Bring a coil carrying a high-frequency current close to a piece of metal and something invisible happens: small swirling currents spring up in the metal, and they push back on the coil in a way that depends precisely on how far away the metal is. That feedback is the whole basis of eddy-current proximity sensing, the physics that lets a probe measure a gap it never touches. This guide explains how induced eddy currents change a coil's impedance with distance, and why that effect is what turns a proximity probe's gap voltage into the shaft displacement a machinery-monitoring SCADA system trends.
Eddy-current sensing in one line: Eddy-current proximity sensing measures the distance to a conductive target without touching it. A coil driven at high frequency creates an alternating magnetic field that induces circulating eddy currents in any nearby conductive metal; those currents create their own opposing field that changes the coil's electrical impedance. Because the strength of the induced currents depends on how close the target is, the coil's impedance - and the voltage derived from it - varies with the gap, giving a precise, continuous measurement of a tiny distance.
The sensor is built around a small coil driven with a high-frequency alternating current, which surrounds the coil with a rapidly changing magnetic field. When a piece of electrically conductive metal - the target - enters that field, the changing field induces small circulating currents within the surface of the metal. These are the eddy currents, so named because they swirl like eddies in water. By the basic law of induction, they arise to oppose the change that created them, and in doing so they generate their own magnetic field pointing back against the coil's.
That opposing field reflects back onto the coil and alters its electrical characteristics - specifically its impedance, the combination of resistance and inductance the driving circuit sees. The eddy currents both extract a little energy from the coil, which looks like added resistance, and oppose its magnetic field, which changes its effective inductance. The sensor's electronics detect this shift in impedance. The coil, in effect, feels the presence of the metal through the eddy currents, without any physical contact and without needing the target to be magnetic - only electrically conductive.
The crucial point for measurement is that the strength of the induced eddy currents depends strongly on the distance between the coil and the target. When the target is very close, the field it sees is intense, the eddy currents are strong, and their effect on the coil's impedance is large. As the gap widens, the field reaching the target weakens, the eddy currents diminish, and their influence on the coil fades. So the coil's impedance is a direct, continuous function of the gap, and the electronics convert that impedance into an output that represents distance.
In a machinery proximity probe, the electronics turn the impedance change into a DC voltage that is proportional to the gap between the probe tip and the shaft it faces, mounted a hair's breadth away. This is why the measurement is often called gap voltage: within the probe's linear range, a larger gap produces a proportionally larger voltage and a smaller gap a smaller one, following a calibrated slope. A signal conditioner, the driver, supplies the high-frequency drive and delivers the gap-proportional output to the monitoring system, so the probe reports the shaft's position as a voltage.
Because a rotating shaft is never perfectly centered and vibrates as it turns, that gap is not fixed - it changes continuously as the shaft orbits and moves within its bearing clearance. The gap voltage therefore has two useful parts. Its steady, average value represents the shaft's mean position, which reveals slow changes like a shaft settling as a bearing wears. Its fluctuating, alternating part represents the shaft's vibration, the rapid back-and-forth as the shaft whirls, and it is this dynamic component that machinery protection systems watch for signs of imbalance, misalignment, or a developing bearing problem.
The same underlying effect powers the simpler inductive proximity switches found throughout automation, though they use it as a threshold rather than a continuous measure. There, the presence of a metal target close enough to load the coil trips the switch, giving a clean, contactless detection of position for a machine guard, an end-of-travel, or a passing part. Whether used as a continuous displacement measurement on a turbine shaft or a simple on-off detector on a conveyor, the physics is identical: eddy currents induced in a conductor loading a coil in proportion to proximity.
On critical rotating equipment - turbines, large compressors, big pumps - proximity probes feed the vibration and position values that a machinery-monitoring system watches to protect the machine. Understanding that gap voltage is an eddy-current effect explains why the reading maps to shaft displacement and why its two components carry different information. The average gap voltage tracks the shaft's mean position over time, while the dynamic component measures vibration cycle by cycle, and both are quantities a SCADA or condition-monitoring layer historizes as the equipment runs.
A cloud SCADA such as Merobix can carry these values as trended tags, turning a moment-by-moment protection signal into a long-horizon record of how a machine is aging. A slow drift in the average gap voltage over weeks can reveal a shaft gradually changing position as a bearing wears or a foundation shifts, a trend no instantaneous alarm would surface. Rising vibration amplitude trended over time is the classic early warning of a developing imbalance or looseness, letting maintenance be planned before the machine reaches a trip point rather than after.
Interpreting these tags well means remembering what the eddy-current measurement depends on. The output assumes a consistent, conductive target surface, so a scratch, a plated repair, or a change in shaft material can shift the calibration and bias the reading even when nothing mechanical has changed - a phenomenon sometimes seen as electrical or mechanical runout. Historizing the gap voltage lets an engineer recognize a step change tied to a maintenance event as a calibration artifact rather than a real shaft move, keeping the machinery-protection data trustworthy over the life of the equipment.
A coil driven at high frequency creates an alternating magnetic field that induces circulating eddy currents in a nearby conductive target. Those currents change the coil's impedance by an amount that depends on how close the target is. The electronics convert that impedance change into a voltage proportional to the gap, giving a continuous, non-contact distance measurement.
The probe outputs a voltage proportional to the gap between its tip and the shaft, and a rotating shaft does not hold that gap constant - it orbits and vibrates within its bearing clearance. The steady part of the gap voltage represents the shaft's average position, while the fluctuating part represents its vibration cycle by cycle. Machinery protection systems watch that dynamic component for imbalance, misalignment, and bearing problems.
No, it only needs an electrically conductive target. The measurement works by inducing eddy currents in the metal, which happens in any conductor regardless of whether it is magnetic. The reading does depend on the target's conductivity and surface condition, though, so a change in shaft material, plating, or a scratch can shift the calibration.
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