Ankle Prosthesis Force and Moment Estimation Using Wireless Strain Gauge Pyramid Adapter

Abstract Force and moment measurements are critical for powered ankle-foot prostheses, with over 73% of prototypes incorporating such sensors. Hardware complexity, compatibility limitations, and cost barriers currently limit widespread clinical adoption. This work presents four contributions using the commercial strain gauge based instrumented pyramid adapter Europa+: (1) hardware integration compatible with standard prosthetic components and minimal added bulk (275g, 37.5mm height); (2) an adaptive zero drift compensation algorithm achieving stable calibration within 2-3 steps for continuous long-term operation; (3) physics-informed linear regression models for real-time ankle force and moment estimation; and (4) experimental validation with 8 non-amputee participants using a passive prosthesis, and a preliminary single-participant evaluation with a two-degree-of-freedom (2-DOF) powered ankle-foot prosthesis in variable impedance closed loop operation. Results demonstrate exceptional axial force estimation (RMSE = 47+-20N, R^2 = 0.97+-0.03) and strong dorsi-plantar (DP) moment estimation (RMSE = 5.7+-2.3Nm, R^2 = 0.87+-0.06) across passive prosthesis participants, and these remain valid for the powered prosthesis. Inversion-eversion (IE) moment estimation achieves RMSE = 0.8+-0.2Nm and R^2 = 0.44+-0.16 on passive data, but performance improves when model parameters are derived from powered prosthesis gait with larger active IE range. This approach establishes a generalizable methodology applicable to strain gauge based instrumented pyramid adapters, offering a practical alternative to custom sensors while significantly reducing barriers to clinical implementation of ankle dynamics estimation in prosthetic applications.