PDC Sensor for Diameter Measurement - Ultrasonic Non-Contact Measurement of Roll and Coil Diameter in Material Handling
This technical article examines the use of PDC sensors for diameter measurement of rolls and coils in material handling applications. It covers the ultrasonic non-contact measurement principle, the calculation of diameter from distance measurements, the integration with tension control systems for constant surface speed, and the strategies for accurate measurement of opaque and reflective materials.
Ultrasonic diameter measurement is a key application in material handling, used to monitor the size of rolls and coils on unwinding or rewinding stands. The sensor is mounted at a fixed distance from the roll's center (or from a reference point). The sensor measures the distance to the surface of the roll. By knowing the distance to the roll surface and the roll's center position, the radius (and hence the diameter) of the roll can be calculated. The sensor's non-contact measurement capability is essential for applications where contact-based methods (e.g., mechanical arms) would be impractical or could damage the material. The typical measurement range for diameter sensors is 0.5 to 5 meters, with an accuracy of ±0.1% of range, which translates to a diameter accuracy of ±1 mm to ±5 mm. This accuracy is sufficient for most process control applications. The sensor operates at a frequency of 40-80 kHz, with a narrow beam angle to ensure the measurement is from the correct point on the roll surface.
PDC Sensor
The diameter calculation from distance measurements is based on a geometric model. The sensor is mounted at a fixed position, with its beam aimed at the roll's surface. The distance from the sensor to the roll's center (D_center) is known and fixed. The sensor measures the distance to the surface (D_surface). The radius (R) of the roll is then calculated as R = D_center - D_surface. The diameter (D) is 2 * R. The calculation requires accurate knowledge of the sensor's mounting position, which is usually established during the installation. The system compensates for any thermal expansion of the sensor mount or the roll stand by using a reference point or by calibrating the sensor periodically. The system typically provides an analog output (4-20 mA) that is proportional to the diameter, or a digital output via fieldbus. The diameter measurement is used for various control functions, such as calculating the surface speed of the roll.
The integration with tension control systems is a primary application of roll diameter measurement. In an unwinding system, the roll diameter decreases over time. To maintain a constant web tension, the unwinding speed must increase as the diameter decreases to keep the surface speed constant. The ultrasonic diameter sensor provides real-time measurement of the roll diameter, which is used to calculate the required unwinding speed. The speed is calculated as V = (Roll Surface Speed) / (π * D). The control system adjusts the drive speed to match the calculated value, maintaining constant tension. This is crucial for processes like printing or lamination where tension variations can cause quality defects. The diameter measurement also helps in detecting the end of the roll, triggering a splice or a roll change sequence.
Strategies for accurate measurement of different materials are based on the ultrasonic sensor's ability to detect the surface regardless of reflectivity. Ultrasonic sensors are not affected by the color or transparency of the material, making them suitable for measuring rolls of paper, plastic film, metal foil, or fabric. However, for very porous materials (e.g., certain non-wovens), the ultrasonic signal may penetrate the surface, causing a measurement error. In such cases, a lower frequency (e.g., 40 kHz) with a longer wavelength is used to reduce penetration. The sensor's gain is also adjusted to ensure the echo from the surface is strong enough. The sensor's mounting distance and angle must be optimized for the specific material to ensure the strongest reflection. The system also includes a filtering algorithm to smooth out measurement noise caused by surface irregularities.
The future of ultrasonic diameter measurement is moving toward higher accuracy and integration with digital systems. The use of dual-frequency sensors (e.g., 40 kHz and 125 kHz) is being explored to optimize the measurement for different materials automatically. The integration of the diameter sensor with edge detection sensors for web alignment is also common. The sensors are being connected to industrial Ethernet networks (e.g., Profinet, EtherNet/IP) for seamless data integration with production management systems. Additionally, the use of ultrasonic sensors in combination with laser distance sensors is being developed for applications where extreme accuracy is required, providing a redundant and more reliable measurement. The ultrasonic diameter sensor remains a robust and cost-effective solution for roll diameter monitoring in a wide range of material handling applications.
