Automatic needle thread control apparatus

In a sewing machine having a thread supply, a thread carrying needle, a thread path between the supply and the needle, and a looptaker seizing thread from the supply for forming a loop of fixed size for forming stitches in cooperation with the needle, an automatic thread handling system for controlling the thread comprising;

• thread take-up means comprising reverse feed roller means positioned along said path and operable to positively move the thread in a direction for taking up the thread of said loop released from said looptaker only during discrete time periods,
• controllable driving means operative to impart movement to said thread take-up roller means and to controllably vary the length of thread taken up,
• sensing means for sensing the thickness of a workpiece being sewn and for generating thickness data corresponding to the sensed thickness,
• means for generating stitch data corresponding to the distance between successive stitches to be formed in said workpiece,
• means for generating a thread take-up start signal in timed relation to termination of the expansion of said loop, and
• data processing means for calculating a desired length of thread to be taken up based on said thickness data and said stitch data, said data processing means being responsive to said thread take-up start signal for controlling the operation of said driving means to positively take-up the thread for a determined time period according to the desired length calculated thereby.