Abstract:
A continuous liquid level measurement system is disclosed which is based upon a differential pressure measurement. The pressure of the liquid level to be measured is calibrated with a second pressure measurement of the pressure above the liquid in order to get a more accurate pressure measurement which is related in a predetermined manner to the level of the liquid. For instances in which the liquid to be measured is in a harsh environment, the invention provides for a means for mounting the sensitive pressure measurement devices remotely from the liquid.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention generally relates to measurement systems and, more particularly, to a continuous liquid level measurement system. 
     BACKGROUND OF THE INVENTION 
     In the field of internal combustion engines, it is often desirable to measure the level of various engine liquid levels, such as engine oil, fuel, coolant, etc. In the case of engine oil, for example, engines are nearly always provided with an oil dipstick which is extended down a tube and into the engine oil pan. By withdrawing the dipstick, the amount of oil in the oil pan may be roughly determined by visually inspecting the distance from the distal end of the dipstick to the uppermost reach of the oil clinging thereto. 
     This method for measuring oil exhibits several drawbacks. First, the visual measurement is an extremely vague measurement in terms of quantity of oil. In some instances, for example, it is desired to measure the amount of oil consumed by an engine versus time, in a long-term test in which the engine is operated continuously over an extended period. In such instances, a dipstick does not provide satisfactory measurement resolution or repeatability. Also, dipsticks are inconvenient from a measurement time standpoint, such as for use in fleet vehicles where it is desired to measure the oil level on a daily basis. 
     Attempts have therefore been made in the prior art to provide for automated electronic sensing of fluid levels such as engine oil levels. Such measurements can be problematic due to the harsh environment in which the measurement must be made. For example, the use of capacitive sensors in the oil pan are generally unreliable due to the high soot environment of the engine oil. Furthermore, oil pans experience high vibration levels when the engine is operating, making it difficult to maintain pressure transducer based measurement systems in the oil pan. Also, oil pan durability can be compromised when mounting sensors therein. 
     Another problem encountered when trying to measure oil level in the oil pan by measuring the pressure exerted by the oil on a pressure sensor is the inaccuracy of such measurements caused by varying head pressure above the oil (caused by blowby gases and the dynamics of the moving engine parts). Varying head pressure can also be a problem in other closed vessels, such as fuel tanks and coolant tanks. 
     There is therefore a need for an accurate liquid level measurement system that may be read automatically, that avoids reliability problems associated with a harsh measurement environment, and that produces an accurate level measurement, even with varying head pressures. The present invention is directed toward meeting this need. 
     SUMMARY OF THE INVENTION 
     A continuous liquid level measurement system is disclosed which is based upon a differential pressure measurement. The pressure of the liquid level to be measured is calibrated with a second pressure measurement of the pressure above the liquid in order to get a more accurate pressure measurement which is related in a predetermined manner to the level of the liquid. For instances in which the liquid to be measured is in a harsh environment, the invention provides for a means for mounting the sensitive pressure measurement devices remotely from the liquid. 
     In one form of the present invention, a continuous liquid level measurement system is disclosed, comprising a shaft having a proximal end and a distal end; a differential pressure transducer disposed at the proximal end and having a first pressure input and a second pressure input; a flexible bladder disposed at the distal end; a first pressure transmission tube disposed within the shaft and coupling the bladder to the first pressure input for substantial equalization of pressure therebetween; a reference pressure port formed in the shaft proximal of the bladder; and a second pressure transmission tube disposed within the shaft and coupling the reference pressure port to the second pressure input for substantial equalization of pressure therebetween; wherein when the bladder is placed below the liquid level and the reference pressure port is placed above the liquid level, the differential pressure transducer will measure a pressure of the liquid on the bladder, and thus the liquid level. 
     In another form of the present invention, a continuous liquid level measurement system is disclosed, comprising a shaft having a proximal end and a distal end; a first pressure transducer disposed at the proximal end and having a first pressure input and a first pressure output signal; a second pressure transducer disposed at the proximal end and having a second pressure input and a second pressure output signal; a signal processor coupled to the first and second pressure output signals and operative to produce a difference output signal substantially equal to a difference between the first and second pressure output signals; a flexible bladder disposed at the distal end; a first pressure transmission tube disposed within the shaft and coupling the bladder to the first pressure input for substantial equalization of pressure therebetween; a reference pressure port formed in the shaft proximal of the bladder; and a second pressure transmission tube disposed within the shaft and coupling the reference pressure port to the second pressure input for substantial equalization of pressure therebetween; wherein when the bladder is placed below the liquid level and the reference pressure port is placed above the liquid level, the difference output signal will be proportional to a pressure of the liquid on the bladder, and thus the liquid level. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A-D illustrate several views of a preferred embodiment continuous liquid level measurement device of the present invention. 
     FIG. 2 is a partial cut-away view of a portion of the device of FIG.  1 A. 
     FIG. 3A is a side-elevational view of the distal end of a second embodiment of the present invention. 
     FIG. 3B is a cross-sectional view of the distal end of FIG.  3 A. 
     FIG. 4 is a partial cross-sectional view of a third embodiment of the present invention mounted in an engine. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     For the purposes of promoting an understanding of the principles of the invention, reference will now be made to the embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, and alterations and modifications in the illustrated device, and further applications of the principles of the invention as illustrated therein are herein contemplated as would normally occur to one skilled in the art to which the invention relates. 
     Using the case of measurement of the level of oil in an engine oil pan as an example, several reasons have been detailed hereinabove as to why it is undesirable to mount a sensitive measurement transducer directly in or through the engine oil pan. Therefore, FIGS. 1A-D and FIG. 2 illustrate a preferred embodiment continuous liquid level measurement system of the present invention, indicated generally at  10 . The device  10  is designed to physically isolate the sensitive pressure sensor from the liquid that is being measured, in order to place the pressure sensor in a lower vibration and more convenient mounting position. In the preferred embodiment of the present invention, the device  10  is designed to mount into a dipstick port of an engine, thereby providing a low impact of incorporating the present invention to engine platforms at the design integration stage. 
     The device  10  incorporates the pressure transducer measurement device at the proximal end  12 , which is located remotely from the liquid  14  being measured. A flexible bladder or bellows  16  is provided at the distal end of the device  10  and is designed to extend into the liquid  14  to be measured. A shaft  17  extends between the proximal and distal ends. The bladder  16  is submerged in the liquid  14  to a desired depth to achieve the pressure range suitable for sensing with a pressure transducer  18  incorporated into the device  10 . A first pressure transmission tube  20  is provided within the shaft  17  in order to transmit the pressure experienced by the bellows  16  to one side of the pressure transducer  18 . 
     In some applications, the liquid  14  may be subjected to a varying head pressure (as in a closed vessel such as a crankcase, fuel tank, coolant tank, etc.). Therefore, in order to obtain a pressure measurement that accurately represents the liquid  14  level, a differential pressure measurement may be made by the device  10 . The differential pressure measurement is made possible by placing a reference pressure port  22  in the shaft  17  at a level above the liquid  14 . A second pressure transmission tube  24  transmits the pressure at reference pressure port  22  to a second side of the pressure transducer  18 . 
     The bladder  16  is preferably partially filled with air (or liquid) such that there is no elastic tension exerted by the bladder  16  membrane on the air (or liquid) within it (which would cause the air (or liquid) to be at a different pressure than external pressure). The bladder  16  is preferably made from a compliant material as is known in the art that offers virtually no resistance to forces imposed upon its walls from the exterior (i.e. from the liquid  14 ). The bladder  16  is volume sized according to the volume of the first pressure transmission tube  20  and the cavity  26  within the pressure transducer  18 . 
     The pressure transducer  18  includes a flexible transducer diaphragm  28  which will flex by varying amounts depending upon the differential pressure on either side of the diaphragm  28 . The reference pressure appearing at reference pressure port  22  is applied to a first side of the diaphragm  28 , while the pressure exerted upon bladder  16  by the liquid  14  is applied to a second side of the diaphragm  28 . The bladder  16 , first pressure transmission tube  20  and transducer diaphragm cavity  26  comprise a sealed pressure transmission system, filled to a predetermined volume with air (or liquid). The bladder  16  is preferably designed to compress in one or two dimensions only (I, J) in order to minimize errors due to displacement which may be encountered if displacement is allowed along the height axis K of the fluid level. 
     In the preferred embodiment, the pressure transducer  18  comprises a differential pressure transducer as is known in the art. The effect of the reference pressure port  22  is to cause flexing of the transducer diaphragm  28  by an amount that is only related to the pressure caused by the liquid level and not by any head pressure existing above the liquid  14 . This differential pressure is sensed by the transducer  18  and an electrical signal proportional thereto is supplied to the electrical conductors  30  at the top of the device  10 . The pressure transducer output appearing at the pins  30  may be coupled to a separate control or indicator system (not shown) as will be readily appreciated by those having ordinary skill in the art. 
     The housing at the proximal end  12  of the device  10  preferably includes a threaded surface  32  for mounting the device  10  to the engine block, but may also be a flange mount design or may incorporate any other convenient means for mounting. For applications having a closed vessel with a head pressure, a seal  34 , such as an O-ring seal, may be designed into the housing. 
     It will be appreciated from the above description that the device  10  may be conveniently mounted to a system, such as an engine, in such a manner that the sensitive pressure measuring devices are located remotely from the liquid to be measured. For example, in the application of measuring the oil level in an oil pan, the proximal end  12  of the device  10  may be conveniently mounted to the dipstick port of the engine head, thereby allowing the shaft  17  to extend down into the oil pan in order to place the bellows  16  below the surface of the oil  14 . This isolates the pressure transducer  18  from the liquid  14  (and the more severe vibrations of the oil pan environment), making the device  10  design easier while extending its life and durability. Furthermore, no modifications are necessary to the oil pan in order to incorporate the device  10 , and normally either no modifications or only minor modifications will be necessary to the dipstick port in order to integrate the device  10  therein. 
     The preferred embodiment of the present invention described hereinabove may be modified in order to provide different features or to meet different design criteria. For example, in systems that do not experience varying head pressure above the liquid  14  when the measurement is being taken, it is not necessary to provide the reference pressure port  22  in such cases and the pressure transducer  18  may then comprise a standard gauge or absolute pressure measurement device. 
     In another modification, the flexible bladder  16  may be physically protected by a rigid shroud  34 , such as illustrated in FIGS. 3A-B. The shroud may incorporate one or more vent holes  36  therein in order to minimize the holding of trapped air and debris within the shroud  34 . 
     As another option for the present invention, the differential pressure sensing function does not have to performed by a single pressure transducer  18 . For example, the main pressure signal provided by the bladder  16  may be coupled to a single gauge or absolute pressure transducer (not shown), while the reference pressure signal may be coupled to a second like transducer. The output signals to the two pressure transducers can then be subtracted by any convenient means (such as by an electronic comparator microprocessor, etc.) in order to create a reference pressure signal. 
     Another embodiment of the present invention is illustrated in FIG.  4  and indicated generally at  50 . The device  50  is very similar to the device  10 , except that the bladder  16  may be coupled to the pressure transducer  18  via a flexible pressure transmission tube  52  where installation of the device  50  imposes indirect access to the liquid  14  level to be sensed. For example, the proximal end  12  of the device  50  may be mounted into the head  54  of an engine block at an angle to the oil pan  56 . In order to extend the bladder  16  into the liquid  14  in the pan  56 , it is necessary to bend the device  50  after insertion through the head  54 . Optional features of the device  50  include the provision of a flexible tube or outer spring  58  for rigidity and protection. When using a spring for the tube  58 , free rotation of the curved tube  58  is allowed within the guide tube  60  (such as those used with some engine dipstick systems) during the threaded installation and removal of the device  50 . 
     As a further modification to the device  10 , the sensor could be mounted in an inverted direction such that the liquid  14  pressure is measured directly at the pressure transducer and the head pressure is measured remotely by the bladder  16  and transmitted to the pressure transducer. 
     In any of the above configurations, it will be appreciated by those having ordinary skill in the art that the present invention provides an extremely convenient and reliable means for accurately measuring the level of a liquid in a system where there might be constraints upon where the measurement apparatus may be located. The present invention has particular applicability in the measurement in the level of oil in an engine oil pan due to the ease with which the device may be mounted to the engine through an existing or slightly modified dipstick opening. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.