Sensor Sense: Ultrasonic Double-Sheet Detectors

Edited by Robert Repas

The sensor operates by beaming ultrasonic sound waves through the material from one side and measuring the intensity of the sound detected on the other side.

Technically speaking, double-sheet detectors are thru-beam sensors with separate emitters and receivers. The emitter sends the ultrasonic sound wave while the receiver senses the amount of sound received. With no sheet present the receiver hears almost 100% of the signal. A single sheet placed between the emitter receiver pair attenuates the ultrasonic signal. For example, a single metal foil sheet can reduce audio intensity up to 80% at the receiver. If two or more sheets are present, the small air gap between the sheets dampens the signal further. An embedded microprocessor evaluates the sound levels at the receiver to trigger one of three output signals for no sheet, individual sheet, and double sheet.

The emitter uses a piezoceramic element that generates an ultrasonic beam at 400 kHz. The decoupling layer that isolates the element from the sensor housing is an epoxy-resin/silica composite. The receiver uses a similar piezoceramic element to detect the ultrasonic vibrations. Manufacturing tolerances require factory matching of the emitter and receiver for optimum operation. Therefore, they must not be used separately or exchanged with other devices of the same type.

Two or more sensors used in the same vicinity may produce mutual interference, disrupting the operation of both sensors. Likewise, the ultrasonic signal should not pass around the detected material. The most common cause of pass around is via reflections of the ultrasonic beam from large surfaces on the machine. Those surfaces may need a layer of sound-absorbing material.

Other applications for these sensors include ultrasonic splice detection that senses a splice in high-speed web applications. Since the splice is typically taped or glued, there is no air gap to detect. The sensor must detect a much smaller change in received signal strength to properly detect a splice.

Likewise, label detectors can sense self-stick labels on a carrier material. Because there is no air gap, the sensor detects the small change in received signal between carrier only and carrier plus label.

Pepperl+Fuchs Inc. (pepperl-fuchs.com) supplied information for this column.

Ultrasonic double-sheet detectors monitor the intensity of an ultrasonic sound wave that makes it through the sensed material. The thicker the material, the lower the ultrasonic level received. The receiver then triggers one of three outputs for no sheet (high level), single sheet (calibrated level), or double sheet (low level).