Amplified active-material actuators

Electrical generator or motor structure – Non-dynamoelectric – Piezoelectric elements and devices

Reexamination Certificate

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C310S328000, C310S368000

Reexamination Certificate

active

06268682

ABSTRACT:

The present invention relates to amplified actuators using active materials of the piezoelectric, electrostrictive, or magnetostrictive type.
BACKGROUND OF THE INVENTION
There are two types of flight control on board aircraft:
primary controls which serve to control the immediate movements of the airplane; these are generally moving surfaces situated at the trailing edges of the wings;
secondary flight controls which serve to trim the aerodynamic configuration of the aircraft to match different stages of flight. This category includes the tail plane and high-lift moving surfaces and tips.
The characteristics required of actuators for such surfaces are very different.
The primary controls must be capable of accommodating a passband that is greater than the spectrum width of the movements that the aircraft can perform, they must be capable of operating at all times, and they must be suitable for returning to a neutral position if there is a loss of power.
In contrast, secondary controls operate intermittently so they need a small passband only, but they must be capable of remaining in the most recent position in the event of a power failure.
These controls are generally implemented as hydraulic devices which, for primary controls, act directly, while for secondary controls they act via mechanical stepdown means. It is the mechanical stepdown means used in the second case that provide the required non-reversibility.
For various reasons (maintenance, pollution, fire risk, etc.) aircraft manufacturers are seeking to reduce the hydraulic contribution in controls and to promote electric control. Unfortunately, the technology of electromagnetic motors associated with stepdown means leads to equipment of excessive mass.
OBJECTS AND SUMMARY OF THE INVENTION
An object of the invention is to propose motors based on piezoelectric, electrostrictive, or magnetostrictive materials that are suitable for presenting high energy densities, that are capable of withstanding high stresses, and that consequently constitute good candidates for primary control.
Proposals have already been made to provide actuators based on vibration motors in which tangential and normal vibration generated on a stator is transformed into continuous motion by mechanical contact friction between the stator and the rotor.
For a general description of applications for that type of motor to secondary flight controls, reference can be made, for example, to:
“Actionneurs—Des matériaux piézoélectriques pour les commandes du futur” [Actuators—piezoelectric materials for future controls], published in Usine Nouvelle, Oct. 31, 1996, No. 2568; and
“Des commandes de vol piézoélectriques” [Piezoelectric flight controls], published in Air et Cosmos/Aviation International, No. 1602, Feb. 28, 1997.
Nevertheless, that type of motor is not suitable for use in making primary controls, given that continuous operation leads to the interface wearing too quickly, and to the last-reached position being maintained in the event of a power failure.
Another solution that has also been proposed consists in using piezoelectric displacement directly to achieve a limited amount of movement of a control surface. Since piezoelectric materials are capable of providing very high levels of force but can provide only very small amounts of displacement, it is appropriate to include therein structures which increase displacement so as to make them compatible with the amount of movement required for control surfaces. Such devices are commonly referred to as “amplifiers” even though power input is always greater than power output.
Amplified actuator structures are described, for example in: “A new amplifier piezoelectric actuator for precise positioning and semi-passive damping” by R. Le Letty, F. Claeyssen, G. Thomin, 2nd Space Microdynamics and Accurate Control Symposium, May 13-16, 1997, Toulouse.
In that article, it is proposed to use an elastic mechanical amplifier at the outlet from a piezoelectric actuator. Others have proposed using hydraulic conversion means as an amplifier (cf. above-mentioned article published in Usine Nouvelle).
Nevertheless, those solutions are not satisfactory. The structure that performs the conversion must be more rigid than the basic actuator, otherwise the power delivered by the actuator serves to deform the conversion structure to the detriment of delivering power output. This rigidity is often obtained by using parts that are massive, thereby considerably reducing the initial advantage of lightness and high energy density.
An object of the invention is thus to propose a structure that is rigid and lightweight for converting small driving piezoelectric displacements into large displacements.
To this end, the invention provides an actuator comprising a plurality of stacks of unit blocks of an active material of the piezoelectric, electrostrictive, or magnetostrictive type, which blocks are distributed so as to form a tubular structure, together with means enabling an electric or magnetic field to be applied to said unit blocks to cause said unit blocks to deform in such a manner that the tubular structure twists.
In first variant embodiments, the unit blocks are piezoelectric or electrostrictive, and the means for deforming said blocks comprise means forming electrodes enabling electric fields to be applied to said blocks.
Such an actuator advantageously further includes the various following characteristics taken singly or in any possible combination:
the unit blocks work in elongation/retraction and they are stacked with polarization that alternates in the height direction of the tubular structure;
the stacks are separated in pairs by separator means extending along the height direction of the structure, and the separator elements are constituted by a succession of slabs each being of a height that corresponds at least to the height of two unit blocks, the slabs being suitable for sliding relative to one another and being of stiffness that is greater than that of the unit blocks, the separator zones between superposed slabs at the same height being offset in the height direction of the structure from one separator means to the next in the circumferential direction;
the separator means are constituted by strips presenting a plurality of slots that, in pairs, define the slabs;
the separator means are constituted by a plurality of separator elements which are superposed, each of which constitutes a unit slab;
the electrode-forming means are constituted by the separator means;
the unit blocks operate in shear;
two successive unit blocks in the height direction of a given stack present electrode-forming metallizations on their facing faces;
the stacks of unit blocks are separated by electrically conductive separator means to which the electrode-forming metallizations are connected;
the unit blocks are distributed in washer-like layers in the height direction of the tubular structure;
the actuator includes a prestress envelope in which the tubular structure of unit blocks is placed;
the prestress envelope comprises a plurality of rings each providing prestress to a washer-like layer of unit blocks of the tubular structure;
the rings are electrically conductive and the actuator comprises a plurality of contact areas distributed in the height direction of the structure between the separator means and the prestress rings;
the contact areas are distributed so as to make contact with every other slab;
the rings are separated by electrically insulating washers; and
the envelope or the prestress rings is/are made of an alloy having shape memory.
Such an actuator is advantageously made as follows:
a) insulating washers and prestress rings in the low temperature phase are stacked in alternation inside an outer tube;
b) split metal strips are stuck along generator lines of a cylindrical inner core having an insulating surface, adjacent pairs of strips being offset relative to one another in the length direction of said core by half the thickness of the core;
c) the core is inserted together with its strips into the tubu

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