Processing textile materials is generally very difficult due to the flexible nature of the material. In industries using sewing as assembly process, most processes rely on human labor, being difficult or even impossible to automate. The relations between machine configuration and adjustment, material properties, and the resulting product quality are also complex. This paper describes current work using an instrumented lockstitch sewing machine to study the dynamics and variations of one of the important process parameters during high-speed sewing of shirts: thread tensions. The objective is study the principles that may allow for an automatic setting of the machines, quality control and for real-time process control. It has been found that differences in material properties result in measurable features of the thread tension signals acquired.
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 mass-production 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 themachines 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 , who studied the interface between the machine and the material feeding system, Stylios  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
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 off-line 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 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 . However, an automatic system for setting thread tensions online is still missing. Wang and Ma  describe thread tension control in embroidery machines. click here to view full site