Idiothetic Verticality Estimation Through Head Stabilization Strategy

Abstract

The knowledge of the gravitational vertical is fundamental for the autonomous control of humanoids and other freemoving robotic systems such as rovers and drones. This article deals with the hypothesis that the so-called 'head stabilization strategy' observed in humans and animals facilitates the estimation of the true vertical from inertial sensing only. This problem is difficult because inertial measurements respond to a combination of gravity and fictitious forces that are hard to disentangle. From simulations and experiments, we found that the angular stabilization of a platform bearing inertial sensors enables the application of the separation principle. This principle, which permits one to design estimators and controllers independently from each other, typically applies to linear systems, but rarely to nonlinear systems. We found empirically that, given inertial measurements, the angular regulation of a platform results in a system that is stable and robust and which provides true vertical estimates as a byproduct of the feedback. We conclude that angularly stabilized inertial measurement platforms could liberate robots from ground-based measurements for postural control, locomotion, and other functions, leading to a true idiothetic sensing modality, that is, not based on any external reference but the gravity field. Index Terms-Biologically-Inspired Robots; Biomimetics; Sensor-based Control I. INTRODUCTION F OR humans, as for other living creatures, it is substantial to know the spatial orientation of our body with respect to the external world. Most of our sensory systems can contribute to this task. Interestingly, visual, auditory, tactile, proprioceptive, or olfactory sensory inputs can be easily put out of action, but the vestibular inputs are always available, even in the absence of gravity [1]. In artificial systems, robots in particular, inertial measurement units (IMUs) often play the role of a 'robotic vestibular system'. These IMUs sense the movements of the robot and provide its control system Manuscript