A Bench-top Prototype of a Variable Stiffness Prosthesis

A prototype of a variable stiffness prosthetic joint has been constructed and tested. The joint is based on two actuator subsystems arranged in an antagonistic configuration. Each actuator subsystem is composed of a small, high-speed electric motor, a single stage, worm-gear based transmission and a nonlinear stiffness element. Each nonlinear stiffness element is composed of a set of short sections of elastic tube that was chosen based on its stiffness characteristics. The system is powered by batteries and can be controlled in a number of ways. The paper will present the basis used for selecting the nonlinear stiffness elements, the details of the design as constructed and a comparison of the actual performance of the prototype to predictions based on design calculations and simulations. The prototype has performance that is comparable to commercially available prostheses and showed good correspondence between simulation and prototype. The prototype was able to lift a 2 kg load through 135 degrees in 1.42 seconds and to vary its stiffness from 14 to 24 Nm/rad. INTRODUCTION A variable stiffness elbow prosthesis should look and feel more natural and allow the amputee to perform constrained or contact movements with the arm at a low stiffness, and fast, precision movements at a high stiffness. Previous research suggests that it should simplify control issues, provide increased dexterity and allow greatly improved energy efficiency when interacting with the environment [1, 2, 3, 4, 5]. Antagonistic nonlinear compliant actuators acting about the joint provide stiffness variation capabilities. A nonbackdrivable transmission allows external forces to be statically supported without using the motors. A variable stiffness prosthetic arm bench top prototype was designed, constructed, mathematically modelled and tested. The configuration consists of two actuators arranged antagonistically around the elbow joint. Each actuator is made up of a dc motor, gearhead, nonbackdrivable transmission, nonlinear spring and two pulleys. Figures 1 and 2 show the resulting prototype. Figure 1: Bench top prototype 1 From “MEC '05 Intergrating Prosthetics and Medicine,” Proceedings of the 2005 MyoElectric Controls/Powered Prosthetics Symposium, held in Fredericton, New Brunswick, Canada, August 17-19, 2005. Distributed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License by UNB and the Institute of Biomedical Engineering, through a partnership with Duke University and the Open Prosthetics Project. Figure 2: Bench top prototype motor, transmission and spring