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Super powered shock absorbers
NEWS
Super powered shock absorbers MECHANICAL PROPERTIES
A prototype shock absorber capable of significantly reducing vibrations, such as those experienced while driving, has been developed by German researchers. The device can also convert vibrations into energy, meaning it has the potential to power inaccessible sensors. Shock absorbers are devices that dampen unwanted vibrations. Most are passive in nature and made of materials called elastomers that are yielding and malleable. An alternative approach is to use an active shock absorber that works to counteract the vibrations: hence improving the dampening effect. Researchers at the Fraunhofer Institute for Structural Durability and System Reliability have made an active shock absorber containing an electroactive elastomer. “These are elastic substances that change their form when exposed to an electrical field,” team member William Kaal explains. An alternating current is applied to the electroactive elastomer, causing it to vibrate. If the elastomer
vibrations are tuned to complement those of the external vibrations they can effectively cancel each other out. “By sensing the vibrations and feeding the signal back to the electronics an active absorber can virtually be tuned in its stiffness and damping properties,” Kaal told Materials Today. The team built a prototype dielectric stack actuator consisting of 40 alternating layers of thin films of natural rubber (the electroactive elastomer) and nickel electrodes. The key part of this work is the design of the electrode layers. “The challenge was the design of the electrodes with which we apply the electric field to the elastomer layers,” explains Kaal‘s colleague Jan Hansmann. As metals are rigid they hinder the vibration of the elastomer layers. To overcome this “we put microscopic-sized holes in the electrodes”, says Hansmann. “If an electric voltage deforms the elastomer, then the elastomer can disperse into these holes.” To test the prototype, a vibrating oscillator was attached. After a short period of time the
vibrations stopped, as the frequency of the stack actuator’s vibrations adjusted to counteract those coming from the oscillator. One potential use for this is in passenger vehicles. “The vibrations [of a car’s engine] are channelled through the chassis into the car‘s interior, where the passengers start to feel them,” says Kaal. “Active elastomers may help further reduce vibrations in the car.” The team also demonstrated that it is possible to change the function of the stack actuator, so it can produce energy through the absorption of external vibrations. When an electromagnetic oscillator was attached to their prototype, they demonstrated that the vibrations were converted into power. “That would be of interest if you wanted to monitor inaccessible sites where there are vibrations but no power connections,” says Hansmann. One example given is for the temperature and vibration sensors that monitor the condition of bridges. Nina Notman
Underwater solar cells ENERGY Solar cells that work nine metres under the sea have been developed by US scientists. These could be used to power autonomous electronic sensing systems. Electricity is currently supplied to under-water sensors by on-shore sources, batteries or solar cells on platforms above the water. However, the development of stand-alone devices is desirable. “The use of autonomous systems to provide situational awareness and long-term environment monitoring underwater is increasing,” says Phillip Jenkins, an expert in solar cell R&D at the Naval Research Laboratory where this research was carried out. Earlier attempts used silicon-based solar cells and researchers struggled with their performance due to the type of light available at this depth. “Solar cells based on gallium indium phosphide were shown to produce 2 – 3 times more power than conventional silicon solar cells when used underwater,” explains Jenkins. At a depth of 9.1 m, the GaInP cells were able to produce 7 watts of electricity per square metre: enough power to operate electronic sensor systems. “Although water absorbs sunlight, the technical challenge was to develop a solar cell that can efficiently convert these under-water photons to electricity,”
Jenkins says. “The spectral response of GaInP solar cells is well matched to sunlight transmitted underwater and converts this light more efficiently than conventional silicon solar cells,” he adds. The sun is filtered by the water and more of the blue/green part of spectrum gets through. “Virtually all photons two metres and below the surface, have an energy greater than 1.8 electron volts (eV). The most efficient way to convert this radiation is with a solar cell having a band gap energy close to this cut-off energy. GaInP has a band gap close to 1.8e V - much higher than silicon’s band gap of 1.1 eV,” Jenkins explains. “Additionally, high quality GaInP solar cells have very low parasitic losses and thus
operate better at low intensities (found underwater) compared to silicon.” Next, the team are planning an in situ study: “We intend to field an underwater power supply to understand issues of lifetime and reliability of long term deployment,” Jenkins told Materials Today.
Nina Notman
JULY-AUGUST 2012 | VOLUME 15 | NUMBER 7-8
301
Super powered shock absorbers MECHANICAL PROPERTIES
A prototype shock absorber capable of significantly reducing vibrations, such as those experienced while driving, has been developed by German researchers. The device can also convert vibrations into energy, meaning it has the potential to power inaccessible sensors. Shock absorbers are devices that dampen unwanted vibrations. Most are passive in nature and made of materials called elastomers that are yielding and malleable. An alternative approach is to use an active shock absorber that works to counteract the vibrations: hence improving the dampening effect. Researchers at the Fraunhofer Institute for Structural Durability and System Reliability have made an active shock absorber containing an electroactive elastomer. “These are elastic substances that change their form when exposed to an electrical field,” team member William Kaal explains. An alternating current is applied to the electroactive elastomer, causing it to vibrate. If the elastomer
vibrations are tuned to complement those of the external vibrations they can effectively cancel each other out. “By sensing the vibrations and feeding the signal back to the electronics an active absorber can virtually be tuned in its stiffness and damping properties,” Kaal told Materials Today. The team built a prototype dielectric stack actuator consisting of 40 alternating layers of thin films of natural rubber (the electroactive elastomer) and nickel electrodes. The key part of this work is the design of the electrode layers. “The challenge was the design of the electrodes with which we apply the electric field to the elastomer layers,” explains Kaal‘s colleague Jan Hansmann. As metals are rigid they hinder the vibration of the elastomer layers. To overcome this “we put microscopic-sized holes in the electrodes”, says Hansmann. “If an electric voltage deforms the elastomer, then the elastomer can disperse into these holes.” To test the prototype, a vibrating oscillator was attached. After a short period of time the
vibrations stopped, as the frequency of the stack actuator’s vibrations adjusted to counteract those coming from the oscillator. One potential use for this is in passenger vehicles. “The vibrations [of a car’s engine] are channelled through the chassis into the car‘s interior, where the passengers start to feel them,” says Kaal. “Active elastomers may help further reduce vibrations in the car.” The team also demonstrated that it is possible to change the function of the stack actuator, so it can produce energy through the absorption of external vibrations. When an electromagnetic oscillator was attached to their prototype, they demonstrated that the vibrations were converted into power. “That would be of interest if you wanted to monitor inaccessible sites where there are vibrations but no power connections,” says Hansmann. One example given is for the temperature and vibration sensors that monitor the condition of bridges. Nina Notman
Underwater solar cells ENERGY Solar cells that work nine metres under the sea have been developed by US scientists. These could be used to power autonomous electronic sensing systems. Electricity is currently supplied to under-water sensors by on-shore sources, batteries or solar cells on platforms above the water. However, the development of stand-alone devices is desirable. “The use of autonomous systems to provide situational awareness and long-term environment monitoring underwater is increasing,” says Phillip Jenkins, an expert in solar cell R&D at the Naval Research Laboratory where this research was carried out. Earlier attempts used silicon-based solar cells and researchers struggled with their performance due to the type of light available at this depth. “Solar cells based on gallium indium phosphide were shown to produce 2 – 3 times more power than conventional silicon solar cells when used underwater,” explains Jenkins. At a depth of 9.1 m, the GaInP cells were able to produce 7 watts of electricity per square metre: enough power to operate electronic sensor systems. “Although water absorbs sunlight, the technical challenge was to develop a solar cell that can efficiently convert these under-water photons to electricity,”
Jenkins says. “The spectral response of GaInP solar cells is well matched to sunlight transmitted underwater and converts this light more efficiently than conventional silicon solar cells,” he adds. The sun is filtered by the water and more of the blue/green part of spectrum gets through. “Virtually all photons two metres and below the surface, have an energy greater than 1.8 electron volts (eV). The most efficient way to convert this radiation is with a solar cell having a band gap energy close to this cut-off energy. GaInP has a band gap close to 1.8e V - much higher than silicon’s band gap of 1.1 eV,” Jenkins explains. “Additionally, high quality GaInP solar cells have very low parasitic losses and thus
operate better at low intensities (found underwater) compared to silicon.” Next, the team are planning an in situ study: “We intend to field an underwater power supply to understand issues of lifetime and reliability of long term deployment,” Jenkins told Materials Today.
Nina Notman
JULY-AUGUST 2012 | VOLUME 15 | NUMBER 7-8
301