Toy Story

Prized for their high force-to-weight ratio, shape memory alloys have long perplexed engineers because of the relatively small displacements achieved and their limited lifespan. Now, engineers at NanoMuscle say they've solved those problems-leading to what they hope will be a plethora of new motion control applications for SMA actuators ranging from repetitive motion in toys to opening and closing vents in cars.Antioch, CA-When Engineer Katie Broughton was a kid, she never played with dolls. She was too busy taking apart her other toys and trying to figure out how they worked. But now the graduate of MIT (BSME) and Stanford (MSME) is making up for lost time. A principal engineer at NanoMuscle Inc. (www.nanomuscle.com), her job is to figure out how to use a new motor technology based on shape memory alloys (SMAs) to make toy eyeballs move realistically. Experimenting with several different prototypes, she's identified the particular sequence of eye motions that

makes a doll look sleepy, happy, even cranky (no toy company has expressed interest in that particular one so far). And a la Tim Burton, she has ideas for creating special-effect eyeballs that pop out or spin. Her efforts are paying off: Baby Bright Eyes, the first production doll to employ NanoMuscle technology, is due out in stores this Fall.

To provide the kind of smooth, silent actuation Broughton needs to mimic human eye motion, the NanoMuscle actuator employs wires made of shape memory alloys (SMAs) as the motive force. These materials undergo a solid-state phase change when cooled below a specific transition temperature, then return to their original geometry when heated-providing constant power output across the entire stroke length. Like a heat engine, they efficiently turn thermal energy directly into work.

SMAs are prized for their impressively high force-to-weight ratio-on the order of 15 Kgf/m². That's a big reason the technology has been of particular interest to engineers targeting the $12B small electric motor market. And there's ever-increasing demand for smaller motors for applications ranging from portable consumer electronics to toys to medical applications. But as they shrink, electromagnetic motors and solenoids can get costly. They also run into some practical limitations, such as bearing contamination and noise.

At 5.32 mm, the NanoMuscle is smaller than most, if not all, dc brush motors.

With its tiny actuator, NanoMuscle is specifically targeting some markets that are notoriously price-sensitive. In fact, one of the first mass-market applications for the NanoMuscle actuator is in the toy industry. "You're just not going to sell them a $300 actuator," says Danielle Fowler, co-founder and general manager of NanoMuscle.

That's certainly true. Playmates' Baby Bright Eyes, the first doll ever to feature a NanoMuscle, will retail for $49.99. According to Playmates, the NanoMuscle and associated electronics account for approximately 70% of the doll's total cost.

The company is also currently in discussions with automakers, another industry that's legendary for its penny-pinching tactics. Applications here will be limited to the interior, where temperatures do not exceed the 90C transition temperature of SMAs, and include pneumatic valve and air vent actuation.

The very first commercial application for the technology is in optical networking, where the tiny actuators move lenses, filters, and covers.

NanoMuscle's President and Founder Rod MacGregor declined to quote an exact price for the NanoMuscle, pointing out that the cost varies significantly depending on volume. The company's website, however, claims that the technology is "orders of magnitude less expensive than comparably sized electromagnetic motors."

Although it's certainly easy to find vendors who sell small-size electromagnetic motors for lots of money, there are competitively priced alternatives. For example, Portescap, a maker of motors in frame sizes as small as 10 mm, is best known for its mid- to high-level precision miniature motion solutions. But engineers say that customers could expect to pay as little as $5 to $7 for a motor/acme leadscrew combination targeted at repetitive motion applications in the automotive, home appliance, and toy industries. Furthermore, for engineers who are willing to accept a frame size beginning at around 10 mm, Portescap's motors deliver higher force (up to 600 gf or more) and extended travel capabilities.

Overcoming the Limitations of SMAs

Despite the benefits, and even though engineers have been experimenting with SMAs for decades, they have seen limited use in commercial applications to date. "Actually I am surprised that the material hasn't moved into the marketplace a little more quickly-it is a very intriguing technology," says Marlene Bourne, a senior analyst at In-Stat who has been following SMA technology for several years.

One of the stickier aspects of SMAs-and an obstacle to achieving that prized tiny size-has been the relatively small displacements produced. According to information published by NanoMuscle, a straight SMA wire has a maximum contraction of around 5% of its total length. "So an application that requires a stroke length of 4 mm would require a device that's 10 cm long-that's way too big for many applications," says Broughton.

To increase the stroke length, coils or levers have been tried successfully, but the trade-off is usually a decrease in available force. NanoMuscle engineers came up with a patented design that they claim mechanically amplifies the displacement, without any corresponding decrease in force. They refer to their design as a "folded construction" geometry (see figure, above right).

It consists of a series of SMA wires attached to a stack of rigid plates. The plates are coated with a low-friction material so that they slide easily against one another as the wires heat up and contract. Given that the plates are mechanically connected in series, the stroke length of the actuator is (n-1) times the contraction of a single wire, where n is the number of plates. A spring provides the return force.

Brougton claims that the patented design addresses two other limitations of SMA technology, namely low bandwidth (cycle time) and limited life. Since SMAs operate on a cycle of heating, cooling, and deformation, the bandwidth depends on the heat transfer capabilities of the system.

NanoMuscle engineers would not divulge what specific aspects of the design helped them achieve cycle times ranging from 320 to 1,600 msec, depending on the actuator model. But it's likely their design increases heat dissipation and improves cooling rates.

SMAs are especially notorious for their limited life. "These alloys tend to degrade very quickly without some type of control. Over 1,000 cycles, you can lose two-thirds of the stroke. And since the fatigue buildup is not linear, it's hard to predict exactly what you're dealing with," says MacGregor.

"Thermal degradation of shape memory materials has long been a major hurdle to widespread commercialization of the technology. Attempts to address this shortcoming in the past have often led to a compromise in some other desired characteristic of the technology, whether it be cost, weight, or size," says Joel Clark, a Professor of Materials Systems at MIT who studied the material while working in industry. His research focuses on the relationship between materials technology and economics. "If this issue can be addressed adequately, it would certainly open up interesting opportunities."

MacGregor claims that NanoMuscle engineers have overcome this limitation through a combination of thermal management techniques and a feedback control system that reduces thermal stresses. The use of pulse-width modulation control, whereby the power supply is switched on and off rapidly, also creates a more even heat distribution that he says eliminates the potential for overheating.

As a consequence, NanoMuscle motors are rated to more than 1 million cycles-more than sufficient for applications such as toys, which undergo heavy abuse at the hands of children and often break long before the components actually wear out. However, many industrial applications demand 100 million cycles or more from an actuator-a limiting factor to widespread adoption of the technology.

"This kind of device is typically very specialized, very niche-oriented and not really competitive with standard electromagnetic motion technology, except possibly on the very fringe," says George Gulalo, president of Motion Tech Trends (www.mttusa.com), a consulting company that specializes in technical analysis and market research in the area of motors, drives, and motion systems. "That's not to say, though, that they won't get a fair amount of volume out of some of these applications."

NanoMuscle's VP of Sales & Marketing Scott Osterman acknowledges that the company is not looking to go up against electromagnetic motors across the board. "What we're offering is a motor with a size and weight advantage, and some unusual characteristics such as smooth motion," he says, which all adds up to entirely new market opportunities-like startlingly real eye motion for toys.

The Eyes Have It

"Playmates has always held a leadership position with high-tech dolls, and there is this ongoing struggle to find the next big thing," says Nancy Goldblum, the company's VP of marketing for girls' toys. "While lots of dolls have motors that move their eyes, it was the silence and range of motion and the potential for the NanoMuscle to deliver realistic eye motion that made it so interesting to us."

Playmates' Hong Kong-based engineers worked closely with NanoMuscle's Broughton to come up with an assembly that would produce the desired eye motion (the doll has three different "play patterns") and fit into the prescribed envelope within the doll's head. They rejected several early configurations, including one that controlled each eyeball with a separate motor. "The problem was that the eyes didn't move completely in tandem, which gave the effect of a lazy eye," recalls Fowler. Not exactly the type of effect they were shooting for.

Though Broughton was not able to divulge exact details of the mechanism, she says that it consists of two NanoMuscle rotary actuators, one controlling both oversize (28-mm) eyeballs and lids along the x-axis and one controlling them along the y-axis. The actuators rotate a maximum of 60 degrees, translating that motion in the y-axis into 30 degrees for the eyeballs and 55 degrees for the lids. A pulse-width modulation scheme, whereby the voltage level to the motor is pulsed, is used to hold the actuator at intermediate positions. PWM also reduces the power requirements of the doll, which operates on four AA batteries. Goldblum says the doll operates "well beyond" the target five-hour play period.

Power to the motor is controlled by inputs from three switches in the doll (see diagram at right). Placing a teddy bear or bottle in the doll's hand activates two of the switches, and a tilt sensor activates the third based on the doll's position. Software determines which state the doll is in and initiates a specific play pattern, including an audio track.

The decision to use a new motor technology in a new doll did not come without some potential risk. "This motor had never been applied before in a toy, and we were really flying on a wing and a prayer that it was going to deliver," says Goldblum.

The fact that Playmates had no experience with the technology was particularly challenging when it came to product demos. Goldblum recalls the trials of traveling the country with a fragile prototype in a Tupperware container, and "it acting really persnickety at times." She finally insisted on having an engineer travel with her to all the major toy fairs so that the prototype would work as planned.

All that's behind her now as the company gears up for a product launch in September. So just how will Playmates measure whether this new application for the NanoMuscle is a successful toy story? "I'd love to sell a couple hundred thousand pieces," Goldblum says. "And that really depends on whether kids think it is as cool as we do."

Contact Chief Editor Karen Auguston Field at [email protected].