The most simple attitude control system for cubesats

[achile]

The most simple attitude control system for cubesats

What would be the most simple attitude control system able to keep a cubesat with one face always down toward the earth. I want to install a camera on that face and be sure that no matter when I decide to take a picture the mini satellite will always photograph an area on the ground straight below the cubesat.

[thomas]

There is no simple attitude control system for doing what you want to do. However, if you put a permanent magnetic bar inside your satellite and align it with the axis that passes through the middles of two opposite faces of the cubesat then the magnet together with the satellite will tend to align with the direction of earth's local magnetic field. In this way there will be predetermined windows of time when the cubesat will point the face, having the camera, directly toward the earth. At least in theory, it should work.

[joseygnacyo]

You should use a Passive Magnetic Attitude Stabilization System. It consists of one permanent magnet aligned on one CubeSat body axis, normally Z-axis and two hysteresis bars in X-axis and Z-axis. Permanent magnet aligns Z-axis with local Earth magnetic field direction. Magnet permits CubeSat rotation about Z-axis, so hysteresis bars are used to damp rotation.
You can see the effect in this picture:


ISIS sells a PMASS: http://www.cubesatshop.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=29&category_id=7&option=com_virtuemart&Itemid=69

[neon]

Yes joseygnacyo, but that cubesat attitude stabilization system proposed by you has a few catches, the most important being that you can not really control well the orientation in space of the satellite. It will still spin and try to align to the earth's magnetic lines of force. Because the cubesat moves around the planet quickly, an always changing force will act on it which will result in a complicated behavior of oscillations. The cubesat do not stay one hour in a place to have enough time to align with the local magnetic line of force.

"Passive Magnetic Attitude Stabilization System
Price: 4500 Euro

Cubesat Passive Magnetic Attitude Stabilization System from ISIS. Price 4500 Euro.

Cubesat Passive Magnetic Attitude Stabilization System.JPG (5.77 KiB) Viewed 1850 times

Overview

The Passive Magnetic Attitude Stabilization System (PMASS) provides attitude stabilization and motion damping for CubeSats. After deployment of the satellite the PMASS will enforce a semi-fixed attitude with respect to the local geomagnetic field. By using a permanent magnet and two orthogonal hysteresis rods only, there is no need for any peripheral subsystems and is 100% stand-alone. As such, the PMASS system is a highly reliable attitude control solution, as long as there are no requirements on the specific orientation of the satellite.

Features

Extremely simple and reliable system
Cheapest solution for attitude control
No software, no electronics, no power use
1 permanent magnet aligned to Z-axis to lock CubeSat on the Earth magnetic field.
2 hysteresis rods aligned to X- and Y- axis to dissipate kinetic (rotational) energy through the magnetic hysteresis effect.
Standard PC/104 form factor PCB
Fits between two boards of the CubeSat
Design based on Heritage from the Delfi-C3 satellite (2008)
Compatible with ISIS and CubeSatShop products
Compliant to latest CubeSat standard

Available options

Custom form factor PCB
Parts only, with mounting provisions for integrations by customer

Product Properties

Mass (depends on configuration): 60 to 90g
Form Factor: PC/104 format PCB
Stacking Height: < 15 mm
Thermal range: -50 to +100°C"

Source: http://www.cubesatshop.com

[neon]

Just do an experiment. Take a magnet and a compass and put them on the table. Position the compass at a certain distance from the compass and keep it there till the needle stabilizes. If our magnet looks like one of the two in the attached pictures, the orientation of your compass needle should agree with the direction as shown in the images.

Now try to move the compass slowly around the magnet (or better the magnet around the stationary compass to avoid abrupt mechanical shocks or accelerations induced by your hand onto the compass needle). You will see that the needle oscillates continuously if the compass is turned relatively fast. I would be curious to see what happens if one moves the compass or magnet so slowly that it performs a full rotation in 90 - 100 minutes (the time needed by a cubesat to complete a full revolution around the earth.

Bar Magnet and Compass.jpg (23 KiB) Viewed 1847 times

Horseshoe-magnet-compass-readings.jpg (7.91 KiB) Viewed 1847 times

Sources:
http://www.stmary.ws/highschool/physics ... Fields.htm
http://peswiki.com/index.php/OS:MYLOW:Plans

[joseygnacyo]

I know it thomas, but there are many CubeSats using PMASS. I'd use it if a CubeSat is intended to take pics of Earth and doesn't need pointing accuracy...
Hysteresis rods don't dissipate all rotational energy but they damp rotation.

[joseygnacyo]

Can a CubeSat remain in a dawn/dusk Sun synchronous polar orbit with a PMASS or is needed a more sophisticated Attitude Control System?

[talisman]

Can a CubeSat remain in a dawn/dusk Sun synchronous polar orbit with a PMASS or is needed a more sophisticated Attitude Control System?

1) The Passive Magnetic Attitude Stabilization System (PMASS) has nothing to do with controlling the orbits of cubesats. In general, an attitude stabilization system just orients the satellite in a convenient position, for instance with its camera always facing the Earth or with its solar panels always pointed to the Sun. However, the gravity center of the cubesat will continue to move along its orbit as if the attitude stabilization system wouldn't exist.

2) Once placed by a space rocket in an Sun synchronous dawn to dusk orbit, the cubesat will ride the border between day and night for a few months. Eventually, the accumulated drift will be so great that the satellite will start to enter, little by little, the earth's shadow.
Without engines or solar sails the cubesat can not keep its dawn/dusk orbit for years.