Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
2000-02-16
2001-07-03
Poon, Peter M. (Department: 3643)
Aeronautics and astronautics
Spacecraft
Attitude control
C244S164000, C244S166000
Reexamination Certificate
active
06254036
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to satellite attitude control and more particularly to a method and apparatus for using magnetic torquers to perform reaction wheel desaturation without the need for magnetometers or magnetic field models.
2. Description of Related Art
Spacecraft attitude control is required to maintain proper orientation of the spacecraft relative to earth. To maintain a desired position, the attitude control system must account for the earth's movement and counteract both internal and external disturbances that cause undesired attitudinal movement of the craft.
The use of reaction or momentum wheels to effect precision attitude control is well known in the art. The reaction wheels are positioned within and rotate independently of the spacecraft. This rotation causes a torque to act on the inertial mass of the spacecraft, thereby inducing a radial acceleration that causes attitudinal movement of the craft. In this way the torque created by the reaction wheels is used to effect attitude control.
Reaction wheels are especially effective at offsetting internal disturbances that are typically periodic in nature and are due to mechanical device movement or vibration onboard the spacecraft. However, reaction wheels acting alone are not sufficient to counteract the external disturbance torques that are typically non-periodic in nature and result from phenomenon such as nearby space debris, passing comets, etc. To offset the effect of these disturbances, magnetic torquers are often employed to limit the speed at which the reaction wheels rotate, an operation that is referred to in the art as wheel desaturation. By creating a torque that opposes the wheel torque, the magnetic torquers cause a decrease in the wheel speed. In the simplest form, the torquers are solenoids that are energized to cause current flow therein. When energized, a magnetic field is induced having an intensity, {right arrow over (H)}, given by the following expression:
H
⇀
=
n
·
i
L
·
a
⇀
H
where
L is the length of the current path,
n is the number of turns in the solenoid,
i is the current; and
{right arrow over (a)}
H
represents a unit vector having a direction that is perpendicular to the direction of the current flow within the solenoid.
This resulting magnetic field seeks to align itself with the earth's magnetic field causing a torque to act on the body of the craft. Since the direction of the induced magnetic field, {right arrow over (H)}, is perpendicular to the direction of the current flow, the polarity of the power source applied to the coil determines the direction of the induced field. Once the direction and strength of the earth's magnetic field is known, a power source of proper polarity applied to the solenoid induces a magnetic field that produces the desired torque. Thus, the key to creating a desired torque is determining the characteristics of the earth's magnetic field with respect to the position of the craft.
Conventional systems use magnetometers or magnetic field models to determine the earth's magnetic field with respect to the spacecraft. However, determination of the earth's field using magnetometers involves measurement of the earth's field requiring a full complement of weight-intensive magnetic field sensors. Magnetic field models that allow estimation of the earth's magnetic field given the position of the craft with respect to the earth and sun are also disadvantageous because the estimation performed with the magnetic field model requires complex circuitry that increases the complexity of the spacecraft design. The added weight and design complexity inherent in using either magnetometers or magnetic field models increase the cost and energy consumption of the spacecraft. Therefore, it is desirable to have a method and apparatus for performing magnetic torquer control without the additional weight, cost, power consumption and operational complexity ordinarily associated with magnetometers and/or magnetic field models.
SUMMARY OF THE INVENTION
The present invention provides an attitude control method and apparatus that uses magnetic torquers to perform reaction wheel desaturation without the need for magnetometers or magnetic field models.
In accordance with one aspect of the present invention an apparatus is provided for use in connection with controlling the attitude of a spacecraft. The apparatus includes at least one magnetic torquer that, when energized, creates a torque that acts upon the spacecraft, and at least one sensor that monitors at least one spacecraft variable and generates sensor signals containing information about the spacecraft variable. In addition, a control device applies a test signal to the magnetic torquer, evaluates the sensor signals to determine the impact of the test signal, and controls the operation of the magnetic torquer based on the evaluation of the sensor signals.
According to another aspect of the invention, a spacecraft having attitude sensors that generate sensor measurements related to the attitude of the spacecraft, reaction wheels that control the attitude of the spacecraft and magnetic torquers that affect the speed of said reaction wheels are managed via a control device. The control device applies a test signal to the magnetic torquer, and uses at least some of the sensor measurements to determine the impact of the test signal. The operation of the magnetic torquer is then controlled using the evaluation of the sensor measurements.
In accordance with yet another aspect of the present invention, a method for use in connection with controlling the attitude of a spacecraft is provided. The method steps include applying a test signal to a magnetic torquer, generating sensor signals containing information about at least one spacecraft variable, and evaluating the sensor signals to determine the impact of the test signal. Thereafter, the operation of the magnetic torquer is controlled using the evaluation of the sensor signals.
REFERENCES:
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patent: 5123617 (1992-06-01), Linder et al.
patent: 5248118 (1993-09-01), Cohen et al.
patent: 5343398 (1994-08-01), Goodzeit et al.
patent: 5608634 (1997-03-01), Goodzeit et al.
patent: 5765780 (1998-06-01), Barskey et al.
patent: 5787368 (1998-07-01), Gamble et al.
Gudmestad T.
Hughes Electronics Corporation
Jakel Kevin
Poon Peter M.
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