Handheld metal detectors detect metal objects through the principle of electromagnetic induction, but the specific implementation method varies depending on the technical route. This article will provide a detailed introduction to its core principles and list various technical implementation methods, focusing only on the principle description.
1、 Basic principles of electromagnetic induction
Handheld metal detectors operate based on electromagnetic induction phenomena. When a metal object enters the detection area, the emitting coil inside the detector generates an alternating electromagnetic field. Metal objects will induce eddy currents (vortices) in an alternating magnetic field, which in turn generate new electromagnetic fields that interact with the original magnetic field. The receiving coil detects this change and triggers an alarm mechanism through signal processing unit analysis. The difference in conductivity and magnetic permeability of metals leads to different responses of different metals to magnetic fields, thereby affecting the detection effect.
2、 Multi technology implementation principle
1. Very Low Frequency (VLF) technology
VLF technology uses two coils: one as the transmitter and the other as the receiver. The transmitting coil emits electromagnetic waves with lower frequencies (usually 3-30kHz). When encountering a metal object, eddy currents are generated inside the metal, which in turn emit electromagnetic waves that are received by the receiving coil. VLF detectors determine the type of metal by analyzing the phase difference between the transmitted and received waves. The change in phase difference is related to the conductivity and permeability of metals, thus distinguishing between ferrous and non-ferrous metals.
2. Pulse Induction (PI) Technology
PI technology utilizes a single coil or three coils working together. The transmitting coil sends high-energy, short-term current pulses (shocks) to generate an instantaneous magnetic field. After the pulse ends, the magnetic field polarity reverses and rapidly decays, producing a reflected pulse. When a metal object exists, it will form a reverse magnetic field inside, prolonging the decay time of the reflected pulse. By measuring the duration of the reflected pulse, the detector can determine the presence and depth of the metal. PI technology is suitable for deep buried metal detection as it can penetrate highly mineralized soils.
3. Beat frequency oscillation (BFO) technology
BFO technology uses two oscillators of different frequencies: one fixed in the detector and the other fixed in the detection head. When these two frequencies are mixed, an audio signal is generated. When a metal object approaches the detection head, it will interfere with the frequency of the radio frequency waves emitted by the detection head, causing changes in the audio signal. Operators determine the presence of metal objects by listening to changes in audio signals. BFO technology is simple and easy to implement, with low cost, but relatively low accuracy.
4. Balanced coil technology
The balanced coil technology is an improved version of VLF technology, using two coils, one as a transmitter and the other as a receiver. These two coils are located in the same position, but with opposite polarities, forming an equilibrium state. When a metal object enters the detection area, it will interfere with the balance state of the balance coil, causing the receiving coil to receive signals. By comparing the differences between the transmitted and received signals, the detector can improve its anti-interference ability and detection accuracy.
5. Flux Gate Technology
Flux gate technology is based on flux gate sensors, which detect metal objects by measuring changes in magnetic fields. The sensor consists of two identical magnetic cores, wound with an excitation coil and a measurement coil. Apply alternating current to the excitation coil to alternately saturate the magnetic core. When an external magnetic field exists, the output signal of the measuring coil changes, reflecting the strength of the magnetic field. This technology is sensitive to weak magnetic metals and is suitable for high-precision detection scenarios.
6. Frequency modulation technology
Frequency modulation technology detects metal objects by modulating the transmission frequency. The transmitting coil generates an electromagnetic field with adjustable frequency, and when a metal object enters, eddy currents cause a frequency shift. The receiving coil detects frequency changes and analyzes them through the signal processing unit to determine the presence of metal. This technology can adapt to different metal characteristics and improve detection flexibility.