纽扣孔脚（4,15）在框架状托架（10）中与织物（14）一起在纵向上可移动地啮合。 在滑架（10）上有纵向张紧的绳索（19），其缠绕在安装在缝纫用脚轴（4b）上的轴（17）的下端。 在轴的上端是光束（22），光束（24）从光源（25）通过该叶轮（22）到达光电转换器（26）。 在缝纫过程中，托架与织物一起移动并驱动叶轮，由此产生脉冲，每个脉冲对应于材料或托架的移位步长。 为了测量毛虫的长度，计算一定数量的步骤或脉冲。 因此可以精确地设计轨道长度。

The Knopflochnähfuss (4.15) engages in a longitudinally displaceable in a frame-like carriage (10), is what carried with the workpiece (14). On the carriage (10) there is a taut longitudinally string (19), the shaft, the lower end one at Nähfussschaft (4b) mounted loops (17). At the upper end of the shaft is an impeller (22) through which passes a light beam (24) from a light source (25) to an opto-electrical converter (26). During the sewing operation, the carriage moves with the workpiece and drives the impeller, which pulses are generated, each of which corresponds to a displacement step of the fabric or of the carriage. To measure the length of the track is ever a certain number of Steps or pulses gezhlt. It is thus a reliable accurate measurement of bead length possible.