Six-degree-of-freedom generalized displacements measurement based on binocular vision

Due to their large flexibility and small damping ratios, harmful vibrations could happen on cables under some external excitations, as for instance the rain–wind-induced vibration, which comes with a translational and rotational coupling. An experimental study of the phenomenon would be quite important in view of an appropriate design. However, the available rotational measurement approaches are not suitable for measuring the rotation of cable, because the installation of such devices alters the structural mechanical properties. Recently, low-cost vision-based structural displacement measurement methods have been widely used due to their flexible deployment and the noncontact measurement feature. But they cannot measure the rotational displacements directly. This paper proposes a rotational generalized displacement measurement method based on a binocular vision assisted by a coded cylinder. The three-dimensional coordinates of the coded points are acquired according to photogrammetry schemes. The cylinder centroid and its direction vector are obtained by a cylindrical fitting. Eventually, the translational and rotational generalized displacements are derived. In the cylinder fitting stage, one optimizes the cylinder model parameters via differential search. Experimental verifications are carried out to demonstrate that the proposed six-degree-of-freedom (6-DOF) generalized displacement measurement is feasible.