Control of thread feed for a sewing machine

Abstract

A sewing machine including an upper thread supply to a needle, a loop taker accommodating a bobbin for a bottom thread, a drive unit which through coupling elements affects a number of mechanical elements, of which the needle is one of the mechanical elements, to perform synchronous cyclic movements between themselves, wherein the needle during its movement in cooperation with the loop taker performs stitches on a sewing material which is transported between upper thread and bottom thread, by means of a take-up lever which in each stitch pulls tight a knot, which is formed in the sewing material by the upper thread and the bottom thread in cooperation, wherein the sewing machine includes a sensor function that detects a deviation between a set value for a point of time at which a predetermined value for a tensile force in the upper thread is reached at pull tight of a knot and an actual value at the time of point where the predetermined value of the tensile force in the upper thread is reached at pull tight of the knot, and wherein the sewing machine includes control means for the control of the means being used to supply upper thread to the needle, so that the deviation between a first position and a second position is brought to zero.