Christophe Grand

Stability and Traction Optimization of a Reconfigurable Wheel-Legged Robot

Actively articulated locomotion systems such as hybrid wheel-legged vehicles are a possible way to enhance the locomotion performance of an autonomous mobile robot. In this paper, we address the control of the wheel-legged robot Hylos traveling on irregular sloping terrain. The redundancy of such a system is used to optimize both the balance of traction forces and the tipover stability. The general formulation of this optimization problem is presented, and a suboptimal but computationally efficient solution is proposed. Then, an algorithm to control the robot posture, based on a velocity model, is described. Finally, this algorithm is validated through simulations and experiments that show the capabilities of such a redundantly actuated vehicle to enhance its own safety and autonomy in critical environments.

A biologically inspired meta-control navigation system for the Psikharpax rat robot

A biologically inspired navigation system for the mobile rat-like robot named Psikharpax is presented, allowing for self-localization and autonomous navigation in an initially unknown environment. The ability of parts of the model (e.g. the strategy selection mechanism) to reproduce rat behavioral data in various maze tasks has been validated before in simulations. But the capacity of the model to work on a real robot platform had not been tested. This paper presents our work on the implementation on the Psikharpax robot of two independent navigation strategies (a place-based planning strategy and a cue-guided taxon strategy) and a strategy selection meta-controller. We show how our robot can memorize which was the optimal strategy in each situation, by means of a reinforcement learning algorithm. Moreover, a context detector enables the controller to quickly adapt to changes in the environment-recognized as new contexts-and to restore previously acquired strategy preferences when a previously experienced context is recognized. This produces adaptivity closer to rat behavioral performance and constitutes a computational proposition of the role of the rat prefrontal cortex in strategy shifting. Moreover, such a brain-inspired meta-controller may provide an advancement for learning architectures in robotics.

Stabilization algorithm for a high speed car-like robot achieving steering maneuver

This paper deals with design and implementation of a stabilization algorithm for a car like robot performing high speed turns. The control of such a kind of system is rather difficult because of the complexity of the physical wheel- soil interaction model. In this paper, it is planned to analyze the complex dynamic model of this process to elaborate a stabilization algorithm only based on the measurement of the system yaw rate. Finally, a 3D simulation is performed to evaluate the efficiency of this designed stabilization algorithm.

Kinematic analysis and stability optimization of a reconfigurable legged-wheeled mini-rover

This paper deals with the optimization of locomotion performances of vehicle used for planetary exploration. The design of an innovative reconfigurable mini-rover is presented. Then, a control process that optimize the stability and the global traction performances is developed. A method to identify in-situ the wheel-ground mechanical contact properties is proposed and used to determine an optimal traction torque. Results on experiments and simulations show that the rover stability is significantly enhanced by using the proposed control method.

Robust obstacle crossing of a wheel-legged mobile robot using minimax force distribution and self-reconfiguration

This paper focuses on the problem of robust obstacles crossing for a high mobility wheel-legged robot. To improve the obstacle clearance capability, a method dealing with the contact stability optimization is developed. A specific stability criterion taking the friction into account is proposed. The optimization algorithm uses both the kinematic redundancy in order to modify the position of the Center of Mass (CoM), modifying the resulting distribution of contact forces, and the actuation redundancy to improve the stability of frictional contacts by adapting the internal forces. We show that the choice of this particular criterion allows us to maximize the robustness of contacts stability relatively to the modeling errors affecting force control (friction in mechanical transmission). Performances of this algorithm are evaluated in simulation and the necessity for a CoM trajectory planning is highlighted by an analysis of obstacle crossing using this criterion.

Dynamic yaw and velocity control of the 6WD skid-steering mobile robot RobuROC6 using sliding mode technique

A robust dynamic feedback controller is designed and implemented, based on the dynamic model of the six-wheel skid-steering RobuROC6 robot, performing high speed turns. The control inputs are respectively the linear velocity and the yaw angle. The main object of this paper is to elaborate a sliding mode controller, proved to be robust enough to ignore the knowledge of the forces within the wheel-soil interaction, in the presence of sliding phenomena and ground level fluctuations. Finally, a 3D simulation is performed with an accurate physical engine to evaluate the efficiency of this designed control law.

Sliding-Mode Velocity and Yaw Control of a 4WD Skid-Steering Mobile Robot

The subject of this paper is the design and implementation of a robust dynamic feedback controller, based on the dynamic model of the four-wheel skidsteering RobuFAST A robot, undergoing high-speed turns. The control inputs are respectively the linear velocity and the yaw angle. The main objective of this paper is to formulate a sliding mode controller, robust enough to obviate the knowledge of the forces within the wheel-soil interaction, in the presence of sliding phenomena and ground-level fluctuations. Finally, experiments are conduced on a slippery ground to ascertain the efficiency of the control law.

Motion control of a compliant wheel-leg robot for rough terrain crossing

In this paper, we propose the use of compliant elements in the actuation of a wheel-legged robot in order to improve its locomotion properties on unknown and irregular terrains. Detection of the obstacles is achieved by a synergistic use of the structural compliances. The robots capabilities to surmount steep obstacles is thus improved thanks to the inertia of the chassis and flexibility in postural control. In the proposed robots kinematics, the four wheels are attached to the main body through vertical series elastic actuators (SEA) and with a passive horizontal compliant mechanism subject to a specific wheel speed control. The overall control relies on postural servoing and a local reactive loop which adapts the vertical forces applied by the SEA on each wheel according to the detected obstacle and the stability margin. The resulting system is evaluated with physical simulations for two case studies: a canonical steep obstacle on one wheel at a time and multiple random rough terrains.

Flapping-Wing Mechanism for a Bird-Sized UAVs: Design, Modeling and Control

Birds daily execute complex maneuvers out of reach of current UAVs of comparable size. These capabalities are at least partly linked to the efficient flapping kinematics. This article describes the flapping wing mechanism developed within the ROBUR project to create a bird-sized UAV relying on such advanced kinematics.

Quasi-Static Motion Simulation and Slip Prediction of Articulated Planetary Rovers Using a Kinematic Approach

Wheel slips are unavoidable when moving on a 3D rough surface. They are mainly due to geometrical features of contact surfaces. In this paper, we propose a model for predicting rover motion and contact slips by using a kineto-static model coupled with a linear contact model derived from semiempirical tire/terramechanics approaches. The paper also introduces a coherent approach for motion simulation of uneven articulated rovers which is computationally efficient and can then be used for autonomous on-line path planning. Model results are compared to another numerical model based on a multibody dynamic model including frictional contacts. The well-known rocker-bogie chassis, a highly articulated structure, is chosen to illustrate results and their comparison. Results demonstrate that for a slow motion, the proposed model approximates with a good accuracy the general behavior of the robot with a minimal time computation.