Trajectory Tracking and Stabilization of Nonholonomic Wheeled Mobile Robot Using Recursive Integral Backstepping Control

In this paper, a generalized nontriangular normal form is presented to facilitate designing a recursive integral backstepping control for the class of underactuated nonholonomic systems, i.e., wheeled mobile robots (WMRs) that perform posture stabilization and trajectory tracking in environments without obstacles. Based on the differential geometry theory, we develop a multiple input multiple output (MINO) generalization of normal form using the input-output feedback linearization technique. Then, the change of variables (diffeomorphism) transform the state-space model of WMR, incorporating both kinematic and dynamic models into nontriangular normal form. As a result, the system dynamics can be represented as internal and external dynamics. The nonlinear internal dynamics of WMR pose serious challenges to design a suitable controller due to its internal dynamics being not minimum phase and non-strict feedback form structure. The proposed backstepping controller is designed in two steps. First, a standard integral backstepping controller is designed to stabilize the robot’s orientation angle. Then, a recursive integral backstepping control technique is applied to achieve asymptotic convergence of position error to zero. Hence, both asymptotic posture stabilization and trajectory tracking are achieved in semi-global regions, except the nonzero initial condition of the orientation angle. The asymptotic stability of the entire closed-loop system is shown using the Lyapunov criteria.

Efficient Actor-Critic Reinforcement Learning With Embodiment of Muscle Tone for Posture Stabilization of the Human Arm

This letter proposes a new idea to improve learning efficiency in reinforcement learning (RL) with the actor-critic method used as a muscle controller for posture stabilization of the human arm. Actor-critic RL (ACRL) is used for simulations to realize posture controls in humans or robots using muscle tension control. However, it requires very high computational costs to acquire a better muscle control policy for desirable postures. For efficient ACRL, we focused on embodiment that is supposed to potentially achieve efficient controls in research fields of artificial intelligence or robotics. According to the neurophysiology of motion control obtained from experimental studies using animals or humans, the pedunculopontine tegmental nucleus (PPTn) induces muscle tone suppression, and the midbrain locomotor region (MLR) induces muscle tone promotion. PPTn and MLR modulate the activation levels of mutually antagonizing muscles such as flexors and extensors in a process through which control signals are translated from the substantia nigra reticulata to the brain stem. Therefore, we hypothesized that the PPTn and MLR could control muscle tone, that is, the maximum values of activation levels of mutually antagonizing muscles using different sigmoidal functions for each muscle; then we introduced antagonism function models (AFMs) of PPTn and MLR for individual muscles, incorporating the hypothesis into the process to determine the activation level of each muscle based on the output of the actor in ACRL. ACRL with AFMs representing the embodiment of muscle tone successfully achieved posture stabilization in five joint motions of the right arm of a human adult male under gravity in predetermined target angles at an earlier period of learning than the learning methods without AFMs. The results obtained from this study suggest that the introduction of embodiment of muscle tone can enhance learning efficiency in posture stabilization disorders of humans or humanoid robots.

Acute consequences of a unilateral VIIIth nerve transection on vestibulo-ocular and optokinetic reflexes in Xenopus laevis tadpoles

Loss of peripheral vestibular function provokes severe impairments of gaze and posture stabilization in humans and animals. However, relatively little is known about the extent of the instantaneous deficits. This is mostly due to the fact that in humans a spontaneous loss often goes unnoticed initially and targeted lesions in animals are performed under deep anesthesia, which prevents immediate evaluation of behavioral deficits. Here, we use isolated preparations of Xenopus laevis tadpoles with functionally intact vestibulo-ocular (VOR) and optokinetic reflexes (OKR) to evaluate the acute consequences of unilateral VIIIth nerve sections. Such in vitro preparations allow lesions to be performed in the absence of anesthetics with the advantage to instantly evaluate behavioral deficits. Eye movements, evoked by horizontal sinusoidal head/table rotation in darkness and in light, became reduced by 30% immediately after the lesion and were diminished by 50% at 1.5 h postlesion. In contrast, the sinusoidal horizontal OKR, evoked by large-field visual scene motion, remained unaltered instantaneously but was reduced by more than 50% from 1.5 h postlesion onwards. The further impairment of the VOR beyond the instantaneous effect, along with the delayed decrease of OKR performance, suggests that the immediate impact of the sensory loss is superseded by secondary consequences. These potentially involve homeostatic neuronal plasticity among shared VOR-OKR neuronal elements that are triggered by the ongoing asymmetric activity. Provided that this assumption is correct, a rehabilitative reduction of the vestibular asymmetry might restrict the extent of the secondary detrimental effect evoked by the principal peripheral impairment.

Quick compensatory mechanisms for tongue posture stabilization during speech production

This study presents evidence of quick compensatory mechanisms in tongue motor control for speech production. The tongue posture is controlled not in relation to a specific tongue position, but to the shape of the tongue contour to achieve specific speech sounds. Modulation of compensatory responses due to task demands and mathematical simulations support the idea that the quick compensatory response is driven by a reflex mechanism.