EEC Based Digital Control for Magnetic Measurements
Résumé
Magnetic properties of soft magnetic materials are high sensitive to the excitation conditions and
especially the imposed waveform of the flux density B(t) or the magnetic field H(t). Their practical
use and modeling often require to perform measurements under different B(t) (e.g., sine, triangular,
trapezoidal, high order harmonics signal, etc.). These experiments are usually performed by means
of the standardized Epstein frame device (or Single Sheet Tester SST) associated to a suitable
supply and data acquisition bench. At high induction levels, such measurements become more
difficult given, on one hand, the high non-linear nature of soft magnetic materials and on the other
hand, the imperfection of the electrical circuit of the Epstein frame itself (resistance and leakage
inductance). Thus, in that conditions, keeping a form factor FF < 1% becomes very complicated;
the latter being the parameter which quantifies the harmonic distortion of the signal as established
by the international standard.
In order to control the induction waveform B(t) (via the secondary voltage vB), it is necessary to
resort to an analog or digital control. The digital control is often privileged because it ensures a
good reproducibility together with low self-oscillations. Indeed, for high induction levels, the
analog control fails given the oscillations in the feedback loop. In addition, the digital method
allows controlling the real B(t) in the material considering the air flux leakage. It makes also the
bench more modular.
In this paper, a digital control of the waveforms of a magnetic characterization setup is proposed.
This method uses the electric equivalent circuit (EEC) of the bench to calculate the correct
waveform to be applied in order to obtain the desired induction in the material. The parameters of
the EEC as the gain, the total electrical resistance, etc. are identified during the measurement and an
output reference signal is calculated to achieve the desired B(t) waveform. The EEC schema and its
related equations will be presented and discussed in the full paper as well as the compensation
procedure. To illustrate the effectiveness of the digital control, comparison between experimental
measurements performed with and without feedback will be reported. Different representative cases
will be studied in which the high distortion is deliberately introduced: High B and low frequency,
high flux leakage, triangular or trapezoidal waveform.