June 2006, Issue 191

Earth Field Magnetometer
Cypress PSoC High Integration Challenge 2004 Winner

IN THE FIELD

There are several methods for measuring the strength of the Earth’s magnetic field. I found one technique in which an electromagnet is used to cancel out the field while a feedback loop adjusts its drive current to achieve a null reading from a magnetic compass. An optical sensor detects the compass needle position.^[2] This seemed like a reasonable method, but I wanted to have the field strength known by design rather than having to calibrate it with a commercial magnetometer.

Instead of the solenoid coil electromagnet in the original design, I used a Helmholtz coil, which consists of a pair of windings designed to create an extremely uniform magnetic field at its center. The field strength can be calculated given its mechanical dimensions and coil current. I built one to experiment with. I placed a compass at its center, but I wasn’t able to get enough sensitivity for an accurate null. Then I remembered having once played with a remote-reading car compass that used a fluxgate sensor and thought it might give better results. After finding it among a pile of old projects, I mounted its sensor at the center of the Helmholtz coil and fired it up. I found a signal along the sensor amplifier chain that varied with the field strength, which was exactly what I needed.