GB2491257A

GB2491257A - Sound wave based sensor

Info

Publication number: GB2491257A
Application number: GB201209207A
Authority: GB
Inventors: Andre Gerlach; Michael Helmle
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2011-05-25
Filing date: 2012-05-22
Publication date: 2012-11-28
Anticipated expiration: 2032-05-22
Also published as: FR2975859A1; GB2491257B; GB201209207D0; CN102798852A; DE102011076430A1; ITMI20120848A1; FR2975859B1

Abstract

The invention relates to a sound wave based sensor, comprising an electromechanical film (7) for transmitting and receiving sound waves, an earth electrode (5) being provided on a surface of the electromechanical film (7) and at least one sensor electrode (3) being provided on the surface that is opposite the earth electrode (5). The at least one sensor electrode (3) comprises a plurality of electrode regions (13; 17; 25, fig 2), each of which can be electrically contacted via at least one web (15).

Description

Description Title

Sound wave based sensor

Prior art

The invention is based on a sound wave based sensor according to the preamble of Claim I. in automobile construction, for example, sound wave based sensors are used in motor vehicles in order to sensethe surroundings of the vehicle. For this purpose, a sound signal, normally an ultrasound signal, is first transmitted, the sound sigial being reflectedby objects pre'sent in the transmission region. The reflected echo is then received by at least one sensor. The'distance to an object is determined from the propagation time of the signal from transmission to receiving of the echo. It is possible in this case for the sensor transmitting the signal and the sensor sending the signal to be the same.

In this case, the sensor operates as a so-called transceivet. The latter fitst emits a signal and, after the membrane has decayed, the incoming echo can be received. Alternatively, it is also possible to separate the transmitter and receiver, such that onesensor operates only as a transmitter and a second sensor oprates only as a receiver.

At present, ultrasound sensors, in particular based on piezoelectric crystals, are used as sensors in motor vehicles for the purpose of sensing the surroundings. in addition to such ultrasound sensors based on piezoelectric crystals, so-called electromechanical films are also known, in which pores are contained in a polymer matrix and electrostatic charges are applied permanently at the boundary surfaces of the pores. Upon application of a voltage, the height of the pores changes and the film begins to vibrate. Correspondingly, when a sound signal is received, the height of the pores is altered because of the incident sound waves, thereby generating a voltage.

The structure of an electromechanical film and a method for producing such a film are described, for example, in US 4,654,546. It is also described that the films can be used as sound sources. In this case, reference is made, in particular, to their use as a loudspeaker or microphone, for which purpose the film is applied, for example, to a surface, for example a wail. PS 4,654,546 does not * disclose that it is possible for the films to be used also as ultrasound sensors for sensing the surroundingsof a * motor vehicle.

In the case of such a film based sensor, there is the possibility of voltage punctures occurring because of defects in the film. This can result in a short circuit, and consequently in permanent damage and, in the least * * favourable case, in complete destruction of the sensor.

* Disclosure àf the invention

Advantages of the invention A sound wave based sensor reali±ed according to the invention comprises an electromechanical film for transmitting and receiving sound waves, an earth electrode being realized on the surface of the electromechanical film and at least one sensor electrode being realized on the surface that is opposite the earth electrode. The at least one sensor electrode comprises a plurality of electrode regions, each of which can be electrically contacted via at least one web.

The web via which the electrode regions can be electrically contacted is used to realize fuse regions on the sound wave based sensor. In the case of a defect in the electromechanical film,. which can result in a voltage puncture from the sensor electrode to the earth electrode, th i;h fnccsc -in t-hcs rninn nf thp i1frt,nri my nnc-ihIi --------, -"---t Ar -----2 vaporize. This has the result that, in the region in which a defect àists, the flow of current from the sensor electrode to the earth electrode is interrupted. There is only local damage to the sensor, but the functioningof the.

sensor is not impaired as a result. The individual webs through which the electrode regions are connected to each other thus act as a fuse.

In a first embodiment of the invention, respectively adjacent electrode regions are connected to each other by the at least one web. This nákes it possible, for example, to divide a large-area electrode into individual regions that are each electrically contacted to each other via the webs. In the event of a short circuit, for example resulting from a defect location in the electromechanical film, it is possible, for example, for all webs to which an electrode regiOn is connected to fuse, and possibly to vaporize, such that this one electrode region is no longer contacted. However, the remaining regions, into which the senso* electrode is divided, continue to be supplied with voltage, such that the functioning of the sensor continues to be ensured.

In an alternative embodiment, the sensor electrode is realized in the form of a meander-shaped structure, which comprises a qentral conductor path, from which electrode fingers extend out laterally,. the web in each case being realized as a constriction on the side of the electrode finger that faces towards. the conductor path. In this case, if a defect occurs in the electromechanical film, only single fingers are cut off from the supply of current in eadh case, in that the web connecting the electrode ti central conductor path fuses, or vaporizes.

The earth electrode and the sensor electrode, by theans of which the sound Wave based sensor is operated, are preferably each realized as. a metallic layer on the electromechanical film. Alternatively, it is also possible for either the earth electrode or, preferably, the sensor elec1rode, to be realized as condubtor paths on a p±inted circuit board, and for the electromechanical film tO be bonded to the printed circuit board comprising the sensor electrodes. It is preferred, hot'iever, for at least one sensor electrode likewise to be realized as a metallic layer on the electromechanical film. The metallic layer can be produced by any deposition method known to persons skilled in the art. Thus, the metallic layer carl be produced, for example, by chemical vapour deposition (CVD) or physical vapour deposition (PVD). It is also possible for the metallic layer that constitutes the sensor -5.-electrode, or the earth electrode, to be produced by a galvanic coating method. In addition to the methods described above, any other method that is known to persons skilled in the art and that can be used to produce a metallic layer is also suitable.

If, in particular, the sound wave based sensor is used in a motor vehicle for the purpose of sensing surroundings, it is advantageous if objects can be sensed by the sound wave based se'nsor, not only in respect of their distance,. but also in respect of.their direction. At least two sensor elements are necessary in order to achieve unambiguous direction sensing. In a preferred embodiment, therefore, a plurality of sensor electrodes are realized on the surface of, the' electromechanical film, which sensor electrodes are separated from each other by a nonraetalIic dielectric.

The plurality of sensor electrodes in this case each constitute a sound transducer, which can act as an independent sensor. The electromechanical film, for example, serves as a non-metallic dielectric by which the sensor electrodes are separated from each other. It is also possible for the sensor electrodes on the electromechanical film to be additiohally coated, such that the additional coating, which is used, for example, to protect the sensor electrodes, acts as a non-ruetallib dielectric.

If the sensor electrodes are realized on a printed circuit board on which the electromechanical film is applied, it is possible, for example, for the adhesive layer, by which the electromechanical film is bonded to the printed circuit board, to act as a non-metallic dielectric.

A coating that can be used to coat the sensor electrodes is usually a coating that provides protection against en'vironrnental influences. in particular, a coating is used that can prevent damage to the sensor electrodes and to the entire sound wave based sensor resulting from weather influences, for example rain, ice or snow, pr froth mechaniáal influences such as, for example, in *the case of slight impacts.

If a plurality of sensor electrodes, separated from each other by a non-metallic dielectric, are realized on the electromechanical film, it is particularly preferred if the sensor electrodes have a meander-shaped structure and the electrode fingers of adjacent sensor electrodes engage in each other, the individual electrode fingers of adjacent sensor electrodes being in each case separated from each other by the non-metallic dielectric. The engagement of the electrode fingers of adjacent sensor electrodes in each other makes it possible for a sensor, having a plurality of sound transducers, to be realized in a space-saving manner.

in order to realize the webs on the individual electrode fingers, in the reali±ation of a sensor electrode having.a meander-shaped structure, in one embodiment the electrode fingers are configured such that the width of the electrode fingers increases as the distance from the conductor path increases. In this case, the narrowest region of the electrode fingers is connected to the conductor path and serves as a web, which, in the event of a defect in the electromechanical film, which can result in.a voltage puncture, serves as a fuse. Alternatively, however, it is * also possible for the fingers each to be contacted to a web on the conductor path, and for an electrode surface to be provided, which constitutes the end of the finger and which is connected to the web on the side that is opposite the conductor path. In this case, the electrode surface can assume any form, and can be, for example, circular, rectangular, square, triangular, or in the form of any other polygon having any number of corners. If the electrode region is circular or oval in shape, it is preferred if the webs have an additional constrictiOn According to the invention, the electrically conductive webs, via which the electrode regions are electrically contacted, are dimensioned such that these webs fuse or vaporize in the case of an overvoltage occurring as a result qf a defect in the electromechanical film. The webs are destroyed as a result of the fusing.or vaporizing, and can thus serve.asa fuse element. -

Brief description of the drawings

Exemplary embodimens of the invention are represented in the figures and are explained more fully in the description that follows.

In the figures: -Figure 1 shows a three-dimensional representation of a sound wave based sensor, Figure 2 shows a top view of the sensor represented in Figure 1, Figure 3 shows a detail of a sensor.surface having meander-shaped sensor electrodes. -& Exemplary embodiments of the invention A sound wave based sensor is shown in a three-dimensional view in Figure 1. A sound wave based sensor 1 comprises a sensor electrode 3 and an earth electrode 5. Located between the sensor electrode 3 and,the earth electrode 5 there is an electromechanical material, which, upon application of a voltage to the sensor electrode 3 and the earth electrode 5, alters the extent perpendicularly in relation to the earth electrode 5 and the sensor electrode 3. In this way, application of an alternating voltage to the sensor electrode 3 can cause a vibration to be generated.

According tO tb inventicnT, the electromechanical material is an electromechanical film 7.. A membrane, composed of a ferroelectret material, is normally used as an electromechànicl film. Designated as ferroelectret materials in this case are closed-pore polymer foams, in which positive and negative electrostatic charges are permanently located at the boundary surface of the individual pores. Upon application of an electric voltage, the impressed electrostatic charges cause the individual pores to dilate, or to contract in the case of a reversed polarisation. It is thereby possible, through -corresponding excitation, to generate a vibration that can * be transmitted as a sound pulse. correspondingly, when sound waves are incident, the mechanical alteration of the size of the pores causes a voltage to be generated, which can be tapped..

Particularly suitable as polymer materials for producing the foams for the ferroelectret material are, in particular, fluorine based polymers, for example * polytetrafluoroethylene, polyfluoroethylene propylene and polyvinylidene fluoride. Also suitable, for example, is polyethylene terephthalate. In addition to the polymers * mentioned, copolymers of these polymers are also suitable.

To enable the electromechanical film 7 to be used as a transmittei or receiver foi sound signals, the earth electrode 5 is preferably applied as a full-surface metallic cbating on the electromechanical film 7. Sensor electrodes 3 are applied on the side of the * electromechanical film 7 that is opposite the earth electrode 5. The sensor electrodes 3 in this case are each electromechanical film at which the sensors are to be located.

If only a single sensor is to be configured, the electromechanical film, as represented in Figure 1, -can likewise have the shape of the subsequent sensor.

Alternatively, it. is also possibIe for the electromechanical film 7 to be configured as a continuous film, and for the sensor electrodes 3 each to be applied at the positions at which sensors are to be located.

The sensor electrodes 3 and the earth, electrode 5 can be applied by any deposition method known to persons skilled * in the art, for example electroless and/or galvapic deposition, chemical deposition methods, physidal deposition methods or vapour deposition onto the.

electromechanical film 7, particularly suitable for * applying the sensor electrdde 3 and the earth electrode 5 --10--are deposition methods in which the metal for the sensor electrode 3 and the earth electrode 5 are vapour-deposited, for example CVD methods or PVD methods.

As an alternative to a full-surface configuration of the earth electrode 5, it is also possible for the earth electrode 5 in each case to be provided only at the positions at which sensor electrodes 3 are also applied.

The earth electrode 5 in this case preferably has the same geometric shape as the sensor electrode 3 and is positioned opposite the sensor electrode bn the electromechanical film 7.

To enable the sound wave based sensor 1 to be operated, the earth electrode 5 is connected to an earth contact 9, and the sensor electrode 3 is connected to a sensor contact 11.

Here, the earth contact 9 and the sensor contact 11 are shown only schematically in Figure 1. Normally, the sensor contact ii is realized, for example, as a conductor path on the electromechanical film serving as a dielectric The electromechanical film 7 that is used to produce the sound wave based sensor 1 may have defects, for example because of: excessively large pores. In the region of the defects, a voltage puncture may occur between the sensor * electrode 3 and the earth electrode 5. Such a voltage puncture results in a short circuit that, in the case of conventional sensors, can result in. damage to the sensor, to the extent of causing the sensor to fail.

In order to prevent such damage to the sensor, according to the invention the sensor electrode 3 is divided into a -11 -plurality of electrode regions, each of which can be electrically contacted via at least one web.

Such a sensor electrode is represented exemplarily in Figure 2.

A sensor electrode 3 realized according to the invention has a plurality of electrode regions 13, in each case electrically connected to each other by webs 15. In addition, the embodiment of a sensor electrode 3 represented in Figure 2 has an annular electrode region 17 surrounding the electrode regions 13. The annular electrode region 17 surrounds the electrode regions 13 and is likewise contacted to the electrode regions 13 via webs 15. Owing to the annular electrode region 17, a defined gnnl-ar r11im4i-tinn nf th nnr 1rtrnd ic rhitcar The electrical contacting of the sensor electrode 3 is preferably effected via the annular electrode region 17.

To enable objects to be sensed, not only in respect of distance, but also in respect of direction in the case of the sound wave based'sensor being used, for example, as a distance sensor' in a motor vehicle, a plurality of sound wave' based sensors are used. For this purpose, it is possible to use, for' example, an array of sound wave based sensors 1. For this purpose, a plurality of senãor electrodes 3 can each be applied, at a defined distance in relation to each other, on an electrbrnechaniqal film. The electromechanical film in this case serves as a dielectric between the individual sensor electrodes, such that in *each case a single sound wave based sensor is defined by the sensor electrodes 3. In this case, the earth electrode opposite the sensor electrodes 3 can be realized as a single earth electrode for all sound wave based sensors, or a dediqated earth electrode, which is opposite the sensor electrode 3, is provided for each sensor electrode. It is preferred to provide a common earth electrode for all sensor electrodes on the electromechanical film.

Between the individual electrode regions 13 that are connected to each other by the web 15 there are dielectric regions 19. The dielectric ±egions 19 are, for example, non-coated regions of the electromechanical film 7.

If a defect then occurs in the electromechanical film 7, Which detect can result in a voltage puncture, and consequently in a-short circuit, the web 15 fuses in the region of the voltage puncture and thus breaks the r'nfl na nt-i inn)nOt-TJCnfln -ht,rn rl -4 z nn ont-rnAc rcrr4 inn c After the web 15 has fused and the material possibly has vaporized, and the conhection between the electrode regions 13 has thus been broken, no further voltage puncture can occur at that location. The sensor therefore remains capable of functioning, and the entire sensor electrode 3 is not destroyed because of the voltage puncture. -An alternative embodiment is represented as a detail in Figure 3. - -Unlike the embodiment represented in Figure 2, Figure 3 shows a meander-shaped sensor electrode 21.

Such a meander-shaped sensor electrode 21 has a central conductor path 23, from which electrode fingers 25 extend out laterally. The individual electrode fingers 25 each have a constriction 27 of the hide that faces towards the -13 -conductor path. The constriction 27. in this case serves as a web, which, in the event of a defect in the electromechanical film 7, fuses and vaporizes in. the event of a short circuit. In this way, the individual electrode fingers 25 are separated fran the conductor path 23, and the functioning of the meander-shaped sensor 21 as a whole is not impaired.

On the side facing away from the central conductor path 23, the width of the electrode fingers 25 increases and forms, for example, a circular termination 29,. as represented in Figure 3. Besides a circular termination 29, the electrode finger 25 can also terminate in an oval termination, a triangular termination, a square or rectangular termination, or in a polygon having any optional number of corners. In the case of a triangular configuration of the electrode finger 25, it is also possible for the triangle to be connected, by one vertex, to the central conductor path 23.

If am array composed of a plurality of meander-shaped sensor electrodes 21 is used, it is possible for the individual electrode fingers 25 to be configured such that the electrode fingers 25 of adjacent sensor electrodes 21 engage in each other. In this case, the individual electrode fingers 25 of the respective meander-shaped sensor. electrodes 21 are separated from each other by a electromechanical film 7. The distance between the.

electrode fingers 25 is in this case selected such that a voltage crossover from one meander-shaped sensor electrode 21 to an adjacent meander-shaped sensor electrode is not possible.

The use of meander-shaped sensor electrodes 21 is particularly suitable in the case of elongate sensàrs. The individual segments can then be operated separately, with controlled phase displacement, in the direction of greatest extent, such that the membrane does notvibrate.inphase.