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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 19:27, 24 August 2016
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# 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.
# Follow the same instructions as above to null the fields. Continue to apply the 20 kHz modulation fields using the '''room coils'''. The three large knobs next to the channel selector knobs will adjust the fields X, Y, and Z for the selected channel. While rotating the knobs, the LCD screen will output a 16 bit number proportional to the current being applied. The range [-32768,32768] is mapped to [-5 V, +5 V], and the output current is simply that voltage divide by the output resistance [100, 500, 1000, 5000] Ω. '''Note: this is different from the Wyllie supply where the current was the "monitor voltage" over ''twice'' the output resistor'''. You will note that because of the coupling, adjusting one current (say, channel 2Y) will also change the currents in Channels (1Y, 3Y, 4Y), but by a much smaller amount.
# Once fields on channel 2 are relatively close to nulled, it's helpful to iterate between the room coils the local coils to null channels 1 and 2, respectively. I've found a good strategy is something along the lines of the following. One iteration is generally good enough to bring both magnetometers into line.
## Adjust room Y to bring the DC value of Channel 1 to 0; adjust Ch 2Y to bring the DC value of Channel 2 to 0.
## Apply a Z modulation field to the room coils; adjust room X to minimize on channel 1; immediately adjust Ch 2X to minimize Channel 2.
## Apply a X modulation field to the room coils; adjust room Z to minimize on channel 1; immediately adjust Ch 2Z to minimize Channel 2.
# Repeat all of the above steps for any other channels that need to be nulled.
'''The good thing is once the magnetometers are all nulled relative to ''each other'', most stray fields (cars driving by, elevators, MRI machines) introduce ''global fields'' that can be adjusted for on all channels using the room coils. The field gradients from channel to channel are pretty much constant.'''
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