“We’ve come to the determination that there’s no limit to how lazy people can be,” an auto-industry insider said recently. Drivers and passengers want more control at their fingertips. Intelligent, agile material-based actuators are giving it to them and even anticipating their desires.
Although shape-memory alloys (SMAs), piezoelectrics, and magnetostrictive materials have been in practical use since the middle of the 20th century, engineers are finding new ways to work these smart actuators into automobile systems, aerospace, and manufacturing.
“We’re systematically looking at all the things that are actuated in a car to see if we can replace them with a shape-memoryalloy wire instead of an electric motor. And then we’re looking at all the things that aren’t actuated because it was previously too expensive or too difficult to place an actuator in that spot,” said Jan Aase, head of General Motors’ Vehicle Development Research Lab.
The capabilities of SMA actuators vary with wire composition, actuator geometry, and material processing. For applications like GM’s, the wire is usually prestrained up to 5% in its more malleable low-temperature state. The amount of prestrain and the wire’s length determine the actuator’s stroke. When heated above its transition temperature, the material’s crystal structure shifts into a higher-stiffness form, contracting the wire and relieving the applied strain. (See box below.)
Stroke speed is determined by how fast the wire heats above its transition temperature and cools below it. Most applications use an electrical current of 4 A or less to heat the wire. Thinner wires and higher amperages get the actuator to its transition temperature faster. The alloy formulation can also be tweaked to set the transition temperature anywhere from 200 to above 300°C.
Once the heat source is removed, the wire must cool below its transition temperature to return to malleability. Opposing pairs of wires, springs, or weights must be used as bias loads to stretch the cooled wire back to its original prestrain level.
This heating, cooling, and rebias cycle means SMA actuators cannot operate faster than about 0.5 Hz unless special cooling accommodations are made. Depending on the envelope available, cooling with forced air, heat-conductive grease, circulating oil, or a glycolwater mixture can speed cycle time. The alloy is corrosion resistant in general and tolerates any of these cooling fluids equally well.
Multiple small-diameter wires cool more quickly than a single thick actuator. The wire diameter can vary from 0.001 to 0.020 in. The cumulative cross-sectional area dictates the force of the contraction which can reach 25 ksi for a single wire.
Efficiency and comfort
In Aase’s lab, GM is taking advantage of the binary nature of SMA wires to actuate simple openclosed or up-down systems, starting with engine cooling air intake.
Air-intake louvers controlled by SMA actuators could improve aerodynamics and fuel economy. At highway speeds, the engine only needs a fraction of the cooling air rushing through the intake. But the lack of space in the engine compartment has made it tough to cut the flow down to just what’s needed.
The system developed by Aase’s lab kicks in when the vehicle is traveling at higher speeds. The bodycontrol module (BCM) sends out a low-amperage current that heats and contracts a bundle of SMA wires. The resulting force pulls on a rack attached to the louvers and cuts the airflow.
Air dams under the front bumper can also improve aerodynamics. Most mass-market cars have to compromise between performance and the beating the dams take from steep driveways and parking blocks. SMAs tied into the BCM would lower a drag-reducing air dam into place at highway speeds and retract it for in-town driving and parking.
Other SMA applications in the works could make the vehicle more comfortable and accessible. Passenger grab handles inside the door frame are often forced into awkward angles or out-of-the-way spots by door geometry and headimpact criteria. Aase’s lab has designed a handle that lies flat until the door is opened.
Movement of the door latch signals an SMA wire to contract, releasing the handle. The handle then springs out from the door frame to assist the passenger entering or exiting the car. Once the door is closed again, the handle automatically stows in the flush position. The passenger can also swing it back manually into the flush position.
Passenger comfort and safety are also behind Professor Marcelo Dapino’s efforts at the Ohio State University Smart Vehicle Concepts Center. Supported by a Honda Initiation grant, Dapino is seeking to incorporate piezoelectric devices into seat belts. Piezoelectric materials change shape in response to electrical signals and generate charge when strained. (See box below.) Their extreme precision has been used for years in sensors, positioning applications, and actuation where rapid, precise, small magnitude motion is required.
Without mechanical amplification, piezos have an upper limit of about 0.1% strain. They operate on 100 to 1,000 V and milliamperelevel currents. More recent work, including Dapino’s, has focused on incorporating the precision and rapid reaction time of piezos into systems that can efficiently accomplish tasks requiring greater movement.
Current seat belts protect the occupant during a crash, but not without trade-offs. When the car’s sensors detect a rapid deceleration, as from a frontal impact, they trigger pretensioners in the buckle or belt retractor. The pretensioners are electric servos or pyrotechniccharge- triggered racks that rapidly remove slack from the belt and place the occupant in a safe position.
Mechanical load limiters, like torsion bars in the retractor, keep a constant force on the occupant during a crash. Forces can reach 4,000 N in the shoulder belt and 2,500 N in the lap belt. The devices are effective, but designed around a narrow window of occupant size and weight.
Dapino’s group wants to streamline the entire seat-belt system while ensuring it can optimally restrain any occupant. They plan to place solid-state piezoelectric actuators in the seat-belt’s D-ring to control the force on the belt. Active nanofiber sensors in the belt webbing would measure forces as a crash unfolds.
The sensor inputs, routed through a 2 6 1-in. 100-W power supply, would instruct the actuator to change the effective friction coefficient between the D-ring and seat belt to best restrain the occupant. The net restraining force, the difference between the shoulder-belt and lap-belt forces, is about 1,500 N and would come from the piezo’s action.
“This adaptive approach to seatbelt design will ultimately eliminate the trade-offs of existing passive or semiactive systems, and will lead to an unprecedented degree of occupant safety while simultaneously offering design simplicity and flexibility, compact operation, and reduced mass,” Dapino said.
Piezoelectric devices can produce motion on an even larger scale when coupled with mechanical or hydraulic systems. CSA Engineering Inc. used a 40-mm piezoelectric stack to supply hydraulic pressure to a Uninhabited Aerial Vehicle’s morphing wing.
In electrohydraulic actuators, the hydraulic fluid can be confined to a closed loop surrounding each actuator, cutting fluid requirements, eliminating hydraulic lines, and removing weight. The highfrequency motion of the piezoelectric stack can also speed response time.
In CSA’s design, the lead lanthanum zirconium titanate piezoelectric crystal was oscillated at 750 to 1,500 Hz with 600 to 1,200 VA supplied. The resulting 0.13% strain was enough to pressurize the hydraulic fluid when coupled with an accumulator, an output piston, and a microcontroller to activate the system’s four valves. To accommodate the high-frequency movement of the piezoelectric device, these valves were driven at up to 1 kHz by lower-power piezos.
One of the prototype hybrid solid-fluid actuators CSA produced had a peak output power of 42 W at about 15 Hz.
Magnets in Motion
More recently, hybrid actuators using magnetostrictive materials have shown promise. Instead of being driven directly by electricity, their shape change is triggered by a magnetic field, usually from an electrified coil surrounding the magnetostrictive core (See box.). Their rapid shape change makes them attractive for high-speed precision machining as well as aerospace applications.
Under the Darpa Compact Hybrid Actuator Program, CSA swapped the piezoelectric stack in previous prototypes for a magnetostrictive core of Terfenol-D and a coil. Program constraints dictated a centralized actuation system for morphing the wing surface from 9 to 16 ft in span with an actuated scissorlike structure.
The central system had to power eight actuators, each with a stroke of 8.5 in. and a bore of 0.75 in. CSA analysis showed they would need to move 30 cu-in. of fluid in 30 sec to meet the morphing requirements. This boiled down to a 0.26-gpm flow rate at 1,000 psi.
A 400-turn coil powered by a 45-A switching amplifier drove the magnetostrictive actuator that pressurized the hydraulic system. With a 1,000-psi prepressure, the magnetostrictive pump enabled a power output as high as 300 W. Flight tests may occur later this year.
Magnetic machining Etrema Inc. has a more earthbound application for Terfenol-D. Its magnetostrictive actuators produce forces as high as 10 ksi, but the materials are most often evaluated by their maximum displacement. Terfenol-D is considered a giant magnetostrictor because 2 to 4 A into the surrounding coils can deform it up to 1,000 microstrain (0.1%) up to 30,000 times/sec.
Etrema’s Advanced Machining System (AMS) uses Terfenol-D to move lathe tooling inserts at about 20,000 Hz. The rapid motion lets a high-speed lathe machine parts with noncircular cross sections and with details that usually require secondary machining operations.
One application of the AMS is small-engine pistons. The piston heads are designed to be slightly oval to minimize efficiency losses from thermal expansion at high temperatures. The heads are also barrel-shaped relative to the piston’s axis and require ring grooves to be machined in.
Signals from the lathe’s rotary encoder feed into a control module that, in turn, routes power to the coil surrounding the magnetostrictive material. Expansion of the Terfenol-D moves the cutting tool in and out to create the desired piston profile.
“Traditional technologies are looking at cycle times on the order of a couple of minutes for these pistons. We machine them to micron tolerances in two passes in 7 seconds,” said Etrema Executive Vice President and Chief Scientist Jon Snodgrass.
CSA Engineering Inc. csaengineering.com
Etrema Inc. etrema-usa.com
General Motors gm.com
Ohio State University Smart Vehicle Concepts Center smartvehiclecenter.org
Etrema’s in-depth article in Machine Design (Nov. 18, 2004): tinyurl.com/47thep
Materials that move
Thinking about using smart material actuators? Here are some material basics:
These metals are formulated to undergo a crystalline phase change at a given temperature. The most common alloy is about 50% nickel, 50% titanium. It’s commonly referred to as NiTi or Nitinol, reflecting its initial development at the former Naval Ordnance Laboratory.
Above the transition temperature, atoms in the metal are arranged in a crystal structure called austenite. As the material cools below the transition temperature, the atoms shift into multiple self-accommodated martensitic domains. Multiple domain orientations keep the alloy in the same bulk shape even though the martensite structure fills the space differently than the austenite does.
The martensite changes shape easily under stress by allowing some grains to grow and others to shrink until most of the grains are oriented in the same direction.
When the metal is again heated above the transition temperature, it returns to the austenitic state. The deformation that was accomplished in the martensitic phase is almost completely undone. This reversibility is its shape-memory property and the reason a bias load may need to be reapplied after each contraction.
Piezoelectricity causes some ceramic crystals to produce a charge when stressed and to strain in response to an applied voltage. Lead zirconium titanate (PZT) doped with lanthanum is a widely used piezoelectric crystal. The lanthanum atoms do not fit neatly into the lattice of the lead zirconium titanate crystal.
When a stress is applied, the structure of the lattice shifts slightly. Positive ions tend to shift in one direction and negative ions in the other. The charge inequality generates a measurable voltage. Similarly, when a voltage is applied, the lattice shifts to equalize the charge and the crystal produces a measurable shape change.
The strength of the ionic forces holding the crystal together means a high applied voltage is needed to produce a small shape change. Piezoelectric crystals are often stacked to magnify their effect or to produce off-axis movement like bending or shear.
Terfenol-D is a magnetostrictive alloy of terbium, iron, and dysprosium that was originally developed for sonar applications by the former Naval Ordnance Laboratory. The most-efficient forms of Terfenol-D are composed of a single metallic crystal. When the material is exposed to a magnetic field, the magnetic domains within it align, shifting the crystal structure and changing the bulk shape.
As with SMAs, the shift in the crystal lattice is entirely reversible. The material returns to its baseline volume when the magnetic field is removed. The magnetic-field direction does not influence the direction of expansion.