Acknowledgements: The research described here has been supported by NSF grant DMI-94-20373, NSF grant DMI-9800187, NSF Young Investigator Award IRI-92-57269, Army Research Office contract DAAH04-94-C-0055, the Office of Naval Research contract N00014-95-1-0107, The Cleveland Advanced Manufacturing Program (CAMP,Inc.), and the Center for Automation and Intelligent Systems Research (CAISR) at CWRU. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the sponsors.