Magnetorquer
Encyclopedia
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
.
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.
, 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 wheel
s and control moment gyroscope
s is the absence of moving parts
and therefore significantly higher reliability.
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 orbit
s 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.
Any spinning satellite made of a conductive material will lose rotational momentum in Earth's magnetic field due to generation of eddy current
s in its body and the corresponding braking force proportional to its spin rate. 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.
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
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
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
, so that the counter-forces produced provide useful torque
Torque
Torque, moment or moment of force , is the tendency of a force to rotate an object about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be thought of as a twist....
.
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 currentElectric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
flow through the coils on or off, usually under computerized feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
control
Control theory
Control theory is an interdisciplinary branch of engineering and mathematics that deals with the behavior of dynamical systems. The desired output of a system is called the reference...
. 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
Center of gravity
In physics, a center of gravity of a material body is a point that may be used for a summary description of gravitational interactions. In a uniform gravitational field, the center of mass serves as the center of gravity...
. This makes it possible to freely pivot the craft around in a known local gradient
Gradient
In vector calculus, the gradient of a scalar field is a vector field that points in the direction of the greatest rate of increase of the scalar field, and whose magnitude is the greatest rate of change....
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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...
allow underactuated control
Underactuation
Underactuation is a technical term used in robotics and control theory to describe mechanical devices that have a lower number of actuators than degrees of freedom....
. 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
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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 thrustersRocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
, they do not require expendable propellant
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...
either, so they could in theory work indefinitely as long as a sufficient power
Electric power
Electric power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is the watt.-Circuits:Electric power, like mechanical power, is represented by the letter P in electrical equations...
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 wheel
Momentum wheel
A reaction wheel is a type of flywheel used primarily by spacecraft for attitude control without using fuel for rockets or other reaction devices....
s and control moment gyroscope
Control moment gyroscope
A control momentum gyroscope is an attitude control device generally used in spacecraft attitude control systems. A CMG consists of a spinning rotor and one or more motorized gimbals that tilt the rotor’s angular momentum. As the rotor tilts, the changing angular momentum causes a gyroscopic...
s is the absence of moving parts
Moving parts
The moving parts of a machine are those parts of it that move. Machines comprise both moving and fixed parts. The moving parts have controlled and constrained motions....
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 orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
s 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.
Any spinning satellite made of a conductive material will lose rotational momentum in Earth's magnetic field due to generation of eddy current
Eddy current
Eddy currents are electric currents induced in conductors when a conductor is exposed to a changing magnetic field; due to relative motion of the field source and conductor or due to variations of the field with time. This can cause a circulating flow of electrons, or current, within the body of...
s in its body and the corresponding braking force proportional to its spin rate. Aerodynamic
Aerodynamics
Aerodynamics is a branch of dynamics concerned with studying the motion of air, particularly when it interacts with a moving object. Aerodynamics is a subfield of fluid dynamics and gas dynamics, with much theory shared between them. Aerodynamics is often used synonymously with gas dynamics, with...
friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of 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.