Early Results in the Development of the Prisma Hand III: Design Solutions for Stiffness Variation

The human hand exhibits remarkable adaptability and dexterity, primarily due to its ability to adjust stiffness according to task requirements. However, traditional prosthetic hands often lack this capability, limiting their effectiveness in dynamic and unstructured environments. This paper presents the design of a variable stiffness robotic hand for prosthetic applications, developed as an enhancement of the existing and functional PRISMA Hand II prosthesis. The primary objective is to introduce a resilient prosthetic hand capable of modulating contact force with external objects by adjusting finger joints stiffness. The proposed design follows current trends in robotic hand development, adopting a simplified architecture and a cable-driven transmission system. This approach enhances dexterity and environmental adaptability while maintaining a lightweight and cost-effective structure. The prototype includes an agonist-antagonist cable-driven actuation system, enabling stiffness variation. All actuation components, including cables, motors, and supporting elements, are integrated within the palm. The actuation system is composed of four motors: one dedicated to thumb adduction, one for thumb flexion, one for flexion of the other fingers, and one specifically for stiffness modulation. Early experimental results validate the effectiveness of the stiffness variation mechanism.