PDC Sensor for Double Sheet Detection - Ultrasonic Attenuation Measurement and Adaptive Thresholding for Reliable Layer Monitoring
This technical article explores the ultrasonic attenuation measurement and adaptive thresholding techniques for reliable double sheet detection, covering the operating principle of acoustic attenuation through materials, the teach-in procedures for different material types, the handling of challenging materials, and the integration with high-speed production machinery.
The operating principle of ultrasonic double sheet detection is based on the measurement of acoustic signal attenuation through the material. The ultrasonic transmitter emits a high-frequency acoustic signal (typically 400 kHz) that propagates through the material. The receiver measures the amplitude of the acoustic signal that passes through the material. The attenuation of the signal depends on the number of sheets in the gap: a single sheet attenuates the signal by a certain amount, two sheets attenuate it more (due to the extra layer and the small air gap between them), and no sheet results in a strong signal. The sensor's electronics evaluate the received signal amplitude against pre-set thresholds to determine the state: no sheet, single sheet, or double sheet. The decoupling layer that isolates the piezoceramic element from the sensor housing is typically an epoxy-resin/silica composite, ensuring stable acoustic performance. The response time is typically fast enough for detection at high speeds, making it suitable for printing presses and paper processing machines.
PDC Sensor
The teach-in procedures for different material types enable the sensor to adapt to varying material properties. The sensor needs a teach-in beforehand for the material used in the application. The configurations for different materials are based on the grammage of the sheets or plates that need to be detected. The standard configuration handles materials with a grammage from 20 up to 1200 g/m², while the 'thin' configuration is for materials less than 20 g/m², and the 'thick' configuration is for materials starting from 1200 g/m². The sensor's three teach-in inputs allow configuration of three different settings, enabling the sensor to handle multiple material types in a single production line. It is possible to switch between configurations during production, providing flexibility for applications handling different materials. In very critical applications, a custom configuration can be taught-in, enabling the sensor to be optimized for specific material characteristics.
The handling of challenging materials such as shiny metal, transparent plastic, and cardboard requires careful sensor configuration. The sensor must be mounted with a special inclined angle toward the material when working with thicker plates. The recommended mounting angle for cardboard types is ≥ 35°, for plastic foils it is 27°, and for (semi-conductive) wafers it is 11°. The angled mounting ensures that the ultrasonic signal penetrates the material at the optimal angle for attenuation measurement, improving detection reliability for challenging materials. The sensor's ability to detect double layers of paper, cardboard, shiny metal, and transparent plastic makes it suitable for a wide range of applications. The sensor also handles materials with high paper dust pollution, where optical sensors would fail. The robust construction and sealed housing ensure reliable operation in demanding industrial environments.
The integration with high-speed production machinery requires fast response times and reliable detection. The sensor's output signal is connected to the machine's PLC or control system, which triggers a rejection mechanism when a double sheet is detected. The sensor's fast response time, typically less than 1 ms, ensures that the machine can react before the faulty sheet causes a jam or damage. The sensor's ability to detect double sheets regardless of the material's color, transparency, or surface reflectivity eliminates the need for sensor changes when switching materials. The sensor also provides diagnostics to monitor its health, detecting signal degradation due to contamination and generating cleaning or calibration alerts. The robust design ensures reliable operation in the dusty, high-vibration environment of printing and paper processing machinery.
The future of ultrasonic double sheet detection is moving toward higher intelligence and connectivity. The integration of machine learning algorithms is enabling the sensor to learn the characteristic attenuation patterns for different materials and automatically adapt its detection thresholds, reducing false positives. The development of sensors with IO-Link communication is enabling remote monitoring, diagnostics, and parameterization, supporting predictive maintenance and reducing downtime. The use of higher frequencies (up to 500 kHz) is being explored to improve sensitivity to thin papers and to better distinguish between single and double sheets. The ultrasonic double sheet sensor remains a cornerstone of quality control in the printing and paper industry, providing reliable, non-contact layer monitoring for efficient, high-quality production.
