Method and apparatus for measuring the straightness error of slender bodies, with compensation of gravity deformation

Measuring method to measure the straightness errors of elongated-shape elements, named bars, by a measuring apparatus comprising a supporting system static during the measurement cycle, on which the bar to be measured is arranged, a first detecting system to detect the development of the longitudinal axis of the bar, a second detecting system to detect the forces acting on the bar and combined with said supporting system, acquiring means to acquire the values of the physical parameters of the bar under measuring, said physical parameters being able to comprise an elasticity module of the bar, and a central control unit, the method comprising the steps of:

• a) positioning a bar on the supporting system so that it is arranged in a position of static equilibrium;
• b) acquiring the values of the physical parameters of the bar by the acquiring means;
• c) detecting, by the second detecting system, the vector of the forces the bar applies to the supporting system;
• d) detecting, by the first detecting system, the coordinates of a plurality of points of the surface of the bar next to at least three cross sections of the bar;
• e) calculating the coordinates of the longitudinal axis of the bar next to said at least three cross sections, on the basis of the detection of step d);
• f) using the data detected by said first and second detecting systems and the values of the physical parameters of the bar detected by the acquiring means to estimate the geometry of the bar in the absence of applied forces;
• g) determining any real straightness error of the bar, on the basis of the data detected by said first and second detecting systems and of the geometry of the bar estimated in step f), so that the error is not influenced by the constraints of the supporting system;
• wherein step f) provides for using a modeling of the theoretical deformation of the bar in accordance with the elastic curve method, which can be applied to deformable slender bodies, on the basis of the values of the physical parameters acquired in step b) and of the vector of the forces applied detected in step c) and for subtracting such theoretical deformation from the deformation calculated in step e) to determine any real straightness error of the bar.