Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering http://pid.sagepub.com/
Cooperative control of regenerative braking and hydraulic braking of an electrified passenger car Junzhi Zhang, Chen Lv, Jinfang Gou and Decong Kong Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering 2012 226: 1289 originally published online 25 April 2012 DOI: 10.1177/0954407012441884 The online version of this article can be found at: http://pid.sagepub.com/content/226/10/1289
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Original Article
Cooperative control of regenerative braking and hydraulic braking of an electrified passenger car
Proc IMechE Part D: J Automobile Engineering 226(10) 1289–1302 Ó IMechE 2012 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954407012441884 pid.sagepub.com
Junzhi Zhang, Chen Lv, Jinfang Gou and Decong Kong
Abstract With the aims of regeneration efficiency and brake comfort, three different control strategies, namely the maximumregeneration-efficiency strategy, the good-pedal-feel strategy and the coordination strategy for regenerative braking of an electrified passenger car are researched in this paper. The models of the main components related to the regenerative brake and the frictional blending brake of the electric passenger car are built in MATLAB/Simulink. The control effects and regeneration efficiencies of the control strategies in a typical deceleration process are simulated and analysed. Road tests under normal deceleration braking and an ECE driving cycle are carried out. The simulation and road test results show that the maximum-regeneration-efficiency strategy, which causes issues on brake comfort and safety, could hardly be utilized in the regenerative braking system adopted. The good-pedal-feel strategy and coordination strategy are advantageous over the first strategy with respect to the brake comfort and regeneration efficiency. The fuel economy enhanced by the regenerative braking system developed is more than 25% under the ECE driving cycle.
Keywords Regenerative braking, cooperative control, brake comfort, regeneration efficiency, electrified passenger car, road test
Date received: 6 December 2011; accepted: 17 February 2012
Introduction As one of the key technologies of electrified vehicles, regenerative braking, which has been applied in various types of electric vehicle (EV), hybrid electric vehicle (HEV) and plug-in hybrid electric vehicle, results in an effective improvement in the fuel economy by recuperating the braking energy.1 All road vehicles, including electrified vehicles, should be equipped with a frictional brake system to guarantee the vehicle’s deceleration performance. Thus, for electrified vehicles, coordination of the regenerative brake and the frictional brake is crucial for brake comfort and brake safety as well as for regeneration efficiency. Based on those issues above, a series of solutions in regenerative braking system design and control have been carried out. In the system design of the regenerative brake system, automakers and parts manufacturers worldwide have proposed several implementations, which can be categorized into two types. 1.
One type is based on the brake pressure modulator. The regenerative braking system which has been applied in the Toyota Prius is based on an electric
2.
hydraulic brake system.2 TRW’s solution which has been employed in commercialized hybrid vehicles is based on an electric stability control system.3 The other type is based on an improved master cylinder. Honda adopted a new master cylinder which has been employed in the Civic Hybrid.4 Continental Teves5 and Hyundai6 have also developed this type of regenerative braking system.
In regenerative braking control, present research mainly concentrates on two different braking scenarios. One is the normal deceleration process, with the aims of improving the regeneration efficiency and the coordinated control effect between the regenerative brake and the frictional brake. Gao7 and Gao et al.8 put forward two regenerative braking strategies. However, neither of the two strategies can achieve good brake stability State Key Laboratory of Automotive Safety and Energy, Tsinghua University, People’s Republic of China Corresponding author: Junzhi Zhang, State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, People’s Republic of China. Email:
[email protected]
1290 and high regenerative brake efficiency simultaneously in various driving conditions. Aoki et al.9 used the sliding-mode control method, modulating the hydraulic braking force to obtain the maximum regeneration efficiency and good brake comfort. Targeting the brake intention of the driver and the regeneration efficiency and adopting the optimum control strategy, Luo et al.10 designed a distribution model of the regenerative brake and the frictional brake. The other focus is the emergency braking process. Taking advantage of the quick response and accurate control of the motor torque, researchers worldwide have explored a way to introduce the motor torque into anti-lock brake system (ABS) control, expecting a better control effect. Tur and Ustun,11 Okano et al.,12 Chu et al.13 and others have carried out research studies in this field. Nevertheless, the existing research on regenerative braking control is mainly in the stage of simulation and bench tests; a road test study has rarely been seen. The present authors have made much effort over a long time to research and develop regenerative braking. A regenerative braking system based on a pneumatic ABS has been developed and applied in three fuel cell city buses serving in Beijing;14 another regenerative braking system for an electrified passenger car based on an electronic stability program (ESP) modulator has been proposed and hardware-in-the-loop (HIL) tested.15 Integrated and optimized control strategies for regenerative braking with a pneumatic ABS during emergency braking have been proposed and HIL tested.16,17 Further study on regenerative braking technology is ongoing for an electrified passenger car with a hydraulic braking system. Distinguished from other technologies and studies cited above, comprehensive research on a combination of system design and control was carried out, and fuel economy enhancement by regenerative braking under a driving cycle was measured and is reported in this article. With the aims of brake comfort and regeneration efficiency, three different cooperative control strategies, namely the maximum-regeneration-efficiency strategy (control strategy 1), the good-pedal-feel strategy (control strategy 2) and the coordination strategy (control strategy 3), were researched. The vehicle, the motor, the battery and the hydraulic brake system were modelled for simulating typical deceleration processes in order to verify the control effects and the regeneration efficiencies. A realtime brake controller was developed and applied in a demonstration electric vehicle with the regenerative braking system developed on the basis of the ESP modulator. Road tests under normal deceleration braking and an ECE driving cycle were carried out. Some simulation and road test results are presented in this article.
Proc IMechE Part D: J Automobile Engineering 226(10) based on an ESP modulator was developed and applied in a front-wheel-drive electric vehicle with a hydraulic brake system. The overall scenario of the regenerative braking system is illustrated in Figure 1. The hardware of the regenerative braking system developed is based on the ESP modulator, which is installed between the master cylinder and the wheel cylinder in an electric vehicle. Figure 2 shows the configuration of the regenerative braking system in the regular mode. Two main route valves, which are normally open, are set between the master cylinder and the wheel cylinders. Two pumps are driven by a motor for applying or releasing hydraulic pressures quickly. Four inlet valves which are normally open and four outlet valves which are normally closed are installed corresponding to the four wheel cylinders. Two accumulators are set for quickly releasing the wheel pressures and storing fluid during the pressure-decrease process. Also, two
Figure 1. Scenario diagram of the regenerative braking system.
Regenerative braking control strategy Hydraulic brake system with regenerative braking As mentioned in the introduction, a new type of regenerative braking system for an electrified passenger car
Figure 2. Configuration of the regenerative braking system based on the ESP modulator. RR: rear right: FL: front left; FR: front right; RL: rear left.
Zhang et al. bypass valves installed in the ESP modulator, which are normally closed, are not utilized in the regeneration mode. Three pressure sensors monitoring the master cylinder pressure and two front-wheel pressures are installed respectively. With the integrated hardware mentioned previously and the software for regenerative braking to be introduced, the regenerative braking system featuring a frictional brake, a regenerative brake and an anti-lock brake is developed.
Regenerative braking control strategy The regenerative braking control strategy is crucial to the regeneration efficiency and braking performance of the vehicle. For the blending brake system which coordinates the frictional brake with the regenerative brake, consistent modulation of the frictional brake pressure may cause fluctuation in the master cylinder pressure, worsening the brake pedal feel and even causing the driver to panic. Many of the regenerative braking systems applied in commercialized vehicles are equipped with a brake pedal stroke simulator, which decouples the brake pedal force with the wheel cylinder pressure; this avoids the problems mentioned above but will increase the manufacturing cost and the complexity of the system. The system adopted in this paper employs no pedal stroke simulator. In order to guarantee brake safety and comfort, appropriate control strategies should be developed to minimize the impact that the wheel cylinder pressure exerts on the brake pedal. Based on the regeneration efficiency and brake comfort, three different braking force distribution strategies are designed, as shown in Figure 3. In all the three strategies to be introduced, the front– rear brake force allocation is not changed, i.e. the b line of the vehicle, which is a fixed value, is not modified.
Figure 3. Diagram of the regenerative braking control strategies.
1291 As the strategies are designed for front-wheel-drive electrified vehicles, the rear braking forces are not modulated and are still provided by hydraulic fluid. In addition, the vehicle stability control, e.g. an ABS, anti-slip regulation and an ESP, is not involved in the present research on regenerative brake control strategies. When entering critical driving situations, the regenerative braking torque will be controlled, decreasing to zero rapidly with the hydraulic brake recovered, i.e. only the frictional brake works under critical driving situations. As shown in Figure 3(a), control strategy 1 (the maximum-regeneration-efficiency strategy) targets the maximum regeneration efficiency. The regenerative braking torque is utilized to its maximum extent, and the hydraulic force will not be exerted on the front wheel until the motor torque cannot meet the deceleration request. Theoretically, this strategy can maximize the use of the regenerative braking torque, achieving the maximum regeneration efficiency on the premise of the original front–rear brake force allocation. However, when the hydraulic braking force needs to be supplemented, the filling of the front-wheel cylinders may draw a significant amount of brake fluid from the pressurized master cylinder and lead to a sudden drop in the master cylinder pressure, causing subsidence of the brake pedal, which affects the brake comfort. In addition, when this regenerative braking system is adopted, the braking demand of the driver is derived from interpreting the master cylinder pressure measured by a sensor. The sudden decrease in the master cylinder pressure would lead to an indicated braking intention which deviates from the real intention of the driver and results in a severe change in the vehicle’s deceleration, affecting the braking safety. Control strategy 2 (the good-pedal-feel strategy), shown in Figure 3(b), is to guarantee the good feel of the brake pedal. At the beginning of the braking procedure, only the frictional brake force is exerted. Once the
1292 front-wheel pressures tend to be stable, the regenerative braking torque would be exerted. Meanwhile, the front inlet valves would be closed and the hydraulic fluid of the front wheels would be extracted and stored in the accumulator. When the hydraulic braking force needs to be supplemented, the fluid stored in the accumulator will be pumped into the wheel cylinder through the inlet valve with the main route valves turned off and the main cylinder insulated from the wheel cylinder to increase the wheel cylinder pressure. Thus the master cylinder pressure would not fluctuate while the wheel cylinder pressure changes. The stability of the pedal feel and indicated driver braking intention are guaranteed at the cost of the regeneration efficiency by exerting a hydraulic braking force at the beginning of the braking procedure. As shown in Figure 3(c), control strategy 3 (coordination strategy) is a combination of the former two strategies which coordinates the regenerative efficiency with the brake pedal feel. Within the free travel of the brake pedal, a slight motor braking torque is exerted on the front axle to imitate the engine brake of the conventional internal combustion engine (ICE) vehicle. Once entering the operational travel of the brake pedal, the front-wheel cylinder pressure is increased under control slowly, providing part of the overall brake request, while the regenerative braking torque supplies the remaining braking request. After the braking pressure reaches a certain threshold, the regenerative braking torque is utilized to its maximum extent, the front inlet valves will be closed and the excessive fluid of the front-wheel cylinder will be extracted to the accumulator. When the front hydraulic brake needs to be supplemented, the fluid stored in accumulator will be pumped into the frontwheel cylinders. Therefore, a good brake pedal feel, a stability of the indicated braking intention and a relatively high regeneration efficiency are all obtained.
Hydraulic pressure control algorithm The modulating effect of the hydraulic pressure influences the regeneration efficiency, the brake comfort and the brake safety simultaneously. Thus, for regenerative braking, the hydraulic pressure control algorithm is of great importance. The hydraulic pressure control algorithm developed is shown in Figure 4. The inlet and outlet solenoid valves of the front wheels and the pump motor are pulse width modulated (PWM), while the two main route valves are on–off controlled and the inlet and outlet valves of the rear wheels are not controlled. pm is the master cylinder pressure, pw act is the actual wheel cylinder pressure and pw tgt is the target value of the wheel cylinder pressure. MV cmd is the on–off command value of the main route values. pump cmd is the PWM command value of the pump motor. Inlet PWM and Outlet PWM are the PWM control signals of the front-wheel inlet valve and front-wheel outlet valve respectively, which are calculated in real time in the
Proc IMechE Part D: J Automobile Engineering 226(10)
Figure 4. Diagram of the hydraulic pressure control algorithm. Outlet cmd: control command value of the front-wheel outlet valve; Inlet cmd: control command value of the front-wheel inlet valve; Inlet_PD: inlet proportional–derivative controller; Outlet_PD: outlet proportional–derivative controller; Inlet PWM: pulse-width-modulated control signal of the front-wheel inlet valve; Outlet PWM: pulse-widthmodulated control signal of the front-wheel outlet valve; MV cmd: on–off command value of the main route values; pump cmd: pulse-widthmodulated command value of the pump motor.
pressure-increase mode and the pressure-decrease mode respectively. Inlet cmd and Outlet cmd are the control command values of the front-wheel inlet valve and front-wheel outlet valve respectively in the whole regeneration procedure. In the pressure-increase mode, Inlet cmd is derived from Inlet PWM. Similarly, in the pressure-decrease mode, Outlet PWM would be assigned to Outlet cmd. To calculate Inlet PWM and Outlet PWM, look up tables of pw act first and obtain an appropriate basic value of the rate of pressure increase or decrease under different wheel cylinder pressures. Then both of the proportional–derivative (PD) feedback controls for the pressure increase and decrease are carried out on the basis of the difference between the master cylinder pressure and the wheel cylinder pressure. The sum of the outputs of Inlet_PD controller and look-up Table 1 (not included in this paper) give the value of Inlet PWM, and the sum of Outlet_PD controller and look-up table 2 give the value of Outlet PWM. The hydraulic braking force is threshold controlled, covering seven phases including pressure increase, pressure hold and pressure decrease. The detailed state flow of the hydraulic pressure control algorithm developed is shown in Figure 5. The default phase of the algorithm is ‘Pressure Hold’, meaning the front-wheel cylinder pressure holds. Both of the other two phases of ‘Pressure Increase’ and ‘Pressure Decrease’ have their two subphases denoted as Inc 1 and Inc 2, and Dec 1 and Dec 2 respectively, which can realize the different increase or decrease rates in the hydraulic braking pressures. These phases can transfer to each other by judging the logic thresholds, including eight thresholds en (n = 1, 2, . . . , 8), which are the various differences between pw act and pw tgt , and four thresholds of the changing rate P_ mn (n = 1, 2, 3, 4) of the master cylinder
Zhang et al.
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Table 1. Comparison of the simulation results for the three control strategies. Control strategy
Recoverable energy (kJ)
Regenerated energy (kJ)
Regeneration efficiency (%)
Maximum-regeneration-efficiency strategy Good-pedal-feel strategy Coordination strategy
39.688 39.868 39.842
26.33 22.48 25.87
65.92 56.39 64.94
while Outlet1 and Outlet2 are fixed values and are used in the other phases.
Table 2. Parameters of the test electric vehicle. Parameter
Value
Total mass m Wheelbase L Frontal area A Coefficient of air resistance CD Nominal radius r of a tyre Final drive ratio i0 Transmission ratio ig Front–rear braking force distribution coefficient b
1060 kg 2.33 m 2.142 m 0.32 0.275 m 3.79 2.08 7.881
System modelling In order to test and verify the regenerative braking control strategies and to evaluate the control effects and regeneration efficiency before road tests, simulation is required. To carry out the simulation, appropriate models including those of the vehicle (overall dynamics), the battery, the tyre, the electric motor and the hydraulic brake system were built.
Vehicle dynamics A model of vehicle dynamics with eight degrees of freedom was built in MATLAB/Simulink. The OXYZ coordinate system and the vehicle model, shown in Figure 6, have their origins at the centre of gravity of the vehicle. OX is in the longitudinal direction of the vehicle, pointing forwards. The plane OXY is parallel to the ground with OZ perpendicular to OXY, pointing upwards. The eight degrees of freedom of the vehicle model are as follows: the displacements of the vehicle along OX and OY, which are denoted by x and y respectively; the yaw angle of the vehicle around OZ, which is represented by g; the rotations of the four wheels and the steering angle d of the front wheels. The equations of the motions are as follows. The longitudinal motion of the vehicle is expressed as Figure 5. State flow of the hydraulic braking logic threshold control algorithm. MV cmd: on–off command value of the main route values; MV1 , MV2 : two fixed control signal values of the main route valves; pump cmd: pulsewidth-modulated command value of the pump motor; pump1 , pump2 : two different control signal values of the pump motor; Inlet1 , Inlet cmd: control command values of the front-wheel inlet valve; Outlet cmd, Outlet1 , Outlet2 : control command values of the front-wheel outlet valve.
m(u_ vg) = (Fx11 + Fx12 ) cos d (Fy11 + Fy12 ) sin d CD A (3:6u)2 + Fx21 + Fx22 21:15 ð1Þ
The transverse motion of the vehicle is expressed as m(v_ + ug) = (Fx11 + Fx12 ) sin d
pressure. MV1 and MV2 are two fixed control signal values of the main route valves. pump1 and pump2 are two different control signal values of the pump motor. Both Inlet1 and Inlet PWM are the control signal values of the front-wheel inlet valves. Inlet PWM is used in pressure-increase phases and calculated in real time, while Inlet1 is a fixed value and is used in the other phases. Similarly with the inlet valves, the Outlet1 , Outlet2 and Outlet PWM are the control signal values of the front outlet valves. Outlet PWM is used in the pressure-increase phases and is calculated in real time,
+ (Fy11 + Fy12 ) cos d + Fy21 + Fy22 ð2Þ
The yaw of the vehicle is expressed as JZ g_ =
BF ½(Fx12 Fx11 ) cos d + (Fy11 Fy12 ) sin d 2 + a½(Fx12 + Fx11 ) sin d + (Fy12 + Fy11 ) cos d BR (Fx22 Fx21 ) b(Fy22 + Fy21 ) + 2 ð3Þ
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Proc IMechE Part D: J Automobile Engineering 226(10) the tyre and Tdij and Tbij are the driving and braking torques respectively exerted on the wheel.
The tyre The tyre model, which is of great importance for research on braking, should be able to simulate the real tyre in both adhesion and sliding. There have been various theories developed to estimate the tyre–road force. In this article, the ‘magic formula’ model is adopted.18 In the single condition, the tyre’s longitudinal force can be expressed as Fx0 = D sin½C arctan (Bf)
ð6Þ
where D is the peak factor, C is the shape factor and B is the stiffness factor which are obtained from the equations f = (1 E)sx +
E arctan (Bsx ) B
D = a1 F2z + a2 Fz C = 1:65 a3 F2z + a4 Fz CDea5 Fz E = a6 F2z + a7 Fz + a8
B=
ð7Þ
In the single condition, the tyre’s lateral force can be expressed as Figure 6. Diagram of the OXYZ coordinate system and the vehicle model.
Fy0 = D sin½C arctan (Bf) + DSv
The motion of the tyre of a drive wheel can be expressed as (Jw + 12Jpt )v_ ij = Tdij Tbij +Fxij r
(i=1,j= 1, 2)
ð4Þ
The motion of the tyre of an idle wheel can be represented as Jw v_ ij = Tbij + fFzij r (i = 2, j = 1, 2)
ð5Þ
In the equations above, m is the overall mass of the vehicle, u is the velocity along OX and v is the velocity along OY. Fxij is the tangential force of the wheel ij, Fyij is the transverse force of the wheel ij, Fzij is the vertical force of the wheel ij, where ij = 11 for the front left wheel, ij = 12 for the front right wheel, ij = 21 for the rear left wheel and ij = 22 for the rear right wheel. CD is the coefficient of the air resistance, A is the frontal area and JZ is the moment of inertia of the vehicle in the OZ direction. a is the longitudinal distance from the centre of gravity of the vehicle to the front axle, b is the longitudinal distance from the centre of gravity of the vehicle to the rear axle and L is the wheelbase; thus, a + b = L. BF and BR are the widths of the front track and the rear track respectively, Jw is the moment of inertia of tyre, Jpt is the moment of inertia of the powertrain which is shared by the two drive wheels, vij is the rotational speed of the wheel, r is the nominal radius of
ð8Þ
where Sv is the shifting value. The factors in equation (8) are obtained from f = (1 E)(a + DSh ) +
E arctan½B(a + DSh ) B
D = a1 F2z + a2 Fz C = 1:30 a3 sin½a4 arctan (a5 Fz ) (1 a12 juj) CD E = a6 F2z + a7 Fz + a8
B=
DSh = a9 u DSv = (a10 F2z + a11 Fz )u
ð9Þ
where u is the camber angle of the wheel. In the combined condition, restricted by the friction circle which is the comprehensive limit both longitudinally and laterally, the longitudinal force and the lateral force can be represented as Fx =
dx Fx0 d
ð10Þ
dy Fy0 d
ð11Þ
and Fy =
respectively where d, dx and dy can be calculated from the equations
Zhang et al. d=
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qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi d2x + d2y
dx =
jsx j , 1 + jsx j
dy =
j tan aj 1 + jsx j
ð12Þ
The slip angle a and longitudinal slip ratio sx of the tyre can be expressed as vyij aij = arctan (i, j = 1, 2) vxij vij r vxij (i, j = 1, 2) ð13Þ sxij = max (vij r, vxij ) where vxij and vyij can be obtained from the equations s ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi BF BF 2 b1 = arctan + a2 , l1 = 2a 2 sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 BR BR b2 = arctan + b2 , l2 = 2b 2
Figure 7. The efficiency map of the electric motor system. .
vx11 = u cos d + v sin d l1 g sin (b1 d) vy11 = v cos d u sin d + l1 g cos (b1 d) vx12 = u cos d + v sin d + l1 g sin (b1 + d) vy12 = v cos d u sin d + l1 g cos (b1 + d) vx21 = u l2 g sin b2 , vy21 = v l2 g cos b2 vx22 = u + l2 g sin b2 ,
vy22 = v l2 g cos b2
ð14Þ
The vertical loads on the tyres can be expressed as _ 0:5mgb 0:5muh 0:5mh(v_ + ug) L L BF _ 0:5mgb 0:5muh 0:5mh(v_ + ug) + Fz12 = L L BF _ 0:5mga 0:5muh 0:5mh(v_ + ug) + Fz21 = L L BR _ 0:5mga 0:5muh 0:5mh(v_ + ug) + + Fz22 = L L BR
battery provided by the Chery Motor Company. Its capacity is 45 A h and the nominal voltage is 336 V. In the simulation, look-up tables are compiled on the basis of the state of charge (SOC) and temperature data for the battery, producing its charging–discharging internal resistance. The model’s input is the power required by the electric motor. Its output includes the SOC, the voltage at the output port of battery, the current and the temperature of the battery.
Fz11 =
Hydraulic brake system
ð15Þ
Tb f =
Electric motor The model of the electric motor is built according to the data for the electric motor applied in the Chery electric vehicle supplied by the Chery Motor Company. The overall efficiency of the electric drive system including the electric motor and its power electronics components while driving is given in an efficiency map in Figure 7. The peak torque profile of the electric drive system is also shown in the figure. The efficiency map in the regenerative braking mode has almost the same symmetry as that in the driving mode with respect to the axis of rotational speed. The transfer function of the electric motor of the first order can be written as G(s) =
1 0:05s + 1
There are disc brakes at the front wheels and drum brakes at the rear wheels for the target vehicle. The frictional braking torque of a front wheel can be obtained from
ð16Þ
The battery The battery model is built as an open-circuit voltage– resistance model based on the data for the lithiumi-ion
2mb ppr2w rb r
ð17Þ
Thus, the braking torque of the vehicle is Tb =
4pmb pw r2w rb r=b
ð18Þ
where mb is the friction coefficient of the brake disc, pw is the pressure of the wheel cylinder, rw is the radius of the piston of the wheel cylinder, rb is the effective friction radius of the brake disc, ris the nominal radius of the tyre and b is the braking force distribution coefficient between the front and rear axles (which is set at 7.881). In order to simulate the modulating procedure of the wheel cylinder pressures, the model of the hydraulic pressure increase and hydraulic pressure decrease needs to be built. The schematic diagram of the hydraulic brake system is shown in Figure 8. The inlet valve and outlet valve are PWM controlled respectively. pm is the
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Proc IMechE Part D: J Automobile Engineering 226(10)
Simulation Scenarios of simulation To evaluate the control effect of the strategies during normal deceleration braking, simulations are carried out in MATLAB/Simulink with the models introduced in the third section. Simulations mainly focus on two aspects as follows: (a)
the control effect of the coordination between the frictional brake and the regenerative brake and the brake comfort of the three control strategies; (b) a comparison of the regeneration efficiencies of the three control strategies.
Figure 8. Schematic diagram of the hydraulic brake system.
input pressure of the inlet valve and p0 is the pressure of the accumulator. The structure of the wheel cylinder is simplified to a piston and a spring. k is the stiffness of the spring and x is the displacement of the spring. The hydraulic fluid flow of a valve can be expressed as sffiffiffiffiffiffiffiffiffiffi 2 Dp ð19Þ Q = Cd A r where Cd is the flow coefficient of the inlet of the valve (which is taken as 0.7), A is the cross-sectional area of the valve opening and r is the density of the hydraulic fluid. Dp is the difference between the pressures at the inlet and the outlet of the valve, which can be calculated from the following equations: for the pressure-increase process Dp = pm pw
In simulations, the initial braking speed is set at 30 km/h, the braking pressure is taken as a ramp input stabilizing at 3 MPa, and the road is assumed to have a dry surface with a high adhesion coefficient. The degree of jerk is selected as an evaluation parameter to evaluate the brake comfort. Jerk, known as the rate of change in the acceleration, can be expressed as j=
da d2 v = 2 dt dt
ð25Þ
where a is the longitude deceleration of vehicle and v is the vehicle’s velocity. The simulation results of the three control strategies are shown in Figure 9, Figure 10, and Figure 11 respectively.
Simulation of three control strategies Based on the maximum-regeneration-efficiency strategy introduced in the second section, the fluid for the wheel
ð20Þ
for the pressure-decrease process Dp = pw p0
ð21Þ
and, for the wheel cylinder pr2w dx = Q dt
ð22Þ
k dx pr2w
ð23Þ
dpw =
Combining equations (22) and (23), pw can be represented as sffiffiffiffiffiffiffiffiffiffi dpw k 2 Dp ð24Þ = 2 4 Cd A p rw r dt
Figure 9. Simulation of the maximum-regeneration-efficiency strategy.
Zhang et al.
Figure 10. Simulation of the good-pedal-feel strategy.
Figure 11. Simulation of the coordination strategy.
pressure supplement is provided by the main cylinder. From the simulation results of the maximum-regeneration-efficiency strategy shown in Figure 9, the frontwheel pressure has a severe increase at 0.5 s; this leads to a drastic decrease in the main cylinder pressure, causing a sudden subsidence of the brake pedal, which affects the brake comfort. In addition, since the total braking demand of the driver is indicated by the master cylinder pressure, the sudden decrease in the master cylinder pressure described above causes the indicated
1297 braking intention to deviate from the real intention of the driver and results in a severe change in the vehicle’s deceleration and jerk, affecting the braking safety. The simulation results of the good-pedal-feel strategy is shown in Figure 10. At the beginning of the braking procedure, based on the strategy, only the frictional brake force is exerted; the front-wheel pressure increases synchronously with the master cylinder pressure. Once the changing rate of the front-wheel pressure reaches a stable value at 0.5 s, the regenerative braking torque starts to be exerted and the front-wheel pressure decreases gradually. At this time, the front inlet valve is closed and the fluid of the front wheels is extracted into the accumulator. After the regenerative braking torque increases to its maximum extent at about 1 s, which can meet most of the braking demand, the front-wheel pressure is kept at a low value of 0.5 MPa. When the regenerative braking torque decreases at about 2.4 s, which is restricted by its fullload characteristic, the hydraulic braking force needs to be supplemented, and the fluid stored in the accumulator is pumped into the wheel cylinder through the inlet valve with the main route valves turned off. Thus the master cylinder pressure is maintained while the front-wheel cylinder pressure increases. The stability of the pedal feel and the indicated brake intention of the driver are guaranteed, and the deceleration and jerk of the vehicle are kept stable. The simulation results of the coordination strategy are shown in Figure 11. At the beginning of the braking procedure at 0.1 s, the front-wheel cylinder pressure is controlled to increase slowly, providing part of the overall brake request. Meanwhile, the motor braking torque is exerted on the front axle, coordinating with the front-wheel cylinder pressure and supplying the remaining braking request. At about 0.25 s, after reaching a certain threshold, the front-wheel pressure is controlled to decrease, and the regenerative braking torque is increased gradually to its maximum extent. The excessive fluid of the front-wheel cylinder is extracted to the accumulator. Afterwards, during the braking procedure, when the front hydraulic brake needs to be supplemented, the fluid stored in the accumulator is pumped into the front-wheel cylinders through the inlet valve with the main route valves turned off. Thus the main cylinder pressure is kept stable at a relatively high level. Therefore, the brake pedal feel, the stability of the indicated braking intention and a relatively high regeneration efficiency are all obtained.
Comparisons of the three control strategies in the simulation Regarding the simulation results of the three strategies, the maximum-regeneration-efficiency strategy would lead to a drastic decrease in the main cylinder pressure when the front-wheel pressure increases, causing a sudden subsidence of the brake pedal and a deviation of
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the indicated brake demand, which would affect the brake comfort and brake safety. However, the goodpedal-feel and coordination strategies are more advantageous in their cooperative control effects between the regenerative and the frictional brakes than the maximum-regeneration-efficiency strategy is. The wheel cylinder pressures are modulated gradually and accurately; a relatively stable master cylinder pressure is guaranteed, indicating that a good brake pedal feel and indicated brake intention can be obtained. In addition, the overall brake forces of the front wheel combining the regenerative and the frictional brakes in these two strategies remain consistent with the change in the master cylinder pressure; this demonstrates the validity and feasibility of the hydraulic braking control algorithm. In order to evaluate the regeneration capabilities of the three control strategies, an evaluation parameter of regeneration braking efficiency hreg is adopted according to hreg =
Ereg 3100% EBK
ð26Þ
Test vehicle The vehicle utilized in the road tests is the electric passenger car shown in Figure 12, and its parameters are presented in Table 2. The front wheels of this test electric vehicle are driven by a permanent-magnet synchronous motor which can work in two states as a driving motor or a generator. The battery, which is connected to the motor through the d.c. bus, can be discharged or charged for motoring or absorbing the regenerative power during the braking process respectively.
Brake controller To carry out road tests on the regenerative braking system, a real-time brake control unit is developed. The brake controller functions in an integrated way with a normal hydraulic brake, a regenerative brake and an anti-lock brake. The inputs, outputs and communication interface of the developed controller are shown in Figure 13. The
where Ereg is the energy regenerated by the electric motor during regenerative braking and EBK is the maximum value of the recoverable energy, i.e. the kinetic energy left after subtracting all the energy that would be dissipated by the road drag and the air resistance. The regenerated energy is expressed by ð t1 Ereg =
ð27Þ
UI dt t0
and the recoverable energy by 1 EBK = mv2 2
ð t1 t0
fmgv dt
ð t1 t0
CD A (3:6v)2 v dt 21:15 ð28Þ
where t0 is the initial braking time, t1 is the final braking time, U is the output voltage of battery pack, I is the charging current of battery, m is the total mass of vehicle, f is the coefficient of rolling resistance (taken as equal to 0.012), CD is the coefficient of air resistance and A is the frontal area of the vehicle. The simulation results of the regenerative braking efficiency are shown in Table 1. Comparing the simulation results, the regeneration efficiency of the maximum-regeneration-efficiency strategy is the highest, followed by the coordination strategy, and then the good-pedal-feel strategy.
Figure 12. The test electric vehicle.
Road tests To study further the influences of the regenerative braking on the vehicle braking performance and the fuel economy enhancement, road tests under both normal deceleration braking and the ECE driving cycle are carried out.
Figure 13. The configuration of the real-time brake controller. ABS: anti-lock brake system; PWM: pulse-width-modulated signal; ECT: electronic control transmission; CAN: controller area network; CANH: high-side bus output driver of the controller area network; CANL: lowside bus output driver of the controller area network.
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Figure 14. The brake controller.
controller consists of a microcontroller, its peripheral circuits and some processing–driving circuits. The microcontroller chosen is the Freescale MC9S12XDP512, which has the function modules of an enhanced capture timer, PWM output and controller area network (CAN) communication. The most important processing circuit is the hydraulic pressure signal-processing circuit, while the most important driving circuit is the solenoid-modulating valve-driving circuit with a diagnosing function. The inputs and outputs (namely the rotational speed of the electric motor, the limit of the electric motor torque and the regenerative motor torque command) for the regenerative brake are carried out by a CAN mainly with the vehicle control unit and the motor control unit. Figure 14 is a photograph of the developed brake control unit.
1299 As the road test results of the maximum-regeneration-efficiency strategy in normal deceleration braking shown in Figure 15 demonstrate, the deceleration phase 1 lasts for 0–2 s. At 2 s the brake pedal is depressed. According to the strategy, only the regenerative braking torque is exerted on the front wheels initially. At the same time, the inlet valves are turned off and the pressure of the front wheel is nearly zero, while the master cylinder pressure reaches a relatively high value. After 2.5 s, the motor torque cannot meet the requirement, and so the hydraulic brake fluid of wheel cylinder is supplemented from the master cylinder. However, because of the large difference between the pressures in the front-wheel cylinder and the master cylinder, the front-wheel cylinder pressure increases suddenly and the master cylinder pressure decreases abruptly, causing subsidence of the brake pedal. Meanwhile, the brake intensity indicated by the master cylinder pressure drops unexpectedly, which leads to an inevitable decrease in the regenerative braking torque, resulting in a positive jerk of vehicle for 2–3 s. These issues analysed above affect the brake comfort and brake safety severely, validating the simulation results in the fourth section. Therefore, based on the regenerative braking system adopted, this
Normal deceleration test To verify the control effects and to analyse the brake comfort and regeneration efficiencies of a vehicle, road tests in normal deceleration are carried out. The test track has a dry surface with a relatively high adhesion coefficient. The test vehicle’s deceleration process is divided into two phases. Phase 1 occurs when the accelerator pedal is released; a small regenerative braking torque controlled by the vehicle controller is exerted to imitate the engine brake of the conventional ICE vehicle and, once the brake pedal is pressed, the deceleration phase 2 is entered, while the regenerative braking torque exerted is controlled by the brake controller based on the regenerative braking control strategy. The initial braking velocity of vehicle in a normal deceleration test is about 40 km/h, corresponding to a rotational speed of the electric motor of about 3100 r/min. The targeted main cylinder pressure for phase 2 is about 3–4 MPa.
Figure 15. Road test of the maximum-regeneration-efficiency strategy.
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Figure 16. Road test of the good-pedal-feel strategy.
Figure 17. Road test of the coordination strategy.
maximum-regeneration-efficiency strategy could hardly be applied in vehicle and will not be discussed further. Figure 16 shows the road test results of the goodpedal-feel strategy in normal deceleration braking. For 0–1.9 s the regenerative braking torque of phase 1 is exerted. When entering phase 2 at 1.9 s, according to the strategy, only the hydraulic braking force is exerted and the regenerative braking torque decreases to zero immediately. As the motor torque responds much more quickly than the hydraulic braking pressure does, however, the braking pressure is not established correspondingly. Therefore, the vehicle deceleration decreases, causing a slight positive jerk for 2–2.5 s. After the braking pressure has established stably, the regenerative brake and the frictional brake cooperate well and the braking deceleration changes smoothly. Figure 17 shows the road test results of the coordination strategy in normal deceleration braking. For 0–1.5 s the regenerative braking torque of phase 1 is exerted. When entering the deceleration phase 2 at 1.5 s, as the strategy defined, the overall braking force of the front wheel is provided by both the motor torque and the frictional force. As the figure shows, the regenerative braking torque transits smoothly from deceleration phase 1 to phase 2. Therefore, the jerk of the vehicle remains stable and good brake comfort is obtained. After the
front-wheel pressure reaches a relatively high value, the regenerative braking torque starts to increase to its maximum limit, and the front hydraulic braking force is controlled to decrease in cooperation with the regenerative braking torque, meeting the overall brake requirement of the vehicle. Based on the road test data shown in Table 3, the regeneration efficiencies with both the good-pedal-feel strategy and the coordination strategy reach a relatively high level, while the efficiency of the good-pedal-feel strategy is slightly lower than that of the coordination strategy, which validates the simulation results.
ECE driving cycle test As the average speed of the ECE driving cycle is close to the condition of an urban area in the People’s Republic of China, the ECE driving cycle is adopted to carry out the road tests for studying the fuel economy of an electric vehicle with a regenerative braking system, which is of practical significance. Figure 18 shows the situation in one of the road tests under the ECE driving cycle. To evaluate the fuel economy of the vehicle improved by a regenerative braking system, the contribution rate d is proposed as an evaluation parameter, which can be expressed as
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Table 3. Comparison of the normal deceleration test results for the three control strategies. Control strategy
Recoverable energy (kJ)
Regenerated energy (kJ)
Regeneration efficiency (%)
Maximum -regeneration-efficiency strategy Good-pedal-feel strategy Coordination strategy
— 80.253 84.015
— 37.846 43.342
— 47.16 52.32
Conclusion
Figure 18. Road test under the ECE driving cycle. SOC: state of charge.
Table 4. Comparison of the test results for the ECE driving cycle. Control strategy
Consumed energy (kJ)
Regenerated energy (kJ)
Contribution rate (%)
Good-pedal-feel strategy Coordination strategy
450.82
129.71
24.88%
456.98
138.31
26.18%
d=
Ereg hcharge hdischarge 3100% Edrive
where Ereg is the regenerated energy at the d.c. bus of the whole driving cycle, Edrive is the consumed energy at the d.c. bus of the whole driving cycle, hcharge is the charge efficiency of the battery and hdischarge is the discharge efficiency of the battery. As the range of battery SOCs is 70–55% during the road tests, the maximum discharge rate is about 1.3 C, the maximum charge rate is about 0.5 C and the temperature of the battery is 25–30 °C; the average charge and discharge efficiency of battery is taken to be 0.93.19 The road test results of the ECE driving cycle are shown in Table 4. Based on equation (29), the fuel economy contribution rates of the good-pedal-feel and coordination strategies are above 24%; in particular, for the coordination strategy, the fuel economy contribution rate even reaches 26.18%, which is a relatively high level.
With the aims of regeneration efficiency and brake comfort, three different cooperative control strategies, namely the maximum-regeneration-efficiency strategy, the good-pedal-feel strategy and the coordination strategy, were researched. Simulations were carried out on models built in MATLAB/Simulink. A real-time brake controller was developed. Road tests under normal deceleration braking and the ECE driving cycle were carried out in a demonstration electric vehicle with the developed regenerative braking system. The results of simulations and road tests show that the maximum-regeneration-efficiency strategy, which would cause issues in brake comfort and brake safety, could hardly be used with the regenerative braking system adopted. The good-pedal-feel and coordination strategies are advantageous with respect to brake comfort and regeneration efficiency, ensuring the stability and brake safety of the vehicle. The test results of the ECE driving cycle also demonstrate that the improvement in the fuel economy of the electric vehicle enhanced by the regenerative braking system developed is greater than 25%. Further studies will be carried out in some areas such as the following: road tests of different control strategies under different driving cycles; life tests of the regenerative braking system; design and manufacturing of the new system with a pedal stroke simulator; the influence of the electric motor on the vehicle dynamic control of the electric powertrain and chassis. Funding This work was supported by the National Natural Science Foundation of China (project number 51075225). References 1. Sovran G and Blaser D. Quantifying the potential impacts of regenerative braking on a vehicle’s tractivefuel consumption for the U.S., European, and Japanese driving schedules. SAE paper 2006-01-0664, 2006. 2. Tanaka Y, Nakaoka H, Mizutani Y and Nakamura E. Brake control device and brake control method. US Patent 7630815, 2009. 3. TRW. Cognitive safety systems, news releases, http:// trw.mediaroom.com/index.php?s=43&item=336 (2012). 4. Tagata K, Sakai K and Aoki Y. Vehicle brake device. US Patent 7360360, 2007.
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