Abstract:
An apparatus is disclosed for cutting a workpiece. A laser beam is directed at successive points along a workpiece surface to be cut and a sensor emits a sensing beam directed at the same successive points as the cutting beam. A beam combining device receives both the sensor beam and the cutting beam and causes downstream beam segments to be collinear with each other as they impinge the workpiece surface. The cutting is thereby able to be carried out in a single pass, and is precise, repeatable and independent of cutting depth, angle of cutting, scoring patterns, material inconsistency, material color, and surface grain variability.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. Ser. No. 09/811,152, filed Mar. 16, 2001, now U.S. Pat. No. 6,423,933. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention concerns apparatus for cutting workpieces by a laser beam to remove material by vaporization. In such apparatus, the laser beam is advanced along points forming a predetermined cutting pattern on a workpiece surface. 
     A widely used method for determining the extent of material removed during controlled cutting involves the use of triangulation type sensors as described in U.S. Pat. No. 5,883,356. These sensors, however, due to their triangulation operating principle, are limited in their ability to reach the bottom of the scoring produced by the cutting device. This is particularly so for narrow, deep penetrations which may be produced by cutting devices such as lasers and cutting knives. Furthermore, due to their offset mounting, these sensors are not well suited to measure the varying penetration depth that occurs during scoring at a specific location. This is especially true if the scoring penetration is in the form of partial perforations or slots. As such, the process does not lend itself to scoring the workpiece in an adaptive control mode, where both depth sensing and cutting are in registry with each other to impinge the same point on the workpiece, during the progression of scoring of the piece. 
     Accordingly it is an object of this invention to provide an apparatus for laser beam cutting in a manner that provides accurate adaptive process control, single-pass processing, and lower manufacturing costs. 
     SUMMARY OF THE INVENTION 
     According to the invention, the cutting of the workpiece is accomplished by apparatus including a source of a controllable cutting laser beam, which, based on feedback obtained from at least one sensor emitting a sensing beam, is controlled in intensity together with controlled relative movement between the laser and the workpiece, producing a precise, predetermined cutting penetration into the workpiece along a predetermined pattern. 
     In this apparatus, the laser cutting beam and sensing beam emitted from a first sensor are both directed at a surface on one side of the workpiece. A second sensor may also be positioned on the opposite side of the piece emitting a second sensor beam in opposition to the cutting laser beam. A beam combining device combines the laser cutting beam and first sensor beam together so as to have collinear segments directed at exactly the same point on the workpiece. The cutting of the workpiece is carried out by the laser beam while the piece is moved in a predetermined pattern relative to the laser. The depth of cutting of the work piece by the laser beam is controlled by real time feedback signals corresponding to the depth of the cut provided by the first sensor. To determine material thickness remaining during cutting of each point along the predetermined pattern, real time feedback from the second sensor can be provided combined with the feedback signals from the first sensor. The sensor feedback can also be utilized to control the movement of the workpiece relative to the laser beam to enhance the cutting process control. 
     This apparatus, due to the collinear arrangement of the impinging segments of the sensor and cutting beams, affords several advantages, including single-pass adaptive processing, cutting precision, and superior piece-to-piece repeatability. The cutting achieved is also independent of cutting depth, angle of cutting, scoring patterns, material inconsistency, material color, and surface variations. 
     Relative motion between the workpiece and the cutting beam to cut the piece in a predetermined pattern can be provided by different means including actuators, robots and X-Y tables. 
     The workpiece can have a monolayer, multilayer, or composite construction and can be scored on either side. The cutting can be continuous, intermittent or be a combination of both, and extend completely through one or more layers of the piece. The piece can be a finished part or a component which is subsequently integrated into a finished part. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of one form of the apparatus according to the invention including two sensors 
     FIGS. 2,  2 A and  2 B are fragmentary enlarged views of several alternative designs of the beam combining device incorporated in the apparatus shown in FIG.  1 . 
     FIG. 3 is a perspective view of an automotive instrument panel with an integrated airbag deployment door formed by in a U pattern scoring carried out by the apparatus of the present invention. 
     FIGS. 4 through 6 are cross sectional views of sample monolayer and multilayer trim piece constructions on which various types of trim piece weakening scorings can be made by the present apparatus. 
     FIG. 7 is a diagrammatic view of a second embodiment of the apparatus according to the invention incorporating only a single sensor. 
     FIG. 8 is a diagrammatic view of another form of second embodiment of the apparatus according to the invention incorporating only a single sensor. 
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, certain specific terminology will be employed for the sake of clarity and particular embodiments described, but it is to be understood that the same is not intended to be limiting and should not be so construed inasmuch as the invention is capable of taking many forms and variations within the scope of the appended claims. 
     This invention describes an improved apparatus for cutting a workpiece with a laser cutting beam. 
     FIG. 1 shows a first embodiment of a workpiece cutting apparatus  10  according to the invention. This includes a laser cutting beam source  12  which generates a laser cutting beam which can be used to carry out controlled scoring of a surface  14  on one side of a workpiece such as an instrument panel trim piece  16  that would overlie an airbag installation when installed. 
     The trim piece  16  is positioned on a holder which can comprise a fixture  18 . A first sensor  20  is provided to determine the depth of scoring produced by the laser cutting beam B impinging onto the surface  14  of the trim piece  16  to score the same. The first sensor  20  and the laser cutting beam generator  12  are connected to a beam combining device  22 . The beam combining device  22  (shown in detail in FIG. 2) is designed to combine the separately generated electromagnetic sensing beam A emanating from the first sensor  20  and the laser cutting beam B from source  12  so as to be in a collinear relationship and to direct the combined segments of the sensor beam A and cutting beam B so as to impinge the same precise spot on the trim piece surface  14 . This beam combining device  22  will also redirect any reflected beam or beams required for sensor operation from the trim piece surface  14  back to the first sensor  20  as necessary in carrying out the process. 
     The trim piece  16  is moved relative to the cutting laser beam source  12 , as well as the first sensor  20  and the beam combining device  22  via a motion actuator  24  drivingly engaged with the holder comprised of fixture  18  to cause tracing of a particular scoring pattern on the surface  14  and to achieve a precisely controlled rate of scoring. The motion actuator  24  can itself directly hold and move the trim piece  16  itself or move the optional fixture  18  onto which the trim piece  16  is mounted. Alternatively, the motion actuator  24  could be used to move the laser beam source  12  and the first sensor  20  relative to the trim piece  16 . 
     A second sensor  26  may be located on the side of the trim piece  16  opposite the first sensor  20 , a second sensor beam emanating therefrom, directed at the outer surface  28  of the trim piece  16  and aligned opposite the same trim piece point as is the laser cutting beam and the first sensor beam or beams are directed in order to control the scoring so as to produce a programmed thickness of material remaining after scoring. This is done by combining signals generated by both sensors  20 ,  26  to create a feedback signal corresponding to the thickness of the remaining material. 
     The apparatus  10  is operated via one or more industrial controllers  30  that control the scoring effected by the laser and/or the movement of the motion actuator based on a particular program and feedback signals provided by the sensor  20 ,  26 . 
     Lasers that in particularly desirable for carrying out this type of scoring processes are of the carbon dioxide, excimer, solid state, argon gas, or diode type. However, based on the workpiece materials utilized (polymers, fabrics, wood, leather), the carbon dioxide laser is likely to be the most preferable in terms of operability, efficiency and cost. The laser can be operated either continuously or in a pulsed mode. 
     Different type of sensors can be utilized to measure the extent of material removed or remaining during scoring of the trim piece. For the first sensor  20 , connected to the beam combining device  22 , a preferred type is a closed loop device that sends and receives a specific beam of electromagnetic radiation in order to determine the depth of scoring effected by the laser. The Conoprobe sensors offered by Optimet and based on the technique of conoscopic holography, is one such sensor commercially available. In this type of sensor, an emitted laser beam and reflected return beams of visible light have segments also traveling in a collinear relationship with each other and the laser beam. Another type of sensor that could be utilized is one that detects reflected light beams such as a high speed CCD camera. In this application, the reflected beam will be reflected from the trim piece surface being scored by the cutting beam. 
     For the second sensor  26  aimed at the outside surface of the trim piece, which is generally smooth and accessible, there are more numerous options than electromagnetic beams and including infrared, laser, ultrasonic, conoscopic, CCD camera, proximity and contact type sensors. 
     The signal spot size of the first sensor beam selected can vary significantly. Generally the smaller the spot size the better. For the first sensor, the preferred size would not exceed the size of the scoring produced on the trim piece by the cutting laser beam. For the second sensor, if surface finish variations, so called grain, are significant, its spot size should preferably not exceed 300 microns. 
     There are numerous ways for combining the separately originated cutting laser beam B and sensor beam A to create collinear segments thereof. FIG. 2 shows the inner details of the beam combining device  22  which combines the separate laser beam B and the first sensor beam A to create collinear segments which impinge the trim piece surface  14 . The beam combining device  22  includes a reflector  32  having coatings causing reflection of light of the wavelength of the sensor beam A from its inclined surface while allowing the cutting laser beam B to be transmitted. 
     Such coated selective reflectors are commercially available. This of course requires that the laser and sensor beams be of different wavelengths. 
     A side entrance tube  29  directed at the reflector  32  is connected to the first sensor  20 . The main tube  31  mounts the reflector  32 , main tube  31  having an end opening  33  directed at the trim piece  16 . 
     The segment of the sensor beam A reflected from the reflector  32  is caused to be collinear, i.e., aligned and coextensive with the segment of the laser beam B past the reflector  32 , with both collinear segments then impinging the surface  14  at the same precise point. 
     FIG. 2A shows a second form of a beam combining device  22 A having an inclined reflector  32 A having coatings causing reflection of a beam having the wavelength of the cutting laser beam B, while allowing transmission of the beam having wavelengths of the sensor beam A to be transmitted therethrough to reverse the relationship shown in FIG.  2 . 
     FIG. 2B is a simplified diagrammatic view of another form of the beam combining device  22 B combining the cutting laser beam B and the first sensor beam A to produce collinear downstream segments thereof. This embodiment includes a simple mirror reflector  36  having a through hole  34 . The hole  34  is small in diameter relative to the diameter of the cutting laser beam B in order to minimize or eliminate the effect that the presence of the hole  34  may have on reflecting the cutting laser beam from the mirror reflector  36  to redirect the cutting laser beam B. Such a mirror does not require coatings that are wavelength-selective such as those shown in FIGS. 2 and 2A in order to combine segments of the beams into a collinear relationship. In this particular arrangement, the first sensor  20  could be a CCD camera receiving beams reflected from the trim piece surface being scored by the laser beam. 
     In order to apply the complete scoring pattern, the trim workpiece is preferably moved relative to the laser beam and/or the sensors. The relative motion can be applied by a various types of motion actuators including robots and X-Y tables. During cutting, the sensor thickness data can also be used to control the movement of the motion device in order to apply the scoring along the predetermined pattern. The workpiece may be held directly by the motion device or be attached to a holding fixture held by the motion device. The holding fixture may be shaped to match the shape of the workpiece and/or be designed to register specific surface features of the workpiece. Vacuum or clamps could also be applied to the holding fixture to hold the trim piece surface in better contact with the fixture  18 . The fixture  18  can be designed to allow the second sensor  26  to have physical and/or optical access to the surface  28  of the workpiece (i.e., transparent fixture wall, opening in fixture wall, etc.). 
     The process controller  30  is designed to control the operation of the laser and/or motion actuator based on the feedback signals provided by the two sensors  20 ,  26  which, from opposites sides or surfaces of the trim piece  16 , monitor the location being scored. The two sensors  20 ,  26  working in tandem determine the remaining thickness of the trim piece  16  at any point they are directed to. During laser scoring at a given point, the two sensors  20 ,  26  provide signals from which a measurement of the material thickness remaining after the scoring can be derived by the control device  30 . Based on this real-time thickness determination, the process control device  30  controls the operation of the cutting beam source  12  to effect only the desired extent of material removal intended for any given point on the workpiece  16 . The remaining thickness data can also be used to control the motion actuator  24  to move the workpiece  16  to the next desired location along the predetermined scoring pattern. 
     Due to the collinearity of the impinging segments of the first sensor beam and the cutting beam, several advantages are realized that could not be attained by any of the existing processes. Since the first sensor beam and the laser beam are always impinging on the same point on the trim piece, the process becomes insensitive to a large number of key variables, including the angle of cutting, the depth of the penetration, the trim piece thickness, the configuration of the weakening pattern and, to a large extent, the speed of cutting. Also, the combination of the two sensors provides for a direct remaining thickness measurement, superior scoring precision and excellent part to part repeatability. In addition, the process enables the user to overcome variations in trim piece thickness, material properties such as density, color, voids and surface grain. These and other benefits are obtained while operating with rapid adaptive control in a single-pass mode. 
     This apparatus can be used in various ways, such as to cut or score a workpiece continuously or to form discontinuous cuts such as slots, grooves, etc., therein. 
     A second embodiment of the apparatus  44  according to the invention is shown in FIG. 7 where the outer surface  42  of the trim piece  16  is in intimate contact with the inner fixture wall  46 . In this arrangement, the distance between the first sensor  48  and the fixture inner wall  46 , along the predetermined scoring pattern, can be measured prior to starting the scoring operation. If this distance can be maintained constant from pass to pass, then the second outside sensor would not be necessary while still running the process in a single-pass, adaptive control mode. 
     FIG. 8 shows another embodiment of the apparatus  50  where the first sensor  52  is mounted immediately alongside the cutting beam source  12  so that both beams A, B are substantially collinear with each other to approximate the effect of using the beam combining device  22  described. 
     The laser cutting beam may also function as the sensor. This arrangement also maintains the collinear configuration as the sensing signals and the laser beam are generated by the same laser. Under this approach, the laser beam characteristics and control would be manipulated to conduct sensing measurements during or between cutting intervals (i.e., sensing after a preset number of cutting pulses).