Patent Abstract:
The present invention relates to a level sensing spray applicator. In one embodiment, the spray applicator includes a gun that receives a liquid and a supply vessel coupled to the gun. The vessel includes a level sensor responsive to the volume retained by the supply vessel. In another aspect, a level-sensing supply vessel includes a level sensor responsive to a volume of liquid retained by the supply vessel, the sensor including a sensor element to detect the volume by sensing a resistance property of the liquid. In still a further aspect, a method of sensing a level of a liquid retained within a storage vessel includes sensing a first liquid volume, removing a portion of the first volume to define a second volume, determining if the second volume is less than a minimum volume, and generating an alarm signal if the second volume is less than the minimum volume.

Full Description:
TECHNICAL FIELD 
   The present invention relates generally to equipment for applying a liquid to surfaces, and more particularly, to fluid level sensing for a spray applicator device. 
   BACKGROUND OF THE INVENTION 
   A wide variety of spray application devices for applying liquids such as paint, varnish, cleaning solvents, or other liquid materials to a surface are known. Typically, such spray applicator devices include a supply vessel that contains a volume of the liquid to be applied to the surface. The liquid is transferred from the supply vessel to a spray gun that atomizes the liquid and projects the atomized liquid towards the surface. In one example of a spray applicator device, the supply vessel is positioned above the spray gun so that the liquid is transferred to the spray gun by a gravity-feed system. In another example of an applicator device, the supply vessel may be positioned below the spray gun and internally pressurized to transfer the liquid upwardly into the gun. In still other examples, the vessel may be positioned remotely relative to the gun so that the liquid is transferred from the vessel to the spray gun through a flexible hose. 
   In all of these spray applicator devices, determining the volume of the liquid remaining in the supply vessel as the application of the liquid proceeds constitutes a significant problem. If the liquid volume in the supply vessel is reduced to a low value, the gun may be supplied with liquid only intermittently, so that the gun emits the atomized liquid on an interrupted basis. As a consequence, the spray applicator device fails to apply the liquid uniformly to the surface. In particular, when the spray applicator device is used to apply a paint material to a surface, surface imperfections in the paint finish may result when non-atomized paint is projected, or “sputtered” onto the surface, thus necessitating time consuming surface rework and re-painting. Since supply vessels commonly used with spray guns are comprised of materials that are substantially non-transparent, a visual indication of the liquid level in the supply vessel is not generally possible. 
   Accordingly, there is a need in the art for a level-sensing device for spray applicators to provide a user of the spray applicator with an audible or visual indication when the volume of liquid in the supply vessel has been reduced to a predetermined level. 
   SUMMARY OF THE INVENTION 
   The present invention relates generally to a spray applicator device for applying a liquid to surfaces, and more particularly, to a fluid level sensing apparatus and method for a spray applicator device. In one aspect, a spray applicator apparatus includes a gun configured to receive a liquid and atomize the liquid, and a supply vessel coupled to the gun. The supply vessel retains a volume of the liquid and includes a level sensor responsive to the volume retained by the supply vessel. In another aspect, a level-sensing supply vessel for a spray applicator includes a level sensor responsive to a volume of liquid retained by the supply vessel, the sensor including a sensor element configured to detect the volume by sensing a resistance property of the liquid. In still a further aspect, a method of sensing a level of a liquid retained within a storage vessel of a spray applicator includes sensing a first volume retained within the vessel, removing a portion of the first volume to define a second volume, determining if the second volume is less than a predetermined minimum volume, and generating an alarm signal if the second volume is less than the predetermined minimum volume. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial diagrammatic view of a spray applicator according to an embodiment of the invention. 
       FIG. 2  is a schematic view of a control system for a spray applicator having a level sensor according to an embodiment of the invention. 
       FIG. 3  is a is a schematic view of a control system for a spray applicator having a level sensor according to another embodiment of the invention. 
       FIG. 4  is a partial cross-sectional view of a supply vessel for a spray applicator having a level sensor according to another embodiment of the invention. 
       FIG. 5  is a partial cross-sectional view of a supply vessel for a spray applicator having a level sensor according to still another embodiment of the invention. 
       FIG. 6  is a partial cross-sectional view of a supply vessel for a spray applicator having a level sensor according to still yet another embodiment of the invention. 
       FIG. 7  is a cross sectional portion of a sensor element according to still yet another embodiment of the invention. 
       FIG. 8  is a partial cross-sectional view of a supply vessel for a spray applicator having a level sensor according to a further embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention is generally directed to equipment for applying liquid coating materials to surfaces, and in particular, to fluid level sensing for a spray applicator. Many of the specific details of certain embodiments of the invention are set forth in the following description and in  FIGS. 1–8  to provide a thorough understanding of such embodiments. One skilled in the art will understand, however, that the present invention may be practiced without several of the details described in the following description. Moreover, in the description that follows, it is understood that the figures related to the various embodiments are not to be interpreted as conveying any specific or relative physical dimension. Instead, it is understood that specific or relative dimensions related to the embodiments, if stated, are not to be considered limiting unless the claims expressly state otherwise. 
     FIG. 1  is a partial diagrammatic view of a spray applicator  10  according to an embodiment of the invention. The spray applicator  10  includes a supply vessel  12  that contains a volume of a liquid  14 . A spray gun  16  is coupled to the supply vessel  12 , and transfers the liquid  14  to the spray gun  16  so that an atomized spray pattern may be developed at a nozzle  18 . As the spray pattern is emitted from the gun  16 , the liquid  14  retained within the supply vessel  12  is gradually depleted, so that a surface level of the liquid  14  gradually descends as the spraying operation is conducted. Operational details of the spray gun  16  are well known in the art, and need not be discussed in further detail. Although a gravity-feed type spray applicator is shown in  FIG. 1 , it is understood that the various embodiments of the present invention may also be employed in other types of spray applicators, and are therefore not limited to the spray gun shown in  FIG. 1 . For example, the supply vessel  12  may be positioned below the spray gun  16 , or alternately may be located remotely from the spray gun  16  so that the liquid  14  is transferred to the spray gun  16  by a flexible hose, or other similar devices (not shown). 
   Still referring to  FIG. 1 , the spray applicator  10  further includes a sensor element  20  that extends into the liquid  14  within the supply vessel  12 . The sensor element  20  is configured to allow an electrical resistance property of the liquid  14  to be measured that corresponds to a volume of liquid  14  retained within the vessel  12 . For example, when the volume of liquid  14  defines a first level  22  within the supply vessel  12 , a first resistance quantity may be measured by the sensor element  20 . As the volume of the liquid  14  is gradually reduced to define a relatively lower intermediate level  24 , a second resistance quantity that differs from the first resistance quantity may be read by the sensor element  20 . As the level of the volume of the liquid  14  is reduced still further to define a second level  26 , so that the sensor element  20  is no longer immersed in the liquid  14 , the sensor element  20  ceases to measure a resistance quantity related to the liquid  14 , so that only a resistance property related to the sensor element  20  is measurable. The sensor element  20  will be described in further detail below. 
   A control system  28  is operatively coupled to the sensor element  20 . The control system  28  includes circuitry that is configured to measure electrical resistance quantities sensed by the sensor element  20 , and to output a control signal when a predetermined resistance quantity is measured. Referring now to  FIG. 2 , a particular embodiment for the control system  28  of  FIG. 1  is shown. The control system  28  includes a voltage source  30  that is selectively coupled to a current-sensing network  32  by a switch  34 . The system  28  is coupled to the sensor element  20 , which has a resistance value (R L ) that depends upon the volume of the liquid  14  retained by the supply vessel  12 , as previously described. With the switch  34  closed, the system  28  is energized and current flows in the resistance R L . The current-sensing network  32  measures a current flowing in the control system  28  and generates an output signal  36  when a predetermined current level corresponding to a predetermined volume of the liquid  14  is reached. In order to adjustably control the current flowing in the system  28 , a potentiometer  38  may be serially coupled to the sensor element  20 . In an alternative particular embodiment, the potentiometer  38  may be coupled in parallel with the sensor element  20 . 
   Turning now to  FIG. 3 , another particular embodiment for the control system  28  of  FIG. 1  is shown. In this embodiment, the control system  28  includes a current source  40  that is selectively coupled to a voltage-sensing network  42  by the switch  34 . The system  28  is coupled to the sensor element  20 . With the switch  34  closed, the system  28  is energized and a constant current flows in the resistance R L . The voltage-sensing network  42  measures a voltage across the resistance R L  and generates an output signal  36  when a predetermined voltage corresponding to a predetermined volume of the liquid  14  is reached. To adjustably control the voltage appearing across the resistance R L , a potentiometer  38  may be serially coupled to the sensor element  20 . In still another alternative particular embodiment, the potentiometer  38  may be coupled in parallel with the sensor element  20 . 
   Returning to  FIG. 1 , the control system  28  is coupled to an alarm device  29  that is configured to emit an alarm indication when a suitable output signal  36  is received from the control system  28 . In one specific embodiment of the invention, the alarm device  29  may be a visual alarm device, such as an incandescent light bulb, light emitting diode (LED), or other similar devices that illuminate when the output signal  36  is received. In another specific embodiment, the alarm device may be an audible alarm device, such as a piezoelectric speaker device, or other similar device capable of emitting acoustic energy that may be perceived by an operator of the spray applicator  10 . The alarm device  29  may also include a source of electrical energy to energize the audible or visual alarm device, or alternately, the alarm device  29  may receive electrical energy from the electrical energy source located within the control system  28 . 
   The operation of the spray applicator  10  will now be described in detail. Still referring to  FIG. 1 , the supply vessel  12  on the spray applicator  10  is supplied with an initial volume of the liquid  14  that defines the first level  22 . At this point, the sensor element  20  senses a first resistance quantity corresponding to the initial volume of the liquid  14 . Since the sensed first resistance quantity corresponds to a supply vessel  12  that contains a suitable amount of the liquid  14 , the control system  28  does not generate an output signal  36  (as shown in  FIGS. 2 and 3 ) so that the alarm device  29  does not generate an audible or visual signal. As the liquid  14  is drawn from the supply vessel  12 , the volume of the liquid  14  decreases to the second level  26 , so that a different resistance quantity is sensed. Since the sensor  20  is no longer exposed to the liquid  14 , the output signal  36  is generated by the control system  28  in response to the different resistance quantity. Accordingly, the alarm device  29  generates the audible or visual alarm that alerts the operator of the spray applicator  10  that the liquid  14  in the supply vessel has been depleted. Although the foregoing describes the output signal  36  as generated when the supply vessel  12  is substantially depleted, it is understood that the control system  28  may be configured to generate the output signal  36  when the volume of liquid  14  has decreased to a value that is intermediate between a full and a depleted state. For example, and referring still to  FIG. 1 , the control system  28  may be configured to generate the output signal  36  when the volume of liquid  14  falls to the intermediate level  24 . 
     FIG. 4  is a partial cross sectional view of the supply vessel  12  of  FIG. 1  that shows a sensor element  50  according to another embodiment of the invention. The sensor element  50  includes a first electrode  52  and a second electrode  54  that is spaced apart from the first electrode  52 . The first electrode  52  and the second electrode  54  are comprised of an electrically conductive material, and are structured to extend through a wall of the supply vessel  12  and to extend downwardly into the liquid  14  within the supply vessel  12 . In order to electrically isolate the first electrode  52  and the second electrode  54  from the wall of the supply vessel  12 , insulators  56  are interposed between the first electrode  52 , the second electrode  54  and the wall of the supply vessel  12 . The first electrode  52  and the second electrode  54  may further include extended portions  58  to permit the sensor element  50  to be coupled to the control system  28 . 
     FIG. 5  is a partial cross sectional view of the supply vessel  12  of  FIG. 1  that shows a sensor element  60  according to still another embodiment of the invention. As in the previous embodiment, the sensor element  60  includes a first electrode  62  and a second electrode  64  that is spaced apart from the first electrode  62 . The first electrode  62  and the second electrode  64  are similarly comprised of an electrically conductive material, and positioned adjacent to the wall of the supply vessel  12  and spaced apart from the wall by respective insulating layers  63  and  65 . The first electrode  62  and the second electrode  64  are electrically isolated from the wall of the supply vessel  12  by insulators  56  that extend through the wall of the supply vessel  12 . The first electrode  62  and the second electrode  64  also may include extended portions  58  to permit the sensor element  50  to be coupled to the control system  28 . 
     FIG. 6  is a partial cross sectional view of the supply vessel  12  of  FIG. 1  that shows a sensor element  70  according to still yet another embodiment of the invention. The sensor element  70  includes a first electrode  72  and a second electrode  74  that are formed on an insulating substrate  76  that extends downwardly into the supply vessel  12 . Referring briefly to  FIG. 7 , a cross sectional portion of the sensor element  70  is shown along the section line  7 — 7  of  FIG. 6 . The first electrode  72  and the second electrode  74  may be formed from a relatively thin and electrically conductive foil that is cladded onto the insulating substrate  76 . The substrate  76  may be comprised of a generally rigid, nonconductive material such as a rigid polymer. The sensor element  70  is suspended within the liquid  14  by an insulating support means (not shown) that is coupled to the wall of the supply vessel  12 . As in the previous embodiments, the first electrode  72  and the second electrode  74  may be coupled to the control system  28  by extended portions  58  that project through the wall of the supply vessel  12 . 
     FIG. 8  is a partial cross sectional view of the supply vessel  12  of  FIG. 1  that shows a sensor element  70  according to a further embodiment of the invention. The sensor element  70  includes an electrode  82  formed from an electrically conductive material, which extends downwardly into the supply vessel  12  and is spaced apart from the wall of the supply vessel  12 . Instead of an electrode that opposes electrode  82 , as in the previous embodiments, the wall of the supply vessel  12  forms an electrode opposite the first electrode  62 . The first electrode  82  is coupled to the control system  28  by an extended portion  58  that extends through the wall of the supply vessel  12 , and is electrically insulated from the wall by an insulator  56 . The supply vessel  12  is also coupled to the control system  28  by a lead  84  that is conductively coupled to the supply vessel  12 . 
   From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. For example, certain features shown in the context of one embodiment of the invention may be incorporated into other embodiments as well. Accordingly, the invention is not limited by the foregoing description of embodiments except as by the following claims.

Technology Classification (CPC): 8