Electroactive Elastomers – Artificial Muscles for Shock Absorption
There are many classes of artificial muscle that have been created by our science, and each serves a different purpose. One of these sets is known as electroactive elastomers, and a few examples are already in use.
The basic principle they work on is that they change their form when exposed to an electrical field. Like most shape memory substances, this means they return to their original shape when the current is turned off. This makes them perfect for use as muscles, as when the current is received, they stretch out, pulling anything within a certain weight range with them.
But, some scientists have thought up a very different way to use their properties. Rather than employing them as muscles, the same properties of electroactive elastomers can be used to absorb shocks and blows.
To do this is very tricky. You require a dedicated microprocessor, and some way of sensing exactly what force is going to hit and when. The clever part is in the activation of the elastomer. By using an alternating current, the material starts to stretch then contract, stretch then contract very quickly. It vibrates extremely quickly in response to the level of current applied.
If a smart system is controlling this application of force, then the rapid expansion and contraction of the muscle can be placed in tune with the incoming force. This dampens down the effect of the impact of the force, and almost dissipates it completely. Now, with a force of an unknown strength, such as a punch, this is useless. However, for a force of a known strength, such as the impact with a bumpy road, where the bump height can be scanned as the vehicle passes over, it is ideal. Other applications include the vibrating of a car engine, where the vibration intensity is measurable and so can be damped before the passengers feel it.
The researchers working on the system created what they call a stack actuator to test their theories. It is made of 40 layers of electroactive elastomer, placed on top of one another, and connected up to separate currents. The entire thing is slightly thinner than an ordinary mathbox, and allows each layer of elastomer to vibrate and dampen the force that the previous layer let through. On and on, down through each of the 40 layers, until there is nothing left to dampen.
However, to create this was a serious engineering challenge. The electric fields had to be applied to each layer by electrodes, and electrodes are tiny metallic structures which are not known for their ability to absorb vibrations and stay put. So, they had to stay on the far side of the structure, whilst simultaneously being able to energise each layer.
The answer lay in the deformation process itself.
Jan Hansmann of the Fraunhofer Institute for Structural Durability and System Reliability, one of the researchers of the project said they were able to find a solution by use of perfectly aligned punched holes. We put microscopic-sized holes in the electrodes. If an electric voltage deforms the elastomer, then the elastomer can disperse into these holes. The result is an actuator that can rise or fall a few tenths of a centimetre upon command several times a second.
That's all it needs. As each layer deforms, it moves off the electrode and current ceases. The next layer continues to deform fractionally longer before it too moves off the electrode, and then the next and the next. A wave is formed, with each layer only receiving enough current to deform, and then slipping back. This also means they are perfectly aligned to catch each force wave as it arrives, and spend the interval before the next one, slipping back into standby.
When switched on, the prototype sits and waits for a collision. Tap it firmly with your hand, and it deforms into a shock absorption pattern. A detection instrument directly under the prototype, never detected any sign of your hand's impact.
The researchers believe one potential application for their stack actuator can be found in vehicle construction. An engines vibrations can be really disruptive, says another of the lead researchers, William Kaal. The vibrations are channelled through the chassis into the cars interior, where the passengers start to feel them. Of course, engines are installed meticulously, and yet: Active elastomers may help further reduce vibrations in the car, Kaal asserts.
As a side-effect of the device's function, it is also capable of absorbing impact energy to create power after all, the absorbed energy had to go somewhere. When the researchers placed an electromagnetic oscillator on their stack actuator, it converted the vibrations into power.
It offers another alternative to active power systems for remote sensor arrays in areas that are geologically unstable. The very act of the earth moving, powers up the sensor that detects the extent of that movement, enabling it to broadcast a warning.
The stack actuator technology has been largely perfected: The manufacturing process can be readily automated. That is important for industrial mass production, thinks Kaal. Nevertheless, endurance tests still have to show what the long-term viability of the intelligent actuators is like. Ultimately, they must be able to withstand harsh environments of the kind found in the engine compartment of a car.