Characterization and clinical implications of ankle impedance during walking in chronic stroke

Individuals post-stroke experience persisting gait deficits due to altered joint mechanics, known clinically as spasticity, hypertonia, and paresis. In engineering, these concepts are described as stiffness and damping, or collectively as joint mechanical impedance, when considered with limb inertia. Typical clinical assessments of these properties are obtained while the patient is at rest using qualitative measures, and the link between the assessments and functional outcomes and mobility is unclear. In this study we quantify ankle mechanical impedance dynamically during walking in individuals post-stroke and in age-speed matched control subjects, and examine the relationships between mechanical impedance and clinical measures of mobility and impairment. Perturbations were applied to the ankle joint during the stance phase of walking, and least-squares system identification techniques were used to estimate mechanical impedance. Stiffness of the paretic ankle was decreased during mid-stance when compared to the non-paretic side; a change independent of muscle activity. Inter-limb differences in ankle joint damping, but not joint stiffness or passive clinical assessments, strongly predicted walking speed and distance. This work provides the first insights into how stroke alters joint mechanical impedance during walking, as well as how these changes relate to existing outcome measures. Our results inform clinical care, suggesting a focus on correcting stance phase mechanics could potentially improve mobility of chronic stroke survivors.

An experimental approach on energy dissipation of bolted joints using a modified oscillator device

A modified and simplified oscillator device is designed and manufactured to investigate the effects of contact roughness and bolt arrangements on energy dissipation caused by joint damping. Three types of surface roughness and five different bolt arrangements are discussed. Identification of Iwan model based on experimental data is carried out. It is indicated that rough surface dissipates more energy than smooth one. Specimens with more bolts dissipate relatively more energy. When bolt number is consistent, the series type dissipates more energy than the parallel type. Experimental data show a good stability and accuracy of the proposed oscillator device. The identified Iwan model provides a good conformity with the reconstructed hysteresis loop. The four-parameter Iwan model can well describe the damping property of joints with different surface roughness and bolt arrangements.

Design, Modeling, and Optimization of a Hopping Robot Platform

Laminate devices have the potential to lower the cost and complexity of robots. Taking advantage of laminate materials’ flexibility, a high-performance jumping platform has been developed with the goal of optimizing jump ground clearance. Four simulations are compared in order to understand which dynamic model elements (leg flexibility, motor dynamics, contact, joint damping, etc.) must be included to accurately model jumping performance. The resulting simulations have been validated with experimental data gathered from a small set of physical leg prototypes spanning design considerations such as gear ratio and leg length, and one in particular was selected for the fidelity of performance trends against experimental results. This simulation has subsequently been used to predict the performance of new leg designs outside the initial design set. The design predicted to achieve the highest jump ground clearance was then built and tested as a demonstration of the usefulness of this simulation.

Joints flexibility effect on the dynamic performance of robots

SUMMARYThis paper studies the effect of joint flexibility on the dynamic performance of a serial spatial robot arm of rigid links. Three models are developed in this paper. The first and the third models are developed using the multibody dynamics approach, while the second using the classical robotics approach. A numerical algorithm and an experimental test-rig are developed to test the final model. The links' inertial parameters are estimated numerically. Empirical formulae with assumption models are used to estimate the flexibility coefficients. The simulation results show that the joint damping is a major source of inaccuracies, causing trajectory error without a proper feedback controller.

Modal space decoupled optimal design for a class of symmetric spatial parallel mechanisms with consideration of passive joint damping

SUMMARYIn this study, we analyze the influence of passive joint viscous friction (PJVF) on modal space decoupling for a class of symmetric spatial parallel mechanisms (SSPM). The Jacobian matrix relating the platform movements to each passive joint velocity is first gained by vector analysis and the passive joint damping matrix is then derived by applying the Kane method. Next, an analytic formula index measuring the degree of coupling effects between the damping terms in the modal coordinates is proposed using classical modal analysis of dynamic equations in task space. Based on the index, a new optimal design method is found which establishes the kinematics parameters for minimizing the coupling degree of damping and achieves optimal fault tolerance for modal space decoupling when all struts have identical damping and stiffness coefficients in their axial directions. To illustrate the effectiveness of the theory, the new method was used to redesign two configurations of a specific manipulator.