Research on Regenerative Braking of Pure Electric Mining Dump Truck

Article Research on Regenerative Braking of Pure Electric Mining Dump Truck

Wei Zhang * , Jue Yang , Wenming Zhang and Fei Ma Department of Vehicle Engineering, School of Mechanical Engineering, Beijing University of Science and Technology, Beijing 100083, China; [email protected] (J.Y.); [email protected] (W.Z.); [email protected] (F.M.) * Correspondence: [email protected]

Received: 10 May 2019; Accepted: 5 June 2019; Published: 8 June 2019

Abstract: When the pure electric mining dump truck is working, it mainly ascends the slope at full load and descends the slope at no load. The loading state of the vehicle and the slope of the road will directly affect its axle load distribution and braking force distribution. In this paper, the slope dynamics analysis of the pure electric double-axle four-wheel drive mining dump truck was carried out. Based on the regenerative braking priority strategy, four regenerative braking control methods were developed based on the Matlab/Simulink platform and ADVISOR 2002 vehicle simulation software to study the ability of regenerative braking energy recovery and its impact on vehicle economic performance. The simulation results show that the regenerative braking priority control strategy used can maximize the regenerative braking force of the vehicle; the regenerative energy recovery capability of pure electric mining dump truck is proportional to the regenerative braking force that can be provided during braking; the two-axis braking strategy based on the I curve and the β line can make full use of the front and rear axle regenerative braking force when the braking intensity is large, and recover more braking energy; under road drive cycle, the single-axis braking force required to the braking strategy based on the maximized front axle braking force is the largest among all strategies, the motor braking efficiency is the highest, and the recovered braking energy is the most. For the studied drive cycle, the regenerative braking technology can reduce the vehicle energy consumption by 1.06%–1.56%. If appropriate measures are taken to improve the road surface condition and reduce the rolling resistance coefficient from f = 0.04 to f = 0.02, the regenerative braking technology can further reduce the vehicle energy consumption to 4.76%–5.73%. The economic performance of the vehicle is improved compared to no regenerative braking. In addition, the vehicle loading state and the driving motor working efficiency also directly affect the regenerative braking energy recovery capability of the pure electric mining truck.

Keywords: mining dump truck; pure electric vehicle; regenerative braking; control strategy

1. Introduction Pure electric mining dump truck (pure electric mining truck) is fully driven by electricity which is supplied by on-board batteries or power wiring. It does not need to burn diesel oil, and can recover regenerative braking energy when braking with high eﬃciency and no pollution. The traditional electric wheel mining dump truck adopts diesel-electric drive technology. When driving, the chemical energy is converted into mechanical energy by the diesel engine to drive the generator to generate electric energy to be transmitted to the motor to drive the wheels to rotate. When braking, the vehicle kinetic energy is transmitted from the wheel to the motor. The electric motor acts as a generator to convert the braking energy into electrical energy and then it is converted to heat by the braking resistor [1]. Regenerative braking is an important feature of pure electric mining trucks that distinguish

World Electric Vehicle Journal 2019, 10, 39; doi:10.3390/wevj10020039 www.mdpi.com/journal/wevj World Electric Vehicle Journal 2019, 10, 39 2 of 17 trucksWorld Electricthat distinguish Vehicle Journal traditional2019, 10, 39 mining trucks. When braking, the drive motor is switched to2 the of 17 power generation state to generate the braking torque. The vehicle kinetic energy is recovered and storedtraditional in the mining vehicle trucks. energy When system, braking, which the has drive a significant motor is switchedeffect on toimproving the power the generation driving staterange to andgenerate economic the brakingperformance torque. of Thethe vehicle vehicle [2]. kinetic energy is recovered and stored in the vehicle energy system,There which are many has a studies signiﬁcant on regenerative eﬀect on improving braking theof pure driving electric range vehicles and economic [3–13], hybrid performance vehicles of [14,15],the vehicle electric [2]. buses [16,17] or electric light trucks [18]. However, there are few studies on regenerativeThere arebraking many of studiespure electric on regenerative mining dump braking trucks. ofThe pure existing electric literatures vehicles basically [3–13], take hybrid a single-axisvehicles [14 driven,15], electric vehicle buses as the [ 16research,17] or object electric [3–10,14,15], light trucks and [18]. analyzes However, it based there on are the few horizontal studies on roadregenerative driving mechanics braking of model pure electric[3,5–7], miningignoring dump the vehicle trucks. wind The existingresistance, literatures rolling resistance basically take and a slopesingle-axis resistance driven during vehicle driving. as the researchThese resistance object [3–s,10 ,especially14,15], and the analyzes rolling it basedresistance on the and horizontal slope resistance,road driving are often mechanics not negligible model [3 for,5– 7large-load], ignoring mi thening vehicle dump wind trucks. resistance, In this paper, rolling a self-weight resistance and45 t slopewith full resistance load 100 during t pure driving. electric These double-axle resistances, four-wheel especially drive the rollingmining resistance dump truck and is slope taken resistance, as the researchare often object. not negligible Through for the large-load vehicle slope mining dynamics dump trucks. analysis, In this the paper, front aand self-weight rear axle 45 load t with and full brakingload 100 force t pure distribution electric double-axle models are four-wheel established. drive The mining regenerative dump truckbraking is taken priority as the control research strategy object. isThrough adopted the to vehicleestablish slope four dynamics regenerative analysis, braking the front strategies: and rear Based axle loadon vehicle and braking speed, force I-curve, distribution β-line andmodels front are axle established. braking force The regenerative maximization. braking The prioritycharacteristics control strategyof regenerative is adopted energy to establish recovery four storageregenerative control braking are studied strategies: on the Based Matlab/Simulink on vehicle speed, platformI-curve, combinedβ-line with and frontthe vehicle axle braking simulation force softwaremaximization. ADVISOR The 2002. characteristics of regenerative energy recovery storage control are studied on the Matlab/Simulink platform combined with the vehicle simulation software ADVISOR 2002. 2. Principle Analysis of Regenerative Braking Control System 2. Principle Analysis of Regenerative Braking Control System In order to ensure the stability and safety of the brake, the pure electric mining truck brake controlIn system order tostudied ensure thein this stability paper and keeps safety th ofe mechanical the brake, thebrake pure system electric [19] mining and truckadds brakethe regenerativecontrol system brake studied system in this on paperthe front keeps and the rear mechanical axles, as brake shown system in Figure [19] and 1. During adds the braking, regenerative the brakebrake ECU system calculates on the the front braking and rearforce axles, demand as shown according in Figure to the1 brake. During pedal braking, signal and the transmits brake ECU it tocalculates the vehicle the brakingECU. The force vehicle demand ECU according performs to regenerative the brake pedal brake signal through and transmits the motor it to ECU, the vehicle and feedbacksECU. The the vehicle regenerative ECU performs braking regenerativeforce to the brake brake ECU. through The thebrake motor ECUECU, compares and feedbacksthe required the brakingregenerative force brakingwith the force regenerative to the brake braking ECU. force, The brake determines ECU compares the magnitude the required of the braking friction forcebraking with force,the regenerative and adjusts brakingthe hydraulic force, determinespressure to theachieve magnitude compound of the braking friction of braking regenerative force, and braking adjusts and the frictionhydraulic braking. pressure to achieve compound braking of regenerative braking and friction braking.

Brake pedal

Hydraulic Wheel speed sensor control unit Brake ECU Vehicle Transmission Transmission Motor ECU ECU

Converter Motor Motor

Battery

Oil Current Signal

Figure 1. Regenerative braking control system.

3. System Model and ControlFigure Strategy 1. Regenerative braking control system.

3.3.1. System Drive Model Motor, and Battery Control and Regenerative Strategy Braking Force Model

3.1. DriveThe Motor, drive motorBattery andand batteryRegenerative parameters Braking directlyForce Model determine the vehicle’s regenerative braking force and regenerative energy recovery and storage capacity [20]. The drive motor and battery parameters directly determine the vehicle's regenerative braking force and regenerative energy recovery and storage capacity [20]. World Electric Vehicle Journal 2019, 10, 39 3 of 17 World Electric Vehicle Journal 2019, 10, 39 3 of 17 3.1.1. Drive Motor Mechanics Model The maximum torque that the motor can provide at a certain speed is determined by the 3.1.1. Drive Motor Mechanics Model external characteristics of the motor. The rated power of the motor and the base speed of the motor determineThe maximum the power torque characteristics that the motor of the can motor. provide In atthis a certainpaper, speedthe permanent is determined magnet by thesynchronous external characteristicsmotor is used, of the the maximum motor. The speed rated is power 3600 r/min, of the motorand the and torque-speed the base speed relationship of the motor curve determine is shown thein powerFigure characteristics2. The positive of part the of motor. the torque In this indi paper,cates the the permanent drive, and magnetthe negative synchronous part indicates motor the is used,brake. the maximum speed is 3600 r/min, and the torque-speed relationship curve is shown in Figure2. The positiveWhen partthe motor of the is torque working, indicates whether the drive,it is in and driving the negative or braking part operation, indicates there the brake. will be power lossWhen due to the iron motor loss, is copper working, loss whether and friction it is in lo drivingss. This or loss braking can be operation, measured there on willthe bemotor power test lossbench. due The to iron specific loss, coppermethod loss is to and let friction the motor loss. run This stably loss can at a be certain measured angular on the velocity motor and test bench.torque, Theand speciﬁc measure method the power is to let of the the motor motor, run so stablythat the at arelationship certain angular between velocity the and motor torque, efficiency and measure and the theangular power velocity of the motor, and torque so that of the the relationship motor can be between obtained, the as motor shown eﬃ ciencyin Figure and 2. the angular velocity and torque of the motor can be obtained, as shown in Figure2.

2800

2100 0.75 1400 0.85 0.81 0.93 700

） 0.91 0.75 N·m 0 · 0.75 0.77 0.77 （ 0.91 -700

Torque 0.81 0.85 0.93 -1400

-2100 0.75

-2800 0 500 1000 1500 2000 2500 3000 3500 Rotating speed（r/min) Figure 2. Motor characteristic curve. Figure 2. Motor characteristic curve.

When the motor speed is lower than the base speed ωb, the constant torque is maintained, and the powerWhen is proportional the motor to speed the speed; is lower when than the the motor base speed speed is 𝜔 above, the theconstant base speed, torque the is maintained, constant power and isthe maintained, power is proportional and the output to torquethe speed; decreases when asthe the motor speed speed increases: is above the base speed, the constant power is maintained, and the output torque decreases as the speed increases:   T ω 𝑇m 𝜔ω ≤𝜔 (1) (1)  N ≤ b =  P 𝑃 Tmmax 𝑇 =N  ωm >𝜔 ωb>𝜔 (2) (2)  ωm 𝜔

where 𝑇 is the maximum torque that the motor can provide; 𝑇 = 2500N · m is the rated where Tmmax is the maximum torque that the motor can provide; TN = 2500N m is the rated torque of torque of the motor; 𝑃200kW is the rated power of the motor; 𝜔 is the angular· velocity. the motor; P 200kW is the rated power of the motor; ω is the angular velocity. From FigureN 2, we can clearly see that when the mmotor rotates at a certain speed, whether it is From Figure2, we can clearly see that when the motor rotates at a certain speed, whether it is the the driving state or the braking state, when the motor torque is too small, and the motor operating driving state or the braking state, when the motor torque is too small, and the motor operating point point will fall in the low efficiency region of the figure. At this time, the motor efficiency is low. will fall in the low efficiency region of the figure. At this time, the motor efficiency is low. 3.1.2. Battery Model 3.1.2. Battery Model Lithium batteries are currently the most mature energy devices available for pure electric Lithium batteries are currently the most mature energy devices available for pure electric vehicles. The truck uses a large-capacity lithium iron phosphate battery (LiFePO4), rated voltage 2.7 vehicles. The truck uses a large-capacity lithium iron phosphate battery (LiFePO4), rated voltage 2.7 V, V, AC internal resistance ≤ 0.7 mΩ, capacity 100 Ah, weight 3.15 ± 0.1 Kg. Figure 3 shows its internal AC internal resistance 0.7 mΩ, capacity 100 Ah, weight 3.15 0.1 Kg. Figure3 shows its internal resistance model. ≤ ± resistance model. WorldWorld Electric Electric Vehicle Vehicle Journal Journal2019 2019, 10, 10 39, 39 4 of4 17of 17

Figure 3. Battery internal resistance model. Figure 3. Battery internal resistance model.

During regenerative braking, the relationship between battery charging power Pbreg , During regenerative braking, the relationship between battery charging power 𝑃 , motor motor charging power Pmreg , battery charging current I, battery resistance Rint, and vehicle accessory powerchargingPacc is:power 𝑃 , battery charging current 𝐼, battery resistance 𝑅 , and vehicle accessory 2 power 𝑃 is: Pbreg = Pmreg I Rint Pacc (3) − − In the above formula, the power generated𝑃 =𝑃 by−𝐼 the𝑅 motor −𝑃 should be less than the sum of the(3) allowable charging power of the battery and the power loss of all accessories and circuit lines. When the In the above formula, the power generated by the motor should be less than the sum of the battery is fully charged, if the output power is negative (charging), limit the battery charging power allowable charging power of the battery and the power loss of all accessories and circuit lines. When to zero, do not charge the battery. The vehicle brake is realized by mechanical braking or energy the battery is fully charged, if the output power is negative (charging), limit the battery charging consumption braking. power to zero, do not charge the battery. The vehicle brake is realized by mechanical braking or 3.1.3.energy Regenerative consumption Braking braking. Force Model

3.1.2.When Regenerative the vehicle Braking brake speed Force is Model low or the wheel is locked, the motor has a small electromotive force due to the low motor speed, which is insufficient to charge the battery. In order to prevent the back electromotiveWhen the vehicle force frombrake being speed too is smalllow or at the low wheel speeds is locked, to provide the sumotorfficient has regenerative a small electromotive braking force,force mechanical due to the friction low motor braking speed, should which be is used. insufficie Therefore,nt to charge for reliable the battery. braking, In the order speed to influenceprevent the back electromotive force from being too small at low speeds to provide sufficient regenerative factor Kωm [21] is introduced. At the same time, in order to prevent overcharging of the battery, it isbraking necessary force, to mechanical consider the friction influence braking of the should battery be SOC. used. This Therefore, paper assumesfor reliable that braking, the maximum the speed 𝐾 allowableinfluence charging factor power[21] and is introduced. charging current At the of same the battery time, canin order meet to the prevent actual generatingovercharging current of the andbattery, generating it is necessary power of theto consider motor. Thethe regenerativeinfluence of the braking battery torque SOC. that This the paper motor assumes can generate that the duringmaximum regenerative allowable braking charging is: power and charging current of the battery can meet the actual generating current and generating power of the motor. The regenerative braking torque that the Tmreg_avail = Tmmax Kωm (4) motor can generate during regenerative braking is: where Tm is the braking torque that the motor can provide, the regenerative braking force that reg_avail 𝑇 =𝑇 𝐾 the motor can provide at the axle is: _ (4)

where 𝑇 is the braking torque that the motor can provide, the regenerative braking force _ Tm i ig reg_avail · 0· that the motor can provide at the axleFm reg_availis: = (5) ηt r · 𝑇 ∙𝑖 ∙𝑖 𝐹 = _ (5) where Fm is the axle braking force that_ the motor can provide; i0 is the main reducer ratio; ig is reg_avail 𝜂 ∙𝑟 the transmission ratio; ηt is the transmission system eﬃciency; r is the wheel radius. where 𝐹_ is the axle braking force that the motor can provide; 𝑖 is the main reducer ratio; 𝑖 3.2. Vehicle Dynamics Model is the transmission ratio; 𝜂 is the transmission system efficiency; 𝑟 is the wheel radius.

3.2.1.3.2. NormalVehicle Dynamics Reaction ForceModel on the Front and Rear Wheels of the Vehicle when Braking Figure4 shows the distribution of braking force during braking. According to the balance condition,3.2.1. Normal the torque Reaction is obtained Force on for the the Front grounding and Rear points Wheels of the of frontthe Vehicle and rear when wheels Braking respectively: Figure 4 shows the distribution of braking force during braking. According to the balance  G condition, the torque is obtained FforZ1 the= groundinb cos α +g hpointsg(z sin of theα) front and rear wheels respectively:(6)  L −  𝐺  𝐹 =G 𝑏 cos 𝛼 +ℎ (𝑧−sin𝛼 ) (6)  F = a cos α + h (sin α z) (7) Z2 𝐿 g L 𝐺 − 𝐹 = 𝑎 cos 𝛼 +ℎ (sin 𝛼 − 𝑧) (7) 𝐿 WorldWorld Electric Vehicle Journal 2019, 10, 39 55 of of 17 17 where 𝐹 is the front axle ground normal force; 𝐹 is the rear axle ground normal force; 𝐺=𝑚𝑔 iswhere the vehicleFZ1 is the gravity, front axle𝑚 is ground the vehicle normal mass, force; 𝑔 F isZ2 theis the gravity rear axle acceleration; ground normal 𝐿 is the force; wheelbase;G = mg is𝑎 theis thevehicle distance gravity, betweenm is the the vehicle truck mass, centerg isof themass gravity to the acceleration; front axle center;L is the 𝑏 wheelbase; is the distancea is the from distance the centerbetween of themass truck of the center truck of to mass the torear the axle front center; axle center; ℎ is btheis theheight distance of the from truck's the center ofof massmass; of 𝑧 theis thetruck braking to the strength. rear axle center; hg is the height of the truck’s center of mass; z is the braking strength.

Figure 4. Vehicle force diagram. Figure 4. Vehicle force diagram. It can be seen from Equations (6) and (7) that as the braking strength increases, the normal reaction forceIt acting can be on seen the frontfrom axleequations by the (6) inertial and force(7) that increases, as the braking and the normalstrength reaction increases, force the acting normal on reactionthe rear axleforce decreases. acting on Inthe order front to axle make by full the useiner oftial the force ground increases, adhesion, and the the front normal axle reaction braking force force actingdemand on will the increase,rear axle and decreases. the rear In axle order braking to ma forceke full demand use of will the decrease. ground adhesion, Studies have the shownfront axle that brakingon horizontal force demand roads, for will single-shaft increase, and driven the purerear axle electric braking vehicles, force using demand the frontwill decrease. axle to drive Studies can haverecover shown more that regenerative on horizontal braking road energys, for single-shaft than using the driven rear axlepure to electric drive [vehicles,22]. using the front axle to drive can recover more regenerative braking energy than using the rear axle to drive [22] . 3.2.2. Front and Rear Axle Braking Force Distribution 3.2.2. Front and Rear Axle Braking Force Distribution When the brake system braking force is sufficient, the braking process may occur in the following threeWhen situations: the brake system braking force is sufficient, the braking process may occur in the following three situations: (1) The front and rear wheels are both locked and dragged. 1) The front and rear wheels are both locked and dragged. (2) The front wheels are locked and dragged first, and then the rear wheels are locked and dragged. 2) The front wheels are locked and dragged first, and then the rear wheels are locked and dragged.(3) The rear wheels are locked and dragged first, and then the front wheels are locked and dragged. 3)Situation The rear 1: Thewheels ground are adhesionlocked and utilization dragged is thefirst, best, and it isthen the the ideal front braking wheels situation; are locked situation and 2: dragged.It is a stable situation, but the steering ability is lost, and its ground adhesion utilization is not as goodSituation as situation 1： 1;The situation ground 3: adhesion The rear axleutilization may have is the a side best, slip, it whichis the isideal unstable braking and shouldsituation; be situationavoided [223：].It is a stable situation, but the steering ability is lost, and its ground adhesion utilization is not as good as situation 1; situation 3：The rear axle may have a side slip, which is unstable and should1. I curve be avoided (ideal braking [23]. force distribution curve) 1. IThe curve braking (ideal forcebraking distribution force distribution curve that curve) satisfies situation 1 is called the ideal braking force distributionThe braking curve force (I curve), distribution as shown curve in Figure that 5satisfies. The I curvesituation is expressed 1 is called as: the ideal braking force distribution curve (I curve), as shown in Figure 5. The I curve is expressed as:   Fµ𝐹1 ++𝐹Fµ2 ==𝐺z−𝐹Gz Fr −𝐹Fi (8)(8)  − −  Fµ𝐹 F𝐹 𝑏b cos cosα + 𝛼h +ℎg((𝑧−sin𝛼z sin α)  1 z1  == = − (9)(9)  Fµ𝐹2 F𝐹z2 𝑎a cos cosα + 𝛼h +ℎg((sinsin 𝛼α z −) 𝑧 − where 𝐹 =𝜑𝐹 is the front axle braking force; 𝐹 =𝜑𝐹 is the rear axle braking force, and 𝜑 is thewhere adhesionFµ1 = coefficient.ϕFz1 is the front axle braking force; Fµ2 = ϕFz2 is the rear axle braking force, and ϕ is the adhesionThe solid coeffi linecient. in Figure 5 is the horizontal road surface I curve with α = 0. It can be known from equationsThe solid (8) and line (9) in that Figure compared5 is the horizontal with the horizontal road surface road,I curve the slope with slidingα = 0. Itforce can will be known lead to from the frontEquations and rear (8) andaxle (9)load that transfer. compared When with the the slope horizontal is uphill, road, the front the slope axle load sliding is reduced force will and lead the to rear the axlefront load and is rear increased. axle load The transfer. I curve When is offset the upwards slope is uphill, around the the front coordinate axle load origin, is reduced and the and downhill the rear World Electric Vehicle Journal 2019, 10, 39 6 of 17 axle load is increased. The I curve is offset upwards around the coordinate origin, and the downhill is just the opposite. When the front and rear axle braking force distribution points are above the I curve, it will cause dangerous conditions in which the rear axle is first locked and dragged. World Electric Vehicle Journal 2019, 10, 39 6 of 17 2. f -line is just the opposite. When the front and rear axle braking force distribution points are above the I The braking force distribution curve that satisfies situation 2, called the f -line, is expressed as: curve, it will cause dangerous conditions in which the rear axle is first locked and dragged.  2. f-line  G  Fµ = ϕ b cos α + h (z sin α) (10) The braking force distribution curve1 thatL satisfies situationg 2, called the f -line, is expressed as:  −  F = Gz𝐺 F F F (11) 𝐹µ2 =𝜑 𝑏r cosi 𝛼µ +ℎ1(𝑧−sin𝛼) (10） 𝐿− − − AsAs shown shown in in Figure Figure 5,5, like the I𝐹I curve,curve,=𝐺𝑧−𝐹 compared compared −𝐹 −𝐹 with with the the horizontal horizontal road road ff curve,curve, the the braking braking (11） forceforce that that causes causes the the front axle locklock becomesbecomes smallsmall duedue to to the the slope slope sliding sliding force. force. The Thef -line f-line will will shift shift to tothe the left left as as a wholea whole when when the the vehicle vehicle is is uphill, uphill, and and to to the the right right when when downhill. downhill. TheThe UNECE UNECE brake brake regulations regulations (developed (developed by by the the United United Nations Nations Economic Economic Commission Commission for for Europe)Europe) do notnot covercover vehicles vehicles with with design design speed speed<25 <25 km /h.km/h. The maximumThe maximu speedm speed of the of vehicle the vehicle studied studiedin this paperin this is paper 15 km is/h, 15 so km/h, this paperso this does paper not does discuss not discuss the UNECE the UNECE curve. curve. AssumeAssume that that the the maximum maximum gradient gradient of of the the pure pure el electricectric mining mining vehicle vehicle road road drive drive cycle cycle is is 10%. 10%. AccordingAccording to (8)–(11), thetheI Icurve curve and andf linef line of of the the vehicle vehicle on theon the horizontal, horizontal, uphill uphill and downhilland downhill roads roadscan be can drawn, be drawn, as shown as shown in Figure in Figure5. In the5. In figure, the figure, the braking the braking force force is the is ratio the ratio of the of frontthe front and and rear rearaxle axle braking braking force force to the to vehiclethe vehicle load, load, which which has nohas unit. no unit. So do So Figuresdo Figure6 and 6 and7. Figure 7.

Horizontal Horizontal 0.6 f Uphill 0.6 Uphill Downhill Downhill 0.5 f 0.4

0.3 0.3 I

0.2

Rear wheel braking force braking wheel Rear force braking wheel Rear 0.1 I

0.0 0.0 0.0 0.3 0.6 0.0 0.3 0.6 Front wheel braking force Front wheel braking force

(a) Full load (b) No load

FigureFigure 5. 5. VehicleVehicle front front and and rea rearr axle axle braking braking force. force. (a (a) )Full Full load; load; ( (bb)) no no load. load.

3.3.3.3. Regenerative Regenerative Braking Braking Strategy Strategy ThisThis paper paper adopts adopts the the regenerati regenerativeve braking braking priority priority strategy, strategy, that that is: is: Under Under a a certain certain braking braking intensity,intensity, regardless regardless of of how how the the front front and and rear rear axle axle braking braking forces forces are are distributed, distributed, each each axle axle is is given given prioritypriority to to use use the the regenerative regenerative braking braking force: force: Comparing Comparing the theaxle axle demands demands braking braking force force with withthe regenerativethe regenerative braking braking force that force the that motor the motorcan provide, can provide, if the braking if the brakingforce provided force provided by the motor by the is greatermotor isthan greater or equal than orto equalthe braking to the braking force requir force requireded for the for shaft, the shaft, the theregenerative regenerative braking braking is completelyis completely used, used, and and the thefriction friction braking braking force force is 0; isif 0;the if braking the braking force force provided provided by the by motor the motor is less is thanless thanthe required the required braking braking force forceof the of shaft, the shaft, the electromechanical the electromechanical composite composite braking braking is adopted, is adopted, the electricthe electric braking braking force force takes takes the thebraking braking force force that that the the motor motor can can provide, provide, and and the the insufficient insuﬃcient part part is is supplementedsupplemented by thethe frictionfriction braking braking force; force; when when the the vehicle vehicle speed speed is slow is orslow the or emergency the emergency braking brakingis performed, is performed, in order in to order ensure to theensure reliable the reliable braking, braking, the regenerative the regenerative braking braking is turned is oturnedﬀ, and off, the andmechanical the mechanical braking braking is completely is completely adopted. adopted. Based on the above principles, this paper develops the following four regenerative braking priority braking strategies.

3.3.1. Vehicle Speed based Braking Strategy The vehicle speed based braking strategy is a parallel braking control strategy based on vehicle speed, using regenerative braking and mechanical braking combined braking. The higher the vehicle speed, the more regenerative braking force is assigned, as shown in Figure 6. The axle regenerative World Electric Vehicle Journal 2019, 10, 39 7 of 17

Based on the above principles, this paper develops the following four regenerative braking priority braking strategies.

3.3.1. Vehicle Speed based Braking Strategy The vehicle speed based braking strategy is a parallel braking control strategy based on vehicle speed, using regenerative braking and mechanical braking combined braking. The higher the vehicle World Electric Vehicle Journal 2019, 10, 39 7 of 17 speed, the more regenerative braking force is assigned, as shown in Figure6. The axle regenerative braking force distribution coeﬃcient and friction braking force distribution coeﬃcient are determined braking force distribution coefficient and friction braking force distribution coefficient are by the vehicle speed. determined by the vehicle speed.

Regenerative braking force 1.0 Friction braking force

0.8

0.6

0.4

0.2 Braking force distribution coefficient

0.0 02468101214 Vehicle speed（km/h）

Figure 6. Brake force distribution map based on vehicle speed. Figure 6. Brake force distribution map based on vehicle speed. 3.3.2. I Curve based Braking Strategy 3.3.2.Compared I Curve based with Braking the single-shaft Strategy drive electric vehicle, the two-axis four-wheel drive pure electric miningCompared dump truck with studied the single-shaft in this paper drive can fullyelectric utilize vehicle, the fast the response two-axis characteristics four-wheel ofdrive the motorpure electricand freely mining distribute dump the truck front studied and rear in axlethis paper braking can forces fully [ 24utilize], and the easily fast distributeresponse characteristics the braking force of theaccording motor toand the freelyI curve distribute [25]. As the shown front in and Figure rear5, axle when braking the vehicle forces is going[24], and uphill, easily if thedistribute horizontal the brakingroad I curve force is according used for braking, to the I thecurve curve [25]. is belowAs shown theactual in FigureI curve, 5, when which the makes vehicle the is front going axle uphill, use the if theadhesion horizontal coeffi roadcient I to curve be large, is used and for the braking, rear axle the uses curve the adhesionis below the coe ffiactualcient Ito curve, be small, which this makes situation the frontdoes notaxle make use the good adhesion use of groundcoefficient adhesion; to be large, when and the the vehicle rear isaxle going uses downhill, the adhesion the curve coefficient is above to bethe small, actual thisI curve, situation when does the ground not make adhesion good coeusffie ofcient ground is small, adhesion; there will when be unstable the vehicle conditions is going in downhill,which the the rear curve axle isis locked above andthe actual dragged I curve, first. Therefore,when the ground the I curve adhesion regenerative coefficient braking is small, strategy there is willdeveloped be unstable in this conditions paper, the frontin which and rearthe rear axle axle load is transfer locked caused and dragged by the slope first. slipTherefore, force is the introduced, I curve regenerativeand the front braking and rear strategy axle braking is developed force distribution in this paper, is performed the front and according rear axle to theload actual transferI curve. caused by the slope slip force is introduced, and the front and rear axle braking force distribution is 3.3.3. Regeneration Braking Strategy based on β Line performed according to the actual I curve. In this strategy, the ratio of the front and rear braking force of the strategy is a fixed value, which is 3.3.3.usually Regeneration expressed as Braking the ratio Strategyβ of the based front on axle β brakeLine braking force Fµ1 to the total brake force Fµ of the vehicle, and is called the front axle brake braking force distribution coefficient: In this strategy, the ratio of the front and rear braking force of the strategy is a fixed value, which is usually expressed as the ratio β of the front axle brake braking force 𝐹 to the total brake Fµ1 𝐹 β = (12) force of the vehicle, and is called the front axle brakeFµ braking force distribution coefficient:

𝐹 The β line is a straight line passing through𝛽= the coordinate origin on the braking force distribution(12) 1 β 𝐹 − map, the slope is tan θ = β , and the intersection point ϕ0 of the β line and the I curve is the synchronousThe β line adhesion is a straight coefficient line as passing shownin through Figure7 .the coordinate origin on the braking force distribution map, the slope is tan 𝜃 = , and the intersection point 𝜑 of the β line and the I curve β of the studied vehicle is 0.55 when fully loaded on horizontal road, and the corresponding issynchronous the synchronous adhesion adhesion coefficient coefficientϕ0_load =as0.45. shown It can in Figure be seen 7. from Figure7 that if the vehicle is braked according to the β line of the horizontal road, the synchronous adhesion coefficient ϕ0 is bigger when the vehicle is going uphill, the β line is far away from the actual I curve, the vehicle braking efficiency World Electric Vehicle Journal 2019, 10, 39 8 of 17

is low; when going downhill, the synchronous adhesion coefficient is smaller, and the β line will be closer to the actual I curve, however, if the ground adhesion coefficient of the road is low, the rear axle will always be locked first when braking, and it is easy to skid. Here, the variable β line is used, and the

Worldsynchronous Electric Vehicle adhesion Journal 2019 coe, 10ﬃ, cient39 ϕ0 remains unchanged regardless of the slope during the running8 of 17 of the vehicle, ϕ0_load= 0.45 at full load and ϕ0_unload= 0.47 at no load.

0.6 0.2

β_up z=0.4709 β_horizontal β_up 0.4 β_down β_horizontal z=0.4509 β_down 0.1

0 0.2 φ

φ0 Rear wheel braking force braking wheel Rear Rear braking force Rear wheel

0.0 0.0 0.0 0.2 0.4 0.6 0.0 0.3 0.6 Front wheel braking force Front wheel braking force

(a) Full load (b) No load

FigureFigure 7. 7.β βlineline and and I curveI curve & &f line.f line. (a) ( aFull) Full load; load; (b) ( bno) no load. load.

3.3.4.β of Maximizing the studied Front vehicle Axle is Braking0.55 when Force fully Strategy loaded F fmaxon horizontal road, and the corresponding synchronousFor the adhesion mining truck coefficient under study,𝜑_ when= 0.45. the It frontcan be wheel seen is from independently Figure 7 that braking if the without vehicle being is brakedlocked, according the braking to the strength β linez ofis greaterthe horizontal than 0.2, road, which the satisﬁes synchronous the braking adhesion demand coefficient of the vehicle𝜑 is biggerroad drivewhen cycle the vehicle (Figure 8is). going Therefore, uphill, when the theβ line braking is far strength away from is less the than actual 0.2, theI curve, front the axle vehicle braking brakingforce is efficiency used only, is and low; the when rear going axle braking downhill, force the is synchronous zero. When the adhesion braking coefficient strength is is greater smaller, than and 0.2, thethe β frontline will and be rear closer axles to are the used actual for I commoncurve, however, braking. if the ground adhesion coefficient of the road is low, the rear axle will always be locked first when braking, and it is easy to skid. Here, the variable

β 4.line Simulation is used, and Analysis the synchronous adhesion coefficient 𝜑 remains unchanged regardless of the slope during the running of the vehicle, 𝜑 = 0.45 at full load and 𝜑 = 0.47 at no load. The above different control strategies_ are established on the Matlab_/Simulink platform and embedded in the vehicle model of the pure electric double-axis four-wheel drive mining dump truck 3.3.4. Maximizing Front Axle Braking Force Strategy F established in the vehicle simulation software ADVISOR 2002. The main parameters of the vehicle are shownFor inthe Table mining1. The truck maximum under study, vehicle when speed the is 15 front km /wheelh and theis independently maximum climbing braking ability without is 15% being(3.5 kmlocked,/h). the braking strength z is greater than 0.2, which satisfies the braking demand of the vehicle road drive cycle (Figure 8). Therefore, when the braking strength is less than 0.2, the front axle braking force is used only, andTable the 1. rearMain axle parameters braking of force mining is zero. truck. When the braking strength is greater than 0.2, the front and rear axles are used for common braking. Parameters Value 4. Simulation Analysis Vehicle curb mass (m1) 45,000 kg (Include batteries) Load capacity (m2) 55,000 kg The above differentCenter control of mass strategies height (h gare) established 1.8 m (Full on load),the Matlab/Simulink 1.5 m (No load) platform and embedded in the vehicle modelWheelbase of the (pureL) electric double-axis four-wheel 6 m drive mining dump truck a established in the vehicleFront simulation wheelbase software ( ) ADVISOR 2.45 m 2002. (Full load),The main 1.78 mparameters (No load) of the vehicle Rear wheelbase (b) 3.55 m (Full load), 4.72 m (No load) are shown in Table 1. The maximumWheel radius vehicle (r) speed is 15 km/h and 0.95 the m maximum climbing ability is 15% (3.5 km/h). Rolling resistance coefficient ( f ) 0.04 Ground adhesion coefficient (ϕ) 0.6 Motor ratedTable power 1. Main (Pe )parameters of mining truck. 200 kW Number of motors (N) 2 (Four-wheel drive) Parameters Value Vehicle curb mass (𝑚1) 45,000 kg(Include batteries) In addition to the influence of vehicle parameters, the vehicle’s potential regenerative energy Load capacity (𝑚2) 55,000 kg recovery capability is also affected by road drive cycles [26]. Figure8 shows the road drive cycle during Center of mass height (ℎ) 1.8m(Full load),1.5m (No load) Wheelbase (𝐿) 6m Front wheelbase (𝑎) 2.45m (Full load),1.78m (No load） Rear wheelbase (𝑏) 3.55m (Full load), 4.72m（No load） Wheel radius (𝑟) 0.95m Rolling resistance coefficient (𝑓) 0.04 Ground adhesion coefficient (𝜑) 0.6

Motor rated power (𝑃) 200kW World Electric Vehicle Journal 2019, 10, 39 9 of 17

World Electric Vehicle JournalNumber 2019, 10, of39 motors (𝑁) 2 (Four-wheel drive) 9 of 17 World ElectricIn addition Vehicle Journalto the2019 influence, 10, 39 of vehicle parameters, the vehicle's potential regenerative energy9 of 17 In addition to the influence of vehicle parameters, the vehicle's potential regenerative energy recovery capability is also affected by road drive cycles [26]. Figure 8 shows the road drive cycle recovery capability is also affected by road drive cycles [26]. Figure 8 shows the road drive cycle during operation. When going uphill, the mining truck is fully loaded and away from the loading duringoperation. operation. When When going uphill,going uphill, the mining the mining truck is truck fully is loaded fully loaded and away and from away the from loading the loading location, location, after 799 s, it climbs to an unloading location with a relative altitude of 34.6 m, the average location,after 799 after s, it 799 climbs s, it to climbs an unloading to an unloading location location with a relative with a relative altitude altitude of 34.6 m,of the34.6 average m, the average speed is speed is 9.64 km/h, and the maximum speed is 14.48 km/h, the maximum slope is 8%, the driving speed9.64 kmis 9.64/h, and km/h, the and maximum the maximum speed is speed 14.48 is km 14/h,.48 the km/h, maximum the maximum slope is slope 8%, the is 8%, driving the driving distance distance is 2.14 km. When going downhill, it takes 735 s to return to the loading location after distanceis 2.14 km.is 2.14 When km. going When downhill, going downhill, it takes 735it takes s to return735 s to to return the loading to the location loading after location unloading, after unloading, the average speed is 10.79 km/h, the maximum speed is 14.99 km/h, the maximum slope unloading,the average the speed average is 10.79 speed km /ish, 10.79 the maximum km/h, the speed maximum is 14.99 speed km/ h,is the14.99 maximum km/h, the slope maximum is 8%, and slope the is 8%, and the driving distance is 2.21 km. isdriving 8%, and distance the driving is 2.21 distance km. is 2.21 km.

Vehicle speed Height Slope 20 50 10 Height Slope Vehicle speed 50 10 20 Full load No load 8 40 Full load No load Full load No load 8 6 15 40 Full load No load 6 4 30 15 4 30 2 10 20 2 0

10 20 0 -2 Slope [%] Slope

Height [m] 10 -2 -4 Slope [%] Slope

5 Height [m] 10 -4 -6 Vehicle speed [km/h] speed Vehicle 5 0 Vehicle speedVehicle [km/h] -6 -8 0 -8 0 -10 -10 0 0 400 800 1200 1600 0 200 400 600 800 1000 1200 1400 1600 0 1000 2000 3000 4000 5000 0 400 800 1200 1600 -10 -10 t [s] 0 200 400 600 800t 1000 [s] 1200 1400 1600 0 1000 2000Distance 3000 [m] 4000 5000 t [s] t [s] Distance [m]

(a) Vehicle speed (b) Elevation (c) Slope (a) Vehicle speed (b) Elevation (c) Slope Figure 8. Road drive cycle. (a) Vehicle speed; (b) elevation; (c) slope. FigureFigure 8.8. Road drive cycle. (a) Vehicle speed;speed; ((bb)) elevation;elevation; ((cc)) slope.slope. 4.1.4.1. Simulation Simulation of DiDifferentﬀerent Braking Braking Strength Strength on on Horizontal Horizontal Road Road at Maximum at Maximum Vehicle Vehicle Speed Speed 4.1. Simulation of Different Braking Strength on Horizontal Road at Maximum Vehicle Speed AssumeAssume that that the the vehicle vehicle is is running running at at maximum maximum speed speed vv0 0= =1515 km/h km/ hat at full full load load and and no no load, load, Assume that the vehicle is running at maximum speed v0 = 15 km/h at full load and no load, brakingbraking with with braking strengthstrengthz z= =0.05, 0.05,z =z 0.1= 0.1 and andz = z0.15 = 0.15 (Because (Because the vehicle the vehicle speed speed is low, is the low, braking the braking with braking strength z = 0.05, z = 0.1 and z = 0.15 (Because the vehicle speed is low, the brakingstrength strength of the road of the drive road cycle drive is smaller cycle is than smalle 0.15,r thethan vehicle 0.15, reservesthe vehicle the reserves mechanical the brake, mechanical during braking strength of the road drive cycle is smaller than 0.15, the vehicle reserves the mechanical brake,emergency during braking, emergency pure mechanicalbraking, pure braking mechanical is adopted braking and theis adopted brake is safeand andthe brake reliable. is Therefore,safe and brake, during emergency braking, pure mechanical braking is adopted and the brake is safe and reliable.the case Therefore, which the the braking case which strength thez braking> 0.15 is strength not discussed). z > 0.15 is not discussed). reliable. Therefore, the case which the braking strength z > 0.15 is not discussed). 4.1.1.4.1.1. Comparison Comparison of of Braking Braking Energy Energy Recovery Recovery 4.1.1. Comparison of Braking Energy Recovery ItIt can can be be seen seen from from Figure Figure 99 thatthat when when the the vehi vehiclecle is is fully fully loaded, loaded, when when the the braking braking intensity intensity is is It can be seen from Figure 9 that when the vehicle is fully loaded, when the braking intensity is z z = 0.05, z = 0.1, the braking strategy based on Ffmax recovers the most energy, and the second is the I z = 0.05, z = 0.1, the braking strategy based on 𝐹 recovers the most energy, and the second is the I = 0.05, z = 0.1, the braking strategy based on 𝐹 recovers the most energy, and the second is the I curvecurve based based and and the the ββ-line-line based based braking braking strategy; strategy; when when zz == 0.15,0.15, the the energy energy recovered recovered by by braking braking curve based and the β-line based braking strategy; when z = 0.15, the energy recovered by braking strategiesstrategies based based on on the the II curve and the ββ lineline is is the the most, most, followed followed by by the the braking braking strategy strategy based based on on the strategies based on the I curve and the β line is the most, followed by the braking strategy based on vehicle speed, the Ffmax strategy recovers the least energy. the vehicle speed, the F strategy recovers the least energy. the vehicle speed, the F strategy recovers the least energy. z=0.05 z=0.05 800 z=0.1 z=0.05 800 z=0.1 z=0.05 800 800 z=0.15 z=0.1 z=0.15 z=0.1 z=0.15 z=0.15

600 600 600 600

400 400 400 400 Energy recovered [kJ] recovered Energy Energey recovered [kJ] recovered Energey 200 200 Energy recovered [kJ] recovered Energy Energey recovered [kJ] recovered Energey 200 200

0 0 0123456780 0123456780 012345678 012345678 t [s] t [s] t [s] t [s] (a) Based on vehicle speed (b) Based on I curve

(a) Based on vehicle speed (b) Based on I curve

Figure 9. Cont. World Electric Vehicle Journal 2019, 10, 39 10 of 17

z=0.05 z=0.05 800 z=0.1 800 z=0.1 z=0.15 z=0.15 World Electric Vehicle Journal 2019, 10, 39 700 10 of 17

World Electric600 Vehicle Journal 2019, 10, 39 600 10 of 17

500 z=0.05 z=0.05 400 400 800 z=0.1 800 z=0.1 300 z=0.15 700 z=0.15

Energy recovered [kJ] recovered Energy 200 [kJ] recovered Energy 200 600 600

100 500

0 0 400012345678 400012345678

t [s] 300 t [s]

Energy recovered [kJ] recovered Energy 200 [kJ] recovered Energy 200 100 (c) Based on β line (d) Based on 𝐹 0 0 012345678 012345678 Figure 9. Different strategies recover energy at full load. (a) Based on vehicle speed; (b) based on I t [s] t [s] curve; (c) based on β line; (d) based on 𝐹.

It can be seen(c) from Based Figure on β line 10 that when the vehicle is under no( dload,) Based the on braking 𝐹 strategy based on 𝐹 recovers the most energy under the three braking intensities, then it is the strategy based FigureFigure 9.9. DiDifferentﬀerent strategiesstrategies recoverrecover energyenergy atat fullfull load.load. ((aa)) BasedBased onon vehiclevehicle speed;speed; ((bb)) basedbased onon II on the I curve and β line, the strategy based on vehicle speed recovers the least energy recovery. curve;curve; ((cc)) based based on onβ βline; line; ( d(d)) based based on onF 𝐹. . Regardless of the braking strategy used,fmax the braking energy recovered by the vehicle increases withIt Itthe cancan increase bebe seenseen of fromfrom the Figure Figurebraking 10 10 strength.that that whenwhen This thethe vehicleisve mainlyhicle isis underunderdue to nono the load,load, large thethe rolling brakingbraking resistance strategystrategy basedofbased the mining vehicle. When the braking intensity is small, the required braking force is small, the braking ononF 𝐹fmaxrecovers recovers the the most most energy energy under under the the three three braking brakin intensities,g intensities, then then it is it the is the strategy strategy based based on thetimeon Ithe curveand I curve the and braking andβ line, β line, thedistance strategythe strategy are basedlong, based and on vehicle onthe vehicle vehicle speed speed needs recovers recovers more the energy leastthe least energy to overcomeenergy recovery. recovery. the rolling resistance,Regardless so that of less the regenerativebraking strategy braking used, energy the braking can be recovered.energy recovered by the vehicle increases with the increase of the braking strength. z=0.05 This is mainly due to the large rolling resistance z=0.05 of the 300 z=0.1 300 z=0.1 mining vehicle. When the braking intensity z=0.15 is small, the required braking force is small, the z=0.15braking time and the braking distance are long, and the vehicle needs more energy to overcome the rolling resistance,200 so that less regenerative braking energy can be200 recovered.

z=0.05 z=0.05 300 z=0.1 300 z=0.1

100 z=0.15 100 z=0.15 Energy recovered [kJ] recovered Energy [kJ] recovered Energy

200 200

0 0 012345678 012345678 t [s] t [s] 100 100

Energy recovered [kJ] recovered Energy [kJ] recovered Energy (a) Based on vehicle speed (b) Based on I curve 0 0 012345678 z=0.05 012345678 z=0.05 300 t [s] z=0.1 300 t [s] z=0.1 z=0.15 z=0.15

200 (a) Based on vehicle speed 200 (b) Based on I curve

z=0.05 z=0.05 300 z=0.1 300 z=0.1

100 z=0.15 100 z=0.15 Energy recovered [kJ] recovered Energy [kJ] recovered Energy

200 200

0 0 012345678 012345678 t [s] t [s] 100 100

Energy recovered [kJ] recovered Energy [kJ] recovered Energy

(c) Based on β line (d) Based on F 0 0 012345678 012345678 Figure 10. Diﬀerent strategiest [s] recover energy at no load. (a) Based on vehicle speed;t [s] (b) based on I curve; (c) based on β line; (d) based on Ffmax.

Regardless of the braking strategy used, the braking energy recovered by the vehicle increases with the increase of the braking strength. This is mainly due to the large rolling resistance of the mining vehicle. When the braking intensity is small, the required braking force is small, the braking time and the braking distance are long, and the vehicle needs more energy to overcome the rolling resistance, so that less regenerative braking energy can be recovered. Table2 shows the energy recovered of the same braking strategy under di fferent braking strengths. It can be seen that, except for the Ffmax strategy, the vehicle’s braking energy recovery efficiency increases with the increase of braking intensity, regardless of the full load or no load. The energy recovery rate can reach up to 59.6%. For the Ffmax strategy, when the braking strength is large (z = 0.15), the regenerative braking force cannot meet the full load braking force requirement, and the energy recovery efficiency decreases. However, at no load, its regenerative braking force can meet the braking demand, and the energy recovery efficiency is still the highest.

Table 2. Energy recovery under diﬀerent braking strengths [kJ].

Braking Strength

Strategy z = 0.05 z = 0.1 z = 0.15 Full Load No Load Full Load No Load Full Load No Load Energy covered 17.7 0.0 310.1 117.9 392.3 163.4 Speed Vehicle Kinetic energy * 868.1 390.6 868.1 390.6 868.1 390.6 based Proportion [%] 2.0% 0.0% 35.7% 30.2% 45.2% 41.8% Energy covered 56.5 0.0 411.4 165.4 517.5 221.7 I Vehicle Kinetic energy * 868.1 390.6 868.1 390.6 868.1 390.6 curve Proportion [%] 6.5% 0.0% 47.4% 42.3% 59.6% 56.8% Energy covered 56.5 0.0 411.3 165.0 517.4 221.0 β line Vehicle Kinetic energy * 868.1 390.6 868.1 390.6 868.1 390.6 Proportion [%] 6.5% 0.0% 47.4% 42.2% 59.6% 56.6% Energy covered 105.0 21.4 445.3 199.9 383.1 232.9 Ffmax Vehicle Kinetic energy * 868.1 390.6 868.1 390.6 868.1 390.6 Proportion [%] 12.1% 5.5% 51.3% 51.2% 44.1% 59.6% = 1 2 * Vehicle kinetic energy calculation formula: Ek 2 mv0.

4.1.2. Comparison of Braking Force Distribution Taking the braking strength of z = 0.15 as an example, the braking force and regenerative braking force of the front and rear axles of diﬀerent braking strategies are studied when the vehicle is full and no load, as shown in Figure 11. For I curve and β line based braking strategy, the braking force is simultaneously provided by the front and rear axles, the braking force can be provided by the regenerative braking force totally regardless of the loading conditions. For the strategy, the braking force is all provided by the front axle. When braking starts, the motor speed is high, it can be known from Equation (2) that the motor is in the constant power operation phase, the torque that the motor can provide is small. When the vehicle is fully loaded, the required braking force is large, the motor torque is not enough to provide the required braking force (Figure 11d full load); when the vehicle is in no load, the braking force required is small, and the regenerative braking force can meet the front axle braking force requirement (Figure 11d no load). The braking force required for the vehicle speed based strategy is always provided by the front and rear axle regenerative braking force and mechanical braking force simultaneously. When the vehicle speed decreases, the distributed and provided regenerative braking force decreases. It can be seen that the more the regenerative braking force distributed during braking and the regenerative braking force that the regenerative braking system can provide, the more regenerative braking energy the vehicle can recover. World Electric Vehicle Journal 2019, 10, 39 12 of 17 World Electric Vehicle Journal 2019, 10, 39 12 of 17

Front wheel braking force Front wheel braking force 120000 Rear wheel braking force 60000 Rear wheel braking force Front wheel regenerative braking force Front wheel regenerative braking force Rear wheel regenerrative braking force Rear wheel regenerrative braking force 90000 40000

60000

20000 Braking force [N] force Braking Braking force [N] force Braking 30000

0 0 0123 0123 t [s] t [s]

Full load No load (a) Based on vehicle speed

60000

20000

Braking force [N] force Braking 30000 [N] force Braking

0 0 0123 0123 t [s] t [s]

Full load No load (b) Based on I curve

60000

20000

Braking force [N] force Braking 30000 [N] force Braking

0 0 0123 0123 t [s] t [s]

Full load No load (c) Based on β line

Figure 11. Cont. World Electric Vehicle Journal 2019, 10, 39 13 of 17 World Electric Vehicle Journal 2019, 10, 39 13 of 17

Front wheel braking force Front wheel braking force 180000 Rear wheel braking force 90000 Rear wheel braking force Front wheel regenerative braking force Front wheel regenerative braking force Rear wheel regenerrative braking force Rear wheel regenerrative braking force 135000 60000

90000

30000 45000 Braking force [N] force Braking [N] force Braking

0 0 0123 0123 t [s] t [s]

Full load No load

(d) Based on 𝐹

Figure 11. DifferentDiﬀerent strategy braking force distribution at full load and no load. ( a) Based on vehicle

speed; ( b) based on I curve; ( c) based on β line; (d) based on F𝐹fmax..

4.2. Simulation Simulation of of Road Road Drive Cycle 4.2.1. Comparison of Braking Energy Recovery 4.2.1. Comparison of Braking Energy Recovery Table3 shows the recovery of braking energy when the vehicle is fully loaded and unloaded Table 3 shows the recovery of braking energy when the vehicle is fully loaded and unloaded under road drive cycle with different braking strategies. For the road drive cycle, the braking strategy under road drive cycle with different braking strategies. For the road drive cycle, the braking based on F recovers the most energy and has the highest energy recovery efficiency. The strategies strategy basedfmax on 𝐹 recovers the most energy and has the highest energy recovery efficiency. based I curve and β line are the second, and the parallel braking strategy based on the vehicle speed is The strategies based I curve and β line are the second, and the parallel braking strategy based on the the worst. Table3 also shows that for the same braking strategy, the braking energy recovery rate at vehicle speed is the worst. Table 3 also shows that for the same braking strategy, the braking energy full load is greater than that at no load. This is mainly due to the fact that under the same braking recovery rate at full load is greater than that at no load. This is mainly due to the fact that under the intensity, the required braking force is small when the vehicle is under no load, and the required motor same braking intensity, the required braking force is small when the vehicle is under no load, and regenerative torque is also small. At this time, the motor works at a low efficiency (Figure2), resulting the required motor regenerative torque is also small. At this time, the motor works at a low in a decrease in energy recovery efficiency. efficiency (Figure 2), resulting in a decrease in energy recovery efficiency. Table 3. Braking energy recovered of different braking strategies [kJ]. Table 3. Braking energy recovered of different braking strategies [kJ]. Working Condition Strategy Working Condition Strategy FullFull Load Load No No Load Load Total Total Energy coveredcovered 361.4 361.4 420.7 420.7 782.1 782.1 SpeedSpeed based based Vehicle Kinetic Kinetic 740.0 740.0 2778.0 2778.0 3518.0 3518.0 Proportion [%] 48.8% 15.1% 22.2% Proportion [%] 48.8% 15.1% 22.2% Energy coveredcovered 501.6 501.6 783.5 783.5 1285.1 1285.1 I curve Vehicle Kinetic 740.0 2778.0 3518.0 I curve VehicleProportion Kinetic [%] 67.8% 740.0 28.2% 2778.0 36.5% 3518.0 Proportion [%] 67.8% 28.2% 36.5% Energy covered 501.6 783.0 1284.6 β line EnergyVehicle Kineticcovered 740.0 501.6 2778.0 783.0 3518.0 1284.6 β line VehicleProportion Kinetic [%] 67.8% 740.0 28.2% 2778.0 36.5% 3518.0 EnergyProportion covered [%] 591.8 67.8% 1464.3 28.2% 2056.1 36.5% Ffmax EnergyVehicle Kineticcovered 740.0 591.8 2778.0 1464.3 3518.0 2056.1 𝐹 VehicleProportion Kinetic [%] 80.0% 740.0 52.7% 2778.0 58.4% 3518.0 Proportion [%] 80.0% 52.7% 58.4% Table4 4shows shows the the braking braking e ffi efficiencyciency of theof frontthe fr andont rearand wheelrear wheel drive motorsdrive motors for diff erentfor different braking braking strategies. It can be seen that the 𝐹 braking strategy uses the front axle to brake strategies. It can be seen that the Ffmax braking strategy uses the front axle to brake independently, independently,compared with compared the two-axis with braking, the two-axis its motor braking, efficiency its motor is higher. efficiency When is higher. under When road driveundercycle, road drivethe vehicle cycle, speed the vehicle is low, thespeed braking is low, strength the brakin is alsog small, strength and theis also front small, axle regenerativeand the front braking axle regenerative braking force can fully meet the braking force demand. Therefore, the 𝐹 strategy can recover the most braking energy and has the highest energy recovery efficiency. Table 4 also World Electric Vehicle Journal 2019, 10, 39 14 of 17 World Electric Vehicle Journal 2019, 10, 39 14 of 17 showsforce canthat fullyfor the meet same the braking braking strategy, force demand. the motor Therefore, regenerative the Ffmax brakingstrategy efficiency can recover at full theload most is greaterbraking than energy that at and no has load. the highest energy recovery efficiency. Table4 also shows that for the same braking strategy, the motor regenerative braking efficiency at full load is greater than that at no load. Table 4. Front and rear motor braking efficiency. Table 4. Front and rear motor braking efficiency. Motor Regenerative Braking Efficiency Strategy MotorFull RegenerativeLoad Braking No ELoadfficiency Strategy FrontFull LoadRear Front No LoadRear Vehicle speed 60.76%Front 69.79% Rear 36.02% Front 2.41% Rear IVehicle curve speed 68.16% 60.76% 75.42% 69.79% 44.95% 36.02% 2.41% 8.11% β lineI curve 74.68% 68.16% 69.43% 75.42% 50.60% 44.95% 8.11% 0.42%

𝐹β line 85.53% 74.68% 0.00% 69.43% 57.65% 50.60% 0.42%0.00% Ffmax 85.53% 0.00% 57.65% 0.00% 4.2.2. Vehicle Energy Consumption and Battery SOC Changes 4.2.2. Vehicle Energy Consumption and Battery SOC Changes Table 5 shows the energy consumption of the vehicle when the vehicle is fully loaded and unloadedTable at5 roadshows drive the energy cycle. consumptionIt can be seen of that the if vehicle the vehicle when theuses vehicle regenerative is fully loadedbraking, and compared unloaded withat road no regenerative drive cycle. Itbraking, can be seenthe energy that if theconsum vehicleption uses of regenerativea complete braking,drive cycle compared is reduced with by no 1.06%–1.56%.regenerative braking,Compared the to energy passenger consumption cars, the of vehicles a complete studied drive cyclehave islower reduced speeds by 1.06%–1.56%.and lower brakingCompared strength, to passenger as well as cars, greater the vehiclesrolling resist studiedance. have However, lower the speeds use of and regenerative lower braking braking strength, can stillas wellreduce as greaterthe energy rolling consumption resistance. of However, the vehicle the and use improve of regenerative the economic braking performance can still reduce of the the vehicle.energy consumption of the vehicle and improve the economic performance of the vehicle.

TableTable 5. 5. EnergyEnergy consumption consumption of of vehicle vehicle ro roadad drive drive cycle cycle (f=0.04) (f = 0.04) [kJ]. [kJ].

Energy Consumed ImprovementImprovement Ratio (Compared ratio Strategy Energy Consumed Strategy Full Load (Uphill) No Load (Downhill) Total to No(compared Regeneration)% to no Full Load(uphill) No Load(downhill) Total No Regenerative 145850 42795 188645 regeneration)%0.00% NoVehicle Regenerative speed 145630 145850 41011 42795 186641 188645 1.06% 0.00% VehicleI curve speed 145630 145630 40811 41011 186441 186641 1.17% 1.06% β line 145630 40811 186441 1.17% IF fmaxcurve 145580 145630 40128 40811 185708 186441 1.56% 1.17% β line 145630 40811 186441 1.17% 𝐹 145580 40128 185708 1.56% Figure 12 shows the changes of the battery SOC when the vehicle is traveling once under the road driveFigure cycle. 12 It shows can be seenthe changes that when of therethe battery is no regenerative SOC when braking,the vehicle the is battery traveling SOC once drops under the fastest, the road drive cycle. It can be seen that when there is no regenerative braking, the battery SOC drops the followed by the braking strategy based on the vehicle speed, strategy based on the Ffmax drops the F fastest,slowest followed and the energyby the savingbraking e ﬀstrategyect is the base best.d on the vehicle speed, strategy based on the drops the slowest and the energy saving effect is the best.

No regenerative braking 1.0 Vehicle speed I curve β line 0.9 0.62 Ffmax

0.60 0.8 SOC

SOC 0.58

0.7 1250 1300 1350 1400 1450 1500 1550 t [s]

0.6

0 400 800 1200 1600 t [s]

Figure 12. Comparison of SOC changes at road drive cycle. Figure 12. Comparison of SOC changes at road drive cycle. If appropriate measures are taken on the road surface for improvement and maintenance, it is assumedIf appropriate that the measures rolling resistance are taken coe onﬃ thecient road of surface the road for surface improvement is reduced and to maintenance,f = 0.02. Table it is6 assumed that the rolling resistance coefficient of the road surface is reduced to f = 0.02. Table 6 shows the energy consumption of the pure electric mining truck under study in this road drive WorldWorld Electric Electric Vehicle Vehicle Journal Journal 20192019, ,1010, ,39 39 1515 of of 17 17 cycle. Obviously, compared with the road rolling resistance coefficient f = 0.04, the maximum shows the energy consumption of the pure electric mining truck under study in this road drive cycle. vehicle energy consumption decreased from 188,645 kJ to 129,142 kJ, a decrease of 31.54%; the Obviously, compared with the road rolling resistance coeﬃcient f = 0.04, the maximum vehicle energy minimum value decreased from 185,708 kJ to 121,743 kJ, a decrease of 34.44%. After the road consumption decreased from 188,645 kJ to 129,142 kJ, a decrease of 31.54%; the minimum value surface is improved, the regenerative braking technology can reduce the energy consumption of the decreased from 185,708 kJ to 121,743 kJ, a decrease of 34.44%. After the road surface is improved, whole vehicle by 4.76%–5.73%. the regenerative braking technology can reduce the energy consumption of the whole vehicle by 4.76%–5.73%. Table 6. Energy consumption of vehicle road drive cycle（f = 0.02）[kJ].

Table 6. Energy consumptionEnergy ofConsumed vehicle road drive cycle (f = 0.02) Improvement [kJ]. Ratio (Compared Strategy Energy Consumed Improvement Ratio (Compared Strategy to No FullFull Load Load (Uphill) (uphill) No LoadNo Load (Downhill) (downhill) Total Total to No Regeneration) % Regeneration) % No regenerative 100290 28852 129142 0.00% NoVehicle regenerative speed 98902 100290 24096 28852 122998 129142 4.76% 0.00% VehicleI curve speed 98812 98902 23785 24096 122597 122998 5.07% 4.76% βI curveline 98812 98812 23785 23785 122597 122597 5.07% 5.07% F 98665 23078 121743 5.73% βfmax line 98812 23785 122597 5.07%

𝐹 98665 23078 121743 5.73% InIn fact, fact, when when the the road road rolling rolling resistance resistance coeffici coefficientent is is large, large, the the power power used used by by the the vehicle vehicle to to F overcomeovercome the the road road rolling rolling resistance resistance (F ( r)r )is is large. large. Figure Figure 13 13 shows shows the the braking braking power power demand demand when when thethe vehicle vehicle is is fully fully loaded loaded on on roads roads with with different different rolling rolling resistance resistance coefficients. coefficients. It It can can be be seen seen that that F F whenwhen the the rolling rolling resistance coecoefficientfficient isis large,large, the the vehicle vehicle kinetic kinetic energy energy ( a(F)a and) and potential potential energy energy ( i ) (Farei) are mainly mainly used used to overcometo overcome the the road road rolling rolling resistance resistance (Fr) ( whenFr) when braking braking or downhill. or downhill. At this At time,this time,the recoverable the recoverable energy energy is limited, is limited, as shown as shown in Figure in Figure 13a. 13a. When When the roadthe road surface surface is improved is improved and andthe the rolling rolling resistance resistance coeffi coefficientcient is reduced, is reduced, more more vehicle vehicle kinetic kinetic energy energy and potential and potential energy energy can be canrecovered. be recovered. See Figure See Figure 13b. Wind 13b. Wind resistance resistance (Fw) is (F negligiblew) is negligible compared compared to rolling to rolling resistance. resistance.

Fa Fa 400 F w 400 Fw

Fi Fi 300 300 Fr Fr Total Total 200 200

100 100

0 0 Power f=0.04 [kW] f=0.04Power Power f=0.02 [kW] f=0.02 Power

-100 -100

-200 -200 0 200 400 600 0 100 200 300 400 500 600 time [s] time [s]

(a) f = 0.04 (b) f = 0.02

FigureFigure 13. 13. PowerPower requirements requirements for braking withwith didiﬀfferenterent rolling rolling resistance resistance coe coefficientsﬃcients (full（full load, load uphill)， uphill(a) f =）0.04;(a) f = (b 0.04;) f = (0.02.b) f = 0.02.

ImprovingImproving the the road road surface surface can can not not only only reduce reduce the the energy energy consumption consumption of of the the whole whole vehicle, vehicle, butbut also also protectprotect the the vehicle tires,tires, extendextend thethe use use of of the the tires, tires, and and reduce reduce the the maintenance maintenance and and use use cost costof the of the vehicles. vehicles. Therefore, Therefore, it is it necessary is necessary for thefor the mine mine to improve to improve and and maintain maintain the the haul haul roads. roads. InIn addition, addition, the the vehiclevehicle loadingloading state state and and the the driving driving motor motor working working effi ciencyefficiency also also directly directly affect affectthe regenerative the regenerative braking braking energy energy recovery recovery capability capability of the of pure the pure electric electric mining mining truck. truck. Tables Tables3 and 43 andshow 4 show that whenthat when the vehicle the vehicle is fully is loaded,fully loaded, the motor the motor efficiency efficiency is higher is higher than that than at that no load, at no and load, the andregenerative the regenerative energy recoveryenergy recovery rate is also rate high. is also It can high. be seenIt can that be the seen vehicle that fullthe loadvehicle downhill full load can downhillrecover morecan recover regenerative more regenerative braking energy braking than theenergy no-load than downhill. the no-load downhill.

5. Conclusion This paper adopts the regenerative braking priority strategy to develop four braking strategies based on the Matlab/Simulink platform, embeds them in the simulation model of pure electric World Electric Vehicle Journal 2019, 10, 39 16 of 17

5. Conclusions This paper adopts the regenerative braking priority strategy to develop four braking strategies based on the Matlab/Simulink platform, embeds them in the simulation model of pure electric double-axis four-wheel drive mining dump truck developed by ADVISOR 2002 to study the characteristics of regenerative braking energy recovery of the pure electric mining dump truck. The simulation results show that the regenerative braking priority strategy adopted can make full use of the regenerative braking force that the vehicle can provide and recover more braking energy. The regenerative braking energy that the vehicle can recover is proportional to the regenerative braking force it is assigned to and can provide. For the pure electric double-shaft four-wheel drive mining dump truck studied in this paper, the working speed is low, the braking strength is not so big, the required braking force is small (especially when the truck is empty and downhill), which leads to low regeneration eﬃciency of the motor. Adopting the Ffmax strategy based on the front axle braking force maximizing, compared with the I curve and the β-line braking strategy which front and rear axle are braking simultaneously, the braking eﬃciency of the motor can be improved and more braking energy can be recovered. For the low-speed pure electric mining truck, the rolling resistance is much bigger than passenger cars and it will consume more vehicle kinetic energy and potential energy during the small braking intensity of the vehicle and the long downhill process, so, the recoverable braking energy is reduced. In this case, adopting regenerative braking strategy, the vehicle can still reduce the energy consumption by 1.06%–1.56% (when the rolling resistance f = 0.04) or 4.76%–5.73% (when the rolling resistance f = 0.02), which makes the economic performance of the vehicle improve. Of course, if the vehicle is fully loaded downhill, the regenerative braking energy recovered will be more.

Author Contributions: Conceptualization, W.Z. (Wenming Zhang); Methodology, W.Z. (Wenming Zhang) and J.Y.; Software, W.Z. (Wei Zhang); Validation, W.Z. (Wenming Zhang), J.Y. and W.Z. (Wei Zhang); Formal analysis, W.Z. (Wenming Zhang); Investigation, W.Z. (Wei Zhang); Resources, J.Y.; Data curation, W.Z. (Wei Zhang); Writing—original draft preparation, W.Z. (Wei Zhang); Writing—review and editing, W.Z. (Wei Zhang) and J.Y.; Visualization, W.Z. (Wei Zhang); Supervision, W.Z. (Wenming Zhang); Project administration, F.M. Funding: This paper is supported by the China Mobile Multi-functional Emergency Rescue Vehicle Key Technology Research and Application Demonstration Project (Project No. 2016YFC0802905). Conﬂicts of Interest: The authors declare no conﬂict of interest.

References

1. Feng, Y. Performance Cost-Benefit Analysis of a Series Hybrid. Electric Mining Truck; University of Science and Technology: Beijing, China, 2017. 2. Zhou, M.; Gao, Z.; Zhang, H. Research on regenerative braking control strategy of hybrid electric vehicle. In Proceedings of the 6th International Forum on Strategic Technology, Harbin, China, 22–24 August 2011; pp. 300–303. 3. Guo, J.; Wang, J.; Cao, B. Brake-force distribution strategy for electric vehicle based on maximum energy recovery. J. Xi’an Jiaotong Univ. 2008, 5, 607–611. 4. Wei, Z.; Xu, J.; Halim, D. Braking force control strategy for electric vehicles with load variation and wheel slip considerations. IET Electr. Syst. Transp. 2017, 7, 41–47. [CrossRef] 5. Liu, L.; Ji, F.; Yang, S.; Xu, B. Control strategy for electro-mechanical braking based on curves of ECE regulations and ideal braking force. J. Beijing Univ. Aeronaut. Astronaut. 2013, 39, 138–142. 6. Liu, Z.; Guo, X. Electronic-hydraulic-compound regenerative braking control for electric vehicles. J. Cent. South. Univ. (Sci. Technol.) 2011, 42, 2687–2691. 7. Lv, C.; Zhang, J.; Li, Y.; Yuan, Y. Mechanism analysis and evaluation methodology of regenerative braking contribution to energy efficiency improvement of electrified vehicles. Energy Convers. Manag. 2015, 92, 469–482. [CrossRef] 8. Zhang, J.; Li, Y.; Chen, L.; Ye, Y. New regenerative braking control strategy for rear-driven electrified minivans. Energy Convers. Manag. 2014, 92, 469–482. World Electric Vehicle Journal 2019, 10, 39 17 of 17

9. Qiu, C.; Wang, G.; Meng, M.; Shen, Y. A novel control strategy of regenerative braking system for electric vehicles under safety critical driving situations. Energy 2018, 149, 329–340. [CrossRef] 10. Guo, Z.J.; Yue, D.D.; Wu, J.B. Optimization of Regenerative Braking Control Strategy for Pure Electric Vehicle. Appl. Mech. Mater. 2017, 872. [CrossRef] 11. Zhang, X.; Göhlich, D.; Li, J. Energy-Efficient Toque Allocation Design of Traction and Regenerative Braking for Distributed Drive Electric Vehicles. IEEE Trans. Veh. Technol. 2017, 99, 1. [CrossRef] 12. Yu, H.; Wood, A.S.; Huang, M.; Qi, H. Optimal Braking Control for an Independent four Wheel-Motor-Driven Electric Vehicle. In Proceedings of the Hybrid & Electric Vehicles Conference, Prague, Czech Republic, 23–24 March 2017. 13. Zhang, J.; Song, B.; Niu, X. Optimization of Parallel Regenerative Braking Control Strategy. In Proceedings of the IEEE Vehicle Power & Propulsion Conference, Harbin, China, 3–5 September 2008; pp. 1–4. 14. Chu, L.; Shang, M.; Fang, Y.; Guo, J.; Zhou, F. Braking Force Distribution Strategy for HEV Based on Braking Strength. In Proceedings of the International Conference on Measuring Technology and Mechatronics Automation, Changsha, China, 13–14 March 2010. 15. Li, P. Regenerative braking control strategy for a mild HEV. Automot. Eng. 2005, 5, 67–103. 16. Zhao, X.; Ma, J.; Wang, G.P. Composite braking control strategy of pure electric bus based on brake driving intention recognition. J. Traffic Transp. Eng. 2014, 4, 64–75. 17. Cai, J.W.; Chu, L.; Fu, Z.C.; Ren, L.P. Regenerative Braking System for a Pure Electric Bus. Appl. Mech. Mater. 2014, 543, 1405–1408. [CrossRef] 18. Song, B.L.; Zhou, X.S.; Li, J.; Wang, X.; Chen, C.; Sun, S.L. Modeling and simulation of regenerative braking system for pure electric light truck. J. Automot. Saf. Energy 2015, 6, 85–89. 19. Sun, F.; Liu, W.; He, H.; Guo, H. An integrated control strategy for the composite braking system of an electric vehicle with independently driven axles. Veh. Syst. Dyn. 2016, 54, 22. [CrossRef] 20. Wang, M.; Sun, Z.; Zhuo, G.; Cheng, P. Maximum braking energy recovery of electric vehicles and its influencing factors. J. Tongji Univ. (Nat. Sci.) 2012, 40, 583–588. 21. Guo, J.; Wang, J.; Cao, B. Regenerative Braking Strategy for Electric Vehicles. In Proceedings of the 2009 IEEE Intelligent Vehicles Symposium, Xi’an, China, 3–5 June 2009. 22. Zhang, W. Automotive Theory, 2nd ed.; China Machine Press: Beijing, China, 2010. 23. Yu, Z. Automotive Theory, 5th ed.; China Machine Press: Beijing, China, 2009. 24. Philipp, S.; Tim, B.; Constantinos, S. Comparison of Electric Vehicles with Single Drive and Four Wheel Drive System Concerning Regenerative Braking. In Proceedings of the Twelfth International Conference on Ecological Vehicles & Renewable Energies, Monte-Carlo, Monaco, 11–13 April 2017. 25. Sun, D.; Lan, F.; Chen, J. A study on the braking energy recovery strategy for a 4WD battery electric vehicle based on ideal braking force distribution (Curve I). Automot. Eng. 2013, 35, 1057–1061. 26. Yao, L.; Chu, L.; Zhou, F.K.; Liu, M.H.; Zhang, Y.S.; Wei, W.R. Simulation and analysis of potential of energy-saving from braking energy recovery of electric vehicle. J. Jilin Univ. (Eng. Technol. Ed.) 2013, 43, 6–11.

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