Minimally Actuated Four-Bar Linkages for Upper Limb Rehabilitation

In the previous years the scientific community dealt extensively with designing, developing and testing robot-based rehabilitation systems. Besides the benefits that resulted for disabled people, this twenty-year endeavor has helped us improve our understanding of the neuroplasticity mechanisms in the Central Nervous System (CNS) and how these are triggered through the interaction with the physical world and especially through interaction with robots. In these systems, most of the state-of-the-art arrangements are based on multi-Degree of Freedom (DOF) open kinematic chains. They also employ sophisticated control hardware as well as high-profile actuators and sensors. The state-of-the-art technology that is integrated in these arrangements, increases the cost and at the same time requires the presence of trained employees that are able to maintain and operate such systems. Another option, are mechanisms that are based on four- and six-bar linkages. These are closed kinematic chain designs that can generate a variety of paths, yet they can do so with much less flexibility and adaptation possibilities. Despite the reduced flexibility over their robotic counterparts these mechanisms are attractive due to their reduced cost, simplicity and low external power requirement. This paper elaborates on the synthesis, analysis, simulation and passive control of four-bar linkages that can be used in upper limb rehabilitation and extends previous work by simulating the mechanism-impaired user interaction using a dynamic multibody system model. The emphasis in this work has been on straight-line trajectory generation, but this established methodology can be applied for developing mechanisms with higher complexity and more complex trajectories.