Improved 7-DOF Control of ISS Robotic Manipulators

Canada’s contribution to the International Space Station (ISS) is the Mobile Servicing System (MSS). Its first component, the 7-degree-of-feedom (7-DOF) robotic manipulator Canadarm2 has been used for ISS assembly since 2001, extracting and installing new components from the space shuttle. In mission planning, the robotics trajectories for assembly operations are extensively analyzed, with particular concern for proximity to structure. Even with prepared trajectories, however, there can be unexpected deviations, resulting from unplanned events, or the use of seven joints. Due to the extra (7th) DOF in Canadarm2, any tip pose (position and orientation) has an infinite number of joint solutions, (since only 6DOF are needed to resolve the position and orientation in space). In Canadarm2, this redundancy allows for a special maneuver called the Pitch Plane Change, where the tip maintains its pose, while the arm plane is rotated. Mission planners take advantage of the Pitch Plane Change feature by repositioning the arm to maximize distance from structure. For 7-DOF Manual-mode flying, when the tip is commanded in Cartesian space, Canadarm2 computes the joint trajectory based on the Pseudo-Inverse algorithm, which uses a “minimum-energy” criterion to resolve the kinematic redundancy. The disadvantage with this control algorithm is that the final joint solution depends on the input hand-controller profile, which is never identical for two operators. This lack of determinism is a safety concern, because unplanned arm trajectories can result in collision with structure. As a result, mission planning community has decided to always fly Canadarm2 with one joint locked during Manual operations. With this approach, it becomes a 6-DOF manipulator, and a unique joint solution exists for a given tip pose. This approach is acceptable for robotics assembly tasks, but problems remain with unscripted tasks, such as robot-assisted extravehicular activities (EVA), where an astronaut at the arm tip indicates the required motion. In fact, restricting Canadarm2 to 6-DOF during EVA tasks can cause the arm to swing worse than with 7-DOF control, creating more dangerous situations. This paper presents an alternate 7-DOF control strategy, which maintains both determinism and flexibility, for increased safety. In this solution, we resolve the kinematic redundancy, not by minimum-energy, but by simultaneously maintaining the arm’s Pitch Plane as the tip is commanded in free space. With this method, we control the arm configuration and minimize dangerous elbow swing, without sacrificing any determinism, all of which contributes to a safer operational environment. In this paper, we will present the circumstances leading to our search for improved 7-DOF control, as well as our implementation, testing and analysis of the Pitch Plane Control solution in the specific case of Canadarm2, using the real-time MSS Operations and Training Simulator (MOTS). In conclusion, we will extrapolate other 7-DOF solutions that could further improve collision avoidance management in Canadarm2 and in Dextre, the next MSS robotics component.