July 17, 2016

Augmenting Humans with an Electroadhesive Boost: An Ankle Exoskeleton 30 times lighter and 500 times more energy efficient

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.

This is from an Carnegie Mellon article written by Lisa Kulick.

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. The team presented their work at the Human 2.0 workshop at IEEE’s International Conference of Robotics and Automation in Stockholm, Sweden, to demonstrate how wearable robotics can assist and improve physical ability.

Collins, an associate professor of mechanical engineering at Carnegie Mellon University, has a record of making the seemingly impossible happen. Last year, his team developed a lightweight, unpowered, wearable exoskeleton—the walking assist clutch—that reduced the energy expended in walking, a feat that scientists have been attempting unsuccessfully for more than a century. The work was featured in the journal Nature in 2015.

Building on that research, Collins’ team 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.

They succeeded. Their new device has impressive features: it 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.



2016 IEEE International Conference on Robotics and Automation (ICRA)- A lightweight, low-power electroadhesive clutch and spring for exoskeleton actuation



Exoskeleton walking with a lightweight, low power electroadhesive clutch and spring



How did they do it? “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.”

Enter collaborator 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 who co-authored the study, 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.”



So how does the electroadhesive clutch work? 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 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 most exciting aspect of this device, though, may be its 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.”

Abstract

Clutches can be used to enhance the functionality of springs or actuators in robotic devices. Here we describe a lightweight, low-power clutch used to control spring engagement in an ankle exoskeleton. The clutch is based on electrostatic adhesion between thin electrode sheets coated with a dielectric material. Each electrode pair weighs 1.5 g, bears up to 100 N, and changes states in less than 30 ms. We placed clutches in series with elastomer springs to allow control of spring engagement, and placed several clutched springs in parallel to discretely adjust stiffness. By engaging different numbers of springs, the system produced six different levels of stiffness. Force at peak displacement ranged from 14 to 501 N, and the device returned 95% of stored mechanical energy. Each clutched spring element weighed 26 g. We attached one clutched spring to an ankle exoskeleton and used it to engage the spring only while the foot was on the ground during 150 consecutive walking steps. Peak torque was 7.3 N·m on an average step, and the device consumed 0.6 mW of electricity. Compared to other electrically-controllable clutches, this approach results in three times higher torque density and two orders of magnitude lower power consumption per unit torque. We anticipate this technology will be incorporated into exoskeletons that tune stiffness online and into new actuator designs that utilize many lightweight, low-power clutches acting in concert.


SOURCES- Youtube, Carnegie Mellon by Lisa Kulick, IEEE 2016 IEEE International Conference on Robotics and Automation (ICRA)

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