Abstract:
An electro-optical level includes a bubble level having a tube with a liquid therein and a bubble formed by the liquid in the tube. A substantially linear light source is directed toward and through the tube. A linear lens array focuses light passing through the bubble tube onto a linear lens array for producing an image of the bubble in the bubble level. The lens array employs the focused light to identify precisely end points of the bubble in the tube and then employs locational data for the end points to assess levelness. The electro-optical level further includes an out put display that is pivotally mounted into an optimal alignment for easy viewing by a user.

Description:
[0001]    This application claims priority on U.S. Provisional Patent Appl. No. 60/420,804, filed Oct. 23, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The invention relates to a bubble level that employs electro-optical components for achieving high precision.  
           [0004]    2. Description of the Related Art  
           [0005]    A typical carpenter&#39;s level is an elongated generally rectangular structure having opposed first and second ends. Planar top and bottom faces are aligned parallel to one another and extend between the ends. The top and bottom faces are the portions of the prior art level that will be placed against another surface for assessing horizontal or vertical alignment.  
           [0006]    A typical prior art level includes a glass tube partly filled with a liquid spirit. The portion of the tube that is not filled by the liquid forms a bubble. Movement of the level will cause the liquid to shift within the tube, and hence will cause a repositioning of the bubble. The tube includes a pair of lines that are spaced apart by a distance approximately equal to the length of the bubble. When the tube is aligned horizontally, the liquid will be disposed symmetrically in the tube, and the bubble will be positioned precisely between the lines.  
           [0007]    The prior art level can be used by placing the top or bottom face of the level on a substantially horizontal surface. The relative position of the bubble in the tube provides an indication of the closeness of the level to a horizontal alignment. The surface on which the level is supported may be adjusted to position the bubble between the lines of the tube, and hence to achieve a fairly exact horizontal alignment of the surface on which the level is supported.  
           [0008]    The precision that can be obtained with the prior art level is dependent upon the eyesight of the user and the ability of the user to determine the positions of the ends of the bubble relative to the lines on the glass tube. This ability will depend at least partly upon the optical alignment of the user of the level relative to the bubble tube. Precision can be fairly good if the user has good eyesight and if the user can be positioned so that the line of sight is perpendicular to the axis of the tube. However, the realities of a construction or manufacturing site often do not permit the user of a level to be positioned perfectly relative to the bubble tube. Thus, the level may be positioned at a location significantly above the head of the user or at a position where the line of sight is at an acute angle to the axis of the bubble tube. Either of these fairly common uses of a level significantly reduce accuracies that can be obtained by the level.  
           [0009]    The boundary between the gas and the liquid in the bubble tube is fairly precise. However, the bubble is an elliptoid with curved boundaries. A viewer necessarily sees the curved boundaries between the gas and the liquid in the bubble tube, and hence the boundary between the bubble and the liquid appears as a fairly dark broad line. The apparent width of the line that defines the bubble can lead to a lack of precision by the user. In particular, accuracy will vary depending upon which part of the apparently broad boundary of the bubble aligns with the positioning indicia on the glass tube.  
           [0010]    The prior art includes the use of electro-optical devices that attempt to determine the position of a bubble in a level. These devices have taken several forms, but rely primarily on attempts to identify an end or midpoint of the bubble or to identify an angle of the surface of the liquid. The known electro-optical devices used with a bubble level do not provide the desired degree of precision.  
           [0011]    In view of the above, an object of the subject invention is to provide a bubble level with significantly increased accuracy.  
           [0012]    Another object of the subject invention is to provide a bubble level where accuracy is not dependent upon the angle of viewing.  
         SUMMARY OF THE INVENTION  
         [0013]    The subject invention is directed to a level that includes electronic and optical components for providing a high degree of accuracy independent of the angle of viewing and independent of the eyesight of the viewer.  
           [0014]    The apparatus of the subject invention includes a bubble tube partly filled with a liquid so that a generally elliptoid bubble remains in the tube. The apparatus further includes a light source in proximity to the bubble tube and operative to direct light toward and through the bubble tube. The apparatus further includes a light sensor on a side of the bubble tube substantially diametrically opposite the light source and aligned for receiving light emitted from the light source and passing through the bubble. The light sensor is operative to generate electrical signals indicative of centers of energy impinging thereon. The apparatus also includes a focusing means between the bubble and the light sensor for focusing light passing through the bubble tube onto the sensor. The focusing means is operative to effectively image the bubble and adjacent areas of the bubble tube onto the sensor. The apparatus also may include a display for providing a clear visual indication of the output of the light sensor. The display may be a screen with a graph for identifying energy levels. Alternatively, the display may be a precise graphic depiction of a bubble tube. Still further, the display may provide a numerical quantification of the degree of levelness. The display may be mounted to the apparatus for pivoting or swiveling into an alignment that facilitates observation by a user. Thus, in contrast to prior art levels, accuracy and readability are entirely independent of the relevant positions of the level and the user.  
           [0015]    The light source may comprise a substantially linear light source, such as a cold cathode illuminator, aligned parallel to the bubble tube.  
           [0016]    The focusing means may include a lens array extending substantially linearly and parallel to the bubble tube. Alternatively, the focusing means may include a conventional cylindrical lens aligned substantially parallel to the axis of the bubble tube.  
           [0017]    The light sensor may be a CCD array aligned substantially parallel to the bubble tube. The light sensor preferably is spaced approximately {fraction (1/16)}-⅛″ from the focusing means.  
           [0018]    In operation, the transmission of the light through the bubble tube will vary depending upon whether the light is being transmitted through the bubble or through the liquid and is significantly differently at the boundaries between the liquid and the bubble. In particular, light will be reflected or refracted differently by the bubble than by the liquid. These differences result in inverted peaks or “divots” in the output signals of the light sensor at locations substantially aligned with the ends of the bubble. The divots will be substantially identical and at known positions when the bubble is centered in the bubble tube. However, the divots will shift and/or change in size when the bubble moves in the tube.  
           [0019]    The apparatus may be used with a differential screw adjusting means for moving the position of the level relative to the object that is being assessed. Hence, the differential screw adjuster may be moved until the light sensor and any output means associated therewith produce signals to indicate a level state for the level. Thus, amounts of adjustments can be measured.  
           [0020]    The apparatus has several significant advantages over more complex electronic levels. In particular, the apparatus is very rugged and very compact. Furthermore, the apparatus can be packaged within a version of a conventional carpenter&#39;s level that is familiar to skilled technicians. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a front elevational view of a level apparatus in accordance with the subject invention.  
         [0022]    [0022]FIG. 2 is a front elevational view similar to FIG. 1, but showing the display in a different orientation.  
         [0023]    [0023]FIG. 3 is an exploded schematic view of the bubble level and the electronic and optical components used therewith.  
         [0024]    [0024]FIG. 4 is a schematic illustration of the output of the level apparatus.  
         [0025]    [0025]FIG. 5 is a graphic depiction of a portion of the readout for precisely identifying a position of an end of the bubble. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    A level apparatus in accordance with the subject invention is identified generally by the numeral  10  in FIGS. 1 and 2. The apparatus  10  includes an elongate rectangular housing  12 , an electro-optical level sensor  14  (see FIG. 3) and an output display  16 . The output display  16  is mounted pivotally to the housing  12  to achieve an optimum viewing angle for a user. The pivotal connection permits 360° of rotation of the display  16  relative to the housing  12 . The display  16  incorporates a carrying handle  17  configured and dimensioned to be gripped by hand for carrying and positioning the apparatus  10 . The apparatus  10  further includes a base  18  removably mounted to the housing  12  at a side substantially opposite the display  16 . A plurality of differently configured bases  18  can be provided in accordance with the size and shape of the surface on which the apparatus  10  will be supported for measuring levelness.  
         [0027]    The electro-optical level sensor  14  includes a bubble level  20  having an elongate tube  22  with a central axis  24 , as shown in FIG. 3. The tube  22  may be curved convexly up or may be ground or otherwise formed to have an internal curve with a central high point, so that the interior of the level is generally barrel-shaped. The tube  22  is filled partly with a liquid  26 , and portions of the tube  22  that are not filled with the liquid  26  define a bubble  28 . The bubble  28  will move longitudinally within the tube  22  depending upon the orientation of the axis  24  relative to the gravitational axis. In particular, the bubble  28  will be centrally disposed in the tube  22  when the axis  24  of the tube  22  is normal to the gravitational axis. However, the bubble  28  will shift toward the high end of the tube  22  when the axis  24  of the tube  22  is not perpendicular to the gravitational axis. The amount of shifting of the bubble  28  will depend upon the angle between the axis  24  of the tube  22  and the gravitational axis.  
         [0028]    The electro-optical level sensor  14  further includes a substantially linear light source  30  aligned substantially parallel to the axis  24  of the tube  22  and operative to emit light toward and through the tube  22 . The light source  30  preferably is a cold cathode illuminator.  
         [0029]    The electro-optical level sensor  14  illustrated in FIG. 3 further includes a lens  32  disposed on a side of the bubble level  20  opposite the light source  30 . The lens  32  may be a cylindrical lens, a single spherical lens or a multiple lens array of cylindrical or spherical lenses arranged linearly, such as lens arrays sold under the trademark SELFOC® by NSG America, Inc. The lens array  32  focuses light that has been emitted from the light source  30  and passed through the tube  22  substantially along the axis of the tube.  
         [0030]    The electro-optical level sensor  14  further includes a light sensitive receiver  34  aligned substantially parallel to the axis  24  of the tube  22  and disposed on a side of the lens array  32  opposite to the tube  22 . Thus, the lens array  32  focuses an image of the liquid  26  and the bubble  28  onto the light sensitive receiver  34 . The light sensitive receiver  34  may be a CCD array or other receiver that is operative to identify the location and magnitude of peaks of light energy impinging thereon. The light sensitive receiver  34  preferably defines a length in excess of the length of the bubble  28 . However, the light receiver  34  need not extend the entire length of the tube  22 . The light sensitive receiver  34  is connected operatively to a processor  36  that is in the housing  12  connected to the display  16 . Thus, the display  16  is operative to produce an output indicative of energy peaks identified by the CCD array or other such light sensitive receiver  34 . The apparatus  10  may further include an aperture plate between the tube  22  and the light sensitive receiver  34 .  
         [0031]    The liquid  26  and the bubble  28  in the tube  22  alter the level of energy of illumination passing through the tube  22  and focused by the lens array  32  onto the CCD array or other such light sensitive receiver  34 . The variation in output voltage is measured by variations in voltage level outputted from the light sensitive receiver  34 . The variation in illumination is most prominent at the edges of the bubble  28 . Thus, the bubble  28  takes the uniform illumination produced by the light source  30  and focused by the lens  32  and couples energy from the position of the bubble  28  and beyond the ends of the bubble  28  to produce an energy distribution as shown in FIGS. 4 and 5.  
         [0032]    The energy distribution shown in FIG. 4 depicts two substantially equal inverse peaks or “divots” indicative of a bubble  28  that is centrally disposed relative to the light sensitive receiver  34  and hence indicating a horizontal alignment of the axis  24 . The bubble  28  shifts to the left or right if the axis  24  is not perfectly horizontal. As a result, the divots also shift, and one of the divots may become larger and the other may become smaller in proportion. The divots are read by the CCD array  34  and associated electronics. More particularly, the input current is adjusted by the processor  36  to achieve a brightness or output voltage for each pixel of the CCD array  34  that is in a readable range. The output voltage of each pixel in the CCD array  34  then is read by the processor  36  of the electro-optical sensor  14 . As shown in FIG. 4, the array output includes a first set of pixel output voltages  50  on a first side of the bubble  28 , a second set of pixel output voltages  52  on a second side of the bubble  28  and an array of pixel output voltages  54  corresponding to the bubble  28 . A divot  56  is defined between the first array of pixel output voltages  50  to the left of the bubble  28  and the array of pixel output voltages  54  aligned with the bubble. The divot  56  has a slope  58  extending from the first array of pixel output voltages  50  to the divot  56  and a slope  60  extending from the divot  56  to the array of pixel output voltages  54  corresponding to bubble  28 . The slope  58  is longer and steeper than the slope  60 . The array output further includes a divot  66  corresponding to the right end of the bubble  28 . A slope  68  extends from the divot  66  to the array of pixel output voltages  52  and a slope  70  extending from the array of pixel output voltages  54  corresponding to the bubble  28  to the divot  66 . The slope  68  is steeper and longer than the slope  70 . The processor  36  of the apparatus  10  establishes a threshold voltage, as shown in FIG. 4, at a position that is known to intersect the slopes  58  and  68  approximately at their midpoint. The controller then identifies voltage levels for specific pixel positions on the respective slopes  58  and  68  and through a simple algebraic linear regression calculation identifies relative pixel positions  62  and  72  where the respective slopes  58  and  68  intersect the threshold voltage, as shown in FIG. 5. The positions  62  and  72  are symmetrically disposed with great precision relative to opposite ends of the bubble  28 . Hence, the center of the bubble  28  can be defined with great precision as the midpoint between the respective threshold intersections  62  and  72  respectively. The longer steeper slopes  58  and  68  provide more precision.  
         [0033]    [0033]FIG. 4 shows a bubble length of 189 pixels in this example. That bubble length will remain substantially constant for a broad array of angular orientations of the level  20  and can be determined during a calibration of the apparatus  10 . As shown in FIG. 4, the divots  56  and  66  have nearly identical output levels. However, as the level  20  is tilted more, the optics associated with the bubble  28  will cause the output voltages of the respective peaks to vary significantly from one another. Within a range of angular alignments normally encountered, the above-described algorithm can be applied with great accuracy despite the different output voltage levels for the respective divots  56  and  66 . However, at a certain angle for the level, one or the other of the divots  56  or  66  may move above the threshold voltage. Thus, the above described algebraic procedure cannot be used for identifying the two intersections  62  and  72  of the threshold voltage. However, one intersection  62  or  72  can be identified. Additionally, the bubble length will remain constant. As a result, the one intersection  62  or  72  for slope  58  or  68  with the threshold voltage can be used with the knowledge of the bubble length to identify the center of the bubble  28 . Thus, in this situation, at least two and preferably three pixel readings along the slope  58  or  68  will be identified so that the readings bridge the threshold voltage. An algebraic linear regression equation is used to identify the precise intersection with the threshold voltage and one-half of the bubble length is added to or subtracted from pixel position for the intersection  62  or  72  of the threshold voltage to identify the precise pixel position for the center of the bubble  28  relative to the CCD array  34 .  
         [0034]    The apparatus  10  is used by placing the base  18  on a surface that is to be tested for levelness. The apparatus  10  then may be powered on by using a control switch  74  connected to an internal power source  76 , such as a rechargeable lithium-ion battery. The display  16  is pivoted relative to the base  18  to be readily visually observable to a user. The bubble  28  will move in the tube  22  depending upon the alignment of the surface. The apparatus  10  functions by directing light from the light source  30  through the bubble level  20 . The lens array  32  focuses the light onto the CCD array or other sensor  34 , and hence produces an image of the bubble  28  on the CCD array  34 . The processor  36  of the apparatus  10  then identifies at least one of the divots  56  and  66  corresponding to the ends of the bubble  28  and calculates the intersection  62  and/or  72  of at least one of the slopes  58  and  68 . The pixel positions for the intersections of the slopes  58  and/or  68  with the threshold voltage are determined with great precision and the midpoint of the bubble  28  then is determined based on those calculated pixel positions. More particularly, the midpoint of the bubble  28  is midway between the threshold intersections  62  and  72  in those situations where both slopes  58  and  68  intersect the threshold voltage. Alternatively, the midpoint of the bubble  28  can be determined by adding a previously calibrated bubble length to the intersection  62  or  72  of the threshold voltage with one of the slopes  58  or  68 . As noted above, the level  12  can be used with differential screw adjusters to alter the relative vertical position of either end of the level  12 . Thus, the user can observe the changes in the peaks on the display  16  shown in FIGS. 1 and 2 as the differential screw adjusters are being turned. As a result, real-time adjustments and calibration can be carried out. The display  16  can output information on levelness in terms of arc seconds or distance (inches or millimeters) based on trigonometric calculations performed by the processor  36 . The display  16  also can indicate whether the output data is positive or negative relative to the “+” and “−” indicia applied to the housing  12 .  
         [0035]    The level  20  is described herein as a bubble tube  22 . However the level also can be a bulls eye level where the bubble can move relative to two axes to measure levelness in plural directions. With this embodiment, it is necessary to focus the image onto two perpendicularly arranged linear arrays of sensors or onto a two axis sensor.