  The use of the Series Elastic Actuator (SEA) system as an actuator system
equipped with a compliant element has contributed not only to advances in human
interacting robots but also to a wide range of improvements in the robotics
area. Nevertheless, there are still limitations in its performance; the elastic
spring that is adopted to provide compliance is considered to limit the
actuator performance thus lowering the frequency bandwidth of force/torque
generation, and the bandwidth decreases even more when it is supposed to
provide large torque.
  This weakness is in turn owing to the limitations of motor and motor drives
such as torque and velocity limits. In this paper, mathematical tools to
analyze the impact of these limitations on the performance of SEA as a
transmission system are provided. A novel criterion called Maximum Torque
Transmissibility (MTT)is defined to assess the ability of SEA to fully utilize
maximum continuous motor torque. Moreover, an original frequency bandwidth
concept, maximum torque frequency bandwidth, which can indicate the maximum
frequency up to which the SEA can generate the maximum torque, is proposed
based on the proposed MTT. The proposed MTT can be utilized as a unique
criterion of the performance, and thus various design parameters including the
load condition, mechanical design parameters, and controller parameters of a
SEA can be evaluated with its use. Experimental results under various
conditions verify that MTT can precisely indicate the limitation of the
performance of SEA, and that it can be utilized to accurately analyze the
limitation of the controller of SEA.
