Rear Wheel Steering - A Study on Low-Speed Maneuverability and Highway Lateral Comfort
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Typ
Examensarbete för masterexamen
Master Thesis
Master Thesis
Program
Automotive engineering (MPAUT), MSc
Publicerad
2017
Författare
Utbult, Juliette
Modellbyggare
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Utgivare
Sammanfattning
Rear wheel steering is a well-known technology that has been researched for many years but have only had limited market acceptance in niche applications. Increased urbanization and the advent of autonomous drive could change the impact of rear wheel steering in favor of greater market acceptance but in what way needs to be better understood. Low-speed maneuverability and highway lateral comfort are two attributes believed to be affected by the introduction of rear wheel steering. Lateral comfort includes comfort in the general sense with vibrations and high-frequency oscillations, but also low-frequency oscillations which may lead to motion sickness. A literature study was conducted in order to better understand the nature of motion which causes discomfort in automobiles, and means of evaluating comfort. A frequency weighting according to ISO-standard was used for comfort and a weighting found in literature was used for motion sickness in the lateral direction as this is still an unexplored area. Maneuverability was assessed using two analytical cases; curvature and swept path width, and two driving scenarios; rear-end parking and driving through a T-crossing. Two control laws were used; one feedforward and one feedback. The feedforward control law was designed to keep the body side slip angle zero at steady state cornering, and the feedback control was taken from literature with the aim of dampening yaw rate. This control law is mainly designed for stability, and was thus not used in low-speed maneuvers and only used for comparison in highway maneuvers. The angle on the rear wheels was limited to +/-5°. Rear wheel steering performed well at low speeds, improving the maneuverability by increasing the maximum curvature for a given steering wheel angle by 12% and decreasing the swept path width up to 14%. The vehicle with rear wheel steering also performed better in the driving scenarios by occupying less space during the maneuver. Overtaking and a sinus steer were the two maneuvers used to assess comfort and motion sickness. Rear wheel steering at high speeds decreased the measures of discomfort by 8.17% in the sinus steer maneuver and with 18.16% for a passenger in the rear seat, and MSDV is decreased by 7.32% in the sinus steer and by 4.04% in the overtaking maneuver for a passenger in the rear seat, implying that a vehicle equipped with rear wheel steering is more comfortable. Yaw stability was assessed as a final objective where it was shown that rear wheel steering, both the feedforward and feedback control, improved stability leading to a safer vehicle. To conclude, the results of this study implies that rear wheel steering can lead to improved maneuverability in low-speeds, improved comfort during highway driving, and improved safety in terms of stability.
Beskrivning
Ämne/nyckelord
Farkostteknik , Transport , Vehicle Engineering , Transport