Testbed for on-orbit servicing and formation flying dynamics emulation

Kurzfassung

The testbed for on-orbit servicing and formation flying dynamics emulation is a facility to emulate the force and momentum-free dynamics of multi-spacecraft missions on ground. The facility consists of a large granite table and several air cushion vehicles that float on this table. The granite table has a size of 4m x 2.5m and a thickness of 0.6m with a very high evenness. The air-cushion vehicles consist of two parts: a lower translation platform and an upper attitude platform. Both platforms are connected by a spherical air bearing. The translation platform carries the flat air bearings which enable the vehicle to float on the table, high pressure air tanks to support the flat and the spherical air bearings and a vertical linear actuator. With this actuator it is possible to adjust the altitude of the spherical air bearing and thereby the altitude of the attitude platform. All components needed for a control loop to actuate the air cushion vehicles are mounted on the attitude platform. These are 12 proportional cold gas thrusters with high pressure air tanks, 3 reaction wheels, an onboard computer with real-time operating system and Wireless LAN for communications and software upload, an inertial measurement unit (IMU) and a sensor for an infrared tracking system. In addition the attitude platform carries a power system with batteries to support all components with electrical energy and a balancing system to move the center of mass in the center of the spherical air bearing. Due to the linear air bearing the system can emulate 2 translational degrees of freedom. The third translational degree of freedom (parallel to gravity vector) can be actuated by the vertical linear actuator. As the attitude platform can tilt and rotate, 3 rotational degrees of freedom are emulated. So there is a total of 5+1 degrees of freedom that can be emulated by the testbed. One purpose of this testbed is the testing of control algorithms for multi-spacecraft missions. It can be used for precise formation control as well as path-planning and formation acquisition. In addition to the usage of real hardware as in Hardware-in-the-Loop simulations, the force and momentum-free dynamics are not simulated but are real (emulation). This is also a foundation for the second task of the testbed: the emulation of contact dynamics for Rendezvous and Docking missions. Models of docking adapters can be mounted on the attitude platform of the air-cushion vehicles and docking maneuvers with real contact dynamics can be tested on ground. A further purpose of the testbed is the qualification of dedicated relative navigation sensors and inter-spacecraft communication systems. These components can also be mounted on the attitude platform and tested in an agile environment. This paper describes the design and configuration of the testbed for on-orbit servicing and formation flying dynamics emulation. Results showing the quality of the force and momentum-free environment are presented. First control, navigation and thruster actuation algorithms have been developed and their design and the result of their usage within the testbed are also shown. Finally possible scenarios for the usage of this testbed are presented and an outlook on further developments and improvements is given.