Abstract
A mechanical transducer for the detection of acoustic and/or seismic signals is indicated, comprising a continuous or discrete coupled mass-spring network with varying masses and/or spring constants. The mass-spring network is adapted to transform a comparatively small-dimensioned motion parameter of a first mass element into a comparatively large-dimensioned motion parameter of a further mass element. Between the first mass element and the further mass element, the mass-spring network comprises one or more intermediate mass elements, which are coupled to the first mass element and the further mass element by means of spring elements.
Claims
1. A mechanical transducer for detection of acoustic and/or seismic signals, comprising a continuous or discrete coupled mass-spring network with varying masses and/or spring constants, the mass-spring network configured to transform a comparatively small-dimensioned motion parameter of a first mass element into a comparatively large-dimensioned motion parameter of a further mass element, wherein the mass-spring network comprises one or more intermediate mass elements arranged between the first mass element and the further mass element and coupled to the first mass element and the further mass element by spring elements.
2. The transducer as claimed in claim 1, wherein the mass-spring network is configured such that a change in motion of the first mass element is transformed into a change in motion of the intermediate mass elements and into a change in motion of the further mass element in such a way that the magnitude of a specific motion parameter is gradually increased from mass element to mass element.
3. The transducer as claimed in claim 1, wherein the masses of the first mass element, the intermediate mass elements and the further mass elements gradually decrease from mass element to mass element in a direction from the first mass element to the further mass element.
4. The transducer as claimed in claim 1, wherein the spring constants of the spring elements gradually decrease in a direction from the first mass element to the further mass element.
5. The transducer as claimed in claim 1, wherein ratios of the masses to the spring constants are essentially constant over all pairs of mass element and spring element being directly connected to each other.
6. The transducer as claimed in claim 1, wherein the mass-spring network comprises at least three intermediate mass elements.
7. The transducer as claimed in claim 1, wherein the mass-spring network comprises discretely arranged masses and springs and is designed such with respect to the masses and the spring constants, that a spectral transfer function is achieved that has the same number of essentially regularly distributed spectral peaks as the number of discrete masses.
8. The transducer as claimed in claim 1, wherein the mass-spring network is a continuous mass-spring network formed as a membrane with varying thickness and/or tension.
9. The transducer as claimed in claim 1, wherein the first mass element, the further mass element and the intermediate mass elements are essentially all arranged in a common plane and are adapted to be displaced out of the common plane upon external excitation.
10. The transducer as claimed in claim 1, wherein the first mass element, the further mass element and the intermediate mass elements are concentrically arranged.
11. The transducer as claimed in claim 1, wherein the mass-spring is formed as a two-dimensional or three-dimensional structure.
12. The transducer as claimed in claim 1 being produced from a silicon-on-insulator (SOI) wafer.
13. The transducer as claimed in claim 12, wherein the SOI-wafer has a bulk layer that forms the mass elements and a device layer that forms the spring elements.
14. An apparatus for measuring acoustic and/or seismic signals, comprising a mechanical transducer comprising a continuous or discrete coupled mass-spring network with varying masses and/or spring constants, the mass-spring network configured to transform a comparatively small-dimensioned motion parameter of a first mass element into a comparatively large-dimensioned motion parameter of a further mass element, wherein the mass-spring network comprises one or more intermediate mass elements arranged between the first mass element and the further mass element and coupled to the first mass element and the further mass element by spring elements; and a measurement device for measuring at least one motion parameter of the further mass element and/or of any intermediate mass elements.
15. A method for measuring acoustic and/or seismic signals by an apparatus comprising a mechanical transducer comprising a continuous or discrete coupled mass-spring network with varying masses and/or spring constants, the mass-spring network configured to transform a comparatively small-dimensioned motion parameter of a first mass element into a comparatively large-dimensioned motion parameter of a further mass element, wherein the mass-spring network comprises one or more intermediate mass elements arranged between the first mass element and the further mass element and coupled to the first mass element and the further mass element by spring elements, and a measurement device for measuring at least one motion parameter of the further mass element and/or of any intermediate mass elements wherein the first mass element of the transducer is exposed to the signal to be measured, and wherein motion data of the further mass element and/or of any intermediate mass elements of the transducer are measured by means of the measurement device.
16. The method as claimed in claim 15, wherein a spectrum of the measured motion data is analyzed, and wherein a type of acoustic and/or seismic signal is determined based on this analysis.
17. The transducer as claimed in claim 6, wherein the mass-spring network comprises at least seven intermediate mass elements.
Description
SHORT DESCRIPTION OF THE FIGURES
[0029] Preferred embodiments of the invention are described in the following with reference to the drawings, which only serve for illustration purposes, but have no limiting effects. In the drawings it is shown:
[0030] FIG. 1 a schematic cross-sectional view of a first embodiment of an inventive mechanical transducer, illustrating the basic principle of the invention;
[0031] FIG. 2 a cross-sectional view of a second embodiment of an inventive mechanical transducer;
[0032] FIG. 3 a perspective view of a third embodiment of an inventive mechanical transducer;
[0033] FIG. 4 a plane view from above on the mechanical transducer shown in FIG. 3;
[0034] FIG. 5 a plane view from above on a fourth embodiment of an inventive mechanical transducer;
[0035] FIG. 6a a first step of the process for producing a fifth embodiment of an inventive mechanical transducer, in cross-sectional view and with a legend;
[0036] FIG. 6b a second step of the process for producing a fifth embodiment of an inventive mechanical transducer, in cross-sectional view;
[0037] FIG. 6c a third step of the process for producing a fifth embodiment of an inventive mechanical transducer, in cross-sectional view;
[0038] FIG. 6d a fourth step of the process for producing a fifth embodiment of an inventive mechanical transducer, in cross-sectional view;
[0039] FIG. 6e a fifth step of the process for producing a fifth embodiment of an inventive mechanical transducer, in cross-sectional view;
[0040] FIG. 6f a sixth step of the process for producing a fifth embodiment of an inventive mechanical transducer, in cross-sectional view;
[0041] FIG. 7 a diagram illustrating the transient displacement response of an inventive mechanical transducer having eight mass elements upon harmonic excitation near its first Eigenfrequency mode;
[0042] FIG. 8 a diagram illustrating the displacement amplification (displacement mass n/displacement mass l) vs. number of coupled masses n;
[0043] FIG. 9 a diagram illustrating the spectral amplification achieved with 16 coupled masses, for two distinct damping coefficients; and
[0044] FIG. 10 a diagram illustrating the spectral amplification achieved with the same total mass, but with a different number of coupled masses.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0045] Various different embodiments of mechanical transducers are shown in FIGS. 1 to 6f. Identical or similar elements of the various embodiments or elements having the same or a similar function are marked with the same reference numerals throughout all FIGS. 1 to 6f.
[0046] FIG. 1 schematically shows a first inventive embodiment of a mechanical transducer for the detection, of weak acoustic and/or seismic signals. The mechanical transducer which here comprises a discrete coupled mass-spring network illustrates particularly well the basic principle of the invention.
[0047] The mass-spring network of the transducer shown in FIG. 1 comprises a total of n discrete mass elements 21 to 25 which are coupled to each other and to a support structure 10 in a chain-like manner, i.e. in series, by means of n spring elements 31 to 35. Spring elements 31 to 35 can be spiral springs or any other type of springs such as beams. The first mass element 21 has a mass m.sub.1 and is coupled to the support structure 10 by means of the first spring element 31 having spring constant k.sub.1. Via the second spring element 32 with spring constant k.sub.2, the first mass element 21 is coupled to the second mass element 22 which has a mass m.sub.2. An arbitrary number of further mass elements being interconnected by respective spring elements can be coupled in a chain-like manner to the second mass element 22 via the third spring element 33. An n.sup.th mass element 25 is coupled to the end of the chain by means of the n.sup.th spring element 35 having spring constant k.sub.n.
[0048] In the embodiment as shown in FIG. 1, the respective masses of mass elements 21 to 25 are consecutively decreased from the first mass element 21 to the n.sup.th mass element 25. Thus, the following relationship holds with respect to the masses m.sub.1 to m.sub.n of mass elements 21 to 25:
m.sub.1>m.sub.2> . . . m.sub.n.
[0049] In order to achieve an efficient motional amplification, in this case displacement amplification, from the first mass element 21 to the n.sup.th mass element 25, the spring constants of spring elements 31 to 35 are likewise consecutively decreased from the first mass element 21 to the n.sup.th mass element 25. The relationship between the spring constants k.sub.1 to k.sub.n of spring elements 31 to 35 is as follows:
k.sub.1>k.sub.2>k.sub.3> . . . >k.sub.n.
[0050] A large motional amplification, particularly displacement amplification, from the first mass element 21 to the n.sub.th mass element 25 is particularly achieved, if the ratio of mass to spring constant remains essentially constant for each pair of mass element and spring element:
[00001]
[0051] The support structure 10 which is only shown in part in FIG. 1 can for example form a housing or a frame of the mechanical transducer. The support structure 10 is preferably directly coupled to the acoustic and/or seismic signal source, such that in the event of an external excitation of the mechanical transducer by means of the acoustic and/or seismic signal source the support structure 10 is moved along displacement direction a. The motion of the support structure 10 is usually of such little extent, that it cannot easily be detected with the usual equipment for measuring small motional amplitudes. The motion of the support structure 10, however, is transferred into a respective displacement of the first mass element 21 due to the coupling with the first spring element 31 and further, via the second spring element 32 to the second mass element 22 until the n.sup.th mass element 25. Since the mass and spring constant is gradually decreased in this process, the kinetic energy of the support structure 10 is transformed from a comparatively small displacement of the support structure 10 and of the first mass element 21 into a comparatively large displacement of the n.sup.th mass element 25 in direction b.
[0052] A measurement device not shown in FIG. 1 is used to measure displacements of the n.sup.th mass element 25. The measurement device can for example be a capacitive displacement sensor in which the n.sup.th mass element 25 forms one of the electrodes. An optical measurement device or a laser interferometer can also be used for this purpose. Further possibilities include e.g. the use of a measurement device that utilizes the piezoresistive or piezoelectric effect. Due to the purely mechanical amplification of the motion of the support structure 10 into a comparatively large displacement of the n.sup.th mass element, it is possible to even detect motion in the pm-range for displacement, in the μm/s-range for velocity and in the mg-range for acceleration by the apparatus comprising the mechanical transducer and the measurement device. Apart from the usual energy needed by the measurement device, no further energy needs to be provided to the apparatus and particularly to the mechanical transducer.
[0053] FIG. 2 shows a second embodiment of a mechanical transducer which is based on the same principle as the transducer shown in FIG. 1. The transducer according to FIG. 2 has a total of four mass elements 21 to 24 which are connected by respective spring elements 31 to 34 (thus, n=4). Each of the spring elements 31 to 34 here has the form of a leaf or a beam spring. The main oscillation direction of the mass elements 21 to 24 is perpendicular to the longitudinal direction of the springs, i.e. along the top-down direction in FIG. 2. The embodiment as show in FIG. 2 has the advantage that the coupled mass-spring network can be made in one piece.
[0054] FIGS. 3 and 4 show a third embodiment in which the mass elements 21 to 25 are concentrically arranged, with the smallest, 8.sup.th mass element 25 being arranged in the center (thus, n=8). The support structure 10 is here designed as a square frame that forms the periphery of the mechanical transducer. In the plane view from above as shown in FIG. 4, the n.sup.th or in this case 8.sup.th mass element 25 has the form of a filled square. The first to the 7.sup.th mass elements are concentrically arranged between the support structure 10 and the 8.sup.th mass element 25. The radial outer surface of the first mass element 21 is connected to the radial inner surface of the support structure 10 by means of four first spring elements 31 regularly distributed along the periphery of the first mass element 21. The first mass element 21 is connected to the second mass element 22 by means of four second spring elements 32 being attached to the radial inner surface of the first mass element 21 and the radial outer surface of the second mass element 22. The second mass element 22 is coupled to the third mass element in the same manner by means of four third spring elements. The 4.sup.th, 5.sup.th, 6.sup.th, 7.sup.th and 8.sup.th mass elements are coupled to each other in an analogous way by means of respective spring elements.
[0055] Thus, the support structure 10, the first mass element 21, the n.sup.th mass element 25 as well as the intermediate mass elements provided between the first and the n.sup.th mass element are all arranged in a common plane. Upon external excitation of the support structure 10, the mass elements 21 to 25 are displaced in an essentially perpendicular direction out of this common plane.
[0056] The spring elements 31 to 35 of the embodiment shown in FIGS. 3 and 4 can for example be designed as leaf springs or as elastic bands, such as rubber bands.
[0057] The embodiment of FIGS. 3 and 4 has the advantage that the mechanical transducer as a whole has a very flat and therefore space-saving design. Furthermore, the mass elements 21 to 25 are effectively protected by the support structure 10 with regard to external mechanical influences e.g. during transportation of the transducer. Due to geometric reasons with the given concentric arrangement, the masses of mass elements 21 to 25 gradually decrease from the radially outermost first mass element 21 to the centrally arranged n.sup.th mass element 25, if each of mass elements 21 to 25 is made from the same material and has the same thickness.
[0058] The embodiment shown in FIG. 5 differs from the embodiment of FIGS. 3 and 4 by the circular shape of the mass elements 21 to 25. The first mass element 21 as well as the intermediate mass elements each has the form of a ring being concentrically arranged around the n.sup.th mass element 25 having the form of a filled circular plate. A large number of first, second, third, etc. and n.sup.th spring elements are provided to couple each pair of neighboring mass elements. The spring elements are distributed in regular distances along the periphery of each mass element. If the spring elements are identical, their number has to increase gradually from the n.sup.th mass element 25 towards the support structure 10, if a change of the effective spring constants between the mass elements is required.
[0059] FIGS. 6a to 6f show a particular method for producing a mechanical transducer according to a fifth embodiment (FIG. 6f). The bulk layer is used to form the mass elements and the device layer to form the spring elements. Photolithography is used to transfer the respective patterns onto the wafer.
[0060] According to FIG. 6a, it is the first step of the method to provide a layered silicon-on-insulator (SOI) wafer with a silicon dioxide (SiO.sub.2)-layer 42 being sandwiched between two silicon (Si)-layers that form a bulk layer 41 and a device layer 43, respectively. A photoresist 44 is then applied to the device layer 43 of the wafer such that gaps are provided in the photoresist 44 at the intended positions of the spring elements. By means of deep reactive ion etching (DRIE) or wet etching techniques, the part of the Si-layer in the regions of the gaps is removed down to the SiO.sub.2-layer 42. Thus, a patterned device layer 43 is obtained (FIG. 6b).
[0061] In the next step, the device layer 43 is spin-coated by a protective layer of photoresist 44 (FIG. 6c). Subsequently, a photoresist is applied to the bulk layer 41 at the intended positions of the mass elements (FIG. 6d). With DRIE or wet etching techniques, the parts of the bulk layer 41 which are not covered by the photoresist 44 are removed down to the SiO.sub.2-layer 42, results in a patterning of the bulk layer 41 (FIG. 6d). The etching is carried out, while the wafer is placed and attached by means of an adhesive 45 on a Si-carrier 46.
[0062] As shown in FIG. 6f, the spring elements 31 to 34 are released from the SiO.sub.2-layer 42 with wet etching using hydrofluoric acid or with dry etching using vapor hydrogen fluoride (HF) or in a reactive ion etching step (RIB).
[0063] Thus, by means of the proposed method the mechanical transducer can be produced from a single SOI-wafer. As shown in FIG. 6f, the bulk layer 41 forms the support structure 10 as well as the first, second, third and fourth mass element 21, 22, 23 and 24 of the transducer. The first, second, third and fourth spring elements 31, 32, 33 and 34, each in the form of a leaf spring, are formed by the device layer. 43. The mechanical transducer as shown in FIG. 6f has a rotationally symmetric design, similar as the one shown in FIG. 5.
[0064] FIG. 7 illustrates the shoaling-like transient response of a mechanical transducer comprising eight mass elements under harmonic external excitation near the first Eigenfrequency mode of the transducer. It is clearly discernible, how the kinetic energy is transformed from the first mass element via the intermediate mass elements to the eighth mass element. During this process, the displacement gradually increases with each mass element, such that a comparatively large displacement of the eighth mass element is obtained.
[0065] FIG. 8 shows a diagram illustrating the relationship of signal amplification (displacement mass n/displacement mass l) vs. the number of coupled masses n. Obviously, large amplifications can be achieved with a high number of masses in the mass-spring network. The dashed continuous curve represents a simple mathematical scaling law obtained by assuming equipartition of kinetic energy of the different masses in steady-state.
[0066] In FIG. 9, the spectral amplification achieved with 16 coupled masses is shown, for two distinct damping coefficients c. It can be seen, that an amplification over a broad frequency range is achieved. FIG. 10 shows that the frequency range of the amplification is directly dependent on the number n of mass elements. With eight mass elements, the spectral transfer function allows for a displacement amplification in a considerably broader range of frequencies as compared to the situation with four mass elements. It can also be seen from FIGS. 9 and 10, that a spectral transfer function is achieved that has the same number of essentially regularly distributed spectral peaks as the number n of discrete masses. Thus, by carefully choosing the masses m.sub.1, m.sub.2, . . . , m.sub.n of the mass elements 31 to 35 and the spring constants k.sub.1, k.sub.2, . . . , k.sub.n of the spring elements 21 to 25, the spectral transfer function of displacement amplification can arbitrarily be designed with regard to the expected signals to be detected and with regard to the measurement requirements.
[0067] The location of the peaks in the spectrum and thus the frequencies with maximal amplification are directly dependent on the ratios of the masses to the respective spring constants of the mass-spring network. In other words, the masses and spring constants of the transducer can be chosen such that the maximal amplification occurs at certain frequencies or over a certain frequency range. An amplification over a broad range of frequencies can particularly be obtained by means of the mechanical transducers shown in FIGS. 1 to 6d by varying the ratio of mass to spring constant from mass element to mass element.
[0068] The coupled mass-spring networks according to the embodiments shown in FIGS. 1 to 6f do not necessarily be discrete, but could alternatively also be continuous. In a continuous mass-spring network, the transitions between the mass elements and the spring elements are not sudden, but smooth. In combination with electrostatic spring softening and/or electrostatic damping, the bandwidth and shape of the spectral transfer function is further adaptable. Thus, the spectral transfer function can be freely shaped within the bandwidth, e.g. also to not have less and/or less distinct peaks as the transfer functions shown in FIGS. 9 and 10. The continuous mass-spring network could particularly .be designed as a membrane with varying thickness and/or tension in the radial direction. The mass elements of such a continuous mass-spring network would in this case at least partly also act as springs and the spring elements as masses. While a purely discrete mass-spring network is shown in FIG. 1, a certain smoothness in the transitions between the mass elements and the spring elements is present in the embodiment according to FIG. 2. An embodiment comprising a continuous membrane instead of discrete masses and springs can be obtained by replacing the mass elements 21 to 25 and the spring elements 31 to 35 of the embodiments shown in FIGS. 3 to 5 by a respective membrane.
[0069] The invention is of course not limited to the preceding presented embodiments and a plurality of modifications is possible. For example, the coupled mass-spring network does not necessarily need to have a one-dimensional structure as in the embodiments of FIGS. 1 and 2 or a two-dimensional structure as in the embodiments of FIGS. 3 to 5, but could also have a three-dimensional structure, in order to measure input signals independently of their directions. Furthermore, the mass elements 21 to 25 of the embodiments shown in FIGS. 1 to 6f do not necessarily need to vary in mass, but could also have constant masses. Instead of varying the masses, the spring constants of the spring elements 31 to 35 could be varied in these embodiments. It would also be possible to design the mass-spring network such, that not the displacement of the mass elements is amplified, but their velocity or acceleration. The spring elements 31 to 35 shown in FIG. 1 could of course also be designed as torsional springs, in order to amplify a torsional motion. A plurality of further modifications is possible.
TABLE-US-00001 REFERENCE NUMERALS 10 Support structure 21 First mass element 22 Second mass element 23 Third mass element 24 Fourth mass element 25 N.sup.th mass element 31 First spring element 32 Second spring element 33 Third spring element 34 Fourth spring element 35 N.sup.th spring element 41 Bulk layer 42 SiO.sub.2-layer 43 Device layer 44 Photoresist 45 Adhesive 46 Carrier a, b Displacement directions
