Machine Design

Proximity Sensors

Proximity sensors detect objects without physical contact. Most proximity sensors emit an electromagnetic field or beam and look for changes in the field. The object being sensed is called a target. Different targets demand different sensors. For example, a capacitive or photoelectric sensor might be suitable for a plastic target; an inductive sensor requires a metal target.

In capacitive proximity sensors, the sensed object changes the dielectric constant between two plates. Sensing range is usually quoted relative to water. Because changes in capacitance take a relatively long time to detect, the upper switching range of these devices is about 50 Hz. Primary applications are in bulk-handling machines, level detectors, and package detection. One advantage of capacitive sensors is that they are unaffected by dust or opaque containers, allowing them to replace optical devices.

A typical capacitive proximity sensor has a 10-mm sensing range and is 30 mm in diameter. The device incorporates a potentiometer to allow fine tuning of the sensing range and can repetitively detect objects within 0.01 mm of the set point. Switching frequency is 10 Hz, and operating temperature range is -14 to 158°F.

Conditioning the output of position sensors has always been difficult. Designers must confront linearity, hysteresis, excitation voltage instability, and voltage offset.

Circuits that measure current flow between the sensing electrode and the target provide readouts in appropriate engineering units. Usually, one side of the voltage source or oscillator connects to the sensing electrode, and the other side connects through a current-measuring circuit to the target, which generally is a metal part at earth or ground potential.

Probes used with capacitive proximity sensors have either a flat disc or rectangular sensing element surrounded by a guard electrode that provides electrical isolation between the sensing electrode and its housing. The guard also ensures that the lines of electrostatic field emanating from the probe are parallel and perpendicular to the surface of the sensing electrode.

Capacitance systems can make measurements in 100 ∝sec with resolutions to 10-7 in. (0.001 micron). Probe diameters range from a few thousandths of an inch to several feet for corresponding measurements ranging from thousandths of an inch to several feet.

Probe selection depends on the material to be sensed. 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 probe 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, probe-tip size should be small when compared to the conducting surface for maximum linearity and accuracy. If the probe 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 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 probe possible is used. However, little is gained by increasing probe size once 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, sensors are 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. Electronic modules automatically scan the outputs of probes, indicating average deviation from standard, the highest and lowest deviation from the average, and the difference between them.

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