As Corvette has become more technically advanced, this once-elemental sports car has seen a proliferation of onboard actuators installed throughout its body and driveline. Today’s C6, for example, includes a plethora of motor- and solenoid-driven systems to control myriad functions—from the remote-unlocking features on the rear hatch and door locks, to the adjustment of the side-view mirrors, to the various small motors that direct airflow in the HVAC system. Imagine replacing one-third or more of these devices with shape-memory alloys (SMA)—or smart metals, as they are also known—to reduce weight, increase durability, enhance driver convenience, and even improve track-day performance.
That future will become reality on the Corvette as early as later this year, when the next-generation C7 Stingray coupe becomes the first vehicle in GM’s history to utilize shape-memory alloy in lieu of a small-motor-driven actuator system.
“The Corvette is a perfect platform to roll out this material,” says Dr. Paul Alexander of GM’s Vehicle Systems Research Laboratory’s Smart Materials and Research Group. “Chevy’s two-seater has a long track record of introducing cutting-edge material, starting with fiberglass back in 1953, and carbon fiber in the last decade, for example. Whereas those materials are static structural advanced materials, shape-memory alloy is what we call a dynamic advanced material.”
The discovery of shape-memory alloy dates to 1962, when the U.S. Navy’s Naval Ordnance Laboratory observed the properties of nickel titanium (NiTi), and branded it Nitinol. The story goes that the Navy presented a badly bent sample of the alloy at a laboratory management meeting. A technical director wanted to see what would happen if the sample was subjected to heat, so he held his pipe lighter underneath it. To the amazement of those attending the meeting, the sample stretched back to its original shape.
By the late ’90s, OE suppliers had advanced the methods by which SMA ingots were transformed into wire forms. That’s when General Motors became interested in the potential benefits the alloy offered. “Once it was commercially viable for automotive applications, our research-and-development teams started tinkering with it as part of safety research,” Alexander says. “In the past five or six years, smart metals have become a hot topic within our department. [We] set out to prototype SMA systems that could last the 10-year lifespan of a vehicle and perform well across a temperature range of -30 to 85 degrees Celsius [-22 to 185 degrees F].”
Not long after the release of the sixth- generation Corvette Z06 in 2006, the car’s development team came to Alexander with a problem. Specifically, some new Z owners were complaining that pressure built up inside the cabin was making it difficult for them to close the rear hatch.
Alexander explains: “For the base Corvette coupe, GM engineers had implemented a massive cinching latch to address the problem. The hatch drops into the primary latch position, and then a large motor yanks it down, compresses all the seals and struts, and gets it into final latching position. That’s a very bulky mass the team didn’t like, so the mechanism was initially omitted from C6 Z06 models. [It was ultimately added to both the Z06 and the ZR1 for MY2009.]
“Team Corvette asked me to eliminate the parasitic mass of the cinching latch and create a shape-memory-alloy system to remove the excessive cabin pressure. That led me to design a patent-pending SMA actuator in the C6 coupe’s rear fascia called the Active Hatch Vent, or AHV, [which] facilitates the same ease of closing as the cinching latch.”
Shape-memory-alloy wire is very thin—approximately 350 microns in diameter, or about the width of a high E string on an electric guitar—so the mass of a given length of wire is very low. That makes weight savings one of the key features of GM’s new SMA actuators. But that’s just the start of this amazing metal’s benefits.
“Since the wires are so thin, we can make very-low-profile actuator packages, which conform or ride along surfaces of existing components. In comparison, we look at a small motor as an extra brick of material that has to be packaged in. We can put SMA actuation in locations where a conventional motor or solenoid is too large.
“SMA actuator systems use solid-state technology, and that makes them more reliable than traditional actuators, which are composed of rotors, stators, brushes, windings, and bearing surfaces.
“SMA [also] produces a very quiet operation. Unlike a motor- or solenoid-driven actuator, which can annoy a Corvette owner with a lot of whirring and clunking, an SMA actuator produces an audibly quiet motion without external noise generation. More so, it’s completely electromechanically quiet. In other words, there is no signal interference of the kind a conventional motor produces. That means we can put this technology next to antennas, which is exactly what we did on the 2014 Stingray.”
So is shape-memory alloy simply a fad, or does it signal the future of automotive-actuator technology?
“I definitely think it’s the future.” Alexander says. “There’s a large number of opportunities where shape-memory-alloy actuation really does make sense. One of our jobs now is to educate the supply base and the designers within GM, so that they’re comfortable employing it.
“As far as the C7 Corvette goes, the vehicle is definitely blazing a trail here with SMA technology. I’m confident this Active Hatch Vent application will be successful, and I hope you’ll see some of the projects we’re working on [for Corvette] come to fruition a little bit quicker. Looking out further, I think shape-memory-alloy actuators will proliferate across all sorts of applications—both on-demand triggered applications and passively actuated applications where SMA is both a sensor and an actuator. I think SMA’s successes will start adding up very quickly.
“This is the initial rollout of this device, so it’s specifically tied to rear-hatch opening and closing,” Alexander reiterates. “But as you can imagine, almost anywhere that small-motors and solenoids are currently utilized on the Corvette, shape-memory alloys can potentially take their place.”
How the C7’s Active Hatch Vent (AHV) Works
1. A Corvette owner opens the rear hatch.
2. The action triggers the “ajar” sensor in the hatch latch.
3. The Corvette’s body control module (BCM) receives this input and enables the Active Hatch Vent (AHV).
4. The vent’s onboard controller receives the enable signal and begins sending power (Pulse Width Modulated [PWM] 12-volt/1.0-amp) to the wire to heat it.
5. As the wire transforms, it contracts and opens the vent within about four seconds of the enable signal being sent to the actuator.
6. Once the vent is open, it stays open as long as the hatch is ajar. At this point, the onboard controller has regulated the power to maintain the target opening angle for the vent, trickling just enough current through the wire to keep it above its transition temperature (around 300 milliamps).
7. When the rear hatch is shut, the pressurized air rushes out of the cabin through the open vent to allow for easy closing. GM’s in-vehicle testing has shown a 25 to 30 percent reduction in required input closing energy.
8. Once the latch sensor reads “closed,” the BCM shuts off the enable signal, and the onboard controller shuts off power to the SMA wire.
9. The wire cools naturally and transforms to its more malleable state. A bias spring within the device then stretches the wire to its initial length and seals the vanes of the vent against its housing. This sealing step takes less than a second in most ambient conditions.