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The photo on the left shows the Fieldmill
head assembly mounted in its' aluminium screening can.
The rotating, earthed brass vane can be seen with the
brass measuring electrode (almost completely exposed)
directly below. The number of vanes has been doubled from
that of the original prototype. Thus for the same motor
speed, the signal frequency will have been doubled. At
the top left corner of the picture, can be seen the opto
electronic sensor which senses the position of the
rotating vane. An infra red emitter and an infra red
sensor are positioned such, that when the rotating vane
passes between them, the infra red beam is broken and the
infra red detector senses this and outputs a signal to
the electronic signal processing board. It is important
to be able to determine the position of the rotating
vane, in order for the polarity of the incident field to
be determined. The overall diameter of both the vane and
the measuring electrode is 67mm.
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The
motor is directly mounted to the acrylic base plate and
the ground plane is also supported from the same base
plate, supported on brass pillars. This form of
construction, the motor and the motor drive pcb are very
similar to that used on the protype version.There are
three cables to be seen exiting from the head assembly.
At the top of the picture is a 2 pin connector which is
the supply connection to the motor control pcb. The gold
plated coaxial connector is connected to the sense
electrode with the screen connected to the ground plane
immediately below the sense or measuring electrode. The
cable takes the signal from the sense electrode to the
signal processing board. The 5 way ribbon cable (lower
rhs of picture) is connected to the opto electronic
sensor at the top of the assembly. The opto sensor senses
the position of the rotating vane, and is positioned such
that a negative signal is generated when the rotary vane
is clear of the sense electrode. Conversely, when the
rotary vane is shielding the sense electrode, a positive
signal is generated. Thus a bipolar squarewave is
generated, phase locked to the rotary position of the
vane. |
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The photo left shows the
general assembly of the fieldmill in its aluminium
screening can. On the RHS at the front can be seen the
opto assembly used to synchronise the electronics to the
position of the rotating vane. Mounted on the rear is the plastic enclosure
housing the fieldmill electronics. The single 9 pin
"D" connector is used to connect the dc supply,
the output signal and also the poolarity select switch
wiring.
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This view
shows the electronics housing. The uppermost board is the
signal processing pcb which carries all the circuitry
necessary to amplify the low level signal from the sense
electrode. It also uses the signal from the opto sensor
to synchronise the electronics to the rotary position of
the vane. Centre right is the coaxial connector which
connects the amplifier input to the sense electrode.
Located top right on the pcb is the DIL switch which
selects the front end gain, along with the gain setting
resistors. Bottom right is the dual op amp which
comprises the front end amplifier and the second stage
amplifier. Centre top is the quad analogue switch IC
which is used to rectify the processed signal. Lower
centre is another dual op amp, one half of which is used
with the synchronous rectifier, the other half forms a
low pass filter. Below this and out of sight is the power
supply pcb. |
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See Next Page for
circuit details.
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