## 3 Experimental Results and Discussion
### Experimental Result
#### 3.2.5 Comparative Analysis  
A. Comparison with previous work [25]  
A test of an oil sample with \(<\)13 \(\upmu\)m ferromagnetic particles was carried out. The drive current was 0.5 A, and the amplification factor was 30. A gear metering pump was used to flow the oil through the sensor at a rate of 3.75 L/min. As shown in Figure 15a, when there was no particle in the oil sample, the induced voltage was a random noise with a peak-to-peak value of 0.4 mV. When the oil contained particles, there were many substantial large surge voltages in the induced voltage signal, as shown in Figure 15b. The sensor proposed in this paper can detect 13 \(\upmu\)m ferromagnetic particles. The previous sensor is able to identify 25 \(\upmu\)m ferromagnetic particles at an amplification of 440 and a drive current of 0.4 A. Therefore, compared with previous sensors, the large-diameter sensor's ability of detecting ferromagnetic particles is not significantly diminished. In previous work, the diameter of the oil path was only 1.5 mm, the change of the magnetic field strength in the oil path was negligible, the radial position of the particle had negligible influence on the induced voltage, and all the particles flowing through the sensor can be detected. However, the smaller oil path increased the flow resistance of the lubricating oil, which easily causes oil path jam. The large-diameter sensor proposed in this paper can be used for monitoring large diameter lubricants  
Figure 14: Inductive voltages of two wear debris particles separated by different distances: (**a**) 10 mm; (**b**) 20 mm; (**c**) 25 mm; and (**d**) 30 mm. Figure 15: Results of the oil sample (**a**) without debris and (**b**) with \(<\)13 \(\upmu\)m debris. without affecting the flow of lubricant. In particular, it is proposed that the diameter of the sensor can be changed according to the diameter of the oil path. When the diameter of the oil path exceeded 40 mm, a local strong high gradient magnetic field was formed in the oil path, thereby ensuring a strong induced voltage when the particles pass through. On the other hand, increasing the diameter of the oil path led to an important problem: when the diameter is small, the trajectory of the particles can be considered as a straight line. When the diameter is increased, the particles no longer move in a straight line, but a complex curve motion under the coupling of gravity, magnetic field and lubricating oil, which has a large influence on the induced signal. This issue will be studied in future work. B. Comparison with other Sensors  
Currently, many inductive debris sensors exist. Among them, the three-coil debris sensor is the most widely used due to its simple structure. The sensor is driven by high-frequency voltage, and its recognition accuracy is positively correlated with the frequency of the drive voltage [26]. When the diameter of the oil path increases, the drive voltage frequency must be increased to ensure the recognition accuracy, but this will increase the skin effect of the excitation coil, resulting in an increasing temperature of the sensor, which will affect the alternating magnetic field in the oil path. [PERSON] et al. [22] proposed a new sensor structure, which includes multiple sensing coils inside an energizing coil. Although this sensor has a large diameter of 34 mm, the induction coils will invade into the inside of the oil path, which will increase the flow resistance of the lubricating oil and adversely affect the lubrication of the equipment. [PERSON] et al. [24] proposed an innovative inductive sensor structure with a diameter of 20 mm. The magnetic field strength inside the pipe is about 100 uT, and the induced voltage signal is weak. If the diameter of the oil path is further increased, the magnetic field will be further weakened. The sensor proposed in this paper is driven by a constant current, which is not affected by the skin effect and does not change the flow resistance.