| 摘要 |
The button hole presser foot engages longitudinally slidably in a frame like slide which is taken along with the sewing material. The slide comprises a cord stretched in the longitudinal direction which encircles the lower end of a shaft of the presser foot shank. At the top end of the shaft is provided an impeller through which a light beam of a light source emerges toward a optoelectric transducer. During the sewing operation the slide moves with the sewing material and it drives the impeller which generates pulses each corresponding to a step of the motion of the sewing material, respectively of the slide. For determining the length of the cords, a determined number of steps, respectively of pulses is counted each time. This permits a reliable and precise determination of the length of the cords. |
