Kinematics and kinetics of a knee assistance exoskeleton for sit-to-stand and stand-to-sit motions with energy storage and regeneration: A bond graph approach

A bond graph model for the kinematics and kinetics of a knee assistance exoskeleton for sit-to-stand (SiTSt) or stand-to-sit (StTSi) motions is presented. The exoskeleton is actuated by a wire rope connected to a lead screw and slider nut assembly driven by an electric motor. An energy storage spring (ESS) is used for energy storage and regeneration. The ESS and wire rope apply opposite forces on the slider nut resulting in switching of contact surfaces. The kinematics of the lead screw and slider nut assembly is discussed and a bond graph model for the same is developed considering switching of contact surfaces and friction. Kinetics equations are derived from the model and verified by free body diagram (FBD) analysis. The efficacy of the developed model is demonstrated through simulations, where the switching and effects of friction are captured elegantly. Further, mechanics of wrapping is discussed considering the friction between the wire rope and the pulley. Finally, the bond graph model of the proposed exoskeleton is developed. Simulations are carried out by varying the stiffness of ESS and friction coefficients. A reduction in the assistance motor torque, power and energy requirements due to ESS and wrapping is demonstrated.