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Diamond actuators suit aerospace applications

Edited by the Engineeringtalk editorial team Oct 28, 2009

Noliac has announced that an actuator design has been developed for active rotor control through the use of trailing edge flaps.

The actuator design is diamond shaped providing improved performance for a minimum mass.

Although it is well recognised that active rotor control will reduce the vibration levels by up to 80 per cent, up to now, no application has reached commercial success.

In the UK Technology Strategy Board (TSB) funded React project, Noliac Motion and helicopter company Agustawestland, part of the Finmeccanica Group, teamed up to provide a more effective solution.

The solution was presented at the Piezoelectric Actuation conference on 22 October 2009 at the Institute of Mechanical Engineers (IMechE) in London.

So far, all designs devised in the literature include amplifying mechanisms that have two important drawbacks.

First, they represent an important inactive mass, reducing the energy density achievable with the actuator.

Secondly, due to the inevitable thermal expansion mismatch between the piezoelectric element and the amplifying structure, a small relative expansion of the active element results in an amplified movement of the output.

The new design was developed to address the limitations in terms of energy density and temperature stability.

It is based on four piezoelectric stacks, connected in pairs.

Each stack is hinged at its ends and maintained in place with a small angle.

The whole assembly is preloaded through the use of a tension member maintaining the fixed members in place.

This ensures that the piezoelectric stacks operate in optimal conditions.

The actuator is operated as follows: when the applied voltage is increased on one pair of stacks, it is decreased on the other pair.

This contributes to a movement of the output member in one direction.

It should be noted that in the case of a free displacement, the tension in the piezoelectric stacks as well as in the tension members (therefore the preload) remains almost constant.

Upon temperature change, differential thermal expansion between the ceramic and the other materials in the assembly will lead to a change in force repartition.

This will result in a change in the internal preload.

However, unlike most of the existing schemes presented in the literature, this will not result in a movement of the output member.

Thanks to the compact design and the large proportion of active material, this design was expected to provide high performance levels as well as high energy density.

A prototype has been manufactured and tested.

It demonstrates a 35 per cent increase in energy density compared to the literature.

In other words, the actuator would be capable of the same performance for 74 per cent of the mass of the best existing solution.

It is thought that this figure can be further improved through the use of lightweight materials.

This amplified actuator will find applications in systems requiring large controllable displacements together with high forces for a minimal weight.

The diamond frame is also more compact than many commercially available amplification systems.

In addition, the lower mass and optimised stiffness imply higher mechanical resonance compared to other solutions, allowing operation at higher frequency.

This could be in other fields such as aeronautics, space, optics, military applications and others where high stroke, high force and low weight are required.