Polyurethane is a plastic, which was used, for example, to make the football for the World Cup in Brazil in 2014. In research at Festo, polyurethane is used in combination with graphite as a new drive element (actuator).
An actuator turns electrical signals into mechanical movements, for example to open and close a valve. In classic valves the actuator is an electromagnet. In the DIELASTAR (dielectric elastomers for control actuators) project, research is being carried out into so-called dielectric elastomer actuators (DEA) that are made of several layers of polyurethane and graphite.
300 wafer-thin layers
Festo uses the DEA in the research project as switching elements in valves that control the air flow in pneumatic installations. “A DEA of this type is made up of 300 wafer-thin polyurethane layers stacked on top of each other,” explains Dr. Metin Giousouf, head of the research project at Festo. “Polyurethane is an elastomer that we use as a specially modified plastic foil.”
One layer is approx. 0.05 millimetres, in other words around half as thick as a human hair. The stacked layers results in a small cuboid the size of a sugar cube that packs a punch because ultra-thin electrodes made of graphite are sprayed on between the polyurethane layers. “The electrodes are arranged alternately so that an electric field can build up in the elastomer when an electric voltage is present,” explains Tobias Pointner, who also works on the research project. Due to the flexibility of the polyurethane layers, the electrodes under different charges move towards each other. This causes the whole actuator to shorten and opens the valve.
Unlike with solenoid operated valves, no heat is generated during this process, because the current only flows briefly to charge the electrodes. A DEA-controlled valve therefore does not need any current to hold its position (holding current), which means that valves fitted with DEAs work very energy efficiently. However, a high voltage of around 2,000 Volts is currently necessary to activate the DEA. The electrical voltage is around 80 times as high as the operating voltage required for standard industrial components. One of the main focuses in the research activities is therefore on the reduction of the electrical voltage.
Benefits of dielectric elastomer actuators
By controlling the electrical voltage applied to the DEA, the air flow in the pneumatic valve can be changed. This means that positions between “open” and “closed” are also possible (proportional operation). Conventional valves with solenoid actuators can either be completely opened or closed. A further benefit of DEA-controlled valves lies in the low material costs for polyurethane and graphite. A demonstrator of the research project makes these positive properties clear: it shows the proportional DEA drive and its energy efficiency compared to a conventional valve with solenoid drive.
You can see the demonstrator here in the video.