Validation of an Instrumented Walkway Designed for Estimation of the Ankle Impedance in Sagittal and Frontal Planes

Recently, the authors designed and fabricated an Instrumented Walkway for the estimation of the ankle mechanical impedance in the sagittal and frontal planes during walking in arbitrary directions [1]. It consists of a powered platform; therefore, the users do not need to wear or carry any measurement device or actuation system other than reflective markers used to record the ankle kinematics with a motion capture camera system. This paper describes the continuous development of the Instrumented Walkway and presents an experimental preliminary validation of its capability to estimate the impedance of a system with time-varying dynamics. To validate the system, a mockup with mechanical characteristics similar to a human lower-leg and controllable time-varying stiffness was used. The stiffness of the mockup was estimated with fixed and time-varying stiffness. With fixed stiffness, a stochastic system identification method was used to estimate the mockup’s impedance. When the mockup presented a timevarying stiffness, a second order parametric model was used. The RMS error between the two methods was 2.81 Nm/rad (maximum 4.12 Nm/rad and minimum of -3.41 Nm/rad). The results show that the proposed approach can estimate the stiffness of systems with time-varying dynamics or static dynamics with similar accuracy. Since the setup was already validated for systems with time-invariant dynamics, it concluded the system’s applicability for time-varying systems such as the human ankle-foot during the stance phase. INTRODUCTION The estimation of the mechanical impedance of the human ankle has been the focus of research since it provides insight on the mechanical properties of the ankle and how it interacts with the environment. The ankle is the first major joint to interact with the ground during walking and it plays a major role in propulsion and stability, while generating torques and angles in all anatomical planes. The impedance of the ankle continually changes during walking to accomplish its task, and the way it changes depends on the type of gait. The impedance of a system correlates the output torque due to an input disturbance, and for a second order system, it is a function of the system’s mass, damping, and inertia. The human ankle is capable of impedance modulation by muscle contraction of the muscles involved on its operation. Impedance controllers are often used in prosthesis, as it allows the device to mimic the mechanical properties of the human limb. For an ankle-foot prosthesis, the quasi-static impedance of the ankle in the sagittal plane has been used [2-4]; however, for proper control of the prosthesis, there is strong evidence that time-varying and task-dependent impedance modulation of the ankle is necessary [5, 6]. Activities of daily living (ADLs) include gait scenarios that require significant modulation of the ankle impedance in all anatomical planes, mainly in Inversion-Eversion (IE) and DorsiflexionPlantarflexion (DP) [5, 6]. These activities include but are not limited to turning, traversing slopes, and adapting to uneven Proceedings of the ASME 2016 Dynamic Systems and Control Conference DSCC2016 October 12-14, 2016, Minneapolis, Minnesota, USA