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→Physics of Magnetorquers
[[File:Equation5.png|frame|center]]
Magnetic torquer is a current carrying insulated loop which can be mounted on satellite body. The outer space has magnetic field due to the earth. In presence of this field, torquer can exert torque on the satellite when current is passed through it. Since magnetic field doesn’t vary over the length scale of dimension of torquer, it can be assumed to be uniform and above formula is applicable.
[[File:Torquer1.jpg|frame|center| Magnetorquer used in Pratham]]
== Modeling of Torquer ==
[[File:Torquer2LR.png|frame|thumb| Torquer as an L-R circuit.<ref>Image reproduced from [https://wwwcommons.electrical4uwikimedia.comorg/rlwiki/File:Series-series-circuit/ </ref>RL.svg here]]]
The actuator is a part of control system which implements the control law on our plant. In case of satellite, the required control torque is determined by control law. This torque is generated by passing the electric current determined by the above equation, through torquer. But it should be noted that microcontroller can provide voltage signal. Thus it is necessary to study the relation between voltage applied to torquer and current generated in it. <br \>
The torquer coil has some specific resistance R. Since it forms a loop it will also have inductance L. Thus torquer can be modelled as series L-R circuit.<br \>
== Effect of the ferromagnetic core ==
[[File:TorqueRodTorqueRod2.pnggif|frame|thumb|Torque Rod <ref>A representative image for torque rod. Image reproduced from [https://wwwcommons.cubesatshopwikimedia.comorg/productwiki/nss-magnetorquer-rod/</ref>File:Simple_electromagnet2.gif here]]]
The magnetorquers in shape of coil take too much space and have larger mass. In order to generate larger moment with stringent mass and size constraints, ferromagnetic core torquers can be used. <br \>
These are called torque rods. It is a solenoid of length ‘l’ and radius ‘r’ with ferromagnetic core of relative permeability ‘µ_r’. The magnetic moment for cored solenoid in steady state is given by : <ref>http://www.aerospades.com/uploads/3/7/3/2/37325123/cadre_torquers.pdf </ref>
[[File:Equation7.JPG|frame|center]]
It can be easily seen that moment is scaled up by constant greater than 1 which depends on geometry and material properties of ferromagnetic core. This is because of generation of magnetization (magnetic moment per unit volume) in the core due free current. This extra moment adds up with original magnetic moment of bare solenoid. <br \>
#It can exert external torques on the satellite body.
#In low earth orbit, magnetic field strength is enough to generate necessary control torques.
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If you are done reading this page, you can go back to [[Attitude Determination and Control Subsystem]]
==References==
