An industrial PFAFF 1183 lockstitch (stitch 301 according to ISO 4915) machine (Fig.1) has been instrumented with a thread tension sensor (Fig.2) connected to a signal conditioning circuit which in turn plugs to a National Instruments PCI-MIO-16E-1 data acquisition board (although often called thread tension, the parameter measured is actually a thread pulling force). The machine’s “synchronizer” (a rotary optical encoder) provides 512 pulses per rotation of the machine, which is used as sample clock for signal acquisition. It is thus possible to determine the exact angle at which each signal sample is acquired, allowing relating the signal directly with the events during the stitch cycle. Signals are thus represented on a continuous angle rather than a time scale, in which the rotation N of the machine corresponds to the angles between 360º·(N-1) and 360º·N. The sensor (custom-designed by Petr Skop) is a cantilever beam with semiconductor strain gauges at the base, configured as a complete Wheatstone bridge.
A glass sphere with a rounded slot allows a low-friction interface with the sewing thread. A thread guide with two ceramic O-rings has been designed to guide the thread around the thread sensor. The thread pulling force produces deformation on the cantilever sensor that is picked up by the strain gauges. Thread tension is imposed to sewing threads by a device called a tensioner (partially visible in Fig.2). This device consists of two disks between which the thread passes. A spring holds the two disks together. The pre-tension of this spring can be adjusted and is called in this context static thread tension.
A software application has been developed in Labview allowing the acquisition and processing of the resulting signals. The signal processing functions of this software have been reported elsewhere . The most important one is splitting the thread tension signals into stitch cycles (each cycle corresponding to one rotation of the machine’s main shaft) and in turn dividing each stitch cycle into phases, which are associated to specific events of stitch formation. For each one of these phases, that will be described later, features such as peak values, power, energy or average of the signal is computed. In the current experimental work, thread force waveforms throughout the stitch cycle are being analysed when varying parameters such as static thread tension adjustment, number of fabric layers, mass per unit area and thickness of fabric, needle size and sewing speed. Both the effect of the machine settings and process variables on the thread tensions, as well as the effect of the material properties are investigated. In this paper, the effect of static thread tension and the influence of the fabric on the dynamic tension signals are analysed. The first step was to observe the resulting thread tension signals and interpret their relation to the stitch formation process. Some trials with the adjustment of the needle thread pre-tensions were made. Afterwards, a more comprehensive experiment was set up to investigate on the influence of the material being sewn. https://www.vssewingmachine.in/