A group of researchers at the University of Washington have figured out a way to make 3D-printed objects emit WiFi signals containing status information, all without using electrical power.The technique uses what’s called WiFi backscattering. The method is to make the 3D-printed device amplitude-modulate a WiFi signal coming from a conventional source such as a wireless router. Then a separate receiver is used to detect the amplitude modulation pattern that the 3D-printed device superimposes on the WiFi emissions.
Researchers implemented the backscattering idea by 3D-printing a WiFi antenna, then connecting and disconnecting it as a way to reflect and not-reflect the WiFi signal to form a coded pattern. Then a receiver — they used a MAX2829 802.11a/b/g transceiver — decodes the backscatter information from the amplitude variations in the received Wi-Fi signal across packets.To make the scheme work without using electrical power, the researchers came up with a mechanical means of breaking and making connections to the 3D-printed antenna in a way such that a broken connection would drastically detune it. Thus the antenna would reflect WiFi signals when connected, not reflect when disconnected. To accomplish this, they devised a 3D-printed bowtie type antenna with a 2-mm gap at the center. A mechanical switch either connects or disconnects the two halves.
To generate a digital code without using external power, the group devised a spring-actuated switch actuated by a coded gear. When a gear tooth pressed down on the switch, it would make a connection between the two halves of the bow tie antenna. They tried two kinds of coding schemes: one where the presence of a gear tooth indicated a 1 bit and the absence of a tooth indicated a 0 bit, and a second where a gear encoded 1s and 0s by doubling the tooth width in a manner analogous to Morse code.
Researchers tried several kinds of springs but got good results using a planar coil spring orthogonal to the contact surface. A slot guides the contact to ensure it stays parallel to the contact surface. The coded gear couples to a 3D-printed, tightly coiled planar spring where the outer edge of the spring is held at a fixed point, and the center couples to a gear using a square axle.
The spring gets wound up to “charge” the device, then applies torque to the square axle and therefore to the connected gear as it unwinds. The gear to which the coil spring attaches, in turn, actuates a circular gear with the encoded bits. By controlling the ratio between the size of the primary and circular gears, researchers control the speed at which the switch toggles between the two states.In operation, the device would backscatter a WiFi signal when something turned the gear — in one demonstration an anemometer turned it as a way of recording wind speed. Researchers have tried their apparatus and detected the backscatter effect it generates over a distance of 15 cm using a WiFi source as far away as about 16 m. The bit rates involved are, of course, relatively slow, ranging from 16 to 45 bps.
The researchers described their work at the recent ACM Siggraph Conference and Exhibition on Computer Graphics and Interactive Techniques in Asia.