Method and apparatus for accurately controlling the seam margins produced by a semi-automatic sewing machine

In a sewing machine having a material feed and a reciprocable needle for stitching seams in material, a drive motor and a stitch length variance mechanism having a finite activation time delay, seam length control apparatus comprising:

• means for detecting a material discontinuity in advance of the end of a seam;
• means for controlling the reciprocable needle to sew a predetermined number of stitches after said detecting means detects said material discontinuity;
• and
• means responsive to said detecting means for operating the stitch length variance mechanism to reduce the length of the last stitch to a desired percentage of the normal stitch length at an angle of drive motor rotation computed according to the formula:
• B-(360*(SPM*RT)/60))+((X/100)*(C-B)),
• where B is the drive motor angle at which stitch formation begins, SPM is the speed of the drive motor while the last stitich is formed in revolutions per minute, RT is the activation time delay in seconds, X is the desired Percentage of the normal stitch length, and C is the drive motor angle at which stitch formation ends.

A method of operating a sewing maching having a reciprocable needle for stitching seams in material, a drive motor, and a stitch length variance mechanism having an activation time delay comprising:

• detecting a material discontinuity in advance of the end of a seam being sewn by the sewing machine;
• controlling the reciprocable needle to sew a predetermined number of stitches after detecting said material discontinuity;
• and
• operating the stitch length variance mechanism at an angle of drive motor rotation which compensates for the time delay associated with operating the stitch length variance mechanism to precisely control the amount of completion of the last stitch of said predetermined number of stitches in response to the detection of said material discontinuity to thus precisely control the length of the seam.

A method of operating a sewing machine having a reciprocable needle for stitching seams in material, a drive motor, and a stitch length variance mechanism having an activation time delay comprising:

• detecting a material discontinuity in advance of the end of a seam being sewn by the sewing machine;
• controlling the reciprocable needle to sew a predetermined number of stitches after detecting said material discontinuity;
• and
• operating the stitch length variance mechanism at an angle of drive motor rotation computed according to the formula and which compensates for the time delay associated with operating the stitch length variance mechanism and the material feed characteristics of the sewing machine:
• (B-(360*(SPM*RT)/60)) .+-.((X/100)*(C-B)),
• where B is the motor angle at which stitch formation begins, SPM is the speed of the motor while the last stitch is formed in revolutions per minute, RT is the activation time delay in seconds, X is the desired percentage of the normal stitch length, and C is the motor angle at which stitch formation ends.