GB2270198A

GB2270198A - A piezoelectric actuator.

Info

Publication number: GB2270198A
Application number: GB9317311A
Authority: GB
Inventors: Michael Brill
Original assignee: Diehl GmbH and Co
Current assignee: Diehl Verwaltungs Stiftung
Priority date: 1992-08-31
Filing date: 1993-08-19
Publication date: 1994-03-02
Anticipated expiration: 2013-08-19
Also published as: DE4228974A1; GB9317311D0; FR2695521B1; FR2695521A1; DE4228974C2; GB2270198B

Abstract

A piezoelectric actuator 10 has piezoelectric ring shaped elements 12 which are combined by means of a fastening device 14 into a mechanically prestressed column of piezoelectric elements. The fastening device 14 has a rod element 18 which extends through the central space 16 of the column of piezoelectric elements, and which has on its two end portions 20, 22 fastening members 24 pressing against the adjacent piezoelectric elements 12. At least one of the fastening members 24 is provided with a prestressing element 28 which consists of a shape memory alloy and which upon suitable temperature change T changes shape mechanically prestressing the column of piezoelectric elements. <IMAGE>

Description

2270198 A PIEZOELECTRIC ACTUATbR The invention relates to a piezoelectric

actuator.

A piezoelectric actuator having mutually abutting elements made of piezoceramic material, which are held by means of a fastening device in a mechanically prestressed column of piezoelectric elements described in the information or data sheet of Messrs. Physik Instrumente (PI) GmbH & Co., 7517 Waldbronn, June 1987. The prestressing of the piezoelectric elements is effected by means of a screwable fastening device. Such screwing necessarily requires the application of mechanical torque to bring about the mechanical prestressing.

However, we have found that this torque can act on the piezoelectric elements detrimentally because such piezoelectric elements are normally stressed only in compression, and that such a mechanical torque can have a disadvantageous effect on the bonding between adjacent piezoelectric elements of the column of piezoelectric elements of such an actuator.

The problem underlying the invention is, therefore, to provide a piezoelectric actuator in which no mechanical torque takes effect for the mechanical prestressing of the column of piezoelectric elements.

In order to solve this problem the present invention 2 provides a piezoelectric actuator having mutually abutting elements made of piezoceramic material, which are held by means of a fastening device in a mechanically prestressed column of piezoelectric elements, characterised in that the piezoelectric elements are of ring form, and in that through a central space of the column of piezoelectric elements there extends a rod element, which has on its two end portions fastening members pressing against the corresponding piezoelectric elements, at least one of the fastening members being provided with a prestressing element made from a shape memory alloy, which upon suitable temperature change changes its axial dimension to mechanically prestress the column of piezoelectric elements.

is With the aid of the at least one prestressing element consisting of a shape memory-material or memory-metal alloy the mechanical prestressing of the column of piezoelectric elements is effected solely by a suitable change of shape of the prestressing element, i.e. by varying the axial dimension of the at least one prestressing element. For the mechanical prestressing of the column of piezoelectric elements no mechanical torque is exerted thereon, so that torque-ocassioned damage to Individual piezoelectric elements or damage to the bonding between adjacent piezoelectric elements is advantageously precluded.

It is advantageous if the column of piezoelectric 3 elements is mechanically prestreesed in' such a way that in each loading or activation state of the column a mechanical prestressing persists. Such an actuator is particularly suitable for temporally rapid switching uses. As a result of this possibility of use, the actuator in accordance with the invention is particularly suitable for use In deformable mirrors for telescopes, such as are described e.g. in the DE-Z "Phys.Bl.44 (1988) No. 12", pages 439 to 446. If, in the case of such known mirrors, side by side and spaced apart from one another several known actuators are used, then an actuation of a specific actuator may generated unintentionally actuation of adjacent actuators, i.e. such actuators may become virtually stressed In the wrong direction. The aforementioned advantageous mechanical prestressing of the column of piezoelectric elements of the/each actuator serves to avoid such a stressing in the wrong direction, the mechanical prestressing being selected so great that a loading of a corresponding actuator against this mechanical prestressing results in a mechanical state of stress which still acts in the direction of the said prestressing. By this means a stressing in the wrong direction is prevented.

Preferably, an intermediate ring is provided between each of the two fastening members and the adjacent piezoelectric element, the, or at least one, prestressing element being arranged between the fastening member and the adjacent intermediate ring. The two intermediate rings at 4 the end portions of the column of piezoelectric elements of the actuator serve to avoid mechanical damage more especially of the sensitive piezoelectric elements adjacent to the fastening members. For this purpose the intermediate rings can consist of a suitable metal or of a suitable metal alloy. The intermediate rings also serve to space the prestressing elements apart from the adjacent piezoelectric elements of the column of piezoelectric elements, to protect the said piezoelectric elements from being damaged when the actuator is mechanically prestressed with the aid of the prestressing element.

For the same purpose, i.e. avoidance of damage to the said piezoelectric elements at the end portions of the column of piezoelectric elements, it is helpful if each of the two intermediate rings is bonded to the associated, i.e. to the directly adjacent, piezoelectric element. For this purpose, i.e. for the bonding of each intermediate ring to the directly adjacent piezoelectric element, the same adhesive can be used as is used for the bonding of the individual piezoelectric elements to form the column of piezoelectric elements.

A simple design of the actuator is achieved, if the, or at least one, prestressing element is formed by at least one arched plate ring. The prestressing element or arched plate ring can consist of an NiTi shape memory alloy. Such a shape memory alloy is described, for example, in the DE-Z "Magazin Neue Werkstoffe" 1/87, pages - 20 to 22, in the journal "JEW'. February 1984, pages 96 et sequ. or in the DE-Z of Messrs. G. Rau GmbH & Co., "Memory ef fect and technically usable alloys", so that it is is unnecessary to go into this in detail. The said shape memory alloys result by reason of a thermoelastic structural transformation between austenitic and martensitic structure after a mechanical treatment In a temperature- dependent change of shape. Thus, as a result of temperature change, switching to and fro between two different shape states of the corresponding prestressing element can be effected. If a prestressing element deformed in the martensitic state is prevented from returning upon a heating above the so-called austenite starting temperature into its original shape, then one speaks of the above-mentioned suppressed shape memory. In particular, in the case of prestressing elements consisting of a NiTi shape memory alloy it has transpired that as a result of the said suppressed shape memory a considerable torque-free prestressing force can be produced, which upon one-sided application can lie orderof -magnitudewise at 800 N and upon two-sided application can consequently lie orderof-magnitude-wise at 1600 N.

In order to be able to subject the prestressing 25 element in a simple manner to a corresponding temperature change, it has proved to be advantageous if the or at least one prestressing element has an electric resistance heating element. This resistance heating element can be a known 6 leper se" heating foil, which is fastened on the prestressing element or to the arched plate ring on the one basal surface thereof.

An actuator of relatively high power capacity can be achieved if a central space is provided in the column of piezoelectric elements and is employed for cooling the piezoelectric elements.

To avoid electrical flashovers along the column of piezoelectric elements it is advantageous if the column of piezoelectric elements has at Its periphery, along which the individual piezoelectric elements are suitably electrically contacted with one another, an electrically insulating sheathing.

Further details, features and advantages of the invention will become apparent from the following description of an exemplified embodiment, shown in the accompanying diagrammatic drawings, of a piezoelectric actuator in accordance with the invention.

In the drawings:- FIGURE 1 shows a partially cut-open side view of the actuator, and FIGURE 2 is a diagram representing the dependency of the 7 shape of the prestressing element as -a function of the temperature T of the same.

Ilk, FIGURE 1 shows partially cut-open a piezoelectric actuator 10 having piezoelectric elements 12 which are of ring form and which are bonded together to form a column of piezoelectric elements. A fastening device 14 serves for prestressing the column of piezoelectric elements mechanically in compression. For this purpose there extends through the central space 16 of the column of piezoelectric elements a rod element 18 of the fastening device 14 which has on end of its two end portions 20 and 22 a fastening member 24. Provided between each of the two fastening members 24 and the piezoelectric element 12 respectively adjacent to this is an Intermediate ring 26 made of a suitable metal or metal alloy. Provided between the lower intermediate ring 26 in FIGURE 1 and the fastening member 24, associated with this, of the fastening device 14 is a prestressing element 28 consisting of a shape memory alloy, which in FIGURE 1 is shown in the active position bringing about the mechanical prestressing of the column of piezoelectric elements.

It is evident from FIGURE 1 that the prestressing element 28 in the active, i.e. prestressing shape, forms an arched plate ring. In the inactive shape the prestressing element 28 forms a flat ring. Provided on the prestressing element 28 is an electric resistance heating element 30, 8 which can be applied by means of connection elements 23 to a source of voltage, in order with the aid of the resistance heating element 30 traversed by current to heat the prestressing element 28, as will be further explained 5 hereinunder in conjunction with FIGURE 2.

Designated by the reference number 34 are the electrical connection elements of the column of piezoelectric elements, the piezoelectric elements 12 lo forming the column of piezoelectric elements being suitably connected together by means of contact elements 36. An electrical voltage applied to the connection elements 34 brings about, as a result of utilisation of the inverse piezoelectric effect, a change in the corresponding dimensions of the individual piezoelectric elements 12. By suitable polarisation of the piezoelectric elements 12, as a result of the electrical voltage applied to the connection elements 34 a change in thickness of the piezoelectric elements 12 is brought about. In this way it is possible to carry out with the actuator 10 very fine adjusting movements in the region of a few nanometers up to several micrometres.

As a function of the cross-sectional dimensions of the actuator or respectively of the piezoelectric elements of the column of piezoelectric elements it is possible to record with the actuator 10 compressive forces up to some 103N, whilst tensile forces remain restricted to smaller 9 values. The mechanical prestressing of the actuator 10 brings about in an advantageous manner an increase in the possible tensile forces which can act on the actuator 10. This means that the actuator 10 in accordance with the 5 invention is particularly suitable for dynamic applications e. g. in the case of mirrors for mirror telescopes, in which comparatively great tensile stresses can occur.

FIGURE 2 shows the relationship between the shape of the prestressing element 28 and the respective temperature T at the prestressing element 28 (see FIGURE 1). The prestressing element 28 in the form of a ring has in the initial state at room temperature TR a flat shape A, i.e. small axial dimensions. If the ring-shaped prestressing element 28 is heated with the aid of the resistance heating element 30 to the temperature T2,1, then the shape memory alloy begins to become austenitic. If the prestressing element 28 is further heated, then at the temperature T2,2 the entire structure of the shape memory alloy is austenitic, in which respect the shape of the prestressing element 28 as a result of the shape memory changes into the dished, arched or indented ring form shown in FIGURE 1. In this way at the same time a corresponding mechanical prestressing of the column of piezoelectric elements of the actuator 10 is brought about. The temperature T2,2 lies in the case of a NiTi shape memory alloy at about 1600C.

If the prestressing element 28 after this heating to the temperature T2,2 is cooled down to foom temperature TR, then the dished ring shape of the prestressing element, which is designated by B in FIGURE 2 is maintained.

A mechanical relaxation of the column of piezoelectric elements of the actuator 10 is possible If the prestressing element 28 is cooled down from the room temperature TR by way of a temperature T3,1 to a temperature T:3,2, in which respect the structure of the shape memory alloy at the temperature T3,1 begins to become martensitic and at the temperature T3,2 the entire structure is martensitic. In this respect, in turn the result is a change of shape from the dished shape B bringing about the mechanical prestressing of the column to the flat ring shape A, in which then no mechanical prestressing is present. This shape is maintained when the prestressing element 28 is heated from the low temperature T3,2 to room temperature TR. The temperature T3,2 amounts, for the said NiTi shape memory alloy, to about - 60C. A cooling down of the prestressing element 28 to the lastly mentioned low temperature of about -60C can be effected by means of liquid nitrogen. Moreover, as a result of the intermediate ring 26 adjacent to the prestressing element 28 upon such a cooling damage to the temperature- sensitive piezoelectric elements 12 adjacent to the prestressing element 28 is prevented.

An electrically insulating sheathing 38 covers 11 periphery of the column of piezoelectric elements, i.e. the outer jacket of the actuator 10, and protects this against 1111 I voltage flashovers and creeping currents. c 1 1 is

Claims

12 CLAIMS bl A piezoelectric actuator having mutually abutting elements

made of piezoceramic material, which are held by means of a fastening device in a mechanically prestressed column of piezoelectric elements, characterised in that the piezoelectric elements are of ring form, and in that through a central space of the column of piezoelectric elements there extends a rod element, which has on its two lo end portions fastening members pressing against the corresponding piezoelectric elements, at least one of the fastening members being provided with a prestressing element made from a shape memory alloy, which upon suitable temperature change changes its axial dimension to is mechanically prestressing of the column of piezoelectric elements.

2. An actuator according to Claim 1, characterised in that the column of piezoelectric elements is mechanically prestressed in such a way that in each loading or respectively activation state of the column a mechanical prestressing persists.

3. An actuator according to Claim 1 or 2, characterised 2.5 in that an intermediate ring is provided between each of the two fastening members and the adjacent piezoelectric element, the, or at least one, prestressing element being arranged between the fastening member and the adjacent 13 intermediate ring.

4. An actuator according to Claim 3, characterised in that each intermediate ring is bonded to the associated 5 piezoelectric element.

5. An actuator according to any one of the preceding claims, characterised In that the, or at least one, prestressing element is formed by at least one ring of 10 dished, arched or indented form.

6. An actuator according to any one of the preceding claims, characterised in that the, or at least one, prestressing element consists of a NiTi shape memory alloy.

is

7. An actuator according to any one of the preceding claims, characterised in that the, or at least one prestressing element has an electric resistance heating element. 20

8. An actuator according to any one of the preceding claims, characterised in that a space within the column of piezoelectric elements is provided for the cooling of the piezoelectric elements. 25

9. An actuator according to any one of the preceding claims, characterised in that the column of piezoelectric elements has on its periphery an electrically insulating 14 sheath or jacket.

bl t

10. An actuator substantially as hereinbef ore described with reference to the accompanying drawings.