Ultrasonic sensor system for automatic depth measurement of surface opening cracks in concrete by means of a robotic arm

The paper reports about the development of an automatic tool that can be used to measure the depth of cracks with millimetric precision on board of a structural inspection robot. The system is based on two piezoelectric ultrasonic transducers with central frequency of 50 kHz that are utilized to take Time-of-Flight (ToF) ultrasound measurements across the crack to determine its depth. In order to cope with the intrinsic inhomogeneity of concrete, the measurement method adopted in the automatic system is based on the simultaneous determination of ultrasound speed propagation and crack depth in one measurement series over the crack. To this purpose, the automatic system is designed to be able to position the two transducers in the required way on a concrete surface by using two stepper positioning systems mounted at 90° with respect to each other, able to displace the sensors with millimetric precision over the concrete wall, as needed in the measurement. The complete system, constituted by the ultrasonic sensors, the 2-dimensional positioning tool and a protection frame with steel supporting pins, can be displaced over the surface of a concrete tunnel by a robotic arm and placed across a crack in order to measure its depth automatically. The design, implementation, laboratory testing and early on-field testing of the system are presented, showing its capability to perform automatic crack depth measurements with millimetric precision.