Insert the copper rod into the bolt hole of the workpiece to be tested, and turn the double-throw switch to the position /10. Then the DC stabilized current supply charges the storage capacitor. When the charging voltage indicates 20V, the double-throw switch is turned to the position/ 20, the transfer switch is turned to position /2, 40, and the storage capacitor is discharged through the resistor R4, the inductor L1, and the copper rod. The low-frequency fading oscillation generated by the RLC circuit causes the current applied to the copper rod to gradually decrease, and the pulse magnetized DC magnetizer is zero, thereby realizing demagnetization. After demagnetization, the remanence in the bolt holes is less than 3 Gauss.
The magnetic field range is generally 25 times of the defect. Taking the crack width of 0101mm as an example, the width of the leakage magnetic field is only 01020105mm. There is a certain dynamic range. With the detection speed, the signal corresponding to the leakage magnetic field can reach several hundred hertz. Hall sensor <7> is the most widely used sensor in today's magnetic flux leakage detection method. It can be made very small, such as 25@25Lm2 or less, suitable for measuring non-uniform magnetic fields. It has a wide response bandwidth and can measure mS-LS. Class of pulsed magnetic field; better temperature adaptability than magnetic sensitive diode; large measuring range, from 10-6T to dozens of Tesla.
Airspace equidistant sampling takes into account the variation of the rotational speed of the magnetic sensing device along the hole, which causes the sampling points to be unevenly distributed over a period of time. To this end, the instrument adopts the airspace isometric sampling method, that is, a circular grating associated with the detector is mounted on the probe head. The circular grating controls the computer to sample once every time a pulse is emitted. When the probe moves along the surface of the workpiece, the magnetic sensor can detect The crack information to many parts of the test piece, the use of the circular grating eliminates the influence of the uneven movement speed of the probe on the sampling. The driving mechanism is mainly composed of a motor, a speed reducer, etc., and the function is to move the probe along the wall of the bolt hole. The motor is a permanent magnet DC torque motor with a speed of 1000r/min (12V). The motor rotates in positive and negative directions under computer control. The diameter of the bolt hole is 56, and the grating emits 2000 pulses per revolution. Therefore, when the probe rotates once, the grating emits 9600 sampling pulses, that is, the probe moves 1196Lm to sample once. For the crack of 30Lm width, it can be known from the calculation that the crack signal part can be Take 800 points, which meet the sampling requirements and quantitative analysis requirements. Considering the reliability and feasibility of the test, the Hall chip array is used for crack detection of bolt holes, which can cover a large area.
Detection system consists of bolt hole leakage magnetic crack detection system consists of magnetic sensor device, adaptive scanning device, magnetizer, sampling controller, signal preprocessor, A/D converter, computer, drive device, etc. Show.
The magnetic sensor probe magnetic flap level gauge is mounted on the driving device and moves with the driving device, so that the automatic scanning of the device under test can be realized. Magnetic flux leakage detection is a contact detection method. The sensor needs to be attached to the surface of the workpiece to be inspected, otherwise it will affect the detection effect. The actual surface of the workpiece is varied, so setting up the adaptive scanning device allows the sensor to adapt to changes in the surface of the workpiece, ensuring a good fit between the two.
The sampling controller is composed of a grating, an optoelectronic signal isolator, and a controller. The grating is linked to the magnetic sensing probe. When the probe starts to move, the grating sends a sampling signal, and the signal is sent to the controller through the photoelectric isolator, and the controller outputs a computer sampling trigger signal, so the computer sends a sampling instruction to perform sampling; when the probe stops moving, the grating does not emit a signal, and the sampling is also performed. stop. Thereby, the airspace isometric sampling is realized, and the problem that the sampling points are unevenly distributed due to the change of the moving speed of the detecting head is eliminated.
Conclusion Based on pulse magnetization DC magnetizer and Hall sensing device and airspace isometric sampling, the bolt hole crack magnetic flux leakage detection system can perform non-destructive crack quantitative detection on bolt hole cracks without disassembly or a small number of simple disassembly conditions. It can detect the minimum crack and other defects such as width 25Lm, depth 100Lm and length 350Lm, which can show the position and azimuth of the crack; record the detection signal, analyze the processing and state judgment, and give the visual detection result, avoiding the manual judgment Introduced leak detection; crack detection reliability is higher than magnetic powder, ultrasonic and other methods more than twice; according to the actual detection of the workpiece modified probe shape can be used for shaft, tube, hole, flat and other parts of the crack, pitting Defect detection such as inclusions has been widely used for defect detection of steel components in aviation, machinery and other industries.
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