I. Introduction
Series Elastic Actuators are a recent development in the field of force-controlled robotics [1], [2]. They have several advantageous properties, including reliable force output, simplicity, robustness of design, and use of traditional robotic actuators (electromechanical DC brushless motors). Series elastic actuators have been used in walking robots, and their underlying concept, introducing compliance, has been applied to such robots as the Honda P2 [3]. The ability to modulate the effective stiffness of the actuator is appealing in areas such as Impedance Control and Admittance control, which have application in human machine interfaces and prosthetics [4], [5].