ELECTROMECHANICAL FORCE TRANSDUCER

Description

TECHNICAL FIELD

[0001] The invention relates to electromechanical force transducers, actuators, exciters and the like devices and more particularly but not exclusively, to such devices for use in acoustic apparatus, e.g. loudspeakers and microphones.

BACKGROUND ART

[0002] The invention relates particularly, but not exclusively, to electromechanical force transducers of the kind described in International patent application WO01/54450 to the present applicants, and comprising one or more resonant elements or beams having a frequency distribution of modes in the operative frequency range of the transducer. Such transducers are known as "distributed mode actuators" or DMA for short.

[0003] It is an object of the invention to provide a transducer in which damping is provided to result in a reduction of Q of the modes and a reduction in the severity of cancellation between modes to give an increased smoothness of acoustic pressure.

[0004] It is also an object of the invention to improve the robustness of the transducer e.g. to give a reduction of chance of failure during drop or impact tests.

[0005] Another object of the invention is to reduce the first resonant mode frequency of an actuator or transducer, e.g. a DMA transducer.

DISCLOSURE OF INVENTION

[0006] From one aspect, the invention is a transducer of the kind described wherein a low stiffness layer is inserted between, and bonded to the adjacent faces of a plurality of resonant elements. We have found that simply adding a damping layer to one face of a resonant element or beam gives poor damping performance as the layer stretches with the element as the element face changes dimensions. However using a flexible reference layer with a high resistance to dimensional change, such as a foil, on the other side of the damping layer results in an improvement in damping as the damping layer now shears between the changing element face dimension and the non-stretching foil. If the reference layer can be made to change dimension in opposition to the damped face, the damping effect will be doubled. This is the effect gained by adhering the damping layer between adjacent element faces.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which:-

Figure 1 is a side view of a first embodiment of electromechanical force transducer of the present invention;

Figure 2a is a side view of part of an electromechanical force transducer;

Figure 2b is a side view of a first embodiment of electromechanical force transducer of the present invention;

Figure 3 is a graph comparing blocked force of a single beam transducer and the transducer of Figure 1;

Figure 4 is a graph comparing acoustic pressure between an undamped double beam DMA, a ½ damped DMA (that is with damping material bonded between the resonant elements over half the lengths of the resonant elements) and a fully damped double beam DMA transducer;

Figure 5 is a side view of a single beam actuator;

Figure 6 is a side view of an example of an electromechanical force transducer which is useful for understanding the invention;

Figure 7 is a graph comparing blocked force under different conditions;

Figure 8a is a graph comparing acoustic pressure under different conditions;

Figure 8b is a perspective view of a transducer of the kind shown in Figure 6 mounted at a panel edge, and

Figure 9 is a graph comparing blocked force with different compliant stubs.

BEST MODES FOR CARRYING OUT THE INVENTION

[0008] Figure 1 shows a double beam transducer of the kind generally described in WO01/54450. The transducer (1) comprises a first piezoelectric beam (2) on the back of which is mounted a second piezoelectric beam (3) by connecting means in the form of a rigid stub (4) located near to the centre of both beams. Each beam is a bimorph.

[0009] The transducer (1) is mounted on a structure (5), e.g. a bending-wave loudspeaker panel, e.g. a distributed mode loudspeaker (DML), by coupling means in the form of a rigid stub (6) located near to the centre of the first beam.

[0010] In the present invention a low stiffness layer (7) of foamed plastics is bonded between adjacent faces of the two beams (2,3). The bonded layer may cover substantially the whole of the adjacent faces or may be discontinuous, e.g. to damp certain modes.

[0011] The following sets out some parameters for one suitable foam damping material

[0012] "Poron" slow rebound foam polyurethane plastics material.
Type: 4790-92-25041-04S
Thickness : 1.05mm (we have also tried 1.0mm with success)
Density : 400kg/m3.
Compressional E (Young's Modulus with the foam in compression)= 2MPa at 1kHz.

[0013] The measured resistance, R, is approx 8 x 10⁵ Ns/m3. These figures are the measured 'real' part of the mechanical resistance when in compression, not shear. Shear figures are not available.

[0014] Use of a thinner foam (0.6mm) also gave good results. A thicker foam, say up to 1.5mm would be expected to give good results with this material. We suggest thickness limits between 0.3 and 2.0 mm.

[0015] The density (in isolation from E and R) is expected to be irrelevant, and could vary by a factor of 100 and have little effect. E is important but the shearing that is occurring makes the importance of E difficult to identify. We suggest a factor of 4 increase in E would start to stiffen the beam, so is to be avoided. A reduction of E would have little effect as it appears the system stiffness is not being affected too much by the addition of the foam. The R figure is important. Reducing R is expected to effect damping in a linear fashion. We suggest that it is not reduced by more than a factor of say 4. Increasing R is good but cannot be achieved without affecting the other parameters.

[0016] Figure 2 shows the effect of bonding to one face or to both faces of multibeam transducer. Figure 2a shows the case where the damping layer (7) is only bonded to one beam (2). When the other beam (3) moves in relation to (2), it slides over the upper surface of the damping layer, which therefore does not deform and adds little damping to the bending resonances. However, in Figure 2b, the damping layer is bonded to both beams, and so is forced into shear by the relative movement of beam (3) in relation to beam (2). It is this shearing which applies damping.

[0017] The beam lengths need not be the same but maximum damping effect is expected if they are. The measured effect of adding a damping layer between two beams on the blocked force of a centrally mounted transducer is shown in Figure 3. The Q of all modes is reduced and the natural frequencies have not changed implying extremely low stiffness of bond material (7). Adding the damping layer increases output when cancellation inside the transducer is occurring, such as between the resonances of dissimilar length beams.

[0018] Figure 4 shows the simulated effect on acoustic pressure of adding a damping between the faces of a 36mm/34mm beam length DMA transducer. Output at the transducer fundamental is slightly reduced, but a broad increase in output occurs in 3-4kHz region. This is the region of internal cancellation in the transducer. The acoustic pressure response is also smoother.

[0019] Drop test failure rates are expected to be reduced. At impact most of the energy will be present in the exciter at its fundamental resonance. Since the damping reduces the Q of this resonance, the instantaneous maximum displacement will be reduced, resulting in reduced stress in the beam. This stress reduction is expected to improve drop test reliability. In addition, the build height of the transducer can be reduced by the present invention.

[0020] The stub used to couple a transducer of the kind described above to its load is stiff in all 3 cartesian axes and rotational stiffness is usually ignored, and is assumed to be high. For the case of a beam with stub position halfway along its length, 0 rotation occurs at the stub for the beam fundamental resonance. If this 0 rotation boundary condition is replicated at the end of a half length beam the fundamental will occur at the same frequency as the full length beam, with half the force. This is the cantilever condition, see Figure 5. Figure 5 is a diagram showing fundamental mode shape of a cantilever beam (that is an extreme offset stub). The displaced shape shows pure bending motion.

[0021] However by reducing the stub rotational stiffness from this high value to a lower one, the f0 of the beam drops and becomes less dependent on bending motion of the beam and more rigid body like, see Figure 6. Figure 6 is a diagram of a modeshape of a beam coupled to a panel with a soft stub allowing rotation of the beam, the modeshape showing some bending in the beam and some rotational translations. In the limiting case of a rotational stiffness of 0, the mode drops to 0 Hz and is a rigid body mode. Reference (9) represents a trapped air layer behind the panel (5), which in the simulation couples to the panel and affects the modal set of resonances in the panel, and reference (10) represents the body of a cell phone containing a loudspeaker formed by the panel (5) and transducer (1). It will be noted that the deflection of the beam (2) is greatly exaggerated so that it is visible.

[0022] By choosing this rotational compliance the f0 of the beam can be lower than the f0 of a beam twice its length, mounted at its centre - FE analysis has been used to show this effect, see Figure 7. Figure 7 is a graph of simulated blocked force generated by 3 conditions: a 36mm beam centrally mounted, a half length beam with stiff stub at end and half length beam with compliant stub at the end. The hard stub case causes a stiffening of the beam, effectively reducing its length slightly.

[0023] A solid stub will have the same stiffness in the 3 translational and rotational axes. By suitably profiling the cross-sectional shape of the stub, different stiffnesses in the 6 different axes can be generated. The result is that modes in the different axes occur at different frequencies. If the load impedance is asymmetric, modes involving movement in directions other than normal to the beam surface can couple into the panel, providing increased modal density, see Figure 8. Figure 8a is a graph of simulated effect on acoustic pressure generated by changing stub stiffness. Figure 8b is a perspective view of a panel-form loudspeaker having a panel(5) with an attached transducer mounted on a soft stub (6) of I-beam section and showing the DMA moving in-plane. In the case of the in-plane mode illustrated in Figure 8, this mode is not present if the rotational stiffness around the axis (8) normal to the plane of the panel is ignored. In this case the first mode is partly due to rotational stiffness around the axis along the short edge of the beam, the second mode is due to the stiffness around the axis normal to the beam. The last rotational axis, around the axis moving along the length of the beam will also generate a mode.

[0024] An example of a stub shape giving different stiffnesses in different axes is an I-section, see Figure 9. Figure 9 is a graph of simulated effect on blocked force of polycarbonate I-section stub with varying vertical bar lengths. The stub is 3mm wide in total with inner bar of 1mm width, bar length being specified on the plot.

[0025] By changing the fundamental resonance from a purely bending motion in the beam to a partly translatory motion, the stress in the beam is reduced at the fundamental. Since the fundamental resonance will receive the most energy during impact, the beam is more likely to survive without damage as most of the deformation will occur in the stub.

[0026] Although a stub of I-beam section has been described, many other stub cross-sections could be used, for example, trapezoidal, cylindrical and so forth.

Claims

1. An electromechanical force transducer comprising a plurality of resonant elements, a damping layer coupled between the adjacent faces of at least two adjacent resonant elements, and a stub member on which the resonant elements are supported and for coupling the transducer to a site to which force is to be applied, characterised in that the damping layer is selected so that output is increased in the frequency region of internal cancellation in the transducer.

2. A transducer according to claim 1, wherein the damping layer is of foamed plastics.

3. A transducer according to claim 2, wherein the foamed plastics is of slow rebound characteristic.

4. A transducer according to any one of claims 1 to 3, wherein the damping layer is in the form of a layer bonded to the whole of, or to a substantial part of, the adjacent faces of the resonant elements.

5. A transducer according to any preceding claim, wherein the resonant elements are beam-like.

6. A transducer according to any preceding claim, wherein the stub is of low rotational stiffness whereby the fundamental resonance of the transducer becomes less dependent on bending motion of the transducer and more rigid body-like.

7. A transducer according to claim 6, wherein the stub has different stiffnesses in the translational and rotational axes whereby the modes in different axes occur at different frequencies.

8. A transducer according to any preceding claim, wherein the parameters of the resonant element are selected to enhance the distribution of modes in the element in the operative frequency range with the parameters being selected from the group consisting of aspect ratio, isotropy of bending stiffness, isotropy of thickness and geometry.

9. A transducer according to any preceding claim, wherein at least one of the resonant elements is active, e.g. of piezo material.

10. A transducer according to any preceding claim, wherein the low stiffness member is coupled between substantially the whole of the adjacent faces.

11. An electromechanical force transducer according to any preceding claim, wherein the resonant elements have a frequency distribution of modes in the operative frequency range of the transducer.

12. A loudspeaker comprising a transducer as claimed in any preceding claim and a bending-wave panel-form acoustic radiator to which the transducer is coupled.

Ansprüche

1. Elektromechanischer Kraftwandler mit einer Mehrzahl von Resonanzelementen, einer Dämpfungsschicht, die zwischen benachbarten Flächen von zumindest zwei benachbarten Resonanzelementen gekoppelt ist, und einem Stumpfelement, auf dem die Resonanzelemente abgestützt sind und zum Koppeln des Wandlers mit einer Stelle, an die eine Kraft angelegt werden soll, dadurch gekennzeichnet, dass die Dämpfungsschicht derart ausgewählt ist, dass eine Abgabe im Frequenzbereich der internen Auslöschung im Wandler erhöht wird.

2. Wandler nach Anspruch 1, bei dem die Dämpfungsschicht aus Schaumkunststoff besteht.

3. Wandler nach Anspruch 2, bei dem der Schaumkunststoff eine langsam zurückfedernde Charakteristik hat.

4. Wandler nach einem der Ansprüche 1 bis 3, bei dem die Dämpfungsschicht die Gestalt einer Schicht hat, die an die gesamten oder an einen wesentlichen Teil der benachbarten Flächen der Resonanzelemente geklebt ist.

5. Wandler nach einem der vorhergehenden Ansprüche, bei dem die Resonanzelemente balkenartig sind.

6. Wandler nach einem der vorhergehenden Ansprüche, bei dem der Stumpf eine niedrige Drehsteifigkeit aufweist, wodurch die Grundresonanz des Wandlers weniger von der Biegebewegung des Wandlers abhängig und starrkörperähnlicher wird.

7. Wandler nach Anspruch 6, bei dem der Stumpf in der Translationsachse und der Rotationsachse eine unterschiedliche Steifigkeit aufweist, wodurch die Moden in den unterschiedlichen Achsen bei unterschiedlichen Frequenzen auftreten.

8. Wandler nach einem der vorhergehenden Ansprüche, bei dem die Parameter des Resonanzelements so ausgewählt sind, dass sie die Verteilung der Moden im Element im Betriebsfrequenzbereich verbessern, wobei die Parameter aus der Gruppe ausgewählt sind, die aus dem Seitenverhältnis, der Biegesteifigkeitsisotropie, der Dickenisotropie und der Geometrie besteht.

9. Wandler nach einem der vorhergehenden Ansprüche, bei dem zumindest eines der Resonanzelemente aktiv ist, z. B. aus Piezomaterial.

10. Wandler nach einem der vorhergehenden Ansprüche, bei dem das Element mit der niedrigen Steifigkeit im Wesentlichen zwischen die gesamten benachbarten Flächen gekoppelt ist.

11. Elektromechanischer Kraftwandler nach einem der vorhergehenden Ansprüche, bei dem die Resonanzelemente eine Frequenzverteilung der Moden im Betriebsfrequenzbereich des Wandlers aufweisen.

12. Lautsprecher mit einem Wandler nach einem der vorhergehenden Ansprüche und einem plattenförmigen akustischen Biegungswellenstrahler, mit dem der Wandler gekoppelt ist.

Revendications

1. Transducteur à force électromécanique comprenant une pluralité d'éléments résonants, une couche d'amortissement couplée entre les faces contiguës d'au moins deux éléments résonants contigus, et un élément de tronçon sur lequel les éléments résonants sont supportés et pour coupler le transducteur à un site sur lequel est appliquée une force, caractérisé en ce que la couche d'amortissement est choisie de telle sorte que la sortie est augmentée dans la région de fréquence d'annulation interne du transducteur.

2. Transducteur selon la revendication 1, dans lequel la couche d'amortissement est en plastique alvéolaire.

3. Transducteur selon la revendication 2, dans lequel le plastique alvéolaire présente des caractéristiques de faible rebond.

4. Transducteur selon l'une quelconque des revendications 1 à 3, dans lequel la couche d'amortissement se présente sous la forme d'une couche collée sur l'ensemble ou sur une partie substantielle des faces contiguës des éléments résonants.

5. Transducteur selon l'une quelconque des revendications précédentes, dans lequel les éléments résonants sont en forme de poutre.

6. Transducteur selon l'une quelconque des revendications précédentes, dans lequel le tronçon présente une rigidité de rotation faible, de telle sorte que la résonance fondamentale du transducteur devient moins dépendante du mouvement de flexion du transducteur et plus rigide dans l'ensemble.

7. Transducteur selon la revendication 6, dans lequel le tronçon présente des rigidités différentes dans les axes de translation et de rotation, de telle sorte que les modes dans les différents axes se produisent à des fréquences différentes.

8. Transducteur selon l'une quelconque des revendications précédentes, dans lequel les paramètres de l'élément résonant sont choisis pour accroître la répartition des modes dans l'élément, dans la plage de fréquences opérantes, avec les paramètres qui sont choisis à partir du groupe consistant en rapport d'aspect, isotropie de rigidité de flexion, isotropie d'épaisseur et géométrie.

9. Transducteur selon l'une quelconque des revendications précédentes, dans lequel au moins l'un des éléments résonants est actif, par exemple en piézomatière.

10. Transducteur selon l'une quelconque des revendications précédentes, dans lequel l'élément à faible rigidité est couplé entre sensiblement l'ensemble des faces contiguës.

11. Transducteur de force électromécanique selon l'une quelconque des revendications précédentes, dans lequel les éléments résonants ont une répartition de fréquence des modes dans la plage de fréquences opérantes du transducteur.

12. Haut-parleur comprenant un transducteur selon l'une quelconque des revendications précédentes, et un élément de radiation acoustique en forme de panneau à ondes fléchissantes sur lequel est couplé le transducteur.

Drawing