Abstract:
During a liquid spray pattern detection procedure, the sensor ( 50, 52 ) is operable to detect the presence or absence of an edge of the dispensed liquid spray pattern. The nozzle ( 18 ) of the liquid dispensing system is moved to a predetermined position (A) relative to the sensor ( 50, 52 ) to determine the presence or absence of the spray pattern and is then moved to a second predetermined position (B) relative to the sensor ( 50, 52 ) to again determine the presence or absence of the spray pattern. Based on these determinations, the width of the spray pattern can be verified. This provides a quick means to verify that the width of the dispensed pattern is within certain tolerances.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of PCT/US03/01414, filed Jan. 16, 2003, now pending, which claims the benefit of U.S. Provisional Application No. 60/350,827 filed on Jan. 22, 2002, and the disclosures of which are hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to control systems for liquid dispensers and, more particularly, to a control system and method for controlling the dispensed liquid spray pattern of a liquid dispensing system.  
       BACKGROUND OF THE INVENTION  
       [0003]     Various liquid dispensing systems have been developed to dispense a liquid spray pattern from a nozzle outlet toward a surface of a substrate. For example, in conformal coating applications, liquid dispensing systems have been designed to dispense flat, fan-like liquid spray patterns of conformal coating material onto a surface of the substrate, such as a printed circuit board. The dispensing nozzle of these systems may be a cross-cut, slit or air-assisted slot nozzle, for example, that is configured to dispense the coating material as a spray, continuous band or sheet, or fibrous web of a predetermined pattern width toward the circuit board. The dispensing nozzle is typically moved by a robotic movement platform in opposite back-and-forth directions relative to the circuit board to dispense side-by-side tracks or bands of conformal coating onto the circuit board and thereby obtain a uniform moisture resistant barrier on the board surface. Alternatively, the circuit board may be moved relative to the liquid dispenser which may be fixed.  
         [0004]     During the conformal coating process, it is important that the tracks or bands of coating join or converge along their adjacent edges, or even slightly overlap, to ensure that a full surface coating is provided on the board. Otherwise, the circuit board will be left vulnerable to undesirable chemical or moisture attack by any gaps left between the coating tracks or bands. On the other hand, too much overlap of the adjacent edges may cause undesirable bubbles in the thickened coating areas at the overlap that may also jeopardize quality control. Thus, a consistent width of the liquid spray pattern is generally necessary to obtain a uniform layer of conformal coating on circuit boards.  
         [0005]     Unfortunately, while a liquid material dispenser may adequately and reliably dispense uniform conformal coating layers on circuit boards in one production run, changes in the viscosity and/or fluid pressure of the material will often lead to undesirable variations in the liquid spray pattern width. Additionally, contamination or partial blockage of the nozzle outlet will cause undesirable changes in the spray pattern. If these changes are not detected prior to a production run, improperly coated boards must be reworked and costly downtime of the conformal coating system is typically required to identify the problem and manually adjust the liquid dispensing system to obtain the desired pattern width and minimal offset. As those skilled in the art will readily appreciate, pattern width control is also critical in other liquid dispensing applications as well, such as in paint, flux and adhesive dispensing environments. In each of these applications, very often the edge positions of the dispensed liquid pattern relative to a substrate surface or to an adjacent liquid pattern dispensed on the substrate must be properly adjusted and set to obtain the desired material application.  
         [0006]     Thus, there is a need for a control system for use in a liquid dispensing system that verifies the width of the dispensed liquid spray pattern.  
         [0007]     There is also a need for a control system for use in a liquid dispensing system that improves the operator&#39;s ability to readily identify problems in the dispensed liquid spray pattern.  
       SUMMARY OF INVENTION  
       [0008]     The present invention overcomes the foregoing and other shortcomings and drawbacks of liquid dispenser control systems and methods heretofore known. While the invention will be described in connection with certain embodiments, it will be understood that the invention is not limited to these embodiments. On the contrary, the invention includes all alternatives, modifications and equivalents as may be included within the spirit and scope of the present invention.  
         [0009]     The control system of the present invention is operable to provide a quick check to determine if the liquid spray pattern dispensed from a liquid dispensing system is within prescribed limits.  
         [0010]     In one embodiment of the present invention the control system includes a sensor, such as, but not limited to fiber optic sensor, located in the vicinity of the liquid dispensing system. Other types of sensors can include, for example, laser sensors.  
         [0011]     The sensor may be mounted as part of a sensor fixture, which also includes a drain pan or receptacle for receiving and draining liquid material that is dispensed from the liquid dispensing system during a pattern measurement and adjustment procedure. The drain pan is fluidly connected to a fluid reservoir that is connected to main system vent air. The fiber optic sensor includes a transmitter and receiver which are mounted in confronting relationship to one another, on opposite sides of the drain pan. The transmitter and receiver are preferably each mounted in a cover or shroud that is releasably engageable with the sensor fixture.  
         [0012]     During a liquid spray pattern detection procedure, the sensor of the sensor fixture is operable to detect the presence or absence of an edge of the dispensed liquid spray pattern. In accordance with one aspect of the present invention, the nozzle of the liquid dispensing system is moved to a predetermined position relative to the sensor to determine the presence or absence of the spray pattern and is then moved to at least a second predetermined position relative to the sensor to again determine the presence or absence of the spray pattern. Based on these determinations, the width of the spray pattern can be verified. This provides a quick means to verify that the width of the dispensed pattern is within certain tolerances. The exact width may be measured periodically. This measurement may be as set forth in U.S. Pat. No. 6,457,655 B1, the disclosure of which is incorporated by reference herein.  
         [0013]     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 DRAWINGS  
       [0014]     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with a general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
         [0015]      FIG. 1  is a partial schematic diagram view of a liquid spray control system in accordance with the principles of the present invention for use in a liquid dispensing system that dispenses a liquid spray pattern toward a substrate;  
         [0016]      FIG. 2  is a schematic diagram of the edge detection of a spray pattern;  
         [0017]      FIG. 3  is an elevational view of one type of spray pattern;  
         [0018]      FIG. 4  is a cross-sectional view of the spray pattern taken generally along the lines of line  4 - 4  of  FIG. 3 ;  
         [0019]     FIGS.  5 A-C are possible sensor signal responses of a spray pattern of  FIGS. 3-4 ;  
         [0020]      FIG. 6  illustrates one embodiment of a sensor fixture;  
         [0021]      FIG. 7  illustrates an enlarged view of the sensor fixture of  FIG. 6  wherein the cover of the mounting bracket assembly has been removed; and  
         [0022]      FIG. 8  is an exploded view of the sensor fixture of  FIG. 6 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0023]     With reference to the figures, and to  FIG. 1  in particular, a liquid spray control system, indicated generally at  10 , is illustrated for use in a liquid spray dispensing system, indicated generally at  12 . As described in the exemplary embodiment herein, liquid spray dispensing system  12  includes a supply of liquid material  14  that is fluidly connected to a pneumatically controlled regulator  16 . The regulator  16  controls the fluid pressure of liquid material within dispensing barrel and nozzle  18  of dispenser  19  so that the liquid material may be dispensed in a flat, fan-like spray pattern. Liquid dispensing system  12  may be a Nordson® Select Coat Dispensing System manufactured and sold by Nordson Corporation of Westlake, Ohio. The barrel and nozzle  18  of the liquid dispensing system may be moved in X-Y-Z directions relative to the substrate under the control of an X-Y-Z robotic movement platform to dispense side-by-side, partially overlapping tracks of liquid material onto a surface of the substrate, such as in a conformal coating process, to provide a uniformly coated moisture proof barrier on the substrate as is well known in the art.  
         [0024]     However, as used herein, the term “liquid spray dispensing system” is not limited in any way to liquid dispensing systems for use in a conformal coating process or to liquid dispensing systems that atomize the dispensed liquid. Rather, the term “liquid spray dispensing system” as used herein more broadly refers to any liquid material dispensing system that is operable to dispense liquid material with a predetermined pattern width in a direction toward a substrate. Thus, for example, the liquid spray control system  10  of the present invention may be used with liquid material dispensers that spray paint, adhesives, sealants or fluxes, by way of example, onto a substrate that is positioned in facing relationship to a dispensing nozzle of the dispensing system. The spray pattern dispensed from the liquid dispenser may be atomized or, alternatively, may comprise a continuous band or ribbon of dispensed liquid material, as well as a fibrous web or band of liquid material that emits from a dispensing nozzle with a predetermined pattern width. Moreover, the liquid dispense pattern may be a flat, fan-like pattern as illustrated in the exemplary embodiment described in detail herein or, alternatively, may have an oval, circular, rectangular, square or other cross-section as taken in a plane generally parallel to a plane of the substrate. Thus, as will be described in detail below, the liquid spray control system  10  of the present invention is particularly adapted to the dispensed pattern of liquid material emitting from the nozzle  18 .  
         [0025]     A sensor fixture  26  may include a drain pan or receptacle  30  that includes a recessed chamber or well  32  for receiving and draining liquid material that is dispensed by the liquid spray dispensing system  12  during the detection procedure as described in detail below. The drain pan  30  is capable of mounting to a line conveyor (not shown) or another convenient location in the workcell area. The drain pan  30  may be coupled to a fluid reservoir (not shown) that is connected to main system vent air through tubing (not shown).  
         [0026]     The liquid spray control system  10  includes a sensor which, in the preferred embodiment, includes a fiber optic emitter or transmitter  50  and a fiber optic receiver  52  mounted in confronting relationship to one another on the opposite of the spray pattern. The transmitter  50  is operable to emit a beam of light that is received by the receiver  52  as is well known in the art.  
         [0027]     With continued reference to the drawings and in particular,  FIG. 2 , the control system determines if the width (W) of the spray pattern is within specific predetermined limits by checking one half of the spray pattern, W/2. It will be appreciated that the full width W of the pattern may be checked instead. During the detection operation, a quick check is made by moving the nozzle  18  to a first position, nozzle position A, and activating the dispenser to cause the nozzle  18  to dispense the material. The nozzle is then moved at least to a second position, nozzle position B, and the material is again dispensed.  
         [0028]     Nozzle positions A and B are both located to one side of the beam  54  being transmitted from the transmitter  50  to the receiver  52  and are both preferably located at the same predetermined distance or height (H) above the sensor.  
         [0029]     During normal operations, the sensor will not see the spray pattern when the nozzle is at position A but should see the pattern when the nozzle is at position B.  
         [0030]     If the nozzle becomes clogged, the pattern may become smaller due to the reduced flow of material. As a result, the sensor will not detect the edge of the fan pattern when the nozzle is moved to position B. On the other hand, if the nozzle becomes partially clogged, the spray pattern may become skewed to one side, thereby increasing its width at that side. If this happens, the sensor will detect the edge of the spray pattern at both positions A and B. Similarly, if the spray pattern is skewed in the opposite direction, then the opposite side of the spray pattern will become smaller and will not be detected at position B and may be not even be detected at position A. To reduce or eliminate such problems, the nozzle may be moved additional positions, for example, between positions A and B. The number of positions at which the spray pattern is checked can be based on the desired tolerance and the minimum width W expected in the particular application.  
         [0031]     With reference to  FIGS. 3-4 , the fluid fan pattern  60 , in general, is heavier at the edges  62 , and thinner in the center section or web  64 . This provides a sensing challenge for those applications utilizing a fluid that is generally transparent to the sensor. FIGS.  5 A-C illustrate possible sensor responses to the fluid as a fan pattern is moved through the beam of the sensor. In  FIG. 5A , the sensor detects the edges  62  generating an output  66 , but does not detect the center section or web  64  and therefore does not generate an output  67 . In  FIG. 5B , the sensor detects the entire fan pattern, including both edges  62  and the web  64  and generates an output  68 . In  FIG. 5C , the sensor detects the edges  62 , generating an output  70 , but the web area  64  results in an unstable sensor reading or output  72 . As a result, while the setting of the sensor threshold can be adjusted to give adequate detection of the edges of the fan pattern, due to sensor limitations, the web of the pattern may not be able to be detected reliably.  
         [0032]     Because the control expects the sensor to be detected at position B, there is a limit on the distance between positions A and B. For those materials that are difficult to detect, such as those that are generally transparent. The maximum for this distance T should be the same as the width of the edge on the fan that can be detected reliably by the sensor beam  54 . The distance qctol between positions A and B from the sensor beam  54  should therefore be no greater than T/2, wherein qctol is the quick check tolerance value.  
         [0033]     For example, if it is desirable to have a 12 mm fan pattern width W with a +/−1 mm tolerance, then the total error band difference is therefore 2 mm (11-13 mm). Since the quick check looks at one side of the fan, Positions A and B need to be separated by no more than 1 mm. This means the desired qctol is 0.5 mm and the width of the edge T of the spray pattern must be at least 1.0 mm. If, however, the combination of fluid, nozzle, pressure, and sensor combine to give a T that is only 0.4 mm, then the maximum allowed value for qctol is 0.2 mm for proper operation of the quick check detection of the spray pattern. This means that positions A and B should be separated by 0.4 mm, and the detection system will be looking for a fan pattern of 12 mm+/−0.4 mm.  
         [0034]     With reference to  FIGS. 6-8 , the transmitter  50  preferably includes a fiber optic cable  79  having an end  81  having a round barrel and two flats  80 . Similarly, the receiver  52  includes a fiber optic cable  79 ′ having an end  81 ′ having a round barrel and two flats  80 ′. The fiber optic cables  79 ,  79 ′, may be mounted to a mounting bracket assembly  82 . Each mounting bracket assembly  82  includes a bracket  84  and a clamp assembly  86 . The clamp assembly  86  includes a threaded bolt  88  or screw, a washer  90 , and O-rings  92 . The bracket  84  includes a milled slot  94 , a stop pin  96  mounted at one end of the milled slot  94  and a threaded bore  98  in which the clamp assembly  86  is retained therein.  
         [0035]     The end  81  of the fiber optic cable is inserted into the opening formed by the milled slot  94  of the bracket  84  and the washer  90  of the clamp assembly  86 . The washer of the clamp assembly pushes down the top flat  80  of the end  81  and holds the bottom flat (not shown) against the milled slot maintaining the flat alignment of the fiber optic cable. The O-rings  92  of the clamp assembly push the barrel of the fiber optic cable against a flat in the slot and by rotation of the screw the end  81  of the barrel pushes against the stop pin  96  in the slot, aligning the light transmitting portion of the barrel with an aperture  100  in the side wall of the bracket  84 .  
         [0036]     When a measurement of the fan pattern is made the dispenser sprays the pattern into the air. With some materials with a low surface tension the spray will break away from the main pattern at the nozzle and/or as the fan or leaf comes together again. As the conformal coating material breaks apart, it splashes onto equipment in the vicinity of the measurement area. If the coating gets on the sensor the ambient light level of the fiber optic sensor is reduced. If the light level decreases too much the ambient light level drops below the pre-set trip point used to measure the fan width edge.  
         [0037]     In these circumstances, a baffle assembly  110  may be used to minimize the splashing and help block splashed droplets from contacting the mounting bracket assemblies  82 ,  82 ′ and in particular, the aperture openings  100  therein. The baffle assembly  110  includes two spaced apart baffles  112 . During dispensing, the fan pattern of the dispensed material will be dispensed between the two baffles  112 . The baffles  112  help to block airborne droplets from being deposited on the apertures  100 . Additional sensor protection can be created, by adding clear lenses  114  and protective covers  115  to the mounting bracket assemblies  82 ,  82 ′. The clear lens  114  can be easily cleaned with solvent and a cotton swab. Holes  117  in the covers  115  and holes  116  in the baffles  112  allow the light beam to pass from the emitter to the receiver.  
         [0038]     The baffles  112  are attached to a base  118 . This base may be in the shape of a W. The W shape or cross section helps in reducing the splashing of the dispensed material in the vicinity of the baffle holes  116 . The base  118  includes openings  120  which allows the dispensed material to pass through to the drain pan  30 .  
         [0039]     The baffle assembly  110  may be mounted to a drain pan mounting plate  122  by retaining arms  124 . The drain pan  30  may be also attached to the drain pan mounting plate  122 . The drain pan mounting plate  122  may be mounted to the conveyor rail of the work station or some other convenient location.  
         [0040]     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. 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 applicants&#39; general inventive concept.