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Current navigation systems for agricultural vehicles rely on GPS as the primary sensor for steering control. In citrus groves, where the tree canopy frequently blocks the satellite signals to the GPS receiver, an alternative method is required.

This paper discusses the development of an autonomous steering system for use in a citrus grove. The vehicle used was a common tractor. The navigation sensor system consisted of machine vision, laser radar and rotary encoder. Machine vision was used primarily for guidance, while laser radar can be used for both guidance and obstacle detection. A rotary encoder was used to feedback steering angle information. A proportional-integral-derivative controller was developed to minimize the path error. A common PC was used to process the sensor information and execute the control.

The vehicle was tested in flexible test paths constructed of common hay bales. Path tracking performance was observed. Performance was estimated using RMS error and average instantaneous error. The guidance system guided the tractor automatically through straight and curved paths. An average error of 2.8 cm using machine vision guidance and an average error of 2.5 cm using ladar guidance was observed, when the vehicle was tested in a curved path at a speed of 3.1 m/s. The guidance system has successfully guided the vehicle in a citrus grove alleyway.