Double Sheet Views

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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.

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).

A high-frequency ultrasonic transmitter beams against the sheet from the underside. The beamed signal induces the material to vibrate. The effect of these vibrations is a very small sonic wave on the other side of the sheet being spread. This wave is evaluated by the ultrasonic receiver opposite. The signal from the stacked sheet ( double sheet ) is so weak that it hardly gets to the receiver. The dbk+4 detects missing, single and double sheet.

The teach-in function is additionally available for materials which cannot be scanned with one of the 3 working ranges. A material teach-in is done by inserting a single sheet into the double sheet control. The C3 control input is then placed at high-level for at least 3 seconds. Materials with non-homogeneous elements should be moved during the teach-in to permit them to be detected by the dbk+4. Success with a teach-in operation is shown by a green LED. The material can now be scanned. The teach-in allows thin Washi up to wafers glued with a water film to be scanned.