Powertrain, steering and control components for the NMBU Agricultural Mobile Robotic Platform

Abstract

The first part of this thesis addresses the design of the NMBU Mobile Agricultural Robot's drivetrain and steering assembly, while the second part discusses aspects related to controlling these systems.

Part I: First, power requirements are calculated based on the expected \textit{worst case} scenario. Second, the principle design of the systems is determined and parts are selected. For the drivetrain, a hub reduction system is used. The system comprises of four separate \textit{wheel modules}, each with one brushless DC motor installed over the module's wheel. Power is transferred to an in-wheel planetary gearbox via a timing belt. For the steering assembly, brushless integrated servomotors with planetary gearboxes are used. The power distribution system is also discussed, and two designs are proposed, one for future and one for immediate implementation.

All parts are selected based on availability, quality and cost. As a result, the components for the drivetrain are not perfectly matched. However, they allow for testing of the basic principle. Preliminary tests of the powertrain and steering assembly show promising results.

Part II: Different components related to communication and control of the robot are presented, and the principle design of the motor control system is determined with interoperability between current and future systems in mind. \textit{ROS} (Robotic Operating System) is selected to be used as the robotic framework, and the main computer and the various motor controllers are to communicate using the \textit{CANopen} protocol. The process of configuring the propulsion motor controllers to match the propulsion motors is described, as is the process of setting up the servomotors. Problems were faced using the propulsion motors built in \textit{Hall sensors} for feedback in \textit{closed loop speed mode}, as the sensor assembly’s resolution proved to be too coarse. Alternative feedback sensors are then proposed.