Active-Damping Papers

This section is relevant to Drag-Free Satellites because a high-precision gyroscope in a Drag-Free Satellite with an optical readout must be Actively Damped and because spherical proof masses in a LISA experiment can be accurately managed using Active-Damping techniques.

Definitions

1. Free-Rotor Gyroscope: A gyroscope with a rotor in the form of a sphere without a fixed mechanical bearing axis which is suspended by forces applied to the surface of the sphere.

2. Electrically Supported Gyroscope (ESG): A free-rotor gyroscope with a spherical solid or hollow rotor which is levitated against gravity by applying an electric field in the cavity between the rotor and the housing. ESG's are in use today, and represent the most accurate gyroscopes on the earth's surface.

3. Air-Bearing Gyroscope: A free-rotor gyroscope supported by the flow of air over its surface. The air supply may come from a source of pressurized air or may come from the motion of the rotor. Air-bearing gyroscopes are not very accurate but are very convenient to work with in the laboratory when measuring a rotor's inertial properties or testing features such as Active Damping.

4. Unsupported Gyroscope: A free-rotor gyroscope in a Drag-Free Satellite. Since the satellite chases the gyro rotor without ever touching it, no support forces or torques are deliberated applied to the rotor. This method gives the most accurate gyroscope possible.

Need For Active Dampers

Electrically-supported, air-bearing, and unsupported gyroscopes have no fixed spinup axis in the rotor. Because of residual damping, however, the spin axis will eventually wind up aligned with the rotor maximum axis of inertia. If the gyro readout depends on rotor-fixed markings, it is necessary to have the spin axis parallel to the maximum axis of inertia before the gyro can be used. Moving the spin axis in the rotor to the rotor's maximum axis of inertia is known as damping.

Active Damping was first demonstrated in the laboratory at Stanford by Bradford Parkinson who is now much more famous for his work with the Global Positioning System (GPS). Parkinson came to Stanford as a student in 1969 with a Masters from MIT and a time limit from the USAF to finish his Ph.D. in one year or not get it at all. He completed all of the Stanford requirements for a Ph.D. in one year including inventing the method of hemispheric torquing for coarse Active Damping and the laboratory demonstration of both coarse and fine Active Damping, an amazing feat. The results of his work are published in two papers in the AIAA Journal of Spacecraft and Rockets:

1. Bradford W. Parkinson and Benjamin Lange, Active Damping of Free-Rotor Gyroscopes during Initial Spin-Up, AIAA J. Spacecraft, Vol. 7, No. 6, June 1970, pp. 667-675.

2. Bradford W. Parkinson and Benjamin O. Lange, Experimental Studies of the Active Damping of Free-Rotor Gyroscopes, AIAA J. Spacecraft, Vol. 7, No. 6, June 1970, pp. 677-684.

Active Damping Without Marking the Rotor

Parkinson's method of Active Damping required marking the rotor. For high-precision ESG's, no marks are allowed on the rotor. A short time later Benjamin Lange published a method for Actively Damping a solid ESG rotor with no marks. The method used the fact that the Center of Mass of the Autonetics' ESG rotor was deliberately offset for the readout. Autonetics went on to mechanize the method in their Micron ESG.

Benjamin Lange, Active Damping of ESG Rotors with Mass-Unbalance Readout, AIAA J. Spacecraft, Vol. 9, No. 2, February 1972, p. 96.