A Passive Elastic-Folding Mechanism for Stackable Airdrop Sensors

Abstract

Air-dispersed sensor networks deployed from aerial robotic systems (e.g., UAVs) provide a low-cost approach to wide-area environmental monitoring. However, existing methods often rely on active actuators for mid-air shape or trajectory control, increasing both power consumption and system cost. Here, we introduce a passive elastic-folding hinge mechanism that transforms sensors from a flat, stackable form into a three-dimensional structure upon release. Hinges are fabricated by laminating commercial sheet materials with rigid printed circuit boards (PCBs) and programming fold angles through a single oven-heating step, enabling scalable production without specialized equipment. Our geometric model links laminate geometry, hinge mechanics, and resulting fold angle, providing a predictive design methodology for target configurations. Laboratory tests confirmed fold angles between 10 degrees and 100 degrees, with a standard deviation of 4 degrees and high repeatability. Field trials further demonstrated reliable data collection and LoRa transmission during dispersion, while the Horizontal Wind Model (HWM)-based trajectory simulations indicated strong potential for wide-area sensing exceeding 10 km.

Figures (8)

• Figure 1: Elastic-folding sensor concept and transformation process. (a) The sensor stacked in planar form before deployment. (b) The sensor passively transforms into a 3D glider after dispersion.
• Figure 2: Cross-sectional view of the three-layered elastic hinge before and after heating. A heat-shrinkable PO sheet is laminated with a gapped FR4 substrate and a polyimide film. Upon heating, the exposed PO sheet shrinks, causing the hinge to fold.
• Figure 3: Fabrication process and passive transformation of the elastic-folding sensor using the proposed hinge mechanism. (a) The fabrication process begins with a rigid PCB patterned with a predesigned gap at the target hinge location. A heat-shrink polyolefin sheet and a polyimide sheet are laminated to the substrate and then heated in a reflow oven. (b) During heating, the exposed portion of the polyolefin sheet contracts. The strain mismatch between this layer and the polyimide layer generates a bending moment that folds the hinge to an angle determined by the PCB gap geometry. This heat-driven self-folding process occurs during fabrication, prior to stacking and loading onto the balloon. (c) After fabrication, the hinge can be flattened under pressure for stacking. Upon release during deployment, its elasticity enables passive recovery into the designed 3D glider configuration.
• Figure 4: Experimental validation of the heating conditions and hinge angle model. (a) Relationship between heating temperature and fold angle, with heating time fixed at 90s. (b) Relationship between heating time and fold angle, with heating temperature fixed at 110℃. (c) Relationship between the board gap ($g$) and the fold angle. In addition to the measured data, theoretical values calculated using (\ref{['eq:theta=']}) are shown.
• Figure 5: Elastic hinge evaluation under cyclic deformation. (a) The tensile test setup and a diagram of the stress in the hinge. A force is applied to unfold the hinge. This results in greater extension and higher stress on the inner side of the hinge. (b) Load-displacement curve from the tensile test. The graph shows the results for the 1st and 10th cycles, along with the theoretical curve. (c) Relationship between elapsed time (in minutes) after release from compression and the angle recovery rate. Samples were held in a planar state under pressure for 1 to 3 hours before release.