PDC Sensor for Double Sheet Detection - Ultrasonic Through-Beam Attenuation Measurement for Sheet Layer Monitoring
This in-depth technical article examines the application of PDC sensors for double sheet detection, covering the ultrasonic through-beam attenuation measurement principle, the signal evaluation for single versus double sheets, the sensor configuration with separate transmitter and receiver units, and the integration with printing and paper processing machinery for quality control.
Ultrasonic double sheet detection sensors are specialized through-beam devices designed to determine whether one, two, or no sheets of material are passing between a transmitter and receiver. 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. The transmitter sends out an ultrasonic soundwave from one side of the material, and the sound wave reaches the sheet. The signal is weakened as it passes through the sheet and reaches the receiver on the other side. With no sheet present, the receiver detects almost 100% of the signal. A single sheet placed between the emitter and receiver 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, providing a clear distinction between single and double sheets. An embedded microprocessor evaluates the sound levels at the receiver to trigger one of three output signals: no sheet, single sheet, or double sheet.
PDC Sensor
The sensor configuration typically consists of a separate emitter and receiver unit placed on opposite sides of the material path. The emitter uses a piezoceramic element that generates an ultrasonic beam, typically at 400 kHz. 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. The recommended mounting distance from transmitter to receiver is typically 40 mm, with the ability to deviate within the range of 20 to 60 mm depending on the installation. For thicker plates made from materials such as cardboard, metal, plastic, or foil, the sensor must be mounted with a special inclined angle toward the material. The recommended mounting angle for cardboard types is ≥ 35°, while for plastic foils it is 27°. This angled mounting improves the signal attenuation characteristics for thicker materials, ensuring reliable detection.
The signal evaluation for single versus double sheets is based on the measured attenuation of the ultrasonic signal. The sensor's embedded microprocessor continuously monitors the received signal amplitude and compares it to pre-set thresholds. The standard configuration is applied to materials with a grammage from 20 up to 1200 g/m². For thin sheets (less than 20 g/m²), a 'thin' configuration is used, while for thicker plates and sheets (starting from 1200 g/m²), a 'thick' configuration is available. The sensor is equipped with three inputs for teach-in, making it possible to configure three different settings for different materials and thicknesses. It is possible to switch between configurations during production, providing flexibility for applications handling multiple material types. In critical applications, it is possible to teach-in a custom configuration, enabling the sensor to be optimized for specific material characteristics.
The integration with printing and paper processing machinery enables automated quality control and error prevention. Double sheet detection sensors are used in applications where sheets or plates are processed, including label processing, printing machines, and folding machines. Creases, wrinkles, and accumulation of sheets can cause production stoppages, but production errors can be detected early with ultrasonic double sheet sensors. When a double sheet is detected, the sensor triggers an output signal that can be used to stop the machine, reject the faulty sheet, or alert the operator. The sensor's ability to detect double sheets regardless of the material's color, transparency, or surface reflectivity makes it suitable for a wide range of applications, including paper, plastic, and metal foil processing. The sensor also supports ultrasonic splice detection, sensing a splice in high-speed web applications by detecting the small change in received signal strength, and label detection on carrier materials, providing a versatile solution for material layer monitoring.
The future of ultrasonic double sheet detection is focused on higher frequency operation and improved signal processing. The use of frequencies up to 500 kHz is being explored to improve sensitivity to thin papers and to better distinguish between single and double sheets. The integration of digital signal processing and machine learning is enabling more sophisticated classification of material layers, reducing false positives and improving detection reliability. The development of sensors with IO-Link communication is enabling remote monitoring, diagnostics, and parameterization, supporting predictive maintenance and reducing downtime. The ultrasonic double sheet sensor remains an essential component in printing and paper processing, providing reliable, non-contact detection of material layers for quality control and process efficiency.
