Aeronautics and astronautics – Spacecraft – Attitude control
Reexamination Certificate
1999-10-11
2001-11-20
Jordan, Charles T. (Department: 3644)
Aeronautics and astronautics
Spacecraft
Attitude control
Reexamination Certificate
active
06318675
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the invention
The present invention relates to systems and methods of controlling three axis stabilized spacecraft, and in particular to a method and system for performing solar wing thermal shock compensation using a solar wing position actuator.
2. Description of the Related Art
Thermal shock disturbance is a common problem experienced by earth orbit spacecraft. When such spacecraft enter and exit earth shadow, abrupt temperature changes cause rapid deformation of spacecraft solar wing panels, which results in significant induced spacecraft attitude disturbances.
There are a number of methods that have been employed to solve this problem, many of which are outlined in “An Evaluation of Thermally-Induced Structural Disturbances of Spacecraft Solar Arrays” by J. D. Johnston and E. A. Thornton, August 1996, which is hereby incorporated by reference herein. These conventional solutions to the thermal shock disturbance problem generally fall into two categories.
The first category of conventional solutions relates to the mechanical design of the solar wing panels. Here, critical structures are designed to minimize temperature gradients and the thermal deformation and resulting induced attitude disturbances. Such designs are disclosed in U.S. Pat. No. 5,720,453, entitled “Solar Panel Parallel Mounting Configuration,” issued Feb. 24, 1998 to Mutschler et al, U.S. Pat. No. 5,620,529, entitled “Low Disturbance Solar Array,” issued Apr. 15, 1997 to Bassily et al., which references are hereby incorporated by reference herein. One significant problem with this category of solutions is that they can significantly increase the cost of the spacecraft.
The second solution relies instead on the spacecraft attitude control system to compensate for the induced solar disturbances. These systems use control actuators to actively counteract disturbance torques resulting from thermal deformation of solar wing panels. Typically, this is accomplished by using traditional control actuators such as reaction wheels to compensate thermal shock disturbance.
However, controlling the spacecraft eclipse thermal transient becomes a significant challenge because of the high magnitude of solar wing thermal shock disturbance. Traditional control actuators such as reaction wheels are limited by their control torque capabilities, and are ineffective in the presence of such high magnitude disturbance. Thrusters can provide high control torque, but it costs propellant, requires complicated procedure to transit from wheel control to thruster control and back to wheel control, and changes spacecraft momentum state. Developing a dedicated actuator of high torque capability only for thermal shock is undoubtedly very costly.
An example of such a control system is disclosed in U.S. Pat. No. 5,211,360, entitled “Spacecraft Thermal Disturbance Control System, issued May 18, 1993 to Darrell F. Zimbleman, which is hereby incorporated by reference herein. This thermal disturbance control system comprises a network of distributed temperature sensors located on solar wing surfaces and a reaction wheel assembly mounted on a solar wing yoke. This is a relatively costly scheme because a dedicated control system including control electronics and microprocessors (in addition to the distributed sensor network and reaction wheel assembly) is needed to implement this scheme.
Another example of spacecraft attitude control system for compensating for thermal shock disturbance is disclosed in U.S. Pat. No. 5,517,418, entitled “Spacecraft Disturbance Compensation Using Feedforward Control,” issued May 14, 1996 to Green et al., which is hereby incorporated by reference herein. During the thermal transient, this scheme feeds a predicted thermal control torque profile to the attitude control actuator to counteract thermal disturbance.
A third thermal disturbance compensation scheme is disclosed in U.S. Pat. No. 5,563,794, entitled “Repetitive Control of Thermal Shock Disturbance,” issued Oct. 8, 1996 to Cosner et al., which is incorporated by reference herein. This reference discloses a learning procedure that allows the spacecraft attitude control system to learn control errors due to thermal disturbance over several eclipse thermal shock cycles. Using the information thus obtained, the control system maintains precise pointing in the presence of thermal shock disturbances.
One limitation of the foregoing techniques for minimizing thermal shock disturbances is that they are typically expensive and/or ineffectual to compensate for large disturbances.
SUMMARY OF THE INVENTION
As can be seen from the foregoing, there is a need for an inexpensive yet effective system and method for compensating for solar wing thermal shock. The present invention satisfies that need with a method and apparatus using solar wing position actuators to compensate for solar wing thermal shock. The invention provides a spacecraft thermal disturbance control system that effectively compensates solar wing thermal shock disturbance of very high magnitude.
The present invention comprises a control system and method for controlling a spacecraft in the presence of predictable and unpredictable solar wing thermal shock disturbances. The present invention uses solar wing position actuators as well as traditional control actuators such as reaction wheels to compensate for solar wing disturbances. A solar wing position actuator is a gimbaled actuator that controls solar wing elevation angles with respect to spacecraft bus.
Solar wing position actuators normally have order of magnitude higher torque capability than traditional attitude control actuators such as reaction wheels. They also locate in a unique interface position between solar wing and spacecraft bus that is at middle of thermal shock disturbance path from solar wing to spacecraft bus. These make them much more effective in compensating solar wing thermal shock disturbance of very high magnitude than traditional attitude control actuators. In addition, solar wing position actuators already exist in many spacecraft product lines, since they are often used to deploy solar wings and to adjust solar wing positions to follow the Sun in elevation. They can therefore be incorporated in to spacecraft thermal shock control systems without much additional cost or complexity.
In one embodiment, the present invention comprises both feedforward open loop control and feedback closed loop control. The open loop portion of the control system compensates for predictable thermal shock disturbances using the position actuators, while the closed loop portion controls unpredictable disturbances using the position actuators as well as traditional attitude control actuators.
When measurements of solar wing temperatures are available, the control method uses these measurements to adjust solar wing thermal shock control so that it can effectively compensate unpredictable thermal shock disturbance as well. When measurements of the solar wing temperatures are not available, the thermal shock control uses a predicted solar wing position profile to compensate the disturbance. This profile is developed based on analytical prediction before the launch of spacecraft and will be calibrated in initial operation phase of the spacecraft on orbit. If temperature sensors are available on the solar wing, their temperature measurement is used to adjust the solar wing position through a nonlinear function that maps the temperature measurements to position steps of the position actuator.
The closed loop portion of the control system uses the solar wing position actuator in concert with traditional attitude control actuators. The portion of control torques that exceeds the capacity of these traditional attitude control actuators is converted to solar wing position command and sent to the solar wing position actuators. The position actuators then provide high level compensation torques for the disturbances of very high magnitudes.
More specifically, the technique described by the present invention begins by determining t
Gates & Cooper LLP
Hughes Electronics Corporation
Jordan Charles T.
Steele George L.
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