Practical flow examples

For testing of fine yarns the problems are particularly acute, but it has been possible to develop an Optical Extensometer Analyser around the basic processes of SHV which can be operated using a simple (but well-calibrated) WebCam (typically 320 x 240 pixels), the use of higher resolution cameras being precluded because of the run lengths necessary for typical tests (of the order of at least several hundred frames).

On the low resolution images the markers can typically only subtend about three or four pixels in each dimension. However, by use of the sub-pixel capabilities of SHV, such a camera is nevertheless capable of automatically sensing and analysing frame to frame yarn extensions to better than 0.05% over framing runs of at least 750 frames. To the right is shown a typical full output plot of the motions of the four marker edges over an entire typical test, together with a sub-sample from the entire test plotted at a much enlarged scale for a single marker edge. This latter plot clearly shows that the trend of marker edge position is determined to a small fraction of a pixel on a frame to frame basis.