A Bond Graph Model for the Estimation of Torque Requirements at the Knee Joint During Sit-to-Stand and Stand-to-Sit Motions

Sit-to-stand (SiTSt) and stand-to-sit (StTSi) are some of the frequent but crucial motions executed by the human body during daily life activities. During these motions, information of torque required at the knee joint is necessary for the development of knee assistance devices. In this work, a bond graph model has been developed to estimate torque requirements at the knee joint during SiTSt and StTSi motions. The bond graph offers graphical representation of a system, causality and power transactions between the subsystems, and algorithmic derivation of system equations. Word Bond Graph Objects (WBGOs) have been used for their compact representations of large systems. The model comprises two seven-link sub-models, one is virtual and the other is actual. The virtual sub-model is considered to be a part of the central nervous system (CNS). It generates the required joint angles for SiTSt and StTSi motions for desired movement trajectories. Based on these joint angles required, joint torques are provided to the actual sub-model through proportional-derivative (PD) controllers. Variations in the torque requirements have been estimated during SiTSt and StTSi motions, and the simulation results are presented.