Flexible film works with force

Tom Shelley reports on a commercial polymer that can be used to make sensors, actuators, and energy harvesters

Thin, textured, silicone rubber film with an electrode surface sputtered onto its top and bottom surfaces can be used to make sensors, produce movement, or extract electrical energy from vibrations. The electrodes allow it to function as a large area capacitor.

The useful effects derive from changes in capacitance caused by stretching the film or by applying voltage to make it change shape.

While the basic idea is not new, it is now possible to produce large quantities of it at an acceptable price, and manufacture it into commercially viable components with suitable driving electronics.

Jørgen Mads Clausen, the president and CEO of Danfoss originally conceived the concept. He came up with the idea of making a polymer fibre structure that could work like a human muscle back in 1995.

The material is described as a dielectric electroactive polymer, but Danfoss simply call it PolyPower. Although it has been under development for sometime, it was officially launched at the Hannover Fair that took place in April.

Danfoss PolyPower general manager Michael Hamann says: "As the material is corrugated, it stretches up to 30% without destroying the mechanical properties while being stiff in the other direction."

When the material is pulled in a direction transverse to the corrugations, these pull out, increasing the capacitance. It this manner it can be used as a displacement or force sensor with no moving parts.

Conversely, if a voltage is applied to the electrodes, these are attracted towards each other making the material expand. If the voltage is reduced the material pushes the electrodes away from each other and the material contracts.

An 'actuator' made from four sheets of PolyPower, called the Inlastor Pull, weighs just 160g and pre-stretched by 10% it can reciprocate a 10kg load over a 13mm.

Maximum bias voltage is 2.5KV, and not very much happens below 500V. These voltages may seem high, but the currents involved are microscopically small and so are not particularly hazardous.

As well as being able to be used in sheet form, the material can be rolled up and used as an actuator with the ability to push. The company call this, the Inlastor Push.

A unit with the material rolled up to form a hollow pillar can support and reciprocate a 1kg load. "The material can lift a hundred times its own weight with no noise and no backlash," says Hamann.

An Inlastor Push sample, 100mm high by 40mm across, has a blocking force of 6.5N, and a stroke of 1mm. The Push Inlastor elongates 2.5% at 2500V, while the folded Pull Inlastor elongates 3.5% at the same voltage.

But most remarkable is its possible use as an energy harvester, turning repetitive movements and forces into electric power. These could be small, extracting power from the movements of the human body to power medical devices, or larger, extracting energy from wave power. It also allows the extraction of energy from random movements and forces, such as those generated by wind acting on tensioned ropes or wires, or even flags. This is possible because the material shows a large hysteresis loop in its charge, discharge, stretch and relax cycle.

It is charged while stretched at maximum capacitance value, maximising electrode area and minimising electrode spacing. Relaxing it decreases the electrode area and increases the electrode spacing, reducing the capacitance. Since the amount of charge remains the same, reducing the capacitance makes the stored voltage rise, increasing the electrical energy content, which may then be harvested.

This requires a certain amount of electronics to manage the charge and discharge cycle, and electronics are also required to work with the material as sensors and actuators.

"The breakthrough," says Hamann, "is in production. We now have a roll to roll process that will produce kilometres per week."

The company is currently selling samples and evaluation kits, which contains PolyPower film kit, an Inlastor Push, an Inlastor Pull, and a 10W electronic controller. This is priced at €2,000.

The company has already identified a large number of potential applications, including proportional valves that use combined actuators and position sensors, direct acting dosing pumps, with variable frequency and stroke and active noise and vibration dampers.

Smart grippers using the actuators could be particularly suitable for handling delicate items. In cars, suitable applications are seen in continuous headlight adjustment, ventilation control, and mirror adjusters. Because of its flexibility, the material is particularly suited for incorporation into smart clothing, whether to allow control of electronic devices, to provide sensor input for sports or rehabilitation training. It could also allow interaction with video games. Small pieces of film may also prove to be of crucial benefit as the active parts of microphones, headphones and loudspeakers.

In case anybody has a particularly good, killer application that Danfoss has not thought of, the company is offering a €10,000 prize to the team of university students that comes up with the most innovative device that demonstrates the unique features of PolyPower and has potential for commercial utilisation.

To be eligible, participants must be teams of two to six bachelors, or masters' degree students from accredited universities.

Pointers

* The crucial material is a silicone rubber film, with a corrugated surface. Electrode metallisation is sputtered onto top and bottom surfaces

* When the material is stretched it changes its capacitance. Conversely, when it is energised, it changes its dimensions

* The material can be used to make sensors, both pulling and pushing actuators, and energy harvesting devices

* The Push Inlastor elongates 2.5% at 2500V, while the folded pull Inlastor elongates 3.5% at the same voltage

Author
Tom Shelley

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