Edge tracing sewing machine capable of automatically adjusting needle position

A sewing machine having means for tracing the edge of a workpiece, including (a) a sewing needle supported by a frame such that the needle is endwise reciprocable in a vertical direction, (b) a feeding device for feeding the workpiece on the work bed in a feed direction, in synchronization with the endwise reciprocation of the needle, (c) an edge sensor for detecting the edge position of the workpiece in a lateral direction perpendicular to said feed direction, (d) a control motor operated within a drive range defined by a first and a second limit position, for changing a relative position between said edge position of the workpiece and a position of said edge sensor, (e) position control means for producing position control signals indicative of a plurality of positions within said drive range of said control motor, and thereby regulating a stop position of said control motor, such that the edge position of the workpiece is detected by said edge sensor, and (f) a tracing control device for controlling a relative position between said needle and said workpiece in said lateral direction, so as to form successive stitches on said workpiece, along a line which is away from the edge of the workpiece by a predetermined distance in said lateral direction inwardly of the workpiece, said sewing machine comprising:

• first scanning means for commanding said position control means to activate said control motor from said first limit position toward said second limit position, and thereby obtaining a plurality of first output signals of said edge sensor which correspond to said plurality of position control signals;
• memory means for storing values of said plurality of first output signals of the edge sensor, in relation with said plurality of position control signals;
• calculating means for calculating an average value of a maximum value and a minimum value of the values of said plurality of output signals of said edge sensor;
• second scanning means for commanding said position control means to activate said control motor from said second limit position toward said first limit position, and thereby obtaining a plurality of second output signals of said edge sensor which correspond to said plurality of position control signals;
• and
• determining means for determining a first average position signal which consists of one of said plurality of position control signals that corresponds to one of said plurality of first output signals of said edge sensor which is nearest to said average value, and a second average position signal which consists of one of said plurality of position control signals that corresponds to one of said plurality of second output signals which is nearest to said average value, and obtaining a compensation value which is a half of a difference between values of said first and second average position signals,
• said tracing control device being operable to adjust said relative position between said needle and said workpiece in said lateral direction, depending upon said compensation value.