Arc-Second Pointer for Balloon-Borne Astronomical Instrument

NASA Tech Briefs, Jun 2004

A notable innovation would eliminate effects of static friction in bearings.

Goddard Space Flight Center, Greenbelt, Maryland

A control system has been designed to keep a balloon-borne scientific instrument pointed toward a celestial object within an angular error of the order of an arc sec ond. The design is intended to be adaptable to a large range of instrument payloads. The initial payload to which the design nominally applies is considered to be a telescope, modeled as a simple thin-walled cylinder 24 ft ([asymptotically =]7.3 m) long, 3 ft ([asymptotically =]0.91 m) in diameter, weighing 1,500 Ib (having a mass of [asymptotically =]680 kg).

The instrument would be mounted on a set of motor-driven gimbals in pitch-yaw configuration. The motors on the gimbals would apply the control torques needed for fine adjustments of the instrument in pitch and yaw. The pitch-yaw mount would, in turn, be suspended from a motor mount at the lower end of a pair of cables hanging down from the balloon (see figure). The motor in this mount would be used to effect coarse azimuth control of the pitch-yaw mount.

A notable innovation incorporated in the design is a provision for keeping the gimbal bearings in constant motion. This innovation would eliminate the deleterious effects of static friction - something that must be done in order to achieve the desired arc-second precision.

Another notable innovation is the use of linear accelerometers to provide feedback that would facilitate the early detection and counteraction of disturbance torques before they could integrate into significant angular-velocity and angular-position errors. The control software processing die sensor data would be capable of distinguishing between translational and rotational accelerations. The output of the accelerometers is combined with that of angular position and angular-velocity sensors into a proportional integral derivative acceleration control law for die pitch and yaw torque motors. Preliminary calculations have shown that with appropriate gains, the power demand of the control system would be low enough to be satisfiable by means of storage batteries charged by solar batteries during the day.

This work was done by Phuip R. Ward and Keith DeWeese of Wallops Right Facility for Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at urww.techbriefs.com/tsp under the Machinery/Automation category.

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