Flux Gate Compass

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The soft iron frame of the flux valve accepts the flux from the Earth’s magnetic field each time the current in the center coil reverses. This flux causes current to flow in the three pickup coils.

A Flux Gate Compass drives slaved gyros using the characteristic of current induction. The flux valve is a small, segmented ring, made of soft iron that readily accepts lines of magnetic flux. An electrical coil is wound around each of the three legs to accept the current induced in this ring by the Earth’s magnetic field. A coil wound around the iron spacer in the center of the frame has 400 Hz alternating current (AC) flowing through it. During the times when this current reaches its peak, twice during each cycle, there is so much magnetism produced by this coil that the frame cannot accept the lines of flux from the Earth’s field.

The current in each of the three pickup coils changes with the heading of the aircraft.

As the current reverses between the peaks, it demagnetizes the frame so it can accept the flux from the Earth’s field. As this flux cuts across the windings in the three coils, it causes current to flow in them. These three coils are connected in such a way that the current flowing in them changes as the heading of the aircraft changes. The three coils are connected to three similar but smaller coils in a synchro inside the instrument case. The synchro rotates the dial of a radio magnetic indicator (RMI) or a HSI.