Abstract:
A method for verifying the validity of an output of a particulate matter sensor mounted in an engine exhaust system downstream of a diesel particulate filter, the particulate matter sensor including a pair of electrodes spaced apart from each other, includes initiating regeneration of the diesel particulate filter, applying and maintaining a higher than nominal voltage across the electrodes following the step of initiating regeneration of the diesel particulate filter, and measuring an electrical parameter across the electrodes while the higher voltage is applied across the electrodes, where the electrical parameter is indicative of an amount of soot accumulated on the sensor. The reading of accumulated soot is evaluated to determine whether the sensor is indicating that the amount of accumulated soot is within an expected range based on a clean diesel particulate filter and the elevated applied voltage.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to sensors for detecting electrically conductive particulate matter, such as soot, and more particularly to a method and system for diagnosing potential failure modes in such sensors. 
     Incomplete combustion of certain heavy hydrocarbon compounds, such as heavy oils, diesel fuel, and the like may lead to particulate formation (e.g., soot). In the operation of internal combustion engines, excessive particulate formation can lead to “smoking” of the engine, which causes air pollution even though the carbon monoxide, hydrocarbons, and other pollutant components of the gaseous state exhaust emissions may be relatively low. Emission regulations require many engines to limit the levels of particulate emissions, and various control technologies such as diesel particulate filters (DPF) have been employed for this purpose. 
     In order to monitor the emission of particulate matter in the exhaust streams of certain types of internal combustion engines, e.g., to assess the effectiveness of DPF&#39;s, it is known to provide a particulate sensor system for detecting the level of particulate concentration emitted from an exhaust gas. Various particulate sensors have been proposed, including those shown in U.S. Pat. No. 4,656,832 issued to Yukihisa et al., U.S. Pat. No. 6,634,210 issued to Bosch et al., U.S. Pat. No. 7,954,230 issued to Nelson et al., U.S. Pat. Publ. No. 2008/0283398 A1, U.S. Pat. Publ. No. 2008/0282769 A1, U.S. Patent Application Publication No. 2012/0119759 A1, and U.S. Patent Application Publication No. 2013/0002271 A1, the disclosures of each of which are hereby incorporated by reference in their entirety. 
     Particulate sensors such as those described above generally have a pair of spaced apart sensing electrodes disposed on a substrate. The sensing electrodes are coupled to a measurement circuit by way of electrically conductive leads. The operating principle of the particulate sensor is based on the conductivity of the particulates (e.g., soot) deposited between the sensing electrodes. The electrical resistance between the sensing electrodes is relatively high when the sensor is clean but such resistance decreases as soot particulates accumulate. These sensors also have a heater that can be selectively activated to burn off the soot particulates to “reset” the sensor to a known, base “clean” state. 
     Regulatory agencies may require that a particulate sensor system has self-diagnostic capability to identify a failure of the particulate sensor to perform its primary function of measuring soot. However, for diagnostic purposes, it can be difficult to distinguish between various states that may occur during various engine operating conditions, such as between: (i) a faulty state such as when the sensor is “poisoned” by a non-conductive or semi-conductive contaminant deposited on the electrodes preventing soot from contacting the electrodes, which presents as a very high resistance between the sensing electrodes, and (ii) a normal state, such as when a sensor has just been cleaned and the DPF is working properly (i.e. preventing soot from passing through to the sensor), which also presents as a very high resistance. 
     Accordingly, there is a need for particulate sensor diagnostics that can accurately verify particulate sensor operation with a properly operating DPF. 
     BRIEF SUMMARY OF THE INVENTION 
     In a first aspect of the invention, a method for verifying the proper operation of a particulate matter sensor is presented. The method comprises applying an elevated voltage to the sensor after regeneration of a diesel particulate filter in an engine exhaust stream located upstream of the sensor has commenced, monitoring the indicated resistance across the sensing electrodes of the sensor with the elevated voltage applied, and evaluating the behavior of the indicated resistance across the sensing electrodes to determine if the behavior is consistent with the behavior that would be expected from a properly-operating sensor. After a period of time, the voltage applied to sensor is reduced to a nominal voltage, the nominal voltage being of a lower magnitude than the elevated voltage. 
     In a further aspect of the invention, control of the engine is perturbed so as to increase soot emissions from the engine for a short period of time while the sensor is being operated at the elevated voltage level. The soot accumulation determined by the sensor is compared to a predetermined estimate of soot rate downstream of a properly operating diesel particulate filter under these special engine conditions to verify that the sensor is able to measure the soot. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified schematic diagram showing a particulate control system in an engine exhaust system. 
         FIG. 2  is an electrical schematic of a particulate matter sensing system. 
         FIG. 3  is a chart showing the dependence of particulate sensor response time on applied voltage across the electrodes of the particulate sensor. 
         FIG. 4  is a chart showing a soot mass rate downstream of a diesel particulate filter in a time interval around the regeneration time of the diesel particulate filter. 
         FIG. 5  is a flowchart depicting elements of a first embodiment of the present invention. 
         FIG. 6  is a flowchart depicting elements of a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     At the outset of the description, it should be noted that the terms “first,” “second,” and the like, herein do not denote any order, quantity, or importance, but rather are used to distinguish one element from another, and the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced items. The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity). It is noted that the terms “left”, “right”, “horizontal”, “vertical”, “bottom”, and “top” are used herein, unless otherwise noted, merely for convenience of description, and are not limited to any one position or spatial orientation. Finally, unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs. 
     As described above, diesel particulate filter (DPF) is commonly used to prevent soot from exiting the tailpipe of an exhaust system used with an internal combustion engine. A simplified schematic diagram of a particulate control system in an engine exhaust system is presented in  FIG. 1 . Air and fuel enter an engine  112 , where combustion of an air/fuel mixture takes place. Combustion byproducts from the engine  112  are exhausted through a first exhaust pipe  114  to a DPF  116 . After passing through the DPF, the exhaust gas passes through a second exhaust pipe  118  to be exhausted to atmosphere. A particulate matter (PM) sensor  240  is located so as to be exposed to the exhaust gas downstream of the DPF  116 . The PM sensor  240  is used to detect if the exhaust gas in the second exhaust pipe  118  includes an amount of soot that would be indicative of a failure, such as a crack, of the DPF  116 . A controller  122  is also depicted in FIG.  1  with an interface to the PM sensor  240 . The controller  122  measures the resistance across the electrodes of the PM sensor  240  and controls the heater used to periodically clean soot from the PM sensor  240 . While shown as a single controller  122  that also interfaces with the engine  112 , it will be appreciated that the functions related to engine control and the functions related to PM sensor interface may be partitioned differently, e.g. using a plurality of controllers including one or more separate controllers dedicated to control and measurement functions associated with the PM sensor  240  and communicating with an engine controller  122 . 
     The DPF  116  typically includes a porous element through which exhaust gas is passed. Pore size is selected so as to trap soot particles in the DPF  116 . As soot accumulates in the DPF  116 , the pores become clogged and flow restriction (backpressure) through the DPF increases. The DPF  116  must periodically be cleaned to remove accumulated soot particles. This cleaning process, known as regeneration, typically involves controlling the engine so as to increase the temperature of the exhaust gas through the DPF  116  to cause combustion of the soot that has accumulated in the DPF. 
     As described above, a PM sensor  240  is used in an exhaust system to diagnose a failed DPF  116 . The basic technology utilizes a resistance based device that has parallel electrodes where particulate matter or soot accumulates in a gap in between the electrodes. Since the soot is conductive, as it accumulates the measured resistance of the sensor will decrease with increasing soot content. 
     Three main effects contribute to the accumulation of soot on the sensor. These are electrophoretic, thermophoretic, and direct impact of soot on the sensor. The electrophoretic effect is due to applied voltage across the electrodes which attract the charged soot particles. As this applied voltage increases attraction of soot will also increase. The thermophoretic effect describes the response of soot to a thermal gradient, with a tendency for increased accumulation of soot when the sensor temperature is lower than the soot temperature. Direct impact is a mechanical accumulation such that the soot is adhered to the sensor when it impinges on the sensor. 
     It has been observed that as a DPF  116  accumulates soot the filtering efficiency of the DPF  116  (i.e. the ability of the DPF  116  to prevent soot from passing through) increases. Without being bound to a theory, it is believed that this is due to accumulated soot in the DPF  116  lowering the effective pore size of the porous element in the DPF  116 . Conversely, it has been observed that a DPF  116  is less efficient (i.e. the DPF  116  allows more soot to pass through) just after regeneration of the DPF  116  occurs because the effective pore size is larger when the DPF  116  is clean. 
       FIG. 2  is an electrical schematic of a particulate matter sensing system  200 . The system  200  may be generally considered as partitioned as indicated into a controller portion  20 , a wiring harness portion  30 , and a sensing element portion  240 . The system may also include means for controlling a heater disposed on the sensing element to allow cleaning of the sensing element, not shown in  FIG. 2 . The function performed by the controller portion  20  of  FIG. 2  may be embodied in a separate controller or may be included in the controller  122  depicted in  FIG. 1 . 
     The controller portion  20  comprises a means for measuring the impedance of a circuit connected thereto. In the exemplary controller portion  20  in  FIG. 2 , the impedance measurement means includes a voltage source  22  that provides a voltage value V supply , a pull-up resistor  24  having a resistance value R pullup , and a voltage measurement means  26 . While voltage source  22  is depicted in  FIG. 2  as a DC source with a given polarity, it will be appreciated that voltage source  22  can alternatively be an AC source, a DC source having opposite polarity from what is depicted, or a source providing both an AC and a DC voltage component, without departing from the inventive concept described herein. The controller portion  20  electrically interfaces to the wiring harness portion  30  by connection means  27  and  28 . The wiring harness portion  30  includes conductors  32  and  34 . The wiring harness portion  30  electrically interfaces to the sensing element portion  240  by connection means  37  and  38 . The sensing element portion  240  includes a first electrode  242  electrically connected by conductor  246  to connection means  37 , and a second electrode  244  electrically connected by conductor  248  to connection means  38 . 
     The sensing element portion  240  in  FIG. 2  contains an additional bias resistor  250  having a resistance value of R bias  electrically connected between conductors  246  and  248 . The resistance of the sensing element R sensor  as measured between connection means  37  and connection means  38  is the parallel combination of R bias  and R particulate , the resistance resulting from particulate matter bridging the gap between the first electrode  242  and the second electrode  244 . R sensor  can be represented mathematically as: 
     
       
         
           
             
               R 
               sensor 
             
             = 
             
               
                 
                   R 
                   bias 
                 
                 × 
                 
                   R 
                   particulate 
                 
               
               
                 
                   R 
                   bias 
                 
                 + 
                 
                   R 
                   particulate 
                 
               
             
           
         
       
     
     In the absence of particulate matter on sensing element  240 , the term R particulate  is very large compared to R bias , and the effective sensor resistance R sensor  is essentially equal to R bias . This condition provides the maximum resistance value of R sensor . As particulate matter accumulates so as to bridge the gap between the first electrode  242  and the second electrode  244 , the effective sensor resistance R sensor  will decrease from its maximum value of R bias . 
     For the particulate matter sensing system  200  depicted in  FIG. 2 , the voltage measured by measurement means  26  will be: 
     
       
         
           
             
               V 
               measured 
             
             = 
             
               
                 V 
                 supply 
               
               ⁢ 
               
                 
                   R 
                   sensor 
                 
                 
                   
                     R 
                     pullup 
                   
                   + 
                   
                     R 
                     sensor 
                   
                 
               
             
           
         
       
     
     In the absence of particulate matter, the value of R sensor  will be at its maximum and will essentially equal R bias . Under this condition, the voltage measured by measurement means  26  will be: 
     
       
         
           
             
               V 
               measured 
             
             = 
             
               
                 V 
                 supply 
               
               ⁢ 
               
                 
                   R 
                   bias 
                 
                 
                   
                     R 
                     pullup 
                   
                   + 
                   
                     R 
                     bias 
                   
                 
               
             
           
         
       
     
     One of the major challenges with resistive PM sensor technology is the ability to prove that the sensor is working properly when the DPF  116  is still good, as there is essentially no soot coming down the exhaust pipe  118  downstream of the DPF. A sensor may exhibit a failure mode (e.g. electrically non-conductive contamination, internal open circuit) that results in a resistance reading that is indistinguishable from a properly operating sensor in the absence of soot. In an aspect of the present invention, readings from the PM sensor  240  are evaluated at times when soot levels in the exhaust pipe  118  are likely to be elevated, for example when the DPF  116  has just been cleaned. 
     The voltage imposed across the sensing electrodes of a PM sensor  240  depends on the voltage V supply  provided by the voltage source  22 , which is typically selected to be 5 volts. Initial testing has shown that soot is not easily measurable using a typical sensor reference voltage (i.e. 5 volts), even after a DPF cleaning event when the filtering efficiency of the DPF is at its lowest. 
     One way to improve the ability to measure soot is to increase the applied voltage across the electrodes, thus increasing the electrophoretic effect. This would require a controller that has the ability to change the voltage applied across the sensor element during operation. To evaluate this approach, testing was performed on a population of soot sensors mounted in a gas stream that contained a controlled concentration of soot. For each sensor, a response time was determined, where the response time is defined as the elapsed time from the end of a sensor cleaning event until the total sensor resistance R sensor  (i.e. the parallel combination of R bias  and R particulate ) decreased to a specific percentage of the bias resistance R bias . Testing was repeated using a number of different values of V supply . Results of this testing are shown in Table 1 below, and are presented graphically in  FIG. 3 . 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Supply Voltage 
                 Average Total Response Time 
               
               
                   
                 (volts) 
                 (seconds) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 5 
                 899.5 
               
               
                   
                 10 
                 341.6 
               
               
                   
                 12 
                 264.8 
               
               
                   
                 14 
                 213.5 
               
               
                   
                 16 
                 177.1 
               
               
                   
                 20 
                 129.7 
               
               
                   
                 24 
                 100.5 
               
               
                   
                   
               
             
          
         
       
     
     The results presented in Table 1 and in  FIG. 3  illustrate the electrophoretic effect on a PM sensor. While each sensor was exposed to the same soot concentration in a gas stream, increasing the supply voltage resulted in more of the soot in the gas stream adhering to the sensor with a resultant decrease in the amount of time required to capture sufficient soot to reduce the sensor resistance to the same predetermined percentage of the bias resistance used in the response time definition. This effect is used in an aspect of the present invention by operating the sensor at a voltage of higher magnitude than the nominal operating voltage for a period of time, to enhance the ability of a diagnostic system to capture sufficient soot to verify proper operation of the sensor in a low soot environment such as would be present with a properly functioning DPF. 
       FIG. 4  is a chart showing a soot mass rate downstream of a diesel particulate filter in a time interval around the regeneration time of the diesel particulate filter. Trace  402  is a logic trace that has a high level when a DPF regeneration event is commanded and a low level otherwise. When a DPF regeneration event is commanded, a controller adjusts engine operating conditions so as to raise the temperature of the exhaust from the engine to a level sufficient to cause spontaneous combustion (also known as auto-regeneration) of soot that has accumulated in the DPF  116 . Trace  404  represents an exhaust temperature, which can be seen to rise and fall in response to the DPF regeneration flag shown in trace  402 . Trace  406  is the output of an independent instrument disposed to measure the actual soot mass rate in the exhaust pipe  118 . Trace  408  represents the output resistance of a PM sensor  240  disposed in the exhaust pipe  118 , with a supply voltage of 12 volts applied to the PM sensor  240 . It is to be understood that the 12 volt level used in this demonstration is an example of an elevated voltage level and is not to be construed to limit practice of the invention to any particular voltage level. 
     With continued reference to  FIG. 4 , a DPF regeneration event is commanded at the time indicated as  410 . During the time interval immediately following time  410 , the exhaust temperature  404  increases and soot in the DPF  116  is combusted. At the time indicated as  412 , sufficient soot in the pores of the DPF  116  has burned to reduce the filtering efficiency of the DPF  116  such that soot is detectable downstream of the DPF, as shown by the trace  406  representing the output of a soot sensing instrument. As shown by the decrease in the resistance across the PM sensor  240  as shown in trace  408 , with a supply voltage of 12 volts the PM sensor  240  is also able to respond to low levels of soot passing through a normally operating DPF  116  after regeneration of the DPF  116 . 
     While operating a PM sensor  240  at an elevated voltage (e.g. 12 volts) improves the ability to recognize low levels of soot, which allows proper operation of the PM sensor  240  to be verified in the absence of a DPF fault, the timing of this voltage shift after the DPF regeneration event is also critical as the higher voltage may also attract contamination, which is undesirable. As used herein, contamination refers to electrically nonconductive material that may be present in the combustion byproducts in the exhaust stream, where said nonconductive material would degrade the functionality of the PM sensor if deposited on the PM sensor. The post-DPF regeneration increase in soot concentration has been seen for a limited time after the DPF regeneration is complete. The higher voltage would only need to be applied for a short duration at which the sensor validity has been proven. The sensor may then be cleaned to prove that the measured resistance was due to removable material (i.e. soot) on the sensing element. Then the applied voltage to the PM sensor  240  would be returned to a lower value (e.g. 5 volts) or turned off until an appropriate time to minimize the possibility of attracting contamination. 
     During or after a DPF regeneration event if soot concentration is insufficient some additional measures could be demanded to increase soot emissions for a short time, such as increasing EGR rate or reducing injection rail pressure. A specific soot emission model which estimates soot mass behind a proper DPF during these special conditions could be compared to the sensor soot mass. Another option is to use the sensor accumulation time to calculate accumulated soot mass and compare this to a limit soot mass to determine whether the sensor is working correctly. 
       FIG. 5  is a flowchart of a non-limiting embodiment of a diagnostic method  500  that incorporates aspects discussed above. In step  502 , a DPF regeneration event is initiated so as to take advantage of the reduction of filtering efficiency exhibited by a clean DPF. In step  504  a timer is initialized. As shown in step  506 , the PM sensor is operated at a high voltage relative to the nominal operating voltage of the PM sensor. In step  508 , the amount of soot accumulation indicated by the PM sensor is evaluated to determine if the PM sensor is recognizing a soot level that is expected for the present conditions of clean DPF and high PM sensor voltage. If the determination in step  508  is that indicated soot accumulation is less than expected, the method passes to step  510  where the timer is checked. If the timer has not expired, the method returns to step  506 . 
     If the determination in step  508  is that the indication of soot accumulation is as expected, that is to say that the PM sensor is capable of recognizing soot, the method passes to step  512 . In step  512  the operating voltage of the PM sensor is reduced to a nominal voltage level. In this way, the PM sensor is less likely to attract undesirable contamination that may impair its performance. In step  514 , cleaning of the PM sensor is commanded. In step  516 , the PM sensor output is evaluated, perhaps after a time delay, to determine whether the PM sensor indicates expected soot removal performance. 
     Returning to step  510 , if the timer has expired without an indication in step  508  that soot accumulation is as expected, this condition may be indicative of a PM sensor fault, and the method proceeds to step  518 . Likewise, if the determination in step  516  is that the PM sensor did not indicate soot removal as expected from a sensor cleaning event in step  514 , this condition may also be indicative of a PM sensor fault, and the method proceeds to  518 . 
     Upon reaching step  518  as a result of detection of a fault, a course of action may be selected from several possibilities. For example, a flag may be set in a controller and/or an indicator lamp may be illuminated. Alternatively, a more aggressive PM sensor diagnostic routine may be initiated. A non-limiting example of a more aggressive diagnostic routine is presented in  FIG. 6 . 
     The PM sensor diagnostic method depicted in the flowchart of  FIG. 6  is similar to the PM sensor diagnostic method in  FIG. 5 , and steps having the same function use the same numbering as in  FIG. 5 . The method depicted in the flowchart of  FIG. 6  includes an additional step  602 . In step  602 , the engine is controlled so as to increase the soot generation from the engine while the PM sensor evaluation is taking place as described in the discussion of  FIG. 5 . Step  602  may include any known method of increasing engine soot generation, including but not limited to controlling EGR or controlling fuel rail pressure. 
     It may be desirable to follow the method depicted in  FIG. 6  in lieu of the method depicted in  FIG. 5 . Alternatively, it may be desirable to follow the method depicted in  FIG. 5  initially, and only employ the method of  FIG. 6  if the determination in step  518  of  FIG. 5  is that the PM sensor has not detected the expected soot accumulation in step  508 . In such a way, the likelihood is reduced of falsely indicating a PM sensor fault when in reality insufficient soot was available for the PM sensor to detect. 
     While this invention has been described in terms of embodiments thereof, it is not intended to be so limited, but rather only to the extent set forth in the claims that follow.