Active damping of free-rotor gyroscopes during initial spin-up

Abstract

The free-rotor gyro is so named because the nearly spherical gyrorotor is not mechanically constrained to rotate about a particular axis. Therefore, it may be initially spun up about any rotor fixed axis. As seen in the rotor, the subsequent motion of the angular velocity vector after spin-up is the well-known classical polhode motion about either the maximum or the minimum principal axes of inertia. The maximum axis of inertia is the desired spin axis direction because it is the only stable free-rotor spin direction in the presence of minute energy losses during flexures of the rotor. Consequently, the rotor-fixed markings used for spin axis readout of many of these gyros are referenced to the maximum axis of inertia. The principle delays in activating typical gyros are 1) waiting for a passive d.c. field to damp the gyro spin axis into the maximum axis of inertia of the rotor after it has been spun up (this requires typically five times the spin-up time) and 2) waiting for the thermal transient (of spin-up and damping) to subside. This paper describes a method of actively controlling the spin axis in the rotor, while leaving it unchanged with respect to a case-fixed reference. A control algorithm, called Hemispheric Torquing is used to actively damp the spin axis to the proper rotor fixed direction. It offers the advantage of being able to reduce damping times to less than is required for initial spin-up.