On a good day, the rock quarry in central Texas is about 370,000 kilometers (230,000 miles) from the moon. But last February, when Rishi Jangale watched his 1.8-meter-wide, 150-kilogram inflatable robot roll effortlessly over rocks, gravel, and wet clay, his imagination turned the quarry into the lunar surface instead.

Jangale is an upbeat mechanical engineer nearing the end of his Ph.D. at Texas A&M University, in College Station, Texas. He and his labmates have been working on this big tan “RoboBall” for about five years. Their goal: create a vehicle capable of exploring some of the most inaccessible terrain in our solar system, such as the 21 km-wide Shackleton crater on the moon’s south pole.

The Shackleton crater is 4 km deep and contains many smaller, deeper craters within. Some parts of the crater never see the sun, and within these perpetually dark, frigid pockets lie mysterious substances that planetary scientists have long struggled to examine, including layers of ancient lunar geology and stores of frozen water that could potentially support a future lunar base.

“The moon is like an archive of what happened to the Earth,” says Sara Russell, a cosmic mineralogist with London’s Natural History Museum who is not involved with Texas A&M’s work. “Robotic collection works brilliantly well, and it’s great to see that being explored more in this context.”

“NASA is not going to let astronauts get anywhere near these craters, because if someone falls in, you’re not going to be able to get them out,” Jangale says. “So we thought, what better shape to roll down a hill than a ball?” In a recent paper published in IEEE Transactions on Field Robotics, Jangale’s team reports on the design of RoboBall, a hypothetical lunar mission, and the results from initial tests in the Texas quarry.

Getting Rolling

RoboBall is the brainchild of Jangale’s advisor, former NASA robotics engineer Robert Ambrose, who first conceived of the design in 2003.

Ambrose figured that a ball could address a pesky mobility risk that robots face on lunar terrain, especially in low gravity: tipping over. An inflatable sphere can’t tip over, and the form factor also insulates its internal components from sharp rocks, dust, and the huge temperature swings from over 93 °C in sunlit spots to minus 240 °C in the shade inside lunar craters. Ambrose imagined a wheeled rover parking at a crater’s edge and releasing a RoboBall to explore its depths. Unlike a small tethered rover, the RoboBall wouldn’t roll its way back up—but with no strings attached, it would have a far greater range to collect geological samples, and then be able to launch them back to the rover outside the crater with small rockets.

A robotic rover would ferry RoboBall across the lunar surface to the edge of a crater.R. Jangale, D. Pravecek, et al.

NASA hasn’t brought lunar samples back to Earth in over 50 years. Some morsels of moon geology find their way to Earth as meteorites, but Russell says these lack the “gold standard” field work—context about where that sample actually came from. Even as NASA reboots its crewed moon missions, many lunar sites remain inaccessible.

In 2022, Ambrose’s lab finished a proof of concept, the 0.5-meter-wide RoboBall II. Creating the full-size RoboBall III then took about 11 months. “We were building these robots really quickly,” Jangale says. “Ambrose really encourages us to use and break these robots.” And designs did go awry. Jangale remembers software errors and a drivetrain that proved too weak to roll over soft bumps in initial tests.

Texas A&M RAD Lab/YouTube

How to Slow Your Roll

RoboBall drives by moving a pendulum within its shell, shifting its entire center of mass. On flat ground, if the pendulum’s arm points forward, the shell rolls forward to compensate, and as long as the pendulum keeps the center of mass in front of the center of the ball, RoboBall will keep rolling forward. If the pendulum leans a few degrees to the left or right, RoboBall steers left or right to match. “The robot wants to go where you point the pendulum,” Jangale says. The 340-pound ball is just soft enough to bounce lightly over bumpy obstacles, but its slight overpressure keeps it relatively firm. On steep slopes, this mechanism also lets the ball control its downhill speed by simply angling the pendulum uphill.

“The beauty here is the simplicity,” says Hiro Ono, an aerospace engineer who worked on robot mobility at NASA’s Jet Propulsion Lab for 13 years before joining Georgia Tech. For space robots, Ono describes simplicity in terms of the number of actuators. RoboBall has just two, and both are fully inside of the shell, shielded from environmental risk factors like dust, a feature that Ono describes as “unique.”

After the first quarry tests, it took about seven months for the team to design and build an upgraded RoboBall III with 2.5 times more torque—enough to fling itself over small obstacles and roll up 20 degree slopes.

A small rocket can launch out of the center of the RoboBall to return a sample back to a rover outside the crater.R. Jangale, D. Pravecek, et al.

Getting Mission Ready

In the new paper, the remotely operated RoboBall III descended the quarry’s slopes, navigated soft terrain, and launched hypothetical payloads back out of the crater with small rockets. Powered by a large battery, the robot would inflate itself during a lunar mission and charge up from a robotic rover at the edge of a crater before heading out on its own.

RoboBall is probably not the right platform for all kinds of missions—its slightly bumbling nature means that it’s not ideal if you want to collect a sample from a very specific rock, for example. For now, the team hopes to work with scientists designing lunar science instruments to physically fit into RoboBall’s carry-on-luggage-size interior, while also fitting in with how RoboBall operates. “The robot is not the mission,” Jangale says. “The robot is a way for you to complete the mission.”

The current version of RoboBall cost roughly $250,000 to build, but is not quite ready for the moon in its current form. While its gold-treated aluminum parts are appropriate for space missions, its other materials are not. Space-grade electronics will cost more, and the ball’s shell—made from a material tough enough to roll over steel shards and withstand minus 184 °C temperatures—has never been evaluated in lunar extremes. Aside from materials questions, the team plans to engineer the ball to adapt how it drives on varying slopes autonomously. They also hope to collaborate with government agencies or spaceflight contractors to keep refining the robot’s design for a real, eventual mission.

Later this year, Texas A&M will open a new facility in Houston with the world’s largest indoor simulated moon and Mars landscapes. The facility is only 190 km from Jangale’s quarry. It’s still about 370,000 km from the moon, but it’s going to help Jangale get his robot quite a bit closer.

Imagine running your fingertip over the surface of a U.S. penny. You would feel the ridges of the raised letters and numbers, Abe Lincoln’s bearded side profile, and, if it’s tails, the fluted columns of the Lincoln Memorial. Getting a robot to sense the same things is an imposing task, often requiring gathering data on pressure and force at many spatial locations at once. But that’s just what a team of scientists in Europe has now managed to do, using an unusual, colorful robotic skin that provides high-resolution sensing in real time.

“To be honest, when they showed us this, we thought it was, and pardon my French, [expletive] cool, because it’s a distinctly different approach,” recalled Rich Walker, director of Shadow Robot, the U.K.’s longest-running robot company, which primarily focuses on robotic hands.

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The research team, which hails from Queen Mary University of London, the University of Florence, the University of Trieste, and the University of Trento, designed a robotic fingertip with a synthetic skin that reflects different colors of light in response to mechanical deformation. By reading the light reflected off the skin, the fingertip generates maps of topology, strain, and contact pressure. The team has already used the sensor to generate maps of a human fingertip, a penny, and a leaf.

Giacomo Sasso, a postdoctoral research associate in the lab of Federico Carpi at Queen Mary University of London, came up with the idea for the sensor. He had been researching optics when he stumbled upon an interesting paper published in the journal Nature. It described the “mechanochromic material” that would eventually make up the reflector in the skin.

Following the method described in Nature, Sasso exposed a light-sensitive film to a 5-megawatt, 635-nanometer (red) laser for seven minutes. The laser beam creates an interference pattern which causes the film to polymerize in alternating densities, creating layers with different refractive indices.

This structure is called a Bragg reflector. The alternating densities and refractive indices in the polymer cause specific wavelengths of light to be reflected. When the reflector is deformed by contact with an object, its layers are stretched, becoming thinner and reflecting light of a different wavelength.

It took Sasso less than a week to re-create the material in the lab. “From there, we started seeing how we could translate these color patterns into something that was useful for us,” he says. Soon, they realized that the color produced by the material was all they needed to be able to sense the topology of objects.

In the robotic finger, the Bragg reflector is sandwiched between a layer of silicone, which protects it from the outside, and a transparent, fingertip-shaped silicone finger with a camera and LED light embedded inside of it.

The light from the LED shines through the clear polymer of the finger. When the fingertip is deformed by an object, the reflector bounces light back to the camera, with wavelengths depending on the level of deformation—red for least deformation, shifting to green, and then to blue when most deformed.

The team also made adjustments to increase the sensitivity of the skin and help the camera to better read color differences. The silicone of the outer layer of the fingertip is colored black to increase the color contrast, allowing the camera to better translate color into the morphology. The rigidity of the camera inside the finger also enhances the deformation of the reflector, producing greater differences in reflected wavelengths.

After all that optimization, the finger provided 100-micrometer resolution with no computational latency, the researchers determined.

What robot fingers need

Human skin takes in a variety of tactile information in order to successfully move and manipulate objects, including temperature, texture, pressure, and vibration. But engineering a robot to do the same is challenging because of spatial constraints. There often isn’t enough room in a robotic fingertip to incorporate more than one type of sensor. The question then becomes: Which type of sensor should be used?

“And the answer to that is…that’s a really hard question. No one knows yet,” says Walker. Carpi’s team’s robotic finger is exciting because it presents yet another option for roboticists to experiment with, Walker says.

Although Carpi’s team isn’t the first to use soft materials for tactile sensing, its technology is unique because it is able to extract quantitative information about depth and size from the topologic maps it generates. According to Walker, most sensors can only generate topological maps, which reveal the relative sizes of object features.

To Sasso, another key advantage of this robotic finger is that it embeds tactile sensing directly into the material of the finger, rather than using taxels, or pixels that measure force or pressure at specific spatial points.

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“The core aspect of the sensor is that we’re essentially [moving toward] having the sensing element at the material level,” he says. “The camera, which is a very highly optimized electronic component, is translating whatever the material is already doing directly into digital signals.”

Michael Wang, co-founder and chief scientist at Daimon Robotics, which, unlike Shadow Robot, primarily uses vision-based sensing, echoed Walker’s sentiment that it’s beneficial to explore new methods of sensing, which may bring unique advantages. But he also explained that soft materials often face challenges with durability, and that the significance of the team’s work would be revealed when the finger is integrated into real robot hands.

“The practical and useful benefits, especially in the context of robot hands, remain to be tested and validated,” he says.

When the materials of soft sensors, like the silicone in Carpi’s team’s fingertip, become eroded or damaged after repeated use, the signals measured by the sensors may not reflect objects’ topography as well.

“Especially if you have the electronics embedded into the material layer itself, that becomes a very challenging engineering problem. And I haven’t seen [many] good soft electronics materials that really can undergo long periods of usage,” Wang says.

However, because the Bragg reflector isn’t in direct contact with objects itself, the outer layer of silicone material acts as a protective barrier, Sasso says. The silicone can also be made more durable using certain chemical coatings, according to Wang.

The team has already been talking to companies that could potentially employ the new sensor. They also hope to improve the sensor so that it can sense objects that don’t lie flat on surfaces. That could open up its use in surgical instruments that require precise contact mapping of tissues and organs, Carpi says.

“There are significant developments that we expect with a clear path toward transition to real world applications,” he says.