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The sample verification of anti metal magnetic plates is usually carried out on a metal sample that is easy to operate. Take a 5 cm x 5 cm or 10 cm x 10 cm aluminum plate, stainless steel sheet, or galvanized steel sheet, attach the label and magnetic sheet, and measure the reading distance with a reader. The reading distance has been restored from 'completely unreadable' to 'close to free space reading distance', and the data looks very good. The magnetic disk scheme has passed.
This plan has been transplanted into actual products. The outer shell of the product is a single piece of aluminum magnesium alloy frame, or the installation surface is a complete metal cabinet wall panel. The same label, the same magnetic flux, the same mounting method, but the reading distance cannot be restored - sometimes even worse than not attaching the magnetic flux. The problem lies not in the magnetic permeability of the magnetic sheet, nor in the bonding state, but in a variable that has been "hidden" by the sample verification: the area of the metal substrate and the eddy current scale determined by the area.
Eddy current is not a phenomenon of "one point", it has its own spatial scale
The mechanism by which metal causes label failure is eddy current. The alternating magnetic field induces eddy currents on the metal surface, and the reverse magnetic field of the eddy currents cancels out the incident magnetic flux. But this description can easily be simplified as' there is a layer of eddy current on the metal surface ', as if the eddy current only occurs in a small area directly below the label.
Spatial distribution of eddy current circuit
The actual vortex distribution has a clear spatial scale. Eddy currents form a closed loop on the surface of metals, and the size of the loop depends on the geometric boundaries of the metal, the distribution range of the magnetic field, and the conductivity of the metal. Directly below the label, the eddy current density is the highest; As the distance increases, the eddy current density gradually decreases, but the scale of attenuation depends on the area and shape of the metal. The eddy current circuit on a small metal sheet is "cut off" by the boundary, with a small circuit area and a concentrated reverse magnetic field near the label; The eddy current circuit on a large metal plate can extend far away from the tag, and the spatial distribution of the reverse magnetic field is wider, which weakens the tag antenna more strongly and is more difficult to be "pressed" by local materials.
The 'jurisdiction' of magnetic plates is limited
The function of a magnetic flux sheet is to provide a low magnetic resistance bypass for magnetic field lines. This bypass only exists in the area covered by the magnetic sheet. Beyond the coverage range of the magnetic flux sheet, the magnetic field lines still face the eddy current environment of the metal.
Small metal area: eddy currents are 'wrapped'
If the metal area is small, the eddy current circuit basically falls within the coverage range or edge margin of the magnetic plate, and the magnetic plate can "take over" most of the magnetic flux path, effectively suppressing the eddy current.
Large metal area: eddy current 'overflow jurisdiction'
If the metal area is large and the eddy current circuit extends far beyond the coverage area of the magnetic plate, the magnetic plate can only change the local magnetic flux distribution within its coverage area, and eddy currents outside the coverage area still exist. The reverse magnetic field generated by these "uncontrollable" eddy currents will "seep back" to the tag antenna from the edges and periphery, weakening the overall effect of the magnetic flux sheet.
This is the physical reason why "small samples are effective, but large metal surfaces fail": it is not that the magnetic flux has deteriorated, but rather that the eddy current scale of the metal exceeds the range that the magnetic flux can affect.
The 'bypass distance' of the edge margin is compressed on the large metal surface
On the small sample, there is a margin of 1.0-1.5 mm on each side of the magnetic plate, which is "sufficient" compared to the size of the metal sample - magnetic field lines can quickly wrap around the edge of the magnetic plate and reach outside the boundary of the metal sample, naturally cutting off the eddy current path.
On a large metal surface, the same 1.0-1.5 mm margin can be almost negligible relative to the scale of the metal. After the magnetic field lines "wrap out" from the edge of the magnetic plate, they face a continuous piece of metal, and the eddy current path is not cut off, but instead forms a loop over a larger area. The "surrounding space" at the edge of the magnetic plate is "swallowed up" by the large metal surface, and the magnetic flux leakage at the edge is more severe than on the small sample.
Three coping paths on large metal surfaces
There are three engineering approaches to address the issue of eddy current scales exceeding the coverage range of magnetic plates, each with its own boundary.
The first method is to increase the coverage area of the magnetic flux sheet
Match the size of the magnetic flux with the eddy current scale, and incorporate more eddy current circuits into the influence range of the magnetic flux. The cost of this path is area and cost, and the benefits decrease as the area increases - after the area of the magnetic plate increases to a certain extent, eddy currents outside the edge still exist, and the marginal benefits that continue to increase decrease.
The second method is to perform "isolation" on the metal to cut off the eddy current circuit
Slotting, milling or insulating the metal around the label, cutting the large metal surface into several small pieces to physically cut off the eddy current circuit. The effect of this path is direct, but it requires modifications to the metal structure, which is often not feasible on already formed cabinets and enclosures.
The third method is to change the relative position between the label and the metal
Move the label to the metal edge, opening or non-metallic area, and use the metal boundary to naturally cut off the eddy current circuit. This path does not require any modifications to the magnetic sheet or metal structure, but is limited by the structural layout of the product.
Selection and verification judgment based on "metal area x magnetic sheet size x eddy current scale"
Firstly, sample verification must use a metal substrate of comparable size to the actual application, rather than small sample pieces that are convenient to operate
The recovery data of the read distance on the small sample cannot be directly extrapolated to the large metal surface. If the actual metal substrate used is a whole wall panel or shell, sample verification should be carried out using metal plates of the same material and similar area, or at least on metal structures close to the actual area. If only sample verification can be used, the sample size should be clearly recorded and sufficient margin should be retained when extrapolating data.
Secondly, evaluate the proportional relationship between the size of the magnetic flux sheet and the metal area, rather than just looking at the coverage rate
The rule with a coverage rate of 90% describes the ratio of "magnetic flux sheet to tag antenna", not the ratio of "magnetic flux sheet to metal substrate". When the metal area is much larger than the area of the magnetic sheet, even if the coverage meets the standard, the ability of the magnetic sheet to suppress the overall eddy current may still be insufficient. When selecting, the area scale of the metal substrate should be confirmed at the same time to determine whether the coverage range of the magnetic plate is sufficient to "wrap" the main eddy current circuit.
Thirdly, in the scenario of large metal surfaces, priority should be given to evaluating the feasibility of "isolation" or "displacement" schemes, rather than simply increasing the magnetic flux sheet
If the metal structure allows for slotting or splitting, the effect of cutting off the eddy current circuit is usually better than increasing the area of the magnetic flux sheet. If the structure does not allow for changes, evaluate whether the label can be moved to the metal edge or near the opening, and use the boundary to intercept the eddy current. Increasing the magnetic flux is the final choice, and it is necessary to confirm whether the benefits match the costs.
Fourthly, in the sample stage, compare the read distance data between the "small sample" and the "actual metal surface" to quantify the impact of scale effects
The difference in read distance between the same magnetic plate on two metal substrates is a direct measure of the eddy current scale effect. If the difference is significant, it indicates that the coverage area of the magnetic sheet is not sufficient to cope with the actual eddy current scale of the metal surface, and the plan needs to be adjusted.
Advanced Institute (Shenzhen) Technology Co., LtdThe anti metal magnetic sheet product line covers ferrite composite sheets and nanocrystalline composite sheets, providing evaluation data for read distance recovery under different metal substrate areas, as well as suggestions for matching the size of the magnetic sheet with the metal area.
The product supports matching the coverage area and mounting position based on the actual area, shape, and boundary conditions of the metal structure, helping engineers extend "small sample verification" to "usable large metal surfaces". For more technical information, please visithttp://www.avanzado.cn/.