INTRODUCTION In modern society, traffic congestion is becoming a major problem in every metropolis, in particular, for the cities like Shanghai, Tokyo, and London, where the population is more than 10 million. According to Schrank, et al. [1], the cost of delays induced by the traffic congestion has risen from $24 billion in 1982 to $115 billion in 2009 in US, which becomes a serious economic cost. Verhoef and Rouwendal [ 2] stated that traffic congestion will have a significant impact on vehicle speed, traffic safety, and time cost. Shinar [3] also showed that driver aggression is increasing because of traffic congestion. An innovative omni-directional vehicle has been suggested as a means of solving the traffic congestion problem because it has four independent steering wheels such that each wheel can have different steering angle to perform a small radius turn and thus save space. Omni-directional vehicles can also have enhanced stability and handling performance due to the available four wheels' driving, braking, and steering functions. The concept of omni-directional vehicle is initially referred to the research of advanced robotics, where many focused on the use of an omni-directional vehicle in situations [ 4,5,6] with slow speed and running on the smooth ground. In [ 7], the omni-directional steering kinematic constraints were developed based on a simple four wheel vehicle dynamics model, and in [8], [9], and [10], a comprehensive vehicle dynamics model for the four wheel independent steering and four wheel independent driving road vehicle with relative high velocity was developed. With the development of electric technology, the in-wheel driving and steering motors have been widely applied in the electric vehicles [11 ]. The omni-directional steering mechanism can be easily implemented in the electric vehicle with in-wheel steering motors and driving motors, and the comprehensive dynamics analysis of this kind of vehicle has been studied in [ 12]. Because the four wheel steering angles can be set differently for the omni-directional vehicle, the improved vehicle mobility, handling and stability performance can be realized and various control strategies have been proposed, like the active steering control and the direct yaw moment control, where the information about the side slip angle is critical to the performance of these control strategies. Due to the difficulty in directly measuring the side slip angle, accurate estimation of the side slip angle via available on-board sensors is becoming more important.A Novel Method for Side Slip Angle Estimation of Omni-Directional Vehicles Boyuan Li, Haiping Du, and Weihua Li University of Wollongong ABSTRACT The omni-directional vehicle is an innovative vehicle, in which the in-wheel steering motor and in-wheel driving motor are integrated into each wheel of the vehicle so that each wheel can be independently controlled to have traction, braking, and turning motions to improve the vehicle's mobility, handling and stability. To realize good performance, various control strategies have been proposed, like the active steering control and the direct yaw moment control, where the accurate slip angle information is critical to these control strategies. However , in practice, the side slip angle is hard or expensive to be measured for a passenger vehicle, therefore, different estimation methods have been proposed in the literature. In this paper, a novel side slip angle estimation method is proposed for the omni-directional vehicle that has four independent steering motors. This method includes the estimation of total alignment torque and side slip angle, and only needs the measurements of steering angles and steering motor currents, which are available by using angle sensors and current sensors embedded with the in-wheel steering motors. Numerical simulations are used to validate the ef fectiveness of the proposed side slip angle estimation approach. CITATION: Li, B., Du,