Magnetorquer

A Magnettorquer, also Magnetorquer or Torqrods, is a component of the attitude control of satellites. By means of a solenoid coil generating a magnetic dipole which interacts with the Earth's magnetic field, and thus a torque is transmitted to the satellite. Applications are active attitude control of the satellite and the balance of angular momentum during the unloading of momentum wheels or reaction flywheels.

Principle of operation

Magnettorquer are essentially solenoids which generate a magnetic dipole. The strength and direction of the moment is dependent on the current flow direction, the winding number and the diameter of the coil as well as the permeability of the core. The dipole produced reacts with the earth's magnetic field and thus generates a mechanical torque.

The Magnettorquer is firmly connected to the satellite and thus transmits the torque about its center. With a suitable orthogonal array of three Magnettorquern the satellite can thus be freely rotated around all axes. Under certain conditions, less Magnettorquer can be used, for example, if a full position control is not necessary to allow under certain rules or unbalanced faults.

Construction

The efficiency of the electromagnetic characteristic, the dipole moment is determined by the construction. There are two different construction methods.

Core coils

-Core coils are coils a core cylinder of highly permeable soft magnetic material. They are characterized by a low ratio of mass to the dipole moment, which is important for space applications. Remanence in the core can be avoided by a convenient choice of materials or suppressed by additional polarity reversal.

Air coils

With air coils, the coil is wound on a frame made of non-magnetic material. The lack of permeability is compensated for by additional area or higher current. This is used when the remanence of the core material would interfere with instruments. The lack of permeability causes air coils with the same dipole moment many times heavier and larger than the core coil. The principle of the air coils is often used for microsatellites and CubeSats where parts of the structure can be used as a frame.

Redundancy can be provided by a bifiliare or superimposed coil winding in two designs. This can be achieved by suitable dimensioning of either "cold redundancy " ( two separate coils equivalent to a core ), or "hot redundant " (two coils, each of which generates one half of the power).

Pros and Cons

Magnettorquer have a small mass in relation to the satellite and is deemed reliable and energy efficient. Unlike thrusters they consume no fuel and can be used as long as desired so long as electrical power is available. In Earth orbit, the energy is the sun reliably available.

Since there are no moving parts, tools are not disturbed by vibrations which are generated by the reaction wheels or momentum wheels. The absence of moving parts is also the reason for the high reliability.

The biggest disadvantage is that an external magnetic field is needed, therefore, the use is limited to orbits around the planet with its own magnetic field. In Earth orbit, the field strength limits the use of LEO to MEO.

For fast rotating high magnetic field strength is required. It is for this requires either a high current or the field strength in orbit would have to be higher. High rotation rates are only achieved through longer cycles, which are usually too long for accurate position control.