Nonlinear Analysis of the Von Mises Truss and Its Application in Assistive Rehabilitation Technologies

Resumo

This work investigates the nonlinear behavior of the Von Mises truss and its applicability in adaptive biomechanical systems, focusing on the development of assistive devices for motor rehabilitation. The Von Mises truss, composed of two articulated bars subjected to large displacements, is a paradigmatic model for studying structural instabilities such as snap-through and bifurcation, with its response strongly influenced by geometric nonlinearity associated with angular variation of the bars. In this study, the truss will be modeled using the Finite Element Method with a geometrically nonlinear positional formulation, adopting the Saint Venant–Kirchhoff constitutive law to describe the material behavior. The modeling will allow for a more accurate assessment of the system's response under different loading conditions, identifying critical transition points and stability regimes. Based on the theoretical and numerical results, the integration of the truss into variable compliance mechanisms in robotic exoskeletons is proposed. The bistable structure of the truss is coupled with flexible pneumatic actuators (PAMs) arranged antagonistically, simulating the action of biological muscles and enabling efficient control of joint stiffness. This integration allows controlled transitions between high and low stiffness states with reduced energy consumption, ensuring safety and performance during use. Experimental tests confirmed the system's effectiveness in assistive applications, highlighting its ability to store and release elastic energy efficiently. The results demonstrate the potential of the Von Mises truss as a key component in adaptive robotic devices for physical rehabilitation.