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UBC Theses and Dissertations

Navigation system for semi-autonomous tracked machines Kusalovic, Dejan

Abstract

The issues researched in this thesis work relate to the theoretical and practical problems in analysis, design and implementation of mobile, tracked, semi-autonomous machines in out-of- door environments (particularly the work sites of small to medium size, 100x100m). A complete navigation system is analyzed and proposed while the focus has been placed on developing an efficient path planning subsystem. Mechanisms of ensuring that the paths are safe are also proposed. The path planner is based on searching an elevation map of the work space. The search algorithm is based upon the A* search with a novel cost function. The cost function optimizes the path with regard to the cost map that reflects length of the path and relative altitude change along the path. These objectives contribute toward improving path efficiency and satisfying environmental standards which are general objectives for heavy mobile machines in off-road environments. Multiple versions of the cost function are used to provide multiple paths which allow an operator to have final control over the choice. Having multiple paths increases the chances that the operator (or supervisor) will be able to find a path that fits a variety of other objectives that have to be considered in real world conditions (for example moving obstacles, soil type etc.). One special case of terrain is studied in great detail. When travelling slopes it is necessary to enforce switchbacking (climbing in a zig-zag fashion) to account for mechanical limitations of the vehicle and to reduce the soil disturbance on these specially sensitive areas. The additional features for the search algorithm are developed to produce such paths when travelling slopes. This has been achieved by choosing suitable cost functions and placing additional constraints on the number and nature of turns along the path generated by the search algorithm. This is the area that no other existing grid search path planner covers. The path planner was implemented and tested on grids representing different terrain types that range from flat to severely hilly. The 30x30 grid required an average of 1.55 sec CPU time to perform which is suitable for the intended usage. The off-line implementation of the path planner is accompanied by a custom graphical interface for path presentation. This GUI is also a prototype of the operator's interface planned for the final version of the navigation system. A partial integration of the navigation system was achieved and evaluated on a mini-excavator. An already existing control system was adjusted for use in leading the mini-excavator along the path determined by the path planner. The control strategy was very simple, yet we demonstrated good precision of path following (position error was less then 3% on an 8x8m field), based on the fact that a path is made up of links between neighbouring nodes in a regular grid. Ways of reducing potentially accrued errors on bigger fields are proposed.

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