U.S. Army Researchers Develop Innovative Sensor Inspired by Elephants

Throughout time, seismic sensing – biological as well as technology-based – have aided warfighters in making critical decisions in combat. Inspired by elephants’ unique seismic sense, U.S. Army researchers are now working on a new and more efficient sensor for today’s multi-domain battlespace.

“No single communication technology can be the only path for the Army,” said Dr. Brendan Hanrahan, materials engineer for the U.S. Army Combat Capabilities Development Command’s Army Research Laboratory. “We believe that improved seismic communications could provide the Army with an option in the most challenging operating environments.”

According to Hanrahan, very low-frequency sensors are currently used in the field, but they’re not small or light enough to make them truly Soldier-portable systems.

Meanwhile, mammals, including elephants, have ultra-sensitive vibration sensors in their fingertips, the Pacinian corpuscle, which served as the early warning system for animals during the Southeast Asian Tsunami in 2004. There are many other examples of nature sensing seismic signals, such as blind moles communicating through tunnel systems, or kangaroo rats thumping predator-specific warnings.

“These cells can sense a vibratory displacement of less than 10 nanometers, which is sufficient to detect small earthquakes and underground explosions, and in the Army’s case, small-amplitude manmade seismic signals,” Hanrahan said. “The inspiration for this research came from elephants, which use seismic signals to communicate information up to 20 kilometers.”

There are critical capabilities of seismic sensing that can be of benefit to the U.S. Army, Hanrahan noted.

These capabilities include non-line-of-sight-sensing, e.g., vibrations can be propagated up walls and through floors in dense urban terrain; far standoff, e.g., small underground explosions can propagate several kilometers; sensing underground activities inside well-protected hard facilities; passive sensing that lends to low power and bandwidth requirements; and good time resolution because of the enhanced speed of sound through solids vs. air.

Hanrahan and his fellow researchers propose sensing and sense-making of low frequency vibrations through solids as a form of seismic communications. Their prototype sensor is shown to be low power and detects signals at significant standoff for non-line-of-sight and through-ground propagation.

He has been drumming up the concept for this sensor for a few years now, after his involvement in a Department of Defense-affiliated hackathon for humanitarian aid and disaster relief that was put on by the now National Security Innovation Network in Austin, Texas, in 2017.

Hanrahan then won a Director’s Research Award for the Transformative Research Challenge program in 2018, which is a program at CCDC ARL intended to on-ramp new, possibly transformational capabilities. His program was based on modelling and making a sensor based on the structure of the vibration sensing cell in the skin of mammals.

The program provides an opportunity to enable transformative research using complementary mechanisms for the liberal pursuit of innovative research ideas, a diversification of the research culture, enhanced collaborations, on-ramps and off-ramps for research areas and heightened technical excellence. It provides seedlings for high-risk, highly innovative efforts with transformative potential.

The lab then hired two co-op students and a team made up of laboratory researchers Dr. Kirk Alberts and Dr. Nathan Lazarus, and Hanrahan built the sensors.

Hanrahan is a recognized expert in the fabrication and testing of sensor materials. Lazarus has designed and patented numerous soft/fluidic electronic systems and will lead the fabrication of the lamellar structure. Alberts currently fields and evaluates Army seismic and infrasound sensors around the world and will lead the modelling and testing of the proposed sensor.

The lab augmented the original team with Dr. Joydeep Bhattacharyya, who brings both seismic wave propagation modelling expertise as well as Army-relevant field testing experience with acoustic and seismic devices. Additionally, a post-doctoral fellow with experience in sensor integration will be hired to support this effort.

This research directly supports the U.S. Army Modernization Priority Long-Range Precision Fires.

The long-range precisions fires priority includes a goal for precision munitions that can travel about 500 km. This means we expect to engage the enemy from great distances, Hanrahan said. When we have the opportunity to place sensors in enemy territory, we don’t want to place many and we don’t want to have to go back to change the batteries. This means each sensor needs to cover a large area, provide information about what’s nearby, and run on low power. One way to accomplish this is with seismic sensing.

“We know that a low size, weight and power seismic sensor, paired with a compact, broadband generator, could provide high-specificity targeting to long-range fires teams, hard-to-jam alternatives for positioning and non-line-of-sight, low-fidelity communication, as well as better situational awareness of subterranean operations,” Hanrahan said. “Each of these potential benefits is salient in the multi-domain operations concept, where long range fires can “shoot further than we can see,” and where position, navigation and timing are under constant intrusion from our adversaries.”

One of the modern-day lab’s predecessors, the U.S. Army Electronics Research and Development Laboratory, conducted similar research in the 1960s and 1970s, and scientists are revisiting this research with the goal to make an impact on Soldier technology.

“Ultimately, this technique was not pursued because of poor suitability to the target environments (the jungles of Vietnam) and the limited state of technology both in terms of seismic generators and receivers as well as more limited adversarial jamming of radio frequency communications,” Hanrahan said. “The previous studies showed us that seismic energy propagates out/in from/to a mine and thus supports our proposed concept of operations; however, when we have a cluttered battlefield environment, where multiple groups of adversaries are engaged at close quarters and covertness is a requirement, novel techniques that exploit coded messages and seismic signal processing is required.”

While progress has been made, the next steps of this research will define the sensor’s success moving forward.

To improve the functionality of the sensor, the team’s research will begin by transitioning their single, dip-coated sensors to 3-D printed arrays.

“Fluid encapsulation within 3-D printing has been a recent focus for ARL, and we believe we can take it to never-before attempted size scales using our Nanoscribe 3-D printer, which would improve the sensor performance by increasing the capacitance and more consistently tailoring the mechanical properties,” Hanrahan said.

The team will be testing these sensors on multi-axis shaker tables with various surface treatments to better mimic field properties within the lab, especially as compared to their initial shaker table tests. The electronics to support arrayed sensors while maintaining low power requirements will also be investigated as part of this phase.

The second year will focus on the field testing of the bioinspired, 3-D printed sensor.

“We expect that our field tests will help us identify both the capabilities and limitations of the artificial Pacinian corpuscle for manmade signals,” Hanrahan said. “We will research ways to package the sensor and analyze the signals to improve sensitivity and specificity. If successful, this work will on-ramp a more significant effort in seismic sensor development.”

At the conclusion of this effort, Hanrahan noted, the team will have a good understanding of the potential for seismic communication given current and future technology and operating environments. They then plan on using this knowledge to construct a transmitter/receiver system and perform the first encoded proof-of-concept message transfer.

Hanrahan recently presented this research virtually with the Mid-Atlantic Micro/Nano Alliance, a local non-profit for researchers who make small devices. The key members include, but are not limited to, the laboratory, the Naval Research Laboratory, Johns Hopkins University Applied Physics Lab and the National Institute of Standards and Technology.


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