In the related art, as a method for detecting a normal-line direction of a curved surface such as in a processing surface of a three-dimensional work piece, a method disclosed in PTL 1 is known.
In the method disclosed in PTL 1, distance measurement means for measuring at least three positions in the vicinity of a reference point on a curved surface of an object is provided, a virtual plane including each measurement position is set based on each measurement position, and a normal-line direction of the virtual plane is detected.
In the method disclosed in PTL 1, when there is unevenness such as a protrusion or a step on a measuring object, it is not possible to detect a correct normal-line. In addition, in PTL 1, a plane approximation method in a case where four or more measurement positions are used is not specifically disclosed.
Meanwhile, PTL 2 discloses a method in which distance sensors are arranged so as to be deviated by an eccentric amount R in a radial direction from the rotation (processing) center of a measurement axis, the distance sensors are swiveled about the measurement axis, information related to the distance to a continuous measured surface is acquired, and a normal-line is detected by removing a step from the distance information.