Electro active devices

Details Electro active devices Electro active devices

2002-10-10

2004-12-21

Budd, Mark (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S328000, C310S339000, C310S800000

Reexamination Certificate

active

06833656

ABSTRACT:

This invention relates to electro-active devices, and uses therefor. More particularly, the invention concerns novel constructions of electro-active (such as piezoelectric and piezoresistive) devices, some with integral positioning and control mechanisms. The electro-active devices may be used as electromechanical drivers, sensors or generators.
Electro-active devices are those which make use of components that display electro-active properties—that is, those in which a component changes shape in response to a change of the appropriate electrical conditions in which the component exists. Equally, of course, the component may produce electrical signals in response to a shape change. The best known, and most developed, of these devices are piezoelectric devices. However, it will be understood that there are a number of other sorts of electro-active device, including those that are electrostrictive (made from a material which contracts on the application of an electric field) or piezoresistive (this latter group being those the electrical resistance of which changes as they change shape). The devices of the invention include those with components that display effects based on such other types of electro-activity.
Early piezoelectric devices, and indeed many in use today, were merely simple blocks of piezoelectric material. If compressed in some direction they produce a voltage across opposite faces in a relevant direction; if, alternatively, a voltage is applied across them then they very slightly change their dimensions—typically by considerably less than a micron (1×10
−6
m).
Devices operating in this manner have found considerable use in various fields. However, there are many occasions when it is desirable for the application of an electric voltage to produce a much greater change in dimensions, of the order of several millimetres, and vice versa. Attempts to achieve this have focussed on a type of device known as a “bender”.
A bender is a construction of piezoelectric device wherein the piezoelectric material is physically in the form of an elongate but relatively thin bar, rather like a ruler, with its associated electrodes along the surface of the bar, and this operating bar is fixedly attached, face to face, onto a substrate in the form of a like bar (which may itself be either of a piezoelectric material or of a non-piezoelectric material). For example,
FIGS. 1 and 2
show a known piezoelectric unimorph bender. The bender comprises a flat, uniform layer of active piezoelectric material
1
(shown hatched) bonded face-to-face to a like flat, uniform layer of inactive non-piezoelectric material
2
(shown plain).
When an appropriate electric field is applied across the piezoelectric layer
1
by means of suitably-placed electrodes (not shown, but on either main face of the piezoelectric layer
1
), the dimensions of the layer
1
change. In particular, the layer lengthens very slightly. The substrate bar is left undisturbed and so its length is unchanged, or perhaps is made to change in the opposite sense, a bimorph. Expansion of the piezoelectric material, coupled with the restriction placed on it by the unchanged inactive layer
2
, causes significant bending of the entire bar in a direction normal to the plane of the bar, as shown in FIG.
2
. The movement of one end of the bar relative to the other may be considerable even though the length change is small; it may be many times the length change. For example, using a dual-bar structure 5 cm long, a length change of a fraction of a micron may manifest itself as a tip movement of up to 0.1 mm, or as much as a hundred times the length change. However, the path of the displacement is not linear, because the tip of the device follows a curved path in space.
As already described, on activation a plane bender bends forming a curve which can be described by a radius of curvature and the angle subtended by the ends of the bender. The average length of a bimorph bender does not change, as one part extends while the other part contracts, leaving a neutral axis along the central part of the bender which is the same length as in the inactivated state curved benders are also known, and are typified by that type known as a ‘Rainbow’. They are shaped such that the thickness of the device is radial, the bender tape being curved about an axis parallel to its width direction. Such a curved bender also bends on activation. The curve becomes tighter, which is equivalent to a smaller radius of curvature, while the subtended angle increases. Further, if the curvature of such a curved bender is circular (that is, the inactivated bender is in the shape of a circle or an arc of a circle), then on activation it bends to give a larger arc of a circle of smaller radius; the angle subtended increases. The radial change is small (microns for radii of curvature of millimetres or centimetres), and independent of bender length. The angle change, however, increases as the bender length increases, and can be quite significant. Thus, if one end of the bender is fixed, the apparent motion of the other end is primarily a rotation. For a circular bender of radius a few centimetres, this rotation may be about one degree or so.
In an extension of this circular geometry, helical benders are also known. In these, the bender is in the shape of a helix, rather like a strip of paper flat-wound around and along a cylinder (a tape-wound helix). As with the circular geometry bender, there is a small radial change, independent of tape length. And also as with the circular geometry case, there is with the helical case a rotational displacement about the axis of the helix, but with a helix the relative displacement of the ends follows a helical, rather than circular, path. There is thus also a small change in the axial length of the helix, dependent on the helix pitch angle. The amount of rotation and hence axial length change increases with bender length, resulting in quite significant rotations and axial displacements in long tape-wound helices. For instance, in a helix with a diameter of about 1 cm, an axial length of several centimetres, and having several helical turns of a bender tape several millimetres wide, the radial change is of the order of microns while the axial length change may be around 1 mm and the rotation may be several degrees.
It would be desirable to provide an electro-active device having a form which allows for large displacement relative to the size and/or weight of the device.
It would further be desirable to provide an electro-active device having a form which provides displacement which is linear in space, or can follow a path which is selectable by design of the device.
According to the present invention, there is provided an electro-active device having an electro-active structure extending along a minor axis which is curved with a total curvature of at least 30°, the electro-active structure comprising successive electro-active portions extending around said minor axis and arranged with electrodes to bend, when activated, around the minor axis such that bending of the successive portions is concomitant with rotation of the electro-active structure about the minor axis adding incrementally along the minor axis.
Such an electro-active device on activation is displaced out of the plane of the curve. On mechanical activation, the displacement creates an electrical signal on the electrodes, and vice versa on electrical activation. The displacement of the electro-active structure is concomitant with the rotation of the structure and can be understood as follows.
The displacement derives from (a) rotation of the structure around the minor axis and (b) the curve of the minor axis along which the structure extends (hereinafter called the major curve, for ease of reference).
The rotation occurs as follows. Because the successive electro-active portions bend around the minor axis, bending of each portion relatively rotates the adjacent portions around the minor axis. In this way bending of the electro-active portions is converted into

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