Patent Publication Number: US-2005121460-A1

Title: Fluid dispenser with automatic compensation and method

Description:
This application is a divisional of application Ser. No. 09/999,058, filed on Oct. 31, 2001 the entirety of which is incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION  
      The present invention generally relates to a liquid dispenser and a method for dispensing fluids and more specifically, to a fluid dispenser having an automatic compensation that improves performance.  
     BACKGROUND OF THE INVENTION  
      The ability to precisely dispense a fluid, for example, a hot melt or cold adhesive or glue, is a necessity for manufacturers engaged in the packaging and plastics industries. Various fluid dispensers have been developed for the placement of fluids, for example, adhesives, coatings, etc., onto a substrate, for example, a carton flap, being supported by a moving conveyor. The speed of the conveyor, or line speed, is set according to such factors as the complexity of the dispensing pattern and the configuration of the gun. Adhesive is normally supplied to the dispensing gun under pressure by a motor driven pump. In such applications, it is important that fluids be dispensed and applied at precise locations or positions on the moving substrate. Fluid that is dispensed too soon or too late and therefore dispensed at other than a desired location can adversely impact subsequent operations on the product and/or result in a lower quality or scrap product.  
      The time required to open and close the fluid dispensing gun, that is, the dispensing gun switching time, creates a delay in the fluid dispensing process that can cause inaccuracies in the fluid dispensing process. For example, a conveyor moving at 500 feet per minute will move 0.008 inches in one millisecond (ms). If a pneumatic solenoid-operated dispensing gun takes 25 ms to open, the substrate will have moved 0.200 inches after the dispensing gun is commanded to open but before any fluid is dispensed from the dispensing gun. Thus, the adhesive is deposited onto the substrate at a different location than anticipated, and such shifts in the location of the adhesive reduces the quality of the fluid dispensing process and may result in scrap product.  
      The quality of the fluid dispensing process is also adversely affected by variations in the dispensing gun switching time when the dispensing gun is commanded to close. At the end of a dispensing process, a lengthening of the switching time of the dispensing gun results in adhesive being dispensed for a longer period of time than desired and hence, at a different location than anticipated. Similarly, a shortened switching time can result in a lower quality fluid dispensing process and a scrap part or product.  
      In order to improve the speed and reliability of the fluid dispensing process, more recent years have seen the development of an electrically operated fluid dispenser or gun. Generally, electrically operated fluid dispensers have an electromagnetic coil surrounding an armature that is energized to produce an electromagnetic field with respect to a magnetic pole. The electromagnetic field is selectively controlled to open and close a dispensing valve by moving a valve stem connected to the armature. More specifically, the forces of magnetic attraction between the armature and the magnetic pole move the armature and valve stem toward the pole, thereby opening the dispensing valve. At the end of a dispensing cycle, the electromagnet is de-energized, and a return spring returns the armature and valve stem to their original positions, thereby closing the dispensing valve. By operating a dispensing gun coil at higher voltages, for example, over 40 VAC, the operational speed of the electric fluid dispensing gun is increased.  
      However, even with a greater speed of operation, a finite period of time, for example, 10 ms, is required to energize a magnetic field with the gun coil and move the valve to its open position. That period of time represents a delay in the application of fluid onto the moving substrate; and depending on the conveyor speed, that short delay also causes inaccuracies in the desired placement of fluid on the substrate. There is a continuing market pressure to provide faster conveyor speeds, for example, 1000 feet per minute and more, without any loss of quality in the fluid dispensing process. Clearly, as conveyor speeds increase, the effect of variations in the gun switching time becomes more important. Therefore, known controls for fluid dispensing guns have a manually adjustable input that is used by an operator to provide a fixed, gun on compensation value. For example, the gun coil switching time can be measured and used as a compensation value that is entered by the operator before initiating a fluid dispensing cycle. The gun control uses the gun on compensation value to advance a start of a fluid dispensing cycle, that is, the time at which the gun coil is turned on or energized. Thus, after the delay caused by the gun coil switching time, fluid is dispensed from the gun at a time that results in a more accurate deposition of fluid onto the substrate.  
      In many applications, that fixed compensation value provides a satisfactory fluid dispensing process. However, in some applications, the operator may observe that the placement of the fluid is not accurate. In those applications, the operator can again use the manually adjustable input to change the compensation value and thus, more accurately locate the placement of the fluid on the substrate.  
      The same issues arise when the fluid dispensing gun is turned off. It should be noted that the fluid dispensing valve is opened by operation of the gun coil, whereas the fluid dispensing valve is closed by the operation of a return spring. Therefore, the switching times required to open and shut the fluid dispensing valve are often different. The increment of time required for the magnetic field in the gun coil to dissipate and the return spring to shut off the valve is measurable and can be manually input into the fluid dispensing control as a fixed, gun off compensation value. The gun control uses that compensation value to advance an ending of the fluid dispensing cycle, that is, the time at which the gun coil is turned off or de-energized. Thus, after the delay to shut the dispensing valve off, fluid ceases to be dispensed from the gun at a time that results in an accurate termination of the fluid dispensing process.  
      Although known fluid dispensing systems operate satisfactorily in many applications, the dispensing gun switching time can be adversely impacted by many different factors. For example, variations in the switching time of the dispensing gun can be caused by variations in fluid viscosity or variations in line voltage being supplied to the dispensing system control. Further, mechanical wear and aging of components within the dispensing gun can impact gun switching time. For example, a return spring is often used to move the dispensing valve in opposition to a solenoid. Over its life, the spring constant of the return spring changes, thereby changing the rate at which the dispensing valve opens and closes and hence, the location of dispensed adhesive on a substrate. Further, the accumulation of charred adhesive within the dispensing gun over its life often increases frictional forces on the dispensing valve, thereby changing gun actuation time. Thus, for the above and other reasons, the operation of the dispensing gun is subject to many changing physical forces and environmental conditions that cause variations in the actuation time of the dispensing gun. Such variations in dispensing gun switching times produce variations from desired locations of adhesive deposits on the moving substrate.  
      Thus, known compensation techniques for fluid dispensing systems have several disadvantages. First, if the initial compensation value is not accurate, a better compensation value requires that production be run in a trial and error process until the desired compensation is determined. Such a process is an inefficient and uneconomical use of the production line, and scrap product is often being produced during this tuning process. Second, if, during production, there are any changes in the components of the fluid dispensing gun that change its operating time, the placement of the fluid on the substrate will drift. Any drift in the switching time of the fluid dispensing gun often results in a less accurate fluid dispensing process and hence, a poorer quality product.  
      Thus, there is need for a fluid dispensing system that automatically corrects for any variations in the switching time of the fluid dispensing gun.  
     SUMMARY OF THE INVENTION  
      The present invention provides a fluid dispensing system that automatically provides a more accurate fluid dispensing process. The fluid dispensing system of the present invention continuously monitors the operation of the fluid dispensing gun and continuously adjusts the dispensing process so that fluid is accurately dispensed onto the substrate. Thus, the fluid dispensing system of the present invention automatically and consistently dispenses fluid at a desired location on a moving substrate independent of changes in the switching times of the dispensing gun that would otherwise adversely impact the quality of the fluid dispensing process. The capability of automatically monitoring the switching time of the fluid dispensing gun and compensating for changes in the gun switching time also permits a wider variety of fluid dispensing guns to be used to accurately dispense fluid onto a moving substrate. For example, with the present invention, fluid dispensing guns having slower gun switching times can be used to more accurately dispense fluid onto a moving substrate. Slower switching fluid dispensing guns are often less expensive, and therefore, the present invention has a further advantage of obtaining a higher quality fluid dispensing process from a lower cost fluid dispensing system. In addition, the capability of quantifying in real time gun switching time is also a useful input to diagnostic and quality control systems.  
      According to the principles of the present invention and in accordance with the described embodiments, the invention provides an apparatus for operating a fluid dispensing gun to dispense a fluid onto a substrate moving relative to the dispensing gun. The apparatus has a control that provides first signals causing the fluid dispensing gun to change operating states, and a sensor produces a sensor feedback signal in response to the fluid dispensing gun changing operating states. The control has a detector producing a compensation signal representing a difference between the occurrences of one of the first signals and the sensor feedback signal. The control then adjusts a subsequent first signal in response to the compensation signal.  
      In one aspect of this invention, the sensor senses a presence of fluid deposited on the substrate. In another aspect of this invention, a coil having an armature operates the fluid dispensing gun; and the sensor produces the sensor feedback signal in response to motion of the armature causing the change of dispensing gun operating state. In a still further aspect of this invention, the sensor produces the sensor feedback signal in response to a change in current flow in the coil representing the change of dispensing gun operating state.  
      In another embodiment of the invention, a method is provided for operating a fluid dispensing gun to dispense a fluid onto a substrate moving relative to the dispensing gun. The dispensing gun changes operating states in response to signals from a fluid dispensing control. With the method, a first signal is applied to the dispensing gun to command a change of operating state. Next, the change of operating state of the fluid dispensing gun is detected. A difference is then detected between the application of the first signal and the detection of a change in the operating state of the fluid dispensing gun. An application of a subsequent signal to the dispensing gun is then adjusted in response to the difference.  
      In one aspect of this invention, a physical characteristic produced by the dispensing gun changing state is detected. In another aspect of this invention, a feature of a fluid deposit applied to the moving substrate in detected. In a further aspect of this invention, a coil having an armature operates the fluid dispensing gun; and motion of the armature is detected. In a still further aspect of this invention, changes in a current flow in the coil caused by the fluid dispensing gun changing state is detected.  
      These and other objects and advantages of the present invention will become more readily apparent during the following detailed description taken in conjunction with the drawings herein. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic block diagram of one embodiment of a fluid dispensing system having a compensation system in accordance with the principles of the present invention.  
       FIGS. 2A and 2B  are waveform timing diagrams illustrating an operation of the compensation system of  FIG. 1 .  
       FIG. 3  is a schematic block diagram of another embodiment of the compensation system for the fluid dispensing system of  FIG. 1 .  
       FIG. 4  is a schematic block diagram of a further embodiment of the compensation system for the fluid dispensing system of  FIG. 1 .  
       FIG. 5  is a waveform timing diagram illustrating the operation of the gun actuation sensors of  FIGS. 3 and 4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Referring to  FIG. 1 , a fluid dispensing system  20  is comprised of a fluid dispensing gun  22  having a nozzle  24  for dispensing a fluid  26 , for example, a hot melt or cold adhesive or glue, onto a part or substrate  28 . A conveyor  30  carries the substrate  28  past the dispensing gun  22 . The conveyor  30  is mechanically coupled to a conveyor drive having a conveyor motor  32 . A conveyor feedback device  34 , for example, an encoder, resolver, etc., is mechanically coupled to the conveyor  30  and detects conveyor motion. The feedback device  34  has an output  36  providing a feedback signal that changes as a function of changes in the conveyor position. For example, the feedback signal may provide a discrete pulse for each incremental displacement of the conveyor  30 .  
      A fluid dispensing control  40  has a system control  42  that generally functions to coordinate the operation of the overall fluid dispensing system  20 . For example, the system control  42  often controls the operation of the conveyor motor  32  and also provides a system user input/output interface (not shown) in a known manner. Further, the system control  42  operates in conjunction with a pattern control  44  that controls the operation of the fluid dispensing gun  22  as a function of a particular application and/or part being run. The pattern control  44  receives, on an input  46 , a part present or trigger signal from a trigger sensor  38 . The trigger sensor is positioned to detect a feature, for example, a leading edge, of the part  28  moving on the conveyor  30 . The trigger sensor  38  often provides a signal transition upon detecting the part feature and thus, provides an ability to synchronize other operations with the motion of the part  28  on the conveyor  30 .  
      In response to the trigger signal, the pattern control  44  provides a sequence of gun on/off signals in the form of pulses to a gun control or driver  48  via an input  50 . In the described embodiment, each of the gun on/off signals has respective leading and trailing edges representing desired changes in the operating state of the dispensing gun  22 . The leading edges command or initiate a change of state that turns on or opens the fluid dispensing gun  22 , and the trailing edges command or initiate a change of state that turns off or closes the fluid dispensing gun  22 . Thus, the leading and trailing edges of the gun on/off signals from the pattern control  44  represent, respectively, gun on and gun off operating state transitions of the dispensing gun  22 .  
      A power control  52  within a gun driver  48  is responsive to the gun on/off signals and provides output signals to a dispensing gun coil  54  via an output  56 . The switching time of the power control  52  is very small when compared to the switching time of the fluid dispensing gun  22 ; and therefore, for purposes of this invention, the switching time of the power control  52  can be ignored. The output signals energize and de-energize the gun coil  54  to operate the dispensing gun  22  as a function of the timing and duration of the gun on/off pulses from the pattern control  44 . Thus, the output signals also command or cause the dispensing gun to change states. The dispensing valve  60  is fluidly connected to a pump  62 ; and the pump  62  receives fluid, for example, an adhesive, from a reservoir (not shown). Upon the dispensing valve  60  opening, pressurized adhesive in the dispensing gun  22  passes through the nozzle  24  and is applied to the substrate  28  as a fluid deposit  64 , for example, a dot, bead, strip, etc.  
      The dispensing valve  60  remains open for the duration of the gun on/off pulse; and in response to the trailing edge of a gun on/off pulse, that is, a gun OFF transition, the gun driver  48  terminates current flow through the gun coil  54 . The magnetic field around the armature  58  collapses, and the dispensing valve  50  is closed by a return spring (not shown) in a known manner.  
      The pattern control  44  has a pattern store and compensator  66  that receives and stores a fluid dispensing pattern from the system control  42  via input  68 . The fluid dispensing pattern is entered into the system control  42  in a known manner. The fluid dispensing pattern represents a series of fluid dispensing cycles associated with a part  28  that result in a desired pattern of fluid deposits  64  thereon. The fluid dispensing pattern is often represented by numerical quantities or values in the pattern store  66  that are a measure of distances on the part  28  from a feature such as its leading edge  70  to leading and trailing edges  72 ,  73 , respectively, of a fluid deposit  64 . A counter  74  within the pattern control  44  is electrically connected to the conveyor feedback device  34  and the trigger sensor  38  and accumulates a numerical value representing motion of the substrate  28  after its leading edge  70  has been detected.  
      Assuming no pattern compensation, a comparator  76  is responsive to a first numerical value from the pattern store  66  representing a distance from the leading edge  70  of the substrate  28  to a first leading edge  72   a  of the first adhesive deposit  64   a.  The comparator  76  is responsive to a second numerical value in the counter  74  representing motion of the substrate  28  after its leading edge  70  has been detected. When the comparator detects a relationship between those two values, for example, a substantial equality, the comparator  76  provides a gun ON transition from the pattern control  44  to the gun driver  48 . The gun driver  48  turns on or opens the fluid dispensing gun  22 , and fluid is deposited onto the substrate  28 . The counter  74  continues to count the feedback pulses from the conveyor feedback device  34 , and the pattern store  66  presents the next stored value to the comparator. That next value determines the time at which the fluid dispensing gun should be turned off and represents the location of the trailing edge  73   a  of the first fluid deposit  64   a  as measured from the leading edge  70  of the substrate  28 . When the comparator  76  detects a relationship between those two quantities, for example, a substantial equality, it provides a gun OFF transition to the gun driver  48 ; and the gun driver  48  causes the fluid dispensing gun  22  to shut off or close, thereby terminating the dispensing of fluid onto the moving substrate  28 .  
      The fluid dispensing system of  FIG. 1  has a compensation system that includes a sensor  80  and a switching time detector  82 . The sensor  80  is mounted with respect to the conveyor  30  so that the sensor  80  can sense, and provides a sensed or sensor feedback signal representative of, one or more edges  72 ,  73  of respective adhesive deposits  64  as the conveyor  30  moves the substrate  28 . The sensor  80  is any sensor capable of reliably providing a high speed indication of the one or more edges  72 ,  73 , for example, an infrared sensor, dielectric sensor, laser sensor, etc. The switching time detector  82  has inputs  84 ,  86  electrically connected to respective outputs of the sensor  80  and the comparator  76  and is used to detect or measure the switching time or delay of the fluid dispensing gun  22 . As will be appreciated, the switching time detector input  86  can alternatively be responsive to output  56  of the gun driver  48 ; however, the signal on the output  56  is a high current signal and therefore, is more difficult to use as a transition reference. The detector  82  provides a compensation signal or value representing the detected delay that is used by the pattern store  66  to compensate the numerical values presented to the comparator  76 , so that gun ON/OFF transitions are automatically and continuously shifted in real time to eliminate the adverse effects of dispensing gun switching time. Therefore, fluid is more reliably and accurately deposited on the moving substrate  28 .  
      In use, a user enters a particular pattern of fluid deposits  64  utilizing the system control  42 . That pattern is then downloaded via line  68  to the pattern store  66 . The capability of the pattern control  44  to store one or more patterns over one or more dispensing cycles will depend on the application and the type of fluid dispensing control  40  being utilized. The user then, via the system control  42 , commands the conveyor motor  32  to start, thereby moving the substrate  28  on the conveyor  30  toward the fluid dispensing gun  22 . When the trigger sensor  38  detects the leading edge  70  of the substrate  28 , a trigger signal  87  of  FIG. 2A  is provided to the counter  74 . The counter  74  then begins to accumulate pulses  89  from the conveyor feedback device  34  and thus, the counter  74  accumulates a numerical value representing the displacement of the conveyor  30  with respect to the leading edge  70  of the substrate  28 .  
      The pattern store  66  presents a first numerical value to the comparator  76  representing the distance from the leading edge  70  of the substrate  28  to the leading edge  72   a  of the first deposit  64   a.  When the comparator  76  determines that the substrate  28  has moved through a displacement substantially equal to the first numerical value, the comparator  76  provides a leading edge of a gun on/off pulse, that is, a gun ON transition to the power control  52 . The power control  52  provides an output signal that energizes and changes the state of the gun coil  54 . A leading edge of an output signal from the gun driver  48  creates current flow through the gun coil  54 , thereby building up a magnetic field that lifts an armature  58  and a dispensing valve  60  connected thereto. As noted, a finite time is required to open the dispensing valve  60  and apply a fluid  26  as a leading edge  72   a  of the deposit  64   a  on the moving substrate  28 .  
      The deposit  64   a  ( FIG. 2A ) can be represented as a waveform  90   a  that has respective leading and trailing edges  92   a,    94   a  that correspond to the respective leading and trailing edges  72   a,    73   a  of the fluid deposit  64   a.  If the fluid dispensing gun switching time were zero, then the gun ON transition  96   a  would correspond to the leading edge  92   a  and thus, produce a leading edge  72  of fluid on the substrate  28 . However, the delay between the actuation of a dispensing valve  60  and the deposition of the leading edge  72   a  onto the substrate  28  changes the desired location of the leading edge  72   a.    
      That delay is detected or measured by the switching time detector  82 . Upon detecting the leading edge  72   a  of the fluid deposit  64   a,  the sensor  82  provides an edge feedback signal represented by transition  98   a  to the switching time detector  82 . The detector  82  is also responsive to the gun ON transition  96   a  provided by the comparator  76 . Thus, the switching time detector  82  detects or measures a difference or delay between the transitions  96   a  and  98   a.  That delay can be represented in a time domain by a pulse  100   a  or represented in a spatial domain by a count of feedback pulse transitions  102   a  from the conveyor feedback device  34  occurring between the transitions of the pulse  100   a.    
      That measured delay or difference represents a real time delay between a command to open the dispensing gun  22  and the deposit of fluid onto the moving substrate  28 . The measured delay in either of its forms  100   a,    102   a  is provided to the pattern store  66  where it is used to adjust or modify the values representing the desired fluid dispensing pattern. In the present example, a stored pattern value representing the next leading edge  72   b  of the substrate  64   b  is compensated by the detected delay  100   a,    102   a.  Therefore, the pattern store  66  presents a numerical value to the comparator  76  that, in essence, advances the location of the leading edge  72   b  by the measured delay time  100   a,    102   a.  Therefore, the comparator  76  produces a gun ON transition  96   b  that is advanced by the measured delay  100   a,    102   a.  The, current is applied to the dispensing gun coil  54  in advance; and assuming that the gun switching time has not changed appreciably since the prior operation, the sensor  80  detects the leading edge  72   b  of the fluid deposit  64   b  at a time represented by the transition  98   b.  Thus, the deposition of fluid  26  onto the substrate  28  occurs at its desired time or location as represented by the transition  92   b.  The measured delay  100   a,    102   a  for each gun ON transition is used by the pattern store  66  to compensate a subsequent gun ON transition, thereby depositing or placing the leading edges  72  of subsequent respective fluid deposits  64  to their respective desired locations on the moving substrate  28 .  
      As discussed earlier, in many applications, environmental and other factors cause the gun switching time to vary or drift with time, and that, in turn, causes leading edges  72  of respective fluid deposits  64  to also change or drift. That drift in the location of the leading edges  72  on the substrate  28  is detected by the switching time detector  82  and used by the pattern store  66  as earlier described to continuously shift the gun ON transition  96 . Thus, the location of the leading edges  72  of subsequent respective fluid deposits  64  are maintained at their desired relative locations on the moving substrate  28 .  
      Referring to  FIG. 2A , the initial gun ON transition  96   a  results in a shift in the location of the leading edge  72   a  of the fluid deposit  64   a  from its desired location as represented by the transition  92   a  to a location represented by the transition  98   a.  Thus, the shifted fluid deposit  64   a  is an example of a poorer quality fluid deposit and may result in a scrap product. In order to minimize that shift, the user can input, via the system control  42 , a fixed compensation value representing an estimate of the switching time of the dispensing gun  22 . That initial compensation value C 1  is provided to the pattern control  44  via input  104  where it is stored. Further, referring to  FIG. 2B , that initial compensation value is utilized by the pattern store  66  to advance the leading edge  72   a  of the first fluid deposit  64   a.  Therefore, the comparator  76  provides a gun ON transition  96   c  that is also advanced by the amount of the initial compensation value C 1 .  
      The advanced gun ON transition  96   c  results in the edge sensor  80  providing a transition  98   c  representing the leading edge  72   a  at a point that is closer to the desired location as represented by the transition  92   a.  Further, the switching time detector  82  provides a pulse  100   c  representing the time between the transitions  96   c  and  98   c;  and as indicated at  102   c,  that time delay can be represented in terms of encoder pulse transitions. Thus, with an initial fixed compensation value, the initial leading edge  72   a  can be placed closer to its desired location. Further, in the example of  FIG. 2B , the initial compensation value C 1  is not equal to the gun switching time. However, the switching time detector  82  measures a delay that does represent the gun switching time; and that delay is used to compensate the next leading edge  72   b  as earlier described.  
      The above examples illustrated in  FIGS. 2A and 2B  provide a compensation for leading edges  72  of fluid deposits  64  arising from variations in the dispensing gun switching time. As will be appreciated, the sensor  80 , switching time detector  82  and pattern store  66  can be used to provide a similar compensation to the gun OFF transition so that the trailing edges  73  of respective deposits  64  are precisely located on the moving substrate  28 . For example, referring to  FIG. 2A , an initial gun OFF transition  106   a  is provided at a time representing the desired location of the trailing edge as represented by the transition  94   a.  Upon detecting the trailing edge  73   a  on the moving substrate  28 , the sensor  80  provides a feedback signal represented by the edge  108   a.  The switching time detector  80  measures the turn off delay of the fluid dispenser  22  and provides a delay signal to the pattern store  66  as represented by the waveforms  110   a,    112   a.    
      The pattern store  66  then compensates the next trailing edge  73   b  by compensating or advancing the numerical value representing the trailing edge  73   b  stored therein. In a manner similar to that earlier described, the comparator then advances the gun OFF transition  106   b  by an amount substantially equal to the measured delay  1101 ,  112   a.  Therefore, assuming the switching time has not changed, the sensor  80  detects an occurrence of the trailing edge  73   b  at a time corresponding to its desire location. Thus, the sensor  80  produces an edge feedback signal represented by the transition  108   b  that corresponds to the desired edge location as represented by transition  94   b.    
      As with the leading edge of the initial deposition  64   a,  the initial trailing edge  108   a  is shifted from its desired position as represented by the transition  94   a.  Therefore, referring to  FIG. 2B , a user defined and input fixed compensation value C 2  can be used to provide an initial compensation for the trailing edge  73   a.  Thus, the gun OFF transition  106   c  is advanced by the magnitude of the initial compensation C 2 , and the resulting trailing edge is placed at a location closer to the position  94   a  as represented by the transition  108   c.  Further, the measured delay as represented by the waveforms  110   c,    112   c  accounts for the full turn off switching time of the dispensing gun  22 , so that the subsequent trailing edge  73   b  is placed at its desired location as represented by the transition  108   b.    
      With some fluid dispensing guns, the turn-on and turn-off switching times may be substantially equal, and therefore, the gun on switching time can be used to compensate the gun OFF transition. Similarly, the measured delay in turning the dispensing gun off may be used to compensate the gun ON transition. However, with many fluid dispensers the turn on switching time will be substantially different from the turn off switching time. In those applications, the pattern store  66  is used to separately store the turn on and turn off switching times or delays. With any of the embodiments, during production runs, any changes caused by a drifting of the switching times, may be used to compensate the gun ON and gun OFF transitions as appropriate.  
      In some applications, it may not be practical to use an edge sensor  80 , and therefore, other devices and methods may be used to detect and measure the switching time of the dispensing gun  22 . For example, referring to  FIG. 3 , a gun actuation or switching sensor  130  may be used to detect the mechanical actuation of the dispensing valve  60  in switching from its off state to its on state. The gun actuation sensor  130  may be implemented using an accelerometer, for example, that detects motion of the armature  58  and/or valve stem (not shown) connected to the armature  58  within the dispensing valve  60 . When the gun ON transition causes the gun driver  48  to provide current to the coil  54 , a magnetic field builds up and shifts the armature  58  in a direction causing the dispensing valve  60  to open. The armature moves through a short linear stroke. Upon the magnetic field causing the armature to move, the gun actuation sensor  130  provides a sensed or sensor feedback signal to an input  84  of the switching time detector  82  as represented by the waveform  114  of  FIG. 5 .  
      When the armature  58  reaches the end of its stroke and its velocity is zero, the output from the gun actuation sensor  130  drops rapidly back to its initial state. Signal conditioning in the gun actuation sensor  130  or the switching time detector may use a peak detector to detect the maximum amplitude of the waveform  114  ( FIG. 5 ). The peak value of the waveform  114  occurs instantaneously before the armature  58  reaches the end of its stroke. Thus, the peak value of the accelerometer signal in essence detects when the dispensing valve is open. Further signal conditioning can be used to create a transition  116 . The switching time detector  82 , in a manner similar to that described before, detects or measures the delay between the initiation of the gun on signal from the comparator  76  and the occurrence of the transition  116 . That delay is used by the pattern store  66  to adjust or compensate the values representing the leading and/or trailing edges  72 ,  73  of the fluid deposit  64  and hence, the occurrence of the gun ON/gun OFF transitions. In a similar manner, the gun actuation sensor can be used to measure a delay caused by the fluid dispensing gun  22  being switched from its on state to its off state.  
      Other applications may lend themselves to a further alternative embodiment. Referring to  FIG. 4 , many gun drivers  48  contain a current sensor  134  that provides a sensed current feedback signal representing current flow in the coil. The current feedback signal from the sensor  134  is often provided to the power control  52  of the gun driver  48  for current control purposes. In this embodiment, the current feedback signal is also provided to a signal conditioner  136  that, in turn, is connected to the input  84  of the switching time detector  82 . The current in the coil  54  has a unique waveform  118  ( FIG. 5 ) in which the magnitude of the current reaches a peak  120  and then drops to a null  122  before increasing again. The null  122  in current magnitude is caused by the magnetic field pulling the armature  58  away from a pole (not shown). The separation of the armature  58  from the pole effectively changes the inductance of the coil  54 , thereby producing the null  122 . The signal conditioner  136  often provides some filtering and in addition, detects the null  122  and provides a transition as represented by the transition  116 . As will be appreciated, the signal condition may be provided in the switching time detector  82 . The null  122  can be detected in any appropriate manner. However, in one embodiment, the derivative of the current feedback signal can be continuously monitored, and the null  122  is represented by a second occurrence of a zero value of that derivative.  
      The fluid dispensing system  20  continuously monitors the switching time of the fluid dispensing gun and automatically adjusts the operation of the gun driver  48  in real time, so that fluid is accurately dispensed onto the moving substrate  28 . This consistency in the fluid dispensing process reliably provides a high quality finished product. The capability of automatically measuring and compensating of variations in the switching time of the fluid dispensing gun also permits a wider variety of fluid dispensing guns to be used to accurately dispense fluid onto a moving substrate. For example, with the compensation described herein, fluid dispensing guns having slower gun switching times can be used to more accurately dispense fluid onto a moving substrate. In some applications, low voltage solenoid-operated guns can be considered for use when such was not possible without the compensation system described herein. This advantage is significant because slower switching, low voltage fluid dispensing guns are often less expensive.  
      The compensation system described herein has a further advantage in that it allows more flexibility in connecting a particular pattern control with different gun drivers. Further, since the compensation system provides more flexibility to a pattern control and gun driver combination, it is now feasible to integrate the design of a pattern control and gun driver into a single unit.  
      A still further advantage of the compensation system herein is that it is no longer necessary to design fluid dispensing guns having shorter and shorter switching times in order to adapt to ever increasing conveyor speeds. In addition, the capability of quantifying in real time gun switching time is also a useful input to diagnostic and quality control systems. Thus, the capability of a switching time compensation system to continuously adjust the fluid dispensing process in real time presents unique opportunities to improve the quality and economy of a fluid dispensing process.  
      While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail in order to describe a mode of practicing the invention, it is not the intention of Applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the spirit and scope of the invention will readily appear to those skilled in the art. For example, in the described embodiments, the switching time detector is located in pattern control  44 ; however, as will be appreciated, in an alternative embodiment, the switching time detector may be integrated into the gun driver  48  or any other part of the system control  40 .  
      In the described embodiments, the frequency of computation of the compensation signal or value and adjustment of the signals from the control system  40  is not specified. As will be appreciated, the frequency of signal adjustments can vary from application to application. For example, a compensation value can be computed and an output signal from the gun driver  48  adjusted with each change of state of the fluid dispensing gun  22 . In other applications, the output signal from the gun driver can be adjusted at a different rate than the determination of compensation values. Further, the determination of compensation values and/or adjustment of output signals can occur after timed periods, after measured conveyor displacements, after a number of dispensing cycles, etc. In other applications, the output signals may be adjusted only after detecting a particular magnitude of change in the compensation value.  
      In the described embodiments, the examples used result in the gun ON/OFF transition and corresponding output signals being advanced in time. As will be appreciated, environmental or other changes in the operation of the dispensing gun may result in the gun switching time in one fluid dispensing cycle decreasing from what it was in a prior dispensing cycle. In that event, the gun ON/OFF transition and output signal from the gun driver  48  are adjusted in an opposite direction or retarded in time in response to the compensation signal.  
      In the described embodiments, each embodiment has a sensor providing a sensor feedback signal that can be used to compensate for the dispensing gun switching time in turning the dispensing gun on and off. As will be appreciated each sensor has its benefits and drawbacks. For example, in the embodiment of  FIG. 4 , a coil current sensor  134  is used to provide a feedback signal with which the compensation value is determined. A current sensor may prove satisfactory in determining a dispensing gun ON switching time because the coil current causes the dispensing valve to open. However, the dispensing valve is often closed by a return spring; and in those applications, current sensing may be less reliable. It is within the scope of the claimed invention to use different and multiple sensors to detect a changes of state of the dispensing gun  22  where each sensor is particularly suited to detect a particular change of state.  
      Therefore, the invention in its broadest aspects is not limited to the specific details shown and described. Consequently, departures may be made from the details described herein without departing from the spirit and scope of the claims that follow.