Sewing – Elements – Starting or stopping
Patent
1991-06-25
1993-08-24
Nerbun, Peter
Sewing
Elements
Starting or stopping
112278, D05B 6936
Patent
active
052379442
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention pertains generally to stitch-forming machines and more particularly to a thread monitor for indicating missed stitches.
BACKGROUND OF THE INVENTION
A thread monitor, known from U.S. Pat. No. 4,170,951, is arranged on a sewing machine in the path of the needle thread and is provided with a transducer with a spring clip, to which a wire strain gauge element, hereinafter called a WSG element, is fastened. The WSG element produces an electrical voltage that is proportional to its mechanical deformation caused by the deflecting movement of the spring clip. The electrical voltage is fed into an evaluating electronic unit following the transducer.
In the course of a stitch, a first, lower tension level is produced during the expansion of the needle thread loop, and a second, higher tension level is generated during the knotting. To detect missed stitches, the tension levels are monitored in measuring windows, whose positioning and size are predetermined by two signal generators that monitor the position of the arm shaft.
A plurality of actual values are determined from the lower tension level, and compared with a threshold value, whose value depends on the maximum of the higher tension level generated during the preceding stitch. If all actual values are below this threshold value, a warning signal is sent by the evaluating electronic unit to indicate a missed stitch.
Since the maximum of the higher tension level may differ from the corresponding value of the preceding stitch during each stitch, and the threshold value depends on this maximum, the threshold value is to be determined anew for each stitch. Such a signal evaluation is problematic especially at high sewing speeds and requires the use of an expensive evaluating electronic unit.
Due to its dependence on the preceding higher tension level, the threshold value cannot be formed during the first stitch performed with ordinary tension after start-up of the sewing machine, which causes a delay in the monitoring process. To prevent this disadvantage, additional control elements not disclosed in U.S. Pat. No. 4,170,951 are necessary. This increases the circuit complexity of the evaluating electronic unit, which is suitable exclusively for indicating missed stitches caused by lack of expansion of the needle thread loop.
SUMMARY AND OBJECTS OF THE INVENTION
It is a primary object of the present invention to design a control device of a stitch-forming machine equipped with a transducer, so that the control device is able to evaluate the measured values sent by the transducer, at low circuit complexity, beginning from the first stitch performed with ordinary tension to detect a majority of different missed stitches as well as thread disturbances on the thread being monitored and on the threads to be connected to this by stitch formation.
According to the invention, a stitch forming machine is provided including a transducer for determining the tension present in a thread, wherein the thread tension assumes a higher value during stitch formation and the transducer provides a signal representing the tension level. Control means are provided for evaluating the signal corresponding to the tension level. The control means includes a comparator device for comparing a peak of the signal representing the tension level, which peak can be used to detect a malfunction, with a limit signal, corresponding to a limit tension. The comparator sends a signal to a switching device when a signal peak drops below the limit signal. The switching device may be connected to a shut-off device of the drive motor of the machine as well as one or more display elements. In this way, the machine may be stopped and the display element associated with a limit tension, below which the tension dropped by a switching device.
According to another aspect of the invention, a stitch forming a machine is provided included a transducer for determining the tension in a thread and for outputting a signal representing the tension of the thread. Control me
REFERENCES:
patent: 4628847 (1986-12-01), Rydborn
patent: 4763588 (1988-08-01), Rydborn
patent: 4793273 (1988-12-01), Hara et al.
patent: 4938159 (1990-07-01), Shibata
Mertel Bernhard
Sinn Walter
Spickermann Rainer
Willenbacher Erich
G. M. Pfaff Aktiengesellschaft
Nerbun Peter
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