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2014 IEEE International Conference on Systems, Man, and Cybernetics October 5-8, 2014, San Diego, CA, USA Electromagnetic Reegenerative Suspension System for G Ground Vehicles Peng Li, Lei Zuo* Jianbo Lu, Li Xu Department of Mechanical Engineeering SUNY at Stony Brook, NY 11794, U USA Ford Motor Company, Dearrborn, MI 48124, USA Abstract—This paper considers an eelectromagnetic regenerative suspension system (ERSS) thatt recovers the kinetic energy originated from vehicle vibraation, which is previously dissipated in traditional shock absorb bers It can also be used as a controllable damper that can n improve the vehicle’s ride and handling performance The proposed electromagnetic regenerative shock absorbers ((ERSAs) utilize a linear or a rotational electromagnetic generaator to convert the kinetic energy from suspension vibration into electricity, which can be used to reduce the load on the alteernator so as to improve fuel efficiency A complete ERSS is discussed here that includes the regenerative shock absorb ber, the power electronics for power regulation and suspensioon control, and an electronic control unit (ECU) Different sshock absorber designs are proposed and compared for simpliicity, efficiency, energy density, and controlled suspension perfoormances Both simulation and experiment results are presented d and discussed Keywords— energy harvesting; vibration; veh hicle dynamics; suspension; power electronics; control I INTRODUCTION In the past, the vehicle suspensions arre designed to control the vehicle vibration by dissipatingg the vibration energy into heat, mostly by hydraulic dam mpers If such dissipated energy can be recuperated, the esstimated power gain would be an average of 100-400 watts for a mid-size passenger car on an average road at 60 mph [1] Taking into account the engine and alternator efficiency (aabout 30% and 55%), this can lead to 2-9% fuel efficiency increase for conventional vehicles Regenerative shock absorbers that are capable of recuperating energy from vibration in susppensions have attracted a lot of interests from both academia and industry in recent years Various concepts and prototyppes have been proposed [2-14] Among all the concepts, eelectromagnetic designs are the most popular for their high eneergy conversion efficiency and design simplicity In addition, [99-14] indicated that the damping force of electromagnetic shhock absorbers can be controlled to serve as active/semi-activee suspensions In this paper, an electromagnetic regenerattive suspension system (ERSS) is proposed The designs and m modeling of the proposed electromagnetic regenerative shoock absorbers (ERSAs) are discussed in Section II Section III studies the energy harvesting potential of such shock absoorbers based on simulation and experiment results Section IV considers using such regenerative shock absorbers ffor suspension control purpose Section V concludes the paperr 978-1-4799-3840-7/14/$31.00 ©2014 IEEE II DESIGN AND D MODELING The electromagnetic regenerative shock absorbers utilize certain mechanisms to drive an electromagnetic generator, either linear or rotational, in respo onse to suspension vibration so that the kinetic energy of the vibration is converted into electricity In this paper, we focu us on two types: linear and rotational regenerative shock abso orbers A Linear Electromagnetic Regen nerative Shock Absorber A linear electromagnetic regenerative r (LER) shock absorber considered here has perrmanent magnets and coils assemblies installed inside, and generate g electricity from the linear relative motion between theem, as shown in Fig [1518] provided both theoretical an nalysis and experiments of such LER shock absorbers Whille the design is simple and relatively durable, two major drawbacks d need attention: limited damping capability and limited l power density The first drawback can be overcome by simply adding a hydraulic damper in parallel to expand its damping capability In this case, the combin nation of the supplementary hydraulic damper and the LER R shock absorber should achieve similar performance to a single traditional hydraulic shock absorber Efforts have been n made to solve the second problem in [19, 20] where optimal design of LER shock absorber with finite element anallysis and experiments were conducted The power density y of the optimized LER transducers can be increased to times, compared with the previous designs [9, 10] The new n design can achieve a damping coefficient of 1680-214 42 Ns/m which can harvest an average 26-33 watts of electriicity at 0.25 m/s root mean square (RMS) suspension velocity y Fig Diagram of the linear electromagneetic shock absorber B Rotational Electromagnetic Regenerative R Shock Absorber Besides linear generators, rotational electromagnetic a also used as energy regenerative (RER) devices are converters To generate electricall energy, mechanisms such as ball screw, rack-pinion, and hydraulic h motor, are needed so as to convert linear vibration in nto rotational motion 1) Traditional design 2513 (a) Li et al [21] designed and built an RER shock absorber (Fig 2) based on rack-pinion mechanism and demonstrated in a test bench and on-road vehicle tests The linear vibration is converted into rotation through the rack-pinion and then is magnified through the bevel gears and a planetary gearbox Bench test results showed the mechanical efficiency ranged from 33% to 63% with various vibration frequencies and electrical loads (a) (b) Fig (a) Design and (b) schematic of the MMR based RER shock absorber (b) A Energy harvesting potential The disturbances due to the unevenness of road surface are random and distributed over a wide frequency range, which can be approximated as stationary Gaussian stochastic process described using displacement power spectrum density (PSD), in m2/(cycle/m) Measured data is smoothed and fitted by a straight line using least-mean-square method in certain spatial frequency range, as given in (1) Fig (a) Design and (b) schematic of non-MMR type of RER shock absorber RER shock absorbers are more compact and cost effective with larger power density compared with LER shock absorbers However, due to the irregular oscillation of the suspension vibration and the inertia of the rotor of the generator, such designs may suffer from large impact force on the mechanical components which results in poor reliability and short service life For the prototype built by the authors for passenger car, the equivalent mass due to the rotor inertia is about 300 kg Simulation results showed the degraded performance in comparison with constant dampers [22] In addition, the irregular bidirectional rotation of the generator will result in irregular AC output that needs an additional rectifier which leads to additional energy loss ܩௗ௦௧ ሺ݊ሻ ൌ ܩ ݊ఉ where ܩௗ௦௧ is one-sided PSD in m2/(cycle/m), ݊ is the spatial frequency in cycles/m, ܩ is the road-roughness coefficient in m2cycle/m The exponent ߚ is usually approximated as -2, so that the road displacement profile can be represented as a unit-intensity white noise signal passing through a first order filter given by equation (2) [24] ܩሺݏሻ ൌ ሺଶగ ீೝሻభȀమ ௦ାఠబ (2) where ߱ ൌ ʹߨݒ , ݒ is the cutoff frequency which is between 0.001 and 0.02 cycles/m, is the vehicle traveling velocity in m/sec, ܩ was chosen to be 25.6e-7 m2cycle/m, corresponding to the Class C road (average condition road), as suggested in ISO 8608:1995 [25] (2) also indicates that the ground velocity excitation input is a white noise with a one-sided PSD of ʹߨܩ Then the average power dissipated in the suspension can be computed as 2) Mechanical Motion Rectifier Based Design To resolve the issues in traditional rotational designs, a mechanical motion rectifier (MMR) based shock absorber has been proposed [23] The MMR converts the irregular bidirectional suspension vibration into unidirectional rotation of the generator, through the bevel gears and two one-way roller clutches underneath, as shown in Fig This unidirectional rotation is then magnified by a planetary gearbox and transmitted to a rotational DC generator, which produces a relatively stable DC output instead of an irregular AC output The MMR design ssignificantly improves mechanical reliability due to impact force reduction and both the mechanical and electrical efficiency In bench tests, the MMR based shock absorber prototype achieved over 60% mechanical efficiency with no obvious backlash effect III (1) ܲ௩ ൌ ܿܧሾ ݒଶ ሿ ൌ ߨܩ ݇ଵ (3) where ݇ଵ is the tire stiffness Then the energy harvesting potential in the suspension of a passenger car can be estimated from ܲ௩ [1] The suspension vibration on a compact passenger vehicle was measured in [1] which has a curb weight of 2398 lb (1088 kg) The instant power dissipated in one shock absorber when the car was driven on the campus road at 25 mph is shown in Fig Based on the vibration data measured, the average power dissipated by one shock absorber was 14.57 W ENERGY HARVESTING On a typical vehicle, power demand is fulfilled through electrical power generated by alternator driven by engine The extra energy generated by the regenerative shock absorbers can reduce the amount of power drawn from the alternator so that the engine’s load can be reduced 2514 500 is shown in Fig The peak output power was 12 watts and 22 watts with 94 ȍ and 42 ȍ external loads respectively at root mean square suspension velocity of 0.067 m/s 450 Instant Power (Watts) 400 350 300 250 200 150 100 50 0 10 20 30 40 Time (Sec) 50 60 Fig The measured power dissipation of one shock absorber on a super compact vehicle when driven on compus road at 25 mph The instant power of MMR, non-MMR ERSAs and the instant power dissipation of a 1425 Ns/m constant damper on quarter passenger car when traveling on simulated Class C road at 40 mph are shown in Fig 900 800 Fig Bench test output power of non-MMR ERSA with different load resistances under 0.5 Hz, 30 mm amplitude displacement excitation The instant output power of the MMR ERSA in bench test is as shown in Fig It can be seen that at root mean square suspension velocity of 0.067 m/s, the average output power were 25.6 watts and 40.4 watts at 100 ȍ and 30 ȍ electrical loads, respectively, with a peak power of 62.9 watts and 104.3 watts- non-MMR ms=80, R=357 MMR ms=80, R=357 Constant Damper 700 Power (W) 600 500 400 300 200 100 0 Time (sec) Fig Output power of non-MMR, MMR ERSAs and power dissipation of a 1425 Ns/m constant damper on quarter passenger car when traveling on ISO Class C road at 40 mph Fig Bench test output power of MMR ERSA with different load resistances under Hz, mm amplitude displacement excitation The average output power of the regenerative shock absorbers and average power dissipation of 1425 Ns/m constant damper on a quarter passenger car traveling on simulated ISO Class C road at a speed up to 90 mph are shown in Fig C On-road test results In addition to bench tests presented in previous sections, the prototypes of both non-MMR and MMR designs were tested on the road to verify the feasibility of the principle and design The prototypes were installed on the rear left wheel of a 2002 Chevy Suburban, as shown in Fig 10, replacing the original hydraulic shock absorber The vehicle was then driven on the paved campus road of State University of New York at Stony Brook to measure the suspension vibration and the output of the regenerative shock absorbers 250 Average Power (W) 200 non-MMR ms=80, R=357 MMR ms=80, R=357 Constant Damper 150 100 50 0 10 20 Vehicle Speed (m/s) 30 40 Fig Average output power of non-MMR, MMR ERSAs and dissipation of 1425 Ns/m constant damper on quarter car on Class C road B Bench test results The instant output power of the non-MMR ERSA in bench test with 0.5 Hz, 30 mm amplitude sinusoid excitation Fig 10 Setup of the road tests 2515 recuperating process Hence the ERSAs can serve as a semiactive or mild active suspensions For non-MMR shock absorbers, the ends of the shock absorber are directly connected to the generator and the force can be in either the same or opposite direction of the suspension velocity, which means the shock absorber can work as an actuator by consuming additional energy from the battery For MMR shock absorbers, due the existence of the one-way roller clutches, the shock absorber force can only be in the opposite direction of the suspension velocity, just as a damper In other words, the shock absorber can only be dissipative Although the MMR shock absorber cannot work as an actuator, feeding energy to the generator inside is still feasible to achieve active braking on the generator to control the generator velocity, which will be necessary for precise control of the damping force The non-MMR shock absorber displacement and output voltage on an external electrical load of 30 ȍ at 20 mph and 30 mph are shown in Fig 11 The average output power on the external resistor were 3.3 watts and 4.8 watts, respectively, or 13.2 watts and 19.2 watts can be harvested from all four shock absorbers at 20 mph and 30 mph, while the peak powers were up to 58.2 and 67.5 watts A Suspension performance with passive load resistance When connected to a passive load resistance, the shock absorbers are dissipative and the total force ݏܨis not only related to the suspension velocity ݏݒ, but also its derivative ܽ ݏ, which is given as: Fig 11 Displacement and voltage measured at 30 mph on a paved campus road The displacement and output electrical power of the MMR regenerative shock absorber on a 20 ȍ external electrical load is shown in Fig 12 The average output power was 15.4 watts for the recorded data when the vehicle was driven at 15 mph ܨ௦ ൌ ݉௦ ܽ௦ ܿ ݒ௦ where ܿ ൌ is the equivalent electromagnetic damping ோ coefficient, ܭis a constant related to the mechanical and electrical design parameters of the shock absorber, ܴ is the total load resistance (including the generator’s internal resistance), ݉ ݏis the equivalent mass calculated from the generator inertia (a) The power demand in a modern vehicle is fulfilled by the alternator driven by the engine The extra energy recuperating from the ERSAs need to be integrated into the existing on-board electrical system so as to make up certain amount of power drawn from the alternator leading to engine load reduction Otherwise a separate energy storage unit can be used to store the harvested energy Energy management strategy can be developed to balance the power flow between the regenerative shock absorber and the energy storage (b) 3.5 R.M.S Dynamic/Static Tire-Ground Contact Force D Power electronics for energy harvesting To power onboard electrical devices, stable DC electrical power is needed Hence a power electronics interface circuit is needed to convert the unstable output from the ERSA into stable DC power at certain voltage level Typical circuit topologies have been reviewed and summarized in [27] R.M.S Weighted Acceleration (m/s2) Simulation results showed that large inertia can greatly degrade the suspension performance The comparison of the root mean square values of the weighted (ISO 2631) sprung mass acceleration and dynamic/static tire contact force of the quarter passenger car with MMR, non-MMR ERSAs (݉௦ ൌ ͺͲ ݇݃ǡ ܴ ൌ ͵ͷ ȳ) of 1425 Ns/m constant damper traveling on an ISO Class C road are shown in Fig 13 Improvements on both the ride comfort and road handling performances were observed with MMR electromagnetic shock absorbers over the non-MMR based regenerative absorber Similar results were observed with other values of ݉௦ and ܴ Simulation results showed that MMR can significantly reduce the negative effect of the inertia on the performance of regenerative shock absorbers Fig 12 The suspension displacement and output electrical power of MMR shock absorber at 15 mph on campus road IV (6) 2.5 1.5 0.5 0 10 20 30 Vehicle Speed (m/s) 40 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 10 20 30 Vehicle Speed (m/s) 40 Fig 13 Comparison of (a) weighted vehicle body acceleration and (b) dynamic/static tire contact force with non-MMR (blue), MMR (red) and constant damper (black) on Class C road at different vehicle speeds SUSPENSION CONTROL The equivalent damping of ERSA is inversely proportional to the load resistance Thus, design of the load The damping force of the ERSAs presented in previous sections can be controlled by reversing the energy 2516 interface circuit with variable resistance to the shock absorber The buck-boost DC-DC converter can be a good solution to achieve r damping control, due to that its average input resistance can be controlled by adjusting the duty cycle when working in discontinuous current mode [28, 29] resistance is critical in designing the damping of the shock absorber Fig 14 shows the effects of load resistance of MMR shock absorber on vehicle performance compared with the performance of constant damper Suspension Displacement Dynamic/Static Tire-Ground Contact Force Weighted Acceleration Nominal Damping Performance For rotational electromagnetic shock absorbers, the rotor inertia of the generator makes it more difficult to control the damping force To achieve the desired shock absorber force, both the acceleration and velocity of the suspension need to be taken into consideration In the case of MMR shock absorber control, the non-linear behavior caused by the disengagement of the roller clutches also needs to be considered for precise control of the damping force Normalized Performance 2.5 1.5 0.5 non-MMR Damper MMR Damper 3000 Normalized Equivalent Damping 10 12 Equivalent Damping Coefficient (Ns/m) 0 Fig 14 Normalized performances of MMR regenerative suspension with various external electrical loads, where ms=80 kg and V=20 m/s Asymmetric damping coefficients for compression and rebound of the suspension help to keep good road contact and reduce shock to the vehicle body This can be realized on the regenerative shock absorber without MMR by connecting two different resistors with diodes to the shock absorber The bench test force-velocity relationship of the regenerative shock absorber with different loads for compression and rebound is shown in Fig 15 2500 2000 1500 1000 500 0 20 40 60 Excitation Frequency (Hz) 80 100 Fig 16 Equivalent damping coefficients of MMR and non-MMR ERSAs under Hz to 100 Hz sinusoid displacement excitations, with ms=200 kg, total electrical resistance R=357 Ω D Energy balance for energy harvesting and suspension control Both suspension control and energy regeneration functions can be achieved in the proposed ERSS The shock absorbers can be switched between consuming energy for suspension control purpose and harvesting energy for improving fuel economy Through a properly designed energy storage unit together with an efficient energy management controller, the controlled suspension based on the ERSS can be self-powered Fig 15 Force-velocity relationship of the regenerative shock absorber with different damping coefficients for compression and rebound Either one direction power flow (from shock absorber to the battery) or bi-direction power flow is needed to be achieved by the power electronics interface circuit between the regenerative shock absorber and the battery Besides functionality and efficiency, fast response speed is also a key for such circuits to regulate the desired suspension force B Nonlinearity of MMR For MMR based shock absorber, because of the one-way roller clutches mounted on the pinion shaft, the pinion shaft and the large bevel gear would disengage from each other when the rotation speed of the pinion shaft is smaller than the rotation speed of the large bevel gear In such case, the torque transmitted would fall to zero, resulting zero damping force of the shock absorber Due to this nonlinearity, the equivalent damping of the MMR shock absorber, which is calculated based on the energy dissipated per cycle, is sensitive to the excitation frequency, as shown in Fig 16 The influence of such non-linear damping and inertia with both traditional and MMR rotational shock absorbers on the vehicle dynamics were investigated in [26] V CONCLUSION Electromagnetic regenerative suspension can serve as an energy recuperating device and a controllable suspension device The electromagnetic regenerative suspension utilizing a rotational generator and mechanical motion rectifier (MMR) showed the advantages of larger power density over the linear designs, improved mechanical structure reliability, and great energy harvesting efficiency through both bench tests and on-road vehicle tests This electromagnetic regenerative shock absorber also has the capability to serve as a semi-active suspension and a mild active suspension The semi-active suspension control functions can be achieved through controlling the resistance C Suspension performance with controlled load resistance For the regenerative shock absorbers described in previous 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