Patent Publication Number: US-6658345-B2

Title: Temperature compensation system for minimizing sensor offset variations

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to temperature compensation systems, and more specifically to temperature compensation systems for minimizing offset variations in a sensor sensing an operating condition of an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     Modern electronic control systems for internal combustion engines include a number of sensors and/or sensing systems for determining various engine operating conditions. Many of these sensors are located in harsh environments and are subjected to widely varying operating conditions throughout their lives. Despite potentially harsh operating conditions, however, such sensors are typically required to produce consistent results over their entire operating range. 
     An example of one varying environmental condition that many engine operating condition sensors are subject to is temperature. Typically, many engine operating condition sensors are required to operate consistently over a wide temperature range that may include temperatures as low as −40° C. and as high as 150° C. While some engine operating condition sensors tend to operate substantially consistently over a required operating temperature ranges, others do not, Even with those that do not, performance specifications of some such sensors may allow for wide variations in sensor operation over temperature, and in such cases, temperature compensation of the resultant sensor signal is typically not warranted. 
     One solution to the problem of varying sensor operation over temperature is to design the sensor to be robust over temperature and therefore less susceptible to temperature fluctuations. This, however, is typically a costly solution, and designers of engine control systems have accordingly opted for less costly solutions such as temperature compensation of the raw sensor signal. Although typically less costly, conventional temperature compensation schemes for engine operating condition sensors have their own drawbacks. For example, the sensor may exhibit a complicated temperature response that is difficult to model or to counteract with temperature compensation circuitry. Further, the sensor temperature response may vary widely from sensor to sensor. Further still, only a portion of the sensor signal; i.e., either a sensitivity (signal gain) term or a DC offset term, may be susceptible to temperature-induced variations while other portions of the signal are substantially temperature independent. What is therefore needed is a temperature compensation system for minimizing sensor signal variations that addresses these and other drawbacks associated with known sensor compensation strategies. 
     SUMMARY OF THE INVENTION 
     The foregoing shortcomings of the prior art are addressed by the present invention. In accordance with one aspect of the present invention, a temperature compensation system for minimizing sensor offset variations comprising: a sensor producing a sensor signal indicative of an operating condition of an internal combustion engine, means for determining a temperature of said sensor and producing a temperature signal corresponding thereto, a key switch for starting and stopping said engine, said key switch having at least an on position and an off position, and an engine controller responsive to a transition of said key switch to said on position to determine a first temperature signal value and an associated first sensor signal value, said controller responsive to a transition of said key switch to said off position to determine a second temperature signal value and an associated second sensor signal value, said controller defining an offset value associated with said sensor as a function of said first and second temperature signal values and of said first and second sensor signal values. 
     In accordance with another aspect of the present invention, a temperature compensation system for minimizing sensor offset variations comprises a sensor producing a sensor signal indicative of an operating condition of an internal combustion engine, a memory having stored therein a model of said operating condition, said model defining a temperature dependent offset term, means for determining a temperature of said sensor and producing a temperature signal corresponding thereto, a key switch for starting and stopping said engine, said key switch having at least an on position and an off position, and an engine controller monitoring said key switch, said controller responsive to said temperature signal and said sensor signal to determine a first temperature and a first signal value associated with said sensor if said key switch switches to either of said off and on positions, said controller updating said temperature dependent offset term based on said first temperature and said first signal value. 
     In accordance with a further aspect of the present invention, a temperature compensation method of minimizing sensor offset variations comprises the steps of sensing an operating condition of an internal combustion engine with an engine operating condition sensor, computing a value of said engine operating condition based on a model defining a response of said engine operating condition sensor, said model including a temperature dependent offset term, monitoring a key switch for starting and stopping said engine, determining a first operating temperature of said engine operating condition sensor and an associated first sensor value if said key switch switches to either of an off and an on position thereof, and updating said offset term of said model based on said first operating temperature and said first sensor value. 
     One object of the present invention is to provide a temperature compensation system for minimizing variations in a sensor offset parameter. 
     Another object of the present invention is to provide such a system for temperature compensating an offset term of an engine operating condition sensor. 
     A further object of the present invention is to provide such a system for temperature compensating an offset term of a differential pressure sensor in particular, wherein the sensor is disposed across a flow restriction mechanism disposed between an exhaust manifold and an intake manifold of the engine. 
     These and other objects of the present invention will become more apparent from the following description of the preferred embodiments. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic illustration of one preferred embodiment of a temperature compensation system for minimizing sensor offset variations, in accordance with the present invention. 
     FIG. 2 is a flowchart illustrating one preferred embodiment of a software algorithm for adaptively updating a sensor transfer function, in accordance with the present invention. 
     FIG. 3 is a flowchart illustrating an alternate embodiment of a software algorithm for adaptively updating a sensor transfer function, in accordance with the present invention. 
     FIG. 4 is a flowchart illustrating one preferred embodiment of a software algorithm for executing the routine illustrated in the dashed-line blocks of the algorithms of FIGS. 2 and 3. 
     FIG. 5 is a plot of ΔP sensor error vs. ΔP signal value illustrating performance benefits of the present invention with a ΔP sensor over those of conventional ΔP sensors signal processing techniques. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to a number of preferred embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated embodiments, and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Referring now to FIG. 1, one preferred embodiment of a temperature compensation system  10  for minimizing sensor offset variations, in accordance with the present invention, is shown. System  10  includes an internal combustion engine  12  having an intake manifold  14  fluidly coupled to ambient via intake conduit  16 . An exhaust manifold  18  is fluidly coupled to ambient via exhaust manifold  20 , and an exhaust gas recirculation (EGR) conduit  22  has a first end fluidly coupled to the exhaust manifold  18  and a second end fluidly coupled to the intake manifold  14 . EGR conduit  22  preferably includes a flow restriction mechanism  24  disposed in line therewith, and may optionally include an EGR cooler  26  disposed between the flow restriction mechanism  24  and the intake manifold  14 , as shown in phantom, for cooling the exhaust gas supplied to intake manifold  14 . System  10  may further include other air handling components (not shown) that are commonly known and used in the automotive and diesel engine industries including, but not limited to, a turbocharger, wastegate and/or exhaust throttle. 
     Central to system  10  is an engine controller  28  that is preferably microprocessor-based and is generally operable to control and manage the overall operation of engine  12 . Engine controller  28  includes a memory unit  64  as well as a number of inputs and outputs for interfacing with various sensors and systems coupled to engine  12 . Controller  28 , in one embodiment, may be a known control unit sometimes referred to as an electronic or engine control module (ECM), electronic or engine control unit (ECU) or the like, or may alternatively be a general control circuit capable of operation as described hereinafter. 
     In accordance with the present invention, engine controller  28  includes a sensor offset compensation block  38  receiving a number of inputs from various sensors and/or control mechanisms associated with the operation of internal combustion engine  12 . For example, system  10  includes a differential pressure sensor (so-called ΔP sensor)  30  having one end fluidly coupled to the EGR conduit  22  downstream of the flow restriction mechanism  24  via conduit  32 , and an opposite end fluidly coupled to EGR conduit  22  upstream of flow restriction mechanism  24  via conduit  34 . Sensor  30  is electrically connected to a ΔP input of sensor offset compensation block  38  via signal path  36 , wherein sensor  30  is operable to supply compensation block  38  with a signal indicative of a pressure difference across flow restriction mechanism  24 . It is to be understood that although FIG. 1 is illustrated as including a temperature compensation strategy for minimizing temperature variations in a ΔP sensor signal, the present invention contemplates that the sensor  30  may alternatively be another engine operating condition sensor for which temperature compensation of the sensor signal is desired. Those skilled in the art will recognize known engine operating condition sensors wherein it would be desirable to temperature compensate signals produced thereby, and such other engine operating condition sensors are intended to fall within the scope of the present invention. While temperature compensation of such other sensors is contemplated, however, the following description will be limited to a ΔP sensor  30  for brevity. 
     In accordance with one aspect of the present invention, the operating temperature of ΔP sensor  30  is preferably determined by thermally coupling sensor  30  to a structural component of engine  12  having a known or readily ascertainable operating temperature. In one preferred embodiment, as shown by example in FIG. 1, engine  12  includes a cooling system  40  having a coolant temperature sensor  42  in fluid communication therewith and electrically connected to a temperature input (TMP) of sensor offset compensation block  38  via signal path  44 . Engine coolant temperature is generally believed to be the most stable and well understood fluid temperature of engine  12 , and by thermally coupling the ΔP sensor  30  to the cooling system  40  and monitoring the coolant temperature sensor  42 , the temperature of the ΔP sensor  30  may be accurately determined. In one embodiment, sensor  30  is thermally coupled to cooling system  40  via a suitable heat sink arrangement so that sensor  30  is at substantially the same temperature as the coolant fluid contained within cooling system  40 . Alternatively, sensor  30  may be designed with a coolant passage therethrough such that coolant fluid from system  40  may be directed through sensor  30  to maintain it at substantially the same temperature as that of cooling system  40 . In any case, the thermal coupling of sensor  30  to cooling system  40  is preferably made in such a manner that the operating temperature of sensor  30  is substantially the same as that of cooling system  40 , and any known technique for accomplishing this goal is intended to fall within the scope of the present invention. 
     As an alternative to cooling system  40 , the present invention contemplates thermally coupling sensor  30  either directly to the engine  12 , wherein system  10  preferably includes an engine temperature sensor of known construction that is operable to provide sensor compensation block  38  with a temperature signal indicative of engine operating temperature. Alternatively still, the present invention contemplates thermally coupling sensor  30  to a structural component of engine  12  having an operating temperature that is either known of readily ascertainable. For example, sensor  30  may be thermally coupled to intake manifold  14 , wherein manifold  14  typically includes an intake manifold temperature sensor operable to produce a signal indicative of intake manifold temperature. Alternatively, engine controller  28  may include a so-called “virtual” intake manifold temperature sensor in the form of a software algorithm that is operable to estimate the temperature of the intake manifold  14  as a function of other engine operating conditions. In either case, sensor  30  may be thermally coupled to, or disposed in fluid communications with, intake manifold  14  such that the operating temperature of sensor  30  is substantially the same as that of the intake manifold  14 . As another example, system  10  may include a turbocharger (not shown) having a turbocharger compressor supplying fresh air from ambient to the intake manifold  14  as is known in the art. In this case, sensor  30  may be thermally coupled to an air outlet of the turbocharger compressor, in which case engine controller  28  may include a “virtual” compressor outlet temperature sensor in the form of a software algorithm that is operable to estimate a compressor outlet temperature based on other engine operating signals. In this case, sensor  30  is preferably thermally coupled to, or disposed in fluid communications with, the compressor outlet such that the operating temperature of sensor  30  is substantially the same as that of the turbocharger compressor outlet. It is to be understood, however, that while the intake manifold and/or turbocharger compressor outlet temperature sensors will generally produce temperature signals substantially indicative of the operating temperature of sensor  30  if coupled thereto, these temperatures may vary widely, and are therefore less preferred over operating temperatures that stabilize over a much narrower operating temperature range. Moreover, the actual operating temperature of sensor  30  may in some cases be significantly greater than that of the intake manifold  14  and/or turbocharger compressor outlet due to exposure of the sensor  30  to high temperature exhaust gases, and care must therefore be taken to ensure that the thermal coupling of sensor  30  to either the intake manifold or turbocharger compressor outlet is adequate to regulate the operating temperature of sensor  30  to that of its underlying structure. 
     Regardless of the location of sensor  30  in relation to any structural component of engine  12 , the present invention contemplates that the operating temperature of sensor  30  may alternatively be determined by a temperature sensor  46  thermally coupled to sensor  30  and providing a corresponding temperature signal to the temperature input (TMP) of block  38  via signal path  48 . In one embodiment, temperature sensor  46  is a thermocouple operable to produce a temperature signal indicative of the operating temperature of sensor  30 , although the present invention contemplates using other known temperature sensors. 
     System  10  further includes a key switch  50  of known construction and electrically connected to a key switch input (K) of sensor offset compensation block  38  via signal path  52 . Key switch  50 , as is known in the art, includes an “off” position, an “on” position and a “crank” position, and signal path  52  preferably carries a signal indicative of the operational state of key switch  50  as just described. 
     Optionally, as will be described in further detail hereinafter, system  10  may include an ambient temperature sensor  54  that is electrically connected to an ambient temperature input (AT) of sensor offset compensation block  38  via signal path  56 , as shown in phantom in FIG.  1 . In operation, sensor  54  is operable to produce a temperature signal indicative of the ambient temperature about system  10 . Engine controller  28  may optionally include a timer  62  connected to a timer input (T) of sensor offset compensation block  38 . In operation, compensation block  38  may reset timer  62 , and timer  62  is otherwise operable to provide compensation block  38  with a time signal indicative of an elapsed time since its most recent reset. 
     In the embodiment shown in FIG. 1, the flow restriction mechanism  24  is preferably an EGR valve of known construction, wherein sensor offset compensation block  38  includes an EGR output electrically connected to an EGR valve actuator  58  via signal path  60 . In this embodiment, EGR valve  24  defines a variable cross-sectional flow area therethrough, and the sensor offset compensation block  38  is operable, as will be described in greater detail hereinafter, to control the position of EGR valve  24  to ensure that valve  24  is open during data gathering operation of the sensor offset compensation block  38 . In an alternative embodiment, the flow restriction mechanism  24  may be a passive flow restriction mechanism defining a fixed cross-sectional flow area therethrough. In this case, the EGR output of sensor offset compensation block  38  may be omitted. 
     In accordance with another aspect of the present invention, the sensor offset compensation block  38  of engine controller  28  preferably includes a software algorithm for gathering data relating to the operation of sensor  30  for a number of operating temperature conditions under known zero ΔP conditions, for the purpose of defining the relationship between the sensor&#39;s offset voltage and the sensor&#39;s operating temperature. In one preferred embodiment, low temperature (at zero ΔP) data are gathered at key-on, prior to engine start up, and high temperature (at zero ΔP) data are gathered at key-off (engine shutdown), preferably after engine and turbocharger speed have reached zero. 
     For systems wherein ΔP is measured across an EGR valve  24  as illustrated in FIG. 1, the EGR valve  24  is preferably controlled by block  38  to a fully open position during the data gathering operations to ensure that the sensor voltage measurements are not corrupted by any residual pressures acting upon sensor  30  from either its fresh air side or its exhaust gas side. Opening the EGR valve  24  under data gathering operations reduces the impact of any such static pressures by allowing the pressure across the valve  24  to substantially equalize. In any case, at least cold start and hot shutdown data are preferably gathered over the life of the engine  12  to provide for continual temperature offset calibration of sensor  30  as well as for diagnostic trending purposes. In its simplest form, the sensor offset compensation block  38  of the present invention is operable to gather one cold (pre-start) temperature operational value for sensor  30  under zero ΔP conditions and one hot (post-shutdown) temperature operational value for sensor  30  under zero ΔP conditions, and to establish a linear relationship therebetween defining the offset signal behavior of sensor  30  as a function of its operating temperature. Alternatively, additional operational values for sensor  30  under zero ΔP conditions may be gathered as the sensor  30  cools following engine shutdown to thereby allow more accurate modeling of the offset signal behavior of sensor  30  as a function of its operating temperature. 
     In one embodiment of engine controller  28 , the sensor offset compensation block  38  includes a model of the differential pressure across flow restriction mechanism  24 , wherein the model preferably includes a temperature-dependent offset term and a substantially temperature-independent gain or sensitivity term. In one embodiment, the ΔP model stored in memory  64  is preferably defined by a transfer function of the form: 
     
       
         Δ P=[a+b×T   ΔP   ]+c×ΔPV,   
       
     
     where, 
     ΔP is the true differential pressure across flow restriction mechanism  24 , 
     “a” is a constant defining a base pressure offset (in psid), 
     “b” is a constant defining an offset temperature gain (in psid/°F.), 
     T ΔP  is the temperature of the ΔP sensor  30  (in °F.), 
     c is a constant defining a mean pressure gain (in psid/VDC), and 
     ΔPV is the operating voltage produced by ΔP sensor  30 . 
     The sensor offset compensation block  38  is operable, in accordance with the present invention, to continually compute at least some of the constants in the foregoing ΔP transfer function based on readings of the sensor voltage and sensor temperature. Preferably, the transfer function constants are computed as a function of such readings taken at different temperatures under operating conditions wherein it is known that ΔP=0 (e.g., when engine  12  is not running). As described briefly hereinabove, the sensor offset compensation block  38  is preferably responsive to transitions of the key switch  50  between “off” and “on” positions to conduct voltage and temperature measurements for sensor  30 . In one embodiment, “c” is a predetermined mean population pressure gain constant stored in memory  64  and based on an established sensor population mean, and constants “a” and “b” are determined by taking measurements under cold; i.e., engine pre-start, conditions and “hot”; i.e., engine shutdown, conditions. In this embodiment, constants “a” and “b” may therefore be determined by solving the transfer function under 0 ΔP conditions at the two temperature extremes which yields the equations: 
     
       
           b=c ( V   C   −V   H )/( T   H   −T   C ) 
       
     
     and, 
     
       
           a=−c×V   C   −b×T   C , 
       
     
     where, 
     V C  is the (cold) signal voltage produced by ΔP sensor  30  when the key switch  50  transitions from the “off” to the “on” position (e.g., engine pre-start), 
     V H  is the (hot) voltage signal produced by ΔP sensor  30  when key switch  50  transitions from its “on” to its “off” state (e.g., at engine shutdown), 
     T H  is the (hot) temperature of the ΔP sensor  30  when the key switch  50  transitions from its “on” state to its “off” state, and 
     T C  is the (cold) temperature of the ΔP sensor  30  when the key switch  50  transitions from its “off” state to its “on” state. 
     It will be noted that the foregoing equations define the offset term of the ΔP transfer function as a linear function of temperature, although the present invention contemplates embodiments of the sensor offset compensation block  38  wherein a number of additional voltage/temperature readings may be made after the engine  12  has been shut down and as the temperature of the ΔP sensor  30  ramps down from its hot operating temperature (e.g., engine coolant temperature) to ambient. Moreover, the sensor offset compensation block  38  is preferably only operational after extended non-operational periods of engine  12  so as to ensure reasonably isothermal conditions between the ΔP sensor  30  and the sensor producing the signal indicative of the operating temperature of the ΔP sensor  30 . 
     Referring now to FIG. 2, a flowchart is shown illustrating one preferred embodiment of a software algorithm  100  for adaptively updating the sensor transfer function described hereinabove. Algorithm  100  is preferably stored within the memory unit  64  of engine controller  28 , and is executed by the engine controller  28  to update the constants of the ΔP sensor transfer function as described above. Preferably, constants “a” and “b” are initially (i.e., when the engine is new and/or when engine controller  28  is newly calibrated) preset to reasonable values therefore, and are updated at each transition of key switch  50  as will be described in greater detail hereinafter. 
     Algorithm  100  begins at step  102 , and at step  104  engine controller  28  is operable to monitor the key switch  50 . Thereafter at step  106 , if engine controller  28  determines that the key switch  50  has been activated, algorithm execution advances to step  108 . Otherwise, algorithm  100  loops back to step  104 . If, at step  106 , engine controller  28  determines that the key switch  50  has been activated, engine controller  28  is operable at step  108  to open the EGR valve if the EGR flow restriction mechanism  24  is embodied as an EGR valve. If the EGR flow restriction mechanism  24  is instead embodied as a fixed cross-sectional flow area mechanism, step  108  may be omitted. In any case, algorithm execution continues at step  110  where engine controller  28  is operable to sense the temperature of the ΔP sensor  30  using any of the techniques discussed hereinabove with respect to FIG.  1 . Thereafter at step  112 , engine controller  28  is operable to sense ambient temperature, preferably via ambient temperature sensor  54 . Following step  112 , algorithm execution advances to step  114  where controller  28  is operable to determine a temperature difference ΔT as an absolute value of the difference between the sensor temperature value determined at step  110  and the ambient temperature value determined at step  112 . 
     Following step  114 , engine controller  28  is operable at step  116  to determine the state of the key switch resulting from the key switch activity detected at step  106 . If the key switch activity detected at step  106  corresponded to a switch from its “on” position to its crank position, algorithm execution loops back to step  104 . If engine controller  28  determines at step  116  that the key switch  50  has switched from its “off” position to its “on” position, this corresponds to an engine pre-start condition and engine controller  28  is operable thereafter at step  118  to compare the ΔT value determined at step  114  with a temperature threshold value T 1 . If, at step  118 , engine controller  28  determines that ΔT is less than T 1 , algorithm execution advances to step  120  where engine controller  28  is operable to set a low temperature term (T L ) to the sensor temperature value TMP determined at step  110 . Thereafter at step  122 , engine controller  28  is operable to determine the current operating voltage (ΔPV) of the ΔP sensor  30  and to set a low temperature voltage value (V L ) to the ΔPV value at step  122 . 
     If, at step  116 , engine controller  28  determines that the key switch activity detected at step  106  corresponds to a switch of the key position from its “on” position to its “off” position, algorithm execution advances to step  128  where engine controller  28  is operable to compare the sensor temperature value (TMP) determined at step  110  with another temperature threshold value T 2 . If engine controller  28  determines that the sensor temperature value TMP is greater than T 2 , algorithm execution advances to step  130  where engine controller  28  is operable to set a high temperature value (T H ) to the temperature value TMP of the sensor determined at step  110 . Thereafter at step  132 , engine controller  28  is operable to sense the operating voltage (ΔPV) of the ΔP sensor  30 , and thereafter at step  134  to set a high temperature voltage value (V H ) to the ΔPV value. Algorithm  100  may optionally include a step  136  wherein engine controller  28  may be operable to gather additional temperature and voltage information relating to the ΔP sensor  30  as it cools following engine shutdown, and details of one preferred embodiment of step  136  will be described hereinafter with respect to FIG.  4 . In any case, algorithm execution advances from step  124  or step  136  to step  126  where engine controller  28  is operable to update the values of the ΔP transfer function constants. 
     In one embodiment, wherein engine controller  28  is operable to determine the ΔP transfer function constants based on two temperature extremes T L  and T H , engine controller  28  is preferably operable at step  126  to update the ΔP transfer function constants “a” and “b” based on an application of the equations described hereinabove. It should be apparent that in this embodiment, any single traversal of algorithm  100  produces only a single “set” of sensor temperature and sensor voltage data; i.e., either T H  and V H  or T L  and V L . In this case, engine controller  28  is preferably operable to update constants “a” and “b” using the sensor temperature and voltage values just obtained along with most recent values of the opposite sensor and temperature and voltage values. In this manner, the transfer function constants “a” and “b” will reflect operating conditions including those relating to the most recent key switch transition. 
     In an alternate embodiment, wherein the engine controller  28  is operable to determine the ΔP transfer function constants based on sensor voltage and temperature information at more than two operating temperatures, engine controller  28  is preferably operable at step  126  to update the ΔP transfer function constants based on any known data fitting technique such, for example, known least squares methods. As with the previous embodiment, engine controller  28  is preferably operable to update constants “a”, “b” and “c”) using the sensor temperature and voltage values just obtained along with most recent values of the opposite sensor and temperature and voltage values. In this manner, the transfer function constants “a”, “b” and “c” will reflect operating conditions including those relating to the most recent key switch transition. 
     Step  126 , as well as the “no” branches of steps  116  and  128 , advance to step  138  where engine controller  28  is operable to compute a compensated ΔP value (ΔP C ) as a function of the current ΔP transfer function. Algorithm execution advances from step  138  to step  104 . 
     It should be apparent that algorithm  100  illustrated and described with respect to FIG. 2 is operable to measure both the operating temperature of sensor  30  and the output voltage produced by sensor  30  after the engine is turned off and prior to engine start up. In order to ensure that the engine has been running sufficiently long to bring the engine temperature (and hence the engine coolant temperature) up to a typical operating temperature prior to measuring “hot” data, step  128  is included to compare the sensor temperature TMP to a temperature threshold T 2 . Preferably, T 2  is set to a temperature above which is considered a normal operating temperature of engine  12 , and “hot” data relating to sensor  30  is only gathered if TMP is above T 2 . Likewise, it is preferable to ensure that the engine  12  has cooled sufficiently following shutdown to allow the temperature to decay to ambient temperature prior to measuring “cold” data. Steps  112 ,  114  and  118  are included to accomplish this goal wherein ΔT represents the difference between the current sensor temperature TMP and the current ambient temperature AT, and wherein T 1  is a temperature threshold below which TMP is considered to be sufficiently close to AT to allow the gathering of “cold” data. Those skilled in the art will recognize that the numerical values of T 1  and T 2  are a matter of design choice, and any values selected for T 1  and T 2  are intended to fall within the scope of the present invention. 
     Referring now to FIG. 3, a flowchart is shown illustrating an alternate embodiment of a software algorithm  200  for adaptively updating the sensor transfer function described hereinabove. Algorithm  200  is preferably stored within the memory unit  64  of engine controller  28 , and is executed by the engine controller  28  to update the constants of the ΔP sensor transfer function as described hereinabove. As with algorithm  100 , algorithm  200  preferably requires constants “a” and “b” to be initially (i.e., when the engine is new and/or when engine controller  28  is newly calibrated) preset to reasonable values therefore, and are thereafter updated at each on/off transition of key switch  50  as will be described in greater detail hereinafter. 
     Algorithm  200  begins at step  202 , and at step  204  engine controller  28  is operable to monitor the key switch  50 . Thereafter at step  206 , if engine controller  28  determines that the key switch  50  has been activated, algorithm execution advances to step  208 . Otherwise, algorithm  200  loops back to step  204 . If, at step  206 , engine controller  28  determines that the key switch  50  has been activated, engine controller  28  is operable at step  208  to open the EGR valve if the EGR flow restriction mechanism  24  is embodied as an EGR valve. If the EGR flow restriction mechanism  24  is instead embodied as a fixed cross-sectional flow area mechanism, step  208  may be omitted. In any case, algorithm execution continues at step  210  where engine controller  28  is operable to determine the state of the key switch resulting from the key switch activity detected at step  206 . If the key switch activity detected at step  206  corresponds to a switch from its “on” position to its crank position, algorithm execution loops back to step  204 . 
     If engine controller  28  determines at step  210  that the key switch  50  has switched from its “off” position to its “on” position, this corresponds to an engine pre-start condition and engine controller  28  is operable thereafter at step  212  to compare a time value (TIMER) of timer  62  (FIG. 1) to a predefined time value T 1 . If engine controller  28  determines that TIMER is greater than T 1 , algorithm execution advances to step  214  where engine controller  28  is operable to determine an operating temperature (TMP) of sensor  30  using any one or more of the techniques described hereinabove with respect to FIG.  1 . Thereafter at step  216 , engine controller  28  is operable to set a low temperature term (T L ) to the sensor temperature value TMP determined at step  214 . Thereafter at step  218 , engine controller  28  is operable to determine the current operating voltage (ΔPV) of the ΔP sensor  30 , and to set a low temperature voltage value (V L ) to the ΔPV value at step  220 . Following step  220 , algorithm execution advances to step  224  where engine controller  28  is operable to reset the timer  62  to a default value; e.g., zero. 
     If, at step  210 , engine controller  28  determines that the key switch activity detected at step  206  corresponds to a switch of the key position from its “on” position to its “off” position, algorithm execution advances to step  228  where engine controller  28  is operable to compare the time value (TIMER) of timer  62  to a second predefined time threshold T 2 . If engine controller  28  determines that TIMER is greater than T 2 , algorithm execution advances to step  230  where engine controller  28  is operable to determine an operating temperature (TMP) of sensor  30  using any one or more of the techniques described hereinabove with respect to FIG.  1 . Thereafter at step  232 , engine controller  28  is operable to set a high temperature term (T H ) to the sensor temperature value TMP determined at step  230 . Thereafter at step  234 , engine controller  28  is operable to determine the current operating voltage (ΔPV) of the ΔP sensor  30 , and to set a high temperature voltage value (V H ) to the ΔPV value at step  236 . Following step  236 , algorithm execution advances to step  238  where engine controller  28  is operable to reset the timer  62  to its default value; e.g., zero. 
     Algorithm  200  may optionally include a step  240  wherein engine controller  28  may be operable to gather additional temperature and voltage information relating to the ΔP sensor  30  as it cools following engine shutdown, and details of one preferred embodiment of step  240  will be described hereinafter with respect to FIG.  4 . In any case, algorithm execution advances from step  224  or step  240  to step  226  where engine controller  28  is operable to update the values of the ΔP transfer function constants. 
     In one embodiment, wherein engine controller  28  is operable to determine the ΔP transfer function constants based on two temperature extremes T L  and T H , engine controller  28  is preferably operable at step  226  to update the ΔP transfer function constants “a” and “b” based on an application of the equations described hereinabove. It should be apparent that in this embodiment, any single traversal of algorithm  200  produces only a single “set” of sensor temperature and sensor voltage data; i.e., either T H  and V H  or T L  and V L . In this case, engine controller  28  is preferably operable to update constants “a” and “b” using the sensor temperature and voltage values just obtained along with most recent values of the opposite sensor and temperature and voltage values. In this manner, the transfer function constants “a” and “b” will reflect operating conditions including those relating to the most recent key switch transition. 
     In an alternate embodiment, wherein the engine controller  28  is operable to determine the ΔP transfer function constants based on sensor voltage and temperature information at more than two operating temperatures, engine controller  28  is preferably operable at step  226  to update the ΔP transfer function constants (optionally including constant “c”) based on any known data fitting technique such, for example, known least squares methods. As with the previous embodiment, engine controller  28  is preferably operable to update constants “a”, “b” and “c”) using the sensor temperature and voltage values just obtained along with most recent values of the opposite sensor and temperature and voltage values. In this manner, the transfer function constants “a”, “b” and “c” will reflect operating conditions including those relating to the most recent key switch transition. 
     Step  226 , as well as the “no” branches of steps  212  and  228 , advance to step  242  where engine controller  28  is operable to compute a compensated ΔP value (ΔP C ) as a function of the current ΔP transfer function. Algorithm execution advances from step  242  back to step  104 . 
     It should be apparent that, like algorithm  100 , algorithm  200  illustrated and described with respect to FIG. 3 is operable to measure both the operating temperature of sensor  30  and the output voltage produced by sensor  30  after the engine is turned off and prior to engine start up. However, in order to ensure that the engine has been running sufficiently long to bring the engine temperature (and hence the engine coolant temperature) up to a typical operating temperature prior to measuring “hot” data, step  228  is included to compare the time value (TIMER) of timer  62  to a timer threshold T 2 . Preferably, T 2  is set to a time value above which is considered a sufficient time for engine  12  to reach a normal operating temperature, and “hot” data relating to sensor  30  is only gathered if TIMER is above T 2 . Likewise, it is preferable to ensure that the engine  12  has cooled sufficiently following shutdown to allow the temperature to decay to ambient temperature prior to measuring “cold” data. Step  212  is included to accomplish this goal wherein T 1  represents a time value above which is considered a sufficient time for engine  12  to cool to near ambient temperature, and “cold” data relating to sensor  30  is only gathered if TIMER is above T 1 . Those skilled in the art will recognize that the numerical values of T 1  and T 2  are a matter of design choice, and any values selected for T 1  and T 2  are intended to fall within the scope of the present invention. 
     Referring now to FIG. 4, one preferred embodiment of a software routine for executing step  136  of algorithm  100  or step  240  of algorithm  200 , in accordance with the present invention, is shown. The software routine begins at step  300  wherein engine controller  28  is operable to monitor the operating temperature (TMP) of sensor  30  using any of the techniques described hereinabove. Thereafter at step  302 , engine controller  28  is operable to compare the sensor operating temperature value TMP with a first mid-temperature value T MID1 , wherein T MID1  represents a temperature between low temperature T L  and high temperature T H . As long as TMP is not equal to T MID1 , step  302  loops back to step  300 . However, as the operating temperature of sensor  30  slowly cools, its temperature TMP will eventually reach T MID1 , and when it does algorithm execution advances to step  304  where engine controller  28  is operable to set a first mid-temperature term (T MID1 ) to the sensor temperature value TMP determined at step  300 . Thereafter at step  306 , engine controller  28  is operable to determine the current operating voltage (ΔPV) of the ΔP sensor  30 , and to set a first mid-temperature voltage value (V MID1 ) to the ΔPV value at step  308 . Following step  308 , the software routine illustrated in FIG. 4 may include steps  310 - 318  that are identical to steps  300 - 308  except that they are configured for gathering sensor operating temperature and sensor operating voltage at a second mid-temperature value T MID2 , wherein T MID2 &lt;T MID1 . Thus, as the operating temperature of sensor  30  cools below T MID1 , it will eventually reach T MID2  wherein engine controller  28  may optionally be operable to gather operating information relating to sensor  30 . In fact, the present invention contemplates that the software routine illustrated in FIG. 4 may include any desired number of sets of steps  310 - 318  for gathering operational information relating to sensor  30  at a corresponding number of temperature values between T H  and T L . Either of algorithms  100  and  200  may then use this additional information in a known manner to provide a more accurate definition of the sensor model offset term. 
     Referring now to FIG. 5, a plot of ΔP error (in % of value) vs. ΔP value (in psid) is shown comparing results of conventional ΔP measuring techniques with that of the present invention over a temperature range of −40° C. to 125° C. Curves  400  and  402  represent the maximum and minimum error envelopes respectively of the conventional ΔP measuring technique over a range of ΔP from 0.0 to 5.0 psid. In comparison, curves  404  and  406  represent the maximum and minimum error envelopes respectively of the ΔP measuring technique of the present invention over the same ΔP pressure range. Inspection of FIG. 5 reveals that the concepts of the present invention yield a substantial increase in accuracy over conventional ΔP measurement techniques. While the invention has been illustrated and described in detail in the foregoing drawings and description, the same is to be considered as illustrative and not restrictive in character, it being understood that only preferred embodiments thereof have been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.