Abstract:
A linear measurement device may be formed from a first tube axially translatable with respect to a second tube. Inside the first tube may be placed a sensor capable of sensing a magnetic field. A magnet may also be found within the first tube and produce a magnetic field sensible by the sensor. The second tube may comprise a plurality of deviations disposed therealong capable of altering the magnetic field when near the magnet. As the first tube is axially translated with respect to the second tube, the sensor may sense alterations in the magnetic field due to the plurality of deviations thus allowing for a linear displacement to be determined.

Description:
BACKGROUND 
       [0001]    Many endeavors call for measuring a position of one object relative to another. Measuring the linear movement of one object relative to another may also be desirable in a great variety of situations. One mechanism capable of measuring such positioning or linear movement is known as a linear variable differential transformer (LVDT). LVDTs generally operate by driving an electrical current through a primary solenoid coil that may cause an induction current to be generated in secondary solenoid coils disposed axially on either side of the primary coil. A cylindrical ferromagnetic core, attached to the object whose position is to be measured, may slide along an axis between the primary and secondary coils and alter the induced current as it moves. When the core is displaced toward one of the secondary coils, the voltage in that secondary coil may increase as the voltage in the other secondary coil decreases and vice versa. While this design may have a variety of advantages, the length that may be measured may be limited given that it is the proximity to edges of the core the causes the induced currents to rise and fall. 
         [0002]    Another mechanism for measuring linear displacement, having a longer possible stroke than previously described LVDTs, may comprise a tube with ferromagnetic ball bearings disposed therein. This series of ball bearings may act as a scale around which a plurality of coils may pass. As in a traditional LVDT, an electrical current may be driven through one of the coils while a number of other spaced pickup coils detect variations in induced magnetic fields. However, in this case, the ball bearings may create a repeating differentiation in the induced magnetic fields. While this design may allow for longer measurement stroke, it still requires coils of wire spaced around a center, just like traditional LVDTs, which may add to its size, complexity, cost and structural weakness. 
         [0003]    Thus, while conventional LVDTs and other known linear position sensors have many advantages, a linear measurement device comprising fewer parts, more robust construction, smaller size, simplified circuitry, or reduced cost may be desirable. Further, while conventional LVDTs may require alternating current that may draw significant power, a linear measurement device with reduced power demands may be desirable. Additionally, the relatively short measurement stroke of conventional LVDTs often requires a scaling of the measured signals. A linear measurement device comprising a longer stroke may not require such scaling and, thus, may be desirable. 
       BRIEF DESCRIPTION 
       [0004]    A relatively small linear measurement device may comprise few working parts, a robust construction and simple electrical circuitry. Such a linear measurement device may be formed from a first tube axially translatable with respect to a second tube. Inside the first tube may be placed a sensor capable of sensing a magnetic field. A magnet may also be found within the first tube and produce a magnetic field sensible by the sensor. The second tube may comprise a plurality of deviations disposed therealong capable of altering the magnetic field when near the magnet. As the first tube is axially translated with respect to the second tube, the sensor may sense alterations in the magnetic field due to the plurality of deviations thus allowing for a linear displacement to be determined. 
     
    
     
       DRAWINGS 
         [0005]      FIGS. 1-1 and 1-2  are an orthogonal view and a longitude-sectional view respectively of an embodiment of a linear measurement device comprising two tubes with  FIG. 1-2  showing a magnified view of a magnet and sensor pairing within one of the tubes.  FIG. 1-3  is a perspective view of the magnet and sensor pairing shown in  FIG. 1-2 . 
           [0006]      FIG. 2  is a perspective view of a sectioned embodiment of a tube comprising a plurality of holes disposed in a sidewall thereof that could be used in conjunction with a linear measurement device. 
           [0007]      FIG. 3  is a perspective view of another sectioned embodiment of a tube comprising radial fluctuations disposed thereon that could be used in conjunction with a linear measurement device. 
           [0008]      FIG. 4  is a perspective view of another sectioned embodiment of a tube comprising alternating materials that could be used in conjunction with a linear measurement device.  FIGS. 4-1, 4-2 and 4-3  are orthogonal and longitude-sectional views of various embodiments of annular forms comprising different internal shapes that could be stacked to form a tube. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]      FIGS. 1-1 and 1-2  show an embodiment of linear measurement device  100  comprising two tubes. A first tube  101  may be disposed within a second tube  110  such that they may translate axially with respect to one another. The first tube  101  may comprise at least one magnet  102  and sensor  103  pairing. The magnet  102  may comprise any of a variety of permanent magnets or electromagnets. As shown in a magnified view of  FIG. 1-2  and  FIG. 1-3 , the magnet  102  may be attached to a circuit board  104  disposed within the first tube  101  axially adjacent the sensor  103 . The circuit board  104  may provide a practical, convenient and efficient platform that may be inserted into the first tube  101  after manufacture. However, other embodiments of similar linear measurement devices may be constructed differently while achieving similar results. As also shown in the present embodiment, the circuit board  104  may be disposed on a central axis of the first tube  101  with a second magnet  105  disposed radially opposite the magnet  102  on an opposing face of the circuit board  104 . It has been found that positioning two magnets opposite one another on either side of a circuit board may help to balance magnetic fields emanating therefrom. However, two magnets are not necessary and one may suffice. 
         [0010]    The magnet  102  may produce a magnetic field  106  capable of being sensed by the sensor  103 . Further, the second tube  110  may comprise a plurality of deviations  111  disposed thereon capable of altering the magnetic field  106  when in proximity thereto. Not only may the sensor  103  sense the magnetic field  106 , but it may also be capable of sensing alterations in the magnetic field  106  due to the deviations  111 . Additionally, while the present embodiment shows the sensor  103  positioned axially adjacent the magnet  102 , such sensors could also be placed in various positions, such as off axis, relative to magnets based on where they are likely to experience substantial changes in magnetic field due to interactions with a second tube. Further, if deviations disposed on a second tube are not symmetric about an axis thereof then it may be advantageous to specifically orient such sensors in relation to the deviations. 
         [0011]    The second tube  110  may be formed of a material comprising a relative permeability significantly greater than unity. As such, physical variations in a sidewall  112  of the second tube  110  may form the plurality of deviations  111 . For example, in the embodiment shown, the plurality of deviations  111  may comprise a plurality of holes  113  disposed in the sidewall  112  of the second tube  110 . As shown, the plurality of holes  113  may each be substantially identical in shape and evenly spaced axially along the second tube  110 . This plurality of holes  113  may be formed by any of a variety of machining or cutting methods. While such a configuration may be desirable in many situations due to its axial consistency, other embodiments comprising uneven configurations could provide a variation in resolution along the displacement. 
         [0012]    In the magnified view of  FIG. 1-2 , a first pairing of magnet  102  and sensor  103  is shown disposed proximate one end of the first tube  101 . This single pairing may be sufficient in many applications. Other axial positions of the first pairing may also function just as well as that shown. In the present embodiment however, this positioning makes room for additional magnet and sensor pairings  107  disposed along the circuit board  104  of the first tube  101 . It is believed that these additional magnet and sensor pairings  107  may increase signal-to-noise ratio and minimize the noise amplification inherent at zero-amplitude crossings. As an example of one such additional magnet and sensor pairing, a second magnet and sensor pairing  108  may be disposed at some axial distance  109  along the first tube  101  from the first pairing. The axial distance  109  between the first pairing and the second pairing  108  may be substantially different from a distance  115  between each of the plurality of deviations  111 . It is believed that a desirable distance  109  between the first pairing and the second pairing  108  may be generally N/4 times the distance  115  between each of the plurality of deviations  111  where N is an odd number. This is because even values of N may actually create a redundancy in the design and result in a measurement equivalent to just one sensor. In the present embodiment, while not shown exactly to scale, N is represented as 15 for reference. 
         [0013]    The circuit board  104  may comprise electronics capable of interpreting data from the sensors and calculating linear displacement of the first tube  101  relative to the second tube  110 . The electronics may further comprise a counter capable of counting repetitive magnetic field alterations sensed by the sensors. A wire  116  extending from the circuit board  104  along the first tube  101  may electrically connect the sensors to further electronics outside the first tube  101 . 
         [0014]    In addition, while the present embodiment shows magnets and sensors disposed within an inner tube and magnetic field altering deviations disposed on an outer tube, a reverse configuration comprising magnets and sensors on an outer tube and deviations on an inner tube may function similarly. 
         [0015]      FIG. 2  shows an embodiment of a tube  210  similar to the second tube  110  discussed in reference to  FIGS. 1-1 and 1-2 .  FIG. 2  shows clearly how a plurality of deviations  211  may comprise a second series of holes  214  disposed radially opposite a first plurality of holes  213  on the tube  210 . 
         [0016]      FIG. 3  shows another embodiment of a tube  310  that could be employed in a similar manner to the tube  210  discussed in reference to  FIG. 2 . In this embodiment, a plurality of deviations  311  comprises a plurality of radial fluctuations  313  shaped like annular grooves cut into an interior surface  322  of a sidewall  312  of the tube  310 . It is believed that annular grooves cut into the interior surface  322  of the tube  310  may be capable of altering a magnetic field when in proximity thereto while providing more rigidity to the tube  310  than the plurality of holes  213  shown in  FIG. 2 . In addition, by forming the annular grooves completely around the interior surface  322 , sensors forming part of a related linear measurement device may not need to be specifically oriented in relation to the plurality of deviations  311 . 
         [0017]      FIG. 4  shows yet another embodiment of a tube  410  that could be employed in a similar manner to the tubes  210 ,  310  discussed previously. Tube  410  may comprise a stack of annular forms  440  held together by an outer sleeve  441 . The annular forms  440  may alternate between those constructed of materials comprising a relative permeability significantly greater than unity  442  and those constructed of materials comprising a relative permeability approximately unity  443 . It is believed that the alternating materials may be capable of altering a magnetic field when in proximity to a magnet. Additionally, as the annular forms  440  completely surround the tube  410 , sensors forming part of a related linear measurement device may not need to be specifically oriented. 
         [0018]      FIGS. 4-1, 4-2 and 4-3  show various possible embodiments of annular forms  440 - 1 ,  440 - 2  and  440 - 3  that could be used to construct a tube similar to that shown in  FIG. 4 . Inner shapes of the annular forms  440 - 1 ,  440 - 2  and  440 - 3  may differ to alter a magnetic field in different ways. For instance, annular form  440 - 1  comprises a generally rectangular cross section  444 - 1 , annular form  440 - 2  comprises a generally trapezoidal cross section  444 - 2 , and annular form  440 - 3  comprises a generally triangular cross section  444 - 3 . 
         [0019]    Whereas the present invention has been described in particular relation to the drawings attached hereto, it should be understood that other and further modifications apart from those shown or suggested herein, may be made within the scope and spirit of the present invention.