Abstract Summary
One way to know the state of health of a cable and its force in operation under varying environmental conditions is to estimate its modal properties. In the present work, the vibration of a nonlinear polymer cable is studied in the laboratory using computer visual techniques (videos) and compare traditional sensors. Two types of excitations are used to make the cable vibrate: broad band ambient vibrations and with initial conditions (deformation or impact). Tests are carried out with various cable tensions in order to cover a greater range of fundamental frequencies and to evaluate the relaxation process that occurred in this type of material. The oscillation is measured simultaneously with traditional sensors and video cameras (including smartphones). The traditional sensors correspond to 6 ultrasonic sensors equidistantly located along the cable, a load cell that allows the cable tension to be measured, and an accelerometer near one end. With the measurement of each sensor, the frequencies and modal damping can be estimated, and by joining the 6 channels of ultrasonic sensors, the modal shape can be estimated. Additionally, 8 cameras are installed. 5 frontal, which record the entire cable, but from different perspectives, which mainly allow observing the vertical oscillation of the cable; 3 cameras that record parts of the cable (local measurement), and one camera that records the cable from above (oscillation out of the vertical plane). In each video obtained, the tracking of multiple feature points of the cable is performed using the KLT algorithm without the use of artificial targets. With this, the displacement records are obtained in pixels/second. The modal properties are obtained using parametric techniques like NeXT ERA, SSI-COV and ERA. In order to know the mode shapes perspective correction due to the angle of the camera with respect to the cable must be performed. Various methodologies are studied that allow solving this problem which include calibrated and non-calibrated cameras. The mode shape estimation is studied considering each camera separately, and also considering 2 or more simultaneous cameras. The latter allows a 3D reconstruction of the movement of certain discrete points of the cable when it is possible to synchronize the movement of a point from multiple views. This additionally allows performing an in-plane and out-of-plane vibration study simultaneously. A comparison of the results obtained with each method is made and its advantages and disadvantages are studied. Finally, the problems and difficulties that may arise in camera calibration, video synchronization, video sampling frequency instability, point tracking, among others, are presented.
