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Brian Larsen
The purpose of attitude control is to properly orient the satellite in space. The scientific experiment on MEROPE requires the Geiger tube to be perpendicular to the earth’s magnetic field. With the needs of the scientific experiment in mind we decided to control MEROPE’s orientation with a passive magnetic system. Using two magnets MEROPE will align with earth’s magnetic field like the needle of a compass. Unfortunately, MEROPE has a much larger moment of inertia and less friction than the needle of a compass. The result is that MEROPE will harmonically oscillate about its proper orientation. A damping system must therefore me included in the attitude control system to stabilize MEROPE’s orientation relative to earth’s magnetic field.
MEROPE will be launched into a polar sun-synchronous orbit. As
MEROPE goes through one orbit it will flip over twice. The greatest
angular velocity is expected to occur as MEROPE passes over the poles.
Approximately 15 minutes after passing over the North Pole MEROPE will
fly over the ground station in Bozeman Montana. One of the main goals
of attitude control is to stabilize the satellite before it fly’s over
ground control to facilitate communication.
Two damping systems are being tested at this time. The most affective system or combination that weighs less than 100 grams will be flown in MEROPE. To test the affect of different damping systems an engineering model has been constructed complete with magnets and interchangeable damping systems. The model is suspended from fishing line and a laser is bounced off one side to measure the amplitude of oscillation of the model. By measuring amplitude and time for a given damping system and configuration the most affective system can be determined experimentally.
One damping system uses a copper rod to create induced eddy currents.
When the rod rotates through earth’s magnetic field the magnetic flux through
it changes. A change in magnetic flux induces a current in a conductor.
These eddy currents create a magnetic field that apposes the original current
producing field. All of this produces a torque apposing the rods
rotation through the earth’s magnetic field. Rods of this type are being
tested in different orientations and with different shapes.
The other damping system under consideration uses a low coercive
force material called HyMu 80, an alloy of nickel molybdenum and iron.
With this material the affect to be exploited is hysteresis loss when aligning
the magnetic domains. HyMu 80 magnetic domains are easily aligned
with an external magnetic field (earth’s magnetic field) but the alignment
requires energy. An oscillating rod in a magnetic field will continuously
realign its domains with the magnetic field converting kinetic energy to
heat until the oscillation damps out. Rods of this type are being
tested in different shapes, with different heat treatments and in different
orientations within the satellite.
From an attitude control perspective the stronger the magnets the better.
The logic behind that statement is that a stronger magnet caused the satellite
to oscillate with a smaller period. A smaller period of oscillation
is analogies with a larger frequency and implies a greater angular velocity;
all of these improve the effectiveness of the damping system. Neodymium
iron boron (NIB) magnets are the most powerful permanent magnets available.
The scientific experiment is the only system that has a problem with a powerful magnetic field being generated on the satellite. Our scientific experiment is trying to capture electrons that are spiraling around earth’s magnetic field, specifically those electrons that are near their mirror point (traveling in a circle around earth’s magnetic field.) A powerful magnetic field would alter the direction those electrons are traveling and could cause none of the desired electrons to enter the Geiger tube. Calculations are being performed to determine the strongest possible magnets that can be used without destroying the experiment.
Three magnet positioning schemes are being
investigated. As the magnet positioning becomes more complicated
so do the calculations of the deflection of the electrons. The
magnetic field that must be passed through is weaker and more aligned with
the velocity of the electrons in the more complicated models. At
this time the most favored placement for the magnets is in the corners
of the satellite.
