June
20065, Issue 191
Earth
Field Magnetometer
Cypress
PSoC High Integration Challenge 2004 Winner
Richard’s
CY8C22213-based Earth Field Magnetometer measures
the strength of the Earth’s magnetic field. The sensitive
system calculates variations in the magnetic field
that result from solar storms and aurora activity.
by
Richard Wotiz
ANALOG
PROCESSING
The
signal from the fluxgate sensor is less than 1 mV when
it’s near its null point. This needs to be amplified,
band-pass filtered, and integrated before it’s fed to
the CY8C22213 microcontroller. It also must be gated
off while the drive coil signal is switching. There
are two reasons for this. First, there is a large noise
spike during the transition that would swamp out the
sensor signal. More importantly, you want to pick up
only the signal as the drive coil enters saturation.
This allows the peak’s polarity to indicate the direction
of magnetic flux. Photo 2 shows the sensor signals in
detail.
|
(Click
here to enlarge)
|
Photo
2—The top trace is the drive coil current (measured
as the voltage drop across R110). Notice the increase
in slope as the coil saturates. Next is the output
of the band-pass filter. I had to disconnect the
Helmholtz coil so this would be visible because
the feedback loop normally drives it to zero. The
bottom trace is the sync signal, which gates the
band-pass filter output. |
A
typical magnetic storm can range from 50 to 1,000 nT
in a 30,000-nT field. I wanted at least a 5-nT resolution,
which would have required 13 bits. Instead, I fed a
fixed current to the Helmholtz coil to generate 28,500
nT. A 6-bit DAC (DAC6) inside the CY8C22213 microcontroller
drives an additional 0- to 2,048-nT offset to compensate
for long-term drift. The integrator output then supplies
a 0- to 1,024-nT feedback signal. It also drives a 12-bit
ADC (ADCINC12) inside the CY8C22213 to give a 0.25-nT
resolution.
