Detection of missed stitches for sewing machines involves thread tension sensor to compare tension profile with expected pattern

摘要

The needle thread (17) on a sewing machine is taken from the moving thread guide arm to the needle through a tension transducer (21), which provides a continuous electrical tension signal to a micro-processor. The tension profile through a full rotation of the drive shaft is then compared with the profile to be expected with a correct stitch to provide a fault signal. The transducer (21) has an eyelet (26) at the end of a spring arm (23) that moves through the field of a permanent magnet (32). The movement is detected by a sensor (36), preferably a Hall generator.