The practical way to generate a reference path in path tracking is to follow an adjacent swath. However, if the adjacent swath contains sharp turnings, the reference path will eventually contain sharper turn than the tractor is able to follow. This occurs especially in the corner of a field plot when the field is driven around.

In the headland, the objective is to minimize the time to reach the next swath. The commonly known method to generate the shortest path between two arbitrary positions is to use Dubins’ Curves. The Dubins’ Curves consist of curves with minimum turning radius and line segments connected together. The problem is that at the junction point of the path, the vehicle would have to either stop or turn wheels infinitely fast, which is not practical. Another approach is to use mathematical optimization methods to calculate the shortest path between two positions. Constraints are met, but the computational cost is high.

To overcome the problems above, a Spiral Connection method is proposed in the paper. The Spiral Connection method meets the constraints of the mechatronical steering system. If the steering rate is limited and tractor curvature is changed from the other extreme position to another, the tractor drives along a spiral. The Dubins’ Curves method is modified to support spirals between curves and lines. Furthermore, in adjacent swath tracking the Spiral Connection method is used to limit the curvature of the desired path.

With the proposed method, the desired path is always feasible with respect to the constraints of the steering system. The method was implemented in an experimental guidance system, which uses Nonlinear Model Predictive Control (NMPC) to realize path tracking. The feasibility of the path is crucial to reduce the calculation time of the NMPC. In the paper, the field experiments with a tractor and a seeder are presented. The results show that the method works both in theory and in practice.