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Small-scale, shape-changing origami sensors

Small-scale, shape-changing origami sensors

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Robotic platforms can alter the trajectory of their descent midair for greater dispersal.

The microflier is shown in its folded state. Only its four corners touch the table, and its body supports the circuitry and actuator.
This microflier robotic device can change how it moves through the air by snapping into a folded position during its descent. It has an onboard battery-free actuator (orange rod) powered by a solar circuit. Credit: Mark Stone/University of Washington

Inspired by leaves flying in the wind, Vikram Iyer and colleagues at the University of Washington have built a system that deploys a fleet of small sensors from a drone. Each “microflier” is made of a foldable sheet of material integrated with a lightweight circuit, which measures temperature and air pressure and transmits the data via Bluetooth. The sensors—one of which is shown in the photograph—are solar powered and have a mass of 414 mg each. Iyer and his team wanted to go beyond random dispersal in the wind, so they created programmable fliers that could change their shape in midair to control their descent behavior.

The final design has two states: flat and folded. Initial wind-tunnel measurements (seen in the figure below) show that when the flier was flat, it met the incoming air with sharp edges. The device tumbled in the air, buffeted by an uneven wake. The accompanying rapidly changing center of pressure created a large aerodynamic torque with respect to the microflier’s center of mass, causing it to tilt and start the tumbling. In that state, the microflier travels far in the wind. In contrast, the flier with the folded state met the incoming air with beveled corners, which resulted in a narrower and more symmetrical wake. That shape allows the microflier to fall straight down. The video shows both states in free fall.

The wave behind the flat state shows a curve of high-velocity air, while the stable state has a relatively constant wake with a much smaller spread in velocity.
Time-averaged 2D velocity profiles show the wake behind the microflier in both the flat state and the stable state. The face of the sheet is parallel to the y-axis at x = 0. The asymmetric wake in the flat state is responsible for the tumbling flight. Credit: Adapted from K. Johnson et al., Sci. Robot. 8, eadg4276 (2023)

To change the shape of the fliers midflight, Iyer’s team turned to origami to minimize the energy needed for the transformation. Specifically, the researchers use the Miura-ori fold, a type of rigid origami where the faces of the structure do not contort during the folding; the only changes occur along the creases. That structure can snap between two stable states and requires only a small amount of energy to make the change. The team designed a lightweight, battery-less actuator and circuit that harvests energy from an array of small solar cells to charge a 100 μF capacitor and then releases a short pulse into the actuator to snap the structure into its folded state. The microflier is the first device at this scale to change shape midair.

In the tests, the flat microfliers dispersed more than three times the distance of the folded ones. Dropped from 40 m, they traveled up to 98 m in a light breeze. In the flat, tumbling configuration, the devices landed upright only 52% of the time; they need to be in an upright position for the solar cells to continue powering them. In the folded, stable state, they landed upright 87% of the time. Many of the remaining 13% subsequently bounced or collided with something and came to a rest upright.

The transition between the flat state and the folded state lasts only 25 ms. The folding action switches the microflier’s trajectory from a tumbling path to a stable descent. Credit: Adapted from the University of Washington

The microfliers currently measure only pressure and temperature, which they use to trigger a change in shape. Iyer and his team, however, envision their work to be used as a platform to study properties beyond their initial atmospheric uses.

The current design can only trigger the transition from the flat state to the folded one, but the team is developing a way for the fliers to revert back into the flat state for increased control in their flight path. To minimize the fliers’ environmental impact, the researchers are also working to integrate their previous work on biodegradable circuits. (K. Johnson et al., Sci. Robot. 8, eadg4276, 2023.)

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