Abstract:
A system and method for positioning a fluid stream for cutting a double contour workpiece includes a compensation module configured to receive information regarding a contour path in at least five degrees of freedom for cutting the double contour workpiece and a velocity of movement of the fluid stream during cutting and configured to provide as an output a modified contour path of said at least five degrees of freedom based on Kerf compensation errors. A motion controller is adapted to receive the modified contour path of said at least five degrees of freedom and the velocity and is configured to provide control signals. A positioner is configured to receive the control signals and position a fluid stream adjacent the workpiece accordingly.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]     The present application is based on and claims the benefit of U.S. provisional patent applications Ser. No. 60/705,684, filed Aug. 4, 2005, and Ser. No. 60/815,032, filed Jun. 20, 2006, the contents of which are hereby incorporated by reference in their entirety. 
     
    
     BACKGROUND  
       [0002]     The discussion below is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.  
         [0003]     Systems using fluid such as water to cut material precisely are well known. Typically, such systems place the fluid under extreme pressure (e.g. 30,000 psi or higher) and force the fluid through an aperture or orifice so as to be discharged at a high velocity upon the material to be cut through an erosion process. In many applications, an abrasive is also introduced into the fluid stream and discharged with the fluid to improve the efficiency of the cutting action by enhancing the erosion process.  
         [0004]     Using a fluid stream to cut material produces cuts with characteristics different than those made with conventional cutters. Both  FIGS. 1 and 2  illustrate a fluid stream  10  exiting an orifice  12  of a nozzle  14  to cut a workpiece  16 . Typically, more than a hole is desired in the workpiece  16  so the nozzle  14  and hence the fluid stream  10  are moved along a desired path  15  relative to the workpiece  16 . In  FIG. 1 , the nozzle  14  moves in and out of the page, while in  FIG. 2  the nozzle  14  moves in the direction indicated by arrow  15 .  
         [0005]     Referring to  FIG. 1 , the resulting cut  20  made by the fluid stream  10  has a width on a top surface  22  (facing the nozzle  14 ) that differs in width from the bottom surface  24  (facing away from the nozzle  14 ). The resulting taper  28  due to the difference in widths is referred to as the “Kerf angle”  30 . Stated another way, the Kerf angle  30  is the angle the cut face  32  is out of parallel from the fluid stream axis (the stream is often not normal to the material surface by design). The taper  28  is a function of material thickness, but also is a function of cutting speed or movement of the nozzle  14 . In general, the taper  28  becomes less as cutting speed slows, and then as cutting speed further slows beyond a point, the taper  28  reverses from that illustrated in  FIG. 1  becoming narrower toward the surface  22 . Compensation for the taper  28  typically includes tilting the nozzle  14  relative to the workpiece  16  about the axis of motion of the nozzle  14 .  
         [0006]     In addition to the taper  28  present in the cut, a “lag” is present due again to the thickness of the material and movement of the nozzle  14 . Referring to  FIG. 2 , the faster the nozzle  14  moves, the more the fluid stream  10  is deflected by the material of the workpiece  16 . As illustrated, a deflection distance  32  is defined as the difference in length between the point where the fluid stream  10  impinges the top surface  22  and where the stream  10  exits the bottom surface  24 , whereas a “Kerf lag” can be defined as an angle  34  using a straight line  36  formed between these points. Typically, the Kerf lag  34  does not affect cutting accuracy when cutting a straight line since the exiting portion of the fluid stream  10  follows the impact point. However, on corners, for example, the deflection of the fluid stream  10  can cause cutting errors as it flares to the outside of a corner leaving behind or cutting undesirable deflection tapers. Furthermore, the finish of even straight line cuts is affected by the speed of the nozzle  14 . However, unlike the taper  28 , the lag  34  may be reduced by slowing the motion of the nozzle  14  across the workpiece  16 . Like the taper  28 , tilting of the nozzle  14 , in this case, about an axis transverse to the direction of motion can also provide some compensation for the lag  34 .  
         [0007]     Systems have been advanced using compensation for Kerf errors, nevertheless improvements are desired.  
       SUMMARY  
       [0008]     This Summary and the Abstract are provided to introduce some concepts in a simplified form that are further described below in the Detailed Description. The Summary and Abstract are not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter. In addition, the description herein provided and the claimed subject matter should not be interpreted as being directed to addressing any of the short-comings discussed in the Background.  
         [0009]     A system and method for positioning a fluid stream for cutting a double contour workpiece includes a compensation module configured to receive information regarding a contour path in at least five degrees of freedom for cutting the double contour workpiece and a velocity of movement of the fluid stream during cutting and configured to provide as an output a modified contour path of said at least five degrees of freedom based on Kerf compensation errors. A motion controller is adapted to receive the modified contour path of said at least five degrees of freedom and the velocity and is configured to provide control signals. A positioner is configured to receive the control signals and position a fluid stream adjacent the workpiece accordingly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is schematic illustration of a taper present in fluid stream cutting of the prior art.  
         [0011]      FIG. 2  is schematic illustration of fluid stream lag present in fluid stream cutting of the prior art.  
         [0012]      FIG. 3  is a flow diagram illustrating exemplary operation of a fluid stream cutting system.  
         [0013]      FIG. 4  is a pictorial representation of a cutting path provided with compensation.  
         [0014]      FIGS. 5A, 5B  and  5 C are pictorial representation of a polynomial based compensation for an exemplary material.  
         [0015]      FIG. 6  is an exemplary schematic illustration of a taper present in fluid stream cutting of the present invention.  
         [0016]      FIG. 7  is an exemplary schematic illustration of fluid stream lag present in fluid stream cutting of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0017]      FIG. 3  is a block/flow diagram illustrating exemplary operation of a fluid stream cutting system  100 . Generally, material is cut using a fluid stream cutting apparatus (also commonly referred to as a water jet system although other types of “fluids”, which is defined herein as including liquids, plasma, particles, gases or combinations thereof, can be used)  102 , which are well known and therefore is shown schematically. Referring to  FIGS. 6 and 7 , apparatus  102  includes nozzle  14 ′. At this point it should be noted prime numbers are used to indicated similar concepts above; however, the workpiece to be cut and the cutting process itself is different in that a complex workpiece that can have double contours and/or varying thickness is cut.  
         [0018]     In the present embodiment, the cutting nozzle  14 ′ of cutting apparatus  102  is moved relative to the material to be cut or workpiece by a multi-axis positioner (e.g. 5 or 6 axis control)  104 . Like the cutting apparatus  102 , such positioners are well known and need not be discussed in detail for purposes of understanding the concepts herein described.  
         [0019]     Briefly, the typical technique for fluid stream cutting is to mount the workpiece (sometimes also referred to as the “material being cut”) in a suitable jig. The fluid stream or fluid-jet is typically directed onto the workpiece to accomplish the desired cutting to produce a target piece having a shape and is generally under computer or robotic control. The cutting power is typically generated by means of a high-pressure pump connected to the cutting head through high-pressure tubing, hose, piping, accumulators, and filters. It is not necessary to keep the workpiece stationary and to manipulate the fluid-jet cutting tool. The workpiece can be manipulated under a stationary cutting jet, or both the fluid-jet and the workpiece can be manipulated to facilitate cutting. As will be described below, specifications of the desired workpiece to be cut are received by system  100  wherein cutting parameters such as but not limited to a cutting velocity or speed of the nozzle, its cutting path including orientation of the nozzle are determined in order to generate the desired workpiece with requisite compensation taking into account characteristics of the cutting process.  
         [0020]     In the exemplary embodiment illustrated, workpiece specifications are embodied in a Computer-Aided Design (“CAD”) program or model  106 . CAD models are well known and can be developed for the desired workpiece using a computer workstation (not shown) that is separate from or part of system  100 .  
         [0021]     The CAD model  106  is provided to a Computer-Aided Machining (CAM) system  108  that is used to determine initial machining parameters in order to generate the desired the workpiece including but not limited to the cutting path (i.e. motion profile), which can then be “post processed,” if necessary, into a format for a specific positioner or cutting apparatus.  
         [0022]     With reference to  FIG. 4 , in the exemplary embodiment described herein and for purposes of understanding, a cutting path  200  for a portion of a desired workpiece can be described in terms of a sequence of datasets  202  comprising coordinates in five degrees of freedom (X,Y,Z,C,B), e.g., three translations (X,Y,Z) and two angles of tilt or surface normal vectors (B,C) in a reference coordinate system  202 . It should be noted a cutting path having six degrees of freedom could also be used, where the sixth coordinate (A) relates to rotation of the cutting head about an axis orthogonal to the other mutually orthogonal axes of tilt (B,C).  
         [0023]     At this point it should be noted that the modules illustrated in  FIG. 3  and discussed below are presented for purposes of understanding and should not be considered limiting in that additional modules may be used to perform some of the functions of the modules herein described. Likewise, functions can be divided or combined in other ways between the modules. The modules can be implemented with digital and/or analog computational devices such as a computer.  
         [0024]     A compensation module  113  illustrated generally by dashed lines is illustrated for purposes of understanding as decision block  112 , path compensation assembly  140  and/or Kerf compensation component  160  and as described below provides a modified contour cutting path in at least 5 degrees of freedom and velocity.  
         [0025]     In addition to cutting path  200 , a velocity of the nozzle as a function of the cutting path can also be provided by CAM system  108  to form a “motion profile”, which is represented in  FIG. 3  at  110 . In addition to the cutting path or contour path, input  110  can include velocity indications or criteria (e.g. maximum velocity) Nevertheless, any initial velocity, if given, may not be optimal given the cutting conditions such as but not limited to the shape of the desired workpiece. Accordingly, the velocity may be adjusted as represented by decision block  112 .  
         [0026]     A model steady state velocity input  114  to block  112  is provided from a processing component  116  using known cutting models such as that described by J. Zeng in “Mechanisms of Brittle Material Erosion Associated With High Pressure Abrasive Waterjet Processing,” Doctoral Dissertation, University of Rhode Island, Kingston, R.I., 1992. In particular, Zeng describes that the cutting velocity can be determined using an equation of the form:  
       u   =       (         f   a     *     N   m     *     P   w   1.594     *     d   0   1.374     *     M   a   0.343         C   *   q   *   h   *     d   m   0.618         )     1.15         
 
 where 
    u: the cutting speed (mm/min or inch/min)     f a : abrasive factor (1 for garnet)     N m : machinability number     P w : water pressure (MPa or kps)     d 0 : orifice diameter (mm or inch)     M a : abrasive flow rate (g/min or lb/min)     q: quality level index     h: workpiece thickness (mm or inch)     d m : mixing tube diameter (mm or inch)     C: system constant (788 for Metric units or 163 for English units).    
 
         [0037]     In general, component  116  receives as input the type of material being cut  118 , a qualitative measure of the “quality” of the desired cut  120  and the thickness of the material  122 , and other parameters as indicated above in the equation above to determine the model steady state velocity  114 .  
         [0038]     However, a further velocity effect input  126  (also referred to as “transient look-ahead velocity effect”) provided herein allows the resulting velocity  128  from block  112  to be further modified based on constraints imposed by the physical movements of the nozzle. The velocity effect input  126  originates from a motion controller  148  for positioner  104 , which can include a module  149  that looks for conditions of needed velocity reductions. For example, and without limitation, it may be necessary to depart from the model steady state velocity  114  when approaching a sharp corner to be cut in the workpiece, where for instance, the velocity of the nozzle must be slowed down prior to reaching the actual corner to be cut. In yet another situation, velocity reduction would be necessary if the operator operates a “stop” switch during cutting. However, other motion modules  151  can also affect velocity such as motion of the nozzle to or away from the top surface  22  as monitored, for example, by a suitable sensor. In short, transient look-ahead velocity effect input  126  is based on any motion to be performed by the cutting nozzle that causes it to depart from velocity  114 .  
         [0039]     The velocity  128  ascertained at block  112  however does not compensate for the errors contributed by Kerf width  28 ′, taper  30 ′ and lag  34 ′ as discussed above, as illustrated in  FIGS. 6 and 7 . Path compensation assembly  140  is provided to address some of these errors. Path compensation assembly  140  is based on the use of polynomial equations or models  143  for each of the Kerf errors, Kerf width (Kw), Kerf angle (Ka) and Kerf lag (Kl) using empirical data  142  from actual cuts for various materials and material characterization data of the materials  144  along with inputs pertaining to the actual material being used, its thickness and the desired quality and the resulting velocity  128  from block  112 . Steady-state (constant operating conditions including but not limited to velocity) Kerf error compensation for Kerf width (Kw), Kerf angle (Ka) and Kerf lag (Kl) is provided. However, prior techniques did not include a dynamic aspect for such compensation, which is provided by the feedback of velocity input  126  from a motion controller  148  for positioner  104 . In yet a further embodiment, such compensation, either static (without input  126 ) or dynamic (with input  126 ), is provided when cutting a workpiece requiring at least 5 degrees of freedom, that is, cutting a workpiece that can have a double contour, which is a significantly different and more complex operating environment than cutting a workpiece in a plane, yet allowing the nozzle to provide at least two degrees of tilt for Kerf compensation. Stated another way, since the dynamic constraints of the motion controller  148  as provided by the feedback of transient look-ahead velocity effect input  126  reduces the resulting velocity  128  from that which would otherwise be used, path compensation assembly  140  can calculate, in a dynamic sense, the compensation required for the Kerf based errors. Using the example of reducing the velocity for an upcoming sharp corner that needs to be cut, Kerf based errors are dynamically compensated due to the over-eroding cutting nature of fluid stream cutting as velocity of the nozzle reduces.  
         [0040]     It should be noted that since the polynomial models for Kerf errors can also be based on the thickness of the material being cut, thickness values can be provided from a cross-section analyzer  154  based on the known geometry of the material/workpiece. However, in a further embodiment, in addition or in the alternative to cross-section analyzer  154 , a cross-section analyzer sensor  156  can provide a signal related to thickness as actually measured during cutting. Examples of suitable sensors include but are not limited to mechanical, optical, electric ultrasonic based sensors. This feature of cutting material to desired shape as well as quality specifications for a constantly varying thickness is particularly useful in complex, arbitrary double contour workpieces such as airplane wing components that commonly vary in thickness.  
         [0041]     In view that the polynomial models  143  are typically based on a family of curves, a model interpolation component  150  is provided for operating points between stored curves.  FIGS. 5A-5C  are representations of polynomial based Kerf error compensation for an exemplary material.  
         [0042]     A Kerf compensation component  160  accepts the Kerf width, Kerf angle, Kerf lag based errors calculated from path compensation assembly  140  as well as the velocity and the contour path datasets (X,Y,Z,C,B) for five dimensional control cutting and (X,Y,Z,C,B,A) for six dimensions, if desired, from CAM system  108 . The Kerf compensation component  160  applies the Kerf compensation errors calculated by path compensation assembly  140  to the specific location of the actual contour being cut. In other words, the Kerf compensation error information provided by path compensation assembly  140  by itself is not enough to move the nozzle  14 ′. The Kerf compensation component  160  includes an instantaneous tool path vector calculator  162  that computes an instantaneous motion path vector from the part program point in the neighborhood of the current position so as to determine which way compensation needs to be provided in view of what side at any given position is part of the desired workpiece versus the waste, salvage or drop material. In the illustrated embodiment, the 5 or 6 axes part program and the computed motion vector are then used to compute the instantaneous 5D or 6D motion command or tool frame by component  166 . In a dynamic mode, other linear, angular, and/or velocity effects determined by the motion planner are incorporated simultaneously. The total compensation, consisting of Kerf width, Kerf angle, Kerf lag, and motion planner effects, are applied to the command frame by component  170 . The resultant modified path and velocity can be stored at  168  and, if desired, a summary report containing relevant information pertaining to the cutting process can also be generated and stored also at  168  such as how long the workpiece took to be cut. It is noteworthy to realize that this report can be based on simulated cutting because given the known cutting path and the dynamic velocity changes, actual overall cutting time can then be estimated, or other problems can be detected prior to actual cutting. However, in addition, or in the alternative, in a real-time cutting mode, the modified path and velocity data is submitted, for execution by the motion controller  148 .  
         [0043]     Referring back to cutting or tool path  200  in  FIG. 4 , the form of compensation provided can also be explained. Path  200  is defined relative to some reference or command coordinate system  204 ; however, in view that at least five degrees of motion control define the cutting path  200 , two degrees of tilt (surface normal vectors) are also provided. Accordingly, as indicated above, defined points  202  on the cutting path are represented (by way of example with five degrees of control) as (X,Y,Z,C,B).  
         [0044]     At each point in the tool path  200 , the adjacent points before and after the current point under consideration are examined in order to determine a instantaneous motion vector  206  at the current point (point  202 A by way of example). The instantaneous motion vector  206  is then used in order to ascertain the cross-section  208  of the cut being made ( FIG. 1 ), which is orthogonal to the instantaneous motion vector  206 , as well as the cross-section along the cut ( FIG. 2 ), which is along the instantaneous motion vector  206 . Thus, the Kerf corrections are made relative to the instantaneous coordinate frame at the current position  202 A and translated back into the reference coordinate system  204  as (X′,Y′,Z′,B′,C′) where no velocity feedback effect  126  is provided, or as (X″,Y″,Z″,B″,C″) when velocity feedback effect  126  is present.  
         [0045]     Kerf compensation component  160  can also factor in other process variables monitored by a process monitoring module  182  such as but not limited to the changing diameter of the orifice as the nozzle wears (due for example to “Jet-on” time), abrasive rate, pressure, etc. This is illustrated by signal line  180 , the input of which can also be applied to path compensation assembly  140 . Although not directly pertinent to the Kerf compensation, a module  184  can be provided to signal when the nozzle requires replacement or when other process variables require attention.  
         [0046]     In summary, some aspects herein described include Kerf compensation in a true five dimensional or more cutting environment, the compensation of which can further include dynamic compensation based on constraints or desired motion of the nozzle for other reasons besides cut quality, as well as workpieces having a constantly vary thickness. However, it should be noted the compensation herein provided is not limited to a static cutting path/orientation based on post processing of the initial cutting path (relative to CAM system  108 ) or compensation provided during dynamic motion control (during actual cutting), but rather a compensation mechanism that can be used in each one separately, or a combination of the foregoing situations.  
         [0047]     Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not limited to the specific features or acts described above as has been held by the courts. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.