Input / Output Devices
Proximity sensor selection depends on the material to be sensed. Proximity sensor probes for sensing nonconducting surfaces or insulators vary slightly from those for conducting surfaces. Nonconducting probe signals are also more difficult to linearize.
As standoff distance decreases, required proximity sensor size decreases. But if the ratio of sensor area to gap width is too small, signal current will be too weak to measure accurately. A probe-to-target capacitance of about 0.25 pF corresponds to a safe maximum standoff distance. This is the capacitance produced by a 1 sq-in. plate spaced 1 in. from a large conducting surface. If plate area is 0.1 in.2 (0.33 in. on a side), then a 0.1-in. spacing produces the same capacitance, and so forth.
When measuring the distance to a conducting surface, proximity sensor tip size should be small when compared to the conducting surface for maximum linearity and accuracy. If the proximity sensor tip is approximately the same size as the measured surface, capacitance changes caused by gap-width variations are not distinguishable from those caused by target surface-area changes between samples. Therefore, the distance between the target surface edges and proximity sensor edges should be at least three times the gap length.
If the target surface cannot extend beyond the probe-tip edges, then probe tip dimensions may exceed those of the target surface. However, the measurement will be affected somewhat by variations in part size. In general, if the target surface is large and stationary, the largest proximity sensor possible is used. However, little is gained by increasing probe size once the proximity sensor capacitance exceeds 1 pF.
In addition to round and square probe shapes, a variety of configurations are available for specific tasks. For example, if required resolution for a moving target surface is finer in one direction of motion than that for a direction normal to the first, then a rectangular probe can be used. Here, the narrow dimension is aligned in the direction requiring the best resolution. This technique is used to measure radial wave crests on a rotating disc, or radial and axial edge runout of hollow cylindrical objects.
For measuring internal conical surfaces, a proximity sensor is often mounted inside cone-shaped probes. This is useful for measuring valve-seat runout. A probe array can measure surface flatness. Here, each probe is electronically zeroed against a known flat surface. A proximity sensor automatically scans the outputs of probes, indicating average deviation from standard, the highest and lowest deviation from the average, and the difference between them.
Sensors: Proximity Sensor Information
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