| Semi-trailer tractor plays a vital role in the material transportation industry with its high specialization,strong flexibility and low cost.However,its special articulated structure leads to problems such as large visual blind zone and easy-folding when reversing vehicles.As the intelligent level of vehicles develops,using machines instead of humans to complete autonomous perception,decision-making,planning and controlling can achieve satisfactory results.In order to reduce the difficulty of manual parking and improve the efficiency of cargo transportation,it is necessary to establish a mature automatic parking system for semi-trailer tractor.Concentrated on driving stability and safety of semi-trailer tractor in the parking process,a path planning method based on articulation angle stability constraints and collision constraints is proposed in this paper.Two control modes are adopted to control the forward and reverse motion of vehicles in order to realize autonomous parking of semi-trailer tractor in narrow vertical parking space.The main research contents in this paper are as follows:(1)The realization idea of this paper is briefly summarized by analyzing parking and driving behavior of vehicles.The standard size of the corresponding vertical parking space is determined by the dimension parameters of semi-trailer tractor,and then the vertical parking yard is built.Two kinematic models are established based on the forward and backward motion states of vehicles,that is,the forward kinematic model and the reverse kinematic model.The forward kinematic model is mainly focused on the rear axle center of tractor and the reverse kinematic model is centered on the rear axle center of trailer.Through simulation comparison,the reliability of two models and the feasibility of their employment in later path planning and the research of motion-control method are verified.(2)Based on the motion regularities of semi-trailer tractor and the kinematic models,the stable region of articulation angle is determined when it’s moving forward and backward.The constraints of articulation angle stability are established and then are added to the hybrid A* algorithm to expend nodes by using the tractor which restricts the maximum turning angle of front wheel.The conditions of anti-collision constraints of the trailer are established and the feasible region of the starting point which could enable the trailer to park safely is solved based on the reversing-stability constraints.The feasible region is selected by collision situation where the tractor parking at the points in this region,and the parking path is smoothed by cubic uniform B-spline.The searching mechanism and smoothing effect are compared and verified,and the design of three parking routes of three targeted parking spaces in the parking scene is completed.(3)Based on the model predictive control theory,the linearized discrete error models of semi-trailer tractor driving in different directions,are derived.Concentrated on the safety and stability,the objective function and constraints are established to complete the design of forward and reverse MPC controllers.A co-simulation model of Truck Sim and Simulink is built to verify the controlling effect of controllers,and the automatic parking of semi-trailer tractor to each targeted parking space is realized. |
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