Development of the Cable Tension Testing Instrument Based on the Permanent Magnetizer

Steel cables are widely used in cable-stayed bridges, suspension bridges and so on. It is important to test them periodically for their safe operation. The cable tension is one of the key testing parameters. Magnetoelastic methods are usually adopted to measure the cable tension due to their non-contact features. However, it is difficult to install the exciting and receiving encircling coil on the cable in existing magnetoelastic instruments. In order to overcome this limitation, in this paper, the testing instrument based on the permanent magnetizer for testing of cable tension is proposed. When the permanent magnetizer is installed on the cable, a spatially varying magnetic field is induced along the length of cable. The reciprocal value of initial differential susceptibility, which is linear to the cable tension, can be obtained from the radial and axial magnetic flux densities measured on the surface of the cable according to the Gauss’s law for magnetism and continuity of tangential magnetic field strength. Based on the above analysis, the cable tension testing instrument is developed, which includes permanent magnetizer, Hall elements, signal amplifier, A/D converter and software. Under the magnetization of the permanent magnetizer, the Hall elements array pick up the magnetic flux densities along the length of the cable and transform into voltage signals. The voltage signals are amplified and converted to digital signals, then uploaded to the portable computer through USB interface. By a series of calculations, the reciprocal value of initial differential susceptibility used for the cable tension testing can be obtained. The laboratory tests indicate that this instrument can test of the ultimate tension with 5% accuracy. One of the biggest advantages of the technique is that it is easy to operate in the field.