Session: 07-05-01: Biomedical Devices, Sensors, and Actuators

Paper Number: 198367

Design and Dynamic Modeling of a Bio-Inspired Compliant Elbow Joint Actuated by Shape Memory Alloy 

Upper limb prosthetic and orthotic devices require actuation strategies that balance lightweight design, sufficient force output, and structural compliance for safe human interaction. Conventional actuators such as electric motors and hydraulic systems are often bulky, noisy, and poorly suited to replicating the smooth, adaptive motion of biological joints. Shape Memory Alloy (SMA) actuators have emerged as an attractive alternative due to their compact size, silent operation, high force-to-weight ratio, and muscle-like actuation characteristics. However, the relatively limited stroke and nonlinear force–displacement characteristics of SMA springs often require an appropriate mechanical transmission or compliant joint mechanism to efficiently convert the actuator output into smooth, biomimetic elbow motion while reducing stress concentrations and improving energy efficiency. Furthermore, the nonlinear thermo-mechanical behaviour and hysteresis associated with SMA materials make the development of accurate dynamic models for wearable prosthetic systems particularly challenging. Limited work has investigated the integration of compliant joints with SMA actuation within a unified physics-based modeling framework. This work presents the design, fabrication, assembly and dynamic modeling of a bio-inspired single link manipulator which incorporates a compliant elbow joint actuated by SMA springs. The joint is 3D printed using flexible thermoplastic polyurethane (TPU 95A) via fused deposition modeling, based on an auxetic-inspired metamaterial architecture. Triangular truss reinforcements are incorporated on one side to introduce directional stiffness, giving the joint a compliant low-stiffness side that permits large deformation during flexion and a reinforced high-stiffness side that resists deformation to prevent hyperextension, replicating the asymmetric stiffness of the human elbow. The stiffness contrast between the two sides is validated through quasi-static finite element analysis in ANSYS and compression testing. Within the operating range, the joint response is linear, allowing it to be represented in the dynamic model by a single equivalent torsional stiffness which is obtained  through quasi-static finite element analysis in ANSYS, relating applied angular displacement to reaction moment. A proof-of-concept prototype is assembled to demonstrate the physical integration of the compliant joint with forearm and upper-arm links along with the SMA actuation system. Actuation is achieved using a pair of SMA springs mounted unilaterally, with flexion driven by thermally induced phase transformation upon electrical activation and extension achieved passively through the elastic restoring force of the compliant structure, eliminating the need for a second antagonistic actuator. A phenomenological dynamic model is developed by adapting the Liang–Rogers constitutive framework, with extensions inspired by the Brinson formulation, to capture the coupled thermo-mechanical response of SMA springs under combined torsional and axial loading. The complete model integrates SMA constitutive behaviour, thermal dynamics, and joint mechanics into a unified simulation framework implemented in MATLAB, where the input supplied is the current and output is the torque and angular deflection. Open-loop simulation results show the joint achieving an angular motion range of approximately 0° to 70°, consistent with the physical range of the fabricated mechanism. Comparison against experimental actuation data shows agreement in the overall transformation trend and settling behaviour, indicating that the model captures the coupled thermo-mechanical response of the SMA-driven compliant joint and provides a reliable basis for subsequent closed-loop control development.

Presenting Author: Arsala Nissar Malik Indian Institute of Technology, Kanpur

Presenting Author Biography: Arsala Nissar Malik is a Ph.D. scholar in the Department of Mechanical Engineering at the Indian Institute of Technology Kanpur, India. Her research focuses on the design and dynamic modeling of Shape Memory Alloy (SMA)-actuated compliant mechanisms for upper-limb prosthetic applications. Her research interests include smart materials, biomechanics, wearable assistive devices and rehabilitation robotics,.

Authors:

Arsala Nissar Malik Indian Institute of Technology, Kanpur
Akshat Nama Indian Institute of Technology, Kanpur
Kanhaiya Lal Chaurasiya Indian Institute of Technology, Kanpur
Bishakh Bhattacharya Indian Institute of Technology, Kanpur