MOSAR-WM: A relocatable robotic arm demonstrator for future on-orbit applications

In the past few years, the raise of space robotics yielded novel potential applications. The utilization of more advanced and capable robotic manipulators opens a whole new horizon of possibilities for future space missions, ranging from On-Orbit Servicing (OOS) of existing satellites (for refueling, Orbital replacement unit (ORU)or de-orbiting)to On-Orbit Assembly (OOA) and reconfiguration of modular spacecraft. This paper deals with the design and primary Manufacturing, Assembly, Integration and Testing (MAIT)activities of a novel robotic manipulator demonstrator for such on-orbit applications. MOSAR-WM is a 7 degree of freedom(DOF)manipulator, 1.6-meter long, symmetrical and relocatable (aka. “walking” capable). Its overall structure is human-like with asymmetric joints. Manipulator joints are hollow-shaft for internal cable routing, and include cutting-edge space-compatible technologies. Each joint embeds a torque sensor in addition to position sensors (incremental and absolute encoders). The kinematic architecture of MOSAR-WM offers a wide end effector workspace, and its stiff structure guarantees a high accuracy and repeatability while allowing compactness for launching and storing purposes. Each extremity of MOSAR-WM is equipped with a HOTDOCK standard interface that allows for mechanical connection, powering and controlling the arm. Manipulator avionics consists in seven joint controllers (one per joint) and an embedded computer called Walking manipulator controller(WMC) running a real time operating system. The WMC receives high-level commands from the external computing unit through the connected HOTDOCK interface. It also calculates the dynamic model of the robot to provide proper feed-forward terms for the joint control. Depending on the desired behaviour, the gains of the joint control loop are adaptive for optimal performance in position control. In addition, a Cartesian impedance control is implemented to allow for compliant operations. The joint controllers are daisy-chained through EtherCAT, while the control of each HOTDOCK is performed through a CAN bus managed by the internal WMC. MOSAR-WM is developed in the context of the European Commission’s Space Robotic H2020 MOSAR project. It aims to validate the developed technologies at Technology Readiness level (TRL)4 in a space representative scenario.