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Electroactive polymer actuators feature microstructured surface

21st February 2013


For more than 50 years the potential for electroactive polymers to be used as artificial muscles has been appreciated, but it is only now that commercialisation appears feasible. Alistair Rae reports on a new development in electroactive polymer actuators

Conventional actuators that use fluid power or electricity are extremely versatile, yet engineers have always wanted a technology that mimics animal muscles. Festo has created pneumatically actuated Fluidic Muscles that have been applied to a wide variety of industrial and artistic projects (Fig.1), but an alternative is electroactive polymers (EAPs) - materials that change shape in response to the application of an electric field. Now Danfoss says its new Polypower technology will unlock the potential of electroactive polymers, with applications well beyond simple artificial muscles.

Danfoss describes the Polypower film as intelligent elastic, with applications encompassing sensors, actuators, combined sensor-actuators and energy-harvesting systems. Among the many applications that Danfoss has identified are industrial robots, noise cancellation systems, vibration-reduction systems, and wave buoys for generating electricity.

Polypower is a dielectric electroactive polymer (DEAP) in the form of a film that can be rolled into almost any shape, creating what Danfoss calls an Inlastor. The technology has a number of key characteristics, being dynamic, precise, lightweight, flexible, proportional, sensitive and completely silent. Just as important is the fact that it can be produced cheaply and in large quantities using conventional roll-to-roll manufacturing processes as used throughout industry to produce standard plastics.

The principle of operation of a DEAP is simple: an elastomeric film is coated on both sides with electrodes, which are connected to a circuit. Applying a voltage generates an electrostatic pressure. This mechanical compression on the incompressible elastomer film causes it to contract in the thickness direction and expand in the film plane directions. When the applied voltage is removed, the film reverts to its original dimensions.

It has been known for a long time that certain types of polymers can change shape in response to electrical stimulation. Indeed, historical awareness can be traced back to 1880, when an experiment was carried out to charge and discharge a rubber band holding a suspended mass. A milestone in the technology was the discovery in 1925 of a piezoelectric polymer called electret, formed when carnauba wax, rosin and beeswax were solidified by cooling while subjected to a DC bias field. This led to ongoing work with new polymers and new activation methods (including chemical, thermal, pneumatic, optical and magnetic).

Early developments on synthetic electroactive polymers focused on exploiting the shape-changing response to develop artificial muscles. Many scientists in the field believe that the technology could one day be used to replace damaged human muscles. Furthermore, EAPs became an area of research within the field of engineering that became known as biomimetics, wherein mechanisms are developed based on biologically inspired models. Scientists in this field are particularly excited about the way EAPs can be applied to mimic animal and insect movements.

But despite much work over the last 50 years, and the discovery since the 1990s of numerous new electroactive polymers, many challenges remained. The actuation forces that could be generated by EAP materials were limited, their mechanical energy density was too low for practical applications, and their robustness was questionable. Operation typically required voltages higher than 150 MV/m, with activation fields close to the breakdown level of the material. They also required a compromise between strain and stress. And, of course, there was the biggest challenge of them all, namely cost.

Nevertheless, it was clear that electrically activated polymer actuators had the potential to induce relatively large actuation forces, hold strain under DC activation, offer rapid responses (at millisecond levels) and operate in ambient conditions over long periods of time. These exciting capabilities drove many pioneering research projects through the 1990s, not least of which was a study at Danfoss, whose president and chief executive Jorgen Mads Clausen was convinced that it should be possible to make polymer fibres that would mimic a human muscle.

Technological breakthrough

After some feasibility studies and initial research projects at Danfoss in the late 1990s, work continued at the company as a fairly low-key project until what Danfoss describes as a major breakthrough in 2006 gave it new impetus and the company scaled up its efforts. The result is the Polypower dielectric EAP (DEAP) film.

A DEAP takes the form of an electroactive polymer with electrodes on the upper and lower surfaces (Fig.2). Actuation is caused by electrostatic forces between two electrodes that squeeze the polymer. Operation requires high voltages - several thousand volts - but the currents are miniscule, so the technology is safe and consumes very low power.

With Polypower, Danfoss says it has made a breakthrough in two important areas. First, the patented design of the metal electrodes sputtered onto the thin, textured, silicone rubber elastomer brings significantly higher performance than alternative designs of DEAP material. Secondly, the material has been designed to enable low-cost, high-volume manufacturing.

A most important feature of the Polypower material that is claimed to be unique is its microstructured surface design that is moulded onto the surface of the thin silicone film. The elastomer is stiff along the micro-corrugations due to the high stiffness of the metal, so it is only allowed to elongate in the compliant direction. This unidirectional deformation and actuation of the film gives significantly higher efficiency than alternative designs wherein the film elongates in two directions. Rolled into an Inlastor, it can provide high-force push or pull actuators that are many times more energy-efficient that traditional actuator designs. At a demonstration at Copenmind, a three-day conference hoping to link up university researchers with corporations and investors, scientists showed how a roll of Polypower about half the diameter of a full roll of paper towels could lift a 5kg weight several millimetres.

With Danfoss well known for its valve and pump product ranges, it is no surprise that the company should have identified valve actuation and direct-acting dosing pumps as potential applications, along with robot actuators and other exciting push/pull tasks. But the potential of Polypower goes well beyond the actuator applications that have driven the developments of EAPs previously.

Sensing applications

The design of the Polypower material is essentially a capacitor; and when the material is stretched, its capacitance changes. With suitable electronics, this variable capacitance can form the basis of a new sensing technology, and Danfoss is exploring the use of the material in position-sensing devices (Fig.3).

Even more interesting is the potential for the sensor and actuator effects to be combined to create a whole new class of devices. Danfoss has identified the likes of smart grippers for handling delicate items, as well as numerous potential applications in the automotive industry, such as continuous headlight adjustment, ventilation control and mirror adjusters. In addition, a PhD project to model the static and dynamic properties of the Polypower film for potential use within active noise and vibration control systems. The project, which is a collaboration between Danfoss, the University of Southern Denmark and the UK's University of Southampton, will run until September 2011.

Within the consumer electronics sector, the dynamic properties of Polypower come into play. A single sheet of Polypower film can handle frequencies that the human ear can detect, making it suitable for new classes of microphone, and loudspeakers that can be round, flat or practically any other shape. It also offers the potential for the design of voltage-controlled variable capacitors for filters or tuning circuits. Many toys and computer gaming systems could also benefit from new sensors and actuators, with examples being force feedback joysticks where position sensor and force feedback are combined in one unit, or virtual reality animation systems based on sensors in a battle suit.

In sport and leisure, where there is a continuous focus on achieving better performance, the role of sensors has come to the fore, but conventional sensors can be something of a hindrance for athletes. Polypower, on the other hand, offers some innovative possibilities for integrating sensors and actuators; Patches of Polypower material could be used to detect angle and speed, allowing, for example, computer animation of movement, while sports clothing could be produced with built-in sensors.

Another key sector is healthcare and medical, in areas where patient rehabilitation often involves a lot of training with specialised personnel and equipment. But battery-powered clothing could be used either to support movements or add pre-programmed friction as part of a personal rehabilitation program. Specific applications include gloves with finger position sensing and built-in actuators, and active compression bandages.

An interesting subject is the energy harvesting potential of Polypower, turning repetitive movements and forces into electrical power. Because Polypower is a capacitor, it can store electrical energy; when the capacitance of a DEAP element changes as a result of mechanical force on the element, the amount of stored energy changes. This feature of the material can be used to convert mechanical energy directly into electrical energy (Fig.4). As a renewable energy source, Polypower could have a high conversion capability, with the potential for high energy density and no wear.

The direct conversion of random frequency linear motions into electrical energy is a good fit for the likes of wave power and wind power generators, but Danfoss is also looking at energy harvesting systems that could perhaps use the movement of the human body to power medical devices.

 







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