Abstract:
An object of the present invention is to improve the transportation efficiency of an automatic transport system for transporting an article. In order to attain the above object, the automatic transport system of the present invention comprises a front detecting device which detects an obstruction in a non-contact state in an area through which an automatic transport vehicle passes and a projection surface of said automatic transport vehicle, and when said front detecting device detects the obstruction in said area, the running speed of said automatic transport vehicle is reduced or said automatic transport vehicle is stopped.

Description:
The present application is a divisional of application Ser. No. 09/741,398 filed on Dec. 20, 2000, now U.S. Pat. No. 6,443,400. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an automatic transport system for transporting articles by an automatic transport vehicle at an assembly location in a plant and the like without human attendance, and particularly to an automatic transport system which detects by a sensor an obstruction located ahead of the automatic transport vehicle in its moving direction to control the operation of the automatic transport vehicle. 
     2. Description of Related Art 
     Automatic transport vehicles (hereinafter referred to as vehicles) are advantageously used in transporting parts in an assembly process in a plant and the like. Particularly, in a manufacturing semiconductor process, vehicles are used in transferring and assembling semiconductor wafers in the clean room without human intervention for preventing the contamination with dust and the like. For example, an Overhead Hoist Transport vehicle (hereinafter referred to as “OHT vehicle”), which travels along a ceiling rail in the clean room is used in the assembly process of semiconductor wafers and the liquid crystal devices. 
     Moreover, an optical beam reflection sensor (hereinafter referred to as “optical sensor”) such as an infrared type sensor, serving as a non-contact obstruction detecting apparatus of vehicles. The optical sensor detects an obstruction ahead in the moving direction by emitting an optical beam which is conical-shaped. If a long range detection sensor is provided at the front of the vehicle as a front detection sensor, the vehicle is stopped when the long range detection sensor is triggered while it is traveling. If a vehicle has two front detection sensor as a front detection sensor, detection may be carried out in two steps by two front detection sensors. 
     FIG. 9 is an operation conceptual view of an OHT system used in the semiconductor wafer manufacturing process or the like. The right portion of FIG. 9 is a side view of an OHT vehicle and the left portion of FIG. 9 is a view showing a projection of the OHT vehicle ahead in the moving direction at a predetermined position in the moving direction. 
     In FIG. 9, a rail  21  is laid down on a ceiling of a clean room (not shown) along the process line, and a part of the rail  21  is shown therein. An OHT vehicle  22  movably hangs on a lower portion of the rail  21 . For example, the OHT vehicle  22  is constituted such that it has a box-like frame and can hold a wafer cassette  23  in this frame and runs along the rail  21 . 
     Moreover, a front detection sensor  24  is attached to the front portion of the OHP vehicle  22  in the moving direction. As the front detection sensor  24 , an optical sensor of such as an infrared sensor is generally used such that an obstruction in the moving direction of the OHT vehicle  22  can be detected in a non-contact state. In other words, an obstruction ahead is detected by optical beams emitted in a conical shape from the front detection sensor  24 . Then, when the front detection sensor  24  detects the obstruction ahead, the OHT vehicle  22  is designed to automatically stop. 
     Additionally, in FIG. 9, although the front detection sensor  24  at the left side surface of the OHT vehicle  22  is provided for movement to the left side of the figure, the OHT vehicle  22  normally moves in two directions. In such a case, the front detection sensor  24  IS also provided at the right side surface of the OHT vehicle  22 . 
     However, in some cases, an associated manufacturing apparatus may be present very close to the periphery of the rail  21 , the door of the manufacturing apparatus may be opened, or parts being processed are located close to the periphery of the rail  21 . Further, in other cases, in locations outside of the passage of the OHT vehicle  22 , there may be a stepladder, a workbench or the like for maintenance, or a person. For this reason, in order to prevent the OHT vehicle  22  from colliding with them, the front detection sensor  24  on the OHT vehicle  22  detect obstructions ahead. However, as shown in FIG. 9, if the detection area of light emitted from the front detection sensor  24  is widened as shown in a detection area A in order to detect obstructions located in the passage area of the OHT vehicle  22 , there is a possibility that objects which are at the periphery of the running path will unnecessarily be detected and that the OHT vehicle  22  will not run. 
     In other words, the left side of the drawing indicates the passage area C of the OHT vehicle  22 , as seen from the front of the OHT vehicle  22  in the moving direction, by a solid line. Also, a wide detection area A where the entire passage area C of the OHT vehicle  22  can be detected is indicated by a broken line. This large detection area A is the bottom surface of the cone of the light beam emitted by the front detection sensor  24  at a predetermined position. 
     Unlike the wide circular detection area A that is the bottom of a conical surface, if the front shape of the OHT vehicle  22  that is the passage area C of the OHT vehicle  22  is rectangular, for example, as illustrated in the figure, excess detection area D, in which the wide detection area A lies outside of the passage area C to be detected will occur. If an object is located in this excess detection area D, the OHT vehicle  22  will stopped even though the object is not actually obstructing the passage of the OHT vehicle  22 . 
     On the other hand, if the detection area is narrowed as in the narrow detection area B indicated by a broken line, the corner portions of the passage area C of the OHT vehicle  22  cannot be detected, and form a non-detection area E. In such a case, there is the concern that the OHT vehicle  22  will collide with an object in the non-detection area E which is in the corner portion of the vehicle, when the vehicle passes the object. 
     FIG. 10 is an explanatory view showing the front detection sensor of the OHT vehicle  22  and an example of an obstruction. As illustrated in this figure, a stepladder  25  is placed in front, in the moving direction, of the OHT vehicle  22 . In this case, if the detection area at the front detection sensor  24  is wide, as in the wide detection area A, the stepladder  25  is detected as an obstruction and the OHT vehicle  22  is stopped even though the OHT vehicle  22  will not collide with the stepladder  25 . Furthermore, if the detection area is narrowed as in the narrow detection area B, the stepladder  25  is not detected. However, if workpieses or the like are placed at a location very close to the OHT vehicle  22 , there is the concern that the OHT vehicle  22  will collide with them and break them since they cannot be detected. 
     FIG. 11 is a conceptual view showing an OHT vehicle used in a semiconductor manufacturing apparatus. As illustrated in this figure, for example, in the apparatus for manufacturing a 300 mm wafer, a distance P between the end surface of the OHT vehicle  22  that transports the wafer and the front surface of the semiconductor manufacturing apparatus  26  is set to about 30 mm on the basis of a standard distance. It is assumed that working is carried out in such small distances. If the detection area is too wide, the front detection sensor  24  will detect the door of the semiconductor manufacturing apparatus  26 , so that the OHT vehicle  22  will not operates well and workcannot be carried out. Moreover, if the detection area is narrowed, there is the concern that the corner of the OHT vehicle  22  will contact semiconductor wafers (not shown) mounted on the semiconductor manufacturing apparatus  26  and these semiconductor wafers will be broken. 
     FIG. 12 is a conceptual view showing a vehicle using two front detection sensors for long range and medium range detection. The vehicle  111  is provided with a medium range detection sensor (not shown) which can detect over a medium detection range  113  and a long range detection sensor (not shown) which can detect over a long detection range  112 . The vehicle  111  detect an obstruction which is loaded ahead in moving direction by switching the respective sensors. Then, control is performed so that the speed of the vehicle  111  is reduced when the long range detection sensor works and makes a detection within the long detection range  112  and the vehicle  111  is stopped when the medium range detection sensor makes a detection in the medium detection range  113 . 
     FIG. 13 is a conceptual view showing an example of the operation state of a plurality of vehicles in a general OHT system. This figure is a conceptual view to explain a system in which a transport apparatus comprising a plurality of vehicles operating between the assembly apparatuses such as a plurality of semiconductor manufacturing apparatuses. In this figure, a rail  124  is provided along a plurality of assembly apparatuses  121 ,  122 ,  123 , and a plurality of vehicles  125  and  126  travel on the rail  124 . Then, in the case of operating the transport apparatus comprising a plurality of vehicles  125  and  126  which detect an obstruction which is located ahead by the front detection sensor as shown in FIG. 12, described above, it is effective for the respective vehicles  125  and  126  to made to be as close as possible to the vehicle in front when stopping in order to increase the transport efficiency of the system. 
     The transport efficiency of the transport system largely differs depending on whether the trailing vehicle  126  can move to a position G or only to a position H when the front vehicle  125  is placed at a position F as shown in FIG.  13 . For example, it is assumed that there are requests for transfer from transfer ports  127  at positions F and G simultaneously in the assembly apparatus  121 . If the trailing vehicle  126  can move to the position G when the front vehicle  125  is stopped at the position F, the simultaneous transfer can be carried out at the positions F and G. However, if the trailing vehicle  126  can move to only the position H, the trailing vehicle  126  cannot move to the position G until the front vehicle  125  finishes transferring at the position F and leaves the position F. Therefore, the transfer efficiency of the trailing vehicle  126  at the position G is decreased. 
     On the other hand, the conventional use of the general front detection sensors is described below. Specifically, as explained in the FIG. 12, when the vehicle moves close to the obstruction, the long range detection sensor detects the obstruction located the long detection range  112 , firstly. Next, the sensor is changed to the medium range detection sensor or the detection range of the long range detection sensor is shortened to carry out the detection of the obstruction in the medium detection range  113 . In this way, the detection range of sensor is shortened in two steps, the speed of the vehicle  111  is reduced, and then the vehicle stops at a predetermined position. In order to stop a vehicle  111  moving at a high speed before colliding with an obstruction, it is necessary to allow for a braking distance to start braking. For this reason, the detection occur in two steps for the long detection range  112  and the medium detection range  113  in the operation control of the vehicle  111 . 
     However, in this case as described above, the front vehicle, which is regarded as an obstruction, may move forwards and is no longer regarded as an obstruction in some cases. This results in unnecessary braking, which reduces the operation efficiency of the entire OHT system. There is a method of preventing the unnecessary braking, that is to reduce the moving speed of the vehicle and to shorten the braking distance. However, this results in a reduction in the operating speed, so that the operation efficiency of the entirety of the OHT system is reduced after all. 
     For this reason, in the general OHT system, the speed of the vehicle is reduced in the long detection range or the medium detection range based on the detection result of the long range detection sensor, and the vehicle is stopped in the short detection range, which is very close to the front vehicle. The switching between the long detection range and the medium detection range using the long range detection sensor is generally decided based on the size of the vehicle and the speed, or the degree of the speed reduction or the like and the switching is decided such that after the operation of the long range detection sensor, the speed reduction of the vehicle and the stopping thereof are completed before the trailing vehicle contacts the obstruction. For example, when executing long range detection, the vehicle is operated if the distance between the obstruction ahead and the vehicle is 2 to 3 m. When executing medium range detection, the vehicle continues to operate when the distance between the obstruction ahead and the vehicle is 0.5 to 1.5 m. When executing short range detection, which covers shorter distances than the above, the vehicle is stopped. In this way, the distance between the obstruction ahead and the vehicle is predetermined in each detective range 
     If the vehicle is moving at high speed, it is necessary to reduce the detection range as little as possible after switching the detection range to the medium detection range from the long distance detection in order to stop the vehicle safely by the short range detection sensor after the operation of the long range detection sensor. However, the reduction of the detection range is limited to the braking distance of the vehicle, so that the medium detection range cannot be shortened much. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an objective of the present invention is to provide an automatic transport vehicle providing sensors that can detect an obstruction present an area through which the automatic transport vehicle passes without losing the operation efficiency of the transport system. 
     Moreover, in an automatic transport vehicle comprising a plurality of vehicles, it is determined whether or not an obstacle ahead is a vehicle, and when the obstruction ahead is a vehicle, the distance up to the vehicle is shortened and the trailing vehicle is stopped, which makes it possible to improve the operation efficiency of an OHT system. 
     In order to solve the above-described problems and attain the above described objectives, the present invention provides an automatic transport system for transporting articles, comprising a front detecting device which detects an obstruction in a non-contact state in an area through which an automatic-transport vehicle passes, and a projection surface of said automatic transport vehicle, and when said front detecting device detects the obstruction in said area, the running speed of said automatic transport vehicle is reduced or said automatic transport vehicle is stopped. 
     Since the front detecting device of the present invention detects an obstruction which is located only the vehicle pass area of the actual passage region of the automatic transport vehicle. Therefore, in this transport system, only an object which located in the vehicle pass area is detected, and parts or the like in a position very close to a vehicle, but which does not impede the running of the vehicle, are not detected. In an automatic transport system, which is used in the assembly process of a semiconductor manufacturing apparatus, it is necessary to transport the workpieces or the like in the extremely narrow range to run the automatic transport vehicle. For this reason, the use of the automatic transport system of the present invention further improves the work efficiency. 
     Moreover, according to the automatic transport system, in the above-described invention, said front detecting device is an optical sensor, which emits an optical beam so as to irradiate an entire outer periphery of a projection surface of said automatic transport vehicle, and said optical sensor detects an obstruction in said area. Then, only the outer periphery of the area where the automatic transport vehicle passes is irradiated with the optical beam to detect the reflected light of this optical beam, making it possible to easily carry out detection in only the passage area of the automatic transport vehicle. 
     Furthermore, according to the automatic transport system of the above-described invention, a plurality of said optical sensors are provided near the outer periphery of a front surface of said automatic: transport vehicle, said optical sensors respectively emit the optical beams that irradiate a area throughout an entire outer periphery of the projection surface of said automatic transport vehicle, and said optical beams are fan-shaped. 
     The plurality of optical sensors are provided near the outer periphery of a front surface of the automatic transport vehicle in the moving direction. Then, the entire outer periphery of the running area is irradiated in the shape of a strip with the optical beams emitted from the respective optical sensors. As a specific method, for example, in the case of the automatic transport vehicle whose front surface in the moving direction is rectangular, if the strip slits are provided along the respective sides of the rectangle and the optical beams are emitted from the interior of these slits in the shape of a fan, the entire corresponding side of the rectangle, which is equivalent to a passage area, is irradiated with the optical beams. Therefore, the strip irradiation areas of the respective sides are combined with one another, making it possible to irradiate the outer periphery of the entire vehicle moving area the shape of a strip with the optical beams. 
     Moreover, according to the automatic transport system in the above-described invention, wherein the area irradiated by said optical beams lies partially outside of the outer area of said projection surface. Then, it is desirable that a slight allowance be provided in the width of the detection area such that erroneous detection of obstacles and a miss of detection can be prevented by mechanical shifts occurring when the automatic transport vehicle moves. 
     Still further, according to the automatic transport system of the present invention, the automatic transport vehicles constituting the automatic transport system of earn invention as-described above can be used in precision work, such as in a semiconductor manufacturing apparatus, and it can be employed in an Automatic Guided Vehicle (hereinafter referred to as “AGV”) running on the floor, a Rail Guided Vehicle (hereinafter referred to as “RGV”) running on a rail on the floor, which transport materials, parts, products or the like in automated plants or the like, other than an OHT, which runs on a ceiling rail. 
     The automatic transport system of the present invention is an automatic transport system, which comprises a plurality of vehicles running on a rail. The vehicles detect obstructions ahead in the moving direction and whether or not the obstruction is an automatic transport vehicle that runs in the front, so as to perform running control. According to the automatic transport system of the present invention, the running control differs depending on whether the obstruction ahead is a vehicle, and if the obstruction ahead is the vehicle, the vehicle is moved forward as much as possible to improve the entire transportation efficiency. In addition, the rail to which the present invention refers is not limited to a rail whose running route is physically constrained and the like. For example, a running route that runs on the floor and the like are also included therein. 
     Furthermore, according to the automatic transport system of the present invention, in the above invention, each of said plurality of vehicles comprises front detecting device for detecting whether at least two kinds of obstructions are present ahead and obstruction determining device which determine whether the obstructions detected by the detecting of the front detecting device are vehicles running ahead, and running control of the vehicles is performed based on the detection result of the front detecting device and the identification result of the obstruction determining device. 
     According to the automatic transport system of the present invention identification of whether an obstruction ahead is not a vehicle or is a vehicle running ahead is correctly performed. Then, the stopping of the trailing vehicle or the effective forward movement are carried out based on the identification result. This makes it possible to further improve the productivity of the entire system as compared with the conventional OHT transport system. Thus, running control can be carried out so that obstacles located in an area through which the vehicle passes can be detected with more reliability without losing the transportation efficiency of the system. 
     Still further, according to the automatic transport system of the present invention, in the above-described invention, said front detecting device comprises a long range detection sensor which detects an obstruction located in a long range, and a short range detection sensor which detects an obstruction located in a short range, said obstruction determining device determines whether or not an obstruction ahead detected by said long range detection sensor is an automatic transport vehicle running ahead, and running control of said automatic transport vehicle is performed based on a detection result of said long range detection sensor, an determining result of said obstruction determining device, and detection result of said short range detection sensor. 
     According to the automatic transport system of the present invention, the long range detection sensor, which has a relatively long detection range, detects obstructions, and the obstruction determining device identifies whether the detected obstruction is a vehicle. Then, the short range detection sensor, which has a short detection range, performs the stopping of the vehicle and control of the speed reduction based on the identification result of whether or not the detected obstruction is a vehicle, and on the distance to the obstruction. 
     In addition, according to the automatic transport system of the present invention, in the above-described invention, when the long range detection sensor detects an obstruction and the obstruction determining device identifies that the obstruction detected by the long range detection sensor is a vehicle running ahead, the vehicle moves ahead until the short range detection sensor detects the vehicle, and when the short range detection sensor detects the vehicle, the vehicle is stopped. 
     Still further, when the long range detection sensor detects an obstruction and the obstruction determining device identifies that the obstruction detected by the long range detection sensor is not a vehicle running ahead, the vehicle is immediately stopped, or when the short range detection sensor detects the vehicle, the vehicle is stopped. 
     According to the automatic transport system of the present invention, different and detailed operation control is performed depending on whether the obstruction ahead is a vehicle. If the obstruction ahead is a vehicle, the forward movement is effectively performed to improve the operation efficiency. Moreover, if the obstruction ahead is not a vehicle, the trailing vehicle is stopped at a safe distance and can be set to a standby state. For example, when a worker is working on the transportation rail, the worker is not erroneously recognized as a vehicle even if the worker is detected as an obstruction. For this reason, the trailing vehicle can be promptly stopped as required by the operation, which is different from the forward movement of the vehicle. As a result, the vehicle waits at a distance without approaching the worker, and this makes it possible to ease any concern that the worker may feel if the vehicle approaches the worker. 
     Furthermore, according to the automatic transport system of the present invention, in the above-described invention, the obstruction determining device comprises a light emitting device, which is provided at a rear portion of a vehicle running ahead, and a light receiving device, which-is provided at a front portion of a trailing vehicle. Alternatively, the obstruction determining device may comprise a reflector, which is provided at a rear portion of the vehicle running ahead, and a reflection sensor for receiving a reflected light, which is provided at a front portion of a trailing vehicle. 
     Still further, according to the automatic transport system of the present invention, in the above-described invention, the front detecting device is a plurality of optical sensors, which are provided over a predetermined periphery at a front portion of the vehicle, and the obstruction determining device comprises a logic circuit for signals from the plurality of optical sensors. 
     According to the automatic transport system of the present invention, the plurality of optical sensors are arranged around a predetermined periphery near an outer peripheral of the front surface of the vehicle in the moving direction, that is the entire periphery. Then, the entire outer periphery of the vehicle moving area is irradiated in the shape of a strip with the optical beams emitted from the respective optical sensors. As a specific method, for example, in the case of a vehicle whose front surface in the moving direction is rectangular, if the strip slits are provided along the respective sides of the rectangle and fan-shaped optical beams are emitted from these slits, the entire corresponding side of the rectangle, serving as a passage area, is irradiated with the optical beams. Therefore, the strip irradiation areas of the respective sides are combined with one another, making it possible to irradiate the outer periphery of the entire vehicle moving area in the shape of a strip with the optical beams. Additionally, if a logical calculation based on the signals from the plurality of optical sensors, for example, a logical sum, is performed, it is possible to detect that the obstruction is the vehicle only when the obstruction ahead is the vehicle. 
     Still further, in an automatic transport system that comprises a plurality of vehicles according to the present invention, the vehicles can be used in an AGV, RGV, or the like other than an OHT, which runs on ceiling rails. 
     As explained above, according to the automatic transport system of the present invention, since the automatic transport vehicle detects only substantially the area though which the automatic transport vehicle moved, only actual obstructions will be detected, without fail. There is no concern that an object or a person, which do not actually impede the running of the apparatus, will be detected, or that art obstruction will not be detected, causing unnecessary stopping and damage of objects. Therefore, the automatic transport vehicle can be run safely and efficiently, so that a safe and efficient automatic production system can be constructed. 
     According to the automatic transport system of the present invention, an obstruction present at in the area though which the vehicle of the moving direction will pass can be detected with more reliability without losing the transportation efficiency of the system. Also, identification of whether a obstruction ahead is a vehicle running ahead or not is correctly performed. Then, stopping of the trailing vehicle and the effective forward movement are carried out based on the identification result. This makes it possible to further improve the productivity of the entire system as compared with a conventional OHT transport system. Moreover, when a worker is working on the transportation rail, the worker is not erroneously recognized as a vehicle even if the worker is detected as an obstruction. For this reason, the trailing vehicle can be promptly stopped as required by the operation, which is different from the forward movement of the vehicle. As a result, the vehicle waits at a distance without approaching the worker, and this makes it possible to ease any concern that the worker may feel if the vehicle approaches the worker. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, in which. 
     FIG. 1 is an outline perspective view of an OHT vehicle according to the embodiment of the present invention; 
     FIG. 2 is a conceptual view showing a state in which the front in the moving direction is detected using the OHT vehicle of FIG. 1; 
     FIG. 3 is a perspective view showing one example of a semiconductor manufacturing apparatus using an OHT vehicle of the present invention; 
     FIG. 4 is a conceptual view showing a state in which a long range detection sensor and a short range detection sensor detect an implement in an OHT system of the present invention; 
     FIG. 5A is a view showing one example of a detection range when conical beam sensors as sensor S 1  and S 2  of FIG. 1 are used, and a view showing the detection range when the vehicle is seen from the side; 
     FIG. 5B is a view showing one example of a detection range when conical beam sensors as sensor S 1  and S 2  of FIG. 1 are used, and a view showing the detection range when the vehicle is seen from the plane; 
     FIG. 6A is a view showing one example of a preferable detection range of a sensor provided to correspond to the sensor shown in FIG. 5, and a view showing the detection range when the vehicle is seen from the side-surface; 
     FIG. 6B is a view showing one example of a preferable detection range of a sensor provided to correspond to the sensor shown in FIG. 5, and a view showing the detection range when the vehicle is seen from the plane; 
     FIG. 7 is a view showing one example of a detection range when the conical beam sensors are provided around the vehicle; 
     FIG. 8 is a schematic view showing one example of a beam scan sensor; 
     FIG. 9 is an operation conceptual view of the OHT system used in the semiconductor wafer manufacturing process or the like; 
     FIG. 10 is an explanatory view showing a front detection sensor of the OHT vehicle and one example of an obstruction; 
     FIG. 11 is a conceptual view showing an OHT vehicle used in a semiconductor manufacturing apparatus; 
     FIG. 12 is a conceptual view showing a state in which the vehicle uses two front detection sensors to detect a long distance range and a middle distance range; and 
     FIG. 13 is a conceptual view showing one example of an operation state of a plurality of vehicles in the general OHT system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following will specifically describe the preferred embodiment of the automatic transport system, according to the present invention with reference to the drawings. Additionally, in the following description, the rail for running is omitted, and an OHT vehicle with a rectangular cross-sectional shape of its front surface, which is considered to be the-passage-area, is explained as an example. 
     FIG. 1 is an outline perspective view of an OHT vehicle according to an embodiment of the present invention. In the FIG. 1, at the front surface portion  2  of an OHT vehicle  1  in the moving direction, four optical sensors S 1 , S 2 , S 3 , and S 4  are arranged along the respective sides of the front surface portion  2  in the moving direction as front detection sensors. 
     In order to reserve a minimum area through which the OHT vehicle  1  passes, the optical sensor S 1 , which emits a fan-shaped beam of light, is placed along the side L 1 . Similarly, the optical sensors S 2 , S 3 , and S 4  are arranged along the sides L 2 , L 3 , and L 4 , respectively. 
     Each of the optical sensors S 1 , S 2 , S 3 , and S 4  is constituted to have a thin and rectangular slit, for example, along the portion close to each of the sides L 1 , L 2 , L 3 , and L 4 , and an infrared light source is provided in each slit, a light beam from the infrared light source (not shown) is emitted from each slit. Therefore, the respective optical beams are emitted from the respective optical sensors S 1 , S 2 , S 3 , and S 4  in the shapes of fans, and the irradiated light has a cross-sectional shape that is similar to the shape of each slit on a projection surface at a predetermined position. 
     FIG. 2 is a conceptual view showing a state in which the front in the moving direction is detected using the OHT vehicle of FIG.  1 . The same figure shows a state in which an imaginary OHT vehicle  1 ′ having the same shape as the OHT vehicle  1  is located ahead in the moving direction of the OHT vehicle  1 . 
     On the front surface portion  2  of the OHT vehicle  1  in the moving direction, the optical sensors S 1 , S 2 , S 3 , and S 4  are arranged along the sides L 1 , L 2 , L 3 , and L 4 . Then, an irradiation area m 1  is irradiated with the light beam emitted from the optical sensor S 1  along a side L 1 ′ of the Imaginary OHT vehicle  1 ′. Also, an irradiation area m 2  is irradiated with the light beam emitted from the optical sensor S 2  along a side L 2 ′ of the imaginary OHT vehicle  1 . Moreover, an irradiation area m 3  is irradiated with the light beam emitted from the optical sensor S 3  along a side L 3 ′ of the imaginary OHT vehicle  1 ′ Then, an irradiation area m 4  is irradiated with the light beam emitted from the optical sensor S 4  along a side L 4 ′ of the imaginary OHT vehicle  1 ′. 
     Moreover, lights from the optical beams reflected by these irradiation areas m 1 , m 2 , m 3  and m 4  are detected by the optical sensors S 1 , S 2 , S 3 , and S 4 , respectively. A detection area which an obstruction is detected is the strip irradiation areas m 1 , m 2 , m 3  and m 4  of the beam light expanded in the shape of fan from the respective optical sensors S 1 , S 2 , S 3 , and S 4 . 
     This makes it possible to detect obstructions in the area which is surrounded by the sides L 1 ′, L 2 ′, L 3 ′ and L 4 ′ of the imaginary OHT vehicle  1 ′ and in the passage area which is surrounded with the sides L 1 , L 2 , L 3 , and L 4  of the front surface portion  2  of the OHT vehicle  1  in the moving direction. Therefore detection can be carried out without failures. In addition, the detection area is the area of the front surface portion  2  of the OHT vehicle  1  in the moving direction and the detection of obstructions in the OHT system can be carried out extremely efficiently without the occurrence of a detection leakage or an excessive detection. 
     Additionally, in performing the actual detection of obstructions, the setting of the direction of irradiation of the optical beams emitted by the respective optical sensors S 1 , S 2 , S 3  and S 4  is contrived to as to irradiate areas which are a little wider than that passage area of the OHT vehicle  1 . However, it is desirable to avoid detection of the peripheral manufacturing apparatuses. Also, it is desirable to prevent unnecessary detections and due to mechanical shift caused by vibration lichen the OHT vehicle  1  moves. 
     Regarding the specific method for setting the optical beams, for example, the respective optical sensors S 1 , S 2 , S 3 , and S 4  can be provided with an optical guide cylinder for restricting the direction in which light is emitted. Then, the directions of the optical beams emitted from the respective optical guide cylinder are controlled to be directed slightly to the outside of the outer periphery of the imaginary OHT vehicle  1 ′. If the irradiation areas m 1 , m 2 , m 3 , and m 4  of FIG. 2 are extended to slightly outside of the sides L 1 ′, L 2 ′, L 3 ′ and L 4 ′ of the imaginary OHT vehicle  1 ′, an areas which is a little wider than the passage area of the OHT vehicle  1  can be detected. 
     In this way, if the optical sensors for emitting the fan-shaped optical beams are arranged around the front surface of the OHT vehicle  1  in the moving direction and detect only the passage area of the OHT vehicle  1  efficiently, unnecessary stops of the OHT vehicle  1  and unexpected collisions with parts or the like can be prevented, and the OHT vehicle  1  can be efficiently operated. 
     The above embodiment describes an OHT vehicle  1  whose cross-section in the moving direction is rectangular. However, the cross-sectional shape in the moving direction is not limited to a rectangle, and the present invention can be applied to any cross-sectional shape. For example, if the cross-section of the OHT vehicle in the moving direction is polygonal, the irradiation of strips of light may be provided such that the respective sides are connected to one another to made a polygonal shape. Moreover, if the cross-section of the OHT vehicle in the moving direction is an elliptical shape, irradiation of the strips of light may be provided at the entire the outer periphery of the elliptical shape. 
     Next, a description is given of the actual-use of an OHT vehicle having the aforementioned front detection sensor. FIG. 3 is a perspective view showing one example of a semiconductor manufacturing apparatus using the OHT vehicle of the present invention. 
     In the case of manufacturing a semiconductor device by the semiconductor manufacturing device illustrated in FIG. 3, the aforementioned OHT vehicle is used to automatically transport semiconductor craters among various kinds of apparatuses. Generally, semiconductor wafers such as silicon wafers are transported by moving the OHT vehicle back and forth among various kinds of semiconductor manufacturing apparatuses (for example, a wafer processing apparatus, a storage apparatus, a workbench, a buffer apparatus, and so on), whereby the semiconductor devices are manufactured via numerous processes. 
     The process in which the OHT vehicle transports the semiconductor wafers is explained with reference to FIG.  3 . An OHT vehicle  12 , which hangs on a rail  11  mounted on a ceiling of a clean room (not shown), runs freely, and a wafer carrier  14  on which semiconductor wafers  13  are leaded is transferred between the respective semiconductor manufacturing apparatuses  15  or between a semiconductor manufacturing apparatus  15  and a stocker  16 , and various kinds of processes are carried out on the wafers. 
     The OHT vehicle  12  shown in this figure comprises a ruining section  12   a  that runs along the rail  11 , a hanging section  12   b  that is provided at a lower portion of the running section  12   a , and a hand  12   c  that hangs from the hanging section  12   b  to be movable up and down. Specifically, the wafer carrier  14  that is placed on a load port  15   a  of the semiconductor manufacturing apparatus  15  is held by the hand  12   c . Then, the hanging section  12   b  moves up the hand  12   c , thereafter the OHR vehicle  12  runs along the rail  11  by the running section  12   a.    
     In manufacturing the semiconductor device, a plurality of OHT vehicles  12  move back and forth between the plurality of semiconductor manufacturing apparatuses  15  arranged in parallel along the rail  11 , and hold the wafer carrier  14  from the load port  15   a  of each semiconductor manufacturing apparatus  15  to be transferred to the load port  15   a  of another semiconductor manufacturing apparatus  15 . 
     In transporting the wafer carrier  14 , the OHT vehicle  12  first runs along the rail  11  and is stopped at the portion above the load port  15   a  having the wafer carrier  14  to be transported thereon. Then, the hand hanging section  12   b  is lowered to move the hand  12   c  down, and this hand  12   c  holds the wafer carrier  14 . Then, the hand hanging section  12   b  is hoisted up to remove the wafer carrier  14  from the load port  15   a  and to be the highest position. Thereafter, the OHT vehicle  12  is run again. 
     Then, the OHT vehicle  12  is stopped at another semiconductor manufacturing apparatus  15 , which performs the next process, or the load port  15   a  of the stocker  16 . Then, the hand hanging section  12   b  is lowered to lower the hand  12   c  so that the wafer carrier  14  is mounted on the load port  15   a . Thereafter, the hand  12   c  releases the wafer carrier  14 . Then, the hand hanging section  12   b  is hoisted up to raise the hand  12   c , and the operation proceeds to the a next transporting operation. 
     Incidentally, the aforementioned transport system has a vehicle providing the front detection sensor (not shown) which detects an obstruction in the minimum range with no obstruction to movement of the OHT vehicle  12 . Therefore in the transport system can prevent the OHT vehicle  12  from contacting the doors of various kinds of apparatuses placed in the moving direction of the OHT vehicle  12 , adjacent parts or the like, and from being stopped after detecting doors and parts even though they are not obstructing the movement of the OHT vehicle  12 , since the transport work carry out in a small area. The detecting by the front detection sensor (not shown) allows the OHT system of the semiconductor manufacturing apparatus to perform efficient processing of the semiconductor wafer. This makes it possible to further improve the production efficiency of semiconductor devices or the like. 
     The aforementioned embodiment is one example to describe the present invention. However, the present invention is not limited to the above embodiment, and various modifications may be possible within the gist, of the invention. Namely, the aforementioned embodiment described the case in which the front detection sensor is provided on an OHT vehicle that runs along a ceiling rail. However, the present invention is not limited to this. For example, it is possible to provide the front detection sensor on an AGV that runs on the floor or an RGV that runs on a rail. The AGV and the RGV are used in process lines in which materials are transported and finished products are moved without human intervention in an automated factory. The provision of the front detection sensor of the present invention prevents the AGV and the RGV from being stopped unnecessarily and from colliding with the other parts and breaking them. 
     Next, an explanation is given of the operation system of the present invention in the case that a plurality of vehicles, each having the aforementioned optical sensors, run on the rail. FIG. 4 is a conceptual view showing a state in which a long range detection sensor and a short range detection sensor detect an obstruction in the OHT system of the present invention. In addition, the long range detection sensor device a sensor which has the longer detection range than that of the short range detection sensor. 
     In FIG. 4, in the OHT system of the present invention, a-front vehicle  4  and a trailing vehicle  5  hang on a rail  3  and run in the advancing direction indicated by the arrow in the figure. Moreover, a stepladder  6  with a height which does not obstruct the movement of vehicles  4  and  5 , is placed in pass of the respective vehicles  4  and  5 . Moreover, each of the vehicles  4  and  5  has the optical sensors at its front surface as shown in FIGS. 1 and 2, although these sensors are not illustrated in FIG.  4 . Further, the trailing vehicle  5  has a vehicle determination sensor (light receiving device)  7   a  as an obstruction determining device, which determines whether the obstruction ahead is a vehicle or not, on its front surface. The front vehicle  4  has a vehicle determination sensor (light emitting device)  7   b  on its rear surface. 
     Now, a description is given of a case in which the trailing vehicle  5  is running while detecting ahead using the optical sensor (not shown). The optical sensor of the vehicle  5  has a long range detection sensor and a short range detection sensor. The long range detection sensor switches among two range, i.e., of the long range P 1  and the medium range P 2 , making it possible to detect an obstruction. For example, the long range P 1  can be used to detect obstructions at a distance of 2 to 3 m, and the medium range P 2  can be used to detect obstructions at a distance of 0.5 to 1.5 m. Moreover, a short range detection sensor can detect obstructions in a short range P 3 , which is shorter than the middle range P 2  (that is, 0.5 to 1.5 m). 
     Firstly, when the trailing vehicle  5  advances, the long range detection sensor, which is provided on the vehicle  5 , detects the obstruction (that is, front vehicle  4 ) with in the long range PI. As a result, if the vehicle  5  continues to advance while reducing its speed, the long range detection sensor detects the obstruction (that is, vehicle  4 ) within the middle range P 2 . Thereafter, when the vehicle determination sensor (light receiving device)  7   a , which the vehicle  5  has, receives an optical signal from the vehicle determination sensor (light emitter)  7   b  of the vehicle  4 , which is the obstruction ahead, and thereby confirms that the obstruction ahead is the vehicle  4 , and the rear vehicle  5  further reduces its speed. Then, the vehicle  5  advances until the short range detection sensor of the vehicle  5  detects the vehicle  4  within the short range P 3 . Thereafter, at the point when the short range detection sensor of the vehicle  5  detects the vehicle  4  at the short range P 3 , the vehicle  5  stops. For example, the short range P 3  is set to about 0.2 to 0.1 m such that the back vehicle  5  is stopped at the shortest range at which the vehicle  5  does not collide with the front vehicle  4 . 
     If there is a station (transfer port of assembly apparatus) which has made a transfer request to the vehicle  5  which is located at a position which is before the vehicle reaches within the short range P 3 , the vehicle  5  can be stopped at the position of the station. When the vehicle  5  reaches the station which has made the transfer request to the vehicle 0.5 while the long range detection sensor is detecting in long range P 1  or the medium range P 2  and braking is performed, it is possible to stop the vehicle  5  at the corresponding station before the short range detection sensor detects in the short range P 3 . 
     Moreover, if the vehicle determination sensor (light receiving device)  7   a  provided on vehicle  5 , cannot confirm that an obstruction ahead is a vehicle  4  when the long range detection sensor provided on the vehicle  5  detects the obstruction within the long range P 1  or the middle distance P 2  while the trailing vehicle  5  is advancing. When no optical signal is received from the vehicle determination sensor (light emitting device)  7   b  of the front vehicle  4 , it is determined that the obstruction ahead is not a vehicle. 
     In this case, since the detected obstruction is, for example, the stepladder  6 , the vehicle  5  can be immediately stopped or the vehicle  5  can be stopped after advancing the vehicle  5  close to the stepladder  6  according to the presetting of the OHT system. The above embodiment describes the case in which the long range detection sensor is operated in the two steps of the long distance P 1  and middle distance P 2 . However, the long range detection sensor may be operated to detect a predetermined distance in only one step. Moreover, the number of long range detection sensors provided at the front surface of the vehicle is not limited to one. Namely, a plurality of sensors may be provided as the optical sensors shown in the aforementioned FIG.  1 . 
     Herein, specific embodiments of the vehicle determination sensor, which is an obstruction determining device, will be described in more detail. Regarding the first embodiment, as illustrated in FIG. 4, a vehicle determination sensor (light receiving device)  7   a  is provided at the front portion of each vehicle and a vehicle determination sensor (light emitting device)  7   b  is provided at the rear portion. When the vehicle determination sensor (light receiving device)  7   a  of the front portion of the trailing vehicle  5  receives an optical signal from the vehicle determination sensor (light emitting device)  7   b  of the rear portion of the front vehicle  4 , it is determined that the obstruction ahead is a vehicle. 
     Moreover, in the second embodiment of the vehicle determination sensor, a reflector is provided at the rear portion of the front vehicle  4 , and a reflection sensor, which receives an optical signal from the reflector, is provided at the front portion of the trailing vehicle  5 . When the reflection sensor of the trailing vehicle  5  receives the optical signal, it is determined that the obstruction ahead is a vehicle. When the reflecting sensor of the trailing vehicle  5  receives no optical signal, it is determined that the obstruction ahead is not a vehicle. 
     Furthermore, regarding the third embodiment of the vehicle determination sensor, as described in the aforementioned FIG. 1, the plurality of sensors are arranged along the outer periphery of the vehicle and the plurality of sensors operate on the principle of an AND operation making it possible to more reliably recognize that the obstruction ahead is the vehicle. In other words, as mentioned in FIG.  1  and FIG.  2 . concerning the optical sensors, that is, obstruction detection sensors, four optical sensors S 1 , S 2 , S 3 , and S 4  are provided along the respective sides of the front surface portions in the moving direction of each vehicle. Then, the sensors are designed to detect the outer peripheral area of a vehicle running ahead. Therefore, when an area different from this area is detected, it is determined that the obstruction ahead is not the vehicle. 
     In other words, based on the AND condition applied to the four optical sensors S 1 , S 2 , SB, and S 4 , only when the signals are sent by all optical sensors S 1 , S 2 , S 3 , and S 4 , it is determined that the obstruction ahead is a vehicle. Then, when no signal is sent by any one of the optical sensors, it is determined that the obstruction ahead is not the vehicle. Furthermore, the detection of a logical sum using a plurality of sensors in this way leads to the effect that the trailing vehicle is effectively moved forward. In addition, even if the plurality of vehicle determination sensors  7   a  and  7   b  of FIG.  4  and logic such as an OR condition are used, there is the effect that the trailing vehicle is effectively moved forward. 
     Note that, as specific embodiments of the above-described long range detection sensor and the short range detection sensor, there are conical beam sensors, which output long conical beams, beam scanning sensors, which scans beams, and the like. 
     FIG. 5 is a view showing one example of using conical beam sensors as sensors S 1  and S 2  of FIG. 1 to detect the upper end and lower end. FIG. 5A is a view showing the detection range when the vehicle is seen from the side surface, and FIG. 5B is a view showing the detection range when the vehicle is shown upper side. 
     Thus, when the conical beam sensors are used as sensors S 1  and S 2 , the detecting range has a conical shape expanding widely in a width direction and thinly in a height direction. 
     Moreover, FIG. 7 shows one example of a detection range. In this figure, the conical beam sensors shown in FIG. 5 are arranged near the outer periphery of the vehicle  41 . 
     Also, FIG.  6 A and FIG. 6B are views, each showing examples of the detection range of the conical beam sensor. As illustrated in these figures, it is preferably that the sensor has a wide detection range in which obstructions near the vehicle are detected. 
     FIG. 8 is a schematic view showing one example of the beam scan sensor. This figure shows a state in which scanning with rays of light emitted from an LED  42 , which is provided at the front surface of an AGV  41  that runs on the floor surface, is performed for tong range and short range detection. For example, a semi-circular field  43  is scanned with rays of light with a wavelength of λ=870 nm emitted from the LED  42  at a step  9  (angle (162°). Then, coordinates are calculated based on a distance measurement and the step angle so as to detect an obstruction. Also, the detection area can be optionally selected, and the setting of the detection area can be carried out by any method such as a volume operation or an operation of a personal computer. For example, the detection area is set by the operation of a personal computer, making it possible to optionally switch the areas from among seven patterns. 
     The aforementioned embodiment is one example to describe the present invention. However, the present invention is not limited to the above embodiment, and various modifications may be possible within the gist of the invention. Namely, the aforementioned embodiment describes the case in which the front detection sensor is provided on an OHT vehicle that runs along a ceiling rail. However, the present invention is not limited to this. For example, it is possible to provide the front detection sensor on an AGV that runs on the floor or on an RGV that runs on a rail. AGV and RGV&#39;s are used in process lines in which materials are transported and finished products are moved without human intervention in an automated factory. The provision of the front detection sensor of the present invention prevents AGV&#39;s and RGV&#39;s from being stopped unnecessarily and from colliding with the other parts and breaking them. 
     Although the preferred embodiments of the present invention have been described in detail, it should be understood that various changes, substitutions and alternations can be made thereto without departing from spirit and scope of the inventions as defined by the appended claims.