Fluxgate compass how does it work

Gyrocompasses do indicate heading, but they have moving parts and their technical performance is better suited for shorter-term navigation rather than long-term.

While IMUs are absolutely immune to any sort of external interference after all, they are inertial , they also excel at showing motion velocity and acceleration rather than heading, whether they use the mechanical gyroscope, fiber-optical laser, or MEMS-based techniques. The only way to keep an instrument from sensing the field is to create a special shielded room. Of course, the field is distorted b the metal of a ship, for example, so a one-time correction and calibration is needed for a given installation location in that particular ship.

A: Actually, no. Further, the field alignment actually drifts over time, so regular corrections are sent to the navigation system. At the point of saturation the core permeability falls away to that of vacuum causing the flux to collapse. During the next half cycle of the excitation waveform the core recovers from saturation and the flux due to the ambient field is once again at a high level until the core saturates in the opposite direction; the cycle then repeats.

Despite the magnetisation reversals due to the excitation, the flux from the ambient field operates in the same direction throughout. A sense coil placed around the core will pick up these flux changes the sign of the induced voltage indicating flux collapse or recovery. The name fluxgate clearly derives from the action of the core gating flux in and out of the sense coil.

This process is shown in the figure on the left as idealised waveforms, and it can clearly be seen that the sense voltage is twice the frequency of the excitation. One other advantage over a traditional compass is that the fluxgate can be placed in remote locations. This is a major plus because the magnetic influence of other equipment can erroneously alter the reading of either type of compass.

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Hillary Flynn. Please enter the following code:. A suitable coil of about 1, turns using 0. A three-layer sandwich form, glued together, to form a long central cavity to accommodate the strip core, is suggested. The picture here shows just such a coil made from a business card. It has a flattened H shape, the bar dimension being 45x10mm to take the winding. This is to locate the core centrally with overlap at the ends to ensure complete magnetization.

This winding is scramble-wound by hand, covering about three layers; it should be evenly distributed along the length. A quick rough and ready test is to power the coil with core fitted from a variable DC power supply, at the same time measuring the magnetic field created at the end.

The core saturation point is clearly shown as a slope change at about 30mA. The voltage used for this test was limited to about 5V to avoid overheating this 55 Ohm coil.

The resistance could be beneficially reduced with thicker wire than the 0. This can be adjusted to look like the Excitation Current of figure 1 above, by varying the frequency and voltage drive, V. To reach core saturation, the frequency must be low enough to allow the coil current to reach core saturation well inside each half cycle.

The coil example above had an inductance of about 4mH, measured. Meaning it will take about 0. All these parameters, including coil turns, can be adjusted to optimize the design. It should also be noted that a twin core device needs to have well-matched cores and coils, for good elimination of odd harmonics.

A gradiometer will need two fluxgates, so possibly as many as four cores. The EAS strips are likely to be reasonably well-matched, at least in one tag, and hopefully from one tag to another of the same manufacturer, but not perhaps between manufacturers. The coil can be used to measure individual strips with some imagination. The width of the strips is quite large for fluxgate use, with a high mass, potentially affecting noise and resolution.

Only testing will reveal performance. Suffice it to say that smaller cores would probably improve things, and could be addressed by cutting the strips into thinner pieces, which is possible with a sharp pair of scissors.

However, it might now be difficult to maintain good matching. Also note that only low temperature mechanical working of these materials is advisable. Of course different sizes will require a revised coil design, which should fit as closely to the core as possible, and appropriate electronics adjustments. The use of mu-metal and similar alloys has been somewhat ignored thus far, but a quick comment is in order. To prepare, for example, wire cores of perhaps 0.

After this, mechanical damage such as bending or cutting will degrade them, so they need to be protected until installed into the fluxgate.

It is possible to achieve some limited success by heating these cores in a naked flame, but that will require considerable experimentation. Amorphous material is much more tolerable of mechanical damage, so it can be bent and cut without too much concern. The condition of the strips discussed above is unknown, but may well already be in this state for its EAS application.

For use in a fluxgate the cardboard coil former may work for a quick prototype, but is not a good long-term prospect for repeatable performance, which requires well defined, matched and stable coil forms. One possible approach is to employ a similar design to the cardboard, but using thin Alumina or Macor sheet, possibly even thin plastic.

While the latter is the least preferred choice, this general approach could be used to produce a twin core fluxgate by bonding two coil assemblies together.

Application of varnish to the coils can also help improve stability, but is difficult to optimize in terms of flexibility and expansion. Winding directly over the paired coil assemblies as in the Vacquier design, figure 2, is possibly the easiest approach.

The coefficient of expansion of this coil with temperature is the primary factor controlling the temperature coefficient of the fluxgate, as it expands, moving the turns apart creating a negative temperature coefficient. This is probably of less concern for a gradiometer than a fluxgate used alone, because the differencing should reduce the effect. The coil will again need to be many hundreds of turns, subject to optimization. The use of a transformer drive from the electronics could eliminate the need for this extra coil, for the inconvenience of this extra custom component.

Once built and connected to the electronics, failure to operate properly can often be traced to reversed or otherwise wrongly connected coil connections. On a more practical note it is all too easy to incorporate materials into the fluxgate design which have small and not very obvious magnetic moments, which can then introduce mysterious offsets. A prime candidate for this must be connection pins, where plating often conceals a magnetic nickel layer; test items with a Neodymium Iron Boron magnet.

Good results for excitation of the core can be obtained with a square wave, and this is certainly easier to generate than a sine, so is to be recommended. Digital generation of square wave excitation, together with a 2f reference frequency, is conveniently accomplished by means of a microcontroller or a few discrete logic devices. To ensure good low noise, the core material should be driven well into saturation by 10 to times its saturation field.