External Force Estimation in Linear Series Elastic Actuator without Load-side Encoder

This article presents a novel estimation method of the external force in linear series elastic actuators (SEAs). Conventionally, such force is estimated through the deflection of an elastic element, placed between motor and end-effector in SEAs (measured using two encoders). However, in some applications (e.g., wearable devices), it is difficult to mount a load-side encoder, due to cost or manufacturing issue. Moreover, methods based on encoders have limitations in terms of bandwidth, and, in turn, in estimating fast-varying impact forces. Additionally, SEAs may have a nonnegligible end-effector mass, therefore gravity (or accelerations of the actuator support) may add a contribution to the external force, not accounted by deflection-based estimation methods. To address these issues, we propose a method based on the Kalman filter in combination with the use of two MEMS accelerometers, mounted on the load-side and on the support of the SEA, for external force estimation and gravity (or support acceleration) compensation, respectively. We tested the proposed method even in a reduced version, with a single load-side accelerometer. Finally, it is shown that using the acceleration signal also improves the robustness against spring stiffness variations, compared to conventional methods. The experimental results show the effectiveness of the proposed methods.