A powered prosthetic intervention for bilateral transfemoral amputees

Abstract

This paper presents the design and validation of a control system for a pair of powered knee and ankle prostheses to be used as a prosthetic intervention for bilateral transfemoral amputees. The control system leverages communication between the prostheses for enhanced awareness and stability, along with power generation at the knee and ankle joints to better restore biomechanical functionality in level ground walking. The control methodology employed is a combination of an impedance-based framework for weight-bearing portions of gait and a trajectory-based approach for the nonweight-bearing portions. The control system was implemented on a pair of self-contained powered knee and ankle prostheses, and the ability of the prostheses and control approach to provide walking functionality was assessed in a set of experimental trials with a bilateral transfemoral amputee subject. Specifically, experimental data from these trials indicate that the powered prostheses and bilateral control architecture provide gait kinematics that reproduce healthy gait kinematics to a greater extent than the subject's daily-use passive prostheses.

Conflict of interest statement

Competing interests: B.E.L., A.H.S., and M.G. hold patent applications through Vanderbilt University that have been licensed to Freedom Innovations, a United States-based prosthetics manufacturer.

Figures

Fig. 1

The pair of powered prostheses used in the experiments.

Fig. 2

The finite state machines executed by the prostheses. θa is the ankle joint angle, which is compared to a predetermined threshold, θth, to trigger the transition into the ankle push off state. Fs is the axial load in the prosthetic shank, which is compared to a predetermined threshold, Fth, to trigger the transition into swing. The swing state executes a trajectory and automatically reverts to the stance state when the trajectory ends, which corresponds to the percentage of stride, ρs, reaching 100%. The red conditions on the transition from State 0 to State 1 are safety conditions that are dependent upon the contralateral signals cρs and cS, which are the contralateral percentage of stride and contralateral state, respectively.

Fig. 3

Healthy subject joint behavior reprinted from [43].

Fig. 4

The approximately linear relationship between knee angle and ankle angle during level walking in healthy subjects between 45% an 60% of stride.

Fig. 5

The three reference trajectories and an estimated trajectory for the swing phase of walking at the knee and ankle joints.

Fig. 6

Joint angles for the right side hip, knee, and ankle for able bodied subjects (reprinted from [43]), the amputee subject using the powered prostheses, and the amputee subject using his daily use, passive prostheses. The knee and ankle joint plots for the powered prostheses include the internally measured joint angles from the powered prosthesis in lighter blue.

Fig. 7

The subject walking with the powered prostheses.

Fig. 8

The powered ankle behavior for a characteristic stride. The top plot shows the axial load in the shank as measured by the onboard magnetic load cell. This signal is roughly proportional to the vertical component of the ground reaction force. The next two plots show the ankle angle and angular velocity, respectively. In the fourth plot, the black line represents the output torque of the ankle given 100% transmission efficiency, and the blue line represents the output torque assuming the transmission friction model described. The final plot shows the estimated power output of the ankle without (black line) and with (blue line) the transmission friction model. The three colored regions of the plots denote the three phases of the controller: stance, push off, and swing. With the transmission friction model, the net energy contribution in the stance phase was −7.67 J, while the net energy contribution in the push off phase was 11.47 J.