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Eddy Current Proximity Probes

Last Modified: February 16, 2016

Proximity probes are sensors that measure relative proximity. They use changes in voltage to measure shaft surfaces that rotate or reciprocate. Because they are non-contacting transducers, proximity probes are mounted on a reasonably stationary mechanical structure, such as a bearing housing. From the mounting point, they measure the static and dynamic displacement behavior of the moving machinery. Use the proximity probe measurement type when you want to measure a dynamic position, such as an air gap between parts of moving machinery.

Eddy current proximity probes contain a driver and a monitor or regulated DC supply. The monitor or DC supply applies a power input of -24 VDC to the driver. The driver's internal oscillator converts some of the energy into a high-frequency radio signal. The signal is directed to the probe coil through a coaxial cable. The coil at the tip of the probe broadcasts the signal as a magnetic field into the surrounding area. If a conductive material intercepts the magnetic field, eddy currents are generated and the high-frequency radio signal loses power. The closer the conductive material is to the probe tip, the more power the signal loses. This power loss triggers a change in the voltage of the driver.

Proximity probe sensitivity is usually defined as the slope of a calibration curve as follows:

You can calculate the sensitivity by measuring two points in the sensor's dynamic range, using the following formula:


Proximity probes can differ in sensitivity. Refer to the documentation provided by the proximity probe manufacturer for information specific to your probe.

Offset voltage can occur when the tip of the probe touches the conductive material. Use the following formula to determine the offset voltage where offset is the output voltage of the sensor when the tip contacts the conductive material. In most cases, offset should be 0 V.

After you calibrate your proximity probe, you need to update the sensitivity and offset attributes. You can determine the physical distance between the sensor's tip and the target material by using the following formula:


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