Abstract:
A method and apparatus for feedback control of a laser, which includes outputting a laser beam from the laser to an optical bundle. The intensity of the laser beam is measured at a point between the laser and the optical bundle and a raw feedback signal is output in response thereto. The raw feedback signal is multiplied by a laser calibration factor and an optical bundle calibration factor and output as an adjusted feedback signal. A controller is then used to control the intensity of the laser in response to the adjusted feedback signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to laser welding and, more particularly, relates to a dual parameter feedback system for calibrating laser output energy. 
   BACKGROUND AND SUMMARY OF THE INVENTION 
   Laser welding is commonly used to join plastic or resinous parts, such as thermoplastic parts, at a welding zone. An example of such use of lasers can be found in U.S. Pat. No. 4,636,609, which is expressly incorporated herein by reference. 
   As is well known, lasers provide a semi-focused beam of electromagnetic radiation at a specified frequency (i.e., coherent monochromatic radiation). There are a number of types of lasers available; however, infrared lasers or non-coherent sources provide a relatively economical source of radiative energy for use in heating a welding zone. One particular example of infrared welding is known as Through-Transmission Infrared (TTIr) Welding. TTIr welding employs an infrared laser capable of producing infrared radiation that is directed by lenses, diffractive optics, fiber optics, waveguides, lightpipes, or lightguides through a first plastic part and into a second plastic part. This first plastic part is often referred to as the transmissive piece, since it generally permits the laser beam from the laser to pass therethrough. However, the second plastic part is often referred to as absorptive piece, since this piece generally absorbs the radiative energy of the laser beam to produce heat in the welding zone. This heat in the welding zone causes the transmissive piece and the absorptive piece to be melted and, with intimate contact, welded together. 
   Often times, multiple laser beams produced from multiple laser and optical bundle assemblies may be used to weld a single zone. That is, in simultaneous plunge welding, multiple laser sources are used to simultaneously produce a single weld at a specific weld zone. The laser beam from each of the multiple laser sources is delivered to the weld zone by an individual fiber optic bundle, lightguide, and or waveguide (hereinafter collectively referred to an optical bundle). It should be understood that each specific laser source and bundle assembly produces a unique laser output intensity that is a byproduct of the unique characteristics of the laser source and bundles used. 
   It is frequently desirable to interchange various bundles with different laser sources to produce a desired effect. However, as one skilled in the art will recognize, interchanging a bundle with a different laser source will again produce a unique combination have a different laser output. Typically, it is required that this new combination of laser source and bundle be recalibrated so the exact laser output intensity is known so as to avoid undesired welds. Thus, it is desirable to be able to readily swap bundles with individual laser sources without having to recalibrate each bundle to each laser. This would permit quick and simplified tooling modifications, without the need for costly down time. 
   It is known to use a single optical sensor positioned near the laser source to provide output energy information. However, this output energy information is only useful when calibrated relative to the laser and bundle being used. Therefore, with a single sensor arrangement, the laser source and bundle assembly still require lengthy delays during calibration. 
   If one chooses not to calibrate the laser source and bundle assembly, excessive statistical error is introduced. As seen in  FIG. 1 , when one uses a single optical sensor positioned near the laser source and swaps between bundles, the standard deviation of the raw feedback signal without a calibration factor has been found to be about 57%, as shown in  FIG. 1 . Obviously, for most applications, this standard deviation is unacceptable. As mentioned above, this deviation could be eliminated by calibration of each laser source and bundle combination. But given the number of interchangeable bundles between various laser sources and machines in a typical commercial application, it is not practical to calculate the calibration factor for every possible combination. 
   Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
       FIG. 1  is a graph illustrating the standard deviation of a conventional feedback control system for various laser and bundle combinations; 
       FIG. 2  is a schematic view illustrating a feedback control system using a dual feedback parameter system according to the principles of the present invention; 
       FIG. 3  is a Ven diagram of a laser and bundle combination illustrating that the laser calibration factor includes both laser efficiency and coupling and the bundle calibration factor includes both bundle throughput and coupling; and 
       FIG. 4  is a graph illustrating the standard deviation of a dual parameter feedback control system for various laser and bundle combinations according to the principles of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of the preferred embodiment is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses. However, it should be understood that although the present invention is discussed in connection with specific types of equipment, such as lasers, photo detectors, and bundles, the present invention should not be interpreted as being so limited. The dual parameter feedback control of the present invention may be used with any laser, any photo detector, or any circuit. Additionally, the present invention may be used for laser plastics welding or for control of a laser in any other application. 
   Referring to  FIG. 2 , a feedback control device  10  is illustrated for use with a conventional laser source  12 , such as a laser diode, and optical bundle  14 . Feedback control device  10  is feedback loop capable of controlling the output of laser source  12 . Laser source  12  may be any conventional laser. Similarly, optical bundle  14  may include any number or combination of conventional optical members or laser light transmitters/reflectors, such as, but not limited to, fiber optics, waveguides, and lightguides. The present invention should not be regarded as being limited to the specific optical members or lasers used. 
   Feedback control device  10  employs a single optical sensor  16 , such as a solid state infrared photodiode, positioned between the outlet of laser source  12  and the inlet of optical bundle  14 . Optical sensor  16  is operable to measure the intensity of laser energy output from laser source  12 . Optical sensor  16  outputs a raw feedback signal  17 , specifically an analog signal, in response to the measured laser energy intensity. 
   A processing unit  18  receives raw feedback signal  17  and multiplies raw feedback signal  17  by a pair of calibration factors to produce an adjusted feedback signal  19 . The pair of calibration factors include a laser calibration factor, C L , and a bundle calibration factor, C B . The adjusted feedback signal is then sent back to a controller  20 , preferably a digital IC controller, which adjusts the output of laser source  12  through voltage pulsing of the DC current supply to the laser source  12  in response to the adjusted feedback signal. An external or user control  22  is also operably coupled to controller  20  to vary a preferred laser output  24 . 
   The following procedure and equations are used to determine the value of C L  and C B . 
                     m   B     _     =       ∑     L   =   1     n     ⁢       m   LB     n                       m   L     _     =       ∑     B   =   1     p     ⁢       m   LB           m   B     _     ⁢   p                       C   B     =     1       m   B     _                     C   L     =     1       m   L     _                   s   =         1       (     n   -   1     )     ⁢     (     p   -   1     )         ⁢       ∑     L   =   1     n     ⁢     (       ∑     B   =   1     p     ⁢       (         m   LB         m   B     _       -       m   L     _       )     2       )                       
Where:
     n=Number of Lasers   p=Number of Bundles   L=Laser Index   B=Bundle Index   M LB =Slope of Individual Laser and Bundle Combination Responses   C B =Bundle Calibration Factor for Each Bundle   C L =Laser Calibration Factor for Each Laser   s=Standard Deviation of Irresolvable Noise
 
Using the above equations, a matrix of slopes of various laser and bundle combinations, m LB , are measured experimentally for a statistical sample. An external light power meter, used for calibration only, located at laser light output  24 , is used to determine the measured laser power output of the system. The voltage of the measured raw feedback signal  17  is divided by the measured laser power output of the system to get the slope of the response, m LB . Then the calibration factors C L  and C B  can be calculated by using the equations above. As schematically illustrated in  FIG. 3 , both C L  and C B  are statistically independent, but both include part of the information about the coupling between the laser sources and bundles. That is, part of the calibration factor of the laser source accounts for the efficiency of the laser, while part of the calibration factor of the bundle accounts for the throughput of the bundle. However, by using this statistical method, each calibration factor also accounts for the dependence of the lasers and bundles to the coupling efficiency between the lasers and bundles. The statistically dependent part of the coupling is irresolvable noise, which can be calculated using the final equation above.
   
   Once a statistical sample of laser source and bundle combinations has been measured to determine C L  and C B , any individual bundle or laser from the original statistical sample can be used to calibrate bundles and laser sources that were not originally in the statistical sample. It has been found that instead of having a single calibration factor as in the prior art, but having two calibration factors as in the present invention, the standard deviation has dropped from about 57% to 7%, as illustrated in comparing  FIGS. 1 and 4 . The remaining 7% standard deviation represents the noise of the variability of the coupling between different laser and bundle combinations that cannot be correlated to just a laser or to a bundle. 
   To achieve separate calibrations of additional lasers and bundles, after the statistical sample of combinations has been taken, any one of the original samples that has been calibrated can be used as a “gold standard” to calibrate others. Therefore a calibrated “gold standard” laser source can be used to calibrate any additional bundles outside the sample. Likewise, a precalibrated “gold standard” bundle can then be used to calibrate any additional laser source. The result is a laser intensity feedback system that allows for freely interchangeable tooling that varies by only up to about 7% from the true laser intensity delivered to the actual weld zone. 
   The present invention of statistical extraction of semi-dependent parameters in a feedback control system for use with a laser can be applied to two, three, or more parameters and can also be applied to any feedback system used so long as the parameters represent the overall throughput of some quantity such as, but not limited to, heat, light, power, fluid flow, mechanical motion, and the like. This extraction of parameters allows for free swapping of components that the parameters represent with a minimum of noise. This process eliminates the need to individually calibrate every combination of laser source and bundle, without introducing excessive statistical noise and/or standard deviation. In other words, laser sources and optic bundles can be swapped in an infrared laser plastics welding system using positive feedback control. Not all laser and bundle combinations need to be measured to allow for the initial calibration. This enables the use of different laser machines and laser bundle tools at different times and in different places, yet still allows laser bundle tooling swapping between machines. On a given single machine, the laser bundles from a tool can also be swapped between different laser channels allowing for reconfiguration of the tool without recalibration. 
   The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.