MilliMobile, The World’s Smallest EV, Has No Battery

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“The gates of history turn on tiny hinges,” my high school history teacher, Mrs. Monahon (NOT Monahan!) liked to say. If that’s true, the EV revolution may one day owe a debt of gratitude to MilliMobile — the smallest EV of all.

MilliMobile is just 10 mm (0.4 in) square. It was created by researchers at the University of Washington, who will be presenting it to the world at the 29th Annual International Conference On Mobile Computing And Networking in Madrid in early October. In a paper published by the university, they write:

“We present MilliMobile: a first of its kind battery-free autonomous robot capable of operating on harvested solar and RF power. We challenge the conventional assumption that motion and actuation are beyond the capabilities of battery-free devices and demonstrate completely untethered autonomous operation in realistic indoor and outdoor lighting as well as RF power delivery scenarios.

“We show first that through miniaturizing a robot to gram scale, we can significantly reduce the energy required to move it. Second, we develop methods to produce intermittent motion by discharging a small capacitor (47-150 µF) to move a motor in discrete steps, enabling motion from as little as 50 µW of power or less. We further develop software defined techniques for maximizing power harvesting. MilliMobile operates in the optimal part of the charging curve by varying the charging time to achieve maximum speeds of up to 5.5 mm/s.

“The MilliMobile prototype has a 10×10 mm chassis and weighs less than 1.1 g. Our robot can carry payloads 3 times its own weight, and only experiences a 25% reduction in speed when carrying a 1 g payload. We demonstrate operation on 10 different surfaces ranging from wood and concrete to compact soil. We further show the ability to cold-start and move in light conditions as low as 20 W/m2 and -10 dBm of RF power.

“In addition to operating on harvested power, our robot demonstrates sensor and control autonomy by seeking light using onboard photodiodes, and can transmit sensor data wirelessly to a base station over 200 m away.”

MilliMobile Is Slow But Steady

According to New Atlas, there are numerous situations in which a robot doesn’t need to move quickly, but it does need to cover long distances (“long” be a relative term) without having to recharge its batteries. The minuscule energy-harvesting MilliMobile robot was designed for just such scenarios.

It has no batteries but does incorporate two motors, a carbon fiber chassis, a foldable printed circuit board, a light sensor, solar cells and an antenna. It can carry up to three times its own weight (sort of like an ant) in the form of cargo such as cameras and environmental sensors.

As the MilliMobile moves along flat surfaces, it scavenges energy from ambient light and radio waves. Even on a cloudy day, it can reportedly cover a distance of approximately 30 feet (9 m) in one hour. It does so by moving in increments instead of trying to store up enough power to make the whole trip in one continuous go.

“We took inspiration from ‘intermittent computing,’ which breaks complex programs into small steps, so a device with very limited power can work incrementally, as energy is available,” said doctoral student Kyle Johnson, co-lead author of the study. “With MilliMobile, we applied this concept to motion.”

Utilizing its light sensor, the robot can autonomously steer itself towards a specified light source. It can also transmit sensor data via Bluetooth. In tests, it has successfully relayed data from onboard light, temperature, and humidity sensors.

MilliMobiles working in collaborative swarms could wirelessly share data with one another. (If you are getting a Minority Report vibe here, you’re not alone.) Potential applications for the technology include monitoring soil moisture at farms, performing inspections of machinery in factories, or seeking out the source of gas leaks.

What Can MilliMobile Do?

The paper makes for fascinating reading for those who enjoy a deep dive into technical details. Here is how the authors summarize their work:

“The ability to carry a wide variety of sensor payloads enables numerous application scenarios. In comparison to a static sensor node, MilliMobile can actively seek out and localize sensory signals. For example, MilliMobile could seek out gas or chemical leaks using gas sensors, metal objects with a magnetometer, RF sources using an antenna and receiver or envelope detector, temperature for detecting fires or sources of heat, and much more. The ability to move also enables sampling of spatial gradients which is important for many of these applications.

“When combined with sensors like cameras, our robots could be used to automate a wide variety of inspection tasks, especially in scenarios dangerous to humans. Industrial equipment and infrastructure such as high powered radio transmitters and other devices that generate strong electromagnetic fields are harmful to human health, but provides the perfect scenario for power harvesting robots to automate dangerous tasks.

“Our robots are not constrained by battery life and could in theory operate for infinite lifetimes outdoors for inspection tasks on roads, railroad tracks,etc. Similarly our robots could be used for space or interplanetary exploration due to their small size and ability to operate on harvested energy. The ability to move in such a small form factor could also be combined with recent developments in airborne sensor release to enable precise large scale deployment of sensor networks.

“The ability to perform battery-free actuation opens up a variety of new research directions for low power wireless systems. For example, the ability to move a whole sensor node or parts of it permits incorporating remote feedback from an edge device to create a dynamically controllable sensor network.

“For example, a base station could coordinate the network to steer directional sensors such as cameras to focus on a sensing target of interest, or nodes could use this ability to move themselves or their antenna to optimize connectivity. These same principles could also be extended to other domains as well such as HCI to enable battery-free haptic interfaces. Our robot uses vibration motors which are designed for haptic interfaces.

“Additionally these techniques could be extended to use other actuators such as solenoids or piezos and combined with alternative energy harvesting modalities such as harvesting power from button presses or user motion for wearables.

“Improving the power harvesting of our robot could further improve performance. A custom circuit with maximum power point tracking (MPPT) and boost converter could be used to increase efficiency in high payload scenarios, and combining RF and solar harvesting, could further increase power delivery. Multi-band power harvesting, beamforming to specific nodes, and improved antennas could further improve RF harvesting.

“While our robot is capable of moving towards a target like a light source, it currently lacks navigation and feedback control. Integrating an accelerometer or employing some of the many wireless localization techniques developed by the mobile systems community could enable the robots to travel to precise locations. Additionally, there are significant opportunities to improve networking capabilities that establish connectivity between nodes to enable large scale swarms of robotic sensor nodes.”

The Takeaway

It’s easy to dismiss the MilliMobile as being slightly silly, but doing so would be a mistake. Think of this as similar to the first solar panels. The possibilities for improvements are enormous and the use cases nearly infinite. Remember the name MilliMobile. Odds are you will be hearing about it a lot in the years to come.