Patent Publication Number: US-7912608-B2

Title: Arrangement for triggering a vehicle safety device

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to GB 0414675.9, filed Jun. 30, 2004 and PCT/SE2005/000742, filed May 20, 2005. 
     FIELD OF THE INVENTION 
     The present invention relates to an arrangement for triggering a vehicle safety device such as, for example, an airbag or a pretensioner. 
     BACKGROUND OF THE INVENTION 
     To avoid inadvertent triggering of a safety device it has been proposed to utilise two units, namely an arming unit and a crash sensing unit, to produce, respectively, an arming signal and a crash signal, the safety device being triggered only when the arming signal and crash signal are present simultaneously. An arrangement of this type is intended to reduce the risk that a single mechanical or electrical error might lead to an inadvertent triggering of the safety device. 
     The arming unit may comprise a simple acceleration sensor which generates the arming signal if the acceleration exceeds a predetermined level. Alternatively the arming unit may include one or more sensors and a processing unit which performs a predetermined algorithm, the arming algorithm, on the signal or signals received from one or more sensors to provide the appropriate output. 
     The crash sensing unit may again comprise a simple acceleration sensor which generates a signal if the acceleration exceeds a predetermined level, but again the crash sensing unit may comprise one or more sensors associated with a processor which is configured to perform an algorithm, the crash algorithm, on the signal or signals received from one or more sensors to provide the appropriate output. 
     The crash sensing unit typically includes a sensor which is directionally sensitive, the sensitive direction of the accelerometer being aligned with the crash direction which is to be sensed by the sensor. The crash algorithm may be determined by whether or not the acceleration sensed by the accelerometer, acceleration “a” or its integrated value, Δv exceed a predetermined threshold. 
     It has been proposed to use the signal from a proximity sensor configured to provide an output related to the distance to an object and/or the relative speed between the object and the sensor, such as a doppler radar, to improve the ability to discriminate between different potential crashes, so as, for example, to discriminate between a potentially dangerous crash and a potentially not dangerous crash. If a doppler radar is utilised, the output represents the relative speed v rel  between the sensor and a nearby obstacle. 
     It has been suggested, for example in U.S. Pat. No. 5,835,007, to utilise an anticipatory crash sensor of the radar type in conjunction with an accelerometer, with the threshold value of acceleration being calculated in dependence upon the signal from the anticipatory crash sensor. 
     Whilst a sophisticated arrangement incorporating a high number of sensors of different types may provide great accuracy in triggering a safety device at an appropriate instant, the provision of a large number of sensors leads to a relatively high cost. 
     The present invention seeks to provide an improved arrangement for triggering a vehicle safety device. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is provided an arrangement for triggering a vehicle safety device, the arrangement incorporating a proximity sensor configured to provide an output related to the distance to an object and/or the relative speed between the object and the sensor, and an accelerometer, the arrangement further comprising a first processing arrangement to perform a crash algorithm on signals received from the proximity sensor and the accelerometer, and a second processing arrangement to perform an arming algorithm, on signals received from the proximity sensor and the accelerometer, the arrangement incorporating a signal generator to generate a signal in response to simultaneous output of a signal from the first processing arrangement and a signal from the second processing arrangement. 
     Preferably, the first processing arrangement and the second processing arrangement are constituted by separate hardware units. In one embodiment of the invention, the first processing arrangement and the second processing arrangement are constituted by discrete hardware components. In another embodiment of the invention, the first processing arrangement and the second processing arrangement are constituted by integrated circuits. 
     Advantageously, the first processing arrangement and the second processing arrangement are each mounted on a respective printed circuit board. 
     In another embodiment the first processing arrangement and the second processing arrangement are constituted by separate software controlled processors. Conveniently, the proximity sensor is a radar sensor. In one embodiment, the radar is a Doppler radar sensor. 
     Advantageously, the accelerometer is sensitive to acceleration in a predetermined direction. 
     Preferably the processing sensor is oriented to determine the distance in a predetermined direction to an object. 
     Conveniently, the arrangement is mounted in a vehicle, with the said predetermined direction of the accelerometer being aligned with the longitudinal axis of the vehicle. 
     Alternatively, the arrangement may be mounted in a vehicle, with the predetermined direction of the accelerometer being parallel with the lateral axis of the vehicle. 
     Conveniently, the signal from the proximity sensor represents the relative velocity (v rel ) between the sensor and an object. 
     Preferably, the signal from the accelerometer is processed by the first processing unit which produces an output when the value exceeds a predetermined threshold and when the relative velocity v rel  exceeds a separate predetermined threshold. 
     Advantageously, the first processing unit which performs the arming algorithm produces an output when the sensed acceleration exceeds a first predetermined threshold and is lower than a second predetermined threshold and when the sensed relative velocity v rel  is greater than a first predetermined threshold and lower than a second predetermined threshold. 
     Conveniently, the output from the accelerometer and the second processing arrangement which performs the crash algorithm compares a function of a and v rel , i.e. f(a,v rel ) with a constant. 
     Preferably, the function of a and v rel  is a function of a multiplied by a function of v rel  so that:
 
 f ( a,v   rel )= f   1 ( a )· f   2 ( v   rel ).
 
     Alternatively, the function of acceleration and relative velocity is acceleration multiplied by the relative velocity raised to a power q and divided by a time period over which acceleration and relative velocity are measured, so that 
     
       
         
           
             
               
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       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the invention may more readily be understood, and so that further features thereof may be appreciated, embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings in which: 
         FIG. 1  is a block diagram illustrating one embodiment of the invention; and 
         FIG. 2  is a block diagram of a more detailed embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1  of the accompanying drawings, an arrangement for triggering a vehicle safety device comprises a proximity sensor  1 , which is configured to provide an output related to the distance to an object and/or the relative speed between the object and the sensor. The sensor may be for example, an optical sensor or an ultrasonic sensor but in the preferred embodiment of the invention the proximity sensor is a radar sensor. A radar sensor can provide an output related to the distance to an object sensed by the radar by measuring the time delay of the reflected pulse. In one embodiment a doppler radar is used to provide an output indicative of the relative speed between the object and the sensor, by using the doppler effect. The relative speed could also be provided by differentiating the distance. 
     The arrangement of the invention also includes an accelerometer  2 . In the preferred embodiment the accelerometer is especially sensitive to acceleration in a predetermined direction, the accelerometer being oriented so that the predetermined direction is aligned with the direction of a crash which is to be sensed. Thus, if the accelerometer  2  is to sense a frontal impact, the accelerometer is oriented so that the redetermined direction of the accelerometer is aligned with the longitudinal axis of the vehicle. On the other hand, if the accelerometer  2  is to sense a side impact, the accelerometer will be located in position with the said direction aligned with the transverse axis of the vehicle. 
     The arrangement of the invention also incorporates a first processing unit  3 . The processing unit  3  is a unit which is programmed to perform a “crash” algorithm. While the preferred embodiment is described with reference to specific units to perform functions, the functions may be performed in many different ways, for example using processors controlled by appropriate software, by specific hardware structures or by discrete hardware components in integrated circuits like transistors, diodes and capacitors. The first processing unit  3  is connected to receive a signal from the proximity sensor  1  and is also connected to receive a signal from the accelerometer  2 . The processing unit  3  has an output which is connected to an AND gate  4 , the AND gate  4  having an output  5  which is the trigger signal to trigger the vehicle safety device. Again, while an AND gate  4  is described, the AND function may be achieved in many different ways. 
     A second processing unit  6  is also provided, the second processing unit  6  again being connected to receive a signal from the proximity sensor  1  and a signal from the accelerometer  2 . The second processing unit  6  is a programmed unit which is programmed to perform an “arming” algorithm. The output from the unit  6  is connected to an input to the AND gate  4 . 
     It is to be understood, at this stage, that the AND gate  4  will only produce the trigger signal  5  when it simultaneously receives signals from the first processing unit  3 , which performs the arming algorithm, and the second processing unit  6 , which performs the crash algorithm. 
     Thus the crash algorithm and the arming algorithm and the AND function may be realised by separate hardware units as described, or by a single complex hardware unit. Each unit could comprise several discrete components or a single integrated circuit. Alternatively the functionality could be provided by a software controlled processor. 
     Turning now to  FIG. 2 , a more detailed embodiment will be described. In this embodiment the proximity sensor  1  is a doppler radar, the doppler radar producing an output signal v rel , indicative to the relative speed of an external object relative to the sensor in a predetermined direction, for objects within a predetermined distance of the sensor. The accelerometer  2  produces an output signal “a” representative of the acceleration of the sensor. 
     The second processing unit  6  which performs the arming algorithm receives, consequently, the signals v rel  and a. The signal v rel  is initially compared with at least a first reference value v 1 . In the described embodiment the comparison is effected by a comparator  7 , but the comparison function may be performed in any way. The comparator  7  will only provide an output if the input signal v rel  is greater than the first predetermined value v 1 . In the described embodiment of the invention, however, the comparator  7  is more sophisticated and will only produce an output signal if v rel  exceeds the first reference value v 1  but is less than a second reference value v 2 . 
     Similarly the signal a from the accelerometer  2  is processed by a comparator  8 , and the comparator  8  will only provide an output if the value a is greater than a first reference value a 1 . Again, while a comparator  8  is described, the comparison function may be performed in any way. In the described embodiment, the comparator  8  is more sophisticated and will only produce an output if the signal a is greater than a first reference value a 1  but less than a second reference value a 2 . 
     The outputs of the two comparators  7  and  8  present within the first processing unit  3 , are passed to an AND gate  9 , the output of the AND gate  9  being the output of the second processing unit  6 . Again, while an AND gate  9  is described, the AND function may be preferred in any way. 
     Consequently the second processing unit  6  which performs the arming algorithm will only produce an output signal when the relative velocity sensed by the doppler radar proximity sensor  1  is greater than a predetermined threshold and the acceleration a sensed by the accelerometer  2  is greater than a predetermined threshold but preferably only produces an output when the sensed relative velocity is greater than a first threshold value and lower than the second threshold value and also when the sensed acceleration is greater than a first threshold value and lower than a second threshold value. 
     The first processor unit  3 , which performs the crash algorithm, again receives the signal v rel  from the doppler radar proximity sensor  1  and signal a from the accelerometer  2 . The first processor unit  3  incorporates a comparator  10  which compares a function of relative velocity and acceleration (f(a, v rel ) with a constant, only producing an output if the function exceeds the constant. Again, while a comparator  10  is described the comparison function may be performed in any way. 
     The function utilized within comparator  10  may be a function of acceleration multiplied by a function of relative velocity, thus:
 
 f ( a,v   rel )= f   1 ( a )· f   2 ( v   rel ).
 
     Alternatively the function may be a more complicated function in which the signal for acceleration a is multiplied by the relative velocity v rel  raised to a predetermined power q, greater than zero divided by the period of time over which the acceleration and the relative velocity have been measured. Thus: 
                 f   ⁡     (     a   ,     v   rel       )       =       a   ·     v   rel   q       t       ,     q   &gt;   0           
where t is the period of time over which acceleration and relative velocity are measured.
 
     It is to be understood that in the preferred embodiments of the invention the first processing unit  3  and the second processing unit  6  are preferably formed in totally discrete and separate units, which may be separate software-controlled processors, or which may be separate hardware processing units  3 ,  6 , which may, for example, each be mounted on its own separate printed circuit board. This is to minimise the risk of a single fault within a single processor, a single hardware unit or on a single printed circuit board leading to inadvertent failure. 
     While the above description constitutes the preferred embodiment of the present invention, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope and fair meaning of the accompanying claims.