Proximity Probes & Bode Plots
From Concept to Actual Sensors
Module 4.1 introduced displacement, velocity, acceleration, and frequency content in the abstract. This module covers the actual sensors that make shaft displacement measurement possible — proximity probes — and the diagnostic tool built specifically around startup and shutdown speed ramps: the Bode plot.
X-Y Probe Pairs and the Keyphasor
At each journal bearing (Module 2.4), a pair of non-contact proximity probes is typically mounted 90 degrees apart, continuously measuring shaft displacement in two perpendicular directions. Using two probes rather than one lets the system reconstruct the shaft's complete orbital motion within the bearing clearance — a single probe would only capture motion along one axis, missing the full picture of how the shaft is actually moving through space.
A separate Keyphasor probe reads a physical notch or projection on the shaft once per revolution, providing a precise timing reference every other vibration measurement gets compared against. Without this once-per-revolution reference, there'd be no way to define phase angle — the timing relationship between the Keyphasor signal and the peak of the vibration signal, essentially indicating where around the shaft's circumference the vibration's "heavy point" currently sits.
Phase angle is often more diagnostically valuable than amplitude alone. Unbalance, misalignment, and a bowed rotor can each produce distinctive phase behavior that amplitude readings by themselves can't distinguish between.
DC Gap Voltage — A Separate Piece of Information
Beyond the AC vibration signal itself, proximity probes also output a DC gap voltage proportional to the average distance between the probe tip and the shaft — indicating the shaft's average radial position within the bearing clearance, distinct from vibration. Trending this DC component can reveal a shifting average shaft position — from bearing wear, thermal growth, or a developing problem — even when AC vibration amplitude itself looks completely unremarkable, making it a complementary rather than redundant piece of diagnostic information.
The Bode Plot — Amplitude and Phase Across a Speed Range
A Bode plot graphs both vibration amplitude and phase angle against shaft speed throughout an entire startup or shutdown speed ramp, revealing how the rotor's vibration behavior changes as speed changes. This is how critical speeds (Module 2.1) are actually identified and confirmed on a real machine — a Bode plot makes visible exactly what speed a resonance occurs at and how sharp or broad that resonant response is, information that isn't available from any single steady-state vibration reading.
On a Bode plot, a true critical speed shows up as an amplitude peak accompanied by a roughly 90-degree phase shift as speed passes through the resonant frequency. Both signatures together — not amplitude alone — are what confirm genuine resonance rather than some other coincidental vibration source. The phase shift is what distinguishes a real critical speed crossing from an amplitude bump that happens to occur near that speed for an unrelated reason.
Amplitude alone, without the accompanying phase behavior, isn't enough to confirm a critical speed crossing. This is exactly why Bode plots — not just simple amplitude trend logs — are the standard diagnostic tool for characterizing critical speeds during commissioning, testing, and after major maintenance affecting rotor dynamics.