Design of a three-axis monolithic flexure-based GRF sensor with balanced axial sensitivity for assessment of walking, running, and jumping

Abstract

In this paper, a monolithic flexure-based three-axis ground reaction force (GRF) sensor was developed to analyze various gaits, including walking, running, and jumping. The GRF sensor should measure the extensively large normal force in a compact and light size. Moreover, since the role of shear force is crucial for feature analysis of dynamic gaits, the sensor should be sensitive in shear force significantly small compared to the normal force. Previous studies have attempted to increase the sensitivity for shear force by designing a mechanically decoupled metallic structure. Due to the complicated structure, they have disadvantages in durability, size, weight, and cost. In this paper, a simple circular cross-section flexure was designed to satisfy the challenging design requirements. A monolithic flexure allows the sensor to have broad measurement range and balanced axial sensitivity with increasing durability in a very compact, simple and light structure. The deformation of the flexure under applied force is measured through capacitance changes and converted into the three-axis force. The proposed sensor can measure normal force up to 2400 N and shear force up to 240 N. The performance of the sensor was evaluated through several experiments. An outsole GRF measurement system was designed by embedding two developed sensors. The resultant GRF data during several in-place motions were analyzed with the developed system.