Passive joint stiffness in the hip and knee increases the energy efficiency of leg swinging

A bioinspired robotic knee with controlled joint surfaces and adjustable ligaments

The knee joint plays a key role in kinematic and kinetic performances of pedestrain locomotion. The key role of meniscus with matched ligaments in joint stability and movability has not been fully explored in current robotic knee designs. We fabricate a bioinspired robotic knee based on a kinematic model of an anatomical knee in order to reveal the relationship between the meniscus, ligaments and their stability and movability, respectively. The kinematic model was built from magnetic resonance imaging of the human knee with generated contact profiles and customized ligament fibers. Then, the bioinspired knee was designed, and its dynamic stability was maintained by ligaments and specific contact profiles, which were acquired based on the kinematic model. Finally, a monopod robot with the bioinspired knee assembled was developed for dynamic testing. The results show that (1) a smooth rolling-sliding motion can be achieved with the addition of menisci and compatible ligaments; and (2) joint stiffness can be adjusted by changing the springs and activation lengths of ligament fibers. This study gives biomimetic insights into a new design of knee joint for a robotic/prosthetic leg.

Singular Configurations and Underactuated Balance Control for Biped Robot with Point Contact

This paper presents a hierarchical controller and extracts the singular configurations based on underactuated system and optimal distribution of forces. Unlike multi-legged and large-sole robots, biped robot with point contact cannot maintain a stable standing state, much less to adjust attitude and resist external impact. The support domain of point-foot bipedal robot degenerates into a line segment consisting of two footholds, introducing the underdactuated characteristic that challenges traditional algorithms based on polygonal domains and full variable optimization. To fully exploit the dynamic connection between support line and balance control, the accurate model is established as feedforward terms using virtual leg and floating reference system. The dynamics model is decomposed into an underactuated module and a force distribution module for hierarchical control, in which the former determines the control forces of base and the singularity corresponding to robot configuration, and the latter distributes forces on each leg according to its capability by solving a quadratic programming with constraints. The results verify the advanced stability of attitude adjustment and impact from external force of biped robot with point contact comparing to model predictive control, which is improved based on robot’s singular configuration.

Leg Joint Stiffness Affects Dynamics of Backward Falling From Standing Height: A Simulation Work

Falling backward can lead to injuries including hip fracture, back injury, and traumatic brain impact among older adults. A loss of consciousness is associated with falling backward and accounts for about 13% of all falls among older adults. Little is known about the dynamics of backward falls, such as the falling duration, the impact severity, and how the fall dynamics are affected by the biomechanical properties of the lower limb joints, particularly the rotational stiffness. The purpose of this study was to investigate the influence of the stiffness of individual leg joints on the dynamics of backward falls after losing consciousness in terms of the falling duration and impact velocities. Based on a 15-segment human model, we simulated the process of falling backward by sweeping the parameter space of ankle, knee, and hip's stiffnesses varying from 0 to 8.73 N·m·deg-1 (or 500 N·m·rad-1). The results revealed that the falling duration and impact speeds of the head and hip ranged from 0.27 to 0.63 s, 2.65 to 7.88 m·s-1, and 0.35 to 3.36 m·s-1, respectively, when the stiffness of the leg joints changed within their limits. Overall, the influence of the joint stiffness on the falling dynamics (falling duration and impact speed) is comparable between hip and knee joints, whereas ankle stiffness showed little influence on the backward falling dynamics. Our findings could provide references for designing protective devices to prevent impact-induced injuries after a backward fall.

A design concept of parallel elasticity extracted from biological muscles for engineered actuators

Series elastic actuation that takes inspiration from biological muscle-tendon units has been extensively studied and used to address the challenges (e.g. energy efficiency, robustness) existing in purely stiff robots. However, there also exists another form of passive property in biological actuation, parallel elasticity within muscles themselves, and our knowledge of it is limited: for example, there is still no general design strategy for the elasticity profile. When we look at nature, on the other hand, there seems a universal agreement in biological systems: experimental evidence has suggested that a concave-upward elasticity behaviour is exhibited within the muscles of animals. Seeking to draw possible design clues for elasticity in parallel with actuators, we use a simplified joint model to investigate the mechanisms behind this biologically universal preference of muscles. Actuation of the model is identified from general biological joints and further reduced with a specific focus on muscle elasticity aspects, for the sake of easy implementation. By examining various elasticity scenarios, one without elasticity and three with elasticity of different profiles, we find that parallel elasticity generally exerts contradictory influences on energy efficiency and disturbance rejection, due to the mechanical impedance shift thus caused. The trade-off analysis between them also reveals that concave parallel elasticity is able to achieve a more advantageous balance than linear and convex ones. It is expected that the results could contribute to our further understanding of muscle elasticity and provide a theoretical guideline on how to properly design parallel elasticity behaviours for engineering systems such as artificial actuators and robotic joints.