Integration of the Formation Flying Testbed with the European Proximity Operations Simulator

Kurzfassung

The European Proximity Operations Simulator EPOS (part of GSOC, Oberpfaffenhofen near Munich) allows to simulate Rendezvous and Docking scenarios involving two spacecraft and integrating rendezvous sensors (Hardware-in-the-Loop). Mockups of the spacecraft are mounted to two industrial robots. Both spacecraft can be moved in six degrees of freedom each, thus simulating relative orientation. A real-time control system operates the robots, involving the Real-Time Operating System VxWorks combined with Matlab/Simulink Real-Time Workshop. This environment demands that any customer adapt his simulation code, satellite simulator etc. to EPOS' needs. In detail, this involves considerable effort and time which may obliterate the benefits of a Hardware-in-the-Loop simulation with EPOS. It is the aim of this thesis to reduce this effort distinctly by designing, implementing and testing a software package which connects any external satellite simulator via ethernet to the EPOS real-time control system. Although this stategy solves many problems, new ones are created, like the fact that a non-deterministic network, like ethernet, is used in a real-time environment. To cope with these problems, an application layer communication protocol is developed, specifically tailored to meet EPOS' needs. It comprises two subprotocols, the Simulation Connection Protocol (SCP) and the Remote Simulation Protocol (RSP). Among many other tasks, these protocols realize a data connection between two Simulink models, monitor packet delay, manage the interaction between a remote simulator and EPOS (realization of starting conditions, timing...) and interpolate the robot trajectory in-between remote simulator commands. These communication protocols are then implemented in the form of Simulink S-Functions, not only compatible to Windows but also to the Real-Time Operating System VxWorks. The remote control system is tested using a demo scenario running on a Formation-Flying-Testbed. This simulation environment is a multi-satellite simulator developed by the Formation-Flying group at GSOC. It runs Formation-Flying algorithms with the possibility to use real GPS receivers (in combination with a GPS signal simulator) and a SPARC based real-time computer to serve as On-Board Computer. Evaluation of this demo-scenario is presented. It proofs, that the remote control system designed and implemented in this thesis indeed works and that it fullfills the requirements. It is illustrated, that connection quality in the local EPOS network allows to couple a remote simulation with EPOS via ethernet, as long as the remote simulator's sample frequency is not too large. Moreover, drift, e.g. the time differential between the remote simulator's clock and the EPOS real-time clock, shows to be in the limits of timer hardware precision. As a result, remote simulations can be run for many hours before drift becomes a problem. The remote simulation software adds to the flexibility of EPOS. Before, an initial speed and angular velocity different from zero was inconvenient to realize. SCP/RSP carries out this task automatically, by determining an initial trajectory when needed. Software running on the EPOS real-time control system has to run at a frequency of $250Hz$ obligatorily. SCP/RSP allows to run a remote simulation at a much lower frequency by translational and rotational interpolation. And SCP/RSP simplifies the simulation process. Different remote simulations can be started and stopped without the need to restart the EPOS real-time simulation.