Research on thread tension behavior in lockstitch machines

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Research on thread tension behavior in lockstitch machines

The processing of textile products by sewing them together
is a very complicated process. This may not be apparent at first
glance, but a closer look at the process reveals that, due to the
flexible, often extensible nature of the materials, their handling
is a procedure that in almost all cases requires human hand.
Another important aspect is setting the machines for the great
variety of materials used currently. This can only be
accomplished by experienced sewing technicians. Machine
configuration and adjustment is an empirical, time-consuming
process that is more and more significant considering that textile
industry has been constantly moving away from massproduction
to small orders with varying materials and styles.
Machines should be able to set themselves up when the data
regarding material properties and desired process parameters is
known. During the process, it would be ideal if they could adapt
themselves and detect defects or malfunction automatically.
This would reduce set-up times, increase flexibility of the
machines and increase product quality and process reliability,
avoiding defects and rejected products.
Research in this direction has been carried out by several
investigators, such as Clapp [1], who studied the interface
between the machine and the material feeding system, Stylios
[2] who proposed the principles of intelligent sewing machines,
amongst others. Within our team, previous work has been
carried out on thread tensions, material feeding and needle
penetration forces in overlock machines [3-5]. Other studies
targeted needle and bobbin thread tension measurement on
lockstitch machines [8-10].
The sewing process is a cyclic process in which several
occurrences take place. The objective is to interlace thread(s)
with each other and through a fabric, for the purpose of joining,
finishing, protecting or decorating. Three main “sub”-processes
can be identified that ideally should be monitored and/or
controlled automatically:
-Material feeding. Seams are produced on the fabric with a
certain pattern, which is, in the simplest case, a straight line, but
may also be a complicated form such as the ones used in
embroidery operations. To form these patterns, the material has
to be transported-“fed” by a distance that is called the stitch
length. Given that industrial machines operate at very high
speeds (some of them attaining 10 000 stitches per minute), the
dynamics involved is complex and there are very often problems
with material deformation and irregular stitch length. Some of
these aspects have been addressed in [1-3, 5];
-Needle penetration. Considering again the high sewing
speeds that occur, problems with needle penetration can arise
due to the mechanical and thermal interaction between needle
and fabric. Fabric yarns may be torn by the forces acting during
needle penetration or they may fuse due to the high needle
penetration produced by friction. Systems to monitor needle
penetration forces during the process to detect defects and offline
systems to support the choice of needles and fine-tune fabric
structures and finishing to avoid these problems, would be of
high value to the industry. This kind of approach has been
studies by several authors, such as in [4-8].

Research on thread tension behavior in lockstitch machines

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. VS sewing machines
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.https://www.vssewingmachine.in/
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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