eFunda: Introduction to Eddy Current Transducers
engineering fundamentals Eddy Current Transducers: Introduction
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Overview

The Eddy Current Transducer uses the effect of eddy (circular) currents to sense the proximity of non-magnetic but conductive materials. A typical eddy current transducer contains two coils: an active coil (main coil) and a balance coil. The active coil senses the presence of a nearby conductive object, and balance coil is used to balance the output bridge circuit and for temperature compensation.

Typical Eddy Current Transducer

Common Specifications
Common specifications for commercially available eddy current transducers are listed below:

  Size: about 2 to 75 mm (0.079 to 3 in) in diameter, 20 to 40 mm (0.79 to 1.58 in) long
  Range: 0.25 to 30 mm (0.0098 to 1.2 in)
  Resolution: Up to 0.1 µm (3.9 µin)
  Nonlinearity: 0.5%
  Bridge Circuit
Frequency
50 kHz to 10 MHz

Pros and Cons

  • Pros:
  - Non-contacting measurement.
  - High resolution.
  - High frequency response.
  • Cons:
  - Effective distance is limited to close range.
  - The relationship between the distance and the impedance of the coil is nonlinear and temperature dependent. Fortunately, a balance coil can compensate for the temperature effect. As for the nonlinearity, careful calibrations can ease its drawback.
  - Only works on conductive materials with sufficient thickness. It can not be used for detecting the displacement of non-conductive materials or thin metalized films. However, a piece of conductive material with sufficient thickness can be mounted on non-conductive targets to overcome this drawback. A self-adhesive aluminum-foil tape is commercially available for this purpose. However, this practice is not always possible.
  - Calibration is generally required, since the shape and conductivity of the target material can affect the sensor response.

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