Proximity Probe

A proximity probe is a non-contact displacement sensor that measures the gap between a probe tip and a nearby target. In rotating machinery, the target is often a shaft observed by an eddy-current probe. The probe, extension cable and driver convert shaft proximity into a voltage that can be interpreted as shaft-relative position or vibration.

The basic calibration relation is:

where \Delta x is displacement change, \Delta V is voltage change and S is probe sensitivity. A common convention reports sensitivity in millivolts per micrometre or volts per mil, but the actual value must come from the calibrated probe system and shaft material.

Engineering Role

Proximity probes are used when shaft motion relative to a bearing or seal matters more than casing acceleration. They support orbit plots, shaft centreline plots, runout checks, oil-whirl diagnostics, critical-speed testing, trip protection and rotor-dynamic validation.

The signal often has two parts:

• a DC component related to average probe gap and shaft centreline position;
• an AC component related to vibration, runout and dynamic shaft motion.

Both parts can be useful, but they must not be mixed casually. Removing DC offset may make an orbit plot easier to read, but it can hide shaft centreline movement, thermal growth, bearing load change or probe gap problems.

Calibration Chain and Linear Range

A proximity-probe measurement is a system measurement, not only a probe-tip measurement. The probe, extension cable, driver, power supply, grounding, target material and acquisition input all affect the reported displacement. The calibrated sensitivity should match the installed chain:

If nominal sensitivity S_{nom} is used instead, the scale error can be screened as:

This matters when comparing trip levels, orbit size, shaft centerline movement or slow-roll compensation.

The installed DC gap must also leave room for motion inside the probe’s linear range. A simple range check is:

where x_{gap} is the operating gap, x_{pk} is expected peak motion and x_{min} to x_{max} is the calibrated linear range. If the shaft motion drives the probe near the edge of its range, the waveform can flatten, phase can shift and orbit shape can become misleading.

Target material is part of the calibration. An eddy-current probe calibrated on one steel alloy may not read the same on a plated, magnetized, repaired or different-conductivity shaft. That is why slow-roll checks, electrical-runout review and material documentation are part of a defensible proximity-probe setup.

Worked Example: Convert Gap Voltage to Shaft Motion

A proximity probe system has calibrated sensitivity:

During a run at steady speed, the 1x filtered AC voltage is:

Convert voltage to millivolts:

The peak-to-peak shaft-relative displacement is:

The peak displacement amplitude is:

Now suppose the DC gap voltage changes from:

to:

The magnitude of the voltage change is:

The equivalent shaft centreline shift magnitude is:

Engineering comment: the AC vibration amplitude and DC gap shift tell different stories. The AC component describes dynamic motion at the selected frequency. The DC shift may indicate thermal growth, bearing load change, shaft lift, probe movement or a change in operating condition. The sign must be interpreted using the probe manufacturer’s convention and the installed probe orientation.

Distinction from Related Terms

Proximity probe is not an accelerometer. An accelerometer measures casing or structural acceleration. A proximity probe measures relative displacement between probe and target.

Proximity probe is not a tachometer. A proximity probe can sometimes detect a once-per-revolution key or toothed target, but in rotor vibration work it usually measures displacement, not speed.

Proximity probe is not an orbit plot. Two orthogonal proximity-probe channels can create an orbit plot, but the orbit plot is the display and the probes are the sensors.

Proximity probe is not runout. Runout is the measured geometric or electrical variation associated with rotation. A proximity probe may measure runout, but it can also measure dynamic shaft motion, centreline position and probe-gap changes.

Validation and Common Mistakes

A defensible proximity-probe measurement states probe type, driver type, extension cable, target material, calibration curve, sensitivity, linear range, installed gap voltage, probe orientation, shaft surface condition, filtering, sampling rate, grounding, temperature exposure and uncertainty.

Common mistakes include:

• using nominal catalog sensitivity without checking the installed probe, cable, driver and shaft material;
• operating outside the linear gap range;
• confusing electrical runout with true shaft motion;
• ignoring probe angle when combining orthogonal channels;
• mounting the probe bracket on a flexible or vibrating support;
• removing DC gap information without documenting it;
• comparing proximity-probe displacement directly with casing acceleration without considering the bearing and support transfer path.