Patent Document

FIELD OF THE INVENTION 
     The present invention relates generally to sensor circuits, and more particularly, to a sensor circuit having an open load detector circuit. 
     BACKGROUND OF THE INVENTION 
     Sensors are being used in a wide variety of automotive electronics applications. The performance of automobile engines, and in many instances the operability and performance of automobile safety systems, depends on sensors. When failures occur, such as open circuits to the sensor power supply, the controlling module cannot operate in accordance with the fault unless the open circuit has been detected. Currently, there is not an easy and cost-effective way to detect the presence of a fault such as, for example, an open circuit to the sensor. 
     A number of ways exist in the prior art to determine an open circuit in the sensor power supply. For example, one prior art approach is to place a current sense resistor in series with the load (i.e., the sensor) and measure the voltage drop across the resistor. This approach is both burdensome and expensive, in that an operational amplifier and a sense resistor are required. Moreover, power dissipation of the resistor can be very large if a short to ground exists. 
     Another prior approach to assessing the condition of the sensor circuit has been to remove power from the sensor supply and monitor the decay of the output of the sensor. Such an approach needs to look at the output of the sensor supply and thus requires permanent external circuitry which may ultimately affect the output performance. 
     Accordingly, there is a continuing need for improved approaches for determining the presence of an open circuit to electronic components such as sensors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a sensor supply open load detector circuit assembled in accordance with the teachings of a preferred embodiment of the present invention; 
     FIG. 2 is a chart illustrating discharge voltage over time and further illustrating the manner by which a voltage feedback signal may be generated in accordance with a preferred embodiment of the present invention; 
     FIG. 3 is a chart illustrating a control signal waveform generated in accordance with the teachings of a preferred embodiment of the present invention; 
     FIG. 4 is a chart illustrating discharge voltage over time and illustrating one manner of generating an output signal based on the amount of time it takes the detector circuit to discharge a known voltage; and 
     FIG. 5 is a chart illustrating discharge voltage over time and illustrating another manner of generating an output signal based on the amount of voltage discharged from the detector circuit at a known time. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The following description of the preferred embodiment is not intended to limit the scope of the invention to the precise form disclosed, but instead is intended to be illustrative of the principles of the invention so that others may follow its teachings. 
     Referring now to the drawings, a sensor circuit  10  constructed in accordance with the teachings of the present invention includes a detector circuit  12  coupled to a charging circuit  14 . A processing system  16  is coupled to both the detector circuit  12  and the charging circuit  14 . A load circuit  18  includes an electronic component  20 , and also includes one or more potential points of open circuit  21 ,  23 . The electronic component is preferably a sensor  22 . The sensor  22  may include one or more sensors, which sensors may include any active, passive or resistive load. The sensor is powered from a conventional power supply, such as a power supply  34 , as would be known to those skilled in the art. 
     The detector circuit  12  includes an energy storage device  24 , which is preferably a capacitor  26 . The detector circuit  12  also includes a resistive network  28 , which in the preferred embodiment comprises a first resistor  30  and a second resistor  32  arranged in series. The resistive network  28  is in parallel with the energy storage device  24 . It will be noted that the resistance of the resistors  30 ,  32  is preferably about four (4) times greater than the equivalent resistance of the electronic component  20 . 
     The charging circuit  14  includes the power supply  34 , and further includes a switch  36 . The processing system  16 , which is preferably a microcontroller, includes a voltage feedback pin  38  at Node B which is connected to the detector circuit  12 , preferably between the resistors  30 ,  32 . The processing system  16  is arranged to generate a control signal  40 , such as the pulsed waveform shown in FIG. 3, and to communicate the control signal  40  to the switch  36 . As shown in FIG. 3, the control signal  40  closes the switch  36  for the duration of the time interval T 0 -T 1 , enabling the charging circuit  14  to charge the energy storage component  24  as will be explained in greater detail below. The processing system  16  is further arranged to generate an output signal  42  which is indicative of the load drawn by the electronic component  20 . 
     In operation, the voltage at Node A is the sensor supply voltage, which is initially zero volts (0 volts). Thus, the energy storage device  24  should be fully discharged. This condition should be verified by a reading of zero (0) at the voltage feedback pin  38 . The processing system  16  generates the pulsed signal  40  shown in FIG. 3, which in turn enables the switch  36  for the duration of the T 0 -T 1  time interval, which time interval is sufficient to fully charge the energy storage device  24 . When the control signal  40  passes through T 1 , the switch  36  is turned off, thus removing the power supply  34  and deactivating the charging circuit  14 . At this point, the energy storage device  24  begins to discharge. 
     Referring now to FIGS. 2,  4  and  5 , with a normal load on the electronic component  20 , the energy storage device  24  will have a nominal discharge time constant which is dependent on the current draw of the sensor  20 . If the load is not present, such as due to an open circuit at either or both of points  21  and  23 , then the discharge time constant for the energy storage device  24  will be dependent solely on the resistors  30 ,  32 . Because the resistance of the resistors  30 ,  32  are greater than the resistance or equivalent current draw of the electronic component  20 , if the load of the electronic component  20  is not present, then the discharge time for such an open circuit condition will be vastly different. The processing system  16  determines the presence or absence of the load based on the difference in the discharge times (i.e., a relatively fast discharge time indicates the presence of the load offered by the electronic component  20 , while a relatively slow discharge time indicates the absence of the load offered by the electronic component  20 ). 
     The load condition is assessed by monitoring the voltage at Node A. It will be understood that the voltage at Node B is proportional to the voltage at Node A. Because this proportion is known, the sensor supply voltage can be inferred from the voltage at Node B. 
     Thus, the load condition can be assessed in at least two ways. As shown in FIG. 4, the load condition may be determined by assessing the amount of time the voltage at Node B stays above a reference voltage V R  (i.e., by monitoring the amount of time it takes for the energy storage device  24  to discharge to a predetermined reference voltage). In other words, if the energy storage device discharges from an initial voltage V 1 ; to the reference voltage V R  within the time interval T 2  to T 3 , then the load is normal. Similarly, if the discharge to V R  does not occur until T 4  or later, then there is an open load. If the voltage in the energy storage device  24  discharges to V R  between T 1  and T 2 , there is a short circuit to ground or a marginally indeterminate shorted sensor. Further, a time to discharge between T 3  and T 4  is indicative of an abnormal condition. The processing system  16  then determines the fault status of the sensor and, if required by the specific application, may generate an appropriate output signal  42 . 
     Alternatively, as shown in FIG. 5, the load condition may be determined by assessing the amount of voltage at Node B at a sample time T sample  (i.e., by monitoring the amount of voltage discharged by the energy storage device  24  by the time a predetermined time interval has been reached). In other words, if the voltage discharged by the energy storage device  24  at T sample  falls between V 3  and V 4 , then the load is normal. If the voltage discharged by the energy storage device  24  at T sample  falls between V 1 , and V 2 , then the circuit is open. If the voltage discharged by the energy storage device  24  at T sample  falls between V 2  and V 3 , then there is an abnormal condition. If the voltage at T sample  has fallen below V 4 , then this would be indicative of a sensor shorted to ground and/or a marginally indeterminate shorted sensor. The processing system  16  then generates the appropriate output signal  42 . 
     As would be understood by those skilled in the art, using the above methodology it would be possible to discriminate open, normal, abnormal and short to ground occurrences on one or more sensors/loads connected to a single supply. Once the processing system determines the fault status of the sensor, the appropriate response would be determined by the specific application. 
     It will be noted that a sensor circuit  10  constructed in accordance with the teachings of the present invention allows the sensor to be monitored non-intrusively so that the act of monitoring does not impinge on normal circuit performance. 
     Those skilled in the art will appreciate that, although the teachings of the invention have been illustrated in connection with a certain embodiment, there is no intent to limit the invention to such an embodiment. On the contrary, the intention of this application is to cover all modifications and embodiments fairly falling within the scope of the appended claims either literally or under the doctrine of equivalents.

Technology Category: y