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Modeling and design of control algorithms for the mobile manipulator

Equations of motion have been derived for a 4-wheel, skid steering mobile platform Rex whose wheels can either be driven independently or coupled sidewise. Classical tools of analytic mechanics are used, including the Lagrange-d'Alembert Principle. Special attention is paid to the platform with coupled side wheels rolling without longitudinal slips. The equations of motion incorporate the kinematic and the dynamic part, and account for both the wheel rolling resistance forces as well as the wheel lateral slip counteraction forces. They have been given a form of the control affine system steered by the torques applied to the platform's wheels. The computation of the dynamic parameters of the system is supported by the Autodesk Inventor 3D CAD package. The parameters of the rolling resistances and the slip counteractions are determined experimentally. Two control problems of the Rex platform have been studied: the motion planning problem and the trajectory tracking problem. The former problem consists in designing a control that steers the platform through prescribed intermediate points to a desired position and orientation, and halts it there. The motion planning through waypoints algorithm has been employed to this objective. Complementarily, a reference trajectory can also be generated by driving the Rex platform manually, using a PS3 pad. The latter problem, that of tracking the reference trajectory of motion of the platform, has been solved by means of the Nonlinear Model Predictive Control (NMPC) method implemented in the ACADO (Automatic Control and Dynamic Optimization) software package. Position and orientation as well as velocity of the Rex platform are estimated by employing the receding horizon estimation method relying on the data provided by a Motion Capture system, also included in the ACADO package. A remarkable robustness of the predictive control algorithm against model uncertainties has been observed. The model based trajectory planning is realized of-line, while the predictive control and the state estimation run in real time. A mobile manipulator RobRex has been devised, built of the platform Rex and a 4R lightweight on board manipulator controlled kinematically. As an alternative to the predictive control algorithm a control algorithm of the mobile manipulator utilizing the concept of the artificial force has been developed.

Implementation and demonstration

In order to achieve this objective an integrated experimental environment has been created including the mobile manipulator RobRex and the Motion Capture system Optitrack. This mobile manipulator plays the role of the demonstrator of technology serving to test the program architecture and control algorithms to be transferred later to the mobile manipulator Scout manufactured by the Industrial Research Institute for Automation and Measurements (IRIAM). In order to implement the control algorithms an integrated programming framework has been designed. Its core ingredients are constituted by the programming frameworks ROS and OROCOS completed by real time extensions Xenomai Linux and RTnet. For demonstration purposes a laboratory inspection scenario has been created that encompasses delivering the video camera from the base station to a prescribed region, performing its inspection and sending video images to the operator station, and finally returning to the base station. The video images can also be gathered and transmitted during the robot motion. The accomplishment of the inspection task requires motion planning and trajectory tracking of the Rex platform, and control of the on board manipulator. The motion planning has been done using the dynamics model of the platform by means of the planning through the waypoints algorithm, then the tracking control is realized by the predictive control algorithm coupled with the receding horizon estimation of the platform's state based on the data from the Motion Capture system. The manipulator control is purely kinematic. The behavior of the demonstrator of technology proved to be satisfactory and instructive. The same inspection scenario has been implemented on the IRIAM Scout mobile manipulator. Results of experiments with Scout call for a further improvement in the aspects of model identification, motion planning, and tracking control. Technically this improvement may be achieved by increasing the throughput of Scout's information channels transmitting control signals.