The present invention relates generally to satellite maneuvering systems, and more particularly, to a magnetic dipole tractor beam control system.
Spacecraft are directed through various maneuvers to guide them along a variety of celestial paths (e.g., interplanetary voyages and orbits around astronomical bodies such as the Earth) to perform a variety of functions (e.g., weather or planet surface monitoring, commercial communications and scientific observations). These spacecraft maneuvers include ones which initially place a spacecraft in a predetermined celestial path, ones which maintain a desired spacecraft station in the celestial path and ones which maintain a desired spacecraft attitude in the celestial path. In an exemplary case in which the celestial path is a geosynchronous Earth orbit, these spacecraft maneuvers are typically referred to as orbital transferring, stationkeeping and attitude controlling.
Spacecraft maneuvers are accomplished by the application of forces and torques to the spacecraft. Typically, orbital transferring and stationkeeeping are achieved by directing force vectors through the center of mass of spacecraft so as to obtain changes in spacecraft position without disturbing spacecraft attitude. In contrast, attitude controlling is typically achieved by spacing force vectors from the center of mass to thereby generate torque vectors which realize attitude changes.
Conventional spacecraft structures for application of spacecraft forces and torques include thrusters, momentum and reaction wheels, surfaces which receive solar pressure (e.g., solar cell arrays), extended masses which interact with ambient gravity gradients, mechanical inter-spacecraft control structures (e.g., mechanical arms) and magnetic torquing coils which interact with ambient magnetic fields (e.g., the Earth""s magnetic field).
Although all of these control structures have been used to effect spacecraft maneuvers, each has characteristics that limit their usefulness. Thruster systems are typically bulky, heavy and expel propellant products which can coat and degrade sensitive spacecraft structures (e.g., electro-optical instruments and solar cell arrays). In addition, fuel is an expendable substance of limited supply and, hence, its lack routinely produces the effective end of useful spacecraft life. Momentum and reaction wheels are restricted to the application of torques and their momentum must be periodically xe2x80x9cdumpedxe2x80x9d with other control structures (e.g., limited-fuel thrusters) when it approaches the design limit of the wheels.
Generation of spacecraft forces and spacecraft torques by use of solar pressure and ambient gravity gradients typically requires the arrangement or deployment of mechanical structures (e.g., selective rotation of solar cell arrays or extension of gravitational masses from the spacecraft on booms or tethers). Alternatively, generation of spacecraft forces and spacecraft torques can be effected with the limited quantity of thruster fuel.
Inter-spacecraft mechanical control structures are typically bulky and heavy which means they use a significant amount of spacecraft volume and weight (quantities which are always in short supply). Such structures can only be applied when the spacecraft spacing is less than the maximum reach of the control structures. In addition, direct-contact mechanical control may initiate a damaging electrostatic discharge because of spacecraft potential differences, may cause mechanical damage and may respond to electrical or mechanical failure by failing to uncouple the spacecrafts.
The application of magnetic fields to spacecraft maneuvers has typically been directed to the use of ambient magnetic fields (e.g., the Earth""s magnetic field) or to the theoretical use of magnetic structures that have been previously distributed in orbit about a celestial body (e.g., see Lebon, Benoit A., xe2x80x9cMagnetic Propulsion along an Orbiting Grain Streamxe2x80x9d, Journal of Spacecraft and Rockets, Vol. 23, March-April, 1986, pp. 141-143).
The forces and torques generated in magnetic structures by an ambient magnetic field have been generally described by many investigators (e.g., see Boyer, Timothy H., xe2x80x9cThe Force on a Magnetic Dipolexe2x80x9d, American Journal of Physics, August 1988, Vol. 56, No. 8, pp. 688-692; Brownstein, K. R., xe2x80x9cForce Exerted on a Magnetic Dipolexe2x80x9d, American Journal of Physics, October 1993, Vol. 61, No. 10, pp. 940-941); Greene, Jack B., et al., xe2x80x9cForce on a Magnetic Dipolexe2x80x9d, American Journal of Physics, February 1971, Vol. 39, pp. 172-175; Hnizdo, V., xe2x80x9cHidden Momentum and the Force on a Magnetic Dipolexe2x80x9d, Magnetic and Electrical Separation, 1992, Vol. 3, pp. 259-265; and Vaidman, Lev, xe2x80x9cTorque and Force on a Magnetic Dipolexe2x80x9d, American Journal of Physics, October 1990, Vol. 58, No. 10, pp. 978-983).
Magnetic forces on neutrally charged objects are not induced by uniform magnetic fields. Accordingly, ambient magnetic fields cannot be used to generate forces on spacecraft because they are essentially uniform at the spatial scale of spacecraft. In addition, the generation of torques with an ambient magnetic field is limited in application because the direction of the ambient magnetic field gradient cannot be selected.
U.S. Pat. No. 6,089,510 teaches using a magnetic dipole for spacecraft maneuvers. In the ""510 patent, spacecraft maneuvers of a first spacecraft are realized with conventional force and torque generators. Spacecraft maneuvers of a second spacecraft are realized through magnetic interaction between the first and second spacecraft. In particular, a magnetic moment vector m1 of a magnetic system of the first spacecraft and a magnetic moment vector m2 of a magnetic system of the second spacecraft are adjusted to apply selected force vectors and torque vectors to the first and second spacecraft. Unfortunately, the system described in the ""510 patent uses open loop control.
The disadvantages associated with these conventional magnetic maneuvering techniques have made it apparent that a new technique for control of a magnetic dipole tractor beam is needed. Preferably, the new technique would provide closed loop control of a magnetic dipole tractor beam. The present invention is directed to these ends.
It is, therefore, an object of this invention to provide an improved and reliable magnetic dipole tractor beam control system. Another object of the invention is to provide closed loop control of a magnetic dipole tractor beam.
In accordance with the objects of this invention, a magnetic dipole tractor beam control system is provided. In one embodiment of the invention, spacecraft maneuvers (e.g., orbit transferring, stationkeeping and attitude controlling) of a first spacecraft are realized with conventional force and torque generators (e.g., thrusters and momentum wheels). Spacecraft maneuvers of a second spacecraft are realized through magnetic interaction between the first and second spacecraft using a closed loop control system. In particular, a magnetic moment vector m1 of a magnetic system of the first spacecraft and a magnetic moment vector m2 of a magnetic system of the second spacecraft are adjusted to apply selected force vectors and torque vectors to the first and second spacecraft using a closed loop control system.
The present invention thus achieves an improved magnetic dipole tractor beam control system. The present invention is advantageous in that it is capable of providing closed loop control of a magnetic dipole tractor beam.
Additional advantages and features of the present invention will become apparent from the description that follows and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims taken in conjunction with the accompanying drawings.