Passive control on the negative unsprung-mass effects with in-wheel motor driving vehicles

In-wheel motor drive systems offer significant advantages for electric vehicles, including large chassis space, high transmission efficiency, and great control flexibility. However, in current mainstream in-wheel motor driving vehicles, the unsprung mass is significantly increased because the motor or the driving unit is rigidly connected to the wheel hub. The increased unsprung mass not only deteriorates vehicle ride comfort and road holding performance, but also results in heavy motor vibration. To mitigate these negative effects, configurations with suspended motor or driving unit have been proposed. It is thus desirable to explore the potential of these new configurations in this regard.

This paper aims to mitigate the negative effects of unsprung mass by optimizing vehicle and motor suspension parameters simultaneously. To this end, it examines two typical in-wheel motor drive configurations with motor suspension: the dynamic vibration absorber configuration and the two-stage suspension configuration. Half-vehicle models are established respectively for both configurations, and key indices for vehicle dynamic performance are selected or defined. Drawing on earlier studies on how the increased unsprung mass impacts vehicle performance at various speeds, and considering the trade-off among ride comfort, road holding, and motor vibration, a multiobjective optimization strategy is proposed for parameter optimization of vehicle suspension and motor suspension. In the strategy, the goal is to minimize body vertical acceleration, wheel dynamic load, and motor acceleration at medium speeds while reducing body pitch acceleration, wheel dynamic load, and motor acceleration at high speeds. Constraints include the natural frequency and dynamic deflection of the vehicle suspension. Using the NSGA-Ⅱ algorithm, Pareto optimal solution sets are derived respectively for the two configurations. The entropy weight method is then applied to determine the optimal parameters for vehicle and motor suspensions. With the optimal suspension parameters, dynamic simulations are conducted on a random road, and the dynamic performance is evaluated based on the predefined indices.

The results indicate that, compared to the fixed hub motor configuration, both motor suspension configurations achieve a substantial performance enhancement in vehicle ride comfort, road holding, and motor vibration. Specifically, the dynamic vibration absorber configuration delivers greater enhancements in vehicle body vertical and pitch vibrations, as well as wheel dynamic load. Specifically, it reduces body vertical and pitch accelerations by 36.9% and 33.09%, respectively, at medium and high speeds. The wheel dynamic load is decreased by 18.42% and 18.55% at medium and high speeds, respectively. By contrast, the two-stage suspension configuration excels in reducing motor vertical vibration. It reduces motor vertical acceleration by 67.48% and 65.43% at medium and high speeds, respectively.

This paper presents a passive control approach to address the negative effects of unsprung mass by utilizing motor suspension configurations. The in-wheel motor drive configurations with motor suspension demonstrate significant potential for improving vehicle dynamic performance. This research serves as a valuable resource for the design of in-wheel motor driving vehicles.