Abstract:
A control system adapted to be mounted on a motor vehicle for control of a motor vehicle system in accordance with the inertial state of the motor vehicle. The control system includes an inertial sensor providing an inertial measurement output in accordance with the inertial state of the motor vehicle, where the inertial measurement output is referenced to a reference voltage. A controller is provided for controlling the motor vehicle system at least partially in accordance with the inertial measurement output. The controller includes a circuit for comparing the reference voltage used by the inertial sensor to a nominal voltage. The circuit causes the controller to discontinue use of the inertial measurement output when the reference voltage deviates from the nominal voltage.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention is directed to a diagnostic method and apparatus for use with inertia sensors, particularly for use in motor vehicles. 
       BACKGROUND 
       [0002]    Increasing numbers of inertia sensors are being installed in motor vehicle systems. Such sensors provide valuable vehicle state information to controllers for such diverse systems as occupant restraints (e.g., airbags, seat belt pretensioners, etc.) and vehicle stability control (e.g. braking, steering systems, etc.). If the inertia sensors fail to operate properly, then the associated system may be degraded as well. 
         [0003]    Inertia sensors sometimes include built in self test (“BIST”) functions for assessing the operation of the sensor. BIST functions are helpful in detecting failures of the sensors. However, BIST functions may fail to reveal some types of degradation of the sensor operation, particularly those types of degradation that are associated with interaction between the sensor and the rest of the system. 
       SUMMARY OF THE INVENTION 
       [0004]    Electronic systems in motor vehicles have single-ended power supplies, since the vehicle battery itself is single-ended. Inertial sensors, whether accelerometers or gyros, often must allow their inertial measurement outputs to swing positive (above a reference) or negative (below a reference), respectively representing positive or negative angular velocity (angular “rate”), for example. Therefore, conventionally, the inertial sensor will define some reference level that is intermediate between ground and the voltage of the available power supply. The reference level must be steady, as any change in the reference level will be mirrored by a change in the output level, and a consequent degradation in the accuracy of the inertial measurement output. 
         [0005]    The present invention provides a method and operation for detection of spurious outputs from an inertial sensor by detecting a deviation of a reference voltage from its design level. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The foregoing and other features and advantages of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which: 
           [0007]      FIG. 1  is a block diagram of a vehicle system in accordance with one example embodiment of the present invention; and 
           [0008]      FIG. 2  is a flow chart showing a control process in accordance with one example embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    Referring to  FIG. 1 , an electronic control system  10  for a motor vehicle, such as a passenger car, truck, or SUV, includes a controller  12 . The controller  12  may be a microcomputer and is illustrated as such in the figure. The microcomputer is not described in detail herein, but is understood to contain the elements typical of such microcomputers, including a single or multiple core microprocessor, appropriate read-only and random access memories, input/output controllers, digital and analog inputs, analog to digital convertors, and so on. The microprocessor(s) operates under control of the software/firmware program stored in the memory of the microcontroller. The controller may also be embodied using discrete circuitry, an application-specific integrated circuit, etc., designed to accomplish the desired functions. 
         [0010]    Microcomputer  12  is connected to the vehicle systems  14  via various control lines  16 , and controls the operation of such systems via those control lines. The controller  12  may be designed and programmed as a vehicle stability controller, in which case the controlled vehicle systems  14  will be such things as electro-hydraulic braking systems and steering systems. Alternately, the controller  12  may be designed and programmed as a vehicle safety system controller, in which case the controlled vehicle systems  14  will be such things air bag initiators and seat belt pretensioners. 
         [0011]    The microcomputer  12  is connected to various sensors that provide the controller with information reflecting the present state of the motor vehicle. The sensor set may include linear accelerometers such as single, double, or triple axis accelerometers, and may also include so called “rate sensors” or “gyros” which respond to angular rate of the vehicle and provide to the microcomputer  12  measurements of such angular rate. In the illustrated example embodiment, only one sensor, a gyro  18 , is shown, connected to the microcomputer via a number of input/output lines  20 . Other sensors may, however, also be included. 
         [0012]    The electronic control system  10  is powered by a DC voltage supplied by a power source  22 . The power source receives power from the vehicle electrical system, usually a wet-cell battery and appropriate charging circuitry. Power from power source  22  is single-ended, meaning that the power source only supplies power at voltage levels that are on one side of vehicle ground. That voltage will typically be derived from the 12 volt battery voltage, and may be, for example, 3 volts DC. The supply voltage V dd  will be provided via a supply lead  24 , and of course the power source will be grounded to signal or vehicle ground, as indicated by ground lead  26 . In  FIG. 1 , for convenience of illustration, both the microcomputer  12  and gyro  18  are shown as connected across the same power leads  24  and  26 . In a particular embodiment, however, the source  22  may in fact supply different supply voltages to the gyro and the microcomputer via associated supply outputs. In either case, the gyro  18  is powered by a single-ended power supply. 
         [0013]    Gyro  18  is an integrated circuit having various elements contained in a single sealed package with input/output pins for connection to external circuit elements. Gyro  18  is of generally conventional construction, and includes a sensor element and control circuitry. An example of one such a gyro is the Pinpoint CRM100 gyroscope manufactured by Silicon Sensing Systems Limited. (The content and operation of the CRM100 gyro is described in a datasheet available on the manufacturer&#39;s public website.) Microcomputer  12  triggers gyro  18  via control lines  20  to measure vehicle angular rate and provide the resulting angular rate measurement to the microcomputer as an analog signal via lines  20 . 
         [0014]    Proper functioning of gyro  18  is important to the proper functioning of the electronic control system  10 . Sensors such as gyro  18  therefore often include a built-in self test (“BIST”) function for testing operation of the sensor. In  FIG. 1 , the microcomputer  12  has an output line  32  (illustrated as separate from lines  20  only for convenience of description) for triggering the BIST function of the sensor. The BIST function is triggered, for example, each time the vehicle is started up. The results of the BIST are supplied to microcomputer  12  via one of the input/output lines  20   
         [0015]    In any given plane (roll, pitch, yaw), the vehicle may rotate in two different directions, clockwise or counterclockwise. The analog output of the gyro must therefore indicate not only the magnitude of the angular rate, but also the direction. The gyro  18  does this by establishing an artificial reference signal VREF midway between the supply voltage V dd  and ground. In the present example embodiment, V dd  is 3 volts and VREF is 1.5 volts. If the analog output of the gyro is above VREF, then the rate is assumed to be in one direction (e.g. clockwise). If the analog output of the gyro is below VREF, then the rate is assumed to be in the opposite direction (e.g. counterclockwise). 
         [0016]    The accuracy of the analog output of the gyro  18  is dependent upon the accuracy of the reference voltage VREF. To stabilize VREF, the internal reference voltage line of the gyro  18  is attached to an output pin  28 , adapted for connection to an external capacitor  30 . Despite the stabilizing effect of the external capacitor  30 , it is still possible that the reference voltage will drift or otherwise deviate from the preferred, nominal voltage. Moreover, a shift in VREF may indicate other problems with the gyro  18 , e.g. a bad capacitor, an open ground connection, or an open connection from V dd  to the gyro  18 . 
         [0017]    In accordance with the present invention, the circuit is designed to measure the reference voltage (sometimes referred to as “reading” the reference voltage) each time the analog output of gryo  18  is read. If VREF deviates from the nominal level, the gryo reading is discarded and an error flag is set. Thus, bad readings due to reference voltage changes are invalidated and not used. 
         [0018]    To this end, the electronic control system  10  further may include a buffer amplifier  34  providing a buffered signal equal to VREF. The buffered reference signal is supplied to an input of microcomputer  12  on analog input line  36 . The microcomputer  12 , as stated previously, includes an analog to digital convertor. The microcomputer also includes a multiplexer that allows it to connect any one of several inputs to the input of the analog to digital convertor. When microcomputer  12  requires a measurement of angular rate, it will first connect the buffered VREF signal on analog input line  36  to the analog to digital convertor, thereby taking a measurement of the reference value, and then will connect the analog output of gyro  18  to the analog to digital convertor, thereby taking a measurement of the gyro output. The digitalized version of the reference signal VREF will be compared with upper and lower thresholds, respectively above and below the nominal reference signal value. If the digitized version of the reference signal is above the upper threshold or below the lower threshold, the gyro output reading will be flagged as invalid and will not be used in the vehicle stability control algorithms or restraint control algorithms implemented in the microcomputer. 
         [0019]    The microcomputer control process in accordance with an example embodiment of the present invention is shown in flowchart form in  FIG. 2 . The flowchart shows the measurement process as part of a larger software program performed by microcomputer  12 . The program generally includes an initialization step  100  performed on key-on of the vehicle (ignition switch in the start or run position) in which initial flags are set, memories cleared, etc., and a main loop  102  that is cycled through repeatedly until key-off of the vehicle (ignition switch in the off position). Interrupt-driven processes will exist as well but these are not shown in  FIG. 2 . The measurement subroutine  104  is shown here as part of the main loop  102 , however it could instead be part of an interrupt driven process performed periodically under timer control. 
         [0020]    In the main loop, the measurement subroutine is preceded by other processes, collectively represented by upstream processes  106 , and is also followed by other processes, collectively represented by downstream processes  108 . The upstream processes may include housekeeping functions, diagnostics, preliminary algorithms, and capture of data from other sensors and systems. The downstream processes may include control algorithms employing the sensor data collected in measurement subroutine  104  and other upstream processes, interaction with controlled elements on the vehicle systems  14  (including controls of dashboard displays), and error processing routines. 
         [0021]    Each time the main loop  102  cycles through the measurement subroutine  104 , the microcomputer reads the output of the gryo  18  at step  110  and reads the reference voltage VREF provided by buffer amplifier  34  at step  112 . In the succeeding evaluation step  114 , the reference voltage VREF is evaluated. As described above, the evaluation step  114  comprises a comparison of the reference voltage VREF with an acceptable range or “window”, where the threshold window is defined by upper and lower limits against which VREF is separately compared. The design nominal value for VREF will typically be halfway between the upper and lower limits. If VREF is within the acceptable window (below the upper limit AND above the lower limit), the sensor output reading is validated in step  116  by resetting an error flag. Otherwise (VREF is EITHER above the upper limit OR below the lower limit), the sensor reading is invalidated in step  118  by setting same error flag. 
         [0022]    In either case, program flow continues with downstream processes  108 , where actions will be taken conditional upon the logic status of the error flag. Specifically, the gyro sensor reading will be used in further algorithmic processes only if the error flag is not set (i.e., is reset). If the error flag is set, however, then the downstream processes  108  will sense the flag status and will react by not using the sensor output reading. Further, the error flag will trigger the downstream processes  108  to (a) alert the vehicle operator of the error by illuminating a warning lamp, and (b) mitigate the effect of the unreliability of the sensor output by substitution of other sensors readings or by other modification of the algorithmic control processes. 
         [0023]    A method and apparatus have thus been described for detection of spurious outputs from an inertial sensor by detecting a deviation of a reference voltage from its design level. Although described with specific reference to a gyro, which measures angular rate, the approach will be of equal value with any other sensors using VREF architectures similar to the example architecture outlined above. The approach will work with digital sensors as well as analog sensors, so long as the internal VREF signal may be accessed. 
         [0024]    From the above description of the invention, those skilled in the art will perceive improvements, changes and modifications. Such improvements, changes and modifications within the skill of the art are intended to be covered by the appended claims.