The Spacecraft Robotics Engineering and Controls Laboratory

G. Creamer
Spacecraft Engineering Department

S. Hollander
Space Systems Associates

Introduction: Over the past decade, significant interest has arisen in both the military and civil space communities in multi-spacecraft mission operations. These operations are used for autonomous rendezvous and capture, on-orbit servicing, remote inspection and surveillance, formation flying, and on-orbit robotic assembly. Critical enabling technologies necessary for these exciting missions include relative sensing for navigation, imaging, and communication; autonomous control logic for hands-off relative maneuvering and capture; and robotic mechanisms for autonomous fluids transfer, electronics upgrade, and structural assembly. The Robotics Engineering and Controls Laboratory (RECL) has recently been completed within the Naval Center for Space Technology to serve as a national testbed for real-time hardware-in-the-loop and software-in-the-loop research, development, and validation of these critical technologies.

Description of the Facility: The RECL is home to the dual-platform spacecraft Dynamic Motion Simulator (DMS), a full-scale realistic test environment for verification of sensor and control technologies using a 6 degree-of-freedom (DOF) servicer platform and a 4 DOF target platform; a local area network architecture for real-time ground-to-platform and platform- to-ground communication; and software to emulate spacecraft mass properties, thruster and reaction wheel actuators, and on-orbit environmental disturbances. Figure 1 shows the RECL and its DMS testbed. The servicer platform consists of an X-Y trolley assembly and a 6-DOF robotic manipulator arm capable of carrying up to 400 kg of payload. The target platform consists of an X-Y trolley assembly and a 2-DOF gimballed robotic mechanism capable of carrying up to 1300 kg of payload (a 6-DOF upgrade to the target platform will be completed by the end of FY02). The effective DMS workspace is approximately 30 m long X 13 m wide X 5 m high, with a control resolution throughout the workspace of approximately 1 mm in position and 0.1 deg in orientation. Both platforms are controlled through a central Dell Pentium 3 computer running real-time Visual C++ software. As depicted in Fig. 2, the communications architecture consists of RS-422 serial links to each platform trolley assembly and a 10BaseT ethernet link to the robotic manipulator arm. Additional user-defined payload links are available through dedicated serial and high-speed ethernet lines.

FIGURE 1
The Robotics Engineering and Controls Laboratory (RECL) and the dual-platform Dynamic Motion Simulator (DMS).

Fig2 Image

FIGURE 2
DMS local network communication architecture.

User Interface and Operation: The DMS facility is designed to allow convenient, user-friendly interfacing for both hardware-in-the-loop and software- in-the-loop test and verification. The user's sensor suite can easily be integrated onto the servicer and target platforms, and C-based control logic can be augmented to the existing real-time base software program. The control block diagram provided in Fig. 3 demonstrates the interaction between the servicer motion, the target motion, and the relative sensing and control system. Thruster and reaction wheel commands, generated from the relative sensor hardware and software blocks, are used to drive the servicer vehicle based on computer-emulated spacecraft geometry and mass properties. The DMS permits validation of concepts for a variety of complex robotic space missions, along with rigorous performance testing of flight sensors, mechanisms, and software under realistic dynamic and lighting conditions.

FIGURE 3
DMS platform-to-platform relative navigation and control block diagram.

Summary: The Robotics Engineering and Controls Laboratory represents a new, state-of-the-art national testbed for evaluation and verification of critical enabling technologies for a variety of future multiple- spacecraft missions. This facility provides an important and necessary step in improving the technology readiness level of sensor hardware and software associated with autonomous spacecraft-tospacecraft navigation, imaging, communication, and servicing.

[Sponsored by ONR]