Unfortunately we are not allowed to reproduce the Bosch standard 3 139 918 950 exactly for copyright reasons. Thus we offer a considerable enlarged version, which in most parts is identical to the description in the manual of the Robograph 2.

The Bosch standard 3 139 918 950 is a test instruction for non destructive flux measurement of hard magnetic magnet segments made from ferrite for small motors. Using this measurement, finished magnet segments are subjected to quality tests at random after production.

The basic idea of this test instruction is, that in the motor only the real magnetic flux is decisive, and not how it was realized. The flux is depending on: Length, width and thickness of the segment, the remanence induction B_r of the ferrite material as well as the radial orientation inside of the segment. The flux measurement is performed as hysteresis measurement, since in a motor not only the remanence flux Φ^*_R but especially the stability of the flux under the influence of opposing fields is important. From the hysteresis curve 2 or 3 numerical results are evaluated. By this the complete operation curve of the magnet in the motor is tested. These results are compared to limit values.

To test the magnet segment a measuring insert is needed, that resembles the situation in the motor as exactly as possible. The measuring insert has an extremely exact air gap between two soft iron radiuses. The larger lower radius, where the segment lies on, corresponds to the inner radius of the motor housing. The smaller upper radius corresponds to the rotor. Between magnet and upper radius there is a standard air gap of 0.7 mm.

The magnet segment is surrounded by a flux measurement coil with n turns that is embedded in the upper part of the insert, and only fills part of the length of the air gap in the measurement insert. Behind the magnet segment a Hall sensor is positioned in the center of the air gap to measure the field strength.

In order to carry out the flux measurement the measuring insert is placed in the yoke, which is then screwed shut without any force. The measuring coil of the measuring insert is now connected and the Hall sensor is pushed into the bracket with the plastic cover upwards and then tightened by means of the knurled screw. Ensure that the bracket is pushed in until it contacts the locating pin, as it acts as magnet end stop and so determines the position of the magnet.

The corresponding flux tolerance containing the limits and the number of windings is loaded before the first measurement with this measurement insert.

Every magnet must be pushed centrally right up to the end stop and securely clamped with the plastic strips. Enormous forces act during the measuring process, so, if the magnet can move during the measuring process, it is possible that the result is false or even that a coding error is produced.
During measurement the segments are magnetised by the yoke in sequence in both directions well above the saturation level.

The voltage induced in the coil is calculated as follows:

u_Φ = -n * dΦ/dt

where

Φ = magnetic flux
dt = sampling time interval
dΦ/dt = change in magnetic flux per time interval

The induced voltage is integrated and divided by the number of turns. This produces the following flux:

Φ = -1/n * ∫u_Φ * dt

Since the measurement insert is made of soft magnetic material, the field strength H can be regarded as homogeneous over the area. At the rear of the air gap there is a Hall sensor for measuring the magnetic field strength H.

By the simultaneous measurement of the field strength and the magnetic flux the complete hysteresis curve Φ against H of the segment is recorded.It can be viewed under graph
Φ and supplies the value of the remanence flux Φ^*_R bei H = 0.

Furthermore the remanence flux Φ^*_RG is to be calculated after the influence of a specified opposing field H^*_G to find how much the segment has been demagnetised by this opposing field. For this purpose a tangent to the hysteresis curve is placed through the point Φ^*_R Then a straight line is drawn through the point of the hysteresis curve at the opposing field H^*_G parallel to this tangent. The point at which these straight line intersects the Y axis at H = 0 gives the desired value Φ^*_RG. The value Φ^*_RG is the flux of the magnet segment after partial demagnetisation by H^*_G and return to the field H = 0.

Additionally if desired a value H_GF(80) can be evaluated. H_GF(80) the field strength that is necessary to demagnetize the segment to 80% of Φ^*_R.

Transformation of the Φ to Ψ hysteresis:

The method mentioned above for calculation of Φ^*_RG is difficult to understand visually from the Φ curve. Therefore the Φ curve is transformed to the Ψ curve. The idea behind this method is to obtain a statement on the pure material characteristics of the segment at
H^*_G which cannot be read direct from the Φ curve.

In physical terms the flux can be calculated as follows:

Φ = B * A

and with B = J + μ₀ * H
the result is Φ = A * J + A * μ₀ * H

where

B = magnetic induction within the coil
J = magnetic polarisation of the segment
A = area of the coil
μ₀ = magnetic field constant

The magnetic properties of the segment material are shown in the polarisation J. In the flux Φ, however, there is still the linear part A * μ₀ * H which distorts the hysteresis curve diagonally. The polarisation is not linear and has the desired hysteresis form. Multiplied by the area A of the segment we obtain the polarisation flux Ψ.

Ψ = A * J

To obtain Ψ the linear part A * μ₀ * H is subtracted from Φ.

Ψ = Φ - A * μ₀ * H

Since the area A of the segment is not known, the following method is used: Since this measurement is only permitted for hard magnetic materials, it is assumed that the polarisation J of the segment material in the area around H = 0 is constant, and therefore that the hysteresis curve of Ψ runs horizontally. Therefore the gradient of the tangent of the Φ curve corresponds to the value A * μ₀. By subtracting a straight line through the zero point with this gradient from the Φ curve, we can obtain the Ψ curve.

Please note that the measurement material must be sufficiently hard magnetic to ensure that this method produces the physically correct result.

The hysteresis curve of the polarisation flux Ψ represents the pure material properties of the segment and is much more informative in its form than the Φ curve. The parallel straight line through the flux Φ at opposing field H^*_G now become a horizontal line so that the value of Ψ^*_G at the opposing field H^*_G which can be read directly off the Ψ demagnetisation curve is identical to the required value Φ^*_RG. This also applies if the segment is not sufficiently hard magnetic.
The rectangular shape of this demagnetisation characteristic curve is decisive for the stability of the permanent magnets under the influence of strong opposing fields. It corresponds to the from material measurement well known J curve when using a J compensated coil. At the Robograph 2 the results Φ^*_R, Φ^*_RG and H_GF(80) are automatically evaluated and compared to given limits.

The results are evaluated as follows:
Φ^*_R must lie within the tolerances Φ^*_Rmin and Φ^*_Rmax.
Φ^*_RG must be at least 0.94 * Φ^*_Rmin.
H_GF(80) must be at least H_GF(80)min.

If no 100% test is performed, an evaluation by the Cpk method is necessary.