## 2 Magnetizer Modeling and Simulation Analysis  
### Open Permanent Magnetizer  
The principle of MFL detection consists of exposing the wire rope to a strong magnetic field in the axial direction. The wire rope has a higher magnetic permeability than the surrounding air, which increases the density of magnetic lines of flux flowing through the wire rope to those flowing outside. Hence, a surface-breaking defect presents high reluctance to the flow of magnetic lines of flux, thereby causing the flux lines to leak out from the wire rope around the defect. This magnetic flux leakage field is postulated to contain information about the shape and size of the defect, as well as the permeability of the wire rope and the strength of the applied axial field. MFL signals are the MFL field values measured by magnetic sensors placed over the wire rope close to the defect. In practice, the MFL technique is more effective if the applied magnetic field is strong enough to saturate the wire rope [18]. In the MFL method, magnetic sensors such as Hall effect sensors, giant magneto resistances, and induction coils are used to collect MFL distribution from the wire rope's defect. The MFL signal is converted into the electric signal by magnetic sensors and it is convenient to process in the corresponding computer software. Induction coils used Faraday law, and it consists of copper. The Hall sensor is based on the hall effect. As the magnetoresistive (MR) effect appears, various MR sensors have been proposed, such as the giant magnetoresistance sensor, tunneling magnetoresistance, and anisotropic magnetoresistance sensor. A MR sensor is composed of a first and a second thin film layer of magnetic material separated by a layer of non-magnetic metallic material. Magnetizing is the primary step in the magnetic flux leakage testing method. The existing ways of magnetizing wire rope are divided into two types: one is based on the principle of electromagnets to magnetize the wire rope with an energized excitation coil, and the other uses a permanent magnet magnetizing steel wire rope connected by the yoke with high permeability. As shown in Figure 1a, to generalize a stable straight-line magnetic field, the exciting coil should be supplied with strong power and high stability. In this way, exciting coils with a large number of turns and big volumes are required to increase the magnetic field. For this reason, it will take too much time and manpower to wind the exciting coil to encircle a new wire rope [24]. It is also extremely inconvenient when applied in practical working sites. According to the second method, yoke type magnetizer containing two permanent magnets and a high permeability yoke with the detected wire rope compose a magnetic bridge circuit, as shown in Figure 1b. Six sets of the yoke type magnetizer arranged in the circumference direction of the wire rope compose an integral magnetizer. However, for the effect of magnetizing, a small lift-off distance has to consider as an important factor that could rub magnetic sensors which are placed between the wire rope and the yoke, inside the integral magnetizer, resulting in an abnormal signal with anomaly noise.  
Considering the above two methods, an open permanent magnetizer is proposed, shown in Figure 2. The ring permanent magnets are magnetized along the wire rope axial direction; by using the ring permanent magnets, induction lines can enter into the detected wire rope evenly. Meanwhile, an open structure consisting of 2 semicircular magnets has been designed to encircle the detected wire rope from the middle position by opening the 2 semicircular magnets. Therefore, the symmetric ring structure magnetizer magnetizes the detected wire rope evenly and makes a large lift-off distance detection possible. Once a large lift-off distance is applied in the detecting procedure, the eccentric problem of the wire rope caused by magnets' attraction can be eliminated [25]. The magnetic flux lines distribute more evenly under open magnetizer conditions than under yoke type magnetizer [6].