Compact piezoelectric fluidic air supply pump
BACKGROUND OF THE INVENTION 
     The present invention relates generally to fluid pumping devices, and more particularly provides a uniquely constructed and operative piezoelectric air pump used to deliver supply air to fluidic devices or systems. 
     Various types of air pumping mechanisms have heretofore been utilized to supply pressurized air to &#34;fluidic&#34; devices--devices which use small, high velocity air jets to perform various control and sensing functions, as opposed to more conventional mechanical or electrical control and sensing devices. However, for a variety of reasons, none of these prior pumping mechanisms has proven to be entirely satisfactory in fluidic air supply applications. 
     As an example, solenoid-operated reciprocating diaphragm pumps have been utilized. This type of pump, though fairly simple in construction, has the disadvantages, in fluidic applications, of undesirable low frequency operation pressure ripple, and acceleration sensitivity due to the relatively high diaphragm mass required. 
     Piezoelectrically driven diaphragm pumps have, for some time, been considered as possible alternatives to electromagnetically driven pumps due to the well-known piezoelectric drive characteristics of lighter weight, greater frequency response and considerably smaller size. However, in practice, the utilization of piezoelectric drives in fluidic air supply pumps has heretofore not resulted in wholly satisfactory structural simplicity, compactness, supply flow characteristics or pumping efficiency. 
     Accordingly, it is an object of the present invention to provide a piezoelectric fluidic air supply pump having, compared to conventional piezoelectric air pumps, improved structural and functional characteristics. 
     SUMMARY OF THE INVENTION 
     In carrying out principles of the present invention, in accordance with a preferred embodiment thereof, a compact piezoelectric fluidic air supply pump is provided which comprises first and second housing members, and a diaphragm member having piezoelectric elements secured, in an opposite polarity relationship, to opposite side surfaces thereof. Each of the housing members has a centrally positioned surface depression formed therein and an outlet channel extending between the depression and the exterior surface of the housing member. 
     With these depressions and channels of the two housing members facing each other in an aligned relationship, the diaphragm member is clamped between the housing members in a position such that it completely separates the depression and outlet channel of one housing member from the depression and outlet channel of the other housing member. 
     When an alternating electrical current is impressed upon the oppositely disposed piezoelectric elements the diaphragm member is caused to laterally vibrate within the assembled housing. This vibration creates alternate outward and inward air pulses through each of the diaphragm-separated outlet channels. An air receiver is provided to capture and collect the outward air pulses, and create therefrom a pulsating air supply stream for delivery to a fluidic device or system. 
     According to a feature of the invention, a peripheral portion of the diaphragm member is extended outwardly of the housing, between the facing outlet channels, and serves as a baffle to prevent air being expelled through either channel from being drawn into the housing through the other channel. 
     According to another feature of the invention, the performance of the pump is substantially improved by clamping the diaphragm member between the housing members only around a peripheral portion of the diaphragm member spaced inwardly from its peripheral edge. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram depicting a piezoelectric air pump embodying principles of the present invention, and a current inverter used to power the pump; 
     FIG. 2 is a simplified circuit diagram illustrating the electrical connections to the diaphragm and piezoelectric portions of the pump; 
     FIG. 3 is a top perspective view of the assembled pump; 
     FIG. 4 is an exploded perspective view of the pump; 
     FIG. 5 is a smaller scale exploded perspective view of the two-piece pump housing; 
     FIG. 6 is a bottom perspective view of the assembled pump depicted in FIG. 3; 
     FIG. 7 is a reduced scale top plan view of the pump; 
     FIG. 8 is a greatly enlarged scale cross-sectional view taken through the pump along line 8--8 of FIG. 7; 
     FIG. 9 is a greatly enlarged scale cross-sectional view taken through the pump along line 9--9 of FIG. 7; 
     FIG. 10 is a partially schematic, greatly enlarged scale cross-sectional view taken through the pump along line 10--10 of FIG. 7, with certain interior portions of the pump being omitted for descriptive purposes, and illustrates the vibratory deflections of the diaphragm member within the pump housing; 
     FIG. 11 is a cross-sectional enlargement of area &#34;11&#34; in FIG. 10; and 
     FIG. 12 is an exploded cross-sectional enlargement of area &#34;A&#34; in FIG. 8. 
    
    
     DETAILED DESCRIPTION 
     Schematically illustrated in FIG. 1 is a compact piezoelectric air pump 20 which embodies principles of the present invention and is used to supply a rapidly pulsating air stream 22 to a fluidic device or system 24. Pump 20 is electrically driven by a suitable current inverter 26 which receives direct current, via leads 28 and 30, and supplies alternating current to the pump via leads 32 and 34, lead 34 having a tuned inductor 36 therein. The inverter is utiIized in typical fluidic applications where only DC electrical power is available. If, however, AC electrical power is available, the inverter 26 may be omitted and AC power can be supplied directly to the pump through the leads 32, 34. 
     As subsequently described in greater detail, the pump 20 includes a thin, disc-shaped metal diaphragm member 38 (FIGS. 2 and 4) having smaller diameter piezoelectric discs 40 and 42 coaxially secured in mutually reversed polarity to its opposite side surfaces. Via other structural elements, AC lead 34 is connected to the piezoelectric discs 40, 42, and AC lead 32 is connected to the diaphragm 38 (FIG. 2). 
     STRUCTURE OF THE PUMP 20 
     Referring now to FIGS. 3 and 4, in addition to the diaphragm 38 and piezoelectric discs 40 and 42, the pump 20 also includes a compact metal housing 44 having a generally disc-shaped upper member 44a and a generally disc-shaped lower member 44b. Housing member 44a is truncated to define a flattened edge 46, while an upper end portion of housing member 44b is similarly truncated to define a flattened edge 48 which projects upwardly from a ledge portion 50 of the lower housing member. 
     The two housing members are clamped together, with the flattened edges 46, 48 in alignment, by suitable fastening means such as rivets 52 that extend through openings 54, 56 respectively formed in the housing members 44a, 44b. Alignment of the flattened edges 46, 48 is facilitated by a circumferentially spaced series of alignment pins 58 which are received in corresponding openings 60 formed in the upper housing member 44a.  Pins 58 project upwardly from the inner surface 62 of the lower housing member 44b. 
     A cylindrical boss 64 also projects upwardly from inner surface 62, along a generally central portion thereof. At the periphery of the upper end of boss 64 is a narrow, upwardly facing annular flattened edge 66 (FIGS. 4 and 12). From the edge 64 the upper end of boss 64 is recessed along a sloping, annular surface 68 which terminates at a circular flat surface 70. A channel 72 is cut into the upper end of boss 64 and extends from the flat surface 70, upwardly along the sloped surface 68, and opens outwardly through the flattened edge 48 via a necked channel outlet 74 of rectangular configuration. 
     Referring now to FIGS. 9 and 12, the upper housing member 44a also has cylindrical boss 76 which projects downwardly from its inner surface 78 along a central portion thereof. Boss 76 is of identical configuration to, and aligned with, the lower boss 64, having an annular flattened edge 80, a sloping annular surface 82, and a circular flat central surface 84. As in the case of the lower boss 64, the upper boss 76 has a channel (not illustrated) which extends from the central surface 84, downwardly along the sloped annular surface 82 and opens outwardly through the flattened edge 46 (FIGS. 4 and 11) via a necked channel outlet 86 of rectangular configuration. 
     The piezoelectric disc 38 is coaxially clamped between the aligned end surfaces 66, 80 of the bosses 64, 76 in a unique manner which significantly enhances the air delivery and various other operating characteristics of the pump 20. This coaxial relation between disc 38 and the boss surfaces 64, 80 is maintained by the alignment pins 58 which prevent the disc from shifting relative to the boss ends. 
     Specifically, the disc 38 is clamped only around an annular portion positioned inwardly of its peripheral edge 88. As can be seen in FIGS. 8-10 and 11, the diameter of disc 38 is slightly larger than the diameters of bosses 64, 76 so that an annular portion 90 of the disc projects outwardly of the side surfaces of the bosses. This peripheral portion of the piezoelectric disc is totally unrestrained, being disposed within an annular housing void 92 positioned between the spaced apart inner side surfaces 78, 62 of the upper and lower housing members 44a, 44b. Similarly, the entire central portion of disc 38 is unrestrained, being positioned inwardly of the boss clamping surfaces 64, 76 in the facing boss recesses defined by surfaces 82, 84 and 68, 70. 
     With the pump housing assembled as shown, the piezolectric disc 38 divides and separates the facing boss end recesses (which collectively define an interior housing chamber) into an upper subchamber 94 and a lower subchamber 96 (FIGS. 10 and 11). Moreover, as can be best seen in FIGS. 3 and 11, the disc 38 divides and separates the facing channel outlets 74, 86 (which collectively define a chamber outlet) and has a portion 98 which projects outwardly of the aligned housing member truncated surfaces 46, 48. 
     Closely adjacent this projecting disc portion 98 is an air receiving member 100 (FIGS. 3, 4 and 11) which is secured to the lower housing member ledge 50 by screws 102 received in threaded openings 104 in the ledge 50. Air receiver 100 has an inlet 106 and an outlet 108 (FIG. 10) which is in registry with an outlet opening 110 (FIGS. 4 and 10) that extends through the housing ledge 50. The receiver inlet 106 faces the aligned channel outlets 74, 86 and extends axially beyond each of the opposite sides surfaces of disc 38. 
     To receive alternating current from the conversion system 26, terminals are provided in the form of a power pin 112 and a ground pin 114, pin 112 being connected to AC lead 34, and pin 114 being connected to AC lead 32. As can best be seen FIG. 9, power pin 112 extends downwardly through a relatively large diameter opening 116 formed in the upper housing member 44a, and is anchored at its lower end to a bushing 118. The bushing 118 is carried by the lower housing member 44b and insulates the power pin 112 from the metal housing. 
     The lower end of the power pin 112 is connected to one end of a flat insulated wire 122 (FIG. 9). From its end connection to power pin 112 the wire 122 is extended along a recess 124 which begins at the lower end of power pin and continues along the underside of housing member 44b, upwardly through each housing member adjacent their peripheries and across the upper side surface of housing member 44a. The opposite end of wire 122 is connected to a metal stud 126, while a central portion of the wire is connected to a metal stud 128. A portion of the wire 122 adjacent its upper end (FIG. 9) is folded over on itself, as at 122a, to facilitate separation of the housing members 44a, 44b during disassembly, as best illustrated in FIG. 5. 
     Stud 126 is received in a bushing 130, carried by a central portion of upper housing member 44a (FIG. 9), which insulates the stud from the metal housing. In a similar manner, the stud 128 is received in a centrally disposed bushing 132 carried by the lower housing member 44b. The inner end of stud 126 is connected to a central portion of an elongated, flexible metal conductor element 134 disposed within subchamber 94, and the inner end of stud 128 is connected to a central portion of an elongated, flexible metal conductor element 136 disposed within subchamber 96. The ends of conductor 134 are bent downwardly into biased engagement with piezoelectric element 40, while the ends of conductor 136 are bent upwardly into biased engagement with piezoelectric element 42. 
     As can be see in FIGS. 8 and 9, the foregoing structure defines between the power and ground pins 112, 114 an electrical current path extending from power pin 112 through the wire 122 to the studs 126, 128, from the studs to the piezoelectric elements 40, 42 through the conductors 134, 136, and from the piezoelectric elements to the ground pin via the metallic disc 38 and the housing members 44a, 44b. Such structure also provides for uniquely advantageous operation of the pump 20 which will now be described. 
     OPERATION OF THE PUMP 20 
     Referring now to FIGS. 8-10, when alternating current is supplied to the power and ground pins 112, 114 via leads 32, 34, the opposite polarity piezoelectric elements 40, 42 cause rapid vibratory lateral deflection of the disc 38 (also referred to herein as &#34;diaphragm means&#34;) within the pump housing interior between the upper and lower deflected positions 38a, 38b indicated by dashed lines in FIG. 10. It should be noted that during such lateral vibration of the disc 38, constant contact is maintained between the flexible conductors 134, 136 and the piezoelectric elements 40, 42 which they engage. Specifically, as the disc 38 is deflected upwardly, the opposite ends of conductor 134 are forced further apart while sliding along piezoelectric element 40, and the opposite ends of conductor 136 move closer together while sliding along piezoelectric element 42. The sliding movement of the conductors is reversed as the disc is deflected toward its downward position 38b. 
     The described vibration of the disc 38 causes alternate compression and expansion of the housing subchambers 94, 96. As the disc deflects upwardly, a high velocity burst of air 138 (FIG. 11) is expelled outwardly through the channel outlet 86 from the subchamber 94. Due to its relatively high kinetic energy, the air 138 is forced directly into the closely adjacent receiver inlet 106. Simultaneously, ambient air 140 is drawn into the expanding subchamber 96. As the deflective direction of the disc 38 reverses, the direction of air flow through the channel outlets 74, 86 is also reversed, causing a high velocity burst of air 142 to be expelled from channel outlet 86. Like its alternating counterpart 138, the air burst 142 is forced into the receiver outlet 106, such air bursts 138, 142 collectively forming the pulsating air stream 22 used as supply air for the fluidic device or system depicted in FIG. 1. 
     At this point several very advantageous features of the pump 20 should be noted. First, the vibrating diaphragm means 38 create two usable supply air streams (138 and 142) during each complete vibrational cycle of such diaphragm means. This is, of course, far more efficient than a variety of conventional diaphragm pumps which can generate a supply air flow only when the particular diaphragm is moving in a single one of its two deflectional direction (i.e., creating only a single burst of supply air during its entire vibrational cycle). 
     Secondly, the use of each of the outlets 74, 76 to supply air to the receiver 100 is achieved without the use of check valve mechanisms of any sort - each of the channels 74, 86 is totally unrestricted. This significant structural simplification vis a vis conventional diaphragm pump construction is achieved in part by a unique dual use of the disc 38. Specifically, the disc is not only used to divide and separate the subchambers 94, 96 and the air outlets 74, 86, but its projecting portion 98 also serves as an air flow baffle interposed between the channel outlets 74, 86 and the receiver inlet 106. Such baffle substantially prevents the supply air burst 138 from being drawn back into outlet 74, and the supply air burst 142 from being drawn back into outlet 86. It also causes the receiver 100 to function, in effect, as a simple fluidic rectifier, helping to guide the air bursts 138, 142 into the receiver inlet 106 while assisting in preventing reverse flow outwardly through such inlet 106. 
     In developing the present invention it was discovered that by clamping the disc 38 only around an annular area positioned inwardly of its peripheral edge, a surprisingly large performance improvement was achieved in the pump 20 in comparison to the conventional method of simply clamping the disc at its periphery. The cause of this unexpected performance enhancement is believed to be that such inward clamping, along the very narrow annular boss end surfaces 66, 80, provides at least a very limited degree of flexural freedom for the unclamped peripheral area of the disc 38 relative to the unrestrained central portion of the disc. In developing the present invention it was also discovered that this performance improvement could be maintained over a wide temperature range by closely matching the thermal coefficient of expansion of the metal housing members 44a, 44b, to that of the metal disc 38. 
     From the foregoing it can be seen that the present invention provides a fluidic air supply pump which eliminates or minimizes a variety of problems and limitations commonly associated with conventional diaphragm and other type pumps proposed for use in fluidic applications. The pump 20 is very compact, relatively simple and inexpensive in construction, light in weight, rugged and efficient--all of which make it particularly well suited to the fluidic air supply applications for which it is intended. 
     The foregoing detailed description is to be clearly understood as given by way of illustration and example only, the spirit and scope of this invention being limited solely by the appended claims.