Process for laser cutting a workpiece

[0001] This application is a divisional of U.S. Ser. No. 10/195,000, filed Jul. 12, 2002, now U.S. Pat. No. 6,737,607, which is a divisional of U.S. Ser. No. 09/811,152, filed Mar. 16, 2001, now U.S. Pat. No. 6,423,933, issued Jul. 23, 2002.

[0002] This invention concerns a process 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.

[0003] 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.

[0004] Accordingly it is an object of this invention to provide a process for laser beam cutting in a manner that provides accurate adaptive process control, single-pass processing, and lower manufacturing costs.

[0005] According to the invention, the cutting of the workpiece is accomplished by a process using 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.

[0006] In this process, 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.

[0007] This process, 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.

[0008] 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.

[0009] 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.

[0016] 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.

[0017] This invention describes an improved apparatus for cutting a workpiece with a laser cutting beam.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] Lasers that are 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Such coated selective reflectors are commercially available. This of course requires that the laser and sensor beams be of different wavelengths.

[0029] 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.

[0030] 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.

[0031] FIG. 2A shows a second form of a beam combining device 22A having an inclined reflector 32A 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.

[0032] FIG. 2B is a simplified diagrammatic view of another form of the beam combining device 22B 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.

[0033] 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.).

[0034] 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.

[0035] 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.

[0036] This apparatus can be used in various processes, such as to cut or score a workpiece continuously or to form discontinuous cuts such as slots, grooves, etc., therein.

[0037] 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 20 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.

[0038] 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.

[0039] 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).