Full-Field Out-of-Plane Displacement Measurement Using Microwave Holographic Interferometry

Full-field displacement measurement via microwave sensing has shown appealing advantages over the laser and vision-based approaches, such as long-range monitoring and environmental adaptability. However, the existing microwave radar-based approaches have limitations in monitoring out-of-plane displacement of surfaces, including reliance on artificial markers, limited measurement points and trade-offs between number of points and sensitivity. In this paper, we present a method of full-field out-of-plane displacement measurement using microwave holographic interferometry, which extends the traditional two-dimensional imaging radar-based displacement measurement method to three-dimensions. First, we describe the principle of microwave holography and build the mathematical model between the phase of holographic interferometry and out-of-plane displacement. Second, using a CAE software and a built prototype of a microwave holographic interferometry system, we evaluate our method with both simulations and experiments. The simulations and experiments of different target size, motion pattern and displacement amplitudes show that our approach can simultaneously monitor hundreds of points without artificial markers and the root mean square error of displacement measurement is less than 30 μm in all displacement scales, offering a novel approach for contactless displacement monitoring in structural health monitoring, especially for surface structures.