FI ELD OF THE I NVENTI ON

The present invention relates to a vibration sensor where m echanical vibrations of the vibration sensor are detected via detection of pressure variations within the vibration sensor. The present invention relates, in particular, to a vibration sensor where a pressure detecting arrangem ent and a pressure generating arrangem ent are arranged in a space-saving m anner without comprom ising the sensitivity, and thus the perform ance, of the vibration sensor.

BACKGROUND OF THE I NVENTION

The integration of m icro-electrom echanical transducers in the form of vibration sensors into hearing devices, such as hearing aids and earbuds, has increased significantly over the past decade. The benefits of integrating m icro-electromechanical transducers (vibration sensors) into hearing devices are m any - including noise reduction via bone conduction voice pick-up where it is intended to detect voice induced vibrations in the skull of the user of the hearing device as well as control of the hearing device via tapping.

An exam ple of a prior art m icro-electrom echanical transducer m ay for exam ple be found in EP 3 342 749 A2 which relates to a m icro-electromechanical transducer in the form of a vibration sensor com prising a stand-alone and self-contained MEMS m icrophone and a subassem bly attached thereto. MEMS is an abbreviation for a m icro-electrom echanical system . The sub-assembly is adapted to generate pressure variations when the m icroelectrom echanical transducer is exposed to vibrations, whereas the MEMS m icrophone is adapted to detect and convert detected pressure variations to an electrical output signal. The m icro-electrom echanical transducer suggested in EP 3 342 749 A2 is disadvantageous due to its relative significant height.

Other examples of prior art m icro-electromechanical transducers m ay for exam ple be found in GN 1 134471 15, CN 1 10972045 and US 2021 /364346 A1 , but also these transducers suffer from their respective heights.

It m ay therefore be seen as an object of em bodim ents of the present invention to provide a compact vibration sensor with both a small footprint and a m inim al height.

It m ay be seen as a further object of embodim ents of the present invention to provide a compact vibration sensor with m inimal dim ensions without com prom ising the sensitivity, and thus the perform ance, of the vibration sensor. 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 comprising 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 wherein the MEMS die comprises oppositely arranged first and second surfaces, 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 comprising an indentation, a suspension m em ber com prising first and second surfaces and a moveable mass secured to at least part of the first or second surfaces of the suspension mem ber, and a PCB com prising a first surface, and a housing secured to the first surface of the PCB using an adhesive, wherein the housing and the first surface of the PCB define, in com bination, a volume within which volum e the pressure detecting arrangement and the pressure generating arrangem ent are arranged, wherein the coupling volum e is defined by the indentation of the fram e structure and at least part of the second surface of the suspension mem ber, and wherein said coupling volume is acoustically connected to the MEMS cartridge of the MEMS die via an acoustical opening in the fram e structure, and wherein the first surface of the MEMS die is secured to at least part of the fram e structure, and wherein at least part of the second surface of the MEMS die is secured to the first surface of the PCB.

Thus, the vibration sensor according to the first aspect comprises a pressure generating arrangement 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, where 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 when the vibration sensor is exposed to external mechanical vibrations. The vibrations of the suspension m em ber and the moveable mass generate pressure variations in the coupling volume defined by the indentation of the fram e structure and at least part of the second surface of the suspension m em ber. The generated pressure variations are allowed to enter the MEMS die via the acoustical opening in the frame structure and thus be detected by for example a biased capacitive read-out mechanism (MEMS cartridge) form ed by a moveable membrane and a rigid back-plate in com bination. The MEMS cartridge m ay also involve other detection schem es, such as piezoresistive, piezoelectric and charged plate capacitor detection schem es. The size of the acoustical opening m ay be used to provide an acoustical impedance in order to dampen the resonance peak. Alternatively, the acoustical opening m ay comprise an acoustical filter in the form of for example a m esh grid to provide the same effect. Moreover, the MEMS die m ay 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 the PCB in com bination. This opening defines the low-frequency cutoff of the vibration sensor.

The resonance frequency of the vibration sensor m ay be within the frequency range 1 - 10 kHz. It is advantageous that the height of the gap of the volum e defined by the indentation of the frame 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 frame 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 m echanical shocks. I n addition, these heights or distances also lim it the generated pressure inside the vibration sensor due to such shocks.

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 m ade 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 m ay be m utually connected via a printed circuit board ( PCB) .

At least part of the frame structure may be secured to the first surface of the MEMS die using a compliant adhesive in a m anner so that the acoustical opening of 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. Securing at least part of the fram e structure directly to the first surface of the MEMS die is also advantageous in that it reduces the overall height of the vibration sensor. As it will be disclosed in further details below the frame structure may be secured to the MEMS die at the front volume so that the frame structure, with the exception of the acoustical opening, closes the front volum e of the MEMS die and thus separates the com bination of the coupling volume and the front volum e of the MEMS die from the remaining volumes of the vibration sensor.

The coupling volume defined by the indentation of the frame structure and at least part of the second surface of the suspension m em ber m ay be acoustically connected to a first surface of the moveable m em brane of the MEMS die. More particularly, said coupling volume may be acoustically connected to a first surface of the moveable membrane of the MEMS die via the front volume of the MEMS die in case the first surface of the moveable m em brane faces the front volume of the MEMS die.

With respect to securing the MEMS die to the PCB, at least part of the second surface of the MEMS die may be secured to the first surface of the PCB using one or more electrically conducting connection pads arranged on the second surface of the MEMS die, and a volum e (formed by the gap between the MEMS die and the PCB) may exist between the second surface of the MEMS die and the first surface of the PCB, said volume being acoustically connected to a second surface of the moveable m embrane of the MEMS cartridge. The one or more electrically conducting connection pads m ay comprise one or more solder bum ps, one or more solder pads or one or more gold bumps (in com bination with a conductive adhesive) having a certain height which determ ines the distance, and affects the volum e, between the second surface of the MEMS die and the first surface of the PCB.

Preferably, the volume between the second surface of the MEMS die and the first surface of the PCB is acoustically connected to the volume defined by the housing and the first surface of the PCB. Thus, the volume defined by the housing and the first surface of the PCB may be considered a coherent and continuous volume which is not divided into separate compartments.

The frame structure and its indentation may extend beyond the dimensions of the MEMS die in order to maxim ise the dim ensions of the suspension m em ber. Thus, the dimensions of the frame structure m ay be significantly larger than the dimensions of the MEMS die at the first surface of the MEMS die where the fram e structure is secured to the MEMS die. This is advantageous in that the dimensions of the suspension member, and thus the sensitivity of the vibration sensor, m ay be optim ised.

The indentation in the frame structure m ay be surrounded by a projecting peripheral rim , and at least part of the second surface of the suspension mem ber m ay be secured to the projecting peripheral rim of the frame structure using a compliant adhesive. The height of the projecting peripheral rim sets the height of the gap of the volume defined by the indentation of the fram e structure and at least part of the second surface of the suspension m em ber.

One or more electrical contact pads for electrically connecting the vibration sensor to external electronic devices m ay be arranged on a second surface of the PCB. The first and second surfaces of the PCB m ay form opposite surfaces of the PCB, and the one or more electrical contact pads m ay com prise one or more solder bum ps.

The vibration sensor com prises a housing secured to the first surface of the PCB using an adhesive. Preferably, this adhesive com prises a conductive adhesive, including soldering. Using a conductive adhesive is advantageous in that the housing can then be electrically connected to the PCB and thus form a shielding against electrical interference. As already mentioned the housing and the first surface of the PCB define, in combination, a volume within which volum e the pressure detecting arrangem ent and the pressure generating arrangem ent are arranged. Thus, the volume defined by housing and the first surface of the PCB in combination houses the MEMS die, the signal processor, the fram e structure, the suspension m em ber and the moveable m ass secured to the suspension mem ber. The primary role of the housing is to protect the active elements of the vibration sensor against for exam ple intrusive moisture and ear vax. Moreover, the housing also m inim ises the risk of cross- sensitivity to surrounding sound.

I n terms of im plementation the housing m ay be implem ented as a stainless steel housing, or it may be im plem ented using a lam inated structure of PCB materials. I n case of a stainless steel housing the shape of the housing may be provided using for example deep-drawing or punching. The housing m ay comprise a through-going opening adapted to vent the volum e defined by the housing and the first surface of the PCB. The purpose of this opening is to prevent excessive pressure inside the housing during reflow soldering. The opening needs to be very sm all in order to provide barom etric compensations and still prevent acoustic leakage. Alternatively, the opening may be sealed with an adhesive or sticky tape after the reflow. The frame structure may also be im plemented in stainless steel. The indentation provided in the frame structure m ay be provided by various m eans, such as etching, punching and deep-drawing. Alternatively, the fram e structure may com prise a plurality of stacked layers, such as a plurality of metal layers arranged on top of each other.

The suspension mem ber may be im plemented 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 . Alternatively, the suspension member m ay com prise 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 m ay be at least partly filled with a flexible sealant, such as a gel. The static and moveable parts of the suspension member m ay be manufactured 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 m ay be implem ented in a high-density material, such as tantalum m ass or stainless steel, and the mass 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 m em ber should be able to withstand typical reflow temperatures, i.e. suspension m em 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 member using a compliant adhesive, or the moveable mass m ay be secured to the second surface of the suspension member using a compliant adhesive. It should though be noted that moveable masses m ay be secured to both the first and second surfaces of the suspension member.

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 .

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 m ay 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 com pared 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 em bodiments described hereinafter.

BRI EF DESCRI PTI ON OF THE DRAWI NGS

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

Fig. 1 illustrates a schem atic view of the principle underlying the vibration sensor of the present invention,

Fig. 2 shows an exploded view of an embodim ent of the vibration sensor, and

Fig. 3 shows a cross-sectional view of an assembled embodim ent of the vibration sensor.

DETAI LED DESCRI PTI ON OF THE I NVENTION

I n general, the present invention relates to a com pact vibration sensor where, in particular, a pressure detecting arrangem ent and a pressure generating arrangement are arranged in a stacked and space-saving arrangem ent.

Referring now to Fig. 1 the general principle underlying the present invention is depicted. Generally, the vibration sensor 1 of the present invention is adapted to generate pressure variations in response to vibrations of the vibration sensor 1 , i.e. generate pressure variations when the vibration sensor 1 is exposed to external m echanical vibrations, and subsequently detect the generated pressure variations. Thus, the detected pressure variations are a m easure for the external mechanical vibrations to which the vibration sensor is exposed. As depicted in Fig. 1 the vibration sensor 1 comprises a moveable mass 2 which is adapted to move up and down as indicated by the arrow 5 in response to m echanical vibrations of the vibration sensor as indicated by the arrow 7. The moveable m ass 2 is suspended in a suspension member 3 whereby the moveable m ass 2 is allowed to move up and down as indicated by the arrow 5. An upward displacem ent of the moveable mass 2 will cause an increase of the air pressure in the volum e 4, and a decrease in the coupling volum e 10. A downward displacement of the moveable m ass 2 has the opposite effect. A pressure detecting arrangem ent 6 is provided for detecting the generated pressure variations, or more particularly, for detecting the generated pressure differences between the volum e 4 and the coupling volume 10. As seen in Fig. 1 the coupling volume 10 is defined by the suspension member 3 and the fram e structure 8, whereas the second volume 4 is defined by the housing 1 1 and the frame structure 8. A vibration sensor of the type depicted in Fig. 1 typically has a resonance frequency around a few kHz, such as between 1 kHz and 10 kHz. Although Fig. 1 only depicts the general principle underlying the present invention actual im plementations of the vibration sensor will be disclosed in relation to Figs. 2 and 3.

Referring now to Fig. 2, an exploded view of an embodim ent of the vibration sensor is depicted. Starting from the bottom the vibration sensor comprises a MEMS die 22 and a signal processor 24 arranged on a first surface 29 of a PCB 21 . Both the MEMS die 22 and the signal processor 24 are electrically connected to the PCB 21 via respective solder pads 25, 26 in a m anner so that respective volum es 27, 28 exist between a second (lower) surface of the MEMS die 22 and the first surface 29 of the PCB 21 , and between the signal processor 24 and the first surface 29 of the PCB 21 , cf. also Fig. 3. The signal processor 24 is adapted to process signals from the MEMS die 22, and the processed signals from the signal processor 24 are subsequently provided on one or more electrical contact pads (not shown in Fig. 2) arranged on a second (lower) surface of the PCB 21 whereby the vibration sensor can be easily connected to external electrical devices.

The MEMS die com prises a front volum e 23 and MEMS cartridge which may involve a capacitor comprising a moveable membrane and a rigid back-plate. The MEMS cartridge is facing the first surface 29 of the PCB 21 and are therefore not visible in Fig. 2.

The MEMS die 22, the signal processor 24 and optionally the PCB 21 m ay be considered the pressure detecting arrangem ent as these elem ents are adapted to detect generated pressure variations in response to vibrations of the vibration sensor. The footprint of the vibration sensor (width and length) is sm aller than 3 m m and 4 m m , respectively, whereas the overall height of the vibration sensor when assem bled is sm aller than 2 m m .

Still referring to Fig. 2 the pressure variations are generated by the pressure generating arrangement which com prises a fram e structure 17, a suspension member 16 and a moveable mass 15 secured to a first (upper) surface of the suspension mem ber 16. As seen in Fig. 2 the frame structure 17 com prises an indentation 19 surrounded by a projecting peripheral rim 18. When assem bled at least part of a second (lower) surface of the suspension m em ber 16 is secured to the projecting peripheral rim 18 of the fram e structure 17. With this arrangem ent a coupling volum e (reference num eral 36 in Fig. 3) is defined by the indentation 19 of the frame structure 17 and at least part of the second (lower) surface of the suspension m em ber 16. When assem bled that the frame structure 17 is secured to the MEMS die 22 in a m anner so that the before-m entioned coupling volum e is acoustically connected to the front volum e 23 of the MEMS die 22 via an acoustical opening 20 in the fram e structure 17. The acoustical connection between the before-mentioned coupling volum e and the front volum e 23 of the MEMS die 22 is provided by physically aligning the acoustical opening 20 with the front volum e 23 so that pressure variations generated in the before-mentioned coupling volum e are allowed to enter the front volume 23 of the MEMS die 22 and thus be detected

It should also be noted that the fram e structure 17, including its indentation 19, extends beyond the dim ensions of the MEMS die 22 thereby the dimensions of the suspension member 16 secured thereto can be m axim ised which also leads to an increased sensitivity of the vibration sensor. When assem bled, and as it will be discussed in further details in relation to Fig. 3, the frame structure 17 overhangs the signal processor 24. The frame structure 17 is im plemented in stainless steel, and the indentation 19 provided therein has been provided by etching. The suspension member 16 is implem ented as a polyim ide film , and the moveable mass 15 secured thereto is a tantalum or stainless steel m ass. As m entioned above other types of suspension members 16 and moveable masses 15 are also applicable.

The em bodiment depicted in Fig. 2 also comprises a housing 13 which, when assem bled, is secured to the PCB 21 using a sealant. Thus, the housing 13 and the PCB 21 forms a volume within which volum e the MEMS die 22, the signal processor 24 and the pressure generating arrangement com prising the fram e structure 17, the suspension m em ber 16 and the moveable mass 15 secured thereto are arranged. The housing 13 is m ade of stainless steel, and a venting opening 14 optionally be provided therein. The venting opening 14 provides a venting passage between the volum e defined by the housing 13 and the PCB 21 .

Turning now to Fig. 3 a cross-sectional view of an assem bled vibration sensor is depicted. As already m entioned in relation to Fig. 2 the vibration sensor com prises a MEMS die 22 and a signal processor 24 arranged on a first surface 29 of a PCB 21 . Both the MEMS die 22 and the signal processor 24 are electrically connected to the PCB 21 via respective solder pads 25, 26 in a m anner so that respective volum es 27, 28 exist between a second (lower) surface of the MEMS die 22 and the first surface 29 of the PCB 21 , and between the signal processor 24 and the first surface 29 of the PCB 21 . I n Fig. 3 the second (lower) surface of the MEMS die is at least partly constituted by the MEMS cartridge 30.

The signal processor 24 is adapted to process signals from the MEMS die 22, and the processed signals from the signal processor 24 are subsequently provided on one or more electrical contact pads 31 , 31 ’, 31 ” arranged on a second surface 37 of the PCB 21 in order to form easy electrical access to external electrical devices.

The MEMS die 22 com prises a front volum e 23 and a MEMS cartridge 30 in the form of a capacitor comprising a moveable membrane and a rigid back-plate. As mentioned above the MEMS cartridge 30 of the MEMS die 22 m ay involve other detection schemes, such as piezoresist ive, piezoelectric or charged plate capacitor detected schemes. As seen in Fig. 3 the MEMS cartridge 30 faces the first surface 29 of the PCB 21 , whereas the front volum e 23 of the MEMS die is facing away from the first surface 29 of the PCB 21 . As also mentioned in relation to Fig. 2 the MEMS die 22, the signal processor 24 and optionally the PCB 21 m ay be considered the pressure detecting arrangement of the vibration sensor as these elem ents are adapted to detect generated pressure variations in response to vibrations of the vibration sensor. Again, the footprint of the vibration sensor (width and length) is sm aller than 3 m m and 4 m m , respectively, whereas the overall height of the vibration sensor is smaller than 2 m m .

The pressure generating arrangem ent of the vibration sensor depicted in Fig. 3 comprises a fram e structure 17, a suspension m em ber 16 and a moveable m ass 15 secured to a first (upper) surface 35 of the suspension member 16 using a com pliant adhesive. The frame structure 17 com prises an indentation 19 surrounded by a projecting peripheral rim 18 to which projecting peripheral rim 18 at least part of a second (lower) surface of the suspension member 16 is secured using a com pliant adhesive. With this arrangem ent a coupling volum e 36 is defined by the indentation 19 of the fram e structure 17 and at least part of the second (lower) surface of the suspension m em ber 16.

As seen in Fig. 3 the fram e structure 17 is secured directly to the first surface 34 of the MEMS die 22 using a com pliant adhesive in order to compensate for different therm al expansion coefficients. As also seen in Fig. 3 the coupling volume 36 is acoustically connected to the front volum e 23 of the MEMS die 22 via an acoustical opening 20 in the fram e structure 17. As already addressed the acoustical connection between the coupling volume 36 and the front volum e 23 of the MEMS die 22 is provided by physically aligning the acoustical opening 20 with the front volum e 23 so that pressure variations generated in the coupling volume 36 are allowed to enter the front volume 23 of the MEMS die 22 and thus be detected

As seen in Fig. 3 the fram e structure 17, including its indentation 19, extends beyond the dim ensions of the MEMS die 22 and thus overhangs the signal processor 24. This is advantageous in that the dim ensions of the suspension m em ber 16 can be maxim ised which also leads to an increased sensitivity of the vibration sensor. The fram e structure 17 is implem ented in stainless steel, and the indentation 19 provided therein has been provided by etching. The suspension member 16 is implem ented as a polyim ide film , and the moveable mass 15 secured thereto is a tantalum or stainless steel m ass.

As seen in Fig. 3 the em bodiment further comprises a housing 13 which is secured to the PCB

21 using a conductive adhesive 32 so that the housing 13 and the PCB 21 form , in com bination, a shield against electric interference. As already mentioned, the housing 13 and the PCB 21 forms a volume 33, 33’ within which volum e 33, 33’ the MEMS die 22, the signal processor 24 and the pressure generating arrangement comprising the frame structure 17, the suspension member 16 and the moveable mass 15 secured thereto are arranged. It should be noted that the two volumes 33, 33’ are m utually acoustically connected as well as acoustically connected to the volumes 27, 28 below the MEMS die 22 and the signal processor 24, respectively. Again, the housing 13 is m ade of stainless steel, and a venting opening 14 m ay optionally be provided therein. The venting opening 14 provides a venting passage between the volum e 33 and the exterior of the vibration sensor.

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 bodim ents 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.