Near-Zero-Power Sensors

The US Department of Defense has a research and development agency, the defense advanced research projects agency or DARPA, which have recently concluded a program N-ZERO—the near-zero power rf and sensor operations. One dramatic outcome of this program is sensors in the battlefield earlier running out of power in weeks or months can now keep operating for more than four years before they need a replacement of their coin batteries.

DARPA’s initiatives began about five years back, with the aim of improving IoT battery power so that sensors could operate in the field without requiring frequent battery replacement. The army deploys sensors in the field for detecting battlefield signals like sound, light, and vibrations.

According to DARPA, it is possible to improve battery lifetimes by using sensors that are idle most of the time, waking up to a triggering event, or periodically monitoring battlefield events as they happen, rather than monitoring them continuously.

In the field, troops can now gather data and intelligence from potential combat zones without venturing there personally. They also do not have to move into hazardous areas frequently for replacing dead batteries for sensors.

According to Benjamin Griffin, program manager of DARPA’s Microsystems Technology Office, unattended, untethered systems can now benefit from the N-ZERO program using idle but continuously alert sensing capabilities. Radiofrequency signatures or specific physical signals can trigger these sensors. As the sensor lifetime now extends to years, deployment of such sensing technologies can now be more cost-effective and safer in areas that lack fixed-energy infrastructure.

With progressive improvements in sensor technology, sensor capabilities are continuously expanding. As explained by Griffin, the N-ZERO program has been successful in developing sensors that wake up with infrared, acoustic, and RF signals. These near-zero-power sensors detect thermal radiation, measure sound levels, and communicate with radio frequency signals.

Even when coin-cell batteries power these sensors, their power consumption was so low the estimated battery life could extend from a few weeks to four years. However, during testing, processing, and communications of confirmed events limited the N-ZERO initiatives. Ultimately, the self-discharging of the batteries would also be another limiting factor.

The DARPA initiative has also been effective in developing an ARM M0N0 processor with ultra-low-power capabilities. The processor consumes only 10 nW when idling, and 20-60 µW/MHz when active, depending on the application. As sensors shutdown, they store the information in read-only memories (ROM), which the users can access without any power penalty, as the data resides in non-volatile memory.

As their power consumption is so low, the ARM processor can run for decades on a single set of batteries. Conventional coin batteries can have a lifetime of several years if the sensors operate in sleep mode, but not much beyond.

Sensors that operate continuously consume a lot of power, but often spend time processing useless data. In contrast, low power consumption processing options such as the ARM processor can cut costs when replacing failed batteries frequently. An example implementation for processing audio files could run continuously for more than 200 days when powered by a single LR44 coin-cell battery.