Spring-loaded electricity

Sensors for the measurement of position, displacement, and proximity may use resistive, capacitive, inductive, or photoelectric methods.

Displacement sensors can mechanically sense the position of an object relative to a fixed reference point. Proximity sensors can also measure linear or angular motion but without making physical contact with the detected object.

Typical applications include the inspection or detection and control of the position of machine tools, manufacturing systems, or moving components of valves and dampers.

They work as safety sensors or operational limit controls in packaging, printing, molding, and the chemical and food processing industries.

Triggering distance depends

Capacitance is a fundamental electrical property. Like a battery, a capacitor has two terminals. Inside the capacitor, the terminals connect to two metal plates separated by a dielectric.

The dielectric can be air, paper, plastic, or anything else that does not conduct electricity and keeps the plates from touching each other. In a proximity sensor the dielectric is air-the distance between the sensor (one plate) and object sensed (the second plate).

The capacitor can store a charge. If one hooks up a battery to a capacitor the plate on the capacitor that attaches to the negative terminal of the battery accepts electrons that the battery is producing.

The plate on the capacitor that attaches to the positive terminal of the battery loses electrons to the battery. Once it's charged, the capacitor has the same voltage as the battery.

In capacitive sensors, a high-frequency oscillator creates a field in the surroundings of the sensor surface. The presence of any capacitive object in these surroundings causes a change in the oscillation amplitude, and the threshold circuit detects that change and generates the output.

The triggering distance depends on the size, shape, and material of the object. If the sensitivity to metals is 1.0, the sensitivity to water is also 1.0, plastic or glass is 0.5, and wood is 0.4. Usually a screw locates on the capacitive sensor, which allows regulation of the operating distance.

Capacitive sensors are widely used for dimensional inspections in large-volume manufacturing operations, such as the filling of containers or the monitoring of the wearing of moving surfaces.

In nonconductive materials such as glass, plastic, or wood, the switch detects the change in dielectric constant; in conductive materials, an additional signal emanates using terminal conductivity.

This proximity switch can detect liquids, glass, plastic, wood, or metallic objects. The sensing distance is between 0.1 and 1.0 inches (3-25 millimeters).

Nicholas Sheble (nsheble@isa.org) writes and edits the Sensors department. The primary source for this month is the Instrument Engineers' Handbook, ISA Press and CRC Press, 2003.

Copyright Instrument Society of America Oct 2004
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