A museum might seem like an unlikely place to see cutting-edge motion control and mechatronic systems in action, but the engineers who design these systems may already suspect that their best creations toss a bit of art in with all of the science behind making things move. So it’s not such a stretch to think a museum exhibit could show motion systems in a new light, one that at once would appear familiar and fresh from an engineering perspective.
The Museum of Modern Art in New York recently staged just such an exhibit. Called Design and the Elastic Mind, it explores what its curator Paola Antonelli describes as “the reciprocal relationship between science and design.” “It’s partly about what happens when scientists and engineers get the design bug,” she says, though the opposite is true, too.
She filled the exhibit with more than 200 objects and displays that highlight what she considers “disruptive or potentially disruptive technologies,” meaning those that can trigger big changes in human behavior or society. The exhibit, which closed last month but can still be seen online, divvied up the diverse group of technologies according to their scale, function and inspiration. Nanotech, for example, had its turn in the spotlight. So did
3-D printing. So did environmental design. Some of the most interesting objects in terms of their mechanical design featured bio-inspired, organic designs. “Nature builds things in a sensible, economical, elegant way,” Antonelli says.
Sophisticated motion control, particularly a mechatronic take on it, cut across several of these the exhibit categories. Here’s a look at three of the standouts.
Museumgoers got a chance to see two of the mechatronic, biologically inspired creatures developed by Festo, a developer supplier of high-end automation and actuation systems. These creatures, a swimming Aqua Ray and flying Air Ray, first appeared at the 2007 Hannover Fair in Germany, so they may not be new to Design News’ readers. Antonelli says, though, that she picked these objects in part to highlight how nature inspired not just the outward form of these creatures but also their propulsion systems.
The Air Ray makes use of Festo’s fluidic muscles, which can serve as an alternative to pneumatic cylinders and other actuators. Consisting of an elastomer tube reinforced by aramid fibers, these actuators contract quickly and exert a pulling force when they’re filled with a blast of compressed air or liquid. Their dynamics approximate biological muscles, Antonelli notes. Like natural muscles, these bionic muscles experience a decay in force as they shorten.
But the initial force and speeds with which that force is reached can be significant. Elias Knubben of Festo’s Corporate Design Center says the company has developed muscles that can exert 6,000N of force when filled to a pressure of 6 bar. They can respond quickly, too — at frequencies up to 100 Hz. At more than 25 percent smaller than comparable pneumatic actuators, they also take up relatively little space.
In the case of the Aqua Ray, the fluidic muscles operate in antagonistic pairs to mimic the propulsive flapping of a real manta ray’s wings. This Aqua Ray system makes use of a compact vane pump to deliver pressure to the fluidic muscles, while Spectra cords serve as “tendons” that transfer forces from the fluidic muscles to the wings. A tendon travel of 55 mm translates into a vertical wing amplitude of more than 550 mm.
The Aqua Ray’s flying cousin, the helium-filled Air Ray, also borrows its propulsion style from a manta’s flapping wings. In this case, though, the movements of the wing are servo driven, by a small Torcman motor.
Another exhibit that made extensive use of motion control was an architectural display created by Chuck Hoberman and his staff at Hoberman Assoc. Made from stainless steel and aluminum, this Emergent Surface structure forms a facade whose shape transforms in response to the environmental conditions or to create a variable “filter” between the interior of a building and the outside world.
This floor-to-ceiling structure consists of seven “stalks,” poles that have been spaced about 3 ft apart. Each of these poles carry three or four slat-like units that can expand or retract through the action of four-bar parallel linkages. “The units move as a ruled surface,” says Ziggy Drozdowski, an electrical engineer and Hoberman’s director of technology. “They translate out over one another following the arc of the linkages’ control arms.”
This unit, intended as a demo but scaleable to architectural proportions, has 24 axes of motion, all direct driven by Quicksilver Controls’ unique servo-stepper system. With the help of a custom controller and encoder, these systems allow ordinary open-loop stepper motors to run as closed-loop servo motors.
Drozdowski picked them because they can offer good holding torque in the stepper mode and good inertial characteristics. Both attributes were important in this job. He says the slats have to be held in position, something his control software does using the stepper mode. “The slats are force balanced, so we don’t use all of the motor’s holding torque capabilities,” he says. At the same time, the 40-50 lb slats impose inertial loads best handled via servo. “We weren’t too concerned with positioning accuracy but we had to be able to stop and start quickly,” he says.
In the exhibit model created for the MoMA, Drozdowski ran Emergent Surface on a loop. Out in the real world, though, the same technology would typically be more mechatronic, incorporating sensors and smarter software that allow the structure to respond to its environment.
Another mechatronic design selected by Antonelli is the powered ankle-foot prosthesis developed by MIT’s Biomechatronics research group. Unlike traditional prosthetics, which offer spring response during motion and are passive during the terminal stance portion of the gait, this new active model uses an arrangement of springs and a small brushless dc motor to generate energy beyond what a spring alone can release.
Energy produced from the forward motion of the person wearing the prosthesis is stored in a tendon-like spring and then released as the foot pushes off. Additional mechanical energy from the motor is also added to help momentum — providing human-like power even at terminal stance.
The powered prosthetic’s creator, MIT Professor Hugh Herr, calls this energy generation a “muscle-like robotic assist” that mimics the action of a biological ankle, even as the wearer travels over uneven terrain.
And that extra energy promises to make a big difference in the gait of amputees. According to Herr, amputees typically expend about 30 percent more metabolic energy on walking than a non-amputee and tend to walk slower for that reason. They can also experience a constellation of joint problems from the asymmetrical nature of their gaits when using passive prosthetics.
Herr, a double amputee himself, has plans to further improve powered prosthetics. He hopes to put small, wireless implants into his muscles near the neuromuscular junction, so when the muscle contracts, the electrical impulse will send a control signal to the artificial limb. Herr is also working with the VA Center for Restorative and Regenerative Medicine and Brown’s Program for Recovery from Trauma, to determine if the foot prosthesis could be mechanically attached directly to the amputee’s residual limb bone. That way, loads would be transmitted directly to the bone structure of the amputee.
Emergent Surface transforms itself into different shapes in reaction to its environment. It’s powered by a collection of hybrid stepper-servo motors from Quicksilver Controls.
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