Soft Sensing Shirt for Shoulder Kinematics Estimation

Soft strain sensors have been explored as an unobtrusive approach for wearable motion tracking. However, accurate tracking of multi degree-of-freedom (DOF) noncyclic joint movements remains a challenge. This paper presents a soft sensing shirt for tracking shoulder kinematics of both cyclic and random arm movements in 3 DOFs: adduction/abduction, horizontal flexion/extension, and internal/external rotation. The sensing shirt consists of 8 textile-based capacitive strain sensors sewn around the shoulder joint that communicate to a customized readout electronics board through sewn micro-coaxial cables. An optimized sensor design includes passive shielding and demonstrates high linearity and low hysteresis, making it suitable for wearable motion tracking. In a study with a single human subject, we evaluated the tracking capability of the integrated shirt in comparison with a ground truth optical motion capture system. An ensemble-based regression algorithm was implemented in post-processing to estimate joint angles and angular velocities from the strain sensor data. Results demonstrated root mean square errors (RMSEs) less than 4.5° for joint angle estimation and normalized root mean square errors (NRMSEs) less than 4% for joint velocity estimation. Furthermore, we applied a recursive feature elimination (RFE)-based sensor selection analysis to down select the number of sensors for future shirt designs. This sensor selection analysis found that 5 sensors out of 8 were sufficient to generate comparable accuracies.