The proximity probe is an electronic substitute for the dial indicator with one important distinction it is noncontact. A proximity probe is shown in Fig. 3.46. These probes come with an oscillator/demodulator and a length of cable. This transducer system must be calibrated as a unit for each specific material. Calibration constants are from 1 millivolt/mil to 1,000 millivolts/mil. Larger-diameter probes have a larger range, up to 2.0 inches. Proximity probes operate on the eddy current principle. The target surface must support an eddy current, that is, be conductive, Fig. 3.47.

Figure 3.46: Proximity probe with oscillator/demodulator. ( Courtesy of Kaman.)

Figure 3.47: Operation of the proximity probe as an electronic distance-measuring device.

The oscillator generates a high-frequency signal to the coil in the probe tip. The coil functions as an antenna and radiates a signal that is mostly directed toward the target. The target reacts with eddy currents with varying current depending on the gap distance to the target. This eddy current radiates a signal of its own that couples to and modifies the original signal. The demodulator detects this change in the original signal. The response is nonlinear, but the demodulator contains a linearizing circuit, in addition to a filter to remove the original oscillating signal, and a voltage conversion. The result is a voltage signal at the oscillator/demodulator, which is proportional to the gap distance, typically 200 mv/mil.

The proximity probe, like the dial indicator, is a relative displacement sensor. It cannot tell if the...