Vibration measurement by radar techniques

Recent advances in radar techniques and systems have led to the development of microwave interferometers, suitable for the non-contact vibration monitoring of large structures. The main characteristic of these radar systems, entirely designed and developed by Italian researchers, is the possibility of simultaneously measuring the (static or dynamic) deflection of several points on a large structure with high accuracy. The main ideas of the microwave-based measurement of deflections are: (a) to employ a radar to take coherent and consecutive images of the investigated structure, with each image being a distance map of the radar echoes intensity coming from the reflecting targets detected on the structure; (b) to evaluate the displacement of each target from the phase of the back-scattered electromagnetic waves collected at different times (microwave interferometry). In the first part of the paper, the main techniques adopted in microwave remote sensing are described, so that advantages and potential issues of these techniques are addressed and discussed. Subsequently, the results of past and recent tests of full-scale structures are presented, in order to demonstrate the reliability and accuracy of microwave remote sensing; furthermore, the simplicity of use of the radar technology is exemplified in practical cases, where the access with conventional techniques is uneasy or even hazardous, such as stay cables or tall structures.