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Unlike the detector element of the simple magnetic compass, the element used in a remote-indicating compass is fixed-in-azimuth, senses the effect of the Earth's magnetic field as an electro-magnetically induced voltage, and operates as follows:

If a highly permeable magnetic bar or coil is exposed to the Earth's field, it acquires a magnetic flux, which is solely dependent on the magnetic latitude at which the system is operating. The amount of flux induced is determined by the strength of the Earth's horizontal ‘H’ component, and the direction of the permeable element relative to the direction of this component.

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Instrumentation

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The diagram below shows the amount of flux induced in a single coil when placed at different orientations to the Earth’s magnetic (H) field. If the coil is placed with its longitudinal axis parallel to the H field, the maximum magnetic flux passes through the coil. If the coil is alternatively rotated through 90°, so that it is at right angles to the field, it produces zero magnetic flux, and if the coil was rotated through a further 90°, it re-aligns itself with the H field, but this time in the reverse direction. In this position, it again produces maximum flux, but is in the opposite algebraic sense. The coil thus shows a cosine relationship (zero flux at 90° and maximum flux at 0°) between the field direction and the coil alignment.

For example if an aeroplane is on a heading of 060° (M), the flux intensity is H cos 60°. Similarly, the flux intensity due to the Earth's magnetic field on a heading of 120° (M) is again H cos 60°, but the polarity of the flux has reversed, since cos 120° is negative. Conversely, on a heading of 300° (M) the induced flux is the same sign and value as for a heading of 060° (M).

A simple system is thus impracticable, because in order to determine the magnetic heading it is first necessary to measure the magnetic flux in the coil, which is difficult to establish, and second, it is subject to an ambiguity in heading, which requires resolution.

If, according to Faraday, “there is a change of flux linked with a circuit, an EMF will be induced in that circuit", then the flux could be easily converted into a measurable electrical current. For an aeroplane, however, at any given position and direction, if a single coil was used the flux produced would be of constant value. It is therefore necessary to convert the steady flux into a changing one, so that a current representing the actual heading would flow. This is achieved in the flux detector unit via a device called a Flux Valve.

Remote Indicating Compass Chapter 13

A flux valve consists of two identical bars or spokes of highly permeable (easily magnetised and de-magnetised) material, mounted on a common hub. The hub is wound with a coil, known as the Primary Coil, and is connected in series to an AC single-phase power source of 23.5volts, at a frequency of 400 Hz. A pick-off or secondary coil is wound around both bars and registers the rate of change of flux in the permeable material. The effect of passing an alternating current through the primary coil has the following affect on the amount of flux produced in each leg:

Time Flux in

Top Leg

Flux in Bottom Leg

The amplitude of the flux produced in each leg is identical, although they are 180° out of phase with each other, so that the algebraic sum of the fluxes, or the total flux, equals zero. This is because at any instant of time the two bars produce flux of equal and opposite (sign) intensity. In practice this situation never occurs, since a bar placed horizontally in the Earth's magnetic field is always subject to an ‘H’ component (unless the aeroplane is near the north or south magnetic pole). This component produces a steady flux in both bars, which when added to their individual fluxes; bias the system by an amount equal to ‘H’, as shown below.

Time Flux in

Top Leg

Flux in Bottom Leg

LAMINATED COLLECTOR HORNS

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The algebraic sum of the fluxes in each leg is no longer equal to zero, and the resultant amplitude is directly proportional to the aeroplane’s heading.

The changing flux in the bars results in an EMF or voltage induced in the pick-up or secondary coil, which is proportional to the ‘H’ component that acts along the axis of the flux valve.

The single flux valve, however, has ambiguity over four headings, although two of these have different algebraic signs, which is resolved in the detector by using three spokes (flux valves), positioned 120° apart, as shown on the next page. In this arrangement, a laminated collector horn is located at the outer end of each flux valve to concentrate the lines of the Earth’s magnetic force along the parent spoke. This increases the overall sensitivity of the detector head and increases the magnitude of the induced voltage in the secondary coil.

It is still possible to align the compass with a 180° error, but the instrument detects this, and immediately starts to precess the gyro unit to the correct heading.