Varying Roll Radius Measurements in a Web Processing Machine using Low-Cost Vision and Sub-Pixel Processing Techniques

In the context of web processing machines, the radial geometry of rolls or bobbins, the input material to the machine, must be measured with precision. The control system of the machine requires a measurement of the varying dimension of the bobbins as an input. A web processing machine (WPM) utilizes the rotational speed to regulate the tension of the web within the machine. Consequently, precise radius measurements, for optimal tension control, are of paramount importance. Traditional sensors, e.g., infrared and ultrasonic sensors, lack precision and are unable to detect anomalies. Additionally, their radius measurement is indirect, as these devices essentially function as distance sensors, and thus necessitate a calibration process. Furthermore, as these sensors only measure a single point on the bobbin, the system is vulnerable to localized irregularities and disturbances that could compromise the integrity of the material. This work proposes a novel methodology for measuring the radius of bobbins. A camera system is used in conjunction with Frequency Domain Zero Padding (FDZP) as a sub-pixel technique to ensure high precision. A correction is applied to compensate for the perspective-induced distortion resulting from the camera’s radial viewpoint with respect to the center of the roll. Moreover, the camera possesses the capability to evaluate multiple points on the bobbin, which enables the detection of the aforementioned anomalies at different positions on the bobbin. The system’s validation was conducted in an industrial setting, where it demonstrated a precision that is 6.2 times better than that of conventional sensors. The method ensures real-time performance while remaining low-cost.