Generating and evaluating stable assembly sequences

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Abstract
High-level assembly planning systems generate plans for the automated assembly of mechanical products by robots. The sequences to be generated underlie several physical and geometrical constraints, and in addition have to be efficient to increase productivity. The challenges still facing the field are to develop efficient and robust analysis tools, and to develop planners capable of finding optimal or near-optimal sequences rather than just feasible sequences. The presented high level assembly planning system High LAP automatically considers physical and geometrical constraints to generate and to evaluate stable assembly sequences. In this paper we propose a relational assembly model including a CAD description and the specification of features and relations of the assembly components. We use an optional specification of an arbitrary hierarchy of assemblies to speed up and guide the generation of sequences. High LAP evaluates all feasible assembly sequences considering several criteria like separability and manipulability of the generated (sub)assemblies. Furthermore, the necessity of reorientation for a mating operation and parallelism during plan execution are considered. Another important criterion is the stability of the generated (sub)assemblies. Most of the assembly planners developed up to date use heuristical or user-defined criteria to determine assembly stability for plan evaluation. The presented system is the first assembly planning system which automatically determines the range of all stable orientations of an assembly for plan evaluation. Therefore, we introduce a stability metric and an algorithm to calculate all stable orientations of an assembly considering friction. Experimental results are presented to demonstrate the efficiency of our assembly planning system.

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