Regenerative active suspension system with residual energy for in-wheel motor driven electric vehicle

Applied Energy 260 (2020) 114180

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Regenerative active suspension system with residual energy for in-wheel motor driven electric vehicle

T

Guimin Longa, Fei Dinga, Nong Zhangb, , Jie Zhanga, An Qina ⁎

a b

State Key Laboratory of Advanced Design and Manufacturing for Vehicle Body, Hunan University, Changsha 410082, China School of Electrical, Mechanical and Mechatronic Systems, The University of Technology, Sydney, Sydney 2131, Australia

HIGHLIGHTS

New renewable energy application scheme was developed for in-wheel motor driven EV. • AEnormous regeneration potential of the suspended driven motor is excavated. • Regardless energy voltage condition, the new DC-DC converter control harvester damping well. • High efficiency strategy is designed for hybrid energy storage system. • Suspension activemanagement control and residual harvested energy are obtained, simultaneously. • ARTICLE INFO

ABSTRACT

Keywords: Active suspension Energy regeneration Electromagnetic damping force control Energy management strategy In-wheel motor driven electric vehicle

The active suspension system is a practical solution to improve vehicle comfort and safety by applying controlled forces to the vehicle body and wheels. However, the widespread application of the system is significantly inhibited by their large power demands. This paper proposes a new regenerative active suspension system for the in-wheel motor driven electric vehicles. In this system, a new advance dynamic-damper mechanism with a suspended driving motor is designed. Two electromagnetic actuators are controlled to imitate the behaviors of skyhook damper and conventional shock absorber for better ride comfort and harvesting energy from the vibration of suspended driven motor, respectively. An improved boost-buck converter is employed to regulate the damping force only utilizing the feedback of current of actuators. To further improve the regenerative efficiency, a variable threshold strategy is designed for the hybrid energy storage system to keep its terminal voltage locating in high-efficiency regions, which are identified through analyzing system performance. The results indicate that the desired damping forces of actuators are precisely tracked regardless of the voltage conditions. The vehicle ride comfort and comprehensive performance are improved by 52% and 14%, respectively. In addition, the variable thresholds strategy shows higher regenerative efficiency than the fixed one. After offsetting the energy consumed by active control, the average regenerated power is 4.9, 17.7, 49.2 and 45.0 W on A, B, C and D class roads, respectively. The proposed system is verified as a practical solution to simultaneously improve the dynamic and energy conservation performances of vehicles.

1. Introduction In-wheel motor (IWM) driven system, functioned as a typical electric vehicle (EV) propulsion configuration, has attracted extensive research interests due to its inherent merits [1], such as fast and precise independent torque control of each wheel [2], simple transmission system [3] and the convenience to implement X-by-wire chassis control [4]. However, the IWM driven system inevitably increases the unsprung mass and hence deteriorates the vehicle ride comfort and road-holding

⁎

performances [5]. To reduce those adverse effects, Wang et al. [6] and Jing et al. [7] applied a dynamic damper on the unsprung mass and respectively designed the finite-frequency and fault-tolerant H∞ controllers to actively control the suspension system. Impressive dynamic performances were achieved by these improvements, but the dynamic damper added extra weight to the vehicle, which was contrary to the demand of vehicle lightweight design. As an important progress, Bridgestone [8] developed the advanced-dynamic-damper mechanism (ADM) driven system, in which the driving motor was served as a

Corresponding author. E-mail address: [email protected] (N. Zhang).

https://doi.org/10.1016/j.apenergy.2019.114180 Received 30 June 2019; Received in revised form 1 November 2019; Accepted 13 November 2019 0306-2619/ © 2019 Elsevier Ltd. All rights reserved.

Applied Energy 260 (2020) 114180

G. Long, et al.

dynamic vibration absorber (DVA). It was reported that this system was potential to reduce the fluctuations of tire contact force and the vibrations of vehicle body and driving motor. Then, Shao et al. [9] employed the linear quadratic regulator (LQR) controller for the active suspension to improve the performances of ADM based EV. Their further studies [10,11] proposed different H∞ controllers in the presence of actuator faults, control input constraints and time delay. Also by using the driven motor as the dynamic vibration absorber, Liu et al. [12,13] proposed a new driving system for IWM driven EV (IWM-EV), and investigated the collaborative control of driving motor and vibration of vehicle body. The achieved dynamic performances of these active suspensions for IWM-EV are admirable, but the energy consumed by these active control systems can be considerable. The large amount of consumed energy may severely limit the widespread commercial application of these active suspension systems [14]. On the other hand, there is amount of energy wasted in suspension shock absorber, especially for the ADM based suspension system, in which the vibration energy of both vehicle body and driving motor is dissipated in thermal form. If these wasted energy can be recycled, it will effectively reduce vehicle energy consumption and promote the applications of active suspension [15]. To actively control vibration and simultaneously harvest energy, regenerative active suspension can be considered as a practical solution to implement an active vibration control with minimum energy consumption. To investigate the idea of regenerative active suspension in hybrid electric vehicles (HEVs), Montazeri-Gh et al. [16] developed a simultaneous simulation in which both HEV powertrain and active suspension systems are simulated in a unified framework. Shi et al. [17] synthetically analyzed the influence of control parameters on regenerative active suspension performance, and designed the energy management strategy of HEV powertrain considering the regenerated energy of suspension. Simulation results of the two studies demonstrated that the application of the active suspension system increased the HEV fuel consumption, but the regenerated energy compensated a large part of energy consumption. In order to implement an active suspension without increasing vehicle energy consumption, different zero energy consumption active suspension systems were proposed. Suda et al. [18] developed a selfpower suspension system by storing the energy regenerated by one motor in the condenser and employing the other motor to achieve active control using the stored energy. Then, they further applied this system on the vibration control of truck cabin [19]. Simulation results demonstrated that better vibration suppression performance than the passive or semi-active system was achieved without utilizing external energy. However, this two actuator self-power suspension system is difficult to be applied in a passenger car due to the limited installation space. Nakano et al. [20] proposed a single electric actuator self-power suspension system by designing the feedback gain of the active controller to have good suppression performance under conditions where regenerated energy exceeded consumed energy. Similarly, Singal et al. [21] proposed an adaptive skyhook gain controller to prevent the selfpower system from running out of regeneration energy. Simulation and experimental results showed that these self-power systems had better dynamic performances than the passive and semi-active suspensions without consuming external energy. However, the presented results also illustrated that the requirement of energy balance lowered the effectiveness of the active controls. To our best knowledge, there has not been any publications showing a regenerative active suspension system that can realize a common active control without increasing vehicle energy consumption. To harvest energy and achieve the desired active control performance, the regenerative circuit should cater to the requirements of actuator damping force adjustment [22]. As a common method adjusting the damping of electromagnetic actuators, variable external resistors are wildly used [20,23,24]. However, this method fails to obtain a continuously controllable electromagnetic damper system. As a

solution for this problem, Ning et al [25] employed a MOSFET switch and exerted pulse width modulation (PWM) signal with different duty cycles on the switch. Another problem for the variable resistors method is the dead zone phenomenon, where the energy can’t be recycled when the generator back electromotive force (EMF) is lower than the voltage of energy storage system (ESS). To eliminate the voltage dead zone of regenerative suspension, Kim et al. [23] applied a step-up chopper at the input side of the rectifier. The experiment result showed that the step-up chopper increased both the dynamic and the energy regeneration performances of the system. Intention to extract the maximum useful power from the vibration, Zuo et al. [26] deduced an analytical expression for the optimal duty cycle of the PWM controlled step-up DC-DC converter. Although more energy can be harvested with this method, the electromagnetic damper is only passively changing as the maximum power duty cycle changes. It may not be the best solution for the vehicle suspension system, where damping control related to comfort and safety should be considered in priori. As another damping control method, Shi et al. [27] employed a bidirectional DC-DC converter to continuously regulate the electromagnetic damping force and overcome the voltage ‘dead zone’ phenomenon. Controllable damping characteristic of the generator is achieved with this method, but the caculation of the duty cycle requires current and voltages feedback from the system. Li et al. [28] suggested that the ratio between the input voltage and current of a boost-buck converter was only related to the duty cycle of the switch pulse width modulation signal when working in discontinuous current mode. By using this property, they proposed an open-loop control method for regulating damping force of electromagnetic actuator. The simulation result showed that this method was capable of providing accurate damping, but there was an upper limit for the available damping value. Being different from above schemes, David et al. [29] employed a gyrator to enable an accurate damping force and energy regeneration. With only the feedback of the battery voltage, the gyrator control signal was computed and the desired skyhook damping was achieved. Without the requirement for voltage measurement, Hsieh et al. [30] presented a switch mode rectifier to achieve the desired skyhook damping force by utilizing the feedback of generator current. The switch mode rectifier worked well in a boost condition, but the damping force might be uncontrollable when generator back electromotive force (EMF) was higher than the voltage of the battery. Based on above analysis, the electromagnetic damping control methods have not been fully studied, and there are still some limitations for a circuit that can only use the current feedback to accurately adjust actuator damping under different voltage conditions. The energy storage system (ESS) is another significant component for the regenerative active suspension system. There are a few articles that have mentioned or discussed the ESS of a vehicle regenerative suspension system. Several studies [26,29,31] have employed a 12 V battery pack as the ESS of the regenerative suspension system. In these studies, batteries were just used as an energy storage device and further discussion about the ESS were not given. Considering the highly fluctuating regenerative power can be harmful to the battery, super-capacitor (SC) with the high power density and rapid response speed [32] is drawn into the ESS of energy regeneration suspension. Shi et al. [27] and Zhang et al. [33] employed the super-capacitors as the ESS of the energy regeneration suspension, and experiment with test benches were carried out. In their studies, batteries endurance was extended by transferring the power fluctuations from the batteries to the super-capacitors, but the energy management between the two energy storage units was not discussed. Montazerigh et al. [16] proposed a battery and super-capacitor hybrid ESS to improve the efficiency and prolong life time of the batteries. In this hybrid ESS, the super-capacitor was directly responsible for the actuator power supply and harvested energy storage, while the battery supplied power to charge the super-capacitor. From above studies, it can be found that super-capacitor is nearly a necessity for the regenerative suspension system, but the benefits of the 2

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application of super-capacitor should not only be giving protection to batteries. Wang et al. [34] investigated the effects of the SC initial voltage on the regenerative suspension system, and demonstrated that it had great impact on the regenerative power. These effects are greatly related to the employed regenerative circuit. Thus, for a specific regenerative suspension system, the energy management strategy of ESS should also consider the regenerative efficiency of the whole system. In order to improve the dynamic and energy conservation performances of IWM-EV, a dual electromagnetic actuators regenerative active suspension system based on ADM is proposed in this paper. The primary actuator is controlled with skyhook strategy to improve vehicle ride comfort, and the second actuator is controlled to be a conventional shock absorber in order to harvest the vibration energy of the suspended driving motor. The contributions of this paper are summarized as follows: (1) proposal of a mechatronic scheme of the dual actuator regenerative active suspension system, which realizes a common active control of vehicle vibration and simultaneously improves the vehicle energy conservation performance; (2) a new boost-buck converter to regulate the generator’s damping force regardless the voltage conditions is presented; (3) the influences of the terminal voltage of ESS is quantitatively analyzed, and a variable threshold energy management strategy for the hybrid ESS is developed to ensure high regenerative efficiency. Simulation under different road excitations is operated to verify the dynamic and energy conservation performances of the proposed system. The rest of this paper is organized as follows. The mechanical design and modelling of this regenerative active suspension are described in Section 2. Then, the electrical circuit with the improved boost-buck converter and the variable threshold ESS is presented in Section 3. In Section 4, the comprehensive controller is presented, and then the results are given and discussed in Section 5 to verify the dynamic and energy conservation performances of the proposed system. The conclusions are provided in the last section.

Fig. 2. Structure of the proposed ADM based suspension system.

connection between the driving motor rotor and wheel. In addition, a hydraulic shock absorber is also employed to guarantee the safety through switching between normal and safe assurances states. Based on the quarter-car model in Fig. 1(c) and the Newton’s second law, the motion equations can be expressed as

ms z¨s = ks (z s z u ) cs (z s z u ) + F1 md z¨d = kd (z d z u ) + F2 z u ) + cs (z s z u ) + kd (z d z u ) kt (z u

mu z¨u = ks (z s

zg)

F1

F2 (1)

where zu, zd, zs and zg are the vertical displacements of the unsprung mass, driving motor, sprung mass and input road excitation, respectively. ms, md and mu respectively are the weights of the vehicle body, driving motor and un-sprung mass when considering the equipment of actuators M1 and M2. ks and kd denote the stiffness of primary and secondary suspensions, respectively. F1 and F2 are the output force of actuators M1 and M2, respectively, and cs is the damping coefficient of the hydraulic shock absorber. By defining the system state vector as

2. Mathematical modelling

x (t )= [ z s (t )

2.1. Quarter-vehicle modelling

z u (t ) z s (t ) z d (t )

z u (t ) z d (t ) z u (t )

z g (t ) z u (t )]

the dynamic equation can be expressed by a state space equation as

The quarter-vehicle model of the IWM configuration with the motor installed in the wheel hub is shown in Fig. 1(a). Fig. 1(b) shows the model of the common ADM based active suspension [10]. Being different from previous studies, the model and structure of the proposed regenerative active suspension system are shown in Fig. 1(c) and Fig. 2, respectively. In this system, a tubular permanent-magnet actuator M1 and a voice coil motor M2 are respectively controlled to imitate the behavior of skyhook damper and passive damper. They are designed for vehicle body vibration control and energy harvesting. The hollow form rotor direct driving motor is attached to the knuckle through the actuator M2 and springs. A translation pair is used to realize the flexible

(2)

x (t )= Ax (t )+ B1U (t )+ B2 z g (t ) 0 ks ms

where A=

0 0

1 cs ms

0 0

0 0

0 0

0 0

0

1 0

0 0

0

0 0

0

1

kd md

0

0

0

ks mu

cs mu

kd mu

kt mu

1

0

cs ms

1

cs mu F2 ]T .

1 ms

, B1 =

0 0 0 1 mu

0 0 0 1 md

0 1 mu

B2 = [0 0 0 0 1 0], U (t )= [ F1 The dynamic performances of this system should include: (1) ride

Fig. 1. Different suspension models of in-wheel motor driven electric vehicle: (a) Conventional IWM-EV; (b) Common ADM based EV; (c) Proposed ADM based EV. 3

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Fig. 3. Comparison of frequency responses between different IWM systems: (a) sprung mass vertical acceleration; (b) tire dynamic deflection; (c) driving motor vibration acceleration; (d) average power dissipated by actuator M2 of the proposed system.

comfort, i.e. the vehicle body vertical acceleration (BVA) as = z¨s ; (2) motor dynamic force, i.e. driving motor vertical acceleration (MVA) am = z¨d ; (3) motor work condition, i.e. secondary suspension deflection (SSD) z du = z d z u ; (4) suspension rattle space, i.e. primary suspension deflection (PSD) z su = z s z u ; and (5) road holding ability, i.e. tire dynamic deflection (TDD) z ug = z u z g . For the hydraulic shock absorber of the proposed suspension system, the safe assurance damping is equivalent to the damping csp = 1496 N m/s of a passive suspension. To design the normal state damping cs , different values are adopted to investigate its influences on the dynamic performances and regenerative potential. It is noted that with the same control strategy, the dynamic response of the proposed system is equivalent to the common ADM based active suspension when cs = csp . The dynamic frequency responses of the conventional IWM-EV, uncontrolled ADM based EV and the proposed system with different cs are displayed in Fig. 3(a–c). Potential energies that can be harvested from the driven motor’s vibration are shown in Fig. 3(d). It can be seen that all the ADM based systems obtains better dynamic performance than the conventional IWM-EV. And in order to achieve better dynamic performance and harvest more energy from the driven motor’s vibration, a smaller cs is preferred. As a result, the parameters of the quartervehicle model are presented in Table 1 [10].

i (a, b, c ) = Iq cos ( z

p

)+ j

2

3+

(3)

(j = 0, 1 and 2)

where and p are the commutation angle and the pole pitch of the quasi Halbach PM array, respectively, and Iq is the amplitude of the three-phase commuted quadrature current. The axial output force and the back electromotive force can be expressed as F1 = Ki Iq and E1 = K e v1, respectively. Factors Ki and K e denote the motor and electromotive force constants, respectively, and v1 is the relative speed between the stator and the slider of actuator M1. 2.2.2. Voice coil motor The back electromotive force and the output force of actuator M2 are respectively expressed as E2 = v2 and F2 = i2 , where and i2 denote the motor constant and the motor current, respectively, and v2 is the relative speed between the stator and the slider of actuator M2. When actuator M2 is powered by the DC bus, the equation of the circuit is described as

La

di2 = udc dt

e2

i2 Ra

(4)

where udc is the voltage of the DC bus, La and Ra denote the inductance and resistance of the armature, respectively. Connecting actuator M2 to a load circuit with equivalent resistance as Rl , the equation of the circuit is

2.2. Actuators modelling

La

2.2.1. Tubular permanent magnet actuator Based on the tubular permanent magnetic actuator presented by Gysen et al. [35], the three phase current i (a, b, c ) of actuator M1 is given by

di2 = e2 dt

i2(Ra

Rl )

(5)

Based on Eq. (5), the steady regenerative current can be expressed as i2 = v2 (Ra + Rl ), and hence the output damping force F2 = i2 can be expressed as

Table 1 Parameters of the quarter-vehicle model of the proposed system. ms (kg)

mu (kg)

md (kg)

kt (N/m)

ks (N/m)

kd (N/m)

cs (N*m/s)

csp (N*m/s)

csky (N*m/s)

cd (N*m/s)

352

45

40

360,000

32,000

41,000

500

1496

5000

1000

4

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In this improved converter, a capacitor C1 = 1 µF is introduced, and the MOS pipeline VT1 is used at the output end of the converter while the inductance LDC = 125 µH is used at the input end. In this section, the mathematical model of the improved boost-buck converter will be derived. During the modelling process, the large capacitance value of the SC makes its terminal voltage can be treated as a constant in each switching cycle. There are boost and buck modes for the converter and the switching period [0, T ] is separated as [0, DT ] and [DT , T ], where T is the period of the switching signal, and D (0 D 1) is the duty ratio of the switching period.

Table 2 Parameters of the two electromagnetic actuators. Actuator M1

Actuator M2

Rph ( )

Lph (mH)

K i (N A)

K e (Vs m)

φ

Ra ( )

La (mH)

1.8

60

115

76.6

98

1.48

17

F2 =

2

Ra + Rl

v2

(6)

From Eq. (6), an equivalent damping coefficient 2 (Ra + Rl ) can be achieved by adjusting load resistance Rl . Typical parameters of the two actuators are listed in Table 2.

3.1.1. Boost mode In boost mode, the MOS pipeline VT1 is kept on, and the MOS pipeline VT2 works as the chopper. When t [0, DT ], the MOS pipeline VT2 is on, and the current flow into SC is zero. The relationship between the regenerative current i and the back electromotive force E is given as

3. Electrical circuit design In this paper, there is no need to employ another converter to transfer the harvested energy to actuator M1, because there are both high and low voltage battery packs, which respectively can power actuator M1 and stored the harvested energy. The topology of the electrical circuit which hybridizes with EV’s existing power system is designed for the proposed system, as shown in Fig. 4. The actuator M1 alternately works in energy-consuming and harvesting modes, because it is controlled by the skyhook control strategy [18]. In order to switch the work mode of actuator M1, switches S1 and S2 are alternately engaged. The actuator M1 works as a generator when switch S1 is engaged. And by that time, the three-phase inverter works as a rectifier and regenerative current is regulated by the improved DC-DC converter. In contrast, actuator M1 works as a motor and the current is controlled by the three-phase inverter when the switch S2 is engaged. For actuator M2, it only works as a generator. Thus, the one-phase inverter works as a rectifier and the regenerative current is regulated by the improved boost-buck converter. From the perspective of energy, all the harvested power is stored in the proposed hybrid ESS which including the onboard 12 V low voltage battery pack, while the energy consumed by actuator M1 is directly supplied by the 340 V battery pack. It is noted that the suspension system can be regarded as a fully self-power system when the total harvested energy is more than the total consumed energy.

(L + LDC )

di =E dt

(R + Rs )i

2VF

(7)

where LDC is the dynamic inductance, Rs is the internal resistance of the MOS pipeline, and VF denotes the forward voltage of diode with 0.2 V [36]. From Eq. (7), the regenerative current i can be obtained as

i (t )=

E 2VF + i0 R + Rs

E 2VF e R + Rs

R + Rs t L + LDC

(8)

where i 0 is the regenerative current when t = 0 . When t [DT , T ], the MOS pipeline VT2 is off, and the current of SC is iSC = i . The relationship of regenerative current i and back electromotive force E can be expressed as

(L + LDC )

di =E dt

(R + Rs )i

USC

4VF

(9)

Therefore, the regenerative current i can be obtained as

i (t )=

E

USC 4VF + iDT R + Rs

E

USC 4VF e R + Rs

R+Rs (t DT ) L + LDC

(10)

where USC is the terminal voltage of the SC, and iDT is the current when t = DT .

3.1. Improved boost-buck converter

3.1.2. Buck mode In buck mode, MOS pipeline VT2 is kept open, while MOS pipeline VT1 serves as a chopper. When t [0, T ] and MOS pipeline VT1 is on, the equations iSC = i + iC1 and IC1 = I (1 - D) D are met, where iC1 is the current of C1, and IC1 denotes the mean value of iC1. Meanwhile, the transient response of the regenerative current is the same as Eq. (9).

As described above, boost-buck converters are employed to regulate the generator damping force. The boost-buck converter, which requires multiple feedback signal to regulate the current, is shown in Fig. 5(a). The improved boost-buck converter is proposed, as shown in Fig. 5(b).

Fig. 4. The topology of the electrical circuit of the proposed system. 5

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Fig. 5. Topologies of boost-buck converters: (a) the conventional converter [27]; (b) the improved converter.

Therefore, the regenerative current i can be obtained as

i (t )=

E

USC 4VF + i0 R + Rs

E

USC 4VF e R + Rs

R + Rs t L + LDC

continuously adjust the generator damping force with only the measuring of the input current and regardless of the voltage conditions between the generator and the battery. It is very practical for voltage varying systems that require precise current control.

(11)

When t [DT , T ] and MOS pipeline VT1 is off, the current of SC is zero and the following equations are met

di (L + LDC ) = e dt

i = iC1 = C1

Ri

UC1

3VF

3.2. Regenerative energy storage system 3.2.1. Influences of the terminal voltage With the obtained SC voltage USC and current ISC , the harvested power can be directly calculated as Ph = ISC USC [38]. Hence the amount of energy harvested by SC directly reveals the regenerative efficiency of the system. Based on the model of the improved boost-buck converter, the root mean square (RMS) of SC current iSC during each switching cycle can be expressed as

(12)

dUC1 dt

(13)

It can be calculated from Eqs. (12) and (13) that

UC1 = a1 e s1 t + a2 e s2 t + E

(14)

3VF

Substituting Eq. (14) into Eq. (13), the current i is derived as (15)

i (t )= C1(as1 e s1 t + a2 s2 e s2 t )

ISCrms =

where s1, s2 , a1 and a2 are defined as

s1 = = a1 =

R C1 + 2 R2C1

4(L + LDC )

2(L + LDC ) C1 R C1

2 R2C1

;

E + 3VF ) s2 s2 s1

T 2 i DT

1 T

DT 0

4(L + LDC ) iDT

a2 =

DI

Boost Mode

(i + ic1) 2 = I

D Buck Mode

1

(16)

;

iDT

=

1

(18)

Therefore, the regenerative efficiency for the two actuators in one switch cycle can be derived as

s2

2(L + LDC ) C1 (UDT

1 T

(UDT s2

E + 3VF ) s1 s1

j

=

ISCrmsj USC |Ziu Fj |

=

D I j USC |z iu Fj |

I j USC |ziu Fj D |

=

=

1 D USC |ziu j |

USC |ziu j D |

Boost Mode

j = 1, j = 2,

Buck Mode

z iu = z su, z iu

or = z du ,

1

= Ki 2

=

(17)

(19)

where UDT = iDT Rs + VF + USC is the voltage of the capacitor C1 at t = DT. As presented in Eqs. (8), (10), (11) and (15), continuously adjustable current for the generator can be ensured in all work conditions of the improved boost-buck converter.

where 1 and 2 respectively denote the regenerative efficiencies of actuators M1 and M2. ISCrms1 and ISCrms2 respectively are the RMS values of currents flow into super-capacitor from the two actuators. I1 and I2 denote the average regenerative currents of the two actuators. On the other hand, the average total regenerative efficiency is expressed as

3.1.3. Performance of the converter The generator damping force is proportional to the regenerative current. Thus, the regenerative current regulating performance of the improved converter is compared with the switch mode rectifier [30,37]. It is noted that both the rectifier and the converter work only based on the current feedback. Diagrams of the two methods are given in Fig. 6, where the rectifier and the converter are controlled with a current hysteresis controller and a switch mode PI controller, respectively. The error band for the hysteresis controller is 0.05 A. As a result, voltages and current tracking performances are shown in Fig. 7, where the source voltage US and the desired input current are given in advance. In Fig. 7(a), battery voltages in both diagrams have the fluctuation due to the charging and discharging processes. Fig. 7(b) shows that both the two methods have good performance in regulating the input current in 0–0.05 s when the source voltage is smaller than the battery voltage. In 0.05–0.15 s, the source voltage fluctuates in larger amplitude than the battery voltage. Simultaneously, the regenerative current controlled by the improved converter is well regulated, while the current regulated by the switch mode rectifier fails to track the desired input current when the source voltage is higher than the battery voltage. Therefore, the improved boost-buck converter can

E = h = EM

Eh1 t 0

USC iSC1 dt +

Eh2 t 0

USC iSC 2 dt

EM1 t 0

z su F1 dt +

EM 2 t 0

z du F2 dt

(20) where t is the time of energy regeneration, EM1 and EM2 are the mechanical energies absorbed by the two actuators. Eh1 and Eh2 are the energies harvested from the two actuators. Eh and EM respectively denote the total energy harvested by super-capacitor and the total mechanical energy absorbed by the two actuators. iSC1 and iSC2 are the currents flow into super-capacitor from the two actuators. Eqs. (19) and (20) illustrate that SC terminal voltage USC has a great influence on regenerative efficiency. Thus, simulations with different SC terminal voltage USC are designed to investigate its influence on the total regenerative efficiency. 3.2.2. Simulation analysis with different voltages In these simulations, the capacitance of the SC is set large enough, so that the SC terminal voltage USC can be treated as a constant. The A, B, C and D class roads classified by ISO 8608 are used as the external excitation, which can be obtained with the equation [39] 6

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Fig. 6. Diagram used to regulate the regenerative current: (a) the switch mode rectifier; (b) the improved converter.

z g (t )+ 2 f0 z g (t )= 2 n Gq(n 0)v (t ) w (t )

Table 3 Road roughness coefficient classified by ISO 8608.

(21)

where z g (t ) is the transverse motions function of the road surface, Gq(n 0) is the road roughness coefficient given in Table 3. The n 0 = 0.1 m 1 and f0 = 0.0628 Hz denote the reference spatial frequency and minimal boundary frequency, respectively. v (t )= 20 m/s is the vehicle driving speed, and w(t ) is a zero-mean white Gaussian noise. As the representation of the system dynamic performances, the RMS value of the vehicle body vertical acceleration is demonstrated in Fig. 8(a). The obtained results indicate that the SC terminal voltage USC has little influence on vehicle dynamic performances due to the improved converter can work regardless the voltage condition. For the total regenerative efficiency η, there is an optimal terminal voltage corresponding to peak efficiency under specific road condition, shown in Fig. 8(b). When SC terminal voltage is smaller than the optimal value, the total regenerative efficiency increases with the higher terminal voltage. Otherwise, it is decreased with the increase of the SC terminal voltage. It can be seen that those optimal voltages are respectively within 12–14, 28–30, 52–60 and 84–92 V on A, B, C and D class roads. The peak efficiencies are around 0.49, 0.42, 0.31 and 0.20, respectively. Therefore, the terminal voltage of ESS should be

Road Class

Gq (n0 )10

6

A

B

C

D

E

F

G

H

16

64

256

1024

4096

16,384

65,536

262,144

maintained in these high-efficiency regions in order to achieve superior regenerative efficiency on different roads. Beside harvesting energy, the proposed system also consumes energy. The average harvested and consumed powers are shown in Fig. 9. Before the peak power is reached, the total recycled power Pt is increased with the growth of both the harvested powers Ph1 and Ph2 from the two actuators. After that, the total harvested power is decreased with the decline of harvested power from actuator M2, while the average power harvested from actuator M1 is slightly increased. It indicates that the total harvested power is mainly determined by the power harvested from actuator M2. From Figs. 8(b) and 9, the values corresponding to the maximum total harvested power and peak regenerative efficiency, average

Fig. 7. Comparison between the switch mode rectifier and the improved DC-DC converter: (a) Voltage signals; (b) current tracking performance. 7

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Fig. 8. Effect of different super-capacitor pack terminal voltage: (a) the sprung mass vertical acceleration; (b) the total regenerative efficiency.

consumed power, fully self-power voltage region and minimum efficiency to realize fully self-power are listed in Table 4. The average consumed powers Pc for the four road conditions nearly equal to 3.3, 10.2, 40.5 and 163.4 W, respectively, because the consumed power is only determined by the requirements from dynamic responses suppression. The average extra power is defined as Pe = Pt Pc , where a positive value means fully self-power is achieved, while a negative value means the system is consuming the energy from an external energy source. It is noted that positive extra power can be achieved when ESS terminal voltages are within the self-power voltage region in Table 4. In addition, the obtained maximum extra powers are around

5.9, 20.1, 53.0 and 80.9 W under the condition of the maximum average regenerative powers and the constant average consumed powers. Therefore, the proposed suspension system is potential to realize the fully self-power and simultaneously achieve the extra regenerated energy. 3.2.3. Energy storage system design To achieve the above described high regenerative efficiency, a new ESS with a wide range of SC terminal voltage USC is proposed, as shown in Fig. 10(a). The new ESS includes the vehicle onboard 12 V low voltage battery pack and series-parallel switched SC packs. In the SC packs,

Fig. 9. Average regenerative powers when with different super-capacitor terminal voltage: (a) A class road; (b) B class road; (c) C class road; (d) D class road. 8

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Table 4 Key information about the analysis of influence of ESS terminal voltage. Road class

Maximum regenerative power Pmax (W)

Maximum regenerative efficiency ηmax

Average consume power Pc (W)

Self-power region Usc (V)

Self-power regenerative efficiency ηself-power

A B C D

9.2 31.5 93.5 241.5

0.495 0.422 0.313 0.204

3.3 10.2 40.5 163.4

3.0–38.0 6.0–60.0 13.5–120.0 40.0–120.0

> 0.177 > 0.137 > 0.136 > 0.138

Fig. 10. The variable thresholds ESS: (a) configuration of the hybrid ESS; (b) high-efficiency energy management state machine.

each sub-pack is composed of twelve series-connected SCs units (150 F, 2.7 V). Furthermore, three switches S6, S7 and S8 are employed to change the connection mode of the sub-packs SC1 and SC2. When the switches S6 and S8 are engaged (disengaged) and S7 is disengaged (engaged), sub-packs SC1 and SC2 are connected in parallel (series). With the knowledge of capacitors series-parallel calculation method, the terminal voltage of the SC packs is limited to USC 32.4 V (USC 64.8 V) when the two sub-packs are connected in parallel (series). Furthermore, in order to regulate the terminal voltage of SC packs precisely, switch S9 is introduced to switch the charging and discharging operation of the SC packs. In this ESS, the SC packs work in charging (discharging) mode when switch S9 is disengaged (engaged). The logic for switch S9 is shown in Fig. 10(b), where the switch S9 is engaged once the terminal voltage of SC packs is higher than the upper threshold, and it keeps engagement until the terminal voltage reaches the lower threshold. In order to achieve the above working mechanism, the SC packs connection modes and charging/discharging thresholds are required and should be pre-defined. Based on the analysis in Section 3.2.2, predefined connection modes and voltage thresholds on different roads are developed for the proposed ESS to satisfy the requirement of high regenerative efficiency, as listed in Table 5. In order to realize the flexible adjustment of the SC packs terminal voltage, the two sub-packs are connected in parallel on A and B class road and in series on C and D class roads. To achieve more extra regenerative energy, terminal voltage thresholds for A, B, C and D class roads are respectively chosen as 14–16, 28–30, 52–60 and 56–64 V. It is noted that when the terminal voltage on D class road is within 84–92 V, regenerative efficiency more than 0.20 and extra power more than 80.9 W are available. However, the threshold on D class road is chosen at 56–64 V, because it is limited by the maximum terminal voltage of the series-parallel switched SC packs. In this voltage region, the corresponding regenerative efficiency is higher than 0.172, the extra power is higher than 41.4 W.

Table 5 Connection modes and thresholds of the high-efficiency energy management strategy. Road class

Connection mode

Low threshold (V)

Upper threshold (V)

A B C D

Parallel Parallel Series Series

14 28 52 56

16 30 60 64

4. Controller design In order to harvest as much energy as possible, a new energy consumption/harvest scheme is proposed for this suspension system, as shown in Fig. 11. It is achieved through the cooperative control of the two actuators and the new ESS. A skyhook control strategy is employed as the external controller of actuator M1 to isolate the vibration of the vehicle body. The external controller of actuator M2 is designed to imitate the behavior of a passive damper and harvest energy all the time. With the desired forces from the external controllers, the inner controllers are used to regulate currents of the two actuators, so that the desired output forces are reached. The inner controller of actuator M1 including both controllers of the three-phase inverter and the boostbuck converter, while the inner controller of actuator M2 only consists of the boost-buck controller. As for the controller of the proposed ESS, the pre-defined control strategy in Section 3.2.3 can be realized by using road information from other advanced technologies such as intelligent transportation systems and road classification technology. In the perspective of energy changes, the actuator M2 only absorbs mechanical power from the quarter-vehicle model. However, for the actuator M1, it not only absorbs the mechanical power, but also exports mechanical power to the quarter-vehicle model by consuming the electrical power from the high voltage battery pack. Thus, the energies flow into the quarter-vehicle model are from both the road excitation 9

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Fig. 11. Integration diagram of the proposed system.

and the actuator M1. These input mechanical energies are then divided into three parts and respectively absorbed by the two actuators and the hydraulic shock absorber. The energy absorbed by the two actuator is partially harvested and finally stored in the onboard low voltage battery pack, while the energy absorbed by hydraulic shock absorber is dissipated in the thermal form.

4.2. Inner loop controller To achieve the output forces required from external controllers, the corresponding inner control diagrams are shown in Fig. 12. It is noted that the Fd denotes the desired force which is defined in Eqs. (25) and (26). Fig. 12(a) shows the inner controller of energy harvesting mode. In this mode, the converter work in boost mode when the real current ireal is smaller than the reference current iref . Meanwhile, the switch VT1 is kept on, and the duty cycle of switch VT2 is controlled with a PI controller. On the contrary, the converter work in buck mode when the real current ireal is larger than the reference current iref . Meanwhile, the switch VT2 is kept off, and the duty cycle of VT1 is controlled by a PI controller. It is noted that the switch frequency of the boost-buck converters is 20 kHz. The corresponding current controller for actuator M1 in energy consuming mode is shown in Fig. 12(b). In this mode, the desired current iq and id are transformed to the desired three-phase currents ia , ib and ic through Park Transform and Clark Transform. The corresponding desired three-phase currents are achieved with three-phase current hysteresis control.

4.1. External loop controller Skyhook control [40] is widely used in vibration control. Its principle is assuming that an imaginary damper is installed on the sprung mass to suppress vibrations. Considering this structure can not be achieved practically, actuator M1 in this study is controlled to imitate the behavior of that imaginary damper. The output damping force of the actuator M1 applied to the vehicle body can be expressed as

Fsky =

(22)

csky z s

Assuming the damping value of actuator M1 is cM1, the output force is written as

F1 =

(23)

cM1 z su

Eqs. (22) and (23) indicate that actuator M1 works in motor mode and releases energy to the vibration system when z su z s 0 . When z su z s 0 , actuator M1 works in the generator mode [18]. It is noted that the maximum electromagnetic damping force of the actuator M1 is limited by

|Fe max| = Ki

e 2VF R + Rs

5. Results and discussion Based on the proposed regenerative active suspension system, several simulations on random road excitations are carried out to show the force control performances of the proposed circuit, dynamic performances of the suspension system and the energy conservation performances of the proposed ESS. In order to evaluate the dynamic performances of the suspension system, a comprehensive dynamic performance index is defined as

(24)

Thus, when z su z s 0 and |Fe max | < |Fsky|, actuator M1 still needs be powered by the high voltage DC bus to achieve the desired active control force. The actuator can be employed to harvest energy when z su z s 0 and |Fe max | > |Fsky|. The corresponding output force and work states of actuator M1 can be expressed as Fd1 =

Csky z s , Csky z s ,

J = w1

As for the actuator M2, it is only designed as a generator and the output damping force is expressed as

cd z du

(27)

where wi (i = 1, 2 , 3, 4 and 5) is the weight factor for each of the five dynamic performances, which are set as w1 = 0.4 , w2 = 0.15, w3 = 0.15, w4 = 0.15 and w5 = 0.15, respectively. z¨s, rms , z¨d, rms , z du, rms , z su, rms and z ug , rms respectively denote the RMS value of the five dynamic P P performances of the active suspension. z¨sP, rms , z¨dP, rms , z du , rms , z su, rms and P z ug are the expressions of the five dynamic performances of the , rms passive suspension, respectively.

z su z s 0 or z su z s 0 |Fe max | < |Fsky| Consuming Energy z su z s 0 |Fe max | > |Fsky| Harvesting Energy

(25)

Fd2 =

z ug , rms z¨s, rms z¨d, rms z du, rms z su, rms + w2 P + w3 P + w4 P + w5 P z du, rms z su, rms z ug , rms z¨sP, rms z¨d, rms

(26) 10

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Fig. 12. Diagram of actuators inner loop current controllers: (a) the improved converter; (b) the three phases inverter.

To investigate the energy-regenerative efficiency of the suspension system in real time, efficiency in specific period is defined as

(t ) =

Eh (t ) EM (t )

Eh (t EM (t

t) (t t)

t)

negative power is 120.1 J and −222.9 J, respectively. It indicates that the consumed energy of actuator M1 is far greater than the harvested energy, due to the energy losses. The proposed control strategy for actuator M2 is designed only to harvest vibration energy, so the power for this actuator is always positive, as shown in Fig. 15(b). The maximum power is over 2.7 kW, and the corresponding energy obtained from the 5-second time integration of the power is 349.9 J. Based on Eq. (26), all the energy is determined by the relative speed between the tire and the driven motor. Fig. 15 indicates that the proposed control strategy effectively harvests power from the primary and secondary suspension. The transient dynamic performances of the passive, the proposed and the common ADM based active suspensions on C class road are further compared. Fig. 16(a) indicates that the proposed system shows the best ride comfort, followed with the common active ADM based suspension system which has a hydraulic damper same as the passive system. Fig. 16(b) demonstrates both the controlled systems almost show the same primary suspension deflection, which is slightly better than the passive system. For the drive motor acceleration and tire deflection in Fig. 16(c) and (d), the proposed system is worse than the other two systems, and the uncontrolled system shows the best performance. From Fig. 16, the proposed method achieved considerable improvement in ride comfort and a slight improvement in primary suspension deflection with acceptable deterioration in the driving motor vertical acceleration and tire deflection. In order to further investigate the dynamic performances of the proposed suspension system on different roads, the comprehensive evaluation index defined by Eq. (27) and the RMS values of each dynamic performance is listed in Table 6. It is clearly shown that the proposed system decreases the vibration of the vehicle body by the ratio of about 52% which is more than the 24–26% improved by the common active ADM based suspension. The RMS value of the primary suspension deformation reduced by the proposed system is nearly 5≃7%. It is inferior to the 19–20% reduced by the common active system. The RMS values of driving motor acceleration, secondary suspension deformation, and tire dynamic deformations are increased by 24%, 25%, and 10%, respectively. As for the common active suspension system, increments in the three performances are only 4–5%, 7–10% and 0 ∼ 4%, respectively. For the comprehensive dynamic performance index defined by Eq. (27), the proposed system is close to 0.86. It is slightly better than the common active system which has a comprehensive index near 0.88–0.90. Based on above analysis, the proposed system can achieve much better ride comfort than the passive system and the common ADM based active suspension system. For the cost, other performances have been deteriorated. It is noted that other control methods can be further applied to improve dynamic performance in different aspects, such as the road-holding performance [41].

(28)

where the designated period t is 1 s. Eh(t ) and EM (t ) represent the total harvested energy and total absorbed mechanical energy in t s, respectively. 5.1. Dynamic performances In order to investigate the performances of the proposed circuit and the dynamic performances benefit from the proposed suspension system, simulations on A, B, C and D class roads with the vehicle speed at 20 m/s are designed. In these simulations, the two SC sub-packs are each at 28 V initial voltages and connected in series. The transient responses on C class road are analyzed in details. Based on Eq. (26), the signal used to control the work mode of actuator M1 is shown in Fig. 13(a). The high-level signal ‘1′ and the low-level signal ‘0′ represent the energy consuming and energy harvesting modes, respectively. The regenerative current of the actuators M1, which is regulated by the improved boost-buck converter, is shown in Fig. 13(b). It is clearly shown that the current regulated by the proposed boostbuck converter precisely tracks the desired current. However, it is also found that a slight and ignorable difference occurs when actuator M1 is switched from energy consuming mode to energy harvesting mode. Moreover, the desired current in each energy harvesting mode is variable due to the random road excitation. The output force of actuator M1 is shown in Fig. 13(c), where force fluctuations within around 20 N are generated by the current difference resulting from mode switching. In addition, the force ripple in energy consuming mode is more obvious than in energy harvesting mode, because of the different current regulating methods. The output force tracking performance of actuator M2 is shown in Fig. 14. It can be seen that the force tracking performance of actuator M2 is better than M1, but there is still trifling force tracking difference. It is because that the damping force of actuator M2 is also limited by the maximum damping force Fd2 max x , which is closely related to the back electromotive force of actuator M2. When the back electromotive force is small enough and the condition |Fd2 max | < |Fd2| is met, these trifling force tracking differences occur. Fig. 15 shows the regenerative powers of actuator M1 and actuator M2 under C class road. In the figure, the positive and negative power values correspond to the energy harvesting and consuming modes, respectively. As it can be seen, the harvested power and the consumed power fluctuate largely, because they are determined by the force requirement and vibration severity. From Fig. 15(a), the positive and negative power for actuator M1 are alternatively generated, and the absolute value of the greatest negative power is almost three times of the greatest positive power. It is because that the work mode of actuator M1 is determined by Eq. (25). Corresponding to Fig. 15(a), the energy obtained from the 5-second time integration of the positive and

5.2. Energy conservation performance In this research, the power limitation for the low voltage battery pack is not considered, and the discharging of the SC packs is assumed 11

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Fig. 13. Work details of actuator M1 of the proposed system: (a) work mode signal; (b) regenerative current tracking performance; (c) force tracking performance.

Fig. 14. Output force tracking performance of actuator M2.

to be an ideal discharging process. In order to investigate the benefits of the proposed variable threshold hybrid ESS, a fixed threshold hybrid ESS with sub-packs SC1 and SC2 connected in series is used for comparison. The charging/discharging thresholds for the fixed threshold hybrid ESS are defined as 14–64 V, and both initial voltages of the two SC sub-packs SC1 and SC2 are 31 V. In addition, energy conservation performance of the common ADM based regenerative active suspension is investigated to show the benefits of the proposed suspension system. It is noted that the actuator and control strategy of the common active suspension is the same as the proposed system. The ESS for the common ADM based system is a 12 V batteries pack, which is wildly used in previous studies [26,29,31]. With vehicle speed at 20 m/s, the road excitation with different road roughness coefficients is shown in Fig. 17. Working mechanisms of variable and fixed threshold ESSs are shown in Fig. 18. In order to reduce the influence of the terminal voltage fluctuation, its mean value in the latest 0.2 s is treated as the steady terminal voltage of the ESS. In Fig. 18(a), symbols ‘0’ and ‘1’ denote the parallel and series connection modes of the proposed ESS, respectively. In Fig. 18(b) and (c), symbols ‘0′ and ‘1′ denote the charging and

discharging working modes of the two ESSs, respectively. The initial terminal voltage for the two ESSs is 62 V. From Fig. 18, when the vehicle is driven on the D class road for 3.6 s, the terminal voltage reaches the pre-defined discharging threshold 64 V, the operation conditions of the two ESSs are switched into discharge mode from initial charge mode. Therefore, the terminal voltage of SC packs starts to decline. When the terminal voltage reaches the low threshold 56 V, the variable thresholds ESS is switched back to charge mode. For the fixed threshold ESS, it will not switch back to charging mode until the terminal voltage reaches the charging threshold 14 V. Furthermore, after each discharging operation, the charge mode for the conventional ESS is always kept until the terminal voltage reaches the discharging threshold 64 V. Under this mechanism, the terminal voltage of the fixed threshold ESS is out of the fully selfpower region in 4–10 s. At 10 s, when the road excitation changes from D to B class road, the pre-defined lower and higher terminal voltage thresholds for high regenerative efficiency are changed to 26 and 28 V, respectively. The connection mode for the two SC sub-packs is switched into parallel. 12

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Fig. 15. Electrical powers of the proposed system on C class road; (a) powers of actuator M1; (b) power of actuator M2.

Then, the variable thresholds ESS can easily reach the high-efficiency region of the B class road after working in discharge mode for a short time. For the conventional ESS, though the terminal voltage (maximum 18 V) is within the fully self-power region in 10–20 s, it is far below the high-efficiency region pre-defined for the proposed ESS. When the road excitation changes to C class road at 20 s, the pre-defined lower and

higher thresholds corresponding to high-efficiency are increased to 48 and 56 V, respectively. The connection mode for the two SC packs is switched into series. Thus, the terminal voltage of the proposed ESS is increased by two times and successfully reaches the high-efficiency region of the C class road immediately. For the terminal voltage of the fixed threshold ESS, it is not over 25 V and far below the high-efficiency

Fig. 16. Comparison of different systems dynamic performances on C class road: (a) sprung mass vertical acceleration; (b) driving motor vertical acceleration; (c) primary suspension deflection; (d) tire dynamic deflection.

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regenerative efficiencies defined by Eq. (20) is further studied. According to the different periods of road excitation, the obtained results are listed in Table 7. Under these road excitations, the variable thresholds ESS recycled much higher power from M2 than the fixed threshold ESS, while the actuator M1 of the variable thresholds ESS harvests less average power during 60–100 s. It is induced by the difference between high regenerative efficiency voltage region of the two actuators, as presented in Section 3.2.1. Due to the above differences of harvested power, the average total regenerative efficiency of the variable thresholds ESS is higher than the fixed threshold ESS under these road excitations. Moreover, even with smaller regenerative efficiency on the worse road condition, the harvested power by both ESSs is still higher than that on the smoother road condition. In addition, the energy harvested from actuator M2 in the variable thresholds ESS is much more than the energy harvested from actuator M1. And when driven on D class road, the variable thresholds ESS harvests 146.7 W average power from actuator M2 in 0–10 s, which is obviously higher than the 128.4 W in 40–60 s. This is because the terminal voltage of the variable thresholds ESS is slightly below the pre-defined high-efficiency region in 40–60 s, as shown in Fig. 18. Analysis of energy flow mechanism inside the regenerative active suspension system is significant for the research of vehicle dynamic and energy conservation performances [42]. Therefore, the obtained kinetic and electrical energy flows are further compared under the same excitations, as shown in Fig. 20. In Fig. 20(a), the proposed ADM suspension systems with different ESSs have the same kinetic energy flows due to the same road excitation. The total kinetic energy of road excitations is 50.30 kJ, it is obtained by integrating the responses of vertical tire loads by its speed. The electrical energy consumed by actuator M1 is 5.98 kJ. With efficiency around 89% in energy consuming mode, the actuator M1 delivers 5.31 kJ kinetic energy to the suspension system. Among above energy injected into the quarter-vehicle suspension, 13.94 kJ is absorbed and dissipated by the hydraulic damper, 9.17 kJ and 32.50 kJ are respectively absorbed by the actuators M1 and M2. Fig. 20(b) shows that the kinetic energy flow of the common active suspension system is much different from the other two system. There are 27.21 and 21.90 kJ energy are respectively dissipated by dampers cs and cd, while the mechanical energy absorbed by the actuator is only 6.50 kJ. For the harvested energy, actuators M1 and M2 respectively deliver 2.68 kJ and 6.16 kJ electrical energy to the variable threshold ESS, and the average regenerative efficiency reaches over 21%. For the fixed threshold ESS, there are only 1.87 kJ and 4.08 kJ electrical energy harvested from actuators M1 and M2, respectively, while the corresponding average regenerative efficiency is about 14%. For the common regenerative active suspension, the harvested energy is only 3.00 kJ. It indicates that the proposed suspension scheme can harvest much more energy than the common configuration. Apart from that, the variable threshold ESS can harvest more energy with higher regenerative efficiency compared with the fixed threshold ESS. In terms of the electrical energy flow, the energy consumed by actuator M1 for active control is defined as Ec. The total harvested energy from M1 and M2 is represented by Eh. It is noted that the consumed

Table 6 Comparison of dynamic performances of different systems. Road class

Suspension

RMS (Z¨s )

RMS (Z¨d)

RMS (Zsu ) mm

RMS (Zug )

J

m s2

RMS (Zdu ) mm

m s2

mm

A

Passive Proposed Common active

0.54 0.25 0.40

2.86 3.56 2.98

1.59 1.85 1.70

3.85 3.64 3.10

1.15 1.27 1.20

1.00 0.86 0.88

B

Passive Proposed Common active

1.05 0.50 0.79

5.71 7.11 5.96

3.18 3.69 3.50

7.71 7.29 6.30

2.31 2.54 2.30

1.00 0.86 0.90

C

Passive Proposed Common active

2.10 1.01 1.59

11.42 14.22 11.91

6.36 7.36 6.90

15.41 14.53 12.50

4.61 5.08 4.60

1.00 0.86 0.89

D

Passive Proposed Common active

4.19 2.03 3.18

22.84 28.40 23.78

12.73 14.68 13.90

30.83 28.89 25.00

9.22 10.15 9.20

1.00 0.86 0.90

region in 20–40 s. At 40 s, when the road excitation changes to D class road, the predefined voltage thresholds of the variable thresholds ESS are further changed to 56 and 64 V, respectively. Because the terminal voltage of the proposed ESS is below the higher threshold voltage 64 V, the ESS keeps working in charging mode. After nearly ten seconds later, the terminal voltage finally reaches the high-efficiency region under D class excitation. During 40 to 60 s, the maximum terminal voltage of the fixed threshold ESS is around 40 V. It is far below the high-efficiency region, and far away from the fully self-power region. For the rest simulation in 60–100 s, the pre-defined thresholds for the high-efficiency region are decreased with smoother road excitations. Meanwhile, the terminal voltage of the variable thresholds ESS is well regulated, as shown Fig. 18. For the voltage of the fixed threshold ESS, it is beyond the pre-defined high-efficiency region in 60–100 s and far away from the fully self-power region in 80–100 s. The results indicate that the voltage of the proposed ESS are effectively kept in the desired region, while the voltage of the fixed threshold ESS is usually far away from the high-efficiency region and occasionally out of the fully self-power region. In order to investigate the difference of regenerative efficiencies of the two ESS in real time, the special period regenerative efficiency defined by Eq. (28) is demonstrated in Fig. 19. This figure shows that the two ESSs have same period regenerative efficiency in near 0–4 s, which is resulted from the same terminal voltages in Fig. 18. Then, the variable thresholds ESS keeps working with the regenerative efficiency higher than the fixed threshold ESS in the remaining simulation time, because of the difference of SC terminal voltages in Fig. 18. On the other hand, the efficiency of the two systems changes with the road roughness coefficients. This result is the same as illustrated in Section 3.2.1, i.e. a smaller regenerative efficiency of the system is obtained under worse road excitations. The average power harvested from the two actuators and total

Fig. 17. Road profile with different roughness coefficients.

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Fig. 18. Comparison of working mechanism of variable and fixed threshold ESSs: (a) connection mode signal of the variable thresholds ESS; (b) discharging signal of the variable thresholds ESS; (c) discharging signal of the fixed threshold ESS; (d) terminal voltages of the two ESSs.

Fig. 19. Comparison of regenerative efficiencies of variable and fixed threshold ESSs. Table 7 Comparison of regenerative performances when with variable and fixed threshold ESS. ESS

Simulation period 0–10 s

10–20 s

20–40 s

40–60 s

60–80 s

80–100 s

Variable threhsold

η Ph1 (W) Ph2 (W)

0.188 65.3 146.7

0.432 6.9 24.8

0.317 24.1 64.8

0.174 64.2 128.4

0.392 7.8 22.2

0.460 1.9 6.8

Fixed threhsold

η Ph1 (W) Ph2 (W)

0.119 41.7 92.8

0.351 5.4 19.7

0.207 14.8 43.4

0.116 44.2 85.2

0.346 8.6 18.1

0.189 2.2 1.3

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2966.5 J of the common active suspension system. The difference between the average consumed power and average harvested power is defined as average extra power to quantitatively analyze the impacts of these systems on vehicle energy consumption. Average extra power of the three systems on A, B, C and D class roads is shown in Fig. 22. It is noted that the average extra power below (above) zero means that the vehicle energy consumption is increased (reduced). Thus, the proposed system with variable threshold can reduce the vehicle energy consumption on all the four road conditions. In more details, the maximum reduction in vehicle energy consumption is achieved on C class road, where 49.2 W average extra power is obtained. The second is the D class road, in which 45.0 W average extra power is calculated by combining the result of 0–10 s and 40–60 s. B and A class roads have average extra power as 17.7 W and 4.9 W, respectively. Being different from the variable threshold ESS, the vehicle energy consumption is increased on A and D class roads when employing the fixed threshold ESS. In addition, its effect in reducing energy consumption is weaker than the proposed ESS when on B and C class roads. For the application of the common active suspension with energy regeneration ability, the quarter-vehicle simulation shows that it increases vehicle energy consumption to 2.9, 7.8, 24 and 99.0 W on A, B, C and D classes road, respectively. In summary, the proposed regenerative active suspension systems with the variable thresholds ESS is more capable of improving vehicles energy conservation performance. 6. Conclusions Based on the advanced-dynamic-damper-mechanism, a new regenerative active suspension system with dual actuators for in-wheel motor driven electric vehicle is developed in this paper. In this system, the primary actuator is actively controlled to improve vehicle ride comfort, while the secondary actuator is controlled to harvest energy from the vibration of the suspended drive motor. An improved boostbuck converter is proposed to adjust electromagnetic damping forces of harvesters with only the feedback of the actuator current, regardless of the voltage conditions. A new hybrid energy storage system with different modes and variable thresholds is presented to harvest more energy. The obtained results indicate that the output forces of actuators are well regulated. Compared to the passive suspension, when the vehicle is driven at 20 m/s on different roads, the ride comfort is improved by around 52%, and the comprehensive dynamic performance is improved by 14%. The variable thresholds energy storage system is able to achieve higher regenerative efficiency compared to the fixed threshold one. Being different from the application of the common ADM based regenerative active suspension, which increases vehicle energy consumption, the proposed system obtains considerable extra power on different roads when considering the energy consumed by suspension active control. The proposed system provides a good solution to implement an active suspension on IWM-EV and simultaneously improves

Fig. 20. Energy flows of different suspension systems: (a) proposed suspension system with variable and fixed threshold ESSs; (b) common ADM based regenerative active suspension system.

energy Ec is only determined by the dynamic response of the system. Both the two kinds of energy of the three systems are shown in Fig. 21. The corresponding values under different periods are listed in Table 8. From Fig. 21, the proposed ADM suspension with different ESSs almost consumes the same energy, which is less than the energy consumed by the common active system. It reveals that the ESS has little influence on the system dynamic responses. For the total harvested energy Eh, the system with variable threshold ESS shows better performance on every stage of road excitation, as listed in Table 8. At the end of the simulation, the variable threshold ESS can harvest energy Eh as 8843.3 J, it is much greater than the 5950.6 J of the fixed threshold ESS and the

Fig. 21. Comparison of electrical energy flows of different regenerative active suspension systems. 16

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Table 8 Electrical energy flows in different stages of road excitation. ESS

Simulation period

Variable Threshold Constant Threshold Common active

Ec (J) Eh (J) Ec (J) Eh (J) Ec (J) Eh (J)

0–10 s

10–20 s

20–40 s

40–60 s

60–80 s

80–100 s

0–100 s

−1635.5 2120.1 −1632.2 1344.6 −1812.7 787.7

−111.9 317.7 −111.2 250.4 −122.3 53.5

−795.7 1779.3 −789.1 1164.9 −908.6 428.1

−3085.4 3950.8 −3079.9 2588.5 −3490.6 1546.8

−274.7 601.2 −276.0 533.4 −293.6 126.1

−77.1 174.4 −79.4 68.9 −81.5 24.3

−5980.3 8843.3 −5967.8 5950.6 −6709.3 2966.5

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Fig. 22. Comparison of average extra powers.

vehicle energy conservation performance. In addition, the improved converter can also be applied to other systems where the input current of the converter needs to be accurate regulated. At the same time, the high efficiency energy management mechanism can be promoted to other energy harvesting systems. The energy management strategy and experiment of the proposed system will be further investigated in our future research work. Acknowledgement This research was supported by the National Natural Science Foundation of China (Grant No. 51805155), the National Natural Science Foundation of China (Grant No. 51675152), and the ‘Fundamental Research Funds for the Central Universities’ of China. References [1] Murata S. Innovation by in-wheel-motor drive unit. Veh Syst Dyn 2012;50:807–30. [2] Li B, Du H, Li W. Fault-tolerant control of electric vehicles with in-wheel motors using actuator-grouping sliding mode controllers. Mech Syst Sig Process 2016;72–73:462–85. [3] Hung Y-H, Wu C-H. A combined optimal sizing and energy management approach for hybrid in-wheel motors of EVs. Appl Energy 2015;139:260–71. [4] Qin Y, He C, Shao X, Du H, Xiang C, Dong M. Vibration mitigation for in-wheel switched reluctance motor driven electric vehicle with dynamic vibration absorbing structures. J Sound Vib 2018;419:249–67. [5] Li Z, Zheng L, Ren Y, Li Y, Xiong Z. Multi-objective optimization of active suspension system in electric vehicle with In-Wheel-Motor against the negative electromechanical coupling effects. Mech Syst Sig Process 2019;116:545–65. [6] Wang R, Jing H, Yan F, Reza Karimi H, Chen N. Optimization and finite-frequency H ∞ control of active suspensions in in-wheel motor driven electric ground vehicles. J Franklin Inst 2015;352:468–84. [7] Hui J, Wang R, Cong L, Wang J. Nan CJIJoVD. Fault-tolerant control of active suspensions in in-wheel motor driven electric vehicles. Int J Veh Des 2015;68:22–36. [8] Go N, Yasumichi W, Akihiko A. Development of an in-wheel drive with advanced dynamic-damper mechanism. JSAE Review. 2003;24:477–81. [9] Shao X, Naghdy F, Du H. Enhanced ride performance of electric vehicle suspension system based on genetic algorithm optimization. 2017 20th International Conference on Electrical Machines and Systems (ICEMS); 2017. [10] Shao X, Naghdy F, Du H. Reliable fuzzy H ∞ control for active suspension of inwheel motor driven electric vehicles with dynamic damping. Mech Syst Sig Process

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