Abstract:
A method and apparatus for detecting the concentration of soot particles in engine oil. A microwave signal having a varying frequency is applied to a transmission line having a probe tip exposed to the engine oil. A voltage is read at a stationary detection point along the length of the transmission line for each applied frequency. The applied frequency when the probe voltage is equal to a null voltage of a standing wave within the transmission line is compared to a reference frequency determined when the probe voltage is equal to a null voltage for a known concentration of soot particles. The concentration is calculated based upon the result of the comparison. Preferably, a reference probe receives the same input signal. A known relationship between the reference probe and the transmission line is then used to eliminate temperature effects on the applied frequency.

Description:
TECHNICAL FIELD 
   The invention relates to sensors and, more particularly, to a method and apparatus for detecting soot in engine oil. 
   BACKGROUND OF THE INVENTION 
   Soot is a product of combustion in the combustion chamber of an engine, and it transfers in small amounts to crankcase oil in an engine, particularly a diesel engine. Thus, during operation of an engine, oil gradually builds up soot. When soot in the oil reaches an unacceptable level, the lubricating ability of the oil is diminished. Thus, measuring the soot content in oil over time is desirable as an indicator of the need to replace the oil. 
   Soot particles are very fine conductive particles. Despite this conductivity, they can increase the relative permittivity or relative dielectric constant of a dielectric fluid, such as oil, by acting as an artificial dielectric. Thus, one method of determining the soot content of oil involves measuring relative permittivity, such as taught in expired U.S. Pat. No. 4,345,202 to Nagy et al., the entire contents of which is incorporated herein by reference. 
     FIG. 5  shows an apparatus used in the method of Nagy et al. A Gunn oscillator  100  receives a supply voltage from a voltage supply line  102  coupled to one end of a sealed coaxial transmission line  104  having outer and inner conductors  104 A and  104 B, respectively. The other end of the transmission line  104  comprises a probe  106  having the same inner conductor  104 B as line  104  and an outer conductor  108  made from a fine mesh screen connected to outer conductor  104 A. The bottom of the probe  106  is shorted by a conductor such as a mesh screen  110 , and the probe  106  is immersed in the engine oil. Microwave energy from the oscillator  100  is reflected back by the short  10  to produce a standing wave  112  within the transmission line  104 . A diode detector is physically moved along a transmission line to find a voltage null in the standing wave, which null changes with the percentage of soot. In  FIG. 5 , a plurality of microwave diode detectors  114 ,  116  and  118  are longitudinally spaced along the transmission line  104  for illustrative purposes. When the location of the null voltage is found, it is compared to the location of the null voltage point with no soot in the oil. From this comparison, the level of soot in the oil is determined. 
   While the teachings of Nagy et al. were applied easily in laboratory settings, their implementation in an actual automotive sensor proved difficult. 
   SUMMARY OF THE INVENTION 
   The present invention detects the level of soot in engine oil. In a preferred embodiment, a diode detector is used, but requirement of Nagy et al. that the diode detector physically move to detect levels has been removed. The apparatus and method described herein also optionally compensates for temperature variations that can effect the null point. 
   Broadly, one embodiment of the present invention is a method for detecting a concentration of soot particles in engine oil. The method comprises the step of applying a microwave signal having a frequency within a range of frequencies to one end of a transmission line having a probe tip at a second end of the transmission line where the probe tip is exposed to the engine oil. The method also includes the steps of detecting a probe voltage at a stationary detection point along an axial length of the transmission line for selected frequencies within the range of frequencies and determining a probe frequency of the selected frequencies when the probe voltage is equal to a null voltage of a standing wave within the transmission line. Further, the method includes the step of comparing the probe frequency to a probe reference frequency, wherein the probe reference frequency is a frequency of the microwave signal when the probe voltage is equal to a null voltage for a known concentration of soot particles in the engine oil. Finally, the method includes the step of calculating the concentration of soot particles in the engine oil based upon a result of the comparing step. 
   A second embodiment of the present invention is an apparatus for apparatus for detecting a concentration of soot particles in engine oil. The apparatus comprises means for applying a microwave signal having a frequency within a range of frequencies to one end of a transmission line having a probe tip at a second end of the transmission line. The probe tip is exposed to the engine oil. The apparatus also includes means for detecting a probe voltage at a stationary detection point along an axial length of the transmission line for selected frequencies within the range of frequencies and means for determining a probe frequency of the selected frequencies when the probe voltage is equal to a null voltage of a standing wave within the transmission line. Finally, the apparatus includes means for comparing the probe frequency to a probe reference frequency, wherein the probe reference frequency is a frequency of the microwave signal when the probe voltage is equal to a null voltage for a known concentration of soot particles in the engine oil, and means for calculating the concentration of soot particles in the engine oil based upon a result of the comparing step. 
   Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     The description herein makes reference to the accompanying drawings wherein like reference numerals refer to like parts throughout the several views, and wherein: 
       FIG. 1  is a schematic diagram of a soot detector with a soot probe in contact with engine oil according to one embodiment of the present invention; 
       FIG. 2  is a schematic diagram of the soot probe according to  FIG. 1  connected to an oscillator; 
       FIG. 3  is a block diagram of a soot detector according to one embodiment of the present invention; 
       FIG. 4  is a graph showing the change in soot content versus the change in input voltage to the oscillator resonate circuit of  FIG. 3  divided by the null voltage; and 
       FIG. 5  is an apparatus used in a prior art method of measuring the relative permittivity of engine oil. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   A soot detector illustrating the apparatus and method according to the present invention is described with reference to  FIGS. 1-4 . One embodiment of the soot detector  10  is shown in relationship with an oil pan  30  in FIG.  1 . The detector  10  includes a soot probe  12  extending from control electronics  14  typically located in the passenger cabin (not shown). Thus, the soot probe  12  can be as long as a meter or more. In the preferred embodiment, a reference probe  16  also extends from the control electronics  14 . The control electronics  14  and the reference probe  16  are described in more detail hereinafter. The soot probe  12  comprises a transmission line  18  and a probe tip  20 . The transmission line  18  can be any transmission line that generally includes a pair of parallel conductors between which the signal voltage is applied, such as metal waveguides, coaxial lines, striplines, including microstrips, etc. and certain dielectric waveguides. Preferably, however, the transmission line  18  is a coaxial cable with, for example, a Teflon® dielectric. 
   The probe tip  20  can be one of any number of configurations. As illustrated, the probe tip  20  is formed by cutting the end of the transmission line  18 , leaving a short extension of the center conductor  25  (shown in FIG.  2 ). The length of this extension is about a couple of centimeters. Of course, the center conductor of the probe tip  20  can be a separate conductor soldered to the center conductor  25  of the transmission line  18 . Four wires  22  are soldered to the shield of the transmission line  18 , preferably one wire every 90 degrees. In the embodiment shown, the wires  22  and the extension of the center conductor  25  are the same length and are soldered to a conductive disk  24 . Alternatively, the four wires  22  are bent toward the center conductor  25 , and all connections are soldered. It is possible for the dielectric of the transmission line  18  to allow the passage of oil. Thus, it is preferably that a small amount of the dielectric be removed and replaced by an epoxy material  26 . The epoxy material  26  in the transmission line  18  prevents the passage of oil into the transmission line  18 . 
   This probe tip  20  is a non-resonate transmission line extension and is similar to probe described in Nagy, et al., U.S. Pat. Nos. 4,345,202, 4,503,384, 4,543,823 and 4,544,880, each of which is incorporated herein in its entirety by reference. The open nature of this probe tip  20  allows the free flow of the engine oil being measured, while containing the generated energy field. An alternative to forming the probe tip  20  in this manner is to use a stripline as the probe tip  20 . 
   The transmission line  18  is preferably a semi-rigid coaxial cable of a length sufficient so the probe tip  20  extends into the oil to be measured. Here, oil  28  fills an oil pan  30  secured to the engine crankcase  34  by known means. The transmission line  18  extends through an opening  32  in the oil pan  30  and is secured by an insulating washer  36 . Of course, other ways of securing the transmission line  18  are possible within the level of skill of one in the art. Also, the use of the soot detector  10  in the oil pan  30  is exemplary only. The soot detector  10  can be used anywhere in the path of the oil through a vehicle. 
     FIG. 2  is a simplified schematic diagram generally illustrating the principles of the operation of the soot detector. In  FIG. 2 , a variable frequency oscillator  40  is coupled to the soot probe  12  at the end opposite the soot probe tip  20 . By example, the oscillator  40  is a Gunn diode oscillator generating signals having a frequency in the X-band, that is a microwave signal having a frequency of eight to twelve GHz. However, higher frequency oscillators, such as oscillators generating signals with a microwave frequency of 24 GHz, are desirable. Frequencies lower than those in the X-band require longer soot probes, which are possible but impractical in automotive applications. Thus, in the application described herein, it is preferable that the oscillator  40  generate a microwave energy having a frequency no lower than an X-band frequency. 
   The oscillator  40  generates the microwave energy in the transmission line  18  and the probe tip  20 . The short between the center conductor  25  and the four or more wires  22 , caused by the conductive disk  24  in the illustrated embodiment, reflects back the energy, forming a standing wave in the soot probe  12  and the transmission line  18 . As is known, the wavelength of a signal depends upon the transmission medium through which it travels. For any given wave frequency, the wavelength of a signal in a vacuum is equal to 300,000,000 m/sec divided by the frequency. The wavelength decreases outside of a vacuum for the same applied frequency. If a transmission line comprises a coaxial cable having a Teflon® dielectric, for example, the wavelength of a standing wave within the transmission line is 0.7 times the wavelength of the signal in a vacuum at the same applied frequency. 
   The wavelength through oil is equal to the wavelength in a vacuum times a velocity factor that varies with the percentage of soot in the oil. Specifically, 
         Wavelength   oil     =         300   ,   000   ,   000   ⁢           ⁢   m   ⁢     /     ⁢   sec     Frequency     *   Velocity   ⁢           ⁢     factor   .           
 
Since the wavelength through a transmission line changes only with frequency according to the known relationship previously described, the only variable when the applied frequency and the wavelength of the standing wave in the entirety of the soot probe  12  are known is the percentage of soot in the oil.
 
   The present invention takes advantage of these relationships as shown in the example of  FIG. 2. A  diode detector  52  is located at a fixed detection, or sampling, point to detect the voltage at the sampling point. Assume that the diode detector  52  detects a null voltage for the first standing wave  38  at the sampling point for a particular percentage of soot. For example, the diode detector  52  can be located at a known position along the axial length of the transmission line  18  so that a null voltage is detected at the sampling point when the percentage of soot is zero. At this first frequency generated by the oscillator  40 , the wavelength of the standing wave  38  within the probe tip  20  is λ 1 . The wavelength of the standing wave  38  in the transmission line  18  can be different. Null point, shows one location of the null voltage for the standing wave  38  in the transmission line  18 . 
   As the percentage of soot in the oil increases, the propagation velocity (and the velocity factor) within the probe tip  20  changes. Thus, wavelength within the probe tip  20  changes from λ 1  even as the frequency remains the same. Of course, the wavelength within the transmission line  18  does not change if the frequency remains the same. However, the null point, i.e., the location of the null voltage, does. As shown in  FIG. 2 , for example, an increase in the percentage of soot results in a standing wave  39  within the probe tip  20  with a wavelength of λ 2 . As the wavelength changes, the null points of the resultant standing wave  39  move axially along the transmission line  18 . Null point 2  is one location of the null voltage for the standing wave  39  in the transmission line  18 . The only way to bring the null point back to the sampling point, i.e., to sense the null voltage with the diode detector  52 , is to change the frequency generated by the oscillator  40 . The change in the frequency necessary to bring the null point back to the sampling point indicates the percentage of soot in the oil. Specifically, and as described in more detail hereinafter, a linear relationship can be developed between the change in frequency and the percentage of soot in the oil. 
   The preferred embodiment of the soot detector  10  is shown in block diagram form in FIG.  3 . The variable frequency oscillator  40  shown is a Gunn diode oscillator operating in the X-band frequency range, selected because of its ready availability and relative low cost. Because such oscillators are typically temperature-sensitive, however, the soot detector  10  includes both the soot probe  12  previously described and a reference probe  16  as mentioned briefly with respect to FIG.  1 . The use of the reference probe  16  for temperature compensation is discussed in more detail hereinafter. A varactor diode is part of the resonate circuit for the oscillator  40 . The frequency of the oscillator  40  varies by the periodic application of a varying voltage, called a sweep voltage, to the varactor diode. As the sweep voltage applied to the varactor diode changes, the capacitance across the varactor diode changes, which in turn changes the resonate circuit and the output of the oscillator  40  to new frequencies according to known mathematical relationships. Although a varactor diode is used as part of the resonate circuit to vary the frequency of the oscillator  40 , any circuit can be used to vary the frequency of the oscillator  40 . 
   The output of the oscillator  40  is bifurcated with a power splitter  42 . Isolators  44  and  46  respectively isolate the electromagnetic signals of the oscillator  40  and the power splitter  42  from the soot probe  12  and the reference probe  16 . More specifically, the isolator  44  is connected at one end to the power splitter  42  and is connected to a first directional coupler  48  coupled to the soot probe  12 . The isolator  46  is connected at one end to the power splitter  42  and is connected to a second directional coupler  50  coupled to the reference probe  16 . 
   A first diode detector  52  for detecting the voltage at the stationary sampling point along the axial length of the transmission line  18  is coupled to the first directional coupler  48  through first parallel isolators  56 . Similarly, a second diode detector  54  is coupled to the second directional coupler  50  through second parallel isolators  58 . The second diode detector  54  detects the voltage at a stationary sampling point along the axial length of the reference probe  16 . The voltage respectively read by each of the detectors  52 ,  54  is supplied to a controller including a processor, such as a microcontroller  60 , which stores the values and determines the percentage of soot using those values. Although the controller is shown as a microcontroller  60 , the controller can be, for example, a microprocessor with external memory or any equivalent circuitry that can perform the functions described herein. 
   The microcontroller  60  preferably generates the sweep voltage in the form of a ramp voltage through a digital-to-analog (D/A) converter, which is connected to the varactor diode frequency control of the oscillator  40 . By example, the ramp voltage is a voltage that periodically ramps down from high of ten to 20 volts to a low of one volt. Alternatively, the ramp voltage is a triangular-shaped waveform that ramps up and down from the high voltage to the low voltage and back. The oscillator  40  sweeps through its frequency range in response to the sweep voltage and supplies microwave energy to each of the soot probe  12  and the reference probe  16 . 
   As previously mentioned, the frequency of certain oscillators, particularly Gunn diode oscillators such as the oscillator  40 , can vary with temperature. While the use of a reference probe  16  is not necessary, its use is desirable when a temperature-sensitive oscillator is used. The reference probe  16  is made of the same conductor as the transmission line  18  of the soot probe  12 , preferably a coaxial cable having a Teflon® dielectric, so that a wave will propagate at the same rate as the transmission line  18 . The reference probe  16  is also shorted at the end opposite the oscillator  40 . Like the soot probe  12 , the reference probe  16  can be shorted by a conductive disk soldered to the center conductor and the shield. In contrast,  FIG. 3  shows the center conductive wire  17  of the reference probe  16  extending past the cut end. The conductive wire  17  is soldered to the shield. Of course, the conductive wire  17  can be a separate wire soldered between the center conductor and the shield of the transmission line that comprises the reference probe  16 . The conductive wire  17  is shown extending from the end of the reference probe  16  only so that it is easily seen. If a conductive wire  17  is used to short the end of the reference probe  16 , the conductive wire  17  should lie flush with the dielectric of the reference probe  16  between the center conductor and the shield. 
   The soot probe  12  as previously described can be as long as a meter or more to extend into the oil. The reference probe  16  can also be long enough to extend into the oil as shown in FIG.  3 . However, the reference probe  16  can also be merely an inch or two, as shown in FIG.  1 . The length of the reference probe  16  is, determined during calibration of the soot detector  10 , which preferably occurs under zero soot conditions. As the sweep voltage applied to the varactor ramps down, the length of the reference probe  16  is adjusted so that a null voltage is detected first on the reference probe  16 , then on the soot probe  12 . The difference between the sweep voltage when a null point along the axial length of the reference probe  16  is detected and the sweep voltage when a null point along the axial length of the soot probe  12  is detected is stored in the microcontroller  60  as a calibration factor. 
   In operation, the oscillator  40  sweeps through its frequency range in response to the sweep voltage as previously described. The oscillator  40  supplies microwave energy to the soot probe  12  and the reference probe  16 . For each voltage in the sweep voltage range, the second detector  54  detects the voltage at the sampling point of the reference probe  16 . The DC reference output signal  64  is sampled through an analog-to-digital (A/D) input of the microcontroller  60  or is supplied to a digital input of the microcontroller  60  after passing through an A/D converter (not shown). Similarly, for each voltage in the sweep voltage range, the first detector  52  detects the voltage at the sampling point of the soot probe  12 . The DC soot probe output signal  66  is sampled through another A/D input of the microcontroller  60 , or is supplied to another digital input of the microcontroller  60  after passing through an A/D converter. 
   As the oscillator  40  sweeps through its frequency range, the microcontroller  60  checks each output signal  64 ,  66  for a null voltage. The difference between the sweep voltage when the second detector  54  senses a null voltage and the sweep voltage when the first detector  52  senses a null voltage at any given percentage of soot does not change due to temperature. Since the ambient temperature in the region of the oscillator  40  affects its output frequency, and an oscillator signal having the same frequency goes to both the soot probe  12  and the reference probe  16 , the only variable is the velocity factor change due to the percentage of soot in contact with the probe tip  20 . Thus, the change in the sweep voltage necessary for the first detector  52  to see a null point along the axial length of the soot probe  12  can indicate the percentage of soot in the oil without errors caused by possible temperature variations in frequencies output by the oscillator  40  when adjusted using the calibration factor previously stored. That is, any difference between the voltage applied to the varactor when the null voltage of the reference probe  16  is detected and the voltage applied to the varactor when the null voltage of the soot probe  12  is detected that is beyond the calibration factor is due solely to soot content. The sweep voltage has a known relationship to frequency, as described previously with respect to  FIG. 2 , so that the change in the sweep voltage to get back to a null point indicates the change in the frequency necessary to bring the null voltage back to the sampling point. 
     FIG. 5  is a graph showing the results of the method according to the invention compared to the standard Thermogravimetric analysis (TGA) method. The data in  FIG. 5  is from a diesel engine run 300 hours with samples collected and measured using the TGA method at certain test points. The other axis of the graph is the change in sweep voltage applied to the varactor needed to get back to a null point as the percentage of soot changes. This difference in voltage is divided by the detected null voltage in volts. The null voltage is not zero; it is a low voltage usually in the millivolt range that is affected by the soot content. By dividing the change in sweep voltage by the null voltage, a linear relationship is revealed through which the soot content can be determined. Of course, an additional calculation can be performed to show the change in frequency instead of the change in voltage, but this is not necessary as the change in voltage represents the change in frequency according to the known relationship of the varactor input voltage to the frequency output of the oscillator. 
   While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims, which scope is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures as is permitted under the law.