Patent Publication Number: US-9415464-B2

Title: Laser machining system utilizing thermal radiation image and method thereof

Description:
CROSS REFERENCE 
     The present application is based on, and claims priority from, Taiwan Application Serial Number 102,143,261, filed on Nov. 27, 2013, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Field 
     The present disclosure generally relates to a laser machining system and a method thereof, and particularly to a laser machining system and a method thereof. 
     2. Related Art 
     Nowadays, the laser machining system is utilized in a variety of manufacturing method originally performed with the conventional mechanism, such as welding, surface quenching, annealing, and other heat treating technologies, as the developing of the laser technology. For example, the glass packaging technology of the organic light-emitting diode, surface hardening of metal, metal welding, semiconductor impurity diffusion annealing, and semiconductor crystallizing are applications with high potential. 
     However, there are some considerations of the aforementioned laser machining technology about the temperature of a material illuminated/heated by a laser beam such as temperature value, temperature gradient, temperature distribution, heating time interval, and etc. As such, it is important for the laser machining technology to accurately control temperature-related parameters. 
     SUMMARY 
     In one or more exemplary embodiments of this disclosure, a laser machining system may comprise a laser generating device, an array photo detecting device, a processing device, and a positioning device. The laser generating device is configured to project a laser beam onto a work piece via a first light path. The array photo detecting device is configured to detect thermal radiation from the work piece illuminated by the laser beam via a second light path to capture a thermal radiation image, wherein the second light path is different from the first light path. The processing device is electrically coupled to the laser generating device and the array photo detecting device and is configured to calculate a temperature centroid of the thermal radiation image and generate a distance control signal according to the temperature centroid. The positioning device is electrically coupled to the processing device and is configured to be controlled by the distance control signal to make a present distance, between the laser machining system and the work piece, equal to a working distance. 
     In one or more exemplary embodiments of this disclosure, a laser machining method may comprise: projecting a laser beam onto a work piece via a first light path, detecting thermal radiation from the work piece illuminated by the laser beam via a second light path to capture a thermal radiation image, wherein the second light path is different from the first light path, calculating a temperature centroid of the thermal radiation image, and controlling the laser machining system according to the temperature centroid so that a present distance between the laser machining system and the work piece is equal to a working distance. 
     In order to make the aforementioned and other features of the present disclosure more comprehensible, several embodiments accompanied with figures are described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description given herein below for illustration only, and thus are not limitative of the present disclosure, and wherein: 
         FIG. 1  is the functional block diagram of a laser machining system according to one embodiment of this disclosure; 
         FIG. 2A  is a normal operation schematic of a laser machining system according to one embodiment of this disclosure; 
         FIG. 2B  is a thermal radiation image detected by the array photo detecting module in  FIG. 2A ; 
         FIG. 2C  is a defocus operation schematic of a laser machining system according to one embodiment of this disclosure; 
         FIG. 2D  is a thermal radiation image detected by the array photo detecting module in  FIG. 2C ; 
         FIG. 2E  is a defocus operation schematic of a laser machining system according to one embodiment of this disclosure; 
         FIG. 2F  is a thermal radiation image detected by the array photo detecting module in  FIG. 2E ; 
         FIG. 3A  is an isotherm schematic according to one embodiment of this disclosure; 
         FIG. 3B  is a temperature distribution schematic related to  FIG. 3A ; 
         FIG. 3C  is a high-temperature area schematic corresponding to  FIG. 3A  in one embodiment of this disclosure; 
         FIG. 4A  is a flow chart of a laser machining method according to one embodiment of this disclosure; 
         FIG. 4B  is a flow chart of the block S 43  in  FIG. 4A  according to one embodiment of this disclosure; 
         FIG. 4C  is a flow chart of the block S 44  in  FIG. 4A  according to one embodiment of this disclosure; and 
         FIG. 4D  is a flow chart of the block S 45  in  FIG. 4A  according to one embodiment of this disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
     Referring to a laser machining system in one embodiment of this disclosure, please refer to  FIG. 1  and  FIG. 2A , wherein  FIG. 1  is the functional block diagram of a laser machining system according to one embodiment of this disclosure while  FIG. 2A  is a normal operation schematic of a laser machining system according to one embodiment of this disclosure. As shown in  FIG. 1A , a laser machining system  1  according to one embodiment of this disclosure may comprises a laser generating device  11 , an array photo detecting device  13 , a processing device  15 , and a positioning device  17 . The processing device  15  is electrically coupled to the laser generating device  11 , the array photo detecting device  13 , and the positioning device  17 . The functionality of each of the aforementioned devices is then explained below. 
     The laser generating device  11  is configured to be controlled by the processing device  15  to project a laser beam onto a work piece  20  via a first light path  101  so that at least a portion of the work piece  20  is heated by the laser beam and emits thermal radiation, and such phenomenon is described in the Wien&#39;s distribution law and the Plank&#39;s law. For example, the work piece  20  is two metal plates put together to be welded. When the laser generating device  11  projects a high energy laser beam onto the work piece  20 , a portion of each metal plate may be heated and melted so that these two metal plates are able to be welded together. In general, the laser generating device  11  is configured to generate laser beam(s) to perform heat treating process such as welding, surface quenching, and/or annealing on the work piece  20 . According to one or more exemplary embodiments, the laser generating device  11  may be, for example but not limited to, a gas laser generator, a chemical laser generator, an excimer laser generator, a solid-state laser generator (for example, Nd-YAG laser generator), a fiber laser generator (for example, Yb-fiber laser generator), a photonic crystal laser generator, a semiconductor laser generator, and any other laser generator capable of generating a high power laser beam. 
     The array photo detecting device  13  is configured to detect thermal radiation from the work piece  20  illuminated by the laser beam via a second light path  102  to obtain a thermal radiation image so as to measure a present distance between a focusing lens  19  and the work piece  20 . As shown in  FIG. 2A , the second light path  102  is different from the first light path  101 , so a first point on the axial line of the first light path  101  is corresponding to a first projected dot on a detecting surface of the array photo detecting device  13  while a second point, different from the first point, on the axial line of the first light path  101  is corresponding to a second projected dot on the detecting surface different from the first projected dot. As a consequence, the present distance, between the focusing lens  19  and the work piece  20 , may be measured since a relationship between the points on the axial line of the first light path  101  and the projected dots on the detecting surface can be described as a bijection function. 
     In an exemplary embodiment, a path angle  8  between the first light path  101  and the second light path  102 , or equivalently defined by the first light path  101  and the second light path  102 , is larger than zero degree. In a further embodiment, the path angle θ is less than sixty degree. According to one or more exemplary embodiments of this disclosure, the array photo detecting device  13  may be, for example but not limited to, a one-dimension charge-coupled device image sensor, a two-dimension charge-coupled device image sensor, an one-dimension metal-oxide semiconductor field effect transistor image sensor, a two-dimension metal-oxide semiconductor field effect transistor image sensor, a photodiode array, a position sensing detector, or any other device applicable for capturing or obtaining the thermal radiation image. 
     Specifically, the array photo detecting device  13  may be used for detecting whether the work piece  20  is located at a position on the focal plane of the laser beam of the laser machining system  1 . For example, please refer to  FIG. 1 ,  FIG. 2A , and  FIG. 2B , wherein  FIG. 2B  is a thermal radiation image detected by the array photo detecting module in  FIG. 2A . It is shown  FIG. 2A  that the work piece  20  is at a position on the focal plane of the laser beam of the laser machining system  1  and a working distance d 0 , which, in one embodiment, is the focal length of the focusing lens  19 , is defined under such circumstance. In other words, the present distance between the focusing lens  19  and the work piece  20  is the working distance d 0  when the work piece  20  is at a position on the focal plane of the laser beam. As shown in  FIG. 2A  and  FIG. 2B , a machining area of the work piece  20 , heated by the laser beam, has a centroid and the centroid of the machining area is corresponding to a temperature centroid of the thermal radiation image shown in  FIG. 2B  when the thermal radiation is captured by the array photo detecting device  13 . Further, the temperature centroid of the thermal radiation image may be defined as a calibration coordinate P 0  when the work piece  20  is at a position on the focal plane of the laser beam (equivalently, the present distance is equal to the working distance d 0 ). 
     Under some operation states, the present distance between work piece  20  and the focusing lens  19  may be less than the working distance d 0 , and such operation states may be discovered from the thermal radiation image. Please refer to  FIG. 2C  and  FIG. 2D , wherein  FIG. 2C  is a defocus operation schematic of a laser machining system according to one embodiment of this disclosure and  FIG. 2D  is a thermal radiation image detected by the array photo detecting module in  FIG. 2C . As shown in  FIG. 2C , a present distance d 1  between the work piece  20  and the focusing lens  19  is less than the working distance d 0 . The centroid of the machining area of the work piece  20  is corresponding to a temperature centroid P 1  of the thermal radiation image shown in  FIG. 2D  when the thermal radiation is captured by the array photo detecting device  13 . As shown in  FIG. 2D , the temperature centroid P 1  is different from the calibration coordinate P 0 . 
     Under another operation state, the present distance between work piece  20  and the focusing lens  19  may be greater than the working distance d 0 , and such operation state may be discovered from the thermal radiation image. Please refer to  FIG. 2E  and  FIG. 2F , wherein  FIG. 2E  is a defocus operation schematic of a laser machining system according to one embodiment of this disclosure and  FIG. 2F  is a thermal radiation image detected by the array photo detecting module in  FIG. 2E . As shown in  FIG. 2E , a present distance d 2  between the work piece  20  and the focusing lens  19  is greater than the working distance d 0 . The centroid of the machining area of the work piece  20  is corresponding to a temperature centroid P 2  of the thermal radiation image shown in  FIG. 2F  when the thermal radiation is captured by the array photo detecting device  13 . As shown in  FIG. 2F , the temperature centroid P 2  is different from the calibration coordinate P 0 . As a conclusion, if the present distance is not equal to the working distance d 0 , the temperature centroid of the thermal radiation image will not equal to the calibration coordinate P 0 . Hence, in one or more exemplary embodiment of this disclosure, the processing device  15  may determine whether the present distance is equal to the working distance d 0  according to the temperature centroid of the thermal radiation image. 
     The processing device  15  is configured to calculate the temperature centroid of the thermal radiation image and generate a distance control signal according to the temperature centroid. In one exemplary embodiment of this disclosure, as shown in  FIG. 1 , the processing device  15  may comprise a temperature computing module  151 , a centroid computing module  153 , a compensating module  155 , and a power controlling module  158 . The temperature computing module  151  is electrically coupled to the array photo detecting device  13 . The centroid computing module  153  is electrically coupled to the temperature computing module  151 . The compensating module  155  is electrically coupled to the centroid computing module  153  and the positioning device  17 . The power controlling module  158  is electrically coupled to the temperature computing module  151  and the laser generating device  11 . In practice, the processing device  15  and the modules therein may be realized by, for example but not limited in, an application-specific integrated circuit, an advanced RISC machine, a central processing unit, a 8051 chip or any other device applicable for performing computing instructions and/or controlling instructions. 
     The temperature computing module  151  is configured to calculate a corresponding temperature value for a pixel among a plurality of pixels of the thermal radiation image according to a grayscale of the pixel. For example, when the temperature computing module  151  is to calculate a first corresponding temperature value of a first pixel among the plurality of the thermal radiation image, the temperature computing module  151  may firstly calculate a first received energy of the first pixel according to a first grayscale of the first pixel. After the first grayscale is calculated, the temperature computing module  151  may calculate the first corresponding temperature corresponding to the first pixel with the Wien&#39;s distribution law or the Plank&#39;s law according to the first grayscale. The first corresponding temperature value is the temperature of a first area on the work piece  20  which is corresponding to the first pixel of the thermal radiation image. 
     For example, take the first received energy value and the first corresponding temperature value into consider, the Plank&#39;s law describes a relationship between the first received energy value, the first corresponding temperature value, and a wavelength as below: 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       1 
                     
                     ⁡ 
                     
                       ( 
                       λ 
                       ) 
                     
                   
                   ≅ 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       hc 
                       2 
                     
                     ⁢ 
                     
                       λ 
                       
                         - 
                         5 
                       
                     
                     ⁢ 
                     
                       ⅇ 
                       
                         
                           - 
                           hc 
                         
                         
                           λ 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             kT 
                             1 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
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                   ) 
                 
               
             
           
         
       
     
     In the equation (1), I 1  is related to the first received energy of the first pixel, h is the Plank constant, c is the speed of light, λ is the wavelength of the thermal radiation received by the first pixel, k is the Boltzmann constant, and T 1  is the first corresponding temperature value, expressed in the form of the absolute temperature. In one exemplary embodiment, as shown in  FIG. 2A , the aforementioned array photo detecting device  13  may comprises an array photo detecting module  131  and a optical filter  135 , located in the second light path  102 . The optical filter  135  may filter the thermal radiation to obtain an output light with a particular spectrum distribution different from an original spectrum distribution of the thermal radiation. The array photo detecting module  131  may obtain the thermal radiation image according to the output light. For example, the output light may be infrared light. If only the infrared light with wavelength between 700 nm and 1000 nm passes the optical filter  135  to be received by the array photo detecting module  131 , the first received energy received by the first pixel equals to a summation of the energy of all infrared light whose wavelength is between 700 nm and 1000 nm. Under such circumstance, the relationship between the first received energy E 1  and the first corresponding temperature value T 1  can be described as an equation as: 
     
       
         
           
             
               
                 
                   
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     The aforementioned equation (2) is an integration of the equation (1) taking the wavelength λ as an integrated variable. It is clear that if the first received energy E 1  is known, one can use the equation (2) to derive the first corresponding temperature value T 1 . 
     The centroid computing module  153  may calculate the temperature centroid according to a corresponding coordinate of each pixel among the plurality of pixels and the corresponding temperature value of each pixel among the plurality of pixels. For example, for each pixel among the plurality of pixels of the thermal radiation image, a coordinate and a corresponding temperature value are known. In one embodiment, a number of the plurality of pixels is n, the coordinate of the i th  pixel among pixels can be expressed as (x i , y i ), wherein x i  and y i  are respectively a x-axis coordinate value and a y-axis coordinate value of the i th  pixel, and the corresponding temperature of the i th  pixel is T i . The x-axis coordinate value of the i th  pixel may be defined as which column of the thermal radiation image the i th  pixel belongs to, and the y-axis coordinate value of the i th  pixel may be defined as which row of the thermal radiation image the i th  pixel belongs to. The temperature centroid can be calculated by: 
     
       
         
           
             
               
                 
                   
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     In the equation (3) and equation (4), X M  is a x-axis coordinate value of the temperature centroid, and Y M  is a y-axis coordinate value of the temperature centroid. 
     The compensating module  155  may generate the distance control signal according to the temperature centroid. Specifically, the compensating module  155  may determine whether the present distance is equal to the working distance d 0  according to the temperature centroid in order to the distance control signal. In one exemplary embodiment, as shown in  FIG. 1 , the processing device  15  may further comprises a storage module  157 . The storage module  157  is electrically coupled to the compensating module  155 . There may be a coordinate-to-distance table stored in the storage module  157 . Hence, the compensating module  155  may determine the present distance according to the temperature centroid and the coordinate-to-distance table so as to generate the distance control signal. After the present distance is derived according to the temperature centroid and the coordinate-to-distance table, the compensating module  155  may calculate a displacement value according to the present distance ant the working distance d 0 . The distance control signal may comprise the displacement value so that the positioning device  17  may adjust the present distance, between the laser machining system  1  (or the focusing lens  19 ) and the work piece  20 , to the working distance d 0  according to the displacement value. 
     In another exemplary embodiment of this disclosure, the storage module  157  may store a grayscale conversion model therein. When the temperature computing module  151  calculates the first corresponding temperature value for the first pixel among the plurality of the thermal radiation image, the temperature computing module  151  may obtain the first corresponding temperature value according to the first grayscale and the grayscale conversion model. The grayscale conversion model comprises information collected from a plurality of experiment and/or a plurality of measurement. In one exemplary embodiment of this disclosure, the grayscale conversion model may be a grayscale-to-temperature table. In another exemplary embodiment of this disclosure, the grayscale conversion model may be a conversion function describing the relationship between the grayscale and the temperature value. The conversion function may be derived from the collected information with a curve fitting or a regression analysis. 
     However, in some cases, the laser machining system  1  may be abnormally operated or be affected by some environmental factor, such as a change of the environment temperature or an earth quake, and the coordinate-to-distance table is not accurate. Under such circumstance, the adjusted present distance is not equal to the working distance d 0 , and there is a distance offset between the adjusted present distance and the working distance d 0 . The compensating module  155  may calculate a compensation parameter according to the distance offset to update the coordinate-to-distance table. 
     In another exemplary embodiment of this disclosure, the storage module  157  may store the calibration coordinate P 0  therein. It is mentioned that if the present distance is not equal to the working distance d 0 , the temperature centroid is not equal to the calibration coordinate P 0 . Hence, the compensating module  155  may determine whether the present distance is equal to the working distance d 0  by determining whether the temperature centroid is equal to the calibration coordinate P 0 . Further, the compensating module  155  controls the positioning device  17  to move the laser machining system  1  or the work piece  20  until the temperature centroid is equal to the calibration coordinate P 0 . 
     The power controlling module  158  may control the laser generating device  11  to project the laser beam. In one exemplary embodiment of this disclosure, the power controlling module  158  further generates a power control signal to control a laser power of the laser beam. Specifically, the power controlling module  158  may generate the power control signal so that the laser generating device  11  may increase or decrease the laser power according to the power control signal. As a consequence, a machining dimension of a machining area on the work piece  20  and the highest temperature in the machining area may be controlled. 
     In one embodiment of this disclosure, the processing device  15  may further comprise a machining area computing module  159 . The machining area computing module  159  is electrically coupled to the temperature computing module  151  and the power controlling module  158 . The machining area computing module  159  may calculate a present machining dimension of the machining area on the work piece  20  according to the thermal radiation image. The machining area computing module  159  may select a plurality of high-temperature pixels from the plurality of pixels of the thermal radiation image and calculate the present machining dimension according to the plurality of high-temperature pixels, wherein the corresponding temperature value of each high-temperature pixel is larger than a temperature threshold value. Specifically, the present machining dimension of the machining area may be calculated according to a system setting or the characteristic of the material of the work piece  20 . For example, the work piece  20  has to be heated to 500° C. in one machining process, so the machining area is the area on the work piece  20  in which a surface temperature is no less than 500° C. Hence, the machining area computing module  159  may find area with a surface temperature no less than 500° C. on the work piece  20  according to the thermal radiation image and calculate the dimension of the found area. The found area is the machining area and the calculated dimension is the present machining dimension. 
     In one exemplary embodiment, the power controlling module  158  may generate the power control signal according to the present machining dimension and a predetermined machining dimension. For example, the power controlling module  158  generates the power control signal to increase the laser power when the present machining dimension is smaller than the predetermined machining dimension so that the highest temperature in the machining area and the present machining dimension are increased. On the contrary, the power controlling module  158  generates the power control signal to decrease the laser power when the present machining dimension is larger than the predetermined machining dimension so that the highest temperature in the machining area and the present machining dimension are decreased. 
     In one exemplary embodiment of this disclosure, please refer to  FIG. 3A  and  FIG. 3B ,  FIG. 3A  is an isotherm schematic according to one embodiment of this disclosure and  FIG. 3B  is a temperature distribution schematic related to  FIG. 3A . As shown in  FIG. 3B , the machining area computing module  159  may select a plurality of high-temperature pixels from a plurality of pixels of the thermal radiation image and calculate the present machining dimension according to the plurality of high-temperature pixels. The high-temperature pixels are those pixels whose corresponding temperature value is larger than a temperature threshold value. As shown in  FIG. 3A , the high-temperature pixels selected by the machining area computing module  159  are enclosed by the isotherm R T . The machining area computing module  159  may identify the area enclosed by the isotherm R T  as the machining area and calculate radius or any other dimension of the machining area. 
     With the concept of the temperature threshold value, the centroid computing module  153  may select a plurality of high-temperature pixels from the plurality of pixels and calculates the temperature centroid according to the plurality of high-temperature pixels, wherein the corresponding temperature value of each high-temperature pixel is larger than a temperature threshold value. Please refer to  FIG. 3C , which is a high-temperature area schematic corresponding to  FIG. 3A  in one embodiment of this disclosure. As shown in  FIG. 3C , the centroid computing module  153  may recognize the black area enclosed by the isotherm R T  as the area composed of the high-temperature pixels. Hence, the centroid computing module  153  may calculate the geometric centroid of the black area and take the calculated geometric centroid as the temperature centroid. 
     The positioning device  17  may controlled by the distance control signal to adjust the present distance, between the laser machining system  1  (or the focusing lens  19 ) and the work piece  20 , to the working distance d 0 . In one embodiment, the work piece  20  is large and hard to be moved, such as the vehicle body, so the positioning device  17  moves the laser machining system  1 . In another embodiment, the work piece  20  is small and easy to be moved, such as one or more metal plates, so the positioning device  17  moves the work piece  20 . According to one ore more embodiment of this disclosure, the positioning device  17  may be, for example but not limited in, a Cartesian robot arm, a selective compliance articulated robot arm, a parallel robot arm, or any other device applicable for controlling the present distance between the laser machining system  1  and the work piece  20 . 
     Referring to the laser machining method according to one exemplary embodiment of this disclosure, please refer to  FIG. 1  and  FIG. 4A , wherein  FIG. 4A  is a flow chart of a laser machining method according to one embodiment of this disclosure. As shown in block S 41 , the laser generating device  11  projects a laser beam onto the work piece  20  via the first light path  101 . As shown in block S 42 , the array photo detecting device  13  detects thermal radiation from the work piece  20  via the second light path  102  to obtain the thermal radiation image. The second light path  102  is different from the first light path  101 , and the path angle, defined by the first light path  101  and the second light path  102 , may be greater than zero degree and less than sixty degree. As shown in block S 43 , the processing device  15  calculates the temperature centroid of the thermal radiation image. As shown in block S 44 , the processing device  15  controls the positioning device  17  according to the temperature centroid in order to adjust the present distance, between the laser machining system  1  (or the focusing lens  19 ) and the work piece  20 , to the working distance d 0  (the focal length of the focusing lens  19 ). 
     Referring to the flow of the block S 43  in one exemplary embodiment of this disclosure, please refer to  FIG. 1  and  FIG. 4B , wherein  FIG. 4B  is a flow chart of the block S 43  in  FIG. 4A  according to one embodiment of this disclosure. As shown in block S 431 , the temperature computing module  151  calculates a corresponding temperature value for a pixel among a plurality of pixels of the thermal radiation image according to a grayscale of the pixel. As shown in block S 432 , the centroid computing module  153  calculates the temperature centroid according to a corresponding coordinate of each pixel among the plurality of pixels and the corresponding temperature value of each pixel among the plurality of pixels. 
     In block S 431  in one exemplary embodiment of this disclosure, the temperature computing module  151  firstly calculates a received energy of a pixel according to a grayscale of the pixel and then calculates the corresponding temperature value of the pixel with the Wien&#39;s distribution law or the Plank&#39;s law according to the received energy. 
     In block S 432  in one exemplary embodiment of this disclosure, the centroid computing module  153  may calculate the temperature centroid according to a corresponding coordinate of each pixel among the plurality of pixels and the corresponding temperature value of each pixel among the plurality of pixels. For example, for each pixel among the plurality of pixels of the thermal radiation image, a coordinate and a corresponding temperature value are known. In one embodiment, a number of the plurality of pixels is n, the coordinate of the i th  pixel among pixels can be expressed as (x i , y i ), wherein x i  and y i  are respectively a x-axis coordinate value and a y-axis coordinate value of the i th  pixel, and the corresponding temperature of the i th  pixel is T i . The x-axis coordinate value of the i th  pixel may be defined as which column of the thermal radiation image the i th  pixel belongs to, and the y-axis coordinate value of the i th  pixel may be defined as which row of the thermal radiation image the i th  pixel belongs to. The temperature centroid can be calculated by the aforementioned equation (3) and equation (4). In another embodiment, the centroid computing module  153  may calculate the geometric centroid of a group comprising all high-temperature pixels and take the calculated geometric centroid as the temperature centroid. 
     Referring to the flow of the block S 44  in one exemplary embodiment of this disclosure, please refer to  FIG. 1  and  FIG. 4C , wherein  FIG. 4C  is a flow chart of the block S 44  in  FIG. 4A  according to one embodiment of this disclosure. As shown in block S 441 , the compensating module  155  may determine whether the present distance is equal to the working distance d 0 . If the present distance is not equal to the working distance d 0 , as shown in block S 442 , the compensating module  155  may generate the distance control signal to control the positioning device  17  to adjust the present distance to the working distance d 0 . 
     In block S 441  in one exemplary embodiment of this disclosure, the compensating module  155  may determine the present distance according to the temperature centroid and the coordinate-to-distance table stored in the storage module  157  and subtract the present distance with the working distance d 0  to obtain the displacement value. The compensating module  155  may generate the distance control signal according to the displacement value to control the positioning device  17  so that the positioning device  17  may adjust the present distance by moving the laser machining system  1  or the work piece  20  according to the control signal (or equivalently, the displacement value). After the present distance is adjusted, the compensating module  155  may compare the adjusted present distance with the working distance d 0  to determine whether the adjusted present distance is equal to the working distance d 0 . If the adjusted present distance is not equal to the working distance d 0 , the compensating module  155  may calculate a distance offset according to the adjusted present distance and the working distance d 0  and update the coordinate-to-distance table according to the distance offset. 
     In block S 441  in another exemplary embodiment of this disclosure, the compensating module  155  may determine whether the present distance is equal to the working distance d 0  by determining whether the temperature centroid is equal to the calibration coordinate P 0 . If the present distance is not equal to the working distance d 0 , the compensating module  155  may generate the distance control signal to control the positioning device  17  to adjust the present distance until the temperature centroid is equal to the calibration coordinate P 0  so that the adjusted present distance is equal to the working distance d 0 . 
     Referring to the laser machining method according to another exemplary embodiment of this disclosure, please refer to  FIG. 1  and  FIG. 4A . The method may further comprises block S 45 , the processing device  15  may control the laser power of the laser beam according to the thermal radiation image. Specifically, please refer to  FIG. 4D , which is a flow chart of the block S 45  in  FIG. 4A  according to one embodiment of this disclosure. As shown in block S 451 , the machining area computing module  159  may calculate the machining area of the work piece  20  according to the thermal radiation image. As shown in block S 452 , the power controlling module  158  may generate a power control signal to control the laser generating device  11  to adjust the laser power according to the present machining dimension of the machining area and the predetermined machining dimension. 
     According to one or more exemplary embodiment of this disclosure, the theorem black body radiation is applicable for the disclosed laser machining system. In the disclosed laser machining system, the first light path via which the laser beam is projected and the second light path via which the thermal radiation is detected/obtained are different. Hence, the temperature parameters of the work piece, such as the temperature value, the temperature distribution, the temperature gradient, etc., and the present distance between the work piece and the laser machining system can be detected with one single device, the array photo detecting device. Further, the disclosed laser machining system may operate directly according to the thermal radiation image. Hence, compared with the conventional laser machining system, the disclosed laser machining system can control the machining dimension on the work piece and the present distance without further complicated detecting method and/or controlling mechanism. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed embodiments. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.