Abstract:
A fluid parameter measuring instrument relies upon a torque transmitting member comprising an integrally joined and substantially concentric shaft within a tube to transfer torsional stress across a pressure differential barrier without a penetration aperture seal. On one environmental side of the barrier, a pair of thin wall beams having strain gauges secured thereto are secured proximate of the beam mid-points to the torque transmitting member and the opposite ends to barrier structure. The strain gauges are connected in an electrical balance circuit to measure torque as a function of stress on the beams.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The priority date benefit of Provisional Application No. 61/270,195 titled Pressure Isolated Strain Gauge Sensor filed Jul. 6, 2009 is claimed for this application. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to a strain gauge type torque sensor for measuring precision values of torque generated as a function of fluid parameters existent within an environment separate from the strain gauge elements such as the hostile environment found in an oil or gas well. Specifically, oil or gas well environments often include exposures to such extreme fluid parameters as high temperature, high pressure, corrosive media, shock and vibration. Additional limitations, restrictions or requirements on or of a downhole sensor usually include a small diametrical size, low power consumption and the ability to make accurate measurements in the presence of all of these factors. 
         [0004]    2. Description of Related Art 
         [0005]    Torque is often measured by the utilization of strain gauges in various configurations. These types of measurement techniques, however, are generally limited to values of torque that are high enough to create measurable strain levels within a shaft or torsion element. Also, these configurations would normally only lend themselves to physical configurations that preclude routing of associated sensor wiring within a fluid media. These criteria are often not met when measurements are to be made below the surface, as in an oil or gas well. Additionally, torque output responses derived from physical measurements often require that the torque should be measured primarily as a force imposed upon a lever of known length rather than as a material displacement. It is an object of this invention, then, to provide a strain gauge type torque sensor, suitable for use with precision physical measurement devices which develop a torque parameter output within a hostile well environment. 
       SUMMARY OF THE INVENTION 
       [0006]    A torque measurement system for fluid medium comprises a frictionless pressure isolator to couple torque from a well fluid environment into an instrument environment, and a strain gauge based torque sensor for measurement within the instrument environment. The input torque is transmitted by means of a shaft, immersed within the well pressure media, to a pressure isolator tube. Torque transmitted by the pressure isolator tube is then coupled to small, thin beams which are designed to support strain gauges which respond solely to tension and/or compression. This approach, as opposed to the more conventional measurement of shear or bending stress, allows very small values of torque, which may be present in a high pressure environment, to be accurately measured. This is also accomplished with a very low resultant input torque displacement response. 
         [0007]    Two embodiments of the design are described with different advantages for each. Additionally, the two embodiments share common design features that allow them to reject external vibration and provide isolation from coupling to the external support housing. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    The advantages and further features of the invention will be readily appreciated by those of ordinary skill in the art as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference characters designate like or similar elements throughout. 
           [0009]      FIG. 1  is an illustration of a pressure isolation tube assembly; 
           [0010]      FIG. 2  is an illustration of the strain gauge mounting structure for measuring torque; 
           [0011]      FIG. 3  is a top view of  FIG. 2 ; 
           [0012]      FIG. 4  is an illustration of the position of each strain gauge within a bridge circuit. 
           [0013]      FIG. 5  is an illustration of the torque sensor with external pressure on the pressure isolator tube. 
           [0014]      FIG. 6  illustrates a structure for isolating mounting stresses for the torque measurement; 
           [0015]      FIG. 7  is a top view of  FIG. 6 ; 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0016]      FIG. 1  depicts a pressure isolation tube,  10 , mounted at its open end  18  to an environmental isolation structure such as a pressure vessel,  14 , and closed at its other end,  16 . The closed end  16  is also integral to a co-axial shaft,  17 . Shaft,  12 , is also mounted to the closed end  16  of the isolation tube  10 . The torque transfer elements of the invention comprising shafts  12  and  17  and isolation tube  10  are all structurally integral and co-axial, any torque  15  applied to shaft  17  will result in a proportional torque  13  on the shaft  12 . Those of ordinary skill will also understand that the shaft  17  may be either solid rod structure or of smaller O.D. tube 
         [0017]    Those of ordinary skill in the art will also understand that the description “structurally integral” does not necessarily mean that all appendages  10 ,  12 , and  17  are all formed from a single material monolith. Each appendage may be formed separately and assembled into a rigid unit. 
         [0018]    Numerous physical parameters may be translated into torque. For example, a turbine stator  35  may be designed to induce a torque  13  upon the shaft  12  that is proportional to the velocity of a fluid F passing through the stator  35 . When the stator is positioned in a conduit of known cross-sectional area, that velocity may be translated by instrument calibration into a rate or volume of fluid flow per unit of time. Hence, the stator  35  is merely one form of torque generating structure for quantifying other fluid parameters. 
         [0019]    FIG.,  1 , also shows a pressure P 1  on one side of pressure vessel,  14 , and pressure, P 2 , on the other. A pressure difference between P 1  and P 2  will result in an insignificant torque component in the output torque if the rotational displacement of isolation tube  10  is kept small with respect to its zero value. Therefore pressure isolation is provided without a friction or pressure effect on the torque measured. It is also apparent that this structure can be designed for either P 1  or P 2  to be a well fluid environment and that the input can be either torque,  13 , or torque,  15 . 
         [0020]    Strain gauge sensors, as the name implies, are electrical devices that respond to strain. Traditionally, strain gauges are intimately bonded to a substrate surface. Strain is the dimensional displacement or distortion of a material that results from the application of a force on a body of the material which constitutes the substrate. The degree of strain on the body is dependent on the magnitude of the applied force, as well as the physical dimensions of the body acted upon by the force. This force, in the case of a low level torque, is also low and will generally also result in corresponding low levels of strain. This, then, will result in strain gauge responses which may be small with respect to other error effects, such as temperature sensitivities or instrumentation inaccuracies. An objective, then, for measuring low level torque is to be able to increase the strain levels related to the measurement. 
         [0021]    Conventional methods of measuring torque with strain gauges do not meet these criteria without also requiring a relatively large rotational displacement of the substrate structure. One such approach, for example, is to mount the strain gauges on a rectangular beam which is axially subjected to the torque to be measured. If the beam is made thin then a relatively large twist is required to get significant strains. This can be largely remedied by making the beam thick but, then, relatively large levels of torque are required to produce the desired output response. The result is that this approach does not lend itself to those measurements which simultaneously require sensitivity to low torque and low displacement. 
         [0022]      FIGS. 2 and 3 , illustrate the basic strain gauge mounting configuration of this invention, which serves to overcome these limitations. As an aid to further illustration the reference numbers or characters of  FIGS. 1 ,  2 ,  3  and  4  all correspond to the same or similar elements. Referring back to  FIG. 2 , the pressure isolation tube  10  is shown attached to a pressure vessel  14  at its base. Additionally, the top of the pressure isolation tube shaft,  17 , is attached, to the midpoints of two strain gauge substrates comprising thin beams  22  and  23 . Representatively, the substrate beams  22  and  23  may be thin strips of stainless steel foil. The substrate beams  22  and  23  are attached to the shaft  17  at its diametrically opposite tangent points  26  and  27  by spot welding, for example. Also, the respective ends of these beams are similarly attached to anchor posts  28  and  29  so that any torque,  15 , will result in tensile and compressive forces  24  and  25  within the beam halves of  22  and  23  attached to post  28  and opposite forces within the beam halves attached to post  29 . These forces are then sensed by strain gauges  20 ,  21 ,  30  and  31  which are applied intimately to the surface of substrate beams  22  and  23  by a suitable bonding agent such as epoxy. The strain gauges  20 ,  21 .  30 . and  31  are electrically connected to form the four legs of a full bridge circuit. This circuit is shown in  FIG. 4 . Each of the gauges, then, will respond to torque to produce a signal which adds to the bridge output. 
         [0023]    This structure offers many advantages. First, the force beams,  22  and  23 , can easily be dimensioned to provide the required level of sensitivity for the torque-to-strain conversion. This feature provides the ability to measure low stress levels. Next, the deflection of the force beams is simply the strain that is being measured, since no bending is involved. This, then, gives the desired low deflection response. Finally, all of the gauges  20 ,  21 ,  30 , and  31  contribute an output to the full bridge circuit for torque measurement but mutually cancel each other for other factors, such as position, temperature or vibration. This has the advantages of providing maximum conversion sensitivity along with rejection of temperature effects while improving long term electrical stability. The stability enhancement occurs because matched strain gauges will tend to have long term drifts that track each other and, therefore, cancel in the output. 
         [0024]    Also, as shown in  FIG. 2 , pressure P is applied within the vessel  14  to the inside of the pressure isolator tube,  10 . This vessel would normally enclose the strain gauge assembly, for measurement within a well bore, but it is shown here, for simplicity of illustration, as enclosing only the torque shaft  12 . The result is the same since, in either case, the external pressure portion of vessel  14  does not envelope the strain gauges  20 ,  21 ,  30  and  31 . This pressure P represents the external pressure which must be isolated from the strain gauges. It is generally consists of well bore fluid and can have any value from atmospheric to very high pressures, such as 20,000 PSI. If it is a high pressure then it will cause a significant axial expansion of the pressure isolator tube  10  but will impose no circuit imbalance on the full bridge connection. 
         [0025]    The configuration illustrated in  FIG. 2  is for the high pressure P to be on the inside of the pressure isolator tube  10 . As discussed for  FIG. 1 , the high pressure can also be configured to be on the outside of the isolation cylinder, as depicted in  FIG. 5 .  FIGS. 1 and 2  share common designator numbers for clarification. 
         [0026]    Both configurations have some advantages and disadvantages and the choice between them is dependent on the application. For example, applying the high pressure to the inside of the pressure isolator tube  10  eliminates the possibility of pressure collapse but it will generally require more clearance between the outside diameter of the internal shaft  12 , ( FIG. 1 ) and the inside diameter of the isolator tube  10 . For example, an internal fluid is often contaminated and the additional annular clearance between the two diameters makes plugging less likely. The effect of this seemingly insignificant requirement is to increase the torsional rigidity difference between that of the internal shaft  12  and the isolation tube  10 . The torsional rigidity of the shaft  12  is automatically less than that of the isolation tube  10  because its outside diameter of the shaft  12  must fit coaxially within the inside diameter of the isolation tube  10 . Even a small difference can be very significant because torsional rigidity varies as the fourth power of the outside diameter of a shaft. The end effect of reducing this shaft diameter will be to increase the input torque displacement required for a given output value. 
         [0027]    Any undesired measurement effects, from non-torque forces, on the torque shaft  17  ( FIGS. 2 ,  6  and  7 ) will tend to cancel in the strain gauge bridge output but this is not necessarily the case for forces, such as  38  and  39  which may be externally induced into the pressure vessel mounting  14 . This issue is addressed in  FIG. 6  where the mounting pylons  28  and  29  of  FIG. 2  have been replaced by a relatively thin walled cylinder  32  that is bridged by a rigid beam structure  33 . These are also seen in a top view in FIG. 7. The purpose of thin walled cylinder  32  is to isolate any stresses  38  and  39  in the mounting  14  from having any significant effect on the strain gauge beams  22  and  23 . It is well known that stresses applied to one end of a long, thin walled cylinder will die out rapidly along the length of the cylinder. The thin walled cylinder  32 , however, is very effective in resisting torque moments about its axis by virtue of its relatively large diameter, as compared to the torque shaft  12 . This is important because the loading of beam,  33 , from the strain gauge beams,  12  and  13 , is primarily a torque for a torque input from shaft  12 , Additionally, the rigid beam,  33 , of  FIGS. 6 and 7  serves to further isolate any stresses transmitted from the mounting through the tube  32  and thus provides a rigid, stress free, mounting base for the strain gauge beams,  12  and  13 . 
         [0028]    It will be apparent to those skilled in the art that various changes may be made in the invention without departing from the spirit and scope thereof and therefore the invention is not limited by that which is disclosed in the drawings and specifications but only as indicated in the appended claims. 
         [0029]    Although the invention disclosed herein has been described in terms of specified and presently preferred embodiments which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the invention are contemplated which may be made without departing from the spirit of the claimed invention.