## 6 Discussion  
In this article, a more than 30 mm detecting distance testing device for wire rope was proposed to solve the problem that the inner surface of the sensor head is close to the wire  
Figure 34: (a)Coils array with permalloy; (b) Coils array with air. Figure 33: Coil array with permalloy. rope leading to the detector scraping off the surface oil of the wire rope or stuck by cut wire in the course of MFL detection. The open permanent magneizer was determined. Moreover, that there is still magnetic leakage flux at a large lift-off distance (>30 mm) was theoretically analyzed using a 3-D simulation model. Furthermore, the detection signal at the 20 mm and 25 mm distances were collected. The feasibility of detecting at a considerable lift-off distance was verified, and the magnetizer was designed. Subsequently, the output signal characteristics of the different types of coils, the influence on the output signal of different kinds of the coupled array, and the different structure parameters of coils were discussed. In addition, the analog circuit was designed to achieve the goal of amplifying the weak signal and removing the noise signal. According to the weak signal output characteristics of the signal source and noise types, an analog signal processing method based on non-inverting amplification was proposed, the circuit was designed, and the relevant electronic components were selected. Finally, by retrofitting the magnetizer and combining magnetic sensing of weak magnetic field and analog processing method of weak signal, a horizontal high-speed test bench was constructed to research the best type of coil arrays and structure parameters. Besides, for the relatively low speed of the detected wire rope, soft magnetic material, namely permalloy, was used to improve the sensitivity of the coil sensors. The testing device in the more than 30 mm detecting distance was determined. By comparing the results from the experiments above, under the same diameter wire rope, the same magnetizer, and the same detection lift-off distance conditions, the detection ability of sensors is focused. A new magnetizer is proposed based on an open permanent magnetizer, and a PCB to process signal is designed. Extensive results carried out show that this method can be used to detect defects at a distance of 30 mm. Notably, different coil arrays lead to different coil sensitivity. This implies that the sensitivity of sensors has something to do with sensors' structure and mutual coupling between coils. We put forward that the different parameters of differential coil sensors result in different sensitivities. Results in Sections 5.2 and 5.3 indicate that a higher sensitivity sensor is required at the large lift-off distance detection. By comparison, sensors' sensitivity should be decreased when the lift-off distance is only less than 15 mm or the wire rope speed is comparatively slow. We confirm that a differential coil sensor can decrease the sensitivity but increase SNR. The present findings confirm that more turns, larger detection areas, or thinner coil wire diameters will add up coils' sensitivity. It is also interesting to note that soft material can add up sensor sensitivity. It means that some magnetic materials may improve the detection ability, and our future work will include finding any material to make crafts of magnetic sensors better. In addition, the simulation result in Figure 17 is also verified through experiments in Section 5.2. We discovered that the concentric array has the lowest background noise, and the stacked array has a larger signal amplitude but easily causes the noise. In contrast, noise collected by the side-by-side array and caused by the rope strand is similar to the defect MFL signal, leading to the defect signal cannot be distinguished. The current work is sufficient to point out that the defect can be detected at a large lift-off (>30 mm) distance using the MFL method.