Abstract:
A collision detection sensor for a waterjet system provides a signal in the event the device approaches to within a selected distance of an obstruction in the plane of the working surface. An annular pressure switch lying in a first plane provides the signal when radial pressure is applied to a perimeter of the pressure switch via an annular trigger skirt, the trigger skirt applying the radial pressure in response to a collision of the device with an obstacle.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 10/922,238, filed Aug. 19, 2004, which issued as U.S. Pat. No. 7,331,842 on Feb. 19, 2008, which patent is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention is related, generally, to waterjet cutting systems, and, in particular, to a method and apparatus for controlling the orientation and position of a waterjet cutting head with respect to a surface. 
     2. Description of the Related Art 
     Waterjet and abrasive-jet cutting systems are used for cutting a wide variety of materials, including stone, glass, ceramics, and various metals, including stainless steel. Such systems are capable of cutting material thicknesses ranging up to and exceeding two inches. Thinner material may be stacked for cutting multiple pieces simultaneously. 
     In a typical fluid jet cutting system, a high-pressure fluid (e.g., water) flows through a cutting head having a cutting nozzle that directs a cutting jet onto a workpiece. The system may draw an abrasive into the high-pressure fluid jet to form an abrasive jet. The cutting nozzle may then be controllably moved across the workpiece to cut the workpiece as desired. After the fluid jet, or abrasive-fluid jet, generically referred to throughout as a cutting jet, passes through the workpiece, the energy of the cutting jet is dissipated and the fluid is collected in a catcher tank for disposal. Waterjet and abrasive-jet cutting systems of this type are shown and described, for example, in U.S. Pat. No. 5,643,058 issued to Erichsen et al., and assigned to Flow International Corporation of Kent, Wash., which patent is incorporated herein by reference, in its entirety. The &#39;058 patent corresponds to Flow International&#39;s Paser 3 abrasive cutting systems. 
       FIG. 1  is an isometric view of a waterjet cutting system  100  in accordance with the prior art. The waterjet cutting system  100  includes a cutting head  120  coupled to a mount assembly  104 . The mount assembly  104  is controllably driven by a control gantry (not shown in detail) having a drive assembly  108  that controllably positions the cutting head  120  throughout an X-Y plane that is substantially parallel to a surface  110  of a workpiece  112 . Typically, the drive assembly  108  may include a pair of ball-screw drives oriented along the X and Y axes, each coupled to an electric drive motor. A Z-axis control mechanism  106  is coupled to the drive assembly and controls the position of the mount assembly in a Z-axis, substantially perpendicular to the surface  110 . 
     Alternatively, the drive assembly  108  may include a five-axis motion system. Two-axis and five-axis control gantries are commercially-available as the WMC (Waterjet Machining Center) and the AF Series Waterjet cutting systems from Flow International of Kent, Wash. 
     The cutting head  120  includes a high-pressure fluid inlet  114  coupled to a high-pressure fluid source  116 , such as a high-pressure or ultrahigh-pressure pump, by a high-pressure line  118 . In this embodiment, the cutting head  120  includes a mixing tube  122  terminating in a jet exit port  124 , from which a high-pressure stream of fluid, i.e., waterjet  126 , is emitted and directed at the workpiece  112 . 
     Although the term “mixing tube” is commonly used to refer to that portion of an abrasive-jet cutting system in which abrasive is mixed with a high-pressure fluid jet to form an abrasive cutting jet, in the following discussion, “mixing tube” may be used to refer to the nozzle through which a jet is discharged, regardless of whether the system uses an abrasive or non-abrasive cutting jet. In addition, the terms “waterjet” or “cutting jet” will be used to refer to the stream of fluid  126 , also regardless of whether or not the stream includes abrasive. 
     A particular challenge in waterjet cutting systems is the provision of an appropriate support for the workpiece, inasmuch as any surface upon which the workpiece is supported will be subjected to the cutting force of the waterjet  126 . A common system includes a grid  128  formed by a plurality of slats  130  positioned across a catcher tank (not shown). Upper edges of the slats  130  lie in a plane that is parallel to the X-Y plane. The workpiece  112  is supported on the grid  128  for cutting. A notch  134  (see  FIG. 5 ) is cut into each slat  130  as the waterjet  126 , penetrating through the workpiece  112 , passes across the slat. The depth of the notch  134  will depend upon factors such as the traverse speed of the waterjet  126 , and the thickness and hardness of the workpiece  112 . The depth D of the slats  130 , as shown in  FIG. 1 , is selected to tolerate significant exposure to the waterjet  126  as it repeatedly passes across the slat during successive cutting operations. Eventually, damage to the grid  128  reaches a level that the grid  128  must be replaced. 
     In operation, ultrahigh-pressure fluid is directed through an orifice (not shown) positioned in the cutting head to form an ultrahigh-pressure fluid jet  126 . As discussed previously, the system may or may not entrain abrasive into the jet. The jet exits the mixing tube  122 , whereby it is directed toward the workpiece  112 . The cutting jet  126  pierces the workpiece  112  and performs the desired cutting. Using the control gantry, the cutting head  120  is traversed across the workpiece  112  in the desired direction or pattern. 
     To maximize the efficiency and quality of the cut, a standoff distance S (see  FIG. 5 ) between the jet exit port  124  of the mixing tube  122  and the surface  110  of the workpiece  112  is controlled. If the standoff distance S is too small, the mixing tube  122  can plug during piercing, causing system shutdown and possibly a damaged workpiece  112 . If the distance is too great, the quality and accuracy of the cut suffers.  FIGS. 2 and 3  illustrate two known devices for determining the position of the workpiece relative to the mixing tube  122 , for the purpose of establishing standoff D. The devices described with reference to  FIGS. 2 and 3  are described in more detail in U.S. Patent Publication No. 2003/0037650 in the name of Knaupp et al. and assigned to Flow International Corporation of Kent, Wash., which publication is incorporated herein by reference, in its entirety. 
     The probe  138  of  FIG. 2  is configured to extend, via actuation of a pneumatic cylinder, until it touches the surface  110  of the workpiece  112 . The height of the surface  110  is thereby ascertained, the probe  138  is then withdrawn, the mixing tube  122  is positioned appropriately in the Z-axis, and cutting commences.  FIG. 2  also shows a shield  136 , configured to capture a significant amount of spray-back that occurs during a piercing operation, as described in more detail below. 
     The contact ring  140  of  FIG. 3  is positioned coaxially with the mixing tube  122  and coupled to an actuator via a cantilevered rod  144 . The contact ring  140  is configured to descend along the axis of the mixing tube  122  until it contacts the surface  110  of the workpiece  112 . The height of the surface  110  having been established, the contact ring  140  may then be withdrawn or may be configured to remain in contact or near contact with the surface  110  during the cutting operation. Because a sensor associated with the contact ring  140  is capable of continuously monitoring the height of the surface  110 , the associated cutting system can correct for changes in height of the workpiece  112 . However, a device such as the shield  136  of  FIG. 2  cannot be used concurrently with the contact ring  140 . 
     When the system  100  is properly configured, and it cuts a continuous line through a workpiece  112 , virtually all of the cutting fluid passes through the workpiece  112  to be captured in the catcher tank below. However, at the beginning of a cut while the waterjet  126  is impinging on a surface, but has not yet penetrated the surface, spray-back occurs, in which some or all of the fluid rebounds upward. Primary spray-back occurs while the waterjet  126  is first piercing the workpiece  112 . In particular, a large portion of the primary spray-back occurs along an angle reciprocal to the angle of the waterjet  126 , and thus, returns directly upward to the cutting device. This high-angle component of the spray-back also retains a significant fraction of the initial energy. Accordingly, it can be very damaging to components of the cutting system, especially in systems employing abrasives in the fluid stream. 
       FIG. 2  illustrates a spray-back shield  136  according to known art (described in more detail in the &#39;650 publication). The shield  136  is configured to block and dampen the high-angle portion of spray-back and substantially prevents damage to components of the cutting system by the spray-back, and potential damage or injury to objects in the path of the spray-back. 
     As previously described, spray-back occurs when the waterjet  126  impinges but does not fully penetrate a surface.  FIG. 4  illustrates a waterjet  126  traveling in direction T and cutting through a workpiece, and into slats  130  of the grid  128 . The waterjet  126  loses energy as it passes through the workpiece, and cuts a notch  134  into the slats  130  to a depth N at which the energy of the waterjet  126  is insufficient to cut any deeper, although the energy remaining in the stream is still substantial. It may be seen that the advancing front  133  of the notch  134  has a curved shape as the waterjet  126  traverses the notch, while the bottom of each notch  134  is substantially horizontal. 
     For the purpose of this description, primary spray-back is that resulting from reflectance of the waterjet by a workpiece, while secondary spray-back results from reflectance of the waterjet by a structure beneath the workpiece. 
     Unlike the primary spray-back of a piercing operation, secondary spray-back, as illustrated in  FIG. 4 , is reflected back by the curved front  133  of the notch  134  in a fan shaped spray, in a direction substantially opposite the direction of travel T. The spray-back shield  136  captures only the highest-angle portion of the secondary spray-back. In some cases, depending on factors such as the speed, direction of travel T, the condition of the slat  130 , the angle of the cut with respect to the slat  130 , etc., a portion of the secondary spray-back can blast back through the kerf  132  of the workpiece  112  at a low angle and travel some distance from the cutting site. This kind of spray-back will be attenuated if the system is cutting a curved line, since the curved wall of the kerf will block some or all of the spray-back. 
     The most powerful secondary spray-back occurs when the direction of travel T is incident to the slat  130 , that is, when the direction of travel T is parallel to the slat  130 , and directly above, such that the waterjet  126  passes through the workpiece  112  and impinges directly on the upper surface of the slat  130  for an extended distance. In this configuration, very little of the cutting fluid can escape downward into the catcher tank, and so is driven upward through the kerf  132 . 
     The mixing tube  122  is typically fabricated of specially formulated carbides to resist wear. Particularly for abrasive cutting systems, the mixing tube  122  suffers extreme wear due to its constant contact with high velocity abrasives. Thus, mixing tubes are a relatively expensive component of the system. The specially formulated carbides may also be brittle, and can easily break if the mixing tube  122  collides with an obstruction during operation of the cutting system  100 , such as fixturing or cut-out portions of the workpiece  112  which may have been kicked up during the cutting operation. Accidental breakage of the mixing tube  122  increases operational costs and downtime of the cutting system  100 . 
     Several collision sensor systems are known in the art. For example, a ring sensor, similar in appearance to the annular sensor  140  of  FIG. 3 , may be positioned in contact with, or just above the surface of a workpiece during a cutting operation. An obstruction will make contact with the ring portion of the sensor prior to contacting the mixing tube  122 . The sensor is configured to respond to contact with the obstruction by initiating a shut down of at least the drive motors of the cutting system, and generally the waterjet  126  as well, to prevent damage to the mixing tube  122 , and minimize damage to the workpiece. 
     Another collision detection system comprises a device having a portion of the cutting head configured to break away without damage to the mixing tube, in the event of a collision. The system is described in detail in U.S. Pat. No. 6,540,586, issued to Felice Sciulli et al., and assigned to Flow International Corporation of Kent, Wash., which patent is incorporated herein by reference, in its entirety. 
     Manipulating a jet in five axes may be useful for a variety of reasons, including, for example, cutting a three-dimensional shape. Such manipulation may also be desired to correct for cutting characteristics of the jet or for the characteristics of the cutting result. More particularly, as understood by one of ordinary skill in the art, a cut produced by a jet, such as the abrasive waterjet  126  of  FIGS. 1-4 , has characteristics that differ from cuts produced by more traditional machining processes. 
     Two of the cut characteristics that may result from use of a high-pressure fluid jet are referred to as “taper” and “trailback.”  FIG. 5  shows an exemplary illustration of taper. The mixing tube  122  of  FIG. 5  is traveling along an X-axis, perpendicular to the plane of the drawing. Taper refers to the angle A of a plane of one wall of the kerf  132  relative to a vertical plane. Taper typically results in a workpiece  112  that has different dimensions on the top surface  110  (where the jet  126  enters the workpiece) and the bottom surface  111  (where the jet  126  exits the workpiece). 
       FIG. 6  shows an example of trailback. The mixing tube  122  of  FIG. 6  is traveling in direction T along the X-axis, parallel to the plane of the drawing. Trailback, also referred to as drag, is a condition in which the high-pressure fluid jet  126  exits the bottom surface  111  of the workpiece  112  at a point behind the point of entry of the jet  126  on the top surface  110  of the workpiece  112 , relative to the direction of travel T. The trailback angle is the angle B of a line extending through a point of entry to a point of exit of the jet  126  relative to a vertical line. 
     These two cut characteristics, namely taper and trailback, may or may not be acceptable, given the desired end product. Taper and trailback vary, depending upon the thickness and hardness of the workpiece  112  and the speed of the cut. Thus, one known way to control excessive taper and/or trailback is to slow down the cutting speed of the system. Alternatively, in situations where it is desirable to minimize or eliminate taper and trailback while operating at higher cutting speeds, five-axis systems may be used to apply taper and lead angle corrections to the jet  126  as it moves along the cutting path, as illustrated in  FIGS. 7 and 8 . 
     It will be assumed, for the purpose of this description, that the portion of the workpiece to the right of the mixing tube  122  of  FIG. 7  comprises the finished product, while the portion to the left is scrap. The mixing tube  122  is rotated around an axis parallel to the X-axis, until the right wall of the kerf  132  is substantially vertical. 
     As shown in  FIG. 8 , the mixing tube  122  is rotated around an axis parallel to the Y-axis, such that the waterjet  126  is angled into the direction of travel T until the trailback is substantially eliminated, as shown. 
     It will be recognized that, as the direction of travel T changes during the course of a cutting operation, the fourth and fifth axis rotations compensating for taper and trailback must change accordingly. A method and system for automated control of waterjet orientation parameters is described in U.S. Pat. No. 6,766,216 issued to Erichsen et al., and assigned to Flow International Corporation of Kent, Wash., which patent is incorporated herein by reference, in its entirety. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an embodiment of the invention, a contour follower device is provided, for use with a tool configured to travel along first, second, and third axes. The tool may be part of a waterjet cutting system or other system in which determination of position, distance or angle of a working surface may be advantageous. In the case of a waterjet cutting system having a cylindrical member with an exit aperture, the device includes a plurality of sensor legs having first and second ends, the first ends coupled to the tool at respective leg positions evenly spaced around the cylindrical member in a first plane perpendicular to an axis of the cylindrical member, each of the sensor legs being configured to change in length in response to variations of displacement of the respective second ends, the second ends of the plurality of sensor legs together defining a second plane. The device also includes a plurality of sensors, each positioned adjacent to a respective one of the plurality of sensor legs and configured to sense a length of the respective sensor leg. 
     According to an embodiment of the invention, each of the plurality of sensor legs comprises a cylinder coupled to the device at the respective leg position and a sensor shaft having a selected length and first and second shaft ends, the first shaft end being positioned in the respective cylinder, the second shaft end extending therefrom, each of the plurality of sensor shafts being configured to move axially within the respective cylinder in response to variations of displacement of the second end of the respective sensor leg. The first end of the sensor shaft of each of the plurality of sensor legs comprises a magnet and each of the plurality of sensors comprises a hall-effect sensor configured to interact with the respective magnet. A bellows, substantially enclosing the respective cylinder and sensor shaft, is provided for each of the sensor legs. Each of the respective bellows is configured to permit travel of the sensor shaft within the cylinder. A hermetic seal between the respective sensor leg and the device is provided, and the device further comprises a gas channel configured to permit passage of gas to and from each of the plurality of sensor legs as each of the respective sensor legs expands or contracts. 
     Processing means is provided for processing a signal provided by at least one of the plurality of sensors and establishing a distance from the second plane to the exit aperture of the cylindrical member, along the axis of the member. The processing means may also include means for controlling movement of the tool in the third axis, and may further include means for establishing an angle of the second plane relative to the first plane, and controlling movement of the tool around fourth and fifth axes lying in a plane parallel to the first and second axes. 
     According to an embodiment of the invention, a plate is provided, coupled to the second end of each of the plurality of sensor legs, an aperture traversing the plate from a first side to a second side in a location corresponding to a position of the exit aperture of the cylindrical member. The plate is configured to block secondary spray-back of the waterjet cutting system. 
     According to an embodiment of the invention, a collision detection sensor is coupled to the device and configured to provide a signal in the event the tool approaches to within a selected distance of an obstruction along the first and second axes. The collision detection sensor may include a plurality of trigger legs arranged in a circle, the circle lying in a plane parallel to the second plane, each of the trigger legs configured to activate the signal when moved inward toward the tool. 
     According to an embodiment of the invention, a brush foot is provided, coupled to the second end of each of the plurality of sensor legs, and having a substantially circular support ring and a plurality of bristles coupled to, and extending from, the support ring with outer ends of each of the plurality of bristles collectively defining a third plane parallel to the second plane. The brush foot is configured to contact a work surface with the outer ends of at least some of the plurality of bristles, such that a change in angle of the work surface relative to the first plane is transmitted, via the brush foot, to the plurality of sensor legs and reflected in a corresponding change in length of each plurality of sensor legs. 
     According to another embodiment of the invention, a method of operation is provided. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       In the drawings, identical reference numbers identify similar elements or acts. The sizes and relative positions of elements in the drawings are not necessarily drawn to scale. 
         FIG. 1  is an isometric view of a waterjet cutting system provided in accordance with prior art. 
         FIGS. 2 and 3  illustrate details of two known waterjet cutting systems. 
         FIG. 4  illustrates a waterjet cutting through a workpiece, according to known art. 
         FIGS. 5 and 6  show typical cutting characteristics of waterjet cutting systems. 
         FIGS. 7 and 8  show methods of compensation for characteristics pictured in  FIGS. 5 and 6 . 
         FIG. 9  is an orthographic view of a contour follower assembly according to an embodiment of the invention. 
         FIG. 10  is a partially exploded view of the contour follower of  FIG. 9 . 
         FIG. 11  is a plan view of the contour follower of  FIG. 9 . 
         FIG. 12  is a cross-section of the contour follower of  FIG. 9 , taken along lines  12 - 12  of  FIG. 11 . 
         FIG. 13  is a cross-section of the contour follower of  FIG. 9 , shown positioned on an angled workpiece. 
         FIG. 14  shows an enlarged view of a small portion of the contour follower of  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While many of the challenges associated with waterjet machining have been described in the background section, there is at least one issue that has not yet been addressed. It has previously been assumed, for the purpose of determining the appropriate position of the mixing tube  122 , that the upper surface  110  of the workpiece  112  will be substantially horizontal, or will lie in a plane that is parallel to the X-Y plane. Most sensors configured to determine the position of the upper surface  110  make that determination prior to the beginning of a cutting operation, such as the sensor described with reference to  FIG. 2 . Even in the case of the sensor described with reference to  FIG. 3 , which can be configured to continually track the surface  110  of the workpiece  112  as the cutting operation progresses, such a sensor can only detect changes in height of the upper surface  110 , and cannot determine the angle of that surface, with respect to the X-Y plane. 
     Accordingly, errors in cutting angle, that is, the angle at which the waterjet  126  impinges the surface  110 , may be introduced into the cutting process because the system is incapable of compensating for variations in surface angle of the workpiece. Some materials that are commonly machined using waterjet processes may be less than perfectly planar. For example, large pieces of sheet metal may have significant changes in elevation and contour over the width and breadth of the piece. Additionally, as a cutting operation progresses, the surface may shift and flex. For example, the balance of internal stresses that are inherent in the crystalline structure of a steel member may suddenly change, causing a portion of a large piece of sheet metal to suddenly flex, altering the relative height and angle of the upper surface thereof. Systems that do not continually monitor the height of the upper surface are subject to a collision of the mixing tube against a suddenly raised portion of the surface, while even systems that do monitor such a height, cannot compensate for the change in surface angle, relative to the cutting angle. 
     Various features and embodiments of the invention will be described now, with reference to  FIGS. 9-14 . 
       FIG. 9  is an orthographic view of a contour follower assembly  150  according to an embodiment of the invention. The contour follower  150  comprises a plurality of subassemblies, including a nozzle nut assembly  152 , a printed circuit board (PCB) assembly  154 , a plurality of sensor leg assemblies  156 , a foot plate assembly  158 , and a collision sensor assembly  160 . The embodiment described herein includes three sensor legs, though the invention is not limited to that number. 
       FIG. 10  is a partially exploded view of the contour follower  150 , providing additional detail, with respect to the various assemblies, and their respective positions.  FIG. 11  is a plan view of the contour follower  150 , showing relative positions of many of the components, including the sensor leg assemblies  156 , in hidden lines. Details of the nozzle nut assembly  152  that would normally be visible in plan view have been omitted to permit a clearer viewing of features of a carbide sleeve  224 , shown in hidden lines. 
     Referring now to  FIG. 12 , a cross-section of the contour follower  150  is shown, taken along lines  12 - 12  of  FIG. 11 . As shown in  FIG. 12 , the cutting head  120  is provided with a collet  121  coupled coaxially to the mixing tube  122 . The nozzle nut assembly  152  includes a nozzle nut  222  configured to engage the collet  121 , thereby coupling the contour follower  150  with the cutting head  120  and the mixing tube  122 . The nozzle nut assembly  152  further includes a carbide sleeve  224  and a resilient sleeve  226 . O-ring  168  provides for an interference fit between the carbide sleeve  224  and the nozzle nut  222 , and is sufficient to hold the carbide sleeve securely during operation. The nozzle nut  222  is further provided with barbs  223  configured to receive the resilient sleeve  226  thereon. The resilient sleeve  226  may be formed of natural or synthetic rubber, or other similar resiliently yielding material. 
     As previously described, high-angle primary spray-back occurs with great force while the waterjet is first piercing the workpiece. The carbide sleeve  224  serves to capture and dampen this spray-back. Fluid relief apertures  228  vent a portion of the fluid through the wall of the carbide sleeve  224 . The plan view of  FIG. 11  shows the carbide sleeve  224  and the fluid relief apertures  228  in hidden lines. It may be seen that the fluid relief apertures  228  are oriented so that abrasive fluid exiting through the apertures  228  is directed between the sensor leg assemblies  156 , preventing possible damage thereto. 
     Resilient sleeve  226  provides final damping to fluid exiting the fluid relief apertures  228 . The resilient sleeve  226  is loosely fitted around the carbide sleeve  224 , such that passage of the fluid is not impeded, but energy is dampened. Some components of the contour follower  150  are described as being formed of a particular material. Such descriptions are for illustration only. For example, the carbide sleeve  224  may be formed of any material capable of withstanding the erosive effects of the spray-back, including other high-hardness metals, resilient materials, or even plastics. Similarly, other references to particular materials in describing an embodiment of the invention should not be considered limiting, with respect to the scope of the invention. 
     The PCB assembly  154  comprises a resin or polymer encased printed circuit board (PCB)  170 . In one embodiment, PCB  170  includes a plurality of hall-effect sensors  172 , each positioned directly above one of the plurality of sensor legs  156 , as shown in  FIG. 11 . The hall-effect sensors together define an upper machine plane U that lies perpendicular to an axis of the mixing tube  122 . 
     The resin or polymer encasement of the PCB  170  renders the PCB impervious to contamination by various materials and substances, especially waterjet cutting fluid and abrasives, such as are ubiquitous during normal cutting operations. Power and control are supplied to the PCB  170  via cable  173 , whose extreme end is also encapsulated with the PCB  170 . A grounding strap  188  grounds the PCB  170  to a housing plate  174 . PCB  170  is mounted on the housing plate  174 , which is in turn coupled to the nozzle nut  222 , and thereby maintained in a plane that lies substantially perpendicular to an axis of the mixing tube  122 . A clamp ring  176  engages a perimeter of the PCB  170 . Fasteners  177 , passing through apertures in the clamp ring  176  and the PCB  170 , engage threaded apertures in the housing plate  174  to retain the clamp ring  176  and the PCB  170 . The shapes of the PCB  170  and the housing plate  174  cooperate with each other so as to fit snugly together. An annular channel  178  is defined by a narrow gap between the PCB  170  and the housing plate  174 . O-rings  171 , positioned in annular grooves formed in the housing plate  174 , seal the annular air channel  178 , preventing the entry of fluid or other contaminants. 
     The housing plate  174  includes a plurality of sensor apertures  184  and a vent aperture  186 , each in fluid communication with the annular channel  178 . An elbow fitting  180  is coupled to the vent aperture  186  and a vent tube  182  is coupled to the elbow fitting  180  as shown. The vent tube  182  has a length sufficient, that a second end thereof is positioned well away from the contour follower  150  and the mixing tube  122 , and thus is not susceptible to the entry of contaminants such as cutting fluids and abrasives. Accordingly, air is free to enter the air channel  178  via the elbow fitting  180  and the vent tube  182  without admitting contaminants therethrough. 
     Due to the density of detail in  FIG. 12 , some of the features of the contour follower  150 , and in particular, of the sensor leg assemblies  156 , are not referenced in  FIG. 12 , but may be seen more clearly in  FIGS. 10 and 13 . 
     Each of the plurality of sensor leg assemblies  156  comprises an upper member  190  and a lower member  196 . The upper member  190  includes an aperture  191  formed coaxially therethrough, and configured to receive a sensor shaft  192 . The upper member  190  further comprises a mounting flange  193  configured to engage a mounting socket  189  formed in a lower surface of the housing plate  174 , in a snap fit. An upper portion of the aperture  191  corresponds in position to a respective one of the plurality of sensor apertures  184  of the housing plate  174 . The upper member  190  also includes a barbed region  195  configured to receive a cylindrical bellows  202  for coupling thereto. 
     The sensor shaft  192  is positioned within the aperture  191  of the upper member  190 , such that it is free to move vertically within the upper member  190 . The sensor shaft  192  includes a magnet  194  received into an aperture formed at one end thereof. Vertical movement of the sensor shaft  192  causes the magnet  194  to move closer to, or further away from, the corresponding hall-effect sensor  172 . A spring  198  is constrained between a lower portion of the upper member  190  at one end and a keeper ring  200  coupled to the sensor shaft  192  at the other end, such that the sensor shaft  192  is biased in a downward direction relative to the upper member  190 . Lower ends  185  of the sensor shafts  192  of each of the sensor leg assemblies  156  together define a lower machine plane L. 
     The lower member  196  includes a bearing surface  197  upon which the lower end  185  of the sensor shaft  192  is configured to bear. The bearing surface  197  has a surface area sufficient to accommodate some lateral movement of the lower end  185  of the sensor shaft  192 . The lower member  196  further includes a mounting flange  199  configured to be received into a mounting socket  201  of a foot plate  218  via a snap fit. The lower member  196  further includes a barbed region  203  configured to receive the cylindrical bellows  202  for coupling thereto. 
     The cylindrical bellows  202  (see  FIG. 12 ) is coupled at a first end to the upper member  190  at the barbed region  195  thereof, and to the lower member  196  at the barbed region  203  thereof. Hose clamps  204 , or the like, serve to secure the bellows  202  in place. Each of the cylindrical bellows  202  is formed of a resilient material and is configured to accommodate expansion or contraction of the sensor leg assembly  156 , as the sensor shaft  192  moves up and down within the aperture  191 . The bellows is also configured to prevent fluids and other contaminants from interfering with the function of the sensor leg assembly  156 . 
     As may be seen in  FIG. 12 , the aperture  191  of the upper member  190  is aligned with, and in fluid communication with, the sensor aperture  184 . Accordingly, as the bellows  202  expands and contracts, air within the bellows  202  is free to pass through the air channel  178  and the vent tube  182 . A passage (not shown) may be provided in the sensor shaft  192  or the upper member  190  to facilitate movement of air past the sensor shaft  192  and magnet  194  in the aperture  191 . Alternatively, the shape of the upper portion of the sensor shaft  192  and magnet  194  may be selected to permit passage of air. 
     The foot plate assembly  158  includes a foot plate  218 , a shield plate  220 , and a foot brush  230 . The foot plate has an annular shape and includes a plurality of mounting sockets  201 , each corresponding in position to one of the plurality of sensor legs  156 , and a central opening. Each mounting socket  201  is configured to receive the mounting flange  199  of the lower member  196  of the respective sensor leg  156  in a snap fit. 
     The shield plate  220  is formed of an abrasive resistant material, such as carbide, for example. The shield plate has an annular shape, with a raised flange  236  at an inner edge thereof. The raised flange  236  is configured to engage the central opening of the annular shaped foot plate  218  in an interference fit. Additionally, a retaining ring  234  may be pressed onto the flange  236  of the shield plate  220  to further secure the shield plate  220  to the foot plate  218 . 
     As has been described with reference to  FIG. 4 , secondary spray-back resulting from passage of the waterjet  126  over a slat  130  can reflect in a fan shaped spray backward from the direction of travel. For several reasons, the energy of the spray-back diminishes in direct relation to the angle of reflectance. Thus, the highest-energy spray-back is the high-angle spray captured by the carbide sleeve  224 . The shield plate  220  has a diameter sufficient to block most of the remaining spray-back that rises above the surface  110  of the workpiece  112 , with a small, relatively low energy, portion being blocked by the foot brush  230 . The flange  236  of the shield plate  220  deflects any spray passing between the carbide sleeve  224  and the shield plate  220 . 
     Most of the primary and secondary spray-back is deflected by various components of the contour follower  150 , as described above. One benefit of this is that the area immediately surrounding a cutting system so equipped is less prone to water spills and damage, and easier to keep dry. 
     Because a high percentage of cutting operations involves linear cuts along the X-axis or the Y-axis, and because the most powerful secondary spray-back occurs in cuts that are in a direction of travel incident to the slats  130 , which are also generally aligned with the X an Y axes, the bottom of the shield plate  220  may wear excessively on lines corresponding to the X and Y axes. Accordingly, the shield plate may be oriented on the foot plate at any angle, and may be rotated periodically to evenly distribute the wear. 
     The foot brush  230  has an annular shape with an inner groove  231  formed around an inner wall thereof, and an outer groove  237 . The groove  231  is configured to engage an outer rim  233  of the foot plate  218 . The annular shaped foot brush  230  has a radial split  235 , which allows the brush  230  to be expanded sufficiently to be positioned with the groove  231  in engagement with the rim  233  of the foot plate  218 . The foot brush  230  further includes a plurality of short bristles  232  extending downward therefrom. During operation, the lower ends of the bristles  232  rest on the upper surface  110  of the workpiece  112 , and so conform to a plane thereof. This plane may be referred to as the working plane, or working surface. It will be recognized that, during cutting operations, the lower machine plane L is parallel to the upper surface  110 . 
     The collision sensor assembly  160  comprises a pressure switch support ring  210  having an annular shape and a ridge  211  formed around an inner surface thereof and configured to engage the outer groove  237  of the foot brush  230 . The support ring  210  is configured to receive a pressure switch  208  therein. The pressure switch  208  is received in a channel of the support ring  210  formed around its circumference, and includes a pressure spine  209 . The pressure switch  208  is configured such that pressure against the pressure spine  209  at any point around its circumference closes a switch. A control cable  216  is configured to electrically couple the pressure switch with collision sensor circuitry (not shown) for detecting closure of the switch. 
     A trigger skirt  206  is positioned over the support ring  210 , locking the components of the collision sensor assembly  160  together, and securing the collision sensor assembly  160 , together with the foot brush  230 , onto the foot plate  218 . The trigger skirt  206  may be configured to snap in place. The trigger skirt  206  comprises a plurality of skirt legs  212  distributed around its perimeter, and formed integral therewith. The skirt legs  212  are configured such that pressure against an outer face of one of the legs  212  will cause the respective leg to flex inward, applying pressure on the pressure spine  209 , and thereby closing the sensor switch. Each of the trigger legs  212  includes a lower face  214  displaced outward, radially, from an upper portion of the leg. This configuration permits the leg  212  to flex inward in response to contact with a sheer vertical surface. 
     The contour follower assembly  150  comprises an upper section  240 , including the nozzle nut assembly  152  and the PCB assembly  154 , and a lower section  250 , including the foot plate assembly  158  and the collision sensor assembly  160  (see  FIG. 10 ). The upper section  240  is rigidly coupled to the collet  121  of the cutting head  120  by the nozzle nut  222 . The lower section  250  is movably coupled to the upper section  240  by the plurality of sensor legs  156 , each having an upper member  190  engaging a respective mounting socket  189  of the housing plate  174 , and having a lower member  196  engaging a mounting socket  201  of the foot plate  218 . The lower section  250  of the contour follower  150  is biased in a downward direction by the springs  198  of each of the sensor leg assemblies  156 . 
     In operation, a workpiece  112  is positioned on the support grid  128  of a waterjet cutting system. The cutting head  120 , with the contour follower assembly  150  coupled thereto, is lowered until the bristles  232  of the foot brush  230  make contact with the upper surface  110  of the workpiece  112 . As the cutting head  120  continues to descend, the bearing surface  197  of each of the lower members  196  presses upward against the respective sensor shafts  192 , moving the shafts  192  upward within the respective apertures  191 , thereby compressing the springs  198 . As the sensor shafts  192  rise within the apertures  191 , the magnets  194  move closer to the hall-effect sensors  172 . Electrical characteristics of the hall-effect sensors  172  change according to the distance of the respective magnet  194  therefrom, in a manner known in the art. The PCB  170  provides a signal via the cable  173  to a position detection circuit (not shown) indicating the position of each of the magnets  194 , relative to the respective hall-effect sensor  172 . 
     According to the embodiment described, the following values are fixed and known: the lower machine plane L, defined by the lower ends  185  of the sensor shafts  192  is a known distance from the upper surface  110  of the workpiece, defined by the bristles  232 ; the exit port  124  of the mixing tube  122  is a known distance, on the Z-axis, from the upper machine plane U, defined by the hall-effect sensors  172 ; and the magnet  194  of each of the sensor shafts  192  is a known distance from the lower machine plane, this distance defined by the length of the sensor shafts  192 . Given these known values, and given the distance between the hall-effect sensors  172  and the respective magnets, which is derived from the sensor signals, the distance of the exit port  124  of the mixing tube  122  to the upper surface of the workpiece  112  can be determined with a high degree of accuracy. 
     The position detection circuit may be configured to provide a variety of calculations, based upon the data provided by the PCB. For example, inasmuch as the lower machine plane L lies parallel to the upper surface  110 , the data from each of the plurality of hall-effect sensors  172  may be processed to establish the angle of the upper surface  110  of the workpiece  112  relative to the upper machine plane U. Alternatively, the data from each of the plurality of hall-effect sensors  172  may be averaged to determine the distance of the upper surface  110  from the exit port  124 . A third alternative calculation may utilize the data from a single one of the sensors  172 , in a case where the upper surface  110  of the workpiece is known to be substantially planar, to determine the distance of the upper surface  110  from the exit port  124 . Design and manufacture of a circuit configured to perform these, and other calculations are within the capabilities of one having ordinary skill in the art. Accordingly, the position detection circuit will not be discussed in detail. 
     As was described previously, one of the challenges that has not heretofore been adequately addressed, with respect to waterjet cutting systems, is the case in which a workpiece does not lay flat on the grid of a cutting system. In the case of a large piece of sheet metal, for example, measuring perhaps many feet on a side, it is not unusual to find that such a piece is non-planar, having some portions that exhibit significant warp. 
     Referring now to  FIG. 13 , a contour follower assembly  150  is shown positioned on a workpiece  112  that is not laying flat on the upper ends of the slats  130  of a support grid. It may be seen that the lower section  250  of the device conforms to the upper surface  110  of the workpiece  112 , conforming thereby to the upper surface  110  of the workpiece  112  over which the cutting head, including the cutting head  120  and the mixing tube  122  must travel. Given the signals provided by the hall-effect sensors  172 , which are directly related to the position of the magnets  194  relative to the sensors  172 , the angle of the upper surface  110 , relative to the X-Y plane, can also be determined with a very high degree of accuracy. 
     In the case of a waterjet cutting system having three axes of control, namely, X, Y, and Z, the position of the mixing tube  122  can be adjusted in the Z-axis to place the exit port  124  at an optimum distance S (see  FIG. 5 ) from the upper surface  110  of the workpiece at the point where the waterjet impacts the workpiece, regardless of the angle of the workpiece  112 . This is not possible with conventional sensors, which measure from a single point, some distance from the mixing tube  122 . 
     In the case of a five-axis system including rotation around X and Y axes, such as that described with reference to  FIGS. 6 and 7 , the axial angle of the mixing tube  122  can be adjusted to compensate for a change in the L plane, and by extension, the plane of the upper surface  110  of the workpiece  112 . Additionally, accurate compensation for taper and trailback can be performed, independent of changes in the upper plane  110 . 
     It will be recognized that, in cases such as that described with reference to  FIG. 13 , for example, the workpiece  112  will be subject to movement in the Z-axis as the cutting process proceeds.  FIG. 13  illustrates a case in which a workpiece does not lie flat on the grid  128 . A segment  113  supporting a raised portion of the workpiece  112  has been cut away, allowing the workpiece  112  to drop. The upper surface  115  of the segment  113  now lies at a different plane than the upper surface  110  of the workpiece  112 . In such a case, not only must the contour follower  150  readjust to a new angle, but there is also a danger of collision, as some cut edges rise above the upper surface  110 . 
       FIG. 14  shows an enlarged view of a small portion of the contour follower  150  in the collision condition described above with reference to  FIG. 13 . As the cutting head, with the contour follower  150  coupled thereto, travels in direction T, a trigger leg  212  contacts a raised segment  113  of the workpiece  112 . The trigger leg  212  flexes inward at a region  213  where the leg  212  joins the trigger skirt  206 . The leg  212  presses against the spine  209  of the pressure switch  208 , causing the switch  208  to close an electrical circuit. Associated collision sensor circuitry is configured to shut down the drive of the cutting system in response to activation of the pressure switch  208 , preventing damage to the system. 
     The embodiment described with reference to  FIGS. 9-14  includes many parts that are coupled via interference or snap fit. This facilitates quick and simple disassembly for servicing or replacement of individual components or assemblies, without the need to remove fasteners, etc. However, other embodiments may incorporate threaded fasteners, retainers, threaded engagements, or any other device or method of connection, without deviating from the scope of the invention. 
     The embodiment described employs hall-effect sensors, which cooperate with magnets coupled to the sensor shafts. An individual having ordinary skill in the art will recognize that many types of sensors or signal generating devices may be used in place of the hall-effect sensors and magnets. For example, configurations employing strain gauges, potentiometers, optical sensors, accelerometers, or other sensing devices may be used. Design and manufacture of such alternate embodiments are within the skill of such an individual, and are within the scope of the invention. 
     An examination of the figures, especially  FIGS. 12 and 13 , will reveal several “O” rings that were not specifically described. One having ordinary skill in the art will recognize the value of providing seals at various points in such a device, and the function of such “O” rings will be clear to such an individual. 
     Alternate embodiments of the invention may not include all of the components described, or may incorporate components or assemblies described herein in systems bearing little obvious resemblance to the embodiment pictured. Such alternate embodiments also fall within the scope of the invention. For example, a cutting or drilling system employing some other cutting method, such as plasma or mechanical saw, for example, might advantageously incorporate some of the principles described with reference to the present embodiment. 
     Alternatively, an embodiment of the invention might incorporate a system in which a tool is required to be oriented with respect to a surface for measuring or cleaning. For example, an automated waterjet cleaning device might be required to be maintained at a precise angle and distance from a surface to effectively remove debris, coatings, or corrosion, without damaging the surface. Other applications may also occur to one having ordinary skill in the art, in which the features described with reference to the disclosed embodiment may be advantageously incorporated. Such applications also fall within the scope of the invention. 
     All of the above U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet, are incorporated herein by reference, in their entirety. 
     From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.