Development of air spindle and triaxial air bearing testbeds for spacecraft dynamics and control experiments

The dynamics and control of spacecraft have been widely studied because of their technological significance. In the classical case the spacecraft is assumed to consist of a single rigid body with three-axis torque inputs and with attitude and rate sensing. In practice, however, the situation may be far more complex. For example, any component of the spacecraft that deforms relative to other components will entail a change in the spacecraft mass distribution; in effect, the spacecraft becomes a multibody system. Similarly, structural flexibility and fuel slosh give rise to vibrational degrees of freedom. Spacecraft control is also exacerbated by sensor and actuator nonlinearities. Traditional actuation devices such as thrusters, reaction wheels, momentum wheels, and control moment gyros entail amplitude and rate saturation constraints, gyroscopic coupling, and coupling between translational and attitude dynamics. Additional difficulties arise when accounting for gravitational effects and external disturbances. All of these issues have technological implications. A fundamental difficulty associated with spacecraft technology is the fact that ground-based testing must occur in a 1-g environment whereas the hardware will operate under zero-g conditions. Consequently, spacecraft control engineering must depend on first-principles analysis as well as extrapolation from 1-g testing. The purpose of this paper is to describe a laboratory-based testbed that will be used to explore various issues and concepts in spacecraft dynamics and control. This testbed is based on a triaxial air bearing to allow experiments involving large-angle, three-axis motion. As a precursor to this testbed, we have also developed an air spindle testbed which allows single-axis rotation.