Toward elevated agrobotics: Development of a scaled-down prototype for visually guided date palm tree sprayer

Agricultural operations at great heights are typically laborious, expensive, and dangerous for human workers. Spraying and pollinating date palm trees, for instance, is currently done manually by a team of several workers from a platform lifted 18 m or more above the ground. This method is extremely unsafe, and many accidents have occurred due to instability when the platform is in a lifted position. In this paper we present the concept of an autonomous field robot that will effectively and accurately spray and pollinate date clusters. A scaled-down prototype has been designed and consists of a visually controlled robotic arm that guides the jet of a mounted sprayer directly to the date clusters completely autonomously and from a short distance. Rather than requiring an expensive dedicated platform, this robotic apparatus can be towed by a standard tractor operated by a single driver, with no human worker operating in the heights. This system can minimize risk of injury, significantly save manpower, and deliver the spray with maximum accuracy, thereby reducing chemical disposal. The spraying guidance system is based on a proportional controller that uses feedback from an image processing system combined with a small dead band. The system was modeled mathematically, and the effect of each component on overall performance was evaluated by simulation. Results were used for tuning the experimental system controller parameters. Experiments were performed to evaluate the tracking performance of the visually guided tracking system on a single tree and a 10-m-long runway at a distance of 6 m. These dimensions were inspired by the conditions at a typical orchard. During the experiment the sprayer was towed along the runway 19 times, during which the speed of the wagon was varied between 0.7 km/h (0.2 m/s) and 12 km/h (3.3 m/s). Experimental results indicate that up to a wagon ve1ocity of 1.25 m/s the tracking error was reasonably low and stayed below 10 deg from the center of the target (typical date spraying speed is 1.1 m/s). However, at higher speeds the tracking quality reduced progressively and some drift (i.e., accumulated error) was noticed in the pan axis due to image processing speed. The simulations and experiments with a scaled-down prototype show feasibility of the presented method and demonstrate how this new approach facilitates more efficient high-altitude agricultural robotics.