June 2006, Issue 191

Earth Field Magnetometer
Cypress PSoC High Integration Challenge 2004 Winner

OPERATION

The magnetometer must be adjusted before it’s ready to acquire data. First, you must press the Forward button twice to enter Continuous Measurement mode. You must then adjust VR101 to maximize the band-pass filter output (U4 pin 8) with the Helmholtz coil disconnected. This adjusts the sensor drive frequency to match the band-pass filter, and it aligns the sync pulse high level with the coil saturation time. You can verify this by looking at the voltage drop across R110 (corresponding to the drive coil current) and the sync signal on an oscilloscope.

The next step is to reconnect the coil and adjust the magnetometer to compensate for the local magnetic field strength. To do so, adjust VR2 to put the measurement value near the center of its range (0x1800) while rotating the magnetometer to find the maximum reading. The unit should be level and at least several feet away from any metallic objects as you do this. If the measurement remains pinned at a constant value, you may have to reverse the Helmholtz coil and sense coil connections.

When I looked at data from the magnetometer’s first week of operation, I found that an unexpected half-scale shift had occurred each day at seemingly random times. The shift’s amount was always the same, but the time intervals were irregular. After spending some time trying to figure out what had been causing this, it suddenly hit me. I had set up the magnetometer directly over my garage. As a result, the system had been picking up a magnetic offset from my car whenever I was home! After I moved the magnetometer to a more suitable location, I started seeing a noticeable difference on days that were more magnetically active than others.