GB2251497A

GB2251497A - A reciprocating laser light deflector for optical bar code readers

Info

Publication number: GB2251497A
Application number: GB9124277A
Authority: GB
Inventors: Landi Giuliano
Original assignee: HAL 9000 INFORMATICA Srl
Current assignee: HAL 9000 INFORMATICA Srl
Priority date: 1990-11-15
Filing date: 1991-11-15
Publication date: 1992-07-08
Also published as: GB9124277D0; DE4137713A1; IT9001222A1; IT1242638B; IT9001222A0; FR2672712A1

Abstract

A combination of a flexible blade 16 of material which displays the phenomenon of piezoelectricity - this blade being provided with a mirror surface 17 on at least one part, and being made to vibrate in a reciprocating manner by an electrical generator connected to it - and a laser generator 1 produces a coherent light scanning line on a reading surface 18. <IMAGE>

Description

-I- 2251497 A RECIPROCATING LASER LIGHT DEFLECTOR FOR OPTICAL BAR CODE

READERS The invention relates to a reciprocating laser light deflector, suitable for optical bar code readers in which, during operation, the coherent light beam emitted by a laser generator repeatedly scans along a line the surface onto which it is directed and which carries the information to be read, expressed by the bar code.

At the present time, bar code readers emit a laser beam which scans by repeated reciprocating scanning the reading surface on which is recorded the information which is read by means of the ref lection of the laser beam onto an optical receiver, the beam being reflected when it strikes a light surface and being absorbed when it strikes a dark surface, such as a black line with a sharp outline like the lines of bar codes.

The reciprocating scanning by the laser beam of the reading screen on which is recorded the information to be read is achieved in two ways: the first makes use of a miniature motor whose output shaft, carrying a mirror, describes an arc of reciprocating motion. The laser beam projected onto the said mirror is reflected, with the angular displacement of the mirror, in continuously varying directions, and therefore describes a continuous line on a screen in one direction and then in the opposite direction when the rotary motion of the mirror is reversed.

The second method makes use of a small motor whose output shaft rotates with a uniform circular motion and causes to rotate a parallelepiped with polygonal bases and mirror outer surfaces onto which the beam emitted by the laser generator is projected. The laser generator and the small motor are positioned with respect to each other in such a way that, with the rotation of the parallelepiped,, the mirror surface struck by the laser beam, as it rotates, causes the direction of reflection of the beam to vary, and the reflected beam is displaced on the screen, describing a continuous line.

Then, following a change in the surface of the polygon struck by the laser beam, the variation in direction is repeated, again causing the point of incidence of the light beam on the screen to be displaced along the same line.

These methods haVe the disadvantage of requiring the use of highly expensive, delicate and easily broken miniature electric motors, making the operation of the optical bar code readers unreliable over a period of time, and finally, in the first case, requiring a device for the reversal of the motion of the motor,and, in the second case, requiring a miniature parallelepiped with mirror outer surfaces for reflection of the laser beam in variable directions, which contribute to an increase in the cost of the optical reader making use of it.

The object of the present invention is to provide a reciprocating laser light deflector capable of eliminating the disadvantages of the currently known deflectors mentioned above.

The invention, as characterised in the claims, solves the problem of creating a reciprocating laser light deflector without rotation and hot making use of miniaturised motors which result in high costs and interruption of operation, but rather makes use of an easily produced and reliably operating device by means of which a plurality of advantages are obtained in optical bar code readers making use of such a device. In particular, - the dimensions of the reader making use of this device may be reduced; - its construction is simplified and its cost is reduced; its fragility is decreased; - the reliability operation is increased; and the'-reading frequency and therefore the frequency of the reciprocating motion of the laser beam projected onto the'surface on which the bar code is reproduced may be varied in any way.

The invention which enables these advantages to be obtained consists of the combination of a flexible blade of material which displays the phenomenon of piezoelectricity - this blade being provided with a mirror surface on at least one part. and being made to vibrate in a reciprocating manner by an electrical generator coniected to it - and a laser generator. so that the blade, when struck by the light beam on its mirror surface, causes the reflection of the beam in continuously variable directions, producing a coherent light scanning line on a reading surface where the information to be read, expressed by a bar code, is provided.

The invention is described in detail below with the aid of the illustrative drawings on the attached sheet, in which Fig. 1 shows in diagrammatic form the laser generator, the deflector unit and the screen or reading surface; Fig. 2 is also a diagrammatic representation-of the invention, comprising an electrical accumulator, an alternating current generator, the deflector unit, the laser generator, the laser power supply unit, and their electrical connections; - Fig. 3 is the plan view of a deflector unit with a central elastic blade carrying a reflective strip and two blades with piezoelectric properties.

However, it should be clearly understood that the drawings and the corresponding parts described are provided solely by way of illustration of the object of the invention and do not in any way constitute a restriction thereof.

In the drawings, 1 indicates the laser generator, 2 indicates the electrical power supply unit of the laser 1, 3 indicates the supporting structure holding the laser generator 1. its power supply unit 2 and carrying elec tri " cal terminals 4, 5, 6, and 7 f or their electrical connections and for connection to the electrical accumu lator 10 which supplies the power for the whole optical reader of which the invention forms part. 8 and 9 indi cate the terminals of the accumulator 10, 14 indicates - 4 the generator producing the non-constant electrical signal which is applied to the plates 15 of the deflector unit, 16 indicates the flexible blade of material displaying the phenomenon of piezoelectricity, 17 indicates a nirror coating capable of reflecting the coherent light beam emitted by the generator 1, and 18 indicates the screen or surface on which the information to be read is reproduced. 19 and 20 indicate two terminals capable of keeping the laser generator 1 in position and 21 indicates its negative terminal. The reciprocating laser light deflector comprises two electrically conductive plates, between which is located an elastic blade of piezoelectric material, the plates being connected to an electrical generator capable of making the electrical potential of the said plates non-constant and causing a variation of their polarisation with time. The potential difference may be a sinusoidal voltage, a square wave voltage, or a sawtooth voltage, but in any case is such that it causes forces of a non-constant mechanical type on the flexible and elastic blade 16 so that the said blade is deflected in a variable manner in the free part not held between the terminals, which are not shown in Figs 1 and 2. and which support the deflector unit. This unit may comprise one or more elastic blades (16) of piezoelectric material, each carrying, on opposite surfaces, coatings of electrically conducting material connected to the electrical generator 14 with a non-constant voltage at the terminals. The conducting coatings or blades may wholly or partially cover each blade (16) which, when subjected to electrical forces, starts mechanical vibration, its motion depending both on its mechanical characteristics, its dimensions, weight and thickness, and on the form, intensity and frequency of the electrical signal applied.

The,movement of the mirror 17, or of the equivalent mirror surface on the blade 16 or on the plate 15 which covers it on one of its lateral surfaces. may thus be finely regulated with respect to frequency and amplitude so that the laser beam emitted from the generator 1 and reflected onto the screen 18 describes a line with a rectilinear reciprocating motion thereon to permit repeated reading of the information shown thereon by means of a bar code. For this purpose, the generator 1, the deflect, or unit 15, 16, 17, the reading surface 18 and the photosensitive 'receiver of the optical reader, which is not shown in the drawings since it does not fall within the scope of the invention, will be positioned with respect to each other in such a way that the sequence of light pulses of the desired length and frequency is received.

In the embodiment shown in Figure 2, the electrical accumulator 10 has its negative terminal 8 earthed and the positive terminal 9 connected to the switch 11 which is capable of de-energising the reader. The output terminal of this switch is connected to the positive terminal 12 of the electrical generator 14 at whose output terminals the variable potential difference to be applied between the plates 15 is established, and to the terminal 4 on the supporting structure 3 connected to the positive input terminal of the laser power supply unit 2, while the positive output terminal 6 of this unit is connected to the positive input terminal 7 of the laser generator 1. When the switch 11 is closed, the generator 14 supplies power to the plates 15 and the blade 16 starts to vibrate. The laser power supply unit 2 supplies power to the generator 1 which emits the coherent light beam directed onto the mirror 17, or onto an equivalent mirror surface, which has a flat or profiled structure according to the reflection effects which are to be obtained, and for the purposes of correction of the scanning speed of the laser beam on the screen 18 and, for the reflected beam, correction of the duration of pulses of light reflected onto the photosensitive receiver not shown in the drawings.

The reciprocating deflector unit constituting the distinctive part of the invention may in practice be produced in a number of ways and in all cases makes use of the mechanical reaction of a material displaying the phenomenon of piezoelectricity when two of its facing surfaces are subjected to an electrical force, following which they bend to cause the 'deflection of a light beam falling on them.

An embodiment considered to be advantageous is shown by way of example in Figure 3 in which 30 indicates a flexible and elastic blade which may be made of any material, 31 indicates a flat mirror coating, 22 and 23 indicate two strips of material with piezoelectric properties, and 24 and 25 indicate the surfaces of electrically conducting material deposited on, or in any way adhering to, the facing surfaces of the strips 22 and 23. 26 and 27 indicate two supports, preferably of insulating material, 28 indicates the insulating or protected bolt which holds together in sandwich form the blade 30 and the strips 22 and 23 towards one of their ends. while 32 and 33 indicate the terminals connected to the conducting surfaces 24 and 25, between which is applied the variable potential difference, particularly in a sinusoidal form capable of making uniform the deflection of the blades 22 and 23 which transmit it to the blade 30, without causing abrupt variations and stresses in the material. The two strips 22 and 23. each having its lateral surfaces covered with a conducting layer 24 and 25 and acting as the plate of a capacitor, are arranged parallel to one another and held together only at one end by the supports 26 and 27, while the remaining parts, in contact with a larger or smaller portion of the elastic blade 30, transmit, as they are charged and discharged, electrical forces to those parts of strips 22 and 23 which are free to oscillate.

The elastic strip 30 has applied to it the mirror 31 which is thus caused to vibrate, thereby producing the variation in the direction of reflecti on of the laser beam projected onto it by the generator 1. This reflected beam is displaced on the reading surface 18 along a line, with a reciprocating motion. thereby permitting repeated reading of the information provided on the said surface and expressed by a bar code.

k 1 - 7 The mirror 17, or the equivalent mirror surface, will then be dimensioned or in any case in contact with the coherent beam over a suf f icient area to provide scanning of the whole length of the surface 18 on which the bars of the code are provided, in such a way that the variations of tangential velocity of the reflection point on the mirror 17 may be compensated f or in the program controlling the reading of the reflected pulses. It is to be understood that the deflector unit is not restricted to the embodiment indicated in Figure 3, but comprises all equivalent solutions in which a mirror surface on an elastic member is caused to vibrate, using materials with piezoelectric properties by applying to the surfaces of these materials a non-constant potential difference so that a laser light beam projected onto the said surface is reflected in variable directions and thereby repeatedly describes a line on a reading surface, consequently producing a sequence of reflected light pulses as a result of the presence of the dark lines of the bar code which absorb the incident beam.

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Claims

1. A reciprocating laser light deflector for optical bar code readers in whichl. during operation. a coherent light beam emitted by a laser generator repeatedly scans along a line the surface onto which it is directed and which carries the information to be read, expressed by the bar code, characterised by a combination of a flexible blade of material which displays the phenomenon of piezoelectricity, a mirror surface associated with the blade such that the blade and the mirror are made to vibrate in a reciprocating manner by an electrical generator connected to the blade, and a laser gene-pator, so positioned that when the mirror surface is struck by the laser light beam, the vibrating blade causes the reflection of the beam to produce a coherent light scanning line on a reading surface.

2. A reciprocating laser light deflector according to Claim 1, wherein the mirror surface is on at least a part of the f lexible blade and the f lexible blade of piezoelectric material is located between two electrically conducting plates, the plates being connected to an electrical generator capable of applying to the plates an electrical potential which varies with time.

3. A reciprocating laser light deflector according to Claim 1, wherein the mirror surface is on at least a part of the flexible blade, and the flexible blade of piezoelectric material carries on its opposite surfaces coatings of electrically conducting material which are connected to an electrical generator which produces a non-constant voltage.

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4. A reciprocating laser light deflector according to any one of Claims 1 to 3, wherein the mirror surface ---isa flat surfce.

5. A reciprocating laser light deflector according to any one of Claims 1 to 3 wherein the mirror surface is a profiled structure capable of reflecting the light beam directed onto it by the laser generator.

6. A reciprocating laser light deflector according to any one of Claims 1 to 5, wherein the mirror surface is formed on an integer affixed to a part of the flexible blade of piezoelectric material.

7. A reciprocating laser light deflector according to Claim 1, wherein the flexible blade of piezoelectric material has its opposite surfaces covered with conducting material, the conducting surfaces are connected to a generator of non-constant electric potential, a further flexible blade, which carries the mirror surface, is located adjacent to the flexible blade of piezoelectric material so as to receive the vibratory motion of the flexible blade of piezoelectric material.

8. A reciprocating laser light deflector according to Claim 9, wherein a portion of the further flexible blade is sandwiched between a pair of flexible blades of piezoelectric material each having their opposite surfaces covered with conducting material, the respective conducting surfaces of the said pair of flexible blades being connected in parallel to the generator of nonconstant electric potential.

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9. A reciprocating laser light deflector according to any one of the preceding claims, wherein a support ----holds the flexible blade or blades of piezoelectric material at one end of the said flexible blade or blades and the other end or ends of the said flexible blade or blades are free to vibrate.

10. A reciprocating laser light deflector according to Claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.

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