Design, Characterization, and Validation of a Variable Stiffness Prosthetic Elbow

TL;DR

This work addresses the need for prosthetic elbows with user-controllable impedance by introducing two variable stiffness elbow architectures that use nonlinear elastic transmissions and dual-motor actuation. The VS-Elbow AA concentrates components below the elbow to fit distal transhumeral cases, while the VS-Elbow D2 distributes mass across forearm and upper arm for comfort in proximal amputations. Both designs meet key targets (ROM 0°–120°, stiffness 2–60 Nm/rad, lift ~3 kg) and exhibit substantial weight reductions compared with prior VSAs, along with validated kinematics, stiffness modulation, and safety in case studies. The study also develops a posture-compensation control and demonstrates EMG-based impedance modulation, highlighting the potential of VSAs to improve safety, adaptability, and quality of life for prosthesis users, while outlining avenues for UI improvements and user-centered evaluations.

Abstract

Intuitively, prostheses with user-controllable stiffness could mimic the intrinsic behavior of the human musculoskeletal system, promoting safe and natural interactions and task adaptability in real-world scenarios. However, prosthetic design often disregards compliance because of the additional complexity, weight, and needed control channels. This paper focuses on designing a Variable Stiffness Actuator (VSA) with weight, size, and performance compatible with prosthetic applications, addressing its implementation for the elbow joint. While a direct biomimetic approach suggests adopting an Agonist-Antagonist (AA) layout to replicate the biceps and triceps brachii with elastic actuation, this solution is not optimal to accommodate the varied morphologies of residual limbs. Instead, we employed the AA layout to craft an elbow prosthesis fully contained in the user's forearm, catering to individuals with distal transhumeral amputations. Additionally, we introduce a variant of this design where the two motors are split in the upper arm and forearm to distribute mass and volume more evenly along the bionic limb, enhancing comfort for patients with more proximal amputation levels. We characterize and validate our approach, demonstrating that both architectures meet the target requirements for an elbow prosthesis. The system attains the desired 120° range of motion, achieves the target stiffness range of [2, 60] Nm/rad, and can actively lift up to 3 kg. Our novel design reduces weight by up to 50% compared to existing VSAs for elbow prostheses while achieving performance comparable to the state of the art. Case studies suggest that passive and variable compliance could enable robust and safe interactions and task adaptability in the real world.

Figures (13)

• Figure 1: The presented variable stiffness elbow and distinct applications in transhumeral prostheses. (a) The bionic limb on the left adopts the VS-Elbow AA to locate all components below the elbow joint, thus being suitable for distal transhumeral amputations. The prosthesis on the right employs the VS-Elbow D2 to achieve a sparser mass distribution, enhancing user comfort in more proximal transhumeral amputations. Panel (b) highlights the two implementations of the variable stiffness elbow: the VS-Elbow AA on the left and the VS-Elbow D2 on the right.
• Figure 2: Schematic representation of diverse VSA architectures and their potential implementation in a VS elbow prosthesis. All the presented architectures feature a non-linear elastic transmission and two motors ($M_1, M_2$) to modulate the elbow joint ($J_e$) stiffness and position by exploiting redundant actuation. Scheme (a) represents the Agonist-Antagonist architecture, whose functioning is inspired by the human musculoskeletal system. Scheme (b) displays the classical implementation of the independent setup, where each motor independently regulates either the position or the stiffness of the elbow joint. Scheme (c) illustrates a novel layout of the independent setup shown in (b), where the position and stiffness motors are placed on opposite elbow segments. Panel (d) shows scheme (a) most intuitive implementation, where the elastic actuation is placed in the upper arm to replace the biceps and triceps brachii biomimetically. The implementation represented in panel (e) leverages scheme (c) to achieve homogeneous mass distribution, thus enhancing user comfort. Panel (f) displays the alternative fitting of the AA implementation that allocates all the components in the user's forearm, thus being suitable for distal transhumeral amputations.
• Figure 3: Different implementations of the elastic transmission mechanism utilized in the VS-Elbow AA (a) and VS-Elbow D2 (b), and working principles of the VS-Elbow AA (c to e) and VS-Elbow D2 (f to h). To regulate the stiffness of the elastic transmission, the system modulates the active length of the belt (in blue), which adheres to the idle pulley (in red). As a result, the lever arm mechanism (in orange) rotates, changing the preload of linear extension springs (in green). Panels (c) and (f) show the system in neutral configuration. Panels (d) and (g) illustrate the regulation of the output position, while panels (e) and (h) display the stiffness modulation.
• Figure 4: CAD views of the two versions of the presented system. Panel (a) portrays the VS-Elbow AA, and panel (b) shows the VS-Elbow D2. The most significant components are highlighted in colors: yellow for worm drives, purple for DC motors, green for springs, red for idle pulleys, and orange for tensioning levers.
• Figure 5: Block diagram schematizing the control strategy implemented.