A METHOD AND AN APPARATUS FOR CHECKING THE CONDITION OF A PROTECTIVE GLASS IN CONNECTION WITH LASER MACHINING

Abstract: The invention relates to a method and an apparatus for checking the condition with respect to dirt contamination in the form of small smoke particles accumulated on a protective glass plate (3) arranged between the workpiece (2) and the laser optics (1) in a laser machining system and through which protective glass plate the machining laser beam (6) is arranged to pass. A thermal detector (10) is arranged, with a good thermal contact with the mechanical part attaching the protective glass plate in a holder (4), for sensing (measuring) the temperature of said mechanical attachment part. Due to an absorption of the laser beam by the small smoke particles an increased temperature is obtained on the protective glass plate and the mechanical parts that are attaching the protective glass in its holder (4). As soon as the detector signal has reached a certain level it can be indicated that it is time to replace the protective glass plate. The thermal detector (10) is preferably combined with an optical detector (8) sensing the radiation scattered by larger dirt particles collected on the protective glass plate during the machining process.

Description: A method and an apparatus for checking the condition of a protective glass in connection with laser machining The present invention relates to a method and an apparatus for checking the condition of a protective glass which is arranged between the workpiece and the laser optics in a laser machining system in which the laser beam, or any other substantially coaxial radiation, is arranged to pass through the glass before it is focused on the workpiece. In order to protect the optical components in a laser machining system, for instance in a laser welding or engraving equipment, it is previously known to use protective glasses. Such protective glasses are then mounted between the laser optics and the object (workpiece) to be machined and prevents dirt and dust particles produced at the machining from entering into the laser optics. During the machining process such protective glasses be- come more and more dirty by particles removed from the workpiece so that the laser beam which passes through the glass is seriously effected and consequently also the machining process. Some of the incident laser radiation is absorbed and scattered by dirt, dust and smoke particles collected on the glass surface. A part of the scattered radiation is collected by the protective glass walls and directed outwards to the peripheral parts of the glass plate. The more dirt and dust particles on the glass surface, the more scattering of the incident laser beam. The absorbed part of the radiation results in an increased temperature. As the incident laser beam also is the working, machining, beam it should be understood that such absorption and scattering of the beam is unsatisfactory as the power density of the focused beam is reduced. This is a disadvantage especially in a laser welding system as the welding capacity then is reduced. But it is also a disadvantage in for instance a laser engraving system of the type illustrated in the Swedish Patent 9403349-5 in which small complex engraving patterns are made by means of the laser beam. If the beam is unsharp due to the scattering effect, the good quality of the pattern or script produced by the laser beam can not be maintained. Furthermore, there is a risk that the protective glass is fractured when it becomes too dirty. For these reasons the protective glasses must be replaced regularly. In automatic production systems it is previously known to check the condition of the protective glasses continuously during the machining process and without any interruption in the machining process. In DE 196 05 018 Al it is illustrated a method for measuring radiation scattered by particles sticked on the protective glass surface. A similar method is described in SE 97.00196-0. In both cases a pho- todetector is arranged at the edge side of the protective glass to sense the radiation which has been spread out to the edge of the glass due to the dirt particles. With respect to laser machining it has turned out that there are two entirely different types of contamination related to the protective glasses. There are large particles collected on the glass surface, but also small smoke particles caused by vaporization of organic material and easily vaporized metals. The first type, the large particles, results in a substantial spread of the radiation and can be easily detected by means of the above-mentioned optical methods. The small smoke particles, however, does not result in any measurable amount of spread of the radiation, but instead a substantial absorption of the laser effect which causes a too high temperature of the protec- tive glass and the mechanical mounting parts for the glass. Both types of contamination problems, however, could mean a deteriorated welding quality and a protective glass fracture and a corresponding optical breakdown. Therefore, there is a need for an improved control of the condition of a protective glass which control can be performed continuously also with respect to small smoke par- tides. According to the invention this is achieved by means of a thermal detector arranged to sense (detect) the temperature of the protective glass itself or the glass support- ing parts, specifically the mechanical protective glass holder. According to a preferred embodiment of the invention the thermal detector is combined with an optical detector of a previously known type, so that both types of dirt contamination can be controlled simultaneously. In the following the invention will be described more in detail with reference to the accompanying drawing which illustrates by means of an example how the detectors could be arranged in connection with a protective glass. Figure 1 illustrates a known optical protective glass control arrangement in connection with laser machining, Figure 2 illustrates schematically an arrangement according to the invention, Figure 3 illustrates a so-called sensor card with an opti- cal as well as a thermal detector, and Figure 4 illustrates more in detail the sensor card mounted in connection with the edge of the protective glass. In figure 1 it is illustrated schematically a focusing optical arrangement, symbolised by a lens 1, which concentrates the incident laser radiation on a workpiece 2. It can be a welding system, an engraving system like the one illustrated in SE 9403349-5, a cutting system or any other laser machining system. In all such machining systems dirt and dust particles are produced from the workpiece during the machining process. To prevent such particles from com- ing into the focusing optics, a protective glass plate 3 is arranged between the workpiece 2 and the focusing lens 1. The protective glass plate is mounted in a holder 4 and it has a circular peripheral edge side. The dirt, dust and smoke particles 5 that are produced during the machining process are instead gathered on the protective glass surface 3. This means that the particles can not enter into the more delicate optical system, but the disadvantage with such a system is that the particles are gradually accumulated on the glass surface and are causing an undesired scattering and absorption of a part of the incident laser radiation 6. Therefore, a photode- tector 8 is arranged at the edge side of the protective glass to sense the scattered radiation 7 on the glass edge 9, and which radiation is an indicia of the contamination layer on the protective glass. The photodetector senses the amount of radiation scattered by particles sticked on the protective glass surface. When the detector signal has reached a certain level, this is an indication that it is time to replace the protective glass. This means that the condition of the protective glass with respect to such particles which generate a scattered radiation can be controlled continuously during the machining process. As already mentioned, however, the laser machining also generates small smoke particles. These particles are not causing any scattered radiation, but instead they are causing an absorption of the laser effect, which can be even more harmful because of the resulting, increased tem- perature on the protective glass and the surrounding mechanical mounting parts, specifically those mechanical parts which are thermally connected to the protective glass. According to the invention a thermal detector 10 is also arranged in connection with the protective glass in order to measure the temperature of the protective glass and the surrounding mechanical parts. When the detector signal has reached a certain level this is an indication that it is time to replace the protective glass. The detector 10 itself could be of a known type and will not be described in any detail here. The thermal detector 10 is mounted together with the optical detector 8 on a sensor card 11, see figure 3. The sensor card 11 is mounted directly onto the mechanical part attaching the protective glass in the holder 4. This means that a good thermal contact is obtained with the holder 4, see figure 4. The optical detector 8 is mounted on the uppermost part of the card so that it is viewing the edge of the protective glass 3 through an optical window 13, i.e. analogous to what has been described in SE 97.00196-0. The card is attached to the holder 4 of the protective glass by means of a screw, bolt 14 or the like. The detectors are, by means of a contacting device 12 and conduits 15, connected to an electronic signal processing circuit, which electronic signal processing circuit in turn is cou- pled to indicating and display means (not shown). As both the electronic signal processing circuit and the indicating and display means could be of a type which is known per se, they will not be described in any detail here. As soon as the signals exceed a certain, predetermined value, this is indicated by means of an audio signal or display means. The invention is not limited to the embodiments which have been illustrated here as examples but can be varied within the scope of the accompanying claims. Also, the invention could be combined with other methods for preventing dirt to accumulate on the protective glass, for instance cross- jet or axial blow methods.

Claims: CLAIMS 1. A method for checking the condition with respect to dirt contamination on a protective glass (3) which is arranged between the workpiece and the laser optics in a laser machining system in which the machining laser beam, or possibly any other radiation which is coaxial with the machining laser beam, is arranged to pass through the pro- tective glass c h a r a c t e r i s e d by arranging a thermal detector (10) in connection with the protective glass (3) or its holder (4) for sensing (measuring) the temperature of the protective glass or the holder (4). 2. A method according to claim l c h a r ac t e r i s e d by combining the thermal detector (10) with an optical detector (8) and arranging both of the detectors in connection with the edge (9) of the protective glass for sensing the temperature of the protective glass (3) or its holder (4) as well as the scattered radiation (7) on the edge (9) of the protective glass. 3. A method according to any of the preceding claims c h a r a c t e r i s e d by mounting the thermal detec- tor (10) directly onto a mechanical part of the holder (4) for providing a good thermal contact with said part. 4. An apparatus for checking the condition with respect to dirt contamination on a protective glass (3) arranged be- tween the workpiece and the laser optics in a laser machining system in which the laser beam, or possibly any other radiation coaxial with the machining laser beam, is arranged to pass through the protective glass c h a r a ct e r i s e d by a thermal detector (10) arranged in con- nection with the protective glass (3) or its holder (4) for sensing (measuring) the temperature of the protective glass (3) or the holder (4). 5. An apparatus according to claim 4 c h a r a c t e ris e d in that the thermal detector (10) is combined with an optical detector (8) and that both of the detectors are arranged in connection with the edge side (9) of the pro- tective glass for sensing the temperature of the protective glass (3) or its holder (4) as well as the radiation (7) scattered to the edge (9) of the protective glass. 6. An apparatus according to claim 4 or 5 c h a r ac - t e r i s e d in that the thermal detector (10) is mounted directly onto a mechanical part of the holder (4) for providing a good thermal contact with said part. 7. An apparatus according to claim 6 c ha r a c t e r i- s e d in that both of the detectors are arranged on the same sensor card (11) which is mounted in such a way that a good thermal contact is obtained with a mechanical part of the holder ( 4) .

Inventors: ROOS SVEN-OLOV, TORSTENSSON PER-ARNE, ROOS, SVEN-OLOV, TORSTENSSON, PER-ARNE
Applicants: PERMANOVA LASERSYSTEM AB, ROOS SVEN OLOV, TORSTENSSON PER ARNE, ROOS, SVEN-OLOV, TORSTENSSON, PER-ARNE
Application year: 1999
Main classification: B23K26
IPC codes: B23K26, H01S3