# MAGNETIC TORQUERS PDF

Go Back Torque on a Current Loop The torque on a current-carrying coil, as in a DC motor , can be related to the characteristics of the coil by the " magnetic moment " or "magnetic dipole moment". The torque exerted by the magnetic force including both sides of the coil is given by The coil characteristics can be grouped as called the magnetic moment of the loop, and the torque written as The direction of the magnetic moment is perpendicular to the current loop in the right-hand-rule direction, the direction of the normal to the loop in the illustration. Considering torque as a vector quantity , this can be written as the vector product Since this torque acts perpendicular to the magnetic moment, then it can cause the magnetic moment to precess around the magnetic field at a characteristic frequency called the Larmor frequency. If you exerted the necessary torque to overcome the magnetic torque and rotate the loop from angle zero to degrees, you would do an amount of rotational work given by the integral The position where the magnetic moment is opposite to the magnetic field is said to have a higher magnetic potential energy.

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Construction[ edit ] 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 kind of magnetorquer 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. Torquerod: this kind of magnetorquer 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.

Advantages[ edit ] Magnetorquers are lightweight, reliable, and energy-efficient. Unlike thrusters , they do not require expendable propellant either, 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. Disadvantages[ edit ] The main disadvantage of magnetorquers is that very high magnetic flux densities are needed if large craft have to be turned very fast.

This either necessitates a very high current in the coils, or much higher ambient flux densities than are available in Earth orbit. To overcome this, magnetorquer are often combined with reaction wheels. 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.

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## Magnetic Torquer for Small Satellites

Construction[ edit ] 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 kind of magnetorquer is constructed creating a spiral trace inside the PCBs of solar panels which generates the effect of the coil.

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