Estimating Modal Properties of Nonlinear Cables Using Targetless Visual Tracking

One way to study the health state and operational force on a cable under varying environmental conditions is to estimate its modal properties. In this study, the vibration of a nonlinear polymer cable is examined in the laboratory using computer vision techniques, such as videos, in comparison with traditional sensors. Two types of excitations are used to induce cable vibration: broad band ambient vibrations and initial conditions (deformation and impact). Displacements are measured simultaneously with traditional sensors and consumer-grade cameras (including smartphones) from different perspectives (full-field and local measurements). The traditional sensors include six ultrasonic sensors equidistantly located along the cable and an accelerometer near one end. For each video obtained, multiple feature points of the cable are tracked using the KLT algorithm without artificial targets, and the displacement records are obtained in pixels. Modal properties are determined using parametric techniques like SSI-COV and ERA. To determine displacement in metric units and mode shapes with perspective correction, a methodology is proposed using camera calibration. The non-linearity of the cable is studied by analyzing the temporal evolution of frequencies over time. The performance of the proposed methodology was then compared to that of conventional sensors.