Abstract:
A method and apparatus for sensing a pierce-through condition of a material made by a piercing force in which a shield surrounding a source of the piercing force is supplied with a gas supply to create a pressure within the shield means. A decrease in pressure caused within the shield by a pierce-through condition created by the piercing force is then detected. A method and apparatus for detecting the distance between a nozzle assembly for a machining process and a workpiece to be machined in which gas is supplied to a shield surrounding a nozzle assembly. An increase in pressure in the shield is detected as an open end of the shield approaches a workpiece to be machined. A method and apparatus for obtaining and maintaining a predetermined gap distance between nozzle assembly and a workpiece for a machining process are provided by further detecting when the pressure within the shield means reaches a pressure corresponding values and ranges. A method and apparatus for determining the thickness of a workpiece is further provided by comparing an established nozzle assembly position with a predetermined reference position.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to sensing methods and apparatus and more particularly to sensing methods and apparatus for monitoring abrasive waterjet machining of engineering materials. 
     Abrasive water jet (AWJ) processes employ abrasive materials entrained into a high-pressure waterjet to perform a variety of cutting and other machining operations on a variety of materials. The high-energy waterjet beam utilized combines a rapid erosion of a workpiece material by high speed solid particle impacts with rapid cooling provided by a waterjet. In AWJ cutting operations an abrasive waterjet pierces through the thickness of and is then moved along a material to be cut. 
     In performing machining operations such as AWJ cutting, various physical dimensions such as workpiece thickness must be measured in order to properly configure the water pressure, abrasive flow rate, and other system parameters for the AWJ apparatus. Additionally, the proximity of and distances between various components of the AWJ apparatus and the workpiece must be monitored. For example, the proximity of an AWJ nozzle to a workpiece must be monitored with respect to establishing and maintaining air gap and stand-off distances within acceptable tolerance ranges. Additionally physical events such as the moment of pierce-through of a workpiece by an AWJ waterjet during a cutting operation must also be detected in order to establish when the relative motion between the workpiece and an AWJ nozzle should be commenced. 
     Although the measuring and monitoring of these and other physical aspects may be done by visual inspection and manual control by an operator, this is generally a cumbersome and not very precise method for controlling such machining operations. The foregoing illustrates limitations known to exist in present machining methods and apparatus. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly a suitable alternative is provided by the multi-functional sensing methods and apparatus of the present invention, which include features more fully disclosed herein. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus for sensing a pierce-through condition of a material made by a piercing force in which a shield surrounding a source of the piercing force is supplied with a gas supply to create a pressure within the shield means. A decrease in pressure caused within the shield by a pierce-through condition created by the piercing force is then detected thereby detecting the pierce-through condition. 
     Also provided are a method and apparatus for detecting the distance between a nozzle assembly for a machining process and a workpiece to be machined in which gas is supplied to a shield surrounding a nozzle assembly. An increase in pressure in the shield is detected as an open end of the shield approaches a workpiece to be machined. 
     A method and apparatus for obtaining and maintaining a predetermined gap distance between a nozzle assembly and a workpiece for a machining process are provided by further detecting when the pressure within the shield means reaches a corresponding pressure value and range, respectively. A method and apparatus for determining the thickness of a workpiece is further provided by comparing an established nozzle assembly position with a predetermined reference position. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention. The foregoing and other aspects will become apparent from the following detailed description when read in conjunction with the accompanying drawing figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general symbolic diagram of the components of an abrasive waterjet system according to the present invention; 
     FIG. 2 is a representation of the pressure signal read by a pressure sensor during a cutting method performed according to one embodiment of the present invention; and 
     FIG. 3 is a program flow chart for a software program resident in the programmable controlling unit of FIG. 1 for performing a cutting method using the sensing apparatus according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The sensing apparatus and method of the present invention are best understood from the following detailed description when read in connection with the drawing figures in which like reference numerals refer to like elements throughout. It is emphasized that according to common practice, the various dimensions of the apparatus shown in the drawings are not to scale. 
     Referring now to the drawing, FIG. 1 shows a broad system diagram of an embodiment of the invention as applied to an abrasive waterjet (AWJ) system. Briefly, shown in FIG. 1 is a nozzle assembly  10  comprised of an orifice  12  and a focusing tube  14  which applies a mixture of high pressure water and abrasive to a moving workpiece  16 . Nozzle assembly  10  is preferably supplied abrasive from an optional vibration feeder  20  and high pressure water from a water source  22 . Although shown using a vibration feeder, it is understood that other types of feeding devices, which are known and will be readily recognized by those having ordinary skill in the art, may be used for this purpose. 
     A controlling unit  30  is typically provided for receiving input on the operating conditions of the AWJ system and controlling the motion of nozzle assembly  10  and workpiece  16 . Controlling unit  30  is preferably a Computerized Numerical Controller (CNC) which is available and known to those in the art and may include, e.g., the Model ACR 2000 motion controller which is available from Acroloop Motion Control Systems, Inc., Chanhassen, Minn. 
     Prior to performing an AWJ cutting or other machining operation, controlling unit  30  is preset by a user with AWJ system operating parameters such as water pressure, abrasive particle size, abrasive flow rate, and the dimensions of the waterjet nozzle orifice. These parameters are varied depending on the type of workpiece material and the type of machining operation to be performed. In operation, controlling unit  30  controls the feed from vibration feeder  20  and the feed supply of high pressure water from water source  22  as is known in the art. As high pressure water and abrasives are supplied to the nozzle, the workpiece  16  is moved back and forth by positioning equipment (not shown) which maneuvers the workpiece at the proper traverse speed for the desired cutting or other machining operation. Preferably, the positioning equipment is responsive to and controlled by a control signal  26  provided by controlling unit  30 , which may be also used to calculate the traverse speed as is known in the art. 
     Multi-Functional Sensing Apparatus 
     Shown in FIG. 1 on nozzle assembly  10  is the multi-functional sensing apparatus according to the present invention which comprises a nozzle shield  15  surrounding the focusing tube  14 . Nozzle shield  15  is connected to and in fluid communication with an air or other gas supply  40  via a conduit  41 . A pressure sensor  42  is connected to conduit  41  and located between nozzle shield  15  and air supply  40  for sensing the pressure conditions inside nozzle shield  15  and providing a pressure sensor signal  43  to controlling unit  30 . 
     Operation for Performing AWJ Cutting Using Multi-Functional Sensing Apparatus 
     Operation of the AWJ apparatus shown in FIG. 1 will now be described with respect to performing an AWJ cutting operation according to the automated sensing method of the present invention. Turning to the flow diagram in FIG. 3, controlling unit  30  is initialized in Step  100  by inputting the specific AWJ system operating parameters required by controlling unit  30  prior to beginning an AWJ cutting cycle, as is known in the art. As described above these operating parameters typically include water pressure, abrasive particle size, abrasive flow rate, and the dimensions of the waterjet nozzle orifice. 
     A. Positioning Nozzle Assembly at a Predetermined Stand-Off Distance 
     Controlling unit  30 , upon receiving a user instruction to begin a cutting sequence, begins a piercing cycle in Step  110  by generating a control signal  39  in Step  110  to air supply  40  thereby initiating airflow into nozzle shield  15  via conduit  41 . Pressure sensor  42  generates and provides to controlling unit  30  an output signal  43  similar to that shown in FIG. 2 indicating the pressure condition inside nozzle shield  15  as a function of time. Controlling unit  30  generates a control signal  11  instructing motion equipment (not shown) to lower nozzle assembly in Step  120  to a form an air gap  17  having a predetermined height. 
     For a cutting operation, the nozzle shield  15  is set to establish a stand-off distance (i.e., the distance between the focusing tube  14  and workpiece  16 ) which is about equal to air gap  17 , once air gap  17  is established. This is accomplished by using pressure sensor  42  as a proximity switch which monitors in Step  130  the pressure increase caused by the restriction created between workpiece  16  and nozzle shield  15  as it moves toward the target surface. As shown in FIG. 2, the pressure inside nozzle shield  15  increases to a predetermined pressure P g  which is programmed into controlling unit  30  and corresponds to the pressure at which the desired air gap  17  is formed. At this point, when Step  130  detects that nozzle assembly  10  is in position, the controlling unit  30  generates a control signal to stop the motion of the nozzle assembly  10  thereby setting the cutting position (i.e., stand-off distance) and controlling unit  30  also records this position. 
     B. Determining Workpiece Thickness 
     According to the present invention, the thickness of workpiece  16  may be measured in Step  140 . This is accomplished by using controlling unit  30  to compare the height of nozzle assembly in the cutting position set in Steps  120  and  130  with a known reference position. The measured thickness of workpiece  16  is inputted into controlling unit  30  for performing calculations necessary for determining the proper operating conditions. For example, the proper traverse cutting speed at which a waterjet cuts through a particular material during an AWJ cutting operation varies indirectly with and may be calculated using workpiece thickness according to the equation disclosed in the article by J. Zeng and J. P. Munoz titled “Intelligent Automation of AWJ Cutting for Efficient Production,” Proceedings of the 12th International Symposium on Jet Cutting Technology, BHRA, Rouen, France, 1994, pp. 401-408, which article is incorporated herein by reference. 
     In Step  150 , controlling unit  30  simultaneously generates control signals  21  and  19  to, respectively, initiate the supply of high pressure water from water source  22  and abrasive from vibration feeder  20  to establish an abrasive water jet in water nozzle assembly  10 . Controlling unit  30  sends a control signal  11  to move nozzle assembly  10  preferably at a constant rate (e.g., 50 inches per minute (ipm)) along a circle which has a radius equal to the focusing tube diameter until workpiece  16  is pierced. 
     C. Detecting Pierce-Through of a Workpiece 
     Prior to commencing a traverse cutting motion of the nozzle assembly  10  across workpiece  16 , the moment the waterjet pierces workpiece  16  is first detected. According to one embodiment of the present invention shown in FIG. 1, this is accomplished by air supply  40  maintaining a steady flow of air to nozzle shield  15  during the time the piercing operation is being performed. Pressure sensor  42  simultaneously monitors and provides a steady output signal  43  to controlling unit  30  as represented by the horizontal signal between “t g ” and “t p ” in FIG.  2 . Upon penetration (i.e., “pierce-through”) of the waterjet through workpiece  16 , a vacuum is created within nozzle shield  15  which, as shown in FIG. 2, causes a virtually instantaneous drop in the pressure detected by pressure sensor  42  at “t p ” which is the moment pierce-through occurs. 
     Upon detecting the decrease in pressure in Step  160  caused upon pierce-through, the desired cutting operation is then initiated in Step  190  by controlling unit  30  which either sends a control signal  11  to begin horizontal movement of nozzle assembly  10 , sends a control signal  26  to begin horizontal movement of workpiece  16 , or both, to laterally move nozzle assembly  10  at the proper cutting speed relative to workpiece  16 , which cutting speed may be calculated as discussed above. The cutting operation is monitored in Step  200  either visually or automatically (e.g., by a mechanical sensor switch as is known in the art) to detect when the cutting operation is complete. 
     D. Stand-Off Distance Monitoring 
     During the cutting operation, air gap  17  between the nozzle assembly  10  and workpiece  16  is preferably monitored in Step  210  for any changes by monitoring the signal provided by pressure sensor  42  for any variation in the signal after time “t c ” which represents the time at which pierce-through is completed and cutting begins as shown in FIG.  2 . Should any variation above or below a predetermined pressure range (represented as “ΔP” in FIG.  2 ), which range corresponds to an acceptable stand-off distance tolerance, an error signal is sent by the controlling unit  30  via connection  11  to implement compensation in Step  220  by the motion equipment to adjust the stand-off distance. Alternatively, controlling unit  30  may be programmed to send an error signal via connections  19  and  21  to respectively stop the flow of abrasive and water to interrupt the AWJ operation being performed. 
     With respect to devices which may be incorporated as pressure sensor  42 , any sensor that can detect the increase and decrease of pressure within nozzle shield  15 , as described above, may be incorporated. An exemplary device includes, but is not limited to, a Model OKC-424 Air Proximity Sensor, available form O&#39;Keefe Controls Co., Monroe, Conn. 
     As a result of the multi-functional sensing method and apparatus of the present invention, a number of advantages in sensing various physical aspects of machining processes such as AWJ cutting processes may be achieved. Among these advantages are the ability to determine and monitor various physical dimensions of and the proximity and distances between various apparatus and workpiece components. For example, various measurement functions may be automatically accomplished using the multi-functional sensing method and apparatus of the present invention including, proximity detection of the waterjet nozzle, measuring the thickness of a workpiece, and real time monitoring and correction of nozzle stand-off distance. Additionally, physical events such as pierce-through of a workpiece may also be detected using the multi-functional sensing method and apparatus of the present invention. 
     Moreover, when used in conjunction with a CNC controlling unit, the multi-functional sensing method and apparatus of the present invention facilitates the automatic programming, calculating, and control of various machining parameters and operating conditions without the need for any user interference or interface while also increasing the accuracy of the operating parameters and conditions so determined. Additionally, changes in process parameters (e.g., cutting speed, changes in water pressure, abrasive flow rate, abrasive type, nozzle diameter, etc.) may also be made automatically based on sensed condition changes (e.g., different workpiece thicknesses) using the multi-functional sensing methods and apparatus of the present invention. 
     Alternative Embodiments to Monitor Additional Machining Technologies 
     Although illustrated and described herein with reference to certain specific embodiments, the present invention is nevertheless not intended to be limited to the details shown. Rather, various modifications may be made in the details in the scope and range of equivalents of the claims without departing from the spirit of the invention. For example, although described above with respect to monitoring an AWJ cutting operation using an AWJ waterjet moved through a circular motion, it is expected that the sensing method and apparatus may be employed to monitor a variety of other machining processes which can incorporate any variety of piercing motions and energy beam machining technologies. 
     Examples of alternate piercing motion patterns which may be used include, but are not limited to, a linear, back-and-forth, star, wiggle or other pattern. Additionally, it is further envisioned that the energy beam processes including AWJ may be used to perform a variety of other AWJ and traditional operations such as piercing, drilling, milling and turning operations. 
     Such other energy beam technologies include those which utilize a concentrated beam energy to effect material removal to cut or otherwise make, shape, prepare, or finish (i.e., machine) a raw stock material into a finished material. By way of example, it is envisioned that the sensing method and apparatus of the present invention may be used with and incorporated into other types of energy beam technologies, including but not limited to, pure waterjet, laser, plasma arc, flame cutting, and electron beam technologies. Although each of these technologies use different physical phenomena to remove material, they behave similarly in nature and methodology to a waterjet energy beam such that the sensing method and apparatus of the present invention may be employed. 
     Furthermore, it is to be understood that the selection of alternative energy beam technologies to which the present invention may be applied is not limited to these specific examples which are merely illustrative. Rather, these energy beam technologies will be readily recognized and may be selected by those having ordinary skill in the art.