A vibration sensor, use of a vibration sensor and method for producing a vibration sensor

Abstract

The invention relates to a vibration sensor, use thereof and method for producing it. The vibration sensor comprises a first plate and a second plate, both having an electrically conductive surface. The plates are movably connected by an elastic substance that holds the plates at a distance from each other, such that a gap exists between the plates.

Description

[0001] A VIBRATION SENSOR, USE OF A VIBRATION SENSOR AND METHOD FOR PRODUCING A VIBRATION SENSOR

[0002] Field of the invention

[0003] The present invention relates to a vibration sensor, a use of a vibration sensor and a method for producing a vibration sensor. In particular, the present invention relates to a vibration sensor comprising a first plate and a second plate that are arranged to move relative to each other.

[0004] Background of the invention

[0005] Vibration sensors are known that are specifically adapted for recording voice. Such vibration sensors may also be referred to as "voice pick-up unit" or "VPU" . Vibration sensors include a movable mass and means for detecting the movement of said mass.

[0006] In a known type of vibration sensor, the movable mass induces pressure variations in a sealed air volume. These pressure variations are detected by a MEMS microphone. The relatively large surface that supports the movable mass is driving a small pickup surface, namely the membrane of the MEMS microphone. This detection principle is also referred to as "acoustic amplification". An example of such a device is described in EP 3 279 621 Al.

[0007] Description of the invention

[0008] A drawback of conventional vibration sensors is their relatively large size, whereas a more compact design would be desirable. Another drawback is that conventional vibration sensors typically have a peak sensitivity in the 4 kHz - 10 kHz range, whereas for some applications a lower frequency for the peak sensitivity is desirable.

[0009] An object of the present invention is to provide a vibration sensor that overcomes one or more of these drawbacks, or at least provides an alternative. This object is achieved by the system and method according to the invention.

[0010] According to the invention, the vibration sensor comprises a first plate, a second plate and an elastic substance that movably connects the first and second plates. The first plate comprises a first surface that is electrically conductive. The second plate comprises a second surface that is electrically conductive. The elastic substance holds the plates at a distance from each other such that a gap exists between the plates.

[0011] The first and second plates are connected to each other by the elastic substance. The elasticity of the substance enables the plates to move relative to each other.

[0012] P352WOOO In an embodiment, the elastic substance is provided at one or more discrete locations along an edge of the first plate and / or the second plate, thereby leaving the gap open along a part of said edge. In other words, only a part of the edge(s) is / are provided with the elastic substance, while another part of the edge(s) is / are free of the elastic substance. The elastic substance is thus restricted to the discrete location(s) along the edge(s) of the first and / or second plate.

[0013] The two plates form a mass-spring system. The undamped natural frequency a>nof a massspring system is given by ojn= wherein k is the spring constant (proportional to stiffness), and m is the mass. By providing the elastic substance only at one or more discrete locations along the edge of the plate(s), a relatively low stiffness k is obtained, which aids in achieving a relatively low frequency for the peak sensitivity. In particular, the vibration sensor requires less mass to be added for tuning the peak sensitivity to lower frequencies, as compared to more stiff systems. This results in a more compact design.

[0014] Moreover, the structure (two plates held apart by an elastic substance) is by itself already quite compact, as it does not require an additional MEMS package such as some types of conventional vibration sensors.

[0015] In addition, the structure is relatively strong, and is thus capable of supporting a relatively large amount of mass. Thus, the peak sensitivity of the vibration sensor can be tuned over a relatively large frequency range. For comparison, conventional MEMS vibration sensors include a silicon structure for supporting a movable element. The frequency of such a vibration sensor can, in theory, be lowered to some extent by adding mass to the silicon structure. However, the strength of the silicone structures of the MEMS sensors is limited, so that only a limited amount of mass can be added without breaking the structure. Moreover, in practice, adding mass to silicon structures is difficult, as compatible materials for MEMS production are light, so that the amount of mass that can be added is relatively small.

[0016] Thus, the low stiffness and the strong construction provide more design freedom to tune the peak sensitivity of the vibration sensor to the desired frequency range, and in particular low frequency ranges.

[0017] Preferably, the Young's modulus of the elastic substance is below 0.5 GPa, more preferably below 100 MPa, even more preferably below 10 MPa.

[0018] In case of a visco-elastic substance, the storage modulus at 1 kHz is preferably smaller than 0.5 GPa, more preferably below 100 MPa, even more preferably below 10 MPa. For example, the storage modulus at 1 kHz is in the range of 10 Pa - 10 MPa. The loss modulus of the visco-elastic substance at 1 kHz is preferably in the range of 100 - 10 MPa at 1 kHz.

[0019] Preferably, the elastic substance is the sole mechanical connection for movably connecting the two plates.

[0020] Preferably, the gap between the plates is substantially free of the elastic substance.

[0021] P352WOOO In one example, the first plate has a larger surface area than the second plate, and the elastic substance is provided at one or more discrete locations along an edge of the second plate.

[0022] In one embodiment, at least one of the plates comprises a structure for capillary action.

[0023] For example, during production the elastic substance is applied at the structure for capillary action. The elastic substance is pulled into the structure by capillary action. This prevents the substance from flowing in between the plate, which would increase the stiffness of the system and negatively affect the frequency response of the sensor. Moreover, the capillary structure prevents the elastic substance from flowing along an edge of the plates. In other words, the structure restricts the elastic substance to the site of application.

[0024] Preferably, each of the one or more discrete locations (the locations where the elastic substance is applied) comprise such a structure for capillary action.

[0025] In an embodiment, the elastic substance is a cured substance. In other words, during production the substance is applied in uncured state, after which one lets the substance cure. After curing, the cured substance forms a mechanical connection between the two plates, that allows movement of the plate relative to each other.

[0026] In an embodiment, the structure comprises an opening or recess dimensioned for capillary action with the elastic substance. In case the elastic substance is a substance that requires curing (such as an elastic adhesive or curable gel), the opening or recess is dimensioned for capillary action with the uncured elastic substance.

[0027] For example, more than one structure for capillary action is provided. For example, both plates comprise one or more such structures.

[0028] In an embodiment, both plates comprise a structure for capillary action. The structures of the first plate and second plate are positioned near each other such that elastic substance provided in the structures forms a bridge between the first plate and the second plate.

[0029] In other words, a pair of capillary structures is provided: the first plate comprises a first structure of the pair and the second plate comprises a second structure of the pair. Notably, more than one pair of capillary structures may be provided. In each pair, the first structure (of the first plate) is positioned near the second structure (of the second plate), such that - when the elastic substance is applied to the pair of capillary structures - the elastic substance forms a bridge between the structures.

[0030] In an embodiment, one of the plates comprises a spacer opening to separate the gap between the plates from the structure for capillary action of the other one of the plates. This prevents the elastic substance from flowing between the plates during production.

[0031] The gap between the plates is small, e.g. 3-30 pm, and thus exhibits capillary behaviour. When during production elastic substance is applied near the gap, it may be drawn into the gap between the plates. This is undesirable, as presence of substance in between the plates leads to a dramatic increase in stiffness of the system. The spacer opening prevents this by

[0032] P352WOOO ensuring that the structure for capillary action (the site for applying the elastic substance) is at a distance from the gap between the plates.

[0033] In a further embodiment, an edge of the spacer opening forms two protrusions that extend into the spacer opening, such that a recess for capillary action is defined between the two protrusions. Thus, a combined structure is formed that comprises both the spacer opening and the structure for capillary action.

[0034] In an embodiment, the elastic substance is provided at one or more corner regions of the first plate and / or second plate. For example, the respective plate has a substantially rectangular shape.

[0035] In an embodiment, the first plate comprises an electrically non-conductive substrate, and the first surface is formed by an electrically conductive material provided on the electrically non- conductive substrate. Preferably, the substrate comprises a printed circuit board (PCB) onto which electrically conductive material is deposited.

[0036] In a first example, also the second plate comprises a non-conductive substrate on which an electrically conductive material is provided to form the second surface. In another example, the second plate comprises an electrically conductive plate.

[0037] In an embodiment, the first plate and the second plate each comprise an electrically conductive plate.

[0038] In the above embodiments, electrically conductive plates preferably comprise metal, such as steel, bronze, zinc or copper. In a current preferred embodiment, the first and / or second plate comprise brass. An advantage of brass is that it is particularly suitable for making flat plates. In addition, brass enables production through laser cutting. Moreover, brass is not ferromagnetic, which is advantageous if the vibration sensor is exposed to magnetic fields during production or use.

[0039] In an embodiment, the elastic substance is provided at only one end of the first plate and / or the second plate, for hinging movement of the first and second plate relative to each other.

[0040] In an alternative embodiment, the elastic substance is provided at opposing ends of the first plate and / or the second plate, for movement of the first and second plate relative to each other in a direction substantially perpendicular to the first and second surface.

[0041] In an embodiment, the elastic substance is provided as at least two discrete amounts of elastic substance. The two amounts of elastic substance are provided at different locations along the edge of the first and / or second plate.

[0042] Preferably, the amount of elastic substance is predetermined. For example, the predetermined amount of elastic substance applied at each of the discrete locations is 1 - 500 nl, such as 3 - 125 nl.

[0043] P352WOOO In an example of a substantially rectangular plate, two discrete amounts of the elastic substance are provided at two corners of one end of the plate, whereas a single discrete amount of elastic substance is provided at an opposing end of the plate, at a substantially central position of the edge of said opposing end.

[0044] In a preferred embodiment, the elastic substance is a viscoelastic substance. In this manner, an amount of damping is achieved, which may be desirable for some applications. For example, the viscoelastic substance comprises a polymer.

[0045] For example, the elastic substance comprises a gel. Preferably, the elastic substance is an elastic adhesive, preferably a viscoelastic adhesive. Adhesives are particularly suitable for holding the plates at a distance to each other.

[0046] In a preferred embodiment, the elastic substance comprises silicone. Preferably, the elastic substance comprises an adhesive comprising silicone. More preferably, the elastic substance comprises a viscoelastic adhesive comprising silicone.

[0047] In an embodiment, one of the first and second plates is provided with a mass element. Adding additional mass to one of the plates lowers the frequency of the sensitivity peak, making the vibration sensor suitable for picking up low frequency sounds.

[0048] Moreover, adding mass to one of the plates improves the signal-to-noise (SNR) ratio of the vibration sensor. One source of noise in the signal is thermal noise originating from the capacitor: temperature induces small movements of the plates that result in a fluctuations in capacitance. By adding mass to one of the plates, the inertia is increased and the temperature induced movement is decreased. Hence, the thermal noise component is decreased.

[0049] Generally, the first plate and second plate are arranged for relative movement. In practice, one of the plates will often be considered fixed, while the other is considered to be movable. For example, one plate is fixed to a housing, which makes the other plate the movable plate. The one or more mass elements are preferably provided to the movable plate.

[0050] Preferably, the mass element comprises a high-density material, such as a metal (e.g. steel or brass). Preferably, the mass element comprises tantalum. For example, the mass element comprises a stack of metal plates, e.g. a stack of tantalum plates.

[0051] Preferably, the mass element comprises a non-magnetic material, such as tantalum, tungsten or brass. This enables using the vibration sensor in presence of a (high) magnetic field.

[0052] In an embodiment, the vibration sensor further comprises a housing arranged to limit movement of the plate with the mass element.

[0053] The housing acts as a shock protection that prevents movement of the plate with the mass element beyond a predefined maximum amplitude. This prevents damage to the vibration sensor.

[0054] P352WOOO For example, a wall of the housing limits the movement of the plate. In another example, the housing comprises a protrusion that limits the movement of the plate.

[0055] The housing may limit movement of the plate in the mass element in one or more directions. Preferably, the housing limits movement in a direction perpendicular to the surface of the plate. Optionally, the housing also limits movement in other directions, e.g. lateral movement of the plate carrying the mass element.

[0056] In a further embodiment, the housing encloses the mass element. The mass element protrudes from said one of the first and second plate towards a part of the housing. Said part is arranged to limit the movement of the plate with the mass element.

[0057] In a rest position (no movement) the mass element does not touch the wall of the housing. Likewise, during normal operation (vibration within predefined limits), the mass element does not touch the wall either. Only when the vibration exceeds a predefined limit (e.g. in case of a shock), will the mass element touch the wall.

[0058] In a further embodiment, said wall of the housing is formed by a substrate (e.g. a PCB) provided with read-out electronics. The read-out electronics are connected to the first and / or second surface. The read-out electronics are preferably arranged to the side of the mass element. This results in a compact design. For example, one of the plates comprises the substrate.

[0059] In an embodiment, the vibration sensor has a sensitivity curve with a peak below 10 kHz, preferably below 5 kHz. Preferably, the peak is at or below 2 kHz, preferably at or below 1 kHz.

[0060] A peak in the range of 1 - 2 kHz is for example achieved by a stiffness k in the range of 900 - 1200 N / m in combination with a mass of the movable plate (and any optional mass elements provided thereon) in the range of 10 - 14 mg. For example, a stiffness k of approximately 1100 N / m with mass of approximately 12 mg.

[0061] Preferably, the vibration sensor is a micro-mechanical vibration sensor. In the context of the invention, "micro-mechanical" refers to dimensions in the micrometre and millimetre range. Notably, micro-mechanical is distinct from "MEMS" which refers to micro-mechanical systems produced using semi-conductor processes (such as etching and deposition). Preferably, the vibration sensor is not a MEMS device.

[0062] The distance between the first and second plate is for example 3-30 pm. Preferably, the distance between the first and second plates is 5-15 pm.

[0063] A smaller gap between the plates increases the capacitance and results in a more sensitive device. In particular, the signal of the capacitor is dominant over the electronic noise from the read-out electronics. However, a smaller gap also increases the amount of squeezed film damping and the thermal noise. The inventors found that a gap of 3-30 pm provides a good signal-to-noise ratio (SNR) while ensuring an acceptable amount of squeezed film damping.

[0064] P352WOOO Moreover, the inventors found that a gap of 5-15 pm provides an even better SNR while still ensuring that the amount of squeezed film damping is acceptable.

[0065] In an embodiment, at least one of the plates is provided with one or more opening for lowering the amount of squeezed film damping.

[0066] In an embodiment, the first and second surfaces form a capacitor. In one example, an electret layer is provided to one of the surfaces. In a further embodiment, the vibration sensor comprises read-out electronics configured to generate a signal in dependence on the electrical capacitance of the capacitor formed by the first and second surfaces.

[0067] In a preferred embodiment, the vibration sensor is a phonocardiogram (PCG) sensor, preferably having a sensitivity curve with a peak at or below 2 kHz, more preferably below 1 kHz, such as below 250-500 Hz or even below 150-200 Hz. The invention further relates to use of the vibration sensor according to any of the embodiments disclosed herein as a PCG sensor. The inventors found that such a PCG sensor has an increased sensitivity and lower noise, as compared to conventional PCG sensors, particularly when tuned (e.g. by adding mass) for a sensitivity peak at or below 2 kHz.

[0068] The invention further relates to a method for producing a vibration sensor. The method comprises providing a first and second plate. The first plate has a first surface that is electrically conductive. The second plate has a second surface that is electrically conductive. A spacer (e.g. of 5-15 pm thickness) is placed on one of the plates. The other plate is placed on the spacer. An elastic substance (e.g. an adhesive comprising silicone) is applied at one or more discrete locations along an edge of the first plate and / or the second plate to movably connect the first and second plates. The spacer is removed to create a gap between the plates. The gap is open along a part of said edge.

[0069] The same technical effects as described above in relation to the vibration sensor apply to its use as a PCG sensor and to the production method. Moreover, any features of the vibration sensor described above can similarly be applied in the method or use, and vice versa.

[0070] Brief description of the drawings

[0071] In the following, example embodiments will be described with reference to the drawings, wherein:

[0072] Figure 1 shows, in perspective view, a vibration sensor according to a first embodiment of the invention;

[0073] Figure 2 shows a detail of the vibration sensor of Figure 1 in top view;

[0074] Figure 3 is a photograph of a vibration sensor according to the first embodiment, showing the elastic adhesive;

[0075] P352WOOO Figure 4 is an exploded view of a vibration sensor according to a second embodiment of the invention;

[0076] Figure 5 is an assembled view of the vibration sensor of Figure 4;

[0077] Figure 6 is a schematic cross-sectional view of the vibration sensor of Figures 4 and 5;

[0078] Figure 7 is a schematic cross-sectional view of an alternative to Figure 6;

[0079] Figure 8 is an exploded view of a vibration sensor according to a third embodiment of the invention;

[0080] Figure 9 is an cross-sectional side view of the vibration sensor of Figure 8 in assembled state;

[0081] Figure 10 is an exploded view of a vibration sensor according to a fourth embodiment of the invention;

[0082] Figure 11 is an assembled view of the vibration sensor of Figure 10, with the top cover removed;

[0083] Figures 12 and 13 illustrate a method for producing a vibration sensor according to an embodiment of the invention;

[0084] Figure 14 shows a graph of sensor sensitivity as function of frequency, for three differently tuned vibration sensors according to an embodiment of the invention; and

[0085] Figure 15 shows the results of modelling signal levels of a vibration sensor according to embodiments of the invention.

[0086] Detailed description of the drawings

[0087] Elements in a figure that correspond to elements in a different figure have been given the same reference numeral, increased by a multiple of 100, and the same description applies to such elements - unless otherwise specified.

[0088] Figure 1 depicts a vibration sensor 2 according to a first embodiment. The vibration sensor 2 comprises two conductive plates: a bottom plate 4 and a top plate 6 that extend parallel to each other and at a distance to each other. The plates 4, 6 are made of a conductive material. Specifically, the plates 4, 6 comprise metal. In this example, plates 4, 6 are brass plates.

[0089] The corner regions of the top plate 6 are each provided with a recess 8. The bottom plate 4 is provided with openings 10. Specifically, as shown in detail in figure 2, each recess 8 of the top plate 6 is flanked by protruding portions 12. The opening 10 in the bottom plate 4 is shaped such that it defines a portion with the same shape, i.e. a portion with a recess 14 flanked by two protruding portions 16. The recess 14 and protruding portions 16 of the bottom plate 4 have substantially the same dimensions as the recess 8 and protruding portions 12 of the top plate 6.

[0090] P352WOOO The plates 4, 6 are positioned such that the recesses 8, 14 of the plates 4, 6 are aligned. The recesses 8, 14 face in opposite directions (as seen in the top view of Figure 2). An elastic substance 18 is provided at each pair of recesses 8, 14 of the plates 4, 6 (see the photograph of Figure 3). In this example, the adhesive 18 is thus applied at four distinct locations (the four corners). At each of these distinct locations, a bridge between the plates 4, 6 is formed by the elastic substance 18.

[0091] In this example, the elastic substance 18 comprises a silicone-based adhesive. Due to capillary action, the adhesive 18 is pulled into the recess 8, 14, which prevents the adhesive 18 from flowing between the plates or along the edge of the plates.

[0092] In figure 2, the presence of opening 10 underneath the second plate 6 is indicated by dashed lines. The fact that the opening 10 in bottom plate 4 extends below the recess 8 of the top plate 6, ensures that the recess 8 is at a distance to the gap between the plates. The opening 10 is therefore also referred to as "spacer opening". By providing a distance between the recess 8 and the gap between the plates, the adhesive 18 is prevented from flowing between the plates due to capillary action of said gap.

[0093] As is clearly seen from the drawing, the elastic substance 18 extends along a very small fraction of the circumferential edge of the top plate 6. In this example, the elastic substance spans less than 2% of circumferential edge of the first plate.

[0094] The plates 4, 6 are arranged at a distance of 5 pm from each other. The adhesive 18 movably connects the plates 4, 6, while holding them at said 5 pm distance from each other. The recesses 8, 14 ensure that the adhesive 18 connecting the plates 4, 6 is located at a well- defined location, and together with opening 10 prevent adhesive 18 from flowing between the plates. The size of the recesses 8, 14 and the thickness of the top plate, determine the dimensions of the bridge formed by adhesive 18. These parameters can be tuned to obtained the desired stiffness and capillary action.

[0095] In addition, the capillary action may contribute to aligning the plates 4, 6. The capillary force exerted by recesses 14 and / or openings 10, pulls the plate 6 into an aligned position in the XY plane (plane parallel to the surface of plate 6).

[0096] As can been seen in Figure 3, the silicone 18 may flow to some extent over the top of top plate 6, which has no significant impact on the stiffness of the connection nor the frequency response.

[0097] In the illustrated example, the adhesive 18 forms the sole mechanical connection between the plates 4, 6 and is the determining factor for the stiffness of the mass-spring system.

[0098] The adhesive 18 leaves the gap between the plates 4, 6 open, i.e. the adhesive does not seal said gap.

[0099] In this example, the recess 8 has a depth of 0.1 mm and a width of 0.15 mm. The height of the recess 8 corresponds to the thickness of plate 6, which in this example is 0.1 mm.

[0100] P352WOOO At one end L of the two conductive plates 4, 6 (left side in the drawing) a feed-through for electric wires is provided for connecting the plates 4,6 to electronics. In figure 1, the feed- through is formed by a through-hole 20 in the bottom plate. The feed-through is optional. Other options exist, e.g. electric wires may extend to a side of the plates 4, 6.

[0101] Figures 4 and 5 show a vibration sensor 102 according to a second embodiment. In this embodiment, three distinct amounts of silicone glue 118 (figure 5) are applied to movably connect the bottom plate 104 to the top plate 106, whereas in the embodiment of Figures 1-3 four distinct amounts of silicone are used. At one end L (left side in figures 4 and 5) of the top plate 106 two distinct amounts of silicone 18 are applied: one at each corner region. This is identical to the left-hand side of figures 1-3. At the opposing end R (right side in figures 4 and 5), a single amount of silicone 118 is applied: at a position substantially central between the two corners of end R. Therefore, less silicone 118 is used in vibration sensor 102 of figures 4 and 5 as compared to vibration sensor 2 of figures 103. This further reduces the stiffness of the system. As before, the silicone glue 118 is the sole mechanical connection between the plates 104, 106.

[0102] Figures 4 and 5 also shows an additional mass 124 that is provided onto the top plate 6. In this example, the mass 124 is formed by a stack of metal plates of substantially equal weight. Although not shown in figures 1-3, also the first embodiment is preferably provided with an additional mass 124, e.g. in the form of a stack of metal plates. In the example of the figures, the mass plates comprise tantalum.

[0103] By increasing the mass of the system, the resonance frequency of the vibration sensor is lowered. In addition, the mass increases the SNR of the sensor.

[0104] In the example, the top plate 6, 106 has a mass of 2.9 mg. The additional mass 124 is 10.0 mg (four metal plates of 2.5 mg each), bringing the total moving mass to 12.9 mg. The stiffness k of the silicone connection between the plates of exemplary vibration sensors 2, 102 is approximately 1145 N / m.

[0105] Figure 4 further shows a venting hole 126 for affecting the damping coefficient of the system. The parallel plates provided intrinsic resonance damping by means of squeezed film damping between the plates 104, 106 (due to the presence of air between the plates). The amount of intrinsic damping may be relatively high, and is reduced by including venting hole 126 in bottom plate 4. Optionally, more than one venting hole 126 is provided in bottom plate 4 to tune the damping coefficient. Additionally or alternatively, the top plate 106 may be provided with venting holes.

[0106] In the embodiment of figures 4-5, a feed-through for electrical wires is formed by a through- hole 120 in the bottom plate 4, and a recess 122 in the top plate 106. The recess 122 is aligned with the through-hole 120 (figure 5) to enable passing electrical wires. As before, the feed-through is optional.

[0107] P352WOOO Figure 6 is a highly schematic cross-sectional view of the vibration sensor 102. Figure 6 is not to scale: in particular the distance between the plates is in reality much smaller than the thickness of the plates. Figure 6 clearly shows that the conductive parallel plates 104, 106 of vibration sensor 102 form a capacitor, i.e. each plate forms one of the electrodes of the capacitor. Each plate 104, 106 of the vibration sensor is connected to electronics 128 via wires 127. The electronics 128 are configured to bias the plates 104, 106, i.e. electrically charge the plates. The electronics 128 include a pre-amplifier to amplify the voltage that varies in response to vibration (as vibration changes the distance between the capacitor plates). The electronics 128 produce an output signal indicative of the amount of vibration. Thus, the vibration sensor 102 is a capacitive sensor that detects movement of the plates relative to each other using the electronics 128. Vibration sensor 2 of figures 1-3 is preferably provided with electronics in a similar manner.

[0108] Figure 7 is a highly schematic cross-sectional view of an alternative embodiment, wherein the vibration sensor 202 is embodied as an electret sensor. In this embodiment, the top plate 206 is an electrically conductive plate that includes a charged electret layer 207, comprising polytetrafluoroethylene (PTFE). The bottom plate 204 is an electrically conductive plate. The plates 204, 206 are connected to electronics 228 for sensing charge differences induced by displacement of the electret layer 207 relative to the bottom plate 204. As before, the electronics 228 generates an output signal indicative of the amount of vibration.

[0109] The vibration sensor preferably includes a housing. This is illustrated in figures 8 and 9 for the vibration sensor 102 of Figures 4-6, but the housing can in a similar manner be used with vibration sensors 2 and 202.

[0110] The housing comprises a PCB 130 onto which the electronics 128 is mounted. In this example, the electronics 128 comprises an integrated circuit (IC), particularly an application specific IC (ASIC). The housing further comprises a case portion 132 with a through-going opening 134. In the example shown, the case portion 132 is formed by a stack of plate-shaped spacers 136. Each spacer 136 comprises a circumferential wall that encloses the opening 134. Alternatively, the case 132 may be provided as single unit, e.g. a deep drawn piece of metal.

[0111] The bottom plate 104 of the vibration sensor 102 rests against a bottom surface B of the case portion 132 (vibration sensor 102 is depicted upside down with respect to figures 4-6). The mass 124 protrudes from the top plate 106 of the vibration sensor 102 and extends into the opening 134 of the case portion 132. In other words, the mass 124 is provided in an inner space of the case portion 132.

[0112] The PCB 130 rests against the top surface of the case portion 132, and thus forms a top cover of the housing. The case 132 is dimensioned such that a gap exists between the mass 124 and the PCB 130. At the same time, the PCB 130 forms a shock protection, as the PCB 130 it determines the maximum displacement for the mass 124. Damage to the vibration sensor 102 due to shocks is thus prevented.

[0113] P352WOOO The housing further comprises a bottom cover 138. In the example, the cover 138 comprises a plate-shaped spacer 136 and a cover plate 140, to form a chamber between the bottom plate 104 and the cover plate 140. Alternatively or additionally, air venting holes may be provided in the cover plate 140.

[0114] The ASIC 128 is electrically connected to the capacitor plates 104, 106. The ASIC 128 is configured for biasing and read-out of the capacitor formed by plates 104, 106. The ASIC 128 is mounted on the PCB 130 in a position aside the mass 124, such that a compact assembly is obtained.

[0115] In this example, the bottom plate 104 is connected to the housing, such that the top plate 106 can be considered as the movable plate, whereas the bottom plate 104 can be considered to be fixed, i.e. at rest with respect to the housing.

[0116] Figure 10 illustrates a vibration sensor 302 according to a fourth embodiment of the invention. The sensor 302 comprises a bottom plate 304 and a top plate 306. The bottom plate 304 comprises a PCB 304a provided with a conductive layer 304b. The top plate 306 is a conductive plate. In this example, top plate 306 is a brass plate. The plates 304, 306 are positioned substantially parallel to each other.

[0117] The top plate 306 comprises two protruding legs 342, extending from one end of the plate 306. The adhesive 318 is applied only to a part of an edge of the plate 306, namely only to the protruding legs 342 (Figure 11). No adhesive is present at the opposing end of the plate 306, such that the adhesive 318 allows a hinging movement of plate 306 with respect to the PCB 304a. The adhesive 318 also holds the top plate 306 at a distance from the bottom plate 304, particularly at a distance from the conductive layer 304b. In this example, the distance between the plates is 5 pm. In this example, the elastic adhesive 318 comprises silicone. The adhesive 318 is the sole mechanical connection between the plates 304, 306.

[0118] The plate 306 and conductive layer 304a form a capacitor. For bias and read-out of the capacitor, electronics 328 are provided and electrically connected to the conductive layer 304b and the conductive plate 306. The electronics 328 are mounted on the PCB 304a. The electronics 328 comprise an IC, particularly an ASIC.

[0119] The conductive top plate 306 comprises a venting hole 326 to fine-tune the damping coefficient of the sensor 302. The sensor 302 further comprises a cover 344 for enclosing the conductive layer 304b, the top plate 306 and the ASIC 328. When assembled, the cover 344 rests on the PCB 304a (not shown).

[0120] The PCB 304a further comprises openings 304c to prevent adhesive 318 from flowing into the gap between the plates 304a, 306 during production. The openings 304c are positioned opposite the location where the legs 342 join the plate 306. In other words, the projection of an opening 304c onto the plate 306 spans the area where the respective leg 342 joins the plate 306.

[0121] P352WOOO During assembly of a vibration sensor according to the invention (e.g. any of sensors 2, 102, 202, 302 described above), a temporary spacer may be used to position the plates at a distance of 5-15 pm from each other. This is schematically illustrated in figures 12 and 13, for an exemplary sensor in which the plates can perform a hinging movement with respect to each other. In this example, a spacer sheet 446 is provided that has a thickness corresponding to the desired distance between the plates (e.g. 5-15 pm). The spacer sheet 446 is placed in between bottom plate 404 and top plate 406. The elastic substance 418 is then applied to movably connect legs 442 of 406 to the bottom plate 404. The vibration sensor thus produced may optionally be provided with additional mass, e.g. by attaching mass - such as a stack of metal plates - to the top plate 406.

[0122] In one example, the temporary spacer comprises a sacrificial layer that is chemically removed after the adhesive is cured. In another example, the temporary spacer is a thin sheet 446 that is removed mechanically after curing of the adhesive.

[0123] A further exemplary embodiment of a method for producing a vibration sensor will be described with reference to figure 1. Bottom plate 4 is provided with spacer openings 10, feed-through opening 20 and venting hole 126. Top plate 6 is provided with recesses 8. A spacer sheet of 5 pm thickness (not shown in figure 1) is placed on bottom plate 4, and top plate 6 is positioned on top of the spacer sheet. The top plate 6 is positioned such that each of its recesses 8 aligns with a corresponding spacer opening 10 of the bottom plate 4. In particular, each recess 8 of the top plate 6 is positioned to face in an opposite direction as compared to a corresponding recess 14 of the bottom plate 4. Seen in a top view (e.g. figure 2), the recesses 8, 14 face each other, even though in the height direction (direction perpendicular to the plates 4, 6) a distance exists between the recesses 8, 14. Then, 4 nl of silicone glue 18 (figure 3) is applied to each pair of recesses 8, 14. The silicone glue 18 is drawn into the recesses 8, 14 by capillary action, and forms a bridge at that location once cured. The bridges form elastic connections between the plates 4, 6. The opening 10 contributes to preventing the silicone glue 18 to flow between the plates during production.

[0124] In the embodiments of figures 1-13, the elastic substance (18, 118, 218, 318, 418) is a non- conductive substance (namely an elastic adhesive comprising silicone). The invention is not limited to a silicone-based adhesive, nor to use of a non-conductive substance. For example, a conductive elastic substance may be used when at least one of the plates comprises a non- conductive substrate. The elastic substance may then combine two functions: mechanically connecting the plates and electrically connecting one or more of the plates to electronics.

[0125] Figure 14 shows sensitivity curves for three prototypes of vibration sensor 102 according to the embodiment of figures 4 and 5. The x-axis represents frequency, and the y-axis represents sensitivity to vibrations (expressed in dB V / g, i.e. the change in output voltage (in decibel) versus the acceleration in g [gravitational acceleration on Earth, 9.82 m / s2] ). Whereas conventional vibration sensors have a peak sensitivity at approximately 4 kHz, the vibration sensors have their peak sensitivity at approximately 1.5 kHz. This makes the

[0126] P352WOOO exemplary sensors particularly suitable for measuring low frequencies. For example, the sensors are particularly suitable for medical diagnostics such as for recording lung or heart sounds. In one particular example, the vibration sensor according to embodiments of the invention is used as a phonocardiogram (PCG) sensors.

[0127] In addition, the equivalent input noise of the vibration sensor 102 was measured, and compared to the equivalent input noise of a conventional vibration sensor (a vibration sensor using a MEMS microphone and acoustical amplification). The inventors found that the noise of the vibration sensor was reduced by 1 to 2 dB g (acceleration g, expressed in decibel) as compared to a conventional sensor (MEMS microphone, acoustical amplification).

[0128] Further, typical PCG signal and noise levels were modelled. For this model, the plate area was 3.25 mm2. The mass of the movable plate (plate + mass elements) was 12 mg. The resonance frequency was 1500 Hz. The bias voltage was 17.7 V and the preamplifier noise was 3 pV (in a band up to 1500 Hz).

[0129] For PCG signals, the acceleration to be measured is typically is in the range of 0.08 to 0.35 m / s2. For the model, the lowest signal to be measured was therefore set to a = 0.01 m / s, which corresponds to -40 dB m / s2.

[0130] The thermal noise and preamplifier noise was modelled for different air gaps, resulting in the graph of Figure 15. The x-axis shows the air gap (in pm, logarithmic scale), the y-axis shows the signal level (expressed in equivalent input acceleration - in dB m / s2). The graphs shows that the thermal noise (short dashes) decreases for increasing gap size, whereas the preamp noise (dotted line) increases for increasing gap size. The total noise (long dashes) shows that, for this model, an acceptable SNR (at least 20 dB) is obtained for a gap size of 3 - 30. This model has an optimum SNR at a gap size of 8 pm. This optimum will be different for different configurations (resonance frequency, mass, plate area, etc). Generally, a gap size of 5- 15 um is found to result in a good SNR.

[0131] EMBODIMENTS

[0132] 1. A vibration sensor, comprising:

[0133] - a first plate comprising a first surface that is electrically conductive;

[0134] - a second plate comprising a second surface that is electrically conductive; and

[0135] - an elastic substance that movably connects the first and second plates and holds the plates at a distance from each other such that a gap exists between the plates.

[0136] 2. The vibration sensor of embodiment 1, wherein the elastic substance is provided at one or more discrete locations along an edge of the first plate and / or the second plate, thereby leaving the gap open along a part of said edge.

[0137] P352WOOO 3. The vibration sensor of embodiment 1 or 2, wherein the elastic substance is provided at only a part of an edge of the first plate and / or the second plate.

[0138] 4. The vibration sensor of any one or more of the embodiments 1-3, wherein the gap between the plates is substantially free of the elastic substance.

[0139] 5. The vibration sensor of any one or more of the embodiments 1-4, wherein the elastic substance is a cured substance.

[0140] 6. The vibration sensor of any one or more of the embodiments 1-5, wherein at least one of the plates comprises a structure for capillary action.

[0141] 7. The vibration sensor of embodiment 6, wherein the structure comprises an opening or recess dimensioned for capillary action.

[0142] 8. The vibration sensor of the combination of embodiments 5 and 7, wherein the opening or recess is dimensioned for capillary action with the elastic substance in an uncured state.

[0143] 9. The vibration sensor of embodiment 8, wherein the elastic substance is provided in said opening or recess.

[0144] 10. The vibration sensor of any one or more of the embodiments 6-9, wherein both plates comprise a structure for capillary action, that are positioned near each other such that elastic substance provided in the structures forms a bridge between the first plate and the second plate.

[0145] 11. The vibration sensor of any one or more of the embodiments 6-10, wherein one of the plates comprises a spacer opening to separate the gap between the plates from the structure for capillary action of the other one of the plates.

[0146] 12. The vibration sensor of embodiment 11, wherein an edge of the spacer opening forms two protrusions that extend into the spacer opening, such that a recess for capillary action is defined between the two protrusions.

[0147] 13. The vibration sensor of any one or more of the embodiments 1-12, wherein the elastic substance is provided at one or more corners of the first plate and / or second plate.

[0148] 14. The vibration sensor of any one or more of the embodiments 1-13, wherein the first plate comprises an electrically non-conductive substrate and the first surface is formed by an electrically conductive material provided on the electrically non-conductive substrate, whereas the second plate comprises an electrically conductive plate.

[0149] 15. The vibration sensor of any one or more of the embodiments 1-13, wherein the first plate and the second plates each comprise an electrically conductive plate.

[0150] 16. The vibration sensor of any one or more of the embodiments 1-15, wherein the elastic substance is provided at only one end of the first plate and / or the second plate, for hinging movement of the first and second plate relative to each other.

[0151] P352WOOO 17. The vibration sensor of any one or more of the embodiments 1-15, wherein the elastic substance is provided at opposing ends of the first plate and / or the second plate, for movement of the first and second plate relative to each other in a direction substantially perpendicular to the first and second surface.

[0152] 18. The vibration sensor of any one or more of the preceding embodiments, wherein the elastic substance is provided as at least two discrete amounts of elastic substance.

[0153] 19. The vibration sensor of the combination of embodiments 17 and 18, wherein at one end of the first plate and / or second plate two discrete amounts of elastic substance are provided at two corners of the respective plate(s), and at the opposing end a single discrete amount of the elastic substance is provided at a substantially central position of the edge of said opposing end.

[0154] 20. The vibration sensor of any one or more of the preceding embodiments, wherein the elastic substance is a viscoelastic substance.

[0155] 21. The vibration sensor of any one or more of the preceding embodiments, wherein the elastic substance is an elastic adhesive, preferably a viscoelastic adhesive.

[0156] 22. The vibration sensor of any one or more of the preceding embodiments, wherein the elastic substance comprises silicone, preferably an adhesive comprising silicone.

[0157] 23. The vibration sensor of any one or more of the preceding embodiments, wherein one of the first and second plates is provided with a mass element.

[0158] 24. The vibration sensor of embodiment 23, the vibration sensor further comprising a housing arranged to limit movement of the plate with the mass element.

[0159] 25. The vibration sensor of embodiment 24, wherein the housing encloses the mass element, wherein the mass element protrudes from said one of the first and second plate towards a part of the housing, and wherein said part is arranged to limit the movement of the plate with the mass element.

[0160] 26. The vibration sensor of embodiment 25, wherein said part of the housing is formed by a substrate provided with read-out electronics connected to the first and / or the second surface, wherein the read-out electronics is preferably arranged to the side of the mass element.

[0161] 27. The vibration sensor of any one or more of the preceding embodiments, wherein the vibration sensor has a sensitivity curve with a peak below 10 kHz, preferably below 5 kHz.

[0162] 28. The vibration sensor of embodiment 27 , wherein the vibration sensor has a sensitivity curve with a peak at or below 2 kHz, preferably at or below 1 kHz.

[0163] 29. The vibration sensor of any one or more of the preceding embodiments, wherein the distance between the first and second plates is 5-15 pm.

[0164] 30. The vibration sensor of any one or more of the preceding embodiments, wherein the first and second surfaces form a capacitor.

[0165] P352WOOO 31. The vibration sensor of any one or more of the embodiments 1-30, wherein an electret layer is provided to the first surface or the second surface.

[0166] 32. The vibration sensor according to embodiment 30 or 31, further comprising read-out electronics configured to generate a signal in dependence on the electrical capacitance of the capacitor.

[0167] 33. The vibration sensor according to any one or more of the embodiments 1-32, wherein the vibration sensor is a medical sensor, preferably a phonocardiogram (PCG) sensor.

[0168] 34. The vibration sensor according to any one or more of the embodiments 1-33, wherein the elastic substance is the only mechanical connection between the plates. 35. Use of a vibration sensor according to any one or more of the preceding embodiments for recording body sounds, such as heart sounds or lung sounds, preferably use of the vibration sensor as a phonocardiogram sensor.

[0169] P352WOOO

Claims

1. CLAIMS1. A vibration sensor, comprising:- a first plate comprising a first surface that is electrically conductive;- a second plate comprising a second surface that is electrically conductive; and- an elastic substance that movably connects the first and second plates and holds the plates at a distance from each other such that a gap exists between the plates, wherein the elastic substance is provided at one or more discrete locations along an edge of the first plate and / or the second plate, thereby leaving the gap open along a part of said edge.

2. The vibration sensor of claim 1, wherein the elastic substance is a viscoelastic substance.

3. The vibration sensor of claim 1 of claim 2, wherein the elastic substance is a cured substance.

4. The vibration sensor of claim 3, wherein at least one of the plates comprises a structure for capillary action, the structure comprising an opening or recess dimensioned for capillary action with the elastic substance in an uncured state, and the elastic substance is provided in said opening or recess.

5. The vibration sensor of claim 4, wherein both plates comprise a structure for capillary action, that are positioned near each other, and the elastic substance provided in the structures forms a bridge between the first plate and the second plate.

6. The vibration sensor of claim 4 or 5, wherein one of the plates comprises a spacer opening to separate the gap between the plates from the structure for capillary action of the other one of the plates.

7. The vibration sensor of any one or more of the claims 1-6, wherein the first plate comprises an electrically non-conductive substrate and the first surface is formed by an electrically conductive material provided on the electrically non-conductive substrate, whereas the second plate comprises an electrically conductive plate.P352WOOO8. The vibration sensor of any one or more of the claims 1-6, wherein the first plate and the second plates each comprise an electrically conductive plate.

9. The vibration sensor of any one or more of the claims 1-8, wherein the elastic substance is provided at only one end of the first plate and / or the second plate, for hinging movement of the first and second plate relative to each other.

10. The vibration sensor of any one or more of the claims 1-8, wherein the elastic substance is provided at opposing ends of the first plate and / or the second plate, for movement of the first and second plate relative to each other in a direction substantially perpendicular to the first and second surface.

11. The vibration sensor of any one or more of the preceding claims, wherein the elastic substance comprises silicone, wherein preferably the elastic substance is an adhesive comprising silicone.

12. The vibration sensor of any one or more of the preceding claims, wherein one of the first and second plates is provided with a mass element.

13. The vibration sensor of claim 12, the vibration sensor further comprising a housing enclosing the mass element, wherein the mass element protrudes from said one of the first and second plate towards a part of the housing, said part being arranged to limit the movement of the plate with the mass element14. The vibration sensor of any one or more of the preceding claims, wherein the distance between the first and second plates is 5-15 pm.

15. The vibration sensor of any one or more of the preceding claims, wherein the first and second surfaces form a capacitor, wherein optionally an electret layer is provided to the first surface or the second surface.

16. The vibration sensor of any one or more of the preceding claims, wherein the elastic substance is the sole mechanical connection for movably connecting the two plates.P352WOOO17. Use of a vibration sensor according to any one or more of the preceding claims as a phonocardiogram sensor.

18. A method for producing a vibration sensor, comprising:- providing a first plate comprising a first surface that is electrically conductive;- providing a second plate comprising a second surface that is electrically conductive;- placing a spacer on one of the plates;- placing the other plate on the spacer;- applying an elastic substance at one or more discrete locations along an edge of the first plate and / or the second plate to movably connect the first and second plates;- removing the spacer between the plates to create a gap between the plates, wherein the gap is open along a part of said edge.

19. The method of claim 18, wherein the elastic substance is a curable substance, and the step of applying the elastic substance comprises applying the elastic substance in an uncured state, and the method further comprising a step of letting the elastic substance cure before the step of removing the spacer.

20. The method of claim 19, wherein at least one of the first and second plates comprises an opening or recess dimensioned for capillary action with the elastic substance in the uncured state, such that, in the step of applying the elastic substance, the elastic substance is drawn into the opening or recess by capillary action.P352WOOO

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