Design and development of a control system for the Robotic Arm Schunk LWA 3

This thesis presents the design of the control system for a robotic arm, the Schunk Lightweight Arm 3. This manipulator is used as part of the on-ground simulation of on-orbit satellite servicing performed at the Rendezvous & Docking facility of Thales Alenia Space in Turin. This area was built in the framework of the STEPS (Sistemi e Tecnologie per l'EsPlorazione Spaziale) program; here, the docking maneuver is simulated on a micrometric black granite floor where two vehicles (the chaser and the target) move floating over a thin air film in order to obtain a frictionless dynamic and have a testing environment able to simulate orbiting conditions. The RV&D facility is also equipped with two robotic manipulators necessary for the capture but more in general for servicing (e.g., refueling, repairing and/or replacement of parts, removing of space debris) of a target. The control development started from the study of the mechanical and electrical configuration of the robotic arm. The technical specifications of actuators and motor drivers were examined and a suitable control architecture has been selected. The final solution consists of a workstation that acts as the central control system, connected via CAN bus to the terminal block of the LWA 3, powered by a 24 VDC voltage. The control system was entirely designed using the Python programming language. The protocol used to exchange data between the master and all the modules of the Schunk LWA 3 is unified and independent from the bus interface used. The module receives serial data, interprets it and acknowledges the command. A command typically is composed by an Identifier, followed by a Command ID, a Parameter ID or a Motion ID, based on the task that the manipulator has to perform, and data bytes if required. The first step of the control design was the creation of a Python script to initialize and enable the CAN communication from the workstation to the robotic arm. The next step was the construction of a CAN message to give commands to the LWA 3: each one of those is the concatenation of a series of strings that correspond to a different section of the CAN message. All the functions for building the string of data were divided in different Python scripts, based on the type of tasks that they perform. They can vary from general commands (e.g., resetting the encoders, halting the motion of the arm or returning to the home configuration) to more complicated ones like the joint control and some direct kinematics functions. Joint control can be performed either choosing the velocity at which each module must move or setting the final position that it must reach. Also, some functions were implemented to directly send the arm in pre-defined configurations from where it is easier to reach objects to be caught. A crucial task was the creation of a direct kinematic function that constantly computes the position and orientation of the end effector and prints them on the screen. The commands were sent to the arm through a Graphical User Interface that replaced its previous version of the Command Line Interface launched from the terminal. The final result is a window that gives the user a more intuitive control system. The GUI has a section for sending commands to the arm on the left and a telemetry one on the right where all the relative positions and velocities of each module, the position and orientation of the end-effector are displayed.