Abstract:
A tensiometer and method for measuring tension measure strain produced by a lateral displacement of a flexible line under tension. A block having a cylindrical cavity following a curved path is used to displace the flexible line. Tension in the line produces compressive stress within the block in the direction of the radius of curvature and tensile stress in the direction opposite the radius of curvature. A resistive strain gauge is used to measure strain due to either the compressive stress or tensile stress and a measurement circuit generates either a graphical display in accordance with standard tension measurement units, an audible or visual alarm when tension exceeds a predetermined threshold, or both.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to mechanical stress measuring devices, and more specifically, to a tensionometer for measuring tension on flexible lines.  
           [0003]    2. Background of the Invention  
           [0004]    It is often necessary to measure tension on ropes, cables, wires and other structural or non-structural flexible lines. Typicaly tensiometers (or tension gauges) for measuring tension are inserted within the force path of the line, by attaching a cable to each end of the tensiometer.  
           [0005]    Tensiometers cannot sometimes be easily inserted in the force path of the flexible line, and doing so might compromise the integrity of the line due to potential failure of the tension gauge. Additionally, in some circumstances it may not be possible to remove tension from a line in order to measure the stress on the line, since the line may be structurally necessary or necessary for safety.  
           [0006]    For example, firefighters and other rescue personnel are required to place a specified tension on lines that are used for rescue operations, prior to commencing rescue. Inserting a tensiometer into the force path of a line used for rescue might compromise the safety of the set-up, and makes for a complicated operation during an emergency.  
           [0007]    Flexible lines also must be tested and tensiometers calibrated periodically. Removing a tensiometer from the force path of the line is inconvenient, and calibration of the tensiometer subject to repeating the proper reference tension on a line.  
           [0008]    Therefore, it would be desirable to provide a tensiometer that may be used to measure line tension quickly and that does not require insertion within the force path of a line. It would further be desirable to provide a tensiometer that is easily and repeatably calibrated.  
         SUMMARY OF THE INVENTION  
         [0009]    The above objective of providing a tensiometer that does not require insertion within the force path of a measured line is achieved in a tensiometer and method for measuring tension on a flexible line. The tensiometer includes a displacing surface for forcing the flexible line to curve around the displacing surface, a measuring surface mechanically coupled to the displacing surface, and a strain gauge mechanically coupled to the measuring surface for measuring a strain within said measuring surface caused by a stress within the displacing surface.  
           [0010]    The foregoing and other objectives, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1A is a pictorial diagram depicting a top view of a tensiometer in accordance with an embodiment of the present invention.  
         [0012]    [0012]FIG. 1B pictorial diagram depicting a side view of a tensiometer in accordance with an embodiment of the present invention.  
         [0013]    [0013]FIG. 1C pictorial diagram depicting a cross-section view of a tensiometer in accordance with an embodiment of the present invention.  
         [0014]    [0014]FIG. 2 is a schematic diagram depicting circuits in accordance with an embodiment of the present invention.  
         [0015]    [0015]FIG. 3A and FIG. 3B are pictorial diagrams depicting alternative embodiments of the present invention  
         [0016]    [0016]FIG. 4 is a pictorial diagram of an embodiment of the invention including an overload protection mechanism.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    Referring now to the figures and in particular to FIG. 1A, a top view of a tensiometer  10  in accordance with an embodiment of the present invention is shown. A rectangular block  11 , generally of metal, but which may be a material selected suitable for a particular range of tension measurement, has a cylindrical channel  12  cut through from one end of block  11  to the other end. The entry and exit (which are interchangeable) of the cylindrical channel lie on the same axis through the block, so that a flexible line introduced to the cylindrical channel will lie on a straight line outside of block  11 . While the illustrative embodiment depicts a cylindrical channel, it should be understood that channels of other cross-section shapes may be used, as well such as rectangular or triangular channel cross-sections.  
         [0018]    A flexible line is inserted within the cylindrical channel, and the displacement wall  13  of the curve (convex wall in the depicted plane) causes the line to be displaced from the axis on which the entry and exit lie. Within block  11 , the lateral displacement around displacement wall  13  causes compressive stress within the region surrounding convex wall  13  and tensile stress is produced in the opposite wall (measurement wall or concave wall in the depicted plane). A resistive strain gauge  15  is mounted to the side of block  11  and measures tensile strain resulting from the tensile stress on the measurement wall.  
         [0019]    While the embodiments depicted herein show a strain gauge mounted on a surface having a tensile strain, configurations mounting the strain gauge on a surface under compressive stress and having a resulting compressive strain are also contemplated by the invention and should be understood as equivalent structures.  
         [0020]    A notch  14  may be cut in the side opposing resistive strain gauge  15 , permitting the compressive stress produced at displacement wall  13  to elastically deform block  11  more readily due to the reduced cross-section at notch  14 , improving the effective sensitivity of resistive strain gauge  15 . A notch may alternatively be cut in the opposing side. The function of the notch is to increase the resulting deformation due to the strain. Considerations for the location of the notch include whether the strain gauge is mounted to measure compressive strain or tensile strain and where the maxima of the strains are located within the block. Positions other than the maxima may also be used to decrease the relative effect of the notch. The strain gauge may be mounted to the bottom or top surface of the block, as well as the side, as tensile or compressive forces are present throughout the block, depending on the configuration of the tensiometer structure.  
         [0021]    Measurement circuits  18  may be embedded within block  11  to couple resistive strain gauge  15  to a display  16 , an alarm buzzer  20 , an alarm indicator  23  or any combination of the above. A battery  17  is used to power measurement circuits  18 , display  16  and alarm buzzer  20 . Alternatively, or in combination, a solar cell may be used to provide power for measurement circuits  18  when ambient light is sufficient.  
         [0022]    Display  16 , which is generally an LCD display, can be customized to a particular set of tension units or measurement indication. Alternatively, displays may be LED or LCD bar graphs, analog gauges, or a binary indication such as a single LED for indicating that a predetermined tension level has been exceeded or has not been exceeded. The display may be a numeric display in units of force to indicate tension, or may be settable or pre-programmed for a material with a particular ultimate tensile strength or cross-sectional area to yield percentage of ultimate stress, or a stress value indication in force/area units. The alarm buzzer may also be set or preprogrammed to produce an audible alarm when any of the above values reach a predetermined threshold. Such an audible alarm is useful for installations where tensiometer  10  may be left in place or with several tensiometers installed on several flexible lines, to produce an indication audible at a distance. Similarly, alarm indicator  23  which is depicted in the illustrations as an LED, but may alternatively be any other form of lamp, provides a visual indication that is readable at a distance.  
         [0023]    Referring now to FIG. 1B, a side view of tensiometer  10  is depicted. Display  16  is mounted on and is visible from the side of block  11  (although other mounting locations are possible such as on the top face). Resistive strain gauge  15  is mounted or deposited on the side surface of block, but may be recessed beneath the surface and covered for protection. Button switch  19  is included for mode selection (e.g., units, force vs. tensile strength percentage, etc.) and other buttons and functions may be included for programming alarm thresholds, line material, clearing stored values in memory, etc. Button switch  19  may serve a dual function such as a power-on switch for use in waking measurement circuits after an automatic turn-off timeout has occurred and then for subsequent use as a mode selection switch.  
         [0024]    Referring now to FIG. 1C, an end view of tensiometer  10  is depicted. Cylindrical channel  12  enters (or exits) through the end wall as shown. Displacement wall  13  is concave in the plane displayed (and above-described as convex in the plane of FIG. 1A) in order to assist retention of a flexible line, but this is not a requirement of the present invention. As tension is applied, the flexible line will be secured to displacement wall  13  by the lip  21  of displacement wall  13 . Alternatively, or in combination, a separate safety latch mechanism may be incorporated in tensiometer to provide retention of a flexible line.  
         [0025]    Referring now to FIG. 2, circuits within tensiometer  10  are depicted. A tensiometer integrated circuit  30  provides the essential functionality of the tensiometer  10  measuring system, but discrete implementations are possible and are contempleted by the present invention. Current source  11  provides a reference current through resistive strain gauge  15 . The voltage across resistive strain gauge  15  is proportional to the strain detected by strain gauge  15  and is sampled by an analog-to-digital (A/D) converter  31 , which may be an A/D converted embedded within an embedded microcontroller. Scaling and display processor  33 , converts the digital output of A/D converter  31  to the signals required to display the desired tension unit outputs to display  16  and values for potential storage in a memory  34 . Button switch  17  is coupled to scaling and display processor  33  to provide user input for mode selection and programming, but any number of buttons might be used to facilitate programming.  
         [0026]    Alarm buzzer  20 , is generally a piezoelectric element driven by a frequency generating circuit (which may be a control pin provided buy scaling and display processor  33 ). Alarm buzzer  20  is coupled to scaling and display processor  33 , so that an alarm may be generated if measured strain exceeds a programmed predetermined level. The level may be user selected via button switch  17 , or may be factory pre-programmed to a predetermined level. The visual alarm lamp  23 , alarm buzzer  20  and/or display  16  may be used to provide a low battery indication.  
         [0027]    Memory  34  is coupled to scaling and display processor  33  and contains non-volatile program instruction storage and volatile data storage. Electrically-alterable memory may be used to provide scaling of particular units, customization for particular flexible line materials, calibration of a particular strain gauge  15  and block  11  combination, and compensation for block material. Memory  34  may be used to store strain values, yielding a history of tension within a flexible line. Use of memory  34  to yield a data-recording tensiometer permits display of such information as number of load cycles encountered in a particular installation, peak tensile load, number of times the peak tensile load is reached, etc. Additionally, an external interface to integrated circuits  30  via an external connection, providing a means for transferring data from tensiometer  10  to a computer or other instrument. The data sample rate may be made adjustable for conservation of battery power and the resolution and sample rate may be made adjustable to conserve storage space within memory  34 .  
         [0028]    Referring now to FIG. 3A, a tensiometer frame  40  in accordance with an alternative embodiment of the invention is depicted. The mechanical details of the frame are shown without a display or any of the electronic components to provide clarity of illustration. In the alternative embodiment of FIG. 3A, one or more pulleys  42 A- 42 C are used to permit a flexible line to slip more freely through frame  40 . Three pulleys may be used as depicted, or a single pulley  42 B may be used at the point of maximum displacement. Alternatively, two pulleys may be used at the exit and entry locations (pulleys  42 A and  42 C). In order to insert a flexible line that is already under tension, one or more of pulleys  42 A- 42 C may be made removable with a slip pin arrangment (generally pulley  42 B) and a lever may be provided to temporarily displace the flexible line, or an external tool may be provided so that the pulley(s) may be inserted easily along with the flexible line.  
         [0029]    Referring now to FIG. 3B, a tensiometer  50  having a clamshell frame is depicted. The frame comprises a first portion  56  including the displacement surface, and a second portion  54  for retaining a flexible line against the displacement surface by completing a cylindrical channel. First portion  56  is attached to second portion  54  by a hinge  52 . A latching mechanism  55  permits securing first portion  56  and second portion  54  together by hooking a recess  58  in latching mechanism  55  over a pin  57  on housing first portion  56 . Latching mechanism is secured to second portion  54  via a nut  53  threaded onto a bolt  51  that attaches latching mechanism  55  to second portion  54 . Alternatively, two latches or other bolting arrangements will be apparent to those of skill in the art so that a clamshell frame for the tensiometer may be implemented and are contemplated by the present invention.  
         [0030]    Referring now to FIG. 4, a tensiometer frame  60  including an overload protection mechanism is depicted. A notch  62  is used to provide a greater deformation in response to compressive stress as described for the embodiments depicted above, but an additional cam  64  is machined as an integral part of frame  60 . At the end of cam  64  is a stop  66  that will contact a wall  68  of a second notch machined in frame  60  if an excessive compressive force is applied across notch. The features of the overload protection mechanism may also me molded or otherwise formed in frame  60  according to the particular materials and processes used to fabricate frame  60 . Alternatively, an externally attached stop might be used, providing a mechanical limit of travel for the walls of notch  62 .  
         [0031]    Within the block tensiometer frame included in various embodiments of the present invention, block material may be varied by application, for example, a polymer block may be used for measuring low tensile strength flexible lines, while a steel block might be used for measuring high tensile strength cables since the more elastic the material chosen, the higher stain level produced at strain gauge  15 . The material should be chosen so that plastic deformation does not occur at the maximum tension levels intended for measurement with a particular tensiometer. The shape of the curvature of displacement wall (generating the amount of displacement) may also be tailored to particular tension levels. For example, in a high-tension application, a low displacement (slight curvature) will yield lower strain within strain gauge  15 .  
         [0032]    While the invention has been, particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form, and details may be made therein without departing from the spirit and scope of the invention.