The robots developed by the team at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and the Wyss Institute for Biologically Inspired Engineering are flat and thin plastic tetrahedrons, with the three outer triangles connected to the central triangle by hinges, and a small circuit on the central triangle. Attached to the hinges are coils made of a shape-memory alloy (SMA) that can recover their original shape by being heated to a certain temperature.
When the hinges lie flat, the SMA coils are stretched out in their “deformed” state; when a current is passed through the circuit and the coils heat up, they move back to their original, relaxed state, contracting like tiny muscles and folding the robots’ outer triangles in toward the centre. When the current stops, the SMA coils are stretched back out due to the stiffness of the flexure hinge, lowering the outer triangles back down.
“This system requires only basic, passive electronic components on the robot to deliver an electric current - the structure of the robot itself takes care of the rest,” said Je-sung Koh who is now an Assistant Professor at Ajou University in South Korea.
The current is generated by coupling to an external coil. In order to control which coils contract, the team built a resonator into each coil unit and tuned it to respond only to a very specific electromagnetic frequency. By changing the frequency of the external magnetic field, they were able to induce each SMA coil to contract independently from the others.
“Not only are our robots’ folding motions repeatable, we can control when and where they happen, which enables more complex movements,” said Mustafa Boyvat, a Postdoctoral Fellow at the Wyss Institute and SEAS.
By changing the frequency of the magnetic field generated by the external coil, the team was able to control the robot’s movements independently.