Advance Modeling of a Skid-Steering Mobile Robot for Remote ...

Introduction 4The robot employed, in this project, for telepresence applications is a Pioneer 3-AT Robot,which is a four-wheeled skid-steering mobile robot (SSMR). A SSMR is a mobile robot withno steering mechanism, in which the motion direction is provided by turning the left-side andright-side wheels at different velocities. The absence of a steering system makes the robotmechanically robust and simple for terrain and outdoor environment navigation. However,the non-holonomic constraint of zero lateral velocity considered for standard differentialdrivenmobile robots, such as unicycles or car-like robots [1], [2], [3], does not hold andthe wheel slip must be taken into account. The wheel slip plays an important role in robotdynamics, as the wheel/ground interactions directly provide traction and braking forces thataffect the motion stability and maneuverability, and they greatly depend on the wheel slip.Moreover, under certain configurations of the robot dynamic properties, for instance the positionof its center of mass (CoM) and the moment of inertia along its principal axes, the tire/-ground dynamics occurring during skidding can produce large amplitude vibrations, whichcan eventually lead to the robot instability. Although such large vibrations never arise whenusing only the platform of the Pioneer 3-AT Robot, they suddenly increase when changingits inertia by adding a vertical structure on it, as it happens for the extended Pioneer 3-ATRobot employed in this project. These vibrations lead to an erratic motion during sharpturns, in particular when the robot is swiveling in place, which causes unacceptable disruptionof the video stream and destabilizes the structure. Because of the robot destabilizationand the video disruption, the remote user can loose the control of the robot for a moment.As the future aim of this project is the telepresence during laboratory class, the user needsfull maneuverability in small environment, therefore the robot jerky motion must be fullycontrolled. In order to provide a stabilization control strategy for the robot jerky motion, anadvance modeling of SSMRs, which reproduces the real robot vibrations, must be provided.In this thesis, we develop a three-dimensional dynamic model of SSMRs including a springdampertire model, which allows to reproduce the real robot jerky motion in a simulation environment.In particular, a complete description of the mobile robot employed in our project,together with its limitations and issues encountered until now [8], [9], is first provided inChapter 1. Then, an introduction to the ¨State of the Art¨ of SSMRs modeling is presentedin Chapter 2. In this section, a kinematic and dynamic model of a four-wheeled skid-steeringmobile robot is presented to characterize the skid-steering properties. In particular, the conceptof wheel slipping is presented and elaborated in order to characterize the wheel/ground