 19 force, where distributionwe can see strategy that the is responded an open-loop motor strategy brake that torque does stopped not have following feedback the correction requested on deceleration.torque at 0.4 s Requestedbecause and the responded regenerative motor current torques, nearly together reached with the current limit of are 50 shown A. As in Figure time progressed,19, where we the canregenerative see that the respondedbraking torque motor increased brake torque slightly stopped due to following a decrease the of requestedthe speed torquewhile the at 0.4current s because almost theremaine regeneratived unchanged, current nearly this can reached be inferred the limit from of 50the A. characteristics As time progressed, of the themotor, regenerative which is brakingshown in torqueFigure increased 8. slightly due to a decrease of the speed while the current almost remained unchanged, this can be inferred from the characteristics of the motor, which is shown in Figure8.

FigureFigure 17. 17Measured. Measured brake brake force force distribution distribution in regenerativein regenerative braking braking mode. mode.

Energies 2018, 11, 378 19 of 24 EnergiesEnergies 20182018,, 1111,, xx 1919 ofof 2424

FFFigureigureigure 1212 18... ChangeChangeChange ofof of brakebrake brake forceforce force andand and decelerationdeceleration duringduring regenerativeregenerative brakingbraking braking...

FFFigureigureigure 1313 19... ChangeChangeChange ofof of motormotor motor torquetorque torque andand and currentcurrent current duringduring regenerativeregenerative brakingbraking braking...

4.4. Simulation and Analysis of Brake Pedal Feel Simulation Mechanism 4.4.4.4. Simulation Simulation and and A Analysisnalysis of of Brake Brake Pedal Pedal Feel Feel Simulation Simulation Mechanism Mechanism Through controlling the solenoid valves, we could connect the master cylinder to the pedal ThroughThrough controllingcontrolling thethesolenoid solenoid valves, valves, we we could could connect connect the themaster master cylinder cylinder to the to pedal the pedal travel travel simulator to directly turn the hydraulic braking system into the pedal travel simulation mode travelsimulator simulator to directly to directly turn the turn hydraulic the hydraulic braking braking system system into the into pedal the travel pedal simulation travel simulation mode and mode then and then input the pedal travel data into the model while collecting the pedal force data output from andinput then the input pedal the travel pedal data travel into thedata model into the while model collecting while thecollect pedaling forcethe pedal data outputforce data from output it. With from the it. With the data collected, a pedal force vs. pedal travel curve that reflected the brake pedal feel it.data With collected, the data a pedalcollected, force a vs.pedal pedal force travel vs. curvepedal thattravel reﬂected curve that the brakereflect pedaled the feel brake characteristics pedal feel characteristics was produced, as shown in Figure 20. This figure also shows the brake pedal force vs. characteristicswas produced, was as shownproduced, in Figure as shown 20. in This Figure ﬁgure 20 also. This shows figure thealso brake shows pedal the brake force pedal vs. travel force data vs. traveltravel data data collected collected from from a a conventionalconventional hydraulic hydraulic braking braking systemsystem with with a a vacuum vacuum booster. booster. The The simulationsimulation resultresultss showshoweded thatthat thethe pedalpedal traveltravel simulatorsimulator ccouldould simulatesimulate thethe brakebrake pedalpedal feelfeel toto makemake thethe systemsystem feelfeel likelike aa conventionalconventional hydraulichydraulic brakingbraking systemsystem withwith vacuumvacuum booster.booster.

Energies 2018, 11, x 20 of 24

Energies 2018The, 11 rel, 378ationship between deceleration and brake pedal force is a basic and important factor for20 of 24 brake feel evaluation [24]. Simulations of the deceleration vs. brake pedal force characteristics were conducted and the results are shown in Figure 21. This characteristic was almost identical in the collecteddifferent from working a conventional modes, hydraulicwith only a braking slight difference system with when a vacuumthe deceleration booster. ex Theceeded simulation 6.5 m/s results2, showedtherefore that the the pedal difference travel was simulator caused couldby the simulateinaccurate the estimation brake pedal of brake feel toforce make in the brake system force feel like a conventionaldistribution hydraulic block. braking system with vacuum booster. Energies 2018, 11, x 20 of 24

The relationship between deceleration and brake pedal force is a basic and important factor for brake feel evaluation [24]. Simulations of the deceleration vs. brake pedal force characteristics were conducted and the results are shown in Figure 21. This characteristic was almost identical in the different working modes, with only a slight difference when the deceleration exceeded 6.5 m/s2, therefore the difference was caused by the inaccurate estimation of brake force in the brake force distribution block.

FigureFigure 20. 14Test. Test of of brakebrake pedal feel feel char characteristics.acteristics.

Figure 15. Deceleration vs. brake pedal force characteristics in different working modes.

FigureFigure 21. Deceleration15. Deceleration vs. vs. brake brake pedal pedal forceforce characteristics in indifferent different working working modes modes..

EnergiesEnergies 20182018, ,1111, ,x 378 2121 of of 24 24

4.5. Energy Recovery Performance Evaluation of the System in Simulation Environment 4.5. Energy Recovery Performance Evaluation of the System in Simulation Environment Simulations tests under New European Drive Cycle (NEDC) conditions with different brake force Simulationsdistribution tests strategies under Newwere Europeancarried out Drive to evaluate Cycle (NEDC) the energy conditions recovery with performance different brake of forcethis system.distribution The simulation strategies were results carried are shown out to evaluatein Figure thes 22 energy–24, which recovery show performance variations of of SOC, this system.motor torqueThe simulation, and current results on the are DC shown bus inunder Figures different 22–24 cases, which, respectively. show variations In Figure of SOC, 22, the motor results torque, show anded thatcurrent the strategy on the DC proposed bus under in this different paper cases,could respectively.produce more In motor Figure braking 22, the torque results than showed the parallel that the strategy.strategy proposedThis means in that this the paper mo couldtor braking produce torque more was motor utilized braking at a torquehigher thanefficiency the parallel and output strategy.ted aThis higher means current that thewhere motor more braking energy torque was wasrecovered, utilized as at shown a higher in efficiencyFigure 23. and The outputted variation aof higher SOC showncurrent in where Figure more 24 indicated energy was that recovered, after finishing as shown an NEDC in Figure cycle, 23. the The SOC variation dropped of SOC from shown 80% to in 55.83%Figure 24 under indicated the pr thatoposed after strategy finishing. When an NEDC compared cycle, the to SOC47.24% dropped under fromthe parallel 80% to 55.83% strategy under and 44.70%the proposed under the strategy. hydraulic When only compared strategy, to the 47.24% difference under was the 8.59% parallel and strategy 11.13% and, respectively 44.70% under. the hydraulicThe comp onlyarison strategy, of SOC the differencehas already was shown 8.59% that and the 11.13%, control respectively. strategy proposed in this paper could improveThe the comparison efficiency of of SOC energy has alreadyrecovery, shown however, that the it can control also strategybe analyzed proposed and evaluated in this paper in more could detail.improve Lv the’s paper efficiency prop ofosed energy two recovery, evaluation however, indexes it canfor alsothe contribution be analyzed and of regenerative evaluated in morebraking detail. to energyLv’s paper efficiency proposed improvement: two evaluation contribution indexes for to thethe contributiondriving range of regenerative extension of brakingthe vehicle to energy, and contributionefficiency improvement: to the energy contribution consumption to reduction the driving of rangethe vehicle extension [25].of Since the vehicle,the energy and consumption contribution wasto the much energy easier consumption to calculate reduction in this case, of thethe vehiclelatter index [25]. was Since used the to energy evaluate consumption the energy was recovery much perfoeasierrmance. to calculate The contribution in this case,the made latter by indexregenerative was used braking to evaluate to energy the energy consumption recovery reduction performance. was definedThe contribution as [25]: made by regenerative braking to energy consumption reduction was defined as [25]:

퐸푟푒푔푒푛Eregen_표푓푓_o f f −−퐸E푟푒푔푒푛regen_on_표푛 훿퐸δ=E = ××100%100% (23 (23)) 퐸E푟푒푔푒푛regen_표푓푓o f f 훿 퐸 퐸 whewherere δE퐸 isis thethe contributioncontribution ratio ratio to to energy energy consumption consumption reduction; reduction; and and E푟푒푔푒푛regen_표푛_on andand E푟푒푔푒푛regen__표푓푓o f f areare thethe energy energy consumed consumed wi withth and and without without regenerative regenerative braking braking under under a a certain certain amount amount of of driving driving rangerange,, respectively. respectively.

FFigureigure 16 22.. VariationVariation of of motor motor torque torque in in the New European Driving Cycle ( (NEDC)NEDC) simulation with with differentdifferent brake brake f forceorce distribution strategies strategies..

Energies 2018, 11, 378 22 of 24 Energies 2018, 11, x 22 of 24 Energies 2018, 11, x 22 of 24

FFigureigureigure 17 23... VariationVariation of the the current current on on the the DC bus in the the NEDC simulation with with different different brake brake force force distributiondistribution strategies strategies...

Figure 18. Variation of SOC in the NEDC simulation with different brake force distribution strategies. FFigureigure 18 24.. VariationVariation of of SOC SOC in in the the NEDC NEDC simulation simulation with with different different brake brake force force distribution distribution strategies strategies..

Energies 2018, 11, 378 23 of 24

Energy efﬁciency and contribution to energy efﬁciency, which are calculated based on the simulation results, are shown in Table2. Based on these results, it appears that the system and the control strategy proposed in this paper were able to recovery kinetic energy during braking more effectively than the traditional parallel distribution strategy.

Table 2. Comparison of energy recovery performance under different brake force distribution strategies.

Energy Efﬁciency Contribution to Energy Strategy (kWh/100 km) Efﬁciency Improvement (%) Hydraulic only 12.5466 - Parallel 11.6438 7.20 Proposed 8.5907 31.53

5. Conclusions In this paper, a regenerative braking system with a cooperative hydraulic braking system and pressure control methods, as well as the brake force distribution strategy were proposed. Then, the system and the electric vehicle platform were modeled, and several basic tests were simulated to evaluate the performance of the pedal feel simulation mechanism, the pressure control method, and the brake force distribution strategy. Finally, simulations under the NEDC driving cycles with different brake force distribution strategies were carried out and the energy recovery performance of the proposed system was evaluated by comparing the energy efﬁciency and contribution to energy efﬁciency improvement under different strategies. The simulations results showed the feasibility and effectiveness of the system proposed. In the future, the authors will focus on the integration of the stability control and regenerative braking control strategy, as well as the implementation of this system on electric cars driven by in-wheel hub motors.

Acknowledgments: The authors gratefully acknowledge the financial support from the China Scholarship Council (No. 201506170105), the Energy Administration of Jilin Province [2016]35, and the Jilin Province Science and Technology Development Fund (20150520115JH). Author Contributions: Di Zhao, Liang Chu and Nan Xu conceived the control method, performed the experiments, and finished the manuscript; Chengwei Sun and Yanwu Xu performed the experiments and revised the paper. Conflicts of Interest: The authors declare no conflict of interest.

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