bulking up

Super-strong mechanical muscles bring us closer to autonomous robots

Self-powered, buff robots are coming.

Futuristic robot man working out with barbell. Very strong cyborg lifting heavy weights or training ...

Unlike their flesh and blood counterparts, robots grow their muscles in labs instead of the gym. Recently scientists developed a new kind of robotic muscle likely capable of powering itself for an extended period of time past its initial charge.

In a paper published Wednesday in the journal Science Robotics, roboticists tested a new robotic muscle design that uses electrostatic bellows to lift objects up to 70 times its own weight. Unlike clunkier past designs, the researchers suggest this muscle would be cheap to build and scale. It could also be used in series to create even greater strength.

Why it matters — Because this muscle can exert as well as reserve and generate its own power, the researchers say it could be used in battery-powered autonomous robots — perhaps those completing search and rescue missions in dangerous terrain — as a means to sustain battery life and power.

This could be another step towards (at least partially) self-reliant autonomous robots.

Here's the background — The first robots were made of primarily hard and potentially dangerous materials like metal, but today's robots can be built much softer. It's a trait that is not only visually endearing but can make them physically safer to interact with. Think BayMax, but in real life.

But to power these soft robots to squeeze through tight spaces or perhaps even to better assist people, they need to have more human-like muscle.

Marco Fontana is the study's lead author and an associate professor of mechanical engineering at TeCIP Institute. He tells Inverse that soft robots are also better at navigating new terrain than stiffer, less pliable robots.

Fontana and his colleagues write that while different artificial muscle designs, such as those powered by fluid pressure or thermo-pneumatics (like hot air) have been proposed in the past to beef up these robots, no design has been right in terms of cost and scalability. For example, a well-powered millimeter-sized robot muscle is no help if it can't be sized to centimeters and beyond.

It may look small at first glance, but this muscle is designed to expand like an accordion.

Sîrbu et al., Sci. Robot. 6, eaaz5796 (2021)

In the study, the team combined the best of a few different worlds to design an electrostatic bellow muscle (EBM) that uses electric charge and pressure to create a lightweight muscle capable of acting as a generator, actuator, and pump all within a single design. They write:

"EBM contractile muscles can operate in generation mode, converting pulsating mechanical energy into electricity, without modifications to their layout and loading mode. Therefore, they could be effectively used to implement power recycling during the passive/breaking phases of actuation."

In other words, even when the muscle isn't moving, it's still working to reserve power for its next action.

"This is of course very important for untethered/battery-powered robots such as active robotic prosthesis or wearable robots, robots for search and rescue operations," Fontana tells Inverse. "It also could improve the use of energy in other applications such as automation industry."

What they did — As its namesake suggests, this robotic muscle is designed to expand and contract like a bellow — an old-fashioned air pump to stoke fires.

To design their bellow structure, the researchers combined layers of donut-shaped electroactive films only a little larger than a Euro with a channel through the center for a dielectric (or charge insulting) oil to be injected. When pressed together, the structure appeared relatively flat a circular, but applying a voltage causes electrodes on the films to become oppositely charged and pull toward each other in a "zipping" motion.

For more power, these muscles can be simply stacked one on top of another.

Sîrbu et al., Sci. Robot. 6, eaaz5796 (2021)

With this design, the EBM can either contract by working against an external force, like a weight, or act hydrodynamically by working against internal fluid pressure.

To test out the armless muscle's strength, the researchers gave it objects of varying weights to lift, ranging from just 1-kg plastic bottles to over 500-kg metal plates.

They also tested how well the EBM was able to achieve its other goals, including acting as a pump and a passive energy harvester.

What they discovered — On its own, the researchers found that the EBM could lift an impressive amount of mass: up to 70 times its own weight.

They also found that when acting as a generator it was capable of converting 20 percent of its own energy back into stored energy to use later. The authors write that these stats are on par with previous muscle designs, but with a more compact and cheaper design.

This robot muscle's PR is lifting up to 70 x its own body weight.

Sîrbu et al., Sci. Robot. 6, eaaz5796 (2021)

They can also all be amplified by simply stacking these EBMs on top of each other, like neatly folding a long winter scarf.

"The EBM could be promising for robotic systems due to its lightness, scalability, and adaptability to different tasks," write the scientists. "The added ability to harvest energy could also be an advantageous feature for autonomous battery-powered robots."

What's next — Before these muscles can bulk up and start filling out human-sized robots, the authors say that the technology will first need to be made even smaller. This, along with fine-tuning the EBMs design and materials, will bring self-powered robots another step closer to reality.

Abstract: Future robotic systems will be pervasive technologies operating autonomously in unknown spaces that are shared with humans. Such complex interactions make it compulsory for them to be lightweight, soft, and efficient in a way to guarantee safety, robustness, and long-term operation. Such a set of qualities can be achieved using soft multipurpose systems that combine, integrate, and commute between conventional electromechanical and fluidic drives, as well as harvest energy during inactive actuation phases for increased energy efficiency. Here, we present an electrostatic actuator made of thin films and liquid dielectrics combined with rigid polymeric stiffening elements to form a circular electrostatic bellow muscle (EBM) unit capable of out-of-plane contraction. These units are easy to manufacture and can be arranged in arrays and stacks, which can be used as a contractile artificial muscle, as a pump for fluid-driven soft robots, or as an energy harvester. As an artificial muscle, EBMs of 20 to 40 millimeters in diameter can exert forces of up to 6 newtons, lift loads over a hundred times their own weight, and reach contractions of over 40% with strain rates over 1200% per second, with a bandwidth over 10 hertz. As a pump driver, these EBMs produce flow rates of up to 0.63 liters per minute and maximum pressure head of 6 kilopascals, whereas as generator, they reach a conversion efficiency close to 20%. The compact shape, low cost, simple assembling procedure, high reliability, and large contractions make the EBM a promising technology for high-performance robotic systems.
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