Abstract:
A segmented, belt-driven conveyor system for use in clean environments. High speed, high density, collision free throughput of work piece carriers is enabled through belt-driven conveyor segments each having co-rotating drive wheels. The drive wheels have a cylindrical profile. Predefined acceleration/deceleration profiles may be employed by a motor controller to affect optimal changes in work piece carrier speed across the respective drive segment. A peripheral groove is formed in idler wheels within a drive segment. A soft, pliant ring of material is disposed in the groove. The ring protrudes slightly beyond the crown of each wheel. The drive belt then remains in contact with the ring when unloaded and the wheel peripheral surface itself when loaded through compression of the pliant ring. By reducing intermittent contact between the belt and the wheels, particulation is reduced.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    N/A 
       BACKGROUND OF THE INVENTION 
       [0002]    Many manufacturing factory environments consist of spatially distributed processing tools, as opposed to sequential tools located along a linearly arranged assembly line. This is especially true for manufacturing environments where work in process, or a “work entity,” re-enters a tool after being processed by another tool or tools. Re-entry into the same tool avoids tool duplication, which is particularly important in environments where the capital cost of the tools is high. 
         [0003]    A semiconductor manufacturing environment is an example of an environment where, due to high tool cost, a work entity enters a given tool, or type of tool, multiple times. Processing tools in a semiconductor manufacturing environment are typically spatially distributed in the factory according to function. Thus, the work flow resembles a chaotic movement of the work entity. With multiple work entities being operated upon and moving between multiple tools at the same time, the respective work flows intersect. 
         [0004]    In modern factories, the progress of multiple work entities through the high number of manufacturing steps and associated tools is enabled by transport networks. Simultaneous processing of plural work entities, necessary to maximize usage of the factory tools and to maximize product output, results in highly complicated logistics. High efficiency and coordination in work entity movement is thus required. Without an efficient transportation network capable of rapid, real time response, bottlenecks in the work flow into or out of some process tools can develop, while other process tools are starved of work. Such an efficient transportation network thus must have high delivery capacity, high speed, and asynchronous capability by which work carriers can move independently of each other. The transport infrastructure is the enabling technology for such efficient logistics. 
         [0005]    In a recursive process flow environment, such as within a semiconductor manufacturing environment, the simultaneous utilization of up to hundreds of individual process tools requires a logistics network that is capable of delivering the right work entity at the right time to each one of the tools. The higher the utilization of each processing tool, the higher the factory output, which simultaneously translates to the increased efficiency of business capital. 
         [0006]    Conveyor systems are one particular type of transportation system used in contemporary factory environments. A conveyor network may be shared by several hundred moving work carriers concurrently dispatched to various tools. Delivery capacity will depend on flow density and conveyor speed. However, flow density and speed are limited by the additional requirement of zero tolerance for collisions between work entities within the conveyor system. Thus, a conflict arises between the above requirements. 
         [0007]    A conveyor network typically has intersections, nodes, and branches to multiple locations in a factory. The open conveyor ends, at work processing locations, are the input and output ports for the conveyor transport domain. At these ports, work entities enter and leave the conveyor domain. When a work entity needs to travel from one of these ports to another in the prior art, a path needs to be cleared for the transit to satisfy the requirement of collision avoidance. Normally, external or centralized dispatch software arranges for such a transit by simultaneously controlling the movement of all other work entities that would otherwise interfere with the work entity in question. This dispatch software is complex, due to the aforementioned throughput requirements. The work entities need be moved concurrently with each other and at maximum rate without collisions. 
         [0008]    In addition to the challenge of highly complex control in dense manufacturing environments, particulate generation by conveyor systems is of great concern in clean room environments. Thus, the efficiency of transport systems in such environments must be weighed against the opportunities for contamination. 
         [0009]    Traditional roller conveyors have achieved extremely low particulate generation. However, such arrangements have not been able to achieve high acceleration of items or carriers transported thereon (generically referred to simply as “carriers” herein) from a stopped condition. This is not due to a lack of torque available for the drive rollers but instead due to the fact that when high starting torque is applied the roller wheels may slip and squeal. This is akin to auto tires squealing when accelerating too rapidly from a stop. 
         [0010]    In certain embodiments, a hysteresis clutch has been utilized in conjunction with synchronous or stepper motor driven rollers or wheels, depending upon the embodiment, to eliminate such slippage. Hysteresis clutches enable asynchronous soft buffering, a process for moving carriers independent of each other and starting and stopping the carriers in a smooth fashion. However, while successful at preventing slippage, hysteresis clutches may make it difficult to achieve high rates of acceleration, including in the multiple g range. Very fast acceleration and deceleration are required in order to increase throughput and thus the density of carriers traveling on the soft buffered conveyor where carriers must never collide. Since the carriers move asynchronously, they need to stop fast and short of a collision with a downstream carrier to achieve increased density in a conveyor environment, as well as start fast so as to minimize interference with upstream carriers. 
         [0011]    Principles of physics dictate that the frictional force required to move an object on a surface is dependent on the normal force and the coefficient of friction for the materials. In other words, it is independent of the area in contact. However, with compressive materials, higher friction forces can be achieved by selectively increasing the surface contact. A result of this realization was the increased utilization of belts for carrier transport, instead of wheels with a rubber drive surface in contact with carriers. This increase in surface area contact in effect increased the coefficient of friction between driving and driven surfaces. 
         [0012]    Unfortunately, simply disposing a driving belt on a respective set of wheels is not clean in terms of particulate generation, particularly with respect to that resulting from the use of driven and idler wheels alone. The particulation of the belts results primarily from interaction of the belt with the wheels below the belt, i.e. those supporting the weight of a carrier. Previous investigations into the source of particulate generation determined that in many cases the belt was not in continuous, full contact with the wheels below it due to machining tolerances in the wheels, the respective axles, and/or the rails that support the wheels. For example, some supporting idler wheels were found to be in constant contact with the overlying belt and thus were turning in concert with the belt while others started and stopped depending on when the belt touched them. The latter contact was haphazard, resulting in frictionally-induced spin up and stops of the supporting idler wheels. This effect was sometimes dependent upon whether a carrier was above the respective portion of belted conveyor. 
         [0013]    In order to impart continuous contact between the belt and all of the wheels in a respective conveyor section, including the idler wheels, it was proposed that the belt be woven in a serpentine path between wheels, such as over two idler wheels and then down under the next. While successful in maintaining contact between the belt and all of the respective wheels, this resulted in an increased motor torque requirement, which also required increased electrical current and thus operational cost. 
         [0014]    There remains the need for an optimized transport solution that results in high density, rapid, flexible, and asynchronous work entity transport, high delivery capacity, avoidance of work entity collisions, and low particulation, particularly for use in clean room environments. 
       BRIEF SUMMARY OF THE INVENTION 
       [0015]    To resolve the inherent conflict between the need for high speed work piece conveyance and the avoidance of work piece collisions and to increase throughput, the conveyor infrastructure as presently disclosed is divided into segments, each having a length substantially equivalent to that of a work entity or work piece carrier. A work piece carrier is prevented from entering a conveyor segment if that segment is already occupied by another work piece carrier. Such collision avoidance is autonomous, embedded in the conveyor itself, allowing a natural, independent flow of dispatched work piece carriers, an approach that is distinct from the centralized control model as practiced in the prior art. By dividing the longer conveyor runs of the prior art into discrete segments and by enabling intelligent, local control of work piece carriers transiting between segments, the capacity-limiting procedure of reserving whole conveyor line runs for dispatched work carriers is avoided. 
         [0016]    Work piece carriers can be sent from port to port autonomously with high flow densities. With the use of localized, segment-based sensing and conveyor control, carriers can occupy adjacent segments, if needed, and can pass through nodes on a first come, first served or “natural” basis. 
         [0017]    How close work piece carriers can be, on consecutive conveyor segments, a concept referred to as “stacking,” depends in part upon work piece carrier travel velocities, i.e. conveyor speed. In the prior art, the prohibition against entry of a work piece carrier into a zone already occupied by another work piece carrier demanded generous spacing of the traveling carriers to ensure sufficient stopping distances to avoid collisions. The higher the speed, the greater the stopping distance, resulting in less flow density. The limitations on stopping (or starting) distance in the prior art is a consequence of using rollers to drive the work piece carriers on a conveyor. Yet such rollers were previously thought to be the only means of achieving clean, particulate free movement. In the pursuit of clean transport, roller conveyors utilized moderate transport speeds to avoid the slipping of the work piece carriers on the rollers when sudden stopping was necessary to avoid collisions with a downstream, stationary work piece carrier. Thus, the physics of the limited contact surface between work piece carriers and the driving conveyor rollers required such moderate speeds. 
         [0018]    With elastic surface contacts, frictional force increases with increasing surface contact. Thus, to increase driving surface contact between the conveyor drive and the work piece carrier, the wheels or rollers of a conveyor in presently disclosed systems and methods are supplemented with belts of high friction coefficients. However, while improving the frictional engagement between work piece carriers and the segmented conveyor, the introduction of belts may introduce new particulate sources, particularly with respect to idler wheels, as discussed above. Overcoming these difficulties, through developments described herein, allows the introduction of high speed, belted, locally controlled segmented conveyors providing high rates of work piece carrier acceleration and deceleration in clean manufacturing environments. High flow density, at high speeds, thus result. 
         [0019]    When velocities are high and stopping and starting distances must be short, the rate of acceleration and deceleration of the work piece carrier must be limited to avoid slippage on the belt, a condition that could create contaminating particulates. Previous control of particulation through limited rates of acceleration and deceleration were achieved through the use of a magnetic hysteresis clutch in conjunction with conveyor segment drive wheels or rollers. The clutch acts as a limiting device on drive roller torque, and can be set to disengage when sudden starting acceleration or rapid stopping of a high speed motor would otherwise cause the frictional force between the conveyor and the work piece carrier to be exceeded. The application of such a clutch allowed masses and velocities of the work piece carrier to be variable (e.g., the weight difference between a full work piece carrier versus an empty work piece carrier) while not exceeding a maximum value of inertia. 
         [0020]    However, it has been discovered that the use of elastic surface contact between a conveyor-driven belt and a work piece carrier provides improved frictional engagement, thus obviating the need for clutch-based techniques for limiting frictional forces. Higher rates of acceleration and deceleration, programmed into local segment controllers, can be employed, thus improving throughput while avoiding collisions. Such motor control can be achieved through servo action or by predefining and limiting open loop stepper motor rates of acceleration or deceleration. Thus, in a particulate-free clean manufacturing environment, segmented conveyors with belts, driven by open loop stepper motors or servo motors with controlled high rates of acceleration and deceleration, results in a collision-free flow of work carriers at high density and high speed, resulting in increased conveyor throughput. 
         [0021]    In one particular embodiment, to achieve improved contact between a drive belt and wheels within a respective conveyor section, and idlers wheels in particular, a peripheral groove is formed in each wheel disposed beneath the belt. A soft, pliant ring of material is then disposed in the groove. The ring protrudes slightly beyond the crown of the idler wheel. 
         [0022]    The slight protrusion of the pliant ring results in improved contact with the drive belt as it passes above the respective idler wheel. The idler wheels turn in coordination with the drive belt at all times. Particulation is thus significantly reduced and drive motor torque requirements are also reduced in comparison to the serpentine belt embodiment previously described. 
         [0023]    Each pliant ring is configured to achieve constant contact with the overlying belt when unloaded by a carrier. When a carrier or other item being transported is adjacent or above a respective wheel, the pliant ring is compressed and the belt comes into contact with the relatively hard wheel crown or periphery itself, increasing the area of contact between the belt and wheel. Thus, the pliant ring material and extent of protrusion above the wheel crown are selected to achieve a high degree of belt contact between the pliant ring and the belt when unloaded and direct contact between the wheel crown and the belt when loaded. Rapid acceleration and deceleration of carriers is achieved with a relatively low degree of required torque and with minimized particulation. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0024]      FIG. 1  is a section view of a wheel according to the present invention disposed from a supporting rail frame; 
           [0025]      FIG. 2  is a detailed view of the wheel of  FIG. 1 ; 
           [0026]      FIG. 3  is a section view of the wheel of  FIG. 1  further illustrating the location of a drive belt atop a pliant ring in the wheel; 
           [0027]      FIG. 4  is a detailed view of the wheel of  FIG. 3 ; 
           [0028]      FIG. 5  is a perspective view of a conveyor drive segment in which is illustrated a drive belt, at least one drive wheel, and a plurality of idler wheels according to the present invention; 
           [0029]      FIG. 6  is a detailed view of the wheel of  FIG. 3  under loaded conditions; 
           [0030]      FIG. 7  is an plan view of one end of a drive shaft shown in  FIG. 5  having planar protrusions on opposite ends; and 
           [0031]      FIG. 8  is a side perspective view of a drive wheel according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    This application claims benefit over U.S. Provisional Application No. 61/894,079, filed Oct. 22, 2013, entitled: CONVEYOR SYSTEM PROVIDING REDUCED PARTICULATION. 
         [0033]      FIG. 1  illustrates an idler wheel hub  10  disposed in relation to a supporting rail frame  20 . The wheel hub, also simply referred to herein as “the wheel,” may be formed of a hard, resilient material that is resistant to particulation, such as polyurethane. One preferred embodiment of the wheel  10  employs 75 Shore D cast electrostatic discharge (ESD) polyurethane rods that are machined to the desired shape and size after casting. Alternatively, a  67 D polyester-type Thermoplastic Polyurethane (TPU) such as ESTANE (™ of Lubrizol Advanced Materials, Inc., Cleveland, Ohio) 58137 TPU. The wheel, in the illustrated embodiment, is substantially cylindrical, though, as can be more clearly seen in  FIG. 2 , has an outer periphery that is inclined with respect to an axis of symmetry  24  centered within the respective axle  18 . Specifically, the radius of the wheel at either a front edge  11  or rear edge  13  is less than the radius measured closer to the middle of the wheel. This difference in radius can be linear or curved, the latter being illustrated in the figures. 
         [0034]    The wheel  10  is disposed upon a bearing assembly  16  of conventional design and configuration. The bearing assembly  16  is disposed about an axle  18  that projects from a drive rail  20 . The axle is shown as being threaded in the figures, and can be mated with a complimentarily threaded bore in the drive rail. However, the axle may be mechanically mated with respect to the drive rail in any conventional manner. The drive rail is shown as being L-shaped in  FIG. 1 , though it can be provided in a variety of shapes. 
         [0035]    Disposed about the wheel outer peripheral surface is a slot  12 . As shown in  FIG. 2 , the slot is continuous about the periphery of the wheel to form a ring-shaped or circular slot into which is provided a ring of pliant material  14 . In a first, illustrated embodiment, the slot and the ring of pliant material are rectangular in cross-section, though in other embodiments, different geometries can be utilized. For example, in another embodiment, the pliant ring may have a circular or ovoid cross-section, while the slot has a complimentary semicircular or semi-ovoid cross-section. The pliant ring is preferably configured to have a maximum thickness, measured in the radial direction of the wheel, that is slightly greater than the maximum depth of the slot. Thus, the pliant ring normally extends a distance x beyond the proximal surface of the wheel itself. The pliant ring is provided of polyurethane in a first embodiment, though other soft, compressible, non-friable materials can be used. Such other materials may include silicone and rubber. In a preferred embodiment, the pliant ring is stretched and forced over the wheel outer periphery and into the slot. The diameter of the pliant ring at rest may be less than the diameter of the slot, such that the pliant ring is held in place through friction fit in one embodiment. In other embodiments, the pliant ring is held in place through an adhesive bond or through mechanical means, including friction fit between the side walls of the pliant ring and the side walls of the slot (not shown). 
         [0036]    In  FIG. 3 , a drive belt  22  is shown in cross-section, disposed across the top of the pliant ring  14 . This is also depicted in greater detail in  FIG. 4 . When there is no carrier or other item being transported on or proximate to the respective wheel, the belt lower surface remains in contact at least with the upper or outer surface of the pliant ring  14 , whereby the respective wheel may respond immediately and without slipping to movement of the belt. Should a wheel have a defect in an outer extent thereof, or if an axle  18  is bent or otherwise not orthogonal to a drive rail, the belt may also at times come into contact with the outer surface of the wheel itself. However, the pliant ring is intended to ensure that the belt is always in contact with the respective wheel, either directly or indirectly, in order to avoid particulation resulting from intermittent contact between the belt and wheel. 
         [0037]    The choice of materials for the drive belt  22  depends in part upon desired values for durometer and electrical conductivity. Pyrathane 83ASD and Stat-Rite S-1107 are typical belt materials. A belt of Pyrathane is somewhat softer and more elastic but simultaneously less electrically conductive. A belt of Stat-Rite is harder and more stiff, but simultaneously more electrically conductive. Preferably, the elastomeric belt is stretched onto the wheels and serves to directly transport overlying work piece carriers through interaction with all of the idler and drive wheels. 
         [0038]    Once a carrier (not shown) is on the belt  22  above or proximate a particular wheel  10 , the weight of the carrier is sufficient to compress the pliant ring  14  such that the belt  22  undersurface comes into direct contact with the relatively hard surface of the wheel outer surface, as shown in  FIG. 6 . The hardness of the wheel ensures that the belt does not dip as the weight of the carrier traverses each wheel and instead provides a level, smooth transition for the carrier. In addition, the increased area of contact between the belt lower extent and the wheel periphery, compared to the area of contact between the belt lower extent and the pliant ring periphery, ensures sufficient frictional force to achieve accurate rotational tracking between the belt and wheel. 
         [0039]    In  FIG. 5 , a perspective view of one embodiment of a drive segment can be seen. The length of a conveyor segment is determined by a number of drive segments it comprises. A drive segment is defined as the length of a work piece carrier plus some margin of free space. Thus, depending upon the embodiment, a conveyor segment may be configured to hold one, two, or more drive segments. With this modular approach, the designer of a conveyor application then constructs the conveyor layout using standard, prefabricated modules of length which hold an integral number of drive segments each. This methodology allows easy conveyor network design and assembly. 
         [0040]    In the figure, a linear array of wheels  10  is provided in relation to a drive rail  20 . In the illustrated embodiment, each such wheel  10  of the array is provided with a peripherally disposed pliant ring  14  to improve the degree of rotational contact between the wheels and an overlying, continuous belt  22 . In this illustrated embodiment, each of the wheels  10  in the linear array across the conveyor segment are idler wheels. In other words, each of the wheels of the linear array are unpowered and are rotated through continuous contact with the overlying belt. Note that in other, more simplified embodiments, the idler wheels are crowned, as shown in  FIGS. 1 and 2 , but are not provided with a slot  12  or pliant ring  14 . Further still, in yet other embodiments, some or all of the idler wheels have a flat outer surface, parallel to the axle  18 , upon which a respective belt  22  rolls. 
         [0041]    At opposite ends of the linear array, the belt  22  extends slightly less than 180 degrees about respective end wheels  10  in substantially the opposite direction towards two lower return idler wheels  26 . The belt extends approximately 90 degrees about these return wheels and thence about the upper surface of a drive rod  28 . Each of the return wheels  26  and the drive rod  28  may also be provided with a respective pliant ring  14  in an alternative embodiment, while in other embodiments, one or both do not have a respective pliant ring. 
         [0042]    In this illustrated embodiment, the drive rod  28  is selectively rotated by a motor  56  ( FIG. 8 ) according to techniques known in the art. By rotating one end of the drive rod by operation of the motor, cooperating belts on opposite sides of the conveyor segment are rotated in unison, thus resulting in linear, even transport of a carrier disposed on an upper surface of the two belts. The drive shaft in one embodiment is a combination of shaft and universal coupling to allow some degree of misalignment between the two sides of the conveyor rail. For example, with respect to  FIG. 7 , the drive shaft  28  is provided with a flat protrusion on each end, with the protrusion  40  on the proximate end in the drawing being orthogonal to the protrusion  42  on the opposite, distal end. The flat protrusion on one end of the drive shaft fits into a slot  52  in the center of a respective drive wheel  50  mounted on one rail frame  20  (not shown in  FIG. 8 ) and to a motor  56  by a spindle  54 , as shown in  FIG. 8 , while the opposite flat protrusion fits into a respective slot in the center of a respective slave wheel on the other, parallel rail frame. The slave wheel is rotatable about a respective spindle through bearing means known in the art. 
         [0043]    Through the use of a common drive shaft, the conveyor belts on both sides of the conveyor segment are synchronized to run at identical speeds, thus avoiding the twisting of work piece carriers on top of the belts as they travel across the conveyor segment. As shown in  FIG. 8 , the drive wheel  50  and slaved drive wheel on the opposite end of the drive shaft have identical cylindrical shapes. Importantly, the radius R of each drive wheel is identical. This assures that the left and right belts are driven at identical speeds, in spite of the normal tendency of the belts to each seek its own highest tension by locating themselves on the highest point of the idler wheels&#39; crowns. 
         [0044]    Due to material variations, conveyor load accelerations, frictional coefficient differences, belt sizes, and mainly the imperfections in wheel shaft alignments, such that not all wheel axes of rotation are not perfectly parallel with each other, the left and right belts normally would otherwise run at slightly different speeds. This would be problematic in clean environments where such speed differentials could lead to friction and particulation. The cylindrically shaped drive wheels counteract this tendency and equalize belt speeds on the two sides. 
         [0045]    In an alternative approach, the conveyor belt is a timing belt, having a flat surface presented upwards towards work piece carriers traveling thereon. The inner surface of the drive belt is provided with mechanical features that cooperate with complimentary mechanical features on the outer periphery of the idler wheels. Specifically, in a first embodiment of such a timing belt, the inner surface of the belt is provided with a linear and continuous array of projections such as pyramidal or frusto-pyramidal projections and the idler wheels are provided with a linear array of complimentarily shaped apertures, each configured to receive a respective belt projection as it passes over the idler wheel. In a second embodiment, the projections, such as pyramidal or frusto-pyramidal projections, are formed in a linear band about the outer periphery of the idler wheels, while the belt is provided with complimentarily shaped and spaced apertures adapted to receive the idler wheel projections as the belt travels over the idler wheels. In this second embodiment, the belt apertures may extend through the belt to the work piece carrier contact surface or, if the belt is of sufficient thickness, may only extend partway through. In any such embodiment, however, the timing belt ensures the idler wheels continuous rotate in sync with the overlying belt and particulates are avoided through the avoidance of intermittent belt/wheel contact. 
         [0046]    Centering wheels  30  are provided to center the carrier on the belts, in the illustrated embodiment. One or more intermediate idler wheels  32  may also be employed where the placement of the drive rod  28  results in a gap between adjacent idler wheels  10  in the linear array. Such intermediate idler wheels may or may not be provided with pliant rings, as disclosed. 
         [0047]    In other embodiments, one of the wheels  10  at either end of the linear array may be powered, or one of the return wheels  26  may be powered, instead of the drive rod as shown. This, however, would require drive elements such as motors on opposite sides of the conveyor segment. Keeping two such motors perfectly synchronized in terms of start or stop times and rotational speed may be a technical challenge. 
         [0048]    Alternatively, the drive rod  28  may replace pairs of wheels  10  on opposite sides of the conveyor segment, such as at one end of the linear array of wheels, or one pair of return wheels  26 . The drive rod as depicted in  FIG. 5  would then be replaced by idler wheels on opposite sides of the conveyor segment. Further still, plural drive rods could be employed, though again this would require accurate synchronization of drive elements associated with each such drive rod. 
         [0049]    In the illustrated embodiment, a hysteresis clutch is not employed in conjunction with the motor  56  for avoidance of slippage between a work piece carrier and the belts. In addition, each drive segment is provided with at least one sensor  60 , and preferably at least two sensors, for detecting the presence of one or more work piece carriers within the conveyor segment. With at least two sensors, one sensor can be provided proximate each end of the respective drive segment such that the respective controller can know whether a work piece carrier occupies the drive segment. Such sensors are of conventional design and can include the use of optical, magnetic, passive resonant circuit, weight, mechanical interference, and inductive sensors. 
         [0050]    The one or more sensors associated with one conveyor drive segment are preferably in communication with a local controller  58  associated with the respective conveyor segment drive motor  56 . The controller is preferably provided with a communications interface and is in communication with the respective controllers of the at least one conveyor segments on either side thereof, such as via a communications bus of conventional design and configuration. In one embodiment, the bus is an industrial Controller Area Network (CAN) bus. Obviously, if the conveyor segment is a port, such as an interface to a process tool, the respective controller would only communicate with the one adjacent conveyor segment controller. 
         [0051]    Multiple segment-specific controllers are in communication with a respective higher-level controller. This higher level controller has a map of the conveyor segment for which it is responsible, and is programmed with the ability to direct how each carrier within this conveyor domain are to be routed. This information is used to control the response of the individual segment-specific controllers. Depending upon the complexity and size of the overall conveyor system, multiple levels of higher-order controllers may be employed. 
         [0052]    The controller for each drive segment is thus capable of detecting the presence of a work piece carrier in an adjacent drive segment and can react to receipt of a new work piece carrier accordingly, such as by decelerating that work piece carrier and bringing it to a stop to avoid a collision with a downstream carrier. The controller is also capable of detecting the movement of a previously stationary work piece carrier in an adjacent drive segment and can respond by accelerating a work piece carrier contained within the respective segment from a stopped condition or can continue transporting the work piece carrier through that drive segment to the next. 
         [0053]    Acceleration and deceleration profiles are preferably stored in a memory  62  associated with the local conveyor segment controller. These profiles may be standard profiles to be used for changing work piece carrier speed, or may be maximum values, whereby the controller is programmed to have flexibility in adjusting work piece carrier speed according to the presence or absence of carriers within the respective conveyor drive segment and/or within adjacent conveyor drive segments. 
         [0054]    The drive segment, as defined above, is approximately the same length as a work piece carrier, plus a small measure of free space. Thus, for a 300 mm wafer carrier found in semiconductor manufacturing environments, a drive segment is 0.5 meter in length. A typical carrier in a semiconductor manufacturing environment has a mass of approximately 8.5 kg and can travel at speeds of approximately 1 meter per second. A deceleration profile must be selected to enable deceleration of this mass to a stop before it enters a downstream, occupied drive segment. This deceleration profile is generally linear in a first embodiment. 
         [0055]    However, it is also envisioned in a further embodiment to use an exponential deceleration profile, where the rate of change in speed is slow at the start but greater at the end, near the stopping point. This takes advantage of the speed-torque characteristic of stepper motors: generally, motor torque in stepper motors is higher at low speeds. 
         [0056]    While deceleration profiles have been discussed in the foregoing, similar profiles can be employed for acceleration to achieve maximum acceleration without slippage. Such controller acceleration and deceleration profiles enable work piece carriers to travel at high speed, in very dense flow environments, without the possibility of collisions. 
         [0057]    While in the foregoing only adjacent drive segments and/or conveyor segments are described as being in mutual communication, controllers of a larger range of nearby drive or conveyor segments can be in mutual communication to enable faster response to segment occupancy changes and to enable predictive response. 
         [0058]    Many changes in the details, materials, and arrangement of parts and steps, herein described and illustrated, can be made by those skilled in the art in light of teachings contained hereinabove. Accordingly, it will be understood that any following claims are not to be limited to the embodiments disclosed herein and can include practices other than those specifically described, and are to be interpreted as broadly as allowed under the law.