Abstract:
In one aspect, the present invention provides a vehicle floor production system, in which a carrier is moved through a loop formed by a return line provided at the top of the system and a welding line provided at the bottom, and the carrier is horizontally moved by the frictional force of a horizontal movement driving means. Preferred systems can reduce the manufacturing cost and required installation area.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims under 35 U.S.C. §119(a) the benefit of Korean Patent Application No. 10-2008-0031400 filed Apr. 3, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     (a) Technical Field 
     The present invention relates to a vehicle floor production system. More particularly, the present invention relates to a vehicle floor production system, in which a working line is disposed at the bottom of the system and a return line is disposed at the top so as to ensure safety and working space. 
     (b) Background Art 
     As shown in exemplary  FIG. 1 , a conventional front floor  100  forming an engine room  101  of a vehicle in general includes a dash panel  110 , left and right fender apron members  112  and  113 , and a radiator support member  114 . The dash panel  110  serves as a partition between the engine room and a passenger room and prevents noise and vibration from being introduced into the passenger room. The left and right fender apron members  112  and  113  are butt-welded at predetermined regions of left and right sides of the dash panel  110  so that the front portions thereof extend toward the vehicle front. Accordingly, the left and right fender apron members  112  and  113  support left and right side portions of the engine and a transmission and fix the upper part of a strut of a front suspension. Both side portions of the radiator support member  114  are butt-welded to the front surfaces of the left and right fender apron members  112  and  113  so as to support the structure of a cooling system including an air conditioner and a radiator. 
     The above-described panel and members (hereinafter referred to as a panel) are welded to each other to form the front floor  100 . 
     Exemplary  FIG. 2  is a configuration diagram showing a conventional front floor production system which generally comprises a linear motor  121 , a stopper  122 , a carrier  123 , a turntable  124 , a welding robot  125 , and an unloading robot  126 . 
     In the conventional front floor production system, the carrier  123  that restricts the panel is moved using the linear motor  121  in a non-contact manner and is rotated by the turntable  124  in a welding process. After completion of the welding process, the panel is unloaded from the carrier  123  by the unloading robot  126  and transferred to the following line. Accordingly, the stopper  122  stops the carrier  123  at a desired position during transfer to the following line, thus regulating the position of the carrier  123 . 
     The operation order of the system having the above-described configuration is described below. 
     (1) The panel is loaded on the carrier  123  in process-A, and the carrier  123  is transferred to process-B. 
     (2) When the turntable  124  is rotated, the carrier in process-B is transferred to process-C and subjected to the welding process, and the carrier  123  of process-C is returned to process-B so that the unloading robot  126  unloads the panel from the carrier  123 . 
     (3) The carrier  123  of process-D is transferred to process-A so that another panel is loaded thereon. 
     (4) The carrier  123  of process-B after completion of unloading is transferred to process-D to stand by. 
     In the conventional linear motor, since the number of the linear motors is large, the manufacturing cost is suitably increased and, since the capacity of inverters for controlling the motors is increased, the manufacturing cost is further increased and the size of a control panel that is needed is thus increased. Accordingly, with the increase in the control panel, the installation area is also increased. 
     Moreover, in the case of an inverter-type linear motor, a stopper is needed during stop, and an impact absorbing means is used to reduce impact force during stop, which is consumable and unfavorable for preservation, and thus causes a cost increase. 
     Furthermore, since the linear motor is driven in a non-contact manner, an error in speed control frequently occurs and, in order to maintain a gap between the linear motor and the carrier, the manufacturing and assembling costs are increased. 
     In addition, because a process has to be added every time a vehicle type is added, the system operation is restrictive, and the manufacturing cost of the turntable is high. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY 
     In one aspect, the present invention is directed to a vehicle floor production system, in which a carrier is moved through a loop formed by a return line provided at the top of the system and a welding line provided at the bottom, and the carrier is horizontally moved by the frictional force of a horizontal movement driving means, such preferred systems can reduce the required manufacturing cost and installation area. 
     In one embodiment, the present invention provides a vehicle floor production system, in which the carrier is handled by a carrier changing robot, and thus a carrier loading stand of the system does not require any separate device for handling the carrier, thus contributing to simplification of the system. 
     In another embodiment, the present invention provides a vehicle floor production system comprising: a return line and a welding line provided parallel or substantially parallel to each other on the top and bottom of a frame so as to move a carrier; a carrier drop means including a guide rail provided on the top thereof to load a panel on the carrier received from the return line and move the carrier to the welding line; a horizontal movement driving means for moving the carrier in the horizontal direction on the return line, the welding line and the carrier drop means by frictional force; a welding robot for welding the panel moved by the horizontal movement driving means in a predetermined section of the welding line; and a carrier changing robot including a carrier handling hanger provided at an end of an arm portion thereof to move the carrier from the welding line to the return line after completion of the panel welding process. 
     In another preferred embodiment, the return line and the welding line include first and second driving wheels provided to move the carrier, and a portion of the first driving wheel of the return line is moved upward and mounted to ensure a working space of a welding robot. 
     In still another preferred embodiment, the carrier drop means comprises: a frame including a guide rail provided on the top thereof; a vertical column for supporting the frame to slidably move in the up and down direction; a horizontal column for connecting the top portion of the vertical column; an up-and-down movement motor provided on the top of the horizontal column; a power transmission means for transmitting the rotational force of the up-and-down movement motor to the frame; and a weight balance connected to an end portion of the power transmission means and moving up and down at the opposite side of the frame to adjusting the balance. 
     In still another preferred embodiment, the power transmission means comprises: a rotation transmission shaft connected to an output shaft of the motor and including a pulley provided on the outer surface thereof; and a belt, of which one end is connected to the frame and a middle portion is engaged with the pulley so as to transmit the rotational force of the motor. 
     In yet another preferred embodiment, the horizontal movement driving means comprises: a friction bar fixedly provided in the longitudinal direction of the carrier; a horizontal movement motor and a decelerator provided on the frame on which the carrier drop means, the return line, and the welding line are provided; a friction wheel driven by the horizontal movement motor and the decelerator and transmitting the rotational force of the motor by being contact with one side of the friction bar; a guide wheel rotatably provided on both sides of the friction bar to be in contact therewith; and an idle wheel provided on the opposite side of the friction bar to be in contact therewith. 
     In still yet another preferred embodiment, the horizontal movement driving means further comprises: a hinge pin rotatably supported to the frame; first and second rotational members, of which center portions are rotatably connected to the hinge pin; and a gap adjusting bolt and a spring inserted between end portions of the first and second rotational members, wherein the motor, the decelerator, and the friction wheel are provided on one side of the first rotational member, and the idle wheel is provided on one side of the second rotational member, and wherein the friction wheel and the idle wheel are in frictional contact with the friction bar by the elastic force of the spring, and a gap between the first and second rotational members is adjusted by the gap adjusting bolt. 
     In a further preferred embodiment, the system further comprises a carrier loading stand capable of loading carriers classified by vehicle type, wherein the carrier loading stand includes a vertical frame provided in the vertical direction and a loading frame provided in the horizontal direction on the top and bottom thereof, and a carrier of the welding line and a carrier of the carrier loading stand are changed by the carrier changing robot. 
     It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like. 
     The above features and advantages of the present invention will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description, which together serve to explain by way of example the principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated by the accompanying drawings which are given hereinafter by way of illustration only, and thus are not limitative of the present invention, and wherein: 
         FIG. 1  is a perspective view showing a conventional front floor forming an engine room of a vehicle. 
         FIG. 2  is a configuration diagram showing a conventional front floor production system. 
         FIG. 3  is a configuration diagram showing a front floor production system in accordance with a preferred embodiment of the present invention. 
         FIG. 4  is a front view of a welding line and a return line of  FIG. 3 . 
         FIG. 5  is a configuration diagram showing a drop lifter of  FIG. 3 . 
         FIG. 6  is a configuration diagram showing a horizontal movement driving means of  FIG. 3 . 
         FIG. 7  is a perspective view showing a state where a friction bar is attached to a carrier of  FIG. 3 . 
         FIG. 8  is a rear view of  FIG. 7 . 
         FIG. 9  is a side view showing a working state of a welding robot of  FIG. 3 . 
         FIG. 10  is a configuration diagram showing a welding section of  FIG. 3 . 
         FIG. 11  is a diagram showing an unloading section and a carrier changing robot. 
         FIG. 12  is a perspective view showing a carrier loading stand. 
     
    
    
     Reference numerals set forth in the Drawings includes reference to the following elements as further discussed below: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 10: drop lifter 
                 11: horizontal movement driving 
               
               
                   
                 means 
               
               
                 12: carrier 
                 13: carrier loading stand 
               
               
                 14: unloading robot 
                 15: carrier changing robot 
               
               
                 16: welding robot 
                 17: welding line 
               
               
                 18: return line 
                 19: guide rail 
               
               
                 20: up-and-down movement frame 
                 21: first driving wheel 
               
               
                 22: electricity and air supply 
                 23: vertical column 
               
               
                 24: horizontal column 
                 25: connection frame 
               
               
                 26: up-and-down movement motor 
                 27: timing belt 
               
               
                 28: weight balance 
                 29: guide roller 
               
               
                 30: decelerator 
                 31: induction motor 
               
               
                 32: friction wheel 
                 33: idle wheel 
               
               
                 34: friction bar 
                 35: first member 
               
               
                 36: second member 
                 37: hinge pin 
               
               
                 38: spring 
                 39: bolt 
               
               
                 40: nut 
                 41: first driving rail 
               
               
                 42: second driving rail 
                 43: jig unit 
               
               
                 44, 56: fixing pin 
                 45: frame 
               
               
                 46: vertically extending frame 
                 47: arm 
               
               
                 48: second driving wheel 
                   
               
               
                 50: carrier handling hanger 
                 52: guide wheel 
               
               
                 55: horizontal frame 
                 54: vertical frame 
               
               
                   
               
             
          
         
       
     
     It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment. 
     DETAILED DESCRIPTION 
     As described herein, the present invention includes a vehicle floor production system comprising a return line and a welding line to move a carrier, a carrier drop means, a horizontal movement driving means, and a welding robot for welding the panel moved by the horizontal movement driving means in a predetermined section of the welding line. 
     In certain embodiments, the vehicle floor production system further comprises a carrier changing robot including a carrier handling hanger provided at an end of an arm portion thereof to move the carrier from the welding line to the return line after completion of the panel welding process. In other embodiments, the horizontal movement driving means moves the carrier in the horizontal direction on the return line, the welding line and the carrier drop means by frictional force. In other embodiments, the return line and a welding line provided parallel or substantially parallel to each other on the top and bottom of a frame so as to move a carrier. In still further embodiments, the carrier drop means includes a guide rail provided on the top thereof to load a panel on the carrier received from the return line and move the carrier to the welding line. 
     The invention also includes a motor vehicle comprising the vehicle floor production system as described in any of the aspects herein. 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the drawings attached hereinafter, wherein like reference numerals refer to like elements throughout. The embodiments are described below so as to explain the present invention by referring to the figures. 
     Exemplary  FIG. 1  is a perspective view showing a conventional front floor forming an engine room of a vehicle,  FIG. 2  is an exemplary configuration diagram showing a conventional front floor production system,  FIG. 3  is an exemplary configuration diagram showing a preferred front floor production system in accordance with a preferred embodiment of the present invention, exemplary  FIG. 4  is a front view of a suitable welding line and a suitable return line of  FIG. 3 , exemplary  FIG. 5  is a configuration diagram showing a suitable drop lifter of  FIG. 3 , exemplary  FIG. 6  is a configuration diagram showing a suitable horizontal movement driving means of  FIG. 3 , exemplary  FIG. 7  is a perspective view showing a state where a friction bar is attached to a carrier of  FIG. 3 , exemplary  FIG. 8  is a rear view of  FIG. 7 , exemplary  FIG. 9  is a side view showing a working state of a suitable welding robot of  FIG. 3 , exemplary  FIG. 10  is a configuration diagram showing a welding section of  FIG. 3 , exemplary  FIG. 11  is a diagram showing an unloading section and a carrier changing robot, and exemplary  FIG. 12  is a perspective view showing a carrier loading stand. 
     A front floor production system in accordance with a preferred embodiment of the present invention suitably comprises a drop lifter  10  (carrier drop means), a horizontal movement driving means  11 , a carrier  12 , a carrier loading stand  13 , an unloading robot  14 , a carrier changing robot  15 , and a welding robot  16 . 
     In certain embodiments according to the present invention, a welding line  17  and a return line  18  are suitably independently provided in consideration of production and working stability. Preferably, the welding line  17  is suitably provided at the bottom of the system and the return line  18  is suitably provided at the top such that the carrier  12  is circulated through the welding line  17  and the return line  18  for the front floor production. 
     Preferably, the drop lifter  10  is an up-and-down movement means for dropping the carrier  12 . Accordingly, the drop lifter  10  receiving the carrier  12  returned to the return line  18  preferably moves down to transfer the carrier  12  to the welding line  17  and then is suitably returned to the original position. 
     Preferably, the drop lifter  10  comprises an up-and-down movement frame  20  including guide rails  19  suitably formed parallel or substantially parallel to each other on the top thereof, a support means for suitably supporting the up-and-down movement frame  20  so as to move up and down, and a carrier drop means and a power transmission means, provided on the support means to suitably move the up-and-down movement frame  20  in the up and down direction. 
     In one embodiment of the invention, the guide rails  19  are preferably provided on the top of the up-and-down movement frame  20  so that a first driving wheel  21  of the carrier  12  returned along the return line  18  may move along the guide rails  19 . In certain examples, the up-and-down movement frame  20  has a lattice structure including a plurality of vertical members suitably formed parallel to each other in the longitudinal direction thereof and a plurality of horizontal members suitably formed in the width direction to substantially connect the vertical members. 
     In further embodiments, a horizontal movement driving means  11  for horizontally moving the carrier  12  is suitably provided in or near the middle of the up-and-down movement frame  20  so as to transfer the carrier  12 , and an electricity and air supply  22  is preferably provided on one side of the guide rail  19  in the width direction so as to suitably supply electric power and air to the carrier  12 . 
     Preferably, the support means includes two vertical columns  23  suitably positioned on one side of the up-and-down movement frame  20  to support the up-and-down movement frame  20  so as to move up and down, and a horizontal column  24  suitably connecting the upper portions of the vertical columns  23 . 
     In other embodiments, the lower portions of the vertical columns  23  are preferably fixed to the ground by means of a bolt, and a vertical movement block, which supports a connection frame  25  suitably extending from the up-and-down frame  20  in the width direction so as to move up and down, and a horizontal bar connecting the vertical movement block are suitably provided on one side of the vertical column  23 . 
     In further preferred embodiments, an up-and-down movement motor  26  as the carrier drop means is suitably provided on the top of the horizontal column  24 , a timing belt  27  as the power transmission means for transmitting the rotational force of the motor  26  is suitably provided, and a weight balance  28  as a balance means for preferably adjusting the weight and balance of the up-and-down movement frame  20  is suitably provided at the bottom thereof. 
     In other preferred embodiments, a pulley shaft including a pulley connected to an output shaft of the motor  6  preferably extends in the longitudinal direction thereof, and at least one end of the timing belt  27  is suitably connected to a connection bar and the other end is suitably connected to the weight balance  28 . Accordingly to preferred embodiments of the invention, when the motor  26  rotates in one direction, the up-and-down movement frame  20  connected to the connection bar moves up, and the weight balance  28  suitably moves down. According to other embodiments of the invention, when the motor  26  rotates in the other direction, the up-and-down movement frame  20  connected to the connection bar moves down, and the weight balance  28  preferably moves up. 
     In other embodiments of the invention, the weight balance  28  is preferably provided on the opposite side of the up-and-down movement frame  20  with the vertical column  23  preferably interposed therebetween, and a guide roller  29  is preferably installed on one side of the weight balance  28  to suitably facilitate the up and down movement of the weight balance  28 . 
     Accordingly, the horizontal movement driving means drives the carrier  12  by friction when the carrier  12  horizontally moves on the welding line  17  and the return line  18 . In particular preferred embodiments, the horizontal movement driving means drives the carrier  12  preferably by using an induction motor  31  suitably equipped with a decelerator  30  and bringing a friction bar  34  of the carrier  12  into contact between a friction wheel  32 , that is, for example, rotatably provided on the top of the decelerator  30 , and an idle wheel  33 , that is in other preferred embodiments rotatably provided on the opposite side thereof. 
     In exemplary embodiments, a rotation shaft of the friction wheel  32  is connected to an output shaft of the decelerator  30 . Accordingly, the induction motor  31 , the decelerator  30 , and the friction wheel  32  are preferably provided on one side of a first member  35 , and the idle wheel  33  is preferably provided on one side of a second member  36 . In exemplary embodiments, center portions of the first and second members  35  and  36  are connected by a hinge pin  37 , and the other sides of the first and second members  35  and  36  are preferably connected by a bolt  39  and a nut  40  with a spring  38  inserted therebetween. 
     According to certain embodiments of the invention, when the bolt  39  is screwed into the nut  40 , the spring  38  is suitably compressed, and thus the gap between the first and second members  35  and  36  on spring side is substantially closed based on the hinge pin  37  and the gap between the friction wheel  32  and the idle wheel  33  on the opposite side is suitably widened. In other exemplary embodiments, when the bolt  39  is unscrewed from the nut  40 , the spring  38  is suitably restored, and thus the gap between the first and second members  35  and  36  on the spring side is widened based on the hinge pin  37  and the gap between the friction wheel  32  and the idle wheel  33  on the opposite side is closed. 
     According to embodiments of the invention as described herein, since the friction wheel  32  and the idle wheel  33  are in frictional contact with the friction bar  34  due to the elastic force of the spring by suitably adjusting the bolt  39  and the nut  40 , the rotational force of the induction motor  31  is suitably transferred to the friction bar  34  through the friction wheel  32  and the idle wheel  33 , thus enabling the carrier  12  to horizontally move. 
     According to other further embodiments, since the present invention enables the carrier  12  to horizontally move in a frictional contact manner, it is possible to suitably reduce the number of motors and the capacity of the inverters for controlling the motors and thus, preferably, to more accurately and rapidly control the speed of the friction bar  34  by the motor control, compared with the conventional non-contact manner. 
     In preferred embodiments, the carrier  12 , preferably including a jig unit  43  for restricting the panel, is a suitably means for transferring the restricted panel to each process. In other further embodiments, the friction bar  34  is suitably attached to the bottom of the carrier  12  so as to transfer the frictional force, i.e., the driving force, by the horizontal movement driving means  11 . For example, in certain exemplary embodiments, the friction bar  34  is detachably mounted to the carrier  12  to suitably improve the handling of the carrier  12  and facilitate the safekeeping of the carrier  12 . For example, in preferred embodiments, the friction bar  34  may be suitably attached to both sides of the bottom portion of the carrier  12  by a fixing pin  44 . 
     In other embodiments, the welding line  17  in accordance with the present invention in which a key welding process is suitably performed by a welding robot  16  is preferably provided on the first floor, and the return line  18  is disposed on the top of the welding line  17  so as to improve the workability of the welding robot  16 . 
     In still other further embodiments, on the welding line  17  and the return line  18 , first and second driving rails  41  and  42  are preferably provided parallel to each other in the horizontal direction on the top and bottom of a plurality of rectangular frames  45  suitably provided at regular intervals such that the carrier  12  may move along the first and second driving rails  41  and  42 . 
     Accordingly, in preferred embodiments, the rectangular frames  45  are preferably provided in the vertical direction, the return line  18  is disposed on the rectangular frames  45 , and the welding line  17  is preferably disposed below the rectangular frames  45 . 
     In further exemplary embodiments, the welding line  17  may be suitably divided into a loading section (B) where the carrier  12  is loaded, a welding section (C) where the panel is welded by the welding robot  16 , and an unloading section (D) where the welded panel is unloaded. In other further embodiments, the return line  18  may be suitably divided into a first return section where the carrier  12 , from which the panel has been unloaded, moves upward, and a second return section where the carrier  12  moves from the first return section to the loading section. 
     In certain embodiments of the invention, the loading section (B), the welding section (C), and the unloading section (D) of the welding line  17  are preferably arranged in a straight line, and the first and second return sections of the return line  18  are suitably provided on different floors. 
     In other embodiments of the invention, a second driving rail  42 ′ of the first return section is preferably supported by a vertically extending frame  46  provided on or near the top of the rectangular frame  45  in the vertical direction, and thus the height of the second driving rail  42 ′ is larger than that of the second return section as much as the height of the vertically extending frame  46 . 
     Accordingly, in certain embodiments of the invention, the second driving rail  42 ′ of the first return section preferably has a greater height than that of the second return section in order to prevent an arm  47  of the welding robot  16  during up and down movement from being interrupted by the second driving rail  42  of the second return section having the same height as the second driving rail  42 ′ of the first return section. 
     For example, the second driving rail  42  of the first return section is preferably moved upward so that the arm  47  of the welding robot  16  may freely move in the welding section (C) and, since the second return section is substantially out of the working radius of the welding robot  16 , the second driving rail  42  is preferably provided on the top of the rectangular frame  45 . 
     In preferred embodiments, the carrier  12  includes, but is not limited to, a second driving wheel  48  preferably mounted on the top side thereof to drive along the second driving rail  42 ′ of the first return section, and a first driving wheel  21  preferably mounted on the bottom side to drive along the introduction portion of the first return section, the second driving rail  42  of the second return section, and the first driving rail  41  of the welding line  17 . 
     In exemplary embodiments, the horizontal movement driving means is preferably provided at each section of the welding line  17  and the return line  18 , and the drop lifter  10  is provided at a line panel loading side (A) so as to move the carrier  12  from the return line  18  to the welding line  17  and preferably employs a carrier changing robot  15  as a carrier lift means to lift the carrier, from which the panel has been unloaded, to the return line  18 . 
     In certain examples, the carrier changing robot  15  includes, but is not limited to, a carrier handling hanger  50  mounted on the arm portion in order to preferably move the carrier  12 , from which the panel has been unloaded, to the return line  18  and move another carrier  12  of a different vehicle type from the carrier loading stand  13  to the return line  18 . 
     Preferably, the carrier loading stand  13  is a place where the carriers  12  classified by vehicle type are loaded and kept and includes a vertical frame  54  provided in the vertical direction and a horizontal frame  55  supported in the horizontal direction on the top and bottom portions of the vertical frame  54 , in which a fixing pin  56  for fixing the carrier  12  is provided on the horizontal frame  55 . 
     According to further embodiments, the carriers  12  are preferably kept on the first and second floors of the carrier loading stand  13 , and thus provide excellent space utilization. According to other embodiments, the carrier  12  is handled by the carrier changing robot  15 , and preferably the carrier loading stand  13  does not require any separate device for handling the carrier  12 , and thus the system is suitably simplified and the manufacturing cost is reduced. 
     According to exemplary embodiments of the invention, the operation order of the floor production system in accordance with the invention as described herein, will be set forth below. 
     (1) After the carrier  12  is returned to the guide rail  19  of the drop lift  10  through the second return section of the return line  18 , the panel to be welded is preferably loaded on the carrier  12  at the A-side. 
     (2) With the operation of the up-and-down movement motor  26 , the timing belt  27  suitably rotates in one direction, and thus the up-and-down frame  20  of the drop lifter  10  moves down. Then, with the operation of the motor  26  in the horizontal movement driving means of the drop lifter  10 , the friction wheel  32  rotates, and thus the carrier  12  is moved to the loading section (B) of the welding line  17  by the frictional force of the friction wheel  32  and the friction bar  34 . Subsequently, the carrier  12  enters the welding section (C) so that the welding robot  16  performs the key welding process, thus completing a front floor. 
     (3) After completion of the key welding process, when the induction motor  31  in the horizontal movement driving means at the welding section (C) operates so that the friction wheel  32  rotates, the carrier  12  is preferably moved to the unloading section (D) of the welding line  17  by the frictional force of the friction wheel  32  and the friction bar  34 . Subsequently, the unloading robot  14  transfers the completed front floor to the following line. 
     (4) After the front floor is unloaded, the carrier  12  is preferably moved up to the first return section of the return line  18  by the carrier changing robot  15  and, when the induction motor  31  in the horizontal movement driving means at the welding section (C) operates so that the friction wheel  32  rotates, the carrier  12  is preferably moved to the second return section of the return line  18  by the frictional force of the friction wheel  32  and the friction bar  34 . At this time, the up-and-down frame  20  of the drop lifter  10  returned to the original position receives the carrier  12  from the second return section and is moved down by the carrier drop means. 
     Here, the carrier changing robot  15  determines the vehicle type of the carrier  12  and, if the carrier  12  is needed to be changed, suitably changes the carrier  12  with another carrier  12  of the corresponding vehicle type in the carrier loading stand  13 . 
     Then, in preferred embodiments of the invention as described herein, the above-described operations in the order of drop lifter  10 →welding line  17  [loading (B), welding (C), and unloading (D)]→return line  18  are continuously repeated to produce the front floors. 
     In certain embodiments, the working line is provided at the top and the return line is provided at the bottom in the conventional system, and accordingly, the working height of the robots is suitably increased and all of the equipment is installed at high position, and thus the maintenance and reliability are lessened. 
     Accordingly, in preferred embodiments, the present invention preferably places the key welding process on the first floor so as to ensure the safety and stability of the system. 
     In further embodiments, the present invention preferably features a center floor and a rear floor besides the front floor. 
     As described herein, the vehicle floor production system in accordance with the present invention has, but is not limited to, the following effects. 
     According to preferred embodiments of the invention, the rotational force of the motor is suitably transferred to the carrier by the frictional contact between the friction bar attached to the carrier and the friction wheel mounted on the line. Accordingly, it is possible to reduce the manufacturing cost, compared with the conventional linear motor by the non-contact power transmission. 
     Moreover, in other further embodiments, since the welding line is preferably provided at the bottom of the system and the return line is preferably provided at the top to reduce the working height, according to other further embodiments, it is possible to ensure the safety and reliability of the system. 
     According to exemplary embodiments, a portion of the return line is moved substantially upward and the driving wheels of the carrier are provided in two stages, thus it is possible in other further embodiments to ensure the working space of the welding robot. 
     In further embodiments, the friction bar is detachably mounted to the carrier, thus it is possible to improve the handling of the carrier and facilitate the safekeeping of the carrier. 
     In other further embodiments, the robot is handled by the robot, thus the carrier loading stand preferably does not require any separate device for handling the carrier, thus simplifying the system and reducing the manufacturing cost. 
     The invention has been described in detail with reference to preferred embodiments thereof. However, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.