Where We’re Going, We Don’t Need Batteries

Sensor networks are a vital part of the modern technological landscape, playing a critical role in a variety of applications across industries such as environmental monitoring, healthcare, agriculture, and industrial automation. These networks are made up of a large number of interconnected sensors that are strategically placed to collect and transmit data from their surroundings. The primary purpose of sensor networks is to gather real-time information, which allows for informed decision-making and improves the efficiency and safety of numerous processes.

One of the primary drawbacks of using fixed sensor nodes in sensor networks is their limited coverage area. Each sensor can only monitor a specific, often small, geographical region, making it necessary to deploy a large number of sensors to effectively cover larger areas. This installation and maintenance can be both expensive and logistically challenging. Additionally, the environmental consequences of producing, powering, and eventually disposing of these sensors can be significant, contributing to electronic waste and resource depletion.

To address some of these challenges, another approach is to attach sensors to mobile platforms such as robots. These mobile sensor platforms have several advantages, like the ability to cover larger areas with fewer sensors. However, this approach also has its own set of limitations. Robots can be expensive to develop, purchase, and maintain. They require a constant supply of energy, which often means limited operating times and distances they can cover before needing to be recharged. Additionally, the energy sources used by these robots, along with their batteries, can have environmental consequences, including the disposal of these components.

Researchers at the University of Washington have defied the conventional assumption that motion and actuation are beyond the capabilities of battery-free robots in developing an autonomous microrobot called MilliMobile. This tiny, ten millimeter square robot weighs in at just one gram, and is powered by an energy harvester that feeds on ambient light or radio waves. MilliMobile was designed for intermittent operations, such that it can perform its tasks as power becomes available, and it can carry three times its own weight to cart around sensors.

The wireless robot rolls on four wheels and maintains battery-free operation by charging capacitors with solar and RF energy harvesters. A Nordic Semiconductor nRF52-series system-on-chip provides both low-power processing resources and wireless connectivity via Bluetooth. To enable intermittent movement, motors are driven in discrete steps by energy stored in capacitors, as it becomes available. MilliMobile is equipped with onboard photodiodes for sensing sources of light, as well as temperature and humidity sensors. It is also possible to add additional sensors to support different applications.

The robot was demonstrated as being able to autonomously chase a light source using its onboard photodiode. It was also shown to be capable of wirelessly transmitting sensor readings to a base station over 650 feet away. It can move at about a quarter of an inch per second under ideal conditions, but can continue to operate even under dim lighting conditions, albeit at a lower speed.

MilliMobile was shown to be versatile, having been tested in a number of scenarios, both indoors and outdoors. Experiments proved that the robot can travel over at least ten different surface types ranging from wood and concrete to compact soil. These factors, in conjunction with the ability of the robot to carry many types of sensors, have the team dreaming of potential future applications for their technology. They envision MilliMobile locating the source of gas leaks with gas sensors, seeking out metal objects with a magnetometer, or tracking down RF sources with an antenna and receiver attached to the mobile platform.

In any case, the battery-free actuation capabilities demonstrated by the researchers are likely to open up a variety of new avenues for future research.