INTRODUCTION Vehicles equipped with in-wheel motors (IWMs) are capable of independently controlling the driving force at each wheel. Therefore, in addition to yaw control, IWMs can also control the roll and pitch of the sprung mass [1]. However, IWMs may also have an adverse effect on ride comfort due to the increased unsprung mass. Figure 1 compares simulated power spectral density (PSD) results for vertical acceleration of a vehicle body driven at 60 km/h (37 mph) over a rough road with and without a 25% increase in unsprung mass (same total vehicle weight) using a 4-wheel full-vehicle model. The increase in the unsprung mass caused vibration to increase, particularly around the 3 to 9 Hz range. Since vibration in the 4 to 8 Hz range is reported to be uncomfortable to vehicle occupants [ 2], this confirms that increasing the unsprung mass is not desirable for maintaining ride comfort. Therefore, this research examined a method of reducing uncomfortable vibrations using IWM driving force control. Figure 1. Effect of increased unsprung mass on vertical acceleration of vehicle body (simulation).RIDE COMFORT CONTROLS TARGETING ROAD SURFACE DISTURBANCES Conventional Controls Before being applied to vehicles, active suspension research originally began as a branch of rail vehicle engineering [ 3]. Skyhook damper control is typical well-established ride comfort control method [4] that has been applied to vehicle suspension controls [ 5], although various suspension control concepts have been proposed [6]. First, to confirm the effect of skyhook damper control, a simulation was carried out using a full-vehicle model equipped with IWMs, which was driven over a rough road at 60 km/h. Skyhook damper control was used to distribute the driving force. Figure 2 shows the PSD results for the vertical acceleration of the vehicle. Although the control was clearly effective in reducing vibration around the sprung mass resonance frequency, it had a slightly adverse effect on the 4 to 8 Hz range targeted in this research (vibration in this frequency range is referred to as “mid-frequency vibration” in this paper). When adopted in an actual vehicle, although the control suppressed major movements, fine hard vibrations were noticeable. The reasons for the adverse effect of the skyhook damper control on mid-frequency vibration were examined theoretically. Figure 3 defines the quarter-car 2-degree-of-freedom (DOF) model used for this purpose. Equations (1) and (2) show the equations of motion for the sprung and unsprung mass, respectively. Here, m 1 and m2 are the unsprung and sprung mass, ks and cs are the spring constant and damping factor of the suspension, and kt is the spring constant of the tire. Fc is the acting force of the suspension due to the control, and reaction force is a positive value. z0, z1, and z2 are the vertical displacements of the road surface, unsprung mass, and sprung mass, respectively. s is the Laplace operator. c sh in Equation (3) is delayed Improvement of Ride Comfort by Unsprung Negative Skyhook Damper Control Using In-Wheel Motors Etsuo Katsuyama and Ayana Omae Toyota Motor Corporation ABSTRACT Vehicles equipped with in-wheel motors (IWMs) are capable of independent control of the driving force at each wheel. These vehicles can also control the motion of the sprung mass by driving force distribution using the suspension reaction force generated by IWM drive. However, one disadvantage of IWMs is an increase in unsprung mass. This has the effect of increasing vibrations in the 4 to 8 Hz range, which is reported to be uncomfortable to vehicle occupants, thereby reducing ride comfort. This research aimed to improve ride comfort through driving force control. Skyhook damper control is a typical ride comfort control method. Although this control is generally capable of reducing vibration around the resonance frequency of the sprung mass, it also has the trade-off effect of worsening vibration in the target