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. 2021 Aug:2021:627-633.
doi: 10.1109/ccta48906.2021.9658844. Epub 2022 Jan 3.

Toward Phase-Variable Control of Sit-to-Stand Motion with a Powered Knee-Ankle Prosthesis

Daphna Raz  1 , Edgar Bolívar-Nieto  1 , Necmiye Ozay  1   2 , Robert D Gregg  1   2
Affiliations

Toward Phase-Variable Control of Sit-to-Stand Motion with a Powered Knee-Ankle Prosthesis

Daphna Raz et al. Control Technol Appl. 2021 Aug.

Abstract

This paper presents a new model and phase-variable controller for sit-to-stand motion in above-knee amputees. The model captures the effect of work done by the sound side and residual limb on the prosthesis, while modeling only the prosthetic knee and ankle with a healthy hip joint that connects the thigh to the torso. The controller is parametrized by a biomechanical phase variable rather than time and is analyzed in simulation using the model. We show that this controller performs well with minimal tuning, under a range of realistic initial conditions and biological parameters such as height and body mass. The controller generates kinematic trajectories that are comparable to experimentally observed trajectories in non-amputees. Furthermore, the torques commanded by the controller are consistent with torque profiles and peak values of normative human sit-to-stand motion. Rise times measured in simulation and in non-amputee experiments are also similar. Finally, we compare the presented controller with a baseline proportional-derivative controller demonstrating the advantages of the phase-based design over a set-point based design.

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Figures

Fig. 1.
Fig. 1.
Depiction of the three phases of the sit-to-stand movement. Figure adapted from [22].
Fig. 2.
Fig. 2.
Three link model of the prosthesis side of a person with amputation performing sit-to-stand. θ1, θ2, θ3, and s correspond to the ankle, knee, hip, and thigh angle, respectively F denotes the approximation of the effects of the sound side, and u1 and u2 denote the prosthesis controller inputs. Under this angle convention, positive knee torque matches knee flexion and positive ankle torque is equal to ankle dorsiflexion. The relationship between s, θ1, and θ2 is illustrated in the bottom third of the figure.
Fig. 3.
Fig. 3.
Normative sit-to-stand kinematics measured in an able-bodied subject. The thigh angle decreases monotonically, making it suitable to be used as a phase variable.
Fig. 4.
Fig. 4.
Plot of sit-to-stand data collected from non-amputees. Individual trials are plotted in gray, while the computed nominal trajectory (mean) is in red. The line of best fit is in black. RMSE for this linear fit was 4.03°.
Fig. 5.
Fig. 5.
Mean torques and powers from the phase-variable controller (PV) for both the knee and ankle, with normative reference curves from a human subject dataset [28]. The dataset includes an unexplained torque offset at standing in the ankle torque. The gray shaded area spans ± one standard deviation across simulated trials.
Fig. 6.
Fig. 6.
Mean torques and powers from the set-point controller (SP) for both knee and ankle, with normative reference curves from human data [28]. Gray shaded area spans ± one standard deviation across simulated trials.
Fig. 7.
Fig. 7.
Representative kinematics of prosthesis with SP and PV control, and corresponding kinematics from one trial. All sets of kinematic trajectories begin from the same initial condition. An ankle angle offset at standing, which is a typical source of error for the PV controller, can be observed. In contrast, the error of the SP controller is consistent throughout the trial.
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