Abstract:
A tilt sensor for use in an automatic leveling device includes a level vial with a level bubble, the vial including a metal base member. The metal base has provisions for mounting the sensor device to an instrument or object to be leveled, and optical as well as capacitive sensing arrangements are disclosed for sensing the position of the level bubble and providing a signal to be used by a motor that brings the instrument or object to level.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to measuring instrumentation, and more specifically to providing electronic tilt information relative to gravity by sensing a bubble position for the purpose of controlling a self-leveling platform.  
           [0002]    In the invention an electrical signal is produced generally proportional to the tilt angle depending on bubble position. The platform is used in precision surveying instruments.  
           [0003]    In the prior art a conventional level vial of glass or plastic contains a low viscosity liquid such as turpentine in a tube. The liquid vessel in most cases is a cylindrical tube with a slight curvature in the vertical plane. As the vessel is tilted in this vertical plane the bubble moves along the cylinder. Automatic detectors in the prior art sensed the bubble location and thus the tilt optically or capacitively. Such methods are described in U.S. Pat. Nos. 4,625,423, 4,956,922, 5,101,570 and 5,953,116. The methods included focusing and refracting the light as well as absorbing with an opaque fluid. In other prior art the bubble is located using the absence of capacity due to the bubble.  
         SUMMARY OF THE INVENTION  
         [0004]    The basic principle controlling the bubble location can be described in terms of the liquid seeking the lowest potential energy. Thus a horizontally held under-filled tube with a curvature in the vertical plane will have a bubble in the center. As the tube is tilted within the vertical plane, and as the liquid seeks the lowest energy level, the bubble will move along the level vial.  
           [0005]    In most precision applications of electronic level vials, the vial is mounted to a metal frame. The glass-metal interface is difficult to control over a large temperature range because of the very different coefficients of linear expansion of glass and metal. By using a metal member as part of the liquid container, this mounting problem is eliminated. An added advantage of a metal vessel is the improved thermal stability due to the high thermal conductivity of the metal. In addition, the use of a metal member is less expensive than making a precision glass tube.  
           [0006]    In the metal vial of the invention the vertical curvature required for the bubble motion is fabricated in the metal forming process. The liquid container is closed using a plastic member which allows for sensing of the bubble location, e.g. optical sensing. This use of a metal vial or at least a metal base member ensures that in all environments the temperature gradient across the chamber is small. The plastic member of the container may be designed to do more than just contain the fluid. Provision for mounting the LED light source and the detectors can be incorporated into the plastic member or container. In addition the plastic surfaces used as optical windows of the housing can be clear while other surfaces can be rough for scattering unwanted light. An alternate sensing method would use electrodes on the plastic member or container to measure the bubble location using a capacitance measurement. Response time of the system is determined by such elements as the bubble curvature, bubble size, viscosity of the liquid, proximity of the container wall and by controlling the cross sectional area of the container. For example, a bubble in a container with a shallow bottom will move more slowly than a bubble in a deep container.  
           [0007]    Another method of providing the needed bubble motion with tilt angle uses the cross section of the vial, with no vertical curvature. In this case, the upper surface containing the liquid is flat and the cross section is wide in the center and narrow on the ends. Since the volume of the bubble is a constant, reducing the width of the channel at the ends lowers the bubble&#39;s center of gravity. Therefore, the locus of the center of gravity of the bubble is an arc in the vertical plane. This is similar to the locus of the center of gravity for the normal curved tubular level vial.  
           [0008]    It is thus an object of the invention to improve construction, thermal insensitivity and reliability in a level bubble vessel in which the bubble&#39;s position is automatically sensed. These and other objects, advantages and features of the invention will be apparent from the following description of a preferred embodiment, considered along with the accompanying drawings. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0009]    [0009]FIG. 1 is an elevation view of a tilt sensor of the invention.  
         [0010]    [0010]FIG. 2 is a bottom view of the tilt sensor of FIG. 1.  
         [0011]    [0011]FIG. 3 is a transverse sectional elevation view of the sensor of FIG. 1, seen along the line  3 - 3  in FIG. 2, with a bubble in the center.  
         [0012]    [0012]FIG. 4 is a transverse section view similar to FIG. 3 but with a bubble at the end.  
         [0013]    [0013]FIG. 5 is a longitudinal section view in elevation of the sensor of FIG. 1.  
         [0014]    [0014]FIGS. 6A, 6B and  6 C are similar bottom views of the sensor of FIG. 1, showing bubble motion.  
         [0015]    [0015]FIG. 7 is a schematic circuit diagram showing conversion of a signal from the sensor to the motor drive into the case of optical detection of the bubble location.  
         [0016]    [0016]FIG. 8 is a bottom plan view of a tilt sensor using variation of the vessel channel width to control the bubble motion with tilt.  
         [0017]    [0017]FIG. 9 is a transverse sectional elevation view of the tilt sensor of FIG. 8 showing bubble sensing as seen along the line  9 - 9  in FIG. 8.  
         [0018]    [0018]FIG. 10 is a schematic bottom plan view of a tilt sensor with electrodes used to sense the bubble position.  
         [0019]    [0019]FIG. 11 is a sectional elevation view of the sensor of FIG. 10 as seen along the line  11 - 11  in FIG. 10.  
         [0020]    [0020]FIG. 12 is a bottom view of a tilt sensor using capacitive sensing of the bubble position and showing the bubble tilt control of FIG. 8.  
         [0021]    [0021]FIG. 13 is a transverse sectional elevation view of the tilt sensor of FIG. 12.  
         [0022]    [0022]FIG. 14 is a schematic circuit diagram showing the conversion of the sensor signal to motor control.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0023]    A preferred embodiment is shown generally as  1  in FIG. 1. An aluminum base  2  has mounting holes  4  and  8 . A liquid  12  is contained by a clear plastic vessel member  10  which is held in place by an adhesive  14 . A screw  16  seals the vessel. Adhesive on the screw, not shown, can be used to assist the sealing. A bubble  18  is formed because the vessel is not completely filled. A light emitting diode is shown at  20 , positioned to shine light through the vessel  10 , generally horizontally.  
         [0024]    A bottom view of the assembly  1  is shown in FIG. 2. LED  20  and photo detectors  22  and  24  are shown in the figure, oriented for horizontal travel of the LED light through the liquid. FIG. 3 is a section view showing the light path when a buble is located in the center of the vessel as in FIG. 2. Most of the light  26  is reflected by total internal reflection so it does not reach the detectors  22  and  24 . A small amount of light  27  is refracted through the bubble and reaches the detectors.  
         [0025]    The section view of FIG. 4 shows light rays  28  reaching one of the detectors in the absence of the bubble in the light path. FIG. 5 is a section view showing a curvature  30  on the roof of the vessel, which controls the motion of the bubble with tilt. This direct liquid contact with the metal base providing the roof or ceiling is an important feature of the invention allowing for high repeatability because of the direct contact to the frame of the platform, high thermal stability, and the thermal conductivity of the aluminum base.  
         [0026]    FIGS.  6 A- 6 C show how the light rays are directed by the position of the bubble  18 . When the left side of the level sensor is high, the bubble prevents most of the light from reaching the detector  22 . Most of the light is reflected by total internal reflection at the bubble liquid interface. On the other hand, the light rays  28  do reach the detector  24 . When the bubble is in the center the light reaches the detectors  22  and  24  with equal intensity, as in FIG. 6B. When the right side of the level sensor is high more light reaches detector  22  and little reaches the detector  24 , as in FIG. 6C.  
         [0027]    [0027]FIG. 7 is a block diagram indicating a circuit and showing how the difference of the detector signal is amplified to drive a motor which tilts a platform (motor and platform represented by a block). To avoid oscillation, a phase shift network may also be incorporated in the amplification path.  
         [0028]    [0028]FIGS. 8 and 9 show the bottom of a level sensor  30  having no vertical curve in the upper surface of its liquid vessel chamber. A base  32  has mounting holes  34  and  36 . Top window  40  and bottom window  42  ( 42  not shown in this figure) are sealed to the sensor base  32  with an adhesive  44 . A channel  38  whose side walls are elliptically curved determines the tilt sensitivity of a bubble  48 . The channel is slightly under-filled with a liquid  46 . A seal screw  50 , which may be in the side as shown, is used to seal the channel. As noted above, due to the elliptical walls and the maximum width at the center, the bubble  48  experiences least bouyancy pressure in the center when level and thus is stable and localized in the centered, leveled position despite the flat upper vessel surface.  
         [0029]    [0029]FIG. 9 is a section view of the tilt sensor of FIG. 8. An LED light source  52  shines light down through the channel  46  and is detected by a photo detector (array)  54  at the underside (the LED  52  and the detector  54  are not shown in FIG. 8). The refraction of the bubble steers the beam to the detectors as would a negative lens. This principle is described in prior patents including some of those listed above.  
         [0030]    In an alternate preferred embodiment of the invention, the bubble location is sensed using the capacitance of the liquid or the absence of capacitance via the bubble as opposed to the optical sensing described above. The bottom plan view of FIG. 10 and the section view of FIG. 11 show a tilt sensor generally designated as  60 . A base  62  and attached plastic vessel member  64  produce a channel for a liquid  66 . The channel is sealed with an adhesive  67  and filled preferably through a tapped hole indicated at  69 . A vessel ceiling surface  70  has a vertical curvature which controls the bubble motion with tilt as described above and shown in FIG. 5. A bubble  72  is formed by underfilling the channel. Electrodes  74  and  76  on the plastic vessel member  64  are used to measure the bubble location. Wires  78  and  80  are attached to the electrodes  74  and  76  for measurement along with a ground wire  82  connected to the metal base.  
         [0031]    In an alternate preferred embodiment of capacitive sensing, the bubble controlling curvature is in the vessel or channel wall shape as was done in FIGS. 8 and 9 and is shown anew in the bottom plan view of FIG. 12 and in the section view of FIG. 13. The tilt sensor is shown generally by  84 . Its metal base  86  has an elliptical channel  88  which controls the bubble motion with tilt. Dielectric covers  90  and  92  complete the channel&#39;s containment of a fluid  94 . The covers are sealed with an adhesive  96 . Electrodes  98  and  100  are used to sense the position of the bubble  104  via the capacitance measurement, via wires  106 ,  108  and ground wire  102 . A seal screw  110  is used to fill the channel with liquid.  
         [0032]    [0032]FIG. 14 is similar to FIG. 7, being a schematic circuit diagram indicating generation of a signal to be fed to a DC motor tilting platform. As in FIG. 7, the motor and platform are not shown, only indicated by a block. The circuit of FIG. 14 shows use of capacitance bubble sensing, for embodiments such as shown in FIGS.  10 - 13 . The electrodes  98  and  100  from the sensor shown in FIGS. 12 and 13 are indicated in FIG. 14. When the bubble is closer to the first electrode than the second, less current flows through the first series resistor because of the high AC impedance. The AC difference voltage at the input of the amplifier is, therefore, proportional to bubble location.  
         [0033]    The above described preferred embodiments are intended to illustrate the principles of the invention, but not to limit its scope. Other embodiments and variations to this preferred embodiment will be apparent to those skilled in the art and may be made without departing from the spirit and scope of the invention as defined in the following claims.