Abstract:
Provided is an acceleration detection apparatus installed in a vehicle and including a plurality of acceleration sensors having different characteristics, a function to input diagnosis signals in order to diagnose the outputs of the acceleration sensors and diagnose the fault detection functions while the vehicle stops, and a function to compare the outputs of the sensors in order to detect a fault while the vehicle runs.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an apparatus for detecting acceleration and, in particular, to a self-diagnostic function of an acceleration sensor to be installed in a vehicle. 
     2. Description of the Related Art 
     JP-H04-203969-A discloses an apparatus as a function that diagnoses an acceleration sensor to be installed in a vehicle. 
     SUMMARY OF THE INVENTION 
     A vehicle is equipped with a plurality of acceleration sensors needed for detecting a collision or a skid of the vehicle in order to ensure the safety during running. However, such sensors needs to be kept highly reliable when being placed and operated in environments such as an engine room where the temperature varies over a wide range, and vibration and electromagnetic noise have significant impacts on the sensors. Thus, a fault diagnosis needs to continuously be performed without interrupting a normal operation of the acceleration sensor during running. To solve such a problem, JP-H04-203969-A discloses an example in which a vehicle includes a high acceleration sensor configured to detect a high acceleration for detecting a collision of the vehicle and a low acceleration sensor configured to detect a low acceleration for detecting a skid of the vehicle to compare the outputs of the two sensors in order to perform a fault diagnosis for determining whether the sensors properly operate. 
     In light of the foregoing, an object of the present invention is to ensure a high fault diagnostic ability. 
     The above object is achieved by an acceleration detection apparatus installed in a vehicle including: a plurality of acceleration sensors having different characteristics; a function to input diagnosis signals to diagnose outputs of the acceleration sensors and diagnose fault detection functions of the acceleration sensors while the vehicle stops; and a function to compare the outputs of the sensors in order to detect a fault while the vehicle runs. 
     Diagnosing the performances and the fault detection functions of two sensors having different characteristics while the vehicle stops and detecting a fault by comparing the outputs of the two acceleration sensors while the vehicle runs can ensure a high fault diagnostic ability. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an acceleration sensor control circuit according to an embodiment; 
         FIG. 2  is a block diagram of a stoppage time diagnosis unit according to an embodiment; 
         FIG. 3  is a flowchart of a sensor determination unit according to an embodiment; 
         FIG. 4  is a flowchart of a fault detection determination unit according to an embodiment; 
         FIG. 5  is a block diagram of a running time diagnosis unit according to an embodiment; 
         FIG. 6  is a configuration diagram of a stoppage time diagnosed result register and a running time fault detected result register according to an embodiment; 
         FIG. 7  is a flowchart of a fault detection determination unit during running according to an embodiment; and 
         FIG. 8  is a timing diagram of a diagnosis voltage generation unit according to an embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to  FIGS. 1 to 8 . 
       FIG. 1  is a block diagram of a control circuit of acceleration sensors (hereinafter, referred to as G sensors) according to a first embodiment. A two-axis high G sensor  101  of the present embodiment includes an oscillator  128 , an oscillator  129 , electrodes  130  and  132 , and electrodes  131  and  133 . The oscillator  128  is displaced when acceleration is added in a horizontal direction (hereinafter, referred to as an X-axis direction). The oscillator  129  is displaced when acceleration is added in a longitudinal direction (hereinafter, referred to as a Y-axis direction). The electrodes  130  and  132  detect the amounts of displacement in the X-axis and the Y-axis directions according to the variation of capacitance. The electrodes  131  and  133  apply voltages in order to forcibly displace the oscillator  128  in the X-axis direction and the oscillator  129  in the Y-axis direction. The two-axis high G sensor  101  further includes capacitance detectors  135  and  136  and AD converters  148  and  149 . The capacitance detectors  135  and  136  detect the variation of capacitance due to the displacement and output the variation as a voltage. The AD converters  148  and  149  convert the detected voltage into a digital signal. 
     Similarly, a two-axis low G sensor  102  includes an oscillator  121 , an oscillator  122 , electrodes  123  and  125 , and electrodes  124  and  126 . The oscillator  121  is displaced when acceleration is added in the X-axis direction. The oscillator  122  is displaced when acceleration is added in the Y-axis direction. The electrodes  123  and  125  detect the amounts of displacement in the X-axis and the Y-axis directions according to the variation of capacitance. The electrodes  124  and  126  apply voltages in order to forcibly displace the oscillator  121  in the X-axis direction and the oscillator  122  in the Y-axis direction. The two-axis low G sensor  102  further includes capacitance detectors  111  and  112  and AD converters  145  and  146 . The capacitance detectors  111  and  112  detect the variation of capacitance due to the displacement and output the variation as a voltage. The AD converters  145  and  146  convert the detected voltage into a digital signal. 
     The control circuit includes a temperature sensor  137  and an AD converter  138 . The temperature sensor  137  detects an ambient temperature and converts the temperature into a voltage in order to output the voltage. The AD converter  138  converts the output voltage into a digital signal. 
     Further, the control circuit includes characteristic correction units  140  and  141  that collect the outputs of the high G acceleration sensor and characteristic correction units  142  and  143  that collect the outputs of the low G acceleration sensor according to the output of the temperature sensor  137 . 
     Further, the control circuit includes a stoppage time diagnosis unit  151  that diagnoses the high G sensor and the low G sensor while the vehicle stops and a running time fault detection unit  152  that diagnoses the two G sensors while the vehicle runs. 
     Further, the control circuit includes a communication unit  153  that outputs the outputs of the sensors to an external device  154 . 
     Next, the operation of the control circuit will be described. Acceleration added in the X-axis direction displaces the oscillator  128  in the two-axis high G sensor  101  and causes the variation of capacitance at a fixed electrode  130  according to the displacement. Then, a signal of the displacement of the oscillator obtained through the capacitance detector  135  and the AD converter  148  is detected as acceleration. The system of the oscillator  129  for detecting the acceleration in the Y-axis direction operates similarly to the system of the oscillator  128 . 
     Next, acceleration added in the X-axis direction displaces the oscillator  121  in the two-axis low G sensor  102  and causes the variation of capacitance at a fixed electrode  123  according to the displacement. Then, a signal of the displacement of the oscillator obtained through the capacitance detector  111  and the AD converter  145  is detected as acceleration. The system of the oscillator  122  for detecting the acceleration in the Y-axis direction operates similarly to the system of the oscillator  121 . 
     Next, the high XG characteristic correction unit  140  and the high YG characteristic correction unit  141  remove the high-frequency noise component from the outputs of the acceleration in the two directions using a temperature correction operation and a low-pass filter according to the value detected by the temperature sensor  137 . 
     Next, the low XG characteristic correction unit  142  and the low YG characteristic correction unit  143  remove the high-frequency noise component from the outputs of the acceleration in the two directions using a temperature correction operation and a low-pass filter according to the value detected by the temperature sensor  137 . 
     Next, the stoppage time diagnosis unit  151  diagnoses the two-axis high G sensor  101  and the two-axis low G sensor  102  while the vehicle stops. Next, the running time fault detection unit  152  detects a fault of the two-axis high G sensor  101  and the two-axis low G sensor  102  while the vehicle runs. 
     Next, the communication unit  153  transmits the values of acceleration detected at the two-axis high G sensor  101  and the two-axis low G sensor  102 , the output of the stoppage time diagnosis unit  151 , and the output of the running time fault detection unit  152  to an external device. 
       FIG. 2  illustrates an embodiment of the stoppage time diagnosis unit  151  of  FIG. 1 . Each of a high G determination threshold unit  211  and a low G determination threshold unit  213  is a register configured to store a threshold for determining a diagnosis. A sensor determination unit  212  is a function to diagnose the output of the acceleration sensor. The operation of the sensor determination unit  212  will be described in detail in  FIG. 3 . An expectation unit  215  is a register configured to store the expectation to be detected for performing a diagnosis of the fault detection function of the running time fault detection unit  152 . A fault detection determination unit  216  is a function to perform a diagnosis of the running time fault detection unit  152 . The operation of the fault detection determination unit  216  will be described in detail in  FIG. 4 . A stoppage time diagnosed result register  202  stores the outputs of two sensor determination units  212  and the output of the fault detection determination unit  216 . A high G diagnosis voltage generation unit  203  is a function to apply constant voltages to the electrodes  131  and  133  in order to forcibly displace the oscillator  128  in the X-axis direction and the oscillator  129  in the Y-axis direction when the acceleration detection function is diagnosed as the process of the stoppage time diagnosis unit  151 . A low G diagnosis voltage generation unit  204  functions to apply constant voltages to the electrodes  124  and  126  to forcibly displace the oscillator  121  in the X-axis direction and the oscillator  122  in the Y-axis direction in order to diagnose the acceleration detection function when the process of the stoppage time diagnosis unit  151  is performed. 
       FIG. 3  illustrates an embodiment of the sensor determination units  212  of  FIG. 2 . First, the diagnosis voltage generation units  203  and  204  apply voltages for displacing the oscillators  121 ,  122 ,  128 , and  129  to the plus side. When each of the output signals from the oscillators  121  and  122  is within the determination threshold on the plus side that is stored in the low G determination threshold unit  213  and each of the output signals from the oscillators  128  and  129  is within the determination threshold on the plus side that is stored in the high G determination threshold unit  211 , “0” is output as a sensor diagnosis flag. On the other hand, when each of the output signals from the oscillators is not within each of the determination thresholds, “one” is output as the sensor diagnosis flag. Next, the diagnosis voltage generation units  203  and  204  apply voltages for displacing the oscillators  121 ,  122 ,  128 , and  129  to the minus side. Then, the same process as the diagnosis on the plus side is performed using each of the determination thresholds on the minus side that is stored in each of the high G determination threshold unit  211  and the low G determination threshold unit  213 . Next, it is determined whether the diagnosis flag value found above has the same value as the signal value output from the sensor determination unit  212 . This process is a function to diagnose the circuit part of the sensor determination unit  212 . 
       FIG. 4  illustrates an embodiment of the operation of the fault detection determination unit  216  which detects the fault of running time fault detection unit  152  during a running time in  FIG. 2 . First, the diagnosis voltage for the low G sensor is turned OFF (0 V). Next, the diagnosis voltage for the high G sensor is turned ON to the plus side (2.5 V). In that condition, it is determined whether the output of the high G sensor is larger than the output of the low G sensor. When the output of the high G sensor is larger than the output of the low G sensor, it is determined that the fault detection is normal and “0” is set at the diagnosis flag. Otherwise, it is determined that the fault detection is abnormal and “1” is set at the diagnosis flag. Next, the diagnosis voltage for the high G sensor is turned ON to the minus side (−2.5 V). In that condition, it is determined whether the output of the high G sensor is smaller than the output of the low G sensor. When the output of the high G sensor is smaller than the output of the low G sensor, it is determined that the fault detection is normal and “0” is set at the diagnosis flag. Otherwise, it is determined that the fault detection is abnormal and “1” is set at the diagnosis flag. Next, the diagnosis voltage for the high G sensor is turned OFF (0 V). Next, the diagnosis voltage for the low G sensor is turned ON to the plus side (2.5 V). In that condition, it is determined whether the output of the low G sensor is larger than the output of the high G sensor. When the output of the low G sensor is larger than the output of the high G sensor, it is determined that the fault detection is normal and “0” is set at the diagnosis flag. Otherwise, it is determined that the fault detection is abnormal and “1” is set at the diagnosis flag. Next, the diagnosis voltage for the low G sensor is turned ON to the minus side (−2.5 V). In that condition, it is determined whether the output of the low G sensor is smaller than the output of the high G sensor. When the output of the low G sensor is smaller than the output of the high G sensor, it is determined that the fault detection is normal and “0” is set at the diagnosis flag. Otherwise, it is determined that the fault detection is abnormal and “1” is set at the diagnosis flag. 
       FIG. 5  illustrates an embodiment of the running time fault detection unit  152 . In a running time fault detection process unit  301 , a low pass filter  311  is a function to adjust the phase of the output of the high G sensor that is displaced fast to the phase of the output of the low G sensor that is displaced slowly. Next, an amplifier  312  is a function to match the output value of the high G sensor with the output value of the low G sensor when acceleration is generated. A subtraction unit  313  is a function to subtract the output of the low G sensor from the output of the high G sensor. Next, a fault detection unit  314  is a function to detect a fault of the high G sensor and the low G sensor from the output of the subtraction unit  313  and the output of the low G sensor. A running time fault detected result register  302  stores fault detected results of the high G sensor and the low G sensor in the X direction and the Y direction. 
       FIG. 6  illustrates exemplary storages of the outputs in the stoppage time diagnosed result register  202  and the running time fault detected result register  302 . 
       FIG. 7  illustrates an embodiment of the fault detection unit  314  in  FIG. 5 . When an absolute value of the output of the subtraction unit  313  is smaller than the upper limit of the detectable value (for example, the detected maximum value of the low G sensor) and is larger than the lower limit of the detectable value (for example, the output of the low G sensor is “0”), the successive process in the fault detection unit  314  is performed. Otherwise, it is determined that the detected value is not, normal and the process in the fault detection unit  314  is not performed. Next, when the absolute value of the difference between the output of the high G sensor and the output of the low G sensor that is output from the subtraction unit  313  is within the range of the upper value and the lower value of the determination, it is determined that there is not a fault and the diagnosis flag is set at “0”. Otherwise, it is determined that there is a fault and the diagnosis flag is set at “1”. 
       FIG. 8  illustrates examples of the output signal of each of the diagnosis voltage generation units  203  and  204  in  FIG. 2  and the output of the acceleration sensor. For example, the high G sensor outputs “0 G” when the diagnosis voltage is 0 V. The high G sensor outputs “50 G” (49 m/s 2 ) when the diagnosis voltage is 2.5 V. The high G sensor is controlled to output “−50 G” (−49 m/s 2 ) when the diagnosis voltage is −2.5 V. Further, the low G sensor outputs “0 G” when the diagnosis voltage is 0 V. The low G sensor outputs “1 G” (0.98 m/s 2 ) when the diagnosis voltage is 2.5 V. The low G sensor is controlled to output “−1 G” (−0.98 m/s 2 ) when the diagnosis voltage is −2.5 V.