A Laboratory Study of Force - Cognitive Excavation Darcy M. Bullock, Intec Controls Corp. Walpole, MA, 02081 Irving J. Oppenheim Departments of Architecture and Civil Engineering Carnegie-Mellon University Pittsburgh, PA, 15213 ABSTRACT Excavation is characterized by the development of unmodelled forces between the bucket and the soil; the subsurface conditions are generally uncertain, and the development of these forces is only revealed during the very act of disturbing the medium. A human operator typically exerts control to keep such forces within limits created by the combined constraints of equipment, geometry, and task. This research effort developed and tested a supervisory control approach of discrete adjustments to the digging trajectory in response to forces encountered during excavation-- essentially constructing a device which can dig by feel. A laboratory manipulator was configured with four actuated degrees of freedom to approximate a backhoe. The motion sequence is represented symbolically as swing-sweep-scoop-raise-swing-dump, with the sweep-scoop-raise motions comprising the actual digging trajectory. Simple rules for supervisory control were programmed and tested in laboratory studies of sand excavation, and were effective in adjusting the digging actions to maintain forces within the target envelope. 1. INTRODUCTION ; PROBLEM STATEMENT The act of digging appears simple only to the most casual observer . On closer inspection, it is extraordinarily complex and difficult. The subsurface conditions, which include soil density, cohesion, internal friction, inclusions and discontinuities, are Clearly unknown, and most digging is accomplished without explicit knowledge or study of them. In fact, the influence of such conditions is generally evidenced only in the very act of irreversibly interfering with the medium while digging . A man digging with a spade adjusts his angle of attack , depth of penetration, and angle of removal in response to these varying characteristics ; those adjustments are essentially done by feel . Moreover , the man with a spade also adjusts his digging mechanics to the force envelope within which his muscles , joints, and limbs can work . As a result , he has extraordinary efficiency in being able to match a very limited work envelope to arbitrarily large excavation tasks. In addition , he processes all the spatial implications of reach and stability , and he embraces the span between an immediate ( tactical ) objective , which is the next spadeful of soil, and the overall 'Formerly graduate student, Department of Civil Engineering, Carnegie-Mellon University. 64