Acoustic pump

An acoustic pump including a resonant member having low internal damping and asymmetrically tapered to a thin edge; a piezoelectric driver mounted on the resonant member; and means for applying an alternating voltage to the piezoelectric driver in the resonant range of the resonant member for vibrating the resonant member and pumping fluid away from the thin edge.

FIELD OF INVENTION 
This invention relates to an acoustic pump, and more particularly to such a 
pump which is ultrasonically piezoelectically driven. 
BACKGROUND OF INVENTION 
Piezoelectric blade blowers are known which are much smaller than the 
smallest rotary fans and are used to cool electronic equipment. These 
blowers are highly efficient, have long life, generate little noise or 
magnetic interference and are approximately two inches by one inch by 
three quarters of an inch in size. However, they too have drawbacks. They 
are not small enough for direct mounting on printed circuit boards and 
they require a 115-volt, 60-line current, which introduces 60 Hz magnetic 
noise in the circuit boards as well as requiring that a 115-volt source be 
made available at the board. Attempts to use a piezoelectric crystal 
directly to pump air by acoustic streaming have also been less than 
successful because large crystals are required which are difficult and 
expensive to obtain in production. Acoustic streaming results from the 
fact that air accelerated by an oscillating surface does not reverse its 
direction when the surface does, due to inertia and compressibility, 
further complicated at higher amplitudes by turbulence and vortex 
formation. 
SUMMARY OF INVENTION 
It is therefore an object of this invention to provide an improved smaller, 
highly efficient, high velocity acoustic pump. 
It is a further object of this invention to provide such a pump which may 
be mounted directly to a printed circuit board and is comparable in size 
to the components it cools. 
It is a further object of this invention to provide such a pump which 
operates on low voltage. 
It is a further object of this invention to provide such a pump which 
operates in the ultrasonic range virtually inaudibly and without 
vibration. 
It is a further object of this invention to provide such a pump which 
produces very high airflow. 
It is a further object of this invention to provide such a pump which has 
virtually unlimited service life, no magnetic disturbance, no heat 
generation and does not draw a high starting current. 
It is a further object of this invention to provide such a a pump which is 
mountable on a printed circuit board and pumps parallel to the board. 
It is a further object of this invention to provide such a a pump which may 
make use of acoustic streaming. 
This invention results from the realization that a truly effective, small, 
high-velocity, high-volume piezoelectric pump can be made by using a 
resonant member with low internal damping and asymmetrically tapered to a 
thin edge driven by a piezoelectric member to vibrate the member in an 
open node pattern that intersects with the thin edge. 
This invention features an acoustic pump which includes a resonant member 
with low internal damping and asymmetrically tapered to a thin edge. A 
piezoelectric driver is mounted on the resonant member. There is some 
means for applying an alternating voltage to the piezoelectric driver in 
the resonant range of the resonant member for vibrating the resonant 
member and pumping fluid away from the thin edge. 
In a preferred embodiment the piezoelectric driver is mounted on the 
resonant member remote from the thin edge and the resonant member vibrates 
in a node line pattern which intersects with the thin edge. The node line 
may be an open node line pattern, may be generally circular and may have 
two inflection points near its intersection with the thin edge. The 
resonant member may be mounted remote from the thin edge. The node line 
pattern may intersect with the thin edge. The perforated plate may be 
mounted on the resonant member above the inflection points. The perforated 
plate may also be mounted below the tapered surface and may be planar or 
have other configurations, such as an inverted V channel. 
Preferably there is a perforated plate spaced above the tapered surface. 
The perforated plate is located at a position of dynamic equilibrium 
between the acoustic pressure exerted away from the surface and the recoil 
pressure exerted toward the tapered surface. The perforated plate may be 
loosely mounted above the tapered surface to permit the plate to seek its 
position of dynamic equilibrium between the acoustic pressure exerted away 
from the surface and the recoil pressure exerted toward the tapered 
surface. 
The resonant member may be asymmetrically tapered to two thin edges and it 
may include a generally planar section from which the tapered portion 
extends. The piezoelectric driver may be mounted on the bottom of the 
resonant member, on the top or on a side. The means for applying may 
include an electrode on the opposite side of the piezoelectric driver from 
the resonant member. The resonant member itself may function as one of the 
electrodes for the piezoelectric driver. 
The perforated plate may be made of metal, may include approximately 270 
holes per square inch, and the holes may be approximately 0.007 to 0.01 
inch in diameter. The perforations may be formed with generally conical 
walls converging away from the tapered surface. The node line pattern is 
generally circular and has two inflection points near its intersection 
with the thin edge. The acoustic pump may be used as an ultrasonic blower.

There is shown in FIG. 1 an acoustic pump 10 in the form of an ultrasonic 
blower having a resonant member 12 with an asymmetrically tapered section 
14 that tapers to a thin edge 16. Member 12 also includes a generally 
planar section 18, FIG. 2. A piezoelectric driver 20 is mounted on the 
resonant member remote from the thin edge 16, although it will work close 
to the edge as well. It may be mounted on the bottom, as shown in FIG. 1, 
or on one of the sides 20a or the top 20aa, as shown in phantom in FIG. 1. 
An electrode 22 is provided on the outer surface of piezoelectric driver 
20 and the resonant member, providing it is sufficiently conductive, may 
act as the other electrode for applying an oscillating electric current to 
the piezoelectric driver 20 by means of an alternating current source 24. 
With the application of the oscillating current, tapered surface 14 
vibrates and causes an acoustic streaming effect which pumps air away from 
thin edge 16, as illustrated by the compressive wave fronts 28. The 
overall size of resonant member 12 may be approximately 1.075 inches in 
width, 1.275 inches in length, and 0.25 inch in thickness or height. 
Piezoelectric driver 20 may be made of PZT-58 piezoceramic supplied by 
Piezo Electric Products, Inc., or the equivalent, approximately 0.98 inch 
in diameter and 0.01 inch in thickness. It may be nickel plated on both 
sides to form electrode 22 on one side and a binding surface for 
attachment to the aluminum resonant member 12 using Locktite Type 404 
cement or the equivalent. 
An amplifying membrane, perforated plate 30 with holes 34, FIG. 3, may be 
applied by attaching it with a flexible hinge 32 to resonant member 12 so 
that it floats over tapered surface 26 at the optimum level. This level is 
self-regulating so that when perforated plate 30 is loosely held in place 
it automatically levitates above the oscillating tapered surface 26 until 
it reaches a position of dynamic equilibrium between the acoustic pressure 
exerted away from the surface 16 and the recoil pressure which is exerted 
toward the surface 16. Although the membrane is shown above the surface 
and of generally planar shape, this is not a necessary limitation of the 
invention. For example the membrane may be mounted spaced from the bottom 
of the tapered surface and may take the form of an inverted "V" channel 
30', FIG. 3A, with holes 34' facing in the direction of air movement. The 
flanges 35 may be secured to surface 26' but the perforated portion with 
holes 34', as in other constructions, is spaced above the surface. The 
effect of the amplifying membrane is not fully understood in detail; 
however, it appears that the levitation of the membrane, as explained, 
occurs at the height at which the downward pressure due to ejected air 
just balances the upward pressure due to the stream of entrained air below 
the membrane. It is found that plate 30 works well with approximately 270 
holes per square inch having a diameter of 0.007-0.01 inch. Holes have 
been constructed by punching through a brass plate 0.002 inch thick, 1.075 
inches long, and 0.65 inch wide. Good results have been found when the 
punched holes 34a, FIG. 4, which have conical protrusions 36 which 
converge away from surface 26 and end in ragged edges 38. The acoustic 
pump 10 of FIG. 1 delivers good performance, but its results are even more 
spectacular when a perforated plate 30 is used in combination with it. 
Resonant member 12 is made of a material having low internal damping, or 
high "Q", in the range of 300 and higher, such as tempered aluminum alloys 
or carbon steel. Using the aluminum alloy 2024T-561 driven at its first 
harmonic with a 34 KHz square wave, 12-volt peak-to-peak source, the 
blower consumes 1.3 watts of power and delivers an air flow of 2 ft..sup.3 
/min. at an average velocity of 475 ft./min., and a peak velocity of 1400 
ft./min. with no significant temperature rise. Under these conditions the 
perforated plate 30 levitates at a height of 0.003 inch above tapered 
surface 26. When the levitation height is known plate 30a, FIG. 5, may be 
fixed in position at that point by being clamped in suitable mountings 
which grip it tightly, as shown in mountings 40, 42, or it could be 
gripped in a mounting which only loosely surrounds the edge of perforated 
plate 30a to enable it to self-regulate its height in the same manner as 
permitted by flexible hinge 32, FIG. 3. 
The invention utilizes a node pattern 50, FIG. 6, which is generally 
circular in shape, is open at the thin edge 16 and contains inflections 
52, 54 near edge 16. The perforated plate is preferably located over the 
inflections. The thin edge is necessary in the configuration of the 
resonant member 12 in order to produce the open node pattern which results 
in the high amplitude pumping action that moves the air through the 
acoustic streaming phenomenon. Resonant member 12 may be mounted to a 
printed circuit board or other environmental structure by means of an arm 
60 mounted to the back side 62 remote from tapered surface 26 and 16; or 
it may be mounted by using a node pattern support 70, FIG. 7, such as a 
half round rubber element formed in the shape of node pattern 50 and 
adhered to the underside of member 12 beneath the node line 50. 
Resonant member 12 is not restricted to the particular shape shown in FIGS. 
1 and 2. For example, it may have a generally elliptical shape 12a, FIG. 
8, which provides the same type of node line pattern 50a when it is 
tapered to a thin edge 16a, FIG. 9, and has the same type of tapered 
surface 26a. Elliptical member 12a, FIG. 9, does not have the extra 
generally planar section 18 but includes only the tapered portion 14a. 
Elliptical resonant member 12a may be 0.125 inch thick with a 1.35 inch 
major axis and a 1.25 inch minor axis. 
The resonant member is not limited to a single thin edge and tapered 
surface; for example, as shown in FIG. 10, member 12b may include a planar 
section or slab 18b which has two tapered surfaces 26b and 26bb 
terminating in thin edges 16b and 16bb, which can be used for similar 
acoustic pumping using similar acoustic techniques. 
Other embodiments will occur to those skilled in the art and are within the 
following claims: