A well-shielded EMAT for on-line ultrasonic monitoring of GMA welding

A hybrid laser electromagnetic acoustic transducer (EMAT) system is currently being developed for pass-by-pass inspection of weldment defects. The laser source is inherently broadband, whereas the EMAT is a narrowband detector. Some of the theory used to develop an optimized EMAT system for use with a laser source is reviewed. The EMAT is used close to a welding torch. The welding system has a 40-kHz (fundamental) switching power supply. This gives rise to severe electromagnetic interference (EMI). The means of temperature- and EMI-hardening used to develop a practical EMAT system for this environment are described. A balanced system of counterwound coil and differential preamplifier which greatly suppressed EMI is presented. Other measures, such as multiple shielding, bandpass filtering, and attention to grounds, were used. The system gives about 32-dB signal-to-noise ratio, when operated 125 mm from a welding torch