The Planning and Control of Space Robotic Systems

In the years ahead, robotic systems are expected to play an increasingly important role in space applications. One broad area of application is in the servicing, construction, and maintenance of satellites and large space structures in orbit. For example, robotic systems may be used to inspect, capture, and repair/refuel damaged satellites. Similarly, coordinated teams of robots might deploy, transport, and assemble structural modules for a large space structure.

The goal of this project is to develop sensing, planning, and control technologies to enable free-flying and free-floating space robots to perform the tasks expected in future missions. This entails the development of:

(*) Sensing strategies to determine the dynamic state, shape, and parameters of unknown/uncertain space targets
(*) Planning algorithms that make use of target knowledge and fundamental mechanics to determine safe approach trajectories and manipulation schemes
(*) Control algorithms to execute these plans in the presence of high joint friction, limited actuation, sensor uncertainty, and dynamic interactions between target and robot
(*) Multi-robot cooperative approaches that enhance the efficiency, reliability, and effectiveness of the above methods

Intelligent Vision-Based Sensing
In order to effectively plan and control robotic interactions with space targets, knowledge regarding the dynamics, inertial properties, and geometric shape of the target must exist. Since a priori knowledge about these targets is often imperfect or incomplete, much of this information must be collected using sensors in orbit. Current work is focused on robustly and efficiently extracting this information using sequences of range image data, taking advantage of fundamental knowledge of mechanics and sensor uncertainty models.

Safe Path Planning
Robots approaching a satellite or space structure must take care not to collide with the target or damage it with exhaust from their thrusters. Trajectory planning methods have been developed to optimize the approach, accounting for the effects of orbital mechanics and physical system constraints such as actuator limits and fuel usage. The trajectory optimization cost function includes safety and collision avoidance through metrics which measure relative velocities and positions of the satellite, robots, and exhaust plumes and penalize movement in dangerous directions. Current work suggests that reasonable approach times and fuel consumption could be achieved using realistic thrusters.

Free-Flying Manipulator Control
Once a robot reaches a target such as a satellite, it needs to grasp and hold on to it while limiting dangerous interactive forces and torques in its manipulators. Unlike a terrestrial robot (whose base is fixed to ground), a space robot cannot ignore the dynamic interactions between the manipulator base and the payload (see Papadopoulos, et al). Current work in the area of grasp execution and relative motion damping includes advanced control techniques (based on the virtual manipulator principal) and cooperative robot strategies including distributed sensing and shared manipulation.

Coordinated Assembly of Large Space Structures
Many unique challenges exist in the robotic assembly of large space structures. Structures on the order of thousands of meters in size experience very low frequency vibrations (period of tens of minutes to hours) and considerable thermal deformations as they move in and out of the earth's shadow in orbit. In order for tasks to be planned effectively, these deformations must be estimated by the robotic systems. The large size of such space structures mandates the use of multiple robots working as a team to perform the tasks of sensing, module transportation, assembly, and maintenance. In order for such missions to be successful, new methods will need to be developed for the sensing, planning, and control of cooperative robotic teams in orbit.