A magnetorquer or magnetic torquer (also known as a torque rod) is a satellite system for attitude control, detumbling, and stabilization built from electromagnetic coils. The magnetorquer creates a magnetic dipole that interfaces with an ambient magnetic field, usually Earth's, so that the counter-forces produced provide useful torque.
Magnetorquers are sets of electromagnets arranged to yield a rotationally asymmetric (anisotropic) magnetic field over an extended area. That field is controlled by switching current flow through the coils on or off, usually under computerized feedbackcontrol. The magnets themselves are mechanically anchored to the craft, so that any magnetic force they exert on the surrounding magnetic field will lead to a magnetic reverse force and result in mechanical torque about the vessel's center of gravity. This makes it possible to freely pivot the craft around in a known local gradient of the magnetic field by only using electrical energy.
The magnetic dipole generated by the magnetorquer is expressed by the formula
where n is the number of turns of the wire, I is the current provided, and A is the vector area of the coil. The dipole interacts with the magnetic field generating a torque
where m is the magnetic dipole vector, B the magnetic field vector (for a spacecraft it is the Earth magnetic field vector), and τ is the generated torque vector.
Constructionedit
The construction of a magnetorquer is based on the realization of a coil with a defined area and number of turns according to the required performances. However, there are different ways to obtain the coil; thus, depending on the construction strategy, it is possible to find three types of magnetorquer, apparently very different from each other but based on the same concept:[1]
Air-core magnetorquer
This comprises the basic concept of magnetorquer, a conductive wire wrapped around a non-conductive support anchored to the satellite. This kind of magnetorquer can provide a consistent magnetic dipole with an acceptable mass and encumbrance.
Embedded coil
This is constructed creating a spiral trace inside the PCBs of solar panels which generates the effect of the coil. This solution is the one with the least impact on the satellite as it is entirely contained within the solar panels. However, due to the physical limit in the board thickness and the presence of other circuits and electronic components, it is not possible to reach a high value of the magnetic dipole.
The Hubble Space Telescope uses four 8 feet (2.4 m) iron torque rods as part of its pointing control system.[2]Torque rod
This is the most efficient solution. A conductive wire is wrapped around a ferromagnetic core which is magnetized when excited by the coil, thus generating a dipole considerably higher than the other solutions. However, the disadvantage is the presence of a residual magnetic dipole that remains even when the coil is turned off because of the hysteresis in the magnetization curve of the core. It is therefore necessary to demagnetize the core with a proper demagnetizing procedure. Normally, the presence of the core (generally consisting of heavy metal) increases the mass of the system.
Typically three coils are used, although reduced configurations of two or even one magnet can suffice where full attitude control is not needed or external forces like asymmetric drag allow underactuated control. The three coil assembly usually takes the form of three perpendicular coils, because this setup equalizes the rotational symmetry of the fields which can be generated; no matter how the external field and the craft are placed with respect to each other, approximately the same torque can always be generated simply by using different amounts of current on the three different coils.
As long as current is passing through the coils and the spacecraft has not yet been stabilized in a fixed orientation with respect to the external field, the craft's spinning will continue.[citation needed]
Magnetorquers are lightweight, reliable, and energy-efficient. Unlike thrusters, they do not require expendable propellant, so they could in theory work indefinitely as long as sufficient power is available to match the resistive load of the coils. In Earth orbit, sunlight is one such practically inexhaustible energy source, using solar panels.
The main disadvantage of magnetorquers is that very high magnetic flux densities are needed if large craft have to be turned quickly. This either necessitates a very high current in the coils, or much higher ambient flux densities than are available in Earth orbit. Consequently, the torques provided are very limited and only serve to accelerate or decelerate the change in a spacecraft's attitude by small amounts. Over time, active control can produce fast spinning even on Earth, but for accurate attitude control and stabilization the torques provided are often insufficient. To overcome this, magnetorquer are often combined with reaction wheels.
A broader disadvantage is the dependence on Earth's magnetic field strength, making this approach unsuitable for deep space missions, and also more suitable for low Earth orbits as opposed to higher ones such as geosynchronous. The dependence on the highly variable intensity of Earth's magnetic field is problematic because then the attitude control problem becomes highly nonlinear[disambiguation needed]. It is also impossible to control attitude in all three axes even if the full three coils are used, because the torque can be generated only perpendicular to the Earth's magnetic field vector.[3][4]
Any spinning satellite made of a conductive material will lose rotational momentum in Earth's magnetic field due to generation of eddy currents in its body and the corresponding braking force proportional to its spin rate.[5]Aerodynamicfriction losses can also play a part. This means that the magnetorquer will have to be continuously operated, and at a power level which is enough to counter the resistive forces present. This is not always possible within the energy constraints of the vessel.
The Michigan Exploration Laboratory (MXL) suspects that the M-CubedCubeSat, a joint project run by MXL and JPL, became magnetically conjoined to Explorer-1 Prime, a second CubeSat released at the same time, via strong onboard magnets used for passive attitude control, after deploying on October 28, 2011.[6] This is the first non-destructive latching of two satellites.[7]
^Niccolò Bellini (2014-09-10). Magnetic actuators for nanosatellite attitude control (pdf) (Report).
^Garner, Rob (2017-12-19). "Observatory - Pointing Control". NASA. Retrieved 2023-03-14.
^Vincent Francois-Lavet (2010-05-31). "Attitude and Determination Control Systems for the OUFTI nanosatellites" (PDF).
^Ping Wang; et al. (21–26 June 1998). (PDF). Proceedings of the 1998 American Control Conference. ACC (IEEE Cat. No.98CH36207). Vol. 1. pp. 222–226. doi:10.1109/ACC.1998.694663. ISBN0-7803-4530-4. S2CID 64318808. Archived from the original (PDF) on 2011-08-21.
magnetorquer, magnetorquer, magnetic, torquer, also, known, torque, satellite, system, attitude, control, detumbling, stabilization, built, from, electromagnetic, coils, magnetorquer, creates, magnetic, dipole, that, interfaces, with, ambient, magnetic, field,. A magnetorquer or magnetic torquer also known as a torque rod is a satellite system for attitude control detumbling and stabilization built from electromagnetic coils The magnetorquer creates a magnetic dipole that interfaces with an ambient magnetic field usually Earth s so that the counter forces produced provide useful torque Contents 1 Functional principle 2 Construction 3 Advantages 4 Disadvantages 5 See also 6 ReferencesFunctional principle editMagnetorquers are sets of electromagnets arranged to yield a rotationally asymmetric anisotropic magnetic field over an extended area That field is controlled by switching current flow through the coils on or off usually under computerized feedback control The magnets themselves are mechanically anchored to the craft so that any magnetic force they exert on the surrounding magnetic field will lead to a magnetic reverse force and result in mechanical torque about the vessel s center of gravity This makes it possible to freely pivot the craft around in a known local gradient of the magnetic field by only using electrical energy The magnetic dipole generated by the magnetorquer is expressed by the formula m n I A displaystyle mathbf m nI mathbf A nbsp where n is the number of turns of the wire I is the current provided and A is the vector area of the coil The dipole interacts with the magnetic field generating a torque t m B displaystyle boldsymbol tau mathbf m times mathbf B nbsp where m is the magnetic dipole vector B the magnetic field vector for a spacecraft it is the Earth magnetic field vector and t is the generated torque vector Construction editThe construction of a magnetorquer is based on the realization of a coil with a defined area and number of turns according to the required performances However there are different ways to obtain the coil thus depending on the construction strategy it is possible to find three types of magnetorquer apparently very different from each other but based on the same concept 1 Air core magnetorquer This comprises the basic concept of magnetorquer a conductive wire wrapped around a non conductive support anchored to the satellite This kind of magnetorquer can provide a consistent magnetic dipole with an acceptable mass and encumbrance Embedded coil This is constructed creating a spiral trace inside the PCBs of solar panels which generates the effect of the coil This solution is the one with the least impact on the satellite as it is entirely contained within the solar panels However due to the physical limit in the board thickness and the presence of other circuits and electronic components it is not possible to reach a high value of the magnetic dipole nbsp The Hubble Space Telescope uses four 8 feet 2 4 m iron torque rods as part of its pointing control system 2 Torque rod This is the most efficient solution A conductive wire is wrapped around a ferromagnetic core which is magnetized when excited by the coil thus generating a dipole considerably higher than the other solutions However the disadvantage is the presence of a residual magnetic dipole that remains even when the coil is turned off because of the hysteresis in the magnetization curve of the core It is therefore necessary to demagnetize the core with a proper demagnetizing procedure Normally the presence of the core generally consisting of heavy metal increases the mass of the system Typically three coils are used although reduced configurations of two or even one magnet can suffice where full attitude control is not needed or external forces like asymmetric drag allow underactuated control The three coil assembly usually takes the form of three perpendicular coils because this setup equalizes the rotational symmetry of the fields which can be generated no matter how the external field and the craft are placed with respect to each other approximately the same torque can always be generated simply by using different amounts of current on the three different coils As long as current is passing through the coils and the spacecraft has not yet been stabilized in a fixed orientation with respect to the external field the craft s spinning will continue citation needed Very small satellites may use permanent magnets instead of coils Advantages editMagnetorquers are lightweight reliable and energy efficient Unlike thrusters they do not require expendable propellant so they could in theory work indefinitely as long as sufficient power is available to match the resistive load of the coils In Earth orbit sunlight is one such practically inexhaustible energy source using solar panels Another advantage over momentum wheels and control moment gyroscopes is the absence of moving parts hence significantly higher reliability Disadvantages editThe main disadvantage of magnetorquers is that very high magnetic flux densities are needed if large craft have to be turned quickly This either necessitates a very high current in the coils or much higher ambient flux densities than are available in Earth orbit Consequently the torques provided are very limited and only serve to accelerate or decelerate the change in a spacecraft s attitude by small amounts Over time active control can produce fast spinning even on Earth but for accurate attitude control and stabilization the torques provided are often insufficient To overcome this magnetorquer are often combined with reaction wheels A broader disadvantage is the dependence on Earth s magnetic field strength making this approach unsuitable for deep space missions and also more suitable for low Earth orbits as opposed to higher ones such as geosynchronous The dependence on the highly variable intensity of Earth s magnetic field is problematic because then the attitude control problem becomes highly nonlinear disambiguation needed It is also impossible to control attitude in all three axes even if the full three coils are used because the torque can be generated only perpendicular to the Earth s magnetic field vector 3 4 Any spinning satellite made of a conductive material will lose rotational momentum in Earth s magnetic field due to generation of eddy currents in its body and the corresponding braking force proportional to its spin rate 5 Aerodynamic friction losses can also play a part This means that the magnetorquer will have to be continuously operated and at a power level which is enough to counter the resistive forces present This is not always possible within the energy constraints of the vessel The Michigan Exploration Laboratory MXL suspects that the M Cubed CubeSat a joint project run by MXL and JPL became magnetically conjoined to Explorer 1 Prime a second CubeSat released at the same time via strong onboard magnets used for passive attitude control after deploying on October 28 2011 6 This is the first non destructive latching of two satellites 7 See also editReaction wheel an active attitude adjustment technique Gravity gradient stabilization a passive stabilization techniqueReferences edit Niccolo Bellini 2014 09 10 Magnetic actuators for nanosatellite attitude control pdf Report Garner Rob 2017 12 19 Observatory Pointing Control NASA Retrieved 2023 03 14 Vincent Francois Lavet 2010 05 31 Attitude and Determination Control Systems for the OUFTI nanosatellites PDF Ping Wang et al 21 26 June 1998 Satellite attitude control using only magnetorquers PDF Proceedings of the 1998 American Control Conference ACC IEEE Cat No 98CH36207 Vol 1 pp 222 226 doi 10 1109 ACC 1998 694663 ISBN 0 7803 4530 4 S2CID 64318808 Archived from the original PDF on 2011 08 21 Magnetorquers Amsat org 2002 11 24 Retrieved 2010 02 08 Michigan Exploration Laboratory Michigan Exploration Laboratory 2011 12 06 Retrieved 2012 12 14 MCubed 2 National Space Science Data Center NASA 2013 08 16 Retrieved 2019 05 27 Retrieved from https en wikipedia org w index php title Magnetorquer amp oldid 1220083065, wikipedia, wiki, book, books, library,
