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Endpoint compliance strategies for precise robot control utilize feedback from a force sensor located near the tool/ workpiece interface. The closed-loop performance of such endpoint force control systems has been observed in the laboratory to be limited and unsatisfactory for industrial applications. This article discusses the particular dynamic properties of robot systems that can lead to instability and limit performance. A series of lumped-parameter models is developed in an effort to predict the closed-loop dynamics of a force-controlled single-axis arm. The models include some effects of robot structural dynamics, sensor compliance, and workpiece dynamics. The qualitative analysis shows that the robot dynamics contribute to force-controlled instability. Recommendations are made for models to be used in control system design.