Cable-driven articulated rehabilitation system for gait training
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Assisted motor therapies play a critical role in enhancing functional musculoskeletal recovery and neurological rehabilitation. Our long term goal is to assist and automate the performance of repetitive motor-therapy of the human lower limbs. Hence, in this dissertation, we examine the viability of a light-weight and reconfigurable hybrid (articulated-multibody and cable) robotic system for assisting lower-extremity rehabilitation and analyze its performance. A hybrid cable-actuated articulated multibody system is formed when multiple cables are attached from a ground-frame to various locations on an articulated-linkage based orthosis. Our efforts initially focus on developing an analysis and simulation framework for the kinematics and dynamics of the cable-driven lower limb orthosis. A Monte Carlo approach is employed to select configuration parameters including cuff sizes, cuff locations, and the position of fixed winches. The desired motions for the rehabilitative exercises are prescribed based upon motion patterns from a normative subject cohort. We examine performance in terms of (i) coordinated control of the redundant system; (ii) reducing internal stresses within the lower-extremity joints; and (iii) continued satisfaction of the unilateral cable-tension constraints throughout the workspace. To eliminate the gravity effects on the human leg during gait training, and to reduce the cable forces and subsequently to reduce the size of the motors due to gravity, we present the perfect and approximates static balancing of variant candidates for articulated leg orthosis. Changing the leg configuration could require considerable actuator power because of the leg weight. Hence, the main focus is to carefully evaluate various means for reducing or eliminating these static forces, principally due to the mass- and inertia-distribution within the system. It is noteworthy that although known apriori, such static forces often are significantly dependent upon the leg configuration. Hence, realizing the static balancing for all possible configurations of leg imposes special set of conditions on the geometry, spring stiffness, and other design parameters. Here, elastic elements such as springs are employed in conjunction with variant mechanisms to achieve the static balancing. The essential principle is to realize that the total potential energy including the elastic potential energy stored in springs and gravitational potential energy becomes constant. Finally, we show that elimination of static torques due to gravity reduces the torque requirements and provides much more efficient design with significant reduction of the actuator sizes. Then, to enhance the motion capabilities of leg orthosis during gait training and reducing the energy requirements, we explore the use of variant candidate articulated leg designs (based on the four-bar mechanism). Multiple leg orthosis parameters, such as kinematic link lengths and static spring stiffnesses and preloads, influence the overall motion performance. Appropriate selection can not only enhance the leg orthosis performance but also reduce the overall energy consumption. In particular, we aim to: (i) achieve the greatest motion-ranges between ankle and hip joint as well as to (ii) reduce the overall actuation requirements by spring assist. Hence, we explore the use of systematic kinetostatic design approaches coupled with optimization to determine the parameters for leg orthosis designs. Further, we also examine enhancement of leg orthosis motion by varying subsystem parameters during gait training via a semi-active elements. Extensive simulation is then employed to evaluate the capabilities of the articulated leg design to track predetermined normal walking profile while reducing actuation requirements. Then, for safe forceful interaction with uncertain environments like human musculoskeleton, we introduce the elasticity to cable-driven system. We move on to investigate the modulation of both task space and joint space stiffness of hybrid cableactuated articulated multibody system for gait training. Elasticity is introduced via (i) springs connected in series with non-extensible cables, (ii) variable stiffness modules. The benefit of series elastic cables include tension control without using force sensors. However, elasticity reduces the positioning accuracy and make the system more prone to disturbances. On the other side, the variable stiffness modules add significant robustness to mechanical systems such as leg orthosis during forceful interaction with uncertain environments like human musculoskeleton, such that by controlling each module’s stiffness, the overall stiffness of the robot can be modulated.