Guidance system for agricultural tractor with four wheel steering

In a typical automatic guidance system for agricultural machines, the guidance system controls only one degree of freedom; either the steering angle of the front wheels of the tractor or other steering equipment is used for steering. In a tractor with the multiple degrees of freedom, like four wheel steering, the guidance system may use both the degrees of freedom for path tracking. However, the implement connected has to support the additional degrees of freedom; otherwise kinematic constraints are addressed. This paper presents a path tracking algorithm for four wheel steered agricultural tractor that is designed to be autonomous. The path tracking algorithm utilizes the kinematic and dynamical model of the vehicle, dynamic prediction and linear control laws to steer both front and rear axles of the tractor. The kinematic model is used to separate the two-times-time control problem into two single-input single-output problems. Conventional linear controllers are used for feedback control together with feedforward part. The guidance system uses real time kinematic GPS receiver for global positioning and additional sensors to measure the attitude of the vehicle. The developed path tracking system was tested in an agricultural field with a mounted seeder; by sowing 2.4 ha winter wheat. The results show that in straight swaths the lateral tracking error was less than 0.05m and the angular tracking error less than 1 degree. The results show also that during the field experiment, the GPS positioning signal was within the quality limits 15% of time that caused number of interrupts for continuous autonomous drive. The longest continous period of valid signal was eighteen minutes while the average of continuous driving was five minutes.