Introduction

In this article, I will explain the principles of an eddy-current displacement sensor. This is the most common sensor used as TSI (Turbine Supervisory Instrumentation). It is also the most frequently used sensor in applications requiring compliance with the API (American Petroleum Institute) 670 Standard.
The eddy-current displacement sensor is described as “proximity probe” in API 670.

2．The principles of eddy-current displacement sensors

As shown in Photograph 2, an eddy-current displacement transducer system is composed of the following parts:

• A sensor with a coil inside the sensor tip
• A converter (driver) with electronic circuits for oscillation and demodulation
• A dedicated co-axial cable (extension cable) that connects the sensor to the converter

As shown in Figure 1, the converter contains an oscillation circuit, a resonance circuit, a demodulation circuit, and a linearizer circuit. The oscillation circuit supplies a high frequency signal (of several MHz) to the sensor coil. This signal causes the sensor coil to generate a high-frequency magnetic flux. When the metal material that is the target of the monitoring comes close to this magnetic field, an eddy current is induced on the surface of the metal material. The magnitude of the eddy current that is induced varies depending on the distance between the sensor coil and the metal. These changes in eddy current magnitude cause changes in the combined impedance of the sensor coil and the target metal. This is the sensor coil impedance as seen from the converter. Therefore, it is possible to use the changes in the sensor impedance to detect the changes in the distance between the sensor and the target. The changes in the impedance can be extracted as changes in the voltage of the output from the resonance circuit. The demodulation circuit converts this output voltage into a direct-current voltage that corresponds to the distance between the sensor and the target. On a typical displacement sensor, a linearizer circuit linearizes the output from the demodulation circuit and then a voltage is output that is proportional to the distance. Eddy-current displacement sensors respond from static displacement (unmoving gap) to dynamic movement (high frequency). Therefore, these sensors can be used to measure displacement, such as in axial position measurements, and they can also be used to measure shaft vibration. Furthermore, if the sensor is installed in line with a gear or a key way, then the sensor can also be used to monitor the rotational speed or to detect the phase reference (phase mark).

Photograph 1 (in the previous article) and Photograph 2 show the FK Series non-contact displacement/vibration transducer. This transducer can be used for the measurements outlined above and is compliant with the API 670.

In basic terms, an eddy-current displacement sensor is a gap sensor that measures the distance between the sensor and the target. The following explains how this gap sensor can be used to measure vibration. The frequency response of the eddy-current displacement sensor covers a wide range from DC to around 10 kHz. In regular shaft vibration measurements, the range measured is from several dozen to several hundred Hz. In this frequency range, the converter output is proportional to the distance (the sensor input). Figure 2(a) shows the static characteristics of an eddy-current displacement sensor. Within the range of use, the voltage that is output is proportional to the distance. In the example, a target is vibrating between x1 and x3. The center of the vibration is x2. The changes in distance over time are as shown in Figure 2(b). The voltage that is output from the converter appears as a voltage waveform with respect to time, as shown in Figure 2(c). At this time, the distances x1, x2, and x3 that correspond to the output voltages y1, y2, and y3 are values that are already known, and the x values are proportional to the y values. It is possible to measure the amplitude of the vibration with a instrument such as a vibration monitor to calculate the deviation between y3 and y1 (y3-y1). This is the value that is usually monitored. Also, the waveform that is output from the converter shows the vibration waveform, so the output waveform is used in waveform monitoring and in vibration analysis.

In the next article, I will explain about the instruments and systems that are used to receive the signal from the sensor, to convert the signal to each monitoring parameter, and to perform the monitoring.