Broken wire detection in coated steel belts using the magnetic flux leakage method

A new kind of coated steel belt (CSB) is becoming widely used in elevator systems, where the reliable and safe use of CSBs is crucial for the elevator operation. It is necessary to develop a reliable inspection method to estimate their safety. The magnetic flux leakage (MFL) method is typically used for the detection of metallic cross-sectional area loss and localised flaw defects in steel wire ropes by arranging a group of magnetic sensors circumferentially around the rope. Unfortunately, this cannot be achieved on a CSB due to its flat structure. This paper presents an optimised MFL method for the detection of broken wires in CSBs, using a pair of sensors for each encapsulated rope to form a differential signal. The matching pursuit method is applied to analyse the noisy signals and decompose them into the expression of normalised vectors. Both the amplitude coefficient and lift-off factor are employed for representing the signals that are obtained from the decomposition and then combined to evaluate the number of broken wires. Experimental results show that the designed set-up could inspect the weak MFL signal at a high lift-off and the matching pursuit method has a robust performance in noise suppression and feature extraction for MFL signals. The estimated number of broken wires is very close to the actual situation.