Patent Publication Number: US-2012038346-A1

Title: Powder flow monitoring using grounded hoses

Description:
TECHNICAL FIELD OF THE INVENTIONS 
     The present disclosure relates generally to the art of applying powder coating material to a work piece using electrostatic spraying technology. More particularly, the disclosure and inventions herein relate to methods and apparatus for monitoring or detecting flow of powder coating material during a powder coating operation. 
     BACKGROUND OF THE DISCLOSURE 
     In many industries, it has become common practice to apply paints and other coatings to products in the form of dry particulate powder. Powder coatings offer a number of advantages over traditional liquid coatings and paints, among them are lower cost, reduced pollution, shorter curing times and improved physical properties. 
     Powder coating materials are applied to workpieces by spraying. The powder is drawn from a container or hopper by a pump that feeds the powder through hoses to one or more spray guns from which it is sprayed towards the workpiece by means of the pneumatic force supplied by the pump. Powder coating application is also commonly although not exclusively done with electrostatic spray guns in which the workpiece is held at ground and the powder is electrostatically charged. The electrostatic charge is applied to the powder coating particles by an electrode supported by the spray gun. Electrostatic charge may alternatively be applied to the powder coating particles using tribo-charging within the spray gun. 
     From the simplest powder coating system that may use only one spray gun and one powder supply, to more complex powder coating systems that may use forty or more spray guns supplied from a plurality of pumps and supply hoppers (for example, hoses that convey about 5 to about 60 or more pounds of powder coating material per hour), it can be advantageous to be able to confirm or verify that powder coating material is in fact flowing through the hoses to the spray guns. By providing an automatic way to detect powder flow, an operator need not spend time manually checking all the spray guns, pumps and hoses. U.S. Pat. No. 5,739,429 (the ‘429 patent), the entire disclosure of which is fully incorporated herein by reference, discloses flow monitoring apparatus using a flow meter and hopper weight change characteristics. U.S. Pat. No. 6,734,679 (the ‘679 patent), the entire disclosure of which is fully incorporated herein by reference, discloses detecting flow of powder particles using a powder transportation line that includes a portion provided with a electrostatically friction charging material that is located upstream or downstream of a powder pump, or within a powder pump. The powder particles flowing through the portion of the transportation line generate an electric charge as a result of friction between the powder and the friction charging material. The electrical charge that is generated is detected in the form of a current by a sensing unit. 
     SUMMARY OF THE DISCLOSURE 
     In accordance with an embodiment of one or more of the inventions presented in this disclosure, an apparatus for monitoring or detecting the flow of powder coating material entrained in an air stream utilizes an electrically conductive material that extends along a portion or length of an electrically non-conductive powder hose or tubing that connects a source of powder coating material to a spray gun. The hose exhibits tribo-charge transfer caused by friction between the powder coating material flowing through the hose and the inside wall of the hose. The electrically conductive material may be connected or coupled to a circuit that detects current produced in the electrically conductive material by the tribo-charge transfer. In a particular embodiment, the electrically conductive material comprises an electrical conductor such as a wire. 
     In various alternative exemplary embodiments, the wire may be embedded in the hose, wrapped around the hose, disposed adjacent the hose, supported on a member that is outside the hose, and disposed as part of a second tube or hose that surrounds at least a portion of the hose, to name a few examples. 
     The use of an electrically conductive material that extends along at least a portion of the length of the hose significantly increases the signal strength of the current produced by the tribo-charge transfer, thus providing an excellent signal that indicates whether powder coating material is flowing through the hose. The inventions herein also include methods for monitoring or detecting powder flow embodied in the use of such apparatus. 
     These and other embodiments of the various inventions disclosed herein will be understood and readily apparent to those skilled in the art based on a reading of the following detailed description of the exemplary embodiments in view of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a functional schematic diagram of an exemplary embodiment of a powder coating system utilizing an exemplary embodiment of one or more of the inventions; 
         FIG. 2  is an embodiment in perspective of a hose illustrating an exemplary embodiment of the inventions; 
         FIG. 3  illustrates another embodiment of a hose and an exemplary embodiment of the inventions; 
         FIG. 4  illustrates additional alternative embodiments of the inventions; and 
         FIGS. 5A and 5B  illustrate additional alternative embodiments of the inventions with  FIG. 5A  being in perspective and  FIG. 5B  being a cross-section taken along the line  5 B- 5 B in  FIG. 5A . 
     
    
    
     DESCRIPTION OF THE EXEMPLARY EMBODIMENTS 
     Although the exemplary embodiments herein are presented in the context of a powder coating material application system for applying powder coating material to a workpiece, the inventions are not limited to powder coating materials but rather will find application for transport or conveyance of any dry particulate material that can exhibit a tribo-charging effect with a portion of the flow path of the material, for example, a hose. The present inventions may be used with dense phase systems utilizing dense phase pumps for transporting particulate material such as the pumps shown in United States Patent Application publication number 2005/0158187 A1, and may also be used with dilute phase systems utilizing Venturi pumps such as the pumps  308  shown in U.S. Pat. No. 7,325,750, the entire disclosures of which are both fully incorporated herein by reference. The inventions are not limited to any particular type of process media or dry particulate material, although the inventions are especially suited for powder coating material as the process media. Still further, while the exemplary embodiments herein illustrate and refer to specific design and configuration features of a powder coating material application system including examples of pumps, hoses and spray guns, those skilled in the art will readily appreciate that the inventions herein will find application beyond just the exemplary embodiments and may be used with application systems having various modifications and components such as pumps and spray guns of many different designs. Although the exemplary embodiments illustrate use of the inventions in corona-type electrostatic charging systems, the inventions may be used with other electrostatic technologies such as, for example, tribo-charging systems. 
     While various inventive aspects, concepts and features of the inventions may be described and illustrated herein as embodied in combination in the exemplary embodiments, these various aspects, concepts and features may be used in many alternative embodiments, either individually or in various combinations and sub-combinations thereof. Unless expressly excluded herein, all such combinations and sub-combinations are intended to be within the scope of the present inventions. Still further, while various alternative embodiments as to the various aspects, concepts and features of the inventions—such as alternative materials, structures, configurations, methods, circuits, devices and components, software, hardware, control logic, alternatives as to form, fit and function, and so on--may be described herein, such descriptions are not intended to be a complete or exhaustive list of available alternative embodiments, whether presently known or later developed. Those skilled in the art may readily adopt one or more of the inventive aspects, concepts or features into additional embodiments and uses within the scope of the present inventions even if such embodiments are not expressly disclosed herein. Additionally, even though some features, concepts or aspects of the inventions may be described herein as being a preferred arrangement or method, such description is not intended to suggest that such feature is required or necessary unless expressly so stated. Still further, exemplary or representative values and ranges may be included to assist in understanding the present disclosure, however, such values and ranges are not to be construed in a limiting sense and are intended to be critical values or ranges only if so expressly stated. Moreover, while various aspects, features and concepts may be expressly identified herein as being inventive or forming part of an invention, such identification is not intended to be exclusive, but rather there may be inventive aspects, concepts and features that are fully described herein without being expressly identified as such or as part of a specific invention, the inventions instead being set forth in the appended claims. Descriptions of exemplary methods or processes are not limited to inclusion of all steps as being required in all cases, nor is the order that the steps are presented to be construed as required or necessary unless expressly so stated. 
     As used herein, we define an “exterior surface” of a hose to mean a radially outermost surface that defines the outside diameter of the hose at least along a portion of the hose, meaning that a hose may have more than one outside diameter although typically hoses are uniform in diameter. We use the terms “outside” and/or “outward” an exterior surface to mean any position or location that is other than embedded in the exterior surface. For example, outside or outward of an exterior surface may mean lying adjacent to or in contact with or attached to or spaced from the exterior surface. We use the term “embedded” in a surface to refer to disposing a member or component sub-surface such as, for example, at least partially below a surface or completely within a body of material. We use the TERM “non-conductive material” when referring to a hose to mean that in normal use the hose material is electrically insulating and does not conduct current. We use the term “electrically conductive” to mean a material, for example a metal wire or composite material, that has a low resistance to current in normal use. 
     With reference to  FIG. 1  then, a powder coating material application system  10  may include a source of powder coating material comprising a supply  12  and a powder pump  14 . The supply  12  may be, for example, a box, hopper, feed center and so forth. The pump  14  may be, for example, a Venturi-type pump or a dense phase powder pump, and is used to draw powder coating material  16  from the supply  12  and convey the powder coating material through one or more hoses  18  to one or more spray guns  20 . The powder coating material may be virgin powder coating material meaning the powder coating material has not been previously sprayed, or reclaimed powder coating material in which powder overspray is collected and recycled for use, or a combination of virgin and reclaimed powder coating material. Although the embodiment of  FIG. 1  illustrates only a single spray gun and hose, many powder coating material application systems utilize many spray guns  20  and hoses  18  being fed by many pumps  14  and possibly more than one supply  12 . 
     The inventions herein provide a way to verify that powder coating material is indeed flowing through the one or more hoses or tubing  18  (we use the terms tubing and hose interchangeably herein), as distinguished from the prior known systems that primarily try to verify flow through the pump or the spray gun. It is not uncommon for the hoses  18  to become kinked or blocked during use, and when there are many hoses it can take a long time to confirm which hose or hoses are blocked. A blocked hose during a powder coating operation can produce a poor finish or no coating on the end product or workpiece. 
     The spray gun  20  may be any suitable corona-type spray gun that uses a high voltage supply (not shown) to produce ionization at an electrode  22 , which applies an electrostatic charge to the powder coating material particles. An example of a spray gun is shown in International Patent Application publication no. 2005/018823 A2, the entire disclosure of which is fully incorporated herein by reference. The workpiece (not shown) is typically maintained at ground potential so that the charged particles are attracted to and adhere to the workpiece. Many different types of corona spray guns are known and commonly used today, and the inventions may also be used with corona type spray guns later developed. The inventions may also be used with other electrostatic spray technologies such as, for example, tribo-charging spray guns, and moreover may find application in systems that are not electrostatic. The inventions herein utilize the powder coating material exhibiting a tribo-charge effect for charge transfer with the hose  18  when the flowing powder coating material contacts the hose interior surface. Whether the hose and powder combination exhibits a stronger or weaker tribo-charge effect is not critical to the present inventions, although a stronger tribo-charging effect may produce a better signal related to flow of powder coating material  16  in the hose  18 . 
     In the embodiment of  FIG. 1 , the hose  18  includes an electrically conductive material  24  that extends along at least a portion of the length of the hose  18 . As shown, the electrically conductive material  24  may extends along substantially the entire length of the hose  18 , however, we have found that shorter lengths may be used, particularly for hoses that exhibit a stronger tribo-charge effect with the powder coating material  16 . 
     A signal wire  26  may be used to form an electrical circuit that includes the electrically conductive material  24 . In the exemplary embodiments herein, the signal wire  26  is electrically coupled or connected at one end to the electrically conductive material  24 , and at another end to a control circuit  27 . The signal wire  26  may be connected at any location to the electrically conductive material  24 , but we have had particularly good success if the signal wire  26  is connected near the outlet of the pump  14 , but such is not required. 
     The control circuit  27  may include a current detection circuit  28 . The control circuit  27  may be an overall control circuit or system for the powder coating material application system  10 , for example, a model i-Control® system available from Nordson Corporation, Westlake, Ohio. However, the inventions may be used with any control system, or may be used as a standalone circuit or a standalone circuit that interfaces with a control system. All that is needed is a current detection circuit  28  or other current detection technique (for example, magnetic coupling induced by the current in the electrically conductive material  24 ) that can detect current in the electrically conductive material  24 , for example via the signal wire  26 , which current is produced by tribo-charge transfer between the powder coating material  16  and the interior surface of the hose  18 . The tribo-charge transfer produces electrical charges that are collected by the electrically conductive material  24 . The circuit  28  may be a standalone circuit that produces a signal  30  that indicates whether powder coating material is flowing through the hose  18 . This signal  30  may be used by a control circuit  27  to decide whether to interrupt a coating operation or at least operation of the associated spray gun. Alternatively, the signal  30  may simply be used to produce a visual or audible warning or alert that there is a problem with powder flow through the associated hose  18 . 
     The current detection circuit  28  may be any circuit that can detect current. For example, the circuit  28  may be an ammeter, a multi-meter, a circuit that converts current to voltage and detects voltage levels, a voltmeter, and so on. Typically, a threshold current level will be selected or predetermined to indicate whether powder coating material is flowing through the hose  18 . The current level may be automatically determined by the circuit  28 , for example using a simple comparator, or an operator might visually confirm current level with an ammeter or other device. The output signal  30  may be used, for example, as a go/no-go signal to indicate whether powder coating material is flowing through the hose  18 . The current produced in the electrically conducive material may be on the order of nanoamps to milliamps but other signal levels, higher or lower, may be used as needed. 
       FIG. 2  illustrates an embodiment of the tubing or hose  18 . In this example, the electrically conductive material  24  is at least partially embedded in an interior surface  32  of the hose  18 . The hose  18  typically may include an annular wall  34  having an interior surface  32  and an exterior surface  36 . The annular wall  34  may comprise an electrically non-conductive material, for example, polyolefin elastomer, but many other suitable materials may alternatively be used. The electrically conductive material  24  may comprise, for example, an electrical conductor such as a wire  38  as part of a conductive cord  40  that is embedded in the interior surface  32 . Tribo-electric charge  42  that is produced by powder coating material particles impacting or contacting the interior surface  32  of the hose  18  (represented by the arrows in  FIG. 2 ) will pass to the wire  38  and produce a current that will be detected by the current detection circuit  28  via the signal wire  26 . 
     It will be noted that in all the embodiments herein, preferably the electrically conductive material  24 , wherever it is disposed, is isolated from contact with electrical ground. For example, the electrically conductive material  24  is preferably not exposed to human contact or contact with a grounded surface or object. 
     In the  FIG. 3  embodiment of the hose  18 , the electrically conductive material  24  may be wrapped around the exterior surface  36  of the hose  18 . The electrically conductive material  24  in this example might simply comprise an electrical conductor such as a wire  38 . The wire  38  may be attached to the exterior surface  36  using a suitable adhesive or tape, for example, or may be embedded in the exterior surface  36 , much like the embedding done in the interior surface  32  example of  FIG. 2 . 
       FIG. 4  illustrates several alternative concepts that may be used in various combinations or alone. In one example, the hose  18  is surrounded by a second or exterior piece of hose or tubing  44 . Powder coating material  16  still flows through the interior space of the annular wall  34 . The exterior hose  44  may extend along substantially the entire length of the interior hose  18 , or only along a portion or portions of the hose  18  (the latter represented by the dashed lines  45  in  FIG. 4 ). In one version, the exterior hose  44  may include an embedded electrically conductive material  46 , much like the hose  18  has in the  FIG. 2  embodiment. Alternatively, a separate loose wire  48  may be captured or otherwise held between an interior surface  50  of the second hose  44  and the exterior surface  36  of the first hose  18 . As still a further alternative, the electrically conductive material  46  or a wire  48  may be wound around the exterior surface  52  of the second hose  44 , similar to the embodiment of  FIG. 3  herein. Depending on the dimensional differences between the outside diameter of the interior hose  18  and the inside diameter of the exterior hose  44 , there may be a gap or space  54  present along a portion or all of the overlaying hoses  44 ,  18 . We have found that this gap  54  does not prevent the detection of the tribo-charge current produced within the first hose  18  although preferably there is contact at some point between the outer wall of the interior hose  18  and the inner wall of the exterior hose  44 . However, one can expect that reducing the size of this gap  54 , or eliminating the gap  54 , may in some cases produce a stronger current in the electrically conductive material  46 . 
       FIGS. 5A and 5B  illustrate additional alternative embodiments for the hose  18 . The hose  18  may include an electrical conductor  56  in the faun of a wire, for example, that is held in place against the exterior surface  36  of the hose  18  using a member  58 , for example, a piece of adhesive tape or other attachment member. As represented by the dashed lines  59 , the wire  56  and supporting member  58  may extend along a portion or substantially all of the length of the hose  18 . The wire  58  may also be wound around the hose  18  exterior surface  36 . 
     Alternatively, the hose  18  may include an embedded wire mesh or braided wire jacket  60  that functions like the conductor  56 . The signal wire  26  ( FIG. 1 ) may be connected to the wire mesh  60  to produce a current that results from the tribo-charge transfer between the powder coating material  16  and the interior surface  32  of the hose  18 . The wire mesh  60  may be completely embedded in the hose  18  or partially embedded. Also, the wire mesh may extend entirely throughout the bulk of the hose  18  material or only partially. 
     One of the improvements we note in our inventions is that the longer the length that the electrically conductive material  24  extends along the length of the hose  18 , or stated another way the greater the amount of electrically conductive material  24  provided along the length of the hose  18 , the more tribo-charge transfer is collected in the electrically conductive material  24 . The more tribo-charge transfer that is collected in the electrically conductive material  24  increases the signal level of the electrical current that can be produced in the electrically conductive material  24  when the electrically conductive material is coupled to a current detection circuit  28 . However, other factors may be taken into account in determining the length that the electrically conductive material  24  should extend along the length of the hose  18  for any particular application, as well as for selecting the threshold current level that will be used to indicate whether powder coating material is flowing through the hose  18 . These factors may include, but are not necessarily limited to nor required in all cases: the length of the hose  18 , the location of the electrically conductive material relative to the hose  18 , type of powder coating material, material from which the annular wall of the hose  18  is constructed, velocity of the powder flow through the hose, flow rates of the powder coating material through the hose, and inside diameter of the hose  18 . 
     The inventive aspects have been described with reference to the exemplary embodiments. Modification and alterations will occur to others upon a reading and understanding of this specification. It is intended to include all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.