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FMCG/Field nulling and Optimization: Difference between revisions
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FMCG/Field nulling and Optimization (view source)
Revision as of 19:14, 24 August 2016
, 24 August 2016→Local Field Nulling
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=Local Field Nulling=
Once the first channel is nulled, the other channels should be ''close'' to nulled: only the small field gradients between the sensors will have to be shimmed away. To accomplish this, we will use the shell-mounted coils. The shell coils are connected to the the '''"Sulai Current Supplies"''' in the main control box. The output currents from these supplies are controlled by an Arduino controller and DAC also located inside the main control box. With the code programmed into the Arduino, adjusting the current in one direction at one of the sensors will automatically adjust the currents at the other sensors to counteract any fields the adjustment to the first sensor may cause. Details of this code are provided '''HERE''' (include link here).
The following will assume that Channel 1 has been nulled with the room coils and Channel 2 will be nulled with its set of local coils. If other channels are in use, they can be nulled sequentially using a similar process.
# On the main control box, turn the '''Channel Selector''' (far left of box) knob to channel 2. The channel selector knob has four positions [12 o'clock, 1 o'clock, 2 o'clock, 3 o'clock] corresponding to [channel 1, channel 2, channel 3, channel 4] respectively.
# Adjust the output resistors appropriately. These are set with the second column of three knobs on the far right of the control box. Each one of these knobs is also 4-position, but often the indicators on the knobs don't correspond very well with the actual position of the switch. Usually best to turn the knob to one of the "end positions" and then select the appropriate output resistor. From "most counterclockwise" to "most clockwise", the output resistors are [100 Ω, 500 Ω, 1000 Ω, 5000 Ω]. Because the residual fields (after adjusting the room coils appropriately) are generally very small, the resistors can generally all be set to the 5000 Ω setting. However, for Z-mode measurements, large Z fields are required and thus the 100 Ω resistor (or 100 Ω) should be utilized. '''Note: the way the program is set up currently, the the coupling coefficients only work correctly if all channels' resistors are the same for a particular direction. That is, it's possible to use a 5 kΩ resistor on channel 2Y and a 500 Ω resistor on channel 1Z since those fields are not coupled with our array geometry. However, it would be inadvisable to use a 5 kΩ resistor on channel 2Y with a 1 kΩ resistor on channel 1Y (or 3Y or 4Y for that matter), since those directions ''are'' coupled.
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