An ankle-foot prosthesis emulator with control of plantarflexion and inversion-eversion torque

Abstract

Ankle inversion-eversion compliance is an important feature of conventional prosthetic feet, and control of inversion, or roll, in robotic prostheses could improve balance for people with amputation. We designed a tethered ankle-foot prosthesis with two independently-actuated toes that are coordinated to provide plantarflexion and inversion-eversion torques. This configuration allows a simple lightweight structure with a total mass of 0.72 kg. Strain gages on the toes measure torque with less than 2.7% RMS error, while compliance in the Bowden cable tether provides series elasticity. Benchtop tests demonstrated a 90% rise time of less than 33 ms and peak torques of 180 N·m in plantarflexion and ±30 N·m in inversion-eversion. The phase-limited closedloop torque bandwidth is 20 Hz with a 90 N·m amplitude chirp in plantarflexion, and 24 Hz with a 20 N·m amplitude chirp in inversion-eversion. The system has low sensitivity to toe position disturbances at frequencies of up to 18 Hz. Walking trials with five values of constant inversion-eversion torque demonstrated RMS torque tracking errors of less than 3.7% in plantarflexion and less than 5.9% in inversion-eversion. These properties make the platform suitable for haptic rendering of virtual devices in experiments with humans, which may reveal strategies for improving balance or allow controlled comparisons of conventional prosthesis features. A similar morphology may be effective for autonomous devices.

Increasing ankle push-off work with a powered prosthesis does not necessarily reduce metabolic rate for transtibial amputees

“…This might help explain findings of powered ankle-foot prostheses not reducing metabolic rate during the work-intensive task of walking uphill (Esposito et al, 2015). Complementarily, the energy cost of walking can be reduced with entirely passive devices (Collins et al, 2015). These results conflict with the hypothesis that augmentation arises in proportion to net energy input, offset by the cost of carried mass (Mooney et al, 2014).…”

Step-to-Step Ankle Inversion/Eversion Torque Modulation Can Reduce Effort Associated with Balance

“…By powering with two off-board motors, this testbed had a worn mass of only 0.72 kg, but it presented a plantarflexion torque of up to 180 N·m and an ankle inversion/eversion torque of up to ± 30 N·m, with a peak power of up to 3 kW. Ankle inversion/eversion torque limits were coupled to plantarflexion torque magnitude, but during most of the stance phase the allowable inversion/eversion torque was higher than seen in the biological ankle (Collins et al, 2015 ). The prosthesis also had a closed-loop torque bandwidth of higher than 20 Hz.…”

“…We used an ankle-foot prosthesis with control of both plantarflexion and inversion/eversion torques to test the effects of once-per-step inversion/eversion torque modulation on below-knee amputees (Figure 1A ). This device had two independently actuated toes (described in detail in Collins et al, 2015 ). The mechanism provided inversion torque when the force of the outer toe was higher than that of the inner toe.…”

Informing ankle-foot prosthesis prescription through haptic emulation of candidate devices

“…It is likely that including passive compliance in the structure of the prosthesis, comparable to what is provided by a DER prosthesis, will improve emulation quality significantly. We are also exploring prosthesis designs with additional controlled degrees of freedom [18] to capture differences across device type. Third, robotic feet with programmable behavior, e.g.…”