Extrusion International 1-2017

27 Extrusion International 1/2017 Technologies for diameter measurement of hoses and tubes For the measurement of the product diameter of hoses and tubes, two established techniques are used: the “Scanning System” as well as the “CCD line sensor tech- nology”. „Scanning System“ The scanning method is based on a rotating mirror or a rotating disk, whereby a laser beam scans across the mea- suring field. In-between the rotating mirror and the light sensor, two lenses are arranged. The first lens diverts the laser beam almost parallel across the measuring field while the second lens directs the light beam onto a light sensitive detector. The product is arranged between these two lenses and disrupts the laser beam, while it is scanned across the measuring field. Therefore, the product diam- eter is calculated by comparing the time the laser beam needs to pass the whole measuring field with the time the laser needs to scan the complete product surface. In this case, time equals the diameter (Fig 1). The measuring rate depends on the rotation speed of the mirror. An increase of the measuring rate is made possible by the use of a polygon mirror. This highlights the problem that the mir- ror surfaces need the exact same perfect surface finish. Often, an averaging from several measurements is neces- sary to achieve a reasonable accuracy. „CCD line sensor systems“ There are two measuring methods for the CCD line sen- sor technology prevalent. The first method is based on a laser beam that is focused on one line sensor using op- tics (lenses). By counting the darkened diodes from the shadow image of the object, the diameter is determined. The advantage of this method is the omission of moving parts, but the costs for the lens are high (Fig 2). The second approach is an intelligent method for which a high-resolution CCD line is directly illuminated by a laser and the diameter is calculated from the diffraction fringe. The measuring rate is extremely high and only limited by the chosen CCD line sensor. The advantages of this sec- ond method are the omission of the expensive lenses as well as moving parts. The main difference between scanning and the line sen- sor technology is that the CCD line sensor technology works solely digitally and does not need moving compo- nents. Therefore, accuracy, repeatability and measuring rate are higher and a calibration is not necessary. Gauge heads that work with the line sensor technology measure the diameter in two or three planes. They are capable of measuring opaque as well as transparent products from all kinds of material with a diameter from 0.05 to 500 mm. In addition, some models offer up to 5,000 mea- surements/per axis/ per second and therefore, as well a reliable detection of lumps and neckdowns. Technologies for the measurement of diameter, wall thickness and eccentricity For applications where a diameter measurement is not suf- ficient, manufacturers of hoses and tubes use measuring systems that additionally measure the wall thickness and Pic. 2: Scanningmethod without rotatingmirror, with CCD-line sensor Pic. 1: Scanning method with rotating mirror Pic. 3: CCD line sensor measuring principle and analysis by diffraction analysis without optics and moving parts