Modeling of dynamics of sit to stand stand to sit motions and design of a human knee assistance exoskeleton using bond graphs

Abstract

The underlying objective of the research work presented in this thesis is to contribute to the understanding of sit-to-stand (SiTSt) and stand-to-sit (StTSi) motions and the development of a knee assistance exoskeleton for these motions. The overall research work can be categorised into three stages. newlineIn the first stage, a generic bond graph model for the dynamics of SiTSt and StTSi motions is developed. The distinguishing feature of the developed model is that it works on a hypothesis that the central nervous system (CNS) controls the trajectory of the centre of mass of the body (COMB) for these motions. The developed model comprises two submodels. One of the submodels emulates the working of the CNS and is called submodel-v. The other submodel represents the human body and is called submodel-a. The submodel-v takes in the reference trajectories of COMB for SiTSt and StTSi motions as commanded input and gives desired joint angle trajectories for these motions as output. The reference trajectories of COMB for SiTSt and StTSi motions are determined through experimentation on a human subject using videos of these motions and a motion analysis software called Tracker. The desired joint angle trajectories are commanded as input to the joint controllers of the submodel-a. These joint controllers act as actuators and produce the equivalent effect of muscles actuation. Based on the error between the required and actual joint angle trajectories, joint controllers provide the required joint torques to the submodel-a. For the development of the model, the concept of word bond graph objects (WBGO) is used, which is advantageous for the compact representation of large systems having multiple interconnected subsystems. Initially, WBGOs of different parts of the considered system are developed and then appropriately assembled to develop the complete bond graph structures for submodel-v and submodel-a. Simulation results show that during SiTSt or StTSi motions, the centre of mass of submodel-a follows the commanded