Extrusion International 5-2018

65 Extrusion International 5/2018 SIKORA AG Bruchweide 2, 28307 Bremen/ Germany www.sikora.net while really present. In the worst case, the specifications might even be violated without this being signaled by the measuring device. The following example of a temperature profile taken over a week in September 2000 shows to which extent averaging of a measuring value can influence the percep- tion (Figure 2). The displayed “real value” results from single measure- ments taken in ten minute intervals. Averaging over a period of one hour only smoothes the extreme values. When averaging the varying temperature for more than 12 hours, the changes in temperature are displayed lower than they actually are. Furthermore, if the mean value is generated over an entire day, the information about the daily temperature variations is completely lost. A device that needs the latter averaging depth will not be suitable for a process where an alarm has to be raised or an ad- justment has to be made depending on the temperature range. A practical example taken from the cable production pro- cess is the diameter measurement based on the shadow projection method with rotating mirrors. Often high measuring rates are indicated, which result from the ro- tation rate multiplied by the number of mirrors’ facets (Zanoni, 1973), (Vossberg, 1981). The specification of ac- curacy, however, usually is based on mean values of up to one second due to a relatively poor single value preci- sion. This has various reasons: Each single measurement is done with a different mirror facet. Product movements during measurement increase or decrease the product di- ameter - depending on the direction of movement - as the measurement of both product edges is not done simulta- neously but sequentially. Lastly, the diameter information is only derived from the very transition from dark to light and light to dark. The rest of the time, the information content of the measurement signal is zero. In contrast to this, for other measuring techniques such as the diffraction method (Blohm, Sikora, & Beining, 2005), (Blohm & Sikora, 2017), line scan cameras are used (pho- to 3 and photo 4). On the one hand, product edges are recorded simultaneously – so product movement is not an issue. On the other hand, each single pixel in the diffraction seam outside the product shadow can be directly linked to the product edges, giving hundreds of reference point per line camera image instead of just two. This leads to a much higher single value precision and consequently, the measuring value has to be averaged nowhere near as long to be used for controlling or characterization of a production process. A mere comparison of measuring rates without considering these circumstances is obvious- ly not sufficient. Hence, for an objective comparison of two measuring devices, first, it is important to clearly define the require- ments of the process. Also, the catalog details given by the manufacturer should be taken into question and brought to a comparable basis using the information needed, so that the investment in a new measuring device leads to an increase in quality, process optimization as well as cost savings. Author: Dr. Hilmar Bolte, Research & Development/ Head of Analysis SIKORA AG Photo 3: Line sensor technology for diffraction analysis in a SIKORA diameter gauge head Photo 4: LASER Series 6000 from SIKORA – absolutely accurate and repeatable