Using Robots for Advanced Rendezvous and Docking Simulation

Abstract

For human spaceflight missions rendezvous and docking (RvD) of two spacecrafts is state of the art today. For future satellite missions this close operation scenario becomes more and more interesting in the last years. These comprise so called on-orbit servicing missions (OLEV, DEOS) as well as explorations missions (Mars Sample Return). One of the critical issues of such mission is to ensure a safe and reliable rendezvous and docking process. Since the RvD process are known to be the most risky part, these operations must be carefully analyzed, simulated and verified before the mission can be launched.
\nRequired by the new type of satellite missions DLR set up a completely new and more advanced RvD simulation facility in 2010. The new facility called EPOS 2.0 will have full test and verification capabilities for on-orbit servicing missions as well as other RvD scenarios. The facility is based on two large industrial robots to deliver the 6-DOF motion in a representative maneuvering space for typical rendezvous and docking operation. The test bed will allow simulation of the last critical phase (ranging from 25m to 0m) of the final approach process including the contact dynamic simulation of the docking process.
\nThe realized simulation concept for the facility is a so called Hybrid Simulator. This name stands for a simulation method where one part of the motion is performed by numerical computations while the other part is executed by hardware. The hardware part comprises the robots as well as the facility monitoring and control system for operator control during simulation. The movement of the robots is a physical representation of the numerically calculated trajectory. This allows stimulating sensors which are mounted on the robots for hardware in the loop simulations. Utilization of industrial robots gives utmost flexibility for different applications in the future. The basis for the software part is a Matlab/Simulink environment. Complex functions can easily be implemented which are then auto-coded with Real Time Workshop and executed under the real-time operating system VxWorks on the EPOS target processor. This method is a model based design approach which helps the developer focussing on its simulation instead of time-consuming hand-coding of the guidance, navigation and control (GNC) algorithms.
\nSince the last two years the facility is continuously extended for different RvD applications. The paper describes the challenges of simulating RvD processes of on-orbit servicing missions and how these challenges are solved by using the new robotics-based simulation system EPOS 2.0. It presents the current status of the facility and provides an outlook of ongoing development activities. In addition the paper describes the implementation of a closed loop rendezvous scenarios based on a camera sensor which is the first RvD application using EPOS 2.0 facility. Furthermore the paper presents the first results for docking simulation.