A Passive Elastic-Folding Mechanism for Stackable Airdrop Sensors

Damyon Kim; Yuichi Honjo; Tatsuya Iizuka; Naomi Okubo; Naoto Endo; Hiroshi Matsubara; Yoshihiro Kawahara; Naoto Morita

cs.ROMar 19, 2026

A Passive Elastic-Folding Mechanism for Stackable Airdrop Sensors

Authors: Damyon Kim, Yuichi Honjo, Tatsuya Iizuka, Naomi Okubo, Naoto Endo, Hiroshi Matsubara, Yoshihiro Kawahara, Naoto Morita,

Organizations: Graduate School of Engineering, The University of Tokyo, Japan · NTT Space Environment and Energy Laboratories, Japan

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 & tables

Fig. 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.

Fig. 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.

Fig. 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.

Fig. 4: Experimental validation of the heating conditions and hinge angle model. (a) Relationship between heating temperature and fold angle, with heating time fixed at $90\text{\,}\mathrm{s}$ . (b) Relationship between heating time and fold angle, with heating temperature fixed at $110\text{\,}\mathrm{\SIUnitSymbolCelsius}$ . (c) Relationship between the board gap ( $g$ ) and the fold angle. In addition to the measured data, theoretical values calculated using ( 2 ) are shown.

Fig. 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.

Material	Specification / Part No.	Thickness ( $\mathrm{\SIUnitSymbolMicro m}$ )
Polyolefin Sheet	THT-14.0 N	350
Polyimide Sheet	200H-A4	50
FR4 Substrate	0.6 mm PCB (JLCPCB)	600

TABLE I: Sheet Materials

Fig. 6: Glider sensor prototypes for environmental sensing. Two versions were developed: a standard design (left) and a cutout design (right), both with $25\text{\,}\mathrm{cm}$ wingspans and integrated PCBs with sensors and LoRa communication.

Fig. 7: Outdoor dispersion test of the glider sensor at the Fukushima Robot Test Field. The airframe integrates a sensor board with IMU, GPS, environmental sensors (temperature and humidity), and a LoRa communication module. (a) Launch setup from a $30\text{\,}\mathrm{m}$ platform using a custom-built frame and rubber band. (b) Onboard IMU pitch-angle data capturing the passive pull-up maneuver after launch.

Fig. 8: Dispersion simulation of glider airframes deployed from high altitude. (a) 3D trajectories of 50 glider airframes released from an altitude of

10\text{\,}\mathrm{km}

, showing descent in turning patterns while drifting with the wind. (b) Top-down view of the simulated trajectories over Mito, Japan, showing the descent from a

10\text{\,}\mathrm{km}

deployment altitude to the simulation’s end altitude of

5\text{\,}\mathrm{km}

. The wind environment combines a mean wind profile from MATLAB’s Horizontal Wind Model ( atmoshwm ) with turbulence generated by the Dryden gust model.

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Mechanical Engineering, University of Washington, Seattle, WA, USA. · Electrical & Computer Engineering, University of Washington, Seattle, WA, USA. · Computer Science and Engineering, University of Notre Dame, Notre Dame, IN, USA.+1

Modular Robotics Robotics Soft Robotics

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We introduce an elastic-driven self-folding approach that fabricates robots directly from flat 3D-printed conductive PLA nets. Elastic bands routed through printed hooks store energy that folds the sheet into programmed 3D geometries, while the flat state allows accurate placement of electronics and magnets before deployment. The same substrate doubles as electrodes for capacitive touch and supports a reusable platform I/O palette with Hall sensors and eccentric rotating mass (ERM) motors for docking detection and vibration actuation. We also derive a closed-form folding model that balances hinge stiffness with elastic band moment to predict equilibrium fold angles; experiments validate the model and yield a design map linking hinge thickness, band size, and hook spacing to target angles. Using this workflow we realize multiple polyhedral modules and demonstrate three applications: a cube that highlights the potential of self-folding for scalable modular robot collectives, a deployable gripper, and a tendon-driven finger. The method is low cost, stimulus-free, and integrates actuation and sensing.

Abstract

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