Patent Publication Number: US-6712906-B2

Title: Control system for metering pump and method

Description:
This application is a Divisional application of U.S. Ser. No. 09/702,427, entitled “Control System for Metering Pump and Method”, filed Oct. 31, 2000, now U.S. Pat. No. 6,517,891 hereby expressly incorporated by reference herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to an apparatus for dispensing viscous fluids and, more particularly, to an apparatus and method for supplying hot melt adhesives to a dispensing gun. 
     BACKGROUND OF THE INVENTION 
     The ability to precisely dispense viscous industrial materials, such as hot melt adhesives, is a necessity for manufacturers engaged in the packaging and plastics industries. Inconsistent application of adhesive onto a substrate translates into unusable and scrap product and increased costs. Therefore, the process of supplying adhesive to a fluid dispensing applicator or gun must be precisely controlled. 
     A typical fluid dispensing operation employs a dispensing gun to apply a fluid, for example, an adhesive, onto a substrate being moved past the dispensing gun by a 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. Fluid adhesive is normally supplied to the dispensing gun by flexible hoses. Adhesive is pumped from a reservoir by a metering pump, for example, a motor-driven positive displacement pump. A metering pump for purposes herein is a pump in which the output volume is directly proportional to the action or displacement of the pump independent of fluid viscosity, except for any fluid leakage within the pump. Therefore, with a metering pump, the flow rate of the adhesive being dispensed from the gun is a function of the speed of the motor driving the pump. 
     The proper application of fluid or adhesive onto a substrate requires that the flowrate of the fluid from the dispensing gun remain as constant as possible throughout the fluid dispensing process. Variations in the flowrate result in different quantities or volumes of fluid being applied at different locations across the substrate. Thus, with too little adhesive, a desired coating thickness is not achieved, and the quality of the adhesive capability is reduced. Similarly, with an excessive quantity of fluid being dispensed, the adhesive may subsequently be displaced to areas of the substrate where it is not wanted; and again, the quality of the substrate product is reduced. In either event scrap product is often the result. 
     In many applications, the speed of the conveyor carrying the substrate is controllable and changed in accordance with the production line&#39;s capability to produce a high quality product. For example, with a first time run of a product, a production line may be operated at a slower speed to ensure a high quality product. But over time, as the production line is tuned, it can operate at a higher conveyor speed and still produce a high quality product. Assume the fluid dispensing system is operating properly with the conveyor operating at a first constant speed. If the speed of the conveyor and the substrate is increased to a higher constant speed, the flowrate of fluid being dispensed through the gun must also be increased in order to maintain a consistent, high quality coating of fluid on the substrate. It is known to use a signal related to the conveyor speed to modify the speed of the pump motor. Hence, when the conveyor is adjusted to the higher constant speed, the speed of the pump motor increases; and the flow of fluid to the gun is increased, thereby causing the pressure within the gun to increase. The increased gun pressure causes the flowrate of fluid from the gun to increase, and thus, the flowrate of the fluid being dispensed is changed as a function of conveyor speed. 
     The above flow control system works relatively well while the conveyor is operating at a constant speed, however, the flow control system does not operate properly during periods when the conveyor is accelerating or decelerating. Such conveyor speed changes occur, for example, when the conveyor is initially started from rest. Known systems are unable to maintain the desired flowrate of the fluid through the dispensing gun during periods of conveyor acceleration and deceleration. 
     FIG. 5A illustrates how the fluid pressure at the dispensing gun changes with respect to an acceleration and deceleration of the conveyor. When the conveyor is at a zero speed ( 500 ), with some systems, for example, those using a pressure relief recirculation valve, the recirculation pressure is higher ( 502 ) than a desired operating pressure ( 504 ) of the dispensing gun. Therefore, when the conveyor line is initially started ( 506 ) and is accelerating, the fluid dispensing occurs at an excessive pressure, thereby depositing excessive fluid and producing scrap product. The production of scrap product will continue as the pressure decreases ( 508 ) and the conveyor accelerates until both the conveyor speed and operating pressure reach their desired values ( 509 ). For purposes of illustration, the desired values of conveyor speed and operating pressure are shown as the common line ( 504 ). Upon being given a deceleration command ( 530 ), the conveyor speed decreases ( 532 ) to a zero velocity ( 534 ). However, upon the dispensing gun closing, the pressure rises ( 536 ) until the pressure relief valve opens and stabilizes the pressure ( 538 ). 
     In other recirculation systems, a solenoid actuated pressure relief valve is in series with a restricted orifice; and upon the recirculation valve opening, the recirculation pressure ( 510 ) is held at a level lower than desired operating pressure. Upon the conveyor accelerating ( 506 ), the gun pressure initially drops to a still lower pressure ( 512 ) faster than the metering pump can increase the pressure. Therefore, for a short period of time after the conveyor line starts, an excessive amount of fluid is dispensed which results in the production of scrap product. As the conveyor line accelerates, at some point ( 514 ), for a current conveyor speed, the correct amount of fluid is being dispensed; but continued conveyor line acceleration ( 516 ) with lower pressure ( 518 ) results in less than the desired flowrate of fluid through the dispensing gun. Thus, scrap product continues to be produced until the conveyor speed and operating pressure both reach their desired values ( 504 ). Upon the conveyor starting a deceleration, the recirculation valve is opened and the pressure decreases until it is stabilized at a value ( 542 ) determined by the restricted orifice. 
     As can be seen in FIG. 5A, with the lower recirculation pressure just described, the conveyor accelerates to its desired speed well before the dispensing gun pressure reaches its desired operating pressure. A significant contributing factor to this extended pressure recovery time is the use of flexible hoses connecting the pump with the dispensing gun. At the desired operating pressure, the hoses expand slightly; and the quantity of fluid being dispensed is small relative to the volume of the hoses. In fact, many times, the quantity of fluid dispensed is no more, and often less, than the expansion, or increased volume, of the hose at the desired operating pressure. Therefore, it takes longer for the pump to restore the desired gun pressure because the pumped fluid has to again expand the hose with fluid in order to achieve the desired operating pressure. As will be appreciated, the graphical representations of the pressure and line speed in FIG. 5 are only exemplary. The acceleration and deceleration of the conveyor often varies nonlinearly and normally is not linear as shown. Further, the acceleration and deceleration of the conveyor may differ from day to day and may be different with different systems. Further, the the exact profile of pressure with respect to time often varies substantially on an instantaneous basis and is not in any respect related to the conveyor speed. 
     Therefore, there is a heed for a fluid dispensing system which maintains a desired flowrate of fluid through the dispensing gun while the speed of the conveyor carrying the substrate is changing, for example, when the conveyor is accelerating from rest to its desired conveying speed. 
     SUMMARY OF THE INVENTION 
     The fluid dispensing system of the present invention addresses the above and other problems associated with known systems in providing a system for pumping a fluid to a dispensing gun. The fluid dispensing system of the present invention minimizes the production of scrap product during periods of changing conveyor speed. The fluid dispensing system of the present invention is especially useful at the beginning of a production run when the conveyor is accelerating from rest to a desired full production speed. In addition, the fluid dispensing system provides the same benefits at the end of a production run when the conveyor is decelerating from its full production speed to rest. Thus, by reducing scrap production, the fluid dispensing system of the present invention reduces scrap product, maintenance, and the product unit cost. 
     In accordance with the principles of the present invention and the described embodiments, the invention in one embodiment provides an apparatus for controlling a speed of a motor of a metering pump providing pressurized fluid at a dispensing gun. The dispensing gun is opened and closed to dispense fluid onto a substrate being carried by a conveyor past the dispensing gun. The apparatus has a pressure control producing first motor speed signals as a function of changing speeds of the conveyor and changing pressures of the fluid in the dispensing gun when the dispensing gun is open. A flow control produces second motor speed signals as a function of the changing speeds of the conveyor. A motor control responds automatically to the first and second motor speed signals to produce speed command signals for the motor. The speed command signals operate the motor at speeds causing the pump to provide fluid to the dispensing gun at pressures changing at a rate tracking a rate of change of the speed of the conveyor. 
     The first motor speed signal from the pressure control operates the pump motor in response to both conveyor speed and fluid pressure at the dispensing gun during an acceleration or deceleration of the conveyor. Thus, the pressure at the dispensing gun changes at a rate that follows the acceleration and deceleration of the conveyor, and the flow of fluid from the dispenser also follows the acceleration and deceleration of the conveyor to dispense the proper amount of fluid on the substrate. When the conveyor reaches a constant full speed, the motor control provides the second motor speed signal to the pump motor, thereby controlling flow of the fluid in accordance with the constant full conveyor speed. 
     In another embodiment, the invention includes a method of providing fluid under pressure to a dispensing gun with a metering pump connected to a motor. The dispensing gun opened and closed to dispense fluid onto a substrate being carried by a conveyor past the dispensing gun. First, a speed of the conveyor is changed. Then, fluid pressures at the dispensing gun are detected while the speed of the conveyor is changing and the dispensing gun is dispensing fluid. In addition, speeds of the conveyor are detected while the speed of the conveyor is changing. In response to detecting the pressures and the speeds, the fluid pressures at the dispensing gun are changed at a rate substantially tracking a rate of change of the speed of the conveyor. Thereafter, the flow of the fluid is automatically controlled as a function of detecting a full speed of the conveyor. 
     In one aspect of the invention, first motor speed signals are generated in response to the detected fluid pressures and conveyor speeds, and a second motor speed signal is generated in response to detecting a full conveyor speeds. The control of motor speed is automatically switched from the first motor speed signals to the second motor speed signal in response to conveyor having the full conveyor speed. 
     In a further aspect of the invention, control of the motor speed is gradually switched from the first motor speed signals to the second motor speed signal utilizing differing proportions of the first and second motor speed signals. 
     The above and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawing, which is incorporated in and constitutes a part of this specification, illustrates embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serves to explain the principles of the invention. 
     FIG. 1 is an overall schematic block diagram of a fluid dispensing system in accordance with the principles of the invention. 
     FIGS. 2A-2B are flowcharts illustrating one embodiment of a process for controlling pump motor speed for the fluid dispensing system of FIG.  1 . 
     FIGS. 3A-3C are flowcharts illustrating another embodiment of a process for controlling pump motor speed for the fluid dispensing system of FIG.  1 . 
     FIG. 4 is a flowchart illustrating a cycle for capturing values of parameters used in the processes of controlling pump motor speed for the fluid dispensing system of FIG.  1 . 
     FIG. 5A is a graphical illustration of known relationships of conveyor speed and fluid dispenser pressure with respect to time. 
     FIG. 5B is a graphical illustration of a new relationship of fluid dispenser pressure with respect to time when using the fluid dispensing system of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, a fluid dispensing system is comprised of a fluid dispensing gun  22  having a nozzle  24  for dispensing a fluid  26 , for example, an adhesive, onto a substrate  28 . The substrate  28  is carried by a conveyor  30  past the dispensing gun  22 . The conveyor  30  is mechanically coupled to a conveyor drive having a conveyor motor  32 . The speed of the conveyor is detected by a conveyor feedback device  34 , for example, an encoder, mechanically coupled to the conveyor  30 . The feedback device  34  has an output  36  connected to a dispensing gun controller  38 , and the feedback device  34  provides a feedback signal that changes as a function of changes in the conveyor speed. 
     A system control  42  generally functions to coordinate the operation of the overall fluid dispensing system. For example, the system control  42  normally provides a user interface for the system and controls the operation of the conveyor motor  32  via signal line  43 . Further, within the system control  42  is a pattern controller  44  that controls the operation of the fluid dispensing gun  22  as a function of the particular application being run. The pattern controller  44  receives, on input  40 , a part present or trigger signal that provides a synchronization with motion of the substrate  28  on the moving conveyor  30 . In response to the trigger signal on an input  40  of a system control  42 , the system control provides a first signal to the gun controller  38  via an input  45  requesting the gun controller to close a recirculation valve  56 . The recirculation valve  56  is used to shunt fluid from the metering pump  52  around the dispensing valve  50  and back to the reservoir  54  during idle periods, for example, between parts. Further, in response to the trigger signal, the pattern controller  44  provides a sequence of gun ON/OFF signals normally in the form of pulses to the gun controller  38  via an input  47 . 
     The gun controller  38  provides output signals to operate the dispensing gun  22  as a function of the timing and duration of the gun ON/OFF signals from the pattern controller  44 . In response to the leading edge of the gun ON/OFF pulse, the gun controller  38  provides a gun command on an output  46  that operates a solenoid  48  within the dispensing gun  22 . The solenoid  48  is mechanically coupled to a dispensing valve  50  that is fluidly connected to a metering pump  52  that, in turn, receives fluid from a fluid reservoir  54 . Upon receiving a signal on output  46  from the gun controller  38 , the solenoid  48  opens the dispensing valve  50 . The pressurized adhesive in the dispensing gun passes through the nozzle  24  and is deposited onto the substrate  28 . The dispensing valve remains open for the duration of the gun ON/OFF pulse; and in response to the trailing edge of a gun ON/OFF pulse, the gun controller changes the state of the solenoid  48  to close the dispensing valve  50 . In most applications, as the substrate  28  is moved past the dispensing gun  22 , a plurality of gun ON/OFF pulses cause the gun controller to rapidly open and close the dispensing valve to deposit the fluid at different locations on the substrate. 
     The pump  52  is a positive displacement pump; and therefore, over a dispensing time period, the volume of fluid supplied to the dispensing valve  50  and dispensed through the nozzle  24  is directly proportional to the speed of the pump motor  58 . A motor speed controller  57  within the gun controller  38  is responsive to the conveyor feedback device  34  and a pressure feedback device  62  for providing motor speed command signals on an output  61  to the pump motor  58 . A flow control  60  within the motor speed controller  57  is responsive to the feedback signal from the feedback device  34  to provide a motor-speed-dependent-on-line-speed (“MS LS ”) motor speed signal. The MS LS  signal is provided by the motor speed control  68  over a signal line  61  to the pump motor  58 . The MS LS  signal changes as a function of the line speed of the conveyor  30 ; and thus, the pump motor  58  is controlled to have a speed that is related to the speed of the conveyor  30 . Consequently, the flow of fluid through the dispensing valve  50  changes as a function of changes in the conveyor speed. 
     As previously described, such a line speed control system has certain disadvantages during periods of acceleration and deceleration of the conveyor. Therefore, the present invention utilizes a pressure transducer  62  that detects pressure at a point immediately upstream of the dispensing nozzle  24 . A pressure control  66  provides a motor-speed-dependent-on-pressure (“MS P ”) motor speed signal in response to the feedback signal from the feedback device  34  and a pressure feedback signal on an output  64 . The motor speed control  68  switches control of the pump motor  58  between the MS LS  signal on an input  70  and the MS P  signal on an input  72 . Essentially, at the beginning of an acceleration or deceleration period, the motor speed selector  68  controls the pump motor  58  as a function of dispensing gun fluid pressure, that is, the MS P  signal from the pressure control  66 . When the dispensing gun pressure is equal to the desired operating pressure with the conveyor at full line speed, the motor speed selector  68  switches control of the pump motor  58  from a pressure control to a flow control using the MS LS  signal from the control  60 . 
     One embodiment of such an operation of the gun controller  38  is illustrated by the flowchart of FIGS. 2A and 2B. Upon initially starting a fluid dispensing system as illustrated in FIG. 1, the pump motor  58  is started before the conveyor motor  32  in order to initially stabilize and pressurize the fluid system comprised of the pump  52 , recirculation valve  56  and fluid reservoir  54 . The motor  58  is operated at a constant recirculation speed such that a known pressure is provided at the output of the pump  52 . The pressure may be created by the recirculation valve  56  being a pressure relief valve. Alternatively, the recirculation valve  56  may be a solenoid valve having a serially connected restricted orifice that provides the desired pressure drop. The pressure at the output of the pump  52  may be higher or lower than the normal operating pressure detected by the transducer  62  immediately upstream of the nozzle  24 . 
     In providing a better control of the speed of the pump motor  58 , the gun controller  38  first, at  202  of FIG. 2A, determines whether a conveyor start command has been given by the system control  42  to the conveyor motor  32 . A signal representing the start of the conveyor line is also provided to the gun controller  38  by the system control  42 . The gun controller  38 , at  204 , switches to pressure control of the pump motor  58  and ends the recirculation control. To end recirculation control, the controller  38  provides a signal over an output  59  causing the recirculation valve  56  to close, thereby terminating the recirculation mode. This step is necessary if the recirculation path includes a solenoid valve. If the recirculation valve is provided by a pressure relief valve, the recirculation mode is terminated by a lesser pressure differential across the relief valve caused by the dispensing valve opening. Thereafter, at  206 , the gun controller  38  samples the feedback signal from the conveyor encoder  34  representing the conveyor speed. The controller  38 , at  208 , then multiplies the recently sampled conveyor speed times a stored pressure scaling constant to determine a target pressure value or setpoint. The stored pressure scaling constant is a fraction having a numerator equal to the desired dispensing pressure and a denominator equal to the full line speed. Thereafter, at  210 , the controller  38  determines whether the target pressure value is greater than a maximum pressure limit, for example, 1500 pounds per square inch (psi); and if it is, the target pressure, at  212 , is set equal to the maximum pressure limit. The controller  38  then determines whether the target pressure value is less than a minimum pressure limit, for example, 25 psi; and if so, at  216 , the target pressure is set to a value equal to the minimum pressure limit. 
     The controller then, at  218 , samples a pressure feedback signal provided from output  64  of the pressure transducer  62 . The pressure control  66  within the controller  38 , at  220 , determines a value for MS P  using the target pressure and the sampled gun operating pressure in a known PID process with acceleration PID constants. With the PID process, depending on the application and desired response, proportional and/or integral and/or derivative terms are determined from the pressure values, and each of the terms has a gain or multiplier that is in the range of from zero to a value that is empirically determined to provide the desired response and stability to the operation of the motor  58  of the pump  52 . At the initiation of a conveyor acceleration cycle, the motor speed selector  68  applies the MS P  signal to the pump motor  58 . 
     The results of utilizing pressure as a pump motor control signal is illustrated in FIG.  5 B. As can be seen with this embodiment, the recirculation pressure ( 550 ) is less than with prior systems. Further, when the line speed provides a target pressure value equal to the recirculation pressure ( 552 ), the controller  38  provides a signal over output  59  to close the recirculation valve  56 . Simultaneously, the controller  38  provides a signal over output  46  to cause the solenoid  48  to open the dispensing valve  50 . The pressure control  66  provides an MS P  signal to the pump motor  58 , so that changes in the dispensing gun pressure ( 554 ) follow changes in the conveyor speed ( 516 ) with respect to time. To provide a desired response, the PID constants are set such that the pressure ( 558 ) slightly overshoots the full line speed ( 504 ). It should be noted that the desired response will differ with different applications and designers. The pressure curve in FIG. 5B at  558  is shown as being slightly underdamped; however, as will be appreciated, the PID process can be adjusted to provide a more critically damped pressure function or even an overdamped pressure function. 
     The controller  38  then, at  222  (FIG.  2 B), determines whether the operating gun pressure is equal to the target pressure at full line speed. The point at which the pressure intersects the constant line speed at  555  is theoretically the ideal pressure to be detected. However, for many reasons, for example, the target pressure is determined from a scaling constant based on noncurrent values, the detection of the pressure at  555  is very difficult. Thus, applicants have chosen to detect when the operating gun pressure has stabilized and thus, has a substantially zero slope for some period of time. As will be appreciated, other methods of detecting pressure at full line speed may be employed. Upon detecting the target pressure at full line speed ( 562  of FIG.  5 B), motor speed controller  57  at  224  switches to flow control the pump motor  58 . Thus, the motor speed control  68  within the motor speed controller  57  switches control of the pump motor  58  from the MS P  motor speed signal to the MS LS  motor speed signal. At this point, the control of the pressure within the dispensing gun  22  transitions ( 564 ) from the switch point ( 562 ) to a flow control ( 566 ) determined by the full line speed of the conveyor. 
     During the time that the conveyor is operating at full line speed, the speed of the pump motor  58  is controlled by the gun controller  38  as a function of the conveyor feedback signal in a known manner. The flow control continues until the controller  38 , at  226  (FIG.  2 B), determines whether a conveyor stop command has been issued by the system control  42 . As with the acceleration mode, controlling the speed of the pump motor  58  with the conveyor feedback signal does not take into account the variations in pressure arising from the fluid dispensing process in a deceleration mode. Therefore, the motor speed selector  68  within the gun controller  38  switches control of the pump motor  58  from the flow control  60  to the pressure control  66 . Once again, a conveyor speed is sampled at  228 , and a target pressure determined, at  230 , in a the same manner as previously described. Also, as previously described, the target pressure is checked against maximum and minimum limits at  232 - 238 . The gun pressure is again sampled at  240 . A motor speed value (MS P ) is determined, at  242 , by the controller  38  using the target pressure and the sampled pressure in a PID loop with deceleration PID constants; and the MS P  value is applied to the pump motor  58 . The gun controller  38  then at  244  detects from the pressure feedback signal on line  64  when the dispensing gun pressure is equal to the desired recirculation pressure. When the recirculation pressure is achieved, the gun controller  38 , at  246 , switches to recirculation control of the pump motor  58 . The controller  38  provides a first signal over line  61  commanding the pump motor  58  to operate at a recirculation speed and a second signal over line  59  commanding the recirculation valve to open. Thereafter, the system control  42  stops the operation of the conveyor motor at the end of the deceleration cycle. 
     Again, referring to FIG. 5B, upon starting a deceleration ( 574 ), the pressure ( 576 ) results from control of the pump motor  58  being switched to the pressure control  66 . Changes in the dispensing gun pressure ( 580 ) generally follow changes in the slowing conveyor line speed ( 532 ) so that the proper amount of fluid is supplied by the pump  52  to the dispensing gun  22  and dispensed on the substrate  28 . Upon reaching the recirculation pressure, the recirculation valve  56  is opened; and the pump motor is operated at the recirculation speed, thereby stabilizing the recirculation pressure. The conveyor comes to rest at a zero velocity ( 534 ). 
     The above system provides a substantially improved relationship of dispensing gun pressure with respect to conveyor line speed during periods of acceleration and deceleration of the conveyor  30 . With the above system, when the conveyor is accelerating or decelerating, a pressure control system is active in which the motor pump speed is under the control of a pressure loop that causes a rate of change in fluid pressure at the gun to follow or track a rate of change in the conveyor speed. However, when the conveyor reaches a full speed condition, control of the pump motor is switched from a pressure control system to a flow control system in which the pump motor speed is controlled exclusively as a function of the conveyor line speed. Such a system is effective in different applications and on different systems where the acceleration and deceleration of the conveyor will vary. Further, with the dispensing system of the present invention, the dispensing of fluid onto the substrate  28  during periods of acceleration and deceleration is within specification; and scrap product is eliminated. 
     However, there is a disadvantage to the operating process described with respect to FIGS. 2A and 2B. Referring to FIG. 5B, control of the pump motor  58  is switched from the pressure control  66  to the flow control  60  at a point in time ( 562 ). However, at the switching point ( 562 ), the motor speed resulting from operation of the pressure control  66  is different from the motor speed resulting from the operation of the flow control  60 . Therefore, the system attempts to provide an instantaneous motor speed change equal to that difference. Such an abrupt switch in motor speed can result in an erratic or jerky operation of the pump motor  58  which creates mechanical stresses on the motor and pump as well as pressure irregularities and inconsistent fluid dispensing within the dispensing gun  22 . 
     FIGS. 3A-3C illustrate an alternative embodiment of the invention in which the transition between pressure control of the pump motor  58  and line speed control of the pump motor  58  is gradual and controlled. In this embodiment, the operation of process steps  302 - 320  are identical to the operation of process steps  202 - 220  previously described with respect to FIGS. 2A-2B. Referring to FIG. 3B, the controller  38 , at  321 , also determines a target line speed value or setpoint by multiplying the current value of the conveyor speed times a motor speed scaling constant. The motor speed scaling constant is a fraction having a numerator equal to the full speed of the pump motor  58  and a denominator equal to the full line speed of a conveyor  30 . The product of the most recently sampled conveyor line speed times the motor speed scaling constant is stored by the controller  38  as an MS LS  value. 
     Again, as previously described with respect to FIG. 2, the motor speed selector  68  within the motor speed controller  57  determines, at  322 , whether the current dispensing gun pressure is equal to the target pressure at full scale line speed. When that switching point is detected, the motor speed selector  68  then gradually shifts control of the speed of the pump motor  58  from the pressure control  66  to the flow control  60 . That shift in control can be performed linearly or nonlinearly with time. Further, the incremental resolution of each step in the transition is selectable in accordance with a particular the application, user preferences, etc. The motor speed selector  68  first, at  324 , sets a transition constant F equal to 1. Thereafter, at  326 , the mode speed selector  68  determines a first increment of the transition in accordance with the following: 
     
       
           MS=F×MS   P +(1 −F )× MS   LS , 
       
     
     and that value of MS is applied to the pump motor  58 . Thereafter, at  328 , the motor speed selector decreases the value of F and, at  330 , determines whether the value of F equals zero. The process of steps  324 - 330  is iterated until the value of F equals zero. With each iteration through steps  324 - 330 , F may be fractionally decreased in equal or nonequal increments. Further any number of increments may be used. When F equals zero, the full value of the MS LS  motor speed signal is being applied to the pump motor  58 , and, at  331 , the motor speed control  57  switches to the flow control of the motor  58 . Thus, the control of the pump motor  58  is gradually shifted from the pressure control  66  to the flow control  60 . Such gradual shifting of control helps to minimize any sudden changes in the motor speed command to the pump motor  58  that may result in abrupt changes in the pressure within the dispensing gun  22 , thereby causing sudden changes in the fluid being dispensed. 
     Thereafter, at  332 , the gun controller  38  is provided with an input from the system control  42  indicating that the conveyor  30  has been commanded to stop. In an identical manner as previously described with respect to steps  306 - 321 , the conveyor speed is sampled at  334 , a target pressure determined and checked against maximum and minimum limits at  336 - 344 . The gun pressure is then sampled at  346 , and a MS P  value determined at  348  and applied to the pump motor. The recirculation pressure is detected at  250 ; and if the pressure is above the recirculation pressure, the process of steps  334 - 350  is iterated. The command of the pump motor  58  remains under the control of the pressure control  66  until the recirculation pressure is reached. Thereafter, in a manner as previously described, and the gun controller  38  switches the system back to recirculation control at  352 . 
     In the embodiments illustrated in FIGS. 2 and 3, various scaling constants are utilized which are based on full dispensing pressure, full line speed and full motor speed. Those values may be determined in advance and manually entered into the system control  42  and passed to the gun controller  38  for storage. Alternatively, those values may be continuously determined and stored by the gun controller  38 . For example, referring to FIG. 4, at  402 , the controller  38  first determines when the conveyor has reached its full line speed. Upon detecting full line speed, the gun controller  38  at  404 , samples the pressure feedback signal, determines the average dispensing pressure and stores that value. Thereafter, at  406 , the controller  38  samples the conveyor feedback signal, determines the average full line speed value and stores that value. At  408 , the controller  38  samples a pump motor feedback signal on line  63 , determines an average motor speed value and stores that value. The process of FIG. 4 may be executed continuously while the conveyor is running at full line speed so that the stored values always represent the most recent full scale values of dispensing gun pressure, conveyor line speed and pump motor speed. Alternatively, the process of FIG. 4 may be run at selected times during the operation of the conveyor, for example, immediately prior to the conveyor being commanded to stop. 
     The fluid dispensing system described above permits an accurate deposition of fluid onto the substrate during periods of conveyor acceleration and conveyor deceleration, thereby permitting the production of good product during the full time of conveyor operation. Thus, the fluid dispensing system described above is effective to reduce scrap as well as maintenance and product unit cost. 
     While the present invention has been illustrated by a description of various embodiments and while these embodiments have been described in considerable detail, it is not the intention of the applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. For example, in the described embodiments, during periods of changing conveyor speed, a pressure feedback signal is used with a target pressure in a PID process to provide motor speed signals operating the motor at speeds causing fluid pressure changes at the dispensing gun to follow changes in conveyor speed over time. As will be appreciated, fuzzy logic, neural nets, model based systems or other processes and systems may be used to provide a motor speed signal as a function of fluid pressure at the dispensing gun. 
     The invention in its broader aspects is therefore not limited to the specific details, representative apparatus and method, and illustrative example shown and described. Accordingly, departures may be made from such details without departing from the spirit or scope of applicant&#39;s general inventive concept.