Magnetorquer

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A magnetorquer also Magnetorquer or Torqrods (of English. Torque = torque ), is a component of the position control of satellite . A magnetic dipole is generated by means of a magnetic coil , which interacts with the earth's magnetic field and thus transmits a torque to the satellite. Applications are the active position control of the satellite as well as the compensation of the angular momentum when untwisting swirl or reaction wheels .

Working principle

Magnet torquers are basically electromagnets that create a magnetic dipole. The strength and direction of the moment depend on the current , the direction of the current , the number of turns and the diameter of the coil and the permeability of the core . The generated dipole reacts with the earth's magnetic field and thus generates a mechanical torque :

The magnetic torquer is firmly connected to the satellite and thus transmits the torque around its center of gravity . With a suitable, orthogonal arrangement of three magnetic gateways, the satellite can be rotated freely around all axes. Under certain conditions, fewer than three magnetic torquers can be used, e.g. B. if a complete position control is not necessary or asymmetrical disturbances allow an underdetermined control.

Construction methods

The efficiency of the electromagnetic parameter, the dipole moment , is determined by the design. There are two types of construction.

Core coils

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

Air coils

In the case of air-core coils , the winding is wound onto 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 disturb instruments. The lack of permeability means that air coils are many times heavier and larger than core coils with the same dipole moment.

The principle of the air core coil is often used in small satellites and CubeSats , where parts of the structure can be used as frames.

Redundancy can be created in both designs by a bifilar or superimposed coil winding. Through suitable dimensioning, either “cold redundancy” (two separate, equivalent coils on one core) or “hot redundancy” (two coils, each generating half the power) can be achieved.

Advantages and disadvantages

Magnet torquers have a low mass in relation to the satellite and are considered reliable and energy efficient. In contrast to thrusters , they do not consume any fuel and can therefore be used as long as you like, as long as electrical energy is available. This is reliably the case in earth orbit due to solar energy . Furthermore, unlike thrusters, they do not emit any particles into the environment, which could interfere with precision measurements in the case of satellites.

Since there are no moving parts, measuring instruments on board the satellite are not disturbed by vibrations such as those generated by reaction or spin wheels when using magnetic gateways. The lack of moving parts is also the reason for the high reliability of the magnetic gate.

The main disadvantage is that an external magnetic field is required, so its use is limited to orbits around planets with their own magnetic field. In earth orbit, the field strength limits use to LEO to MEO .

A high magnetic field strength is required for rapid rotation. A high current is required for this, or the field strength in orbit would have to be higher. High rotation rates are only achieved through longer switching cycles , which are usually too long for precise position control.

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