Standing and Sitting Motion Assistance using a Compliant Knee Exoskeleton

Sit-to-stand movement is the one of most important tasks in daily life for individuals with impaired legs and elderly people who do not have the required physical strength [1], [2]. This work presents an compliant knee exoskeleton (see Fig. 2) which can provide motion assistance to the user during standing-up and sitting-down based on the user intent. The knee exoskeleton is energized by a series elastic actuator with offline reconfigurable stiffness (CompAct-RS) [3], [4]. Introducing passive compliance to the actuation unit decouples the inertia of the motor drive from the output link and thus the exoskeleton presents low impedance to the wearer. Additionally, compliant robots can intrinsically absorb impacts and associated with dedicated control strategies can achieve safety and adaptability of physical human-robot interaction (pHRI). CompAct-RS as mentioned above is a series elastic actuator with the ability to regulate off-line the level of stiffness in as wide a range as needed. This feature permits the experimentation with different compliance levels and the adaptation of the joint to fit specific task requirements. The elimination of active tuning of the spring stiffness through a second motor was performed to reduce the weight and dimensions of the unit. The working principle of the CompAct-RS is based on the CompAct-VSA (Variable Stiffness Actuator) [5], which uses a lever arm mechanism with a variable pivot axis. and experimental data of the human standing-up and sittingdown. Acquiring this data we selected the design variables of the actuator and analysed its performance and stiffness characteristic. An assistive control strategy is proposed which allows the actuator to generate assistive torques towards to the direction of motion based on estimated user torques. Particularly, electromyographic signals are used in order to derive the estimated user torque while the equilibrium position of a virtual spring-damper network is being updated in accordance to this estimated user torque