Impedance and Interaction Control

Abstract

Mechanical interactionwith objects is arguably oneof the fundamentallyimportant robotbehaviors.Many current robot applications require it; for example,mechanical interaction is essential for manipulation, the core task of assembly systems. Future robot applications, such as versatile use of tools or close cooperation with humans, may be enabled by improved control of mechanical interaction. Interaction with the environment may serve sensory or motor functions (or both), and the most may be characterized by mechanical impedance, which may loosely be considered a dynamic extension of stiffness.1 Lower mechanical impedance reduces interaction forces due to encountering an unpredicted object, thereby protecting both the robot and any object it manipulates (interaction forces on each being opposite but equal). Using a human analogy, by this reasoning, tactile exploration and manipulation of fragile objects should evoke the use of our lowest-impedance limb segments, and while we can (and routinely do) interactwith objects using other body parts (the elbow, the knee, the foot, etc.),we naturally tend to use our fingers for gentle, delicate tasks. Conversely, wielding an object as a tool often requires it to be stabilized and that requires higher mechanical impedance. This is especially important if the interaction between the manipulator and the object is destabilizing, as is the case for many common tools. Again using a human analogy, consider, for example, the simple task of pushing on a surface with a rigid stick. If force is exerted on the stick normal to the surface, then the stick is statically unstable; small displacements from the configuration in which stick axis and force vector co-align result in torques that act to drive the stick further from that configuration. Success at this task requires a pushing harder exacerbates the problem (the magnitude of the destabilizing torque is proportional to the applied force), the minimum required impedance grows with the force applied; see Rancourt and Hogan (2001) for a detailed analysis. Simple though this task may be, it is an essential element of the function of many tools (e.g., screwdrivers, power drills) and any manipulatorhuman or roboticmust provide a stabilizing mechanical impedance to operate them. In other applications the robots interactive behaviormay be the main objective of control. For example, to use a robot to serve as a force-reflecting haptic display (Miller et al., 2000) or to deliver physiotherapy (Volpe et al., 2000) requires intimate physical interaction with humans. In these applications the feel of the robot becomesan importantperformance measure, and feel is determined bymechanical interaction dynamics.Versatile interactionwith objects (whether tools or humans or other robots), therefore, requires an ability to modulate and control the dynamics of interaction.