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Ankle exoskeleton algorithm adapts to speed and gait

ankle prosthetics on lower legs in jeans and black boots

A new control algorithm could make ankle exoskeletons automatically adapt to individual users and tasks, say researchers.

Current exoskeletons are limited because they must be tailored to a single user performing a single task, like walking in a straight line. Any changes require a lengthy set of manual readjustments.

The new control algorithm demonstrates the ability to handle different speeds, as well as changes in gait between running and walking. It could pave the way for exoskeletons that are better able to handle the uncertainties of the real world.

โ€œThis particular type of ankle exoskeleton can be used to augment people who have limited mobility,โ€ says Leia Stirling, associate professor of industrial and operations engineering and robotics at the University of Michigan and senior author of the study in PLOS ONE.

โ€œThat could be an older adult who wouldnโ€™t normally be able to walk to the park with their grandkids. But wearing the system, they now have extra assistance that enables them to do more than they could before.โ€

The control algorithm directly measures how quickly muscle fibers are expanding and contracting to determine the amount of chemical energy the muscle is using while doing its work. Then, it compares that measurement with a biological model to determine the best way to assist.

Measuring muscle physiology directly is a key departure from current methods, which use broader measures of motion. Going straight to the source of motion could result in more accurate measurements over a larger range of movements with far less computing power required.

Stirling and first author Paul Pridham, senior research area specialist in industrial and operations engineering, zeroed in on the ankle because it plays a key role in mobility. Assisting the muscles in the ankle could have a dramatic impact on our ability to walk further and faster.

Since the research was done during COVID-19 restrictions, testing with human participants wasnโ€™t possible. Instead, the team used data on existing ankle exoskeleton devices and muscle dynamics from previous studies to simulate, test, and adjust the algorithm to be more responsive to changes in speed and gait.

Human testing is an important next step, and will require the measurement of muscle fibers in real time using ultrasound. While much work and refinement remains, the researchers are confident that the new avenue of research will one day help people on the ground.

โ€œThis has the potential to help just about anyone,โ€ Pridham says. โ€œFrom someone who walks a lot for their job, to individuals in the military that perform tasks for long periods of time, to people with muscular disorders that need some extra assistance, and the elderly who need help day-to-day.โ€

The Under Secretary of Defense for Research and Engineering funded the work.

Source: Jessalyn Tamez for University of Michigan

The post Ankle exoskeleton algorithm adapts to speed and gait appeared first on Futurity.

Souped up Hall thrusters might get people to Mars

glowing blue O in darkness

Running more propellant through a Hall thruster might power a crewed mission to Mars, experiments suggest.

It was believed that Hall thrusters, an efficient kind of electric propulsion widely used in orbit, need to be large to produce a lot of thrust. Now, a new study suggests that smaller Hall thrusters can generate much more thrustโ€”potentially making them candidates for interplanetary missions.

โ€œPeople had previously thought that you could only push a certain amount of current through a thruster area, which in turn translates directly into how much force or thrust you can generate per unit area,โ€ says Benjamin Jorns, associate professor of aerospace engineering at the University of Michigan who led the work.

His team challenged this limit by running a 9 kilowatt Hall thruster up to 45 kilowatts, maintaining roughly 80% of its nominal efficiency. This increased the amount of force generated per unit area by almost a factor of 10.

Whether we call it a plasma thruster or an ion drive, electric propulsion is our best bet for interplanetary travelโ€”but science is at a crossroads. While Hall thrusters are a well-proven technology, an alternative concept, known as a magnetoplasmadynamic thruster, promises to pack much more power into smaller engines. However, they are yet unproven in many ways, including lifetime.

Hall thrusters were believed to be unable to compete because of the way they operate. The propellant, typically a noble gas like xenon, moves through a cylindrical channel where it is accelerated by a powerful electric field. It generates thrust in the forward direction as it departs out the back. But before the propellant can be accelerated, it needs to lose some electrons to give it a positive charge.

Electrons accelerated by a magnetic field to run in a ring around that channelโ€”described as a โ€œbuzz sawโ€ by Jornsโ€”knock electrons off the propellant atoms and turn them into positively charged ions. However, calculations suggested that if a Hall thruster tried to drive more propellant through the engine, the electrons whizzing in a ring would get knocked out of the formation, breaking down that โ€œbuzz sawโ€ function.

โ€œItโ€™s like trying to bite off more than you can chew,โ€ Jorns says. โ€œThe buzz saw canโ€™t work its way through that much material.โ€

In addition, the engine would get extremely hot. Jornsโ€™ team put these beliefs to the test.

โ€œWe named our thruster the H9 MUSCLE because essentially, we took the H9 thruster and made a muscle car out of it by turning it up to 11โ€”really up to a hundred, if weโ€™re going by accurate scaling,โ€ says Leanne Su, a doctoral student in aerospace engineering who presented the study at the AIAA SciTech Forum in National Harbor, Maryland on January 24.

They tackled the heat problem by cooling it with water, which let them see how big a problem the buzz saw breakdown was going to be. Turns out, it wasnโ€™t much trouble. Running with xenon, the conventional propellant, the H9 MUSCLE ran up to 37.5 kilowatts, with an overall efficiency of about 49%, not far off the 62% efficiency at its design power of 9 kilowatts.

Running with krypton, a lighter gas, they maxed out their power supply at 45 kilowatts. At an overall efficiency of 51%, they achieved their maximum thrust of about 1.8 Newtons, on par with the much larger 100-kilowatt-class X3 Hall thruster.

โ€œThis is kind of a crazy result because typically, krypton performs a lot worse than xenon on Hall thrusters. So itโ€™s very cool and an interesting path forward to see that we can actually improve kryptonโ€™s performance relative to xenon by increasing the thruster current density,โ€ Su says.

Nested Hall thrusters like the X3 have been explored for interplanetary cargo transport, but they are much larger and heavier, making it difficult for them to transport humans. Now, ordinary Hall thrusters are back on the table for crewed journeys.

Jorns says that the cooling problem would need a space-worthy solution if Hall thrusters are to run at these high powers. Still, he is optimistic that individual thrusters could run at 100 to 200 kilowatts, arranged into arrays that provide a megawattโ€™s worth of thrust. This could enable crewed missions to reach Mars even on the far side of the sun, traveling a distance of 250 million miles.

The team hopes to pursue the cooling problem as well as challenges in developing both Hall thrusters and magnetoplasmadynamic thrusters on Earth, where few facilities can test Mars-mission-level thrusters. The amount of propellant exhausting from the thruster comes too fast for the vacuum pumps to keep the conditions inside the testing chamber space-like.

The research had partial support from the Joint Advanced Propulsion Institute.

Source: University of Michigan

The post Souped up Hall thrusters might get people to Mars appeared first on Futurity.

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