Abstract:
A sensor having a seismic mass and having an arrangement for detecting the deflection of the mass and converting it into an electrical signal; in at least one operating mode of the sensor, a mechanical stop asymmetrically limiting the deflection of the seismic mass with respect to a vibrational center position. An arrangement for symmetrical limiting of the signal provided on the sensor.

Description:
BACKGROUND INFORMATION 
   Accelerations in motor vehicles, particularly in their airbag control units, are measured using micromechanical inertial sensors. In so doing, the principle of the differential capacitor is used, in which a movable seismic mass, together with fixed reference electrodes, forms two capacitances. If an acceleration acts on the mass, it is deflected and the capacitances change. The difference in the capacitances is converted by an electronic circuit, a so-called capacitance/voltage converter (C/U converter), into a voltage signal essentially proportional to the acceleration. The movable structures and the capacitor-like arrangements, which form the capacitances, are usually implemented as microelectromechanical structures (MEMS). 
   The precise centering of the seismic mass between the fixed electrodes represents a problem when manufacturing the inertial sensors in the construction as MEMS. As a rule, the actual position of the seismic mass deviates from the desired central position due to process uncertainties. This displacement leads to a capacitive signal which is not owing to a deflection of the mass as a result of an influencing acceleration, and which is usually compensated for by an electrical circuit. To that end, the deviation is electrically measured when switching on the sensor and is continually subtracted from the output signal during operation. 
   During normal operation, the seismic mass of the inertial sensor is induced by accelerations to movements having a certain amplitude and in part high frequency. 
   In unusual operating states, e.g. when the sensor is subjected to strong impacts in the provided deflection direction of the seismic mass, the amplitude can be very much greater. To prevent contact of the electrodes and therefore an electrical collapse, mechanical stops are provided which limit the deflection of the seismic mass. Because of the circumstance that the position of rest or vibrational center position of the seismic mass deviates from the geometric center position between the stops, an asymmetrical limiting of the sensor signal occurs, resulting in a faulty signal at the sensor output. 
   SUMMARY OF THE INVENTION 
   The present invention is based on a sensor having a seismic mass and at least one mechanical stop. The sensor has means for detecting the deflection of the vibrating mass and converting it into an electrical signal. At least one operating mode of the sensor exists in which the deflection of the seismic mass is limited asymmetrically by the stop with respect to a vibrational center position. 
   An essence of the present invention is that means are provided for limiting the maximum value of the electrical signal, the limiting being symmetrical with regard to the time average of the signal, and the maximum value of the electrical signal not being greater than the smallest value predefined by the mechanical stop. 
   The sensor of the present invention has the advantage that an asymmetrical limiting of the signal is replaced by a symmetrical limiting. Under certain conditions during operation, such as, for example, the influence of broken stone or spray water, the deflection of the seismic mass represents a periodic signal whose time average is essentially constant. If the deflection were limited asymmetrically, the average value would be changed. After a subsequent filtering in the signal path, this altered value would be interpreted falsely as a very great constant acceleration acting on the sensor. On the other hand, if the signal is limited symmetrically, then the average value is retained in comparison to a vibrator which is not limited by a stop. 
   In one advantageous embodiment of the sensor according to the present invention, means for determining the deviation of the vibrational center position of the seismic mass from the geometric center are provided on the sensor. In principle, the deviation of the position of rest or vibrational center position of the seismic mass from the geometric center between the stops or the detection means can already be determined after the sensor is manufactured. However, this deviation may change due to ageing or changing environmental conditions. Consequently, the time average of the signal in the case of high-frequency, large accelerations also changes. To determine the time average of the signal as precisely as possible, it is determined upon switching on the sensor. Proceeding from that, the limits for the symmetrical signal limiting are then provided. 
   It is also advantageous that the sensor has a micromechanical construction. Based on their construction and their manufacturing technology, owing to process uncertainties, micromechanical sensors exhibit to a considerable degree the above-described deviation of the actual position of rest or center position of the seismic mass, specific to its deflections by outer forces, from the desired, geometric center position. The symmetrical signal correction described is particularly effective here. Moreover, micromechanical structures, electrical sensors and microelectronic evaluation circuits can advantageously be integrated in one common component. 
   One advantageous development of the sensor according to the present invention is that the means for detecting the deflection take the form of electrodes which represent a capacitive detecting element, particularly according to the principle of differential capacitance. This is a simple and proven principle for measuring deflections when working with micromechanical acceleration sensors and rotation-rate sensors. The variable capacitance can be easily measured using an electronic evaluation circuit connected thereto, and the electrical measured values obtained may easily be further processed. 
   In one particularly advantageous refinement, the means for limiting the maximum value of the electrical signal are represented by an electronic evaluation circuit. The electrical limiting of the maximum values of the electrical signal is advantageously carried out in a simple manner in an electronic evaluation circuit. 
   In one advantageous embodiment, a further stop is provided that is able to be variably positioned. In this case, the means for limiting are provided in such a way that they suitably position at least the further stop so that the deflection of the vibrating mass is symmetrically limited. For example, the stop may be positioned using a piezo element by applying a voltage to the latter. 
   Advantageously, the sensor is an inertial sensor, particularly an acceleration sensor or rotation-rate sensor. Sensors of this type are used in vehicles, where they are subjected to particularly high stresses with respect to impacts and temperature changes as well as other environmental influences. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows an inertial sensor according to the related art. 
       FIG. 2A  shows the vibration signal of an inertial sensor in response to high-frequency external excitation. 
       FIG. 2B  shows the vibration signal of an inertial sensor having asymmetrical signal limiting. 
       FIG. 3  shows a specific embodiment of the inertial sensor according to the present invention having symmetrical electrical signal limiting. 
       FIG. 4  shows the vibration signal of an inertial sensor according to the present invention having symmetrical electrical signal limiting. 
       FIG. 5  shows a specific embodiment of the inertial sensor according to the present invention having symmetrical mechanical signal limiting. 
       FIG. 6  shows the vibration signal of an inertial sensor according to the present invention having symmetrical mechanical signal limiting. 
   

   DETAILED DESCRIPTION 
     FIG. 1  shows an inertial sensor according to the related art. A seismic mass  100  is suspended by a system of springs  103 ,  104  in a manner allowing vibration. The deflection or vibration of seismic mass  100  induced by an external force takes place in a direction  107 . The maximum possible deflection of mass  100  is limited in this example by two mechanical stops  101  and  102 . Movable mass  100  represents a movable electrode, and together with two fixed reference electrodes, forms two capacitances  105  and  106 . Capacitances  105  and  106  change as a function of the deflection of the mass along direction  107 . The detecting element operates according to the principle of differential capacitance. The difference of capacitances  105  and  106  is converted in a capacitance/voltage converter (C/U converter)  110  into a voltage signal  111  essentially proportional to the acceleration. Voltage signal  111  is filtered in a low-pass filter  112  which, for example, may be a filter with switched capacitances (S/C filter—switched capacity (capacitance) filter). Filtered voltage signal  113  is amplified in an amplifier stage  114 . An amplified voltage signal  115  is present at the sensor output. Because of the deviation of the vibrational center position of seismic mass  100  from the geometric center position between the reference electrodes, and because of further electrical influences, voltage signal  115 , without the influence of an external acceleration on the sensor, is not zero, but rather has an offset. Compensation is made for this offset by offset-compensation circuit  116 . To that end, voltage signal  115  is supplied to compensation circuit  116 . At the beginning of sensor operation, the offset is determined when there is no external acceleration, and a signal  117  is supplied to amplifier  114  in such a way that signal  115  is compensated to zero. Consequently, the sensor is calibrated for further operation with respect to the offset. 
     FIG. 2A  illustrates the vibration signal of an inertial sensor in response to high-frequency external excitation. The vibration signal results from a high-frequency acceleration. The time is plotted in any units as desired on axis X. Voltage signal  111  at the output of C/U converter  110  is plotted on axis Y. Signal curve  203  results in response to high-frequency excitation of sensor element  100  with low amplitude. This represents a first, proper operating state of the sensor. Function  203  has turning points which identify a vibrational center position  210  of seismic mass  100 . Vibrational center position  210  has an offset  200  with respect to zero line  0  of the diagram. Zero line  0  represents the geometric center position between the reference electrodes. Thus, offset  200  is accounted for in the deviation of vibrational center position  210  of seismic mass  100  from the geometric center position between the reference electrodes. Lines  201  and  202  mark the maximum possible values of function  203  caused by the limitation of the deflection of seismic mass  100  by stops  101  and  102 . In a sensor according to  FIG. 1 , offset  200  and additional electrical influences from low-pass filter  112  and amplifier  114  are compensated for by compensation element  116 , and filtered signal  115  is equal to zero, given the absence of low-frequency external accelerations. 
     FIG. 2B  illustrates by way of example the vibration signal of an inertial sensor having asymmetrical signal limiting. The vibration signal is the result in response to high-frequency external excitation with large amplitude by accelerations acting on the sensor. The time is plotted in any units as desired on axis X. Voltage signal  111  at the output of C/U converter  110  is plotted on axis Y. Signal curve  204  results in response to high-frequency excitation of sensor element  100  with large amplitude. Such a large amplitude comes about, for example, due to the influence of external shock on the sensor with accelerations of up to a few 100 g. These accelerations occur, inter alia, as a result of the effect of spray water or gravel impact on a vehicle in which the sensor is installed. This represents a second, exceptional operating state of the sensor. Function  204  likewise has turning points which identify vibrational center position  210  of seismic mass  100 . In the same way, vibrational center position  210  has offset  200  with respect to zero line  0  of the diagram. The amplitude of function  204  is limited in the regions of greatest deflection  205  to maximum value  201  by stop  101 . 
   The essentially periodic function  204  can be divided into two half waves that are differentiated by vibrational center position  210 . A first half wave has an area  206  that is bounded by center position  210 , curve  204  and the line of maximum value  201 . A second half wave has an area  207  that is bounded by center position  210  and curve  204 . At even greater amplitude, area  207  would also be bounded by the line of maximum value  202 . However, as a result of offset  200  toward maximum value  201  in this example, area  206  is always smaller than area  207 . Function  204  is thus asymmetrically limited and its time average no longer corresponds to offset  200 . Given compensation of offset  200  by compensation element  116  in a sensor according to  FIG. 1 , signal  115  is therefore not equal to zero. Thus, a signal is output which is interpreted as acceleration, although no acceleration to be normally measured is acting on the sensor. 
     FIG. 3  shows a specific embodiment of the inertial sensor according to the present invention having symmetrical electrical signal limiting. In contrast to the sensor according to  FIG. 1 , the sensor of the present invention in this exemplary embodiment has a modified C/U converter  110 A and evaluation electronics  300  which are used to electrically limit signal  111 . To that end, signal  111  is supplied to evaluation electronics  300 . After the sensor is switched on, offset  200  is determined in evaluation electronics  300  from signal  111 , stored and converted into a manipulated variable  301  which is fed to modified C/U converter  110 A. In an output stage of modified C/U converter  110 A, maximum possible positive and negative amplitudes of signal  111  are provided symmetrically relative to signal center position  210 . The maximum amplitude may also be provided in an adjustable manner. Based on signal center position  210 , which is expressed in manipulated variable  301 , and the maximum amplitude relative thereto, absolute positive and negative maximum values which signal  111  is allowed to assume are determined. The output stage of modified C/U converter  110 A limits signal  111  symmetrically on this basis. 
     FIG. 4  shows the vibration signal of an inertial sensor according to the present invention having symmetrical electrical signal limiting. The time is plotted in any units as desired on axis X. Voltage signal  111  at the output of modified C/U converter  110 A is plotted on axis Y. Signal curve  204  results in response to high-frequency excitation of sensor element  100  with large amplitude. Signal curve  204  is limited symmetrically here by electrical limitations  401  and  402 . The limitations represent the maximum possible positive and negative amplitude of signal  111 . Limitations  401  and  402  have the same distance from signal center position  210 , and areas  403  and  404  are therefore of equal size. Electrical limitations  401  and  402  lie within mechanical limitations  201  and  202 . 
     FIG. 5  shows a specific embodiment of the inertial sensor according to the present invention having symmetrical mechanical signal limiting. In contrast to the sensor according to  FIG. 1 , the sensor of the present invention in this exemplary embodiment has evaluation electronics  500  and a modified mechanical stop  102 A which are used to mechanically limit signal  111 . At the beginning of sensor operation, offset  200  is compensated for in the manner described in  FIGS. 1 and 2A . Moreover, signal  111  is supplied to evaluation electronics  500 . From signal  111 —which, given the absence of external forces, such as, for example, at the beginning of operation, is essentially determined by offset  200 —evaluation electronics  500  generate a manipulated variable  501  with the aid, for example, of a mathematical function or a value table stored in a memory. Manipulated variable  501  is routed to an actuator  502 , to which modified stop  102 A is secured. Position  503  of modified stop  102 A is adjusted by actuator  502  as a function of manipulated variable  501 . Actuator  502  is used to variably position modified stop  102 A along the vibration direction of actuating vibration  107  of seismic mass  100 . By adjusting stop  102 A as a function of offset  200 , the maximum possible amplitude of signal  115  is symmetrically limited in a mechanical manner. 
     FIG. 6  shows the vibration signal of an inertial sensor according to the present invention having symmetrical mechanical signal limiting. The time is plotted in any units as desired on axis X. Voltage signal  111  at the output of C/U converter  110  is plotted on axis Y. Signal curve  204  results in response to excitation of sensor element  100  with high frequency and large amplitude by external forces. Signal curve  204  is symmetrically limited here in its maximum value  201  by mechanical stop  101 , and in its maximum value  601  by modified mechanical stop  102 A. The symmetrical limiting is achieved by the shift of maximum value  202  toward maximum value  601 . Shift  600  of the maximum possible amplitude of function  204  is expression of the shift of position  503  of mechanical stop  102 A. The limitations represent the maximum possible positive and negative amplitude of signal  111 . Limitations  201  and  602  have the same distance from signal center position  210 , and areas  403  and  404  are therefore of equal size. Maximum values  202  and  601  of electrical signal  111  are equal to the values predefined by the mechanical stops.