ANTIOCH, Calif. Nanomuscle Inc. has tapped the properties of shape-memory alloys to yield small actuators that it believes could address a large applications niche between macroscopic motors and microelectromechanical systems. Nanomuscle said its actuators can be produced at a fraction of the size and cost of traditional electric motors.
In contrast to a motor-powered actuator, which might use coils, magnets and gears, Nanomuscle's device has one working part: a long, hair-thin wire of nickel-titanium alloy. When a current passes through the wire, the wire contracts because of the resistive heating of the current, providing the basic actuation function.
Although the shape-memory property of NiTi would seem ideal for building small actuators, designs based on that approach have yet to find a mass application. Inherent problems with previous attempts have included "a low cycle life and slow speed of actuation, and it has proved to be very difficult to use inside of a device," said Pete von Behrens, chief engineer for Nanomuscle, based here.
Shape-memory alloys were discovered in 1965 at the Naval Ordinance Laboratory (now called the Naval Surface Weapon Center), which found that the material could be stretched or deformed from its original shape but would spontaneously return to its original shape when heated. The material develops very large forces if it is constrained from returning to its original shape.
Dream come true
"Now, 40 years later, we have been able to overcome the basic problems with the material to create what was always the dream for shape-memory alloy: a practical actuator," said Danielle Fowler, vice president of operations and a cofounder of Nanomuscle. "There is a $12 billion market out there for miniature actuators. We have started with robotic applications and are now moving into consumer electronics."
Two leading toy manufacturers, Jetta Group and Hasbro Inc., have recently signed agreements to use Nanomuscle's actuator in their products, and the company said it is negotiating with automotive and consumer electronics manufacturers as well.
Fowler said the actuator could also find mass-market application as read/write head mechanisms in disk drives, as disk ejection mechanisms in computers, and in such automotive applications as mirror-adjusting mechanisms.
A physical barrier that has prevented the shape-memory alloy from being applied to actuators is the slowness of the full cycle, which depends on heating and cooling the wire. Nanomuscle engineers overcame that problem with a miniature cooling system.
Another basic problem is the small amount of contraction. "A wire will only contract by about 40 percent of its length, so we decided to have a very thin and very long wire. Of course, the thinner the wire, the less force it develops," von Behrens said. The length and diameter of the wire were adjusted so that the actuator moves through a distance of 4 mm and develops a force of 70 grams. The actuator is about the size of a paper clip and operates off of a voltage of 4 volts using 470 milliamps.
"Another inherent problem with this material is the practical problem of precisely controlling its shape change," von Behrens said. The Nanomuscle approach attaches individual filaments to a series of small, stacked metal plates that slide over one another. The movement of the plates is then controlled by a built-in microprocessor to achieve repeatable and smooth movement.
One competitive advantage of the shape-memory actuator is its ability to scale down to even smaller sizes. "With electric motors you begin to run into a whole lot of problems as you reduce size; the magnets have to be rare earths, and you end up having to assemble tiny, watch-sized gears," von Behrens said. We won't have any trouble scaling this design down, however."
Eventually, the design could be scaled down to compete with microelectromechanical systems (MEMS). "We expect that this technology will actually fill an important gap between MEMS and macroscopic motors," he said. "MEMS motors are so small that it is difficult to interface them with the rest of the world."
The Nanomuscle actuator could be made very small but would still be a discrete component that could be mounted on a circuit board.