I. Introduction
High fidelity versions of conceptually classical robots have often been used for interaction with people because they are capable of achieving high torques and speeds. Many term these robots as having low-impedance if their actuator's impedance is low in the controllable bandwidth. At uncontrollable high frequencies that result from unplanned collisions, however, conventional robots - even ones with high force fidelity - have high impedance and become hazardous to people. The problem is exasperated by the low torque densities common in electromagnetic motors, which require high gear ratios to obtain acceptable torque densities. These high gear ratios significantly amplify the inertia of the actuator, creating high impedance systems. Perhaps the most common remedy in conventional robotics is to soften the blow of the robot with a compliant cover. As Zinn et al. have illustrated [1], however, more than five inches of cushioning would be needed to generate sufficient compliance to make a robot such as the Puma 560 safe when interacting with people. This bulkiness is unacceptable for many human-robot applications. Several new approaches have attempted to minimize the impedance of robots, including inertia reduction, passive impedance modulation, joint torque control, and increased compliance. After reviewing these techniques for use in anthropomorphic systems, improvements to increased compliance design will be suggested.