Magnetically Clean Arrays

1
Magnetically Clean Arrays

• A MIMIC Case Study
• 30 June 2004
• L. Richardson GSFC
• Technique formulated by M. Acuna for MGS

2
April 04 Power Requirements

• According to the Tuskegee April 04 Power Design,
  MIMIC will require
• 20 W average power consumed (arrays produce 39.5
  W on worst-case day)
• 12V operating bus voltage with 14V battery and
  16V array (actual is 14.4V battery and 15.5 V
  array)

3
April 04 Power Design

• To meet the previously mentioned requirements
• 33 BOL (29 EOL) Spectrolab 4cmx6cm PV cells
• Two symmetric arrays, each having 20 in series
  and 4 sets of parallel providing 18x8 combined
  configuration (note even numbers required for
  symmetric array placement with magnetic
  cleanliness)

4
Canceling FieldsHere is one set of 18 in series
(not to scale)
Trombone-like adjustment

• Notice the pair of return wires around each set
  of 4 cells
• The PV cells act like a sheet of current or a
  series of closely spaced current carrying wires
• By using Amperes law and very precise placement
  of return wires, the current induced magnetic
  field can be nearly eliminated
• Note that the return wire is slightly longer than
  the array string so error can be tuned

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Twisted pair or Bifilar wire
5
Bifilar Wire
Twisted pairs of wires with opposite currents
produce virtually no magnetic field. However,
the twisting of wire can add mass and volume to
wiring harnesses. Bifilar wire is manufactured
wire that is coated and runs side by side, so no
extra wire mass or volume is needed.
6
MIMIC Array modeled at GSFC
7
MIMIC Array tests
8
MIMIC Array Mesh Return

• One option for a clean array would be to make the
  return current carried by a wire mesh underneath
  the array
• This may become a bonding issue especially if
  high temperatures are an issue (solar, ohmic
  heating, aerobraking)

9
Field predictions
Using the Biot-Savart law and a spreadsheet, an
array can be simulated by breaking it down into a
series of linear line segments.
10
Can the current induced magnetic field be
canceled out from an array to a 0.1nT level?
Yep, you can do it.
11
Array Design Considerations

• If your array is going to hold your precision
  magnetometers, they need to be stiff and as
  magnetically clean as possible.
• Note the thick honeycomb, and array projection to
  hold the magnetometer.

Instead of putting a small boom on the end of the
array, it makes more sense in terms of simplicity
to extend the array material. I used 20cm as the
horizontal seperation between magnetometer and
nearest array segment.
12
More cool MGS array picts
This one is slightly larger than MIMIC slightly.
For scale purposes, a MIMIC skeleton is shown in
red.
13
Can it be done?

• Buy the PV cells, and have the array honeycomb
  made to specifications
• Deployment mechanisms (springs or any bending
  metals) should be made of Elgiloy. If you bend
  normal metals, a B-field will appear once you
  redecorate those metal atoms
• Bring a team of students to Goddard or somewhere
  with a magnetics testing facility to mount the
  cells and tune out the field at the point where
  your magnetometer will live

14
Array vs. Boom

• TRADES to be made
• Cost
• Most of the modifications made to the array would
  need to be made anyhow
• Risk
• Boom is an extra deployable mechanism, and may
  make the parent s/c team nervous
• If array has trouble deploying, could take out
  one of the redundant magnetometers but it would
  also take out the s/c anyhow
• Magnetic contamination
• It should be possible to limit contamination to
  1/10th nT with the array with careful design and
  calibration in a magnetics lab
• The 2 magnetometers on the boom may provide some
  level of error correction. There may be an
  applicable array technique, but more research is
  needed (will be done by July 9th).