A Gravity Balancing Passive Exoskeleton for the Human Leg

Abstract

Abstract-A gravity balancing lower extremity exoskeleton is a simple mechanical device composed of rigid links, joints and springs, which is adjustable to the geometry and inertia of the leg of a human subject wearing it. This passive exoskeleton does not use any motors or controllers, yet can still unload the human leg joints of the gravity load over the full range of motion of the leg. The underlying principle of gravity balancing consists of two steps: (i) Locate the combined system center of mass of the human leg and the exoskeleton, (ii) Add springs to the exoskeleton, one between the center of mass of the combined system and the fixed frame representing the trunk, the others within the links of the exoskeleton so that the potential energy of the combined system is invariant with configuration of the leg. Additionally, parameters of the exoskeleton can be changed to achieve a prescribed level of partial balancing, between 0-gravity and 1-gravity.The goals of this paper are as follows: (i) briefly review the theory for gravity balancing and present laboratory prototypes of gravity balanced machines, (ii) describe the design of a lower extremity exoskeleton that was fabricated using this principle, and (iii) show the performance of the exoskeleton on both healthy human subjects and a stroke patient by comparison of leg muscle EMG recordings, joint range of motion, and measured joint torques. These results strongly suggest that human joints can be unloaded from gravity using these exoskeletons and the human joint range of motion can be significantly increased. Potential applications of gravity balancing exoskeletons include: (i) gait training of stroke patients, (ii) characterization of long-term effects of zero gravity on the human musculature, (iii) human performance augmentation during assembly tasks.

Review of control strategies for robotic movement training after neurologic injury

“…The assistance applied, measured as the amount of arm weight counterbalanced, can be selected by a clinician by adding or removing elastic bands, according to the impairment level exhibited by the participant. A similar approach has been developed for assisting in gait training, counterbalancing the weight of the leg using a gravity-balancing, passive exoskeleton [ 32 ]. Non-exoskeleton passive devices that reduce the amount of weight on the participant lower limbs have been developed to assist participants to train standing-balance [ 90 ], or to keep balance while walking overground [ 91 ].…”

“…For many robots, such "high-level" algorithms are supported by low-level controllers that achieve the force, position, impedance, or admittance control necessary to implement the high-level algorithm. Research in robotic therapy devices has advanced the state-of-art in low-level force control also, for example, in control of pneumatic [ 21 , 22 , 27 ] and cable-based actuators [ 8 , 14 , 18 , 19 , 26 , 32 - 35 ], but these advances are not the focus of this article.…”

Walking of biped with passive exoskeleton: evaluation of energy consumption

“…Vectorẋ − describes the velocities of the biped after the first sub-phase. Torque Γ 5 (t) = I − 5 δ(t−T − ) (see expression (19)) with magnitude I − 5 is applied in the ankle of the stance leg (hind leg) at the instant t = T − . It ensures that the COP is located at abscissa L f under the sole of the massless foot 1 as follows:…”

“…For the human with exoskeleton, the velocity of the inter-link angle in the knee of the stance leg (hind leg) after first sub-phase remains zero through the impulsive brake effort Γ 1 (t) = I − 1 δ(t − T − ) (see expression (19)), therefore:…”

“…The quasi-passive elements of the exoskeleton, that are springs and variable dampers, have been chosen from an analysis of the kinetics and kinematics of human walking. Another quasi-passive exoskeleton has been designed at University of Delaware, Newark, see [19] and [20] for the leg of a human subject. This gravity balancing exoskeleton, also does not use any motors, but through springs and a four bars mechanism is able to partially or fully unload the hip-and knee-joints from the gravity throughout the range-of-motion of the leg.…”