Remote Calibration and Trajectory Replanning for Robot Manipulators Operating in Unstructured Environments Antonio C. Leite ∗ , Fernando Lizarralde ∗ , P˚ al Johan From ∗∗ Ramon R. Costa ∗ , Liu Hsu ∗ ∗ Department of Electrical Engineering - COPPE, Federal University of Rio de Janeiro, Rio de Janeiro, Brazil. (e-mail: [toni, fernando, ramon, liu]@coep.ufrj.br). ∗∗ Department of Mathematical Sciences and Technology, Norwegian University of Life Sciences, ˚ As, Norway. (e-mail: pafr@umb.no). Abstract: In this work the problem of remote calibration and trajectory replanning for a subsea robotic manipulator is considered. Because the trajectory planning is normally done in a structured environment, several uncertainties arise when the robot is placed on the seabed and these need to be compensated for to guarantee that the task specifications are fulfilled. We address the particular problem of configuration errors in the robot base with respect to the configuration used during the off-line trajectory planning. A calibration method based on both internal and external sensors is presented in order to estimate the uncertainty in the location of the robot. Moreover, a trajectory replanning strategy in the Cartesian velocity space is proposed to guarantee that the originally planned trajectory is followed. Simulation and experiments performed with a 6-DoF robot manipulator and a real calibration grid show the viability of the proposed planning and control schemes. Keywords: Robot calibration, Underwater robotics, Trajectory replanning, Robot kinematics. 1. INTRODUCTION Calibration and task planning of robotic systems in remote and unstructured environments is of vital importance for accurate and robust operation of subsea fields (Augustson and Meggiolaro, 2010). In such scenarios, the planning of a reference trajectory is usually performed on the surface or in a structured place on-shore, or even in a simula- tion environment (Bellingham and Rajan, 2007; Trevelyan et al., 2008). However, once the robotic cell is inserted into its workplace, uncertainties will arise, for example in the robot’s actual location with respect to the location used during planning. The difference between the actual robot base location and the location adopted during the planning characterizes the trajectory replanning problem discussed in this paper. Several calibration schemes for kinematic and dynamic calibration are described in literature (Mooring et al., 1991; Beyer and Wulfsberg, 2004), but to the authors best knowledge, the specific problem of robot base con- figuration errors has not previously been addressed in this setting. Due to the nature of the problem the approach presented can be used not only for calibration of fixed- base underwater manipulators, but also to estimate the vehicle pose in underwater vehicle-manipulator systems. Robot calibration refers to a set of procedures for de- termining the real values of the geometric dimensions ⋆ This work was partially supported by the Brazilian Founding Agencies CNPq, FAPERJ, and the Norwegian Research Council. and mechanical characteristics of the robot structure. A taxonomy for robot kinematics calibration methods based on a calibration index which represents the number of independent equations available for calibration was pro- posed in Hollerbach and Wampler (1996). A well known methodology used to calibrate the robot with respect to a known geometric structure is to endow the robot end- effector with a probe. Upon touching the structure sur- face in appropriate points it is possible to estimate the transformation which relates the robot coordinates with the structure coordinates of interest (Ikits and Hollerbach, 1997). Also, open-loop and closed-loop calibration meth- ods have been proposed using different sensors such as cameras, laser beams, and triaxial accelerometers (Canepa et al., 1994; Motta et al., 2001; Lei et al., 2004). However, for all the approaches above, it is assumed and required that the location of the robot base is known in advance. This is not always the case, for example when the robot is operating in an unstructured environment, such as on subsea installations. In this work, a calibration method for estimating the uncertainties in the robot base configuration using a cal- ibration grid is proposed. We present solutions based on both internal and external sensors to solve the calibration problem. A calibration and trajectory replanning strategy in the Cartesian velocity space is presented to solve the problem of trajectory planning in the presence of uncer- tainties. Empirical studies obtained with a 6-DoF robot manipulator show the viability of the proposed scheme. Proceedings of the 2012 IFAC Workshop on Automatic Control in Offshore Oil and Gas Production, Norwegian University of Science and Technology, Trondheim, Norway, May 31 - June 1, 2012 ThA2.5 Copyright held by the International Federation of Automatic Control 59