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Autonomous weeders for Christmas tree plantations - a feasibility study SummaryThe Danish Christmas trees production covers about 31000 ha and has an annual turnover about 500-600 million DKK. About 4100 producers own this area. The majority have less than 1 ha. To obtain good growth and quality frequent weed control is essential. This is mainly done by application of chemicals, which leads to clean stands with poor living conditions for other species and risks of pesticide leaching. To reduce these problems of chemical weed control machinery has been developed for mechanical weeding. The first approximately two years after planting it is possible to use special spring tooth harrows with long flexible tines, which are effective in control of small weed plants without damaging the trees too much. In the following growth phases it is necessary to use row crop weeders. These weeders, however, are generally only able to control weeds between the rows an also less effective and more costly to use than herbicides. In addition the tillage process may cause negative effects of nutrient leaching and soil erosion. This project report presents the feasibility of developing a small autonomous machine for mechanical weed control in a more environmentally friendly way. The study was initiated by defining the minimum requirements to weeding using results from an ongoing investigation at The Danish Forestry and Landscape Research Institute. This investigation has shown, that maximum tree growth and development is achieved by weeding of concentric circle areas of only 40 cm radius around each tree. The left over weeds on the remaining area have beneficial effects on the trees because of shelter effects. On the basis of a literature review the idea and overall concepts of a small autonomous weeding machines is outlined, including suggestions of operation patterns, physical designs of the vehicle and the system architecture with sensors, databases, navigation system, and actuators. Also the likely costs of using such a machine were estimated and compared to currents contractor rates. The study has led to the conclusion, that it from a technical point of view is realistic to develop an autonomous Christmas tree weeder and possible to fulfil the aim of cleaning only part of the area. In addition the machine is considered suitable for collection of tree specific data concerning condition and development for use in plantation management and product documentation. It is estimated that the machine will have significantly lower environmental effects and facilitate better tree growth than present technologies (spraying and mechanical weeding). Further on it is estimated, that the machine as concept will have considerable interest for weed control in other special cultures as fruit and berry plantations. The chosen concept is based on centimetre precision determination of the position of each tree as well as the current position of the ACW by means of a RTH GPS (Real Time Cinematic Global Positioning System). The positions of the trees are to be measures automatically during machine planting by recording of the position where the tree is placed in the soil. The map produced in this way can then be used by the autonomous machine as basis for navigation during weeding over the entire lifetime of the plantation. Apart from the position it would also be necessary to measure a number of other parameters of the conditions around and in the machine itself for navigation, steering and controls, including an integrated safety system, which is to avoid collisions with obstructions, and injury of humans, animals and trees. These steering and control functions are structurized in the system architecture. As weed control tool is chosen a rotor cutter similar to those used in certain lawn mowers, as this needs less energy than tillage, and the effect is nearly the same as for mechanical weeding. As operation pattern is chosen a nearly linear movement along the rows as this was found to be the most economic. It was suggested to control the horizontal fine position of the rotor relative to the single tree by means of an optical or mechanical distance sensor. Two platforms were considered: 1) A small, low machine being able to move below the branches and in between the trees. 2) A portal machine that have a set of wheels and a working tool on either side of the row. The first is estimated to be the simplest and best suited. The system architecture, which is to be built up in several layers, comprises a coordinator, who is the person that runs the vehicle via a PC, a supervisor, which is a computer, that primarily via a "mode changer" controls the machine to behave appropriate, i.e. to navigate, to do route planning, safety check, and secondary supervision at the different stages of the safety behaviour, as well as communicating the situation and conditions to the coordinator. As regards economy it is estimated that the continued price reductions of electronic equipment together with increasing production numbers rather quickly will bring the costs dawn at a level between the present costs of mechanical weeding and spraying. On top of that added values may be achieved in terms of tree specific data for management decisions and costumer information.
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