Abstract:
A feedback control system for controlling a laser source. The feedback control system includes a laser source outputting laser energy and an optical sensor detecting the laser energy. The optical sensor outputs a measured signal in response to a measured amount of the laser energy. The system further includes an optical device receiving the laser energy and directing the laser energy to a predetermined location. The optical device reflects a first portion of the laser energy toward the optical sensor. A controller receives the measured signal from the optical sensor and calculates the amount of the first portion of the laser energy. The controller then adjusts the laser source to correct for the losses associated with the first portion of the laser energy reflecting from the optical device to obtain a predetermined amount of laser energy at the predetermined location.

Description:
FIELD 
     The present disclosure relates generally to plastics welding and, more particularly, relates to automatic part feedback compensation for laser plastics welding. 
     BACKGROUND AND SUMMARY 
     Currently, the art of welding plastic or resinous parts incorporates a variety of techniques including ultrasonic welding, heat welding, and, most recently, Through Transmission Infrared (TTIr) welding. 
     TTIr welding employs infrared light passed through a first plastic part and into a second plastic part. TTIR welding can use either infrared laser light or incoherent infrared light in the current art. Infrared laser light in the current art can be directed by fiber optics, waveguides, or light guides through the 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. 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 thus welded together. However, control of the laser can be difficult and currently requires manual adjustment of the output of the laser source to achieve the desired laser heating effect. This manual adjustment is performed on a trial and error process and can be very laborious and time consuming. 
     According to the principles of the present teachings, it is desirable to control the output of the laser source to insure proper welding and, more particularly, it is desirable to control the output of the laser source through the use of closed loop feedback control. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way. 
         FIG. 1  is a schematic view illustrating a Through Transmission Infrared (TTIr) welding system; 
         FIG. 2  is a schematic view illustrating a Through Transmission Infrared (TTIr) welding system using closed-loop feedback control; 
         FIG. 3  is a perspective view illustrating an infrared welding machine incorporating the teachings of the present disclosure; and 
         FIG. 4  is a perspective view illustrating a laser diode chamber having a photodiode and a laser diode. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. 
     As illustrated in  FIG. 1 , the principles of the present teachings provide a method and apparatus for use in Through Transmission Infrared (TTIr) welding. In general, in TTIr welding, infrared laser light  100  is directed from one or more laser sources  102  through an optical device, such as lightguides, waveguides, and/or fiber optics, to plastic parts to be welded. In this regard, first plastic part  110  is transmissive to infrared light and thus permits the infrared light to pass therethrough. Second plastic part  112  is absorptive to infrared light. Therefore, the laser light passes through first transmissive part  110  to second absorptive part  112 , where it is converted to heat and in turn melts the plastic at weld joint  114  causing the parts to weld together. Alternatively, both parts can be transmissive to infrared light, in which case, a medium absorptive to infrared light can be positioned at weld joint  114  to absorb the infrared light and converts it to heat causing the parts to be welded. However, according to the principles of the present teachings, it is desirable to control the output of laser source  102  to insure proper welding and, more particularly, it is desirable to control the output of laser source  102  through the use of a novel closed loop feedback control. 
     Referring now to  FIG. 2 , in some embodiments, a feedback control system  12  is employed to provide feedback information in a TTIr laser plastics simultaneous plunge welding system  10  to monitor the laser intensity downstream from a laser source  14  (can be similar or identical to laser source  102 ). Feedback control system  12  comprises an optical sensor  16  positioned downstream from laser source  14 , yet upstream from first transmissive part  110  and second absorptive part  112 , and a control module  17 . In some embodiments, optical sensor  16  is a photodiode. Control module  17  is operably coupled in electrical communication with optical sensor  16  for receiving real-time laser intensity information from laser source  14  and operably coupled in electrical communication with laser source  14  for controlling an output intensity of laser source  14 . 
     In some embodiments, optical sensor  16  can be positioned upstream of a fiber optic member  18  and/or a waveguide  20  (illustrated), or can be positioned downstream of one or more of fiber optic member  18  and waveguide  20 . In other words, optical sensor  16  can be positioned at any position between laser source  14  and first transmissive part  110 . However, prior to monitoring, laser source  14  should be calibrated to a set value. Ideally, this calibration process is performed without parts or other tooling in place. In some embodiments, optical feedback sensor  16  is positioned upstream from the tooling, such as fiber optic member  18  and/or waveguide  20 , thereby eliminating the need to change or replace optical feedback sensor  16  or feedback control system  12  during part or tooling changes. 
     The teachings of the present disclosure automatically compensate for such things as part and tool reflectivity in TTIr welding system  10  through the use of closed loop feedback control that enables quick and convenient tooling changes. More particularly, the teachings of the present disclosure permit tooling changes and part changes after feedback control system  12  is initially calibrated without throwing off or adversely effecting the feedback signal. 
     Step 1—In order to initially calibrate feedback control system  12  of TTIr welding system  10 , laser source  14  is first fired in open loop mode without any tooling (i.e. fiber optic member  18  and/or waveguide  20 ) or part to be welded present, at an initial power level percentage, % P initial  verified by an external meter. The signal from optical feedback sensor  16  in this condition is measured as an initial optical feedback signal, V initial , stored electronically, and used as a baseline. This can be performed when TTIr welding system  10  is first manufactured, or at any time later, but only has to be performed once. 
     Step 2—Laser source  14  can then be fired in open loop mode with fiber optic member  18  and/or waveguide  20  in place at some known power percentage level, % P tool . The optical feedback signal is then measured as V tool , and stored electronically. The optical feedback signal with the tool, V tool , will be higher than the initial optical feedback signal, V initial , because of reflected light returning from the tooling. In other words, as light is output from laser source  14 , it will travel down fiber optic member  18  and/or waveguide  20 . Optical feedback sensor  16  will, in part, detect this output light. However, optical feedback sensor  16  will also detect a portion of light that is reflected back at optical feedback sensor  16  from fiber optic member  18  and/or waveguide  20 . Therefore, the optical feedback signal with the tool, V tool , includes the sum of the actual output light from laser source  14  and the amount of light that is reflected back at optical feedback sensor  16  due to the tooling. This step only needs to be performed when the tooling is changed. 
     Step 3—Laser source  14  can then be fired in open loop mode at a percentage of the full power level, % P mirror , with the tooling in place and with a mirror with a known reflectivity, R mirror , placed where parts  110 ,  112  will later be placed. The optical feedback is measured as V mirror , and stored electronically. This step only needs to be performed when the tooling is changed. 
     It should be noted, however, that the measurement of the optical feedback signal in open loop with tooling in place (i.e. Step 2), and in open loop with both the tooling and mirror in place (i.e. Step 3) are not necessary steps for correcting for laser power delivered to the top of part  110 . However, these steps are necessary for compensating for laser power delivered through part  110  down to weld zone  114 . In other words, the reflectivity and/or absorption of part  110  may reduce the amount of laser light getting through to weld joint  114  and, therefore, laser source  14  should be compensated for this effect. 
     Step 4—Finally, laser source  14  is then fired in open loop mode with both the tooling and parts  110 ,  112  in place at some percentage of the full power level, % P part . This optical feedback signal is measured as V part , and once again stored electronically. This step can be performed once before an initial part run, for the new part, or before a series of new parts to account for the variability between part batches, or even before each individual part to account for individual part variation. Because only a percentage of the full laser power is being used, the power can be set below the welding threshold of parts  110 ,  112 , thus allowing the feedback signal to be measured without sacrificing the integrity of parts  110 ,  112 . 
     During actual welding in closed loop mode, the feedback signal, V actual  is modified to a corrected value, V corrected  as follows: 
                     V   corrected     =       V   actual     ⁢       (       V   initial       %   ⁢           ⁢     P   initial         )       (       V   Part       %   ⁢           ⁢     P   part         )                 (   1   )               
where:
         V corrected =corrected feedback signal actually used by the closed loop processor;   V actual =feedback signal read by optical feedback sensor  16  during actual weld cycle;   V initial =feedback signal read initially with no tool and no part;   % P initial =percentage of total power used in open loop with no tool and no part;   V part =feedback signal read initially in open loop with tool and part at a percentage of total power; and   % P part =percentage of total power used in open loop with tool and part.       
     The corrected feedback value used by the closed loop processor, V corrected , will be less than the actual feedback signal, V actual , seen by optical feedback sensor  16 . The actual feedback signal includes additional spurious reflected signal. The corrected feedback has that additional spurious amount cancelled out. This allows the closed loop controlled laser power to be delivered at the top of the part at a known amount specified as per the initial calibration of the machine. 
     Only steps one and four need to be measured to correct the feedback signal so that a known amount of laser power reaches the top of part  110 . Part  110  has some reflectivity that bounces a percentage of the delivered power away from weld joint  114 , which is at a distal surface of part  110 . This can be further compensated for by steps two and three. With a known actual reflectivity of part  110 , laser source  14  power can be boosted to make the delivered power at weld joint  114  equal to the amount requested (minus any dispersion of laser power in part  110 ). 
     The reflectivity of the part, R part , can be calculated as: 
                     R   part     =           (       V   part       %   ⁢           ⁢     P   part         )     -     (       V   tool       %   ⁢           ⁢     P   tool         )           (       V   mirror       %   ⁢           ⁢     P   mirror         )     -     (       V   tool       %   ⁢           ⁢     P   tool         )         ×     R   mirror               (   2   )               
where:
 
     R part =reflectivity of the part; 
     R mirror =reflectivity of a known partially reflective mirror; 
     V tool =feedback signal read in open loop with a tool but without a part;
         % P tool =percentage of total power used in open loop with a tool but no part;   V mirror =feedback signal read in open loop with a tool and a known partially reflective mirror; and   % P mirror =percentage of total power used in open loop with tool and known partially reflective mirror.       

     With part reflectivity, not only is the feedback signal boosted by reflected signals, but also less laser power gets to weld joint  114 . If the output power of laser source  14  is boosted by the amount of reflected power, the reduction of laser power due to reflection is compensated for at weld joint  114 . The new compensated feedback, V compensated , necessary to achieve this new power level is: 
                     V   compensated     =       V   corrected     ×     1     (       R   part     +   1     )                 (   3   )               
where:
         V compensated =feedback signal used by the closed loop microcontroller that boosts laser power to weld joint  114  to compensate part reflectivity.       
     TTIr welding system  10  is now operated in closed loop with the actual optical feedback, V actual , modified to the new compensated feedback, V compensated , so that the requested laser power is now delivered to weld joint  114  automatically. 
     Light absorption within parts  110 ,  112  also reduces the amount of laser power that gets to weld joint  114 . If the power of laser source  14  is boosted by both the amount of reflected power and absorbed power, the power from laser source  14  reaching weld joint  114  will be exactly the amount specified. 
     Equations (1), (2), and (3) above assume that the feedback signal is linear with the light impinging on optical sensor  16 . If in some embodiments the response is non-linear, then an appropriate lookup table can be used for optical sensor  16  so that the signal can be modified to be a linear response. 
     The feedback loop to control laser source  14  can be embedded in electronic hardware, embedded in mechanical hardware, embedded in firmware, embedded in software, or the like. In some embodiments, software and firmware may provide improved flexibility in terms of implementation. 
     The teachings of the present disclosure have been tested on a Branson IRAM L-386FAi infrared laser plastics welding machine (see  FIG. 3 ). As seen in  FIG. 4 , the light from a laser diode  202  is detected in the laser diode chamber  204  by a photodiode  206 , upstream from any fiber optic and waveguide tooling. The various open loop feedback signals are recorded on memory, and the control algorithm for the various machine states needed for the compensation resides in software in the machine controller. 
     Allowing for automatic closed loop feedback signal correction for reflected signals from tooling and parts downstream of optical feedback sensor  16  has a major advantage. The power of laser source  14  reaching the part will be known quantitatively, and the process is automatic. Previous methods required a time consuming iterative approach of manually adjusting power to achieving the desired power level in a closed loop system, because the feedback signal would be altered by reflection off the part. The teachings of the present disclosure provide automatic calibration of the laser source using a precise baseline. The feedback signal is then compensated for this calibration. The precise baseline and closed loop control enable reliable delivery of laser power to the weld zone. 
     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.