AU571334B2

AU571334B2 - Focal plane scanning device

Info

Publication number: AU571334B2
Application number: AU43598/85A
Authority: AU
Inventors: Kevin Charles Liddiard
Original assignee: Australian Government
Current assignee: Australian Government
Priority date: 1984-05-24
Filing date: 1985-05-17
Publication date: 1988-04-14
Anticipated expiration: 2005-05-17
Also published as: AU4359885A

Description

FOCAL PLANE SCANNING DEVICE:

Infrared detection systems which operate over the 2 to 20μm wavelength region, and which are required to have an optical resolution of 1 mrad or less, conventionally employ either a 5. germanium refracting objective or a catadioptric objective such as a Schmidt system. The optical system is used in conjunction with a single infrared detector or array of detectors, associated signal processing electronics, and a suitable target 10. indicator device or display.

In order to detect targets over a substantial field of view, the small instantaneous field of view defined by the focal length of the optical system and the size of individual detectors may 15. be optically scanned through the total search field of view. This may be achieved, .for example, by means of rotating optical components which generate a line-by-line rectangular raster scan.

Alternatively, a large number of detectors 20. may be employed without scanning, i.e. a staring focal plane array system. This technique suffers from various limitations arising from the finite spacing between adjacent detectors.

A disadvantage of refractive and catadioptric 25. systems is their high unit cost. Furthermore, optomechanical scanning techniques which employ motor-driven components moving at high speed to generate the desired search field, add to system complexity and overall cost.

30. The object of the present invention is the construction of a low cost optical system which incorporates a novel scanning technique to fill the gaps in optical coverage between adjacent sensors of a focal plane array of infrared detectors.

In accordance with the invention, the optical 5. system is of the Cassegrain type, both the primary and secondary mirrors being constructed of a metal such as aluminium. The secondary mirror is caused to vibrate by means of a piezoelectric drive unit, the angular displacement of this mirror being 10. of sufficient amplitude to scan the optical image by the desired amount at the focal plane.

Piezoelectric drive means for mirror oscillation have been proposed hertofore, see the specification of U.S. Patent No. 4,436,364, and Offenlegungsschrift

15. DE3035314, both by Lauer and Pfefferle, the first relating to a device comprising a pair of coplanar but oppositely actuated piezoceramic strips coupled to a mirror disposed between facing ends, the second relating to a single piezoceramic strip

20. extending to a mirror anchored on a second flexible strip normal to the first strip to give a modular motion, in each case the mirror being carried by the piezoceramic strip or strips.

The invention comprises a focal plane scanning 25. device arranged to move a mirror in an- optical scanning device such as an infrared scanner wherein the mirror is mounted to be angularly displaced by means of a piezoelectric assembly comprising at least a piezoelectric member connected to the 30. said mirror by means of a first flexure member to support the said mirror on a support bracket to allow the mirror to be tilted about a selected axis relative to the said support bracket and at least a piezoelectric drive element carried on the said support and connected to the said mirror, characterised by a pair of piezoceramic 5. drive elements (4-5) arranged generally parallel to the plane of the said mirror (1) each connected at one end to the said support bracket (2) and extending in opposite direction past a first flexure member (3) which supports the said mirror (1),

10. and by flexure members (8-9) one on the other end of each of said piezoceramic drive elements (4-5) connecting that piezoceramic member to the said mirror (1) remote from the said first flexure member (3), whereby a balanced movement of the said

15. mirror (1) about the said first flexure member (3) results when said piezoceramic members (4-5) are oppositely energised.

To enable the invention to be fully understood an embodiment thereof will be described with reference 20. to the drawings in which -

FIG. 1 shows schematically a general arrangement of an embodiment.

FIG. 2 shows details of a piezoceramic drive element in plan (A) and side elevation (B), and

25. FIG. 3 is a central sectional view of the piezoelectric drive unit.

The primary and secondary mirrors are furnished with aspheric reflecting surfaces by means of computer-controlled machining. The quality of 30. these surfaces are such that for an f/1 system having a focal length of 75mm, the angular aberration blur size is less than 1 mrad. The aspheric shape is either machined directly in the aluminium mirror, or follows an electroless nickel deposition onto a rough-machined aluminium mirror. The final 5. surface may be lightly polished and provided with a vacuum-deposited gold reflecting coating. Invar spacer rods are used to maintain precise separation between the primary and secondary mirrors following optical alignment.

10. Subsequent to the machining of the aspheric surfaces, further mirrors may be prepared by estab¬ lished replication processes.

Whilst various configurations of the piezo¬ electric drive unit have been devised, the basic

15. concept can be understood from FIG. 3. The light¬ weight mirror 1 is mounted on bracket 2 by means of a central flexure member 3. Two piezoceramic drive elements 4 and 5 are held in clamps 6 and 7. The opposite ends of the drive elements are

20. connected to dual flexure members 8 and 9 which in turn are connected to the mirror 1 through a load adjustment bracket 10. Application of a voltage to one or both of the drive elements produces a cantilever action which forces the

25. mirror 1 to turn about the axis of the central flexure member 3. An oscillating voltage causes the mirror to vibrate at the frequency of the applied voltage. The flexure members 8 and 9 have opposed notches 11 at each end portion to allow

30. flexing in a required plane. The central flexure member has only two opposed notches 12 to form a pivot normal to the flexure member 3. It is possible with this general arrangement to produce a forced vibration of the mirror at the desired frequency. However, this method of operation may require a large applied voltage, 5. which could necessitate the use of transformers to step-up the voltage from associated power sources.

A convenient technique is to design and construct the mirror assembly in such a fashion that mechanical resonance is achieved at the desired

10. scan frequency. In this manner an oscillating voltage at the mechanical resonant frequency will produce a large mirror deflection at a low drive voltage. Course mechanical tuning is effected by varying mass loading via the load adjustment

15. bracket 10 in FIG. 3, and fine tuning is achieved by adjusting the effective driv*e length of the piezoceramic elements 4 and 5.

The piezoceramic drive elements are comprised of two lead-zirconate-titanate (PZT) rectangular

20. piezoceramic slabs 15 and 16, bonded one on each side of a metal strip 17 as illustrated in FIG. 2. The metal strip may be aluminium, steel or titanium; however, titanium is preferred for reasons of rigidity and lightness. The dimensions

25. of this trilaminar bender design are chosen according to theory to meet the electro-mechanical drive requirements of the scan unit assembly. Simple bilaminar bender designs, consisting of either two piezoceramic slabs or a single slab bonded

30. onto a metal strip, have also been investigated, but were found to be less suitable than the trilaminar construction. The mirror can be made to vibrate by applying voltages of appropriate phase to both piezoceramic elements in parallel. However, a convenient arrange¬ ment is to drive one element only. The second 5. element then provides mechanical compensation and produces an electrical reference signal for the associated electronic drive circuit.

Scan units constructed as described herein have been operated at frequencies from 50 to 1000 Hz.

10. For a resonant system mechanically tuned to 100 Hz, the typical angular mirror deflection is -_l/3 mrad per volt (rms). Since a scan amplitude in the range 0.5 to 5 mrad would satisfy most requirements concerned with focal plane detector arrays, the drive voltage

15. can be readily derived from microelectronic circuits.

It should be understood that optical systems of the Cassegrain type have been widely u-s-ed. for many years, and methods of generating aspheric surfaces in metal mirrors are well know. The

20. trilaminar piezoceramic bender design has also been previously described. This invention deals with the construction of a low cost optical system, used in conjunction with the novel scan technique described herein to produce maximum optical coverage

25. at the focal plane for large arrays of infrared detectors. It may be noted, however, that the scan concept is not restricted to the simple optical arrangement shown in FIG. 1, but can be employed with other optical designs, and with systems operated

30. in the visible region of the spectrum. From the foregoing it will be seen that the invention comprises an optical system specifically of the Cassegrain type wherein one of the optical components is caused to vibrate by virtue of a 5. piezoelectric drive unit as described herein, thereby causing the optical field of view to execute an oscillating motion at the focal plane.

A feature is an optical scan technique wherein a mirror or lens component of the optical 10. system is caused to vibrate by means of piezoceramic bender elements operated in cantilever fashion and connected to the said component via flexure members, such vibration producing an image scan motion in the focal plane of the optical system.

r-5. Such an optical system can be used with an array of infrared detectors positioned at the focal plane.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

1. A focal plane scanning device arranged to move a mirror in an optical scanning device such as an infrared scanner wherein the mirror is mounted to be angularly displaced by means of

5. a piezoelectric assembly comprising at least a piezoelectric member connected to the said mirror by means of a first flexure member to support the said mirror on a support bracket to allow the mirror to be tilted about a selected axis

10. relative to the said support bracket and at least a piezoelectric drive element carried on the said support and connected to the said mirror characterised by a pair of piezoceramic elements (4-5) arranged generally parallel to the plane of the said mirror (1)

15. each connected at one end to the said support bracket (2) and extending in opposite directions past a first flexure member (3) which supports the said mirror (1), and by flexure members (8-9) one on the other end of each of said piezoceramic elements (4-5) connecting

20. that piezoceramic element to the said mirror (1) remote from the said first flexure member (3), whereby- a balanced movement of the said mirror (1) about the said first flexure member (3) results when at least one of said piezoceramic members (4—5) is energised.

2. A focal plane scanning device according to Claim 1 wherein the said mirror (1) is held against an adjustment bracket (10) and located by a central flexure member (3) secured to a bracket ⁵- (2) forming the said support, and the said piezo¬ ceramic elements (4 and 5) are rigidly held at their one said end to the said bracket (2) by clamps (7 and 8) and are connected at their other said ends by flexure pivots (8 and 9) to the said adjustmen 10. bracket (10).

3. A focal plane scanning device according to claim 2 wherein the said flexure members (8-9) comprise rods threaded at each end and having opposed notches (11) near each end adapted to 5. allow flexing in a plane normal to the axis of the said rod, one of said flexure members (8) being apertured to allow the other piezoceramic element to project therethrough.

4. A focal plane scanning device according to Claim 1, 2 or 3 characterised in that the mirror (1) and adjustment bracket (10) are arranged to have a mechanical resonance at the desired scan 5. frequency whereby to minimise drive required by the said piezoceramic elements (4-5).

5. The method of activating a focal plane scanning mirror as defined in Claim 1 wherein one of the said piezoceramic elements only is electrically driven and the other said piezoceramic drive element is arranged

5. to provide mechanical compensation and optionally produces an electrical reference signal for the associated electronic drive circuit.

6. The method of actuating a focal plane scanning mirror as defined in Claim 1 wherein both of the said piezoceramic elements are electrically driven but in opposite phase.