Sensor Sense: Capacitive sensors

Aug. 10, 2006
Most proximity sensors use an electromagnetic field to sense the presence of a target metal. But there are times when it's necessary to sense nonmetallic materials such as plastics, liquids, or powders.

 

Capacitive proximity sensor block diagram

Capacitive proximity sensors use two metal electrodes or plates to generate a high-frequency electrostatic field. A nonmetallic target entering the field increases the dielectric constant between the plates, raising the amplitude of the oscillations. The higher amplitude triggers a level detection circuit to turn the output of the sensor on. It turns off again when the target leaves and oscillation amplitudes return to normal levels.


Those target materials have little effect on electromagnetic fields, so inductive proximity sensors are useless in these situations. The sensing field must change from the electromagnetic domain to the electrostatic domain; the inductive proximity sensor must become capacitive.

Capacitive proximity sensors detect a wide variety of materials. For instance, capacitive sensors are often used to detect granular or powdered materials like plastic pellets in injection-molding hoppers. And intrinsically safe units sense the level of organic materials like rice, barley malt, corn, and soybeans in possibly explosive areas, such as grain elevators.

Two metallic electrodes or plates create the sensing element in a capacitive proximity detector. The electrodes form a capacitor in the feedback loop of a high-frequency oscillator. The amount of capacitance is a function of the surface area of the two electrodes, the distance between the electrodes, and the dielectric constant of the material between the electrodes. The capacitance is low with no target present, so the oscillation amplitude is small. A target approaching the face of the sensor changes the dielectric constant between the plates and raises the capacitance. The higher capacitance boosts the amplitude of the oscillations being measured by a level-detection circuit. When the amplitude of the oscillations exceeds a specific value, the level detector turns on the output of the sensor. As oscillation amplitude falls below the threshold, the level detector turns the sensor output off.

Conductive targets affect capacitive sensors as well, but in a different way. When a conductive target enters the sensor field it forms a counter electrode to the active face of the sensor. The target effectively reduces the distance between electrodes and boosts the average surface area. The net result is a jump in the capacitance value, the same as for a nonconductive target.

cannot sense through metal. Applications that measure liquid level in a metal container require either a special sight glass or nonmetallic tank well fittings to hold the capacitive sensor.

Turck Inc. (turck.com) provided information for this column.

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