Patent Publication Number: US-8978562-B2

Title: Rail guided vehicle system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a rail guided vehicle system that conveys, along a rail, an article such as a container, in which various kinds of substrates for the production of semiconductor apparatuses, etc., are contained. 
     2. Description of the Related Art 
     A conventional rail guided vehicle system is known to be a rail guided vehicle system that conveys an article such as a FOUP (Front Opening Unified Pod) and that includes an overhead hoist transport (so-called OHT) travelling on a rail arranged on a ceiling. 
     Since such a rail guided vehicle system is often used in a clean room, where the cost of space is expensive, it is necessary to make the rail guided vehicle system smaller. One suggested technique is to make the rail guided vehicle system smaller by shortening the length of a connection rail that connects two main rails. For example, a known technique is to include a forcing guide, configured to force a diverging roller that is used when travelling on the connection rail to shift on the connection rail, in order to shorten the length of the connection rail. See, for example, Japanese Laid-Open Patent Application No. 2008-126743. 
     Here, the case where the vehicle travelling on a N-shaped rail moves from a rail on which the vehicle is travelling, to another rail is considered as an example. First, the vehicle shifts a diverging roller mechanism to the side on which another rail exists. The diverging roller mechanism is arranged in a travelling unit of the vehicle. Then, the vehicle is guided by a diverging guide, which extends from the rail to the middle of a connection rail, and moves to the connection rail. After this, the diverging roller mechanism is shifted to the opposite side in order for the vehicle to be guided by another diverging guide at the merging side. Here, another diverging guide is arranged in the connection rail and at the position opposite to the diverging guide, with a predetermined offset in between (separating area). In this situation, a forcing guide, which is arranged in the separating area, forces the diverging roller mechanism to be shifted toward another diverging guide. Thus, the travelling unit of the vehicle is guided to another rail. 
     In the above-mentioned technique, the diverging roller mechanism can only be shifted in the separating area, where the diverging guide does not exist. However, even if the forcing guide is used, the length of the separating area needs to be equal or greater than the length that is derived by adding the length of one diverging roller mechanism along the travelling direction, to the travelling length in which the vehicle travels during shifting the diverging roller mechanism. 
     For example, in order to shorten the connection rail, it is necessary to shorten the separating area by shortening the length of the forcing guide in the travelling direction. However, when shortening the length of the forcing guide, the diverging roller needs to be shifted rapidly unless the vehicle is slowed down. Shifting the diverging roller rapidly might cause abnormal operation because a large amount of stress is applied to the diverging roller. In addition, since the impulsion generated when the diverging roller abuts the forcing guide is large, the vibration of the vehicle and the damage of the diverging roller may occur. 
     As described above, the conventional technique has a limited benefit even if the connection rail can be shortened, and the conventional technique has a technical problem of causing various disadvantages when attempting to shorten the connection rail further. 
     SUMMARY OF THE INVENTION 
     In view of the above, preferred embodiments of the present invention provide a rail guided vehicle system that is made smaller by effectively shortening the connection rail. 
     A rail guided vehicle system according to a preferred embodiment of the present invention is a rail guided vehicle system that includes a rail and vehicles. The rail includes a first main rail, a second main rail, and a connection rail that connects the first main rail to the second main rail. 
     The vehicles travel while being supported and guided by the rail. A vehicle includes a travelling unit, a diverging roller supporting member, and a position stabilizing roller supporting member. The travelling unit is configured to travel on the rail. The diverging roller supporting member is arranged in the travelling unit. The diverging roller supporting member supports a first diverging roller, and a second diverging roller, that are arranged with a gap in between in a width direction of the travelling unit. The first diverging roller and the second diverging roller shift in the width direction of the travelling unit. 
     The position stabilizing roller supporting member is arranged in the travelling unit. The position stabilizing roller supporting member supports position stabilizing rollers. The position stabilizing rollers are configured to stabilize a travelling position of the travelling unit such that the position stabilizing rollers do not shift in the width direction of the travelling unit. 
     The rail includes a first diverging guide, a second diverging guide, and a position stabilizing roller guide. The first diverging guide extends, on one side of the connection rail in the width direction of the connection rail, from the first main rail along an extending direction of the connection rail. The first diverging guide is configured to guide the first diverging roller. 
     The second diverging guide extends, on another side of the connection rail in the width direction of the connection rail, to the second main rail along the extending direction of the connection rail while having a no-guide zone that does not overlap with the first diverging guide in the extending direction of the connection rail. The second diverging guide is configured to guide the second diverging roller. 
     The position stabilizing roller guide is arranged in the no-guide zone. The position stabilizing roller guide is configured to guide the position stabilizing rollers in the no-guide zone. Here, the width direction of the travelling unit indicates the direction crossing the travelling direction of the vehicle. 
     According to a preferred embodiment of the present invention, the position stabilizing roller guide configured to guide the position stabilizing rollers is arranged in the no-guide zone. The no-guide zone is where the diverging guide of the rail is not provided. By arranging such a position stabilizing roller guide in the no-guide zone, the position stabilizing roller is guided by the position stabilizing roller guide, and thus the position of the vehicle is stabilized. 
     If the position of the vehicle is stabilized in the no-guide zone, the no-guide zone can be extended to the zone where the first diverging guide and the second diverging guide should be arranged in the conventional techniques. This is the zone where the position of the vehicle should be stabilized by guiding the first diverging roller and the second diverging roller to the first diverging guide and the second diverging guide. Using a preferred embodiment of the present invention, while the no-guide zone has a sufficient length, lengthening of the entirety of the connection rail is avoided. The preferable movement of the vehicle between the first main rail and the second main rail is performed, and the shortening of the connection rail is also achieved. 
     As described above, according to the rail guided vehicle system of a preferred embodiment of the present invention, the system is made smaller by shortening the connection rail. 
     The vehicle may include a main body member and a plurality of bogie truck units that are arranged to rotate in a horizontal direction relative to the main body member. The travelling unit, the diverging roller supporting member, and the position stabilizing roller supporting member may be arranged in each of the plurality of bogie truck units. 
     In this structure, since the plurality of bogie truck units are arranged in the vehicle, the vehicle can be larger. Specifically, when the vehicle reaches a curve of the rail, each of the plurality of bogie truck units rotates along the horizontal direction with a different angle from those of other bogie truck units, thus the preferable travelling of the vehicle is achieved. 
     The position stabilizing roller may include a first roller member and a second roller member. The first roller member and the second roller member are attached with a gap in between in the width direction of the travelling unit. The position stabilizing roller guide includes a first roller guide member and a second roller guide member. The first roller guide member extends along the extending direction on one side of the connection rail in the width direction of the connection rail and in the no-guide zone. The first roller guide member is configured to guide the first roller member. The second roller guide member extends along the extending direction on another side of the connection rail in the width direction of the connection rail in the no-guide zone. The second roller guide member is configured to guide the second roller member. 
     In this structure, in the vehicle reaching the no-guide zone from the first diverging guide, the first roller member is first guided by the first roller guide member. Then, after the vehicle moves across the end of the first roller guide member, the second roller member is guided by the second roller guide member. With the first roller member, the second roller member, the first roller guide member, and the second roller guide member, the fluctuations in the right and left directions and the mismatches of the position are prevented. As a result, the position of the vehicle in the no-guide zone is better stabilized. 
     The rail may be arranged on a ceiling or in the vicinity of the ceiling. The rail includes a first travelling plane portion and a second travelling plane portion at both sides of the lower portion of the rail in the width direction of the rail. The first travelling plane portion and the second travelling plane portion are arranged with a gap in between. The first roller guide member rises in a planar manner from a position closer to the gap of the first travelling plane portion of the connection rail. The second roller guide member rises in a planar manner from a position closer to the gap of the second travelling plane portion of the connection rail. The first roller member and the second roller member are guided by abutting the first roller guide member and the second roller guide member, respectively, from the outer sides of the connection rail in the width direction of the connection rail. 
     In this structure, each of the first roller member and the second roller member is guided by abutting the first roller guide member and the second roller guide member, respectively. Specifically, the first roller member is guided while the first roller guide member abuts the first roller member from the inner side of the connection rail in the width direction of the connection rail. On the other hand, the second roller member is guided while the second roller guide member abuts the second roller member from the inner side of the connection rail in the width direction of the connection rail. Therefore, the position of the vehicle in the no-guide zone is better stabilized. 
     The rail, which is arranged on a ceiling or in the vicinity of the ceiling, may include a first travelling plane portion and a second travelling plane portion. The first travelling plane portion and the second travelling plane portion are disposed at both sides of the lower portion of the rail. A gap is defined between the first travelling plane portion and the second travelling plane portion. 
     The rail further includes a first projecting member and a second projecting member. The first projecting member projects toward the gap from the first travelling plane portion in the no-guide zone. The first projecting member extends a predetermined length along the extending direction of the connection rail. The first projecting member includes a first side plane portion at the end side of the first projecting member in a width direction of the first projecting member. The second projecting member projects toward the gap from the second travelling plane portion such that the second projecting member does not overlap with the first projecting member in the extending direction of the connection rail in the no-guide zone. The second projecting member extends a predetermined length along the extending direction of the connection rail. The second projecting member includes a second side plane portion at the end side of the second projecting member in a width direction of the second projecting member. The position stabilizing roller supporting member is arranged at the lower side of the travelling unit. The position stabilizing roller supporting member supports the position stabilizing rollers such that the position stabilizing rollers abut the first side plane portion and the second side plane portion. 
     In this structure, the position stabilizing rollers in the no-guide zone first abut the first side plane portion that exists at the one side of the connection rail in the width direction of the connection rail. Then, the position stabilizing rollers abut the second side plane portion that exists at another side of the connection rail in the width direction of the connection rail. The travelling position of the vehicle in the no-guide zone is stabilized. When the position stabilizing rollers abut the first side plane portion, the position can be better stabilized by a side roller abutting the rail wall at another side. When the position stabilizing rollers abut the second side plane portion, the position can also be better stabilized by a side roller abutting the rail wall at one side. 
     The position stabilizing rollers may be arranged in a line along a travelling direction of the vehicle, in the vicinity of the center portion of the lower side of the travelling unit in the width direction of the travelling unit. The first side plane portion and the second side plane portion are separated from each other by a gap equal or substantially equal to a diameter of the position stabilizing roller in the width direction. 
     In this structure, since plural portions of the position stabilizing rollers abut the first side plane portion and the second side plane portion, fluctuations in the right and left directions and the mismatches of the position are prevented with a smaller number of the position stabilizing rollers. The travelling position of the vehicle in the no-guide zone is stabilized. The gap between the first side plane portion and the second side plane portion does not have to be exactly equal to the diameter of the position stabilizing roller, as long as the gap and the diameter have almost the same length. 
     The above and other elements, features, steps, characteristics and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view showing the overall structure of the rail guided vehicle system according to a first preferred embodiment of the present invention. 
         FIG. 2  is a side view showing the structure of the vehicle according to the first preferred embodiment of the present invention. 
         FIG. 3  is a perspective view showing the transferring operation of the vehicle according to the first preferred embodiment of the present invention. 
         FIG. 4  is a perspective view showing the lateral transferring operation of the vehicle according to the first preferred embodiment of the present invention. 
         FIG. 5  is a plan view showing travelling of the vehicle according to the first preferred embodiment of the present invention on the connection rail. 
         FIG. 6  is a cross-sectional view showing the structure of the travelling unit according to the first preferred embodiment of the present invention together with the rail. 
         FIG. 7  is a cross-sectional view showing the structure of the travelling unit according to the first preferred embodiment of the present invention together with the rail (Part 2). 
         FIG. 8  is a cross-sectional view showing the structure of the travelling unit according to the first preferred embodiment of the present invention together with the rail (Part 3). 
         FIG. 9  is a cross-sectional view showing the structure of the travelling unit according to the first preferred embodiment of the present invention together with the rail (Part 4). 
         FIG. 10  is a cross-sectional view showing the structure of the travelling unit of the vehicle according to the comparative example, together with the rail. 
         FIG. 11  is a plan view showing the zone where the position of the vehicle according to the comparative example is stable and the zone where the position of the vehicle is unstable. 
         FIG. 12  is a plan view showing a diverging roller shifting zone for the vehicle according to the comparative example. 
         FIG. 13  is a cross-sectional view showing the structure of the travelling unit of the vehicle according to a second preferred embodiment of the present invention together with the rail. 
         FIG. 14  is a side view showing the structure of the travelling unit of the vehicle according to the second preferred embodiment of the present invention. 
         FIG. 15  is a plan view showing the structure of the travelling plane of the rail according to the second preferred embodiment of the present invention. 
         FIG. 16  is a plan view showing the travelling of the vehicle according to the second preferred embodiment of the present invention on the connection rail. 
         FIG. 17  is a cross-sectional view showing the structure of the travelling unit of the vehicle according to the second preferred embodiment of the present invention together with the rail. 
         FIG. 18  is a cross-sectional view showing the structure of the travelling unit of the vehicle according to the second preferred embodiment of the present invention together with the rail (Part 2). 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention will be explained below based on the figures. 
     First Preferred Embodiment 
     First, the overall structure of the rail guided vehicle system according to the first preferred embodiment of the present invention will be explained, referring to  FIG. 1 .  FIG. 1  is a plan view showing the overall structure of the rail guided vehicle system according to the first preferred embodiment. 
     In  FIG. 1 , the rail guided vehicle system according to the present preferred embodiment includes a rail  100 , vehicles  200 , and a controller  300 . 
     The rail  100  preferably is arranged on a ceiling and made of metal such as aluminum, stainless steel, etc. The rail  100  includes a first main rail  101  and a second main rail  102 , and a connection rail  105 . The first main rail  101  and the second main rail  102  are arranged parallel or substantially parallel to each other. The connection rail  105  connects the first main rail  101  to the second main rail  102 . Thus, the vehicles  200  travelling on the first main rail  101  can move to the second main rail  102  by travelling on the connection rail  105 . 
     A plurality of vehicles  200  are arranged on the rail  100  and can convey FOUPs, as articles to be conveyed, by travelling along the rail  100 . 
     In addition, each of the vehicles  200  includes an on-board controller  205 . The on-board controller  205  receives conveying instructions from the controller  300 , and controls the travelling of the vehicle  200 . It should be noted that the on-board controller  205  not only is programmed to control the travelling of the vehicle  200 , but also is programmed to control all of the apparatuses included in the vehicle  200 . 
     The controller  300  includes an arithmetic circuit, a memory, etc. and is configured to give the conveying instructions to the vehicle  200  via the on-board controller  205 . 
     Although not shown in the figure, a shelf (for example, a buffer, a port, etc.) storing the FOUP temporarily and a semiconductor fabricating apparatus are arranged in the location along the rail  100 . 
     Next, the further detailed structure of the vehicle  200  will be explained referring to  FIG. 2 .  FIG. 2  is a side view showing the structure of the vehicle according to the first preferred embodiment. 
     In  FIG. 2 , the vehicle  200  includes a travelling unit  210 , a main body member  220 , a moving unit  230 , a hoist unit  235 , a hoist belt  240 , and a gripping unit  250 . Diverging rollers and position stabilizing rollers, which are described later, are omitted in  FIG. 2  for the convenience of this explanation. 
     The vehicle  200  travels along the rail  100  while travelling rollers  215  are rolled by a driving power of the travelling unit  210 , e.g., a linear motor. The main body member  220  is suspended on the bottom plane of the travelling unit  210 . 
     The moving unit  230  is attached to the main body member  220 . The moving unit  230  can move to the side (namely, in the right and left directions in  FIG. 2 ) of the rail  100 . The hoist unit  235  is attached to the bottom plane of the moving unit  230 . 
     The gripping unit  250  that grips the FOUP is attached to the bottom plane of the hoist unit  235  by the hoist belt  240 . The gripping unit  250  can ascend or descend relative to the main body member  220  by winding off or reeling in the hoist belt  240 . 
     Next, the method of transferring the FOUP by the vehicle will be explained referring to  FIGS. 3 and 4 .  FIGS. 3 and 4  are perspective views showing the method of transferring the FOUP by the vehicle according to the first preferred embodiment. 
     In  FIG. 3 , when the vehicle  200  transfers the FOUP  400  on the port  510  located right below the rail  100 , the vehicle  200  first travels on the rail  100  and stops over the FOUP  400  arranged on the port  510 . 
     Next, as shown in  FIG. 3 , the hoist unit  235  winds off the hoist belt  240  and the gripping unit  250  then descends to the location of the FOUP  400 . The gripping unit  250  then grips the FOUP  400 . 
     After the gripping unit  250  grips the FOUP  400 , the hoist unit  235  reels the hoist belt  240  so that the gripping unit  250  and the FOUP  400  ascend to the location of the main body member  220 . Then, the vehicle  200  travels again on the rail  100  and the FOUP  400  is conveyed. 
     The example shown in  FIG. 4  shows the case when the FOUP  400  is arranged at a side buffer  520  located off the side of the rail  100 . In this case, after the moving unit  230  moves to the side of the rail  100 , the hoist unit  235  winds off the hoist belt  240  and the gripping unit  250  descends to the location of the FOUP  400 . With such an operation, it is possible to perform a lateral transfer of the FOUP  400  located off the side of the rail  100 . 
     Next, an example of a method of travelling on the connection rail by the vehicle according to the present preferred embodiment, and the specific structures of the vehicle and the rail that realize preferable travelling on the connection rail, will be explained referring to  FIGS. 5 through 9 .  FIG. 5  is a plan view showing the travelling of the vehicle according to the first preferred embodiment on the connection rail. In addition,  FIGS. 6 through 9  are cross-sectional views showing the structure of the travelling unit according to the first preferred embodiment together with the rail.  FIGS. 6 through 9  show the vehicle viewed from the back side of the vehicle in the travelling direction. 
     The operation performed when the vehicle  200  travels from the first main rail  101  to the connection rail  105  and then moves to the second main rail  102  will be explained below, referring to  FIG. 5 . In the vehicle  200  according to the present preferred embodiment, diverging rollers  610  and position stabilizing rollers  620  are arranged in each of the bogie truck units. 
     As shown in  FIG. 5  and  FIG. 6 , when the vehicle  200   a  travelling on the first main rail  101  reaches the point where a diverging guide  110   a  is arranged, the diverging rollers  610  are guided by the diverging guide  110   a . A pair of the diverging rollers  610  are supported by a diverging roller supporting member  720  that is configured to shift in the direction crossing the travelling direction of the vehicle  200  (namely, the right and left directions in  FIG. 6 ). Here, the vehicle  200   a  is about to move to the connection rail  105 . The connection rail  105  is located to the left side of the first main rail  101  in the travelling direction. The diverging roller supporting member  720  is shifted to the left side at the point where the diverging guide  110   a  is arranged. In such a situation, the left side one of the pair of the diverging rollers  610  is positioned to be guided by the diverging guide  110   a . In this manner, the vehicle  200  is guided from the first main rail  101  to the connection rail  105 . 
     On the other hand, when the vehicle  200  is not guided to the connection rail  105 , the diverging roller supporting member  720  is shifted to the right side. In such a situation, the diverging rollers  610  are not guided by the diverging guide  110   a  at the side of the connection rail  105  but by a diverging guide  110   c  at the side for travelling straight. Thus, the vehicle  200  is kept travelling on the first main rail  101 . 
     As shown in  FIG. 5  and  FIG. 7 , in the vehicle  200   b  having moved to the connection rail  105  and reaching the point where a position stabilizing roller guide  120   a  is arranged, the position stabilizing rollers  620  at the left side are guided by the position stabilizing roller guide  120   a . A pair of the position stabilizing rollers  620  are supported by the position stabilizing roller supporting member  710 , together with a pair of side rollers  650 . The position stabilizing rollers  620  are guided by the position stabilizing roller guide  120   a  and are configured to stabilize the position of the vehicle  200 . 
     As shown in  FIG. 5  and  FIG. 8 , when the vehicle  200   b  reaches the point where the position stabilizing roller guide  120   b  is arranged, the position stabilizing rollers  620  at the right side (namely, the rollers at the side opposite to the side where the rollers that were guided before are arranged) are guided by the position stabilizing roller guide  120   b . Although the position stabilizing roller guides  120   a  and  120   b  do not overlap with each other in the present preferred embodiment, a zone where both the position stabilizing roller guides  120   a  and  120   b  are overlapping may exist. 
     In the zone where the position stabilizing rollers  620  are guided by each of the position stabilizing roller guides  120   a  and  120   b , the diverging roller supporting member  720  is released (namely, the diverging roller supporting member  720  is in the state in which the diverging roller supporting member  720  can be shifted). This is because the diverging guide  110  configured to guide the diverging rollers  610  is not arranged. Therefore, the diverging roller supporting member  720  is shifted to the right side in the zone where the position stabilizing rollers  620  are guided. 
     As shown in  FIG. 5  and  FIG. 9 , when the vehicle  200   c  reaches the point where the diverging guide  110   b  exists after passing through the zone where the position stabilizing roller guide  120   b  exists, the diverging roller supporting member  720  is shifted to the right side. As a result, the diverging rollers  610  at the right side are guided by the diverging guide  110   b . Therefore, the vehicle  200  can move from the connection rail  105  to the second main rail  102 . 
     The movement of the vehicle from the first main rail  101  to the second main rail  102  can be preferably realized with the rail guided vehicle system according to the present preferred embodiment, since the diverging rollers  610  and the position stabilizing rollers  620  are respectively guided by the diverging guide  110  and the position stabilizing roller guide  120 . In the present preferred embodiment, the position stabilizing rollers  620  are guided by the position stabilizing roller guide  120  in the no-guide zone, the no-guide zone being where the diverging guide  110  is not arranged. Therefore, the position of the vehicle  200  is stabilized even in the no-guide zone. 
     The rail guided vehicle system that does not include the above-described position stabilizing rollers  620  and the position stabilizing roller guide  120  will be explained, referring to  FIGS. 10 through 12 .  FIG. 10  is a cross-sectional view showing the structure of the travelling unit of the vehicle together with the rail, according to the comparative example.  FIG. 11  is a plan view showing the zone where the position of the vehicle according to the comparative example is stable, and the zone where the position of the vehicle is unstable.  FIG. 12  is a plan view showing a diverging roller shifting zone for the vehicle according to the comparative example. 
     As shown in  FIG. 10  and  FIG. 11 , in the rail guided vehicle system according to the comparative example, the position stabilizing rollers  620  and the position stabilizing roller guide  120  are absent. Thus, in the zone where both side rollers  650  located at the right and left sides abut rail walls (namely, the inner side plane of the rail  100 ) in the no-guide zone where the diverging guide  110  is absent, the position of the vehicle  200  is stable. However, as shown in  FIG. 11 , in the no-guide zone where the diverging guide  110  is absent, there is a zone where the rail wall exists only at one side in the travelling direction. In this zone, the side rollers  650  abut only one of the rail walls. Thus, in the above-described zone, since the position cannot be stabilized by the side rollers  650 , it is hard to stabilize the position of the vehicle  200 . Therefore, in order to realize the stable travelling of the vehicle  200  in the connection rail  105 , it is required to lengthen the diverging guide  110 . 
     As shown in  FIG. 12 , if the diverging guide  110  is lengthened in the rail guided vehicle system according to the comparative example, the length of the zone where the diverging rollers  610  are shifted (namely, the no-guide zone where the diverging guide  110  is not arranged) becomes shorter. Therefore, there might be the case in which it is hard for the diverging rollers  610  to be shifted properly. In addition, in such a case, the use of a forcing guide, configured to force the diverging rollers  610  to be shifted, is possible. However, even if the forcing guide is used, the diverging rollers  610  need to be shifted rapidly unless the vehicle  200  is slowed down. Shifting the diverging rollers  610  rapidly might cause abnormal operations because the diverging rollers  610  might receive a large force. In addition, since, if the diverging rollers  610  are rapidly shifted, the impulsion generated when the diverging rollers  610  abut the diverging guide  110  becomes large, the vibration of the vehicle  200  and/or the damages of the diverging rollers  610  might occur. 
     On the other hand, the rail guided vehicle system according to the present preferred embodiment shown in  FIGS. 5 through 9  includes the position stabilizing rollers  620  and the position stabilizing roller guide  120 . As a result, the no-guide zone, where the diverging guide  110  is not arranged, is lengthened. Thus, the shifting operation of the diverging rollers  610  is reliably and safely performed. 
     As it can be seen by comparing  FIG. 5  with  FIG. 11  and  FIG. 12 , in the rail guided vehicle system according to the present preferred embodiment, while the no-guide zone is longer than that of the rail guided vehicle system according to the comparative example, the lengths of the connection rail  105  are same. More specifically, the rail guided vehicle system according to the present preferred embodiment prevents the connection rail  105  from becoming longer even if the no-guide zone is lengthened. Thus, the connection rail  105  can be shortened even if the no-guide zone is lengthened. 
     As explained above, with the rail guided vehicle system according to the present preferred embodiment, the position stabilizing rollers  620  makes possible the stable travelling on the connection rail. In addition, the position stabilizing rollers  620  make the system smaller by shortening the length of the connection rail  105 . 
     Second Preferred Embodiment 
     Next, the rail guided vehicle system according to the second preferred embodiment of the present invention will be explained. Only some of the structures in the second preferred embodiment are different from those of the above-described first preferred embodiment and other structures of the second preferred embodiment are almost the same as those of the first preferred embodiment. Therefore, only the structures that are different from those of the first preferred embodiment will be explained below, and the explanation of structures that are the same as those of the first preferred embodiment will be omitted from the following description accordingly. 
     The structure of the vehicle according to the second preferred embodiment will be explained, referring to  FIG. 13  and  FIG. 14 .  FIG. 13  is a cross-sectional view showing the structure of the travelling unit of the vehicle according to the second preferred embodiment together with the rail.  FIG. 14  is a side view showing the structure of the travelling unit of the vehicle according to the second preferred embodiment. 
     As shown in  FIG. 13 , in the vehicle  200  according to the second preferred embodiment, lower position stabilizing rollers  625  are arranged at the lower portion of the travelling unit  210 . The lower position stabilizing rollers  625  are fixed so as not to be shifted, and are arranged at the position closer to the lower plane of the rail  100  (namely, the travelling plane on which the travelling unit  210  travels). 
     As shown in  FIG. 14 , the lower position stabilizing rollers  625  are arranged in a line along the travelling direction of the vehicle  200 . Specifically, two lower position stabilizing rollers  625   a  are arranged longitudinally at the front side of the travelling unit  210   a  in the travelling direction. In addition, two lower position stabilizing rollers  625   b  are arranged longitudinally at the back side of the travelling unit  210   b  in the travelling direction. 
     Next, the structure of the rail in the rail guided vehicle system according to the second preferred embodiment will be explained, referring to  FIG. 15 .  FIG. 15  is a plan view showing the structure of the travelling plane of the rail according to the second preferred embodiment. 
     As shown in  FIG. 15 , the rail  100  according to the second preferred embodiment includes a first projecting member  910  and a second projecting member  920  in the travelling plane of the connection rail  105 . The first projecting member  910  includes a first side plane portion  915 . The second projecting member  920  includes a second side plane portion  925 . As described later, the first side plane portion  915  and the second side plane portion  925  are arranged at the position where they abut the lower position stabilizing rollers  625 . 
     The first projecting member  910  is arranged at the right side of the rail  100  in the travelling direction and extends from the vicinity of the diverging point of the first main rail  101  and the connection rail  105 , to the middle of the connection rail  105 . The second projecting member  920  is arranged to the left side of the rail  100  in the travelling direction and extends from the middle of the connection rail  105  to the vicinity of the merging point of the connection rail  105  and the second main rail  102 . The first projecting member  910  and the second projecting member  920  are arranged at the position where they do not overlap with each other in the travelling direction of the vehicle  200 . 
     The length of the gap between the first side plane portion  915  and the second side plane portion  925  in the travelling direction of the vehicle  200  is long enough to change the rotation direction of the lower position stabilizing roller  625 . In addition, the length of the gap between the first side plane portion  915  and the second side plane portion  925  in the direction crossing the travelling direction of the vehicle  200  is equal to the diameter of the lower position stabilizing roller  625 . 
     Next, the method of travelling on the connection rail  105  by the vehicle  200  according to the second preferred embodiment and the operation of the travelling unit  210  will be explained in detail, referring to  FIGS. 16 through 18 .  FIG. 16  is a plan view showing the travelling of the vehicle according to the second preferred embodiment on the connection rail.  FIG. 17  and  FIG. 18  are cross-sectional views showing the structure of the travelling unit of the vehicle according to the second preferred embodiment together with the rail. 
     The operation that is performed when the vehicle  200   a  travels from the first main rail  101  to the connection rail  105  and then moves to the second main rail  102  will be explained below. Since the guiding operation and the shifting operation of the diverging rollers  610  are the same as those in the above-described first preferred embodiment, here the operation of the lower position stabilizing rollers  625  that is unique in the second preferred embodiment will be explained in detail. 
     As shown in  FIG. 16  and  FIG. 17 , in the vehicle  200   b  reaching the connection rail  105 , the lower position stabilizing rollers  625  abut the first side plane portion  915  of the first projecting member  910  when the vehicle  200   b  moves across the end of the diverging guide  110   a . Thus, the position of the vehicle  200  can be stabilized even if the diverging rollers  610  are not guided by the diverging guide  110   a . Specifically, the position of the vehicle  200  is maintained by the abutting portion of the lower position stabilizing rollers  625  and the first side plane portion  915 , and by the abutting portion of the side rollers  650  at the left side and the rail wall. 
     As shown in  FIG. 16  and  FIG. 18 , when the vehicle  200   b  moves across the end of the first projecting member  910 , the lower position stabilizing rollers  625  then abut the second side plane portion  925  of the second projecting member  920 . Thus, the position of the vehicle  200  can be stabilized even if the diverging rollers  610  are not guided by the diverging guide  110   b  existing at the side of the second main rail  102 . Specifically, the position of the vehicle  200  is maintained by the abutting portion of the lower position stabilizing rollers  625  and the second side plane portion  925 , and by the abutting portion of the side rollers  650  at the right side and the rail wall. 
     As shown in  FIG. 16 , when the vehicle  200  reaches the second main rail  102  and moves across the end of the second projecting member  920 , the diverging rollers  610  are guided by the diverging guide  110   b . Therefore, the position of the vehicle  200  is stabilized even if the lower position stabilizing rollers  625  do not abut the second side plane portion  925 . 
     As described above, with the lower position stabilizing rollers  625 , and the first projecting member  910  and the second projecting member  920 , the travelling position of the vehicle  200  is stabilized even in the zone where the diverging guide  110  does not exist. Therefore, the zone where the diverging guide  110  is arranged can be shortened. By shortening the zone where the diverging guide  110  is arranged, the no-guide zone can be lengthened without changing the length of the connection rail  105 . More specifically, even if the no-guide zone is lengthened, the connection rail  105  does not become longer due to the lengthening of the no-guide zone. As a result, the connection rail  105  can be shortened. 
     As described above, with the rail guided system according to the second preferred embodiment, like the above-described first preferred embodiment, the lower position stabilizing rollers  625  enable the travelling on the connection rail  105  to be stable and the system is made smaller by shortening the length of the connection rail  105 . 
     The present invention is not limited to the above-described preferred embodiments and can be changed without departing from the scope or concept of the present invention that can be read from the claims and the entire specification. The rail guided vehicle system including such changes is also within the scope of the present invention. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing from the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.