It is not easy for us to instruct robots using robot language like AL because its task specification are not familiar to us. If the programming is permitted in natural language, novices will be able to access any robot systems. This system is given as input a set of English sentences in an instruction manual of mechanical assembly, and it regards them as a rough plan which specifiesrelations among mechanical parts to be assembled. As the specification is not precise, the system must refine it by referring to 3-D geometric models of objects and a world model that reflects the current state of assembling environment. Ambiguity in language specification and the assumption that common sense knowledge on assembling operations is available to us makes this refinement process difficult. At first, sentences are analyzed using DCG parser in PROLOG and the results are converted into frame structures. Instructions in the given sentence are converted into OPERATION frame. At this stage, several slots cannot be filled because sentences do not mention conditions in detail. They must be determined referring to object models and a world model. Even if candidates of slot fillers have been found, it is not clear whether the obtained operation will be possible or not. For this purpose, the system must simulate the operation and select an alternative by retrying the wrong decision. Concerning the simulation, it is not always true that a terminal condition of an operation is stated in sentences, and the system should take care of a termination of the operation. For example, START operation in a sentence will be transformed into INSERT operation, but no specification on its completion may be stated there. Then the system must assume the condition, but the system cannot judge the correctness of the assumption withoutestimating the rest of the sentences, so the system tries to find it using backtracking. As such, the system cannot tell whether the assembled object is the same one to be assembled or not. Consequently an estimation is needed for confirming the correctness of the assembled object. As the system is not given any explicit goal statement on the assembly, we have given to it a heuristic estimation.
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