Abstract:
The transmitter for projecting a beam of laser light includes a source of a beam of laser light, and a projection arrangement for directing the beam of laser light at a selected grade. The transmitter further includes a temperature sensor for detecting the temperature of said transmitter, and a temperature correction circuit, including a look-up table, responsive to said temperature sensor, for adjusting said projection arrangement in dependence upon offset grade values that are stored in said look-up table for a plurality of transmitter temperature ranges. These offset grade values are separately determined on an empirical basis for each transmitter, such that temperature induced errors in the direction of the beam of laser light are compensated.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to an improved laser transmitter and, more particularly, to a laser transmitter and method of laser transmitter compensation in which thermally induced errors in the grade of the projected beam of laser light are reduced by monitoring the transmitter temperature and correcting the transmitter operation accordingly.  
           [0003]    Laser transmitters are commonly used in surveying and in the construction industry for measuring or checking elevations, grade, dimensions from off-set lines, and the like. It is well known, for example, to use a laser beam transmitter in place of the level instrument. At the location where elevation is to be measured or checked, a target or laser beam detector is employed to intercept the laser beam from the transmitter. The laser transmitter may include rotating optical components which produce a beam that sweeps in a generally horizontal plane. Some such transmitters incorporate visually readable level vials and manually adjustable screw mechanism to permit the transmitter to be oriented so that the plane defined by the beam is level or is tilted in a desired direction at a desired grade. Other similar transmitters employ a generally conical reflector which intercepts the laser beam and reflects it simultaneously generally perpendicular to the beam in all directions. This stationary reflector has the advantage of simplicity. It will be appreciated, however, that the intensity of the light in the plane will be substantially reduced when it is reflected out of the beam. Still other laser transmitters project a stationary beam that is used as a reference in operations such as for example pipe laying.  
           [0004]    While known laser transmitter systems provide many improvements over conventional level and rod survey equipment, they also present certain disadvantages and limitations. For example, the degree of accuracy in establishing a desired beam orientation is dependent on the operator&#39;s skill and judgement in reading the level vials as he operates the adjusting screws. Moreover, where the operator moves away from the device to tend the target or a beam detector, the laser beam transmitter can move out of adjustment, as from being jarred, without the operator&#39;s knowledge so that subsequent measurements are erroneous.  
           [0005]    A laser transmitter having significant advantages over earlier prior art devices is shown in U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando et al, and assigned to the present invention. The system disclosed in the Rando et al patent is one in which orientation of the laser beam reference plane is accomplished automatically. A support frame for the laser source is pivotally mounted on the base frame of the Rando et al device. The support frame carries electrical sensor vials which sense the orientation of the support frame and provide electrical signals used by a feedback control system. The feedback control system activates electric motors to move the support frame into a position in which the vials are leveled. The vials are mounted on the support frame in such a manner that their positions may be adjusted by separate grade motors. When the reference laser plane is to be oriented at an angle to the horizontal, at least one grade motor is actuated by the operator to tilt a vial with respect to the support frame. The feedback control system then reorients the support frame to bring the vial back into its level position, tilting the frame by the desired amount. Other laser transmitters that incorporate level vials to detect orientation of transmitter components are shown in U.S. Pat. No. 5,852,493, issued Dec. 22, 1998, to Monnin, and in U.S. Pat. No. 6,055,046, issued Apr. 25, 2000, to Cain.  
           [0006]    While also providing a significant improvement over the prior art, it has been found that laser transmitters of this type may experience significant errors as a result of changes in ambient temperature. It has been found that one source of these temperature induced errors are the level vials in these systems. A level vial of the type used in such transmitters typically comprises an electrically nonconductive vial casing, usually made of glass, that defines an elongated, arcuate chamber which curves generally downward toward its opposite ends. A quantity of electrically conductive fluid is provided in the chamber. Such a fluid may, for example, have a ketone component. A pair of end electrodes electrically communicate with the upper portions of the chamber adjacent its opposite ends and extend toward the central portion of said chamber. A common electrode extends substantially the entire length of the chamber along its lower surface. The quantity of electrically conductive fluid in the chamber is such that an air bubble is left in the chamber, rising to whatever portion of the chamber is uppermost. It will be appreciated that, as the vial is tilted in one direction, the electrical impedance of a path from one end electrode through the electrically conductive fluid to the common electrode will increase, while the electrical impedance of a path from the other end electrode to the common electrode will decrease. When the vial is tilted in the opposite direction, the end-electrode-to-common-electrode impedances change in the opposite fashion. When the two end-electrode-to-common-electrode impedances are equal, the vial can be said to be oriented horizontally. It will be appreciated, however, that other impedance ratios might be defined as horizontal, if desired.  
           [0007]    In any event, changes in the ambient temperature of a vial may cause the vial casing to change dimensions and shape. Of particular concern is any asymmetric change in the shape of the chamber, in that this may result in a change in the position of the air bubble and a change in the impedance ratio without any actual change in vial orientation. Vials have, in the past, been thermally insulated. While this reduces short term temperature fluctuations and temperature gradients along the length of the vial, it does not reduce errors stemming from asymmetric changes in chamber shape.  
           [0008]    While the level vials account for a significant portion of the temperature dependent transmitter errors, other transmitter components are also a factor, as well. Most mechanical components will expand with increases in temperature. Some components will also change shape with increases or decreases in temperature. Further complicating matters is the fact that components are made of various materials and the materials may have varying coefficients of thermal expansion. These varying coefficients of thermal expansion may cause components that fit together perfectly at one temperature to bind or otherwise fit improperly at other temperatures. Since the various thermally induced errors can be cumulative so that the resulting errors are compounded, and since such errors can vary significantly from one transmitter to the next, a need exists for a laser transmitter and method of laser transmitter calibration in which such errors are eliminated, or at least minimized.  
         SUMMARY OF THE INVENTION  
         [0009]    This need is met by a transmitter for projecting a beam of laser light and a method according to the present invention in which thermally induced errors are compensated. The transmitter includes a source of a beam of laser light, a projection arrangement for directing the beam of laser light at a selected grade, a temperature sensor for detecting the temperature of said transmitter, and a temperature correction circuit. The temperature correction circuit includes a look-up table. The temperature correction circuit is responsive to the temperature sensor and adjusts the projection arrangement in dependence upon offset grade values that are stored in the look-up table for a plurality of transmitter temperature ranges. By this arrangement temperature induced errors in the direction of the beam of laser light are compensated.  
           [0010]    The projection arrangement includes a level vial. The level vial comprises an electrically nonconductive vial casing defining an elongated chamber which curves generally downward toward opposite ends thereof A quantity of electrically conductive fluid is provided in the chamber. A pair of end electrodes electrically communicate with the upper portions of the chamber adjacent opposite ends and extend toward the central portion of the chamber. A common electrode electrically communicates with the lower portion of the chamber. The vial is presumed to be level when resistances measured at each end are equal. The magnitudes of these resistances change with temperature, thus providing an indication of temperature. The temperature sensor includes a current sensor circuit for sensing the resistivity of the electrically conductive fluid. The current sensor includes a test resistance connected to one of the end electrodes, and a test circuit for determining the voltage across the test resistance in response to the application of a test signal of predetermined voltage and short duration across the end electrodes of the level vial.  
           [0011]    The projection arrangement for directing the laser light at a selected grade comprises a level vial having a quantity of electrically conductive fluid. The temperature sensor includes a circuit for sensing the bulk resistivity of the quantity of electrically conductive fluid. The projection arrangement for directing the laser light at a selected grade includes an arrangement for changing the direction of the beam until the selected grade is reached. The temperature correction circuit includes a circuit for providing the offset grade value thereto. The circuit for providing an offset grade value to the arrangement for changing the direction of the beam includes a look-up table having offset grade values associated with specific temperatures. The offset grade values are specific to the individual transmitter. By this technique temperature induced errors associated with any portion of the transmitter may be compensated. The look-up table has offset grade values associated with at least three specific temperatures. The temperature sensor may alternatively comprise a thermistor.  
           [0012]    The source of a beam of laser light may be a source of a rotating beam of laser light defining a reference plane. The projection arrangement directs the laser light at selected grades in first and second orthogonal axes, and the temperature correction circuit includes first and second look-up tables for adjusting said projection arrangement such that temperature induced errors in the direction of the beam of laser light along the first and second axes are compensated. The projection arrangement for directing the beam of laser light at a selected grade may include a generally conical reflector which reflects the beam radially outward, substantially in a reference plane. The projection arrangement directs the laser light at selected grades in first and second orthogonal axes.  
           [0013]    The projection arrangement for directing the beam of laser light at a selected grade includes a generally conical reflector which reflects the beam radially outward, substantially in a reference plane. The projection arrangement directs the laser light at selected grades in first and second orthogonal axes. The temperature correction circuit includes first and second look-up tables for adjusting the projection arrangement such that temperature induced errors in the direction of the beam of laser light along the first and second axes are compensated.  
           [0014]    A method of calibrating a transmitter for projecting a beam of laser light, the transmitter having a source of laser light, a projection arrangement for directing the laser light at a selected grade, a temperature sensor for detecting the temperature of the transmitter, and a temperature correction circuit, responsive to the temperature sensor, for adjusting the projection arrangement such that temperature induced errors in the direction of the beam of laser light are compensated, and the beam of laser light is thereby directed substantially at the selected grade, comprises the steps of: a.) selecting a plurality of temperatures for which correction will be made; b.) subjecting the transmitter to an ambient temperature equal to the first temperature for a period sufficient to achieve thermal equilibrium; c.) setting the transmitter for projecting the beam of laser light at a specified grade; d.) measuring the actual grade of the beam of laser light; e.) determining the error in the grade of the beam of laser light achieved; f.) determining the grade offset needed to correct for the error in the grade of the beam of laser light; g.) measuring the temperature of the vial at that ambient temperature; h.) storing the grade offset for the first temperature range in a look-up table; and i.) repeating steps a.) through g.) for each of the others of the plurality of temperatures.  
           [0015]    The step of selecting a plurality of temperature ranges for which correction will be made may include the step of selecting three temperatures, or it may include the step of selecting five temperatures. The step of setting the transmitter for projecting the beam of laser light at a specified grade may comprise the step of setting the projecting the beam of laser light at a specified grade in a first direction and setting the projecting the beam of laser light at a specified grade in a second direction, orthogonal to the first direction. The step of measuring the actual grade of the beam of laser light may include the step of measuring the actual grade of the beam of laser light in the first direction and in the second direction. The step of determining the error in the grade of the beam of laser light achieved may include the step of determining the error in the grade of the beam of laser light achieved in the first direction and in the second direction. The step of determining the grade offset needed to correct for the error in the grade of the beam of laser light may include the step of determining the grade offset needed to correct for the error in the grade of the beam of laser light in the first direction and in the second direction. Finally, the step of storing the grade offset for the first temperature in a look-up table may include the step of storing the grade offsets for the first and second directions for the first temperature in a pair of look-up tables.  
           [0016]    A method of calibrating a transmitter for projecting a beam of laser light, the transmitter having a source of laser light, a projection arrangement for directing the laser light at a selected grade, a temperature sensor for detecting the temperature of the transmitter, and a temperature correction circuit, responsive to the temperature sensor, for adjusting the projection arrangement such that temperature induced errors in the direction of the beam of laser light are compensated, and the beam of laser light is thereby directed substantially at the selected grade, comprising the steps of: a.) selecting a plurality of temperature ranges for which correction will be made; b.) subjecting the transmitter to a first ambient temperature for a period sufficient to achieve thermal equilibrium; c.) setting the transmitter for projecting the beam of laser light at a specified grade; d.) measuring the actual grade of the beam of laser light; e.) determining the error in the grade of the beam of laser light; f.) determining the grade offset needed to correct for the error in the grade of the beam of laser light; g.) repeating steps a.) through f.) for each of a plurality of temperature ranges; h.) constructing a grade offset curve; i.) determining from the grade offset curve the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges; and g.) storing the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges.  
           [0017]    The step of selecting a plurality of temperature ranges for which correction will be made may include the step of selecting three temperature ranges, or the step of selecting five temperature ranges. The step of setting the transmitter for projecting the beam of laser light at a specified grade may comprise the step of setting the projecting the beam of laser light at a specified grade in a first direction and setting the projecting the beam of laser light at a specified grade in a second direction, orthogonal to the first direction. The step of measuring the actual grade of the beam of laser light may include the step of measuring the actual grade of the beam of laser light in the first direction and in the second direction. The step of determining the error in the grade of the beam of laser light may include the step of determining the error in the grade of the beam of laser light in the first direction and in the second direction. The step of determining the grade offset needed to correct for the error in the grade of the beam of laser light may include the step of determining the grade offset needed to correct for the error in the grade of the beam of laser light in the first direction and in the second direction. The step of constructing a grade offset curve may include the step of constructing a grade offset curve for the first direction and constructing a grade offset curve for the second direction. The step of determining from the grade offset curves the grade offsets needed for the temperature at the midpoint of each of the plurality of temperature ranges may include the step of determining from the grade offset curve the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges for the first direction and for the second direction. The step of storing the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges may include the step of storing the grade offset needed for the temperature at the midpoint of each of the plurality of temperature ranges for the first direction and for the second direction.  
           [0018]    Accordingly, it is an object of the present invention to provide a laser beam transmitter which corrects effectively for thermally induced grade errors; to provide such a transmitter in which the temperature of the transmitter is monitored and correction is effected through grade offset values that are stored in a look-up table and that are unique to the errors of the specific transmitter; and to provide a method of calibrating such a laser beam transmitter in which grade offset values are determined specifically for the transmitter of interest and stored in one or more look-up tables. Other objects will be apparent from reference to the accompanying description and claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is a perspective view of a laser transmitter according to the present invention, with portions broken away to reveal internal structure;  
         [0020]    [0020]FIGS. 2A, 2B, and  2 C are longitudinal sectional views of a level vial, illustrating the manner in which the vial is used to determine orientation;  
         [0021]    [0021]FIG. 3 is a schematic view of circuitry according to the present invention;  
         [0022]    [0022]FIG. 4 is a schematic view of circuitry according to the present invention;  
         [0023]    [0023]FIG. 5 is a flow chart useful in understand the method of calibration; and  
         [0024]    [0024]FIG. 6 is a diagrammatic representation of a test arrangement for carrying out the method. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0025]    While the present invention is applicable in general to laser transmitters, it will be described herein with reference to a laser transmitter  10  illustrated in FIG. 1. The laser transmitter  10  includes a base  12 , a projection arrangement  14 , an input device  15 , a positioning device  16 , and a processor and temperature correction circuit  19 . The transmitter  10  includes a source of laser light  18 , which cooperates with an optical assembly  20  and an optical projecting device  22 . The optical assembly  20  includes a frame  24  and a gimbal mechanism  26 . The positioning device  16  comprises a first positioning device  28  and a second positioning device  30 . The laser transmitter  10  also includes a cover  31  to enclose and protect the internal components of the transmitter.  
         [0026]    The laser light source  18  is coupled to the frame  24  and generates a beam  32 . In the illustrated embodiment, the light source  18  is a laser diode. It will be appreciated by those skilled in the art, however, that other laser devices may be used to generate the beam of laser light  32 . A collection lens  34  is coupled to the frame  24  and positioned above the light source  18  so as to collect the laser beam  32  and project it in the first direction  35  along a substantially vertical axis  37 . Preferably, the collection lens  34  is a planar convex lens which collimates the laser beam  32 . The projection arrangement  14  may comprise additional components, such as a steering window, to ensure that the laser beam  32  is projected along the axis  37 , or an aperture to improve image quality, as desired.  
         [0027]    The optical projecting device  22  is coupled to the frame  24  and includes a motor  36  and a pentaprism  38  within the spindle  40 . The optical projecting device  22  deflects the laser beam  32  ninety degrees such that it emerges in a direction  43 . The pentaprism  38  is an optical element that is rotated to direct the laser light in a rotating beam. The pentaprism  38  is a structure which deflects incoming light at a ninety-degree angle with respect to the direction of the incoming light, within limits, regardless of the precise orientation of the pentaprism  38 . Consequently, the orientation of the pentaprism can fluctuate slightly due to bearing tolerances and other reasons without the operation of the transmitter being significantly affected.  
         [0028]    The pentaprism  38  is rotated with spindle  40  by the motor  36  at a speed of approximately 600 rpm to define a plane of light. The laser beam  32  is rotated along an arc defined about a central rotational axis  42 . The central rotational axis  42  corresponds to the center of rotation of the pentaprism  38 . The rotational arc is preferably 360 degrees; however, it will be appreciated by those skilled in the art that the laser beam  32  may be dithered back and forth, defining a rotational arc of less than 360 degrees. A speed of 600 rpm is well suited for machine control applications of the laser beam  32 ; however, the pentaprism  38  may be rotated at any speed desired, depending upon the manner in which the transmitter is used. While a pentaprism is used in the illustrated embodiment to deflect the incoming light, other light deflecting devices, such as a pentamirror, mirror, prism, reflector or refractor may also be used. Further, while the laser transmitter  10  has been described with the laser beam  32  being transmitted initially upwards, it will be appreciated by those skilled in the art that various optical components may be shifted  25  appropriately so that the laser beam  32  is transmitted downwards, with the optical projecting device  22  being the lower most component and the light source  18  being the upper most component.  
         [0029]    The frame  24  is coupled to a first portion  26 A of the gimbal mechanism  26  through a second or Y-axis pivot  56 . The first portion  26 A is coupled to a second portion  26 B of the gimbal mechanism  26  through a first or X-axis pivot  54 . In the illustrated embodiment, the first and second axes are substantially orthogonal to each other and correspond to the X and Y axes, respectively, of a standard X-Y coordinate system. The first and second axes, referenced hereafter as the X and Y axes, respectively, are therefore fixed relative to the transmitter.  
         [0030]    The second portion  26 B of the gimbal mechanism  26  is coupled to the base  12  so that the frame  24  is suspended by the gimbal mechanism  26 . Since the frame  24  is suspended from the gimbal mechanism  26 , the frame  24  pivots about the X and Y axes via the pivots  54 ,  56 , respectively. The angular orientation of the frame  24  with respect to the X and Y axes, and hence the laser beam  32 , is dependent on the orientation of the first portion  26 A with respect to the second portion  26 B of the gimbal mechanism  26 , and the orientation of the frame  24  with respect to the first portion  26 A of the gimbal mechanism  26 , respectively.  
         [0031]    The first positioning device  28  is coupled to the first and second portions  26 A,  26 B of the gimbal mechanism  26 . The first positioning device  28  includes a first motor  57 , a first gear  58 , a second gear  60 , and a first shaft  62 . The second gear  60  is coupled to the first portion  26 A of the gimbal mechanism  26  using conventional fasteners (not shown) while the first shaft  62  is coupled to the second gear  60  and the second portion  26 B through the first axis pivot  54 . The first shaft  62  rotates within the first axis pivot  54  so that as the second gear  60  rotates, the first portion  26 A of the gimbal mechanism  26  rotates with respect to the second portion  26 B of the gimbal mechanism  26 . The first gear  58  is coupled to a shaft  57 A of the first motor  57 . The teeth on the first gear  58  correspond to and engage the teeth on the second gear  60  such that as the first motor  57  rotates the first gear  58 , the second gear  60  rotates which effectuates rotation of the first portion  26 A of the gimbal mechanism  26 , and hence the frame  24 , with respect to the second portion  26 B of the gimbal mechanism  26 . Accordingly, the angle of the frame  24  and the laser beam  32  with respect to the X-axis is adjusted.  
         [0032]    The second positioning device  30  is coupled to the first portion  26 A of the gimbal mechanism  26  and the frame  24 . The second positioning device  30  includes a second motor  64 , a third gear  66 , a fourth gear  68 , and a second shaft  70 . The fourth gear  68  is coupled to the frame  24  through conventional fasteners (not shown) while the second shaft  70  is coupled to the fourth gear  68  and the first portion  26 A of the gimbal mechanism  26  through the second axis pivot  56 . The second shaft  70  rotates within the second axis pivot  56  so that as the third gear  66  rotates, the frame  24  rotates with respect to the first portion  26 A of the gimbal mechanism  26 . The third gear  66  is coupled to a shaft  64 A of the motor  64 . The teeth on the third gear  66  correspond to and engage the teeth on the fourth gear  68  such that as the second motor  64  rotates the third gear  66 , the fourth gear  68  rotates which effectuates rotation of the frame  24 . Accordingly, the angle of the frame  24  and the laser beam  32  with respect to the Y-axis is adjusted.  
         [0033]    It will be appreciated by those skilled in the art that other conventional gearing schemes may be used to effectuate a change in the angular orientation of the frame  24 , and hence the laser beam  32  with respect to the X and Y axes. It will be further appreciated by those skilled in the art that belt drive systems or direct drive systems may be used to effectuate a change in the angular orientation of the frame  24 , and hence the laser beam  32  with respect to the X and Y axes. It will be even further appreciated by those skilled in the art that other positioning devices may be used to effectuate a change in the angular orientation of the frame  24 , and hence the laser beam  32  with respect to the X and Y axes.  
         [0034]    The projection arrangement  14  also includes a first level vial  80  and a second level vial  82 . The first and second level vials  80  and  82  are coupled to the frame  24 . The first level vial  80  is positioned so that it senses the angular orientation of the frame  24  relative to the X-axis while the second level vial  82  is positioned so that it senses the angular orientation of the frame  24  relative to the Y-axis. The first and second level vials  80  and  82  provide outputs representative of the respective angular orientations of the vials. The first and second level vials  80  and  82  provide first and second electrical signals, respectively, representative of the angular orientation of the frame  24  with respect to the X and Y axes, respectively, and therefore indicative of the grades in the X and Y axes in which the laser light is being projected.  
         [0035]    Other configurations may be used to sense and position the frame  24  relative to the X and Y axes. For example, the first and second level vials  80 ,  82  may be coupled to a separate mechanism that is coupled to and moves with respect to the frame  24 . A grade mechanism in such an arrangement then causes the separate mechanism to carrying the level vials to shift so that it has a desired angular orientation with respect to the frame  24  and the X and Y axes. The positioning device  16  then adjusts the angular orientation of the frame  24  until the first and second level vials  80 ,  82  are level again. At this point, the frame  24  assumes the desired angular orientation with respect to the X and Y axes. Such an arrangement is shown in U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, herein incorporated by reference. Another example is set forth in U.S. Pat. No. 5,852,493, issued to Monnin on Dec. 22, 1998, also herein incorporated by reference.  
         [0036]    In the illustrated embodiment, the input device  15  comprises a keypad  84  and a display  86 . The keypad  84  includes numeric keys for inputting a first desired slope for the rotating laser beam  32  along the X-axis and a second desired slope for the laser beam  32  along the Y-axis. The display  86  provides a visual indication of the inputted slope along the X and Y axes. It will be appreciated by those skilled in the art that the display  86  may be configured to display additional information as desired. It will be further appreciated by those skilled in the art that other input devices may be used to input the first and second slopes. For example, a pair of mechanical dials may be used to input the first and second slopes with the dials providing the visual indication of the inputted slopes directly. The input device  15  may be removably coupled to the laser transmitter  10  or fixed in position, as desired.  
         [0037]    The input data from the input device  15 , and the first and second level signals from the first and second level vials  80 ,  82 , respectively are transmitted to the processor  19 . The processor  19  is programmed to control the positioning device  16  so that the laser beam  32  forms a plane which is angularly oriented in a desired direction in response to the above signals.  
         [0038]    Reference is now made to FIGS. 2A, 2B, and  2 C, which depict a level vial in greater detail. All three figures are sectional views taken in a longitudinally extending generally vertical plane. The exemplary level vial  100  provides an electrical signal indicating the orientation of the vial relative to gravity. In use on the transmitter, the electrical signal from the level vials indicates whether the laser light projected by the transmitter is oriented at the selected grade. The level vial  100  comprises an electrically nonconductive vial casing  102  defining an elongated chamber  104 . The casing  102  is commonly made of glass, and is thermally insulated by means of an insulating layer, indicated at  106 , which shields the vial from short term thermal fluctuations. The insulating layer, preferably a foam insulation layer, also insulates the vial  100  from the effects of thermal gradients that might otherwise distort the shape of the vial. The casing  102  curves generally downward toward opposite ends  107  and  108 . A quantity of electrically conductive fluid  110  is provided in the chamber  104 . The fluid may be any of a number of known, electrically conductive fluids which are commonly used in level vials. A quantity of air in the form of a bubble  1   18  is also provided in the chamber  104 . A pair of end electrodes  112  and  114  electrically communicate with the upper portions of the chamber adjacent opposite ends  108  and  107 , respectively, and extend toward the central portion of the chamber  104 . A common electrode  116  electrically communicates with the lower portion of the chamber  104 . Electrodes  112 ,  114 , and  116  are electrically connected to electrical leads, labeled A, B, and C, respectively, which extend through the casing  102 .  
         [0039]    As indicated above, the vial  100  provides electrical signals which indicate the orientation of the vial. This electrical signals are a function of the resistances between leads A and C, and between leads B and C. The drawings depict only a single lead C as connected to electrode  116 , although it is also common for level vials to include a pair of leads C emerging from both ends of the vial, connected to opposite ends of electrode  116 . The resistance between leads A and C, (R AC ), substantially equals the resistance between leads B and C, (R BC ), when the vial is in a generally horizontal orientation, as illustrated in FIG. 2A. If the vial is pitched to the left, as shown in FIG. 2B, with end  108  lowered and end  106  raised, the air bubble  118  moves to the right. This results in more of the electrode  112  coming into contact with the electrically conductive fluid  110 , and less of the electrode  114  contacting the electrically conductive fluid  110 . It will be appreciated that this will cause R AC  to decrease, and R BC  to increase. Conversely, if the vial  100  is pitched to the right, as shown in FIG. 2C, with end  106  lowered and end  108  raised, the air bubble  118  moves to the left. This results in more of the electrode  114  coming into contact with the electrically conductive fluid  110 , and less of the electrode  112  contacting the electrically conductive fluid  110 . It will be appreciated that this will cause R BC  to decrease, and R AC  to increase.  
         [0040]    It has been found that changes in the operating temperature of a laser transmitter such as shown in FIG. 1 produce errors in the direction of the beam of laser light produced by the transmitter. This is the result, at least in part, of thermally induced errors in the electrical signals from the level vials. It will be appreciated, for example, that a distortion of the shape of the casing  102  that occurs with a temperature change may well affect the relative areas of the electrodes  112  and  114  in contact with the fluid  110 , thereby affecting the ratio of the resistances R AC  and R BC . It has been found that such thermally induced errors are repeatable with temperature change. The present invention therefore takes these errors into account, and does so based on the temperatures of the vials themselves. Because level vials are significant contributors to temperature in induced errors, and because this invention contemplates using the vials to measure temperature, it is possible to compensate more accurately for the temperature than if another sensor arrangement were used.  
         [0041]    Reference is now made to FIG. 3 which illustrates a level vial  100  connected as a part of a transmitter constructed according to the present invention. The vial  100  has leads A, B, and C connected to a vial circuit  120  which monitors the relative values of resistances R AC  and R BC , thereby assessing the inclination of the vial  100 . Resistances R AC  and R BC  are shown in FIG. 3 as discrete resistors for simplicity.  
         [0042]    A temperature sensor for detecting the temperature of the vial includes a current sensor circuit comprising a test resistance R T  and a test circuit, including amplifier  122 . Periodically, vial circuit  120  applies a test signal of predetermined voltage and short duration across lines  124  and  126 , and therefore across the end electrodes of the level vial  100 . The current produced in response to this test voltage is directly proportional to the conductance of the series connected resistances R AC , R BC , and resistor R T , and is inversely related to the bulk resistivity of the electrically conductive fluid in the vial  100 . The value of resistor R T  is small in comparison to the resistances R AC  and R BC . As a consequence, resistor R T  does not significantly affect the operation of the balance of the circuitry. The current through the series connected resistances R AC , R BC , and resistor R T  produced in response to the test voltage is an indication of the temperature of the vial  100 , as it has been found that resistances R AC  and R BC  vary with the temperature of the vial  100 . The voltage across resistor R T  provides an indication of this current level. This voltage is amplified by amplifier  122  and an output signal applied to processor and temperature correction circuit  19 . As described below in greater detail, processor  19  controls the operation of the transmitter such that temperature induced errors in the orientation of the beam of laser light are compensated. By this arrangement, the beam of laser light  43  is directed substantially at the selected grade.  
         [0043]    Reference is now made to FIG. 4 which shows a circuit arrangement for a pair of vials  200  and  202 . The vial circuite 220  includes a vial drive circuit  233 , level amplifiers  237 , current sense resistor, R T  and coupling capacitors. The coupling capacitors minimize direct current and resulting plating of the vial electrodes. Also shown is a current sense amplifier  222 .  
         [0044]    Outputs  227  and  229  are connected to the common electrode C of the vials  202  and  200 , respectively, and kept separate. The voltages at the A leads of vials  202  and  200  are substantially equal to each other as are the voltages at the B leads of the two vials. Furthermore the voltage at the A leads is substantially opposite with respect to the system ground voltage compared to the voltage at the B leads. When R AC  equals R BC , the voltage at the C lead will be substantially equal to the system ground voltage. Signals on  227  and  229  are amplified to five indications for each X axis and Y axis level errors. A vial is presumed to be level when its amplified signal is substantially equal to the system ground voltage.  
         [0045]    Vial drive circuitry provides current at  225  and  228  such that the voltage between  224  and  226  is a predetermined level. Currents on  224 ,  226 ,  227 , and  228  are kept to substantially zero, so that substantially all of the current that flows through the parallel combination of vials also flows through current sense resistor, R T . Amplifier  222  measures the voltage across R T  and provides that signal to the processor as an indication of current through the vials. This signal is also an indication of the bulk conductance of the two vials, which is an indication of the bulk conductance of the electrolyte in the vials and the size of the bubble and, finally, an indication of the temperature of the vials. It will be appreciated by those skilled in the art that circuits other than that described here could be employed to measure the bulk conductance of the vials. As described in greater detail below, processor  19  controls the operation of the transmitter such that temperature induced errors in the orientation of the beam of laser light are compensated. By this arrangement, the beam of laser light  43  is directed substantially at the selected grade.  
         [0046]    As discussed previously with regard to FIG. 1, the transmitter  10  includes an arrangement for changing the direction of the beam until the selected grade is reached, as indicated by the level vial electrical signal. This arrangement includes the optical assembly  20  having a frame  24  and a gimbal mechanism  26 . The positioning device  16  comprises a first positioning device  28  and a second positioning device  30 . Devices  28  and  30  tilt the projection arrangement, changing the beam direction until the grade selected with keyboard  84  is achieved. In the present invention, the processor  19  provides an offset grade value and corresponding vial temperature. This offset grade value is related to the thermally induced error, and provides a compensation for such error. Preferably, this arrangement includes a look-up table that stores offset grade values and specific associated operating temperatures. It has been found that in some instances it is sufficiently accurate to measure the thermally induced errors at three temperatures—“cold,” “medium,” and “hot”—and to use a set of three offset grade values and corresponding vial temperatures. The specific offset grade values selected will vary from transmitter to transmitter, and are preferably empirically determined after the transmitter  10  is constructed and operational. The offset grade values and vial temperatures are stored in a look-up table having offset grade values associated with specific vial temperatures. The look-up table therefore has offset grade values that are unique to the specific transmitter and the specific level vials incorporated therein.  
         [0047]    In operation, the processor  19  constantly monitors vial temperature and provides grade offsets appropriate for the vial temperature at the time. The processor calculates grade offset during operation by applying vial temperature at the time to a linear interpolation of grade offsets and vial temperatures in the look-up table.  
         [0048]    In one version of the invention (FIG. 3), each of the level vials used in the transmitter will have its own associated current sensor and test resistor R T  connected thereto and a separate test circuit for determining the voltage across the test resistance R T  in response to the application of test signals of predetermined voltage. In another version of the invention (FIG. 4), the level vials are connected in parallel through appropriate switching circuitry during the brief, periodic testing of the temperatures for the two level vials, such that a single temperature for the two vials is detected.  
         [0049]    It will be appreciated that since the error induced by temperature change varies from level vial to level vial, the grade offsets needed to correct for the such errors will also vary from vial to vial. As a consequence, each set of grade offsets are unique, and are established by subjecting a transmitter and the level vials that are incorporated into the transmitter to a series of ambient temperatures and measuring the resulting induced errors in the direction of the beam in the corresponding axes. The method is begun by initially selecting a plurality of temperatures for which correction will be measured. The same number of temperatures will typically be used with all of the transmitters of a given model. It has been found that using only three temperatures is adequate to characterize some transmitter designs.  
         [0050]    Next the transmitter is subjected to an ambient temperature at the first temperature for a period sufficient to achieve thermal equilibrium. Even though the level vials are thermally insulated, the vials will eventually reach ambient temperature. Next the transmitter is set at a specific grade for both axes, such as for example zero grade. The actual grade of the laser beam in each axis is then measured. By comparing the actual grades of the beam and the set, specific grades of the beam, the grade offsets needed to correct for the measured error are determined. The grade offsets are then stored in the look-up table with the vial temperature. Next, the transmitter is subjected to an ambient temperature at the second temperature for a period sufficient to achieve thermal equilibrium. The grade off-sets and vial temperatures are then measured and stored, as described above. These steps are repeated for each of the other temperatures until the look-up tables are completed. The method described for a device under test (DUT) is illustrated by the flow chart of FIG. 5.  
         [0051]    [0051]FIG. 6 is a schematic representation of the test arrangement for carrying out this method. The DUT is placed on a motorized rotary table  300  inside a temperature chamber  302 . A computer  304  controls the temperature setting of the temperature chamber  302 . Computer  304  sends command signals to a motor controller  306  which turns the rotary table  300  and the DUT so that both axes can be readily measured with one optical angle measurement device  308 . The beam from the DUT passes through a window  310  in the wall of the chamber  302  to the optical angle measurement device  308 . The optical measurement device includes a beam detector which is connected an amplifier  310  and analog-to-digital converter  312  converter  312  is connected to the computer  30 . By this method, the computer can monitor the grade error of the DUT.  
         [0052]    Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. For example, while the present invention has been illustrated with respect to a transmitter which produces a reference plane of laser light, either with a rotating beam of laser light or with a static reflection of a beam into diverging fan of light, it will be appreciated that the invention may also be incorporated into a transmitter of the type which projects a substantially stationary beam. Such transmitters find application in trenching, tunneling, and pipe laying operations.