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.
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