Abstract:
A spray generator ( 10 ) including: a membrane ( 40 ) having a perforate portion through which, in use, a fluid is caused to flow when the membrane ( 40 ) is vibrated; an electronically-driven or a piezoelectrically driven actuator for vibrating the membrane ( 40 ); a chamber ( 18 ) for storing fluid for supply to a surface of the membrane ( 40 ); and a sealing element ( 13 ) located in and movable within the chamber ( 18 ) between a first position in which fluid flow from the chamber ( 18 ) through the membrane ( 40 ) is prevented and a second position in which fluid flow from the chamber ( 18 ) through the membrane ( 40 ) is allowed.

Description:
FIELD OF THE INVENTION 
     This invention relates to a spray generator and, in particular, a spray generator having an electronically driven or piezoelectrically driven actuator for vibrating a perforate membrane, typically for use in electronic aerosols and the like. 
     BACKGROUND OF THE INVENTION 
     When piezoelectrically actuated aerosols are used in fast-moving consumer goods such as household and personal care products, they are often required to prevent fluid leakage from the device when it is not in use. Such applications could include, but are not limited to, fragrance dispensers, cosmetic products and household cleaning products. 
     In these types of applications, it is often the case that the droplets which are generated need to be sufficiently large that they will land on a surface, rather than simply evaporate into the atmosphere once they have been dispensed. Typically, these droplets will have an average droplet diameter in the region of 20 to 60 microns and a spray generator that is capable of producing such droplets will typically have nozzle diameters in the region of 10 to 20 microns. In some of the applications referred to above, the fluid to be dispensed has a relatively low surface tension and a relatively low viscosity and, as a result, fluid can easily flow through the nozzles of a membrane when the device is not in use. This fluid flow is driven by a combination of capillary action and pressure differences. This pressure difference comes from the fluid head behind the membrane and, if the chamber containing the fluid cannot maintain equilibrium with the atmosphere, differences caused by changes in atmospheric temperature or pressure. 
     One method of preventing fluid flow through those nozzles when the device is not in use is to apply a negative pressure to the fluid in the region directly behind the perforate membrane. However, maintaining a negative pressure behind the membrane of such a device within a sealed chamber, consisting of a fluid feed and a fluid reservoir, is not a trivial task. In particular, any mechanism for supplying a negative pressure will need to cope with pressure changes resulting from changes in ambient pressure or temperature. 
     Thus, an alternative method of preventing unwanted fluid flow through a perforate membrane is required. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided a spray generator comprising: 
     a membrane having a perforate portion through which, in use, a fluid is caused to flow when the membrane is vibrated; 
     an electronically driven or a piezoelectrically driven actuator for vibrating the membrane; 
     a chamber for storing fluid for supply to a surface of the membrane; and 
     a sealing element located in and moveable within the chamber between a first position in which fluid flow from the chamber to the membrane is prevented and a second position in which fluid flow from the chamber to the membrane is allowed. 
     Thus, the present invention provides a movable sealing element which, in certain embodiments, is preferably compliant, such that it can be moved into direct physical contact with the perforate membrane. By creating a seal around any perforations in the membrane, unwanted fluid flow can be prevented whilst the device is not in use. 
     Preferably, in the present invention the movable sealing element moves principally perpendicular to the membrane (or other sealing) surface. This minimises the required travel of the seal and any biasing force when closed assists in the sealing of the surface. Further, the present invention preferably utilises any pressure difference across the membrane to push the compliant seal against the membrane and improve sealing. This is accomplished by designing the seal such that its back surface is primarily exposed to the pressure that the fluid is under. As its front surface is exposed to atmospheric pressure when sealed this adds a beneficial biasing force. This beneficial force is not present when the back of the seal is not subjected to the same pressure as the fluid. If the seal was on the other side of the membrane this biasing force would be detrimental to sealing rather than beneficial. 
     A further benefit of this design is that the sealing element can be thin and compliant in nature as it does not have to resist bending in order to maintain a complete seal. In a preferential embodiment, the sealing element is, when sealed, unconstrained from moving tangentially relative to the membrane and, when subjected to a typical pressure difference, deforms to seal against the membrane. 
     For the seal to perform it will need to be sealed for pressure differences as low as 1 kPa. For this to occur it needs to both deform to the surface it is sealing against and be compliant enough to seal against surfaces which are not ideally smooth (i.e. have surface roughness). To achieve compliance, the Durometer (Shore A) hardness should be of value 70 or lower and more ideally of value 50 or lower. The seal will ideally deform up to 0.1 mm, more ideally up to 1.0 mm under such pressures to ensure good seal contact. Modelling the seal section in contact with the membrane as a simply supported flat plate under large deflection the following formula approximately relates the pressure difference, q, to the deflection of the seal at its centre, y: 
     
       
         
           
             
               
                 qa 
                 4 
               
               
                 Et 
                 4 
               
             
             ≈ 
             
               
                 1.451 
                 ⁢ 
                 
                   y 
                   t 
                 
               
               + 
               
                 0.376 
                 ⁢ 
                 
                   
                     ( 
                     
                       y 
                       t 
                     
                     ) 
                   
                   3 
                 
               
             
           
         
       
     
     where E is the Young&#39;s Modulus of the material, t is its thickness and Poisson&#39;s ratio has been taken to equal 0.3. ‘a’ is the seal outer radius and depending on spray generator design will typically vary between 2 mm and 6 mm. For a 0.5 mm thick seal the Young&#39;s Modulus should ideally be less than or equal to ˜10 8  or more ideally less than or equal to ˜10 6 . Whilst reducing thickness further allows for increased Young&#39;s Modulus, the seal becomes more fragile. 
     Preferably, the present invention further includes an actuating device for moving the sealing element between its two positions. The sealing element may be mounted on the actuating device. The actuating device may be a plunger which is movable towards and away from the perforate membrane. The actuator may pass through a wall of the chamber and, if this is this case, a seal is preferably provided around the actuating device to prevent fluid flow from the chamber passed the seal. 
     The sealing element preferably forms part of a sealing device having a mounted outer portion, wherein the sealing element is connected to and movable relative to the outer portion. The outer portion of the sealing device may be mounted in or on the chamber. The actuator and the membrane may be mounted in the outer portion of the sealing device. The outer portion of the sealing device may be mounted within an outer wall of the chamber. 
     In an alternative construction, the sealing device may extend through, in at least two locations, a wall defining the chamber such that movement of portions of the sealing device external to the chamber causes movement of the sealing element between the first and second positions. 
     The sealing device may be integrally formed with the walls of the chamber in such a way that a pivoting movement of the sealing device relative to the chamber walls can be achieved. The integral connection between the wall of the chamber and the sealing device may be relatively thin compared to the thickness of the wall to enable movement of the sealing device. 
     The sealing element may be mounted on or be connected to a shape memory alloy (SMA) which, upon activation, causes movement of the sealing element relative to the perforate membrane. Alternatively, the sealing element may be mounted on an arm which passes through a wall of the chamber and has a deformable seal preventing fluid flow between the wall and the arm. 
     Where shape memory alloy is provided to cause movement of the sealing element, it is preferable that activation of the shape memory alloy causes movement of the sealing element away from the membrane, with the deactivation of the shape of any alloy causing movement in the opposite direction and into sealing engagement with the perforate membrane. 
     Biasing means may be provided for urging the sealing element to the desired at rest position, which is preferably in sealing engagement with the perforate membrane. 
     The sealing device may include one or more openings located between the sealing element and the chamber such that fluid can pass through the sealing device within the chamber. The openings may be provided between a plurality of spokes in the sealing device. 
     The sealing element may have a flat sealing face for sealing the perforate portion of the membrane from the rest of the chamber. The flat sealing face is preferably in direct contact with the perforate portion of the membrane when the sealing element is in the first position. 
     The sealing face may, alternatively, include a circumferential bead for contacting the perforate membrane around the perforate portion, so as to prevent fluid flow through the perforate portion. 
     A fluid supply means is preferably provided to allow fluid to enter the chamber to replace that which is dispensed through the perforate membrane in use. 
     The fluid supply means is preferably on the opposite side of the sealing device to the perforate membrane such that fluid can flow through the openings in the seal device in order to reach the perforate membrane. A spacer may be provided on the chamber side on the perforate membrane, the spacer having an opening to permit fluid from the chamber to reach the perforate membrane wherein the sealing element in the first position is arranged to block the opening in the spacer. The sealing element may extend into the opening and the spacer when the sealing element is in the first position so as to prevent fluid flow from the chamber to the membrane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of the present invention will now be described with reference to the accompanying drawings, in which: 
         FIG. 1  shows a first example of a spray generator according to the present invention; 
         FIG. 2  shows one possible design of a spoked sealing device; 
         FIG. 3  shows a further example of a spray generator; 
         FIG. 4  shows a yet further embodiment of a spray generator; 
         FIG. 5  shows another embodiment of a spray generator; 
         FIG. 6  shows a yet further still embodiment of a spray generator; 
         FIG. 7  shows the provision of an internal SMA actuator; 
         FIG. 8  shows the provision of an external SMA actuator; 
         FIG. 9  shows a further construction using an external SMA actuator; and 
         FIG. 10  shows an actuator method using external magnetic actuation. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a first example of a spray generator  10  which is formed in a main body  11 . Although not shown in  FIG. 1 , a perforate membrane would, in use, be located in slot  12  within sealing element  13 , with the perforate portion of the membrane being located substantially at the centre of the membrane such that it aligns with the sealing portion  14  at the centre of element  13 . 
     The sealing element  13  is shown in greater detail in  FIG. 2   a  in which the outer substantially annular section can be seen and it is this outer section which supports the perforate membrane in slot  12 . The central sealing portion  14  can also be seen and it is supported, spaced from the outer portion  15 , by plurality of spokes  16 . Thus, a plurality of openings  17  are provided between adjacent spokes and the outer annular portion  15 . When the perforate membrane is in place as shown in  FIG. 2   a, b  chamber  18  is defined by the perforate membrane, walls of the main body  11  and a rolling seal  19 . The sealing element and, in particular, the central portion  14  of the sealing element is movable within the chamber  18 . A fluid inlet  20 , typically from a bulk reservoir (not shown) is provided into chamber  18  and is located on the opposite side of the sealing element to the perforate membrane. Thus, the fluid flow is able to pass through the opening  17  in the sealing element in order to reach the perforate membrane for dispensing. 
     The rolling sealing  19  is connected between the main body  11  and a plunger portion  22  which is, in turn, connected to the central portion  14  of the sealing element  13 . Movement of the plunger towards and away from the perforate membrane causes flow to be either prevented or permitted through the perforate membrane. The rolling seal  19  has a rolling section  23  which moves with the plunger thereby allowing the plunger to move within the chamber, but the seal maintains the fluid integrity of that chamber. 
     The central portion of the sealing element has a substantially flat sealing face  24  which contacts the perforate membrane over the region of the perforations, such that no fluid flow is permitted through those perforations. Alternatively and/or additionally, a circumferential bead  25  may be provided on the central portion  14  of the sealing element such that this surrounds the region of the perforate membrane having perforations in order to prevent fluid flow from the chamber  18  through those perforations. 
     The sealing element is preferably formed from some compliant material or at least the flat sealing face  24  and/or the circumferential bead  25  are formed from compliant material in order to provide a better seal with the perforate membrane. In order for the central portion  14  of the sealing element  13  to move relative to the perforate membrane, other portions, such as the spokes, of the sealing element  13  must be flexible. 
     The rolling seal  19  is held in place by means of a clamp block  26 . An activation button  27 , biased to an outward position (the right in  FIG. 1 ), is connected to a magnet which, in the at rest position is spaced from a secondary magnet  29  such that the magnets are not attracted to each other. By pressing the activation button  27  such that magnet  28  moves closer to magnet  29 , the two magnets are caused to attract one another such that magnet  29  is caused to move towards magnet  28 . Magnet  29  is, although not shown in  FIG. 1 , connected to the plunger  22  thereby causing the central portion  14  of the sealing element to be moved away from the perforate membrane. This enables fluid to flow through the perforate membrane once it has been actuated. Biasing means  30  and  31  are provided to separate the magnets  28 ,  29 . Biasing means  30  returns the central portion  14  of the sealing element  13  into contact with the perforate membrane, once the activation button  27  has been released by a user. Biasing means  31  is sufficiently strong to overcome the attraction of the magnets and will separate them once a user has released button  27 . 
     When the actuation button  27  is pressed, and activation arm  32  is brought into contact with a switch  21  which activates an actuator for causing the perforate membrane to vibrate. This actuator is typically a piezoelectric actuator or some other electronically driven actuator and can be seen in  FIG. 3 . 
     A simplified schematic of a slightly different design is shown in  FIG. 2   b , but like reference numerals have been included.  FIG. 2   b  does show the provision of and location of a perforate membrane  33  and the central portion  14  of the sealing element  13  can be seen in an at rest position in which the sealing face  24  is in contact with the perforate membrane  33 . 
     In the example in  FIG. 2   b , biasing means  30 , typically taking the form of a spring, is located within chamber  18 , whereas in  FIG. 1  it is external to the chamber. The location of the biasing means is not important. 
     Further simplified mechanisms for causing a sealing element to be moved into and out of engagement with a perforate membrane are shown in the following figures. In the following figures, the perforate membrane is shown having a domed perforate portion, whereas in  FIG. 2   b , the perforate membrane is substantially planar. The exact form of the perforate membrane is not important, but it is important that the sealing face of the sealing element prevents fluid flow through the perforations of the perforate element. Thus, it is preferable for the sealing face to conform to the shape of the perforate portion of the perforate membrane as this will minimise or preclude there being any small retained volume which can then leak from the perforate membrane, but, as described above, the seal may simply be made by way of a circumferential bead extending around the perforate portion of the membrane. 
     Turning now specifically to  FIGS. 3   a  and  3   b , a perforate membrane  40  is mounted in a substrate  41  and, on substrate  41 , a piezoelectric element  42  taking the form of an annulus is provided. Actuation of the piezoelectric annulus causes the substrate and subsequently the membrane to vibrate causing fluid to pass through the perforate membrane. The actuator in its broadest sense takes the form of a composite thin walled structure which is arranged to operate in a bending mode. 
     The perforate membrane and substrate are mounted to the walls  43  of a chamber  44  and a sealing element having a central, membrane sealing portion  45  is provided within chamber  44 . In this example, the sealing element also includes a pair of arms  46  that extend away from the central portion  45  to locations external to the chamber  44 . In this example, the arms are formed integrally with the walls  43  of the chamber and, as such, no additional sealing is required at the point at which the arms  46  pass through the walls  43 . However, the arms may simply pass through holes in the chamber wall  43 , as long as appropriate seals are provided to prevent fluid exiting the chamber at those points. 
     The connection  47  of the arms  46  with the walls  43  is by way of a relatively thin section of wall  43 , such that, as can be seen in  FIG. 3   b , the arms can be flexed at the joint, like a hinge, so as to cause the central membrane sealing element  45  to be moved away from the perforate membrane  40  in order to permit fluid flow from the chamber  44  out through the perforate membrane. In addition, the connection  47   a  between the arms  46  and the central portion  45  is notched so as to form a hinge portion  48 . The seal mechanism shown in these figures also clearly highlights another benefit of this invention, the fact that any pressure difference across the membrane creates a beneficial biasing force that assists in sealing. 
       FIGS. 4   a  and  4   b  show an alternate embodiment in which the seal with the perforate membrane  40  is provided by way of a plunger  50  on which an integrated membrane seal and sliding seal element  51  is mounted by way of a notch  52  in the plunger  50  and a corresponding projection  53  on the inner portion of the seal  51 . The seal  51  is provided with a membrane sealing portion  54  and a sliding seal  55  such that the plunger is movable within channel  56  defined within the main body  11 . The remainder of main body  11  is not shown, but, as with other examples, a chamber into which fluid can be supplied to an inlet is provided and is defined, typically by the main body  11  and the perforate membrane and substrate.  FIG. 4   b  shows the plunger in a position in which it has been moved away from the perforate membrane in order to permit fluid to be dispensed. In this case, the sliding seal  55  has simply slid along the inner wall of the channel  56 , thereby maintaining the fluid tight seal to prevent fluid exiting the chamber past the plunger  50 . 
       FIGS. 5   a  and  5   b  show a similar embodiment to those of  FIG. 4 , but in which, rather than mounting a sliding seal  55  on the membrane sealing element connected to the plunger  50 , a sliding seal  57  is mounted to the wall of the chamber. Thus, as can be seen in  FIG. 5   a , when the plunger is in the at rest position with the membrane sealed, the sliding seal  57  mounted on the wall of channel  56  is in contact with the sealing element mounted on the end of plunger  50 . As the plunger is withdrawn as shown in  FIG. 5   b , the sliding seal  57  runs along the outer portion of the seal mounted on the end of plunger  50  thereby maintaining the fluid tight integrity. 
     In  FIGS. 6   a  and  6   b , a rigid membrane seal plunger  60  is provided and this is movable into and out of engagement with the perforate membrane  40 . The seal plunger  60  does not include a seal (as in  FIG. 4 , item  51 ) and is a metal or plastic part shaped to fit the profile of the membrane  40  and provides a membrane seal with the membrane sealing element  14 . The membrane sealing element  14  is integrated with the head-mount seal to make a single component that can be easily assembled with the spray head. 
     The motion of the seal plunger could be could be constrained by a sliding seal ( 57 ) mounted on the wall of the channel  56  as shown in  FIGS. 5   a  and  5   b , or by other means, as shown in  FIG. 10 . 
       FIG. 7  shows one method of actuating the plunger shown in  FIGS. 3   a  and  3   b . In this example, a shape memory alloy actuator  80  is mounted within the chamber  44  and is connected to an end wall  81  and to at least one of the arms  16 . An external portion of one of the arms  16  is connected to a biasing means, in the form of a spring  82 , which is, at its other end, connected to a mounting surface  83 . Thus, upon actuation of the shape memory alloy, the shape memory alloy  80  contracts, thereby drawing the membrane sealing element  45  away from the perforate membrane  40 , as shown in  FIG. 6   b . This contraction of the shape memory alloy  80  causes spring  82  to extend and apply a restoring force to arm  16  which, upon de-activation of the shape memory alloy, causes the membrane sealing element  45  to return to a position in which it seals against perforate membrane  40 . 
     An alternative actuation method is shown in  FIGS. 8   a  and  8   b  in which the membrane sealing element  45  is mounted to a plunger  50  which extends through end wall  81  of the chamber  44 , a bellows type seal  84  or a rolling seal is provided between plunger  50  and the opening  85  and the end wall  81  of the chamber. The plunger is then connected to a lever arm  86  which can pivot about pivot  87 . A shape memory alloy  88  is connected between a rigid surface and the lever  86  and, on the opposite side of the pivot  87 , a return spring  82  is also connected between the lever and the rigid mounting point  83 . Thus, as with the example in  FIG. 7 , actuation of the SMA causes it to contract, drawing the plunger away from the membrane, thereby permitting flow to exit through perforate membrane  40 . When the SMA is de-activated, the return spring  82  causes the lever to pivot back to the at rest position causing the plunger to be moved into the chamber  44  and for the sealing element to contact the perforate membrane again. 
     In  FIG. 9 , a further construction using an external SMA actuator is shown and in this example the SMA actuator  88  is positioned such that, when un-activated, the spring  82  causes the arm  16  to be in a position in which the sealing element  45  is in contact with the perforate membrane  40 , but upon activation and therefore contraction of the shape memory alloy  88 , the arm  16  is caused to deflect against the biasing force of spring  82  and cause the membrane sealing element  45  to be moved away from the perforate membrane  40 . Upon de-activation of the SMA, the return spring then causes the seal to be reformed. 
       FIGS. 10   a  to  10   c  illustrate a further actuation method similar to that shown in  FIG. 1  in which an external activation button  90  is connected a magnet  91  and is biased outwardly by spring  92 . The sealing element  60 , similar to that shown in  FIG. 6 , is biased into contact with the perforate membrane  40  by way of a spring  30 . A further magnet  93  is provided on the end opposite to the membrane seal and, in the at rest position, magnets  92  and  93  are separated by sufficient distance that they do not attract one another. However, upon activation of button  90 , magnet  91  is brought sufficiently close to magnet  93  that they are attracted and, as magnet  91  is prevented from further movement by way of locking element  94 , magnet  93  is drawn towards magnet  91 . This therefore causes the plunger on which the sealing element is mounted to move away from the perforate membrane in order to permit flow through the perforate membrane  40 .