Axial force measurement method

An axial force measurement method for measuring an axial force of a bolt fastened to an object to be fastened, comprising:

• acquiring a distance image with a height as a pixel value by measuring a height of a head of the bolt with a distance sensor;
• calculating a gravity center position of the head of the bolt by subjecting the distance image to binarization processing to acquire a binarized image and extracting an image of the head of the bolt from the binarized image;
• calculating an average value of a height of pixels positioned inside a minimum circle as a minimum point by subjecting the distance image to mask processing and extracting only pixels positioned inside the minimum circle centered on the gravity center position of the head of the bolt;
• calculating an average value of the height of extracted pixels with a relatively large height as a maximum point by extracting pixels with the relatively large height from the pixels of the distance image on the basis of the pixel value of the pixel;
• calculating a displacement amount of the head of the bolt on the basis of a difference between a calculation result on the minimum point and a calculation result on the maximum point;
• and
• calculating an axial force by substituting the calculation result on the displacement amount of the head of the bolt into a pre-calculated relational expression of an axial force and the displacement amount of the head of the bolt.