A Passive Gait-Based Weight-Support Compliant Lower-Extremity-Exoskeleton

The knee joint is one of the most important joints in the human body, and the knee Osteoarthritis (OA) is the most common joint disorder. Research reveals OA is always associated with mechanical and psychosocial factors, and the knee load has been generally recognized as an important factor. However, the knee joint is highly loaded in daily life, such as walking, stair descending, stair ascending, and two legged stance. To help early prevention from OA and release the painful feeling which prohibits human walking, this dissertation presents the design and analysis of a passive weight-support lower-extremity-exoskeleton (LEE) with compliant joints to relieve compressive load in the knee.Normal human gaits are divided into two phases:stance phase and swing phase. The knee joint transfers the bodyweight (BW) to the ground in the stance phases, and is compliant to free the leg in swing phases. As the internal forces/torque in a human knee cannot be directly measured, a leg-dynamic model and biomechanical model are derived for calculating the knee bone-to-bone reaction forces. With measured plantar forces and joint trajectories, the model has been validated by comparing simulated knee forces with published in-vivo data.Unlike the case of an open kinematic chain (such as human walking with no exoskeleton) experiencing no impulse within the joint, a combined knee-exoskeleton tends to create a residual force if the DOFs of the exoskeleton are insufficient to compromise with that of a human joint to align the motion axis or any human-machine kinematic differences. This dissertation presents a relatively complete analytical model of a knee joint interacting with a two-link exoskeleton for investigating the effects of different exoskeleton designs on the internal joint forces/torque in the knee. Based on the knowledge of a knee-joint kinematics, an adaptive knee-joint exoskeleton has been designed to eliminate negative effects associated with the closed leg-exoskeleton kinematic chain on a human knee.With the gait analyses, a gait-based LEE to support human BW and accommodate bio-joint motion during walking is designed. The bio-joint-like knee decouples human gait into two phases - stance and swing phases. In stance phases, the knee joint transfers the BW to an ankle spring whereas the energy is harvested in a hip spring. In swing phases, the knee joint is free, and the hip joint releases energy to actuate the LEE. The compliant mechanism for the gait-based knee has been analyzed numerically with experimental validation. Experiments were conducted on an LEE prototype to validate the concept feasibility and verify dynamic models. The investigation analyzes the effects of LEE design configurations on supporting the human BW. The excellent agreement in the joint trajectories with/without LEE suggests that the adaptive LEE does not influence the human gait. The reduced plantar forces and knee forces validate LEE’s effectiveness in supporting about 20%of BW. The stored potential energy of hip spring saves 61.5% of energy needed to swing the LEE.To coordinate the movements between the human lower-extremity and the LEE, the mechanical joints of the LEE perceive the human’s angles and angular velocities. For example, hip joint perceives, records, and analyzes the gait’s parameters of human walking, i.e. hip joint maximum angle, hip joint minimal angle, and gait cycle time. The LEE responds to the human’s changed gait, and the left hip motor copies the right hip with half-cycle delay.Based on the above analyses, a passive LEE with compliant (hip, knee and ankle) joints capable of accommodating biological joint motion while supporting the human BW during walking has been designed and presented in this dissertation. Experimental results show that the LEE reduces plantar force, confirming the effectiveness of the LEE in supporting BW in stance phases.