Clutches can be used to enhance the functionality of springs or actuators in robotic devices. A research team headed up by Steve Collins, an associate professor of mechanical engineering at Carnegie Mellon University, created a lightweight, low-power clutch used to control spring engagement in an ankle exoskeleton.

The electrostatic clutch and spring built into an ankle exoskeleton. During 150 consecutive steps of walking, the clutch was used to engage the spring while the foot was on the ground, and disengage it during swing. (Credit: Carnegie Mellon University)

When Steve Collins first envisioned the electroadhesive clutch, he made a prototype with a sandwich bag and a couple of pieces of aluminum foil from his kitchen. Since creating that makeshift prototype, he and his research team have developed a sophisticated, functional device that can be used in exoskeletons that compensate for a person’s disability or enhance their athletic performance.

Last year, Collins’ team developed a lightweight, unpowered, wearable exoskeleton — the walking assist clutch — that reduced the energy expended in walking. Building on that research, they wanted to push the boundaries of the technology even further by creating a general- purpose clutch that offered increased functionality while being lightweight and consuming very little energy. Their new device is three to 30 times lighter than other clutch mechanisms with the same holding force; it consumes 340 to 750 times less energy compared to previous devices; and it operates at four to 20 times lower voltage than previous electrostatic components in robots (see figure).

“We knew that electrostatic adhesion was key to developing a responsive, adjustable device, but our early prototypes were plagued by high voltage requirements and undesired sticking when the clutch was turned off,” said Collins. “We had to look outside of our field to the area of materials for a solution.”

The researchers sought help from Carmel Majidi, also of the mechanical engineering department, who specializes in stretchable electronics, soft robotics, and wearable computing. Working with Stuart Diller, a Ph.D. student, they found the right combination in aluminum, Mylar®, and Luxprint® (an insulating material developed by DuPont™) that could easily integrate into a wearable system.

“Mylar® is thin, flexible, and has a good strength-to-weight ratio — but isn’t conductive,” Majidi explained. “Aluminum foil is conductive but dense, and tears easily. In order to have the flexibility, strength, conductivity, and low mass necessary for an electroadhesive clutch, we used Mylar® coated with an ultra-thin layer of aluminum.” He also noted, “the Luxprint® we used as the dielectric has the properties that are necessary for high adhesion at low voltage.”

How it Works

When the electroadhesive clutch operates, the layer of Luxprint® separates the two sheets of aluminum-coated Mylar®, allowing a strong electric field to develop when voltage is applied. The electrons on one side are attracted to the protons on the other, causing the sheets to stick together and preventing any sliding. The charge can develop quickly, and power consumption is low because the electrons don’t flow across the Luxprint®. When the voltage is removed, the electrons equalize and the coated Mylar sheets just slide against each other.

This leads to a lightweight, low-power clutch that can engage and disengage quickly, setting the stage for devices with many clutches acting together. One application is selectable stiffness exoskeletons.

“Selectable stiffness is important for making a lightweight exoskeleton practical for everyday use,” explained Diller. “The exoskeleton would be able to assist you in the best way for many different activities, such as running, hill climbing, or carrying different loads of weight. Think of how this could improve mobility for the elderly, assist workers carrying heavy loads while traveling on foot, or help athletes train for competition.”

Another application is for energy recycling actuators. These devices can lock in energy and then return it later — similar to the way regenerative braking works in a vehicle. The team is currently working on a new design for this application using the clutches.

The device could play a large role in the future designs of wearable robotics and autonomous robots. “Because prior clutches were so cumbersome, it was infeasible to incorporate more than one in a robotic design. Now, we can use hundreds of individually controlled clutches — each one thin, lightweight, and consuming very little electricity — in a single exoskeleton,” said Collins. “This will completely change how we design robotic systems in the future. We can combine these devices to work in concert in ways we haven’t yet seen in robotics.”

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