Development of Low-Inertia High-Stiffness Manipulator LIMS2 for High-Speed Manipulation of Foldable Objects

In this paper, a dual-arm robot system for high-speed manipulations, which is named LIMS2-AMBIDEX and is developed to compete in the IROS2018 Robotic Challenge, is presented. It has two seven-degrees-of-freedom (DO
$F$
) lightweight arms, a three-DOF head, and a one-DOF gripper to manipulate foldable objects. Because all the heavy actuators are placed at the shoulder, it has remarkably low mass beyond the shoulder (2.63 kg), which guarantees an inherent safety at high speeds. Utilizing tension-amplification mechanisms, the high stiffness and strength are achieved, and thus it has the control performance comparable to conventional industrial manipulators. A unique three-DOF wrist mechanism, whose motions directly represent the quaternion values of the joint orientation, can manipulate objects without singular points in the entire range of motion. In order to utilize the object's inertia during rapid manipulation, the gripper was specially designed: it has a one-DOF finger to grasp the upper rib of the foldable fan and two supporting forks to grasp the bottom rib stably. For real-time performance and increased scalability, a software framework was developed based on Robot Operating System (ROS). The real-time capability is achieved by using the real-time development framework Xenomai and the high-speed communication protocol EtherCAT. As most of the algorithms are implemented in the distributed nodes using ROS, it is convenient to expand, improve, and replace the algorithms. Consequentially, the entire motion of the Fan Robotic Challenge Phase I Modality B required 1.05 s, which is substantially faster than a similar manipulation by most humans.