The formation control testbed

Terrestrial planet finder (TPF) is a space telescope mission which performs spectral analysis of the infrared emissions from extrasolar planets, and which searches for carbon-based life on such planets. One configuration being considered for this mission is a stellar interferometer with several collectors and a combiner on separate spacecraft flying in a tightly controlled formation. The distance to earth for this mission are sufficiently great that having ground in the loop for reconfiguration or collision avoidance maneuvers impractical. Moreover, because of constraints in the orientation of the spacecraft relative to the sun, limitations on the field of view of relative range and bearing sensors, and restrictions on the orientations of thrusters, both the attitude and the relative position of each spacecraft in the formation must be taken into account in the event of a temporary sensing or control fault during maneuvers. These maneuvers include initial deployment of the formation, reconfiguration, and collision avoidance maneuvers. The formation algorithms and simulation testbed (FAST) and the formation control testbed (FCT) at JPL are being built to simulate and demonstrate 6 degree of freedom, autonomous formation flying and reconfiguration for TPF. The testbeds are complementary. Control algorithms simulated in the FAST are tested in the FCT in order to validate the FAST. This paper describes the design and construction of the formation control testbed. The FCT consists of three robots navigating on an air bearing floor, propelled by cold gas thrusters. Each robot contains an attitude platform supported on a spherical air bearing which provides three rotational degrees of freedom. The sixth degree of freedom, vertical translation, is provided by a powered vertical stage, actively controlled to provide a simulated zero-g environment for the attitude platform.