Coordinated Mobile Manipulator Point-Stabilization Using Visual-Servoing Techniques Marco Gilioli, Claudio Melchiorri DEIS - Department of Electronics, Computer Science, and Systems University of Bologna, via Risorgimento 2, 40136 Bologna, Italy Email: {mgilioli, cmelchiorri}@deis.unibo.it Abstract In this paper we consider the problem of stabilizing in a desired configuration a mobile manipulator; only the arm’s joint displacements information and the mea- sures provided by a camera mounted on the end-effector are used to stabilize the system. In particular, no knowledge about the position and the orientation of the mobile base is supposed to be available. An hy- brid control algorithm, based on the concatenation of a sensor-based feedback control and an open-loop strat- egy, is proposed. A 3-dof planar manipulator mounted on a mobile base, modelled as an unicycle, is consid- ered as case study, and simulation results are reported in order to demonstrate the capabilities of the proposed control algorithm. 1 Introduction In this paper, we consider the control problem for mo- bile manipulators, i.e. systems composed by a robotic arm installed on a mobile base. These systems are able to reach and operate over targets which are initially outside the working space of the robot arm. Mobile manipulators can be divided into many categories, de- pending on the tipology of the mobile base. In par- ticular, nonholonomic wheeled vehicles, subject to the rolling-without-slipping constraint, are considered, [1]. As far as the control of these devices is concerned, one can identify two main problems: the control of the mo- bile base and the base/arm coordination. With respect to the mobile base, it is well known that a continuous state feedback law, able to exponentially stabilize the wheeled base to a desired configuration, cannot exist, [2]. This problem has been solved by sev- eral authors with different approaches. For example, in [3], [4] (among may others) a time-varying state feed- back control has been proposed. The base/arm coordination problem has been ad- dressed in different manners. For example, in [1] and [5] the authors, assuming that the manipulator’s end-effector follows a pre-computed trajectory, control the base in order to maintain the arm in a preferred configuration, that is computed on the basis of its manipulability index. Recently, in order to obtain more robust input-output stabilization laws, visual data provided by a camera mounted on the robot have been considered for the design of feedback control loops. In [6], the problem of tracking a curve with a mobile base using only the camera information has been considered. In [4] and [7], a visual-servoing technique able to solve the point- stabilization problem of the mobile manipulator has been proposed. The base is stabilized by an hybrid time-varying technique, and its state is reconstructed on the basis of the camera information, assuming as known the position of a target. The camera measures are also used to stabilize the manipulator. In this paper, the stabilization problem for a mobile manipulator using a vision-based approach is consid- ered. The proposed control strategy takes into consid- eration two typical problems arising in mobile manip- ulator control: the difficulties in the base state recon- struction and the coordination problem. In particular, if the base control inputs are computed by consider- ing only the instantaneous arm joint displacements and the manipulator is controlled using the camera infor- mation, the base reconstruction problems are avoided. Moreover, an extra task must be solved if the camera measures are considered to stabilize the manipulator: the targets must be maintained inside the camera field of view during the whole stabilization process. The structure of this paper is the following. In Section 2, the kinematic model of the mobile manipulator and of the camera measures is reported. In Section 3, the control law, able to stabilize the system in a desired configuration, is discussed, while in Section 4 simula- tion results are reported to validate the proposed ap-