Working Principle, Application Field, and Selection Tips of Eddy Current Probes!

Eddy current probe is a sensor made according to the eddy current effect. It has the advantages of non-contact measurement, fast response, good reliability, strong anti-interference ability, and normal operation in harsh environments such as oil, dirt, and water, etc.

It can monitor the axial displacement, radial vibration, axis trajectory, and rotational speed of large rotating machinery in real time for a long time, and effectively protect and analyze equipment failures.

The eddy current probe is mainly composed of a probe, an extension cable, and a preamplifier. The size and style of the probe can be customized according to user needs; the extension cable has a 5-meter system and a 9-meter system, and can also be equipped with a stainless steel armored to protect the cable. The preamplifier can choose different signal outputs: -2~ -18V, 4~20mA, 1~5V, 1~10V.

Working principle of eddy current probes

If a high-frequency (generally 1MHz) current flows from the oscillator into the sensor coil, the sensor coil generates a high-frequency oscillating magnetic field. If a piece of metal approaches this magnetic field, eddy currents will be generated on the surface of the metal.

The intensity of eddy currents varies with the distance between the sensor coil and the metal. This is because the distance affects the impedance of the sensor coil, and distance measurement can be achieved by measuring impedance. An eddy current sensor outputs a direct current voltage signal that is a monovalued function of the distance.

Applications of eddy current probes

Eddy current displacement sensors can be widely used in the online monitoring and fault diagnosis of axial displacement, radial vibration, thermal expansion, eccentricity, phase discriminator, deflection, and shaft speed of large rotating machinery.

These sensors find extensive applications in various large-scale machinery such as steam turbine units, hydropower units, compressors, large cooling pumps, induced draft fans, gearboxes, coal mills, blowers, water pumps, and wind turbine generators. They are also utilized in fields like material identification, surface roughness measurement, metal plate thickness measurement, and material deformation measurement.

Selection tips for eddy current probes

Choose the probe specifications (Φ5mm, Φ8mm, Φ11mm, Φ25mm) based on the linear range of the sensor, considering the measurement range needed. Then determine the probe type (e.g., standard, reverse, threadless) and size, as well as the installation thread according to the spatial and structural requirements of the on-site mechanical installation. Finally, select the probe cable length (1M, 0.5M).

Determine the total length of the system cable (5M, 9M) from the measurement point to the preamplifier installation point. Choose the extension cable length (4M, 4.5M, 8M, 8.5M) that matches the probe cable length (1M, 0.5M) or opt for a direct lead (5M, 9M).

Select an appropriate signal output (1–5V, 1–10V, 4–20mA, -2–-18V) based on the requirements of the backend signal reception.