Abstract:
The invention relates to a level detecting system for a powder container of a powder-coating apparatus. It contains an inductive proximity sensor ( 22 ) and an electrically insulating membrane ( 28 ) which is displaceable to-and-fro the proximity sensor and which is fitted on its inside surface opposite the proximity sensor with a metallic element ( 32 ) affixed to it and affecting the magnetic properties of the proximity sensor. The level detecting system operates regardless of any powder particles adhering to it.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a level probe, hereafter level detecting system, for a powder container in a powder spray-coating apparatus. 
     BACKGROUND ART 
     The state of the art contains the following: 
     EP 0 185 479 B1, 
     EP 0 452 635 B1, 
     DE 198 08 765 A1, 
     DE 196 11 533 A1, 
     DE 39 05 236 A1. 
     The European patent document 0 185 479 B1 discloses powder spray-coating apparatus for spray-coating objects using a powder coating material, wherein a first pump pneumatically conveys coating powder from a first powder container into a second powder container and a second pump conveys the powder from the second powder container to a spray apparatus. A powder overflow device keeps the powder level constant in the second powder container and returns excess powder into the first powder container. 
     Both powder containers are fitted with a perforated upper bottom crossed from below to top by the fluidizing compressed air into the powder present on this upper bottom and “fluidizing” this powder, that is, the powder particles are kept floating in this fluidizing compressed air. 
     The European patent 0 452 635 discloses a pneumatic fluidizing and evacuation unit which can be immersed into a powder container in order to fluidize powder therein and to evacuate this fluidized powder from the container and convey it to a spray apparatus or another. The fluidizing and evacuation unit is fitted with a level detector generating an electrical signal as a function of depth of immersion into the fluidized powder. The electrical signal controls an advance system by means of which the fluidizing and evacuation unit jointly with the level detector always is kept at the same depth of immersion in the fluidized powder and is made to automatically follow the dropping powder level when the quantity of powder decreases. As a result, identical powder concentration and identical powder/airflow conditions are maintained at the lower, powder-aspirating end of a suction tube of the fluidizing and evacuation unit regardless of the height of the powder in the powder container. Consequently uniform power feed to a spray apparatus is assured. 
     It is known in practice to use so-called capacitive detectors as the level detectors, their capacitance changing as a function of the powder concentration in the detector vicinity. This procedure however entails the substantial drawback of spurious test results caused by powder particles which over the time collect at the detector and adhere to it. 
     SUMMARY OF THE INVENTION 
     The objective of the invention is to solve the problem of designing a level detecting system for powder containers in such manner that it shall operate continuously and accurately regardless of any powder particles adhering to it. Preferably such a level detecting system may be designed to operate not only vertically, but instead in any arbitrary position, for instance also horizontally, when detecting the powder level in a powder container. 
     The expression “powder container” not only includes a “rigid” container, but also a “flexible” one such as a pouch made of plastic or another material. 
     The goal of the invention is attained using an inductive proximity sensor, further a membrane configured opposite this proximity sensor and moved in reciprocating manner by the pressure exerted by fluidized powder on its external side away from the proximity sensor, and a metallic element mounted on the membrane inside surface opposite the proximity sensor and jointly moving with latter and thus affecting the proximity&#39;s sensor magnetic properties. 
     The following advantages in particular are offered by the invention: the test signals are permanently accurate regardless of any adhering particles. Accordingly it is also maintenance-free. Moreover it may be used in any arbitrary direction rather than only vertically, namely for instance also horizontally, when determining the powder level in a powder container. It may be assembled from a few, commercially accessible parts and accordingly both its manufacture and its operation are economical. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is elucidated below in a preferred embodiment and in relation both to the drawings and to preferred applications. 
     FIG. 1 is a side view and in axial section at the lower end of a powder container, 
     FIG. 2 is an enlargement of the lower end of FIG. 1, and 
     FIG. 3 is a powder spray-coating apparatus fitted with level detecting systems of FIG. 1 for powder containers. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The level detecting system  8  sensing the powder level in powder containers substantially consists of a detector head  10 ; of a tube  12  with a thread  14  at one end to affix the detector head  10 ; of a support  16  comprising a clearance through which passes the tube  12 ; of a fastener  18  in the form of a quick-connect for instance, to which the support is affixed and which passes the tube  12  in such a manner that this tube is adjustable in its longitudinal direction relative to the support  16  and can be affixed to it in any arbitrary longitudinal position. 
     The detector head  10  contains an affixation disk  20  fitted with a central clearance receiving an inductive proximity sensor  22  that is mounted on the affixation disk  20  and configured axially to the tube  12 . The affixation disk  20  is mounted on a head element  24  comprising a clearance  26  running axially relative to the tube  12  and being entered by he proximity sensor  22 . 
     A membrane  28  is clamped between an end face of the head element  24  away from the tube  12  and a retaining ring  30  screwed onto the head element. 
     A metallic element  32 , preferably a metal pane  32 , is mounted axially relative to the proximity sensor on the inside of the membrane  28  opposite the proximity sensor  22  and is affixed, preferably bonded or vulcanized to the membrane  28 . As a result the metallic element  32  jointly with the membrane  28  may be displaced to-and-fro the proximity sensor  22  as a function of the pressure exerted by the fluidized powder on the external membrane side  34  radially accessible inside the retaining ring  30  and can be contacted by the fluidized powder. When in a neutral position wherein the external membrane side  34  is unpressurized—except for atmospheric pressure—the membrane  28  and its metallic element  32  are axially away from the proximity sensor  22   
     The metallic element consists of a metal, preferably iron, which shall alter the magnetic properties, in particular its inductance of the proximity sensor  22 , as a function of said element&#39;s distance from this proximity sensor. 
     The gap between the membrane  28  and the affixation disk  20  is vented to preclude air pressure at the membrane inside surface from counteracting in undesired manner axial displacements of the membrane  28  caused by pressure changes on the external membrane side  34 . Preferably this gap pneumatically communicates through at least one axis-parallel borehole  36  in the affixation disk  20  with the tube duct  38  in turn communicating for instance by a vent  42  with the atmosphere at the tube end  40  away from the detector head  10 . 
     Preferably the affixation disk  20  is made of an electrically insulating material such as plastic in order not to affect the inductance of the proximity sensor  22 . The head element  24  and the tube  12  may be made of metal or of plastic. 
     The proximity sensor  22  is connected by a multi-conductor cable  44  to an electronic control unit  46 . This control unit  46  electrically powers the proximity sensor  22  which transmits to it data concerning the distance between the metallic element  32  of the membrane  28  from the sensor  22 . Preferably the electronic control unit  46  contains at least one microprocessor and it is used to control at least one sub-assembly, preferably a powder conveyor, as described below in relation to FIG.  3 . The electronic control unit  46  may be housed in a system separate from the level detecting system  8  or, as in the manner of the embodiments described herein, in a housing  48  which is mounted on one end segment of the tube  12  situated at the other side of the support  16  away from the detector head  10 . Preferably the housing  48  shall be a bush coaxially enclosing the tube  12  and receiving the control unit  46 . Consequently the electronic control unit  46  together with the housing  48 , the tube  12  and the detector head  10  together with the proximity sensor  22  and the membrane constitute one assembly which is operational with or without the support  16  and the fastener  18 . 
     In the preferred embodiment, the proximity sensor  22  is a proximity switch which shall switch when the metallic element  32  approaches. 
     The electronic control unit  46  may contain optical displays of the operational state of the level detecting system  8 . Illustratively an optical display  52 , for instance a green light, may be used to indicate electrical power being fed to the proximity sensor  22 . Also an optical display  54 , for instance a red light, may be used to indicate absence of pressure, or pressure below a predetermined level, on the external membrane side  34 ; again an optical display  56 , for instance a yellow light, may indicate a critical pressure on the external membrane side  34 . Additionally or alternatively, acoustic indicators also may be used. The displays may be protected through a viewing pane  58  in the housing  48  from the ambience outside the housing and hence against soiling. 
     Preferably the membrane  28  shall be an intrinsically bending or flexible disk. The metallic element  32  preferably is a metal pane of a diameter substantially less than that of the membrane  28 , for instance being only one sixth or less that of the membrane diameter. 
     To preclude fluttering of the membrane  28  generating spurious control signals at the control unit  46 , latter preferably shall be fitted with delay or damping means in such a manner that control signals shall be generated only when the membrane states shall be substantially stable. 
     FIG. 3 shows two preferred applications of the level detecting system  8  of FIGS. 1 and 2. 
     In one of the preferred applications, the level detecting system  8  dips by means of its detector head  10  into a powder container  60  in order to maintain fluidized powder  62  therein at a predetermined level  64 . 
     A pneumatic conveying sub-assembly  66  contains a suction tube  68  of which the lower end dips into the powder  62  in the powder container  60  and of which the upper end is affixed to an injection pump  70 . The lower tube end is fitted with a powder suction aperture  72  which is lower by a predetermined vertical distance  74  (or higher or at the same level) than the membrane  28  of the detector head  10 . Powder  62  from the powder container  60  is moved by the injector  70  to a spray apparatus  76  and is sprayed onto an object  78  to be coated. 
     When the powder level  64  drops to a predetermined magnitude, the control unit  46 —by means of schematically indicated lines  80  and  82 —switches ON a second pneumatic conveyor  166  which shall move fluidized coating powder  162  from a second powder container  160  into the other powder container  60  until latter has reached the same level again or another one, namely  64 . Accordingly the level detecting system  8  maintains a constant level  64  of a predetermined magnitude or a predetermined range of magnitudes in the powder container  60 . By keeping the level or the range of levels  64  constant while simultaneously keeping constant the vertical distance  74  between the powder suction aperture  72  and the membrane  28 , the same air/powder pressures always are present at this aperture  72  and therefore only the desired rate of powder shall always be fed from the injector  70  to the spray apparatus  76 . The output from the injector  70  is substantially determined by the air pressure applied to it by an air conveying line  84 , said air pressure being adjustable by an adjusting element  86  contained in said line  84  and illustratively being a pressure control, a flow baffle or a valve. In manner known in the state of the art, compressed air in the form of supplementary air also may be fed through a supplementary air line  88  to the injector  70  and be controlled or regulated by a further adjusting element  90  of a design which may be similar or the same as that of the adjusting element  86 . 
     The second powder conveying system  166  may be designed in the same manner shown in FIG. 3, and is denoted by the same references. The two powder conveying sub-assemblies may be fed from a common source of compressed air  92 . 
     In relation to a second level detecting system  8  affixed to a lift  94  or to the powder conveying sub-assembly  166  for common vertical displacement, the second powder conveying sub-assembly  166  is vertically displaced by the lift  94  in relation to the powder level  164 . This vertical displacement as a function of the powder level  164  is controlled or regulated by the control unit  46  of this powder conveying system  166  as schematically indicated by arrows  81  symbolizing electrical lines. 
     The powder suction apertures  72  must not be sealed by the bottom of the container  60  or  160 . Therefore they must be elevated above the container bottom or they must be constituted in the suction tube  68  or  100  or sideways in the tube wall. 
     In both containers  60  and  160 , the powder  62  or  162  is kept fluidized by feeding in known manner fluidizing compressed air into the powder. The fluidizing compressed air  96  illustratively may be applied through a perforated upper floor  98  schematically indicated for the powder container  60 , or through a line, or in parallel with the suction tube  68 , into the container&#39;s powder, the latter being schematically indicated by a dashed line  100  for the second powder conveying sub-assembly  166 . 
     Preferably the membrane  28  shall be a thin, flexible pane. It may consist of an electrically conducting material, but preferably it shall be an electrically insulating material such as rubber.