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

Time Response Dynamics of Linear Model of Microcantilever-MEMS

Abstract This paper presents analytic derivation of dynamic behavior of a liniearized micro-electro-mechanical resonator. The parametric oscillation results from a displacement-dependent electrostatic force generated by oscillation of a microbeam. The utilized device is a MEMS with a time-varying capacitor. The stability and steady state dynamic behavior of the MEMS has been analyzed without polarization voltage. The main characteristic of the no-polarization model is effects of parameters in stability of the system. A set of stability charts is provided for prediction of the boundary between the stable and unstable domains for the principal resonance. Applying perturbation method, analytical equations are derived to describe both the steady state and time response of the system. INTRODUCTION A microelectromechanical resonator is built by attaching a microplate to the tip of a long microcantilever. The microplate is utilized as a moving electrode of a variable capacitor, whose other electrode is fixed to the frame of the MEMS device. The MEMS can be activated by both DC and AC voltages. The DC voltage is called polarization voltage and is utilized to activate the system statically. The polarization voltage generates an initial attraction due to electromagnetic force field between the electrodes, so increases the sensitivity of the system. However, introducing a polarization voltage destroys the symmetry of the system, and may be dropped in some applications. The AC voltage alternates the electromagnetic field and applies a harmonic force to the microplate. The alternative electromagnetic force excites the movable electrode harmonically. The mechanical stiffness of the microbeam or microcantilever struggles with the active force and produces a vibrating motion. An equivalent viscous damping may be used to simulate most damping phenomena in the syste

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

Effects of nonlinearities on the steady state dynamic behavior of electric actuated microcantilever-based resonators

This paper presents the dynamic behavior of microcantilever-based microresonators and compares their steady state behavior for polarized and nonpolarized systems at different levels of nonlinearities. A microcantilever, equipped with a time-varying capacitor, makes the microresonator system. The capacitor is activated by a constant polarization voltage, and an alternative actuating voltage. The partial differential equation of motion of the vibrating electrode can be reduced to a highly nonlinear parametric second order ordinary differential equation. The steady state behavior of the microresonator has been analyzed with and without polarization voltage. The main characteristic of the non-polarized model is explained by the stability of the system in parameter plane. A set of stability chart is provided to predict the boundary between the stable and unstable domains. Furthermore, the main characteristic of the polarized model is determination by the period-amplitude relationship of the system. Applying perturbation methods, analytical equations are derived to describe the frequency response of the system, which are suitable to be utilized in parameter study and design

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

Active vibration cancellation of tonal disturbance using orthogonal eigenstructure control

Orthogonal Eigenstructure Control (OEC) is a novel control method that can be used for active vibration cancellation. OEC is an output feedback control method applicable to multiple-input, multiple-output linear systems. In this paper, application of OEC for active vibration cancellation in a plate is presented. A steel plate clamped at four edges is used as a test plate and piezoelectric actuators are used as control actuators. Accelerometers are used for measuring the acceleration and displacement at ten locations on the plate. A tonal disturbance with a frequency of 150 Hz is applied to the plate by an electromagnetic actuator. After identification of the state-space model of the plate, orthogonal eigenstructure control is used to find the control gains that decouple the modes of vibrations and reduce transferring of vibrational energy between them. The results show significant vibration suppression throughout the plate