Gyroscope spin axis direction control for the Gravity Probe B satellite

The Gravity Probe B Relativity Experiment (GP-B) is a joint NASA/Stanford University orbiting astrophysics experiment, under development, to test two predictions of Einstein's theory of general relativity, the geodetic and frame-dragging effects, using orbiting, ultra-precise, mechanical, electrically-suspended gyroscopes (ESVG) carefully isolated from Newtonian torques. General relativity predicts that the gyroscopes' spin axes will precess with respect to a distant inertial reference frame at a rate of 6.6 arc-sec/year for the geodetic effect, and 42 marc-sec/year due to frame-dragging in the planned orbit. To achieve the needed levels of measurement precision, the gyroscopes' axes must be aligned to within 10 arc-sec of the line-of-sight to a distant guide star.; Presented is a technique by which the initial orientation of each gyroscope can be controlled through the use of residual torques generated by the gyroscope' s electrostatic suspension system. The electrostatic torques acting on the gyroscope depend on the rotor shape, which is nominally spherical but also contains small manufacturing asphericities and a spin-induced bulge. These torques are averaged by rotor spin to take on a simple form: they cause the gyroscope to precess about the suspension electrode axes.; Orientation control torques are applied by introducing additional suspension voltages to the electrodes in combinations which do not exert a force on the gyroscope, but do generate a torque. A control system was developed to use these torques to drive the spin axis to a desired orientation in minimum-time using a bang-bang actuation scheme. A net torque identification scheme was also created to monitor polhode-induced modulations of the spin-averaged torques. This information was used by the orientation control system to keep the spin axis on a minimum-time trajectory.; Laboratory experiments confirmed the validity of the spin-averaged torque models and gave a proof-of-principle of the effectiveness of the bang-bang spin axis orientation control system. Under active orientation control, the polhode modulations of the spin-averaged torques were readily measured using the proposed identification technique. The net result of the tests confirm that the spin axes of the gyroscope may be oriented using these techniques to the accuracy required for the GP-B experiment.