Technical Documents

Operating principle

Technical Documents
GAP-SENSOR is generally called "an eddy current displacement sensor". A high-frequency current is supplied to the coil inside the sensor head to generate a high-frequency electromagnetic field. When the target (Condutor) approaches this electromagnetic field, an eddy current is generated on the surface of the target and the sensor coil impedance is changed. The sensor system detects the change in oscillation strength resulting from this phenomenon to identify the relationship between displacement and voltage.

Meterial, dimensions, and profile of target.

Change in output characteristics due to material

GAP-SENSOR works as long as the target is metallic. Note that both thesensitivity and the measuring range are dependent on the material of the target.

Dimensions of target

It is recommended that the relative surface area of the target should be at least three as large as that of the sensor coil. When the surface area of the target is less than this size, the sensitivity is decreased. If the target is powder, laminated material (when measured at its cross section), or a wire bundle, the sensitivity is decreased as well, and at worst the measurement may be impossible.

Thickness of target (In the case of PU-05)

When the thickness of the target is more than 0.2mm (iron SCM440), 0.4mm(Alminum A5052P), 0.3mm (copper C1100P) the sensitivity is not affected by the thickness.

Profile of target

When the target is cylindrical (e.g., shaft) and its diameter is at least 3.5 times as large as the sensor coil, the sensitivity is not affected by the profile of the target. Should this ratio be one to one, the sensitivity is decreased to about 70 %.

Run-out phenomenon

In the measurement of axis vibrations, the output that does not correspond to true value may be observed. This event is called "run-out phenomenon". The mechanical run-out attributable to scratches, unevenness and elliptical or eccentric surfaces can be prevented by corrective machining.
For the electrical run-out observed in the measurement of axis vibrations on magnetic substances, particularly steel products, the causes are not simple, and it has been considered that electrical run-out is caused by the remaining magnetism, uneven crystalline structure, and dispersed surface quench hardness of the steel material. Different countermeasures as shown below may be needed depending on the measurement objective.

  • a. Demagnetizing the target axis after machining.
  • b. Lining copper sheet around the target axis and machining the target axis.
  • c. Decreasing the compressive stress from the target axis.
  • d. Changing the target from steel to copper or any other appropriate material.

Sensor mounting

Surrounding environment

When a metallic material other than the target exists close to the sensing surface of GAP-SENSOR or the sensor is embedded into a metallic component, enough distance should be provided between the relevant metal and the sensor as shown below.
Note: This rule does not apply to our PF series sensors. Please contact us for details.
Generally, the distance equivalent to the sensor diameter (i.e., a) is needed in the longitudinal direction and a cutout three times as large as the sensor diameter (i.e., 3a) is needed in the radial direction.

Interference between sensors

Technical Document
When multiple sensors are closely mounted in the multi-channel simultaneous measurement, the sensors interfere with each other to cause errors in the result. It is recommended to provide enough distance between sensors (generally six times as large as the sensor diameter) to avoid interference, to use different types of sensors, or to use an interference preventive converter. Please contact us.

Dead zone (α0)

GAP-SENSOR (except for some models) has a dead zone (α0). The dead zone refers to the distance where the sensor output does not show the linearity on the section between the sensor and target. In our brochures, the measuring distance of 0 mm includes α0.