Development of Ultrasonic Wave Nondestructive Inspection Robot without Coupling Medium Using Emat

The ultrasonic wave is applied as an inspection method for the gas-holder and the pipeline in service. In such an inspection, the application of an automatic inspection system is desirable, because a manual inspection is difficult to accomplish perfectly and exactly due to its enormity, a so-called nondestructive inspection robot is then preferable. However, an ultrasonic nondestructive inspection robot with a piezoelectric oscillator needs to directly touch the material surface to be inspected using the coupling medium. Therefore, a mechanism where the coupling medium is spread thoroughly between the sensor and the material surface and where the sensor is always held at a constant pressure in the same direction on the material side to be inspected, are performances requested for the inspection robot. Actually, it is difficult to overcome this problem and the ultrasonic inspection robot could not be applied widely. We then tried to develop an ultrasonic inspection robot with an electromagnetic acoustic transducer (EMAT) which did not require the coupling medium, which inspected the circumferential parts in the pipe. We especially developed the EMAT to transmit and receive a S0-mode with high sensitivity and a SH0-mode plate wave without the influence of the welding bead alternately as the sensor parts. The fixed method between the pipe and the inspection robot was not especially fabricated, because the magnetic force of the EMAT was very strong and was sufficient for attaching the inspection robot on the surface of the steel pipe. The method in which the inspection robot turned around in the direction of the steel pipe surroundings was executed by observing the tape pasted in the direction of the steel pipe surroundings with an installed CCD camera. In this study, the basic mechanisms of this inspection robot and the examination results are described. Introduction: Generally, an angle beam method is used as the nondestructive inspection method of welded parts, but the precise depth and lateral scanning is absolutely required to cover the entire welded part as shown in Fig.1 (a). In addition we must control any ultrasonic transducers so that they touch with a constant pressure on the surface of the steel pipe. A resolution of these problems is important in order to put an inspection robot into practical use. On the other hand, the inspection by a plate wave does not require depth scanning as shown in Fig.1 (b). In addition, we could supplement each method’s good points and faults if we could simultaneously use a Lamb wave and a SH plate wave. However, it is difficult to use a SH-plate wave for auto-scanning inspection, as we must use a coupling medium with a high viscosity. We then wrestled with the development of an ultrasonic nondestructive inspection robot which carried an EMAT that could alternately drive a Lamb wave and a SH-plate wave, because an EMAT can theoretically transmit and receive ultrasonic waves without any coupling medium. Plate wave Weld parts Transducer Weld parts (b) Plate wave method Scanning direction Angle beam transducer Weld parts Transducer Angle beam Weld parts (a) Angle beam method Angle beam transducer Pipe