Geomagnetic declination refers to the angle between the magnetic north direction and true north direction anywhere on the earth. When the local magnetic north is eastward, the geomagnetic declination is positive, otherwise it is negative. In most areas of mainland China, the geomagnetic declination is between -10° and +2°. In Taiwan, it is about -4°～-3°.

In fact, in permanent magnets, there will be some deviations between the magnetization direction of the magnet and the actual physical direction.

At present, in most magnetic material production processes, the accuracy and consistency of the magnetizing direction of the material are not critical. However, as the application fields of magnetic materials become wider and wider, the accuracy of permanent magnets is getting higher and higher. Compared with the requirements of traditional magnetic materials, the new application technology has more special requirements for the magnetic performance of permanent magnets. Because the final magnetization vector direction (magnetic declination) of permanent magnets has an extremely important impact on the performance of precision magnetic components, permanent magnets the magnetic declination angle of a magnet is an important index to measure the performance of permanent magnets.

The earliest products for magnetic declination measurement of rare-earth permanent magnet devices can be traced back to the 1990s. The "vector magnetic moment test system" produced by the American LDJ company (merged by Laboratorio Elettrofisico) and the product of the German Brockhaus company have been reported, 2010 Years later, the new magnetic declination test system designed by the German matesy company using magnetoresistance matrix, and domestic enterprises still use the traditional coil method for measurement.

1. Helmholtz coil method measurement: single-axis or three-axis Helmholtz coils are used to test the three magnetic moment components Mx, My and Mz respectively, and the total magnetic moment M and magnetic deviation can be obtained through simple mathematical calculations Angle θz.

2. M-axis magnetoresistance matrix method: Use matrix-distributed magnetic field sensors to test the spatial magnetic field distribution B1, B2..., Bn of permanent magnetic devices, and then obtain the total magnetic moment M and magnetic declination θz through more complex mathematical calculations.

Test accuracy: 

A measurement system should not only measure the value, but also ensure its accuracy. Even if any measurement system cannot avoid errors, there are still ways to reduce its range. Under normal circumstances, there are two main types of errors that need to be considered, namely mean deviation and uncertainty. The following will compare two different measurement methods from these two aspects.

1. The main sources of deviation in the Helmholtz coil method are the accuracy of the calibration of the Helmholtz constant and the accuracy of the fluxmeter. Things to note are:

  1-1. Design of the coil structure: If the test sample has a strong magnetic moment signal in the main axis direction, it can be used as the outermost coil of the three-axis to reduce the difficulty of processing. This structure has been imitated by other companies after we used it.

  1-2. Accurate calibration of coil constant: It can be calibrated by high-precision constant current power supply and Gauss meter.

  1-3. Inspection of the uniform area of ​​the coil: Gauss meter can be used for multi-point inspection to meet the size requirements of the tested sample.

  1-4. Inspection of the orthogonality of the coil: use a three-dimensional Gauss meter for inspection, and use a square magnetic steel rotation measurement to fine-tune the platform surface.

  1-5. The flux meter can be calibrated with a volt-second generator or a high-level flux meter.

  1-6. The angle accuracy can be inspected by using standard angle wedge addition or other means.

2. The main source of deviation of the M-axis magnetic moment scalar method is the linearity of the sensor and the positioning accuracy of the installation. The linearity can be calibrated in a standard magnetic field, and the positioning accuracy can be corrected by standard samples. Matsy GmbH is kept by the German PTB. For standard samples, the relevant software can be corrected well to achieve accurate measurement. The angle accuracy inspection method can also be carried out by adding a wedge with a standard angle.