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1. A tilt detecting device in which an inner frame is provided inside an outer frame, a tilt detecting unit having a tilt sensor which detects a tilt from the horizontality is provided inside said inner frame, said inner frame is rotatably supported by said outer frame through a first shaft, said tilt detecting unit is rotatably supported by said inner frame through a second shaft orthogonal to said first shaft, comprising: a first encoder which is provided on said first shaft and detects a rotation angle between said outer frame and said inner frame, a second encoder which is provided on said second shaft and detects a rotation angle between said inner frame and said tilt detecting unit, motors provided on said respective shafts so as to rotate said respective shafts, and an arithmetic processing module which drives and controls said respective motors based on a detection result from said tilt sensor, wherein said arithmetic processing module is configured to drive said respective motors based on a signal issued upon detecting a tilt by said tilt sensor such that said tilt sensor detects the horizontality and to calculate a tilt of said outer frame in real time based on the outputs from said first and second encoders when said tilt sensor detects the horizontality, and said arithmetic processing module is configured to determine a stationary state of a tilt detecting device based on a detection result from said tilt sensor, to drive said respective motors, to reverse said tilt detecting unit 180° at least once based on the detection result from said respective encoders in relation to said first shaft and said second shaft, to obtain ½ of a tilt angle difference before and after the reversal as a tilt angle, and to perform the calibration to remove a drift component based on the detection signals output from said tilt sensor before and after the reversal.
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