Towards balance recovery control for lower body exoskeleton robots with Variable Stiffness Actuators: Spring-loaded flywheel model

Abstract

This paper presents a biologically-inspired realtime balance recovery control strategy that is applied to a lower body exoskeleton with variable physical stiffness actuators at its ankle joints. For this purpose, a torsional spring-loaded flywheel model is presented to encapsulate both approximated angular momentum and variable physical stiffness, which are crucial parameters in describing the postural balance. In particular, the incorporation of physical compliance enables us to provide three main contributions: i) A mathematical formulation is developed to express the relation between the dynamic balance criterion ZMP and the physical ankle joint stiffness. Therefore, balancing control can be interpreted in terms of ankle joint stiffness regulation. ii) 'Variable physical' stiffness is utilized in the bipedal robot balance control task for the first time in the literature, to the authors' knowledge. iii) The variable physical stiffness strategy is compared with the optimal constant stiffness strategy by conducting experiments on our exoskeleton robot. The results indicate that the proposed method provides a favorable balancing control performance to cope with unperceived perturbations, in terms of center of mass position regulation, ZMP error and mechanical power.

Assisting Human Balance in Standing With a Robotic Exoskeleton

“…Existing works on augmenting balance with robotic exoskeletons have important limitations. In [11], [12] a lower limb exoskeleton with variable stiffness actuators was used for balance recovery, however experiments were performed with the robot only. In [13] the influence of passive exoskeleton mechanics on human biomechanics of walking and balancing was investigated.…”

“…The importance of providing pelvis anterior movements in the exoskeleton to improve stability of walking was demonstrated in [14]. Studies [11], [14] indicated that the exoskeleton design, specifically its kinematic structure, can significantly degrade balancing function. In [15] a knee exoskeleton is used for balancing assistance based on mimicking estimated human knee joint impedance control.…”

“…Overall only a few balance-assisting controllers in lower limb robotic exoskeletons have been investigated with humanparticipants: balance recovery with one subject in [15] and with two subjects in [16]; influences on balance during walking with passive exoskeletons in [13], [14]. Most importantly, these studies did not consider human's active behavior in balancing with the exoskeleton [11], [12]. The study on exoskeleton-assisted balance recovery after slippage reported in [17] considered the biomechanical responses of the participants but did not analyse the related muscle activation.…”