Abstract:
A system for detecting aeration in a lubricant includes a sensor having a pair of spaced apart concentric rings forming a first capacitor through which the lubricant flows and a capacitor segment forming a second capacitor with the outer ring. The capacitors are connected in a balanced bridge circuit and the second capacitor is constructed to remove gas entrapped in the lubricant present in the second capacitor so that the bridge becomes unbalanced indicating gas entrapped in the lubricant flowing through the first capacitor.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to an apparatus for detecting gas entrapped in a liquid lubricating system, and more specifically, to an aeration sensing device for detecting gas entrapped in the engine oil of an internal combustion engine. 
   Engine oil has been used in engine systems to lubricate moving parts such as pistons, piston rods, compression rings, and other engine components to reduce friction and heat build up between the moving parts and an engine block. Contaminants or foreign substances found in the engine oil can inherently damage an engine system. Gas, such as air or combustion products, entrapped in the lubricant can also result in improper or inadequate lubrication of the engine components and can damage engine components including the oil pump. 
   Vehicle systems utilize various sensors within a vehicle to monitor whether an engine is operating within normal operating parameters. One type of such sensors is an oil pressure sensor that monitors the oil pressure exiting from the oil pump. If the oil pressure goes below or above a predetermined operating range, a warning indicator is displayed to the operator of a vehicle informing the operator of the improper operating condition that is occurring. However, oil pressure sensors are used only for detecting the oil pressure of the engine system, and such sensors are not indicative of aeration caused by gas entrapped in the engine oil. Aeration within the engine oil may not necessarily affect the oil pressure, but the aeration could still cause damage to the engine components. An aeration detection system as described in U.S. Pat. No. 4,599,888 utilizes a rod encased within a cylinder for monitoring the capacitance with oil flowing between the rod and the cylinder wall. However, other impurities or contaminants within the engine oil could change the capacitance. The system in the referenced patent does not differentiate between contaminants in the lubricant causing a capacitance change and aeration in the system causing a capacitance change. 
   SUMMARY OF THE INVENTION 
   The present invention concerns a system for detecting aeration in a lubricant which system includes a sensor having a pair of spaced apart concentric rings forming a first capacitor through which the lubricant flows and a capacitor segment forming a second capacitor with the outer ring. The capacitors are connected in a balanced bridge circuit and the second capacitor is constructed to remove gas entrapped in the lubricant present in the second capacitor so that the bridge becomes unbalanced indicating gas entrapped in the lubricant flowing through the first capacitor. 
   The aeration sensing system comprises: a non-conductive sensor body having opposed first and second ends; a first capacitor positioned within the sensor body and having spaced apart plates forming a first gap; a lubrication flow path formed in the sensor body between the first and second ends and including the first gap; a second capacitor positioned within the sensor body and having spaced apart plates forming a second gap in fluid communication with the lubrication flow path, the second gap being in the form of a dead-end cavity; a bridge circuit having the first and second capacitors connected in associated legs thereof; and a signal generator connected to and generating an input signal at an input of the bridge circuit, the bridge circuit being balanced when non-aerated lubricating fluid is flowing in the lubrication path and being unbalanced when aerated lubricating fluid is flowing in the lubrication path. 
   The signal generator can be an oscillator and the system includes a demodulator connected to an output of the bridge circuit for generating an output signal. The plates of the first capacitor are first and second conductive rings positioned concentrically in said sensor body and the plates of the second capacitor are the second conductive ring and a conductive segment positioned in the sensor body. The first and second rings and the segment can be formed of copper material and the sensor body can be formed of a plastic phenolic material. 

   
     DESCRIPTION OF THE DRAWINGS 
     The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which: 
       FIG. 1  is a side elevation view of an aeration sensing device according to a first preferred embodiment of the present invention in cross section taken along the line  1 — 1  in  FIG. 2 ; 
       FIG. 2  is an end view of the aeration sensing device shown in  FIG. 1 ; 
       FIG. 3  is a perspective view of the first and second capacitance plates shown in  FIG. 1 ; 
       FIG. 4A  is a view similar to  FIG. 1  of an aeration sensing device according to a second preferred embodiment of the present invention in cross section taken along the line  4 A— 4 A in  FIG. 5 ; 
       FIG. 4B  is an enlarged cross-sectional view taken along the line  4 B— 4 B in  FIG. 5 ; 
       FIG. 5  is a view similar to  FIG. 2  of the aeration sensing device shown in  FIGS. 4A and 4B ; 
       FIG. 6  is a perspective view of the first and second capacitance plates shown in  FIG. 4A ; and 
       FIG. 7  is an electrical diagram of an aeration sensing system incorporating the sensors according to the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring now to the drawings and particularly to  FIGS. 1 and 2 , there is shown an aeration sensing device  10  for detecting gas entrapped in a liquid lubricating system. The aeration sensing device  10  is mounted between a filtration device  14  (e.g. an oil filter) and a filtration device mount  12  (e.g. an engine oil filter mount). The aeration sensing device  10  includes a generally cylindrical sensor body  11  that is produced from a nonconductive material such as plastic phenolic. Alternatively, the sensor body  11  can be made from any other high temperature nonconductive composite material. The filtration device mount  12  has a lubrication outlet circuit or passageway  16  formed therein open to a facing surface  15  of the mount. The circuit  16  allows lubricant to flow from a lubricating area (not shown) to the aeration sensing device  10  and on to the filtration device  14 . A return passageway or lubrication inlet circuit  17  is formed in the mount  12  and is open to the surface  15  for allowing filtered lubricant to return from the filtration device  14  through the sensing device  10  to the lubrication area (not shown). The surface  15  is surrounded by an outwardly extending flange  13 . 
   The return path includes a tubular conduit  18  having an externally threaded first end  18   a  threadably engaged with an internal thread formed in the opening of the inlet circuit  17 . The conduit  18  extends through an axial aperture or central bore  19 , formed in the body  11 , to an externally threaded second end  18   b  that threadably engages the filtration device  14 . Although the conduit  18  is shown and described as threadably engaging the mount  12  and the filtration device  14 , other methods of fastening known in the art may be utilized. An internal diameter of the central bore  19  is slightly larger than an outer diameter of the return conduit  18  to allow the aeration sensing device body  11  to slip fit over the conduit and rotate thereabout. The conduit  18  is first threaded into the open end of the inlet circuit  17  and then receives the sensing device  10 . The sensor body  11  has a first end  11   a  that abuts the flange  13 . Then the filtration device  14  is threaded onto the second end  18   b  of the conduit  18  and rotated to tighten an end  14   a  of the filtration device against a sensor body second end  11   b  to seal the flange  13  to the surface  11   a  and to seal the end  14   a  to the surface  11   b . An annular groove  20  can be formed in the surface  11   a  to retain an O-ring  21  for better sealing at the abutting end  11   a  and the flange  13 . Typically, the filtration device  14  has an annular groove (not shown) formed in the surface  14   a  to retain an O-ring (not shown) for better sealing at the abutting ends  11   b  and  14   a.    
   Alternatively, the conduit  18  can be divided for threaded engagement with the corresponding ends of the central aperture  19 . Thus, as oriented in  FIG. 1 , a left portion of the conduit  18  would threadably engage the aperture  19  at the first end  11   a  and a right portion would threadably engage the aperture  19  at the second end  11   b.    
   A smaller diameter first conductive capacitor ring  22  and a larger diameter second conductive capacitor ring  23  are press fit into the sensor body  11 . Alternatively, the sensor body  11  may be injection molded and the first and second rings  22  and  23  may be overmolded into the sensor body  11 . Both of the first and second rings  22  and  23  are made from a suitable material such as copper tubing and share a common axis with the sensor body  11  and the central aperture  19 . Other types of conductive material may be utilized in place of the copper tubing. 
   In the preferred embodiment, as best shown in  FIG. 3 , the first ring  22  includes an axially extending wall formed of a plurality of arc shaped plates or wall segments  24  extending axially along the common axis with adjacent wall segments separated by one of a plurality of slots or openings  25 . A terminal lug  26  extends radially from the first ring  22 . The second ring  23  is of similar construction to the first ring  22  and has a plurality of arc shaped plates or wall segments  27  extending axially along the common axis with adjacent partition walls separated by one of a plurality of slots or openings  28  and a terminal lug  29  extending radially from the second ring  23 . The first ring  22  is positioned in an annular groove  30  formed in the first end  11   a  and the walls  24  extend through the body  11  to a circular recess  31  formed in the second end  11   b . Each of the walls  24  is aligned with an associated one of the walls  27  in a pair to form a gap  32  therebetween permitting fluid flow between the walls from the groove  30  to the recess  31 . The gaps  32  are arcuate in shape and correspond in number to the number of pairs of walls. Since the lubricating fluid can become electrically conductive through additives that have metallic components and/or combustion debris from engine operation, the surfaces of the rings  22  and  23  exposed to the lubricating fluid can be covered with a suitable non-conductive material,  22   a  and  23   a  respectively, to prevent shorting of the capacitor plates. The non-conductive material,  22   a  and  23   a  can be, for example, a powder coat material. Typical powder coat materials are polyester, epoxy, urethane, and mixtures thereof depending upon the desired characteristics. 
   Preferably, the first and second rings  22  and  23  each include three of the wall segments  24  and  27  respectively. However, more or less segments can be provided. One of the slots  27  is aligned with the terminal lug  29  so that the terminal lug  26  can extend upwardly through that slot when the rings  22  and  23  are inserted into the body  11 . 
   A first terminal  33  extends radially into the sensor body  11  and is threaded into the terminal lug  26  for electrical connection to the first ring  22 . Similarly, a second terminal  34  extends radially into the sensor body  11  and is threaded into the terminal lug  29  for electrical connection to the second ring  23 . Each of the terminals  33  and  34  is sealed at the exterior of the body  11  by an associated O-ring  35  for maintaining a seal against leakage either into or out of the body. 
   In a second preferred embodiment sensing device  10 ′, shown in  FIGS. 4A ,  4 B,  5  and  6 , both the first and second rings  22 ′ and  23 ′ include a substantially continuous wall extending axially along the common axis as shown in  FIG. 6 . The first ring  22 ′ has a wall  24 ′ interrupted by a single slot  25 ′ adjacent to a terminal lug  26 ′. The second ring  23 ′ has a wall  27 ′ interrupted by a single slot  28 ′ adjacent to a terminal lug  29 ′ for receiving the terminal adapter  26 ′. Both of the walls  24 ′ and  27 ′ extend only a portion of the axial length into the sensor body  11 . As shown in  FIG. 4B , the wall  24 ′ terminates short of the bottom surface of the recess  31  and the wall  27 ′ terminates short of the bottom surface of the groove  30 . Otherwise the body  11 ′ could not be formed as an integral one-piece molded structure. The surfaces of the rings  22 ′ and  23 ′ exposed to the lubricating fluid can be covered with a suitable non-conductive material,  22   a ′ and  23   a ′ respectively, to prevent shorting of the capacitor plates. 
   The engine lubricant filtration circuit  16  provides lubricant to the filtration device  14  via the aeration sensing device  10  ( 10 ′) for filtering contaminants from the lubrication. The lubricant is forced under pressure through the arcuate flow passages or gaps  32  ( 32 ′) to the filtration device  14 . The aeration sensing device  10  ( 10 ′) is included in an aeration sensing system  40  (shown in  FIG. 6 ) in accordance with the present invention. A signal generator, such as an oscillator  41 , has an output electrically connected to the sensor second terminal  34  and generates an input signal (e.g. oscillating voltage of fixed amplitude and frequency). The sensor first terminal  33  and the sensor second terminal  34  are connected at opposite ends of a first leg of a bridge circuit  42 . The first leg includes an oil aeration capacitor  43  with plates formed by the walls  24  ( 24 ′) and  27  ( 27 ′). A second leg of the bridge circuit  42  includes a first resistor  44  connected between the sensor first terminal  33  and a ground terminal  45 . A third leg of the bridge circuit  42  includes a second resistor  46  connected between the ground terminal  45  and a sensor third terminal  47 . A fourth leg of the bridge circuit  42  includes a compensating capacitor  48  connected between the sensor second terminal  34  and the sensor third terminal  47 . The terminals  34  and  45  are the inputs to the bridge circuit  42  and the terminals  33  and  47  are the outputs at which an output signal is generated. A demodulator  49  has a pair of inputs connected to the terminals  33  and  47  and an output  50  at which a sensor output signal is generated. 
   As the lubricant passes between the walls  24  ( 24 ′) and  27  ( 27 ′), the first capacitor  43  will have a capacitance value that is proportional to the areas of the plurality of partition walls (e.g., capacitance plates) and the net dielectric properties of the lubricant and the gap  32  between them. The demodulator  49  receives the output signal from the bridge  42  and converts it to an output signal (e.g., a DC signal) that is proportional to the capacitance value associated with the lubricant flowing through the first capacitor  43 . Since the lubricant has a known dielectric constant, changes in the capacitance of the first capacitor  43  will be reflected in the signal at the output  50 . However, any detected changes in the capacitance of the first capacitor  43  can be a direct result of either lubricant specifications, contaminants, aeration entrapped in the lubricant, deterioration of the lubricant, fuel dilution, or temperature affects. Since the change in capacitance may be a direct effect from any one of the sources mentioned previously, aeration entrapped in the lubricant cannot be positively identified as the cause of the change in the capacitance. To differentiate whether the change in the capacitance is a direct result of either the aeration or one of the other sources, the second capacitor  48  is added to the system to compensate for changes to the capacitance of the first capacitor  43  caused by sources other than aeration. 
   As shown in  FIGS. 1 and 2 , a capacitor segment  51  is retained in the body  11  and has an arcuate plate or wall  52  spaced from one of the walls  27 . The segment  51  is provided with a terminal lug  53  extending radially from the wall  52  and electrically connected to the third terminal  47 . The wall  52  can be made of copper and cooperates with the adjacent wall  27  to form the plate of the second capacitor  48 . A gap between the walls  27  and  52  forms a chamber for receiving the lubricant open to the recess  31  at one end and closed by the body  11  at the opposite end to function as a dead-end cavity. The distance between the capacitance plates and the surface area of the capacitance plates of the second capacitor  48  defines a capacitance equal to the capacitance of the first capacitor  43  when non-aerated lubricant is present in each capacitor. The surfaces of the capacitor segment  51  exposed to the lubricating fluid can be coated with a non-conductive material  52   a  in the manner described above. 
   The arc shaped chamber  54  is arranged so that the lubricant within the dead end cavity is in a substantially stagnant area (i.e., no flow in or out). The dead-end cavity is arranged to be filled with lubricant from the main flow, however, the dead-end cavity will hold the lubricant for a sufficient duration to allow the lubricant to de-aerate. During installation of the sensor  10  ( 10 ′), the chamber  54  is radially positioned at the lowest point by rotating the body  11  ( 11 ′) about the conduit  18  so that the ends of the arc are the highest points and any gas entrapped within this chamber can easily escape to the recess  31 . As a result, any difference between the capacitance of the first capacitor  43  and the capacitance of the second capacitor  48  is due to aeration of the lubricant flowing through the first capacitor  43 . 
   The resistors  44  and  46  are of equal value and the bridge  42  is balanced when the capacitance values of the first capacitor  43  and second capacitor  48  are equal. Thus, the terminals  33  and  47  will be at equal potential and there is no output signal for the demodulator  49  to sense. When the capacitance of the first capacitor  43  changes due to aeration of the lubricant, the bridge  42  becomes unbalanced and a bridge output signal is generated to the demodulator  49 . If the dielectric constant of the lubricant changes due to some factor other than aeration, the first capacitor  43  and the second capacitor  48  will change capacitance by an equal amount and the bridge  42  will stay balanced. The first capacitor  43  and the second capacitor  48  cause the sensing device  10  ( 10 ′) to stay in calibration even though the dielectric constant of the lubricating fluid changes during operation. Furthermore, the bridge  42  has a better immunity to the electrical noise generated by an operating vehicle. 
   In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention can be practiced otherwise than as specifically illustrated and described without departing from its spirit or scope.