  Series Elastic Actuators provide many benefits in force control of robots in
unconstrained environments. These benefits include high force fidelity,
extremely low impedance, low friction, and good force control bandwidth. Series
Elastic Actuators employ a novel mechanical design architecture which goes
against the common machine design principal of "stiffer is better". A compliant
element is placed between the gear train and driven load to intentionally
reduce the stiffness of the actuator. A position sensor measures the
deflection, and the force output is accurately calculated using Hooke's Law
(F=Kx). A control loop then servos the actuator to the desired output force.
The resulting actuator has inherent shock tolerance, high force fidelity and
extremely low impedance. These characteristics are desirable in many
applications including legged robots, exoskeletons for human performance
amplification, robotic arms, haptic interfaces, and adaptive suspensions. We
describe several variations of Series Elastic Actuators that have been
developed using both electric and hydraulic components.
