Abstract:
A sensor includes an electrode and a seed structure. The electrode is configured to measure current due to movement of particulate matter relative to the electrode. The seed structure is deposited on the electrode. The seed structure includes a plurality of elongated members extending outward from the surface of the electrode. The elongated members are configured to promote charge transfer to particles and/or agglomerates of the particulate matter during operation of the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. application Ser. No. 14/166,658 (Atty docket no. EMI-P030CIPDIV), which is a divisional of U.S. application Ser. No. 13/853,890 (Atty docket no. EMI-P030CIP), which is a continuation-in-part of U.S. application Ser. No. 13/315,146 (Atty docket no. EMI-P030), which claims the benefit of U.S. Provisional Application No. 61/490,528, filed on May 26, 2011. Each of these applications is incorporated by reference herein in its entirety. 
     
    
     BACKGROUND 
       [0002]    Internal combustion engines (e.g. diesel engines) typically generate an exhaust flow that contains varying amounts of particulate matter (PM). In general, particulate matter is small particles of solid matter (i.e., soot) that are suspended in exhaust gasses. The amount and size distribution of particulate matter in the exhaust flow tends to vary with engine operating conditions, such as fuel injection timing, injection volume, injection pressure, or the engine speed to load relationship. Adjustment of these conditions may be useful in reducing particulate matter emissions and average particle size in the particulate matter from the engine. Reducing particulate matter emissions from internal combustion engines is environmentally favorable. In addition, particulate matter measurements for diesel exhaust is useful for on-board (e.g., mounted on a vehicle) diagnostics of PM filters and reduction of emissions through combustion control. 
         [0003]    On-board (in situ) sensors for particulate matter typically fall into two categories: 
         [0004]    1. Accumulative sensors 
         [0005]    2. Real-time sensors 
         [0006]    Accumulative sensors make use of the effect that exhaust particulates, especially soot particulates, tend to stick to surfaces exposed to exhaust gas. In an accumulative sensor, soot is allowed to accumulate on an exposed surface as a layer. A property of that soot layer is measured, either by vibrating the body (of which the surface is part) at its resonance frequency and then measuring the resonance frequency change that results from the accumulation of soot, or by measuring the resistance of the accumulated soot or its capacitance. When a certain threshold of the measured quantity is reached, the accumulation surface is heated to a high temperature to burn off the accumulated soot and the accumulation process starts anew. The frequency of repetition of this process is then used as a measure of the average particulate (or soot) content of the exhaust gas. Accumulative sensors are simple in construction, small enough to be installed like other exhaust sensors and relatively inexpensive. However, they have, due to their operating principle, a relatively slow response speed (not real-time) and suffer reliability issues when particles are accumulated that cannot be readily burned off. 
         [0007]    Real-time sensors have response speeds in the millisecond range and typically make use of electrostatic effects on particulates, or use optical effects. 
         [0008]    These sensors fall into four categories: 
         [0009]    a. Natural charge detectors 
         [0010]    b. Ionizing induced charge sensors 
         [0011]    c. Contact charge sensors 
         [0012]    d. Optical sensors 
         [0013]    Natural charge detectors try to detect the natural charge of particles produced during the production process. These sensors require very sensitive charge electronics and suffer from the fact that the natural charge of particles and/or their polarity can change on their way through the exhaust system. 
         [0014]    Ionizing induced charge sensors create ions in the exhaust gas path through the sensor using a very high voltage on an electrode of a high surface curvature, like a thin wire or needle tip. Voltages are typically in the 2-15 kV range. This voltage causes a corona discharge in the particle carrying gas. Soot particles flowing through the corona discharge field acquire an electric charge. These charged particles are then collected by a collection electrode and the charge transfer rate of the collection electrode is measured. The collection electrode and the necessity to prevent charged gas ions to also transfer a charge to the collection electrode requires a fairly large and complicated apparatus that cannot be easily reduced to the size of a typical exhaust sensor. 
         [0015]    Contact charge sensors typically use much lower voltages than ionizing induced charge sensors. In contact charge sensors soot particles coming in contact with a high voltage electrode acquire a surface charge that is determined by the surface charge density of the high voltage electrode. Typical voltages for the high voltage electrode are in the 500V to 3 kV range. These charged particles then deposit their acquired charge to grounded parts of the exhaust system or to a secondary detection electrode. The charge loss from the high voltage electrode is typically proportional to the particle concentration in the exhaust gas and is measured. However, because the resulting current (charge transfer per second) is very small, it is very difficult to isolate the high voltage electrode sufficiently to prevent current leakage. Any current that flows through an imperfect isolation to ground also creates a charge loss on the high voltage electrode and therefore causes a false sensor signal. As a way around that problem, a second collection electrode, essentially at ground voltage level, is placed in close proximity to the high voltage electrode and the charge accumulation on that electrode is measured. However, this necessitates an additional electrical connection that has to be well insulated from the high voltage supply to the electrode. For the typical temperatures encountered in the exhaust system of internal combustion engines it is very hard to find insulating materials that can withstand those temperatures and still maintain the high electrical insulating properties required. 
         [0016]    The current detected by contact charge sensors is proportional to the particle content of the exhaust gas, but also proportional to the area of the electrodes. For the areas possible for a typically sized exhaust sensor the currents are in the low picoAmpere range for the typically encountered soot levels, which are very difficult to detect in the electrically noisy environment where internal combustion engines operate. Furthermore, the low currents require the use of electrometer grade amplifiers to detect and amplify the sensor signal. Due to limitations of current semiconductor technology, these amplifiers typically can maintain their specifications only over a narrow temperature range, which is much smaller than the typical temperature range required for vehicular applications (typically −40 to +125 degrees Celsius). 
         [0017]    Optical sensors consist of a light source and a light detector. They measure either the opacity of the gas stream containing particles or measure light that is scattered by particles in the light path of the light source. 
         [0018]    Common to all described real-time sensor methods is that soot accumulation on the sensor parts has a detrimental effect on the sensor performance and it is attempted to be remedied by various methods, depending on the embodiment. Either by diluting the exhaust gas with clean air, flowing compressed filtered air periodically past the electrodes (or optical parts in case of optical sensors) to blow off accumulated particles, or by heating the electrodes (or lenses in case of optical sensors) to a temperature where accumulated soot particles burn off, but not high enough to burn contacting soot particles immediately. 
         [0019]    Further common to the described real-time sensors is that they have reaction times in the low millisecond range and can therefore for example detect changes in the soot concentration of the exhaust gas on a cylinder by cylinder basis for internal combustion engines. 
         [0020]    Because of the above described limitations of the state-of-the-art particulate sensors for in-situ applications there is a need for a sensor that overcomes some of the limitations. 
         [0021]    The sensor described in this invention combines certain aspects of the accumulative and the contact charge sensors in such a way as to increase the measured current by several orders of magnitude compared to a contact charge sensor, and by requiring soot accumulation on the electrodes to operate. 
         [0022]    This allows this sensor to be scaled to the size of a typical exhaust sensor and makes it possible to use common insulators for the electrodes. In addition the sensor described in this invention does not require any special remedies to prevent soot accumulation on the sensing electrode. Compared to other real-time sensors, the sensor described in this invention has a slower response time, typically slower than 100 milliseconds, but faster than 5 seconds, which is sufficient for most applications. 
       SUMMARY 
       [0023]    Embodiments of an apparatus are described herein. In one embodiment, the apparatus includes a sensor to measure particulate matter in an exhaust gas. An embodiment of the sensor includes an electrode and a seed structure. The electrode is configured to measure current due to movement of particulate matter relative to the electrode. The seed structure is deposited on the electrode during manufacturing or otherwise prior to use of the electrode. The seed structure includes a plurality of elongated members extending outward from the surface of the electrode. The elongated members are configured to promote charge transfer to the particulate matter during operation of the sensor. The charge transfer may be to particles and/or agglomerates of the particulate matter. In one embodiment, the seed structure includes dendric rhenium disposed on the surface of the electrode. In another embodiment, the seed structure includes nano-wires disposed on a surface of the electrode. Other embodiments may include other types of elongated members with similar size, geometry, chemical properties, and/or other features similar to dendric rhenium or nano-wires. Other embodiments of the sensor are also described. 
         [0024]    Embodiments of methods are also described herein. In one embodiment, a method for making a sensor includes providing an electrode for measuring current due to movement of particulate matter relative to the electrode. The method also includes depositing a seed structure on the electrode. The seed structure includes a plurality of elongated members extending outward from the surface of the electrode. The elongated members are configured to promote charge transfer to the particulate matter particles and/or agglomerates during operation of the sensor. 
         [0025]    In another embodiment, a method for using a sensor includes passing exhaust by an electrode having a seed structure deposited thereon. The seed structure includes a plurality of elongated members extending outward from the surface of the electrode. The elongated members are configured to promote charge transfer to the particulate matter during operation of the sensor. The method also includes measuring current due to movement of particulate matter relative to the electrode. The method also includes correlating the measured current to a quantity of the particulate matter in the exhaust. Other embodiments of methods are also described. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  depicts a schematic block diagram of one embodiment of an exhaust sensor system. 
           [0027]      FIG. 2A  depicts a schematic diagram of an exterior of one embodiment of the sensor assembly of  FIG. 1 . 
           [0028]      FIG. 2B  depicts a schematic diagram of an interior of one embodiment of the sensor assembly of  FIG. 1 . 
           [0029]      FIG. 2C  depicts a schematic diagram of a flow pattern through the interior of the sensor assembly of  FIG. 2B . 
           [0030]      FIG. 2D  depicts a schematic diagram of another embodiment of the sensor assembly of  FIG. 1 . 
           [0031]      FIG. 3  depicts a schematic circuit diagram of one embodiment of the control circuit of  FIG. 1 . 
           [0032]      FIG. 4  depicts a schematic circuit diagram of another embodiment of the control circuit of  FIG. 1 . 
           [0033]      FIG. 5  depicts a schematic circuit diagram of the sensor assembly of  FIG. 1 . 
           [0034]      FIG. 6A  depicts a schematic circuit diagram of one embodiment of a sensor assembly circuit. 
           [0035]      FIG. 6B  depicts a schematic circuit diagram of another embodiment of a sensor assembly with a guard trace. 
           [0036]      FIG. 7  depicts an exploded view of one embodiment of a heater assembly for use in a sensor assembly such as the sensor assembly of  FIG. 2D . 
           [0037]      FIG. 8A  depicts a perspective side view of an assembled embodiment of the heater insulator assembly of  FIG. 7 . 
           [0038]      FIG. 8B  depicts another perspective side view of an assembled embodiment of the heater insulator assembly of  FIG. 7 . 
           [0039]      FIG. 9  depicts results from modeling of one embodiment of a sensor assembly circuit which includes a guard trace. 
           [0040]      FIG. 10  depicts one embodiment of a sensor electrode with a deposited seed structure of, for example, dendritic rhenium. 
           [0041]      FIG. 11A  depicts an enlarged view of one embodiment of the seed structure of  FIG. 10 . 
           [0042]      FIG. 11B  depicts an enlarged view of another embodiment of the seed structure of  FIG. 10  having additional particle depositions connected to the seed structure. 
           [0043]      FIG. 11C  depicts a further enlarged view of the seed structure of  FIG. 11B . 
           [0044]      FIG. 12  depicts one embodiment of a sensor electrode with a coupled structure of nano-wires. 
           [0045]      FIG. 13  depicts an enlarged view of another structure of nano-wires. 
       
    
    
       [0046]    Throughout the description, similar reference numbers may be used to identify similar elements. 
       DETAILED DESCRIPTION 
       [0047]    It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
         [0048]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
         [0049]    Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
         [0050]    Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
         [0051]    Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
         [0052]    It will be readily understood that the components of the embodiments as generally described herein and illustrated in the appended figures could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of various embodiments, as represented in the figures, is not intended to limit the scope of the present disclosure, but is merely representative of various embodiments. While the various aspects of the embodiments are presented in drawings, the drawings are not necessarily drawn to scale unless specifically indicated. 
         [0053]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative and not restrictive. The scope of the invention is, therefore, indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. Reference throughout this specification to features, advantages, or similar language does not imply that all of the features and advantages that may be realized with the present invention should be or are in any single embodiment of the invention. Rather, language referring to the features and advantages is understood to mean that a specific feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of the features and advantages, and similar language, throughout this specification may, but do not necessarily, refer to the same embodiment. 
         [0054]    Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, in light of the description herein, that the invention can be practiced without one or more of the specific features or advantages of a particular embodiment. In other instances, additional features and advantages may be recognized in certain embodiments that may not be present in all embodiments of the invention. 
         [0055]    Reference throughout this specification to “one embodiment,” “an embodiment,” or similar language means that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the present invention. Thus, the phrases “in one embodiment,” “in an embodiment,” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment. 
         [0056]    The described embodiments include a particulate matter sensor that operates based on the agglomerated charge loss principle. 
         [0057]    An exhaust stream containing small particles is directed toward an electrode connected to the positive connection of an insulated high voltage power supply. These particles will naturally attach themselves to the high voltage electrode and form a layer on that electrode. When, after a few minutes of operation, enough particles are attached, surface irregularities on the electrode and on the accumulated particle layer act as nucleation points where particles preferentially attach. 
         [0058]    The time delay until which enough of a layer is formed depends on the particle concentration in the exhaust. Typically it is between 5 and 10 minutes for typical soot concentrations in the exhaust of a diesel engine. However, this time delay is relevant only for a new electrode that has never been exposed to exhaust gas. The soot layer initially formed will stay permanently on the electrode and can also be artificially created during the production process of the sensor to prevent this initial delay. 
         [0059]    Because of the high electric field, and because the field strength, due to the physical geometry of the sensor, these particles attach preferentially to nucleation sites where the field strength is strongest, typically a surface imperfection or roughness element. Soot particles then continue to deposit in the presence of the electric field and form diffusion-limited aggregates (fractal or dendrite structures). Once the soot aggregates reach a large size (microns), dielectrophoresis may play a role whereby particles suspended in a fluid or gas will move toward the stronger field area in an inhomogeneous electric field, provided their relative permittivity is higher than that of the surrounding medium. The relative permittivity of soot or carbon is typically 5-10, while the relative permittivity of exhaust gas is close to 1. Soot or carbon particles comprise the majority of particles in the exhaust of an internal combustion engine at operating temperature. 
         [0060]    When these soot particles attach to the nucleation points, they attach and are held there by intermolecular forces, like Van der Waals forces. Once they contact, and if they are at least partially conductive, they acquire the same surface charge as the electrode surface. 
         [0061]    Because equal polarity charges repel, they are at the same time repelled by the electrode. However, for individual particles the electric field strength used in this sensor (400-1500 kV/m) is not strong enough to overcome the attachment forces. 
         [0062]    However, because of the electric field, soot particles will agglomerate into branched filamentous 3-dimensional structures extending away from the electrode. Under the influence of the field these structures tend to self-organize such that they maximize their surface area as each individual branch of the structure acts as new nucleation and attachment point for new particles, but at the same time each branch is repelled by the other branches as they have equal charge and polarity. After some time such a structure acquires enough surface charge such that the resulting repelling electrostatic force between the structure and the electrode soot layer is enough to overcome the attachment force at the point where the structure is attached to the layer on the electrode, or in the structure where its attachment forces are weakest. 
         [0063]    When these structures break off from the electrode, they carry part of the charge of the electrode with them and deposit their charge when they contact a grounded part of the sensor or the exhaust system. This charge deposition on the grounded part of the sensor can be detected as a current pulse flowing between electrode and ground, and is in this embodiment measured by a current meter between ground and the negative connection of the high voltage power supply. 
         [0064]    Because of the self-organization of the branched structures in the electric field, these structures can carry a far higher charge per mass unit than individual particles can, given the voltages used. In the embodiment described here, the ratio of charge per mass unit is 100-1000 times higher than what would be expected from a charge transfer by individual particles as used in a contact charge sensor. Other embodiments may have different charge per mass ratios relative to the charge transfer by individual particles. 
         [0065]    The surface to mass ratio is implied from the measurements of current, particle mass flow, and particle size and is a virtual expression for charge per mass unit acquired. The argument for high surface to mass ratio, or high surface charge density to mass ratio, is based on the fact that soot particles are conductive, therefore they can carry charge only on their surface. 
         [0066]    During one example measurement, the mass concentration of particles was 6 mg/m 3  (particle mass per exhaust gas volume). 
         [0067]    The measured particle count was about 6*10 7  particles/cc and the average diameter of the particles was 50 nm. The measured current was about 41 nA, and the gas flow rate through the sensor was about 47 cc/second. This means about 2.82*10 9  particles are flowing through the sensor with a total particle mass flow rate of about 282 ng/second. 
         [0068]    The total surface area of the particles flowing through the sensor, assuming roughly spherical particles is therefore about 2.21*10 −5  m 2 /second. However, when they touch the electrode, what counts is not the total surface area of the particles, but roughly their cross sectional area. Therefore, the approximate effective area is only 5.5*10 −6  m 2 /second for spherical particles. 
         [0069]    The effective surface to mass ratio of those particles is therefore about 19.5 m 2 /g. 
         [0070]    The surface charge density of the electrode with the described sensor at 1000V is about 6.9*10 −5  Coulomb/m 2 . 
         [0071]    If each particle acquires from the electrode the same surface charge density as the electrode itself has, and transfers that charge, there would be a current of about 3.8*10 −10  Coulomb/second or 0.38 nA. This can be expressed also as charge transferred per mass unit which would be in this case about 1.35*10 −3  Coulomb/g. 
         [0072]    In one embodiment, the current actually measured with the sensor is about 41 nA. 
         [0073]    For a current of about 41 nA and assuming the agglomerates are acquiring the same surface charge density as the electrode, the total surface area of agglomerates transferring charge away from the electrode must therefore be about 5.9*10 −4  m 2 /second. Because the total particle mass flowing through the sensor is not changing, therefore the average effective surface to mass ratio for the agglomerates is about 2092 m 2 /g. 
         [0074]    Expressed as charge transferred per mass unit in this case comes to about 0.15 Coulomb/g. However it&#39;s expressed, there is typically more than a two orders of magnitude difference. Other embodiments may exhibit a difference between about 1.5-5 order of magnitude, or greater. 
         [0075]    In operation, for a given particle concentration, an equilibrium is reached where the rate of break-off of agglomerated structures and their buildup balance. Higher soot concentrations will build structures faster, therefore creating more and larger current pulses. 
         [0076]    A larger surface area of the electrode also creates more opportunity to create more structures and therefore creates more current pulses. 
         [0077]    However, the surface area of the electrode has preferentially a positive curvature to create an inhomogeneous field. This physically limits the size of the electrode. Some embodiments may use multiple smaller curved electrodes to create a larger surface area. 
         [0078]    The current pulses are integrated in the electronics and yield an average overall current that is proportional to the particle concentration of the flowing exhaust gas. 
         [0079]    Increasing the voltage will create a higher surface charge density on the electrode and the agglomerating structures, yielding current pulses with higher average amplitude, but the limit of the voltage is the onset of ionizing discharge from the tips of the agglomerated structures, which would create a current from the electrode to ground that is not proportional to the soot concentration. 
         [0080]    The magnitude of this voltage has to be experimentally determined, depending on the specifics of the embodiment. 
         [0081]    Because of the far higher current, this measurement method has far lower demands on the electrical insulation properties of the electrode mounting than a contact charge sensor would need. In addition the sensitivity of the signal amplification circuit can be reduced such that it is possible, without expensive shielding, to deploy it in the electrically noisy environment of a vehicle. 
         [0082]      FIG. 1  depicts a schematic block diagram of one embodiment of an exhaust sensor system  10 . The illustrated exhaust sensor system  10  includes a sensor assembly  12 , an engine  14 , and an exhaust system  16 . The engine  14  produces exhaust which moves through the exhaust system  16 . The exhaust system  16  facilitates flow of the exhaust gases to a gas outlet  18 , typically for emission into the atmosphere. The sensor assembly  12  is at least partially inserted into the exhaust system  16  to detect a parameter within the exhaust stream. As the gas in the exhaust system  16  passes over and/or through the sensor assembly  12 , the sensor assembly  12  detects a condition within the exhaust by measuring chemicals or temperature or other parameters at the sensor assembly  12 , as described herein. In a specific embodiment, the sensor assembly  12  includes a particulate matter sensor to detect conditions indicative of the presence of particulate matter within the exhaust stream. 
         [0083]    The exhaust sensor system  10  also includes an electronic control module  20 . The electronic control module  20  includes a processor  22 , and an electronic memory device  24 . The electronic control module  20  also may include a control circuit  26  to control some or all of the operations of the sensor assembly  12 . Alternatively, some or all of the control circuit  26  functionality may be implemented at the sensor assembly  12  or at another location that is not necessarily proximate the electronic control module  20 . Additionally, in some embodiments, the control circuit  26  may control a peripheral system (not shown). Some examples of peripheral systems that may be implemented at the sensor assembly  12  include, but are not limited to, a heater (not shown) or a chemical neutralizer system (not shown). Instead of or in addition to the chemical neutralizer system, some embodiments may include an emission control element (not shown) to neutralize other aspects of the chemicals and/or substances within the exhaust system  106 , either upstream or downstream from the sensor assembly  10 . In other embodiments, the control circuit  26  may control peripheral systems at other locations within the exhaust sensor system  10 . 
         [0084]    In one embodiment, the sensor assembly  12  relays a sensor signal to the processor  22  of the electronic control module  20 . The processor  22  analyzes the sensor signal from the sensor assembly  12 . If the sensor signal is corrupted, the processor  22  may send a control signal to the control circuit  26 , for example, to shut down the sensor assembly  12 . In this situation, or in other situations, the control circuit  26  may activate one or more heaters inside of or within proximity to the sensor assembly  12  to burn off particulate matter deposits that might corrupt the sensor signal from the sensor assembly  12 . In some embodiments, the processor  22  sends the control signal to the control circuit  26  to activate a chemical injection system to introduce a chemical agent into the exhaust system  16  to remove particulate matter deposits built up on the sensor assembly  12 . Further functionality of embodiments of the control circuit  26  is described below. 
         [0085]    If the sensor signal from the sensor assembly  12  is not corrupt, the processor  22  may compare the sensor signal with data stored in a lookup table  28  on the electronic memory device  24  to determine one or more qualities of the exhaust in the exhaust system  16 . For example, the processor  22  may determine an amount of particulate matter in the exhaust stream. The processor  22  also may compare the sensor signal from the sensor assembly  12  with data from the lookup table  28  to estimate, for example, a mass concentration of particulate matter in the exhaust stream. In other embodiments, the electronic control module  20  facilitates detection of one or more other qualities of the gas in the exhaust system  16 . For example, types of sensors that may detect qualities of the exhaust stream may include but are not limited to, a particulate matter sensor, an oxygen sensor, a thermal sensor, an ammonia sensor, a flow rate sensor, and an air-fuel ratio sensor. 
         [0086]    It should also be noted that embodiments of the sensor assembly  10  may be tolerant of fluctuations of certain gaseous constituents in a gas environment. In this way, the sensor assembly  10  may be calibrated to measure particular chemicals, materials, or other conditions within an exhaust stream, with relatively little or no disruption from one or more other chemical substances and/or operating conditions. 
         [0087]    It should also be noted that the sensor assembly  12  may be used, in some embodiments, to determine a failure in another component of the exhaust sensor system  10 . For example, the sensor assembly  12  may be used to determine a failure of a particulate matter filter (not shown) within the exhaust system  16 . In one embodiment, a failure within the exhaust sensor system  10  may be detected by an elevated signal generated by the sensor assembly  12 . In some embodiments, the exhaust sensor system  10  includes an alarm to indicate a detected failure of the sensor assembly  12  or other component of the exhaust sensor system  10 . In some embodiments, the sensor assembly  12  also could be coupled to another sensor or detector such as a mass flow meter. 
         [0088]      FIG. 2A  depicts a schematic diagram of an exterior of one embodiment of the sensor assembly  12  of  FIG. 1 .  FIG. 2B  depicts a schematic diagram of an interior of one embodiment of the sensor assembly  12  of  FIG. 2A . The illustrated external components of the sensor assembly  12  include a sensor housing  120 , an intermediate bolt portion  122 , and a threaded portion  124 . The depicted external components also include an outer housing  126  with a cap  128 . The outer housing  126  has one or more holes  130  which provide airflow within the outer housing  126 . The cap  128  includes a separate opening  132  to facilitate the airflow within the outer housing  126 . One example of an airflow pattern through the outer housing  126  of the sensor assembly  12  is shown in  FIG. 2C  and described in more detail below. 
         [0089]    For reference, the sensor assembly  12  is installed within the exhaust sensor system  100  so that the sensor housing  120  and the bolt portion  122  of the sensor assembly  12  are typically outside of the exhaust stream through the exhaust system  106 . The outer housing  126  and the cap  128  of the sensor assembly  12  are installed within the exhaust stream that passes through the exhaust system  106 . The dashed line  134  distinguishes, generally, between the parts of the sensor assembly  12  that are outside (left of the dashed line  134 ) of the exhaust system  106  and the parts of the sensor assembly  12  that are inside (right of the dashed line  134 ) of the exhaust system  106 . In one embodiment, the threaded portion  124  allows the sensor assembly  12  to be threaded, or screwed, into a corresponding hole within the exhaust system  106 . Some of the threaded portion  124  may remain outside of the exhaust system  106  or, alternatively, may enter into the exhaust system  106 . Many of the external components of the sensor assembly  12  may be made from metals, such as stainless steel, that are substantially insensitive to typical mechanical and/or chemical conditions within the exhaust sensor system  100 . 
         [0090]    In one embodiment, as exhaust gasses flow by the cap  128 , the associated Venturi effect creates a low pressure which draws exhaust gas out of the interior of the outer housing  126 . A corresponding amount of exhaust gas is drawn from the ambient exhaust stream through the holes  130  into the outer housing  126 . Drawing a portion of the ambient exhaust stream into the outer housing  126  allows a sensor electrode  136  to measure the amount of particulate matter (PM) within the exhaust stream. 
         [0091]    In one embodiment, the large surface area of the electrode  136  is cylindrically shaped. The cylinder can be hollow for weight reduction. The axis of the cylinder may be coincident with the axis of the electrode mounting stem  138 . 
         [0092]      FIG. 2B  shows the inner construction of one embodiment of the sensor assembly  12 . The inner construction of the sensor assembly  12  includes the sensor electrode  136  and the electrode mounting stem  138 , as well as an inner baffle  140  and one or more insulators  142  and  144 . In the depicted embodiment, the electric field extends radically between the surface of the electrode  136  and inner baffle  140 . Because the electrode  136  has a smaller outside diameter than the inside diameter of inner baffle  140 , the electric field is stronger at the surface of the electrode  136 . In general, exhaust gas passing by the sensor assembly  12  enters into the outer housing  126  through the exhaust inlet holes  130 . The exhaust gas flows through the space between inner baffle  140  and the outer housing  126  to one or more baffle holes  146  at the base of the inner baffle  140 . The gas then flows in the space between cylindrical sensor electrode  136  and the cylindrical inner baffle  140  to the exhaust outlet  132  in the Venturi cap  128 . 
         [0093]    Because the flow of exhaust gas has to make a 90 degree turn when flowing from the holes  146  in the inner baffle into the space between the inner baffle  140  and the electrode  136 , particles suspended in the gas stream are accelerated towards the electrode  136  because of their higher inertia compared to gas molecules. This flow pattern further supports the formation of particle agglomerates on the electrode  136 . 
         [0094]    In one embodiment, the inner baffle  140  is metallic and connected to the ground. The Venturi cap  128  is also connected to the inner baffle  140  and grounded. 
         [0095]    Agglomerates that detach from the electrode  136  are repelled by the electrode  136  due to having equal charge and are attracted by the surface of the inner baffle  140 , which has an opposite surface charge to the electrode  136  or agglomerates. Because detached particle agglomerates flow with the gas stream between inner baffle  140  and electrode  136 , and because the gas stream has to execute two more turns to exit through the Venturi hole  132 , these turns in the flow further insure that even agglomerates that have not deposited their charge in the inner baffle  140  are accelerated by their inertia towards a grounded surface part of the Venturi cap  128 . 
         [0096]    The grounded filter baffle  144  and the high voltage filter baffles  148 , which are disk shaped, provide a tortuous path for particles that could migrate to the electrode insulator  142 . The electric field between these filter baffles is highly inhomogeneous at the inner edge of filter baffle  144  and at the outer edges of filter baffles  148 . Therefore, any particulates that migrate along the path between those filter baffles are attracted to these filter baffles themselves and will attach there. The normal gas flow path through the sensor insures that there will not be enough soot particles migrating to create large structures. The filter baffles therefore slow down particle accumulation on the insulator  142 . Particle accumulations on the insulator  142  could form an electrically conductive path that will create a charge loss from electrode  136  to the sensor housing  120 , parallel and in addition to the charge loss by particle agglomerates, which could detrimentally affect sensor performance. In addition, in some embodiments, the insulator  142  could be periodically heated by an embedded heater (not shown) such that any accumulated soot particles on the surface of the insulator  142  are burned off. 
         [0097]    In some embodiments, this burn-off of soot particles may be implemented infrequently. Even in exhaust gas with particle concentrations far above the legal limit for emissions, the burn-off may be required only every few hours of operation. 
         [0098]      FIG. 2D  depicts a schematic diagram of another embodiment of the sensor assembly  12  of  FIG. 1 . In contrast to the sensor assembly of  FIG. 2B , the sensor assembly of  FIG. 2D  includes a heater assembly  162  (also referred to as a heater disk  162 ). The heater assembly  162  may perform some or all of the insulating functions of the insulator  144  of  FIG. 2B . Additionally, the heater assembly  162  may perform one or more of the heater functions described herein. A more detailed depiction of the heater insulator assembly  162  is shown in  FIGS. 7, 8A, and 8B  and described in more detail below. The illustrated sensor assembly of  FIG. 2D  also includes a heater bushing  164 , and separate electrical leads  166  and  168  for the heater and ground traces (see  FIGS. 7, 8A, 8B ). In general, the heater may be controlled to provide heat to the adjacent electrically insulating surfaces so as to burn off deposited particles that may have settled on or otherwise coupled to the insulating surfaces. Additionally, the heater supply heat to the sensor electrode  136 , or a portion thereof, to burn off some or all of the soot or deposited particles at the heated location(s). The heater bushing  164  provides an insulated pathway for the corresponding portion of the sensor electrode  136  to pass through the heater insulator assembly  162 . 
         [0099]    In operation, exhaust gas flowing past the Venturi tip  128  of the sensor assembly  2  creates a low pressure area at the tip. Flow through the sensor assembly  12  is also promoted by a positive pressure that is created at the “inlet holes”  130  because of the velocity head (velocity is reduced to zero at the inlet holes and velocity is converted to pressure at the inlet holes by Bernoulli&#39;s principle). The low pressure at the tip  128  combined with the positive pressure at the inlet holes  130  draw exhaust gas through the sensor in a reproducible manner. The exhaust gas flows up through the space between exhaust housing and inner baffle and through holes  146  in the inner baffle  140  into the space between inner baffle  140  and electrode  136  and back down towards the Venturi hole  132 . Although different operating conditions may be achieved, on example of a model of the illustrated embodiment exhibits a free stream exhaust velocity of 14 m/s and an exhaust gas temperature (EGT) of 200 C. At these conditions, the velocity in the sensor is 0.34 m/s (Re=25), and the volumetric flow rate is 0.82 liters/minute. 
         [0100]    In one embodiment, the sensor body, including the inner baffle  140 , is grounded through the vehicle exhaust system to the vehicle ground (not shown). The cylindrical electrode  136  is held at a potential of approximately 1,000 V relative to ground. In the depicted embodiment, the electrode  136  and its connections are insulated with two alumina discs: the back insulator disk  142 , which also acts as holder for the heater and guard connections, and the heater disk  162 . The heater disk  162  also mechanically fixes the electrode  140  assembly in the sensor  12 . In one embodiment, one or both of the insulator disks  142  and  162  are co-fired. In some embodiments, the heater disk  162  contains a platinum thick-film heating element (see  FIGS. 7, 8A, 8B ) that serves also as resistance temperature detector (RTD) for temperature measurements. The heater element can be used to evaporate moisture that condenses on the heater disk  162  during condensing conditions, and as mentioned above can also be used to burn off soot that accumulates on the exhaust side face of the heater disk  162  over time. Under some operating conditions the function of burning off soot may be used infrequently, as needed. 
         [0101]    In one embodiment, the electrode  140  assembly is brazed to the heater bushing  164 , which acts as the main insulator between the electrode  136  and heater disk  162 . The hole in the heater disk  162  in which the heater bushing  164  is brazed is surrounded on both sides of the heater disk by metalized guard rings (see  FIGS. 7, 8A, 8B ). These guard rings are electrically connected to the brazing metal in the heater disk  162  (e.g., to a heater bushing braze joint) and connected to the “guard” connection  166 . Similar guard rings may be implemented on one or both of the outermost surfaces of the heater disk  162  (See  FIGS. 8A and 8B ). These guard traces are further connected to the inner shield of a triaxial sensor supply cable (guard), while the inner conductor of the triaxial cable supplies the high potential for the electrode  136 . The outer conductor of the triaxial cable is connected on the electronic side to ground to provide RF and capacitive noise shielding. 
         [0102]    The electronics for the sensor  12  hold the guard connection at ground potential while the guard itself is connected to the negative side of the 1,000 V supply. Therefore, any current leakage through either the bulk resistivity of the heater bushing  164 , or due to conducting surface contaminants on heater disk  162  or back insulator between the electrode  136  and the guard trace creates a current load on the HV power supply, but does not contribute to a current measured between the negative of the 1,000 V supply and ground. 
         [0103]      FIG. 3  depicts an electrical schematic block diagram of one embodiment of the control circuit of  FIG. 1 . 
         [0104]    The positive connection of the high voltage source  104  is connected to the electrode  108  of sensor assembly  12 , while the inner baffle  140  and the rest of the sensor assembly  12  are connected to ground. This rest of the sensor assembly  12  is depicted as part  110  in the electrical schematic block diagram. The negative connection of the high voltage source  104  is connected to one side of the filter resistor  114  and to one side of the filter capacitor  112 . The other side of filter capacitor  112  is grounded. The other side of the resistor  114  is connected to one side the current meter  106 , and the other side of the current meter  106  is grounded. 
         [0105]    The resistor  114  and filter capacitor  112  form a low pass filter with a bandwidth of, for example, 5-10 Hz. The current pulses created by the operation of sensor assembly  12  are integrated in this low pass filter. 
         [0106]      FIG. 4  depicts a more detailed electrical schematic of the schematic block diagram depicted in  FIG. 3 . Generally, the illustrated control circuit  26  includes a generator block  150 , a protection block  152 , and a detection and filter block  154 . Other embodiments may include fewer or more blocks and/or fewer or more components within one or more of the illustrated blocks. 
         [0107]    In the depicted embodiment, the generator block  150  includes an impulse generator (IG), a bi-directional Zener diode (Z 1 ), a transformer (Tr), a high voltage diode (D 1 ), and a high voltage capacitor (C 1 ). The illustrated transformer includes primary and secondary windings with a 1:10 winding ratio, although other embodiments may have different winding ratios. The protection block  152  includes the clamping diode D 2 . The detection block  154  includes an initial filter resistor (RF), an initial filter capacitor (CF), an operational amplifier (OA), a gain resistor (RG), a PMOS transistor (Q), and a current to voltage conversion resistor (RS). 
         [0108]    In general, the control circuit  26  generates a relatively high voltage to apply to the sensor electrode  108  and, in turn, generates an output signal that can be correlated with the particulate matter level of the exhaust stream to which the sensor electrode  108  is exposed. Specifically the secondary side of the transformer TR, diode D 1 , and capacitor C 1  form an embodiment of the floating voltage source  108  depicted in  FIG. 3 . While specific circuit components are shown in a particular arrangement, other embodiments may use similar or different circuit components to achieve the same or similar results. Additionally, while the illustrated circuit is implemented substantially in hardware, it may be possible to implement some portions of the control circuit  26  using software instructions that are executed by a central processor or other digital signal processing device. 
         [0109]    In one embodiment, the impulse generator  156  generates periodic, short pulses duration (e.g., about 1-2 μsec) with a particular repeat rate (e.g., about 1400 impulses per second) and a maximum amplitude (e.g., about 1000V). These impulses charge the capacitor C 1  to the same amplitude (e.g., about 1000V). The positively charged side of capacitor C 1  is connected to the sensor electrode  108  of the sensor assembly  100 . Agglomerates that get positively charged at the sensor electrode  108  carry away some of the capacitor charge periodically and, thus, discharge the capacitor C 1  relative to the ground reference (e.g., vehicle ground). This discharge current flows through the initial filter resistor RF and are integrated at the initial filter capacitor CF. The operation amplifier OA together with the gain resistor RG and PMOS transistor Q form an inverting current amplifier with a gain of—RF/RG. This amplified current Tout flows out of the drain of PMOS transistor Q into the voltage conversion resistor RS. The voltage at resistor RS is therefore proportional to RS*Iout, according to Ohms law. Other methods of detecting the discharge current can be employed as well. 
         [0110]    In one embodiment, the impulse generator  156  receives vehicle battery voltage (about 12V) and switches the vehicle battery voltage on the primary winding of the transformer until the current in this winding reaches a primary winding threshold value. In one embodiment, the primary winding threshold value is approximately 3 Amperes, although other embodiments may use a different primary winding threshold value. When the specified current limit is reached, the current is rapidly switched off. In one embodiment, the inductive flyback pulse of the primary winding is limited by the bidirectional Zener diode Z 1  to a value of 100V. Because the transformer has a winding ratio of 10:1 between its secondary and primary windings, the flyback pulse limited to 100V on the primary side translates into a 1000V pulse on the secondary side. Although some embodiments may use 100V on the primary side and 1000V on the secondary side, other embodiments may use different voltages and/or different winding ratios. 
         [0111]    The impulse generator  156  periodically switches the battery on to the primary winding to generate corresponding pulses on the secondary side on a regular basis. In one embodiment, pulses are generated once every 0.7 msec (1.4 kHz), although other embodiments may use a different pulse generation frequency. 
         [0112]    The 1000V pulse on the secondary side of the transformer charges the high voltage capacitor C 1  via the diode D 1  to 1000V. This circuit is a flyback converter with primary voltage limiting. 
         [0113]    The 1000V charge of the high voltage capacitor C 1  is connected to the sensor electrode  108 . 
         [0114]    If soot agglomerates extend all the way from the sensor electrode  108  to the grounded part  110  of the sensor assembly, a short circuit would form that shorts the high voltage source to ground. This creates a high current that would discharge C 1  rapidly while CF would be charged rapidly to a high negative voltage. The protection diode D 2  prevents CF to be charged to a more negative voltage than about −0.7V. The high discharge current of C 1  will heat up the particles of which the shorting soot agglomerate is formed to a high enough temperature to burn them. This way these types of short circuits are self extinguishing. 
         [0115]    The negative side of high voltage capacitor C 1  is connected to the initial filter resistor RF and initial filter capacitor CF. The filter capacitor CF integrates the current pulses caused by dislodged soot agglomerates as described before. Therefore the average current through RF is proportional to the charge loss rate from the electrode caused by dislodging soot agglomerates depositing their charge to a grounded part of the sensor or exhaust system. This integrated current is correlates with the soot concentration of the exhaust gas flowing through the sensor. Because of the need for integration of the current pulses, this sensors response time is determined by the low pass filter bandwidth of the low pass filter formed by RF and CF. For typical implementations, this bandwidth is less than 10 Hz. 
         [0116]    The exhaust sensor system  10  also includes an electronic control module  20 . The electronic control module  20  includes a processor  22 , and an electronic memory device  24 . The electronic control module  20  also may include a control circuit  26  control some or all of the operations of the sensor assembly  12 . Alternatively, some or all of the control circuit  116  functionality may be implemented at the sensor assembly  12  or at another location that is not necessarily proximate the electronic control module  20 . Additionally, in some embodiments, the control circuit  26  may control a peripheral system (not shown). Some examples of peripheral systems that may be implemented at the sensor assembly  12  include, but are not limited to, a heater (not shown) or a chemical neutralizer system (not shown). Instead of or in addition to the chemical neutralizer system, some embodiments may include an emission control element (not shown) to neutralize other aspects of the chemicals and/or substances within the exhaust system  106 , either upstream or downstream from the sensor 
         [0117]      FIG. 5  depicts a schematic circuit diagram of the sensor assembly of  FIG. 1 . The illustrated circuit diagram includes a processor  172 , a high voltage generator  174 , a sensitive current meter  176 , a current meter  178 , and a heater drive  180  to control the heater. The illustrated circuit diagram also includes several control and/or communication lines. In one embodiment, the electrode  136  is connected to the positive side of the high voltage generator  174 , and the guard trace is coupled to the negative side of the high voltage generator  174 . Current measured by the sensitive current meter  176  may be reported to the processor  172 , which can correlate the measured current to a characterization of the particulates in the exhaust gas to which the electrode  136  is exposed. More generally, the control and communications processor coordinates all measurements, does the analog to digital conversions, controls the operation of the HV generator, processes the data, and communicates the results via a CAN bus. 
         [0118]      FIG. 6A  depicts a schematic diagram of one embodiment of a sensor assembly circuit  182 . This embodiment of the sensor assembly circuit  182  does not include a guard trace. Generally, the current ISoot generated by the sensor represents the charge loss current from the sensor electrode  136 . The resistance RLeak represents the leakage resistance between the electrode  136  and the sensor body  126 , which is connected to the exhaust pipe (see  FIG. 1 ). The voltage VGO represents a ground offset voltage between the exhaust pipe and the electronics ground. The measurement resistance Rm corresponds to a measurement current Imeasure. And the variable gain amplifier Amp amplifies the voltage drop over the measurement resistance Rm caused by the measurement current Imeasure. In this embodiment, the measurement current is the sum of the leakage current ILeak and the sensor current ISoot. In some embodiments, this arrangement requires a very large leakage resistance RLeak in order to avoid false positives (e.g., 1 TOhm for &lt;0.25 mg/m 3  error). 
         [0119]      FIG. 6B  depicts a schematic circuit diagram of another embodiment of a sensor assembly with a guard trace. The presence of the guard trace in the illustrated circuit divides the leakage current ILeak over three resistances, which include a first resistance RELeak between the electrode  136  and the guard trace, a second resistance RGLeak between the guard trace and a ground trace, and a third resistance RGG between the ground trace and the sensor body  126  (and the connected exhaust pipe). The third resistance has no known impacts on the measurements of the sensor assembly. 
         [0120]    The first resistance RELeak increases the load on the HV generator  174 , while the sensor gain changes approximately linearly with the electrode voltage between about 800-2,000V. Below about 800 MOhm for RELeak, the electrode voltage will drop. The calculated gain error is only about 5% relative to an RELeak of about 150 MOhm, and goes down to about 0% as the RELeak resistance value increases. In some embodiment, the electrode voltage may be monitored between two thresholds, and known or calculated error rates in the gain change for RELeak within that range may be compensated for. For example, gain changes for RELeak may be compensated for between about 40-800 MOhm. 
         [0121]    The second resistance RGLeak is typically much larger than the measurement resistance Rm, so the impact of the second resistance RGLeak may be negligible for any values over about 100 kOhm. However, an input offset at the simplifier may create a false current through the second resistance RGLeak. Thus, an amplifier with a very low offset may be used. For example, the amplifier may have an offset of about 20-25 μV over the target temperature range. 
         [0122]      FIG. 7  depicts an exploded view of one embodiment of a heater assembly  190  for use in a sensor assembly such as the sensor assembly  12  of  FIG. 2D . The heater assembly  190  is substantially similar to the heater assembly  162  of  FIG. 2D . The illustrated heater assembly  190  includes several layers of insulating materials  192  such as alumina. On the outer surfaces of the insulating layers  162 , one or more ground traces  194  and one or more guard traces  196  are provided. The traces  194  and  196  may be any type of electrically conducting material (e.g., printed conductors) that is applied to the corresponding insulating surfaces. The heater assembly  190  also includes heater conductors  198  and  200 , including one or more contact pad(s) and inner-layer innerconnect trace(s). 
         [0123]    The ground trace  194  on the insulator layer  192  prevents ground offsets between the electronics ground and the sensor body ground. In some embodiments, the ground trace  192  may be electrically coupled to the ground side of the heater. 
         [0124]    As explained above, the guard trace  196  effectively divides the leakage resistance RLeak into three separate resistances. This reduces errors that would otherwise occur due to the leakage current RLeak via bulk conductivity, surface contamination of the exposed insulating layers  192 , or condensation on any insulator layer  192 . The guard trace electrically extends through the system cabling via a shield conductor. In some embodiments, three-dimensional guard traces may be implemented within the sensor insulating layers to provide further protection. 
         [0125]      FIG. 8A  depicts a perspective side view of an assembled embodiment of the heater insulator assembly of  FIG. 7 .  FIG. 8B  depicts another perspective side view of an assembled embodiment of the heater insulator assembly of  FIG. 7 . 
         [0126]      FIG. 9  depicts results from modeling of one embodiment of a sensor assembly circuit which includes a guard trace  196 . This modeling was obtained from simulations performed with a test setup using an RELeak value of 10 GOhm, an RGLeak value of 40 MOhm (and down to 2.2 MOhm), and an RSoot value (representing the sensor) of 100 GOhm. The RELeak resistance exhibited about 100 nA leakage current, and the RSoot exhibited about 10 nA with the guard trace  196  present. This is compared with about 12 nA without the guard trace  196 . 
         [0127]      FIG. 10  depicts one embodiment of a sensor electrode  136  with a deposited seed structure  240  of, for example, dendritic rhenium. A sensor electrode  136  that has not been operated previously may require, or benefit from, several minutes of operation in a gas stream containing particles to reach a constant output with acceptable sensitivity. Initially, the observed current may be close to zero, then rises rapidly to a value below the final sensitivity, and then rises slowly to the final sensitivity value. The timing of these changes in sensitivity are due, at least in part, to the cumulative charge transfer effect of particles depositing on the exposed surface of the electrode  136  and/or the corresponding surface of the inner baffle  140 . Higher soot concentrations during this initial period cause a faster rise to the final sensitivity value. In some embodiments, coating the positive electrode  136  of the sensor  12  with a seed structure during manufacturing can accelerate the ability of particles to deposit on the exposed surface area during operation of the sensor  12 . Dendritic Rhenium (dendrites in the micron scale) is one example of a seed structure that speeds up the startup process. In some embodiments, the use of dendritic rhenium may accelerate reaching final sensitivity by about 40-50%. In other embodiments, other types of seed structures may be used. 
         [0128]      FIG. 11A  depicts an enlarged view of one embodiment of the seed structure of  FIG. 10 .  FIG. 11B  depicts an enlarged view of another embodiment of the seed structure of  FIG. 10  having additional particle depositions connected to the seed structure.  FIG. 11C  depicts a further enlarged view of the seed structure of  FIG. 11B . In particular,  FIGS. 11B and 11C  illustrate field enhancement and preferential soot deposition primarily at the tips of dendritic rhenium. 
         [0129]      FIG. 12  depicts one embodiment of a sensor electrode with a coupled structure of nano-wires. In general, the nano-wires are characterized by material spacing and lengths of nano-wires in approximately the micron range, with tip diameters in approximately the sub-micron range. This coupled structure of nano-wires may serve as an alternative type of seed structure on the electrode  136  of the sensor.  FIG. 13  depicts an enlarged view of another structure of nano-wires. In some embodiments, stable nano-wires are assembled during manufacturing. During operation, these nano-wires create the field enhancement to highly charge soot particles. In this type of assembly, the amplification does not necessarily come from agglomeration and breakoff of particles, but rather from the high charge between the nano-wire coated electrodes. In some embodiments, the field amplification at the tips of the nano-wires is chosen such that the repellant force experienced by a touching particle is always bigger than the intramolecular forces that allow a soot particle to stick to other materials like the nano-wires. The magnitude of the field amplification is determined by the length/radius ratio of the nano-wires and their spacing. In other embodiments, other factors may influence the design and implementation of the nano-wires. Additionally, other embodiments may utilize other nano-structures that are not necessarily classified as nano-wires. 
         [0130]    In the above description, specific details of various embodiments are provided. However, some embodiments may be practiced with less than all of these specific details. In other instances, certain methods, procedures, components, structures, and/or functions are described in no more detail than to enable the various embodiments of the invention, for the sake of brevity and clarity. 
         [0131]    Although the operations of the method(s) herein are shown and described in a particular order, the order of the operations of each method may be altered so that certain operations may be performed in an inverse order or so that certain operations may be performed, at least in part, concurrently with other operations. In another embodiment, instructions or sub-operations of distinct operations may be implemented in an intermittent and/or alternating manner. 
         [0132]    Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.