Astra A5040 Manuel de l'utilisateur - Page 9

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Calibration

4.1 Why the compass needs calibrating

When any compass is installed the magnetic characteristics of the whole installation affect the

way the Earth's field reaches the detector inside the device. Each individual installation

will be different and so the A5040 is equipped with an auto-calibration routine which

corrects for these installation distortions.

A NMEA-0183 command can be sent to initiate the process.

Switch 1 can be pressed to start it.

There are two compass calibration techniques in common use:

Constant angular velocity

Double rotation without constant velocity.

The A5040 uses the first method. The process consists of rotating the whole installation in the

Earth's field so that both permanent magnets and induced magnets are corrected. The

method used is the single turn at a constant rate and this rate should be in range 60 to

180 seconds for the full 400 degrees needed. The whole arrangement is turned at

constant angular velocity because the method relies on applying a correction when the

heading measured is uneven with time.

The second method is not used in the A5040. Astra Yacht has evaluated the method which

uses a mathematical model of an ideal compass and treats errors as arising from

distortion due to magnetic objects. The ideal error plot of the field vector would be a circle

with no offset from the origin. Permanent magnets ('hard') have the effect of shifting the

centre from the origin and induced magnets ('soft') modify the circle into an ellipse. The

results of our investigation show that there is always a residual error due to the fact that

real magnetic objects do not behave exactly according to the model outlined. Astra Yacht

is aware that the need for a constant angular velocity is onerous but the benefit is that it

provides a very low residual error – less than 0.5°.

Note that the calibration is different if the unit is mounted upside-down.

Calibration must always be done with the unit in its final mounted position. The essential point

of the calibration is remove the influence of external magnetic components in the vicinity

– the nearby pieces of iron and steel –so it is essential that these remain unchanged after

calibration. If any significant iron component is altered – such as the installation of new

equipment or replacement of equipment such as winches or engines – then the

calibration must be re-done.