Understanding Proximity Probes (Eddy Current Sensors)

Definition: What is a Proximity Probe?

A proximity probe, also known as an eddy current probe or a displacement transducer, is a non-contact sensor that measures the distance (gap) between the probe’s tip and a conductive target, which is typically a rotating shaft. Unlike accelerometers that are mounted on the machine casing and measure structural vibration, proximity probes are mounted through the bearing housing and measure the actual motion of the shaft itself, relative to its bearing. This makes them the primary sensor for protecting and monitoring critical, high-speed machinery with fluid-film bearings.

The Proximity Probe System Components

A complete proximity probe measurement system consists of three precisely matched components:

1. Probe: A threaded-body probe with a sealed tip containing a coil of wire. This is mounted with a specific gap between it and the shaft.
2. Extension Cable: A specialized cable of a specific length that connects the probe to the driver. Its length is part of the system’s electronic tuning.
3. Proximitor / Driver: An electronic module that generates a high-frequency radio frequency (RF) signal, sends it to the probe, and demodulates the return signal to produce a voltage output directly proportional to the gap.

These three components are calibrated as a set and are not interchangeable with components from other systems.

How Does It Work? The Eddy Current Principle

The proximitor sends an RF signal to the coil in the probe tip, which creates a magnetic field. When the probe is brought near a conductive shaft, this field induces small circulating currents, known as eddy currents, on the surface of the shaft. The eddy currents create their own opposing magnetic field, which is detected by the probe. The strength of this opposing field is directly proportional to the distance between the probe tip and the shaft. The proximitor measures these changes and outputs a DC voltage that represents the average gap and an AC voltage that represents the shaft’s dynamic vibration.

What Do Proximity Probes Measure?

Proximity probes provide a wealth of information about the rotor’s health and dynamic behavior:

• Radial Vibration: By using an X-Y pair (two probes mounted 90° apart), they measure the shaft’s vibration in two dimensions, allowing for the creation of shaft orbit plots.
• Axial (Thrust) Position: A probe mounted at the end of the shaft measures its axial movement. This is critical for protecting machines against thrust bearing failure.
• Shaft Centerline Position: The DC voltage component of the signal indicates the average position of the shaft within its bearing, which is used to monitor bearing wear and alignment changes.

– Rotational Speed: A probe looking at a keyway or notch on the shaft can act as a highly reliable tachometer.

• Runout: A slow-roll measurement that quantifies the combined mechanical and electrical imperfections of the shaft surface.

Advantages and Applications

Proximity probes are the standard for protecting large, critical turbomachinery for several reasons:

• Non-Contact: They do not touch the shaft, making them ideal for high-speed applications.
• Measures Shaft Motion Directly: They see what the shaft is doing inside the machine, which is often more important than what the casing is doing.
• Frequency Response to 0 Hz (DC): They can measure both the dynamic vibration (AC) and the average position (DC), which accelerometers cannot.
• High Reliability: They are robust, sealed sensors designed for harsh industrial environments.

They are almost universally used on machines like large steam and gas turbines, centrifugal and axial compressors, turbo-generators, and large pumps and motors equipped with sleeve or journal bearings.

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