Target-free measurement of cable forces based on computer vision and equivalent frequency difference

Cables are the primary elements of cable-supported structure bridges, and cable force is an important index to evaluate the safety and integrity of bridges. The approach of cable tension estimation using computer vision (CV) technology has many advantages over traditional methods. However, there are still two major challenges with the existing CV-based methods, i.e., it is difficult to identify tiny vibrations of cables under ambient excitation and/or in the presence of dampers, and most CV-based techniques require to install artificial targets on the cable. To address these challenges, the present study develops a target-free method for cable vibration and force measurement based on broadband phase-based video motion magnification (BPVMM) and line tracking algorithm. The BPVMM technique is firstly adopted to amplify the tiny vibration of cables, and then the line tracking algorithm is utilized to measure the cable displacement from the amplified video to obtain frequencies of the cable. In addition, a theoretical formula has been suggested to calculate cable tension in terms of the cable frequencies. The suggested approach is validated through the laboratory and field cable tests. The results demonstrate that the cable force values calculated applying the proposed approach agree very well with the measured values of the cable force, and the maximum error is within 3 %. This study provides a target-free, convenient, and low-cost approach for the measurement of cable vibration and forces.