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Chiu, V. L., Voloshina, A. S., Collins, S. H. (2019) An ankle-foot prosthesis emulator capable of modulating center of pressure. Transactions on Biomedical Engineering, in press.
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A tethered ankle-foot prosthesis emulator consisting of two forefoot digits and a heel digit. Independent control of the digits modulates the ground reaction force and center of pressure, which can be used to improve balance. The device is actuated by offboard motors and computers and connects to the user with a standard prosthesis adapter and pylon. Click for high-resolution version. Image credit: Vince Chiu.


Prosthesis with independent control of three digits. Several powered ankle-foot prostheses have demonstrated moderate reductions in energy expenditure by restoring pushoff work in late stance or by assisting with balance. However, it is possible that center of pressure trajectory modulation could provide even further improvements in user performance. This work describes the design of a prosthesis emulator with two torque-controlled forefoot digits and a torque-controlled heel digit. Independent actuation of these three digits can modulate the origin and magnitude of the total ground reaction force vector. The emulator was designed to be compact and lightweight while exceeding the range of motion and torque requirements of the biological ankle during walking. We ran a series of tests to determine torque measurement accuracy, closed-loop torque control bandwidth, torque tracking error, and center of pressure control accuracy. Each of the three digits demonstrated less than 2 Nm of RMS torque measurement error, a 90% rise time of 19 ms, and a bandwidth of 33 Hz. The untethered end-effector has a mass of 1.2 kg. During walking trials, the emulator demonstrated less than 2 Nm of RMS torque tracking error and was able to maintain full digit ground contact for 56% of stance. In fixed, standing, and walking conditions, the emulator was able to control center of pressure along a prescribed pattern with RMS errors of about 10% the length of the pattern, similar in performance to human control. This emulator system will enable rapid development of controllers designed to enhance user balance and reduce user energy expenditure. Experiments conducted using this emulator could identify beneficial control behaviors that can be implemented on untethered devices, thus improving mobility and quality of life of individuals with amputation. Click for high-resolution version. Image credit: Vince Chiu.


Prosthesis with independent control of three digits. Difficulty balancing is one of the most commonly reported challenges following lower limb amputation. Active prosthetic limbs could be used to assist with balance by better controlling interactions with the ground. This work describes the design of a prosthesis emulator with two forefoot digits and a heel digit. Independent actuation of these digits modulates the origin and magnitude of the ground reaction force vector. During standing and walking, the emulator was able to control center of pressure along a prescribed pattern with errors of about 10% of pattern length, similar to human control. Click for high-resolution version. Image credit: Vince Chiu.


Prosthesis with independent control of three digits. Vertical position, sagittal plane orientation and frontal plane orientation can all be changed independently. Click for high-resolution version. Image credit: Vince Chiu.