Everyone who has visited the supermarket knows that there are always vertical doors outside the entrance of the supermarket. This is the anti-theft door used by the supermarket to prevent the supermarket goods from being stolen. If a thief steals something in the supermarket, he must not leave. If you go out of the supermarket, the anti-theft label on the supermarket product will be detected by the anti-theft door and an alarm will sound so that the supermarket security will check the thief. How about this invention?
There are two types of anti-theft doors commonly used in supermarkets on the market: one is radio frequency anti-theft doors, and the other is acousto-magnetic anti-theft doors. Magnetic anti-theft system can achieve almost zero false alarms, so why can the acousto-magnetic anti-theft system solve the problem that the radio frequency anti-theft system can’t solve and achieve almost zero false alarms? The following Baige decrypts the acousto-magnetic anti-theft system to achieve almost zero false alarms. reason.
1. The working process of the acousto-magnetic anti-theft system is simply to use the resonance phenomenon generated by the tuning fork principle to achieve almost zero false alarm operation. When the frequency of the transmitted signal (alternating magnetic field) is consistent with the oscillating frequency of the acousto-magnetic tag, the acousto-magnetic tag will cause resonance similar to a tuning fork and generate a resonance signal (alternating magnetic field); when the receiver detects it continuously 4-8 times (adjustable) ) After the resonance signal (once every 1/50 second), the receiving system will send out an alarm. The characteristics of the acousto-magnetic system are high anti-theft detection rate, almost zero false alarms, not shielded by metal tin foil, good immunity, and wide protection (the maximum width of a single system can protect 4 meters).
Second, it is the principle used by the acousto-magnetic anti-theft system. This principle involves the magnetic effect of physics. The process may be a bit esoteric, but I hope everyone can understand it.
1. Magnetostrictive effect: under the action of an external magnetic field, the size of a ferromagnetic substance changes; after the external magnetic field is removed, it returns to its original length. Under the action of a magnetic field, the length of the magnetostrictive material changes linearly and shifts; or it changes repeatedly under the action of an alternating magnetic field, resulting in vibration or sound waves; this material can convert electromagnetic energy into mechanical energy or sound energy, and vice versa. Convert mechanical energy into electromagnetic energy; the former is called magnetostrictive effect, and the latter is called piezomagnetic effect.
Under the action of a certain magnetic field strength, the ferrite magnetic metal produces a length change, which can be understood to be a slight change in the distance between atoms due to magnetization. In an alternating magnetic field, you can see the magnetostrictive metal strip vibrating according to the frequency of the alternating magnetic field. If the frequency of the alternating magnetic field is consistent with the resonant frequency of the metal bar, its amplitude is the largest, that is, resonance occurs. This effect is especially obvious for permalloy (or iron-nickel alloy).
On the other hand, this magnetostrictive effect is reversible, that is, piezomagnetic effect. Therefore, when the frequency of the alternating magnetic field is consistent with the resonant frequency of the metal strip in the acousto-magnetic tag, the permalloy strip starts to vibrate. When the alternating magnetic field is turned off, the acousto-magnetic tag will maintain a damped vibration for a certain period of time like a tuning fork, and generate a resonance signal as a spatial extension of the alternating magnetic field, which can be detected by the receiver.
The magnetostriction coefficient λ is used to describe the magnetostriction effect, λ=(LH-L0)/L0, L0 is the original length of the material, and LH is the length of the material after the change under the action of an external magnetic field. Because permalloy has high magnetostriction coefficient, such as: Ni50 permalloy λ=25×10-6, Ni80 permalloy λ=(0.1"0.5)×10-6, so the magnetostriction of permalloy The coefficients are all larger, and the resonance signal generated by the tag is also larger.
2. Magneto-mechanical coupling coefficient k. When the permalloy thin strip is excited by an alternating magnetic field under a bias magnetic field, due to the magnetostrictive effect and piezomagnetic effect, alternate conversion between magnetic energy and mechanical energy occurs in the thin strip. The conversion of energy is called magneto-mechanical coupling. The magneto-mechanical coupling coefficient k is used to measure its size, and the k value is determined by the following method. The core element in the acousto-magnetic tag is a thin strip of permalloy.
According to the phenomenological theory, the magneto-mechanical coupling coefficient k is expressed as: In the above formula, fr is the resonance frequency and fa is the anti-vibration frequency. According to the resonance curve of the acousto-magnetic tag test. When the excitation signal frequency is 57.9kHz, the resonance curve reaches the maximum value, namely fr=57.9kHz; when the excitation signal frequency is 59.7kHz, the resonance curve reaches the minimum value, namely fa=59.7kHz. Therefore, calculate the magneto-mechanical coupling coefficient k=0.251. Obviously, the acousto-magnetic tag has resonance points and anti-vibration points. Under the action of a small excitation magnetic field, it can generate a larger resonance signal, and the voltage difference between the two points is large, indicating that the tag has a large magneto-mechanical coupling coefficient. The sharp resonance curve indicates that the tag has a higher Q value, a narrower bandwidth, and a stronger selectivity. Therefore, if an appropriate bias magnetic field is set to make it work in an area with better characteristics, a higher resonance signal and stronger frequency stability can be obtained.
3. The tuning fork effect acousto-magnetic label is composed of a small plastic box with a length of about 40mm, a width of 8"14mm, and a thickness of 1mm (the existing thinner). In the small box, it is composed of two metal strips similar to a tuning fork The structure of the label is a hard magnetic metal strip fixed on the plastic box, and the other is a soft magnetic permalloy strip that can vibrate freely. According to the special material and structure of the label, it has a certain resonant frequency; when added When the frequency of the alternating magnetic field is consistent with the resonant frequency of the tag, resonance will occur. Due to the magnetostrictive effect and piezomagnetic effect, when the external alternating magnetic field disappears, the tag will still produce damped oscillation, forming a mode of alternating magnetic field energy and mechanical energy conversion. , Produces an attenuated resonance signal, which is an acousto-magnetic composite signal. The working frequency of a typical acousto-magnetic tag is 58kHz, and the tuning fork resonance signal is similar to ultrasound. Therefore, the anti-interference ability and penetrating power are extremely strong, which is different from other The biggest advantage of labels.
In the process of using the tuning fork effect to identify, it is actually a process of mutual conversion between electromagnetic energy and mechanical energy. However, due to the low energy conversion efficiency of magneto-sensitive devices, strong transmission power is required. For example, the typical value of the minimum active magnetic field strength is greater than 16A/m Therefore, the antenna detector of the acousto-magnetic system is relatively large.
3. The false alarm of the current acousto-magnetic anti-theft system is nothing more than the debugging of the machine (such as the sensitivity is too low, just increase the sensitivity of the machine) and quality problems (such as the quality of the machine is not up to standard or the internal parts of the machine are faulty, etc. Quality problems) and installation problems (such as weak installation), there will be almost no false alarms when encountering metal objects.