Proximity Sensors

Proximity sensors are non-contacting devices that measure the relative distance between the probe tip and the conductive surface that it is observing. In operation, a modulator/demodulator generates a high frequency RF signal that is then applied to a coil on the tip of the probe. The signal is radiated into the area surrounding the probe tip. If there is no conductive material within the range of the RF field, the entire signal is returned to the modulator/demodulator. As a conductive material begins to intersect the RF field, eddy currents are set up in the conductive material, resulting in an energy loss and, consequently, a decreasing signal return to the modulator/demodulator. At some point approaching zero gap, the modulator/demodulator drops to zero.

For most applications, the modulator/demodulator is calibrated to a scale of 200 millivolts per mil. This means that for each one mil of gap change, there will be a corresponding 200 millivolt change. Five mils of gap change will produce a one volt change in output of the modulator/ demodulator.

If the observed surface is moving, the modulator/demodulator output varies in direct proportion to the peak-to-peak movement of the observed surface. Having a flat frequency response from DC to 10,000 Hz, the transducer is able to accurately follow motion at frequencies in excess of those typically encountered.

There are several types of information available from each probe and modulator/demodulator. Average gap, or position, data are available from the DC output. Thus, the system can be used for measurement of average position, eccentricity, concentricity, thickness, etc.

The AC component of the modulator/demodulator output is an indica tion of dynamic motion, or vibration. This signal provides data relating to the peak-to-peak amplitude, frequency, and form of the dynamic action of the observed surface.

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