According to Faraday’s principle of electromagnetic induction, when a massive metal conductor is placed in a changing magnetic field or when cutting the magnetic field lines (independent of whether the metal is massive or not, and there is no eddy current when cutting the magnetic field that does not change), a vortex-like induced current will be generated in the conductor, which is called eddy current, and the above phenomenon is called eddy current effect. The sensor made according to eddy current effect is called eddy current sensor.
The high frequency oscillating current in the preprocessor flows into the probe coil through the extension cable and generates an alternating magnetic field in the coil at the head of the probe. When the measured metal body is close to this magnetic field, an induced current is generated on the metal surface. Meanwhile, the eddy current field also generates an alternating magnetic field in the direction opposite to that of the head coil. Due to its reaction, the amplitude and phase of the high-frequency current in the head coil are changed (the effective impedance of the coil). This change is related to the permeability of the metal body, electrical conductivity, coil geometry, geometry size, current frequency and the distance between the head coil and the surface of the metal conductor. The physical properties of coil and metal conductor system can be described by the conductivity of metal conductor, permeability ξ, size factor τ, distance D of head coil and metal conductor surface, current intensity I and frequency ω parameters. The characteristic impedance of coil can be expressed by Z=F(τ, ξ, no, D, I, ω) function. Usually we can control the parameters τ, ξ, squirt, I, ω to be constant within a certain range, then the characteristic impedance Z of the coil will be a single value function of distance D, although its whole function is nonlinear, its function characteristic is “S” curve, but it can be selected to be approximately linear. Here, the change of coil impedance Z, that is, the change of the distance D between the coil of the head body and the metal conductor, is converted into the change of voltage or current through the processing of the electronic circuit of the preprocessor. The size of the output signal varies with the distance between the probe and the surface of the measured body. Eddy current sensor can measure the displacement, vibration and other parameters of the metal object according to this principle.
It is characterized by good long-term reliability, high sensitivity, strong anti-interference ability, non-contact measurement, fast response, not affected by oil and water and other media, often used for large-scale rotating machinery axis displacement, shaft vibration, shaft speed and other parameters for long-term real-time monitoring, can analyze the working condition of the equipment and the cause of failure, effectively protect the equipment and pre-maintenance.

 

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