In with the new
Future fluids 'Controllable' or 'smart' fluids, the properties of which can be changed by applying a magnetic or electrical field, have been known since the 1950s, but until recently we have relied on clunky manual control mechanisms.
Magneto-rheological and electro-rheological (MR and ER) fluids had no real practical applications.
In the late 1990s, this changed: sensor technology improved and computing power became cheaper and physically smaller. US-based vibration-control expert Lord Corporation employs MR fluids to dampen vibrations in lorries, in automatic clutches for automobiles and to reduce vibrations in washing machines. Together with Japanese engineering firm Sanwa Tekki Corporation it has begun to explore the use of MR fluids in seismic-isolation systems in Japan.
The result is two experimental MR seismic-damping systems in an apartment building in Kawasaki and a state of the art distributed system in Japan's National Museum of Emerging Science and Innovation (MeSci), Tokyo.
'These are both demonstration projects, ' says Lord's Lynn Yanyo. 'To see what really happens, we'll have to wait for a real earthquake - simulators can't generate that kind of force.' Meanwhile, Lord's MR-fluid technology has been used to stabilise Dongting Lake Bridge in China's Hunan province. 'The bases of cablestayed bridges can be isolated in the same way as the bases of buildings, using a much smaller version of the dampers, ' says Yanyo.
The dampers handle not just seismic events but even unpredictable motion from rain and wind, sensing such vibrations in individual cables and preventing kinetic energy reaching dangerous levels.
MR-fluid dampers offer many advantages over other damping technologies. They have no mechanical valves, says Sanwa Tekki mechanical engineer, Hiroshi Sodeyama, making them more reliable and cheaper to maintain. Using an organic process to create micron-sized, round particles of pure iron that form like rain droplets as they coalesce, Lord suspends these in oil within the damper.
Additives mean the iron is readily dispersed and keep it from sinking.
Most other damping methods are essentially passive. 'In a situation where there are multiple or unpredicted vibration modes, ' says Yanyo, such as a cable-stayed bridge in a high wind, 'a passive solution is simply a compromise and can only cover a certain range of what you want to protect.' But MR-fluid dampers pre-empt and prevent. 'You don't have to wait for force to build up, ' says Yanyo. 'If you intercept the motion before it gets started, you can slow it before it turns into something bigger than you can control.' Unlike conventional damping, MR-fluid damping can easily be switched on and off as needed. 'If a washing machine damper is on all the time, passive, during a spin cycle it's actually working against the motor so it's less energy efficient, ' says Yanyo.
'A controllable system can take the damper off during a spin cycle.' MR-fluid dampers are completely adjustable. Unlike an ordinary car shock absorber, 'the force of an MR fluid does not depend on speed, ' says Yanyo. 'You can get any force you want at any speed. You can get forces at zero velocity that you'd get only at high speeds using a standard damper.' Compared with a passive damper, which must have the capability to absorb worst-case scenario high forces, MR-fluid dampers are more energy efficient. 'They have a range from not moving at all, if forces are low enough, to very high-force damping, if necessary, 'says Yanyo, thus only using as much energy as is needed at any given moment.
MR-fluid dampers are far smaller than conventional dampers. 'However, MR dampers in civil engineering are not out of the research stage yet', says Sodeyama. MR dampers are costly and sedimentation of fluids remains a serious problem.
ER fluids, too, are still being heavily researched, says Yanyo, but have, as yet, no civil-engineering applications. 'The challenge with ER fluids is that you use a large voltage across a very tiny gap, so they're hazardous to work with.' Lord is working with customers in Asia on other civil-engineering and transportation MR fluid applications, including the proposed Sutong Bridge, to be built in China's Jiangsu province across the Yangtze River - the longest such bridge ever built, at 1,088m.
Lord is also developing MR technology for use in prosthetic knees.
Future applications for MR fluids may include morphing aircraft wings or helicopter blades that allow millisecond changes to tailor their shape for optimum efficiency and aerodynamics, depending on speed and direction of travel.
It is interesting to read that US patent application 20030195623 has been filed for an MR or ER prosthetic penile implant to treat erectile dysfunction. However, says Yanyo dryly, 'Lord is not working on that aspect of the technology'.
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