The security inspection of transportation places such as stations and subways is basically to detect whether the person passing through the security gate is carrying the prohibited metal articles by using the principle that the security gate alarms the metal articles.Through alarm and display, prompt security personnel to carry out further inspection.It is detected by the induction of parasitic current and the digital signal processing method of homogenizing magnetic field.
So since the security gate is detected by magnetic field changes, the earth itself is a large magnetic field, so it will not be limited by the working state?Let's take a look at the factors that affect it.
1. The factors of the security door itself
The field intensity, detection method (continuous field and pulse field), working frequency and detection program are the most important factors that affect detection.For CW metal detectors, the higher the frequency, the higher the detection sensitivity.However, the increase in frequency increases the possibility of false alarms.The main reason for this situation is the skin effect, the higher the frequency, the smaller the skin depth, more field can form flow, thus causing greater field loss.For pulsed field metal detector changes the pulse frequency and the continuous wave metal detector changes the frequency situation is similar.The pulsed field metal detector can also obtain better detection effect by changing the duration of the pulse and the delay time between the disconnection of transmitter current and the opening of receiver.
The detection ability of both continuous wave and pulsed wave metal detectors is related to the eddy current generated in the conductor.Therefore, the factors that determine the size of the eddy current are the main factors that affect the efficiency of the detector.
2. Factors of detection
The quality and shape of the object, the metal type and alloy composition, and the direction of the detection field are the important factors affecting the detection.For example, two metal spheres of the same material and different masses, one solid and one hollow, will respond differently when they pass through the metal detector.If the shape and size of the two spheres are the same, but the mass is different, if the wall thickness of the hollow sphere is greater than five times the skin depth, they enter the field under the same conditions, and the vortex will be basically the same.If the wall thickness of the hollow ball is less than several times the skin depth, the metal detector response will be different.In addition, objects with the same mass and different shapes will pass through the detector with different results.We used two aluminum plates of the same thickness and area, a square with a side length of 3 in. and a rectangle with a length of 1 in. x9 in. to enter the detection area in the same way.We found that the response of the square aluminum plate was greater than that of the rectangle.This is because the square aluminum plate with rectangular aluminum surrounded the area of the field lines are equal, the induced voltage are equal, but the perimeter of the perimeter of the former is less than the latter, therefore, small resistance, eddy current big detection content material properties is also the important factors influencing the detection effect, under the condition of the same conductivity, ferromagnetic metal easier than non ferromagnetic metal detection,Non-ferromagnetic metals with high electrical conductivity are easier to detect than ferromagnetic metals with low electrical conductivity.
3. The tester factor
The physical characteristics of the person, the speed at which the person passes through the metal detector, and the location of the object on the person's body all affect the results.In general, the detection effect will be affected if the detection speed is too fast or too slow. Modern metal detectors have a good detection effect for the detection of objects passing at a normal speed. The very slow movement or galloping past will be stopped by the security personnel.Due to any of the metal detector in the whole detection area is likely to provide an absolute consistent detection sensitivity, so the same detection when different parts by detecting area, can detect the result is distinct, but with the lowest sensitivity to set standards, better solve the problem of the whole detection area sensitivity is inconsistent.
4. Factors of the surrounding environment
There are some metal components, metal furniture, and large metal objects carried by passengers in the use environment, which will affect the background noise and detection sensitivity due to their constant change.In addition, changes in ambient temperature and the surrounding electromagnetic field also affect the function of the detector.
After knowing these influencing factors, we can effectively avoid the restriction of the use of the security door, and fully make the work of the security door achieve the best effect.