Abstract:
A tubular probe having annularly spaced electrodes is immersed in ATF and sequentially excited with an alternating voltage at a relatively high and low frequency. The current is measured at both frequencies and the difference in impedance computed; and, the differential impedance is corrected for temperature and the corresponding value of one of TAN per ASTM D-669, Delta Oxidation per ASTME-168 (ΔOX) and HPDSC induction time per ASTM D-5483 (MIN) determined from a lookup table of values of TAN, ΔOX and MIN versus impedance differential for known fluid conditions. The remaining useful life (RUL) may then be computed from determined present value of TAN, ΔOX or MIN. When the temperature corrected impedance difference ΔZ TC  reaches 6.5×10 5  Ohms, the ATF is considered to have reached the end of its useful life.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to fluid condition monitoring utilizing a sensor providing an electrical signal indicating in real time the chemical condition of the fluid to be monitored. Sensors of this type are known to employ impedance spectroscopy techniques; and, an example of such a sensor is that shown and described in U.S. Pat. No. 6,278,281 Bauer, et al granted Aug. 21, 2001 in which a pair of spaced electrodes are sequentially excited at a relatively low frequency for determining the electrochemical interaction at the electrode surface and at a relatively high frequency for determining the bulk impedance of the fluid. The current is measured at both excitation frequencies and the impedance computed for each current measurement and the impedance differential computed which enables determining the fluid condition by a comparison of the computed impedance differential with that determined for known conditions of the fluid as determined by chemical analysis.  
           [0002]    It has been proposed to use such devices for monitoring fluid condition in power transmissions and for real time monitoring of lubricating oil in combustion engines. A further example of such a sensor application for engines is that shown and described in U.S. Pat. No. 6,377,052, McGinnis, et al granted Apr. 23, 2002 in which the spaced electrodes are spirally wound on a dipstick for insertion into the engine crankcase.  
           [0003]    Devices of the aforesaid type employing impedance spectroscopy may utilize the electrode arrangements of the type employing interdigitated planar arrays of electrodes or the aforementioned spiral arrangement or concentric radially spaced tubular electrodes such as for example those taught in U.S. Pat. No. 6,433,560 issued to Hansen et al. and granted Aug. 13, 2002.  
           [0004]    The aforesaid Bauer, et al. patent describes in FIG. 15 thereof the impedance determined at the aforesaid high and low frequencies for automatic transmission fluid in the new condition and after a limited number of vehicle miles in service.  
           [0005]    However, since actual vehicle service conditions depend upon the type of vehicle operation and the loading and environment during such operation, it has long been desired to provide a sensor which can provide over the service life of the vehicle a real time indication of the fluid condition based upon the actual chemical characteristics of the fluid and to indicate the amount or percentage of estimated remaining useful life (RUL) based upon the current condition of the fluid.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    The present invention provides a technique for generating a continuously varying electrical signal in real time indicative of the chemical condition of the fluid being monitored and employs algorithms based upon sensor readings in samples of fluid of known chemical conditions to provide a database for comparison with the real time electrical signal for providing an indication of the remaining useful life (RUL) based upon the current condition of the fluid.  
           [0007]    The present invention provides algorithms for determining the RUL of automatic transmission fluid, particularly fluid of the type comprising solvent dewaxed paraffinic oil in real time based upon differential impedance techniques. The present invention employs a pair of spaced electrodes configured preferably as concentrically disposed radially spaced annular electrodes for improved dispersion of the fluid over the electrode surfaces. The present invention utilizes any of three parameters derived from chemical analysis of the fluid, namely total acid number (TAN) per ASTM D-664, delta Oxidation per ASTM e-168 (ΔOX) and HPDSC induction time per ASTM D-5483 (MIN).  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a pictorial block diagram of the sensor of the present invention deployed in a fluid filled transmission casing;  
         [0009]    [0009]FIG. 2 is a block flow diagram of the system operation for determining RUL based on TAN;  
         [0010]    [0010]FIG. 3 is a flow diagram similar to FIG. 2 for MIN;  
         [0011]    [0011]FIG. 4 is a block diagram similar to FIG. 2 for ΔOX;  
         [0012]    [0012]FIG. 5 is a perspective view of the probe of the present invention;  
         [0013]    [0013]FIG. 6 is a graph plotting differential impedance values as a function of TAN;  
         [0014]    [0014]FIG. 7 is a graph similar to FIG. 6 plotting values of differential impedance as a function of ΔOX;  
         [0015]    [0015]FIG. 8 is a graph plotting values of differential impedance as a function of MIN and,  
         [0016]    [0016]FIG. 9 is a graph of changes with temperature of values of ΔZ. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    Referring to FIGS. 1 and 5, the sensor of the present invention is indicated generally at  10  and includes a probe assembly indicated generally at  12  immersed in fluid denoted ATF contained in an automatic transmission housing or casing  14 . The present invention is particularly useable with ATF of the type having solvent dewaxed heavy paraffinic oil as the essential ingredient  
         [0018]    The sensor  12  may comprise any of those known in the art, as for example, an interdigitated planar array or spirally configured electrode pair; however, in the presently preferred practice the probe  12  comprises a pair of concentrically disposed radially spaced tubular or annular electrodes  16 ,  18  retained in closely spaced concentric or nested arrangement by end caps  20 ,  22 . Inner tubular electrode  16  has a connector terminal  24  extending axially therefrom and outwardly through a clearance slot  26  formed in header  22 ; and, similarly outer electrode  18  has a connector terminal  28  extending outwardly through slot  30  formed in cap  22 .  
         [0019]    A temperature sensor, which may comprise a thermistor device, indicated by reference numeral  32  is disposed such that the sensing element thereof is exposed to the ATF within the casing  14 .  
         [0020]    The inner electrode terminal  24  is connected along line  34  to receive an excitation signal from driver  36 . The connector terminal  28  is connected along line  36  to a current sensor  40 .  
         [0021]    In the presently preferred practice of the invention, the probe  12  has the concentric electrodes  16 , 18  spaced radially a distance of about 0.15 mm for an inner electrode  18  having a diameter of about 6 mm and a length of about 38 mm. It will be understood however that other diameters and lengths may be employed to provide about the same surface area exposed between the electrodes. In the presently preferred practice the electrodes  16 ,  18  are formed of stainless steel; however, other electrode materials may be employed which are compatible with the ATF. In the present practice, the invention has been found particularly suitable for use with ATF comprising solvent dewaxed heavy paraffinic oil but the invention may be employed with other types of ATF.  
         [0022]    Referring again to FIG. 1, the excitation driver  36  receives an input along line  42  from an oscillator  44  which is powered by an on-board vehicle supply such as the 12 Volt DC supply  46  which also supplies the excitation driver  36  along line  48 . A microcomputer  50  is powered by the power supply  46  along line  52 ; and, the microcomputer receives an input along line  54  from the current sensor  40  and a temperature input along line  56  from sensor  32  and provides an output along line  58  to an alarm or readout device  60 .  
         [0023]    In the present practice of the invention, the oscillator  44  provides a low frequency alternating voltage of a frequency not greater than about 0.1 Hertz (100 milliHertz) and a relatively high frequency alternating voltage at a frequency not less than about 7.5 Hertz.  
         [0024]    The microcomputer  50  is programmed with lookup tables based upon data taken from laboratory chemical aging of the ATF and determining the differential impedance at successive intervals. The fluid samples were tested to determine any one of three known test parameters, namely Total Acid Number per ASTM D-664 (TAN), delta Oxidation per ASTM E168 (ΔOX) and HPDSC induction time per ASTM D-5483 (MIN). The data is then plotted for each of the parameters and curves drawn therebetween as displayed respectively in FIGS. 6 through 8. The graphs include data points taken for ATF stressed by laboratory oxidation aging tests, such as an Aluminum Beaker Oxidation Test (ABOT) per Southwest Research Institute, San Antonio, Tex. procedure BJ110-4 and some ATF fluid drained from vehicles in actual road service. It will be noted from FIGS. 6 through 8 that the linear approximations may be made for the data; and, algorithms for the slope used to calculate the respective chemical parameter for valves of ΔZ TC  are indicated on the graphs.  
         [0025]    Referring to FIG. 2, the operation of the system circuitry is denoted generally by reference numeral  62 ; and, upon user initiation at step  64  the system proceeds to step  68 . At step  68  the system inquires as to whether the fluid temperature T f  is within desired limits T MIN , T MAX ; and, if the answer is negative the system proceeds to abort or Stop. However, if the determination at step  68  is affirmative, the system proceeds to step  70  and excites the probe  32  with an alternating voltage at a relatively high frequency and measures the current I HI . The system then proceeds to step  72  and computes and stores the impedance Z HI  from the measured current I HI .  
         [0026]    The system then excites the probe  32  with a relatively low frequency alternating voltage and measures the current I LO  and proceeds to step  76  and computes and stores the impedance Z LO  from the measured current I LO .  
         [0027]    The system then proceeds to step  78  and computes the vector {right arrow over (ΔZ T )} by subtracting {right arrow over (Z LO )} from {right arrow over (Z HI )} yielding the vector result {right arrow over (ΔZ T )}. The system then proceeds to step  80  and computes the absolute value of ΔZ T  and proceeds to step  82  and determines ΔZ TC  the temperature compensated impedance differential from a lookup table of values of ΔZ TC  versus temperature, which table is compiled by taking data points from a temperature correction curve. Typical curves for such temperature compensation are shown in FIG. 9.  
         [0028]    It will be seen that a family of three plots; namely on upper graph: 
           ΔZ   TC =−1.59 E +04 *T +1.92 E +06, 
         [0029]    a lower graph: 
           ΔZ   TC =−9.98 E +03 *T +1.18 E +06 
         [0030]    and a middle graph: 
           ΔZ   TC =−1.16 E +04 *T +1.39 E +06 
         [0031]    plotted by interpolating between the upper and lower graph are presented in FIG. 9. It will be noted that the shapes and intercepts of the three graphs are similar; and, thus provide a region from which ΔZ TC  may be computed.  
         [0032]    The system then proceeds to store the value determined at step  82  as ΔZ TC     1    at step  84 ; and, after a suitable time delay of not more than about 10 seconds at step  86  the system proceeds to step  88  and repeats steps  70  through  82  and stores the result as ΔZ TC     2    at step  90 .  
         [0033]    The system then proceeds to step  92 , computes the change in ΔZ denoted ΔΔZ by subtracting ΔZ TC     1    from ΔZ TC     2    and proceeds to step  94  and inquires as to whether ΔΔZ is positive.  
         [0034]    If the determination at step  94  is affirmative the system proceeds to step  98  and computes TAN from a lookup table of values of TAN versus ΔZ TC  based upon the algorithm from FIG. 6: 
           ΔZ   TC =2.77 E +05* TAN−2.7 E +05 
         [0035]    If however, the determination at step  94  is negative, the system proceeds to step  96  and repeats steps  70  through  94 .  
         [0036]    After completion of any of step  98 , the system proceeds to step  100  and stores the determined value as TAN 1  and proceeds to step  102  for a time delay ΔT of not less than about one hour. The system then proceeds to step  104  and repeats steps  70  through  102  and stores the result as TAN 2  at step  106 . The system then proceeds to step  108  and computes the rate of decay ψ by subtracting TAN 1  from TAN 2  and dividing the differential by ΔT. The system then proceeds to step  110  and recalls a stored value TAN EOL  and then proceeds to step  112  and computes the remaining useful life in hours (RUL) by subtracting TAN EOL  from TAN 2  and dividing the differential by ψ. The system then displays the computed value of RUL at step  114 .  
         [0037]    Referring to FIG. 3, the flow diagram for determining RUL from the parameter HPDSC induction time per ASTM D-5483 (MIN) is shown wherein the system, upon initiation at step  116  proceeds to read the fluid temperature at step  118  from the sensor  32 . The system then proceeds to step  120  and asks whether the temperature read at T f  read at step  118  is between the limits T MIN , T MAX ; and, if the determination is negative the system proceeds to abort or Stop. However, if the determination at step  120  is affirmative the system proceeds to step  122  and excites the probe at the relatively high frequency alternating voltage and measures the current I HI . The system then proceeds to step  124 , computes and stores the impedance Z HI  computed from the current measured at step  122 .  
         [0038]    The system then proceeds to step  126  and excites the probe at the relatively low frequency alternating voltage and measures the current I LO . The system then proceeds to step  128  and computes and stores the impedance Z LO  from the current measured in step  126 .  
         [0039]    The system then proceeds to step  130 , computes the vector difference {right arrow over (ΔZ t )} by subtracting {right arrow over (Z LO )} from {right arrow over (Z HI )} and proceeds to step  132  and determines the absolute value ΔZ T .  
         [0040]    The system then proceeds to step  134  and determines ΔZ TC  from a lookup table of ΔZ T  versus Temperature which lookup table is determined from data points taken from curves such as those shown in FIG. 9 which identify the change in the differential impedance with temperature for samples of fluid of known condition. The procedure is the same as for step  82 .  
         [0041]    The system then proceeds to step  136  and stores the value ΔZ TC     1    computed at step  134  and proceeds to step  138  for a time delay of not more than about ten seconds. The system then proceeds to step  140  and repeats steps  122  through  134  and stores the computed value ΔZ TC     2    at step  142 .  
         [0042]    The system then proceeds to compute the change in ΔZ TC  denoted ΔΔZ by subtracting ΔZ TC     1    from ΔZ TC     2   . The system then proceeds to step  146  and asks whether ΔΔZ is positive; and, if the answer is affirmative the system proceeds to step  148  and repeats steps  122  through  146 . However, if the determination at step  146  is negative, the system proceeds to step  150  and asks whether ΔZ TC  is equal to or greater than 3.4E+05. If the query in step  150  is answered in the affirmative, the system proceeds to step  152  and determines MIN from a lookup table of values of MIN versus ΔZ TC  compiled from the graph of FIG. 8 using the algorithm: 
           ΔZ   TC =−2.78 E +05*MIN+2.95 E +06. 
         [0043]    However, if the determination at step  150  is negative the system proceeds to step  154  and determines MIN from a lookup table of values of ΔZ TC  versus MIN compiled from the graph of FIG. 8 using the algorithm: 
           ΔZ   TC =−1.98 E +04* MIN+5.26 E +05. 
         [0044]    Upon completion of one of the steps  152  or  154  the system proceeds to step  156  and stores the determined value of MIN as MIN 1  and proceeds to step  158  for a time delay ΔT of not less than about one hour and then proceeds to step  160  and repeats steps  122  through  154 . The value of MIN determined at step  160  is then stored as MIN 2  at step  162  and the system proceeds to step  164  and computed the rate of decay by ψ determined by subtracting MIN 1  from MIN 2  and dividing the differential by ΔT.  
         [0045]    The system then proceeds to step  168  to get a stored value of MIN EOL  and proceeds to step  170  and computed the remaining useful life RUL by subtracting MIN EOL  from MIN 2  and dividing the differential by ψ as determined in step  164 . The system then proceeds to display the computed value of RUL at step  172 .  
         [0046]    Referring to FIG. 4, the flow diagram for determining RUL from the parameter Delta Oxidation per ASTM E-168 (ΔOX) is shown wherein the system, upon user initiation at step  174 , proceeds to read the fluid temperature at step  176  and then proceeds to step  178  to determine if temperature T f  is within the limits T MIN , T MAX . If the determination at step  178  is negative the system aborts or proceeds to Stop. However if the determination at step  178  is affirmative, the system proceeds to step  180  to excite the probe  32  with a relatively high frequency alternating voltage and measures the resultant current I HI . The system then proceeds to step  182 , computes the impedance Z HI  from the measured current and stores the computed value. The system then proceeds to step  184  and excites the probe  12  with a relatively low frequency alternating voltage and measures the resultant current I LO  and proceeds to step  186  and computes and stores the impedance Z LO  from the measured current I LO .  
         [0047]    The system then proceeds to step  190  and computes the impedance vector differential {right arrow over (ΔZ t )} by subtracting {right arrow over (Z LO )} from {right arrow over (Z HI )} and then determines the absolute value of the computed differential ΔZ T  at step  192 .  
         [0048]    The system then proceeds to step  194  and determines the temperature compensated value ΔZ TC , compiled from data points taken from curves such as shown FIG. 9 from the lookup table of values of ΔZ TC  versus temperature and, the system then proceeds to step  196  and stores the computed value as ΔZ TC     1   .  
         [0049]    The system then proceeds to step  198  and provides a time delay of not more than about 10 seconds and then proceeds to step  200  and repeats steps  180  through  194  and stores the computed value as ΔZ TC     2    at step  202 .  
         [0050]    The system then computes the change ΔΔZ in the differential impedance ΔZ by subtracting ΔZ TC     1    from ΔZ TC     2    at step  204  and proceeds to step  206  and asks the question whether ΔΔZ is positive. If the answer to the query in step  206  is negative, the system proceeds to step  208  and repeats steps  180  through  204 . If the query in step  206  is answered in the affirmative, the system proceeds to step  210  and asks whether ΔZ TC  is equal to or less than 3.40E+05. If the determination in step  210  is affirmative, the system proceeds to step  212  and determines from a lookup table the values of ΔOX versus ΔZ TC  compiled from the graph of FIG. 7 using the algorithm: 
           ΔZ   TC =1.35 E +04 *ΔOX +1.32 E +05. 
         [0051]    If the system answers in the negative at step  210 , the system proceeds to step  214  and determines ΔOX from a lookup table of values of ΔOX versus ΔZ TC  based on the graph of FIG. 7 using the algorithm: 
           ΔZ   TC =2.67 E +04 *ΔOX −6.86 E +04. 
         [0052]    After completing one of the operations  214 ,  212 , the system proceeds to step  216  and stores the result as ΔOX 1  and proceeds to execute a time delay of not less than about one hour at step  218 . The system then proceeds to repeat steps  180  through  214  at step  220  and stores the result as ΔOX 2  at step  224 .  
         [0053]    The system then proceeds to step  226  and computes the difference ψ of the values of ΔOX by subtracting ΔOX 1  from ΔOX 2  and dividing the result by ΔT. The system then proceeds to step  228  and recalls a stored value of ΔOX EOL  and proceeds to step  230  to compute the remaining useful life (RUL) by subtracting ΔOX EOL  from ΔOX 2  and dividing the result by the computed value of ψ and proceeds to step  232  to display the value of RUL.  
         [0054]    Irrespective of which of TAN, ΔOX or MIN is calculated, if ΔZ TC  is measured equal or greater than 6.5×10 5 , the ATF fluid is deemed to have reached the end of its useful life.  
         [0055]    Although the invention has hereinabove been described with respect to the illustrated embodiments, it will be understood that the invention is capable of modification and variation and is limited only by the following claims.