Faïz Benamar

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.

Decoupled control of posture and trajectory of the hybrid wheel-legged robot hylos

This paper addresses the control of a hybrid wheel-legged system evolving on rough terrain. First, the posture and trajectory parameters are introduced. Then, a decoupled posture and trajectory control algorithm based on the velocity model of the robot is proposed. Last, the performance and feasibility of the control algorithm are evaluated through simulations and experiments with the Hylos robot.

Design and control of an active anti-roll system for a fast rover

Off-road operational conditions require large sus- pension displacements and a significant clearance between the ground and the main frame, yielding to an elevated position of the vehicle mass center. Consequently, this makes the vehicle more likely to turn over when cornering fast. This paper proposes a new design, and its associated control, of an active device which improves the stability of fast rover moving up to 10 m/s. The proposed design can be equipped on any off-road chassis which has independent suspensions. We propose the using of an active anti-roll system allowing the control of the roll angle and thus improving the vehicle stability, especially when turning or when moving on slopping ground. The proposed system increases the controllability of the vehicle, by giving access to the roll angle which is usually uncontrollable. We develop a model based predictive controller for the roll dynamics, which minimizes the load transfer during cornering and the energy consumed by the actuators. The control model is based on a dynamic model of the rover and on a stability criteria defined by the lateral load transfer. Dynamic simulation, carried out for different rover trajectories with different speeds, show the benefit of the proposed active system and the validity of the control approach.

Analysis of self-reconfigurable modular systems: a design proposal for multi-modes locomotion

This work presents some general considerations on self-reconfigurable robots design, and proposes an original design of mechatronic modules. Geometrical and kinematical features of these modules, offer the ability to be used as well as wheels to produce rolling motion, and as joints for building kinematic chains as legs, arms or snakes.

An Expandable Mechanism for Deployment and Contact Surface Adaptation of Rover Wheels

In this paper, an expandable mechanism for unfolding wheels is proposed. This mechanisms combines 2 elementary mechanisms. One allows the deployment of the rim the other one ensuring the contact shape adaptation.

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.

Pose estimation-based path planning for a tracked mobile robot traversing uneven terrains

A novel path-planning algorithm is proposed for a tracked mobile robot to traverse uneven terrains, which can efficiently search for stability sub-optimal paths. This algorithm consists of combining two RRT-like algorithms (the Transition-based RRT (T-RRT) and the Dynamic-Domain RRT (DD-RRT) algorithms) bidirectionally and of representing the robot-terrain interaction with the robots quasi-static tip-over stability measure (assuming that the robot traverses uneven terrains at low speed for safety). The robots stability is computed by first estimating the robots pose, which in turn is interpreted as a contact problem, formulated as a linear complementarity problem (LCP), and solved using the Lemkes method (which guarantees a fast convergence). The present work compares the performance of the proposed algorithm to other RRT-like algorithms (in terms of planning time, rate of success in finding solutions and the associated cost values) over various uneven terrains and shows that the proposed algorithm can be advantageous over its counterparts in various aspects of the planning performance. Robot pose and its stability are estimated to plan paths over uneven terrains.The path-planning problem over uneven terrains is solved using a novel algorithm.More stable paths can be found with the novel approach than with other approaches.The planning is tested over a real 3D point-cloud map built with a 3D RGB sensor.

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.

Joint angle estimation with accelerometers for dynamic postural analysis.

This paper presents a new accelerometer based method for estimating the posture of a subject standing on a dynamic perturbation platform. The induced perturbation is used to study the control mechanisms as well as the balance requirements that regulate the upright standing. These perturbations are translated into different intensity levels of speed and acceleration along longitudinal and lateral directions of motion. In our method, the human posture is modeled by a tridimensional, three-segment inverted pendulum which simultaneously takes into account both the anterior-posterior and medio-lateral strategies of hip and ankle. Four tri-axial accelerometers are used her, one accelerometer is placed on the platform, and the other three are attached to a human subject. Based on the results, the joint angle estimated compare closely to measurements from magnetic encoders placed on an articulated arm joint. The results were also comparable to those found when using a high-end optical motion capture system coupled with advanced biomechanical simulation software. This paper presents the comparisons of our accelerometer-based method with encoder and optical marker based method of the estimated joint angles under different dynamics perturbations.

Steering behaviour and control of fast wheeled robots

This paper is devoted to mechanical and control analysis of steering systems of fast wheeled robots. Steering performances are analysed by means of a formulation of a path-tracking control problem. The mechanical model takes into account the wheel-sliding and wheel-elasticity which are relevant to the steering behaviour. When the path is a straight line or a circular arc, the tracking offsets are small and the assigned velocity is constant, the proposed tracking scheme leads to a controller using direct state feedback and linear quadratic regulation approach. Four different steering ways are examined, they combine steer and drive actions on a four wheeled robots.