Patent Publication Number: US-7712634-B2

Title: Apparatus and method for dispensing small quantities of particles

Description:
RELATED APPLICATIONS 
     This application is the U.S. National Stage of International Application No. PCT/GB02/04281, filed 20 Sep. 2002, published in English, which application claims priority under 35 U.S.C. §119 or 365 to Great Britain Application No. 0122935.0, filed 24 Sep. 2001, the entire teachings of the above referenced applications are incorporated herein by reference. 
     The present invention relates to an apparatus and method for dispensing small quantities of particles. 
     The flow characteristics of powders have a tendency to prevent flow of the powder through small holes, for example in a sieve containing the powder, under the action of gravity because the powder particles tend to agglomerate into larger particles However it is well known that shaking the hopper causes the powder to flow. It has been shown that applying discrete movements of a well defined nature to the hopper can cause a reproducible amount of powder to flow through the holes. 
     For example, WO-A-01/33176 discloses an apparatus and method for dispensing small quantities of particles, in particular small amounts of medicament especially in a powder form. The apparatus uses a funnel shaped hopper with a plurality of holes in a membrane at the base of the hopper, forming a sieve-like element, through which powder present in the hopper may fall. A preferred method is to tap the hopper horizontally to cause such a movement, thereby controllably dispensing powder through the membrane. The tapping is achieved by an electro-mechanical actuator which delivers impact energy to the hopper, which in turn causes a small number of particles to fall through the sieve-like element and onto a weighing measuring balance. The actuator is a horizontally oriented solenoid which taps the side of the hopper via a rod which supports the hopper at one end and has the solenoid mounted at the other end. A tapping action can also be done with a vertical component to the action of the actuator or the resultant movement of the hopper. 
       FIG. 1  shows schematically the dispensing head of a precision powder metering system as described in WO-A-01/33176. 
     Referring to  FIG. 1 , the device consists of a hopper  1  for a powder material, for example a medicament used for administration to the lungs of a patient via a powder inhaler. The hopper  1  is of generally frusto-conical form with the larger end  2  open and uppermost. The smaller end  3  is closed by a plate  4  in which a plurality of holes  5  are formed, thereby forming a sieve. When a powder  7  is placed in the hopper  1 , some powder  7  may initially fall through the holes  5  but thereafter, in general, the powder flow stops as the powder  7  jams in the holes  5 . The flow of powder  7  through the holes  5  can be made controllable and reproducible by the choice of appropriate dimensions for the holes to match the properties of the powder. Typically, the holes lie in the range of from 10 microns to 1000 microns. 
     In order to use the apparatus for precision dispensing, a receptacle  8  for the powder  7  is placed under the plate  4  and the hopper  1  is tapped on the sidewall  9  thereof at a location  6 . The tap may be in a form that results from the impact of a mass travelling at a controlled velocity. The resulting motion of the hopper  1  and powder  7  causes the powder  7  to flow through the holes  5  in the plate  4  for a small period of time following the impact, after which the powder flow stops. Thus a discrete amount of powder  7  is controllably dispensed into the receptacle  8  as a result of each tap. 
     In order to accurately dispense a desired total amount of the powder  7 , a plurality of taps are used to fill each receptacle  8  and the total weight of powder  7  dispensed into the receptacle  8  is measured in real time so that as soon as the required amount has been dispensed, the tapping can be stopped. 
     This arrangement however results in the movement of the bulk powder  7  remaining in the hopper  1  as shown in  FIG. 2 . It has been observed that the bulk powder  7  remaining in the hopper  1  slowly moves progressively laterally to one side of the hopper  1 . This can occur to such an extent that when only a small amount of powder  7  remains in the hopper  1 , the powder  7  will not cover all of the holes  5  in the plate  4 . 
     This can lead to the technical problems of an increase in the dispense time or spillage of powder, even if the hopper is frusto-conical with the smaller lower end being provided with the sieve. 
     The present invention aims to overcome these problems of the known apparatus and method. 
     GB-A-2360950 discloses a feed assembly for a particular material comprising an apertured screen positioned in a delivery passage, through which material to be delivered passes, and means for hitting screen to assist in the flow of material. The hitting means is centrally disposed in the delivery passage located downstream, with respect to material flow, of the screen. Therefore the hitting means impacts a central part of the screen. The hitting means can move upwardly to repeatedly strike the screen and may comprise a slidable element supported on a cross-member structure or a magnetizable ball movable by means of an electric coil around the delivery passage. The feed assembly is used to deliver material by gravity to a vibra-feeder device and it is disclosed that the material to be dispensed may be toxic material such as plutonium oxide. 
     The feed assembly disclosed would suffer from problems when employed to dispense very fine powders, such as medicaments, using a precision metering system. This is because the powder may become deposited on the hitting means which is disposed beneath the screen and past which the powder is required to pass for delivery onto the vibra-feeder. This would reduce the accuracy of the precision metering of the powder. Moreover, it would be difficult to prevent any contamination of that medicament by material present or trapped on the hitting means. 
     Accordingly, the present invention provides an apparatus for dispensing small quantities of particles, the apparatus comprising a hopper provided with a sieve at a bottom portion thereof, the hopper in use containing powder to be dispensed therefrom through the sieve, a support for the hopper, the support holding a portion of the hopper so that the hopper can in use be held above a container into which the dispensed powder is to be received, and at least one actuator for delivering impact energy to the hopper for causing powder to be dispensed through the sieve when the hopper receives the impact energy, wherein the at least one actuator is arranged to deliver impact energy to the hopper from different directions and/or at different locations on the hopper. 
     The present invention also provides a method of dispensing small quantities of particles, the method comprising the steps of: disposing in a hopper provided with a sieve at a bottom portion thereof a powder to be dispensed therefrom through the sieve; supporting the hopper by holding a portion of the hopper with a support so that the hopper is held above a container into which the dispensed powder is to be received; and delivering impact energy to the hopper by at least one actuator thereby to cause powder to be dispensed through the sieve when the hopper receives the impact energy, the at least one actuator being arranged to deliver impact energy to the hopper from different directions and/or at different locations on the hopper. 
     This invention accordingly provides the advantage that lateral movement of the bulk powder material in the hopper is substantially prevented such that the uppermost surface of the powder remains substantially horizontal throughout the dispensing of the powder through the sieve. 
     In one embodiment of the invention, the excitation motion, achieved through the impact energy of a tap, is implemented equally in opposing or differing directions but separated in time. For example, tapping the hopper twice with the same force at the same position of the hopper using a common tapping device can achieve this excitation motion if the second tap is in the opposite direction to the first tap. Alternatively, tapping from other positions around the hopper by using a plurality of tapping devices can achieve the same technical effect. 
     In another embodiment of the invention, the excitation motion is successively implemented at different positions of the hopper by relative rotation of a tapping device and the hopper between successive taps from the tapping device. 
     This invention accordingly provides the advantage that lateral movement of the bulk powder material in the hopper is substantially prevented such that the uppermost surface of the powder remains substantially horizontal throughout the dispensing of the powder through the sieve. 
     The present invention is predicated on the discovery by the inventors that the lateral movement of the bulk powder across the hopper can be substantially eliminated if it is ensured that the summation of a large number of taps has no cumulative force or motion component laterally across the hopper. 
    
    
     
       Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:— 
         FIG. 1  is a schematic section, from one side, through a hopper of a known powder dispensing apparatus for dispensing powder into a receptacle; 
         FIG. 2  is a schematic section, from one side, through the hopper and receptacle of  FIG. 1  showing the problem of lateral movement of bulk powder in the hopper; 
         FIG. 3  is a schematic section, from one side, through a hopper and a tapping device of a powder dispensing apparatus in accordance with a first embodiment of the present invention for dispensing powder into a receptacle; 
         FIG. 4  is a schematic plan view of a hopper and a plurality of tapping devices of a powder dispensing apparatus in accordance with a second embodiment of the present invention for dispensing powder into a receptacle; and 
         FIG. 5  is a schematic plan view of a hopper and a tapping device of a powder dispensing apparatus in accordance with a third embodiment of the present invention for dispensing powder into a receptacle. 
     
    
    
       FIG. 3  shows a hopper and a tapping device of a powder dispensing apparatus in accordance with a first embodiment of the present invention for dispensing powder into a receptacle. In this embodiment, a frusto-conical hopper  20  has a sieve  21  at its smaller lower end  22  and a larger upper end  23  for receiving bulk powder  24 , such as medicament, to be dispensed through the sieve  21 . The hopper  20  is supported by a cantilever arm  25 , which is attached to or bears against a sidewall  26  of the hopper  20 . Within the cantilever arm  25  is provided a longitudinally directed cavity  27 , and in the cavity  27  are disposed, in a longitudinally mutually spaced configuration, a pair of longitudinally oriented first and second solenoid coils  28 , 29  of a solenoid  30 , comprising an electro-mechanical actuator. The coils  28 , 29  are rigidly attached to the cantilever arm  25 . An armature  31  of the solenoid  30  comprises a longitudinally extended body having a central bush  32  and two opposed first and second projecting portions  33 , 34 , each of the projecting portions  33 ,  34  extending within a respective one of the coils  28 , 29 , and with the bush  32  centrally disposed between the two coils  28 , 29 . If desired, a pair of opposed helical compression springs (not shown) may be provided, with each spring located between the bush  32  and a respective coil  28 , 29 , thereby to urge the armature  31  into a central position in the absence of any actuating force on the armature  31 . The first and second projecting portions  33 , 34  have respective first and second end walls  35 , 36  which are each spaced from a respective first and second end face  37 , 38  of the cavity  27  when the armature  31  is in the central position. 
     When a first current pulse is passed through the first coil  28 , the armature  31  is accelerated towards the second end face  38  of the cavity  27  and the end wall  36  impacts it. The impact momentum is transferred by the cantilever arm  25  to the hopper  20  and the bulk powder  24  therein and causes a discrete amount of the powder  24  to fall into a receptacle  39  located, in use, beneath the sieve  21  of the hopper  20 . Thereafter, when a second current pulse is passed through the second coil  28 , the armature  31  is accelerated towards the first end face  37  of the cavity  27  and the end wall  35  impacts it. The impact momentum is again transferred by the cantilever arm  25  to the hopper  20  and the bulk powder  24  therein and causes a discrete amount of the powder  24  to fall into the receptacle  39 . Accordingly, alternate energising of the two coils  28 , 29  causes the armature  31  to move in opposite directions in an alternating manner. 
     With this arrangement it is possible to tap the hopper  20  in either direction along the cantilever arm  25 . Accordingly, powder dispensing may occur either by alternating the direction of tapping in successive tapping steps corresponding to successive powder dispense actions or alternatively by always using a pair of taps closely separated in time in a single tapping step to achieve a single powder dispense action. 
     The use of a solenoid  30  to generate the impact on the hopper  20  and the bulk powder  24  therein allows the magnitude of the impact to be altered by controlling the voltage driving the first and second coils  28 , 29  of the solenoid  30 . Thus even if the mechanical arrangement causes some difference between the magnitude or effect of the forward and reverse taps associated with the energisation of the two coils  28 , 29 , the overall cumulative effect can be balanced by using different forward and reverse drive voltages. The same effect can be achieved by changing the pulse width, i.e. the period of time during which each coil  28 , 29  is switched on. 
     In other embodiments of the invention, it may be advantageous in some instances to use a different actuator arrangement to stimulate powder flow and as such the means of averaging the direction of excitation would be altered for optimal performance with that arrangement. 
     Accordingly, a hopper and a plurality of tapping devices of a powder dispensing apparatus in accordance with a second embodiment of the present invention for dispensing powder into a receptacle is shown in  FIG. 4 . 
     In this embodiment, the hopper  40  is supported by a plurality of arms, in the illustrated embodiment three arms  41 ,  42  and  43  arranged at 120 degrees to each other and with the hopper  40  located and supported at the radial centre of the arms  41 , 42 , 43 . Each arm  41 , 42   43  has a respective solenoid  44 , 45 , 46  mounted in it. Each solenoid may be a single direction acting solenoid, or alternatively a double direction acting solenoid as described with reference to the first embodiment. In the three arm illustrated embodiment, each arm only needs a single direction acting solenoid which has a modified structure as compared to the double direction action solenoid of the embodiment of  FIG. 3 . In the single direction acting solenoid, only one coil is provided and a helical compression spring is provided to bias the end wall of the projecting portion remote from the coil away from the adjacent end face of the cavity in the arm. The armature is moved against the bias of the spring to cause a tap of the end wall against the end face of the cavity by energising of the coil. After the current pulse has terminated, the spring bias returns the armature back to its original position. 
     Such an arrangement of plural arms, each with a respective solenoid actuator, enables tilt of the powder in any direction to be compensated for by control of the number and amplitude of the taps from the solenoid associated with each arm. This is of benefit if the hopper axis is not vertical. In contrast, if a tilt of the powder surface orthogonal to a single support arm occurs, this tilt would not be correctable by the provision of a single support arm. 
     Alternatively where it is preferred to maintain the single direction tap using a single cantilever arm then it may be advantageous to incorporate a means by which the hopper is rotated with respect to the arm as it is tapped. Accordingly, a hopper and a tapping device of a powder dispensing apparatus in accordance with a third embodiment of the present invention for dispensing powder into a receptacle is shown in  FIG. 5 . 
     In this embodiment, a hopper  50  is free to rotate in a circular hole  51  defined in an annular portion  52  of a cantilever arm  53  by means of bearings  54  which are provided between the hopper  50  and the annular portion  52 . A ratchet mechanism  55  is provided around the annular outer surface  56  of the hopper  50 . The ratchet mechanism  55  comprises a plurality of angularly spaced radial projections  57 . The projections  57  engage with an armature  58  of an actuating solenoid  59  mounted on the arm  53 . The solenoid  59  is a single direction acting solenoid as described hereinabove. Accordingly, when the solenoid  59  is energised it not only delivers the impact energy to dispense the powder through a sieve  60  of the hopper  50  but also rotationally moves the hopper  50  incrementally through an angle defined by the angular separation of the projections  57  of the ratchet mechanism  55 . 
     In this way the direction of impact is averaged over the complete circumference of the hopper  50  once the hopper  50  has completed one revolution. This ensures that the upper surface of the bulk powder in the hopper  50  is substantially level. 
     Although the actuator comprising the hopper impact mechanism is described in the illustrated embodiments as a solenoid, this is only one possible actuator. Alternative actuators may comprise an electric motor and cam; a piezoelectric actuator; or a voice coil linear actuator. Alternative arrangements can include a vertically directed solenoid or linkage such that the horizontal action of the solenoid causes the hopper to have a vertical as well as a horizontal response to the tapping action.