Abstract:
An RF welder, and a method of operation thereof, is disclosed that has circuitry that automatically affects system impedance so that, throughout the welding or embossing cycle, a generator is capable of delivering the same power to a die and fabric, and cabling is capable of dissipating a load that is less than a preselected amount.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/338,026, entitled RF WELDING DEVICE filed on Jan. 24, 2006. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to packaging machines, and more particularly relates to sealers used in horizontal form, fill, and seal packaging machines. 
     Machines that utilize Radio Frequency as the means for welding (RF Welders) are known in the art. RF welders are typically used for sealing and embossing appliqué on RF sealable materials. Such materials are commonly used in processing materials such as PVC, PU, PET, PETG and polyolefin. The welders process these materials in manufacturing, for example, vinyl envelopes and binders having internal pockets. For example, pockets are sealed to the binder on first and second side edges and a bottom edge, leaving a top edge open for egress. RF welding of the edges obviates the need for stitches. 
     The theory and implementation of RF welding is disclosed in U.S. Pat. No. 5,833,915, incorporated herein by reference.  FIG. 2  discloses an RF welder  1  known in the art. A standard generator (not shown) provides power to the welder  1  at an FCC mandated frequency of 27.12 MHz, using standard 50 Ohm coaxial cable  2 . The coaxial cable is used because it is an excellent conveyor of energy and suffers very little loss. The term load in this application should be understood to mean all components seen as a load with respect to the generator. 
     Referencing  FIG. 2 , the welder  1  has a top plate  3  and a bottom plate  4  that are used as electrodes for transferring electrical energy through a subject material  5  and die  6 , where the die  6  has impressions  6   a  used for embossing or welding. 
     The die  6  is attached to the top plate  3  and acts as an electrode in tandem with the top plate  3 . The die  6  has conductive electrical characteristics which alter the load characteristics. The material  5  is non-metallic and acts as a dielectric, absorbing energy passed between the top and bottom plates  3  and  4 , to emboss or weld the material  5 . The dielectric characteristics of the material  5  also alter the electrical characteristics of the load. 
     Accordingly, the impedance of the load is a combination of the impedance of all of the components in the electrical conduction loop plus the material being processed. Since the impedance varies from part to part and during the welding process, optimum power is not delivered to the weld with a manually fixed impedance match. If the matching network or the frequency generator automatically adjust the reactance of the load to maintain the correct impedance at the generator, maximum power is always delivered to the weld during processing giving a better, quicker and more efficient weld. 
     If not enough energy is delivered to the load the material  5  may not weld or fail to become embossed. If too much energy passes through the load, the material may burn and other load components may fail (such as the coaxial cable which can be over-dissipated). 
     If the frequency at which energy is delivered to the material is incorrect, the welding or embossing of that material will suffer lagging or leading, which is known to provide poor quality results. More specifically, the power that transfers through the fabric may rise continuously through the weld cycle, or the power may rise to a maximum value and fall as the die sinks into the material. Such a power fluctuation provides an uneven weld with potential undesirable results in, for example, weld strength or emboss appearance. 
     Accordingly, with differing load impedance characteristics, a result of changing load impedance, there may be a slow reaction by the fabric causing a slow start of the welding or embossing or a complete failure to weld or emboss the material. Other problems include flashing caused by a high voltage arc-over. 
     As a result of the unique impedance characteristic, the RF welder must be electrically tuned, via impedance matching, after placement of the die  6  and material  5  within the welder. After the tuning of the load impedance, the power delivered will be optimum and over-dissipation of the cables and other elements will not occur. Energy will be passed through the platen and fabric at frequency of 27.12 MHz, preventing lagging or leading of the welding or embossing process. 
     Normally, impedance matching occurs as illustrated in  FIG. 4 . As indicated, the die and material must first be installed on the machine at S 1  and S 2 . The user is capable of adjusting the load impedance by manually adjusting a capacitor external to the generator at S 3 . The capacitor is adjusted by attempting to weld a material while adjusting capacitor electrodes towards or away from a sample dielectric. The RF welder  1  is calibrated by running the RF welder  1  and checking the material at S 4  and S 5  to determine if the quality is satisfactory at S 6 . 
     When the material welds appropriately, it is deduced that the RF welder  1  is tuned properly, the power is set correctly, and over-dissipation does not occur. Once the RF welder  1  is tuned, a series of identical materials can be welded or embossed at steps S 7  and S 8 . 
     Various problems normally occur with the manual adjusting of the capacitor. First, it is relatively impossible for a person to adjust the capacitance so that the generator sees exactly 50 ohms due to the inherent sensitivity and robustness of the RF welder  1 . Rather, manual adjusting typically provides at least a 5% error on the frequency adjustment. Also, a sample does not capture the dielectric characteristic for a series of materials since each individual piece of material has unique inconsistencies which affect the electrical characteristics of each weld. To adjust for these problems, the RF welder  1  must constantly be checked for quality at S 9 . Further, if non-identical materials or dies are used, then the RF welder  1  must be continuously retuned. 
     Even if the impedance is adjusted for each unique material  5 , the material capacitance tends to change as the die sinks into the material. As the die sinks the capacitance changes, impedance changes, and optimum power is not transferred to the die and material. Accordingly, the falloff causes a decrease in the ability for the die to weld or emboss the material. 
     BRIEF SUMMARY OF THE INVENTION 
     It is the object of the invention to supply an RF welder that has circuitry which automatically adjusts the impedance of the load so that when welding occurs the same power is applied to the die and material in a consistent manner ensuring that over-dissipation does not occur to material, dies, or other load components. 
     The two exemplary embodiments disclosed adjust the impedance of the load in different manners. The first exemplary embodiment uses a matching network having a variable filter with a motor driven capacitor. The matching network has the capability of measuring the magnitude and phase of a radio frequency signal at its input and varies the filter impedance, by adjusting the motor driven capacitor, to match the impedance of the load to the generator. In the second embodiment, a variable frequency generator measures the reflected power or phase difference between voltage and current of a radio frequency signal at its output and varies the frequency of the power output by the generator thereby adjusting the impedance of the load, to match the impedance of the load to the generator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Accompanying the specification are figures which assist in illustrating the embodiments of the invention, in which: 
         FIG. 1  is a perspective view of the RF Welding Machine; 
         FIG. 2  is a front view of the RF Welding Machine according to the prior art; 
         FIG. 3  is a front view of the RF Welding Machine with the top plate engaging the bottom plate and the generator activated; 
         FIG. 4  is a schematic illustration of the method of operating the RF Welding Machine according to the prior art; 
         FIG. 5  is a block diagram of the method of operating the RF Welding Machine according to a first exemplary embodiment; 
         FIG. 6  is a block diagram of the method of operating the RF Welding Machine according to a second exemplary embodiment; 
         FIG. 7  is an illustration of a sample output from the RF Welding Machine; and 
         FIG. 8  is a schematic diagram of the load and generator of the second exemplary embodiment of the invention using a fixed filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning to  FIG. 1 , an RF welder  1  is disclosed that has circuitry  7  and software that automatically tunes the impedance of the die  10  and material  11  ( FIG. 3 ) to match that of a solid state generator  9 . Alternatively the solid state generator  9  may automatically tune the frequency of its output signal to match the impedance of the load to the generator. The tuning enables a maximum power transfer through the material  11 . 
     Remaining with  FIG. 1 , the RF welder  1  has welding components  12  that include a top platen or plate  13  and bottom plate  14 . Each plate  13  and  14  is manufactured from an electrically conductive material and is adapted to act as an electrode for an RF dielectric heating system. Each plate  13  and  14  is in communication with the high frequency RF generator  9  so that the top plate  13  is electrically hot and the bottom plate  14  serves as an electric ground. 
     The thickness of each plate is sufficient to prevent flexing or fatiguing of the plates by periodic loading of the plates. The top plate  13  is adapted to receive energy from the generator  9  and, as illustrated in  FIG. 3 , the bottom plate  14  is adapted to ground energy traveling from the top plate  13  through a conductive base  27 . The plates  13  and  14  are parallel to each other and large enough to fit the die  10  and material  11 . 
     The lower plate  14  is a stationary base. The top plate  13  is movable between an opened or home position and a closed or press position. The opened position spaces the top plate  13  from the lower plate  14  so that the die  10  can be changed and material  11  can be fed. 
     The top plate  13  is indirectly mounted to a plurality of platen arms  18  that move perpendicular to the pressing surface of the plate  13 . The rate of motion of the plate arms  18  is relatively slow to minimize the injury risk upon inadvertent operator contact with the plate. 
     The range of motion for the top plate  13  is defined by the range of motion for the plate arms  18 . The arms  18  are provided with a bottom plate  19  which defines the top range of motion and a top plate  20  which defines the bottom range of motion. The bottom range of motion, with the absence of the usable die  10 , is 0.010 inches from the top face of the bottom plate  14 . The separation minimum prevents the plates from buckling when power is transferred during operation. The die  10  and material  11  further limit the range of bottom motion that is reachable by the top plate  13  by creating a separation barrier between the top and bottom plates  13  and  14 . 
     Between the arms  18  and the top plate  13  are four spacers  21 . Each spacer  21  is parallel to and the same length as each other spacer. Each spacer is manufactured from a non-conductive material which prevents the power from the generator  9  from being transferred to arms  18 . 
     The electrical control of the arms  18  is achieved through known techniques, such as those defined in U.S. Pat. No. 2,993,3600 incorporated herein by reference. After the press has been actuated by the operator, the operation of the press elements is coordinated via cam-operated micro-switches and limit switches at the ends of the strokes of the various reciprocating components. Alternatively, strain gauges are located in each spacer  21 . The registered strain determines the maximum downward travel of the top plate  13 . 
     The arms  18  are advanced and retracted, to advance or retract the top plate  13 , with the use of activation switches  22 . The switches are powered through cabling  23  that receives power from a standard electrical current, such as a 120 volt wall current, fed through a transformer  24 . 
     Turning again to  FIG. 3 , advancing the top plate  13  maintains pressure on the die  10  for purposes of completing the embossing or welding of the material. The pressure is maintained for a period of time sufficient to ensure both that the material properly fills the die cavity so that the desired overall outer shape is achieved, and so that the complete formation of the embossed indicia is obtained. Dwell times for the press will be on the order of 0.1-5.0 seconds, preferably 1.0 to 3.0 seconds. 
     As compared to manually retracing the top plate  13 , a timing relay switch maybe used for controlling the period of the application of the dielectric sealing current in accordance with the materials being added. 
     Referring again to  FIG. 1 , the welding components  12  include the die  10 . The die  10  is a typical die use for embossing or welding the material  11 .  FIG. 7 , for example, illustrates a sample die having a diamond design about the word “Diamond.” 
     Returning to  FIG. 1 , the die  10  has a cavity with inner walls and a bottom shape that is exactly like the desired final outward shape of the end product being the compressed product. The die  10  is made of material which is capable of withstanding the required welding and embossing pressures. Typically, the principle component of the die  10  is a steel alloy. 
     Another welding component  12  is the material  11 . Material, which maybe welded or embossed, includes PVC, PET, RPET, PU, urethane and vinyl coated materials and other related sealable materials. The material is adapted to be sealed onto other different or identical materials, a.k.a. appliqué on material and material on material. 
     The RF welder  1  has power input components  15  which include the generator  9 . The generator  9  in a first exemplary embodiment, is a solid state generator being, for example, model CX-1000A, 27.12 MHz by Comdel Corporation, of 11 Kondelin Road, Gloucester, Mass. 01930. The “1000A” stands for 1000 Watts, or 1 KW of power. In a second alternative embodiment of the invention a variable frequency generator, such as Comdel model CV1000, may be used as the generator  9 . The amount of power required for a given application is dependent upon type or quantity of material that is subject to the RF welder  1 . 
     A solid state generator is required as compared to an oscillator tube style generator. The oscillator tube style generator has an extensive swing in load impedance during use which renders the tuner incapable of matching the impedance of the platen to the generator. The solid state generator, on the other hand, is capable of operating at a 100% duty cycle and, notably, is capable of remote operation, i.e. the power supply is capable of being located in a separate room, with the efficiency of the RF welder  1  remaining in a high 90 percent efficiency. 
     Separating the power supply from the generator is advantageous for medical applications as the power supply portion can be put outside a clean room environment if needed. Separating the power supply is also advantageous for quality control purposes for allowing a separate control room where the operators of the machines are incapable of changing the setting arbitrarily. 
     Another benefit of a solid state generator is the efficiency at which the generator is capable of supplying power. Some materials weld better with differing frequencies or respond better to variable frequencies, such as frequencies that ramp downwardly through the welding or embossing process to account for material property changes through the welding process. The physical characteristics of alternating frequencies will be apparent, though unobvious, to those knowledgeable in the art. 
     The generator  9  in the first exemplary embodiment provides welding and embossing energy to the load at a fixed frequency such as 27.12 MHz. In the second exemplary embodiment the generator  9  is a variable frequency generator and provides energy at an adjusted frequency in a band advantageously around a predetermined frequency such as 27.12 MHz. The frequency for RF welding and embossing is set by governmental regulation, and it is to be appreciated that other frequencies could be-used where available by law. 
     The power supplied by the generator depends on the material being processed and the processing to the material. A typical generator for an embosser or welder produces 6 kilowatts (KW) or more of power. However, wattage both above and below 6 KW could be supplied, where necessary. 
     The cabling  16  is chosen because it is capable of carrying the current which results and is determined by the RF power supplied by the generator and the impedance of the load without failure due to over-dissipation. This cable is a standard cable for applications in RF Welding and embossing. A sample of the cabling is the type provided with one of the aforementioned generators by Comdel Corporation. It is to be appreciated that cabling having greater or lesser dissipative characteristics could be employed, where necessary. The power input  15  also includes a transducer  26  that transfers power from the generator to the top plate  13 . 
     In the first exemplary embodiment of the invention there is a power regulator  17  which includes the impedance matching system  7  and is connected to the welding components  12 . The impedance matching system  7  and the generator  9 , electronically communicate through the coaxial cable  16 . The impedance matching system  7  is, for example, model CPM-25, air cooled, single phase, 115 volt, vacuum variable caps by Comdel Corporation, 11 Kondelin Road, Gloucester, Mass. 01930, having power supply number CX 27.12 by Comdel, which is a known impedance matching network. 
     The impedance matching system  7  includes an algorithm that gradually slows the adjustment of the matching network capacitors so that precise impedance matching is achieved ensuring that the proper power is delivered to the load during the welding or embossing process. Even with high loads, the impedance matching system  7  is capable of being responsive to the change of material characteristics during the process. 
     In the second exemplary embodiment of the invention frequency tuning is used to match the impedances. The generator  9  is a variable frequency generator that supplies energy at a varying frequency within a band of frequencies that extend above and below a predetermined, advantageously chosen, frequency. The generator  9  has a control circuit  52  that adjusts the frequency of the RF energy output by the generator  9  to match the impedances between the generator  9  and the load. Therefore the impedance matching system  7  may be replaced with a fixed filter  38  that matches the impedance between the generator  9  and the load at the predetermined, advantageously chosen, frequency in the band of frequencies that the generator  9  will operate. 
     An exemplary configuration of the fixed filter  38  is illustrated in  FIG. 8 . As may be seen in  FIG. 8 , the fixed filter  38  is interposed between the generator  9  and all other load components  50 . All other load components  50  may include and is not limited to: the welding components, material  11  and power regulator  17 . The fixed filter  38  impedance should be determined based on the average or mean load impedance that will be experienced during processing. Alternatively the impedance matching system  7  may be used and the impedance matching algorithm is activated for the initial configuration of the impedance matching system  7 . 
     When frequency tuning is activated during processing, the control circuit  52  measures the radio frequency signal at the input nodes  40  and  41  to the fixed filter  38  (i.e. at the generator  9  output), either for the reflected power or phase difference between voltage and current signals. The control circuit  52  then determines that the frequency should be changed in order to provide an impedance match between the generator  9  and the load and makes the necessary adjustment to the frequency of the generator  9 . The control circuit  52  continues during the welding process, to monitor the signal and vary the frequency to maintain the impedance match between the generator  9  and the load. It is also understood by those in the art that other indicia of the electrical state of the RF welder at the generator  9  or another location within the RF welding circuit may be used to determine adjustment to the frequency of the generator  9 . 
     The impedance characteristics of the welding components  12 , such as the plates  13  and  14 , the die  10  and the material  11 , are based upon the solid structure and material properties of the components. The impedance characteristics differ from one piece of material to another and change throughout the welding or embossing processes in a manor which is known in the art. These changes affect the power being transferred through the load and the power required to be dissipated by the load components. 
     Both the first and second exemplary embodiments of the invention are designed to dynamically adjust the impedance that the generator sees so that the power level delivered is controlled and over-dissipation of the cables and other components does not occur. This ensures that the proper power is consistently delivered to the die  10  and material  11 . The disclosed stabilizing technology optimizes amplifier (power supply and generator) performance, reducing power-gain changes caused by material  11  and die  10  impedance fluctuations, both dynamic and static. 
     The power regulator  17  includes cables  25  which are, for example, standard  25  pin peripheral cables adapted to transfer signals indicative of inductance and electrical characteristics of the welding components  12  to the impedance matching system  7  after the top plate  13  is pressed against the bottom plate  14 . Signals traveling through the cables  25  are, for example, digital signals converted from an analog to digital converter (not shown). The a/d converter is connected to receive signals from the impedance matching system  7  in a manor that is readily apparent to one skilled in the art and analyzed and interpreted by the regulator  17  so that the regulator  17  may determine the electrical state of the load. 
     Due to the dynamic adjustments of the impedance matching system  7  in the first exemplary embodiment and the variable frequency generator  9  in the second exemplary embodiment, manual adjustments are unnecessary and iterations are not required to maintain the desired power characteristics. Accordingly, both the first and second exemplary embodiments account for differences in each unique material piece, each unique die, and the changing power characteristics that result during heating. 
     In  FIG. 5 , the operation of the first exemplary embodiment of the RF welder  1  is disclosed. The die  10  is installed on the underside of the top press plate  13  at step S 11 . The material  11  that is the subject of the embossing or welding is secured to the lower plate  14  at step S 12 . The top plate  13  is lowered onto the bottom plate  14  and the impedance matching system  7  is activated at step S 13 . 
     Once the plate  13  is lowered, the impedance matching system  7  reads and, as required, affects the impedance characteristics of the welding components  12 . The adjusting prevents more than a 50 ohm load from being dissipated by the cables  16  and enables power to transfer continuously through the die  10  and material  11  at a frequency of 27.12 MHz. Once the impedance characteristics are tuned, the RF welder  1  is capable of indicating readiness with a visual or audible signal at S 14 . The generator  9  is automatically or manually activated and the impedance matching system  7  dynamically maintains the tuned characteristics throughout the embossing or welding cycle at step S 15 . Then the material is removed at step S 16 . 
     Since the material welding is optimized for each material piece, there is no need to manually re-tune before welding or embossing any individual piece of material. Accordingly, the welding or embossing of the next material pattern can continue at step S 17 . Alternatively, a new die can be installed and the process can begin again at step S 11 . 
     In  FIG. 6 , the operation of the second exemplary embodiment of the RF welder  1  is disclosed. The die  10  is installed on the underside of the top press plate  13  at step S 110 . The material  11  that is the subject of the embossing or welding is secured to the lower plate  14  at step S 120 . The top plate  13  is lowered onto the bottom plate  14  at step S 130  and continues to step S 150 . 
     Alternatively, if the impedance matching system  7  is used, the top plate  13  is lowered onto the bottom plate  14  and the impedance matching system  7  is activated at step S 135 . Then once the plate  13  is lowered, the impedance matching system  7  reads and, as required, affects the impedance characteristics of the welding components  12 . The adjusting prevents more than a 50 ohm load from being dissipated by the cables  16  and enables power to transfer through the die  10  and material  11  at a predetermined, advantageously chosen, frequency such as 27.12 MHz. Once the characteristics are tuned, the RF welder  1  is capable of indicating readiness with a visual or audible signal at S 140  and the process proceeds to step S 150 . 
     Once the plate  13  is lowered, the generator  9  is automatically or manually activated and the generator  9  dynamically maintains the tuned characteristics by varying its output signal frequency throughout the embossing or welding cycle at step S 150 . Then the material is removed at step S 160 . 
     Since the material welding is optimized for each material piece, there is no need to manually re-tune before welding or embossing any individual piece of material. Accordingly, the welding or embossing of the next material pattern can continue at step S 170 . Alternatively, a new die can be installed and the process can begin again at step S 110 . 
     An RF welder  1  has been disclosed with circuitry that automatically affects the impedance of the load so that the generator  9  is capable of transferring maximum power through die  10  and material  11 . 
     The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not as restrictive. The scope of the invention is, therefore, indicated by the appended claims and their combination in whole or in part rather than by the foregoing description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.