Patent Publication Number: US-10331105-B2

Title: Machine for self-adjusting its operation to compensate for part-to-part variations

Description:
TECHNICAL FIELD AND PRIORITY CLAIM 
     This disclosure relates to methods for self-adjusting industrial machinery which performs automated operations on mass-produced parts, and claims priority benefit of App. No. 62/202,231, filed Aug. 7,2015. 
    
    
     BACKGROUND 
     A mass-produced part is commonly manufactured to a specification which defines a master model of the part using specific values of various physical characteristics such as geometric dimensions, characteristics of the part material, etc. A part specification also recognizes that physical characteristics of actual production parts may differ from those of the master model defined in the specification. Some part-to-part differences may be acceptable in a particular mass-production manufacturing process, but others are not. Consequently a specification for a part which is mass-produced in a particular manufacturing process may contain allowable tolerances for various physical characteristics of the part. 
     A plastic part is first manufactured by a molding process, such as blow molding, and is then further processed by various operations which for example, may include boring, drilling, and welding. Part-to-part variations in molded plastic parts may apply to almost any particular physical characteristic, but may be generally said to involve geometric differences, such as wall thickness and flatness for example, which can be caused by variations of melt flow rate, humidity of raw material, ambient temperature, etc. Those factors can cause significant part-to-part variation in finished parts when compared to the master model. 
     A common way of defining a particular dimension of a mass-produced part is by specifying an acceptable range for the particular dimension, such as a length of 100.0 mm.+/−1.5 mm., which defines an acceptable range in length from, and including, 98.5 mm. to, and including, 101.5 mm. Stated another way, the dimension has a nominal length of 100.0 mm. and a tolerance on that length of +/−1.5 mm. Any length within that range is said to be within tolerance. Any length not within that range is said to be out-of-tolerance. 
     The nature of a particular material or materials of a mass-produced part and the nature of the industrial machinery which performs operations on those parts may have significant effect on dimensional tolerances for the part. For example, mass-produced machined steel parts can be fabricated with a greater degree of precision than can mass-produced parts which are fabricated by certain molding processes and whose material is less rigid than steel, such as blow molding of plastic parts. Consequently when further processing operations, such as drilling and boring for example, are performed on those two types of parts, modern machine tools can perform those operations on steel parts with precision which allows a specification for such a part to have very small tolerances while comparable operations on mass-produced molded parts whose material is less rigid than steel do not allow a specification for such a part to have the small tolerances which apply to the steel part. 
     When the process for manufacturing a mass-produced part involves using a machine to perform one or more of operations on a part, the part is placed in a particular three dimensional relationship to the machine. Because of part-to-part dimensional differences, the location where a particular operation is performed on one part may differ from the location where the same operation is performed on another part. As long as each of those two parts is within tolerance before the operation is performed, and the path of motion of an element of a machine which performs the operation is invariant from part-to-part, dimensionally correct parts will be manufactured. 
     A machine which comprises an industrial robot can position an element which performs an operation on a part during a manufacturing process with a high degree of accuracy and repeatability. Consequently, it is essentially part-to-part variation, and not the machine, which is the predominant cause of a finished manufactured part being out-of-tolerance. 
     Certain parts which are fabricated by a molding process such as blow molding may be subject to localized variations in thickness, to variations in locations of certain features such as holes, and to variations in shapes of certain features. For example, a zone which should ideally be flat may be bumpy or it may be tipped out of an imaginary plane which it should occupy. 
     When a machine having an industrial robot for positioning a heated welding plate which is to perform a welding operation at a particular location on a molded part which is subject to significant part-to-part variation, the inability of the machine to self-adjust itself to compensate for part-to-part variation at the location on the part where the operation is to be performed, the thickness of the part at that location, and the shape of the part at that location, may produce an unacceptable number of out-of-tolerance parts. 
     SUMMARY OF THE DISCLOSURE 
     This disclosure introduces a machine which compensates, by self-adjustment of its operations, for part-to-part variation at a location on a part where an operation, or operations, is, or are, to be performed. Variations include variations in the location itself, variations in thickness of the part at the location, and variations in shape of the part at the location. Because of this capability, the disclosed machine can significantly minimize the percentage of finished parts which are out-of-tolerance. 
     The machine can also discover out-of-tolerance parameters which it discovers in a part before it conducts any operations on the part. In that way, a non-compliant part found unsuitable for performing operations on can be returned to its fabricator to see for itself that the part which was sent to be processed by the machine was non-compliant as sent. 
     The foregoing summary, accompanied by further detail of the disclosure, will be presented in the Detailed Description below with reference to the following drawings that are part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic plan diagram of a four-station work station having robots at three of the stations for performing repetitive operations on workpieces. 
         FIGS. 2 and 2A  collectively show a perspective view of a first of three stations which have robots. 
         FIG. 3  is a perspective view of the front of a processing unit which is positioned by a robot at the first station. 
         FIG. 4  is a perspective view of the rear of the processing unit shown in  FIG. 3 . 
         FIGS. 5, 6, and 7  schematically show steps of a hot plate welding process. 
         FIG. 8  is a perspective view of a hot plate assembly shown in  FIG. 3 . 
         FIG. 9  is a legend showing layers of a wall of a co-extruded blow-molded tank for holding volatile liquid fuel. 
         FIG. 10  is a fragmentary perspective view of a master shape for a feature of interest in the wall of the tank of  FIG. 2  including a cross section through the feature of interest. 
         FIG. 11  is a view in the direction of arrow  11  in  FIG. 10  and includes the hot plate assembly of  FIG. 8  melting the feature of interest in the tank wall. 
         FIG. 12  is an enlarged fragmentary view in circle  12  of  FIG. 11 . 
         FIG. 13  is cross section view of the feature of interest after the hot plate assembly has been removed and a tube has been welded to the feature of interest. 
         FIG. 14  is a three dimensional image display resulting from a scan of an area of the tank containing the feature of interest. 
         FIG. 15  is a two dimensional image developed from the three dimensional image display. 
         FIG. 16  is another view of the two dimensional image showing a step in an analysis of the image. 
         FIG. 17  is another view of the two dimensional image showing another step in the analysis. 
         FIGS. 18, 19, 20, 21, 22, and 23  illustrate a series of additional steps in the analysis. 
         FIG. 24  is a perspective view showing diagrammatically an example of a result of the analysis. 
         FIG. 25  is a perspective view showing diagrammatically how the result of the analysis is used to orient a hot plate to the feature of interest. 
         FIG. 26  is a perspective view showing diagrammatically how melted material may be displaced by the hot plate during a matching phase of the melting process. 
         FIGS. 27, 28, and 29  illustrate a sequence of steps in measuring wall thickness of the tank at the feature of interest. 
         FIG. 30  is a perspective view of another type of heating element for melting plastic. 
         FIG. 31  is a side elevation view of another hot plate. 
         FIG. 32  is a perspective view in the general direction of arrows  32 - 32  in  FIG. 31 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a workstation  50  having a loading/unloading station  52  at which a part to be processed (i.e. a workpiece W) is loaded into, and unloaded from, a fixture  54  on a turntable  56 , a first processing station  58  comprising a first robot  60 , a second processing station  62  comprising a second robot  64 , and a third processing station  66  comprising a third robot  68 . 
     Four fixtures  54  are mounted on turntable  56  at 90° increments about a central vertical axis  70  of the turntable, and stations  52 ,  58 ,  62 ,  66  are arranged on a workplace floor  72  at 90° intervals about axis  70 . A prime mover indexes turntable  56  in precise 90° increments of rotation about axis  70  to advance a fixture in which a workpiece is secured from one station to a succeeding station in the following manner. 
     After a workpiece has been loaded into a fixture  54  at station  52 , turntable  56  is indexed to advance the workpiece to station  58  for processing at the latter station while a workpiece which has been processed at station  58  is concurrently advanced to station  62  for processing at the latter station, a workpiece which has been processed at station  62  is advanced to station  66  for processing at the latter station, and a workpiece at station  66  is advanced to station  52  for unloading at the latter station. In this way each of the three stations  58 ,  62 , and  66  performs a specific operation on a workpiece which arrives at the respective station before the workpiece is advanced to a succeeding station. 
       FIGS. 2, 2A, 3, and 4  show first robot  60  and a processing unit  74  which is securely fastened to an arm  76  of first robot  60  for movement by first robot  60  within an orthogonal coordinate system by translation of processing unit  74  along linear X, Y, and Z axes of first robot  60  and by rotation of processing unit  74  angularly in roll, pitch, and yaw about respective axes W, P, and R. First robot  60  is a commercially manufactured device known as a six-axis industrial robot having multiple motors which are operated by a controller  79  for positioning any device fastened to arm  76  within a coordinate system with six degrees of freedom (X, Y, Z, W, P, R). An example of such a robot is a Fanuc™ 125F. 
       FIG. 2  shows a fixture  54  into which a workpiece W has been loaded. Fixture  54  holds the workpiece secure such that the workpiece is stabilized against movement on the fixture although certain portions of the workpiece may be slightly deformed by an operation being performed at any station depending on the nature of the construction of the workpiece and that of the operation being performed on it. 
     The particular example of workpiece W shown in  FIG. 2  is a plastic tank  77  which has been manufactured by a co-extrusion process and which has a feature (feature of interest), common to all such tanks processed by workstation  50 , on which first processing station  58  performs a prescribed operation or operations. 
     Processing unit  74  comprises a three-dimensional vision system, including a three-dimensional scanning camera  78 , for acquiring X, Y, and Z coordinate data defining the feature of interest on a tank  77  which has arrived at first processing station  58 , and a processor  80  for storing and processing data. Processor  80  and controller  79  can communicate with each other via a data link  81 . However, before first processing station  58  is allowed to perform any operation on a succession of what are substantially identical tanks, coordinate data which defines a master location for the feature of interest is developed by what may be called a teaching process for first robot  60 . The vision system has what is known as a “smart” camera which embodies both processor  80  and a scanner. A different vision system may have the processor located remotely from processing unit  74 , such as in an electrical cabinet on workplace floor. 
     The teaching process can be best explained in the following way. 
     To begin, a tank which is to be used in the teaching process is loaded into a fixture  54  and properly secured. The tank can be an exact master model of a tank or an actual tank which has been measured to assure that its relevant dimensions correspond sufficiently closely to the design intent represented by the master model that it can be considered equivalent to the master model. First robot  60  is operated to a coordinate position within the six-axis coordinate system described above which has been determined to be an appropriate starting point for camera  78  to begin a scan of a region of interest of the tank containing the feature of interest on which an operation would be performed. That starting point may be referred to as scan start position. First robot  60  then moves processing unit  74  along a scan path at a constant speed in a straight line, which also moves camera  78  in the same way, while camera  78  scans the region of interest. Processor  80  acquires data about the region of interest as the scan proceeds and from that data, by a process which will be more fully described later, develops X,Y,Z,W,P,R coordinate data which defines the master location for the feature of interest in a coordinate system of processor  80  and records that data in a flash drive of processor  80 . 
     Next, first robot  60  is operated by controller  79  to place a tool  82  of processing unit  74  in a home position whose coordinates in the robot&#39;s coordinate system bear a specific positional relationship to the feature of interest. Tool  82  is an actual tool (or an exact replica) which would be used to perform a prescribed operation on the feature of interest in a fixtured tank. Controller  79  has controls which can be manually operated by a person to place tool  82  in the tool&#39;s home position. The tool&#39;s home position may, for example, be a master home position from which tool  82  would begin to perform a prescribed operation on the feature of interest in a fixtured tank. 
     With tool  82  in its master home position, its coordinates (X,Y,Z,W,P,R) are recorded in controller  79  to define the coordinates of the tool&#39;s master home position. Coordinates of the tool&#39;s master home position may be identical to coordinates of a master home position for first robot  60  if tool  82  is not movable on processing unit  74 . The master home position for first robot  60  may be defined by the coordinates of the end of arm  76  to which processing unit  74  is affixed when first robot  60  is in its master home position. 
     When first robot  60  is positioned at its master home position, the coordinates of tool  82  are different from the coordinates of the end of arm  76  to which processing unit  74  is affixed because tool  82  is distant from the end of arm  76 . When tool  82  is movable on processing unit  74 , as will be eventually explained, the coordinates of tool  82 , when at its master home position are determined by modifying the coordinates of the master home position of first robot  60  to take into account the position of tool  82  on processing unit  74  relative to the end of arm  76  to which processing unit  74  is affixed. 
     Next, tank  77  is removed from fixture  54  after which a second tank is loaded into the fixture and properly secured. First robot  60  is operated to position camera  78  at the same scan start position which was used for the scan of the prior tank. Robot  60  once again moves processing unit  74  from scan start position along the same scan path and in the same manner as it did during the scan of the previous tank to scan the region of interest of the second tank. Processor  80  acquires data about the region of interest as the scan proceeds and from that data develops X,Y,Z,W,P,R coordinate data about the feature of interest in the coordinate system of processor  80  and records that data in the flash drive of processor  80 . 
     Processor  80  then calculates differences between the X,Y,Z,W,P,R coordinate data for the feature of interest developed from the second scan and the X,Y,Z,W,P,R coordinate data for the feature of interest developed from the first scan. 
     Those differences are then transmitted from processor  80  to controller  79  for use in controlling the robot&#39;s manipulation of processing unit  74  and of any movement of tool  82  on processing unit  74  during performance of the same prescribed operation on the second tank by applying the differences to re-locate the master home position to a modified home position from which tool  82  will begin to move when performing the prescribed operation on the second tank. For example, if the X-axis coordinate data for the feature of interest developed from the second scan is more positive than the X-axis coordinate data for the feature of interest developed from the first scan, the magnitude of the difference is added to the X-axis coordinate of the master home position so that any X-axis translation which is imparted to tool  82  during performance of the prescribed operation on the feature of interest in the second tank will begin at a modified home position which is offset from the master home position by the calculated X-axis difference. In that way, the prescribed operation on the feature of interest in the second tank will begin at the same position relative to that feature of interest as the master home position is relative to the feature of interest in tank  77 . Similarly for such differences in the other five axes. If the X-axis coordinate data for the feature of interest developed from the second scan is less positive than the X-axis coordinate data for the feature of interest developed from the first scan, the magnitude of the difference is subtracted from the X-axis coordinate of the master home position to define the X-axis coordinate of the modified home position. Similarly for such differences in the other five axes. 
     Processor  80  compares data from the two scans by comparing the new X, Y, and Z data points which define the image of the feature of interest in the second scan with the saved X, Y, and Z data points which define the image of the feature of interest in the first scan. For each axis, the difference between the data points from the two scans is calculated. Processor  80  then calculates roll, pitch, and yaw data (W, P. R) from each scan by trigonometric calculations and then calculates W, P, and R differences between the scans, as will be more fully explained later. 
     Processing unit  74  as shown in  FIGS. 3 and 4  comprises a frame  84  which has a central plate  86  shown disposed in a generally vertical orientation. Central plate  86  has a front face  88  and a rear face  90 . A robot mounting bracket  92  is disposed against rear face  90  and fastened to central plate  86 . Robot mounting bracket  92  has a circular end plate  94  which is fastened to the distal end of robot arm  76  (robot&#39;s six axis arm). 
     Camera  78  and processor  80  are disposed vertically below and fastened to central plate  86  by a bracket  96  and posts  98  to dispose the camera and processor at an appropriate distance from the location where robot arm  76  is fastened to end plate  94 . 
     A track  100  is disposed against front face  88  and fastened to central plate  86 . A carriage  102  is retained on and guided by track  100  for back and forth travel in a straight line along the track. A pneumatic cylinder  104  has a cylinder body  106  mounted on central plate  86  by a bracket  108  and a cylinder rod  110  which extends out of cylinder body  106  into attachment with carriage  102  via a bracket  112  which is fastened to carriage  102 . Cylinder rod  110  is displaced back and forth to move carriage  102  back and forth along track  100  as suggested by arrow  111 . 
     A face of carriage  102  which is opposite track  100  contains two parallel tracks on which respective carriages  112 ,  114  are retained and guided for back and forth travel in straight lines along the respective tracks as suggested by arrows  116 ,  118 . The tracks cannot be seen in the Figs. because their view is blocked by other parts of processing unit  74 . 
     Pneumatic cylinders  120 ,  122  have respective cylinder bodies  124 ,  126  mounted on central plate  86 . A cylinder rod extends out of cylinder body  124  into attachment with carriage  112 , and a cylinder rod extends out of cylinder body  126  into attachment with carriage  114 . The cylinder rods cannot be seen in the Figs. because their view is blocked by other parts of processing unit  74 . Each cylinder rod can be independently displaced back and forth to move the respective carriage  112 ,  114  independently back and forth along the respective track. 
     A valve mounting panel  128  containing various pneumatic control valves is mounted via posts  130  on central plate  86 . Connections from the valves to the pneumatic cylinders are not shown. Those connections, as well as other connections also not shown, such as electrical ones, are organized to follow travel of each of the three carriages by guidance which is constrained by respective chain links  132 ,  134 ,  136 . Each chain link has a first end attached to frame  84  and a second end attached to a respective carriage. The connections enter at the first end and exit from the second end. 
     The coordinates of the position of tool  82  in the robot&#39;s coordinate system are a function not only of the coordinates of the position of the end of arm  76  but also a function of the position in which carriage  112  is placed by pneumatic cylinders  104 ,  120 . Assuming that the position of arm  76  is defined by coordinates of the end of the arm, the location of carriage  112  on processing unit  74  is, as mentioned earlier, accounted for when the coordinates of the tool are being calculated. Respective sensors associated with the respective cylinders communicate data to controller  79  measuring the distance to which the respective cylinder rod is extended. 
     First processing station  58  performs a hot plate welding process in which tool  82  is a hot plate. A schematic example of hot plate welding is portrayed in  FIGS. 5, 6, and 7 . The process comprises a first phase ( FIG. 5 ) in which a hot plate  138  is used to heat ends of a first plastic part  140  and a second plastic part  142  at which the parts will be welded together. The surface temperature of hot plate  138  and the duration for which it is in contact with the parts&#39; surfaces create sufficient melting of those surfaces to enable the parts to be welded together. In typical plastic welding processes, the hot plate temperature is within a range from about 30° C. to about 100° C. above the temperature at which the plastic will melt. A second phase of the process ( FIG. 6 ) is a changeover phase in which, after an end of each part  140 ,  142  has been melted, the parts are moved out of contact with hot plate  138 . A third phase of the process  FIG. 7 ) is a fusion phase in which the ends of the parts which have been melted are first placed together under pressure to cause the melts to blend together and then are held in that position long enough, at the same or a reduced pressure, for the melts to cool and solidify, thereby completing the weld. 
     The nature of certain processes for mass-producing plastic parts can create dimensional differences from part-to-part, two examples of which are differences in wall thickness and differences in surface flatness. Such differences may be caused by variations in melt flow rate, humidity of raw materials, ambient temperature, etc. Other part-to-part differences involve a feature of interest on which an operation is to be performed and they include the feature&#39;s shape and its location in the robot&#39;s six-axis coordinate system. As long as differences between any given part and the specified design intent, as represented by master dimensions for the part, are within acceptable tolerances, that part is may be considered suitable for being welded to another part as long as that other part itself is also within tolerance of its design intent. Yet if the melted portions of the within-tolerance parts are mismatched when placed together during a welding operation, such mismatch may adversely affect quality of the finished weld. 
     A surface of a plastic part which is to be melted by a hot plate should ideally match the shape of a surface of the hot plate which will contact the part surface, and the hot plate surface and the part surface should be brought into full surface-to-surface contact at the exact location specified by the design intent. Failure to meet such requirements can adversely affect weld quality, and when plastic parts are being welded, part-to-part variations as described above can have such an effect on achieving consistent weld quality. 
     The disclosed system and method can provide more consistent weld quality by performing adjustments which compensate for part-to-part differences such as those described above. 
     When a plastic part is properly secured in a fixture and a hot plate surface is precisely positioned by a robot to the same coordinate location at which it is to make initial contact with a surface of the part intended to be melted, part-to-part differences as described above can result in mismatch between the part surface to be melted and the hot plate surface. If mismatch occurs, force being applied by the hot plate to a part will initially displace some melt before eventually conforming melted plastic to the hot plate geometry. Any melt which is displaced during this conforming, or matching, phase is incorporated into flash beyond the hot plate perimeter. 
     Processing unit  74  has a thermal imaging camera for taking a thermal image of a surface area of a part after it has been heated for a specified length of time to verify thermal distribution of a pool of material melted by a hot plate to assure that sufficient melting has been achieved for the welding process to be completed. The image is taken after the hot plate has been moved away. If the image shows that melt is insufficient, the process is terminated to avoid the possibility of making a bad weld. 
     After the melt has been conformed to the hot plate, the process proceeds as if full surface-to-surface contact had initially occurred. Force being applied to the part by the hot plate can be maintained, or reduced to some minimum, while the hot plate continues to remain in contact with the part. 
     Heat now penetrates into the part without any substantial displacement of melt. The temperature of the melt surface continues to rise slightly and eventually reaches about 20° C. below the surface temperature of the hot plate. 
     At the same time as a feature of the part is being melted, a feature of a second plastic part is being melted in a similar way, as explained earlier. 
     At the end of the melting phase, the change-over phase occurs by moving each part out of contact with the respective hot plate. 
     The fusion phase begins by pressing the parts together at their melts. The magnitude of the applied pressure depends on certain factors, such as melt viscosity and part wall thickness. The pressure can be maintained or reduced as the melts blend together and cool by heat flow from the melts both into the surrounding air and also more deeply into the interiors of the parts. The weld is complete once the temperature of the solidified melts has dropped significantly below the crystalline melting point or below the softening temperature. Quality of the finished weld is affected by the severity of any mismatch occurring when the hot plate initially contacted the plastic. By compensating for mismatch in any one or more of the axes of a six-axis coordinate system as disclosed herein, significant improvement of weld quality of mass-produced parts can be achieved. 
     The example of hot plate welding process which is portrayed by the drawings described above is performed at first processing station  58  where a circular flange  145  of a tube  143  ( FIG. 13 ) is welded to a weld pad  157  ( FIG. 2 ) of a tank  77 . 
     Processing unit  74  comprises a first hot plate assembly  144  ( FIG. 3 ) containing a first hot plate  146  and a second hot plate assembly  148  containing a second hot plate  150 . First hot plate assembly  144  is fastened to carriage  112  for movement on processing unit  74 . First hot plate  146  is used to melt a portion of weld pad  157  and is an example of a tool  82  whose position is controlled by controller  79  during performance of an operation on a workpiece. Second hot plate assembly  148  is mounted on one leg of a right angle bracket  152  whose other leg is fastened to central plate  86 . Consequently second hot plate  150  is immovable, and therefore has a fixed location, on processing unit  74 . 
     First hot plate assembly  144  is shown by itself in  FIG. 8  and comprises a proximal end  156  via which it is securely fastened to carriage  112 . First hot plate  146  is at a distal end of assembly  144 . Electric conductors (not shown) come from an electrical panel, enter and pass through chain link  134 , and then pass out of chain link  134  to end at terminals  158  fastened to a circular side surface of a band heater  155 . The conductors provide an electric current path for electricity to flow to and through band heater  155 , heating hot plate  146  in the process. Second hot plate  150  is heated in a similar manner. Resistance temperature detectors (RFD&#39;s)  160  are fastened 180° apart to a circular side surface of first hot plate  146  and used to monitor hot plate temperature and enable current to band heater  155  to be controlled for proper temperature regulation. A first RTD is used for actual control while the second is used as a redundancy check on the first. 
     A gripping tool  154  is securely fastened to carriage  114  for movement with that carriage on processing unit  74  toward and away from second hot plate  150 . Gripping tool  154  functions to grip a tube  143  and move the tube to contact a flat bottom surface of flange  145  with second hot plate  150  for melting a portion of the flange&#39;s bottom surface. Once sufficient melting has occurred, gripping tool  154  moves tube  143  off second hot plate  150 . 
     Weld pad  157  is located within a region of interest in the wall of tank  77  and comprises a raised formation having a circular annular top surface  159  ( FIG. 10 ) surrounding a circular hole  164  having a center  165 . As will be further explained below, a portion of the wall underneath top surface  159  is melted by contacting surface  159  with a circular annular flat surface  161  of first hot plate  146  while at the same time the bottom of tube flange  145  is being melted by second hot plate  150 . After sufficient melting of both flange and weld pad, gripping tool  154  removes tube  143  from second hot plate  150  and places the melted bottom of flange  145  in contact with the melted top of weld pad  157  so that the melts can blend and cool to completion, after which gripping tool  154  releases tube  143  and moves away. 
     Tank  77  is fabricated by blow molding of multiple plastic coextrusions in accordance with known processes. The fabricated tank comprises a multiple layer wall which has a basic shape which comprises a bottom wall, a side wall, and a top wall. The specific shape of each of these walls is determined by the shape of the interior of the mold cavity within which the blow occurs. 
     Various operations, such as welding and boring for example, may be performed on the blown tank to enable additional components to be assembled to the tank so that the tank can be used in the environment for which it is intended, such as a fuel canister or a fuel tank in a motor vehicle. 
     When a canister or tank is intended to hold a volatile liquid such as gasoline, the multiple layer coextrusion commonly includes one extrusion which in the blown canister or tank provides an EVOH layer  163  which serves as a hydrocarbon barrier for preventing volatile gases from escaping through the wall of the canister or tank. 
       FIG. 10  illustrates a portion of the tank wall at weld pad  157  and its six coextruded layers.  FIG. 9  describes those layers and shows the relative thickness of each layer as a percentage of the total wall thickness. Even though the EVOH layer  163  is closer to the interior surface of the tank wall than the exterior surface, the EVOH layer is potentially at risk of being breached if it should be contacted by hot plate  146  during the melting phase of a welding operation due to excessive mismatch of the hot plate to the wall during melting. An example of angular (W, P, R) mismatch is illustrated in  FIGS. 11, 12, and 13 . 
       FIG. 13  illustrates weld pad  157  and tube  143  after having been welded together as explained above. In this example, the orientation of flange  145  is angularly tipped (i.e. not parallel) relative to the underlying weld pad  157  to which it has been welded. The tipping is the result of angular mismatch between weld pad surface  159  and surface  161  of first hot plate  146  which caused a portion of the circumference of surface  161  to penetrate the wall more deeply than elsewhere around the circumference, and as a consequence breach a portion of EVOH layer  163 . Although a melted portion of tube flange  145  is placed onto a melted portion of weld pad  157 , the finished weld may not seal the breach and consequently be defective. 
       FIG. 12  illustrates an example of how such angular mismatch can potentially damage EVOH layer  163  if hot plate surface  161  comes into contact with the EVOH layer. Locational (X, Y, Z coordinate) mismatch is not present in this example, but if present, could potentially damage weld pad  157  if hot plate  146  were to make inaccurate contact with surface  159 . 
     Due to part-to-part variations, the X, Y, Z coordinates of a feature of interest, such as weld pad  157 , on which a tool, such as first hot plate  146 , is to perform one or more operations, can vary as much as about ±½″ when the workpiece is placed in a fixture associated with a machine that controls the operation of the tool. Also angular mismatch between a feature of interest and a tool in W, P, and R coordinates can vary up to about ±3°. 
     Robot  60  and processing unit  74 , as described above, are capable of compensating for such mismatches and thereby enable a repetitive operation of a mass-production process to be performed with more consistent accuracy from part to part. Such a capability provides higher quality finished parts and can significantly reduce, or even eliminate, production of non-compliant parts. 
     The manner of compensating for both locational (X, Y, and Z coordinate) mismatch and angular (W, P, R coordinate) mismatch has been explained earlier in a general way. Further detail will now be presented with reference to the example involving tank  77  and tube  143 . 
     Robot  60  operates to position processing unit  74  at a starting location and then move processing unit  74  along a defined path while camera  78  scans an area which should contain the region of interest. During a scan, robot  60  moves processing unit  74  along the same path but because of part-to-part variations, the coordinates of the feature of interest can vary. The completed scan contains coordinate data for the region of interest which can be visually portrayed on a two-dimensional screen as a three-dimensional image which can be manipulated for viewing from different directions.  FIG. 14  shows a two-dimensional image of weld pad  157  as viewed from one direction, but the actual scan data from which the image was developed contains X, Y, and Z coordinate data.  FIG. 14  is presented to facilitate the reader&#39;s understanding of how processor  80  functions, and the fact that the scan data can be portrayed as it is in  FIG. 14  should not be construed to imply that such an image is actually created as a matter of necessity. It is from the X, Y, Z scan data that W,P,R coordinate data is obtained as mentioned earlier and as will be more fully explained later. 
     Processor  80  contains stored master data for any one or more geometric aspects which uniquely define the feature of interest. In the present example, those geometric aspects of weld pad  157  are annular surface  159  and hole  164  (an annulus surrounding a hole). For each tank  77 , processor  80  uses the master data for those geometric aspects to identify the feature of interest in the scan data. Once the feature of interest has been identified, processor  80  derives a two-dimensional X, Y image ( FIG. 15 ) from the X, Y, Z scan data. As suggested by  FIG. 2 , the path of the scan is generally parallel with annular surface  159 . Consequently, the X, Y scan data lie in a plane which is parallel with the scan path while Z scan data for each X, Y data point is perpendicular to that plane. Z coordinate data can be obtained by use of the gray scale method. 
     Using the X, Y master data for surface  159  and hole  164 , Processor  80  analyzes the X, Y scan data to locate surface  159  and hole  164 . If they are not located, they are considered too far out of tolerance for surface  159  to be melted. In the present example, the analysis takes place within a defined area circumscribed by an imaginary circle  167  shown in  FIG. 16 . Imaginary circle  167  is defined by data in processor  80  and has a diameter greater than that of hole  164  and can be greater less than an outer perimeter  169  of surface  159  depending on the part geometry and feature of interest. If the analysis discloses that surface  159  and hole  164  lie within the defined area, processor  80  uses X, Y scan data to calculate the hole&#39;s cross sectional area for comparison with the master data to determine if the hole size is within tolerance and to determine X, Y coordinate data for center  165  of hole  164 . 
     Processor  80  also contains X, Y data defining two concentric imaginary circles  173 ,  175  ( FIG. 17 ) whose pre-set diameters bound an annulus  177  representing an area of surface  159  which is to be melted. The center of annulus  177  is centered on a center  166 . Processor  80  places center  166  on X,Y coordinate data for center  165  of hole  164  to cause annulus  177  to overlie a portion of surface  159 , thereby defining the annular zone of surface  159  within which Z coordinate data is then obtained by the gray scale method mentioned above. 
       FIG. 18  shows a curved line  179  representing Z-axis coordinate data obtained for surface  159  at each of a number of X, Y locations around the defined annular zone. The example in  FIG. 19  uses twenty-eight X, Y locations at which Z-axis data is obtained. Using the Z-axis coordinate data at those locations, processor  80  calculates a “best fit” plane  181  for fitting to that data. Some portions of curved line  179  are above plane  181  while other portions are below plane  181 , and while plane  181  is shown horizontal in  FIG. 18 , whether it is or is not horizontal depends on the geometry of surface  159  as determined by the Z-axis data which define it. In any event, X, Y, Z coordinates of any point on plane  181  are established and the Z coordinate value can be calculated from the Z coordinate value for the same X,Y coordinate on curved line  179  by algebraically adding the Z-axis distance between plane  181  and curved line  179  which is negative at an X,Y location where line  179  is below plane  181  and positive at an X,Y location where line  179  is above plane  181 . 
     Now that plane  181  has been defined in X, Y, Z coordinates, W, P, R coordinates can be calculated. How this is done is described with reference to  FIGS. 21-23 . 
     Three points on plane  181  are selected.  FIG. 21  shows a first point  185  at the center of the crosshairs. The X, Y, Z coordinates of that point appear in the accompanying chart  187  to the left. 
       FIG. 22  shows a second point  189  spaced along the Y axis a distance (25 mm in the example) from first point  185 . The X, Y, Z coordinates of second point  189  appear in the accompanying chart  191  to the left. The difference between the Z-axis coordinate of second point  189  and that of first point  185  is calculated. Using principles of trigonometry, the tangent of the ratio of that difference to 25 mm is calculated and that tangent defines a pitch angle. 
       FIG. 23  shows a third point  193  spaced along the X axis a distance (25 mm in the example) from first point  185 . The X, Y, Z coordinates of third point  193  appear in the accompanying chart  195  to the left. The difference between the Z-axis coordinate of third point  193  and that of first point  185  is calculated. Using principles of trigonometry, the tangent of the ratio of that difference to 25 mm is calculated and that tangent defines a roll angle. 
       FIG. 24  shows the best-fit plane  181  passing through weld pad  157 . The two zones  199 ,  201  are portions of surface  159  which overlie plane  181 .  FIG. 25  shows two zones  203 ,  205  which are below surface  159  and which will have to be melted in order to enable a good weld to be achieved. Hot plate surface  161  is positioned by robot  60  to be parallel to best-fit plane  181  as the hot plate moves toward weld pad  157 . During the matching phase, some melted plastic is displaced until surface  159  becomes fully matched to the underlying portion of the weld pad which is to be heated. This is portrayed in a general way in  FIG. 26  where displaced plastic is indicated by the numeral  207 . Once surface  161  is fully matched to the plastic, the hot plate need not be advanced further. 
     Surface flatness is a factor which deserves consideration in a welding process and should be taken into account especially when a weld is to provide a hermetic seal. If its flatness is excessive, a weld surface may be distorted enough to look like a “potato chip”. Curved line  179  and best-fit plane  181  are used to calculate a flatness value for surface  159  in a way analogous to running a depth gauge along the surface and taking depth measurements at a number of locations, as suggested by  FIGS. 19 and 20 . Instead of using a gauge however, the distance between plane  181  and a point on curved line  179  which is farthest above plane  181  and the distance between a point on curved line  179  which is farthest below plane  181  is calculated by processor  80  ( FIG. 19 ). The sum of those distances is defined as the flatness value for surface  159  ( FIG. 20 ). 
     Standard pre-set matching/heating times for properly melting a weld pad  157  may be insufficient to conform the portion to be melted to the hot plate geometry if the flatness value for the weld pad is too great. While a flatness value such as 0.2 mm may be suitable for standard pre-set melt times, a flatness value can often vary between 0.5 and 1.0 mm due to molding warpage. 
     If parameters such as time, temperature and pressure are set for all welds based a nominal 0.5 mm flatness value, surfaces having flatness values close to a 0.5 mm may be properly melted. However, surfaces having significantly smaller flatness values are apt to be melted too much, unnecessarily displacing material. On the other hand, surfaces having significantly greater flatness values would be insufficiently melted. Processor  80  allows the hot plate to perform the prescribed operation on a workpiece when the flatness value lies within a flatness tolerance range and disallows the hot plate from performing the prescribed operation when the flatness value does not lie within the flatness tolerance range. When processor  80  allows the prescribed operation to be performed on workpieces, the flatness value for each surface  159  is used by robot controller  79  to set at least one parameter for the prescribed operation, such as controlling the cycle time of the matching/heating phase so that proper melting occurs. 
     For example, depending on the measured flatness value, for each 0.1 mm value that the flatness varies, another 2 seconds of time can be added or removed (if we consider 0.5 mm as a nominal value) to overcome this. If thirty seconds is required to melt a weld pad having a 0.5 mm flatness value, a weld pad having a 0.7 mm flatness value, the time parameter would be thirty-four seconds. This would provide nominal material displacement and complete surface melt as shown in  FIG. 26  where plastic which has been displaced as flash during the matching phase is indicated at  207  and conformed melting at  209 . Smaller flatness value provide for shorter cycle times since the material does not need to be displaced as much in order to cover the entire weld pad area. Parts out of flatness require more matching time in order to displace all of the material and then heat the material on the bottom of the surface once the matching phase is finished. 
     Thickness of a weld pad wall is another variable inherent in a blow molding process. Measuring wall thickness allows the position of the EVOH layer to be determined. This can be done by a measuring device mounted on processing unit  74 . After processing unit  74  has completed a scan of a weld pad, wall thickness at several locations around the weld pad is measured. Processor  80  is operable to allow the hot plate to perform the prescribed operation on a workpiece when the measures of thickness lies within a thickness tolerance range and to disallow the prescribed operation from being performed when a measure does not lie within the thickness tolerance range 
     The EVOH layer represents approximately 3% of the total wall thickness and if manufactured correctly, it is located at 70% of the wall below surface  159 . Hence, in a wall having a 7.0 mm thickness, the EVOH layer should be at a depth of 4.9 mm from surface  159 . 
     The ability to measure wall thickness and locate the EVOH layer enables accurate calculation of the quantity of material required to be displaced during the matching phase (calculated from flatness value) and the quantity of material in the weld pad to be melted so that the EVOH layer is not breached. 
     If wall thickness is measured at three locations as: 7.0 mm, 6.5 mm, &amp; 6.0 mm (6.0 mm being a minimum value for a worst case scenario), the EVOH layer depth should be 4.2 mm. If the flatness value is 0.5 mm, the depth would be 3.7 mm due to material displacement. This leaves 5.5 mm (6.0 mm−0.5 mm) of the wall thickness available for melting without breaching the EVOH layer. If the heating phase melts material to a depth of 2.0 mm, 1.7 mm of material above the EVOH layer is not melted and the EVOH layer is not breached. 
       FIGS. 27, 28, and 29  show one way to measure wall thickness. A laser sensor  211  is mounted on a bracket  213  which is itself mounted on processing unit  74 . Robot  60  positions bracket  213  such that a top surface of a foot  215  of the bracket is placed against the interior surface of the weld pad wall as in  FIG. 29 . Sensor  211  emits a beam ( FIG. 28 ) which provides a measurement of the distance  217  from the sensor to surface  159  of weld pad  157 . The distance  219  from the sensor to the top surface of foot  215  is known. The difference  221  between distances  219  and  217  is the thickness of the tank wall at the location of the weld pad. 
     Another component which can be used for plastic welding is a laser temperature probe which measures temperature of a component which is to be heated and uses the measurement to automatically re-adjust parameters. Cold components have their own set of parameters which are used for processing and will need longer heat cycle time than warmer components. Temperature of warmer components allows for the heat cycle time to be decreased. For example, if a cold component requires 30 seconds of heat cycle time, a warmer component at 70° C. might require only 25 seconds of heat cycle time because residual heat is already present in the component. Algorithm calculations can provide time parameter based on the temperature measured. 
     Short wave and medium wave infrared (IR) is another welding method which is similar to hot plate welding. The surfaces of the components to be joined are not placed in contact with a hot plate but rather are heated by direct IR exposure for a sufficient length of time to melt portions of the components which are to be joined. Once the surfaces have been sufficiently melted, the IR source is withdrawn front the components, they are then placed together at their melts, and the melts are allowed to solidify. Whether standard or custom IR heating elements are used in order to conform to part geometry, distance at which an element is spaced from a component is controlled. An example of an IR bulb  229  is shown in  FIG. 30 . 
     Standard industry practices do not allow for more than 0.5 mm deviation from a set nominal distance to a surface being heated. The time and power parameters are changed if the deviation is greater. Because IR welding is a non-contact method of plastic joining, flatness values calculated in the manner described above can be used to change the time parameter. Larger flatness values will increase the heating time, while smaller ones will decrease the heating time. The process described above enables an IR element to be properly oriented to the surface to be heated and to be placed at an appropriate distance from that surface. 
     The process described above can be applied to heating a non-planar surface.  FIGS. 31 and 32  show a hot plate  230  which, unlike the flat planar surface  161  of hot plate  146 , has a non-planar heating surface  232  which surrounds a cavity  234 . Surface  232  is electrically heated in the same way as surface  161  of hot plate  146 . by the use of standard industry heaters such as flexible heaters, band heaters or cartridge heaters 
     A component (not shown) has a surface be heated whose shape corresponds to surface  232 . The component&#39;s surface whose shape corresponds to surface  232  is the feature of interest and can be uniquely identified by its shape at certain locations around that surface. 
     The system which has been described can be used in multiple robot workstations, such as in  FIG. 1 , where the positional differences calculated at the first workstation are transmitted to subsequent robots at workstations to which a fixtured workpiece is advanced for additional operations. By advancing a fixtured workpiece from the first workstation to a second workstation with sufficient accuracy to place the fixture at the same location relative to the robot at the second workstation as the fixture was to the robot at the first workstation, the controller at the second workstation can use those differences to modify the master home position and create a modified home position for its tool. Consequently, the second workstation need not have a scanning camera nor perform a scanning process on a workpiece. Other process parameters such as surface flatness and wall thickness can also be transmitted with adjustments being made in real time. 
     While the embodiment which has been illustrated and described performs an operation on a workpiece by moving the tool relative to a stationary workpiece, principles disclosed herein may be applied to an embodiment in which the tool is stationary and the fixtured workpiece is movable relative to the stationary tool. In the embodiment which has been illustrated and described, camera  80  is movable with, but not movable on, processing unit  74 . However camera  80  could be movable on processing unit  74  in the same way as tool  82  is movable on processing unit  74 . In that case, coordinates of the position of camera  80  in the robot&#39;s coordinate system would be a function not only of the coordinates of the position of the end of arm  76  but also a function of the position of camera  80  on processing unit  74 . The ability to move camera  80  on processing unit  74  would allow a scan of a feature of interest on a part to be made by movement of the camera alone while the end of robot arm  76  remains stationary. 
     The principles disclosed herein are adaptable to tooling for performing operations other than hot plate melting of plastic. Examples of other operations include, but are not limited to, assembly, drilling, and cutting operations.