## 3 Experimental Verification
### Experimental Principles and Platform Construction
#### 3.2.3 Data Acquisition and Processing  
The main elements of data acquisition and processing are more complex and can be divided into the following three parts: acquisition and calculation of the magnetic induction and magnetic field strength signals, selection of the reference magnetic induction waveform, and waveform iteration of the output voltage. Acquisition and calculation of magnetic induction and magnetic field strength signals  
According to [PERSON]'s circuital law and [PERSON]'s Law of Electromagnetic Induction, the magnetic field strength \(H\) and magnetic induction strength \(B\) signals are calculated from the collected primary side current and secondary side voltage signals. \[H=\frac{N_{1}u_{1}}{LR} \tag{13}\]  
\[B=\frac{\phi}{S}=\frac{1}{N_{2}S}\int u_{2}dt \tag{14}\]  
where \(N_{1}\) is the number of turns of the excitation winding; \(N_{2}\) is the number of turns of the measurement winding; \(\phi\) is the magnetic flux in the sample; \(S\) is the cross-section of the sample magnetic circuit for the product; \(u_{1}\) is the primary voltage; \(u_{2}\) is the secondary side of the induced electromotive force; \(L\) is the length of the magnetic circuit; and \(R\) is the primary side of the resistance value of the series resistance. It can be seen that \(u_{1}\) and \(u_{2}\) in the formula are the voltage signals to be collected, and after the data acquisition card is set up, Al0 and Al1 are used as the data acquisition channels to collect \(u_{1}\) and \(u_{2}\), respectively, so that the data can be processed in the host computer. Reference Magnetic Induction Waveform Selection  
In order to ensure the accuracy of measurement, it is critical to select the reference magnetic flux density waveform. In the process of determining the reference waveform, it is necessary to determine the proportion of harmonics in the reference magnetic induction intensity waveform. The harmonic proportion properties of the reference waveform are related to the excitation properties of PWM. Compared with the fundamental wave, the impedance of higher-order harmonics is relatively large; therefore, the increase in the amplitude of the fundamental component of the power supply is the main factor leading to saturation of the tested sample. To avoid the effect of core saturation, in the experiments under PWM excitation, the reference waveform of magnetic induction intensity is selected, with the fundamental waveform as a specified sinusoidal waveform, and the proportions of each harmonic component are kept the same as the proportions of each harmonic component to the fundamental waveform in the case where \(B\) is not saturated. The  
Figure 12: Variation of PWM excitation waveform with modulation index: (**a**) \(K=0.9\); (**b**) \(K=1.2\). harmonics are selected to be extracted and superimposed to finally obtain the reference magnetic induction intensity waveform. The reference waveform \(B_{\text{ref}}\) can be expressed in the form of Equation (15). \[B_{\text{ref}}(t)=\sum_{i=1}^{n}B_{n}\cos(2\pi nft+\varphi_{n}) \tag{15}\]  
where \(n\) is the number of harmonics selected for superposition (in this paper the first 30 harmonics are selected for superposition), \(f\) is the fundamental frequency of the magnetic induction intensity, and \(\varphi_{n}\) is the phase angle value of the \(n\)th harmonic of the magnetic induction intensity. Waveform Iteration of Output Voltage  
In this paper, in the PWM excitation magnetic characteristic measurement experiment, the waveform selected for control is the waveform of magnetic induction strength \(B\). Since the NI data acquisition card not only completes the output of the PWM excitation signal but also realizes the acquisition function of the \(B\) and \(H\) signals, the whole system can form a feedback mode; thus, the closed-loop control system is selected for the control of the waveform iteration. In order to measure the magnetism of silicon steel sheet under PWM excitation, it is necessary to iterate the measured waveform until it is close to the reference waveform.