Abstract:
A laser survey instrument, comprising a laser projector for projecting a laser beam onto a plane by rotary irradiation, a tilting unit for tilting the laser projector in two directions running perpendicularly each other and a tilt setting unit for setting a desired tilt angle for the laser projector at least in one direction, wherein the tilt setting unit comprises a tilt sensor for detecting a horizontal position and an angle detector for detecting an angle between the tilt sensor and the laser projector.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a laser survey instrument, which can project a laser beam at an arbitrary tilt angle to a horizontal plane. 
     A type of laser survey instrument is already known, which can form a horizontal reference line by a laser beam by projecting the laser beam in a horizontal direction, or which projects the laser beam by rotary scanning and forms a horizontal reference plane by the laser beam. As one of the laser survey instruments of this type, a survey instrument is concretely realized, which not only projects the laser beam in the horizontal direction but also can project the laser beam at any desired tilt angle with the horizontal plane as reference. 
     Referring to FIG. 5 to FIG. 7, description will be given below on a conventional type laser survey instrument, which can tilt the projecting direction of the laser beam at any desired angle. 
     At the center of a casing  5 , a recessed portion  6  in form of a truncated cone is provided, and a support seat  7  is arranged at the center of the recessed portion  6 . The support seat  7  is provided with a circular bore  8  formed on the recessed portion  6 , and projections  9  are smoothly projected in three-dimensional curved surface and are arranged at three positions equally spaced on inner periphery of the bore  8 . 
     A laser projector  10  for emitting a laser beam is placed into the bore  8 , and a head  11  of the laser projector  10  is engaged with and supported by the support seat  7 . The lower portion of the head  11  is designed in a spherical shape, and this spherical portion  11   a  slidably contacts the three projections  9 . The laser projector  10  is supported in such manner that it can be tilted in any direction with respect to the vertical line. 
     On the head  11 , a motor seat  14  extending in a horizontal direction is provided. A scanning motor  15  is mounted on the motor seat  14 , and a gear  16  is attached on an output shaft of the scanning motor  15 . The gear  16  is engaged with a scanning gear  17  as described later. 
     On the head  11  of the laser projector  10 , a prism holder  13  is rotatably mounted via a bearing  12  on the axis of the laser projector  10 . The scanning gear  17  is attached on the prism holder  13 , and the scanning gear  17  is engaged with the gear  16  as already explained. By the scanning motor  15 , the prism holder  13  is rotated around the vertical axis. The prism holder  13  and the scanning gear  17  make up together a rotator  3 . A pentagonal prism  18  is provided on the prism holder  13 , and the laser beam emitted from the laser projector  10  is deflected and projected in the horizontal direction through a projection window  19 . 
     In the middle portion of the laser projector  10 , a sensor support shelf  63  is mounted in a direction perpendicularly crossing the optical axis of the laser projector  10 . On the sensor support shelf  63 , fixed bubble tubes  20  and  21 , serving as tilt detectors for detecting the horizontal position, are arranged so that these bubble tubes cross perpendicularly each other. The fixed bubble tubes  20  and  21  are electric bubble tubes of capacitance detection type, and each of the bubble tubes outputs an electric signal corresponding to a tilt angle with the horizontal plane as reference. 
     On the lower portion of the laser projector  10 , there is provided a tilt angle setting unit  74 . The tilt angle setting unit  74  comprises arbitrary angle setting bubble tubes  65  and  66 , and these bubble tubes are arranged at positions opposite to the fixed bubble tubes  20  and  21  respectively. 
     At the lower end of the laser projector  10 , a base plate  64  is fixed, which is approximately in shape of a right-angled triangle. A support column  70  is erected at a position near a vertex of the right-angled triangle of the base plate  64 , and a ball  67  is fixed on the upper end of the column  70 . A tilting base plate  62  in L-shaped rectangular form is arranged above the base plate  64 . A conical recessed portion  99  is formed at a vertex of L-shaped rectangle on the back side of the tilting base plate  62 . The ball  67  is engaged in the recessed portion  99 . The column  70  pivotally supports the vertex of the tilting base plate  62  via the ball  67 , and the tilting base plate  62  can be pivotally moved on the ball  67 . Further, a spring  68  is provided between the tilting base plate  62  and the base plate  64 . This spring presses the conical recessed portion  99  against the ball  67  and pushes the tilting base plate  62  clockwise as seen in FIG.  5 . 
     On the tilting base plate  62 , the arbitrary angle setting bubble tubes  65  and  66 , serving as tilt detectors, are placed along lines, which cross perpendicularly each other along the L-shape. 
     A bearing support plate  72  is disposed below the sensor support shelf  63 . The bearing support plate  72  is projected in the horizontal direction from the laser projector  10 . Tilting screws  52  and  53  are rotatably mounted at such positions that these two screws form a triangle with the column  70  as a vertex on the base plate  64 . Upper end of each of the tilting screws  52  and  53  is rotatably and pivotally supported on the bearing support plate  72 . 
     The lower end of the tilting screw  52  is protruded downward from the base plate  64 . A tilting gear  54  is attached on the protruded lower end of the tilting screw  52 , and the tilting gear  54  is engaged with a tilting gear  56  as described later. The lower end of the tilting screw  53  is protruded downward from the base plate  64 . A tilting gear  55  is attached on the protruded end of the tilting screw  53 . The tilting gear  55  is engaged with a tilting gear  57  as described later. 
     A tilting nut  48  is screwed on the tilting screw  52 , and a nut pin  50  with circular cross-section is mounted on the tilting nut  48  in the horizontal direction. From an end surface of the tilting base plate  62  closer to the arbitrary angle setting bubble tube  65 , a tilting pin  60  with circular cross-section is protruded in a direction parallel to the central line of the arbitrary angle setting bubble tube  65 , and the tilting pin  60  is in contact with the nut pin  50 . Further, two parallel guide pins  71  are run between the base plate  64  and the bearing support plate  72 . The tilting pin  60  is slidably supported by the two guide pins  71 , and the rotation of the tilting base plate  62  in the horizontal direction is restricted. Also, it is allowed only to rotate in the up-to-bottom direction of the tilting pin  60  and to rotate around the axis of tilting pin  60 . 
     A tilting nut  49  is screwed on the tilting screw  53 , and a nut pin  51  with circular cross-section is mounted on the tilting nut  49 . From an end surface of the tilting base plate  62  closer to the arbitrary angle setting bubble tube  66 , a tilting pin  61  with circular cross-section is protruded in a direction parallel to the central line of the arbitrary angle setting bubble tube  66 , and the tilting pin  61  is brought into contact with the nut pin  51 . 
     A pedestal column  73  is attached vertically on the lower surface of the base plate  64 , and a tilt detector  23 , which also serves as a motor base, is fixed via the pedestal column  73 . On the upper surface of the tilt detector  23 , tilt angle setting motors  58  and  59 , i.e. pulse motors, are mounted. The tilting gear  56  is engaged with an output shaft of the tilt angle setting motor  58 . The tilting gear  57  is engaged with an output shaft of the tilt angle setting motor  59 , and these tilting gears are engaged with the tilting gears  54  and  55  respectively. 
     On the lower surface of the tilt detector  23 , a ring-like reflection mirror (not shown) is attached. Optical sensors  24   a ,  24   b ,  24   c  and  24   d  ( 24   b  and  24   d  are not shown) comprising four pairs of light emitting element and photodetection element are arranged on a bottom of the casing  5 . The optical sensors  24   a ,  24   b ,  24   c  and  24   d  are disposed at positions facing to the tilt detector  23 , and on the same circumference around the axis of the laser projector  10  when both the casing  5  and the laser projector  10  are at vertical position. 
     In the following, description will be given on a tilting unit for tilting the laser projector  10  in any arbitrary direction. 
     From the head  11  of the laser projector  10 , tilt arms  25  and  26  are extended in two different horizontal directions perpendicularly crossing each other, and these arms are passed through the conical surface of the recessed portion  6  and are positioned within the casing  5 . On the forward end of each of the tilt arms  25  and  26 , engaging pins  27  and  28  are arranged to protrude from the end respectively. The engaging pins  27  and  28  are designed in cylindrical shape. Axes of the cylinders cross perpendicularly each other, and the positions are determined in such manner that these are included in a plane passing through the center of the spherical portion  11   a . With respect to one of these two engaging pins  27  and  28 , e.g. to the engaging pin  27 , the movement in the horizontal direction is restricted, and it is allowed to move only in the vertical direction. 
     Shelf plates  29  and  30  are provided on inner wall of the casing  5 . A tilting motor  31  is provided on the shelf plate  29 , and a tilting motor  32  is arranged on the shelf plate  30 . A driving gear  33  is engaged on the rotation shaft of the tilting motor  31 , and a driving gear  34  is engaged on the rotation shaft of the tilting motor  32 . A screw shaft  35 , which runs perpendicularly to the engaging pin  27  and reaches a ceiling of the casing  5  and the shelf plate  29 , is rotatably mounted. A driven gear  36  is attached on the screw shaft  35 , and the driven gear  36  is engaged with the driving gear  33 . A slide nut  37  is screwed on the screw shaft  35 , and a pin  38  is protruded from the slide nut  37 . The pin  38  and the engaging pin  27  are brought into contact so that these can slide with respect to each other. 
     In similar manner, a screw shaft  39 , which runs perpendicularly to the engaging pin  28  and reaches a ceiling of the casing  5  and the shelf plate  30 , is rotatably mounted. A driven gear  40  is attached on the screw shaft  39 , and the driven gear  40  is engaged with the driving gear  34 . A slide nut  41  is screwed on the screw shaft  39 , and a pin  42  is protruded from the slide nut  41 . The pin  42  and the engaging pin  28  are brought into contact with each other so that these can slide with respect to each other. 
     A spring receiver  43  is provided on the ceiling of the casing  5  and between the screw shafts  35  and  39 . A spring  44  is stretched between the spring receiver  43  and the laser projector  10  so that a force is applied on the laser projector  10  to move it clockwise around the support seat  7  as seen in FIG.  5 . 
     In the figure, reference numeral  45  denotes a battery box to accommodate a battery for driving the laser survey instrument. A main unit of the laser survey instrument is placed on a tripod (not shown) via a leveling bolt  46  for leveling purpose. Reference numeral  47  represents a glass window which encloses the periphery of the prism holder  13 . 
     Referring to FIG. 8, description will be given on operation to tilt the projecting direction of the laser beam in the conventional type laser survey instrument as described above. 
     A tilt value is inputted to a tilt control unit (not shown). 
     Based on the results of detection by the fixed bubble tubes  20  and  21 , the tilting motor  31  is driven. The laser projector is tilted via the screw shaft  35 , the slide nut  37 , and the tilt arm  25 . Also, the tilting motor  32  is driven, and the laser projector  10  is tilted via the screw shaft  39 , the slide nut  41 , and the tilt arm  26 . When the fixed bubble tubes  20  and  21  detect the horizontal position, leveling is performed for the laser projector  10 . 
     Next, the tilt angle setting motors  58  and  59  are driven so that output of each of the arbitrary angle setting bubble tubes  65  and  66  will be identical with output of each of the fixed bubble tubes  20  and  21  respectively. The angle of the tilting base plate  62  is adjusted by adjusting the tilting gear  56 , the tilting gear  54 , the tilting screw  52 , the tilting nut  48 , and the nut pin  50 , and also by adjusting the tilt angle setting motor  59 , the tilting gear  55 , the tilting screw  53 , and the nut pin  51 . As a result, the tilting base plate  62  can be set to the zero position (horizontal position). The horizontal setting of the tilting base plate  62  can also be performed by setting a stopper, etc. to the mechanical zero position, and by touching the base plate on the stopper. 
     Next, the tilting base plate  62  is tilted to the inputted tilt value. Description will be given now on the setting of the tilting direction detected by the arbitrary angle setting bubble tube  65 . 
     From the condition where the output of each of the arbitrary angle setting bubble tubes  65  and  66  is turned to identical with the output of each of the fixed bubble tubes  20  and  21 , the tilt angle setting motor  58  is driven in such number of steps as to correspond to the setting angle, and the tilting base plate  62  is tilted at a setting angle in a direction opposite to the direction, in which the laser projector  10  is to be tilted. 
     When the tilting base plate  62  is tilted at the setting angle, the tilting motor  31  is driven, and the laser projector  10  is tilted in a direction to be tilted. When the condition is reached where the arbitrary angle setting bubble tube  65  detects the horizontal position, tilt setting for the laser projector  10 , i.e. tilting of the laser beam projecting direction at a predetermined angle, is completed. 
     The tilt setting of the arbitrary angle setting bubble tube  66  can also be performed by the same procedure. 
     When the pentagonal prism  18  is rotated via the prism holder  13  using the scanning motor  15 , a reference plane tilted in a predetermined direction can be formed. 
     In the conventional type laser survey instrument as described above, tilting of the laser beam is set only by adjusting the feeding amount of the tilting nut  48  according to the amount of rotation of the pulse motor. As a result, the accuracy of the tilt angle setting depends on fabrication accuracy and assembling accuracy of the components such as the tilting screw  52 , the tilting nut  48 , etc. Therefore, variation occurs in the tilting accuracy due to the variation in fabrication accuracy and assembling accuracy. The difference in thermal expansion due to temperature exerts influence on the accuracy of the tilt setting. 
     Further, in order to meet the specifications and the standards of the product, strict accuracy is required for each of the components, and this results in the higher manufacturing cost. 
     As described above, the tilting base plate  62  is tilted, and angle setting is performed. A distance from the contact point of the ball  67  (the center of tilting of the tilting base plate  62 ) to the contact point of the tilting pin  60  with the nut pin  50  is increased when the base plate  62  is tilted, and an error occurs between the displacement of the tilting nut  48  and the tilt angle of the tilting base plate  62 . This can be corrected by calculation based on a correction formula. However, because of the fabrication error and assembling error of the components as described above, the error cannot be corrected simply by the certain correction formula. When the setting tilt is larger, the error will be larger. 
     Further, in the tilt setting, when it is turned from a certain status to the horizontal position, zero detection is performed to maintain accuracy of the horizontal position. That is, prior to the tilt setting, detection values on the arbitrary angle setting bubble tubes  65  and  66  of the tilting base plate  62  are equalized with those of the fixed bubble tubes  20  and  21 . Or, it is touched to a mechanical zero setting point (more concretely, touched to a component such as a stopper), and zero reset is performed. For this reason, zero reset operation must be performed each time the tilt setting is carried out, and this means that longer time is required until the setting procedure is completed. Further, because driving frequency of the tilting nut  48  and the slide nut  37  is higher, and problem arises in the wear between the tilting screw  52  and the tilting nut  48  and between the slide nut  37  and the screw shaft  35 . 
     Also, zero detection is performed using mechanical components such as stopper, and an error also occurs due to the changes of position caused by the temperature of the stopper or the changes of condition caused by wear of the operating part of the components such as tilting nut  48 . 
     Cumulative results of the various errors as described above increase more over the course of time, and unless re-setting is performed at appropriate time, the errors may exert influence on the accuracy of tilt setting of the laser beam. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a laser survey instrument, for which no strict component accuracy is required and by which it is possible to perform tilt setting for the laser beam projecting direction without causing cumulative errors. 
     To attain the above object, the present invention provides a laser survey instrument, comprising a laser projector for projecting a laser beam onto a plane by rotary irradiation, a tilting unit for tilting the laser projector in two directions running perpendicularly each other and a tilt setting unit for setting a desired tilt angle for the laser projector at least in one direction, wherein the tilt setting unit comprises a tilt sensor for detecting a horizontal position and an angle detector for detecting an angle between the tilt sensor and the laser projector. Also, the present invention provide a laser survey instrument as described above, wherein the tilting unit can tilt the laser projector in two directions running perpendicularly each other and the tilt setting unit is provided so as to set tilting only in one direction. Further, the present invention provide a laser survey instrument as described above, wherein the tilting unit and the tilt setting unit are provided so as to set tilting in two directions running perpendicularly to each other, the tilting unit comprises a first tilting unit and a second tilting unit, and the tilt setting unit comprises a first tilt setting unit and a second tilt setting unit. Also, the present invention provide a laser survey instrument as described above, wherein the tilting unit comprises a first tilting unit and a second tilting unit provided for two directions crossing perpendicularly each other, and the tilt setting unit is provided on one of the first tilting unit and the second tilting unit, and the tilt sensor for detecting the horizontal position is provided on the other. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an elevation view showing a partial cross-section of an essential portion of an embodiment of the present invention; 
     FIG. 2 is a control block diagram of the embodiment of the present invention; 
     FIG.  3 (A), FIG.  3 (B) and FIG.  3 (C) each represents a drawing to explain tilt setting operation of the embodiment of the present invention; 
     FIG. 4 is a flow chart of the tilt setting operation of the embodiment of the present invention; 
     FIG. 5 is a cross-sectional elevation view of a conventional example; 
     FIG. 6 is an arrow diagram along the line A—A in FIG. 5; 
     FIG. 7 is an arrow diagram along the line B—B in FIG. 5; and 
     FIG. 8 is a flow chart of tilt setting operation in the conventional example. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Description will be given below on an embodiment of the present invention referring to the drawings. 
     FIG.  1  and FIG. 2 each represents an essential portion of an embodiment of the present invention. In FIG. 1, the same component as in FIG. 5 is referred by the same symbol. 
     Supporting structure of a laser projector  10  and mechanism of a tilting unit are basically the same as in the conventional example, and detailed description is not given here. In the following, description will be given on a tilt angle setting unit in the present embodiment. 
     A frame  77  is fixed on a laser projector  10 . A tilt setting screw  78  in parallel to the axis of the laser projector  10  is rotatably mounted on the frame  77 , and a setting driven gear  81  is engaged on the lower end which is protruding downward from the frame  77 . A tilt setting nut  79  is screwed on the tilt setting screw  78 . Rotation of the tilt setting nut  79  is restricted, and it can be moved only in the direction of the axis of the tilt setting screw  78 . A power transmission pin  80  protruding in the horizontal direction is provided on the tilt setting nut  79 , and it is brought into contact with a driven pin  90 , which is to be described later. A tilt setting motor  82  is provided on the frame  77 . A setting driving gear  83  is attached on an output shaft of the tilt setting motor  82 , and the setting driving gear  83  is engaged with the setting driven gear  81 . 
     A rotor  85  is provided, which has a rotation axis perpendicularly crossing the axis of the laser projector  10 . An absolute encoder  86  is arranged on the rotor  85  so that the absolute encoder  86  is integrally rotated with the rotor  85 . On the rotor  85 , there are provided an X-axis (horizontal axis in parallel to paper surface) tilt sensor  87  and a Y-axis (horizontal axis perpendicular to paper surface) tilt sensor  88 , and these are integrally rotated with the rotor  85 . A CCD unit  91  is mounted on the laser projector  10 , and an angle of the absolute encoder  86  can be read by the CCD unit  91 . The CCD unit  91  and the absolute encoder  86  make up together an angle detector, and the angle detector detects an angle between the X-axis tilt sensor  87  and the laser projector  10 . Under condition that the absolute encoder  86  detects an angle of 0, also under the condition that the X-axis tilt sensor  87  and the Y-axis tilt sensor  88  detect an angle of 0, the relationship of the laser projector  10  with the absolute encoder  86 , the X-axis tilt sensor  87  and the Y-axis tilt sensor  88  is determined in such manner that the axis of the laser projector  10  is directed in the vertical direction. Here, the Y-axis tilt sensor  88  detects the horizontal position in Y-axis direction as adjusted by the tilt arm  26 . 
     From the rotor  85 , a tilt arm  89  is extended toward the tilt setting nut  79 . At the forward end of the tilt arm  89 , the driven pin  90  is arranged, and the driven pin  90  is brought into contact with the power transmission pin  80  from the below. A spring  92  is stretched between the rotor  85  and the laser projector  10 , and the tilt arm  89  is pushed counterclockwise, and the driven pin  90  is pressed against the power transmission pin  80  with a predetermined force. The spring  92  pushes up the tilt setting nut  79  upward via the tilt arm  89 , the driven pin  90  and the power transmission pin  80 , and prevents backlash of the tilt setting nut  79  and the tilt setting screw  78 . 
     In the figure, reference numeral  76  denotes a laser diode, which emits a laser beam. 
     FIG. 2 is a control block diagram. Reference numeral  93  denotes a control unit, which is typically represented by a CPU. 
     The results of the detection from the X-axis tilt sensor  87  and the Y-axis tilt sensor  88  are inputted to the control unit  93 , and a detection angle of the absolute encoder  86 , i.e. the result of angle detection from the CCD unit  91 , is inputted. The control unit  93  is provided with a storage unit (not shown). In the storage unit, a sequence program for tilt angle setting and an arithmetic operation program necessary for tilt angle setting are set and inputted. The data necessary for tilt angle setting is inputted to the control unit  93  from an input unit  94 . 
     Based on the data inputted from the X-axis tilt sensor  87 , the Y-axis tilt sensor  88 , and the CCD unit  91 , the control unit  93  issues a control driving signal to tilting motor driving units  95  and  97  and to a tilt setting motor driving unit  96 . The tilting motors  31  and  32  and the tilt setting motor  82  are driven via the tilting motor driving units  95  and  97  and via the tilt setting motor driving unit  96 . 
     Referring to FIG.  3  and FIG. 4, description will be given on operation of tilt setting. 
     A tilt angle as desired is inputted from the input unit  94 . 
     The control unit  93  drives the tilt setting motor  82  via the tilt setting motor driving unit  96 , and the tilt setting screw  78  is rotated. When the Y-axis tilt sensor  88  is not at the horizontal position, the tilting motor  32  is driven via the tilting motor driving unit  97  at the same time. By the rotation of the tilt setting screw  78 , the tilt setting nut  79  is moved up or down. And the absolute encoder  86  is integrally rotated with the rotor  85  via the power transmission pin  80 , the driven pin  90 , and the tilt arm  89 . The absolute encoder  86  detects a rotation angle and the rotation angle is turned to a signal by the CCD unit  91 , and it is inputted to the control unit  93 . In this case, when the Y-axis tilt sensor  88  detects the horizontal position, it is turned to tilt setting status, and driving of the tilting motor  32  is stopped. 
     At the control unit  93 , the signal from the CCD unit  91  is compared with the preset value from the input unit  94 . When these values are identical with each other, the tilt setting motor  82  is stopped. 
     Under this condition, the rotor  85  is rotated in opposite direction with respect to the laser projector  10  and at the same angle as the desired tilt angle. On the rotor  85 , the X-axis tilt sensor  87  and the Y-axis tilt sensor  88  are fixed. The X-axis tilt sensor  87  is tilted together with the rotor  85  with respect to the laser projector  10 . The Y-axis tilt sensor  88  is twisted only by the tilt angle, but there is no influence on the detection of the horizontal position in Y direction (See FIG.  3 (B)). 
     Next, until the signal from the X-axis tilt sensor  87  is turned to 0, i.e. until the X-axis tilt sensor  87  detects the horizontal position, the control unit  93  issues a driving control signal to the tilt setting motor driving unit  96  and drives the tilting motor  31 . 
     The signal from the X-axis tilt sensor  87  is inputted to the control unit  93 . At the control unit  93 , it is judged whether the input signal is 0 or not. If it is not 0, the driving of the tilting motor  31  is continued, and the tilting motor  31  is stopped when it is turned to 0. The X-axis tilt sensor  87  is tilted at the same angle as the desired tilt angle and is in the opposite direction with respect to the laser projector  10 . Thus, the laser projector  10  is tilted at the desired tilt angle when the X-axis tilt sensor  87  detects the horizontal position. Thus, the setting of tilt angle is completed (See FIG.  3 (C)). 
     When the scanning motor  15  is driven and the prism holder  13  is rotated via the gear  16  and the scanning gear  17 , a laser beam is projected by rotary irradiation, and a laser reference plane tilted at a predetermined angle in a predetermined direction is formed. 
     When a new tilt angle is to be set, the above procedure is repeated. 
     The tilt setting procedure as described above is based on angle information of the absolute encoder  86 . The absolute encoder  86  outputs an absolute angle between the laser projector  10  and the rotor  85 , i.e. the X-axis tilt sensor  87 . When a new setting angle is inputted, the new setting angle is compared with the detection angle of the absolute encoder  86 , and it should be controlled in such manner that the detection angle of the absolute encoder  86  is equal to the setting angle. Thus, each time the tilt angle setting, there is no need to perform zero re-setting to the horizontal position for the laser projector  10  and the X-axis tilt sensor  87 . 
     In the above embodiment, description has been given on the tilt setting in one direction, while it is needless to say that it is possible to tilt in any direction as desired when there are provided two sets of tilting mechanisms as in the conventional example. In this case, the Y-axis tilt sensor is set to a tilt setting mechanism in Y-axis direction. 
     According to the present invention, the tilt setting unit comprises a tilt sensor for detecting the horizontal position and an angle detector for detecting an angle between the tilt sensor and the projector. As a result, high accuracy is not necessarily required for the components of the tilt setting unit. This contributes to the reduction of the manufacturing cost. Because the actual tilt angle is detected, there is no error caused from mechanical problem. The actual angle is detected, and a tilt angle is set based on the result of detection. Therefore, there is no need to perform zero setting. The operation can be carried out quickly, and movement of the movable components is limited to the minimum movement necessary for the setting. Wearing of components is reduced, and the increase of cumulative errors over time can be prevented. Further, the number of the components can be reduced, and this leads to lower manufacturing cost and simpler assembling and adjusting procedure. As a result, the time required for assembling and adjustment can be extensively shortened.