FR2626078A1 - ACCELERATION SENSOR AND METHOD FOR MANUFACTURING SAME - Google Patents

Abstract

An acceleration sensor, preferably of subminiature construction, serves to measure an acceleration b in more than one direction of the x, y coordinates. It comprises a mass-spring system, in which a deviation of the mass, which is a function of the acceleration, is converted into an electrical signal corresponding to the acceleration vector to be measured b. In order to be able to detect, with maximum reliability and simplicity of construction, in which direction of the coordinates an acceleration threshold has been exceeded, a mass-spring system which can be deflected in an x / y plane, in particular a rod of bending 12 fixed at only one end, is provided with a first electrical contact element and surrounded, in the x, y plane, by several second electrical contact elements, in particular by rigid micro-contact beams 13 to 20. When of exceeding the predetermined acceleration threshold, the first electrical contact element touches one or two of the second electrical contact elements, depending on the direction of the acceleration to be measured b in the x, y plane.

Description

ACCELERATION SENSOR AND METHOD FOR MANUFACTURING SAME

  An acceleration sensor for measuring an acceleration in more than one coordinate direction, provided with a mass which can be deflected in a plane, and which is provided with a first electrical contact element and surrounded in the plane of a plurality of other electrical contact elements, so that, if a predetermined threshold is exceeded, the first electrical contact element actuates one or more of the other contact elements, depending on the direction of the contact. acceleration to be measured in the

plan.

  An acceleration sensor of this type has already

  described in DE OS 21 22 471.

  The invention further relates to a method of manufacturing an acceleration sensor of the aforementioned type. There are many developments of acceleration sensors. Most sensors

  acceleration systems operate with a

  Spring, in which the mass is deflected by the acceleration which is exerted, the deviation being at its

  turn converted into an analog electrical signal.

  There are, moreover, binary output acceleration sensors which indicate only whether a predetermined acceleration threshold has been exceeded or not. For this purpose, on the one hand, analog sensors are connected with electronic switching devices, in the form of a comparator, thereby detecting the overshoot of a device; on the other hand, there are also acceleration sensors which, because of their design, are already used in physical measurements, so that they only produce a logical signal determined in case of

exceeding a given threshold.

  In DE-OS 21 22 471 has already been described a detection device in which a metal sphere is disposed on a conductive support disk. The support is provided in its center with a hole of diameter smaller than that of the sphere. Under the hole is a magnet. At rest, the sphere is in the hole of the bore and is furthermore

  frozen in this rest position by the magnet. Au-

  above the sphere is a ring of symmetry of revolution consisting of contact elements distributed on a frustoconical surface. At rest, these contact elements are at a distance from the sphere. If, now, the sphere is deflected from its rest position under the effect of an acceleration, the attraction between the sphere and the magnet is broken and the sphere rolls on the conductive support in the

  direction of acceleration or deceleration.

  After having traveled a certain path, the upper part of the sphere comes to bear against one of the contact elements, thus closing an electrical circuit between the contact element and the conductive support, since the sphere is also conductive. It is thus possible, according to the contact element touched, to determine the direction of the path of the sphere and hence the direction of

acceleration.

  The disadvantage of this known detection device concurs nevertheless in that the sphere is not guided on its path between the rest position and it is fixed by the magnet and the contact position, so that It can not be ruled out that the sphere is subjected in its path to other forces, which then exert a relatively great influence on the trajectory of the sphere, because it is no longer impeded on its way by no kind of stopping force. In addition, the rest position of the sphere is relatively unstable and, finally, the known detection device allows only one mounting position, in which the conductive support is

found in a horizontal plane.

  A sensor described in EP-OS 251 048 uses a pendulum formed by an optical fiber and a mass. The point of impact of the light beam emitted by

  the optical fiber is monitored in a plane for

  see if it is inside a circular surface

  (isotropic measurement) or inside an over-

  elliptical face (anisotropic measurement).

  In addition, there are spring-mass mechanical acceleration sensors in which the mass is mechanically connected to the environment over the entire travel path.

  the movable element of the sensor. The mass is thus guiding

  mechanically on its path between the rest position and a deflected position. In the sensors of this

  type 'the direction of deviation of the element of de-

  tion is, however, predetermined by construction.

  This is the reason why it has already been proposed, to measure acceleration in more than one direction of coordinates, to use two or more

  three coptcurs, the mass of each system

  mass that can be deflected in a coordinate direction. The produced analog electrical signals are added vectorially in this known vector sensor, by means of a switching device, which makes it possible to display and evaluate

intensity and direction.

  Sensors for one-dimensional or multi-dimensional measurements find application in the most diverse fields of activity. Acceleration sensors with a binary output are mainly used to detect extreme situations, for example in passenger safety systems, that is to say to trigger an air bag or a belt tensioner. It is also common to use binary output acceleration sensors of this type for monitoring machines and installations, for example to detect when the machines or the installations enter their own resonance in a dangerous way. In many cases of application of the above type, it is necessary to monitor whether a determined acceleration threshold has been exceeded in a given direction or in which direction there has been an acceleration whose intensity has exceeded a threshold. As a general rule, acceleration sensors should consist of a small number of components that are as simple as possible and have a minimal footprint, to allow reliable measurements without interference even

  when there is only limited space.

  DE-OS 35 20 383 has also been described a collisicn indicator, which finds its application.

  during vehicle clashes. The indicator of col-

  Known knowledge also uses a spherical contact element, which is arranged in the middle of a cup

  going back to the outside. Under the effect of an

  During the deceleration or deceleration, the sphere rolls in the direction of acceleration or deceleration up and down. The cup is

  provided with a large number of

  divided into sectors, which makes it possible

  finish the direction and movement of the sphere.

  Moreover, this known collision indicator also has the drawbacks that have been cited above in the case of

  detection described in DE-OS 21 22 471.

  Finally, methods of manufacturing acceleration sensors are known, according to which the measuring system itself is not composed of discrete mechanical elements, but is

  rather formed by chemical processes.

  US-Z-IEEE "Transactions on Electron Devices", VOL. ED-26, No. 12, December 1979, Pages 1911 & 1917 describes the obtaining of an acceleration sensor by etching process of a silicon substrate. However, these known methods only make it possible to produce acceleration sensors in silicon technique. Silicon, as a ceramic material, however, has a very high rigidity, so that the mode of response must be studied accordingly. In addition, because of the rigidity of the ceramic material, the sensor

  acceleration may enter vibration.

  The purpose of this invention is to develop an acceleration sensor of the aforementioned type, as well as a method for manufacturing the latter, making it possible to produce acceleration sensors that have maximum operational safety for a very small footprint, which can operate in any mounting position, and who,

finally, are easy to manufacture.

  The acceleration sensor mentioned at the beginning, in accordance with the invention, is obtained by

  so that the mass is part of a system

  Mass in the form of a bending rod fixed on one side and whose free end is

  equipped with an electrical contact element.

  The object of the invention is thus perfectly achieved; indeed the use of a mass-spring system ensures reliable guidance of the contact element over its entire range of displacement, because the moving mass is guided continuously. The same applies when, in accordance with the invention, this is obtained by using a bending microbeam, whose deflected free end is permanently mechanically connected to the plate in which the bending micro-beam is fixed. In addition, the acceleration sensor which is the subject of this invention can be installed in any position, since the micro-beam bending device operates independently of the fact that the micro-beam is mounted horizontally , vertically or in any direction in relation to the center of

Earth.

  Although, normally, it is preferred to arrange the second electrical contact elements in a circle around the first electrical contact element, it is recommended, in another exemplary embodiment of the invention, to arrange the second electrical contact elements on an ellipse around the first

electrical contact element.

  This arrangement has the advantage of allowing, already in the sensor itself, a freely fixed weighting of the acceleration thresholds as a function of their direction in the

  plan, when it is necessary to close

  different contacts in the various directions, in order to connect the electrical contact elements together. It goes without saying that the elliptical shape of the track is only an example here, because it is naturally possible to install, instead of

  elliptical track, any other tracks not

circular.

  In another particularly recommended version of the invention, the mass-spring system consists of a bending rod fixed on one side, whose free end is equipped with the first

contact element.

  This arrangement has the advantage of allowing an extremely simple construction in which the bending rod, the only movable element, is active. The bending stiffness can be adjusted for each application case, especially for extremely small dimensions, by appropriately determining sizing, shaping and material. In one of the improvements of the version cited above, the bending rod is made of a material with high internal friction, in particular of plastic material, and is surrounded, at least with its

  free end of a metallization.

  This provision has the advantage of

  significantly reduce the inherent resonances of the bending rod, due to the high internal friction of the material used. Metallization present

  the advantage of ensuring, by means of

  simple, contact from all sides, although the material used for the bending rod is not

not an electrical conductor.

  In certain versions of the invention, the bending rod consists of a circular cylinder. This provision in turn presents

  the advantage of allowing an aniso-

  trope, because the symmetrical rod of cylindrical flexion fixed at one end presents the same

  rigidity in all directions of deviation.

  Sectional cylindrical bending rod

  elliptical is another alternative.

  This arrangement also makes it possible to achieve a given anisotropy of the response mode; the elliptical section is again only one

  example among a multitude of non-conventional

  cylindrical symmetry of revolution.

  In a particularly recommended version of the invention, the second electrical contact elements are in the form of rigid micro-beams which are arranged on the same substrate as

the flexion rod.

  This arrangement in turn has the advantage of allowing an extremely simple and even in some cases monolithic construction, which, because of its simple structure, is particularly easy to manufacture and away from disturbances. In another version of the acceleration sensor described above, the micro-beams

  are made of a metallic material.

  This arrangement has the advantage of being able to detect a contact between the bending rod and the micro-beams by any voltage drop at the micro-beam without adding contact elements, conductive cables or

others at the level of micro-beams.

  In another version of the invention, the second electrical contact elements are arranged in the form of conductive tracks located on the face

  internal of a tube surrounding the bending rod.

  This arrangement has the advantage that the second electrical contact elements are arranged

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  on a particularly rigid structure as to

  it is particularly opposed to accelerating

  rations. The installation of conductive tracks on the inner face of the tube has the advantage of allowing very precise staggering due to very thin conductive tracks, so that the resolution of the angle is particularly

  high when measuring the acceleration vector.

  In another version of the invention, an electrical value processing unit converts the contacting between the first electrical contact element and the second electrical contact elements into an electrical signal corresponding to a

  deflection angle of the mass-spring system.

  This arrangement has the advantage, thanks to an immediate corresponding decoding, of producing a digital signal, which can be used in current computers, for example on board a vehicle. In an improvement of this variant, the electrical unit for processing the values produces a signal corresponding to an intermediate value, in the event of contact with at least two elements of

electrical contact neighbors.

  This arrangement has the advantage of increasing the accuracy of the resolution of the angle

with simple means.

  In the context of the present invention are particularly recommended acceleration sensors in the subminiature domain, in

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  which the length of the bending rod 12 is equal to 200 to 1000 times, in particular 500 times its thickness, this thickness being between 1 and

10. m, preferably 5 μm.

  In the case of acceleration sensors of the type described above, the fundamental problem of the invention has been solved by the fact that the stem of the invention, the micro-beams rsp. the tube and the substrate are manufactured using a forming process

  Lithographo-galvanotechnical (LIGA).

  Details of the LIGA process are for example described in the report "KfK-Bericht" No. 3995 of November 1985: "Herstellung von Mikrostrukturen .."

  (Manufacturing of microstructures), Kernforschungs-

  zentrum (Nuclear Research Center) in Karlsruhe.

  These methods have the advantage of making it possible to manufacture the structure described above with a high precision and however practically any shape, and this, from a multitude of original materials, in particular the metal, the plastic or ceramic. The multitude of possible shaping allows, in this case, to predetermine certain characteristics; so that it is possible to vary practically & wish the response mode of the acceleration sensor thus manufactured, depending on the

requirements of special cases.

  The description and drawings attached

indicate other advantages.

  It goes without saying that the characteristics mentioned above and those that still remain to be explained

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  can not only be used in each combination indicated, but also in other combinations or alone, without leaving the frame

  of the invention which is the subject of the patent.

  The drawings show examples of execution of the invention, which are explained in more detail

  in the following description, in particular:

  Fig.1 a perspective view of an exemplary embodiment of an acceleration sensor according to the invention; Fig.2 a top view of the acceleration sensor shown in Fig.1, equipped with the necessary wiring; Fig. 3 a representation similar to Fig.2, however showing another example of execution of an acceleration sensor; Fig.4 another representation similar to Fig.2, showing yet another example of execution of an acceleration sensor;

  Fig.5 an extremely schematic modular synoptic

  equipped with a value processing unit installed in accordance with the invention; Fig.6 a truth table for exploiting the measurement signals of the acceleration sensor according to Fig.2;

  In FIG. 1, 10 denotes the whole of

  accelerator, as it can be used to

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  vectorally measure acceleration b. in one

  plane defined by the Cartesian coordinates x and y.

  The acceleration sensor 10 has a common substrate 11, in which. a bending rod 12 of circular cylindrical shape is fixed at one end. Micro-contact beams 13 to 20 are arranged on a circular track around the bending rod 12; these micro-beams have approximately the same length as the rod 12 and eight of them, for example, can be distributed over a circumference of 360 '. It goes without saying that, depending on the application case and the desired resolution, it is possible to have, consequently, more or less micro-beams of

  contact around the bending rod 12.

  The arrow 21 shows that, when subjected to acceleration b, the bending rod 12 is deflected in the direction of the acceleration vector. The module of the deflection of the free end of the bending rod 12 then corresponds to the modulus of the acceleration b. If this modulus of acceleration b exceeds a threshold, which depends on the sizing and the choice of the material of the bending rod 12 as well as the micro-beams 13 to 20, the bending rod 12 is deflected so that its free end touch one or two micro-contact beams 13 to 20. If it happens that the contact between

  the bending rod 12 and one or two of the micro-

  contact beams 13 to 20 causes the closure of an electrical contact, then it is possible to detect

this state selectively.

  It goes without saying that the micro-contact beams 13 to 20 are suitably rigid with respect to the

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  elastic bending strokes 12, so that they do not deform also on their side when they

  are subject to acceleration.

  Fig.2 shows, in the form of a top view of the device of Fig.1, that the plug shaft 12 is preferably made of plastic and is surrounded by a metallization 28. The mr. However, as shown for the micro-beam 15, the contact beams 13 to 20 are preferably metallic, so that the contact of the

  metallization 28 and one or two of the micro-

  contact beams 13 to 20 can detect

  easily closing an electrical contact.

  In addition, the metallization 28 is connected to one of the first connection terminals 30 of the acceleration sensor 10, while the micro-beams 13 to are connected to the connection terminals 31 to 38, as

shows it in detail Fig.2.

  FIG. 2 further illustrates the case where the bending rod 12 is subjected to a b-oblique acceleration directed downwards and to the right and whose module is so large that the bending rod 12 is deflected into a position 12 ', where it comes to rest against the contact micro-beam 15. In this case, it is therefore possible to detect a contact between them.

terminals 30 and 35.

  Fig.3 illustrates a modified acceleration sensor 10a, wherein the bending rod 12a is not cylindrical but cylindrical in shape of elliptical cross section. This means that in the device shown in FIG.

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  acceleration b directed upwards should exceed a small threshold in order to cause contact, for example at the contact micro-beam 20a, as indicated with a deflected bending rod 12a 'in FIG. while in the case of a right or left deviation in Fig. 3, the acceleration threshold to be exceeded, before reaching a contact, is considerably higher. This is because, as we know, in the case of a bending rod of elliptical section fixed at one end, the deviation depends on the geometrical moment of inertia of the latter, which, in the case of 'a bending rod & elliptical section in the directions of the two main axes, is a function of the cube of the length of the

main axes.

  Fig.4 represents another variant of a

  acceleration sensor with anisotropic measurement mode.

  This variant also comprises a circular cylindrical bending rod 12b located in the center of a tube 40 of elliptical section, whose face

  internal is equipped with conductive tracks 41.

  If, in this case, the bending rod 12b is subjected, for example, to an oblique acceleration directed upwards and to the right (FIG. 4), it comes into contact position 12b 'after having reached a certain bending, which is a function of the angle,

  as clearly shown in Fig.4.

  It goes without saying that the forms described so far, namely cylindrical circular and elliptical are cited as an example. It is naturally possible to choose other

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  for the section of the floxion rod 12 with respect to the device of the peripheral electrical contacts, so as to weight the acceleration values in one of the relative directions. at such values

in other directions.

  FIG. 5 illustrates in an extremely zechetic manner a thieves processing unit 50 with inputs 51 and outputs 52. One of the inputs 51 is connected to a voltage source 53, which can, for example, be connected to the terminal of FIG. fitting of the device according to Fig.2. In this case, the metallization 28 of the bending rod 12 conducts a positive electrical voltage, which then appears at one or two other connection terminals 31 to 38, depending on the micro-contact beam 13 to 20 touched

by the bending rod 12.

  There then appears a signal at the outputs 52, if contact is established between the bending rod 12 and micro-contact beams 13 to, when, therefore, one of the predetermined acceleration thresholds has been exceeded; this signal is given at the outputs 52 in a current code, for example a BCD code, a binary sequence, which is a measurement of the angle d whose bending rod 12 has been deflected and which coincides with the direction of

the acceleration b that is exercised.

  Fig.6, finally, still shows a table of truth, in which are inscribed the states of

terminals 31 and following.

  In the first line of the truth table in Fig. 6, the status entered indicates that all

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  connection terminals 31 ... show a negative logic signal. This means that no contact has been established between the positive voltage bending rod 12 and one of the micro-contact beams 13 to 20; it is Adire that the bending rod 12 is not deflected or when it is deflected in one direction

Unmeasurable.

  When a positive logic signal appears only at the connection terminal 31, this means, in the definition of the angle according to FIG. 1, that there is a deflection angle of 0 'with respect to the y-axis. When the angle increases and reaches the value of 22.5, contact is established with two neighboring microporous beams, namely the microptresses 19 and 20, so that the connection terminals 31 and 32 present a signal positive logic. When the angle (increases, this phenomenon continues, a positive logic signal then appears at the connection terminal 32 alone, then terminals 32 and 33, then terminal 34 alone, etc. It is thus possible to locate angles of 45 *, 67.5-, 90 'etc. This corresponds to the case ot the rod of

  flexion 12 is surrounded by only eight micro-

  contact beams 13 to 20. It is naturally possible to increase the resolution of the angular measurements by increasing the number of contacts through the use of very thin conductive tracks 41,

as shown in Fig.4.

  Acceleration sensors of the type described above

  above can be advantageously manufactured in a lithographic-galvanotechnical process (LIGA) or in a chemical etching process in silicon technology. These methods make it possible to produce

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  Bending tigcs of extremely small dimensions, so that it is not necessary to have at the free end of the bending rod additional earthquake masses, which would both reduce the speed of response and increase the tendency to enter. in own resonance. The LIGA process makes it possible to check the material and manufacture the desired structures of metal, plastics

or ceramic.

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Claims (7)

claims   1 / Accelerator sensor for measuring an acceleration (b) in more than one coordinate direction (xéy), equipped with a mass which can be deflected in a plane (x / y), in which the mass is provided with a first electrically contacting element and surrounded, in the plane, by a plurality of second electrical contact elements, such that, when exceeding a predetermined acceleration threshold, the first electrical contact element touches a or a plurality of second electrical contact elements as a function of the direction of acceleration to be measured (b) in the (x / y) plane, characterized in that the mass is part of a mechanical spring-spring system consisting of a bending rod (12) fixed on one side, the free end of which is equipped with the first electrical contact element.   2 / Acceleration sensor, according to the claim   1, characterized in that the second electrical contact elements are arranged on a non-circular track, in particular on an elliptical track   all around the first electrical contact element.   3 / Acceleration sensor, according to the claim   1 or 2, characterized in that the bending rod (12) is made of a material with a high internal friction, in particular a plastic material, and in that the free end of the bending rod at least   is surrounded by a metallization (28).   4 / Acceleration sensor, according to one or   several of claims 1 to 3, characterized in that   that the bending rod (12) has the shape of a circular cylinder. - 20 -   / Acceleration sensor, according to one or   several of claims 1 & 3, characterized in that   that the bending rod (12a) is in the form of a cylinder of non-circular section, of elliptical section. 6 / Accelerator sensor, according to one or   several of claims i to 5, characterized in that   the second electrical contact elements are in the form of rigid micro-beams (13 to), which are arranged on the same substrate (11) as the bending rod (12) 7 / acceleration sensor, according to the   claim 6, characterized in that the micro-   beams (13 to 20) are made of a metallic material. 8 / Acceleration sensor, according to one or   several of claims 1 to 5, characterized in that   that the second electrical contact elements are arranged in the form of conductive tracks (41) on the inner face of a tube surrounding the bending rod. 9 / Acceleration sensor, according to one or   several of claims 1 to 8, characterized in that   an electrical value processing unit (50) produces an electrical signal corresponding to a deflection angle (a) of the WeightRessort system when a contact has been established between the first electrical contact element and one of the second electrical contact elements.   10 / acceleration sensor according to claim 9, characterized in that the unit - 21 -   the processor (50) generates a signal corresponding to an intermediate value, when contact with at least two   Neighboring electrical contact elements.   11 / Acceleration sensor, according to one or   pliers of claims 1 to 10, characterized in that   that the length of the bending rod (12) is tale to 200 L 1000 times, preferably 500 times, its thickness and in that this thickness is included   between 1 and 10, m, preferably 5, m.   12 / A method of manufacturing an acceleration sensor (10) according to one or more of the   Claims 6 to 11, characterized in that the rod   flexion (12), the substrate (11) and the rigid micro-beams (13 to 20) resp. the tube (40) are manufactured according to a forming process   Lithographo-galvanotechnical (LIGA).