Development of an FEM model of a Spherical Dummy Joint-Soft Actuator Complex for Evaluation of Soft Actuators

In recent years, soft robotic rehabilitation devices have increased as alternative devices for recovering hand functions in stroke patients. In the research and development of soft rehabilitation hand gloves, measuring the joint load is an essential step for designing, evaluating performance, and ensuring the safety of soft actuators. Considering that soft actuators behave nonlinearly, the Finite Element Method (FEM) has been applied for its analysis. One important aspect is to develop FEM models considering the interaction between fingers and soft actuators. In our previous study, we proposed a simplified joint structure in the form of a pin joint (pin dummy joint-soft actuator complex). However, it is necessary to progress into geometry-closer anatomical structures to appropriately measure joint loads. In this study, we proposed a spherical dummy joint with spherical contact between phalanx bones based on human finger joint anatomy. Both the simulation (FEM model) and physical devices of the spherical dummy joint-soft actuator complex were built, as well as their pin dummy joint counterparts. The joint stress results of the dummy joint FEM models were also compared to verify which model was more accurate based on the joint stresses verified in previous studies. The FEM models were also validated in terms of bending angle by performing tests on corresponding physical devices. As a result, the FEM model of the spherical dummy joint model showed better agreement than the pin dummy joint model, and the dummy joint FEM models showed better agreement with the physical model with respect to bending angles. Furthermore, bending performance evaluation and joint stress evaluation were conducted using the proposed FEM model of dummy finger and soft actuator complex.