How Smart Structures Will Work

Computer technology, allied with engineering research, has fuelled the growth of smart structures, allowing better, safer, and more reliable structures to be built.

A smart structure can be an earthquake-proof skyscraper, a mile-long bridge, a multi-million dollar aircraft, or a shape conforming $2 foam pillow, and many objects in-between. Computer technology, allied with structural engineering research, has fuelled the growth of smart structures, and they increasingly impact our lives by allowing better, safer, and more reliable products to be built.

Most of us, when we think of smart structures, envisage earthquake-proof skyscrapers and bridges, large structures that can now withstand quite strong earthquakes. Not too long ago, huge structures like these used to be built as rigidly as possible with massive cross-beams. The problem is that rigid structures crack, and, inside a rigid building during an earthquake, the non-rigid contents, including humans, go flying all over the place. Not too long ago, dampers were introduced in this type of structure, massive ball-bearings or other devices that allowed a building to "roll with the punch."

The latest tool in the development of the smart structure is a unique substance, called magnetorheological fluid (MR fluid), a liquid that changes to a near-solid when exposed to a magnetic force, then back to liquid once the magnetic force is removed. It's a gray liquid that looks like dirty oil. Magnetize it, and it gains the consistency of peanut butter. How hard the substance becomes depends on the strength of the magnetic field. Take away the magnet, and the iron particles in this substance unlock immediately, and it's liquid again. The substance was discovered in 1940, but only recently, with the advent of small, fast computer chips, has a practical use been found for it.

Skyscrapers and long bridges resonate under high winds and seismic activity. Large dampers are used in these. Dampers using magnetorheological fluid are classed as semi-active. They counteract motion with a controlled resistive force. The damping level is changed by varying the amount of current supplied to an internal electromagnet. This type of damper is more reliable than the passive damper, which is simple and cheap, but is unable to adapt to changing needs. It's also more reliable than the active damper, which is fully controllable, requires a great deal of power, and has the potential to go out of control. About 5 liters of MR fluid is used to fill the semi-active damper's chamber, which is designed like a piston. During an earthquake, sensors signal the computer to send an electrical charge through the damper, and the earthquake vibration causes the MR fluid to change from liquid to solid thousands of times per second. In a large building, there can be hundreds of dampers, and each damper will help reduce the shaking. With computer control, a building can be left to sway gently and safely during minor quakes, insulating people and equipment inside, or it can be frozen solid during severe quakes, protecting the building from major damage.

Applications for magnetorheological fluid now include dampers for washing machines, shock absorbers for cars and advanced leg prosthetics, and Nautilus exercise equipment, Treadmills and 'stair climber' machines use the liquid to give just the right workout. And it's already being used in some truck seats for a smoother ride for drivers.

A fighter aircraft is another structure that uses smart materials. Capable of flying and maneuvering at speeds from slow to supersonic requires different wing designs for maximum efficiency at different speeds. Smart structural engineering, with wing sensors of differing kinds, is now beginning to allow automatic adjustment of the shape and size of the wings, different sweep and pitch, even variable rigidity. Polymeric smart skins can control acoustic noise, vibration drag and skin friction. Work is also being done on sensitizing aircraft to a range of structural failure modes, dynamic monitoring of the onset of structural faults, and dynamic load monitoring. Aircraft contain sensors for temperature, speed, acceleration, brightness, and humidity, and smart actuators move components, exert forces, inject fluids and gases and reduce brightness. De-icing systems are used widely on aircraft wing leading edges and on engine intakes. They use pressure or ultrasonic sensors and pneumatic or electrical heating actuators. Video cameras monitor flaps, landing gear, and other parameters. Once all of these features can be reliably achieved, pilotless planes will perform many dangerous tasks, in both war and peace, such as reconnaissance, storm analysis, and atmospheric sampling.

Smart structures and structural components have unusual abilities: they can sense temperature and pressure changes; they can identify strain; they can diagnose a problem; initiate an action to preserve structural integrity and continue to perform their intended functions. They can also store processes in memory and learn to repeat actions taken. The design, development, and deployment of smart structures are at the cutting edge of engineering research.

© High Speed Ventures 2011