Abstract:
The invention relates to the idea of not initially multiplying the output signal (a) of an acceleration sensor ( 19 ) with regard to a weighted sum (Σ g ) with a correction factor (k w ) but to alter a test signal (t) by using a weighting means ( 16 ) in such a manner that an already weighted output signal (a g ) can be generated so that during a test operation, the safing algorithm of an evaluating device can be directly fallen back upon, whereby this can be advantageously tested with regard to its ability to operate. The invention is particularly suited for occupant protection systems of a modern motor vehicle.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a control arrangement and a method for testing the operation of a control arrangement of this type for occupant protection means in a motor vehicle. 
   Occupant protection means such as airbags, belt tensioners, etc. represent important safety components in modern motor vehicles. They are not legally required but almost every automobile currently produced somewhere in the world has at least one front airbag for the driver. 
   In addition to the front airbag many automobiles now also have further different airbags, in particular side airbags, head airbags, knee airbags, etc. for the driver, front passenger or other occupants. Every one of these airbag systems uses a plurality of sensors, arranged at different points on the body of the vehicle, which identify the deceleration (negative acceleration) that occurs in the event of a collision. The systems generally used with driver, front passenger and side airbags operate with acceleration sensors arranged in or adjacent to a control arrangement. The control arrangement is located at a central point in the motor vehicle, for example under the driving seat or in the vehicle tunnel. It is therefore also frequently referred to as the central module. To identify a side collision, at least one acceleration sensor—or more progressively at least one pressure sensor—is provided on both sides of the motor vehicle, frequently referred to as so-called satellites due to their non-central position. The specifications for the respective sensors are generally relatively exacting, as they are the first component of an occupant protection system to receive crash information. They have to convert the rapid deceleration of the motor vehicle to a reliable and accurate electrical signal (a). 
   One of the most frequently used methods for measuring acceleration is to measure the action of a force F, which results from the acceleration g acting on a seismic mass m. This force generates mechanical stresses and a change in the length of the seismic mass. The stresses can be determined using the piezo-resistive (or piezo-electric) characteristics of the material used. Length changes are generally measured using a variable capacity. The piezo-resistive effect in semiconductors is utilized to a large degree in pressure sensors, while for acceleration sensors the capacitive measuring principle is preferred in a plurality of technical applications. This design allows very small sensor structures and therefore economical solutions to be produced using surface micromanufacturing. Sensors of capacitive structure are also less susceptible to temperature fluctuations and offer a wide operating temperature range. Therefore in the field of occupant protection systems both the acceleration sensors and the pressure sensors that are increasingly being used are based predominantly on this principle. 
   The actual sensor element or so-called g-cell in particular is a mechanical structure made of solid-state materials. It comprises for example two fixed plates with a movable plate between, which represents the seismic mass. If the g-cell is exposed to acceleration, the central plate moves from its rest position. When the central plate moves, its distance from one fixed plate increases to the same degree that its distance from the other fixed plate decreases. The change in distance is a measure of acceleration. The supports used to suspend the central plates act as springs. A fluid possibly compressed between the plates, for example a specific gas or even just air, cushions the movement. If this is not desirable it is known that a vacuum can be provided. A g-cell generally senses along a sensitivity axis. With an appropriate structure however one mass can be used for two axes, thereby reducing cost. Reference is then made to so-called x-y g-cells or x-y sensors. From an electrical point of view the plates of the g-cell form a linked capacitor pair. When the central plate moves along the sensitivity axis due to acceleration, the distance between the plates changes, as a result of which the capacity of each of the two capacitors also changes. The same also applies to g-cells with for example a plurality of finger-shaped, meshing elements. 
   The g-cells produced by micromanufacturing have very small dimensions. The seismic mass for example weighs only a few hundred picograms (1 picogram=10 −12  grams). When subject to an acceleration of 100 g, the movable plate or finger changes position by less than 400 nm (nanometers). A capacity change ΔC of less than 1 femtofarad (10 −15  F) must be identified to achieve a measuring release of 1 g. To be able to measure such a small capacity, it is necessary to have a dedicated control circuit in the acceleration sensor to convert the capacity to an analyzable output signal (a). 
   The output signal (a) of the sensor is supplied to an evaluation unit, which comprises at least one but currently generally more than one microcontroller, which then execute(s) a crash discrimination algorithm, to differentiate between an actual collision and the normal dynamic vehicle response and if necessary generates a release signal for the restraint means. 
   The release signal is at present frequently only generated as a function of a so-called switch signal, which can in the simplest instance originate from a mechanical acceleration switch. However in many acceleration sensor arrangements today one of the acceleration sensors themselves carries out this task. After executing the so-called saving (sic) algorithm, for which a dedicated microcontroller is now regularly provided in the evaluation unit, such so-called safing sensors are responsible for the releasing or preventing the release of the restraint means, if the acceleration sensor or the evaluation device, i.e. the algorithms executed in the microcontrollers, operate incorrectly and would therefore supply an incorrect release signal. 
   EP 1 149 004—the disclosure of which should be deemed to be specifically included in full—discloses a method and a device for testing the operation of a control arrangement for occupant protection means in a motor vehicle, with which a weighted sum is created from the output signals of the acceleration sensors to test the plausibility of the signals, by multiplying at least the output signal of one acceleration sensor by a correction value. Such a test advantageously allows information to be obtained about the operational capacity of the acceleration sensors, their signal output, levels, etc. But information cannot be obtained about the extent to which the safing algorithm itself operates reliably, because this is not referred back to in test mode. 
   SUMMARY OF THE INVENTION 
   The object of the invention is to provide an improved method for testing the operation of a system of a plurality of acceleration sensors in a control arrangement for occupant protection means in a motor vehicle. It should in particular be possible to test the operational capacity of the safing algorithm as well as the sensors. 
   This object is achieved according to the invention by a circuit arrangement with the features according to claim  1  and by a method for testing its operation with the features according to claim  7 . 
   The idea behind the present invention is not initially to multiply the output signal (a) of an acceleration sensor by a correction value (k w ) in respect of a weighted sum (Σ g ) but to use a weighting means to modify a test signal (t) such that an already weighted output signal (a g ) can be generated, so that the safing algorithm of an evaluation device can be referred back to directly in test mode, as a result of which said safing algorithm itself can advantageously be tested in respect of its operational capacity. 
   Advantageous embodiments and developments, which can be used individually or in combination with each other, are set out in the dependent claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further advantages of the invention and its developments are described in more detail below with reference to exemplary embodiments and the drawing, in which: 
       FIG. 1  shows a schematic illustration of the typical structure of an occupant protection system in a motor vehicle; 
       FIG. 2  shows a schematic illustration of an acceleration sensor comprising a seismic mass, which can be displaced along a sensitivity axis; 
       FIG. 3  shows a schematic illustration of an acceleration sensor, which can be displaced along two sensitivity axes; 
       FIG. 4  shows a schematic illustration of the acceleration sensor according to  FIG. 2  actively connected to a weighting means; 
       FIG. 5  shows a schematic illustration of an arrangement with an x-y acceleration sensor and a further acceleration sensor; 
       FIG. 6  shows a schematic illustration of an alternative sensor alignment in the arrangement according to  FIG. 5 ; and 
       FIG. 7  shows a schematic illustration of a star-shaped arrangement of three acceleration sensors. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The same elements and signals are shown with the same reference characters in all the figures. 
     FIG. 1  shows the typical structure of an occupant protection system in a motor vehicle  1 . A control arrangement  2  is located at the most central point possible in the motor vehicle  1 . It has an evaluation unit  3  for example in the form of at least one microcontroller. In the control arrangement  2  or adjacent to it is a sensor field  5 , in which appropriate sensors  17 ,  18 ,  19 ,  20  are arranged to measure acceleration, e.g. an acceleration g x  along a sensitivity axis in the x-direction or an acceleration gy along a sensitivity axis in the y-direction. The sensitivity axes u, v, w, x, y of the sensors  17 ,  18 ,  19 ,  20  span a plane, which after the control arrangement  2  has been integrated in a motor vehicle  1  is essentially parallel to a plane defined by the longitudinal axis of the vehicle A–A′ and the transverse axis of the vehicle B–B′. Further sensors  6 , in particular to identify a side collision, are arranged toward the sides of the vehicle  1  at a non-central point, for the preferably capacitive measurement of lateral acceleration, e.g. an acceleration g r  from the right or g l  from the left. The sensors  6  for lateral integration are typically acceleration sensors but are now increasingly pressure sensors. The respective output signals a of the sensors are scanned by the microcontrollers arranged in the evaluation unit  3 , one of which then executes a crash discrimination algorithm, to differentiate between an actual collision and the normal dynamic vehicle response. One microcontroller in the evaluation unit  3 , which is preferably independent of crash processing, uses a safing routine to carry out continuous and/or cyclical diagnosis of the system, to ensure that it is operating correctly and is available in the event of an accident. The sensors  17 ,  18 ,  19 ,  20  arranged in the central sensor field  5 , like those arranged at the sides  6 , have to be extremely reliable so that they do not send the microcontroller  3  any incorrect signals a, which could result in unwanted activation of the restraint means. The driver is therefore notified of any fault, e.g. by means of an airbag warning light on the dashboard (not shown) and the restraint function is blocked until the error has been eliminated. If the airbags have to be deployed in the event of a collision, the evaluation unit  3  activates a trigger current switch  4 , so that current flows through the trigger circuits of the trigger for the driver&#39;s front airbag  7 , the trigger for the passenger&#39;s front airbag  8 , the trigger for the side airbag  9 , the trigger for the belt tensioner  10 , etc., thereby activating the belt tensioners and activating the gas generation response within the inflation module. 
     FIG. 2  shows the operating principle of a capacitive acceleration sensor  17 ,  18  or  19  comprising a sensor element  11 —hereafter also referred to as the g-cell  11 —in which a seismic mass is arranged in a fashion such that it can be displaced along a sensitivity axis w. The mode of operation is based for example on micromechanical twin-plate capacitors, which are predominantly finger-shaped. A central plate  14  is attached to the movable, suspended seismic mass  12  between two outer rigid plates  13 . During acceleration the mass  12  moves so that the capacity changes. The arrangement of a plurality of finger-shaped, intermeshing elements one behind the other is also known. With an appropriate structure it is possible to use one mass  12  for two sensitivity axes (x, y), which advantageously reduces costs. 
     FIG. 3  shows such a so-called capacitive x-y acceleration sensor comprising a sensor cell  11 , in which a seismic mass  12  is arranged such that it can be displaced along two sensitivity axes x and y. As in the sensor according to  FIG. 2 , a dedicated control circuit  15  is arranged downstream from the g-cell  11 , to convert the capacity to an analyzable output signal (a). 
   In normal, i.e. crash, mode of the evaluation device  3  all the output signals a u , a v , a w  or a w , a x , a y  of the sensors  17 ,  18 ,  19  or  19 ,  20  are tested for plausibility using a safing algorithm by creating a weighted sum Σ g  from the output signals a u , a v , a w  or a w , a x , a y . Parallel to this for example the output signals a u , a v , a w  or a w , a x , a y  are analyzed using a crash discrimination algorithm, with any release of the restraint means only taking place once plausibility has been determined. According to the invention at least one of the output signals a u , a v , a w  or a w , a x , a y  is preferably compared beforehand with a threshold value SW, so that the safing algorithm is only released if at least one of the output signals a u , a v , a w  or a w , a x , a y  exceeds the threshold value SW. 
   In order to be able to test the error-free operation of the control arrangement  2  when starting and/or during operation of the motor vehicle  1 , it is advantageous to be able to subject the acceleration sensors  17 ,  18 ,  19  or  19 ,  20  to a so-called automatic test. To this end, with the control arrangement  2  in test mode, the evaluation unit  3 , for example of one microcontroller, sends a test signal t to at least two acceleration sensors  17 ,  18 ,  19  or  19 ,  20 , to generate output signals a u , a v , a w  or a w , a x , a y , which serve to test the operation of the sensors  17 ,  18 ,  19  or  19 ,  20 . In the case of a so-called physical test on the sensor(s)  17 ,  18 ,  19 ,  20  the test signal t brings about displacement of the seismic mass  12  along the sensitivity axis u, v, w, x, y. The capacity change in the g-cell  11  is identified in a control circuit  15  of the sensor  17 ,  18 ,  19 ,  20  downstream from the g-cell  11  and this knowledge is converted to an output signal a that can be used by the microcontrollers of the evaluation unit  3 . Such a physical test advantageously allows information to be obtained both about the operational capacity of the g-cell  11  and about the operational capacity of the control circuit  15  of the acceleration sensor  17 ,  18 ,  19 ,  20 . 
   It is also known that a test signal t is supplied only to the control circuit  15  of the acceleration sensor  17 ,  18 ,  19 ,  20 , said test signal t also being used to generate or simulate an usable output signal a. However no information can be obtained about the operational capacity of the g-cell with such a purely electronic test. Information about the electronic operational capacity of the control circuit  15  of the sensor  17 ,  18 ,  19 ,  20  alone may in many instances suffice however, in particular when other mechanisms are provided to detect a defective g-cell, for example measurement of movement or fluctuations in the offset voltage of the sensor, in so far as these are characteristic of a defective g-cell. 
     FIG. 4  shows an acceleration sensor  17 ,  18 ,  19  or  20  according to the invention. This is modified compared with the acceleration sensor according to  FIG. 2  to the extent that it is also actively connected to a weighting means  16 . The weighting means  16  can be part of the sensor  17 ,  18 ,  19 ,  20  and/or part of the control arrangement  2 , in particular part of the evaluation device  3 . This advantageously allows a test signal t from the evaluation unit  3  to be modified such that in the case of a physical test a correspondingly weighted displacement of the seismic mass  12  itself takes place. Weighting means in this instance are in particular so-called test fingers, etc. configured separately in the sensor. During an automatic test only the test fingers experience displacement. Being arranged on the same seismic mass, their displacement also brings about displacement of the fingers provided for crash sensing. A larger or smaller number of test fingers is displaced, depending on the degree of weighting required for an output signal. If then—by way of an illustration—fourteen or even just seven test fingers are displaced instead of the standard ten test fingers for example, this causes a correspondingly larger or smaller displacement of the fingers provided for crash sensing and a correspondingly weighted output signal. An electronic test is also possible, in that the weighting means  16  supplies an attenuated or amplified, i.e. weighted, test signal t g  to the control circuit  15 . The weighting means  16  can therefore be voltage-reducing electronic components such as resistors or voltage-increasing electronic components such as an electronic charging pump, etc. Compared with unmodified acceleration sensors, such an acceleration sensor  17 ,  18 ,  19 ,  20  comprising a weighting means  16  allows a weighted output signal a g  to be output when the same automatic test signal t is received from all sides. 
   The present invention utilizes this consideration, in that it provides at least one sensor  19 , which outputs a weighted output signal a g . The generation of a plurality of weighted output signals a ug , a vg , a wg , a xg , a yg  is expedient depending on the arrangement of the acceleration sensors. Different arrangements, preferred according to the invention, are shown in  FIGS. 5 to 7 . 
     FIG. 5  shows the sensor field  5  of a control arrangement  2  comprising three acceleration sensors  17 ,  18 ,  19 , each with a g-cell, which allow the sensing of an acceleration g along a sensitivity axis w, x, y, aligned differently in each instance. Sensor  17  serves to sense in x-direction, sensor  18  in y-direction. Sensor  19  is arranged at an angle to these. In crash mode the direction and strength of an acceleration g acting on the vehicle  1  can be detected in the downstream evaluation device  3  using only two of the three acceleration sensors  17 ,  18 . The signal a w  of the third acceleration sensor  19  is hereby used to test one of the two calculated variables, the direction or strength of the acceleration acting on the vehicle  1 . The third sensor  19  thereby assumes the function of a safing sensor and can in this manner prevent the triggering of the restraint means  7 ,  8 ,  9 ,  10  at least indirectly, if the value supplied by it differs significantly from a value calculated from the signals a x  and a y  of the two other sensors. 
   In the example the sensors  17  and  18  are unmodified and commercially available, i.e. on receipt of a test signal t with the control arrangement  2  in test mode they generate an unweighted output signal a x  or a y . Sensor  19  is an acceleration sensor  19  structured according to the invention and actively connected to a weighting means  16 . Its sensitivity axis w is arranged in the sensor field  5  at an angle to the sensitivity axis of the x-sensor or y-sensor  18  or  17 , for example at 45° to the transverse vehicle axis B–B′ corresponding to the x-axis. The weighting means  16  modifies the same test signal t according to a presetting such that a specifically weighted output signal a g  is generated. 
   A first test specification for the sensor arrangement according to  FIG. 5  provides for pairs of tests on the sensors, such that an unweighted output signal of either the sensor  17  sensing in the x-direction or the sensor  18  sensing in the y-direction is considered together with the weighted output signal of the so-called safing sensor  19 . In particular the seismic mass  12  of the sensor element  11  of the first acceleration sensor  19  is displaced with weighted force in the opposite direction to its sensitivity axis w or a corresponding signal a wg  is generated electronically. The seismic mass  12  of the sensor element  11  of the second acceleration sensor  17  or  18  is also displaced with unweighted force in the direction of its sensitivity axis x or y or a corresponding signal a x  or a y  is generated electronically. A reverse process is also possible, i.e. the generation of a weighted output signal a x  or a y  in the x-direction or y-direction and an unweighted signal a w  for the w-direction. In the first-mentioned instance the weighting means  16  should preferably modify the test signal t such that the electrical output signal a wg  is output weighted by the, mathematically expressed, weighting factor k w  equal to ½*√2 (corresponding to around 0.707). If the sensors are arranged at different angles to each other, the weighting factor should be adjusted as a function of the angle position. 
   According to a second test specification, all the output signals of the sensors  17 ,  18 ,  19  arranged in the sensor field  5  are considered together at the same time. In particular the seismic mass  12  of the sensor element  11  of the first acceleration sensor  19  is displaced with weighted force in the opposite direction to its sensitivity axis w or a corresponding signal a wg  is generated electronically. The seismic masses  12  of the sensor elements  11  of the second acceleration sensor  17 ,  18  are also displaced with unweighted force in the direction of their sensitivity axis x and y or a corresponding signal a x  and a y  is generated electronically. In this instance the weighting means  16  should preferably modify the test signal t such that the electrical output signal a wg  is output weighted by the, mathematically expressed, factor k w  equal to √2 (corresponding to around 1.41). Possible alternatives should be deemed to be included as well. 
     FIG. 6  shows an alternative preferred sensor arrangement according to the invention. In contrast to the arrangement according to  FIG. 5 , instead of individual sensors  17  and  18  sensing in the x-direction and y-direction, a so-called x-y sensor  20  is provided. The sensitivity axis v of the safing sensor  19  is again arranged at an angle to these axes, in the present instance an angle of 45° to the longitudinal axis of the vehicle A–A′ corresponding to the y-axis. The operational capacity of the control circuit  2  can also be tested in this arrangement according to the two text (sic) specifications described above. 
   Finally  FIG. 7  shows a further preferred sensor arrangement according to the invention. In contrast to the arrangement according to  FIG. 5 , of the at least three individual sensors  17 ,  18 ,  19  sensing along a sensitivity axis u, v or w, a second sensor  18  is also arranged at an angle to the longitudinal axis of the vehicle A–A′ or the transverse axis of the vehicle B–B′, so that the acceleration sensors  17 ,  18 ,  19  each have differently aligned sensitivity axes u, v, w. The sensors  17 ,  18 ,  19  are preferably star-shaped, each arranged with a 120° offset. The operation of this arrangement is tested according to a third test specification, according to which all the sensors  17 ,  18 ,  19  arranged in the sensor field ( 5 ) are considered together at the same time, as described for the second test specification, in this instance the weighting means  16  having to modify the test signal t in respect of the safing sensor  19  such that the electrical output signal is output weighted by the, mathematically expressed, factor k w  equal to 2. 
   Weighted and unweighted output signals from the acceleration sensors  17 ,  18 ,  19 ,  20  are processed in the microcontroller  3  using a safing algorithm, which again itself creates a weighted sum Σ g . If processing produces a predefined value, for example approximately zero, this indicates that the safing algorithm itself is also operating reliably. If the predefined value is to be approximately zero, the factor k w  should preferably be selected according to the invention such that the angle positions between the sensors ultimately find a balance. 
   The present invention therefore advantageously allows not only the operation of the g-cell  11  and/or the control circuit  15  of an acceleration sensor  17 ,  18 ,  19 ,  20  to be tested but also the operational capacity of a safing algorithm associated with the crash algorithm. The present invention is therefore particularly suitable for occupant protection systems in a modern motor vehicle.