Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
1999-12-17
2001-11-06
Jordan, Charles T. (Department: 3643)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S165000
Reexamination Certificate
active
06311931
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to systems and methods for controlling spacecraft, and in particular to a system and method for operating a momentum bias controlled spacecraft in an inverted orientation without requiring any control law, or momentum wheel modifications.
2. Description of the Related Art
Various systems of attitude steering and momentum management for spacecraft are known today. These spacecraft use different systems of momentum wheel arrays for various orbit geometries and mission purposes.
Momentum bias stabilized spacecraft are very desirable due both to the inherent attitude stability afforded by the gyroscopic stiffness associated with the momentum and to the yaw attitude sensing facilitated by orbital coupling with the momentum bias. The orbital coupling with the momentum bias allows yaw attitude (which is difficult to measure with sensors) to be deduced from a roll attitude sensor.
Typically, the momentum bias mechanization is only useful for small attitude motion control about a nominal attitude with the bias nominally orbit normal. However, for selected spacecraft applications, it can be often useful to operate the spacecraft in an upright orientation (pitch axis anti-orbit-normal) at some times during the mission, and in an inverted orientation (pitch axis orbit-normal) during other portions of the mission. For example, this capability is useful in establishing new antenna coverage areas or in providing for other new missions.
Further, there are a large number of momentum bias stabilized spacecraft presently on orbit. As a result of proliferation in the number and uses of spacecraft, coupled with the aging and replacement of such satellites, the desire to move them and invert the attitude to establish new antenna coverage areas is becoming more frequent.
Early momentum bias spacecraft were controlled by thrusters and magnetic torquers that apply external torques. The attitude control of such spacecraft is unstable when inverted because the relative sign of the momentum bias and orbit rate rotation is reversed. Consequently, to stabilize such a spacecraft in the inverted state it is necessary to (1) reverse the direction of the momentum wheel rotation, or (2) design and implement a new control compensation, or (3) both (1) and (2).
SUMMARY OF THE INVENTION
From the foregoing, it is apparent that a system and method is needed for currently deployed momentum bias spacecraft to operate in both a nominal and an inverted orientation. The present invention satisfies that need. In this invention we teach that when a momentum bias stabilized spacecraft is controlled by reaction wheels (or equivalent internal torques) it can be operated in upright or inverted attitude with no change to the control architecture or implementation. This allows the many momentum bias spacecraft of this architecture on orbit to perform new and unforeseen missions.
To address the requirements described above, the present invention discloses a method, apparatus, article of manufacture, and a memory structure for controlling a spacecraft having a first axis. The apparatus comprises a momentum wheel unidirectionally rotatable about a spin axis substantially co-aligned with the first axis; a momentum wheel mount for tilting the momentum wheel about at least one of two axes substantially orthogonal to the spin axis; and a processor in communication with the momentum wheel and the momentum wheel mount. The processor commands either the momentum wheel speed or the momentum wheel tilting to control the attitude of the spacecraft in a first orientation and in a second orientation inverted from the first along the first axis, using the same control law.
The method comprises the steps of tilting the momentum wheel according to the control law as applied to a spacecraft attitude command and a measured attitude of the spacecraft; angularly displacing the spacecraft from a first orientation to a second orientation substantially inverted from the first orientation, and tilting the momentum wheel according to the same control law applied to the spacecraft altitude command or a new command and the measured attitude of the spacecraft. The article of manufacture comprises a program or data storage device tangibly embodying instructions to perform the method steps described above.
REFERENCES:
patent: 3741500 (1973-06-01), Liden
patent: 5112012 (1992-05-01), Yuan et al.
patent: 5261631 (1993-11-01), Bender et al.
patent: 5308024 (1994-05-01), Stetson, Jr.
patent: 5611505 (1997-03-01), Smay
patent: 5692707 (1997-12-01), Smay
Gates & Cooper LLP
Jakel Kevin
Jordan Charles T.
The Boeing Company
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