New Robots with Sticky Feet Can Inspect Inside Complex Machines Without Dismantling Them

Jet engines comprise of up to 25,000 individual components, making standard maintenance a laborious task that can take more than a month per engine. Many parts are positioned deep inside the engine and cannot be checked without taking the machine apart, adding costs and time to maintenance. This problem is not only associated with jet engines, either; numerous complicated, expensive machines like construction equipment, generators, and scientific instruments require large investments of time and money to check and maintain.

Scientists at Harvard University’s Wyss Institute for Biologically Inspired Engineering and John A. Paulson School of Engineering and Applied Sciences (SEAS) have built a micro-robot whose origami ankle joints, electroadhesive foot pads, and specifically engineered walking gait permit it to climb on vertical and upside-down conductive surfaces, like the inside walls of a commercial jet engine. The work is detailed in Science Robotics.

“Now that these robots can explore in three dimensions instead of just moving back and forth on a flat surface, there’s a whole new world that they can move around in and engage with,” said first author Sébastien de Rivaz, a former Research Fellow at the Wyss Institute and SEAS who currently works at Apple. “They could one day enable non-invasive inspection of hard-to-reach areas of large machines, saving companies time and money and making those machines safer.”

The new robot, named HAMR-E (Harvard Ambulatory Micro-Robot with Electroadhesion), was created in response to a challenge put forward to the Harvard Microrobotics Lab by Rolls-Royce, which asked if it would be feasible to design and construct an army of micro-robots that can  climb inside parts of its jet engines which cannot be accessed by human workers. Current climbing robots can handle vertical surfaces, but experience problems when trying to climb upside-down, as they require a large amount of adhesive force to stop them from falling.

The team based HAMR-E on one of its present micro-robots, HAMR, whose four legs allow it to walk on flat surfaces and swim through water. While the rudimentary design of HAMR-E is akin to HAMR, the researchers had to solve a series of challenges to enable HAMR-E to effectively stick to and traverse the inverted, vertical, and curved surfaces that it would come across in a jet engine.

First, they had to make adhesive foot pads that would keep the robot attached to the surface even when upside-down, but also release to enable the robot to “walk” by lifting and positioning its feet. The pads comprise of a polyimide-insulated copper electrode, which enables the formation of electrostatic forces between the pads and the underlying conductive surface. The foot pads can be easily released and re-engaged by turning the electric field on and off, which works at a voltage similar to that necessary to move the robot’s legs, thus requiring very little extra power. The electroadhesive foot pads can produce shear forces of 5.56 g and normal forces of 6.20 g—more than sufficient to keep the 1.48-g robot from falling off or sliding down its climbing surface. Besides providing high adhesive forces, the pads were engineered to be able to flex, thus permitting the robot to climb on uneven or curved surfaces.

The researchers also built new ankle joints for HAMR-E that can rotate in three dimensions to compensate for rotations of its legs as it walks, permitting it to keep its orientation on its climbing surface. The joints were produced out of polyimide and layered fiberglass, and folded into an origami-like structure that enables the ankles of all the legs to rotate easily, and to passively align with the landscape as HAMR-E climbs.

Finally, the scientists developed a special walking pattern for HAMR-E, as it needs to have three-foot pads touching an inverted or vertical surface at all times to stop it from sliding or falling off. One foot releases from the surface, swings forward, and reattaches while the other three feet stay attached to the surface. Simultaneously, a small amount of torque is applied by the foot diagonally across from the lifted foot to keep the robot from moving away from the climbing surface during the leg-swinging period. This process is done again for the three other legs to form a full walking cycle, and is coordinated with the pattern of electric field switching on each foot.

When HAMR-E was tested on inverted and vertical surfaces, it was able to accomplish more than one hundred steps in a row without detaching. It walked at speeds akin to other small climbing robots on inverted surfaces and somewhat slower than other climbing robots on vertical surfaces, but was considerably faster than other robots on horizontal surfaces, making it an ideal candidate for examining environments that have a range of surfaces in varied arrangements in space. It is also able to do 180-degree turns on horizontal surfaces.

HAMR-E also effectively maneuvered around a curved, inverted section of a jet engine while remaining attached, and its adhesive foot pads and passive ankle joints were able to house the rough and uneven features of the engine surface just by increasing the electroadhesion voltage.

The team is continuing to improve HAMR-E, and plans to add sensors into its legs that can detect and compensate for detached foot pads, which will help stop it from falling off of inverted or vertical surfaces. HAMR-E’s payload capacity is also more than its own weight, opening the possibility of holding a power supply and other electronics and sensors to examine various environments. The team is also looking for options for using HAMR-E on non-conductive surfaces.

“This iteration of HAMR-E is the first and most convincing step towards showing that this approach to a centimeter-scale climbing robot is possible, and that such robots could in the future be used to explore any sort of infrastructure, including buildings, pipes, engines, generators, and more,” said corresponding author Robert Wood, Ph.D., who is a Founding Core Faculty member of the Wyss Institute as well as the Charles River Professor of Engineering and Applied Sciences at SEAS.

“While academic scientists are very good at coming up with fundamental questions to explore in the lab, sometimes collaborations with industrial scientists who understand real-world problems are required to develop innovative technologies that can be translated into useful products. We are excited to help catalyze these collaborations here at the Wyss Institute, and to see the breakthrough advances that emerge,” said Wyss Founding Director Donald Ingber, M.D., Ph.D., who is also the Judah Folkman Professor of Vascular Biology at Harvard Medical School and the Vascular Biology Program at Boston Children’s Hospital, and Professor of Bioengineering at SEAS.

This study is co-authored by Benjamin Goldberg, Ph.D., Neel Doshi, Kaushik Jayaram, Ph.D., and Jack Zhou from the Wyss Institute and Harvard SEAS.

This research received support from the Wyss Institute for Biologically Inspired Engineering at Harvard University, Rolls-Royce, and the US Army Research Office.

HAMR-E, created in collaboration with Rolls-Royce, is a micro-robot that uses electroadhesion to scale vertical, inverted, and curved surfaces, allowing it to explore spaces that are too small for humans. HAMR-E could one day be used to inspect jet engines and other complicated machines without requiring them to be taken apart. (Credit: Wyss Institute at Harvard University.)