Design and Testing of a Variable Stiffness Transverse Plane Adaptor for Use in a Lower Limb Prosthesis

Abstract

<bold>Background</bold> The goal of a lower limb prosthesis is to restore the abilities of the intact limb for an amputee. Daily ambulation includes many maneuvers such as turning, pivoting, and uncertain terrain, all of which require a component of transverse plane mobility. It has been shown that the addition of a transverse plane adaptor can help to decrease soft tissue damage, increase mobility, and help reduce the risk of falls in amputees. However, currently available transverse plane adaptors only allow for a single stiffness setting and do not allow for variation to accommodate the maneuvers of everyday ambulation. The aim of this research was to design, build, and test a prototype lower limb prosthetic adaptor that is capable of variable stiffness in the transverse plane. The device will be used to better understand the role transverse stiffness plays in varying daily ambulation activities such as walking and turning. <bold>Design</bold> A variable stiffness torsion device (VSTA) was designed, and built and is capable of controlling stiffness in the transverse plane of a lower limb prosthesis. Design criteria were established to determine structural and functional requirements for the VSTA. A custom spring was designed with a rate of 0.33 Nm/° allowing for VSTA settings between 0.10-1.17 Nm/°, and includes a locked infinitely stiff setting. Refinement of the design was then conducted using a mathematical model, finite element analysis (FEA), and analysis of VSTA kinematics. Following design completion, a prototype was built and tested. <bold>Mechanical Testing</bold> Mechanical bench testing was performed to determine the physical capabilities of the VSTA. The VSTA is actually capable of infinite stiffness variation between 0.12-0.91 Nm/°. Initial designs accounted for the internal spring to be capable of 90° of deflection which would have allowed for infinite stiffness variation between the minimum and infinitely stiff, however, internal spring stresses limited spring deflection to 57°, resulting in the limited range of the VSTA. The bench testing showed that the VSTA as designed and manufactured would be suitable for human subjects testing. <bold>Controller Testing</bold> A proportional-integral-derivative (PID) controller, provided by the motor manufacturer, was used to control step inputs of the spring carrier to adjust the stiffness of the VSTA. The controller, using a 16 volt supply, could accurately perform step inputs, but could not meet rate of performance design goals. When supply voltage was brought up to the ideal 24 volts the controller was able to meet the rate goals of the VSTA, however, over power faults occurred resulting in incomplete step controls. Use of a different control module that would allow for the full 24 volt supply to the motor and with consideration of the reduced functional range of the VSTA it is estimated that the controller would meet all design goals. <bold>Future Work</bold> Structurally the VSTA needs to be improved beyond the factor of safety of one to provide a more robust solution. Mechanical improvements include increasing VSTA ability to apply and sustain higher stiffness settings without being limited by overstress of the spring. It would also be beneficial to test the VSTA for use as an active stiffness generator, modulating stiffness while under load. Additionally, the controller could be developed to better optimize the gains, and possibly increase speed and performance of the system. Lastly, human subjects testing should be conducted to better evaluate the VSTA's ability to function in a real setting, as well as to better understand the role of transverse plane stiffness during ambulation.