Patent Publication Number: US-10761037-B2

Title: Laser processing device for determining the presence of contamination on a protective window

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a new U.S. Patent Application that claims benefit of Japanese Patent Application No. 2017-226147, filed Nov. 24, 2017 for all purposes. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to laser processing devices, and in particular, relates to laser processing devices. warning of contamination of a protective window during laser processing. 
     2. Description of the Related Art 
     Laser processing is performed by presetting the focal position based on, for example, the material to be cut using laser and the thickness. In case of processing defects, an external optical system for applying a light output from a laser oscillator to a workpiece is checked whether the external optical system is contaminated, or contamination of the external optical system is detected using a temperature sensor, a scattered light sensor, or the like attached to the external optical system. 
     As for techniques related to the present invention, documents below, for example, are well-known. In JP 2016-530611 T, a system for evaluating the state of a process is disclosed. The system applies an output laser beam to a workpiece via a semitransparent magic mirror disposed between a collimator and a focal lens, causes a reflected light to reflect from the magic mirror, and receives the light using a camera to evaluate processing quality. 
     In JP 2016-097412 A, a laser welding method capable of readily restraining poor welding in a case where spatters adhere to protective glass is disclosed. In the laser welding method, a low power laser beam for testing is applied to a welding portion, and a return light is received via a mirror disposed upstream of a focusing lens to calculate the decrement of laser output and the amount of focal deviation by comparing the intensity of the return light with a reference intensity to adjust the output and the focal length of a laser oscillator before laser welding. 
     In JP 2002-361452 A, a laser processing system measuring the degree of contamination of protective glass is disclosed. The laser processing system includes a radiation detector measuring the intensity of scatter radiation from protective glass and another radiation detector measuring the intensity of a laser beam via a partially permeable mirror disposed behind a lens mechanism focusing the laser beam to compensate influences of the radiation intensity of the laser beam on measured values of scattered radiation. 
     In JP 2013-233593 A, a laser processing device determining the quality of a processed state is disclosed. The laser processing device includes an optical sensor for detecting the spatial distribution of light emitted from a processing point in at least two directions. 
     SUMMARY OF THE INVENTION 
     An external optical system degrades over time. In particular, a protective window easily gets contaminated during laser processing and needs to be cleaned or replaced when contaminated. Delay in maintenance timing causes the quality of laser processing to significantly deteriorate. 
     Thus, a technique to accurately warn of contamination of the protective window during laser processing has been required. 
     An aspect of the disclosure provides a laser processing device warning of contamination of a protective window during laser processing, the laser processing device including a laser oscillator, an external optical system including a focusing lens configured to focus a light output from the laser oscillator and the protective window disposed downstream of the focusing lens, a beam splitter disposed between the focusing lens and the protective window, a return light measurement unit configured to measure intensity distribution of a return light reflected from a workpiece and returning to the external optical system via the beam splitter, a storage unit configured to store at least one of normal pattern data representing the intensity distribution of the return light from the workpiece when the protective window is in normal condition and abnormal pattern data representing the intensity distribution of the return light from the workpiece when the protective window is contaminated, a processing unit configured to perform a process of detecting contamination of the protective window during laser processing, and a warning unit configured to warn of contamination of the protective window in accordance with the process performed by the processing unit, the processing unit including a contamination detecting section configured to detect contamination of the protective window based on measurement data about the return light and at least one of the normal pattern data and the abnormal pattern data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating the overall configuration of a laser processing device according to one embodiment. 
         FIG. 2A  illustrates longitudinal sections of output lights. 
         FIG. 2B  illustrates plan views of return lights. 
         FIG. 2C  illustrates enlarged views of the return lights during piercing, cutting, and welding. 
         FIG. 3A  illustrates a plan view illustrating normal patterns representing the intensity distribution of the return lights during piercing, cutting, and welding. 
         FIG. 3B  illustrates a plan view illustrating abnormal patterns representing the intensity distribution of the return lights during piercing, cutting, and welding. 
         FIG. 4  is a block diagram illustrating the configuration of a numerical control device according to one embodiment. 
         FIG. 5  is a flowchart illustrating operations of the laser processing device according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present disclosure will be described in detail below with reference to the accompanying drawings. In the drawings, identical or similar constituent elements are given identical or similar reference signs. Additionally, the embodiments described below are not intended to limit the technical scope of the invention or the meaning of terms set forth in the claims. 
       FIG. 1  is a schematic diagram illustrating the overall configuration of a laser processing device according to the present embodiment. A laser processing device  10  includes a laser oscillator  11 , an external optical system  12  guiding a light output from the laser oscillator  11  to apply the light to a surface of a workpiece, a numerical control device  13  controlling the entire laser processing device  10 , and a drive controller  14  controlling driving of the laser processing device  10 . The laser oscillator  11  is, for example, a fiber laser oscillator with a wavelength from 1060 to 1080 nm. The external optical system  12  includes a fiber  20  guiding the light output from the laser oscillator  11 , a collimator lens  21  collimating the light output from the fiber  20 , a focusing lens  22  focusing the output light, a beam splitter  23  partially reflecting and partially passing the output light or a return light, and a protective window  24  disposed downstream of the focusing lens  22 . The drive controller  14  includes an X axis, a Y axis, and a Z axis that move a work table  16 , a V axis that moves the position of the focusing lens  22  in the optical axis direction, an S axis and a T axis that move the protective window  24  in directions orthogonal to the optical axis of the output light, and components such as servo motors and controllers that drive the axes. 
     The laser processing device  10  further includes a return light measurement unit  15  measuring the intensity distribution of the return light reflected from a workpiece W and returning to the external optical system  12  via the beam splitter  23  during laser processing. The beam splitter  23  is disposed inside a linear processing head  25  between the focusing lens  22  and the protective window  24  at an angle of 45° with respect to the optical axis. As there is no optical part other than the beam splitter  23  between the protective window  24  and the return light measurement unit  15 , contamination of the protective window  24  can be accurately detected. The return light measurement unit  15  is disposed at a position forming an angle of 90° with the optical path of the output light in the forward direction, and includes a plurality of sensor elements (e.g., photoelectric elements, etc.) arranged two-dimensionally, a plurality of sensor elements (e.g., thermocouples, etc.) arranged concentrically, or a component such as a CCD camera or a CMOS camera. 
       FIG. 2A  illustrates longitudinal sections of output lights,  FIG. 2B  illustrates plan views of return lights, and  FIG. 2C  illustrates enlarged views of the return lights during piercing, cutting, and welding. During piercing, in the intensity distribution of the return light, the intensity is high at around an optical axis O and decreases toward the periphery of the optical axis O. During cutting, only the light reflected from a cut portion C of the workpiece W constitutes the return light, and thus the intensity distribution of the return light is substantially crescent. During welding, only the light reflected from a melted portion M of the workpiece W constitutes the return light. Thus, the intensity distribution of the return light includes glittering low intensity portions L caused by scattered light and always changes without being stable. In this manner, the intensity distribution of the return light changes depending on the types of laser processing even when the protective window  24  is in normal condition. 
       FIGS. 3A and 3B  illustrate plan views illustrating normal patterns and abnormal patterns representing the intensity, distribution of the return lights during piercing, cutting, and welding. In a case where contamination of the protective window exists, high intensity portions H sparsely appear around the optical axis O. During piercing, the return light has substantially one normal pattern of the intensity distribution, and contamination of the protective window  24  can be detected by simply subtracting the normal pattern data from the return light measurement data. During cutting, although the number of normal patterns of the intensity distribution of the return light increases depending on the cutting direction; contamination of the protective window  24  can be detected by subtracting the normal pattern corresponding to the cutting direction during processing from the return light measurement data. However, during welding, the positions, the sizes, and the number of low intensity portions L caused by scattered light always change, and there is no normal pattern during processing. Thus, contamination of the protective window  24  cannot be detected by the above-described subtraction processing. To solve this, the laser processing device according to the present embodiment detects abnormality in the protective window  24  by pattern recognition (machine learning) using the normal patterns and the abnormal patterns without depending on the types of laser processing. 
     Specifically, a linear discriminant function u (i.e., equation of a straight line) for discriminating contamination of the protective window  24  is defined as follows, where x 1  is the number of sensor elements detecting intensity of the return light higher than a predetermined intensity in a set P 1  of sensor elements surrounding the optical axis O, x 2  is the number of sensor elements detecting intensity of the return light higher than a predetermined intensity in a set P 2  of sensor elements surrounding the set P 1  of the sensor elements, w 1  and w 2  are weights for x 1  and x 2 , respectively, and w 0  is a bias (hereinafter, w 0 , w 1 , and w 2  are simply referred to as “weights”).
 
 u=w   0   +w   1   x   1   +w   2   x   2  
 
     When the discriminant function u&gt;0, it can be determined that contamination of the protective window  24  exists, and when the discriminant function u&lt;0, it can be determined that contamination of the protective window  24  does not exist. To determine the parameters w 0 , w 1 , and w 2  (i.e., weights) of the discriminant function, (1) a random value is set to each of w 0 , w 1 , and w 2 , (2) x 1  and x 2  are input using at least one of the normal pattern data and the abnormal pattern data as teaching data, (3) when the output is not correct (i.e., when it is determined that contamination exists although a normal pattern is read or when it is determined that contamination does not exist although an abnormal pattern is read), the values of the weights are updated in the correct direction, and (4) when there is an update, the steps (1) to (3) are repeated to perform learning. The learning ends when correct outputs are made for all the teaching data. A known gradient descent and the like can be used for the process of updating the values of the weights, and a weight update expression, for example, is defined as follows. 
     
       
         
           
             
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     As the intensity distribution of the return light decreases with increasing distance from the optical axis O, contamination of the protective window  24  can be detected more easily with increasing distance from the optical axis O. Thus, it is highly probable that the weights determined by learning satisfy w 1 &lt;w 2 . The discriminant function u is generated by the above-described learning. In a case where the return light measurement unit  15  is a camera, the number of sensor elements increases, and thus the discriminant function u is defined as follows.
 
 u=w   0   +w   1   x   1   +w   2   x   2    . . . +w   1   x   1  
 
     Moreover, in a case where the return light measurement unit  15  includes a plurality of sensor elements (e.g., thermocouples, etc.) arranged concentrically, the sets P 1  and P 2  of the sensor elements are both one, and thus each of the values of x 1  and x 2  is 0 or 1. 
       FIG. 4  is a block diagram illustrating the configuration of the numerical control device according to the present embodiment. The numerical control device  13  includes a storage unit  30  including a RAM, a ROM, a nonvolatile memory, or the like storing various data, a processing unit  31  including CPUs, ASICs, FPGAs, or the like, and a warning unit  32  including a display panel, a speaker, an output interface, and the like. The storage unit  30 , the processing unit  31 , and the warning unit  32  are connected with each other by buses or the like. The processing unit  31  performs the process of detecting contamination of the protective window  24  during laser processing, and the warning unit  32  warns of contamination of the protective window  24  in accordance with the process by the processing unit  31 . 
     The storage unit  30  prestores at least one of the normal pattern data representing the intensity distribution of the return light when the protective window  24  is in normal condition and the abnormal pattern data representing the intensity distribution of the return light from the workpiece when the protective window is contaminated. In addition, the storage unit  30  stores return light measurement data representing the intensity distribution of the return light measured by the return light measurement unit  15 . 
     The components of the processing unit  31  are implemented by program modules executed by CPUs or integrated circuits including ASICs, FPGAs, or the like. As described above, the processing unit  31  includes a discriminant function generating section  40  generating the discriminant function u for discriminating contamination of the protective window  24  using at least one of the normal pattern data and the abnormal pattern data as the teaching data. The processing unit  31  further includes a drive commanding section  41  issuing commands for the drive controller  14  to move the optical axis of the output light onto the workpiece W in accordance with processing conditions and an output commanding section  42  issuing commands for the laser oscillator  11  to output the laser beam in accordance with the processing conditions. 
     The processing unit  31  further includes a contamination detecting section  43  detecting contamination of the protective window  24  based on the return light measurement data measured by the return light measurement unit  15  and at least one of the normal pattern data and the abnormal pattern data. The contamination detecting section  43  may detect contamination of the protective window  24  by determining the difference between the return light measurement data and at least one of the normal pattern data and the abnormal pattern data, and preferably detects contamination of the protective window  24  based on the return light measurement data and the discriminant function u generated by the discriminant function generating section  40  (i.e., by pattern recognition). 
     The processing unit  31  may further include a first warning commanding section  44  issuing commands for the warning unit  32  to warn of contamination of the protective window  24  and an abnormal pattern storage commanding section  45  issuing commands for the storage unit  30  to store the return light measurement data as an abnormal pattern when contamination of the protective window  24  is detected. 
     The processing unit  31  may further include a window position adjustment amount calculating section  46  calculating the amount of position adjustment for adjusting the position of the protective window  24  when contamination of the protective window  24  is detected and a window position adjustment commanding section  47  issuing commands for the drive controller  14  to move the protective window  24  based on the amount of position adjustment. The processing unit  31  may further include a second warning commanding section  48  issuing commands for the warning unit  32  to warn that excessive contamination of the protective window  24  exists when contamination of the protective window  24  is still detected after the position adjustment of the protective window  24 . 
     The processing unit  31  may further include a discriminant function updating section  49  updating the parameters (i.e., the above-described “weights”) of the discriminant function u when contamination of the protective window  24  detected by the contamination detecting section  43  does not exist. That is, the discriminant function updating section  49  reads the return light measurement data when contamination of the protective window  24  is detected by the contamination detecting section  43  as a normal pattern, and updates the parameters of the discriminant function u. 
       FIG. 5  is a flowchart illustrating operations of the laser processing device  10  according to the present embodiment. When the process of identifying contamination of the protective window  24  is started during laser processing, the discriminant function u is generated from at least one of the normal pattern data and the abnormal pattern data in Step S 10 . In Step S 11 , a command is issued for the drive controller  14  to move the optical axis onto the workpiece W in accordance with processing conditions. In Step S 12 , a command is issued for the laser oscillator to output a laser beam in accordance with the processing conditions. 
     In Step S 13 , contamination of the protective window  24  is detected based on the return light measurement data and the discriminant function u. In Step S 14 , when contamination of the protective window  24  is not detected (NO in Step S 14 ), the process returns to Step S 10 , and the laser processing is continued. On the other hand, when contamination of the protective window  24  is detected in Step S 14  (YES in Step S 14 ), it is determined whether it is the second time in Step S 15  (NO in Step S 15 ), and a command is issued for the warning unit  32  to warn of contamination of the protective window  24  without stopping the laser processing in Step S 16 . In Step S 17 , a command is issued for the storage  30  to store the return light measurement data when contamination of the protective window  24  is detected as an abnormal pattern. 
     In Step S 18 , the amount of position adjustment for adjusting the position of the protective window  24  is calculated such that contamination of the protective window  24  lies outside the area of the output light. In Step S 19 , a command is issued for the drive controller  14  to adjust the position of the protective window  24 . To confirm the position of the protective window  24  is correctly adjusted, the process returns to Step S 10 , and the process of detecting contamination of the protective window  24  is repeated. 
     In Step S 10 , the discriminant function u is again generated from at least one of the normal pattern data and the abnormal pattern data. In a case where the abnormal pattern is stored in Step S 17 , the abnormal pattern is necessary for updating the discriminant function u in Step S 10 . When contamination of the protective window  24  is still detected after the position adjustment of the protective window  24  in Step S 14  (YES in Step S 14 ), it is determined as the second time in Step S 15  (YES in Step S 15 ). Thus, the laser processing is stopped, and a command is issued for the warning unit  32  to warn of excessive contamination of the protective window  24 . When an operator checks the protective window  24  and finds excessive contamination in Step S 21  (YES in Step S 21 ), the operator cleans or replaces the protective window  24 . 
     On the other hand, when the operator checks the protective window  24  and does not find any excessive contamination in Step S 21  (NO in Step S 21 ), the parameters of the discriminant function u are updated in Step S 23 . Repetition of the above-described learning allows contamination of the protective window  24  to be accurately warned of even during laser processing in which the intensity distribution of the return light is unstable. Consequently, the automatic operation can be continued without a large amount of processing defects. In addition, successful correction allows the maintenance period of the protective window  24  to be extended. 
     A program that can be executed by a computer in the above-described embodiment can be provided having been recorded in a computer-readable non-transitory recording medium, a CD-ROM, or the like. Although some embodiments have been described in this specification, the present invention is not intended to be limited to the above-described embodiments, and it is to be understood that many changes can be made without departing from the scope of the appended claims.