Katsuyama, E. and Omae, A., "Improvement of Ride Comfort by Unsprung Negative Skyhook Damper Control Using In-Wheel Motors," SAE Int. J. Alt. Power. 5(1):214-221, 2016, doi:10.4271/2016-01-1678.
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 targeted mid-frequency 4 to 8 Hz range. This research aimed to improve mid-frequency vibration by identifying the cause of this adverse effect through the equations of motion. As a result, a method was derived by analysis that reduced vibration over a wide mid-frequency range by a control that applies unsprung vertical velocity in the direction that enhances that velocity (i.e., a negatively signed skyhook damper control called unsprung negative skyhook damper control). This control was then incorporated into a vehicle equipped with IWMs and the improvement effect on ride comfort was verified.