Abstract:
A system for detecting a characteristic of oil in a reservoir includes a sensor at least partially located in the reservoir. The sensor has a chamber for receiving oil from the reservoir, a detector member in the chamber, and an electromagnetic coil. Application of voltage across the electromagnetic coil moves the detector member and the presence and viscosity of oil in the chamber affects the rate of that movement. The electric current through the electromagnetic coil is measured and analyzed to determine an amount of time that the detector member took to move between two positions in the chamber. That amount of time is used to determine whether there is at least a predefined amount of oil in the reservoir and, if so, the viscosity of the oil.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of U.S. Provisional Patent Application No. 60/938,328 filed on May 16, 2007. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to apparatus for sensing oil in an internal combustion engine, and more particularly to such sensors that detect the level of oil in a reservoir of the engine and that provide a signal indicating the viscosity of the oil. 
     2. Description of the Related Art 
     Internal combustion engines are lubricated by oil that is stored in a reservoir, typically an oil pan located underneath the cylinder block of the engine. An oil pump draws the oil from the reservoir and forces it through passages to the top of the cylinder block. After exiting those passages, the oil lubricates various components of the engine, as it flows downward through the cylinder block by gravity ultimately returning into the reservoir. 
     A sensor often has been used to detect pressure at the outlet of the oil pump to provide an indication to the operator of the engine whether there is sufficient oil for proper lubrication. However, this pressure sensor does not provide an indication of the oil&#39;s viscosity. Engine lubricating oil is commercially available in different viscosities and a particular engine is designed to use a specific type of oil. If oil of an improper viscosity for a given engine is used, the components of that engine may not be properly lubricated and damage to those components may result. 
     Therefore, it is desirable to determine whether there is a sufficient amount of oil within the reservoir and whether that oil is the proper viscosity. 
     Operation of an engine usually is controlled by a microcomputer that monitors the level of engine usage and the operating conditions. From such monitoring the microcomputer is able to determine when the lubricating capability of the oil becomes depleted and the oil needs to be replaced. At that time the microcomputer provides an indication of that need to the engine operator. When the oil is changed, the service technician must manually reset that indication, a process that differs for each make of motor vehicle. Therefore, it is desirable to provide a mechanism by which the microcomputer can detect when the oil has been changed and automatically reset the oil change indication. 
     SUMMARY OF THE INVENTION 
     A sensor is provided to detect a characteristic of oil within a reservoir of an internal combustion engine. The sensor comprises a chamber for receiving oil from the reservoir, a ferromagnetic detector member movably received in the chamber, and an electromagnetic coil that produces a magnetic field. The detector member preferably is biased by a spring. Energizing the electromagnetic coil produces the magnetic field that moves the detector member in one direction through the chamber and deactivation of the electromagnetic coil terminates the magnetic field allowing the spring to drive the detector member in the opposite direction. 
     Oil from the reservoir enters the chamber within the sensor and affects the rate at which the detector member moves. Specifically, the absence of oil within the chamber, which then is filled with air, provides minimal resistance to the motion of the detector member. Because oil is more viscous than air, its presence within the sensor chamber provides a greater resistance to motion of the detector member. In fact, the amount of that resistance is a function of the viscosity of the oil, thus the rate at which the detector member moves is related to the viscosity of the fluid (air or oil) in the sensor chamber. 
     Movement of the ferromagnetic detector member with respect to the electromagnetic coil changes the permeance of the sensor&#39;s magnetic circuit which affects electric current flowing through that coil. By analyzing the waveform of that electric current, the relative speed of the detector member can be determined and then analyzed to determine whether oil is present within the sensor chamber and the viscosity of that oil. Specifically the amount of time that it takes the detector member to move between two positions in the chamber is measured from features of the electric current waveform. That amount of time is employed to determine the characteristic of the oil in the reservoir. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of an oil pan on an internal combustion engine with a sensor according to the present invention mounted in a wall of the oil pan; 
         FIG. 2  is a cross sectional view through the sensor mounted on the oil pan; 
         FIG. 3  is a block schematic diagram of a control circuit for operating the sensor and analyzing the electric current flowing through the sensor; 
         FIG. 4  is an exemplary waveform of the electric current flowing through the sensor; and 
         FIG. 5  is a flowchart of the process by which the control circuit determines the presence or absence of oil and the viscosity of any oil that is present. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates an internal combustion engine  10  that has a cylinder block  12  at the bottom of which an oil pan  14  is attached. The oil pan  14  serves as a reservoir for lubricating oil used in the engine  10 . An oil sensor  16  is located in an aperture on one side wall of the oil pan  14  at a position such that should the oil within the pan drop beneath the location of the sensor, there would be an undesirably small amount of oil in the reservoir. The sensor  16  is an electrically operated device that receives a signal via a cable  18 . 
     With reference to  FIG. 2 , the oil sensor  16  is located within an aperture  20  in the oil pan  14  and has a housing  22  that engages a shoulder  24  in that aperture to limit how far the sensor can be inserted into the aperture. The housing  22  has a cup shaped piece  21  with an open end that is closed in a fluid tight manner by a disk  23 . An O-ring  25  extends within a groove around the exterior of the sensor housing  22  to provide a fluid seal with respect to the oil pan  14 . When the sensor housing  22  abuts the shoulder  24 , a snap ring  26  is inserted in an annular groove  28  around the oil pan aperture  20  to secure the sensor in place. 
     The sensor housing  22  contains a solenoid  30  that has an electromagnetic coil  32  wound on a conventional plastic bobbin  34 . The electromagnetic coil  32  and bobbin  34  are held within a magnetic core  35  formed by components of a ferromagnetic material, such as steel. Specifically the magnetic core  35  comprises a cup  36  and a cylindrical pole piece  38  located centrally within the cup and abutting the flat inside surface of the cup, thereby forming a core that has an E-shaped cross section. The interior of the cup  36  opens into a chamber  40  within the housing  22 . A disc-shaped plate of ferromagnetic material forms a detector member  41  that is located within the chamber  40  and slides along a guide pin  43  that is embedded in a wall of the housing  22 . The detector member  41  is biased away from the solenoid  30  by a coil spring  42 . This biasing forms two working gaps  45  and  46  in the magnetic circuit between the core  35  and the detector member  41 . One is an annular gap  46  around the lip of the cup  36  and the other gap  45  is at the exposed end of the cylindrical pole piece  38 . 
     A first aperture  47  at the bottom of the sensor housing  22  provides a fluid drain passage between the sensor chamber  40  and the interior cavity  39  of the oil pan  14 . The sensor housing  22  and the oil pan aperture  20  are keyed so that the sensor  16  only can be mounted on the oil pan  14  with the first aperture  47  facing downward, so that oil drains through that aperture by gravity. A second aperture  48  near the top of the sensor housing  22  provides another fluid passage between the sensor chamber  40  and the oil pan&#39;s interior cavity  39 . The second aperture  48  extends through a boss  49  on an interior surface of the sensor chamber  40  against which the detector member  41  rests in the de-energized state of the solenoid  30 , thereby closing the fluid passage provided by the second aperture. Two additional bosses  44  (only one being visible in  FIG. 2 ) also are provided on that surface of the sensor chamber  40 , so that the detector member  41  rests perpendicular to the axis of the guide pin  43 . 
     The spring  42  normally biases the detector member  41  away from the solenoid  30  and its electromagnetic coil  32 . When an electric voltage is applied to the solenoid, the electromagnetic  32  generates an magnetic field which attracts the detector member  41  toward the solenoid. The force of the magnetic field overcomes the force of the spring  42 , thereby causing the detector member  41  to abut the open end of the cup  36  of the solenoid core  35 . When the electric voltage is removed from the electromagnetic coil  32 , the magnetic field ceases and the force of the spring  42  moves the detector member  41  away from the solenoid  30  returning that plate to the position illustrated in  FIG. 2 . 
     The speed at which the detector member  41  moves toward the solenoid  30 , each time electric voltage is applied to the electromagnetic coil  32 , is affected by the fluid within the chamber  40 , and particularly the viscosity of that fluid. When there either is no oil within the oil pan  14 , as occurs during an oil change, or the level of that oil is below the position of the sensor  16 , any oil that was previously within the sensor chamber  40  drains out through the first aperture  47  and air enters that chamber. Air has a relatively low viscosity, as compared to conventional lubricating oils, thereby air in the chamber  40  allows more rapid motion of the detector member  41  in response to energizing the solenoid  30 . 
     When the oil pan  14  is refilled with oil, the air in chamber  40  is trapped and prevents the oil from entering through the first aperture  47  at the bottom of the sensor housing  22 . Note that in the de-energized state of the solenoid  30 , the detector member  41  closes the second aperture  48  near the top of the housing. Thereafter, cycling the solenoid  30  on and off repeatedly moves the detector member  41  back and forth within the chamber  40 , thereby intermittently opening the second aperture  48  to allow the air to escape and oil to enter through the first aperture  47 . Typically the solenoid  30  is cycled at a frequency of one Hertz, for example, and five to seven cycles are adequate to exchange the fluid so that the chamber  40  becomes filled with oil. More or less cycles may be necessary depending on the operating frequency, the viscosity of the oil and the volume of the sensor chamber. When the chamber  40  contains oil, the greater viscosity of the oil, as compared to air, causes the detector member  41  to move slower. 
     With reference to  FIG. 3 , the sensor  16 , electrically represented by its electromagnetic coil  32 , is part of an oil detecting system  50  and is connected to a control circuit  52 . The control circuit  52  comprises a coil driver  51 , a controller  53  and a current sensor  54 . The electromagnetic coil  32  is energized by a fixed level of electric voltage produced by a coil driver  51  in response to a command from a controller  53  in the motor vehicle. The controller  53  may be a dedicated controller for the oil sensing or it may be one of the microcomputer based controllers already present for operating other components of the engine or the motor vehicle powered by the engine. The controller  53  executes a software program that is stored along with data in the controller&#39;s internal memory. That program activates the coil driver  51  to apply the fixed voltage across the electromagnetic coil  32 . The resultant electric current flowing through the electromagnetic coil  32  is measured by a current sensor  54 , which may be any of several well known types for sensing a direct current. The current sensor  54  provides an input signal to the controller  53  indicating the magnitude of the electric current flowing through the electromagnetic coil  32 . 
     With reference to the graph  FIG. 4 , when a drive voltage is initially applied to the electromagnetic coil  32 , the resultant electric current begins to rise to a peak  57  and then drops precipitously to a cusp  58  at a time designated T 1  when the detector member  41  strikes the solenoid core  35 . The depth of the cusp, designated ΔI, is a function of the velocity of the detector member and the changing magnetic permeance with the stroke of the solenoid. After the cusp  58  at time T 1 , the current rises again. 
     The controller  53  is able to detect when the input signal from the current sensor  54  indicates the occurrence of the cusp  58 . The length of time ΔT between the initially applied electric current to the electromagnetic coil  32  and the cusp  58  varies depending upon the viscosity of the fluid within the sensor chamber  40 . Therefore, by analyzing the current waveform, as provided by the signal from the current sensor  54 , and particularly measuring the length of period ΔT, the controller  53  is able to determine whether the chamber  40  is filled with air, indicating an abnormally low level of oil in the pan  14 , or has oil therein, which denotes that the oil pan is adequately filled. The duration of period ΔT also varies as a function of the particular viscosity of the oil within the pan, i.e. the greater the viscosity, the longer the period ΔT. Thus the controller  53  also is able to determine whether the oil within the pan has the proper viscosity for this particular engine. The controller  53  provides information about the oil level and viscosity via a communication link  55  to the instrument panel for the motor vehicle in which the engine  10  is located. The communication link  55  also can carry that information to other computers in the motor vehicle. As an alternative or an additional feature, the controller is connected to a separate indicator  56  through which the oil level and viscosity information are presented to a human operator. 
     To make those determinations the controller  53  performs a process  60  implemented by a software routine, such as the one depicted in  FIG. 5 . The process  60  commences at step  62  by the controller initializing the variables, counters and a timer used during the process. The coil driver  51  is signaled to apply a predefined, constant level of electric voltage to the valve&#39;s electromagnetic coil  32  at step  64 . Next at step  66 , the controller starts a software timer that measures how long it takes for the detector member  41  to travel from the position shown in  FIG. 2  to the lip of the cup  36 , at which time the cusp  58  occurs in the current waveform. 
     At step  68 , the controller  53  reads the input signal from the current sensor  54  and determines the magnitude of the electric current (I NEW ) flowing through the electromagnetic coil  32  in the oil sensor  16 . The present level of the electric current (I NEW ) is compared to the previous sensed level, designated I OLD , which is stored temporarily in the controller&#39;s memory. When the oil sensor is initially activated, the current increases, i.e. presently sensed electric current level (I NEW ) is greater than the previously sensed electric current level (I OLD ). If that relationship exists, as determined at step  70 , the process branches to step  72  at which the value (COUNT 1 ) of a first counter stored in memory is reset to zero. Then a check is made whether a software flag is set. During the initial waveform segment  75 , while the current level is increasing the counter remains a zero and the flag is not set. As a result, the process branches from step  74  to step  76  near the beginning of the process at which the value of I OLD  is set equal to the most recently sensed electric current level. The cycle repeats by again reading the input signal from the current sensor  54  to obtain a new electric current measurement (I NEW ). 
     Eventually the coil current reaches a peak  57  (see  FIG. 4 ) and begins decreasing during a subsequent waveform segment  77 . During that waveform segment  77 , the presently sensed electric current level (I NEW ) is less than the previously sensed electric current level (I OLD ), so that at step  70  the process now branches to a section the detects the downward portion of the current waveform after the peak  57 . At step  78 , the value (COUNT 1 ) of a first software counter is incremented and the value COUNT 2  of a second software counter is reset to zero at step  80 . Then the first counter value COUNT 1  is tested to determine if the count is greater than a threshold value X. Until that determination is true the process continues looping through steps  68 ,  70 ,  78 - 82  and  76 . After X number of consecutive passes, COUNT 1  is greater than X and the process branches from step  81  to step  82  where the flag is set. 
     If while the current was initially increasing waveform segment  75  ( FIG. 4 ), a sporadic new current level measurement was less that the previous current level (I NEW &lt;I OLD ), this anomaly also causes the process to branch from step  70  to step  78  even though the current peak  57  has not occurred. As a result the first counter&#39;s value (COUNT 1 ) is incremented during the anomaly, however due to the short duration of that anomaly, the COUNT 1  never reaches a value of X and the flag does not get set at step  82 . Thus when the current begins to increase again, i.e., I NEW &gt;I OLD , in waveform segment  75  and the process advances once more from step  70  to step  74  the flag will not be found set and the process returns back through step  76  to obtain a new electric current measurement at step  68 , just as though the anomaly never occurred. 
     In due course during waveform segment  77  after the current peak  57 , the more than X consecutive electric current samples I NEW  will be acquired that are less than the previously sensed electric current level (I OLD ), so that repeated branching through steps  78  and  80  results in the flag being set at step  82 . Setting the flag indicates that the electric current waveform has reached the peak  57  and begun the downward waveform segment  77  toward the cusp  58 . Until that cusp occurs the process continues to loop through steps  68 ,  70 ,  78 - 82  and  76 . 
     Occurrence of the cusp  58 , causes the coil current to again begin increasing in another waveform segment  79 . The next time thereafter that step  70  is executed the process branches through step  72  to step  74 . Now the controller  53  finds that the flag has been set which causes further advancement to step  84 . A transient increase in the current level that may occur between the current peak  57  the cusp  58  is prevented from erroneously being considered as the cusp, by requiring that the current level remain increasing for a plurality (Y) of consecutive processing cycles. That requirement is implemented by incrementing the value COUNT 2  of the second software counter on each pass through this processing branch and determining that the cusp  58  occurred only after the COUNT 2  is greater than Y. Note that after a transient increase in the current level lasting less than Y consecutive processing cycles, the process again branches from step  70  through step  78  to step  80  at which the value COUNT 2  of the second counter is reset to zero. 
     When the coil current level now increases for more than Y consecutive processing cycles as occurs during waveform segment  79 , a determination is made that the cusp  58  occurred and the process branches to step  88 . At this juncture, the controller  53  stops the timer and at step  90  signals the coil driver  51  to terminate applying the voltage to the sensor&#39;s electromagnetic coil  32 . 
     The operation of the controller  53  enters a phase in which the timer value is analyzed to ascertain whether there is an adequate level of oil in the oil pan  14  and, if so, to determine the viscosity of that oil. Therefore at step  92 , the controller checks whether the timer&#39;s value (ΔT) is less than a period of time T AIR  which indicates that air is present in the oil sensor chamber  40 . As noted previously, the length of time ΔT between when the voltage was initially applied to the electromagnetic coil  32 , at which current began to flow, and the cusp  58  in the current waveform varies depending upon the viscosity of the fluid (oil or air) within the sensor chamber  40 . Because oil is more viscous than air, its presence within the sensor chamber provides a greater resistance to motion of the plate causing a longer time interval ΔT. Therefore, if the timer&#39;s measurement of ΔT is less than T AIR , air is present in the sensor chamber  40  and the amount of oil in the oil pan  14  is below a desired level. Conversely, if the timer&#39;s measurement of ΔT is greater than T AIR  then there is an adequate level of oil, because oil is present in the sensor chamber  40 . The value of T AIR  is a function of the particular design of the oil sensor  16  and is determined empirically. 
     If at step  92 , the timer&#39;s measurement of time interval ΔT is less than the threshold value T AIR , the process  60  provides an indication of a low oil level at step  94 . The controller  53  sends that indication to via a communication link  55  to the instrument panel for the motor vehicle in which the engine  10  is located and activates the indicator  56 . 
     Otherwise, when at step  92 , the timer&#39;s measurement of ΔT is found greater the threshold T AIR , as occurs when there is a desirable amount of oil, the process advances to step  96 . At this point, any low oil indication that might have been set previously now is reset, as occurs after additional oil was added to the engine. Then the time interval ΔT is employed to determine the viscosity of the oil. The duration of time interval ΔT is directly related to the viscosity of the oil, i.e. the greater the viscosity, the longer the period ΔT. The measurement of the time interval ΔT is used to address a lookup table stored in the memory of the controller  53  with the output of the lookup table being the viscosity value for the oil. That viscosity value can be displayed on the indicator  56 . In addition the controller can compare the viscosity value from the lookup table to the known proper oil viscosity for the engine and when an improper viscosity is found a warning indication is provided via indicator  56  and the instrument panel of the motor vehicle. 
     After the oil detecting system  50  determines that the oil level is unacceptably low for proper engine operation, the system also can detect when oil has been added to an acceptable level. When the oil level in the oil pan  14  drops below the oil sensor  16 , the oil drains from the sensor chamber  40  through the first aperture  47  at the bottom of the sensor housing  22 . At that time, air enters the sensor chamber. Adding oil brings the level above the sensor  16 , but does not immediately fill the sensor chamber  40  with oil because air is trapped therein preventing entry of oil. As a consequence, after a low oil indication has been given by the controller  53 , the sensor  16  preferably is cycled by repeatedly energizing and de-energizing the solenoid&#39;s electromagnetic coil  32  to move the detector member  41  back and forth several times. This action to opens the second aperture  48 , allowing the air to escape from the sensor chamber  40  and be replaced with oil, if the pan was refilled. If the oil pan  14  was not refilled, air will remain in the sensor chamber  40 . At the end of the recycling when the sensor chamber  40  is filled with oil, the oil analysis process  60  determines that fact at step  92  as explained above and the previous low oil indication is reset at step  96 . 
     The present oil sensor and signal processing also can be used to indicate when the engine oil has been changed. Many motor vehicles illuminate a light on the instrument panel when the oil should be changed. Presently a service technician must manually reset that light, the process for doing so differs with each make of motor vehicle. A determination by the present oil analysis process  60  that the oil has been changed can be used to turn off that light automatically. When oil is drained from the oil pan  14 , the oil also drains from the sensor chamber  40  and is replaced by air. Refilling the oil pan  14  does not immediately fill the sensor chamber  40  with oil because the air is trapped therein preventing the entry of oil. Therefore while the change oil indicator light is illuminated, the sensor  16  is cycled by repeatedly energizing and de-energizing the solenoid&#39;s electromagnetic coil  32  to move the detector member  41  back and forth several times to open the second aperture  48  to allow the air to escape from the sensor chamber  40  and be replaced with oil, if the pan was refilled. During this cycling of the sensor solenoid  30  after an oil change, the controller  53  observes the time interval ΔT between the inception of the electric current and the cusp  58  getting significantly longer within five to seven cycles, due to the air being replaced by oil. In response to that observation, the controller  53  determines that the oil had been drained from the oil pan  14  and replaced. In response an indication that the oil has been changed is send via the communication link  55  to the controller that governs illumination of the oil change light on the instrument panel. 
     The foregoing description was primarily directed to a preferred embodiment of the invention. Although some attention was given to various alternatives within the scope of the invention, it is anticipated that one skilled in the art will likely realize additional alternatives that are now apparent from disclosure of embodiments of the invention. Accordingly, the scope of the invention should be determined from the following claims and not limited by the above disclosure.