Magnetorquer

In satellite systems, a magnetorquer or magnetic torquer is a system for attitude control, detumbling and stabilization built from electromagnetic coils. The magnetorquer develops a magnetic field which interfaces with an ambient magnetic field, usually the Earth's, so that the counter-forces produced provide useful torque.

Construction

Magnetorquers are essentially sets of electromagnets which are laid out 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.

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 hasn't yet stabilised in a fixed orientation with respect to the external field, the craft's spinning will continue.

Very small satellites may use permanent magnets instead of coils.

Advantages

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 a sufficient power source is available to match the resistive load of the coils. In Earth orbit, sunlight is one such practically inexhaustible energy source, using solar panels.

A further advantage over momentum wheels and control moment gyroscopes is the absence of moving parts and therefore significantly higher reliability.

Disadvantages

The main disadvantage of magnetorquers is that very high magnetic flux densities would be needed if large crafts had to be turned very fast. This would either necessitate very high current in the coils, or much higher ambient flux densities than are available in Earth orbit. Subsequently, the torques provided are very limited and only serve to accelerate or decelerate the change in a spacecraft's attitude by minute amounts. Over time active control can produce very fast spinning even here, but for accurate attitude control and stabilization the torques provided often aren't enough.

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 like the geosynchronous. The dependence on the highly variable intensity of Earth's magnetic field is also problematic because then the attitude control problem becomes highly nonlinear. It is also impossible to control attitude in all three axes even if the full three coils are used, since the torque can be generated only perpendicular to the Earth's magnetic field vector.^[1][2]

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.^[3] 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.

References

1. ^ Vincent Francois-Lavet (2010-05-31). "Attitude and Determination Control Systems for the OUFTI nanosatellites" (pdf). http://vincent.francois-l.be/OUFTI_ADCS_2010_05_31.pdf. 
2. ^ Ping Wang et al. (21-26 Jun 1998). "Satellite altitude control using only magnetorquers" (pdf). American Control Conference 1: 222–226. doi:10.1109/ACC.1998.694663. http://microsat.sm.bmstu.ru/e-library/Algorithms/mss/mag.pdf. 
3. ^ "Magnetorquers". Amsat.org. 2002-11-24. http://www.amsat.org/amsat/archive/amsat-bb/200211/msg00474.html. Retrieved 2010-02-08.