Abstract:
A system to allow track-guided vehicles to avoid collisions with one another in straight track and curved track situations utilizing vehicle mounted devices and without the need for additional active traffic control devices. The system includes track-guided vehicles equipped with a plurality of selective sensors on the front and identifying reflective elements on the rear. Reflective strips mounted on the inner face of curved track allow the selective sensors to detect targets around curves. Retroreflective sensors coupled with corner cube reflective material are a preferred set of sensor and target implementations. The placement of the sensors and reflective elements is specified such that a calibrated system limits the range of the system and is easily maintained.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Applicant claims priority under 35 U.S.C. §119(e) to U.S. provisional application Ser. No. 60/120,509 filed Feb. 18, 1999. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     N/A 
     BACKGROUND OF THE INVENTION 
     Electrically powered vehicles are often used in manufacturing and warehouse environments for transporting and manipulating articles of manufacture. Such vehicles are desirable in such environments due to their clean operation and low noise. Often such vehicles are propelled along a fixed rail or track, allowing precise control of movement along a predetermined path. 
     In particular, computer controlled materials transport systems are known for moving materials among various work stations of a facility. Such systems are employed, as an example, in semiconductor fabrication facilities for moving semiconductor wafers to successive work stations. In such a wafer transport system, a monorail track is routed past the work stations and a plurality of electric vehicles are mounted on the track and moveable there-along for delivering wafers to successive work stations and for removing wafers therefrom after requisite processing operations have been accomplished. The track is composed of interconnected track sections that usually include one or more routing sections or modules that are operative to provide plural paths along the track. 
     The vehicles on the track can operate in two modes—connected or semi-independent. In connected operation, a central controller, usually a computer, assigns destinations to vehicles and monitors operation of the whole system even when the vehicles are not at a station. The central controller monitors for collisions, obstacles and other extraordinary conditions, issuing commands to the vehicles to avoid undesired actions. While this mode allows more complex responses to conditions, it requires constant communication with the vehicles, a more powerful central controller and may have less flexible response to changing conditions. 
     In semi-independent mode, a central controller dispatches the vehicles and controls them when they are at a station but does not monitor the real-time operation of the system. The vehicles and/or track have facilities built in to allow the vehicles to sense their condition and respond it. This system requires some intelligence in the vehicles and may require expensive sensors to detect operational and extraordinary conditions. 
     Even when tracks are mounted overhead, obstacles such as hanger poles, manufacturing equipment, tools, walls and maintenance personnel can be present. The semi-independent vehicles need to sense and protect the payload from collisions with such obstacles. The avoidance of these obstacles is well known in the art. 
     The avoidance of other vehicles on the track has been accomplished in a number of ways; the track has been regarded as a number of zones and only one semi-independent vehicle may occupy a zone at one time, semi-automatic vehicles have been fitted with radar like capabilities and the intelligence to compute when collisions are likely, and semi-independent vehicles have treated obstacle vehicles like any other obstacle and stopped themselves. These alternatives have increased the installation cost of the system and may not allow a tailored response to other vehicles. 
     Curves in the track pose particularly difficult problems for semi-independent operating vehicles. Active traffic control devices have been needed at corners to assure that collisions are avoided near these features. 
     SUMMARY OF THE INVENTION 
     The invention allows track-guided vehicles to avoid collisions with one another in straight track and curved track situations utilizing vehicle mounted devices and without the need for additional active traffic control devices. The system is based on two complementary sensor systems, one to detect all obstacles and act to avoid track obstructions and the other, based on a sensor/target configuration, to detect other vehicles and prevent collisions between vehicles while protecting the payload. The system uses four special polarized retroreflective sensors and tuned targets to detect vehicles. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a plan view of a track-based transport system; 
     FIG. 2 is a top view of a vehicle showing the location of selective sensors, wideband sensors, and target tape; 
     FIG. 3 is a diagram of the operation of a selective sensor; 
     FIG. 4 is detail of the operation of a retroreflective sensor/target combination of the invention; 
     FIG. 5 is a diagram of the retroreflective sensor not detecting an ordinary obstacle; 
     FIG. 6 is a side view of a preferred embodiment of placement of sensors and highly reflective tape. 
     FIG. 7 is a top view of the operation of the system on a curved track; 
     FIG. 8 is a top view illustrating the more complete coverage provided by adding a secondary sensor; and 
     FIG. 9 is a flow chart of the logic utilized in activating and deactivating each secondary sensor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a simplified version of a track-based transport system as used in a manufacturing environment. The track  10  runs past processing stations  12 . Vehicles  14  deliver material to be processed and retrieve the finished article to deliver to the next station. The vehicles must not collide with each other. If vehicle  4  is delivering material to processing station  12 B, vehicle  4  is stopped, thereby blocking the track. Vehicle  3 , following vehicle  4 , must stop before colliding with vehicle  4 . Vehicle  2 , around the curve from vehicle  3 , must detect that vehicle  3  has stopped in sufficient time to prevent colliding with vehicle  3 . Vehicle  1  can travel until in the vicinity of station  12 A before it too will need to stop to avoid a collision with vehicle  2 . 
     Once vehicle  4  has moved out of collision range, vehicle  3  can restart. Similarly vehicles  2  and  1  can restart when their respective obstacle vehicle is out of range. Knowing that possible obstacle vehicles will be on the same track as a following vehicle allows efficient utilization of sensors when the objective is limited to identifying obstacle vehicles rather than all obstacles. 
     The invention uses selective sensors, which respond only to reflections from specific targets to customize the response to obstacle vehicles. All vehicles are marked with the specific reflective material at locations on the vehicle that would be presented to a following vehicle as a collision is imminent. 
     FIG. 2 illustrates a vehicle  14  used in the disclosed system. Vehicle  14  travels along track  10 . Primary selective sensors  22 , placed on the outer third of the front of the vehicle, search for obstacle vehicles. Secondary selective sensors  24 , placed on the inner third of the front of the vehicle, are utilized to further detect obstacle vehicles as will be described further on. General obstacle sensors  20 , placed approximately in the middle of the front of the vehicle, as are known in the industry, are also used to detect all obstructions within a target acquisition area and stop the vehicle  14  if needed. Typically, the general obstacle sensors have a shorter range than the selective sensors and will decelerate the vehicle more quickly than the selective sensors. Therefore the general obstacle sensor can act as a back-up to the selective sensors. All sensors are mounted so they can be angled and tilted as needed for best operation. 
     Identifying reflective tape  26 , which is disposed to work in conjunction with the selective sensors, is affixed to the rear of each vehicle. The tape is affixed to the vehicle rather than the payload carrier to allow a singe calibration even if the payload carrier is changed. The identifying reflective tape extends substantially across the entire width of the vehicle and curves slightly around the vehicle to improve operation. As vehicle  14  travels along the track  10 , its selective sensors  22  will only detect reflections from identifying reflective tape  26 . Therefore, the selective sensors respond only to other vehicles that are within the range of the selective sensors and do not perceive other obstacles. 
     FIG. 3 illustrates the operation of a preferred embodiment of the system in which the selective sensor  22  is a retroreflective sensor and the identifying tape  26  is corner cube reflective tape. The retroreflective sensor  30  transmits polarized light  34 . When the polarized light  34  reflects from the corner cube reflector  32  it is depolarized so that some of the reflected light  36  will be oriented at 90° to the incident light. A normal object  38  will not depolarize the light, so any light reflecting from it will retain its polarization. A detector that is activated only by light polarized at 90° relative to the transmitted light will only “hit” when a corner cube reflector has been the target. 
     The retroreflective sensor used for the invention is configured as shown in FIG.  4 . The sensor  30  contains a light source  40  putting out unpolarized light. A polarizing filter  44  polarizes the light in a single plane. A lens  46  focuses the polarized light. Corner cube reflector  32  depolarizes the polarized light  48  incident on it and reflects the depolarized light  50  back toward the sensor. The depolarized light  50  passes through the lens  58  and only that portion of the light  54  that is parallel to a second polarizing filter  52  (oriented at 90° to the first filter  44 ) passes through to be received by the photodetector. 
     FIG. 5 illustrates why this system doesn&#39;t see objects with ordinary reflective material rather than corner cube reflective material. The polarized light  48  emitted from the sensor strikes an obstacle  60  and is reflected back, still polarized. When this polarized light meets the rotated polarizing filter  52 , no light passes through to be detected by the photodetector. Because the sensor system does not detect other objects which the sensor beams may cross, the sensing distance for these sensors may be relatively large without getting false hits. 
     The operation of the system can be calibrated with knowledge of the application to which it will be applied. If the maximum velocity, v m , of the vehicles is known and the deceleration, a d , that is to be used for obstacle vehicle stops, the time to stop the vehicle, t, and the stopping distance, d, can be calculated. 
     
       
         
           t=v 
           m 
           /a 
           d 
         
       
     
     
       
           d=v   2 /(2 a   d ) 
       
     
     If a longer stopping distance can be allowed, a gentler deceleration can be used. The gentler deceleration may allow bulkier cargoes to be carried by the vehicles. The range of the sensor must be greater than the stopping distance but should not be so great than targets beyond the desired range cause false hits. One way to limit the range of the sensors is to adjust the gain of the sensors. This method could require maintenance as the components age. In a preferred embodiment, the sensors and the identifying tape are disposed at approximately the same height on the vehicles, but the sensors are aimed at an upward angle to limit the distance at which the emitted light can impact the identifying tape. Further, the sensors are angled inward to assure that the light doesn&#39;t disperse beyond the desired region. This method reduces the amount of maintenance versus a gain adjustment and allows factory setting of the distance. In a preferred embodiment, a 30 inch (76 cm) range was reduced to a 20 inch (51 cm) range using this method. 
     When an obstacle vehicle is stopped ahead on a straight track, the primary sensors on both sides of the vehicle will register a hit. However, if one of the vehicles is on a curve, only one primary sensor may register a hit, or neither primary sensor may register a hit. 
     In order to use the system on curved section of track, a highly reflective non-diffusing surface  70  is attached to the inner face of the curved track. FIG. 6 Illustrates that the centerline of the mounting of the sensors  22 , identifying reflectors  26 , and highly reflective non-diffusing surface  70  are approximately aligned. The reflective surface is used whenever the track is non-linear and extends for the entire length of each curve. This surface redirects the light  72  around the curve  74  as illustrated in FIG.  7 . Because the sensing distance for the sensor is relatively large, the arc length distance of the curve can be accommodated. When a second vehicle  14 B is stopped or too close around the bend of the curve, the incident polarized light will reflect off the corner cube reflector  26  on the back of the vehicle and be redirected back to the sensor  22  by the highly reflective non-diffusing surface on the track as unpolarized light. When the sensor  22  detects obstacle vehicle  14 B, the logic associated with the vehicle  14 A decelerates its vehicle to a stop. In a preferred embodiment, the deceleration is at a constant rate. This allows vehicles with a relatively large footprint relative to the radius of the curve and vehicles with a relatively large stopping distance relative to the arc length of the curve to detect an obstacle vehicle which is stopped in or just beyond the curve before the following vehicle enters the curve. 
     During the deceleration, the vehicle  14 A may travel part way through the curve. The vehicle may pass through a “blind spot” where the reflected light from the obstacle vehicle  14 B would not impinge on the sensor  22 . FIG. 8 illustrates the use of a secondary sensor  24  in this situation. When the primary sensor  22  senses an obstacle in its path, it starts the deceleration process and activates the secondary sensor  24 . Both of these sensors send out a beam of polarized light that is redirected around the curve by the reflective surface  70  on the track. The beams are depolarized and reflected by the corner cube reflector  26  and redirected around the curve as they return to the vehicle  14 A. If either the primary sensor  22  or the secondary sensor  24  detects obstacle vehicle  14 B, the deceleration process continues, or if vehicle  14 A has stopped, the vehicle remains stopped. The secondary sensor increases the amount of light in the transmission path and effectively provides a broader target for receipt of reflected light, thereby reducing the effect of “blind spots” on the operation of the collision avoidance system. 
     Because the secondary sensors require excess power, they are operated only when needed and shut off as soon as possible. In a preferred embodiment, the logic of FIG. 9 is used to control the power to the secondary sensor for a single side of the vehicle. If the primary sensor for side one registers a hit  90 , information is sent to the motor control to prevent the collision (where side one could be either the left or the right, with side two being the other side). If the sensor on the other side has not registered a hit  94 , then the reflector returning the light is not straight ahead and the secondary sensor is needed. The secondary sensor on side one is activated  96  in this case. The logic then shifts into a mode of looking to turn off the secondary sensor. As long as either the primary or secondary sensor for side one is registering a hit  98 , while the side two primary sensor is not registering a hit  102 , the side one secondary sensor is maintained on. If both side one sensors are not registering a hit  98  and a suitable delay such as six seconds have passed since the last hit  100 , then the side one secondary sensor is deactivated  104 . Alternately, if a side one sensor and the primary side two sensor register hits  102 , the side one secondary is deactivated because the obstacle has moved to directly in front of the vehicle. 
     In a preferred embodiment, a target velocity, v m , of 100 ft/min and a deceleration, a d , of 0.1 g were accommodated. These factors dictate a 6 inch (15 cm) stopping distance. A sensor range of 20 inches (51 cm) was found sufficient to provide sufficient warning to prevent collisions. In a typical corner for this configuration, the stopping distance equated to a 30° displacement into a curve. When payloads have large diameters, the system needs to be set up to detect the presence of the stopped vehicle before the payloads collide. 
     Having described preferred embodiments of the invention it will now become apparent to those of ordinary skill in the art that other embodiments incorporating these concepts may be used. Accordingly, it is submitted that the invention should not be limited by the described embodiments but rather should only be limited by the spirit and scope of the appended claims.