Patent Publication Number: US-8121744-B2

Title: Control system and method for oxygen sensor heater control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/074,274, filed on Jun. 20, 2008. The disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present disclosure relates to control systems for internal combustion engines, and more particularly, to oxygen sensor heater control. 
     BACKGROUND 
     The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure. 
     Referring now to  FIG. 1 , a functional block diagram of an engine system  100  is presented. The engine system  100  includes an engine  102  that may be used to produce power by combusting fuel in the presence of air. Typically, air is drawn into the engine  102  through an intake manifold  104 . A throttle valve  106  may be used to vary the volume of air drawn into the intake manifold  104 . The air mixes with fuel that may be dispensed by one or more fuel injectors  108  to form an air and fuel (A/F) mixture. The A/F mixture is combusted within one or more cylinders of the engine  102 , such as cylinder  110 . Combustion of the A/F mixture may be initiated by spark provided by a spark plug  112 . Exhaust gas produced during combustion may be expelled from the cylinders to an exhaust system  114 . 
     The exhaust system  114  may include one or more oxygen sensors, such as oxygen sensor  116 , that may be used to measure the amount of oxygen in the exhaust gas. The oxygen sensor  116  may be threaded into a hole provided in the exhaust system  114  and thereby be disposed within a flow of the exhaust gas. The oxygen sensor may output a voltage corresponding to the quantity of oxygen in the exhaust gas. It may be desired to operate the oxygen sensor  116  above a particular temperature, such as a sensitivity temperature, in order to ensure a reliable output voltage. Accordingly, the oxygen sensor  116  may include a heater that receives power from a heater power supply  118 . The heater may be used to supply supplemental heat and thereby bias the oxygen sensor  116  to within an operating temperature range above the sensitivity temperature. 
     An engine control module (ECM)  120  may be used to regulate the operation of the engine system  100 . The ECM  120  may receive the output voltage of the oxygen sensor  116 , along with signals from other sensors  122 . The other sensors  122  may include, for example, a manifold absolute pressure (MAP) sensor and an intake air temperature (IAT) sensor. Based on the output voltage of the oxygen sensor  116 , the ECM  120  may regulate the A/F mixture by regulating the throttle valve  106  and fuel injectors  108 . The ECM  120  may also regulate the A/F mixture based on the signals it receives from the other sensors  122 . 
     The temperature of the oxygen sensor  116  may be below the sensitivity temperature when the engine  102  is started. Accordingly, the output voltage of the oxygen sensor  116  may be unreliable for a period of time after engine startup. While the output voltage of the oxygen sensor  116  is deemed unreliable, the ECM  120  may regulate the A/F mixture independent of the output voltage of the oxygen sensor  116 . 
     Heat provided by the exhaust gas and the heater may be used to bring the temperature of the oxygen sensor  116  above the sensitivity temperature. However, for a period of time after engine startup, water condensate present within the exhaust system  114  may become entrained in the exhaust gas come in contact with the oxygen sensor  116 . Liquid water that comes into contact with the oxygen sensor  116  may cause thermal shock to the oxygen sensor  116 . Repeated thermal shock to the oxygen sensor  116  may induce fractures in the oxygen sensor  116  and result in premature failure. 
     SUMMARY 
     The present disclosure provides a control system and method for detecting liquid water that may have come in contact with an oxygen sensor and operating a heater included with the oxygen sensor at a reduced power to ameliorate thermal shock to the oxygen sensor. 
     In one form, the present disclosure provides a control system for the heating element used in the oxygen sensor comprising a rate module that periodically determines a rate of change of current through the heating element; and a temperature adjustment module that periodically compares the rate of change and a rate value and selectively adjusts an operating temperature of the oxygen sensor between a normal temperature and a remedial temperature lower than the normal temperature based on the comparison of the rate of change and the rate value. In one example, the remedial temperature may be lower than a thermal shock temperature of the oxygen sensor. In another example, the operating temperature may be the operating temperature of a sensing element and the remedial temperature may greater than a sensitivity temperature of the sensing element. 
     In one feature, the control system may further comprise a power supply module that supplies a power to the heating element based on a power control signal, wherein the temperature adjustment module generates the power control signal to adjust the operating temperature. 
     In another feature, the temperature adjustment module adjusts the operating temperature towards the remedial temperature when the rate of change is greater than or equal to the rate value. The temperature adjustment module may adjust the operating temperature towards the remedial temperature when a number (C) of consecutive values of the rate of change are greater than or equal to the rate value, C being an integer greater than zero. 
     In yet another feature, the temperature adjustment module adjusts the operating temperature toward the remedial temperature while the rate of change is positive. In one example, the temperature adjustment module may adjust the operating temperature towards the remedial temperature while a number (Z) of a consecutive number (W) of the most recent values of the rate of change are greater than or equal to the rate value, Z and W being integers greater than zero. In another example, the temperature adjustment module may adjust the operating temperature towards the remedial temperature while at least a number (T) of a consecutive number (S) of the most recent values of the rate of change are positive, T and S being integers greater than zero. 
     In still another feature, the temperature adjustment module waits to compare the rate of change and the rate value until the current is greater than or equal to a first current threshold and less than or equal to a second current threshold, the first current threshold being less than the second current threshold. 
     In another form, the present disclosure provides a control method for a heating element used in an oxygen sensor, the control method comprising periodically determining a rate of change of current through the heating element; periodically comparing the rate of change and a rate value; and selectively adjusting an operating temperature of the oxygen sensor between a normal temperature and a remedial temperature lower than the normal temperature based on the comparing the rate of change and the rate value. 
     In one feature, the selectively adjusting an operating temperature includes selectively supplying a normal power and a remedial power to the heating element. 
     In another feature, the selectively adjusting an operating temperature includes adjusting the operating temperature towards the remedial temperature when the rate of change is greater than or equal to the rate value. In one example, the selectively adjusting an operating temperature may include adjusting the operating temperature towards the remedial temperature when a number (C) of consecutive values of the rate of change are greater than or equal to the rate value, C being an integer greater than zero. 
     In yet another feature, the selectively adjusting an operating temperature includes adjusting the operating temperature toward the remedial temperature while the rate of change is positive. In one example, the selectively adjusting an operating temperature may include adjusting the operating temperature towards the remedial temperature while a number (Z) of a consecutive number (W) of the most recent values of the rate of change are greater than or equal to the rate value, Z and W being integers greater than zero. In another example, the selectively adjusting an operating temperature may include adjusting the operating temperature towards the remedial temperature while at least a number (T) of a consecutive number (S) of the most recent values of the rate of change are positive, T and S being integers greater than zero. 
     In still another feature, the control method further comprises periodically comparing the current and a first current threshold and a second current threshold, the first current threshold being less than the second current threshold; and waiting to begin periodically comparing the rate of change and the rate value until the current is greater than or equal to the first current threshold and less than or equal to a second current threshold, the first current threshold being less than the second current threshold. 
     Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein: 
         FIG. 1  is a functional block diagram of an engine system according to the prior art; 
         FIG. 2  is a partial cross-sectional view of an exemplary oxygen sensor; 
         FIG. 3  is a functional block diagram of an engine system according to the principles of the present disclosure; 
         FIG. 4  is a functional block diagram of the heater control module shown in  FIG. 3 ; and 
         FIG. 5  is a flowchart depicting exemplary control steps performed by a heater control module according to the principles of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure. 
     As used herein, the term module refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. 
     The present disclosure provides a control system and method for detecting liquid water that may have come in contact with an oxygen sensor by monitoring a current supplied to a heater that may be included with the oxygen sensor. The present disclosure also provides a control system and method for operating the heater at a reduced power to ameliorate thermal shock to the oxygen sensor, while maintaining reliable oxygen sensor output. 
     With particular reference to  FIG. 2 , an exemplary oxygen sensor  116  is shown. The oxygen sensor  116  may include a sensor element assembly  130  supported within a housing  132  by one or more support tubes  134 . The sensor element assembly  130  may be of several common types. For example, the sensor element assembly  130  may be of the narrow band type or the wide band type. Narrow band oxygen sensors, such as a conical zirconia sensor, generate a non-linear (i.e. binary) output voltage based on the quantity of oxygen in the exhaust. The output voltage generated by a narrow band oxygen sensor may be used to determine whether the engine  102  is operating in a lean or a rich state. Wide band oxygen sensors, such as a planar zirconia sensor, generate a generally linear output voltage based on the quantity of oxygen in the exhaust. Thus, wide band oxygen sensors may be used to determine the specific oxygen content in the exhaust and whether the engine is operating in a lean or a rich state. As discussed herein, the sensor element assembly  130  is a wide-band oxygen sensor of the planar zirconia sensor type. 
     Accordingly, the sensor element assembly  130  may be a generally flat, elongate member having a sensing element  140  disposed on one end within a sensing cavity  142  defined by housing  132 . The sensing element  140  may include an integral heating element  144 . The heating element  144  may be included to provide supplemental heat to warm the sensing element  140  to within a temperature range above its sensitivity temperature. For example, the heating element  144  may be used to warm the sensing element  140  to a temperature above 350° C. The heating element  144  may be formed of various materials, such as, for example, platinum or tungsten. The choice of material may be based on whether the sensor element assembly  130  is of the narrow band or the wide band type. 
     A contact holder  146  may be disposed on an opposite end to connect electrodes (not shown) of the sensing element  140  and the heating element  144  with wiring  148  of the oxygen sensor  116 . The wiring  148  may include four or more wires, depending on the particular configuration of the sensing element  140  and the heating element  144 . 
     The housing  132  may be generally cylindrical in shape and include a sensor cover  160  press fit on one end and a protective sleeve  162  press fit on an opposite end. The housing  132  may further include external threads  164  that may be used to secure the oxygen sensor  116  to the exhaust system  114  such that the sensing element  140  is in communication with the exhaust gas. The sensor cover  160  may be used to shield the sensing element  140  from direct impingement by the exhaust gases. The sensor cover  160  may include an inner shield  166  and an outer shield  168  that work together to define internal and external openings  170 ,  172  through which exhaust gas may enter cavity  142 . 
     The openings  170 ,  172  may be of varying sizes. The openings  170 ,  172  may be located and sized to produce a particular response of the sensor element assembly  130  to changes in the oxygen content of the exhaust gas. Additionally, the openings  170 ,  172  may be located and sized to affect a thermal response of the sensor element assembly  130  to liquid water impingement. Put another way, the amount of and location where liquid water may contact the sensor element assembly  130  may depend on the location and size of the openings  170 ,  172  and thereby affect the thermal response of the sensor element assembly  130 . 
     Water condensate may be present in the exhaust system  114  for a variety of reasons. For example, water condensate may be present while the exhaust gas temperature is less than a dew point of the exhaust gas. Water condensate may also be present as a result of water that has pooled within portions of the exhaust system  114 , such as within a catalytic converter (not shown), and is carried over from one engine operating cycle to another subsequent engine operating cycle. 
     Water condensate within the exhaust system  114  may become entrained in the exhaust gas during engine operation. Liquid water entrained in the exhaust gas may enter cavity  142  and come in contact with the sensor element assembly  130 , resulting in thermal shock to the sensor element assembly  130 . Repeated thermal shock to the oxygen sensor  116  may induce fractures in the sensor element assembly  130  and result in premature failure. 
     Accordingly, the present disclosure provides a control system and method for detecting liquid water that may be present within cavity  142 . Additionally, the present disclosure provides a control system and method for operating the heating element  144  at a reduced power to ameliorate the thermal shock events to the sensor element assembly  130 , while maintaining proper operation of the oxygen sensor  116 . 
     The foregoing objectives may be achieved by monitoring current supplied to the heating element  144 . More specifically, the presence of liquid water on the sensor element assembly  130  may be detected by monitoring the time rate of change in the current supplied to the heating element  144 . Liquid water contacting the sensor element assembly  130  will have a temporary cooling effect on the sensor element assembly  130  as the liquid water comes into contact with the sensor element assembly  130  and subsequently evaporates. Since the resistance of metals such as the platinum and tungsten used to form the heating element  144  decrease with decreasing temperature, temporary increases in the current supplied to the heating element may result when liquid water contacts the sensor element assembly  130 . 
     By monitoring the current supplied to the heating element  144 , it is possible to detect the presence of liquid water on the sensor element assembly  130  and take remedial control measures to inhibit thermal shock to the various components of the sensor element assembly  130 . Remedial control measures may include temporarily reducing a power (e.g., voltage) supplied to the heating element  144 . The power may be reduced to reduce an operating temperature of the sensor element assembly  130 . More specifically, the power may be reduced to operate the sensor element assembly  130  at a temperature below a thermal shock temperature of the sensor element assembly  130  yet above a sensitivity temperature of the sensing element  140 . In this manner, thermal shock events may be inhibited while ensuring reliable output of the sensing element  140 . 
     With particular reference to  FIG. 3  an exemplary engine system  200  according to the principles of the present disclosure is shown. The engine system  200  may include an engine  102  regulated by an engine control module (ECM)  202  having an improved O 2  sensor control system. 
     Air is drawn into the engine  102  through an intake manifold  104 . A throttle valve  106  may be used to vary the volume of air drawn into the intake manifold  104 . The air mixes with fuel that may be dispensed by one or more fuel injectors  108  to form an air and fuel (A/F) mixture. The A/F mixture is combusted within cylinder  110 . While a single cylinder  110  is shown, the engine  102  may include two or more cylinders. Combustion of the A/F mixture may be initiated by spark provided by a spark plug  112 . Exhaust gas produced during combustion may be expelled from the cylinders to an exhaust system  114 . 
     The exhaust system  114  may include oxygen sensor  116  to measure the amount of oxygen in the exhaust gas. While a single oxygen sensor is shown, the engine system  200  may include two or more oxygen sensors located at various points along the exhaust system  114 . The oxygen sensor  116  outputs a voltage (V O2 ) to the ECM  202  that may be used to determine the quantity of oxygen in the exhaust gas. The oxygen sensor  116  includes heating element  144 . The heating element  144  may receive power from a heater power supply module  204 . 
     The ECM  202  may be used to regulate the operation of the engine system  100 . The ECM  202  may receive the output voltage of the oxygen sensor  116 , along with signals from other sensors  122  of the engine  102 . Based on the output voltage of the oxygen sensor  116  and the signals it receives from the other sensors  122 , the ECM  202  may regulate the A/F mixture by regulating the throttle valve  106  and fuel injectors  108 . 
     The ECM  202  may also be used to regulate the operation of the heating element  144 . More specifically, the ECM  202  may include a heater control module  210  that may be connected to the heater power supply module  204 . The heater control module  210  may output a heater voltage command signal (V h ) to the heater power supply module  204 . The heater control module  210  may vary V h  to raise or lower the temperature of the heating element  144  to ameliorate thermal shock to the sensor element assembly  130 . 
     For example, the heater control module  210  may generate V h  to operate the heating element  144  to maintain the temperature of the sensor element assembly  130  at a first temperature for a period of time after starting the engine  102 . The first temperature may be below a thermal shock temperature of the oxygen sensor  116 . Subsequently, the heater control module  210  may generate V h  to operate the heating element  144  to maintain the temperature of the sensor element assembly  130  at a second temperature higher than the first temperature after a cumulative mass of intake air has been drawn into the engine  102 . The second temperature may be above the thermal shock temperature and/or the sensitivity temperature of the oxygen sensor  116 . A control system and method for the foregoing oxygen sensor heater control strategy is disclosed in Assignee&#39;s commonly owned U.S. Non-provisional application Ser. No. 12/132,653, the disclosure of which is incorporated herein in its entirety by reference. 
     Additionally, the heater control module  210  may generate V h  to operate the heating element  144  at reduced power when the heater control module  210  determines that water condensate has come into contact with the sensor element assembly  130 . In this manner, the heater control module  210  may generate V h  to adjust an operating temperature of the sensor element assembly  130  towards a remedial temperature lower than a normal temperature. More specifically, the heater control module  210  may generate V h  to adjust the operating temperatures of the sensing element  140  and the heating element  144  towards the remedial temperature. 
     With particular reference to  FIG. 4 , the heater control module  210  may include a baseline module  212 , a rate module  214 , a rate comparison module  216 , and a temperature adjustment module  218 . The baseline module  212  receives a current signal (I h,in ) from the heater power supply module  204  and determines whether the sensor element assembly  130  has achieved a baseline operating state. The baseline module  212  may determine whether the sensor element assembly  130  has achieved a baseline operating state in a variety of ways. For example, the baseline module may determine that the sensor element assembly  130  has achieved a baseline operating state when I h,in  is between predetermined limits of a nominal current value associated with the desired operating temperature of the sensor element assembly  130 . The baseline module  212  may generate a BASE signal indicating whether the sensor element assembly  130  has achieved a baseline operating state. The baseline module  212  may output the BASE signal to the temperature adjustment module  218 . 
     The rate module  214  receives I h,in  from the heater power supply module  204  and determines a time rate of change (I h,rate ) in the current supplied to the heating element  144 . The rate module  214  may output I h,rate  to the rate comparison module  216 . 
     The rate comparison module  216  receives I h,rate  from the rate module  214  and determines whether water condensate may have come into contact with the sensor element assembly  130  and may cause a shock event. The rate comparison module  216  may determine that water condensate has contacted the sensor element assembly  130  when I h,rate  is excessive (e.g., above a threshold value). The rate comparison module  216  may generate a SHOCK signal indicating whether I h,rate  is deemed excessive. The rate comparison module  216  may output the SHOCK signal to the temperature adjustment module  218 . 
     The temperature adjustment module  218  receives I h,in  and the BASE and SHOCK signals and determines the heater voltage command signal (V h ) that may be used to adjust the power supplied to the heating element  144  and thereby raise or lower the temperature of the heating element  144 . The temperature adjustment module  218  may determine V h  based on I h,in , BASE, and SHOCK. The temperature adjustment module  218  may also receive other signals from various modules of the ECM  202 . For example, the temperature adjustment module  218  may receive signals, such as, but not limited to, signals indicating a speed and a run time of the engine  102 , a temperature and mass air flow of intake air, and control flags indicating whether the engine system  200  is running properly. The temperature adjustment module  218  may further determine V h  based on the other signals it receives. The temperature adjustment module  218  may output V h  to the heater power supply module  204 . 
     Referring again to  FIG. 3 , the heater power supply module  204  may be used to regulate the power supplied to the heating element  144  based on the heater voltage command signal (V h ) it receives from the ECM  202 . For example, the heater power supply module  204  may regulate one or more of a voltage and a current supplied to the heating element  144 . As discussed herein and shown in the figures, the heater power supply module  204  regulates the voltage supplied to the heating element  144 . 
     Accordingly, the heater power supply module  204  regulates the voltage (V h,in ) supplied to the heating element  144  based on the heater voltage command signal (V h ) it receives from the ECM  202 . The heater power supply module  204  may regulate voltage in a variety of ways. For example, the heater power supply module  204  may regulate a magnitude of the voltage (V h,in ) supplied to the heating element  144 . Alternatively, the heater power supply module  204  may vary a duty cycle of the voltage (V h,in ) supplied to the heating element  144 . In this manner, the heater power supply module  204  may be used to regulate the power supplied to the heating element  144  based on V h . The heater power supply module  204  may also provide a current signal to the ECM  202  indicating the current (I h,in ) supplied to the heating element  144  as previously discussed. 
     With particular reference to  FIG. 5 , an exemplary control method  300  is shown. The control method  300  may be implemented as a supplementary control method to other normal heater power control methods. As used herein, normal heater power control refers to control of the heating element  144  to maintain the sensing element  140  within a desired temperature operating range above the sensitivity temperature of the sensing element  140 . For example, normal heater power control may be used to maintain the temperature of the sensing element  140  to within a few degrees of 650° C. 
     The control method  300  may be implemented using the various modules of the ECM  202  described herein. The control method  300  may be run (i.e. executed) at a periodic interval following starting of the engine  102 . For example, the control method  300  may be run at a periodic interval of six milliseconds or more. Alternatively, the control method  300  may be run based on the occurrence of a particular event (i.e. event based). For example, the control method  300  may be run once a run flag indicating the heating element  144  should be energized is generated by the ECM  202 . As another example, the control method  300  may be run once closed-loop control of the engine  102  has commenced. As discussed herein, the control method  300  is implemented as a supplemental control method to normal heater power control and is run at a periodic interval of six milliseconds following the starting of the engine  102 . 
     Control under the control method  300  begins in step  302  where control initializes control parameters used by the method  300 , such as I h,rate , BASE, SHOCK, and V h . In step  302 , control may set the values of the foregoing parameters to a default value. The default values may correspond to normal heater power control. 
     Control proceeds in step  304  where control determines whether entry conditions are met. If the entry conditions are met, control proceeds in step  306 , otherwise control in the current control loop ends and control loops back as shown. The entry conditions may include various operating conditions of the engine  102  and whether or not a command to operate the heating element  144  has been generated. 
     For example, the entry conditions may depend on whether the engine  102  has achieved a predetermined engine speed (e.g., RPM) and/or a control flag indicating the engine  102  is operating properly has been generated. The entry conditions may depend on whether or not a temperature of the engine and/or intake air is below a predetermined temperature. The entry conditions may depend on whether the engine has been running for a period of time less than a predetermined value of time or has ingested a cumulative amount of intake air less than a predetermined mass. 
     In general, the entry conditions will be met during a period of time following starting of the engine  102  when there is a risk of liquid water coming into contact with the oxygen sensor  116  and operation of the heating element  144  under normal heater power has commenced. Put another way, the general entry conditions may be met when the heating element  144  is being operated above a minimum duty cycle under normal heater power control. 
     In step  306 , control determines whether any exit criterion is met. If the exit criteria are not met, then control proceeds in step  308 , otherwise control proceeds in step  310  where control maintains normal heater power control. The exit criteria may be met when there is an overriding reason to maintain normal heater power control, which may include inhibiting operation of the heating element  144 . For example, the exit criteria may include whether a diagnostic fault related to the oxygen sensor  116  has been generated. 
     In step  308 , control determines a baseline current value based on the I h,in  signal generated by the heater power supply module  204 . The baseline current value may be generated by monitoring the I h,in  signal and applying one or more filtering methods to the value of I h,in . The filtering methods may include a first order lag filter. The filtering methods also may include slow filtering of the I h,in  signal by exponentially weighted moving averages of values of I h,in . In step  308 , control may store the baseline current value in memory of the ECM  202  for retrieval in subsequent control steps. 
     In step  312 , control determines whether stable operation of the heating element  144  has been achieved based on one or more of the baseline current values generated in step  308 . In step  312 , control may generate a BASE signal indicating whether a stable baseline has been achieved. In general, control will determine that a stable baseline has been achieved when the sensing element  140  has been brought to within the desired temperature operating range for a period of time. Control may also determine that a stable baseline has been achieved where an inrush current of the heating element  144  has stabilized. As used herein, inrush current is used to refer to current which rises rapidly during initial operation of the heating element  144 . 
     Control may determine whether a stable baseline has been achieved in a variety of ways. For example, control may determine that the baseline is stable when a number (X) of a number (Y) of successive baseline current values determined in step  308  are within minimum and maximum baseline current values (e.g., I base,min &lt;baseline value&lt;I base,max ). The minimum and maximum baseline current values may be based on a nominal current of the heating element  144  when operating within the desired temperature operating range. The nominal current value may be, for example, between 0.6 and 0.7 amps. The minimum and maximum baseline current values may be based on an expected power of the heating element  144  related to past operation of the engine  102  and the particular operating conditions of the engine  102  when control arrives in step  312 . Values for X, Y, I base,min , and I base,max  may be determined through development testing of the engine system  200  and stored in memory as calibration values used by control method  300 . 
     In step  314 , control determines a time rate of change in the current supplied to the heating element  144  (I h,rate ) based on i h,in . Control may determine the value of I h,rate  in a variety of ways. Control may determine I h,rate  using the I h,in  signal generated by the heater power supply module  204  or using the baseline current values determined in step  308 . The period of time used to determine I h,rate  may be the period of time between successive control cycles (e.g., 6 milliseconds) or may be for a predetermined period of time greater than the period of time between successive control cycles. For example, the period of time used to determine I h,rate  may be around one second. In step  314 , control may store the value of I h,rate  in memory. 
     In step  316 , control determines whether an excessive rise in heater current has occurred, indicating that liquid water may have come into contact with the sensor element assembly  130 . More specifically, control determines whether an excessive rise in heater current has occurred based on a comparison of one or more I h,rate  values determined in step  314  and a threshold current rate value (I rate,thresh ). If control determines an excessive rise in current has occurred, control proceeds in step  318 , otherwise control proceeds in step  320 . In step  316 , control may generate a SHOCK signal indicating whether control has determined an excessive rise in heater current has occurred. 
     Control may determine whether an excessive rise in heater current has occurred in a number of ways. For example, control may compare the most recent I h,rate  value determined in step  314  and I rate,thresh . If the most recent value of I h,rate  is greater than I rate,thresh  then control may determine that an excessive rise in current has occurred. Alternatively, control may compare a consecutive number (W) of the most recent values of I h,rate  and I rate,thresh . If a predetermined number (Z) of the W most recent values of I h,rate  are above I rate,thresh , then control may determine that an excessive rise in current has occurred. Values for W, Z, and I rate,thresh  may be determined through development testing of the engine system  200  and stored in memory as calibration values used by control method  300 . 
     In step  318 , control operates the heating element  144  at a reduced heater power as a remedial measure to lower the temperature of the sensor element assembly  130  and thereby inhibit thermal shock. Control may regulate the power to adjust the operating temperature of the sensor element assembly  130  towards the remedial temperature. Control may further regulate the power to maintain the operating temperature of the sensor element assembly  130  at the remedial temperature. 
     Accordingly, in step  318 , control may generate V h,in  to operate the heating element  144  in order to maintain the temperature of the sensor element assembly  130  below the thermal shock temperature of the sensor element assembly  130 , yet above the sensitivity temperature of the sensing element  140 . Where the thermal shock temperature of the sensor element assembly  130  is below the sensitivity temperature of the sensing element  140 , control may generate V h,in  to maintain the temperature of the sensing element  140  to a temperature at or just above the sensitivity temperature. From step  318 , control in the current control loop ends and control loops back and begins the next control loop in step  314  as shown. 
     In step  320 , control determines whether control is currently operating the heating element  144  at reduced heater power. If control is currently operating the heating element  144  at reduced heater power, control proceeds in step  322 , otherwise control proceeds in step  310 . 
     In step  322 , control determines whether the heater current is continuing to rise, indicating that there may still be liquid water present on the sensor element assembly  130 . More specifically, control determines whether the heater current is continuing to rise based on a comparison of one or more I h,rate  values determined in step  314 . If control determines the heater current is continuing to rise, control proceeds in step  318  where control continues to maintain reduced heater power, otherwise control proceeds in step  310 . 
     Control may determine whether the heater current continues to rise in a number of ways. For example, if the most recent I h,rate  value determined in step  314  is positive (i.e. current value of I h,rate ), control may determine that the heater current is continuing to rise. Alternatively, control may evaluate a consecutive number (S) of the most recent values of I h,rate . If a predetermined number (T) of the S most recent values I h,rate  are positive, then control may determine that the current is continuing to rise. Control may determine that the current is not continuing to rise where a number (U) of the most recent I h,rate  values is not positive. Values for S, T, and U may be determined through development testing of the engine system  200  and stored in memory as calibration values used by control method  300 . 
     In step  310 , control operates the heating element  144  under normal heater power control. From step  310 , control in the current control loop ends and control loops back and begins the next control loop in step  306  as shown. 
     In the foregoing manner, control method  300  may be used to detect the presence of liquid water within the oxygen sensor  116  and regulate the operation of the heating element  144  to ameliorate thermal shock to the various components of the sensor element assembly  130 . Thus, control method  300  may also be used to improve the durability and reliability of the oxygen sensor  116 . 
     Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification, and the following claims.