Minimum energy wheel configurations for energy storage and attitude control

#The present invention is generally directed to energy wheels and, more specifically, to minimum energy wheel configurations for energy storage and attitude control.

#Today, most spacecraft that orbit the Earth contain attitude control, attitude reference and energy storage systems. In general, attitude control of the spacecraft has been accomplished by applying torques to the spacecraft through reaction wheel assemblies (RWAs) or control moment gyros (CMGs), while attitude reference has generally been provided by inertial sensors and devices that provide an absolute reference, such as sun and star sensors. Energy storage for the spacecraft has been performed by batteries that are charged by solar panels and energy wheels, which have performed both energy storage and attitude control.

#In one known configuration 100 (FIG. 1), three pairs of energy wheels 102 & 104, 106 & 108, and 110 & 112 (FIG. 1) are implements within a spacecraft with each pair being located on a mutually orthogonal axis. In this configuration, attitude control is achieved by changing the speeds of each pair of the energy wheels 102 & 104, 106 & 108, and 110 & 112 (FIG. 1) in opposite directions. Such systems require six energy wheels 102 & 104, 106 & 108, and 110 & 112 (FIG. 1) (without redundancy) and when the wheels 102 & 104, 106 & 108, and 110 & 112 (FIG. 1) are storing energy, the speed of each pair is maintained at a constant difference. In order to provide redundancy, many designers have implemented a fourth pair of energy wheels. Another approach to the utilization of an energy wheel to provide both energy storage and attitude control of a spacecraft has been to double gimbal each of the energy wheels.

#These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings.

#FIG. 1 is a diagram of an exemplary spacecraft including a pair of energy wheels mounted on each of three orthogonal axes;

#FIG. 2 is an angular momentum diagram of three energy wheels mounted in a plane such that the angular momentum vectors of the three energy wheels are rotated every one-hundred twenty degrees;

#In general, the angular momentum generated by a set of energy wheels is greater than what is needed for satellite attitude control. For example, a spacecraft, e.g., a satellite, with a maximum power requirement of 2000 watts and a safe mode power requirement of 400 watts is required to store enough power such that 400 watts can be delivered for 24 hours. That is, the set of energy wheels must be capable of providing 9600 watt-hours (or 29,490,000 foot-pounds) of energy. In addition, the satellite may require 100 foot-pound-seconds of angular momentum and 5 foot-pounds of torque for its attitude control system. A satellite whose set of energy wheels can provide a total energy of 29,260,566 foot-pounds can be achieved with an energy wheel set having the following characteristics: a maximum speed of 40,000 RPMs, a turndown speed of one-third of the maximum speed and an energy conversion efficiency of 98 percent. In a typical case, the total momentum of inertia of the wheels may be about 3.3353 foot-pounds-seconds2 and the total angular momentum may be about 13,970 foot-pound-seconds.

#However, if attitude control is to be accomplished simply by changing the speeds of the energy wheels, then the power required to provide 5 foot pounds of torque is about 28,396 Watts, which is about 14 times the power available from one of the energy wheels. Thus, if both attitude control and power are to be accomplished with the same energy wheels, it is generally necessary to gimbal the wheels to provide appropriate torques to the satellite. For example, if 100 foot-pound-seconds of angular momentum transferred to the satellite gives the satellite an angular rate of 1 degree per second, then the total energy transferred to the satellite is 1.18 Joules over a 20 second period. Assuming a 10 percent efficient gimbal drive, the power required to perform such an operation would be under about 0.6 Watts.

#According to the present invention, attitude control using energy wheels is achieved by suspending the energy wheels in a set of double gimbals using a high effective gear ratio with low back drive torques. In this manner, the energy wheels can all be driven at a constant ratio of speeds over their operating range. Thus, according to the present invention, both a non-redundant and a singular redundant minimum energy wheel configuration can be achieved. The configurations can perform attitude control without singularities in the angular momentum from the gimbal angle Jacobian matrix at zero angular momentum.