Design of a Digital Triaxial Force Sensor for Plantar Load Measurements

Measurement of load information on the plantar (lower) surface of the foot can provide valuable insights to help identify pathologies like diabetic foot ulcers. Studies have shown that both plantar pressure and shear stress play an important role in foot disorders, especially ulcer formation. However, in this context shear stress is much less studied in comparison with pressure distribution, mainly due to the lack of reliable measurement technologies. In this paper, we propose a triaxial force sensor for measuring plantar loading. The sensor consists of an array of sensing coils combined with an elastomeric spacer and a conductive target. Under loading, the sensor demonstrates differential variations in inductance which are digitized by built-in conditioning circuitry and decoupled. A 3D finite element (FE) model was developed for the system as a design tool. This was validated experimentally and demonstrated a high agreement to the results. In experimental evaluation with multiaxial loading the sensor showed precise operation over the operating range (RMSE: 0.05 N for shear (-1.5 N - 1.5 N) and 0.70 N for normal force (0-13 N) measurements). The FE model was then used to investigate the effect of undesirable tilting of the target. The results indicated that it is important to minimize the tilting of the target for robust operation in real-world scenarios.