Cartesian control of an advanced tractor's rear hitch

摘要

The movement of a traditional three-point hitch of an agricultural tractor provides one degree of freedom. This limits the use of hitched implements and forces implement manufacturers to build additional hydraulics into the implement. In this research a tractor was instrumented to provide two rotational and two translational degrees of freedom for the rear hitch. The goal was to develop a control system which allows the operator to control the rear hitch in Cartesian space.To allow Cartesian control four different models of the inverse kinematics of the rear hitch were developed. Three of them were chosen for field tests based on feasibility for real-time control. First model was a geometrical one. A hybrid model used the iterative DLS-method for calculating the inverse kinematics for the lower links from the direct kinematic transform and rotation matrix for calculating the upper link control. Third model used DLS-method with a few modifications: the restriction for the lower hitch point span was considered as one state of the system and also the jacobian of the upper link was augmented to the inversed jacobian. Algebraic solution proved too complex and was thus not tested.For controlling the rear hitch a system of two ECUs was made. A PhyCORE MPC555-computer module was used to read analog measurements and control the cylinders of the rear hitch. A Toradex Colibri computer module was used to calculate the inverse kinematics and to handle communications with the user via a joystick and a virtual terminal. In software development Simulink C-code generation was utilized for both computers. A user interface for VT was also developed with PoolEdit.All three kinematical models provided logically correct behavior in the actual test tractor, with minor differences in repeatability and absolute accuracy. The system provides a good solution to situations where controllability of the implement is advantageous without requiring extra effort from the operator.