When it comes to magnetic survey, it is necessary to understand why magnetic survey can be successfully applied in practice. The theoretical basis of magnetic detection method is that when the wire rope is magnetized to saturation by the excitation device, whether there is damage on its surface or inside will cause the change of magnetic field distribution in the magnetic circuit system. Using effective means to detect the change of magnetic field distribution caused by this can reflect the detection signal of wire rope damage information.
First,? What is the classification of wire rope damage?
First of all, let's learn about the damage classification of wire rope, because electromagnetic flaw detectors are classified according to the types of defects that can be detected.
1) Local damage (LF local defect): Discontinuity in steel wire rope, such as broken wires inside and outside, corrosion pit of steel wire, deep wear of steel wire or abnormal local shape of steel wire rope.
2) LMA loss of metal cross-section: The main damages to reduce the sum of metal cross-section of steel wire rope are abrasion, corrosion and rope diameter narrowing. Compared with LF defects, such defects generally change slowly along the axial direction of wire rope. It is a relative measure of material (quality) defects in a specific area of wire rope, which is determined by comparing the detection point with the reference point symbolizing the largest metal cross-sectional area on the wire rope.
Second, what are the classification of wire rope nondestructive detector?
1, AC electromagnetic class
Its working principle is similar to a transformer. The primary and secondary coils are wound on the wire rope, just like the iron core of the transformer (Figure 1). The power supply of the primary (excitation) coil is 10~30Hz low-frequency alternating current, and the secondary (detection) coil measures the magnetic characteristics of the wire rope. Any key change in the magnetic characteristics of the wire rope will cause the voltage change (amplitude and phase) of the secondary coil to be reflected.
Key points: Electromagnetic instruments usually work under the condition of low magnetic field strength, so it is necessary to completely degauss the wire rope before testing.
Detection defect type: LMA defect with changing metal cross-sectional area.
Figure 1 Schematic diagram of electromagnetic instrument sensor
2.DC and permanent magnet (magnetic flux) instruments
DC and permanent magnet provide constant magnetic flux. A section of steel wire rope (see Figure 2) is magnetized by the sensor head (magnetic ring). The total axial magnetic flux in wire rope can be measured by induction coil.
Fig. 2 schematic diagram of the sensing head of permanent magnet equipment for measuring metal cross-sectional area loss by induction coil.
3. Magnetic leakage instrument
DC or permanent magnet instrument magnetizes a section of steel wire rope by providing constant magnetic flux through sensor head (magnetic ring). Magnetic flux leakage caused by discontinuity (such as broken wire) in steel wire rope is detected by different sensors, such as Hall element sensor.
This instrument is used to determine LF defects.
Fig. 3 schematic diagram of magnetic leakage caused by disconnection
4. Residual magnetic instruments
After the steel wire rope is magnetized by DC or permanent magnet magnetizing device, the change of remanence can be effectively measured by using the remanence characteristics of magnetic steel wire rope and adopting appropriate detection device under the condition of ensuring that the external magnetic field has been eliminated or not affected by the external magnetic field, so as to measure the change of remanence in steel wire rope.
This instrument can be used to measure the change of metal cross-sectional area and the existence of local damage.
This method is a newly developed wire rope detection technology, which needs further follow-up research and application verification.
Fig. 4 schematic diagram of metal cross-sectional area damage measured by remanence instrument
A device can have both magnetic flux and magnetic flux leakage detection principles.
Three, two different sensors: induction coil and Hall element.
1, induction coil
Speaking of induction coils, everyone is familiar with transformers. When the coil moves relative to the wire rope, the coil cuts the leakage magnetic field to generate induced electromotive force Uc.
Fig. 5 formula of induced electromotive force
Where: n-number of turns of the coil;
φ-magnetic flux through the coil;
V- speed of wire rope relative to induction coil;
Dφ/ds—— the change rate of magnetic flux in the wire rope relative to the displacement of the wire rope;
When the number of turns n and the moving speed of the coil are constant, the induced electromotive force Uc can reflect the change of magnetic flux in the wire rope along the axial direction, that is, the change of effective metal cross-sectional area of the wire rope along the axial direction.
With the relative motion of the wire rope relative to the induction coil and the exciter, the wire rope will be magnetized to saturation by the exciter gradually. If there is damage, its internal magnetic flux (proportional to the effective metal cross-sectional area of the wire rope) will inevitably decrease, so the induction coil will produce voltage output. The change of metal cross-sectional area can be detected by measuring the output voltage.
The biggest disadvantage of induction coil is that the output of sensor is related to the detection speed. When the detection speed is uneven, the output signal of the sensor is distorted and there is no output at a very low speed. At the same time, the uneven speed will cause the compression and stretching of the detection signal on the time axis, which is not conducive to the subsequent signal processing.
Fig. 6 Principle of full flow detection method
2. Hall element sensor
Principle of Hall element: When a certain current passes through a conductor perpendicular to the magnetic field, there is a magnetic field perpendicular to the current and the magnetic field, and the electromotive force output at both ends becomes the Hall effect.
Hall voltage of Hall element is:
In the formula? Sensitivity coefficient of Kc- Hall element
Control current of Ic input
B- magnetic induction intensity of magnetic field
φ —— the angle between the direction of magnetic induction intensity B and the normal vector of the element.
For a Hall element, Kc is a constant. When the installation position of the element is determined, the φ value remains unchanged, so VH in the formula is proportional to B, which is an important directional response characteristic of the Hall element. By applying this principle, as long as the output voltage VH at both ends of the Hall element is detected, the broken wire damage signal can be obtained.
The biggest advantage of Hall element is that the output signal is not affected by speed, and its volume is small, which has great advantages for magnetic field measurement in small gap space.