Paper Title
Performance Comparison of Linear vs Rotary Actuator in a Knee Exoskeleton During Stand to Sit Motions: A Bond Graph Approach

Abstract
The people suffering from injuries related to knee joint face difficulties in carrying out activities of daily livingparticularly,stand to sit motions (STS) or vice versa. STSis quite crucial activity as the patient is more likely to fall in comparisons to other mobility activities. For stand to sit and vice versa motion, the knee must exert a large amount of torque which the feebler elderly user is incapable to provide by itself without using any external supports. The conventional assistive devices like walking sticks crutches and wheelchairs have not been successful enough in providing complete autonomy to the wearer. Modern assistive devices such as knee exoskeletons specifically targeting the knee joint,unlike the lower body exoskeletons that target the entire lower body,seems a viable solution to the patients suffering from knee impairment. These knee exoskeletons are either actuated by a rotary actuator or by a linear actuator. Thus, this paper presentsa comparative study of four bar mechanism-based knee exoskeleton actuatedby a linear and then by a rotary actuator. The modelling and simulation of both the systems are performed by means ofbond graph(BG) technique using Symbols Shakti simulation software. The application of BG method to model the multidisciplinary exoskeleton proves to be quite useful as it conveniently model the system containing different elements in different energy domains and requires only kinematic equations to develop dynamic model of the system. The paper also presents acomparison which is drawn for the assistive torques provided by the linear and rotary actuatorsindependently, at the knee joint of the exoskeleton,for performing the desired STS motions. Furthermore, the current and torque characteristics of both actuators have also compared and discussed. Keywords - Knee Exoskeleton, Four Bar Mechanism, Rotary Actuator, Linear Actuator, Stand to Sit Motion, Bond Graph Modelling