Preface; Measurement Principles - Allen-Bradley 1442 User Manual

Measurement Principles

This manual describes how to install and use the 1442 Series Eddy Current Probe
System.
The 1442 Series eddy current probe system performs non-contact measurement
of the distance between the sensor and the measured object (target), and outputs
a proportional voltage signal. The static component of the measurement is the
"gap, " the absolute (DC) distance from the target surface to the probe tip. The
dynamic component of the measurement is the "vibration, " the cyclical (AC)
movement of the target toward and away from the probe.
By combining this system with an Allen-Bradley® 1440 or 1444 Series
measurement module, you can measure the vibration of a rotating shaft, its
eccentricity, thrust position and rotating speed. The system is used for
continuous measurement or monitoring of shafts rotating at high speeds, such as
turbines, generators, and compressors.
The gap between the sensor and the target is found according to the following
principles:
• When an approximately 1 MHz high frequency current is supplied from
the oscillator to the sensor, a high frequency magnetic field is created at the
sensor tip.
• The inter-linkage of the high frequency magnetic flux on the target
induces an eddy current that flows on the target surface.
• When the eddy current flows on the target surface, a magnetic field is
created at the target side, and the sensor impedance changes.
• When this change in output of the oscillator is detected, the distance
versus output voltage is made linear by a linearizer circuit, and the result is
output.
Rockwell Automation Publication ICM-UM004C-EN-E - February 2014

Preface

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