Abstract:
A laser joining system deforms a workpiece projection based on collapse of the projection to a predetermined displacement. The laser joining system has a laser system for generating a predetermined level of laser radiation based on radiation control signals. An actuation system directs the laser radiation to the projection and contacts the projection with a laser head based on forced control signals. The actuation system also generates position feedback based on a position of the laser head, wherein the position feedback includes a reference position of the laser head. The joining system further includes a controller communicating with the laser system and the actuation system. The controller generates the radiation control signals and the force control signals based on the position feedback. When the projection collapses to a predetermined displacement with respect to the referenced position, one of the radiation control signals causes the laser system to discontinue generation of the laser radiation. Controlling laser radiation on the basis of collapse distance allows improved consistency and reduced rework costs.

Description:
TECHNICAL FIELD 
     The present invention relates generally to laser staking and welding. More particularly, the invention relates to discontinuing the application of laser radiation to a thermoplastic projection when the projection collapses to a predetermined displacement with respect to a reference position. 
     BACKGROUND ART 
     In many industries it is necessary to deform and shape a thermoplastic projection of a workpiece as a part of a fastening or staking process. For example, in the automotive industry it is common for an emblem to be staked to the center of a steering wheel closeout. While earlier approaches to performing such staking activities involved the use of ultrasonics and hot air, ultrasonics typically produce part marking and hot air often results in damage due to over spray of the hot air. 
     As a result of the above limitations associated with ultrasonics and hot air, laser staking has evolved in many industries. In conventional laser staking approaches, a projection of a workpiece is deformed by applying a predetermined level of laser radiation and a predetermined weld force to the projection with a specialized dye. The predetermined weld force and the predetermined level of laser radiation cause the projection to melt and collapse into the shape of the dye. After a predetermined period of time, the laser radiation and weld force are discontinued, and the projection is allowed to solidify. After solidification, the staking process is complete and the workpiece is fixed to the adjacent part. 
     A particular area of potential improvement for the above laser staking process relates to what parameter is monitored to determine when to discontinue the laser radiation and weld force. Specifically, the above discussed weld time control strategies fail to take into account molding and environmental history variables for the parts being staked together. For example, various projections will exhibit varying amounts of collapse for a given weld force, laser radiation and staking time. The final assemblies would therefore have varying overall physical dimensions due to collapse inconsistencies. The present invention recognizes that the collapse distance of the projection is the parameter of most interest and in large part determines the strength and quality of the part connection. It is therefore highly desirable to provide a mechanism for controlling the laser staking process which takes into consideration the staking parameter of most interest, i.e., collapse distance. Such a mechanism would provide reduced rework costs and improved quality. 
     The difficulties relating to determining what parameter to monitor in order to determine when to discontinue the laser radiation and weld force are equally applicable in other areas of laser welding. For example, in through transmission infrared (TTIr) welding, a first part that is transparent to the laser radiation is welded to a second part that absorbs the radiation. The laser radiation raises the temperature of the absorbent material to a critical melting temperature and the pressure is applied to press the parts together. A weld or bond joins the parts as the melt cools. TTIr welding has widespread application due to its relatively rapid formation of the weld as well as the strength and uniformity of the joint. Thus, in TTIr welding the collapsed distance within the weld zone can be most representative of the strength and quality of the part connection. It is therefore also highly desirable to provide a mechanism for controlling TTIr welding which takes into consideration the welding parameter of most interest, i.e., the collapsed distance. 
     SUMMARY OF THE INVENTION 
     The above and other objectives are provided by a system and method in accordance with the present invention for deforming a projection of or creating a weld within a workpiece to join an assembly of parts. The method includes the steps of applying a predetermined weld force to the assembly, and applying a predetermined level of laser radiation to the assembly. The predetermined weld force and the predetermined level of laser radiation cause the assembly to collapse. The method further provides for discontinuing application of the laser radiation when the assembly collapses to a predetermined displacement with respect to a reference position. In one embodiment of the present invention, application of the weld force is discontinued upon expiration of a predetermined time period after the radiation is discontinued to allow for solidification of the assembly. 
     Further in accordance with the present invention, a method for discontinuing application of laser radiation to a thermoplastic projection when the projection collapses to a predetermined displacement with respect to a reference position is disclosed. The method includes the steps of defining the reference position, and tracking a collapse position for the projection. A difference between the reference position and the collapse position is calculated and compared to the predetermined displacement. 
     The present invention also provides a laser staking system for deforming a projection of a workpiece and a laser joining system for joining an assembly of parts. Each system has a laser system, an actuation system, and a controller. The laser system generates a predetermined level of laser radiation based on radiation control signals. The actuation system directs the predetermined level of radiation to the parts and contacts the parts with a laser head based on forced control signals. The actuation system further generates position feedback based on a position of the laser head, where the position feedback includes a reference position. The controller communicates with the laser system and the actuation system, and generates the radiation control signals and the force control signals based on the position feedback from the actuation system. One of the radiation control signals causes the laser to discontinue generation of the laser radiation when the parts collapse to a predetermined displacement with respect to the referenced position. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to one skilled in the art by reading the following specification and subjoined claims and by referencing the following drawings in which: 
     FIG. 1 is a block diagram of a laser staking system in accordance with the present invention; 
     FIG. 2 is a block diagram of a controller in accordance with the present invention; 
     FIG. 3 is a block diagram of a controller actuation control module in accordance with the present invention; 
     FIG. 4 is a block diagram of a controller laser control module in accordance with the present invention; 
     FIG. 5 is a circuit schematic of a laser system in accordance with the present invention; 
     FIG. 6 is a cross-sectional side view of a laser head at an absolute initial position in accordance with the present invention; 
     FIG. 7 is a cross-sectional side view of a laser head in an initial projection position in accordance with the present invention; 
     FIG. 8 is a cross-sectional side view of a laser head in a position where the projection has collapsed to a predetermined displacement with respect to a reference position; 
     FIG. 9 is a flowchart of a computerized method for deforming a projection of a workpiece in accordance with the present invention; 
     FIG. 10 is a flowchart of a process for discontinuing application of laser radiation to a thermoplastic projection when the projection collapses to a predetermined position in accordance with the present invention; and 
     FIG. 11 is a cross-sectional side view of a laser head with respect to a TTIr welding operation in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, the preferred laser joining system  20  for joining an assembly of parts, or in one embodiment, for deforming a thermoplastic projection  22  of a workpiece is shown. It will be appreciated that while the system  20  is described with respect to a staking process as applied to projection  22 , the present invention can be readily modified for non-staking processes as applied to any assembly of thermoplastic parts. 
     Accordingly, the following description of the deformation of the projection in a staking system and the measurement of the projection collapse should be understood to apply equally to the displacement or collapse that may occur during through transmission welding of a first part that is transparent to laser radiation and a second thermoplastic part that absorbs the radiation. The joining system  20  includes a laser system  40 , an actuation system  60 , and a controller  80 . The laser system  40  generates a predetermined level of laser radiation based on radiation control signals. For the purposes of this invention, the laser radiation may be of any frequency or wavelength sufficient to induce the desired melting and temperature control of the thermoplastic projection  22 . Notwithstanding the general applicability of the invention over a variety of frequencies and wavelengths, for staking applications using thermoplastic projections such as that described herein, the wavelength of the laser radiation is preferably within the range of about 600 to about 1000 nm. The actuation system  60  directs the laser radiation to the projection  22  and contacts the projection  22  with a laser head based on force control signals. Contacting the projection  22  with the laser head results in a predetermined weld force. The combination of the predetermined weld force and the predetermined level of laser radiation causes the projection  22  to collapse such that a workpiece  24  may be staked to a part  26  (FIG.  8 ). 
     The actuation system  60  generates position feedback based on a position of the laser head, where the position feedback includes a reference position of the laser head. It can be seen that the controller  80  communicates with the laser system  40  and the actuation system  60 . The controller  80  generates the radiation control signals and the force control signals based on the position feedback from the actuation system  60 . When the projection  22  collapses to a predetermined displacement with respect to the reference position, one of the radiation cortrol signals from the controller  80  causes the laser system  40  to discontinue generation of the laser radiation. Controlling the laser radiation based on position feedback represents a significant improvement over time-based laser joining approaches. 
     Turning now to FIG. 2, one embodiment of the controller  80  is shown in greater detail. Specifically, the controller  80  can include a reference module  82  for defining the reference position of the head  46 , and a dynamic collapse module  84  for tracking a collapse position for the projection  22  (FIG. 1) by monitoring the position of the head. Thus, the position feedback also includes a dynamic collapse position for the projection  22 . A summation module calculates the difference between the reference position and the collapse position, and a comparison module  86  compares the difference to a predetermined displacement that is specific to the particular application. It is preferred that a displacement database  88  contains the predetermined displacement information required for comparison module  86  to make its comparison. The information in the displacement database  88  can relate to all potential parts and assemblies to be joined by the joining system  20 . The comparison module  86  signals a laser control module  96  to discontinue the radiation when the difference between the reference position and the collapse position equals the predetermined displacement. 
     It will be appreciated that the present invention further provides for various modes of defining the reference position. Thus, a mode selector  89  is included with the controller to provide a mechanism for transitioning between the modes. For example, the reference module  82  can record an initial projection position as the reference position, or an absolute initial position as the reference position. The various modes of defining the reference position will be discussed in greater detail below. 
     It will further be appreciated that the controller  80  can also include an actuation control module  90  for communicating constant weld force data or variable weld force data to the actuation system  60 . As a result, while the radiation and weld forces are referred to herein based upon “predetermined” levels, the magnitude of these values may be constant or variable throughout the weld process. By way of example, FIG. 3 demonstrates that the actuation control module  90  of the controller  80  can have a constant actuation sub-module  92  for generating the constant weld force data and a variable actuation sub-module  94  for generating the variable weld force data. Similarly, FIG. 4 demonstrates that the laser control module  96  of the controller  80  can include a constant radiation sub-module  98  for generating constant radiation data and a variable radiation sub-module  99  for generating variable radiation data. 
     FIG. 5 demonstrates one embodiment of the laser system  40 . Specifically, it can be seen that the laser system  40  uses a diode array  42  to generate the laser radiation. The laser radiation from the diode array  42  can be piped to a laser head  46  (FIGS. 6-8) via optical fibers or other suitable optical transmission mechanism. A laser sub-system  44  acts as a “black box” and provides current to the diode array  42  in response to a radiation control (drive) signal from the controller  80 . 
     As will be discussed below, the laser head of the laser head  46  has a pressure transducer for providing the actuation system  60  (FIG. 1) with force feedback. This allows the actuation system  60  to determine when the projection  22  has been contacted, as well as how much force is being applied. When the force feedback indicates that the projection  22  has been contacted and the joining system  20  is operating in collapse mode (to be described later), the actuation system  60  reports an initial projection position back to the controller  80 . In such case, the initial projection position is defined as the reference position. The actuation system  60  uses an encoder (not shown), which is also mounted in the laser head  46 , to provide the controller  80  with the necessary position data. Both the transducer and the encoder can be commercially available “off-the-shelf” parts and are well known in the art. 
     Turning now to FIGS. 6-8, the joining and laser staking process of the present invention is demonstrated in greater detail. With specific reference to FIG. 6, it can be seen that a workpiece  24  having a projection  22  is to be joined with an adjacent part  26 . The laser head  46  has a die  48  and a pressure sensing mechanism such as a transducer  49 . As already discussed, the reference position can be defined based on either an absolute initial position  50  or an initial projection position  52 . If the reference position is defined based on the absolute initial position  50 , the joining system  20  is said to be operating in the “absolute mode.” 
     FIG. 7 demonstrates movement of the laser head  46  toward the projection  22  until contact is made at an initial projection position  52  which can be defined as the reference position when the system is operating in the “collapse mode”. In the collapse mode, the transducer  49  reports a contact force back to the actuation system  60 . When the contact force reaches a predefined trigger force, the position of the laser head is stored as the initial projection position  52 . It will also be appreciated that the trigger force can serve as a mechanism for beginning the laser radiation  41 . 
     During welding, the laser head  46  applies the predetermined level of laser radiation to the projection  22  until the projection  22  collapses to the predetermined displacement shown in FIG.  8 . If the absolute initial position  50  is used as the reference position (i.e., the absolute mode), the predetermined displacement  54  will serve as the distance for discontinuing application of the laser radiation  41  (FIG.  7 ). On the other hand, if the initial projection position  52  is used as the reference position (i.e., the collapse mode), predetermined displacement  56  will serve as the distance for discontinuing application of the laser radiation  41 . It should be appreciated that the laser head  46  also applies the weld force while the projection  22  is being collapsed, that is, as the laser head  46  moves from its initial projection position to its predetermined displacement. The laser head  46  may be maintained in its predetermined displacement position for a period of time following termination of the laser radiation so as to allow the projection to solidify while being constrained by the die configuration. During this solidification period, which may be programmed by the user, the weld force is generally decreased to maintain the projection position. 
     Turning now to FIG. 9, a computerized method  100  for joining an assembly of parts is shown for programming purposes. It will be appreciated that the present invention can be implemented in either hardware or software, or both, using techniques well known in the art. Specifically, it can be seen that at step  110  the reference position is defined. At step  120  a predetermined weld force is applied to the assembly, and at step  130  a predetermined level of laser radiation is applied to the assembly. As already discussed, the predetermined weld force and the predetermined level of laser radiation cause the assembly to collapse. At step  140  it is determined whether the assembly has collapsed to a predetermined displacement with respect to the reference position. If so, the application of the laser radiation is discontinued at step  150 . If the predetermined displacement has not been reached, the predetermined weld force and laser radiation continue to be applied. One embodiment of the present invention further includes the step  160  of determining whether a predetermined time period has expired for the purposes of discontinuing application of the predetermined weld force at step  170 . This allows the assembly to solidify. 
     FIG. 10 shows the step  140  of determining whether the predetermined displacement has been reached in greater detail. Specifically, at step  142  a collapse position for the assembly is tracked. This can be achieved by merely recording the position data provided by the actuation system encoder. At step  144  a difference between the reference position and the collapse position is calculated. The difference is then compared to the predetermined displacement at step  146 . The laser radiation is terminated and the force discontinued when the difference is greater than or equal to the predetermined displacement. A cool down period may also be applied for solidification. 
     It is important to note that while the present invention has generally been described with respect to thermoplastic material, any material in which the laser radiation can have a frequency sufficient to induce melting of the material can be used. Moreover, as is generally noted above, the distance mode of controlling when the laser radiation and pressure is discontinued may be applied to various other applications including TTIr welding. For completeness, the beginning of a representative TTIr welding application is generally illustrated in FIG.  11 . In this application, the laser radiation is nearly one hundred percent transparent to a first clear part  70  but absorbent relative to a second absorbent part  72 . In most TTIr applications, the second absorbent part  72  is black in color. A series of diodes are commonly positioned in side-by-side relation in a diode array to produce a radiation line that matches the contour of the desired weld line. The laser radiation passes through the first clear part  70  and impacts the second part  72  which is preferably an  25  absorbent polymer. As the second part  72  is heated to a critical melting temperature, the head  46  is displaced to press the two parts together. The distance that the head  46  is displaced is again the parameter that is measured to discontinue the radiation and pressure. The pressure may be maintained as the weld or bond cools to form the joint. It should be appreciated that the above discussed control techniques have equal applicability to the TTIr applications, as well as various other laser welding techniques. 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification and following claims.