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Specifically with the small satellite and cubesat people, there is an rather 'old' idea around: Passive attitude stabilization based on permanent magnets.

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Schematic of the Earth magnetic field lines and camera orientation throughout polar orbit (image credit: University of Michigan)

The concept is rather simple. A magnet, which is mounted on a satellite, will always attempt to align its magnetic field with Earth's magnetic field. This is intriguing, because it does not need computers, power, moving parts or just any form of control.

For me, there are two critical weak spots in this concept. First, if it works, it does stabilize a satellite only on a single axis. Second, Earth's magnetic field is not a clean dipole - with respect to both, its intensity and the orientation of magnetic field lines. But even if you treat it as a simple dipole, the relative rotation of the magnetic field lines along e.g. an polar orbit does not occur in one geometric plane or with a constant angular velocity. For me, intuitively, this whole thing should lead to a rather strange tumbling behaviour of such a satellite.

How is it supposed to work? Are there studies of this concept based on attitude data from satellites actually in space? If yes, what was found?

Results from detailed computer simulations are interesting for me, too, but I clearly prefer results based on flight data. There are a fair number of projects, which intend to use magnets or used them (while the satellite failed), so I find it rather hard to find statements, which go beyond 'it will work'.

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  • $\begingroup$ @ernestopheles UNISAT-4 was going to use magnets for passive attitude stabilization, but the launch failed. $\endgroup$ – called2voyage Sep 10 '13 at 17:24
  • $\begingroup$ I see one problem with this method. As compass needle is swinging back and forth before it stabilizes pointing to the north when its friction and magnetic torque cancel out, a satellite has no friction to neutralize the oscillations. A passive magnetorquer - a coil with resistance - might be used to provide electrodynamic force acting as a friction, but then it would decay the orbit in progressive motion more than canceling the oscillations. $\endgroup$ – SF. Feb 10 '16 at 8:01
  • $\begingroup$ @SF This is why I'd love to have a look at a real data set to understand this better. If I could only get one ... $\endgroup$ – s-m-e Feb 10 '16 at 13:33
  • $\begingroup$ @SF There is some damping of oscilaations by eddy currents. If the magnetic fields varies periodically, the induced currents flow is influenced by elcetric resistance and the energy of oscillations is transformed into to heat. $\endgroup$ – Uwe Oct 9 '17 at 19:09
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Check with Kentucky Space. They have KySat-2 on orbit using passive magnetic control.

Here is a description of the control system from their website:

Passive Magnetic Stabilization: KySat-2 is equipped with a passive attitude control scheme known as Passive Magnetic Stabilization. This passive control technique uses permanent magnets and magnetic hysteresis material fixed to the chassis of KySat-2. The permanent magnets will provide torque in attempts to align with Earth’s magnetic field (in the same fashion that a magnetic compass needle points to magnetic North). The permanent magnets will be mounted to the spacecraft’s chassis in such a way that when aligned with Earth’s magnetic field, KySat-2′s camera will point at Earth while over Northern Hemisphere, and out into space while over the Southern Hemisphere.

Permanent magnets provide the small amount of torque needed to keep the spacecraft oriented correctly, but in the vacuum of space, there is no damping effect (such as air resistance as we have on the surface). Because of this, KySat-2 would oscillate around it’s [sic] targeted orientation rather than settle into it smoothly. Also, permanent magnets can only provide control over two axes while the third axis is uncontrolled and free to rotate. To alleviate these problems, KySat-2 is also equipped with hysteresis (memory) material. This material “memorizes” the current magnetic field and thus resists changes in KySat-2′s orientation. This effect is small, and much less assertive than the permanent magnets, but it provides the needed damping effect to stabilize KySat-2 on the two control axes, and resist changes on the uncontrolled axis as well.

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  • $\begingroup$ I wonder how viable it would be to make the system half-passive: mount the magnet on a motor/actuator. Turn it to max torque position to brake the oscillation, turn it back to neutral when returning to neutral, etc. Similar to using magnetorquers, except mechanical rotation instead of manipulating the field, and zero power requirements once position is set; stabilization still occurs. (plus allows stabilization to any position, not just preset, plus power requirements of a very weak motor instead of quite powerful electromagnet.) $\endgroup$ – SF. Jul 9 '16 at 9:28
  • $\begingroup$ There is a paper somewhat related paper on semi-passive gravity gradient stabilization. I know ChargerSat-1 used fully passive gravity gradient stabilization. $\endgroup$ – Daniel Jul 9 '16 at 14:12
  • $\begingroup$ And for roughly equatorial orbits, it acts in axis which is perpendicular to magnetic field! And that would mean the two provide a full 3-axis stabilization! $\endgroup$ – SF. Jul 9 '16 at 20:24
  • $\begingroup$ I don't know if this would work @SF but your idea has me thinking about mounting a permanent magnet on a coiled spring to provide some sort of damping effect. $\endgroup$ – Daniel Jul 9 '16 at 21:13
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Yes, it does work. The first Australian satellite, Australis OSCAR 5, used a magnet to approximately align one axis with the earth's magnetic field AND rods with a large magnetic hysteresis loop to remove spin energy. AO5 was built in 1966-7 and launched by NASA on a Delta rocket with a Tiros satellite in 1970. Crude, passive and effective.

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    $\begingroup$ Interesting. Do you have a link to a more detailed description of the system? $\endgroup$ – Hobbes Jul 9 '16 at 8:13
  • $\begingroup$ @Hobbes Best I was able to find was on the NASA Space Science Data Coordinated Archive by way of Wikipedia. Neither says much. WinOrbit references ARRL's membership magazine QST, however, including referencing an article titled Australis Oscar - Its Design, Construction and Operation in the July 1969 issue that might be a reasonable reference if one can get a copy. $\endgroup$ – a CVn Jul 9 '16 at 18:16
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If you try this, be sure to coordinate with the other cubesat teams sharing your deployer pod. In 2011 the MCubed and HRBE teams found out the hard way that there can be unintentional interactions between cubesats employing passive magnetic attitude stabilization.

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  • $\begingroup$ I knew I did not like this approach, but what happened to those two satellites just sucks - thanks for sharing. I should add a warning notice at the top of my question ... $\endgroup$ – s-m-e Oct 11 '17 at 7:10
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The Swayam satellite made by students of COEP (College of Engineering), Pune, India works on this principle and is proven stable. It was launched in June or July 2016 (don't remember the exact date now). Try and contact them if you wish.

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  • $\begingroup$ Can you find a link? Sounds interesting! $\endgroup$ – uhoh Aug 18 '16 at 15:26
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I've managed to obtain a scan of ARRL QST July issue of 1969, where operation of Australis-Oscar 5 satellite is described. This includes spin reduction through permalloy bars and stabilization by a bar magnet:

When Australis is placed into orbit, it will be spinning at some 4 revolutions per minute. This spinning will cause fading in the signal. In order to remove the spin energy, a set of permalloy rods with a very large hysteresis loop at the low flux density has been included. Hysteresis loss along with eddy current loss in the case, will tend to remove the spin, allowing orientation along the local magnetic field vector by a small magnet (see fig. 3) which is also included. Since the v.h.f. antenna lies along the same axis as the bar magnet (the X axis) fading of the v.h.f. signals should be reduced. All antennas are made of flexible steel tape.

Australis-Oscar electronics, with a thin bar magnet going along the whole length of X axis

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