Approaches to diminish large unsprung mass negative effects of wheel side drive electric vehicles

Benefit from quick response of motor torque and torque vectoring control, distributed drive electric vehicles (EVs) have a good potential to realize ABS, ESP and Traction control, and thus to achieve better driving performance. The drivetrain layouts of distributed drive EVs can be mainly divided into on-board motor drive system and wheel side drive system. With elimination of classic differential and drive shaft, the transmission chain of the wheel side drive system is relatively short and it doesn’t occupy space of the car body, and thus it features high efficient and compact. Therefore, the wheel side drive system is a prospective application on distributed drive EVs. However, the wheel side drive systems are normally equipped with a low speed hub motor or a high speed motor with reducer. These would bring in a large unsprung mass. Research pointed out that moving the electric propulsion from the vehicle body to the wheels can add up to 50kg or more, per wheel, to the unsprung mass (Van Schalkwyk and Kamper, 2006). The driving characteristics of a vehicle can be greatly improved by reducing the unsprung mass (Heißing and Ersoy, 2010). With the increased unsprung mass, the ride comfort and especially the road holding ability (or in other word driving safety) of a vehicle are worsen (Han, 2002; Guo, 2012; Fang, 2012; Liu, et al., 2012; Rojas, et al., 2010; Ślaski et al., 2014). This is so called “large unsprung mass negative effects”. Researchers and companies are devoting to diminish large unsprung mass negative effects. An axial flux disc motor is used to reduce unsprung mass of distributed drive EVs (Eastham et al., 1995; Hredzak, et al., 1998). As shown in Fig. 1(a), ZF released an early concept study of electric twist bean for Close-to-Wheel drive EVs at the IAA Abstract Ride comfort and road holding ability of distributed drive electric vehicles are worsened due to large unsprung mass introduced by wheel side drive system. To diminish this large unsprung mass negative effects, three approaches (“Suspension and Reducer Integrated” Close-to-Wheel Drive System, “Dynamic Vibration Absorber” Close-to-Wheel Drive System and in-wheel drive system with “Active and Energy Regenerative Suspension”) are introduced. The corresponding dynamics are analyzed and the schemes are provided. Evaluations of suspension performance show the superiorities of these approaches in different magnitude compared with the conventional configuration which is equipped with hub motor and passive suspension. Overall comparisons between three approaches are carried out and the feasibilities are discussed. With proper design

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