Abstract:
A guidance system for guiding an automated guided vehicle along a pathway is disclosed which includes rails for holding the AGV to the generally straight portions of the pathway and a non-rail guidance system for directing the AGV from the end of a first rail segment to the beginning of another. The rails provide the necessary level of control to allow an unmanned vehicle to move at high speeds while the non-rail guidance system eliminates the need for the rail switches that would otherwise be needed to shift a rail guided vehicle from one rail portion to another.

Description:
[0001]    The present invention is directed toward a guidance system for guiding an automated guided vehicle (AGV) along a pathway, and more specifically, toward a guidance system for an AGV that uses rails to guide the AGV through first portions of a pathway and a non-rail control system to guide the AGV through second portions of the pathway.  
         BACKGROUND OF THE INVENTION  
         [0002]    There are two general types of AGV guidance systems, rail systems and non-rail systems. In the first type of system, a pathway is formed out of rails. These rails may support an AGV or merely guide the AGV&#39;s wheels as they roll along the ground. In a non-rail system, the AGV may include detectors for detecting and following a wire in the ground that marks out a pathway, or a controller for following a set of commands to navigate between various types of reference markers. Each of these systems has certain advantages and drawbacks, and the choice of which system to use is normally based on many factors such as the needs of the particular user and the environment in which the system will operate.  
           [0003]    Rail guidance allows for precise control over the position of an AGV. Where only a limited number of pathways are needed, and where these pathways do not need to be changed frequently, rail guidance offers a relatively simple method of keeping an AGV on a selected path. One of the biggest drawbacks to rail guidance, however, is that switches are needed to direct AGV&#39;s from one pathway to another. These switches are relatively costly and include moving parts that can wear out. In addition, each switch must be connected to a power source, a sensor for determining the position of the switch, and a controller for moving the switch from one position to another at appropriate times. The switches are often connected to a controller and to one another by a series of wires that run along the pathways, and these wires are expensive to install and maintain. Furthermore, the wires must be reconfigured each time the system is modified. Another disadvantage to such systems is that the rails themselves are generally raised off the ground and can interfere with the free movement of people and other vehicles.  
           [0004]    Non-rail guidance systems offer increased design flexibility since pathways can be changed by reprogramming the AGV&#39;s or their controllers and without removing and re-laying rails. Moreover, because each vehicle receives or is programmed with instructions concerning the pathway to follow, switches are not needed to shift a vehicle from one path to another. However, because AGV&#39;s in such a system can stray from their pathways, extra care is required to make sure that each AGV is in its intended location and often this entails virtually constant communication with each AGV in the system. The quality of the communication link and the speed at which information about the AGV&#39;s and their positions can be processed also limits the maximum rate of travel of these systems. Moreover, collision avoidance becomes more complicated when AGV&#39;s travel along pathways that are not defined by rails. The need to constantly monitor and control a large number of AGV&#39;s, and to keep them on course and to avoid collisions, requires a significant amount of processing power which can make non-rail guidance systems more complex and expensive to operate than rail systems.  
           [0005]    For high speed transport, that is for speeds in the range of 2200 feet per minute, rail guidance has traditionally been the only practical method for guiding an AGV. This is in part due to a perception that it is unsafe to operate vehicles at high speeds without physical path constraints and partly due to control problems. For example, the servo-control mechanisms used to steer AGV&#39;s often cannot respond quickly enough to the changing location of a guide wire in order to control a fast-moving vehicle. In addition, the signal to noise ratio of the position sensors may be too low to allow them to accurately sense the presence of a wire in the ground or to communicate reliably with a central controller when moving rapidly. Therefore, in applications where high speeds are needed, it has heretofore been necessary to use rail based control with all of its attendant drawbacks.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention addresses the above and other problems by providing an AGV control system that uses rails for controlling an AGV through high speed portions of a pathway and non-rail controls for guiding the AGV through turns and other low speed portions of the pathway. Beneficially, this system eliminates the need to use track or rail switches. The result is a control system for AGV&#39;s that allows vehicles to operate at the same high speeds as rail-based systems with a high degree of safety. In addition, this system does not require the monitoring of switches or constant communication with each vehicle because the vehicles are physically constrained to a guidepath over the majority of the pathway that they traverse.  
           [0007]    In the preferred embodiment, a pathway is designed which connects various loading and unloading sites and which includes straight sections, branches, and curves. Guiding rails are laid out along the straight-aways and gaps are left at the curves and near branches. A non-rail guidance system, such as an in-the-ground wire, is used to continue the pathways between the ends of the rails. The AGV&#39;s are provided with steering arms for engaging and following the rail or track sections and sensors for following the in-the-ground wires. Machine-readable tags are positioned at various points along the rail sections to provide information concerning the identity and length of the rail, and the radii of the turns which might be made at the end of the section. The AGV&#39;s also include sensors for reading the in-the-ground wires.  
           [0008]    In operation, a central controller provides an AGV with instructions for traveling from an origin to a destination which instructions tell the AGV which rails to follow and how to turn at the end of each segment. The AGV is inserted into the system at the entrance end of a straight rail section and moved along the rail until it passes an initial reference marker that tells the vehicle where it is. If this location is consistent with its instructions for reaching a destination, it will accelerate to a high speed and travel along the rail looking for additional reference markers. One of these reference markers will tell the AGV the distance to the end of the rail. If the AGV&#39;s instructions are to continue in a straight line to the next rail section, the AGV will lock its steering wheels, maintain its speed, and travel past the end of the first rail and onto the entrance to a second rail collinear with the first rail. It will then continue along that rail section until another reference marker is encountered. Alternately, if the AGV&#39;s instructions provide that the vehicle is to make a 90 degree turn to the right at the end of the first rail, the AGV will begin to decelerate when the reference marker indicates that the end of the rail is approaching. The AGV will decelerate to a safe turning speed by the time it reaches the end of the rail and then use its onboard sensors to follow the in-the-ground wire that leads from the end of one rail segment to the beginning of the next. The AGV travels onto the second rail and resumes its high speed. This process continues until the vehicle reaches its final destination.  
           [0009]    It is therefore the principal object of the present invention to provide an improved guidance system for automated guided vehicles.  
           [0010]    It is another object of the present invention to provide a switchless rail-based AGV guidance system.  
           [0011]    It is a further object of the present invention to provide an AGV guidance system that uses a first guidance mechanism for guiding the AGV along high speed portions of the pathway and a second guidance mechanism for guiding the AGV along low speed portions of the pathway.  
           [0012]    It is still another object of the present invention to provide an AGV guidance system to which additional branches may be easily added.  
           [0013]    It is still a further object of the present invention to a guidance system for high speed AGV&#39;s which integrates guide rails into a reference marker based system.  
           [0014]    It is yet another object of the present invention to provide an AGV guidance system that uses physically constraining guide members to define the high speed portion of an AGV pathway and reference markers that can be sensed by the AGV to define turns and other low speed portions of the pathway.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    These and other objects and advantages of the subject invention will be better appreciated after a reading of the following detailed description of several preferred embodiments of the invention together with the following drawings of which:  
         [0016]    [0016]FIG. 1 is a plan view of various pathways along which an AGV may be guided by a guidance system according to the present invention;  
         [0017]    [0017]FIG. 2 is a sectional elevational view of an AGV following a rail in one of the pathways shown in FIG. 1;  
         [0018]    [0018]FIG. 3 is a plan view of the entrance end of the rail shown in FIG. 2 and the leading portion of the AGV steering arm about to engage the rail;  
         [0019]    [0019]FIG. 4 is a plan view, partly in section, of the steering system of the AGV of FIG. 2;  
         [0020]    [0020]FIG. 5 is an elevational view of a second embodiment of the present invention wherein the rail of FIG. 2 is equipped with an electrical bus bar for providing power to the AGV;  
         [0021]    [0021]FIG. 6 is a plan view, partly in section, of the front portion of an AGV equipped with contacts for drawing power from the rail shown in FIG. 5;  
         [0022]    [0022]FIG. 7 is a sectional elevational view of a third embodiment of the present invention in which portions of a pathway are defined by a pair of spaced apart guiding rails; and,  
         [0023]    [0023]FIG. 8 is a sectional elevational view of a fourth embodiment of the present invention in which portions of a pathway are defined by a pair of spaced-apart guiding rails equipped with electrical bus bars for providing power to the AGV.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    Referring now to the drawings, wherein the showings are for purposes of illustrating several preferred embodiments of the subject invention only and not for purposes of limiting same, FIG. 1 shows a number of rail segments  10 ,  12 ,  14 , and  16  for guiding an AGV along a pathway on a paved surface  18 . The shape of the pathway will vary depending on the environment in which this system is used; however, each straight portion of the pathway will be defined by a rail segment. Each segment includes an entrance end  20  having a tapered portion  22  and an exit end  24  having a tapered portion  26 . Furthermore, each entrance end includes a flared guide  28  the purpose of which will be described hereinafter. As can be seen in FIG. 2, each rail comprises a base  30  connected to ground  18  by bolts  32  or other suitable means, a vertical web  34  extending perpendicularly from base  30 , and a top strip  36  supported on top of web  34  and including spaced apart side walls  38  generally perpendicular to surface  18 . The rails are preferably formed from steel, although other materials can be used if they possess sufficient strength to maintain their shape and under the operating conditions to be described herein.  
         [0025]    [0025]FIG. 3 shows an AGV  40  having two front wheels  42  supported on axles  44  and two rear wheels  46  supported on an axle  48 . The AGV is powered by an onboard battery (not shown). A steering arm  50  extends forwardly of front axles  44  and is pivotally connected to the frame of the AGV by a pin  52  at its fixed end  54 . The free end  55  of steering arm  50  includes two spaced-apart guide rollers  56  depending therefrom which rollers have parallel axes  58 . Guide rollers  56  are spaced apart by a distance equal to the width of top strip  36  so that the rollers will engage and roll along side walls  38  of strip  36  as steering arm  50  traverses the guide rail. A pair of tie rods  60  are connected between steering arm  50  and front axles  44  so that the movement of arm  50  about pin  52  as the steering arm follows the guide rail causes wheels  42  to turn. In this manner, wheels  42  are kept generally parallel to the rail to cause the AGV to follow the rail. Alternately, an AGV can be provided with steerable rear wheels which are coupled to the front wheels to move therewith or which are connected to a second steering arm which causes the rear wheels to follow the rail in the same manner as the front wheels.  
         [0026]    There are many types of AGV&#39;s and AGV control systems. Some AGV&#39;s follow an in-the-ground wire using a sensor to sense the magnetic field generated by a current in the wire. The steering system of these vehicles is connected to the sensor, and the AGV steers itself to keep the sensor centered over the wire. When the AGV is given a command to start, it travels along the wire until it reaches a branch or receives a command to stop. At a branch, the AGV follows whatever instructions it is given concerning the appropriate branch to follow. Such a system is shown in U.S. Pat. No. 5,434,781, for example, and the disclosure of this patent is incorporated herein by reference. Other AGV&#39;s follow commands sent from central controllers which tell the AGV where it should go and the speed it should travel. Reference markers are sensed by the AGV as it passes near or between them, and these markers are used to determine the position and/or bearing of the AGV to help it follow the proper course. An example of such a control system is disclosed in U.S. Pat. No. 4,866,617 which is also incorporated herein by reference. In the present invention, the exact type of non-rail guidance system is not important. Any such system which is capable of guiding an AGV may be used, as long as it can cause the AGV to start, stop, and travel over particular path. In the preferred embodiment AGV  40  follows a wire buried beneath ground  18  to guide it from the exit end of one rail segment to the entrance end of another, but other guidance systems would work equally well, and the use of a wire-based system should in no manner be read as a limitation to the possible methods of guiding the AGV.  
         [0027]    Each of the rail segments includes at least one informational marker which can be read by a reading device onboard the AGV. In the preferred embodiment, the markers are bar codes and the reading devices is an optical scanner. Alternately, the markers could be magnets or small transponders for communicating with each AGV as it passes by the marker. Rail segment  10  includes a first marker  62  which is read by optical reader  64  onboard the AGV. This marker is located immediately downstream from the entrance end of guide rail  10  and it includes information identifying the particular rail segment that it is attached to. The AGV and/or a central controller for directing the AGV know where on the pathway a given AGV should be at a given time. If the information from the marker indicates that the vehicle is not on the correct rail segment, appropriate corrective action can be taken such as routing the vehicle back to its correct path or shutting down the system if necessary. Assuming that the AGV reads marker  62  and confirms that it is on the correct pathway, it will accelerate to a high speed such as 2200 feet per minute and travel along the rail. Guide rollers  56  straddle top strip  36  of the guide rail as AGV  40  traverses the rail and hold front wheels  42  parallel to the rail. If the pathway marked by the rail shifts slightly to the left or right for example, the guide rollers will cause steering arm  50  to follow the rail and turn wheels  42  in the correct direction to keep the AGV on the pathway. While the pathways marked by guide rails are generally linear, it is possible for them to deviate slightly to the left or the right or to describe very large radius turns, the radius depending on the speed of the vehicle. Generally, any such path segment may be treated as linear for purposes of discussion if it can be traversed safely by AGV  40  at full speed.  
         [0028]    The AGV proceeds along rail  10  until it encounters another rail marker. This marker could be located toward the middle of the rail and provide a positional update for the AGV. By reading this marker, the AGV confirms that it is at a certain location at a certain time and this information can be passed along to the central controller. Eventually, the AGV will reach a terminal marker such as marker  66  on rail  10 . This marker tells the AGV the distance to exit end  24  of the particular rail segment. If the AGV&#39;s instructions require it to execute a turn at the end of rail segment  10 , it will begin a deceleration routine to reduce its speed to a safe speed for negotiating the turn. Once the AGV comes to exit end  24  of rail segment  10 , it will be traveling at a low enough speed that it can be controlled safely by a non-rail guidance system such as one which includes an in-the-ground wire.  
         [0029]    [0029]FIG. 1 shows several in-the-ground wires  68  marking out various paths for an AGV to follow from the exit end of rail segment  10 . A first path a - b extends between rail segment  10  and rail segment  12 ; a second path a - c extends between rail segment  10  and rail segment  14 ; and a third path segment a - d extends between rail segment  10  and rail segment  14 . If the pathway that AGV  40  has been instructed to follow requires it to travel from rail segment  10  to rail segment  14 , it will follow pathway a - c through a 45 degree curve to the right and then through a 45 degree turn to the left until it is facing the entrance of segment  14 . The pathway a - c is shaped such that the AGV approaches segment  14  with free end  55  of steering arm  50  generally aligned with rail  14  and pointing into flared guide  28 . When free end  55  makes contact with guide  28 , the guide centers steering arm  50  on rail  14  and helps ensure a proper engagement between the steering arm and rail  14  even if AGV  40  has deviated slightly from guide path a - c. Tapered end  22  of rail  14  also helps to correct for any minor deviations from the proper path by pushing free end  55  in one direction or the other to center the steering arm over the guide rail. Once the steering arm has engaged rail  14 , AGV  40  proceeds along the rail until it comes to informational marker  70  which contains information identifying rail segment  14 . AGV  40  then accelerates and proceeds along segment  14  until another junction or its final destination is reached, which information will be conveyed by additional markers on the rail. In order to proceed from rail  10  to rail  16 , the AGV would proceed as above but along pathway a - d instead of pathway a - c  
         [0030]    The spacing between the exits and entrances of nearby rails is determined by the width and turning radius of the vehicles using the system. In order to provide for smooth transfer from one rail to another, the in-the-ground wires  68  are arranged so that straight path segments join radiused portions tangentially, and the spacing between the entrance and exits ends of the rail segments must be adequate to accommodate such pathways. In addition, because the rails are raised off ground  18 , the spacing must be sufficient to allow a vehicle to make the required turns without its wheels passing over one or more of the rail sections. While the spacing can be made arbitrarily large, it is preferred to keep it to a minimum for purposes of efficiency because the AGV&#39;s can only travel at high speeds along the rail portions of the system.  
         [0031]    If no turns are to be made at a given junction, such as when an AGV travels pathway a - b from rail  10  to rail  12 , the AGV can be made to slow down as above and follow a linear guide wire. However, it is preferable to keep the vehicle operating at a high speed whenever possible. Therefore, in the preferred embodiment, when AGV  40  is to travel from rail  10  to rail  12 , it will read the information from marker  66  and proceed straight ahead at its current speed. It will also lock steering arm  52  so that the AGV will continue to travel in a straight line once it passes exit end  24  of rail  10 . Because the distance between exit end  24  of segment  10  and entrance end  22  of segment  12  is not great, on the order of twice the AGV&#39;s length for example, the vehicle will not veer off its course significantly over this distance and any minor deviation will be corrected by flared guide  28  and tapered portion  26  of rail  12  interacting with the steering arm. When first marker  72  on rail segment  12  is read to confirm that the AGV is on course, the steering mechanism is unlocked and the AGV proceeds to follow the rail segment as before.  
         [0032]    The vehicle remains in communication with the central controller and/or is programmed to stop if it does not read the initial marker on rail  12  within a predetermined length of time. The vehicle can thus be rapidly stopped if it does not properly engage rail  14 . If this occurs, however, it is likely that the left and right side tires of the vehicle would still be on opposite sides of rail  14  even if the steering arm had not properly engaged the rail. The tires on the vehicle could not easily roll across rail  10  and thus the deviation of the vehicle from its path would be minimal. In this manner, the vehicle is prevented from straying too far from its proper pathway and could safely be stopped once the problem was detected. Of course, in situations where it is absolutely critical to retain accurate control over the position of the vehicle at all times, such as when the AGV could cause significant damage if it missed its connection with rail  14 , the vehicle should be slowed to a speed at which it can follow the in-the-ground wire which defines path a - b and travel from segment  10  to segment  14  by following this pathway.  
         [0033]    [0033]FIGS. 5 and 6 show a second embodiment of the subject invention wherein the same reference numerals are used to identify parts common to the first embodiment. In this embodiment, the AGV receives power from a power bus bar  76  mounted in an insulator  78  that runs along the length of each rail segment. A pair of wiper arms  80  depend from steering arm  50  for drawing power from bus bar  76  in a well known manner. By delivering power to the AGV in this way, the need to carry a large battery onboard the AGV is eliminated. However, a small battery or capacitor (not shown) must be included to provide power for the AGV between rail segments. In addition, the problems associated with routing power bus bars through rail switches is also eliminated by eliminating the switches themselves from the system.  
         [0034]    [0034]FIG. 7 shows a third embodiment of the invention in which the straight portions of the pathway are defined by a pair of spaced apart side rails  82 . In this embodiment, a steering bar  84  extends perpendicularly from either side of steering arm  50  and guide rollers  56  depend from the opposite ends of the steering bar. The length of the steering bar  84  is slightly less than the spacing between rails  82  so that guide rollers  56  engage and roll along the side rails to hold AGV  40  to its path.  
         [0035]    [0035]FIG. 8 shows a fourth embodiment of the subject invention which is identical to the third embodiment described above except that side rails  82  are provided with power bus bars  86  supported in insulators  88  running the length of each of the side rails. Wiper arms  90  depend from steering bar  84  to contact the bus bars and draw power therefrom in a well known manner.  
         [0036]    While the subject invention has been described in terms of several preferred embodiments, it should be understood that various modifications and additions to these described embodiments may be made without exceeding the scope of this invention. For example, the particular rail guidance system can be varied as can the particular non-rail guidance system, as long as two such systems are used together in combination as disclosed and claimed. All such modifications and additions form part of the present invention to the extent that they are covered by the several claims appended hereto.