TunnelSense: Low-power, Non-Contact Sensing using Tunnel Diodes

TL;DR

TunnelSense tackles the energy- and complexity-barrier of noncontact sensing by exploiting the frequency drift of a tunable tunnel diode oscillator caused by nearby motion. It employs a low-power tunnel diode tag operating at 868 MHz and a simple receiver to track carrier frequency changes, enabling sensing with minimal processing. The authors demonstrate breathing-rate detection at up to 30 cm and show possible battery-free operation via ambient energy harvesting. This approach suggests a practical path to ubiquitous, low-cost noncontact sensing in indoor environments and could extend to additional motion modalities.

Abstract

Sensing the motion of physical objects in an environment enables numerous applications, from tracking occupancy in buildings and monitoring vital signs to diagnosing faults in machines. Typically, these application scenarios involve attaching a sensor, such as an accelerometer, to the object of interest, like a wearable device that tracks our steps. However, many of these scenarios require tracking motion in a noncontact manner where the sensor is not in touch with the object. A sensor in such a scenario observes variations in radio, light, acoustic, and infrared fields disturbed by the object's motion. Current noncontact sensing mechanisms often require substantial energy and involve complex processing on sophisticated hardware. We present TunnelSense, a novel mechanism that rethinks noncontact sensing using tunnel diode oscillators. They are highly sensitive to changes in their electromagnetic environments. The motion of an object near a tunnel diode oscillator induces corresponding changes in its resonant frequency and thus in the generated radio waves. Additionally, the low-power characteristics of the tunnel diode allow tags designed using them to operate on less than 100microwatt of power consumption and with a biasing voltage starting at 70 millivolt. This enables prolonged tag operation on a small battery or energy harvested from the environment. Among numerous applications enabled by the TunnelSense system, this work demonstrates its ability to detect breathing at distances up to 30 centimeter between the subject and the TunnelSense tag.

Figures (11)

• Figure 1: TunnelSense enables noncontact sensing of the movement of objects in its vicinity. The movement of macroscopic objects causes changes in the electromagnetic environment of the tunnel diode oscillator, causing changes in its resonant frequency. It operates at under 100µWs of power consumption, and a commodity transceiver can track these changes in the frequency of the radio waves. In this work, we apply TunnelSense to sense a person's breathing.
• Figure 2: TunnelSense is designed with minimal components, which allows the platform in its simplest form to be as compact as a coin cell. The simplicity of the circuit and low-power consumption can also enable the realization of the sensor in novel form factors, such as postage stamp sized stickers.
• Figure 3: Tunnel diode shows negative resistance characteristics. Thus, an increase in the bias voltage across the tunnel diode (1N3712) leads to a drop in the current through the diode (after reaching a threshold voltage). We show a part of this region where TunnelSense operates with the shaded area.
• Figure 4: The tag consists of a tunnel diode oscillator integrated with a resonant circuit. It is configured to generate a carrier signal at the 868MHz band. When motion occurs near the tunnel diode, the reflected signal propagates back to the oscillator. Additionally, the nearby object induces electrical and magnetic interactions with the circuit, resulting in frequency drifts in the generated carrier signal that correlate with the object's motion.
• Figure 5: The choice of material and location of object from the tag impacts the frequency drift of the radio waves generated by the tunnel diode oscillator.