Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Aeronautical vehicle
Reexamination Certificate
1999-07-29
2001-03-20
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Aeronautical vehicle
C701S003000, C701S004000, C701S123000, C701S226000, C342S350000, C342S355000, C060S200100, C060S201000, C060S204000, C244S003210, C244S158700, C244S164000, C244S169000, C244S172200
Reexamination Certificate
active
06205378
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a system for controlling the attitude of a vehicle, particularly a spacecraft, wherein the system employs thrusters operative by an on/off control process to obtain pulsed mass expulsion and, more particularly, to pulsed mass expulsion with adaptive pulse width modulation to minimize a frequency of pulsation.
In the employment of spacecraft in a mission, such as the encirclement of the earth by a communications or scientific satellite, it is necessary to stabilize the attitude of the spacecraft. The stabilization is accomplished by overcoming the destabilizing torques of sources of disturbance to the stabilization of the spacecraft. Sources of destabilizing torque include aerodynamic torques experienced by spacecraft travel in a low orbital path through residual atmosphere, solar torque caused by pressure from the sun, and torque induced by gravity gradient from the earth's gravitational field.
The attitude control system may employ thrusters which, upon activation, develop forces and moments which push the spacecraft back to the desired attitude. A thruster may be constructed to expel mass, such as ionized particles accelerated by an electrostatic field, or molecules of gas expelled from a canister of pressurized gas. In addition, a control system employed for attitude stabilization may employ magnetic forces, as by use of magnetic torquers. The magnetic torquers comprise rods of magnetic material encircled by coils excited with electric current provided by photocells onboard the spacecraft, wherein the magnetic forces of the coils interact with a relatively weak magnetic field of the earth. The interaction of these magnetic forces develops a torque which tends to aid in the attitude stabilization.
However, the magnetic forces are unable, both in terms of magnitude and direction, to overcome the net destabilizing torque of one or more of the foregoing sources of destabilizing torque and, therefore, are useful only in reducing the rate of drift of the spacecraft away from the desired attitude. With respect to the direction of the magnetic forces developed by the magnetic torque rods, it is noted that such forces are available only in two directions, both of which are orthogonal to the earth's magnetic field, while the development of forces in three dimensions, such as the forces of an array of thrusters, is required to stabilize the spacecraft.
Of particular interest herein is the employment of the thruster which operates by expelling gas in the form of pulses of the gas. In such a thruster, the gas is contained compressed in a canister, and the canister of compressed gas communicates via a solenoid-operated valve to an exterior nozzle through which the expelled molecules of gas are directed into the environment outside the spacecraft. Use of the solenoid to open the valve during the time interval of the resulting jet, followed by a closing of the valve to terminate the jet of compressed gas, provides the desired impulse of the expelled gas.
Generally, in a control system, the pulses of the expelled gas have a fixed predetermined duration. The repetition frequency of the pulses is sufficiently low such that information obtained from attitude sensors onboard a spacecraft can be employed to monitor and predict the progress in correction of the spacecraft attitude. Thereby, the pulses of expelled gas can be generated as needed for correction and/or stabilization of the spacecraft attitude.
It has been found that a spacecraft stores sufficient gas for operation of the thrusters during a period of years which constitute the mission of the spacecraft. However, such a thruster experiences a problem in that the use of the valve, with its numerous openings and closings, produces a degradation of the valve seat and/or is mating element with a consequent impairment of the operation of the thruster. Even though a valve may be designed, possibly for a million operations, the duration of the spacecraft mission may have to be curtailed because the valve has exceeded its design lifetime in terms of the number of openings and closures.
SUMMARY OF THE INVENTION
The aforementioned problems are overcome and other advantages are provided by an attitude control system employing a plurality of gas jet thrusters wherein, in accordance with the invention, the duration of pulses of the expelled gas is varied to provide a desired value of impulse while obtaining a minimization of the number of opening and closing operations of the valves of the respective thrusters. In accordance with a feature of the invention, the attitude control system monitors a pattern of drift of the spacecraft attitude about a desired attitude and within an acceptable band of attitude error. This enables the system to determine the magnitude of the impulse required to alter the momentum of the spacecraft, and to urge the spacecraft back towards the desired attitude. The invention provides for both modulation of the duration of a thruster pulse as well as for a variation in repetition frequency of the thruster pulsing. The thrusters are activated in the situations wherein the spacecraft attitude is outside the acceptable band, or is within the acceptable band but is in imminent danger of moving out of the acceptable band.
In accordance with a feature of the invention, the thrusters are not activated in the situation wherein the attitude of the spacecraft is well within the band of acceptable attitude, and is not in imminent danger of moving outside the band of acceptable attitude. Accordingly, for convenience in description, the acceptable band of attitude may be referred to as the dead band. In view of the need for corrective action even in the situation wherein the attitude is in a central region of the dead band but is drifting rapidly away from the desired attitude, the control system of the invention is responsive both to the angular orientation of the spacecraft as well as to the angular velocity, or angular momentum, of the spacecraft. Correspondingly, the system error, in terms of use of the dead band, is a combination of both the angular orientation and the angular velocity of the spacecraft.
The invention provides for two modes of operation of the attitude control system, one mode being preferred in the situation wherein the attitude correction is obtained by use of the gas jet thrusters without augmentation of the magnetic torque rods, and the second mode being preferred in the situation wherein the attitude correction is obtained by use of the magnetic torque rods in conjunction with the gas jet thrusters.
In the first mode of operation, a gas jet having a nominal value of impulse is employed to deflect the spacecraft attitude away from a first edge of the dead band in a direction towards the opposite edge of the dead band. A plot of the attitude error shows, subsequent to the impulse, a diminution in the attitude error and, later, a drifting back towards the first edge of the dead band. The thruster is operated again to deflect the spacecraft attitude away from the first edge of the dead band. A plot of the attitude error then shows a repetitive and predictable pattern. The longest duration pulses, which correspond with the minimum number of opening and closure operations of the valve, are selected to drive the spacecraft attitude from the first edge toward the opposite edge of the dead band without over-stepping the opposite edge. The determination of the requisite duration of a pulse of the gas jet is based on the repetition period in the plot of the attitude error.
In the second mode of operation, the presence of the magnetic torque introduces a somewhat random appearance to the aforementioned plot of the attitude error. As a result, the pattern is not predictable in an accurate fashion and, as a result, the periodicity of the plot is not available for the determination of the optimal duration of a pulse of the gas jet. Accordingly, in the second mode of operation, the plot is observed to determine the displacement of the attitude error as measured b
Carrou Stephane
Rodden John J.
Stevens Homer D.
Beaulieu Yonel
Cuchlinski Jr. William A.
Perman & Green LLP
Space Systems Loral, Inc.
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