GB2289773A

GB2289773A - Piezoelectric scanner

Info

Publication number: GB2289773A
Application number: GB9510536A
Authority: GB
Inventors: Duncan Rowe
Original assignee: GEC Marconi Ltd; Marconi Co Ltd
Current assignee: BAE Systems Electronics Ltd
Priority date: 1994-05-25
Filing date: 1995-05-24
Publication date: 1995-11-29
Anticipated expiration: 2015-05-24
Also published as: GB9510536D0; GB2289773B; GB9410439D0; DE19519161A1; FR2720521B1; FR2720521A1

Description

1 PIEZOELECTRIC SCANNER 2289773 This invention relates to a piezoelectric

scanner, and particularly to such a scanner for scanning a mirror.

Methods for scanning an optical mirror by means of piezoelectric actuators are known.

An example of such a scanner is shown in Figure 1. It consists of four piezoelectric plate elements 1 attached via a flexible hinge 2 to a mirror 3. Each plate element 1 consists of two piezoelectric plates attached such that a voltage applied to the centre contact will cause the plate element to bend. Two axis motion is possible over relatively small scanning angles.

In general the larger the scanning angle required, the larger the plate elements must be, and as a result, to scan large angles the plate elements must often be larger in area than the mirror being scanned. It is often undesirable for the plate element mechanism to cover a greater area than the mirror aperture, due to shortage of space within the optical system and particularly when several such scanners are to be formed into an array. This invention allows larger scanning angles within a limited area.

This invention provides a piezoelectric scanner comprising a member to be scanned, a pair of piezoelectric plate elements extending transversely to the member and acting at one end on a mounting element connected to the member, the plate elements being C arran-ed to bend such that their ends are urged towards or away from the mounting c element when volta Z:1 ge is applied, the positions along the mounting element at which the 2 P160168.GI1PC ends of the plate elements act being offset to cause the member to pivot when the ends are urged towards or away from each other.

The transverse arrangement of the piezoelectric plate elements permits larger angular deflections of the member to be achieved in an arrangement in which the plate elements do not extend beyond the lateral dimensions of the surface eg as would be desirable in the case of an array of elements, since the length of the elements would not be restricted by the lateral dimensions of the surface in that case.

The member to be scanned may be a mirror.

In order to allow the member to be able to pivot about two orthogonal axes, a second pair of plate elements may be provided.

Advantageously, the ends of the plate elements act on the mounting element via resilient members eg of silicon rubber. The mounting element may be a post extending from the rear of the surface. Spacers may be interposed between the ends and the resilient members to permit the use of wider plate elements in a situation where two pairs of plate elements are employed.

Each plate element is desirably composed of two layers of piezoelectric material, one of which expands and the other of which contracts when voltage is applied.

Embodiments of the invention will now be described by way of example with reference 1 11 3 to the accompanying diagrammatic Figures in which:

Figure 2a is a sectional plan view of one form of piezoelectric scanner; Figure 2b is a side view of the scanner of Figure 2a; and Figure 3 is a side view of another form of piezoelectric scanner.

P/60168.GBPC Referring to Figures 2a and 2b, four piezoelectric plate elements 4, 5, 6 and 7 are aligned perpendicular to a mirror 8 which is mounted onto a mounting element 9. Each plate element consists of a pair of piezoelectric plates, one of which expands and the other of which contracts when voltage is applied. The result of this is that each plate bends when voltage is applied. This element 9 is attached to respective corners of the plate elements 4-7 via resilient members 10-13 of silicon rubber. Plate elements 4 and 6 are shown in the side view of Figure 2b; for clarity, plate elements 5 and 7 are not shown in this diagram. The resilient members 10 and 12 attached to the plate elements 4 and 6 respectively are connected at different heights on the mounting element 9. The plate elements 4-7 are the same length and so the base 14 to which they are attached is made so that each plate element penetrates the same depth of base material.

The plate elements 4 and 6 are arranged so that when voltage is applied, the ends acting on the mountin., element 9 bend either towards or away from the mounting element, depending upon the sign of the voltage. The resulting opposing forces set up a turning CI c moment about a point equidistant from the points of action of the forces on the mounting 4 P/60168.GBP^C element. Thus, the nfirror 8 turns about pivot point 15, along the axis 16 of Figure 2a. When voltage is applied to the pair of elements 5 and 7, the ends acting on the mounting element 9 similarly bend towards or away from the mounting element according to the sign of the applied voltage and this action causes the mirror 8 to pivot about axis 17. Thus angular motion of the mirror 8 about either axis or both axes in combination can be achieved.

Figure 3 shows an alternative, one-axis scanner, and in this embodiment only two plate elements 18 and 19 are present and these may be as wide as the mirror 27. Each is attached to mounting post 20 by means of resilient members 21, 22 which are fixed to the ends of rigid spacer members 23, 24. The provision of spacer members permits ease of manufacture of the scanner, as the other ends of the plate elements may be simply fixed to the sides of the base 25. In the scanner of Figure 3, application of voltage to the plate elements 18 and 19 causes the ends mounted to the mounting element 20 to move either towards or away from the mounting element, causing the mirror to tilt about the point 26 as before, the motion of the mirror being along one axis only.

The use of rigid spacer members permits an additional pair of plate elements to be used.

These may be attached to the other two sides of the base 25. The use of these spacer elements permits wider plate elements to be used than for the embodiment of Figures 2a, 2b. The advantage of this is that the volume of the piezoelectric plates is increased, thus increasing the stiffness of the plate elements and thus the speed of operation. Hence the mirror can be scanned more rapidly.

The half scanning angle is given by the equation P160168.GWIMRC E) 3d31 V1 2 4ha 2 where d3, is the piezoelectric charge coefficient of the piezoelectric material, V is the voltage applied, 1 is the active length of the plate elements, a is the thickness of the piezoelectric material and h is half the distance between the flexible members.

Taking the values d3, = 250 x 10712 m/V, Voltage +80V, 1 = 15mm, a = 0. 2min and h l mm we have an expected angle of T.

The resonant frequency of the scanner is given by:

4 = 1 E 211 1 where I is the moment of inertia of the mirror and K, the force constant of the piezoelectric plate elements is given by the equation:

K = 16a'bh 2y 3 13 where Y is the Youngs modulus of the piezoelectric material and b is its width. Taking a 10 x 10 x 1 mm silicon mirror, with moment of inertia I = 1 x 1 ffiKgm', b = 7mm and Youngs modulus Y = 7 x IONInn2, the resonant frequency will be 125Hz.

The scanners will produce some linear vertical motion as well as the scanning motion, 6 but this can be calculated for the above scanners to be less than 30 microns.

P/60168.GBPC The scanners described, thus have large scanning motion ( Y for a voltage of 8OV), a high resonant frequency (125Hz) and their drive mechanisms are completely contained within the area of the mirror aperture. The depth of the scanners, including mirror and mounting, could be made to be less than 25mm, giving a total volume of 3. 6 x 106 d.

Ideally, in order to produce maximum turning moment, the part of the mounting element on which the plate elements act should be thin. This can be achieved either by making the extending post-like part of the mounting element as thin as possible or by cutting into this part of the mounting element and inserting the resilient members into the resulting spaces so that the points of action on the mounting element of the forces produced by the plate elements and the pivot point lie in a straight line, perpendicular to the plane of the mirror.

It will be clear that the scanner mechanisms described could be used to scan devices other than mirrors, for example laser diodes could be used. Whether the scanner carries a mirror or a laser diode however, the invention is particularly applicable to an array of mirrors (or laser diodes etc), because the deflecting mechanism (the plate elements) does not extend beyond the projection of the surface area of the mirror etc. A particular vantage of the invention is that the transverse arrangement of the plate elements permits elements to be used which exceed the lateral dimensions of the mirror. Further alternatives may be made without departing from the scope of the invention. For instance, Figures. -2a, 2b and 3 show the piezoelectric plate elements mounted j A 7 P/60168.GBPC perpendicularly to the plane of the mirror, however each plate element could be mounted obliquely.

8 P/60168.GBPfMRC

Claims

1. A piezoelectric scanner comprising a member to be scanned, a pair of piezoelectric plate elements extending transversely to the member and acting at one end on a mounting element connected to the member, the plate elements being arranged to bend such that their ends are urged towards or away from the mounting element when voltage is applied, the positions along the mounting element at which the ends of the plate elements act being offset to cause the member to pivot when the ends are urged towards or away from each other.

2. A piezoelectric scanner as claimed in claim 1, in which the member is a mirror.

3. A scanner as claimed in claim 1 or claim 2, in which the plate elements are connected to the mounting element by means of resilient members.

4. A scanner as claimed in any one of claims 1 to 3, in which rigid spacer members are interposed between the plate elements and the resilient members.

5. A scanner as claimed in any one of claims 1 to 4, in which the plate elements each consist of two layers of piezoelectric material.

Ir 6. A scanner as claimed in any one of claims 1 to 5, in which the plate elements are mounted perpendicularly to the member to be scanned.

9 P/60168.GBP^C 7. A scanner as claimed in any one of claims 1 to 6, in which a further pair of plate elements extending transversely to the member is provided.

8. A scanner substantially as hereinbefore described with reference to Figures 2a, 2b or 3 of the accompanying drawings.

9. An array of scanners, each scanner being as claimed in any one of claim 1 to 8, wherein the members to be scanned are arranged in an array.