Closed-loop System Identification of Ankle Dynamics With Compliant Loads

Joint stiffness, defined as the relation between the angular position of a joint and the torque acting about it, can be used to describe the dynamic behavior of the human ankle during posture and movement. Joint stiffness can be separated into intrinsic stiffness and reflex stiffness, which are modeled as a linear system and a LNL system, respectively. With a compliant load, joint stiffness can be viewed as being operated in closed-loop because the torque is fed back through the load to change the position. In this paper, we present a new method to estimate the intrinsic and reflex stiffness from the total torque measurement. An EIV (errors-in-variables) subspace system identification method is used to estimate the dynamics of each pathway directly from measured data. Simulation and experiment studies demonstrate that the method produces accurate results.