Sit-to-stand (STS) movement is one of the most common and demanding daily activities which is also a pre-requisite for other functional movements that require ambulation like walking, jogging or jumping. In spite of the importance of sit-to-stand motion, many people have difficulty to do this simple movement; therefore, design of an assistive device, aimed at assisting physically impaired individuals specially elderly people when rising from a sitting to standing position is useful. In this thesis, first, we analyze STS movement kinematically and dynamically to consider the effects of different parameters. This consideration shows that bending the trunk and using of hand as support reduce the required joint torques. Also, it has been shown that the contribution to the joint torques by the gravitational torques is dominant during STS motion. Therefore, we use hybrid gravity balancing method to reduce the required joint torques. All the previous rehabilitative devices designed and manufactured for STS motion are completely passive or active and both of these devices have their advantages and disadvantages. In this thesis, a hybrid active and passive robot is designed to take advantages of both active and passive devices. Passive parts uses auxiliary parallelograms to identify the center of mass of the system and two appropriate springs connected to device to reduce the required joint torques. Auxiliary parallelogram links that locate the center of mass, connect to 3 principal links that connect to the shank, thigh and trunk. Auxiliary parallelogram links and principal links is made to be telescopic to accommodate variability in the leg dimensions and inertia across human subjects. By exerting the passive part on body, the only significant remaining joint torque is in the knee. Most of aged people have muscular weakness in the knee. Based on these facts, we use of an actuator with an appropriate controller for remaining joint torque in the knee. At the end of this thesis, this hybrid active and passive assistive device is compared with active and passive devices. Using this device, user needs insignificant force for STS movement and thus it improves the quality of life for individuals with difficulties in standing up particularly elderly people. Providing the remaining joint torques in hip and ankle by user prevents of reducing the remaining physical strength. This device is able to be portable and connected to a wheelchair. Key Words: Gravity Balance, Rehabilitation device, Sit-to-Stand (STS), Wheelchair, Active rehabilitative device, Passive rehabilitative device.
