Abstract:The Series Elastic Actuator (SEA) has recently been developed by many research groups and applied in various fields. As SEA is the combination of motor, spring, gear and load, various types and configurations of mechanism have been developed as SEAs to satisfy many requirements necessary for the applications. This paper provides a theoretical framework to categorize and compare these various configurations of SEAs. The general structure and model of SEA is provided, and SEA configurations are categorized into Force-sensing Series Elastic Actuator, Reaction Force-sensing Series Elastic Actuator and Transmitted Force-sensing Series Elastic Actuator, based on the relative location of the spring. Criteria such as Force sensitivity, Compliance and Transmissibility of SEA are derived and compared using actual SEAs that have been developed previously.