Automatic thread tension device for a sewing machine

An automatic thread tension adjusting device for a sewing machine including an upper thread supply, a needle (44) vertically reciprocable by rotation of an upper drive shaft (9) to penetrate into a fabric to be sewn to thereby form stitches in the fabric from an upper and lower thread, a thread take-up lever (4) receiving an upper thread from the upper thread supply, a loop taker, a bobbin (8) placed in the loop taker, the lower thread being wound up on the bobbin, the loop taker being rotatable in timed relation with the vertical reciprocating movement of the needle to interlock the upper thread with the lower thread, the upper thread being supplied from the upper thread supply to the needle through at least the thread take-up lever,

• and user-operated means including a start switch (15) operated to produce a start signal, the automatic thread tension adjusting device comprising:
• thread tensioning means (12, 35) positioned between the upper thread supply and the thread take-up lever, said thread tensioning means being responsive to said start signal for giving a predetermined tension to the upper thread;
• detecting means (11) arrangable between the thread take-up lever and the needle for detecting a tension of the upper thread, said detecting means being activated in response to a tension applied to the upper thread at a predetermined angular position of the upper drive shaft as the latter is rotated and producing a detected tension signal;
• first measuring means (37) responsive to said detected tension signal for measuring therefrom the value of the tension applied to the upper thread and producing an output representing the measured value of the tension applied to the upper thread;
• second measuring means (18, 55, 56) for measuring the tension of the lower thread as the lower thread is drawn out from the bobbin as the upper drive shaft is rotated, said second measuring means producing an output based on sensing a measured value of the lower thread tension;
• calculating means (40, 39) responsive to said output of said first measuring means and said output of said second measuring means to produce an output representing a calculated value;
• and
• control means (27) operated in response to said output from said calculating means to set said upper thread tensioning means (42) to said calculated value.