ANTHROPOMORPHIC ROBOTIC ANKLE-FOOT PROSTHESIS WITH ACTIVE DORSIFLEXION- PLANTARFLEXION AND INVERSION-EVERSION

The main goal of the research presented in this paper is the development of a powered ankle-foot prosthesis with anthropomorphic characteristics to facilitate turning, walking on irregular grounds, and reducing secondary injuries on bellow knee amputees. The research includes the study of the gait in unimpaired human subjects that includes the kinetics and kinematics of the ankle during different types of gait, in different gait speeds at different turning maneuvers. The development of a robotic ankle-foot prosthesis with two active degrees of freedom (DOF) controlled using admittance and impedance controllers is presented. Also, a novel testing apparatus for estimation of the ankle mechanical impedance in two DOF is presented. The testing apparatus allows the estimation of the time-varying impedance of the human ankle in stance phase during walking in arbitrary directions. The presented work gives insight on the turning mechanisms of the human ankle and how they can be mimicked by the prosthesis to improve the gait and agility of below-knee amputees

ANKLE IMPEDANCE AND ANKLE ANGLES DURING STEP TURN AND STRAIGHT WALK: IMPLICATIONS FOR THE DESIGN OF A STEERABLE ANKLE-FOOT PROSTHETIC ROBOT

During locomotion, turning is a common and recurring event which is largely neglected in the current state-of-the-art ankle-foot prostheses, forcing amputees to use different steering mechanisms for turning, compared to non-amputees. A better understanding of the complexities surrounding lower limb prostheses will lead to increased health and well-being of amputees. The aim of this research is to develop a steerable ankle-foot prosthesis that mimics the human ankle mechanical properties. Experiments were developed to estimate the mechanical impedance of the ankle and the ankles angles during straight walk and step turn. Next, this information was used in the design of a prototype, powered steerable ankle-foot prosthesis with two controllable degrees of freedom. One of the possible approaches in design of the prosthetic robots is to use the human joints’ parameters, especially their impedance. A series of experiments were conducted to estimate the stochastic mechanical impedance of the human ankle when muscles were fully relaxed and co-contracting antagonistically. A rehabilitation robot for the ankle, Anklebot, was employed to provide torque perturbations to the ankle. The experiments were performed in two different configurations, one with relaxed muscles, and one with 10% of maximum voluntary contraction (MVC). Surface electromyography (sEMG) was used to monitor muscle activation levels and these sEMG signals were displayed to subjects who attempted to maintain them constant. Time histories of ankle torques and angles in the lateral/medial (LM) directions, inversion-eversion (IE), and dorsiflexionplantarflexion (DP) were recorded. Linear time-invariant transfer functions between the measured torques and angles were estimated providing an estimate of ankle mechanical impedance. High coherence was observed over a frequency range up to 30 Hz. The main effect of muscle activation was to increase the magnitude of ankle mechanical impedance in all degrees of freedom of the ankle. Another experiment compared the three-dimensional angles of the ankle during step turn and straight walking. These angles were measured to be used for developing the control strategy of the ankle-foot prosthesis. An infrared camera system was used to track the trajectories and angles of the foot and leg. The combined phases of heel strike and loading response, mid stance, and terminal stance and pre-swing were determined and used to measure the average angles at each combined phase. The Range of motion (ROM) in IE increased during turning while ML rotation decreased and DP changed the least. During the turning step, ankle displacement in DP started with similar angles to straight walk and progressively showed less plantarflexion. In IE, the ankle showed increased inversion leaning the body toward the inside of the turn. ML rotation initiated with an increased medial rotation during the step turn relative to the straight walk transitioning to increased lateral rotation at the toe off. A prototype ankle-foot prosthesis capable of controlling both DP and IE using a cable driven mechanism was developed and assessed as part of a feasibility study. The design is capable of reproducing the angles required for straight walk and step turn; generates 712N of lifting force in plantarflexion, and shows passive stiffness comparable to a nonload bearing ankle impedance. To evaluate the performance of the ankle-foot prosthesis, a circular treadmill was developed to mimic human gait during steering. Preliminary results show that the device can appropriately simulate human gait with loading and unloading the ankle joint during the gait in circular paths

System for powered ankle-foot prosthesis with active control of dorsiflexion-plantarflexion and inversion-eversion

A system and method for operating a prosthesis is provided. The system includes a socket configured to engage a residual limb of a subject and a shaft having a first end connected to the socket and an opposing second end. The system also includes a foot piece connected to the second end of the shaft. The foot piece includes an ankle plate and a sole piece configured to contact a surface. The system also includes at least one computer configured to detect a state of the foot piece and to transmit an indication of the state of the foot. The system further includes a motor assembly configured to receive the indication of the state of the foot and to control a position and impedance of the ankle plate based on the state of the foot.https://digitalcommons.mtu.edu/patents/1136/thumbnail.jp

Preliminary design and evaluation of a multi-axis ankle-foot prosthesis

The human gait shows significant differences in the ankle movements during turning and sidestep cutting compared to straight walking, especially in frontal plane. This suggests that the next advancement in lower extremity assistive devices is to extend their design and control to the frontal plane. In this paper, the concept of a multi-axis powered anklefoot prosthesis is introduced and its feasibility is shown by a proof of concept prototype of a cable-driven, multi-axis anklefoot prosthesis. The design kinematics and its ankle joint\u27s mechanical impedance in non-load bearing conditions are evaluated and discussed. It is shown that the developed prototype is capable of closely mimicking the ankle movements in both sagittal and frontal planes during turning and walking on straight path with passive mechanical impedance in sagittal and frontal planes comparable to the ones of the human ankle

Impedance and admittance controller for a multi-axis powered ankle-foot prosthesis

© 2014 by ASME. This paper introduces a finite state machine to select between impedance and admittance control for a powered anklefoot prosthesis controllable in both Dorsiflexion-Plantarflexion (DP) and Inversion-Eversion (IE). Strain gauges are installed on the prosthesis\u27 foot to measure the strain caused by ground reaction forces, which are correlated to the external torques in DP and IE. The external torques are used for the admittance and impedance controllers. Additionally, the finite state machine uses the strain gauges feedback to detect the heel-strike and switch to admittance control. The admittance control accepts torque feedback to generate motion, this way larger feedback torques effectively reduces the stiffness of the ankle. During push off, the finite state machine switches to impedance control, accepting motion feedback to generate the appropriated torques. The quasi-static stiffness of the prosthesis with impedance control was tested, showing a near linear relationship between the torque feedback gain and the stiffness of the ankle. The finite state machine and controllers were also evaluated using a custom-made circular treadmill and the results were compared to the results of position and passive controllers; showing that the impedance/admittance controller was capable of tracking the desired input trajectory while decreasing the required torque at the ankle joint

Stochastic estimation of human ankle mechanical impedance in lateral/medial rotation

Copyright © 2014 by ASME. This article compares stochastic estimates of human ankle mechanical impedance when ankle muscles were fully relaxed and co-contracting antagonistically. We employed Anklebot, a rehabilitation robot for the ankle to provide torque perturbations. Surface electromyography (EMG) was used to monitor muscle activation levels and these EMG signals were displayed to subjects who attempted to maintain them constant. Time histories of ankle torques and angles in the lateral/medial (LM) directions were recorded. The results also compared with the ankle impedance in inversion-eversion (IE) and dorsiflexionplantarflexion (DP). Linear time-invariant transfer functions between the measured torques and angles were estimated for the Anklebot alone and when a human subject wore it; the difference between these functions provided an estimate of ankle mechanical impedance. High coherence was observed over a frequency range up to 30 Hz. The main effect of muscle activation was to increase the magnitude of ankle mechanical impedance in all degrees of freedom of ankle

Ankle kinematics describing gait agility: Considerations in the design of an agile ankle-foot prosthesis

The designs of available lower extremity powered prostheses are focused on a single degree of freedom (DOF) in sagittal plane, allowing the control of their ankle joints in dorsiflexion and plantarflexion. The human gait however, shows that the ankle movements in both sagittal and frontal planes are significant even during walking on a straight path. Additionally, there is a significant change in the ankle movements during straight walking compared to turning and cutting, especially in frontal plane. A better understanding of the ankle characteristics in both sagittal and frontal planes may result in the design of significantly more effective lower extremity prostheses that mimic the ankle function and improve the agility of gait. In this paper, the ankle rotations are estimated during step turn and cutting to provide evidence for necessity of a multi-axis design while providing the preliminary design parameters for a prototype multi-axis powered ankle-foot prosthesis. It is shown that the proposed cable-driven prototype is capable of closely mimicking the ankle movements in both sagittal and frontal planes during turning and walking on a straight path

Gait emulator for evaluation of a powered ankle-foot prosthesis

Copyright © 2017 ASME. In this paper we present an enhanced gait emulator and a novel hybrid control system to test powered ankle-foot prostheses with two degrees of freedom in the sagittal and frontal planes. The gait emulator is a nonlinear and non-smooth system that has to follow a precisely timed set of phases to achieve a human-like periodic gait. Despite the complexity and parameter uncertainties of this five degrees of freedom system, our proposed hybrid control system simplifies the walking control by use of state triggered kinematic events. The control system works in closed loop with kinematic event detection to ensure robust and repeatable walking tests as design parameters are varied. The developed gait emulator can be used to test the prosthesis under various loading conditions and walking speeds

Dynamic modeling of a 2-DOF cable driven powered ankle-foot prosthesis

Copyright © 2016 by ASME. The first step to study and develop a two Degrees of Freedom (DOF) prosthesis is to derive a dynamic model for simulation and control design. In this paper, the ankle-foot prosthesis has controllable Dorsi-Plantarflexion (DP) and Inversion-Eversion (IE) DOF. We derive a compliant dynamic model for a recently developed ankle-foot prosthesis followed by identification of the actuators, transmission, and prosthetic foot parameters. The resulting model is then verified experimentally and in simulation. Dynamic decoupling of the actuators to the ankle\u27s DP and IE DOF is also investigated using Bode plots. The code used for simulating the prosthesis is provided on GitHub for the community