| Wheeled mobile robots moving on uneven terrain are attracting interest at an impressive pace. In the work reported here two distinct architectures of two-wheeled mobile robots are proposed.; The first architecture, corresponding to the case where the two wheels linked by a frame lie in a vertical plane, constitutes the material of our earlier research and is laid out in the appendix. The system being unilaterally constrained by the environment, slipping or losing contact with the ground can occasionally occur. Therefore, nine distinct topologies are identified and accounted for in the model describing all the possible motion modes of the system. The mathematical model is formulated using the Natural Orthogonal Complement (NOC) and takes into account the terrain geometry. Additionally, the model includes necessary conditions for the switching between the distinct topologies.; The second architecture pertains to an Anti-Tilting Outdoor Mobile robot, ATOM, composed of two spherical wheels and a cylindrical central body. The spherical shape of the wheels allows the robot to restore its posture after flipping over, thus giving it the anti-tilting property. Moreover, this particular shape ensures pure-rolling motion on uneven terrain without resorting to any adaptive structure; i.e., without increasing the complexity of the system. Here, also, the mathematical model is developed using the NOC, while taking into account the terrain geometry. Moreover, constraints on the terrain curvatures are derived in order to ensure pure rolling. Although the design of ATOM is simple, this brings about new challenges in terms of control. According to its structure, ATOM pertains to the class of Mobile Wheeled Pendulums (MWP). A feature common to MWPs, that is not encountered in other wheeled robots, is that their central body, which constitutes the platform of the robot, can rotate about the wheel axis. Therefore, aside the nonholonomy aspects encountered in conventional wheeled robots; the central body stabilization problem is pointed out here and rigorously treated in order to avoid unstable zero-dynamics. For that, an intrinsic dynamcal property, referred to as the natural behaviour of the system, is brought forward and employed to control the heading velocity of the robot using the inclination of the central body. Moreover, a particular selection of the generalized coordinates and the system outputs allows a global stabilization of the system without resorting to any linearization. Furthermore, a posture control (preceded by a velocity and orientation control) is proposed based on sliding mode and Lyapunov function for navigation. Finally, the robust aspect of the controller is underlined by showing the control behaviour versus an over/under estimation of system parameters. |
