Patent Publication Number: US-2013249564-A1

Title: System and method to detect a short to ground of an external sensor return line

Description:
TECHNICAL FIELD OF INVENTION 
     This disclosure generally relates to detecting a short to ground condition of a return line, and more particularly relates to using a ground switch to electrically float the return line and detecting that a short to ground condition exists based on a change of an external sensor signal when the ground switch is momentarily opened. 
     BACKGROUND OF INVENTION 
     Regulations for emission control systems of vehicle engines require that the systems be able to detect wire harness fault conditions such as a short to battery or a short to ground. These regulations are sometimes referred to as On-Board-Diagnostics regulations, or OBD-II when referring to a second revision of those regulations. A short to ground may occur, for example, if the insulation protecting a wire in the wire harness is damaged. If a wire makes electrical contact with the vehicle chassis, an electrical short of the wire to the chassis (ground) may occur. Ways to detect shorts to ground of signal wires expected to have a voltage potential other than ground potential are known. However, detecting a short to ground of a return line, that is a line or wire that is supposed to be at or near ground potential, can be difficult since there is little or no change of the expected potential caused by the short to ground. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment, a controller configured to detect a short to ground condition of a return line electrically coupled to an external sensor is provided. The external sensor is configured to output a sensor signal relative to the return line. The controller includes a ground switch and a processor. The ground switch is operable to a closed state and an open state. The ground switch is configured to electrically connect the return line to a ground potential when the ground switch is in the closed state. The processor is configured to receive the sensor signal, and operate the ground switch to the closed state and the open state. The processor detects a short to ground condition of the return line if a value change of a sensor signal is less than a threshold value when the ground switch is switched from the closed state to the open state. 
     In another embodiment, a system configured to detect a short to ground condition of a return line of the system is provided. The system includes an external sensor, a ground switch, and a processor. The external sensor is electrically coupled to the return line and is configured to output a sensor signal relative to the return line. The ground switch is operable to a closed state and an open state. The ground switch is configured to electrically connect the return line to a ground potential when the ground switch is in the closed state. The processor is configured to receive the sensor signal, and operate the ground switch to the closed state and the open state. The processor detects a short to ground condition of the return line if a value change of a sensor signal is less than a threshold value when the ground switch is switched from the closed state to the open state. 
     In yet another embodiment, a method to detect a short to ground condition of a return line electrically coupling an external sensor to a controller is provided. The method includes the step of providing a ground switch coupled to the return line. The ground switch is operable to a closed state and an open state, and is configured to connect the return line to a ground potential when the ground switch is in the closed state. The method also includes the step of determining a signal first value of a sensor signal from the external sensor while the ground switch is in the closed state. The method also includes the step of determining a signal second value of the sensor signal from the external sensor while the ground switch is in the open state. The method also includes the step of indicating that a short to ground condition of the return line exists if a value change indicative of a difference between the signal first value and the signal second value is less than a threshold value. 
     Further features and advantages will appear more clearly on a reading of the following detailed description of the preferred embodiment, which is given by way of non-limiting example only and with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The present invention will now be described, by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a diagram of a system to detect a short to ground condition of a return line in accordance with a first embodiment; 
         FIG. 2  is diagram of a system to detect a short to ground condition of a return line in accordance with a second embodiment; 
         FIG. 3  is a graph of a signal present in the systems of  FIG. 1  or  2 ; and 
         FIG. 4  is flowchart of a method performed by the systems of  FIG. 1  or  2  in accordance with one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Described herein is a means to detect a short to ground condition of a return line electrically coupled to an external sensor. Typically, the external sensor outputs a sensor signal having a voltage potential or sensor signal value that is measured relative to the return line. If the return line is shorted to ground by, for example, making unexpected electrical contact with a vehicle chassis that differs by some ground shift value, then the sensor signal may be influenced in such a way that the information conveyed by the sensor signal becomes inaccurate or includes undesirable noise. 
       FIG. 1  illustrates a non-limiting example of a system  10  that includes an external sensor  12  electrically coupled to a controller  14  by a return line  16 . In this non-limiting example, the external sensor  12  is an ammonia (NH3) sensor sold by Delphi Corporation having administrative offices in Troy, Mich. However, it is recognized that the detection of a short to ground condition of the return line  16  may be used for other sensor types such as an exhaust oxygen sensor. The NH3 sensor includes a temperature sensor cell  18 , a first gas sensing cell (EMF1)  20 , a second gas sensing cell (EMF2), and a heater element  24 . The temperature sensor cell  18 , the first gas sensing cell (EMF1)  20 , and the second gas sensing cell (EMF2) are all electrically connected to the return line  16  in order to provide a reference voltage to the cells. The external sensor  12  also outputs a sensor signal  26  to the controller  14 , in this non-limiting example a temperature signal  26   a,  a first cell signal  26   b,  and a second cell signal  26   c.    
     The system  10  includes a ground switch  28  operable to a closed state and an open state. The ground switch  28  is illustrated as being within the controller  14 , but it is contemplated that the ground switch  28  could be located elsewhere, such as part of a wiring harness  40  between the external sensor  12  and the controller  14 . In general, the ground switch  28  is configured to electrically connect the return line  16  to a ground potential  30  when the ground switch  28  is operated to the closed state, and to electrically float or isolate the return line  16  when the ground switch  28  is operated to the open state. The ground switch  38  may be a solid-state device such as a transistor, for example a metal oxide semi-conductor field effect transistor (MOSFET), or a relay, or other component suitable to couple and de-couple the return line  16  to the ground potential  30 . As will become apparent in the description that follows, by operating the ground switch  28  to the open state, there may be a value change  34  ( FIG. 3 ) to the sensor signal  26  defined in this example as a difference between a signal first value  36  and a signal second value  38  that can be measured, and if the value change in the sensor signal  26  is not what was expected, then that may be an indication that the return line  16  is shorted to ground at some location other than via the ground switch  28 . 
     The system  10  may include a processor  32  configured to receive the sensor signal  26 . The processor  32  may be a microprocessor or other control circuitry as should be evident to those in the art. The processor  32  may include memory, including non-volatile memory, such as electrically erasable programmable read-only memory (EEPROM) for storing one or more routines, thresholds and captured data. The one or more routines may be executed by the processor  32  to perform steps for operating the ground switch  28 , and determining if sensor signal  26  received by the processor  32  indicates a short to ground of the return line  16  as described herein. By way of an example, the processor  32  may detect a short to ground condition of the return line  16  if a value change  34  of the sensor signal  26  is less than a threshold value when the ground switch  28  is operated or switched from the closed state to the open state. As will become apparent in the description that follows, the threshold value would need to be determined analytically and/or empirically for the specific configuration of the external sensor  12 . 
       FIG. 3  illustrates a non-limiting example of a graph a sensor signal  26 . For the purpose of explanation, it will be assumed that the graph is of the signal output by EMF1  20 , i.e. the first cell signal  26   b.  In this example, the signal first value  36  corresponds to when the ground switch  28  is in the closed state, and the signal second value  38  corresponds to when the ground switch is in the open state. If EMF1 is a typical chemical sensor, the voltage output by EMF1 may be in the range of a few tenths of a volt (V), for example 0V to 0.4V, depending on the composition of gas to which EMF1 is exposed. With the ground switch  28  in the closed state, the signal first value  36  would then be about 0V to 0.4V. With the ground switch  28  in the open state, the signal second value  38  corresponding to this open state would be influenced by the design of the external sensor  12 , and if the wire harness  40  had any shorts to ground or other potentials. For this non-limiting example, the controller  14  may include a test pulse generator  42  configured to output a test signal that feeds through the temperature sensor cell  18  while the ground switch  28  in in the open state and thereby causes the signal second value  38  shown just prior to time t 1 , t 2 , and t 3  on the graph, for example 5.0 volts. However, if the return line  16  is subject to a short to ground, the signal second value  38  may be that shown just prior to time t 4 , for example 0.7V. For this example, a suitable threshold value may be 3.0V. 
     If a short to ground is detected, the controller  14  may send a signal to the vehicle engine control module (ECM)  44  indicating that a fault has been detected, and the ECM  44  may take some remedial action such as illuminating a service engine soon indicator on a vehicle instrument panel. It is recognized that the controller  14  may be incorporated into the ECM  44 , but is shown herein as a separate entity only for the purpose of explanation. 
       FIG. 2  illustrates an alternative embodiment of a system to detect a short to ground condition of a return line of the system that includes a weak voltage source, hereafter the system  200 . As used herein, a weak voltage source is characterized as a voltage source having source resistance that is sufficiently high to prevent damage the ground switch  28  when the ground switch  28  is in the closed state, but sufficiently low to influence the sensor signal  26  when the ground switch is in the open state. As such, it may not be necessary to provide the test pulse generator  42  shown in  FIG. 1 . 
     In this example of the system  200 , a suitable value for Vcc is 5.0V and a suitable voltage divider network includes a 249 kilo-Ohm (kΩ) resistor and a 10.5 kΩ resistor, thereby providing a weak voltage source having an open circuit voltage of about 0.2V and a source resistance of about 10 kΩ. The example given for  FIG. 1  suggests that a substantial voltage is on the order of a volt or two, while this example suggests that a substantial voltage is on the order of a tenth of a volt. It should be appreciated that the threshold voltage and any voltage provided by the weak voltage source characterized as substantially different from the ground potential is selected in view of the design and sensor technology used in the external sensor  12 . 
       FIG. 4  illustrates a non-limiting example of a method  400  to detect a short to ground condition of a return line  16  electrically coupling an external sensor  12  to a controller  14 . 
     Step  410 , PROVIDE GROUND SWITCH, may include providing a ground switch  28 , for example a MOSFET, that is coupled to the return line  16 , the ground potential  39 , and the processor  32  in a manner known in the art. In general, the ground switch  28  is operable to a closed state and an open state. As describe above, the ground switch  28  is configured to connect the return line  16  to a ground potential  30  when the ground switch  28  is in the closed state, and electrically isolate the return line  16  from the ground potential  30  when the ground switch  28  is in the open state. 
     Step  420 , OPEN GROUND SWITCH, may include the controller  14  outputting a control signal  46  that operates the ground switch to an open state. By way of example and not limitation, the ground switch  28  may be periodically opened for a time interval of one hundred milliseconds (100 ms) every three seconds (3 s). With this example timing, the external sensor  12  is operated normally for two point nine seconds (2.9 s), and then a check for a short to ground is done during the 100 ms that the ground switch  28  is in the open state. 
     Step  430 , DETERMINE V2, may include determining a signal second value  38  (V2) of the sensor signal  26  from the external sensor  12  while the ground switch  28  is in the open state. 
     Step  440 , CLOSE GROUND SWITCH, may include the controller  14  outputting a control signal  46  that operates the ground switch  28  to a closed state. 
     Step  450 , DETERMINE V1, may include determining a signal first value  36  (V1) of the sensor signal  26  from the external sensor  12  while the ground switch  28  is in the open state 
     Step  460 , V2−V1&lt;VT, may include determining if a difference between the signal first value  36  and a signal second value  38  is less than a threshold value. If the test result is NO, then the method  400  returns to step  420  and the cycle of operating the ground switch  28  to the open state in order to check for shorts to ground is repeated. If the test result is YES, then there is an indication that the return line  16  may have an unexpected connection to ground, i.e. a short to ground, and so the method  400  proceeds to step  470   
     Step  470 , INDICATE SHORT, may include indicating that a short to ground condition of the return line exists if a difference between the signal first value  36  and a signal second value  38  is less than a threshold value. Indicating that a short to ground condition exists may include illuminating a ‘SERVICE ENGINE SOON’ indicator on the vehicle instrument panel (not shown). 
     Accordingly, a system  10 , a controller  14  for the system  10  and a method  400  to detect a short to ground condition of a return line  16  electrically coupled to an external sensor  12  is provided. In normal operation, the ground switch  28  is closed. The system  10  opens the ground switch  28  in the return line  16  and expects to see a change in response of one or more of the signals from the external sensor  12 . If these changes are not detected, it may be surmised that the return line  16  is shorted to ground outside of the controller  14 . If appropriate changes are detected, the return line  16  is assumed to be properly insulated. 
     While this invention has been described in terms of the preferred embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.