June 2006, Issue 191

Earth Field Magnetometer
Cypress PSoC High Integration Challenge 2004 Winner

FLUXGATE SENSOR

As its name implies, the fluxgate sensor works by gating any externally applied magnetic flux on and off at regular intervals. The most common type is known as a ring-core fluxgate, which uses a toroid core with a drive winding wrapped around it and one or two sense windings wound around the outside of the core.

An external field such as the Earth’s tends to concentrate in the core material. The sense coil will see this flux if it’s perpendicular to the coil. If the drive coil current is increased so that the core is saturated, the external field will no longer concentrate within the core. The sense coil will no longer see the concentrated external field. Nor will it pick up any flux from the drive coil because the circular winding generates equal flux in opposite directions. It cancels itself out.

You can apply an AC signal to the drive coil to cause the core to alternate between these two states. In the process, the sense coil will produce a current that’s proportional to the change in flux. This change, which is caused entirely by the external field, enables you to measure its field strength.

The sense coil signal will be twice the frequency of the drive signal because it appears on both its positive and negative half cycles. This sensor is also known as a second-harmonic fluxgate.

For a compass, two sense coils are at right angles to provide a full 360° of sensing. For this application, the entire assembly has to be oriented north and south for the Helmholtz coil to properly cancel the Earth’s field. As a result, you need only one sense coil. I ignored the magnetic inclination to make alignment easier.