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Gravity Balancing Design of Human Leg using an External Orthosis Khatereh Hajizadeh Khatere hajizade@gmail com June 21 2009 Department of Mechanical Engineering Isfahan University of Technology Isfahan 84156 83111 IranDegree M Sc Language FarsiAssit Prof A Fattah fattah@cc iut ac ir AbstractVarious motorized robotic devices have been developed for rehabilitation of the lower limbs For example the Lokomat is an exoskeleton orthosis instrumented with bilateral hip and knee actuators for locomotiontraining Such motorized devices are expensive and long term benefits from these are still underinvestigation Non motorized devices have also been developed to assist upper extremity movements byeliminating or reducing the effects of gravity in individuals with arm impairments and for the lowerextremity to help in gait training Such devices are relatively inexpensive and indicate that adjustableassistance to the limbs is possible during training to maximize voluntary motor activity In this work usingan external orthosis we present a new design for gravity balancing of the human leg in order to decrease therequired joints toques during walking Gravity balancing is often used in industrial machines to decrease therequired actuator efforts during motion In this proposed design passive gravity balancing orthosis isconnected to the human leg on the shank and its other end is fixed to a walking frame The human leg isassumed to have three DOF for the hip medial and lateral rotation hip abduction and adduction hipflexion extension and one DOF for the flexion extension at the knee The foot is considered as a point mass atthe end of the shank segment The external orthosis connected at the shank together with the human leg creates a kinematic closed loop The kinematic loop constraint can be satisfied during walking by choosingappropriate link lengths for the external orthosis Gravity balancing of the human leg and the externalorthosis is achieved by making the potential energy of the combined system human and the machine to beconfiguration invariant These conditions are satisfied by choosing appropriate inertia parameters of thesegments of the orthosis and addition of proper springs Regarding the possible motions of the human leg two three and four degree of freedom motion of the human leg in the sagittal coronal and transverseplanes feasible designs are then presented The joint torques at the hip and knee joints are derived usinginverse dynamics of closed loop system human leg and external orthosis or open loop system human legwithout orthosis The joint torque trajectories of the human leg during walking are computed and comparedfor the motions with different DOFs for two cases of human leg and the external orthosis and human legwithout external orthosis The results show that the joint torques in all designs with the external orthosis areless than the joint torques of the human leg without the external orthosis at slow walking The mainadvantages of the design of human leg with external orthosis as compared to the existing exoskeletons are asfollows i This design has a better alignment between the human leg and the orthosis ii We can also getthe full extension of the knee with this design iii It is required less hardware such as force torque sensorsbetween human leg and orthosis to compute the joint torques However there are also some drawbacks forthis design such as a It increases the inertia of the system which may be a drawback during fast walking b Modeling of the human leg with the external orthosis which makes a kinematic closed loop is morecomplicated than the modeling of the existing exoskeletons namely open loop system Key WordsRehabilitation Robots Passive external orthosis Human joint torques Gravity balancing

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