Abstract:
A new class of precision powder feeders is disclosed. These feeders provide a precision flow of a wide range of powdered materials, while remaining robust against jamming or damage. These feeders can be precisely controlled by feedback mechanisms.

Description:
This invention was made with Government support under Contract DE-AC04-94DP85000 awarded by the U.S. Department of Energy. The Government has certain rights in the invention. 
    
    
     BACKGROUND 
     The present invention relates generally to devices to deliver powder for a process, and more specifically to a new class of powder feeders which deliver a wide range of powdered materials at a substantially constant rate. 
     Traditional powder feeders incorporate as part of their action the movement of powder between moving surfaces. Examples include screws, gear teeth, rotating perforated disks, or other such mechanisms which move the powder from a reservoir at a substantially constant rate. Tight mechanical tolerances are required so that such devices can provide a controlled feed rate, but these same tolerances render the device susceptible to jamming as powder collects in bearings or between moving surfaces. In addition, if hard or abrasive powders are to be delivered, the rate of wear of the mechanical components of the powder feeder can be unacceptably large. Wear is not only to be avoided for the survival of the powder feeder, but also to avoid contaminating the powder being fed. Alternately, overly soft powders can agglomerate and clog these mechanisms. 
     Jamming in conventional powder feeders is also exacerbated by feeding powder with a range of particle sizes. If the powder size is smaller than the separation between the moving metal surfaces in the feeder, it can collect, e.g., between bearing surfaces, causing damage and contamination of the powder by galling. If the powder size is too large, individual particles can jam and/or break the apparatus. 
     There is a need for a powder feeder which is robust in operation, and can supply a precisely defined and controlled delivery of powder. Desirable characteristics for such a powder feeder, based on the difficulties seen in prior art feeders, would include a minimum of moving parts, a delivery mechanism suited to feedback control of delivery rate, and a mechanism which does not undergo damage when the powder flow becomes jammed. More preferably, the feeder should possess a mechanism through which the feeder can be unjammed with a minimum of interruption. 
     SUMMARY 
     The present invention is of a precision powder feeder in which a powder falls from a powder reservoir through an orifice. The powder flow then impinges on a powder disperser, which can take the form of an inclined ramp. The flow then fans out into a cone-shaped sheet on said ramp. A powder collector is then moved in and out of the path of the sheet of powder, collecting that portion of the powder which falls into the collector and delivering it to the apparatus being served by the powder feeder. The collector position can be controlled by a feedback signal obtained from a powder mass rate sensor, thereby providing a constant rate of flow. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a schematic diagram of a precision powder feeder according to the present invention. 
     FIGS. 2 a  &amp;  2   b  illustrates several implementations of a powder fluidization mechanism according to the present invention. 
     FIG. 3 illustrates a powder flow cutoff mechanism suited to the present invention. 
     FIGS. 4 a ,  4   b  &amp;  4   c  illustrates several implementations of a powder disperser according to the present invention. 
     FIG. 5 illustrates the action of a powder feeder according to the present invention with a powder diverter. 
     FIG. 6 illustrates schematically a feedback control system for the present invention. 
    
    
     DETAILED DESCRIPTION 
     A precision powder feeder according to the present invention is shown in FIG.  1 . Powder reservoir  100  directly feeds orifice  101  by gravity flow aided by fluidization of the powder in said reservoir through the action of vibrator  120  on said reservoir. Powder drops through orifice  101  at a substantially constant rate of flow, thereby forming powder flow  102 . Powder flow  102  is incident on powder disperser  103 , which in this implementation is a substantially flat inclined plane. In the process of sliding down powder disperser  103 , the flow of powder  102  spreads out into a cone-shaped sheet. Studies of such particle flows have shown the resulting distribution of powder to be quite stable over a considerable range of operating conditions. As the flow reaches the edge of the powder disperser, the flow density varies as a function of position across the edge, but is substantially constant at any one position as a function of time. 
     Positioned below the output edge of powder disperser  103  is waste powder collector  104 , positioned so as to collect substantially all portions of the flow of powder  102  which are not diverted to feeder output  116  of the powder feeder. In the implementation shown in FIG. 1, this waste powder is directed into waste powder gas entrainment device  105 , where it is mixed with a carrier gas added through inlet  106 , and then returned to the powder reservoir  100  via waste powder return manifold  107 . 
     A portion of the flow of powder  102  which falls from the output edge of powder disperser  103  is collected through collection aperture  109  by powder collector  108 . The powder collector is mounted on strut  110 , which itself is rotably mounted on shaft  111  and can be rotated through the action of drive/encoder  112 . This allows the powder collector to collect a portion of the flow of powder  102  which varies according to the degree of rotation about shaft  111 . 
     The powder collected by the powder collector is directed to powder entrainment device  113 , where it is thoroughly mixed with carrier gas added through inlet  114 . The resulting powder-gas mixture is eventually directed to the feeder output  116 . 
     One of the goals for powder feeders according to the present invention is to minimize the role of moving parts in the delivery of powder, while maintaining the rate of delivery at a substantially constant value. The powder feeder as described to this point functions remarkably well, despite having no moving parts during routine operation. 
     There are some classes of applications for powder feeders, however, that require a higher degree of control over and uniformity of the powder flow output. To serve such applications, a feedback control system can be added to the powder feeder. 
     The rate at which carrier gas is added to the powder entrainment device is measured by a gas flow sensor (not shown). This powder-gas mixture is directed toward a mass flow sensor  115 , which measures the total mass flow of the powder-gas mixture. From the mass flow sensor  115 , the powder-gas mixture is directed to feeder output  116 . 
     The difference between the total mass flow and the rate of carrier gas addition is the powder flow rate. This data is transferred to feedback controller  118  through cabling means  117 . Feedback controller  118  compares the actual powder flow rate to a target powder flow rate, and sends a correction to drive  112 . If the powder flow rate is too small, the drive rotates shaft  111  in a clockwise direction, so that more of the flow of powder  102  enters collection aperture  109 , and vice versa. This type of feedback control mechanism has proven extremely reliable in powder feeders after the present invention. Note that, although some relative motion of parts is required to implement the feedback control, this motion is not the type that leads to clogging or jamming of the powder feeder. 
     Although powder feeders as described above operate well, they are not the only manner in which the present invention can be implemented. FIG. 2 shows alternate approaches toward fluidization of the powder in the powder reservoir  100 . Of course, one approach is not to attempt to fluidize the powder. Many powders will flow perfectly well without such fluidization. If it is useful, however, other approaches to fluidization than vibration of the reservoir can be used instead of or in addition to vibrator  120 . 
     FIG. 2 a  shows a approach toward injecting gas directly into the powder near the orifice  101  so as to effectively fluidize said powder. Here we have a powder reservoir  100  with a hollow wall, a portion  200  of which is porous. A mass of powder  201  partially fills the reservoir, eventually to pass through orifice  101  under the influence of gravity (hydrodynamic effects can enter as well when gas injection is used). When gas is injected into the interior of reservoir  100  via inlet  202 , some gas escaped through the porous inner wall  200 . This gas fluidizes the powder near the orifice  101 , thereby enabling smoother and more consistent flow. 
     FIG. 2 b  shows a different structure aimed at the same result, that of fluidization of the powder near the orifice  101 . Here powder  201  is held within a simple reservoir  100 , which is closed at the bottom by nozzle  203 . Nozzle  203  comprises an orifice for escape of the powder, a hollow portion fed with gas through inlet  204 , and at least a single nozzle which releases gas into the powder near the orifice. Again, the gas so injected serves to fluidize the powder near the orifice. 
     Although the flow of powder can always be shut off by positioning powder collector  108  so that collection aperture  109  does not intersect the flow of powder  102 , this technique is often too slow for a desired application. It can be useful, therefore, to include a rapidly-acting powder flow shutoff. This function is often served by placing a solenoid valve into the path of the feeder output. In FIG. 3 the precise location is given as between the powder collector and the orifice, but many other locations are suitable. 
     Here a pile of powder  300  sits in a powder reservoir  100 . The operation of the powder feeder requires that this powder eventually flows through orifice  101 . These two components are connected by a solenoid valve, which comprises a solenoid  301  with a plunger  302 , positioned so as to block off access to the orifice  101  when the plunger is thrust into valve seal  303 . Many other approaches toward powder cutoff mechanisms will be clear to one skilled in the art. 
     In the implementation of FIG. 1, the powder disperser  103  is simply an inclined flat plate. Beyond having suitable surface properties (e.g., the surface of the plate should not adhere to the powder particles), this is perhaps the simplest approach to obtaining the required functionality. It is possible to imagine rather complex powder dispersers, such as cones rotating about their axis, flat plates spinning powder off its edge, or shaker tables with a dispersing grid of openings at the bottom. Although such complex dispersers would function in the present invention, they are not described in detail because their complexity detracts from the simple designs made possible by the present invention. 
     There are other powder dispersers which are as simple as that of FIG. 1, but offer a different sort of pattern to the flow of powder. FIG. 4 a  shows a powder disperser in the form of a curved inclined plane  401 . When a flow of powder  400  drops onto powder disperser  401 , the curvature of the surface (shown as convex relative to the powder) serves to spread the flow of powder  402  over a wider area than is seen when a flat plate with the same inclination is used. 
     FIG. 4 b  shows a powder disperser  404  in the form of a (nominally) flat plate comprising a number of vertical shafts  405  positioned so as to interact with the flow of powder  402 . The location of these shafts allows one to influence the distribution of powder over the surface. In the figure, shafts  405  are positioned in a pattern which will form the flow of powder roughly into a binomial distribution. 
     Similarly, FIG. 4 c  shows a powder disperser  406  in the form of a nominally flat plate comprising a number of shallow channels  407 . The purpose is the same as that described above—to control the distribution of powder. 
     Other types of powder collectors can be used while satisfying the essential purpose of this component—to capture an operator-variable portion of the flow of powder. An example is shown in FIG. 5, where the flow of powder falls into a double-chambered receptacle  500 , comprising a waste receptacle  501 , an output receptacle  502  and a pivoting powder diverter  503 . The purpose of the powder diverter is to change the relative amount of the flow of powder which is captured by the waste receptacle and by the output receptacle. In FIG. 5 diverter  503  can take the simple form of a flap whose width essentially equals the internal depth of the double-chambered receptacle, mounted with said receptacle so that any powder falling on the right side of the diverter (as shown) reaches the output receptacle, whereas any powder falling on the left side of the diverter enters the waste stream. Clearly, as the diverter is pivoted, the amount of powder reaching the output changes. This adjustment technique is compatible with remote drive and control. 
     Precision powder feeders according to the present invention can be combined as components in a multiple powder feeder and mixer. Such combination devices can be made in many ways, one of which is illustrated in FIG.  6 . Here, the feeder outlets of three precision powder feeders according to the present invention,  601 ,  602 , and  603  are attached to powder combining manifold  600 . Within powder combining manifold  600 , the streams of powder-gas mixtures output from each of the three powder feeders are mixed together, possibly with the injection of additional carrier gas through inlet  604 , and ejected through manifold outlet  605 . Each of the powder feeders is functionally connected to a mixture controller  606 , which converts an operator-set desired mixture of powders into control values, then sets the powder mass flow rate controllers of the individual powder feeders so as to obtain the desired output mixture. 
     The examples and implementations described above are intended to illustrate various aspects of the present invention, not to limit the scope thereof. The scope of the invention is set by the claims interpreted in view of the specification.