Tina: The Modular Torque Controlled Robotic Arm - A Study For Mars Sample Return

Upcoming space missions, like the Mars Sample Return Mission, increasingly aim to include robots to enable highly skilled tasks and to increase safety. However, the requirements for such robots are high, due to the demanding environment and the high reliability of the system needed to operate independently in space. As part of the ESA project for a Sample Transfer Arm (STA) breadboard study, the German Aerospace Center (DLR) has developed a small modular torque controlled robotic manipulator that complies with the MSR mission requirements. It has 7 degrees of freedom (DOFs) and a total arm length of 2,30 m. The brakes hold the arm in position during no operation and serve as emergency stop. The reachability of the robot was investigated in a workspace study. Each robotic joint incorporates a brushless high torque DC motor combined with a harmonic drive gear stage and a planetary pre-gear stage. A Universal Motor Controller (UMC) is placed in each shoulder of the robotic arm, which consists of an FPGA, phase current measurement, the motor bridge driver and peripherals. To ensure safety and flexibility, a redundant matrix concept is integrated in the UMC, which allows control of more than one joint in case of a failure. The communication between the joints and the OBC (On-board-computer) is using SpaceWire with a 3 kHz cycle time, implemented in the joints FPGA. The motor control loop and the joint torque controller are implemented in the FPGA. The joint power is fed by two separate supplies, logic and motor, for a greater efficiency in power limited missions. The torque estimation analysis proved that the maximum required torques can be generated in each joint. To ensure the absolute high accuracy of the arm position sensing, the motors have a hall-sensor twelve step commutation and in addition a resolver on the link side resolver. An internal torque sensor in each joint is added to measure torques on the end-effector which allows collision avoidance during a task. To allow autonomous tasks, such as moving samples from a rover and inserting them into a sample container, various intelligence features have been included. For the arm control, a RCU (Robot Control Unit) is located in the base of the arm, implementing a Cartesian impedance and position controller. With all these features, the newly developed robotic manipulator meets all the needed requirements of the MSR mission. It is capable of interacting with sensitive spacecraft components, but can also be used as a payload manipulator for rovers.