FI ELD OF THE I NVENTI ON

The present invention relates to a m iniature vibration sensor suitable for being incorporated in hearing devices such as hearing aids, hearables, headsets, earbuds or sim ilar devices. I n particular, the present invention relates to a vibration sensor where undesired vibration modes of a m ass-spring system are suppressed.

BACKGROUND OF THE I NVENTION

Modern vibration sensors typically rely on sym m etric m ass-spring systems. However, such sym m etric m ass-spring systems often suffer from m ultiple vibration modes which are disadvantageous in that m ultiple vibration modes often im pact the fundam ental vibration mode in a negative m anner - for example by extracting energy from the fundam ental vibration mode. When energy is extracted from the fundam ental vibration mode undesired dips in the frequency response curve of the vibration sensor m ay be introduced.

The above-m entioned problem is to at least som e extend addressed in CN 1 13296213 A wherein the response sensitivity of a vibration unit to a vibration in a first direction is higher than the response sensitivity of the vibration unit to a vibration in a second direction, where the first and second directions are perpendicular to each other.

Thus, there seems to be a need for suppressing the undesired vibration modes in vibration sensors.

It may therefore be seen as an object of embodiments of the present invention to provide a vibration sensor where undesired vibration modes in the operating frequency range of a m ass-spring system are suppressed.

It m ay be seen as a further object of embodiments of the present invention to provide a vibration sensor having a pronounced fundamental vibration mode with m aximal energy relative to undesired higher order vibration modes.

DESCRI PTION OF THE I NVENTI ON

The above-m entioned objects are com plied with by providing, in a first aspect, a vibration sensor com prising a pressure detecting arrangement for detecting generated pressure variations, wherein the pressure detecting arrangem ent com prises a MEMS die and a signal processor, wherein the MEMS die comprises a front volum e and a MEMS cartridge, and

5 a pressure generating arrangement for generating pressure variations in a coupling volum e in response to vibrations of the vibration sensor, wherein the pressure generating arrangement com prises a frame structure and a spring-m ass system comprising a suspension mem ber suspending a moveable m ass secured to at least0 part of a first or a second surface of the suspension member, wherein the coupling volum e is at least partly defined by the fram e structure and at least part of the second surface of the suspension mem ber in com bination, and wherein said coupling volum e is acoustically connected to the MEMS cartridge of the MEMS die via an acoustical opening, and wherein the suspension mem ber suspends the moveable mass in an asym m etric m anner.

Thus, the vibration sensor according to the first aspect comprises a pressure generating arrangem ent for generating pressure variations in response to vibrations of the vibration sensor, and a pressure detecting arrangement for detecting these generated pressure variations. The vibration sensor m ay form part of a hearing device, such as ear buds, where0 it is intended to detect voice induced vibrations in the skull of the user of the hearing device when the hearing device is positioned in the ear canals of the user.

I n terms of functioning, the suspension member and the moveable m ass secured thereto are adapted to vibrate at a fundamental vibration mode when the vibration sensor is exposed to external m echanical vibrations. The vibrations of the suspension member and the moveable5 mass generate pressure variations in the coupling volume. The generated pressure variations are allowed to enter the MEMS die via the acoustical opening and thus be detected by for exam ple a biased capacitive read-out mechanism (MEMS cartridge) formed by a moveable membrane and a rigid back-plate in com bination. The MEMS cartridge m ay also involve other detection schemes, such as piezoresistive, piezoelectric and charged plate capacitor detection0 schem es. The size of the acoustical opening m ay be used to provide an acoustical im pedance in order to dam pen the resonance peak. Alternatively, the acoustical opening may comprise an acoustical filter in the form of for example a mesh grid to provide the sam e effect. Moreover, the MEMS die may comprise a sm all opening providing a barom etric com pensation between the front volum e and the volume defined by the housing and the first surface of the5 PCB in combination. This opening defines the low-frequency cut-off of the vibration sensor. The resonance frequency of the vibration sensor may be within the frequency range 1-10 kHz. The signal processor, which may be either an analog or a digital signal processor, is adapted to process signals from the MEMS die. The processed signals from the signal processor are subsequently made available to external electrical devices, such as filters, amplifiers etc. As it will be discussed in further details below the MEMS die and the signal processor may be mutually connected via a printed circuit board (PCB).

The suspension member suspends and optionally surrounds the moveable mass in an asymmetric manner. This asymmetric suspension of the moveable mass is advantageous in that it only allows that the suspension member and the moveable mass secured thereto vibrate at a fundamental vibration mode when the vibration sensor is exposed to external mechanical vibrations. As it will be discussed in further details below the asymmetric suspension of the moveable mass may be established in various ways, such as providing a suspension member having a nonuniform/asym m etric stiffness around the moveable mass, or as phrased differently, that the stiffness of suspension member surrounding the moveable mass may be asymmetric. In general, the asymmetric stiffness of the suspension member may for example be provided by changing the properties of the suspension member itself, or by suspending the moveable mass in an asymmetric manner relative to an active part of the suspension member.

In a first embodiment the asymmetric suspension of the moveable mass may be provided by arranging an attachment region of the moveable mass in an asymmetric manner relative to a peripheral edge of the suspension member. The attachment region of the moveable mass is here to be understood as the region or area of the moveable mass that is secured to the suspension member. The area of the attachment region may be smaller than the projected area of the moveable mass. The peripheral edge of the suspension member is here to be understood as the edge of the active portion of the suspension member. A shift or displacement of the attachment region of the moveable mass away from the centre of the active part of the suspension member may widen the active part of the suspension member on one side of the moveable mass. This widening of the active part of the suspension member may introduce the desired asymmetric properties of the suspension member. Moreover, a shift or displacement of the attachment region of the moveable mass away from the centre of the active part of the suspension member may also narrow the active part of the suspension member on another side of the moveable mass which may also introduce the desired asymmetric properties of the suspension member. An asymmetric arrangement of the attachment region of a moveable mass may be achieved in various ways, such as via shaping of the moveable mass. Thus, by shifting or displacing the attachment region of the moveable mass away from the centre of the active part of the suspension member the stiffness of the suspension member may be altered in a desired direction. Along the same line, but in a second embodiment, the asymmetric suspension of the moveable mass may be provided by arranging the moveable mass in an asymmetric manner relative to a peripheral edge of the suspension member. In this embodiment the moveable mass may be a solid and homogeneous structure which is shifted or displaced relative to a centre of the active part of the suspension member. Again, a shift or displacement of the moveable mass away from the centre of the active part of the suspension member may widen the active part of the suspension member on one side of the moveable mass which may lead to the desired asymmetric properties of the suspension member. A shift or displacement of the moveable mass away from the centre of the active part of the suspension member may also narrow the active part of the suspension member on the opposite side of the moveable mass which may also introduce the desired asymmetric properties of the suspension member. A shift or displacement of the moveable mass that corresponds to at least 50% of the symmetric gap (between the moveable mass and the peripheral edge of the suspension member) is preferred. Again, the peripheral edge of the suspension member is to be understood as the edge of the active portion of the suspension member.

In another embodiment the asymmetric suspension of the moveable mass may be provided by locally increasing or decreasing the stiffness of the suspension member. With respect to increasing the stiffness of the suspension member one or more materials may locally be provided to the suspension member in order to locally increase the stiffness of the suspension member. Such one or more materials locally provided to the suspension member may comprise elastic or viscoelastic materials. The one or more materials may be provided to those portions or regions of the suspension member where a higher stiffness of the suspension member is required, and the one or more materials may be the same material or a combination of materials. The stiffness of the one or more materials is preferably higher than the stiffness of the suspension member itself. With respect to decreasing the stiffness of the suspension member one or more openings may locally be provided in the suspension member in order to locally lower the stiffness of the suspension member. In order to avoid undesired acoustical leakage such one or more openings may though be filled and thus closed with a suitable gel or silicone having lower stiffness than the material of suspension member itself.

In yet another embodiment the frame structure may comprises one or more frame extension elements adapted to locally support the suspension member in order to provide the asymmetric suspension of the moveable mass. This corresponds to one or more local increases of the stiffness of the suspension member. The local support of the suspension member, and thus the local increase of the stiffness of the suspension member, may be provided by securing the one or more frame extension elements to the suspension member using an appropriate adhesive. The one or more fram e extension elements may be implem ented as one or more projections that extend from the frame structure at positions where a higher stiffness of the suspension m em ber is required. The one or more fram e extension elem ents m ay form an integral part of the frame structure.

I n yet another em bodiment the moveable m ass m ay comprise one or more m ass extension elements locally secured to the suspension member in order to provide the asym m etric suspension of the moveable m ass. Again, this corresponds to one or more local increases of the stiffness of the suspension member. The local increase of the stiffness of the suspension member m ay be provided by securing the one or more mass extension elements to the suspension m em ber using an appropriate adhesive. Sim ilar to the frame extension elements discussed above the one or more m ass extension elem ents m ay be implem ented as one or more projections that extend from the moveable mass at positions where a higher stiffness of the suspension member is required. The one or more mass extension elem ents may form an integral part of the moveable mass.

The suspension m em ber may be implem ented as a film , such as a polyim ide (Kapton) or silicone film . I n case of a polyim ide film the thickness of the film m ay be around 5 pm . Also films having lower thicknesses may be applicable. Alternatively, the suspension m em ber may comprise a static part and moveable part being hinged together, and wherein one or more openings exist between the static and the moveable parts. The one or more openings may be at least partly filled with a flexible sealant, such as a gel. The static and moveable parts of the suspension m em ber may be m anufactured is an integrated and one-piece com ponent also including one or more hinges that operatively connects the static and moveable parts. The moveable m ass may be implem ented in a high-density material, such as tantalum mass or stainless steel, and the m ass of the moveable m ass is preferably higher than 3 mg in order to ensure a low self-noise of the vibration sensor. The self-noise of the vibration sensor should preferably be below -75 dB(A) re. 1 g.

The suspension mem ber should be able to withstand typical reflow temperatures, i.e. suspension mem ber should be capable of withstanding temperatures of at least 80°C, such as at least 100°C, such as at least 120°C, such as at least 150°C, such as at least 200°C, such as at least 250°C, such as at least 300°C, such as at least 350°C, such as at least 400°C.

The moveable m ass may be secured to the first surface of the suspension m em ber using a compliant adhesive, or the moveable m ass may be secured to the second surface of the suspension mem ber using a com pliant adhesive. It should though be noted that moveable m asses m ay be secured to both the first and second surfaces of the suspension m em ber. As the vibration sensor m ay form part of a hearing device the dim ensions of the vibration sensor (width, length and height) are sm aller than 3 m m , 4 m m and 2 m m , respectively. Thus, the footprint of the vibration sensor m easures at most 3 m m by 4 m m , whereas the overall height of the vibration sensor is sm aller than 2 m m .

With respect to an assem bled vibration sensor the pressure detecting arrangem ent may com prise a MEMS m icrophone com prising the MEMS die, the signal processor and a first housing within which first housing the MEMS die and the signal processor are provided. The first housing may comprise a PCB comprising the acoustical opening that acoustically connects the coupling volum e and the MEMS cartridge of the MEMS die. The pressure generating arrangement may comprise a second housing secured to a first surface of the PCB using a conductive adhesive in order to avoid electrical interference. The second housing and the first surface of the PCB may define, in combination, a volume within which volume the fram e structure, a suspension m em ber and the moveable m ass secured thereto are arranged. The coupling volum e m ay be defined, in com bination, by the frame structure, at least part of the second surface of the suspension m em ber and at least part of the first surface of the PCB.

Alternatively, the vibration sensor may, when assembled, further comprise a housing secured to a first surface of a PCB using a conductive adhesive in order to avoid electrical interference. The housing and the first surface of the PCB may define, in combination, a volum e within which volume the pressure detecting arrangem ent and the pressure generating arrangement are arranged. The MEMS die and the signal processor m ay be secured to the first surface of the PCB using one or more electrically conducting connection pads in a manner so that a volum e (form ed by the gap between the MEMS die and the PCB) exists at least between the MEMS die and the first surface of the PCB. The coupling volum e may be defined by an indentation in the frame structure and at least part of the second surface of the suspension m em ber, and the acoustical opening may be provided in the frame structure. A first surface of the MEMS die m ay be secured to at least part of the frame structure using a compliant adhesive so that the acoustical opening in the fram e structure is aligned with the front volume of the MEMS die. The use of a compliant adhesive is advantageous in order to prevent that m echanical stress, due to m ism atch in thermal expansion coefficients, propagate to the MEMS die.

It is advantageous that the height of the gap of the volum e defined by the indentation in the fram e structure and at least part of the second surface of the suspension m em ber can be accurately controlled via the depth of the indentation. By properly selecting the gap the resonance peak m ay be reduced via squeeze film damping - the narrower the gap the higher the squeeze film damping. Moreover, the height of the gap between fram e structure and suspension m em ber, and the height of the gap between moveable m ass and the housing are important because these heights or distances lim it the deflection of the suspension m em ber in both directions if the vibration sensor is exposed to severe mechanical shocks. I n addition, these heights or distances also lim it the generated pressure inside the vibration sensor due to such shocks.

I n a second aspect the present invention relates to a hearing device com prising a vibration sensor according to the first aspect, wherein the hearing device comprises a hearing aid, a hearable, a headset, an earbud or a sim ilar device.

I n a third aspect the present invention relates to a use of a vibration sensor according to the first aspect, wherein the vibration sensor is used for detecting voice induced vibrations in the skull of the user of the hearing device, and wherein the detected voice induced vibrations are used for voice recognition of the user’s own voice. The step of recognising of the user’s own voice may be implem ented by using a voice recognition algorithm where predeterm ined characteristics, such as the frequency content, of the detected voice induced vibrations are compared to the sam e characteristics of the user’s own voice.

I n general, the various aspects of the present invention may be combined and coupled in any way possible within the scope of the invention. These and other aspects, features and/or advantages of the present invention will be apparent from and elucidated with reference to the embodim ents described hereinafter.

It should also be noted that the general concept of suspending a moveable m ass in an asym m etric manner in order to suppress undesired vibration modes in a m ass-spring system is also applicable in vibration sensors that do not rely on a vibration-to-acoustic conversion involving a pressure generating arrangem ent and a pressure detecting arrangem ent. Thus, the asym m etric suspension concept is also applicable in for example capacitive vibration sensors, electrostatic vibration sensors, piezoelectric vibration sensor, piezoresistive vibration sensors etc. all having a direct read-out of the relevant physical entity.

BRI EF DESCRI PTI ON OF THE DRAWI NGS

The present invention will now be described with reference to the accom panying drawings where

Figs. 1 A and 1 B illustrate two different embodim ents of the vibration sensor of the present invention, Fig. 2 illustrates various vibration modes of a mass-spring system com prising a suspension m em ber and a moveable m ass secured thereto,

Fig. 3 illustrates an asym metric arrangement of a moveable mass on a suspension m em ber,

Fig. 4 illustrates how the stiffness of a suspension member is increased locally by locally applying an elastic m aterial,

Fig. 5 illustrates how the attachm ent region of the moveable mass is shifted relative to the centre of the suspension m em ber,

Fig. 6 illustrates how the stiffness of a suspension member is increased locally by locally supporting the suspension m em ber with one or more frame extension elem ents,

Fig. 7 illustrates how the stiffness of a suspension member is increased locally by locally securing the suspension member to one or more mass extension elements,

Fig. 8 illustrates the effect of shifting the moveable mass, and

Fig. 9 illustrates the effect of locally increasing the stiffness of the suspension member.

DETAI LED DESCRI PTI ON OF THE I NVENTION

I n general, the present invention relates to a m iniature vibration sensor comprising a pressure generating arrangement for generating pressure variations in a coupling volume in response to vibrations of the vibration sensor, and a pressure detecting arrangem ent for detecting the generated pressure variations. The frequency response of the vibration sensor is optim ised by m axim ising the energy of the fundamental vibration mode of the vibration sensor. This energy maxim ising of the fundamental vibration mode is provided by ensuring that a spring-m ass system comprising the suspension member and the moveable mass secured thereto only have a single allowable vibration mode within the operating frequency range of the vibration sensor. The rem aining additional vibration modes are suppressed.

Figs. 1 A and 1 B show two different em bodiments of the vibration sensor of the present invention. Fig. 1 A shows a cross-sectional view of a vibration sensor 1 according to an embodim ent of the present invention. I n general, the vibration sensor shown in Fig. 1 A comprises a pressure generating arrangem ent secured to an exterior surface of a MEMS m icrophone comprising a MEMS die 5 and a signal processor 6 for processing electrical signals generated by the MEMS die 5. The MEMS die 5 com prises a front volum e 8 and MEMS cartridge 7 in the form of a biased capacitor com prising a moveable m em brane and a rigid back-plate form ing the capacitor in combination. The MEMS cartridge 7 of the MEMS die 5 may involve other detection schemes, such as piezoresistive, piezoelectric or charged plate capacitor detected schem es. Both the MEMS die 5 and the signal processor 6 are arranged in the back volum e 9 of the MEMS m icrophone.

The MEMS m icrophone com prises a housing having a first PCB 2 and a second PCB 3. One or more electrical contact pads 18, 18’ are arranged on a second (lower) surface of the second PCB 3 in order to form easy electrical access to external electrical devices, such as external am plifiers, filters, power supplies etc.. A wall portion 4 is provided between the first PCB 2 and the second PCB 3, and acoustical openings 10, 17 are provided in the first PCB 2. The acoustical opening 10 acoustically connects the coupling volume 15 and the front volume 8 of the MEMS die 5, whereas the acoustical opening 17 acoustically connects the back volum e 16 of the pressure generating arrangem ent and the back volume 9 of the MEMS m icrophone, i.e. the acoustic pressure in back volum e 9 follows the pressure generated in back volume 16 and thus enhances the differential pressure on the MEMS cartridge 5. It should though be noted that the acoustical opening 17 is optional.

The pressure generating arrangem ent is secured to a first (upper) surface of the first PCB 2 of the MEMS m icrophone. The pressure generating arrangement com prises a housing 1 1 , a suspension m em ber 12 in the form of a polyim ide or silicone film and a moveable mass 13 secured thereto, and a frame structure 14. The moveable m ass 13 is a tantalum or stainless steel mass. The housing 1 1 is secured to the first (upper) surface of the first PCB 2 using a conductive adhesive 23 thus form ing a shield against electrical interference. Also the frame structure 14 is secured to the first (upper) surface of the first PCB 2 using an adhesive. The suspension m em ber 12 may be a single layer film or a m ultilayer film , and it may optionally com prise a sm all opening (not shown) providing a barom etric compensation between the coupling volume 15 and the back volum e 16. This opening defines the low-frequency cut-off of the vibration sensor. Moreover, the housing 1 1 may comprise a venting opening (not shown) adapted to vent the back volum e 16 to the exterior of the vibration sensor. The purpose of this venting opening is to prevent excessive pressure inside the housing 1 1 during reflow soldering during manufacturing. The venting opening is dim ensioned to provide barometric com pensation and still prevent acoustical leakage. Alternatively, the venting opening m ay be sealed with an adhesive or sticky tape after manufacturing. The housing 1 1 is im plemented in stainless steel. The MEMS die 5 and the signal processor 6 are electrically connected to a second (lower) surface of the first PCB 2. The first and second PCB’s 2, 3 are electrically connected via one or more via’s (not shown) . The overall functioning of the vibration sensor 1 is as follows: When the vibration sensor 1 is exposed to external m echanical vibrations, the spring-m ass system com prising the suspension m em ber 12 and the moveable mass 13 secured thereto move relative to the fram e structure 14, and the suspension m em ber 12 will generate pressure variations in the coupling volume 15. The generated pressure variations are allowed to enter the front volume 8 of the MEMS die 5 via the acoustical opening 10 in the first PCB 2. The generated pressure variations are subsequently converted to electrical output signals due to changes of the capacitance of the MEMS cartridge 7 form ed by the moveable m em brane and the rigid backplate.

Fig. 1 B shows a cross-sectional view of a vibration sensor according to another embodim ent of the invention. The vibration sensor shown in Fig. 1 B also com prises a MEMS die 5 and a signal processor 6 arranged on a first (upper) surface of a PCB 2. Both the MEMS die 5 and the signal processor 6 are electrically connected to the PCB 2 via respective solder pads 21 in a m anner so that respective volumes 22, 22’ exist between a second (lower) surface of the MEMS die 5 and the first (upper) surface of the PCB 2, and between the signal processor 6 and the first (upper) surface of the PCB 2. I n Fig. 1 B the second (lower) surface of the MEMS die 5 is at least partly constituted by the MEMS cartridge 7 which com prises a moveable membrane and a rigid back-plate.

The signal processor 6 is adapted to process signals from the MEMS die 5, and the processed signals from the signal processor 6 are subsequently provided on one or more electrical contact pads 18, 18’ arranged on a second (lower) surface of the PCB 2 in order to form easy electrical access to external electrical devices, such as external amplifiers, filters, power supplies etc..

The MEMS die 5 com prises a front volum e 8 and a MEMS cartridge 7 in the form of a biased capacitor comprising a moveable membrane and a rigid back-plate form ing the capacitor in com bination. The MEMS cartridge 7 of the MEMS die 5 may involve other detection schem es, such as piezoresistive, piezoelectric or charged plate capacitor detected schemes. As seen in Fig. 1 B the MEMS cartridge 7 faces the first (upper) surface of the PCB 2, whereas the front volum e 8 of the MEMS die 5 is facing away from the first (upper) surface of the PCB 2. The MEMS die 5, the signal processor 6 and optionally the PCB 2 may be considered the pressure detecting arrangem ent of the vibration sensor as these elements are adapted to detect generated pressure variations in response to vibrations of the vibration sensor.

The pressure generating arrangem ent of the vibration sensor depicted in Fig. 1 B com prises a fram e structure 14, a suspension m em ber 12 and a moveable m ass 13 secured to a first (upper) surface of the suspension member 12 using a com pliant adhesive. The fram e structure 14 com prises an indentation surrounded by a projecting peripheral rim to which projecting peripheral rim at least part of a second (lower) surface of the suspension m em ber 12 is secured using a compliant adhesive. With this arrangement a coupling volume 15 is defined by the indentation of the frame structure 14 and at least part of the second (lower) surface of the suspension m em ber 12.

As seen in Fig. 1 B the fram e structure 14 is secured directly to the first (upper) surface of the MEMS die 5 using a compliant adhesive in order to com pensate for different expansion coefficients. As also seen in Fig. 1 B the coupling volume 15 is acoustically connected to the front volume 8 of the MEMS die 5 via an acoustical opening 10 in the fram e structure 14. The acoustical connection between the coupling volum e 15 and the front volum e 8 of the MEMS die 5 is provided by physically aligning the acoustical opening 10 with the front volum e 8 of the MEMS die 5 so that pressure variations generated in the coupling volum e 15 are allowed to enter the front volume 8 of the MEMS die 5 and thus be detected.

As seen in Fig. 1 B the fram e structure 14, including its indentation, extends beyond the dim ensions of the MEMS die 5 and thus overhangs the signal processor 6. This is advantageous in that the dim ensions of the suspension m em ber 12 can be maxim ised which also leads to an increased sensitivity of the vibration sensor. The fram e structure 14 is implem ented in stainless steel, and the indentation provided therein has been provided by for exam ple etching, punching or deep drawing. The suspension mem ber 12 is implem ented as a polyim ide or silicone film , and the moveable m ass 13 secured thereto is a tantalum or stainless steel m ass.

The em bodiment shown in Fig. 1 B further com prises a housing 19 which is secured to the PCB 2 using a conductive adhesive 23 thus form ing a shield against electrical interference. The housing 19 and the PCB 2 form a volum e 20 within which volume 20 the MEMS die 5, the signal processor 6 and the pressure generating arrangem ent comprising the frame structure 14, the suspension m em ber 12 and the moveable m ass 13 secured thereto are arranged. It should be noted that the volum e 20 is acoustically connected to the volum es 22, 22’ below the MEMS die 5 and the signal processor 6, respectively. The housing 19 is made of stainless steel, and a venting opening (not shown) m ay optionally be provided therein. The venting opening prevents excessive pressure inside the housing 19 during reflow soldering during manufacturing. The venting opening is dimensioned to provide barom etric compensation and still prevent acoustical leakage. Alternatively, the venting opening m ay be sealed with an adhesive or sticky tape after m anufacturing.

With respect to the moveable m asses 13, the higher the m ass the lower is the effect of the thermal movem ent noise of the vibration sensor. As already m entioned the moveable mass m ay be implem ented in a high-density material, such as tantalum m ass or stainless steel, and the m ass of the moveable mass is preferably higher than 3 mg in order to ensure a low self-noise of the vibration sensor.

The footprint of the vibration sensor (width and length) of Fig. 1 B is smaller than 2 m m and 3 m m , respectively, whereas the overall height of the vibration sensors is smaller than 1 m m . The vibration sensor of Fig. 1 A is slightly bigger.

I n relation to Figs. 3-8 various optim ised implem entations of the m ass-spring system com prising the suspension member 12 and the moveable m ass 13 will be discussed. All of these optim ised implem entations only have a single allowable vibration mode within the operating frequency range of the vibration sensor. The various optim ised im plem entations shown in Figs. 3-6 are all suitable for being used in the em bodiments shown in Fig. 1 A and 1 B.

Fig. 2 illustrates the various vibration modes of a typical mass-spring system comprising a suspension m em ber 12 and a moveable m ass 13 sym metrically secured thereto. As seen in Fig. 2 the suspension m ember 12 is secured to a frame structure 14. As illustrated in Fig. 2 the mass-spring system m ay vibrate linearly along the axis 24 as indicated by the arrow 24’. Moreover, the m ass-spring system m ay have rotational vibration modes around axes 25, 26 as indicated by the arrows 25’, 26’, respectively. As already discussed, the additional vibration modes are disadvantageous in that they generally extract energy from the fundamental vibration mode which causes undesired dips in the frequency response curve of the vibration sensor.

Referring now to Fig. 3 a first optim ised implem entation of the m ass-spring system is depicted. As seen in the cross-sectional view in Fig. 3 a second (lower) surface of a suspension m em ber 12 is secured to a fram e structure 14, whereas a moveable mass 13 is secured to a first (upper) surface of the suspension m em ber 12. It should though be noted that the moveable mass 13 and the frame structure 14 could alternatively be secured to the same surface of the suspension member 12 - either the first (upper) surface or the second (lower) surface. As also shown in the cross-sectional view in Fig. 3 the moveable mass 13 is asym metrically arranged on the suspension m em ber 12 due to its displacement to the right. I n the top view in Fig. 3 the suspension member 12 is supported by the frame structure 14 outside the dashed line. The supported region of the suspension member 12 is denoted 27 in the top view in Fig. 3. The region inside the dashed line corresponds to the suspended and thus active part of the suspension m em ber 12. As seen in the top view in Fig. 3 the moveable mass 13 is arranged off-centre relative to the suspended and thus active part of the suspension m em ber 12. The displacem ent of the moveable mass 13 away from the centre of active part of the suspension m em ber 12 ensures that only a single vibration mode exists within the operating frequency range of the vibration sensor

I n Fig. 4 a second optim ised im plementation of the mass-spring system is depicted. As seen in the cross-sectional view in Fig. 4 a first (upper) surface of a suspension m em ber 12 is secured to both a fram e structure 14 and a moveable mass 13. It should again be noted that the moveable m ass 13 and the frame structure 14 could alternatively be secured to the second (lower) surface of the suspension m em ber 12, or to different surfaces of the suspension m em ber 12. I n contrast to Fig. 3 the moveable m ass 13 is sym m etrically arranged on the suspension m em ber 12. I n order to ensure that only a single vibration mode exists within the operating frequency range of the vibration sensor the stiffness of the suspension m em ber 12 is locally increased by providing one or more materials 28, 28’ to the suspension m em ber 12. The one or more materials 28, 28’ locally provided to the suspension member 12 m ay be elastic or viscoelastic m aterials. I n the top view shown in Fig. 4 the stiffness of the suspension member 12 has been increased at two locations 28, 28’ along the same side of the moveable m ass 13. Other suitable locations as well as number of locations for increasing the stiffness of the suspension m em ber 12 may be applicable as well.

Referring now to Fig. 5 a third optim ised implem entation of the m ass-spring system is depicted. As seen in the cross-sectional view in Fig. 5 a second (lower) surface of a suspension m em ber 12 is secured to a fram e structure 14, whereas a moveable mass 13, 29 is secured to a first (upper) surface of the suspension m em ber 12. It should be noted that the moveable m ass 13, 29 and the fram e structure 14 could alternatively be secured to the same surface of the suspension member 12 - either the first (upper) surface or the second (lower) surface. As also shown in the cross-sectional view in Fig. 5 the moveable mass 13 has an asym metric cross-sectional shape due to its first (wider) part 13, and its second (narrow) part 29. The asym m etric cross-sectional view of the moveable mass 13, 29 shifts or displaces the attachment region of the moveable mass 13, 29 com pared to a moveable mass having a sym metric cross-sectional shape, cf. for exam ple the cross-sectional views of Figs. 3 and 4.

I n the top view in Fig. 5 the suspension m em ber 12 is supported by the frame structure 14 outside the outer dashed line. The supported region of the suspension m ember 12 is denoted 27 in the top view in Fig. 5. The region inside the outer dashed line corresponds to the suspended and thus active part of the suspension member 12. As seen in the top view in Fig. 5 the first (wider) part of the moveable mass 13 (solid line) has essentially the sam e dimensions as the active part of the suspension m ember 12, whereas the second (narrow) part of the moveable m ass 29 is displaced to the right (inner dashed line) as also depicted in the cross-sectional view of Fig. 5. The shift or displacement of the attachm ent region of the moveable mass 13, 29 away from the centre of the active part of the suspension member 12 widens the active part of the suspension member 12 of one side (left side) . The active part of the suspension member 12 is also widened on the opposite side (right side) although the latter widening is significantly smaller than the widening on the left side. On both the left and right sides of the moveable m ass 13, 29 the widening is defined relative to the projected area of the first (wider) part 13 of the moveable mass on the suspension member 12. As a result the larger active part of the suspension m em ber 12 on the left side of the moveable m ass 13, 29 ensures that the suspension m em ber 12 has an asym m etric stiffness whereby only a single vibration mode exists within the operating frequency range of the vibration sensor.

Turning now to Fig. 6 a fourth optim ised im plementation of the mass-spring system is depicted. As seen in the cross-sectional view in Fig. 6 a second (lower) surface of a suspension m em ber 12 is secured to a fram e structure 14, whereas a moveable mass 13 is secured to a first (upper) surface of the suspension m em ber 12. It should again be noted that the moveable mass 13 and the frame structure 14 could alternatively be secured to the same surface of the suspension member 12. The moveable m ass 13 is sym m etrically arranged on the suspension m em ber 12. I n order to ensure that only a single vibration mode exists within the operating frequency range of the vibration sensor the stiffness of the suspension m em ber 12 is effectively and locally increased by locally supporting the suspension m em ber 12 using one or more frame extension elem ents 30, 30’ which are secured to the suspension m em ber 12 using an appropriate adhesive. The one or more fram e extension elem ents 30, 30’ physically supports the suspension m em ber 12 at selected locations as illustrated in the top view of Fig. 6 where the stiffness of the suspension member 12 is effectively increased at two locations 30, 30’ along the sam e side of the moveable m ass 13. It should be noted that other suitable locations as well as number of frame extension elements may be applicable as well.

Fig. 7 shows fourth optim ised implem entation of the m ass-spring system is depicted. As seen in the cross-sectional view in Fig. 7 a second (lower) surface of a suspension m em ber 12 is secured to a fram e structure 14, whereas a moveable mass 13 is secured to a first (upper) surface of the suspension m em ber 12. It should again be noted that the moveable mass 13 and the fram e structure 14 could alternatively be secured to the sam e surface of the suspension m em ber 12. I n order to ensure that only a single vibration mode exists within the operating frequency range of the vibration sensor the stiffness of the suspension m em ber 12 is effectively and locally increased by locally securing the suspension m em ber 12 to one or more mass extension elements 33, 33’. The one or more mass extension elements 33, 33’ are secured to the suspension member 12 using an appropriate adhesive at selected positions as illustrated in the top view of Fig. 7. At these selected positions the stiffness of the suspension member 12 is effectively increased since the active parts of the suspension member 12 (at these positions) are narrowed. As seen in Fig. 7 the selected positions are along the sam e side of the moveable m ass 13. It should though be noted that other suitable positions as well as the num ber of mass extension elements 33, 33’ m ay be applicable as well.

Fig. 8 illustrates the effect of shifting the moveable mass 13 so that its position becom es asym metric relative to the suspension mem ber 12 and the frame structure 14. I n the upper cross-sectional view in Fig. 8 the moveable m ass 13 is sym m etrically arranged relative to the suspension m em ber 12 and the frame structure 14 as indicated by the unshifted distance 31. I n the m iddle cross-sectional view in Fig. 8 the moveable m ass 13 is shifted a distance 32 to the right and thus becom es asym metrically arranged relative to the suspension member 12 and the fram e structure 14. Shifting the moveable mass 13 to the right has the effect that the suspension member portion 12’ becom es stiffer that the suspension portion 12”, and as a result, the moveable mass 13 will excite an asym metric vibration mode as illustrated in the lower cross-sectional view in Fig. 8. I n this asym metric vibration mode the stiffer portion 12’ of the suspension m em ber will deflect significantly less than the softer portion 12” of the suspension m em ber.

Fig. 9 illustrates the effect of locally increasing the stiffness of the suspension member 12. I n the upper cross-sectional view in Fig. 8 the moveable m ass 13 is sym metrically arranged relative to the suspension m em ber 12 and the fram e structure 14. I n the lower cross- sectional view in Fig. 9 the stiffness of the suspension member 12 is locally increased by providing one or more m aterials 28, 28’ to the suspension member 12 on the right-hand side of the moveable mass 13. As previously m entioned, the one or more materials 28, 28’ m ay be elastic or viscoelastic m aterials. The asym metric stiffness of the suspension m em ber has the effect that the stiffer portion (right- hand side) of the suspension mem ber will deflect significantly less than the softer portion (left-hand side) of the suspension member.

Although the present invention has been discussed in the foregoing with reference to exem plary embodim ents of the invention, the invention is not restricted to these particular em bodiments which can be varied in many ways without departing from the invention. The discussed exem plary em bodiments shall therefore not be used to construe the appended claims strictly in accordance therewith. On the contrary, the em bodiments are m erely intended to explain the wording of the appended claims, without intent to lim it the claims to these exem plary em bodim ents. The scope of protection of the invention shall therefore be construed in accordance with the appended claims only, wherein a possible am biguity in the wording of the claims shall be resolved using these exem plary embodim ents.