Metamorphic substances shape actuator, sensor system design

Signal, Mar 2000 by Kenyon, Henry S

Reactive aerodynamic and hydrodynamic controls form essential components in tomorrow's aircraft, submarines.

Advances in materials and structures research will allow future military and civilian vehicles to move and react to environmental or mechanical stimuli in novel and dynamic ways. By using actuators to move individual parts such as the intake ducts in high-performance aircraft, or altering the shape of entire components such as helicopter blades or wings, performance can be greatly enhanced.

Intelligent structures offer novel ways to control factors such as geometric shape, movement, aerodynamic and hydrodynamic flow, and noise and vibration damping. Research programs are beginning to produce prototype devices that demonstrate some of these new concepts.

Launched by the Defense Advanced Research Projects Agency (DARPA) in 1993, the eight-year program seeks ways to develop new, affordable intelligent substances and bodies and to demonstrate corresponding performance gains by applying these techniques in existing systems. A major focus of the work is aerodynamic and hydrodynamic control, vibration and noise reduction.

Smart materials are usually composites made up of several constituent parts, with active elements either embedded or attached to conventional structural elements. Structural sensors typically consist of fiber optics and piezoelectric ceramics and polymers. These sensors can be embedded in a structure such as a wing or a fuselage to provide structural quality assessment both during production and operation. When combined with actuators, an embedded signal-sensor processing network and a control system, intelligent substances can alter a vehicle's structural performance by making modifications to compensate for damage or new mission requirements.

Actuators act as the smart structures' muscles. The most common types are made of shape memory alloys, piezoelectric and electrostrictive ceramics, electro- and magnetorhelogical fluids and elastomers, and magnetostrictive materials. They can either be dynamic devices-for suppressing vibration, for example-or quasi-static for shape control.

One group conducting research for DARPA is the shape memory alloy consortium, a cost-sharing program led by The Boeing Company, Seattle, Washington that includes ITN Energy Systems Incorporated, Wheat Ridge, Colorado; Metaltex Corporation, Reno, Nevada; the University of Maryland, College Park; the University of Minnesota, St. Paul; the University of Washington, Seattle; and the Massachusetts Institute of Technology, Cambridge. The goal of the project is to study how shape memory alloys (SMAs) work under industrial conditions, to develop SMA actuators for commercial and military applications, and to produce new magnetically activated SMA materials with high dynamic response.

According to Dr. Ephrahim Garcia, project manager for DARPA's Defense Science Office, Arlington, Virginia, the research is important because it provides data on how SMA actuators work. Scientists have conducted experiments such as moving actuators up to 100,000 times to study them for signs of fatigue. This is a fairly complex, nonlinear environment, but scientists now better understand how these materials operate, he adds.

Garcia considers the discovery of a new class of materials, magnetic shape memory alloys, as one of the program's major achievements. These materials are magnetostrictive-they react to magnetic fields in the same way piezoelectric elements react to electricity, by deforming their crystal structures. These new materials demonstrate performance capabilities that allow them some advantages over piezoelectrics or SMAs, he notes. But methods to utilize the fundamental properties of these new materials are currently unknown. "How to engineer with this is really the question," Garcia observes.

Research in smart structures has already yielded promising results in aerodynamics, specifically in wing and helicopter blade design (SIGNAL, July 1997, page 19). Developments are also appearing in areas with marine applications. Two hydrodynamic research projects nearing the test stage are the smart sleeve demonstration and the vortex leveraging tab program.

Smart sleeve seeks to reduce noise caused by torpedo movement through water, which can interfere with the weapon's sensors. Developed by the Lockheed Martin Missiles and Space, Advanced Technology Center, Palo Alto, California, the sleeve consists of a system of integrated actuators, sensors, amplifiers, signal processing hardware and software that cancels out vibrations caused by the turbulent boundary layer of water passing over the torpedo's hull before it reaches the sonar sensor. According to Stephen R. Winzer, smart sleeve program manager at Lockheed Martin, the sleeve does not have to cover the entire torpedo-only the areas screening the weapon's sonar systems to isolate them from vibrations. The sleeve also can reduce noise radiated by the torpedo. As with the sonar, only the skin overlaying the propulsion unit needs to be protected.