  In assistive robots, compliant actuator is a key component in establishing
safe and satisfactory physical human-robot interaction (pHRI). The performance
of compliant actuators largely depends on the stiffness of the elastic element.
Generally, low stiffness is desirable to achieve low impedance, high fidelity
of force control and safe pHRI, while high stiffness is required to ensure
sufficient force bandwidth and output force. These requirements, however, are
contradictory and often vary according to different tasks and conditions. In
order to address the contradiction of stiffness selection and improve
adaptability to different applications, we develop a reconfigurable rotary
series elastic actuator with nonlinear stiffness (RRSEAns) for assistive
robots. In this paper, an accurate model of the reconfigurable rotary series
elastic element (RSEE) is presented and the adjusting principles are
investigated, followed by detailed analysis and experimental validation. The
RRSEAns can provide a wide range of stiffness from 0.095 Nm/deg to 2.33 Nm/deg,
and different stiffness profiles can be yielded with respect to different
configuration of the reconfigurable RSEE. The overall performance of the
RRSEAns is verified by experiments on frequency response, torque control and
pHRI, which is adequate for most applications in assistive robots.
Specifically, the root-mean-square (RMS) error of the interaction torque
results as low as 0.07 Nm in transparent/human-in-charge mode, demonstrating
the advantages of the RRSEAns in pHRI.
