the present invention will be clearer from the following description when viewed together with the accompanying drawings , which show , for purpose of illustrations only , the preferred embodiment in accordance with the present invention . referring to fig1 and 2 , a sewing direction control apparatus for sewing machine in accordance with a preferred embodiment of the present invention comprises a base plate 10 , a transmission element 20 disposed on the base plate 10 , a sewing plate 30 disposed in the transmission element 20 to fix a sewing product a , and a slide rack 40 and a driving element 50 disposed at one side of the base plate 10 . the base plate 10 is rectangular and centrally provided with a hole 11 and a flange 12 around the hole 11 . the base plate 10 is an x - y planar surface with an x direction and a y direction . in this embodiment , as shown in fig1 , the base plate 10 is further provided with two protrusions 13 which are located adjacent to the hole 11 an a corner of the base plate 10 . the transmission element 20 is a circular ring - shaped structure located around the hole 11 of the base plate 10 . as shown in fig2 , the transmission element 20 is centrally provided with a circular cavity 21 , an annular slot 211 around the bottom of the circular cavity 21 , and a plurality of positioning pins 212 disposed at the bottom of the cavity 21 and located around the bottom of the annular slot 211 . an annular engaging portion 22 extending outward from the bottom of the cavity 21 is formed with an annular groove 221 which is located around the periphery of the transmission element 20 . the annular groove 221 and the cavity 21 open in opposite directions and separated from each other by the wall of the cavity 21 . the transmission element 20 is provided with a threaded surface 23 around the outer peripheral surface of the annular engaging portion 22 . the annular groove 221 of the transmission element 20 is located corresponding to the flange 12 of the base plate 10 , namely , the flange 12 is received in the annular groove 221 , and an annular bearing 24 is disposed between the flange 12 and the annular groove 221 to enable the transmission element 20 to rotate with respect to the flange 12 . in this embodiment , the transmission element 20 is provided with four spaced positioning pins 212 . the sewing plate 30 is a circular structure received in the cavity 21 of the transmission element 20 and provided with a plurality of ears 31 around a periphery thereof . the ears 31 each have a pivot hole 311 and are located corresponding to the positioning pins 212 of the transmission element 20 in such a manner that the positioning pins 212 of the transmission element 20 are inserted in the pivot holes 311 of the sewing plate 30 , so as to fix the sewing plate 30 to the annular slot 211 of the transmission element 20 . in this embodiment , the sewing plate 30 is provided with four spaced ears 31 to cooperate with the positioning pins 212 . the slide rack 40 , as shown in fig1 , is disposed on the x - y surface and fixed at one side of the base plate 10 which is perpendicular to the direction y . the slide rack 40 a rectangular structure which is centrally provided at a top surface thereof with a rack plate 41 which is located higher than the x - y surface . the slide rack 40 is provided with an abutting protrusion 43 at one side thereof adjacent the base plate 10 , the abutting protrusion 43 is located corresponding to the protrusions 13 of the base plate 10 and extends in the direction y . the slide rack 40 is further provided with a passage 42 corresponding to the rack plate 41 . in this embodiment , the slide rack 40 is driven by a linear movement device ( not shown ) to move linearly along the direction y of the x - y surface , so as to move close to or away from the base plate 10 . the driving element 50 , as shown in fig1 , is mounted on the slide rack 40 and comprises a servo motor 51 disposed on the rack plate 41 , a driving shaft 511 located below the servo motor 51 and inserted in the rack plate 41 , and a driving unit 52 connected to one end of the driving shaft 511 . the driving unit 52 is a disc structure . in this embodiment , around the outer peripheral surface of the driving unit 52 is formed a threaded surface , and the end of the driving shaft 511 is connected to the center of the driving unit 52 . the driving unit 52 is located on the x - y surface and extends out of the passage 42 of the slide rack 40 to mate with the threaded surface 23 of the transmission element 20 . a control element 60 , as shown in fig1 , 3 a , 3 b , is pivoted to the two protrusions 13 of the base plate 10 and comprises a control unit 61 and an elastic unit 62 . the control unit 61 is reversed u - shaped and includes an operating section 611 , a connecting section 612 and an engaging section 613 . a conjunction between the operating section 611 and the connecting section 612 is pivoted to one of the protrusions 13 adjacent the transmission element 20 , so that the operating section 611 and the engaging section 613 approximately extend in the direction x and toward the transmission element 20 , and the free end of the engaging section 613 is a threaded structure . the elastic unit 62 is approximately l - shaped and includes a stationary section 621 and an abutting section 622 . a connecting hole 623 is formed at the conjunction between the stationary section 621 and the abutting section 622 to enable the elastic unit 62 to be pivoted to the one of the protrusions 13 adjacent the transmission element 20 in such a manner that the end of the stationary section 621 of the elastic unit 62 is pressed against another one of the protrusions 13 which is located farther away from the transmission element 20 , and the end of the abutting section 622 is pressed against the connecting section 612 of the control unit 61 . as shown in fig3 a , when the base plate 10 moves away from the slide rack 40 , the connecting section 612 of the control unit 61 will be pushed by the abutting section 622 of the elastic unit 62 , the engaging section 613 of the control unit 61 will be engaged with the threaded surface 23 of the transmission element 20 , and the driving unit 52 of the driving element 50 will be disengaged from the transmission element 20 to enable the transmission element 20 to be engaged with and fixed by the control unit 61 . as shown in fig3 b , when the base plate 10 move toward the slide rack 40 , the operating section 611 of the control unit 61 will be pushed by the abutting protrusion 43 of the slide rack 40 , so that the engaging section 613 of the control unit 61 will be disengaged from the threaded surface 24 of the transmission element 20 , and the driving unit 52 of the driving element 50 will be engaged with the threaded surface 24 of the transmission element 20 to enable the transmission element 20 to be rotated by the driving unit 52 . the abovementioned are the structural relations of the main components of the first preferred embodiment . it is to be noted that the present invention also provides another embodiment ; its structure is explained as follows . referring to fig4 and 5 , in this embodiment , around an outer peripheral surface of the annular engaging portion 22 of the transmission element 20 is provided a driven belt 25 , and a driving belt 53 winds around the driven belt 25 and the driving unit 52 of the driving element 50 to rotate the transmission element 20 . at a corner of the base plate 10 is disposed a pallet 14 which is higher than the x - y surface . the servo motor 51 of the driving element 50 is inserted in the pallet 14 , the driving shaft 511 of the servo motor 51 is connected to the driving unit 52 , the driven belt 25 of the transmission element 20 located toward the driving unit 52 , and the driving belt 53 winds around the driven belt 25 and the driving unit 52 of the driving element 50 . when the servo motor 51 rotates the driving unit 52 , the driving unit 52 will drive the transmission element 20 to rotate on the base plate 10 via the driving belt 53 . in this embodiment , the driven belt 25 and the driving belt 53 are timing belts , which are engaged with each other via teeth engagement . fig6 shows that the sewing direction control apparatus for sewing machine in accordance with the present invention is used in combination with a needle 71 of a sewing head 70 . as shown in fig6 , when the slide rack 40 moves toward the base plate 10 , the sewing product a is fixed on the sewing plate 30 and located corresponding to the needle 71 of the sewing head 70 , and the preset sewing path of the needle 71 extends along the direction y . the transmission element 20 is rotated by the driving element 50 . meanwhile , the sewing product a is caused to rotate clockwise , so that the sewing direction is maintained tangent to the rotation direction of the sewing product a , thus fixing the sewing direction of the sewing machine , making the sewing machine perform sewing operation by moving along the desired sewing direction , and consequently improving the sewing speed and quality . furthermore , the sewing direction control apparatus for sewing machine in accordance with the present invention uses teeth engagement to perform highly accurate , intermittent and fast movement , therefore , the direction control apparatus of the present invention has low cost , and is suitable for various types of automatic sewing machines . while we have shown and described various embodiments in accordance with the present invention , it is clear to those skilled in the art that further embodiments may be made without departing from the scope of the present invention .