US2305369A - Piezoelectric unit - Google Patents

Description

?atented Dec. 15,1942

PIEZOELECTRIC UNIT Alfred L. W. Williams, Cleveland Heights, Ohio, assignor to The Brush Development Company, Cleveland, Ohio, a corporation of Ohio Application August 2, 1940, Serial No. 350,017 18 Claims. .(Cl.1'71-327) This invention relates to piezo-electric transducer units and relates more particularly to means for extending the frequency range of 'piezo-electric devices.

The principal object of this invention is to provide an improved typeof a piezo-electric transducer unit that is capable of responding to a widerange of frequencies.

A secondary object of this invention is to increase the sensitivity of piezo-electric transducers.

Another object of this invention is to reduce the variation of the impedance of piezo-electric transducer units with changes in frequency.

Another object is to reduce the wave form distortion produced in power amplifiers used in conjunction with piezo-electric motor devices.

Another object of this invention is to extend the frequency range of piezo-electric motor de- One of the principal factors limiting the frequency range of piezo-electric transducers .is mechanical resonance in the crystal unit itself. Since piezo-electric crystal units have a high stiffnessthe first mechanical resonance can be madeto occur at a reasonably high frequency, and therefore crystal devices are usually designed so that this resonance occurs near the high end of the useful frequency range. If a wide frequency range is desired,- the dimensions of ,the crystal unit are chosen so that a high resonant frequency is obtained. However, when this is done, the sensitivity may be comparatively low. in other words, in piezo-electri'c transducers v vices.

constructed in accordance with theprior art methods, it usually has been necessary to sacririce sensitivity if a wide frequency range is desired, or sacrifice frequency range when high sensitivity is desired. The sensitivity of prior art crystal devices is also further limited by the variation in impedance with frequency. The impedance of. a crystal unit is predominantly capacitive reactance except at those frequencies at which the crystal is mechanically resonant;

- operate a piezo-electric generator'device into a prevent a loss of low frequency response. In either case the overall sensitivity of the crystal transducer and its associated circuit is limited because the impedances cannot be matched except for a very narrow band of frequencies. The variation in the impedance of a piezo-electric motor also reduces the handling capacity of an amplifier used to operate the device. I

By constructing a piezo-electric transducer in accordance with this invention, it is possible to extend the frequency range without'sacrificing sensitivity. This object is achieved by connecting suitable electrodes of the crystal unit to the associated circuit in a manner which permitsthe entire crystal unit to function at low frequencies while permitting only a part of the unit to function at the higher frequencies. The part of the crystal unit which is effective throughout the entire frequency range is preferably coupled most closely to the associated mechanical or acoustic system. In most types of piezo-electric transducers this coupling comprises a single drive pad or link or a torque drive wire, and so when a crystal unit constructed in accordance with this invention is used in such a device, the

. coupling to the mechanical or acoustic system is located at a point or zone on the part of the unit which functions throughout the entire frequency range.

is particularly useful in piezo-electric transducers which employ crystals of Rochelle salt or of other piezo-electric material ,having substantially the same characteristics. The invention will there fore be described with reference to devices of this type.; For the purpose of illustratingthe invention a loudspeaker is shown in the accompanying draw ng, but the invention is also applicable to numerous other devices which are not illustrated such as earphones, record cutters, microphones, Oscilloscopes, pen recorders and phonograph pickups.

In the drawing:

Fig. 1 is a rear view of a piezo-electric loudspeaker embodying the invention, part of the high resistance circuit or a circuit whose impedance is high for low frequencies in order to 5 piezo-electricunit l is connected to the sound racase enclosing the crystal um't being broken away to show the interior construction.

Fig. '2 is a sectional side View of the loudspeaker taken along line 2-2 of Fig. 1.

Fig. 3 is a diagrammatic view of a loudspeaker illustrating the application of the invention to a single plate expander type piezo-electric unit.

In the loudspeakershown in Figs. 1 and 2 a This unit, which is of the torque Bimorph type of construction comprises two plates 4, 5 of piezoelectric material of the Rochelle salt type cemented together face to face with an interior electrode 6 interposed between them. This inner electrode is of the conventional low resistance type and is'preferably composed of finely divided conducting material such as colloidal graphite, but metal foil or any other suitable electrode material could be used. The use of metal foil electrodes is described more fully in United States Patent No. 1,995,257 granted to C. B. Sawyer, and the method of applying graphite electrodes is disclosed in my United States Patent No. 2,106,143. Instead of the conventional outer electrodes, however, the outer face of each plate is provided with an electrode having a high electrical resistance. Such an electrode may preferably be obtained by spraying the outer faces with an extremely thin coating of colloidal graphite or by mixing graphite with a finely divided insulating material and applying a coating of normal thickness. The outer electrodes I and 8 are connected to one of the signal input terminals 9 by means of a flexible lead II), which is fastened to the crystal unit at a point as close as possible to the drive link 3. The inner electrode 6 is connected to the other input terminal H by the flexible lead I 2. -However, since the inner electrode is of the low resistance type it is not essential that this lead be located adjacent to the drive point. The connecting lead It! is split at the end which is fastened to the crystal unit, and the connections to the two outer electrodes are held in place by small patches I 3, M of metal foil. The crystal unit is supported at three corners bymeans of mounting pads l5, l6, I! which are preferably constructed of damping material, and the unit is entirely enclosed by the case 18. This method of mounting the crystal unit is disclosed in my United States Patent No. 2,105,011 and therefore forms no part of the present invention. The point at which the drive link extends.

through the wall of the case is sealed by means of the flexible bushing IS.

The crystal unit used in this loudspeaker operates in substantially the same manner as a-conventional Bimorph unit, particularly at low frequencies. to the electrodes produces an electrostatic field in a direction parallel to the electrical axis of the crystal, and this causes 'each plate to expand in a direction perpendicular to the electrical axis and contract in a direction perpendicular to the electrical axis and to the direction of expansion. Since the plates are oriented so that the direction of expansion of one is parallel to the direction of contraction of the other, they mutually constrain each other so that the unit bends about two axes of curvature disposed at right angles to each other on opposite sides of the unit and parallel to the faces of the unit. For a more complete discussion of the operation of piezoelectric Bimorph units of the Rochelle salt type, reference may be made to United States Patents Nos. Re. 20,213 and Re. 20,680, granted to C. B. Sawyer. Bimorph units may also be constructed of other types of piezo-electric material which have at least one axis of expansion and contraction which is at an angle to the electrical axis. When a signal voltage of low frequency is applied to the crystal unit substantially the full voltage is impressed across the unit at all points because of the high impedance ex- The application of signal voltage diating diaphragm 2 by means of a drive link 3.

isting between the electrodes at low frequencies. The entire crystal unit therefore bends, or flexes, in the same manner as when conventional electrodes are used. However, by making the resistance of the outer electrodes suitably high and applying the signal voltage near the drive point the operation of the unit at high frequencies is improved. For practical purposes, the electrode resistance of a unit may be considered to be suitably high when it is effective in applying not vmore than about 50% of the maximum voltage to those portions of the unit which are most remote from the portion where the electrodes apply the maximum voltage, when the voltage which is applied to the unit has a frequency selected from the high frequency end of the useful frequency range of the unit. When a high frequency signal voltage is applied to the terminals of the unit the maximum voltage is impressed only across the part of the crystal unit that is adjacent to the drive link; the voltage across the part of the unit that is farthest from the drive point is much less because the low impedance existing between the electrodes at high frequencies causes a-large voltage. drop in the high resistance outer electrodes. The flexing of the unit is therefore greatest near the drive point. This effect increases the resonant frequency without decreasing the sensitivity, and extends the useful range of the unit to above the resonant frequency. The use of high resistance electrodes also reduces the variation of impedance with changes in frequency, so that more eflicient coupling to the source of signal voltage is obtained, particularly at the higher frequencies. Reducing the variation in impedance also improves the handling capacity of an amplifier used to supply signal voltage to the loudspeaker.

A crystal unit having high resistance electrodes can also be used in a generator device but the operation is somewhat different. For example, when a unit of the type shown in Figs. 1 and 2 is used in a diaphragm type microphone the improvement in the high frequency response is not due to the comparatively high impedance at high frequencies, as in the loudspeaker, but is due to the fact that only the portion of the unit adjacent to the drive point is effective at high frequencies. At low frequencies the vibration of the diaphragm causes the crystal unit to vibrate in such a manner that the voltages generated in the various parts of the unit are approximately equal, and the operation is the same as when conventional low resistance electrodes are used. When the frequency of the sound wave actuating the diaphragm is high, the inertia and mounting arrangement of the crystal unit causes the flexing to be greatest near the drive point, and so the highest voltage is generated in this part of the unit. The high resistance of the electrodes prevents the comparatively inactive part of the unit from acting as an'electrical load on the part where the flexing is greatest. When conventional low resistance electrodes are used the part of the crystal unit that flexes least acts substantially as a capacitive load on the part where the flexing is greatest, and this effect reduces the terminal voltage at high frequencies. However, by separating different parts of the crystal unit electrically throughthe use of high resistance electrodes the inactive part of the unit is effectively disconnected for the high frequencies, and therefore the terminal voltage is not greatly reduced by this loading effect even for crystal unit constructed in accordance with the invention can be controlled by varying either the resistivity or the thickness of the electrode material, and it can also be varied by making the thickness of the electrode material different at various points on the surface of the crystal unit.

The invention is applicable not only to twoplate Bimorph units but also to single plate Bimorphs, single plate expander type units, and multiple plate units such as four-ply and six-ply Bimorphs. A loudspeaker using a single plate expander type unit is shown in diagrammatic form in Fig. 3. One end of the crystal plate 20 is fastened rigidly to a supporting member 2|, and the other end is connected to a sound radiating diaphragm 22 through a rigid drive link 23. High resistance electrodes 24, 25 are applied to the faces of the plate, and the connecting leads 26, 21 are fastened to the unit at points adjacent to the drive link 23. If it is desired, however, one of the electrodes may be of the conventional low resistance type. When a low frequency signal voltage is applied substantially the maximum voltage is impressed across all parts of the unit, and so the expansions and contractions take place throughout the entire length of the crystal unit. However, when a high frequency signal is applied the maximum voltage is impressed across the part of the unit that is adjacent ,to the drive link, while substantially reduced voltages are applied to the more remote portions. Therefore the maximum expansions and contractions occur in the part adjacent the leads. In this manner it is possible to reduce the loss of high frequency response that would occur due to various regions vibrating out of phase with each other if conventional electrodes were used. Also, the loss in high frequency response due to variation in the impedance is reduced.

It is evident from the above description that the invention is applicable to crystal units comprising any desired number of plates. Piezoelectric units constructed in accordance with the invention are applicable to either motor or generator devices, and they can also be used in loudspeakers in which the crystal itself functions as the sound radiator and in microphones in which the crystal functions as the diaphragm. The sound cell is an example of this type of microphone. A loudspeaker of this type is disclosed in United States Patent No. 2,102,668, granted to Stuart Ballantine and sound cell microphones are described in detail in United States Patent No. 2,105,010 to C. B. Sawyer and in my United States Patent No. 2,126,436. Although only loudspeakers have been described in the preceding specification and the accompanying drawing, it will be understood that these embodiments of the invention are to be considered as illustrative and that the scope of the invention is limited only by the following claims. For the purpose of defining the invention, the term "high resistance is used in reference to electrode resistances to mean an electrode resistance which is effective in applying not more than about 50% of the maximum voltage to those portions of the unit which are most remote from the portions where the electrodes apply the maximum voltage, when the voltage which is applied to the unit has a frequency selected from the high frequency end of the useful frequency range of the unit.

What I claim is:

1. A piezo-electric transducer unit comprising the combination of a plate-like member of piezo-electric material; an electrode system associated with said member and including a pair of electrodes secured to opposite faces of said member, at least one electrode of said pair having electrical resistance high with respect to the impedance of the member at the high frequency end of the useful frequency range thereof, said system also including terminal connections which make contact with said pair of electrodes, said electrode system being effective in subjecting a selected referenc portion of the member to a voltage of desired magnitude and in subjecting successively remote portions of the member to voltages of progressively reduced magnitudes when an alternating potential having a frequency selected from the high frequency end of the useful frequency range of the unit is applied to the electrodes through the said terminal connections.

2. A piezo-electric transducer unit as claimed in claim 1 wherein said plate-like member is adapted to be coupled within a selected coupling zone thereof to a mechanical system and wherein said coupling zone is the said reference portion.

3. A piezo-electric transducer unit comprising the combination of: a plate-like member of piezo-electric material having an electrical axis which intersects the faces thereof; electrode means associated with opposite faces of said member and including a pair of electrodes. at least one electrode having electrical resistance high with respect to the impedance of the member at the high frequency end of the useful frequency range thereof, and terminal connections making contact with said pair of electrodes; said electrodes being effective in subjecting a selected reference portion of the member to an electrostatic fleld of desired strength parallel to.the said electrical axis, and in subjecting successively remote portions thereof to field strengths of progressively reduced magnitudes when an alternating potential having a frequency selected from the high frequency end of the useful frequency range of the unit is applied to the electrodes through the said terminal connections.

4. A piezo-electric transducer unit as claimed in claim 3 wherein said piezo-electric member is adapted to be coupled within a selected coupling zone thereof to a mechanical system; and wherein said coupling zone is the reference portion.

5. A piezo-electric transducer unit comprising the combination of: a plate-like member of piezo-electric material having an electrical axis which intersects the faces of the plate-like memher and being adapted to expand in at least one direction perpendicular to said electrical axis when subjected to an electrostatic field of a given polarity substantially parallel to the said electrical axis and to contract in the said direction when the polarity of such field is reversed; constraining means adapted to oppose expansion and contraction of the said member and to cause said member, when subjected to th" action of an electrostatic field of given polarity, to bend;

and conversely to cause the establishment of such an electrostatic field when the member is mechanically bent; an electrode system associated with the said plate-like member and comprising: a pair of electrodes secured to opposite faces of said member, at least one electrode of said pair having electrical resistance high with respect to the impedance of the member at the high frequency end of the useful frequency range thereof; and terminal connections making contact with said pair of electrodes, said electrodes being effective in subjecting a selected reference portion of the member to a voltage of desired magnitude and in subjecting successsively remote portions thereof to voltages of progressively reduced magnitudes when an alternating potential having a frequency selected from the high frequency end of the useful frequency range of the unit is applied to the electrodes through the said terminal connections.

6. A piezo-electric transducer unit as claimed in claim 5 wherein said plate-like member is adapted to be coupled within a selected coupling zone thereof to a mechanical system and wherein said coupling zone is the said reference portion.

7. A piezo-electric transducer unit comprising the combination of: a pair of plate-like members of piezo-electric material, each member having an electrical axis which intersects the faces thereof and each being adapted to expand in at least one direction perpendicular to said electrical axis when subjected to an electrostatic field of a given polarity substantially parallel to the said electrical axis and to contract in the said direction when the polarity of such field is reversed; an electrode system associated with first and second pairs of electrodes to a pair of terminals, each of said first and second pairs of electrodes being effective in subjecting a selected reference portion of one member to a voltage of desired magnitude and in subjecting successively remote portions of that member to voltages of progressively reduced magnitudes when an alternating potential having a frequency selected from the high frequency end of the useful frequency range is applied to the electrodes through the said terminal connections; means securing said pair of plate-like members together face to face to form a multiple assembly, said members being disposed in said assembly with their respective reference portions in substantial register with each other, with their individual'directions of expansion and contraction in substantial alignment, and so oriented with respect to each other that one of said members opposes expansion or contraction of the other when a potential of given polarity is applied to said terminals whereby said multiple assembly is caused to bend, and conversely to establish a potential at said terminals when the assembly is mechanically bent.

pling zone thereof to a mechanical system; and wherein said coupling zone substantially coincides with the said registering reference portions.

9. A piezo-electric' transducer as claimed in claim 'I wherein said first and second pairs of electrodes utilize a common electrode disposed between the two members.

10. A piezo-electric transducer unit as claimed in claim 1 wherein both electrodes of said pair have high electrical resistance.

11. A piezo-electric transducer unit as claimed in claim 1 wherein said high resistance electrode is composed of a finely-divided mixture of conducting and insulating materials.

12. A .piezo-electric transducer unit as claimed in claim 7 wherein the high resistance electrodes of said first and second pairs of electrodes are composed of a finely-divided mixture of conducting and insulating materials.

13. A piezo-electric transducer unit as claimed in claim 1 wherein said unit is a motor unit.

14 A piezo-electric transducer unit as claimed in claim 1 wherein said unit is a generator unit.

15. A piezo-electric transducer unit which comprises, the combination of: a flexible platelike member, the said plate-like member having an electrical axis substantially perpendicular to the faces of the plate and being adapted to expand in one direction perpendicular to said electrical axis and simultaneously to contract in a direction perpendicular to the first direction and to the electrical axis when subjected to an electrostatic field of a given polarity substantially parallel to the said electrical axis, and to contract in the first direction and expand in the second direction when the polarity of such field is reversed; constraining means adapted to oppose expansion and contraction of the flexible member in both of the said directions and to cause said member, when subjected to the action of an electrostatic field of given polarity, to bend about two axes of curvature disposed at right angles to each other on opposite sides of the member and parallel to the faces of the member, and conversely to cause the establishment of such an electrostatic field when the member is mechanically bent; an electrode system associated with the said plate-like member and comprising a pair of electrodes in register with each other and in contact with opposite faces of the said member, at least one electrode of said pair having electrical resistance high with respect'to the impedance of the member at the high frequency end of the useful frequency range thereof, said system also including terminal connections making contact with a selected reference portion of said pair of electrodes; said pairof electrodes being effective in subjecting said selected portion to a voltage of desired magnitude, and in subjecting successively remote portions thereof to voltages of progressively reduced magnitudes when an alternating potential having a frequency selected from the high frequency end of the useful frequency range of the unit is applied to the electrodes through said terminal connections.

8. A piezo-electric transducer unit as claimed 16. A piezo-electric transducer unit as claimed in claim 15, wherein said plate-like member is adapted to be coupled within a selected coupling zone thereof to a mechanical system and wherein said coupling zone is the said reference portion.

1'7. A piezo-electric transducer unit as claimed in claim 15 wherein said high resistance elec- 2,805,369 trode is composed of a finely-divided mixture of conducting and insulating materials.

18.'A piezo-electric transducer-unit comprising the combination of: a plate-like member of piezoelectric material; and high resistance electrode means for subjecting a selected reference portion of said member to a voltage of desired

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