CA1244807A - Solid state blower - Google Patents

Abstract

Applicant: Kolm et al For: Solid State Blower ABSTRACT OF DISCLOSURE

A pumping device comprising: a housing; piezoelectric element having one end mounted to the housing and one end free;
a generally planar impeller blade connected to the free end of the piezoelectric element and having its distal end unconstrained by the housing; the blade having a high Q factor, a high stiffness-to-weight ratio and a low mass per unit area substantially less than that of the piezoelectric element; a voltage is applied to the piezoelectric element for oscillating its free end perpendicular to its plane at or close to resonance and propagating a traveling wave along the blade to generate and shed vortices at the distal end of the blade.

Description

~ ~2~8~7, ~.

FIE~D OF INVENTION
This invention relates ~o a piezoelectric blower and more particularly to such a blower having an improved impeller blade.

RELATED CA$ES

This application corresponds to United States Patent No. 4,498,851 which issued February 12, 1985.

BACKGROUND OF THE INVENTION
Electronic equipment is customarily cooled using rotary fans or blowers, which circulate air through the entire housing to maintain a constant operating temperature. Steady state temperature maintenance of the electronic components is important not only ~o prevent overheating, but also to assure reliable operation.
Most electronic equipment now contains only solid state electronic componentsl such as miniaturized transistors and integrated circuits, and no longer utilizes vacuum tubes and other generally large heat producing components. The amount of cooling required to maintaln stable operating temperatures has therefore been substantially reduced. Also, the cooling .

~ -3 re~uirements have been localized, since only several very small components, typically mounted on printed circuit boards, actually require cooling. Thus-, cooling of the entire cabinet is not required. Nevertheless, even though wasteful, electroni~
equipment has continued to be cooled in this manner, since neither rotary fans nor o~her cooling devices have successfully been miniaturized, and rotary fans, which have been substantially improved over the years, continue to offer the most reliable and efficient method of cooling. Comparatively, however, when used in solid state electronic equipment, rotary ans or blowers stand out as the largest, noisiest, and most short-lived part of the assembly, the only moving component, and the componen~ which most severely limits environmental tolerance specifications.
Another form of blower, using the principle of a vibrating blade, has been proposed in ~he past. Austrian Patent No.
167,983 to Anderle, and U.S. Patent No. 4,063,826 to Riepe are typical of such designs. In the Riepe patent a flexible blade is driven magnetically to deflect from side to side. The blade bends back and forth about a node point. The flapping end of the blade to the outside of the node point is disposed in a pumping duct to pump liquid through the duct. In the Anderle patent, a flexible blade is fixedly mounted at the inlet end of a blower duct and driven magnetically from side to side.
Theoretically, due to the few moving parts, blowers of these types are susceptible of minia~urization; as a practical matter, however, ~hey are generally s~ ine~icient that they are better suited for producing heat ~han for generating cooling air movement, with the result ~hat none has found any significant commercial acceptance.

SI~ RY OF INVENTION
It is therefore an object of this invention to provide an improved, highly efficient, inexpensive, and reliable piezoelectric blower.
It is a further object of this invention to provide such a piezoelectric blower having highly effective impellPr blade motion far in ~xcess of that obtainable from the piezoelectric element alone.
It is a further object of this inven~ion to provide such a piezoelectric blower in which the impeller blade is driven with traveling wave motion.
It is a further object of this invention to provide such a piezoelectric blower in which the traveling wave motion of the impeller blade generates and sheds vortices which move fluid without valves or ducts.
The invention resul~s from ~he realization that an impxoved piezoelectric blower can be achieved using a generally planar impeller blade connected to the free end of a piezoelectric element and having its distal end unconstrained by any P_~ -5-

2~L4~37 surrounding housing, wi~h the blade having a high Q factor, a high stiffness-to-weight ratio and a mass per unit area substantially less than that of ~he piezoelectric element.
The invention features a pumping device including a housing, a piezoelec~ric element having one end mounted for the housing and one end free, and a generally planar impeller blade connected to the free end of the pie~oelectric element. The distal end of the impeller blade is unconstrained by the housing. The blade has a high Q factor, a high stiffness-to-weight ratio and a mass per unit area substantially less than that of the piezoelectric element. There are means of applying a voltage ~o the piezoelectric element for oscillating its free end perpendicular to its plane at or close to the resonance frequency of the cantilevered blade and propagating a traveling wave along the blade to generate and shed vortices at the distal end of the blade where it is unconstrained by the housing.
In the preferred embodiment, the traveling wave propogated along the blade is a ~uadrature wave, the Q factor is at least 10 and the ~tiffness-to-density ratic of the blade i~ more than one mi llion newton-meters per kilogram. The blade and piezoelectric element are of uniform width and thickness and the mass per unit area of the blade is less than sîxty percent of the mass per unit area of the piezoelectric element.

3L2~ 8~7 The piezoelectric element or bilaminat~ applies a sinusoidal driving force to ~he blade for propagating a tr~veling flexure wave along the blade, preferably in a ~uadrature relation. The entire length of the blade is thus free to move la~erally as it is driven back and forth by the piezoelectric element. The piezoelectric bilaminate is a strip consisting of two layers of piezoelectric ceramic polariæed in opposite directions which on their facing sides are separated by a conducting layer and on their ~utside ~aces are surrounded by ~onducting layers. The two outside conducting layers are connected as electrodes to a controlled alternating current supply Since the piezoelectric layers have opposite polarity, voltage applied across the bilaminate strip induces bending of the element. Accordingly, alternating voltage acxoss the piezoelectric element drives the blade back and forth at the point of attachment. More than two layers of ceramic may be used if desired, and connected in parallel to lower the operating voltage.
The blower operates without any substantial mechanical friction to permit hi~h opexating speed, high throughput relative to size, virtually unlimited service life, and it may be minitiarized and still produce a significant flow of air to cool miniature components~ In its miniaturized form, the device may be mounted directly on printed circuit boards, alongside the individual components which require cooling, and due its high efficiency it will provide sufficient cooling air.

~, 7 The blower preferably is constructed with a pair of counter-oscillating blades in parallel so that it is dynamically balanced and vibration free.

BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention, reference may be had to the following detailed description of the preferred embodiments, taken in conjunction with ~he accompanying . drawings, in which:
Fig. 1 is a pictorial view of a solid ~tate blower having a pair of blades driven by piezoelectric ~lements according to the invention;
Fig. 2 is a longitudinal-sectional view of a piezoelectric bilaminate driving elemen~ fro use with the blower of Fig~ l;
Figs. 3A, 3B, 3C, 3D, 3E and 3F are schematic representations of first the blade a~ rest and then the pumping motion of the blade, phased in quadrature, at various points of the oscillation cycle;
Fig. 4 is a pictorial view of a modified form of the solid state blower shown in Fig. l;
Fig. 4A is an enlarged detail view of an alternative interconection between the blade and piezoelectric bender;
Fig. 5 is an axonometric view of an alternative embodiment of the blower according to this invention; and - ~ ~ 8-, ~2~ 7 Figs. 6A-I are a series of schematic illustrations of the generation and shedding of. vortices by the blower of this invention.

DETAILED DESCRIPTION OF PREFEXRED EMBODIMENTS
Referring to Fig. 1, a solid state blower according to the present invention has a housing 10, ~uter walls 12a, 12b and bottom 17a and top 17b lifted out of the way for clarity. A
pair of resilient blades 18 having inlet ends 22 and outlet ends 24 are mounted in housing 10.
A piezoelectric bilaminate 28 is a~tached at one end 40, for example by a plastic holder and screws 41, to each of the housin~ walls 12a, 12b and at the other end 42, by cementing or any other suitable means, to a point on each blade 18 to support the blade in the channel 10, in a manner such that upon lateral movement of the bilaminates the blades 18 are free to undergo simultaneous lateral deflection. This mounting arrangement permits free lateral movement of the blade 18 along the entire length with corresponding lateral movement of the end 42 of the piezoelectric element 28.
A piezoelectric element ~uitable for use in the present invention is marketed by Piezo Electric Product6, Inc., Metuchen, N.J., under ~he name "Piezo Ceramic Bender Element, No. G1195n. Each bilaminate strip 28, Fig. 2, has two layers of piezoelectric ceramic 29 ~eparated by n layer of conducting ' ;. 3 ~ ~ ~Z4~7 material 30, e.g. brass. The outside layers 32, 34 are conducting (e.g., nickel, silver) and connected to the leads 36, 38 of a controlled alternating current supply 3g. The tw~
ceramic layers 29 are polarized in ~pposite directions, so that voltage across the bilaminate induces a bending motion in the strip. Since the bilaminate strip 28 is fixed on the housing at 41, controlled alternating voltage causes the free end 42 of the piezoelectric element 28 to move back and forth at the voltage frequency. The bending movement of the bilaminates 28, in turn, drives the blades 18 back and forth at the point of attachment 42 at a controlled rate.
Although not illustrated in FigO 1~ ~he connec~ions from the piezoelectric elements 28 to the power supply 39, Fig. 2, are conveniently made at the end 40, beneath the holder 41.
When driven back and forth, ~he blade 18 represents a beam subjected to combined bending and shearing loads varying so rapidly that inertial effects dominate to propagate a traveling flexure wave along the impeller or blade from the inlet end to the outlet end. Typically a voltage oscillating in the range of 60-400 hz is applied. ~he most efficient pumping action results when the driving force is applied in quadrature, that is, to produce a 90 degree phase lag in the oscillation cycle between two points along the blade, as illustrated schematically in Figs. 3A-3F. The driving force ~F) is applied at a single point, and within a selected frequency range depending upon the .

~LZ~ 7 resonant frequency of th~ combination of the blade and piezoelectrtic element, such that the blade undergoes both lateral displacement and bending at the point of applied force.
The driving force F on the blade produces the successive blade shapes shown in Figs. 3A-3F and directions of air motion (A) indicated by arrows, as described below.
Referring to Fig. 3A, with the blade 18 at rest, an initial lateral force F is applied (by the piezoelectric element~ to the blade at point 42. Thereafter, the rear portion of the blade 18 moves in the direction shown, with the forward end of the blade lagging, Fig. 3B, due to inertia.
When the rear portion 42 of the blade 18 reaches the maximum deflection, Fig. 3 B, the force F applied by the bilaminate is reversed, Fig. 3C, to move the rear portion of the blade in the other direction 16b, Fig. 3D. The forward end of the blade, however, continues to lag behind by 90 degrees of the oscillation cycle. When the driven point 42 of the blade reaches maximum deflection in the other direction, the force F
is again reversed, Fig. 3~ o move the blade back, with the forward end of the blade again being 90 degrees later in the oscillation cycle, Fig. 3F. Optimum pumping efficiency results when the blade resonance frequency is Bt or near the driving frequency of the piezoelectric bilaminate assembly 28, ~ince this maintains a quadrature relation betwe~n the leading (rear) and lagging (forward end) portions of the blade 18.

In the Fig. 1 embodiment, the blower contains two counter-oscillating blades 18 to operate 180 degrees out of phase with each other. The complementary back and forth motion of the two blades 18 provides dynamic balancing and prevents vibration of the device.
As an example of the efficient operation of the present inven~ion, a miniaturized form of blower constructed in accordance with Fig. 1, having an overall length of about 1.75 inches, a width of 0.75 inches and a height of 0.5 inches, and operated at a frequency of 60 Hz by the piezoelectric bilaminates, produces a sufficient throughput of air and a sufficient output pressure to be capable of blowing out a Zippo wind-proof lighter. Thus the device is very efficient, and in tests has been very stable, with efficiency so high that rises in temperature of the bilaminates have been virtually undetectable.
A modified embodiment of the solid state blower illustrated in Fig. 1 is shown in Fig. 4, where in place of the side mounted piezoelectric element 28, a pair of end-mounted bilaminate piezoelectric elements 128 drive respective ones of a pair of flat resilient blades 118.
The blower assembly includes a housing 110, side walls 112a and 112b and a bottom plate 117a. A top cover may be added if desired, similar to cover 17b shown in Fig. 1. Efficient pumping action i6 achieved without ~he enhanced valving action Trademark .
; ~12-8(~7 produced by the ducts due to the ~uadra~ure traveling wave induced in the blades 118.
The piezoelectric bilaminates 128 ar~ mounted at ~n~ end 140 to a cross member 141 bridging the walls 112a, 112b of the housing 110. The member 141 is provided with a pair of vertical slots 142, each of which is sized to snugly receive the end of the bilaminate 128 and a pair of electrically conductive contact leaves 144, one on either side of the bilaminate. Conductors, not shown, are connec~ed to the leaves for coupling to the alternating voltage supply. The free ends of the bilaminates 128 are attached at junctions 150 to resilient blades 118.
Alternatively, a doiuble-slotted saddle junction bloc~ 152 may be used to attach the resilient blade li8a to the free end of the bilaminates 128a.
In this mounting arrangement, as in the ~ig. 1 embodiment, the blade 118 is not fixed at any point relative to the housing and is free to move laterally (i.e., perpendicular to the flat surface of the blade 118) back and forth along its entire length when driven by the free end o~ the piezoelectric element 128.
As in the case of the blade in Fig. 1, when alternating voltage is applied across the bilaminates 128, a cyclical ba~k and forth movement occurs in ~he free ends of the bilaminates 128 which in turn drives the ends of the blade 118 at junction~
150 back and forth in the housing. Since the entire length of the blade 11~ is free to move back and forth relative to the housing, a traveling 1exure wave is propagated when the blade ~ is driven at an appropriate fre~uency, i.e. to produce P~ 13-~L244807 quadrature similar to that illustrated in Figs. 3A-3F, from the inlet end 124 toward ~he outlet end 122. Since, however, the propagated wave travels along the blade from one end 125 to the other 122, the blower works very efficiently in pumping fluids, especially air, without the need for valving action. To effect dynamic balancing of ~he system, ~he two bilamina~es are driven in opposing phase xelationship, as in the Fig. 1 embodiment.
Although for dynamic balancing purposes, it is prefer~ble to employ a pair of counter oscillating blades, the embodiments of both Figs. 1 and 4 can provide effective air movement with a single oscillating blade.
As recently more ~ully understood, no ducts, walls or valving are required for the operation of the blower according to this invention. In fact, the blades operate best in free air completely unobstructed. Valving action or flow rectification is accomplished with a process of vor ex shedding from the blade tip. In the preferred form, the blower has a hsusing which provides only mechanical protection without obstructing the flow near the vortex shedding tips of the blades. Such a housing 200 is shown in Fig. 5 as having an upper half 202 and lower half 204, which may be permanently fixed together at sonic weld points 205 for example. The rear closed portion 206 of housing 200 holds the piezoelectric driver elements and their electrical connections. Benders 107, 109 extend slightly beyond rear part closed portion 206 through slots 212 and 214 into the open frame ~- ( ~Z~8~7 area 21~, where they j~in with blades 108, 210. Frame area 216 includes upper 21~ and lower 220 rail p~rtions so that the v~rtex shedding areas at the kips of blades 20B and 210 are unconstrained by ~he housing~ ~ail~ 218 and 220 are primarily pr~vided as mechanical protection ~or the blades and, in fact, may be eliminated if desired.
Vortex sheddin~ is a pr~cess whereby air is prevented from being sucked around the blade tip when motion reverses. It is based on ~he ~act that air displaced from the front ~f a moving blade rota~es so rapidly that it is unable to reverse its direction of rotation when the blade reverses its moti~n. If the rotation is not sufficiently rapid, the vortex can reverse its direction of rotation to be sucked around the blade tip instead of leaving the blade. Vortex shedding is enhanced by, but does not require, exact quadrature motion; that is a 90 degree lag between the root and tip of the blade.
The vortex shedding action is illustrated in Fig. 6A-6I.
In Fig. 6A, the blade illustatively re~erred ~o as blade 208 of Fig. 5 is centered and moving upward at maximimum velo~ity as indicated by arrow 250, and air is being ~ucked downward around the blade tip as indicated by arrow 252, while the previou~ly shed vortex 254 is moving to the right below the center line of the blade. In Fig. 6B, the blade is beginning to curve upward at about one quarter amplitude. The air i~ being suc~ed around the blade tip into ~he vacuum on the back side of blade 20~ and the new vortex 2s2a is beginning ~o ~orm while the old vortex 254 is moving farther to the right. The blade nears the end of its travel in Fig. 6C, leaving a fully formed vortex 252b in its wake, with vortex 254 still moving outwardly. ~n Fig~ 6D, blade 2D8 has reached its full excursion and it has s~opped moving and is about to reverse with the fully formed vortex 252b still in its wake and the previously formed vortex 254 still moving to the right. The blade then starts downwardly again, Fig. 6E.
The vortex 252b is rotating too rapidly to reverse this motion and it is therefore expelled from ~he blade area by the new airflow around the blade. The new airflow 256 is moving up around the tip of ~he blade towards its wake, while the blade is moving in the direction as shown by arxow 258. ~pward flow 256 continues to gain speed as i~ is flows into the vacuum behind the blade, Fig. 6F, and the previous vortex 252b is now clear of the blade wake and gaining speed. The blade accelerates towards its center position in Fig. 6G while the air flowing into its wake indicated by arrow 256 is developing a new vortex. In Fig.
6H, with the blade centered and moving downward at maximum velocity as indicated by arrow 258, the air 256 being drawn into the vacuum of the wake has developed into a full vortex 256b.
Finally, in Fig. 6I the blade 208 is moved further dowmward, feeding more air into vortex 256b in its wake. The two previous vortices 252bt 254 are moved toward the right, rotating in opposite directions, one above the axis the other below the axis of blade 208. In this way, a line of opposi~ely rotating vortices is generated r~sulting in a highly direc~ional ~trea~
of air~ If this vor~ex shedding effect is disturbed by obstructions in the area, the air simply flows from the forward surface of the blade around its trailing edge to ~he rearward surface of the blade when the motion reverses. There is then only circulation around the trailing edge and very little outward flow.
While normal piezoelectric elements such as benders have amplitudes of several thousandths of an inch ~ypically from 0.01 inch to 0.02 inches, the blower blades of this invention provide amplitudes on the order of one inch.
The material out of which the blade is constructed must have low internal damping. Internal damping is a measure of thP
material's elasticity, usually expressed in terms of a "Q-factor" which is simply the ratio of peak elastic energy stored to total energy lost during one deformation cycle. For example, once struck, a bell of perfectly elastic material would ring forever. A bell of bronze ring~ audibly; one of lead does not ring at all. Bronze has a higher Q-factor than lead. In quantitative terms, a perfectly elastic tennis ball would rebound to the same height from which it was dropped. If it rebounded to 90~ of the height, it is said to have a Q-factor of 10. One-tenth o~ the peak energy stored is lost during impact.

.

8~37 C

If it rebounds to half the height, i~s Q-factor would be 2, half the energy lost If it landed with a ~hud like a piece of clay and didn ' t bounce at all, i~s Q-factor would be unity. All the stored energy would have been dissipated. For effective blowing action, the blade material in this invention ~hould have a Q-~actor of at least 8 to 10. Various metals satisfy this requirement, i.e. hard brass, phosphor-bronze, beryllium, copper alloy, steel.
The blade material should have a high stiffness-to-weight ratio. The minimum stiffness-to-weight ratio can be defined as a ratio of Young's modulus over density greater than one million newton-meters per kilogram. Young's modulus is defined as the slope of the stress versus strain curve within the elastic range and has the dimensions of stress over strain, notably newton's per sqare meter over meters per meter, while density has the dimensions of kilograms per cubic meter; thus the requirement can be express~d as Young's modulus/density greater than one million newton-meters per kilogram.

.

~L~4~7 C

The blade should also have a low mass compared to the piezoelec~ric bender. If the mass of the blade is too high, it will cause the bender to break when the blade is driven to a high resonant amplitude and there wi~l not be a discontinuity at the point where the blade joins the piezoelectric bender. For a blade of uniform width and thickness, the maximum mass per unit area of the blade is usually no more than 50 to 60 % of the mass per unit area of the bender. Two materials have been found to work very well for the blade, Mylar and G-10. A table showing the stiffness, density and stiffness/density ratio of a number of blade materials, including Mylar and ~-10, is shown below:

BLOWER BLADE MATERIALS
PROPERTY TABULATION

MATERIAL STIF~NESS DENSI~Y STIFF/DENS RATIO
~Nt/M ) (Rg/M ) (NtM/Kq) Steel 20 x 101 7.83 x 103 2/55 x 107 ~rass 9 x 101 8.56 x 103 1.05 x 107 G-10 1.9 x 101 . l.g x 103 1.0 x 107 ~Mylar .379 x 101 1.39 x 103 .272 x 107 * Lexan .199 x 101 1.2 x 103 ~166 x 107 Polyethylene 7 (High Dens.) .11 x 101 .96 x 103 .114 x 10 Polyethylene 7 ~Low Dens.) .026 x 101 .91 x 103 .028 x 10 * Trademarks ~. 19 ,. ~ , J _ _ ~ e ~2~807 The combined system of ~he piezoelectric element and the blade ~hould have its resonant frequency equal or approximately equal to the frequency of the applied voltage to an accuracy typically within plus or minus 2% or within 1-lJ4 Hz. at a resident frequency of 60 Hz. The blades may be attached to the bender by any suitable means such as by means of a cemented lap joint, or by the use of a slotted junction blockO
Other embodiments will occur to those skilled in the art and are within the following claims:

Claims (2)

C L A I M S

1. A pumping device comprising:
a housing;
a piezoelectric element having one end mounted to said housing and one end free;
a generally planar impeller blade connected to the free end of said piezoelectric element and having its distal end unconstrained by said housing; said blade having a Q-factor of at least eight, a stiffness-to-density ratio of more than one million newton-meters per kilogram and a mass per unit area which is less than 60% of the mass per unit area of said piezoelectric element; and means for applying a voltage to said piezoelectric element for oscillating its free end perpendicular to its plane at or close to resonance and propagating a travelling wave along said blade to generate and shed vortices at the distal end of said blade.

2. The pumping device of claim 1 in which the travelling wave propagated along said blade is a quadrature wave.