## 2 Measuring Systems, Materials and Methods
### Measuring System and Samples
#### 2.2.2 Comparison of Measuring Systems
At the same time, reactance is influenced by the excitation frequency (\(f\)), the magnetic permeability of the core (\(\alpha\)), the cross-sectional area of the core (\(A\)), the number of turns (\(N\)), and the length of the winding (\(l\)). Ultimately, the equation takes the form (6):  
\[|I|=\frac{|U|}{\sqrt{\left(\frac{\rho^{-l}}{5}\right)^{2}+\left(2\pi\cdot f \cdot\frac{\mu\cdot N^{2}\cdot A}{l}\right)^{2}}} \tag{6}\]  
EC systems are relatively complex (both in hardware and software), with many factors affecting the final result. Additionally, (since this is a surface method) the signals obtained from evaluating RC structures can be weak and noisy, especially when dealing with a thick concrete cover. Eddy current transducer excitation can be implemented in various ways, ranging from coils driven by single- or multi-frequency signals to pulsed excitation [64; 65] or rotating magnets [66; 67; 68]. However, in all versions, the excitation is married to the  
Figure 12: Waveforms obtained from EC transducers: (**a**) absolute, (**b**) differential. pickup sensor, which poses challenges in densely reinforced RC objects. The interaction between neighboring reinforcing rebars may distort the measurements and result in incorrect classifications. Moreover, it also makes it more challenging to build an area testing system [34]. On the other hand, properties of systems of this kind are straightforward to regulate, and spatial resolution can be high (the EC method can identify defects that are approximately the size of a wavelength [69]). The alternative for EC can be a magnetic system (Figure 13). The main advantage of magnetic nondestructive testing systems is their straightforward design, which is typically much less complex than eddy current designs. The system presented in Figure 13 includes an excitation subsystem, a positioning robot, a magnetic field sensor (HMC5883L, Honeywell, NJ, USA [70]), and a data acquisition subsystem. Although the typical setup for MFL transducers and measurement procedures resembles that of EC, the authors argue that this configuration could be more optimal and requires significant adjustments. In the system, the following modifications have been implemented:  
* The excitation system consists of two neodymium magnets. These strong magnets effectively magnetize the reinforcing bars, eventually producing a significantly stronger signal than the EC method. * The magnets are oriented with the same poles toward the sample (same-pole magnetization--SPM). This configuration allows for a much higher signal-to-noise ratio than the standard opposite-pole magnetization (OPM) [71] and facilitates the construction of area transducers [41];
* The high distance between the magnets and the transducer (500 mm) significantly limits direct interaction. In this method, the only adjustable parameters are the strength of the magnets and the distance between them (Figure 12b). This setup also facilitates the development of an area-testing transducer [41];
* The stationary magnets are placed directly above the tested rebar (no movement during measurement). This minimizes the influence of neighbor rebars in the reinforcement grid on measurement results, allowing for effective measurements even at much greater distances than those achievable with EC. Similarly, like in the case of the EC method, the most typical technology for this method was selected. The AMR sensor (HMC5883L, Honeywell, NJ, USA [70]) satisfies the fundamental requirements (Figure 1) and possesses several advantageous features. The element enables measurement of the spatial components of magnetic induction (\(B_{\mathrm{x}}\), \(B_{\mathrm{y}}\), and \(B_{\mathrm{z}}\)) and can detect negative values, which facilitates polarization recognition. Furthermore, it reduces the risk of damage in cases where the measurement range is exceeded, thereby enhancing system reliability. The design of the measurement system is  
Figure 13: The magnetic system, (**a**) block diagram, and (**b**) elements are arranged on the sample surface. M—magnet, S—sensor.