Integrating the European Proximity Operations Simulator with the Formation Flying Testbed

Kurzfassung

The European Proximity Operations Simulator EPOS (part of GSOC, Oberpfaffenhofen near Munich) allows simulating Rendezvous and Docking scenarios involving two spacecraft and integrating rendezvous sensors (Hardware-in-the-Loop). Mockups of the spacecraft are mounted on 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 (RTOS) VxWorks combined with Matlab/Simulink Real-Time Workshop. This environment demands that any customer adapt his simulation code, satellite simulator etc. to EPOS' real-time control. In detail, this involves considerable effort and time which may obliterate the benefits of a Hardware-in-the-Loop simulation with EPOS. This paper presents the author's diploma thesis in a compressed form. Its aim is to reduce this effort distinctly by designing, implementing and testing a software package which connects any external satellite simulator, "as it is", via Ethernet to the EPOS real-time control system. Although this strategy 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 sub-protocols, 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 an external simulator and EPOS (realization of starting conditions, timing...) and interpolate the robot trajectory in-between external simulator commands. These communication protocols are implemented in the form of Simulink S-Functions, not only compatible to Windows but also to the Real-Time Operating System VxWorks. SCP/RSP 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 is illustrated that connection quality in the local EPOS network allows coupling an external simulation with EPOS via Ethernet, as long as the external simulator's sample frequency is not too large. Moreover, drift, e.g. the time differential between the external simulator's clock and the EPOS real-time clock, shows to be in the limits of timer hardware precision. As a result, simulations can be run for many hours before drift becomes a problem. SCP/RSP 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 obligatorily. SCP/RSP allows running an external simulation at a much lower frequency by translational and rotational interpolation. And SCP/RSP simplifies the simulation process. External simulations can be started and stopped without the need to restart the EPOS real-time simulation.