Micromechanical accelerometer

Abstract

Micromechanical acceleration sensor comprising at least a first seismic mass (2) which is suspended to allow deflection, at least one readout device (3a, 4a; 3b, 4b) for detecting the deflection of the first seismic mass and at least one reset device (5), characterized in that at least the first seismic mass (2) is suspended eccentrically with respect to its center of mass (9) on at least one torsion spring (1).

Description

The invention relates to a micromechanical acceleration sensor, to a method for measuring an acceleration, and to the use of the acceleration sensor in motor vehicles.Acceleration sensors of this type are shown, for example, in US 2008 / 0066561 A1 or US 2004 / 0160232 A1. In both specifications, the seismic masses are eccentrically suspended.The object of the invention is to propose a micromechanical acceleration sensor and a method for measuring accelerations, with which accelerations can be detected relatively precisely.This object is achieved according to the invention by the micromechanical acceleration sensor according to Claim 1 and by the method according to Claim 7.A reset device is preferably understood to mean a capacitive device, in particular acting according to the electrostatic principle, by means of which the deflection of the seismic mass can be influenced and in the process the deflection of the seismic mass is particularly preferably always or continuously regulated back to a defined deflection value, wherein this defined deflection value corresponds very particularly preferably to a rest position of the seismic mass.When the acceleration sensor is designed as a single-axis sensor, i.e. for detecting accelerations in one direction, the center of mass of at least the first seismic mass is particularly preferably offset in a direction with respect to its suspension or torsion axis, the center of mass being very particularly preferably offset or marked below or above the suspension or torsion axis, on a perpendicular to this axis. If the acceleration sensor is designed as a multi-axis sensor, i.e. for detecting accelerations in at least two different directions, the center of mass of at least the first seismic mass is particularly preferably offset or formed in two directions with respect to its suspension or torsion axis, the center of mass being very particularly preferably offset or formed below or above and laterally offset with respect to the suspension or torsion axis.It is preferred that the at least one restoring device comprises at least one electrode, in particular an electrode of substantially flat design, and is substantially designed and arranged relative to the first seismic mass such that a substantially square relationship exists between deflection of the first seismic mass and / or the force acting on it on account of an electrical voltage applied to the restoring device and this electrical voltage. Particularly preferably, the at least one restoring device comprises one or more plate capacitors and very particularly preferably no meandering capacitor structure which has a substantially linear relationship between the deflection of the first seismic mass on the basis of an electrical voltage applied to the restoring device and this electrical voltage. By means of the above-described quadratic relationship or the corresponding configuration of the restoring device, both relatively large and relatively small accelerations can be detected relatively precisely, since the restoring voltage-acceleration characteristic curve is relatively steep in the region of relatively low accelerations and the sensor therefore has a relatively high resolution in this region and this characteristic curve is relatively flat in the region of relatively high accelerations and thus excessively large restoring voltages are not required for these relatively large accelerations. The acceleration sensor is in particular preferably designed such that the restoring voltage-acceleration characteristic curve substantially has the profile or the shape of a root function at least with respect to the first seismic mass and the at least one restoring device assigned to it.The electrode of the at least one reset device is preferably arranged in an encapsulation module of the acceleration sensor, wherein this encapsulation module is in particular designed as a cover.The electrode of the at least one restoring device expediently has an angular amount of less than 20° with a base surface or substrate plane of the acceleration sensor and is arranged in particular substantially parallel to the base surface.It is preferred that the acceleration sensor has at least two or a multiple of two readout devices, which are arranged and / or formed symmetrically with respect to a geometric or mass-related center point and / or a geometric or mass-related center axis of the first seismic mass or of the acceleration sensor.The acceleration sensor preferably has at least two or a multiple of two restoring devices, which are arranged and / or formed symmetrically with respect to a geometric or mass-related center point and / or a geometric or mass-related center axis of the first seismic mass or of the acceleration sensor.The at least one reset device and the at least one read-out device preferably have one or more capacitances with the seismic mass assigned to them. This capacitance is in particular formed as at least one plate capacitor, particularly preferably as comb structures with a plurality of plate capacitors.It is expedient that the two or more restoring devices and / or reading devices of the acceleration sensor are designed such that, when at least the first seismic mass is deflected in a first direction, the at least two restoring devices and / or reading devices undergo changes in capacitance in opposite directions, that is to say changes in plate spacing which are inverse to one another. In particular, the comb structures of opposing reset and / or read-out devices engage in one another in a manner offset with respect to one another. This counter-rotating capacitance configuration particularly preferably also has otherwise symmetrical reset and / or read-out devices described further above.It is expedient for the acceleration sensor to be designed as a three-axis sensor and to have four seismic masses, which are each suspended from at least one torsion spring, wherein the center of mass of the seismic mass is each offset with respect to the suspension axis and in each case two seismic masses are suspended in such a way that the suspension axes are formed by substantially 90° with respect to the suspension axes of the two other seismic masses. The acceleration sensor comprises in particular an electronic evaluation circuit or is connected to such an evaluation circuit which can detect the accelerations in three directions from the deflections and / or restoring voltages of the four seismic masses. The suspension axes are particularly preferably arranged substantially parallel to an x-y substrate plane, wherein the suspension axes of the four seismic masses are aligned in pairs in the x-direction and y-direction, and most particularly preferably the suspension axes or torsion springs are arranged or formed in this case in front of or on the left side of the respective seismic mass and behind or on the right side of the center of mass of the respective seismic mass with respect to the one mass. Above and / or below, i.e. spaced apart in the z direction, the seismic masses are each assigned two reading electrodes, these reading electrodes being assigned or arranged on both sides of the suspension axis or of the corresponding torsion spring. By means of the respectively offset center of mass with respect to the respective suspension axis or by means of the torsion springs eccentrically formed or arranged with respect to the center of mass, one pair of seismic masses is rotatably deflected in antiphase about the y axis upon the action of an acceleration in the x direction and the other pair of seismic masses is rotatably deflected in antiphase about the x axis upon the action of an acceleration in the y direction. Upon the effect of an acceleration in the z-direction, i.e. perpendicular to the substrate plane, all four seismic masses are deflected in phase-twisting about their respective suspension axis.It is expedient for at least two read-out devices to be assigned to at least the first seismic mass, which read-out devices are assigned on this side and beyond this suspension axis and / or on both sides with respect to this suspension axis and are arranged accordingly with respect to a suspension axis of the first seismic mass and / or which are assigned to a central region of the first seismic mass and are arranged accordingly, and wherein the at least one reset device is assigned to the first seismic mass further outwards than the read-out devices with respect to its suspension axis and / or the central region and is arranged accordingly. In particular, a reset device is arranged further outwards than the read-out device, particularly preferably on both sides of the read-out devices. The arrangement of the at least one restoring device in the outer region of the seismic mass has the effect that the required restoring voltage can remain relatively low on account of the relatively large lever, with respect to the suspension axle, that is to say only relatively low electrical restoring voltages are required.The acceleration sensor preferably comprises a control circuit which can adjust the deflection of the seismic mass to a defined deflection value, in particular the deflection value corresponding to a rest position of the seismic mass, by means of at least the resetting device.The at least one read-out device preferably detects the deflection of the seismic mass according to the capacitive principle.It is expedient for the acceleration sensor to have at least two read-out devices which are assigned jointly to the seismic mass, as a result of which differential detection of the deflection of the seismic mass can be carried out and therefore, in particular, an offset capacitance does not have to be taken into account.It is preferred to arrange the at least one read-out device above and / or below or below the seismic mass, relative to the substrate plane, since no additional chip area is required here for read-out or reset structures and the sensor can therefore be made smaller.The acceleration sensor preferably has at least one reset device or at least one reset electrode, in particular in pairs, in front of and behind or above and below at least the first seismic mass, as a result of which the total capacity of the reset devices is increased, in particular doubled, and lower reset voltages, that is to say electrical voltage applied to the respective reset device, are thus required.An advantage of the acceleration sensor with reset device / s is the small design compared to sensors with a plurality of seismic masses which are suspended from springs for different measurement ranges, or compared to a plurality of sensors. A further advantage is that existing sensor designs can be used which only have to be extended by the at least one reset device.By integrating at least one reset device or additional electrodes alone, the measurement range of a low-g sensor can preferably (typically 1-5 g) be extended to an additional higher measurement range (50-100 g). By means of a suitable arrangement in a motor vehicle, a separate high-g acceleration sensor, which was in some cases customary or necessary up to now, can thereby be dispensed with.In particular in comparison with reset devices which are designed as meander-shaped comb structures, at least one reset device comprising at least one parallel plate capacitor permits the non-linear reset of the seismic mass or of the acceleration signal. This makes it clearly easier to realize the opposing requirements for the highest possible resolution in the low-g range and the greatest possible measurement range with the smallest possible reset voltages in the high-g range. The reduction in resolution usually associated with an increase in the measurement range occurs in this solution only at high accelerations. The non-linear profile of the transmission characteristic thus ensures that a relatively high resolution can be achieved in measurements in the low-g measurement range (1-5 g).The method is preferably further developed by the control being carried out continuously.It is preferred that the acceleration detected by the acceleration sensor is calculated at least from the value of an electric voltage which is applied to the restoring device for regulating the deflection of the seismic mass to the defined deflection value within the scope of the regulation.The invention also relates to the use of the micromechanical acceleration sensor in motor vehicles, in particular for the combined detection of relatively low accelerations, for example for ESP applications, and relatively high accelerations, for example for occupant protection and airbag applications.Further preferred embodiments are evident from the dependent claims and the following descriptions of exemplary embodiments with reference to figures.They show in schematic representation: FIG. 1 shows an exemplary control system of an acceleration sensor in the form of a block diagram, wherein the acceleration sensor comprises a resetting control and has a relatively wide measurement range. FIG. 2 a) shows an exemplary embodiment with four reset devices, b) an exemplary acceleration sensor with four read devices, FIG. 3 shows an exemplary embodiment with a first seismic mass, which has a center of mass offset with respect to its suspension axis, FIG. 4 shows an exemplary three-axis acceleration sensor, FIG. 5 shows the cross section of an exemplary acceleration sensor having two seismic masses 2 b, 2 cthat are deflected in phase in the z direction by an acceleration, FIG. 6 is a cross-sectional view of an exemplary acceleration sensor with top and bottom electrodes and reset devices. This requires less reset voltage, since the capacitance increases. Likewise, the signal strength of the readout electrodes is greater for the same reason, FIG. 7 shows an exemplary transfer function of a reset signal and a linearized signal as a function of acceleration, and FIG. 8 shows the exemplary illustration of the resolution of a reset acceleration sensor as a function of the reset voltage at the reset electrodes.FIG. 1 shows, by way of example, the functional principle with the circuit components of an electronic controller connected to sensor element 1, schematically consisting of measuring capacitances C 1 and C 2. These capacitances measure the deflection of the seismic mass. The arrangement of C 1, C 2 is selected such that a deflection of the seismic mass causes an opposite change in the two capacitances C 1 and C 2. The conversion of the capacitance signal into an electrical measured variable is effected by feeding in a constant alternating voltage to pin (carrier). Via the subsequent current-voltage converter consisting of the amplifier block 2 and the two reference capacitances CREF 1 and CREF 2, the changes in capacitance in C 1, C 2 are converted into a proportional voltage signal. Circuit block 3 comprises an A / D converter which converts the analog signal into a digital signal. There are several embodiments for the implementation of the A / D converter. Parallel converters allow direct conversion into a digital bit signal with a predetermined conversion width. Further alternative embodiments are designed, for example, as sigma-delta converters, in which the analog signal is first converted into a pulse-width-modulated signal and then, via at least one subsequent decimation stage, into a parallel digital signal. Circuit block 4 consists of a regulator structure which adjusts the output signal so that the input signal is regulated to 0. This regulator has the effect that the voltage signal fed back to the reset electrodes C 3, C 4 via the D / A converter 7 and the high-voltage converter 8 is adjusted such that the force of the acceleration signal acting on the seismic mass is compensated by the electrostatic force acting in C 3 and C 4. Similar to the A / D converter, a sigma-delta converter can also be used here. A combination of the A / D converter with the D / A converter to form a so-called closed-loop signal delta converter is also possible.The relationship applicable to the parallel plate capacitor, that the electrostatic force acting is proportional to the square of the voltage acting, results in the non-linear dependence of the reset voltage on the acceleration acting, as shown in FIG. 7. In the signal processing block 8, the signal is squared by multiplication and a linear relationship to the acceleration is thus established again. In signal processing block 8, the adjustment of the offset and the sensitivity is then likewise carried out, which is advantageous for sensors in this accuracy class. By means of an additional test input, it is possible to deflect the seismic mass by means of the electrostatic excitation for test purposes. Any loose particles or etching residues present can thus be detected.FIG. 2 shows an exemplary embodiment of a micromechanical acceleration sensor which comprises a seismic mass 2 suspended on a frame by springs 1 aand 1 b, readout devices 3 a, 3 bwith counter electrodes 4 a, 4 bfixed to the substrate, with which a change in capacitance of these comb structures can be differentially detected. In addition, the acceleration sensor has restoring devices 5 aa- 5 bbhaving counterelectrodes 5 a / b-L and 5 a / b-R, respectively, designed as capacitive comb structures, with which forces can be provided or generated, which counteract the movement of seismic mass 2. By applying an electrical voltage in phase with respect to the vibration of seismic mass 2 to 5 a / b-L or 5 a / b-R, an acting force, in particular a force caused by a detected acceleration, can be compensated. The four reset devices 5aa to 5bb are arranged symmetrically with respect to the center of seismic mass 2. The signal readout takes place, for example, in a double differential manner by means of the two readout devices 3 aand 3 b, which are formed and arranged symmetrically with respect to the central axis of seismic mass 2 in the x direction, wherein, however, the comb structures engage one in the other in an offset or opposite manner, whereby, upon deflection of seismic mass 2 in the negative x direction, illustrated by way of example by the arrow, the comb structures of readout device 3 a, 4 amaintain a positive change in capacitance and the comb structures of readout device 3 b, 4 bmuts.FIG. 2 b) shows an exemplary embodiment with four read-out devices 3 a- 3 d, 4 a- 4 d, which are arranged symmetrically in the center point of seismic mass 2, but in each case have comb structures which mutually engage in opposite directions in pairs or in an offset manner, as a result of which an additional differential measurement is made possible. The changes in capacitance c- and c+ of these comb structures upon deflection of seismic mass 2 in the direction indicated by the arrow are likewise illustrated. Four schematically indicated reset devices 5aa to 5bb are arranged in the outer region.FIG. 3 shows the cross section of an exemplary micromechanical acceleration sensor, comprising a seismic mass 2 having a center of mass offset with respect to springs 1, a frame 6, readout devices 4 a, 4 band additional reset devices 5- L, 5- R designed as electrodes. The acceleration sensor is closed by means of a cover or encapsulation module 7, which has electrical vias 8, with which the electrodes can be connected.FIG. 4 illustrates an exemplary three-axis acceleration sensor having four seismic masses 2 a- d, having spring suspensions or torsion springs 1 a- dthat are offset with respect to the center of gravity of masses 9 a- d. Of the four seismic masses 2 a- 2 d, two seismic masses 2 b, 2 care each suspended in such a way that the suspension axes are aligned by substantially 90° with respect to the suspension axes of the two other seismic masses 2 a, 2 d. The acceleration sensor comprises, in particular, an electronic evaluation circuit, not shown, or is connected to such an evaluation circuit, which can detect the accelerations in three directions from the deflections and / or restoring voltages of the four seismic masses 2 ato 2 d. The suspension axes are particularly preferably arranged substantially parallel to an x-y substrate plane, wherein the suspension axes of the four seismic masses are aligned in pairs in the x-direction 1 a, 1 dand y-direction 1 b, 1 cand are arranged or formed in this case, with respect to the one mass in front 1 dor left-side 1 band, with respect to the other mass behind 1 aor right-side 1 c, in each case of the center of mass 9 a- 9 dof the respective seismic mass. Above and / or below, i.e. spaced apart in the z direction, the seismic masses are each assigned two reading electrodes, not shown, these reading electrodes being assigned to both sides of the suspension axis or the corresponding torsion spring. By means of the respectively offset center of mass with respect to the respective suspension axis or by means of the torsion springs eccentrically formed or arranged with respect to the center of mass, one pair of seismic masses is rotatably deflected in anti-phase about the y-axis upon the action of an acceleration in the x-direction and the other pair of seismic masses is rotatably deflected in anti-phase about the x-axis upon the action of an acceleration in the y-direction. Upon the effect of an acceleration in the z-direction, i.e. perpendicular to the substrate plane, all four seismic masses are deflected in phase-twisting about their respective suspension axis.FIG. 5 shows an exemplary embodiment in which two read-out devices 4 aand 4 bare assigned to seismic masses 2 band 2 c, which are eccentrically suspended in each case with respect to their center of mass 9 by means of torsion springs 1, and are situated above seismic mass 2 b, 2 cin a central region of these masses on both sides of the suspension axis. Further to the outside, the restoring device 5 is assigned and arranged in each case to the seismic masses. The arrangement of the restoring devices 5 in the outer region of the seismic masses 2 b, 2 ccauses that the required restoring voltage can remain relatively low on account of the relatively large lever, with respect to the suspension axis, that is to say that only relatively low electrical restoring voltages are required.FIG. 6 shows an exemplary cross section of an acceleration sensor with a seismic mass 2 which is eccentrically suspended on torsion spring 1 with respect to its center of mass. In this case, read-out devices above 4 aa, 4 aband below 4 ba, 4 bbare assigned to seismic mass 2 on both sides of the suspension axis or torsion spring 1, respectively, with respect to the z direction, perpendicular to the x-y substrate plane. With respect to the read-out devices, reset devices 5 are likewise assigned to both sides above and below seismic mass 2 in an outer region and are arranged accordingly. These are electrically contacted by means of plated-through holes 8 a, 8 bof the encapsulation modules or covers 7 a, 7 b.

Claims

Micromechanical acceleration sensor, comprising at least one first seismic mass (2) which is suspended in a deflectable manner, at least one read-out device (3a, 4a; 3b, 4b) for detecting the deflection of the first seismic mass, and at least one reset device (5), characterized in that at least the first seismic mass (2) is suspended eccentrically with respect to its center of mass (9) on at least one torsion spring (1).Acceleration sensor according to Claim 1, characterized in that the at least one resetting device (5) comprises at least one electrode, in particular an electrode of substantially flat design, and is substantially designed and arranged relative to the first seismic mass (2) in such a way that a substantially square relationship exists between deflection of the first seismic mass on account of an electrical voltage applied to the resetting device and this electrical voltage.Acceleration sensor according to Claim 1 or 2, characterized in that the acceleration sensor has at least two or a multiple of two reading devices which are arranged symmetrically with respect to a geometric or mass-related centre point and / or a geometric or mass-related centre axis of the first seismic mass or of the acceleration sensor.Acceleration sensor according to at least one of Claims 1 to 3, characterized in that the acceleration sensor has at least two or a multiple of two restoring devices which are arranged symmetrically with respect to a geometric or mass-related centre point and / or a geometric or mass-related centre axis of the first seismic mass or of the acceleration sensor.Acceleration sensor according to at least one of Claims 1 to 4, characterized in that it comprises a control circuit which adjusts at least the deflection of the first seismic mass to a defined deflection value, in particular the deflection value corresponding to a rest position of the first seismic mass, by means of at least the resetting device.Acceleration sensor according to at least one of Claims 1 to 5, characterized in that at least two read-out devices are assigned to at least the first seismic mass, which read-out devices are assigned on this side and beyond this suspension axis and / or on both sides with respect to this suspension axis and are arranged accordingly with respect to a suspension axis of the first seismic mass and / or which are assigned to a central region of the first seismic mass and are arranged accordingly, and wherein the at least one reset device is assigned to the first seismic mass further outwards than the read-out devices with respect to its suspension axis and / or the central region and is arranged accordingly.Method for measuring an acceleration with a micromechanical acceleration sensor, in particular a sensor according to at least one of Claims 1 to 6, in which the deflection of at least one first seismic mass is detected by means of at least one read-out device and the seismic mass is adjusted, in the course of a control method, by means of an electronic controller which actuates at least one reset device, to a defined deflection value, in particular the deflection value corresponding to a rest position of the seismic mass.Method according to Claim 7, characterized in that the acceleration detected by the acceleration sensor is calculated at least from the value of an electrical voltage which is applied to the resetting device for regulating the deflection of the first seismic mass to the defined value within the scope of the regulation.Use of the micromechanical acceleration sensor according to at least one of Claims 1 to 6 in motor vehicles.

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