The interlacing of the threads itself, which constitutes the actual
stitch formation, cannot be dissociated from the processes of material feeding
and needle penetration. However, there are two variables directly linked to the
thread that most intimately represent it: Thread tensions and thread
consumption. The relationships between fabrics, machine set-up and stitch
formation in lockstitch machines have already been studied in [9-15]. Methods
for defect detection have been developed for overlock machines and presented in
[3]. best sewing machine dealers in chennai. However, an automatic system for setting thread tensions online is still
missing. Wang and Ma [15] describe thread tension control in embroidery
machines, but the work only tackles the issues associated to the control of the
actuator. Setting of the correct references for the controllers to produce a
high-quality product in varying conditions is the key issue, and this has to be
further tackled.
This paper describes current work
on the behavior of thread tensions in an industrial lockstitch sewing machine
using a new measurement set-up. Methods previously investigated for monitoring
of thread tensions and establishing the correct variable references are being
ported and/or re-evaluated. The first step is the study of the relations
between material properties and thread tensions. Some aspects are still not
clear in this regard. In [13], for instance, the authors state that the
thickness of fabric plies does not affect the needle thread tension. This is
one of the aspects to be studied in this work.
An industrial PFAFF 1183
lockstitch (stitch 301 according to ISO 4915) machine 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-MIO16E-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.
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