Development and control of an omnidirectional mobile robot on an etherCAT network

An omnidirectional mobile platform possesses the ability to be maneuvered in any direction instantaneously from any starting position. Control of such, requires mathematical operation that is crucial to occur within a specified time, particularly in autonomous navigation. In this paper, we present a development approach of a four-wheel omnidirectional mobile robot using mecanum wheels for directional flexibility and the recently standardized fieldbus, EtherCAT, for performing control instructions within the network. We implemented an open source EtherCAT master stack with a dual kernel real-time approach, Xenomai, configured to run alongside standard Linux on an embedded board to meet the real-time requirements of a main controller. The main controller schedules and executes multiple periodic tasks such as recognition of the target position, computation of the velocity commands to navigate along a target trajectory, and running the actual control task of servo drives. Feasibility of the EtherCAT solution for the mobile robot controller is determined thorough a statistical analysis in terms of periodicity of control task, corresponding jitter, and execution time. Actuation of the mobile robot is performed by implementing a proven trajectory planning algorithm considering the physical limits of the system to generate velocity commands that could track an S-curve path. Finally, accuracy of the mobile robot is determined by analyzing the encoder feedback and is represented in Cartesian space.