## 3 Magnetic Field Intensity Measurement  
The experimental measurement of the three induced EMVs had to comply with the requirements adopted according to the purpose of this paper, namely, to allow the spatial distribution (magnitude, direction, orientation) of the EMF generated by the WPT-EC. The compliance with these requirements was the key element in the investigations contained in this article. Two aspects of global action were focused on: the accurate positioning of the measuring head, which included the three orthogonal coils (the coil transducers), and the effective measurement of the induced EMV. From the point of view of the measurement operation, the noise, i.e., the parasitic influences that affected the obtained numerical values, could not be neglected. This effect could not be completely eliminated, its existence being mainly due to the behavior of the electronic circuits used in the measurement process and the electromagnetic influences of other perturbations near the active area, which were not related to the measurement. In terms of the electronic components, the noise levels specified in their data sheets had to be considered, and from the point of view of electromagnetic disturbances, some efficient shielding solutions were adopted:  
* The connections from the 3D field sensor were twisted and shielded;
* The analog processing board of the three signals from the sensor coils was encased and shielded;
* The three induced EMVs in the same analog processing board were not processes simultaneously, in order to avoid the interference between signal channels. The measurement of the induced EMVs in the three coils with perpendicular axes (the three OSCs) had to be carried out with sufficient accuracy, so that the determination of the field distribution was not affected. We determined experimentally that it was necessary to measure very low effective values, of the \(\upmu\)V order; consequently, the dynamic range of the induced EMVs had to be large enough. This was necessary both to detect values at long distances from the WPT and to correctly detect values near the transmitter, but in directions with considerable deviation from the axis normal to the coil surfaces. Another technological option would be to switch the elements inserted on the amplification path for range switching, which would solve the effective measurement issue, yet the input circuits issue in the amplification chain would remain. The viable alternative was that the induced EMVs were acquired by means of special amplifiers (designed with dedicated operational amplifiers for precision instrumentation), with the input impedance set as high as possible. Moreover, analog-to-digital converters obviously had to be involved in the signal processing path. Their resolution needed to be as high as possible, but a compromise solution would be that 15 bits were enough (32,768 distinct sequences). The tests performed led us to adopt a compromise solution that ensured sufficiently accurate measurements so that the exact EMF distribution was determined. Figure 1: The 3D field sensor/3 OSCs (orthogonal sensor coils): (**a**) design; (**b**) construction. As a consequence, we designed a measurement solution for the sinusoidal input signal, considering amplification, low-pass filtering, high-frequency sampling and quantization on a large bit number and we inserted for noise reduction, the digital filtering at the path end. The solution involved ADCs working at a sampling rate much higher than the working frequency of the WPT (125 kHz) with sufficient accuracy for the required dynamic range, e.g., 16-bit quantization (15 bits for value and 1 bit for sign). The solution adopted in this paper was a compromise solution, which, as will be seen, ensured that the measurement was accurate enough to determine the structure of the field.