Platypus: Sub-mm Micro-Displacement Sensing with Passive Millimeter-wave Tags As "Phase Carriers"

Micro-displacement measurement is a crucial task in industrial systems such as structural health monitoring, where millimeter-level displacement of specific points on the structure or machinery displace can jeopardize the integrity of the structure and potentially leading to catastrophic damage or collapse. Traditionally, such displacements on large structures are measured using visual sensing platforms or advanced surveying equipment. However, they either fall short in varying weather and lighting conditions or require installation and maintenance of high-power sensing platforms that are expensive to deploy at scale, especially if continuous measurements are desired. In this paper, we explore simultaneous tracking of quasi-static micro-displacements of multiple objects or multiple points from a single vantage point using millimeter-wave (mmWave) radars. We present Platypus, a micro-displacement sensing system that enables sub-millimeter level accuracy by using mmWave backscatter tags and their reflection as phase carriers to shift the phase changes due to tiny displacements to clean frequency bins for precise tracking. It then reconstructs the phase changes with sub-millimeter level accuracy even from extended ranges (over 100m) or in non-line-of-sight (NLoS) situations. While Platypus enables many different applications, we demonstrate the proof-of-concept in structural health monitoring, where mmWave tags are attached to building models and track the structural micro-displacements, achieving a median of 0.3 mm accuracy. [GRAPHICS] .