Trajectory Planning for a Four-Wheel-Steering Vehicle

This paper develops a trajectory planning algorithm for a four-wheel-steering (4WS) vehicle based on vehicle kinematics. The flexibility offered by the steering is utilized fully in the trajectory planning. A two-part trajectory planning algorithm consists of the steering planning and velocity planning. The limits of vehicle mechanism and drive torque are taken into account. Simulation results are presented to illustrate the application of the proposed algorithm. Introduction Research works on Autonomous Guided Vehicles (AGV) have been extensively carried out in the last four decades, and still in the process of rapid development at present. One application of AGV has been explored in harbor automation operation. Figure 1 shows an AGV for transporting cargo containers in harbor area. In order to move in cluster space within the harbor with flexibility, the vehicle structure is designed so that all the four wheels can be driven and steered individually. This kind of vehicle is referred to as a Four-Wheel-Steering (4WS) vehicle. Figure 1 A 4WS vehicle for cargo transport The problem of trajectory planning and generation is to design the configuration of the vehicle motion as a function of time, as well as generate corresponding reference inputs to the trajectory tracking system so that the vehicle will move along a specified path. The vehicle trajectory planning is not as well researched as that of vehicle path planning, and to our knowledge, only several works [2] [4][12] addressed this problem. This paper develops a methodology that consists of so-called rotation planning and translation planning for the trajectory planning for 4WS vehicles. The methodology utilizes the flexibility of the 4WS vehicle to plan the vehicle orientation. Vehicle Kinematic Analysis Under the basic assumptions of planar motion, rigid body and non-slippage of tire, the large size vehicle with four steering wheels as shown in Figure 1 can be approximated using a bicycle model. To describe the vehicle motion, a global coordinate X-Y is fixed on the horizontal plane on which the vehicle moves. The motion status of the vehicle at an arbitrary moment can be described using the Bicycle Model as illustrated in Figure 2.