US 6062074 A abstract the invention relates to a method and apparatus for detecting periodic defects in a test material moved longitudinally in order to enable periodic defects in elongate test material moved longitudinally to be continuously detected with little expenditure at least two measurements of a parameter which follow one another at a comparatively short interval are to be carried out at time intervals application_number US 20624098 A description fig1 shows in diagrammatic form a traveling sensor 1 with two measuring members 2 and 3 which is disposed such that it can travel on a track or bar 4 along a ring rail 5 with the spinning stations 6 7 and 8 as production stations the measuring members 2 3 are connected via a line 9 to an evaluation unit 10 with an output 11 each measuring member 2 3 in the traveling sensor detects the yarn 12 rotating at the spinning station upon traveling past and delivers a signal which is at least approximately proportional to the yarn diameter or yarn cross section or the mass of the yarn or test material periodic fluctuations due to defective front cylinders in the drafting systems are usually of particular interest where ring spinning machines are concerned the spacing a of the measuring members must in these cases be of a magnitude such that the traveling sensor 1 covers a distance corresponding to the spacing a of the measuring members.