A co-ordinate system was introduced to make it possible to describe, in a simple but fast way, the intended route that an agricultural implement should follow in the open field. The co-ordinate system was also used as a reference to compare the actual position of the implement with its intended path, and to calculate lateral position errors of the implement. To measure the actual position of the implement, Real Time Kinematic DGPS was used. The calculated error was used to control a correction device that moved the implement to its predetermined path, within a control band of +/- 20 cm, thereby providing a steering aid for the driver. Field tests with a full-size tractor that followed a winding guide rail with +/- 12 cm lateral undulations, while the implement was programmed to cover a straight track, showed an average error of 2 cm, when moving at a speed of 5.2 km/h. In a second series of experiments, a small four-wheeled cart was pushed along a concrete road, being swung left and right at random. An electrical side-shift actuator was used to write a chalk line on the road that represented the actual path of the imaginary implement. The straight, programmed path to be followed by the implement was represented by a permanent line on the road surface, to be compared with the line of chalk. These tests showed an average error of 1 cm at an average speed of 3.6 km/h. (C) 1999 Elsevier Science B.V. All rights reserved.
