Redundantly actuated mobile robots using conventional wheels need a precise coordination of their actuators in order to guarantee a safe and precise motion without generating high internal forces and slippage. Using the instantaneous centre of rotation (ICR) of the chassis to describe this motion is a well established method. But the ICR is a mathematical concept which is hardly achieved on a real robot. This paper addresses the problem of ICR estimation of a non-holonomic omnidirectional mobile robot using conventional wheels. Instead of estimating the ICR in the working space, our approach estimates it in the actuators' space. The algorithm is presented in its general form and then adapted for a particular robot. The use of the algorithm with other omnidirectional robots is also discussed. Results from extensive testing done in simulation as well as with a real robot are presented, demonstrating the effectiveness of the proposed method.

Omnidirectional mobile robots, i.e., robots that can move in any direction without changing their orientation, offer better manoeuvrability in natural environments. Modeling the kinematics of such robots is a challenging problem and different approaches have been investigated. One of the best approaches for a nonholonomic robot is to model the robot's velocity state as the motion around its instantaneous center of rotation (ICR). In this paper, we present a motion planner designed to compute efficient trajectories for such a robot in an environment with obstacles. The action space is modeled in terms of changes of the ICR and the motion around it. Our motion planner is based on a Rapidly-Exploring Random Trees (RRT) algorithm to sample the action space and find a feasible trajectory from an initial configuration to a goal configuration. To generate fluid paths, we introduce an adaptive sampling technique taking into account constraints related to the ICR-based action space.

AZIMUT-3 is an omnidirectional non-holonomic (or pseudo-omnidirectional) robotic platform intended for safe human-robot interaction. In its wheeled configuration, shown in Fig. 1, AZIMUT-3 uses eight actuators for locomotion: four for propulsion and four for steering the wheels, which can rotate 180 degrees around their steering axis. Propulsion is done using standard DC brushless motors (Bayside K064050-3Y) with optical encoders (US Digital E4-300-157-HUB, 0.3 deg of resolution), capable of reaching 1.47 m/s. The platform uses steerable wheels motorized using differential elastic actuators (DEA), which provide compliance, safety and torque control capabilities. AZIMUT-3's hardware architecture consists of distributed modules for sensing and low-level control, communicating with each other through a 1 Mbps CAN bus. A Mini-ITX computer equipped with a 2.0 GHz Core 2 duo processor running Linux with real-time patches (RT-PREEMPT) is used on-board for high-level control modules. Nickel-metal hydride batteries provide power to the platform for up to 3 hours of autonomy. A passive vertical suspension mechanism (Rosta springs) is used to connect the wheels to AZIMUT-3's chassis, allowing them to keep contact with the ground on uneven surfaces. The platform has a 34 kg payload capacity and weights 35 kg.

Robots are usually built using stiff actuators that can provide impressive motion performances. However, they struggle to control the force, they do not handle collisions graciously and are generally bad at interacting significantly with partially unknown or kinematically constrained environments. One solution is to add a force sensor in the closed-loop control of backdrivable actuators [1], but this is limited in terms of stability, safety and robustness [2] [3]. One of the foremost initiatives using this method was undertaken by the German Aerospace Center (DLR) and resulted in three generations of extensively optimized lightweight robotic arms [4] that can physically interact with people. Performances are impressive but robustness is still an issue.

The most common ground locomotion method to make a mobile robot move is to use two-wheel drive with differential steering and a rear balancing caster. Controlling the two motors independently makes the robot non-holonomic in its motion. Such robots can work well indoor on flat surfaces and in environments adapted for wheelchairs. But the benefit of providing mobility to a robot directly relies on its locomotion capability, for handling different types of terrains (indoors or outdoors) and situations such as moving slowly or rapidly, with or without the presence of moving objects (living or not), climbing over objects and potentially having to deal with hazardous conditions. It is with this objective in mind that we designed AZIMUT. AZIMUT is a legged tracked wheeled robot capable of changing the orientation of its four articulations. Each articulation has three degrees of freedom (DOF): it can rotate 360deg around its point of attachment to the chassis, can change its orientation over 180deg, and rotate to propulse the robot.