Design and Validation of a Semi-Active Variable Stiffness Foot Prosthesis

Abstract

This paper presents the design and validation of a novel lower-limb prosthesis called the Variable-Stiffness Foot (VSF), designed to vary its forefoot stiffness in response to user activity. The VSF is designed as a semi-active device that adjusts its stiffness once per stride during swing phases, in order to minimize size, mass and power consumption. The forefoot keel is designed as an overhung composite beam, whose stiffness is varied by moving a support fulcrum to change the length of the overhang. Stiffness modulation is programmed in response to gait characteristics detected through foot trajectory reconstruction based on an embedded inertial sensor. The prototype VSF has mass of only 649 g including the battery, and build height of 87 mm. Mechanical testing demonstrated a forefoot stiffness range of 10–32 N/mm for the prototype, a threefold range of stiffness variation. The stiffness range can be altered by changing the keel material or geometry. Actuation testing showed that the VSF can make a full-scale stiffness adjustment within three strides, and tracks moderate speed-driven variations within one swing phase. Human subjects testing demonstrated greater energy storage and return with lower stiffness settings. This capability may be useful for modulating prosthesis energy return to better mimic human ankle function. Subjective feedback indicated clear perception by subjects of contrasts among the stiffness settings, including interpretation of scenarios for which different settings may be beneficial. Future applications of the VSF include adapting stiffness to optimize stairs, ramps, turns, and standing.

Use of Dynamic FEA for Design Modification and Energy Analysis of a Variable Stiffness Prosthetic Foot

“…Stiffness change of 51% was demonstrated. Although the change achieved is less than demonstrated in previous work [11,12] it is considerably higher than the reported user perception of 8% [13]. Furthermore, for the given system and boundary conditions, it was presented that the dissipation of a damping element will reach a maximum at a given value of damping coefficient.…”

“…Recent publications demonstrate an increased research interest in prosthetic feet that have adjustable stiffness and the effect of prosthetic stiffness on user in different tasks. Prosthetic feet with modular ankle stiffness have been proposed by Shepard et al [11] and Glanzer et al [12]. These devices alter stiffness by shifting a support in a beam deflection system, allowing a change in forefoot stiffness between steps.…”

The foot and ankle structures reveal emergent properties analogous to passive springs during human walking

“…As our study, and other studies have found [57, 58, 61], the biological limbs have an inherent ability to modulate joint/limb stiffness based on the demands of locomotion. As such, a device that incorporates micro-motors to alter the stiffness characteristics [70, 71] may be a viable solution to replicate salient features of the biological foot and ankle system.…”

A Semi-Powered Ankle Prosthesis and Unified Controller for Level and Sloped Walking

“…In order to provide the adaptability offered by powered prostheses in a potentially smaller, lighter, and quieter package, computer-controlled, primarily passive devices have recently been developed in both the academic and industrial settings [28][29][30][31][32][33]. These primarily passive devices rely on energetically passive mechanisms such as springs and dampers during the load-bearing phases of gait with low-power actuators to modulate the passive parameters of these elements.…”