The VSPA Foot: A Quasi-Passive Ankle-Foot Prosthesis With Continuously Variable Stiffness

Abstract

Most commercially available prosthetic feet do not exhibit a biomimetic torque-angle relationship, and are unable to modulate their mechanics to assist with other mobility tasks, such as stairs and ramps. In this paper, we present a quasi-passive ankle-foot prosthesis with a customizable torque-angle curve and an ability to quickly modulate ankle stiffness between tasks. The customizable torque-angle curve is obtained with a cam-based transmission and a fiberglass leaf spring. To achieve variable stiffness, the leaf spring's support conditions can be actively modulated by a small motor, shifting the torque-angle curve to be more or less stiff. We introduce the design, characterize the available torque-angle curves, and present kinematics from a transtibial amputee subject performing level-ground walking, stair ascent/descent, and ramp ascent/descent. The subject exhibited a more normative range of motion on stairs and ramps at lower stiffness levels, and preferred different stiffness levels for each task. Paired with an appropriate intent recognition system, our novel ankle prosthesis could improve gait biomechanics during walking and many other mobility tasks.

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.…”

Development of a Passive Ankle Foot Prosthesis with Manually Adjustable Ankle Stiffness

“…This change in the stiffness of the spring will result in a corresponding change in ankle stiffness as described by Shepherd and Rouse [10]. The distance x, moved by the slider support upon rotating the screw through an angle θ, is given by…”

“…The changing of the ankle stiffness is based on a system of a cam transmission and a leaf spring whose support can be altered [10]. In this current design, the adjustment mechanism is based on a lead screw which is used to move a support slider for the leaf spring.…”

“…Thus, a device whose stiffness properties can be altered will be of great value to the amputees. To this end, Shepherd and Rouse [10], developed a quasi-passive prosthetic foot, whose stiffness can be altered for different mobility tasks, with the use of a control system and motor. The work of Shephered and Rouse is adopted in this paper, with the aim of developing a passive ankle-foot prosthesis, whose stiffness can be adjusted manually, thereby not requiring controllers or motors that increase the cost, as well as the technical requirements for fitting and maintenance of the device.…”