Multidirectional optical scanner.

This invention generally relates to a multi­directional optical scanner for multidirectionally scanning a light-reflecting target, such as a bar code symbol, and/or the field of view of a detector and, more particularly, to an arrangement for, and a method of, generating a multidirectional scan pattern for read­ing randomly-oriented bar code symbols, as well as for changing the size and/or the shape of the scan pattern.

Targets having indicia of different light reflectivity, such as bar code symbols, have been scanned by directing light along an optical path to a symbol located in the vicinity of a reference plane lying gen­erally normal to the optical path. The light reflected off the symbol is detected by a detector having a finite field of view. The symbols and/or the field of view have been scanned with many different scanning patterns in the reference plane. The pattern has been a single scan line, or a set of generally parallel scan lines extend­ing in one direction lengthwise along the symbol. See U.S. 4,251,798 or 4,387,297. The pattern has also been two sets of intersecting parallel lines extending in two directions relative to the symbol. See U.S. 4,369,361.

In the case of randomly-oriented symbols, it has been proposed to produce a dense scanning pat­tern extending in multiple scanning directions in order to maximize the probability that the symbol will be quickly read, no matter what its orientation. Thus, conventional point-of-sale (POS) scanners, of the type generally found in supermarkets that have been built into and underneath check-out counters, deflect a laser beam in several directions and read symbols oriented in different directions that move across the reference plane, i.e. in the plane of or slightly above the countertop. To deflect the laser beam, a central mir­rored polygon surrounded by a ring of many auxiliary mirrors is mounted below the countertop, each auxiliary mirror corresponding to a different scan line. Such constructions occupy a great deal of space and are easily accommodated under a supermarket countertop where a large amount of space is readily available. See U.S. 3,978,317.

It has also been proposed to generate a Lissa­jous scanning pattern in the reference plane. A pair of scanning elements, one deflecting a laser beam in the X-direction, and the other deflecting the laser beam in the Y-direction, are sinusoidally driven simultane­ously to form a closed curved scan pattern. Although curved, the Lissajous pattern includes an interior central portion characterized by generally linear scan lines es­sentially orthogonally intersecting each other to form an X shape, and exterior portions characterized by sharply curved scan lines having small radii of curvature. Since such lightly curved scan lines are not useful for symbol reading, they are typically cut off, usually by sizing the exit window through which the outgoing laser beam passes en route to a symbol so as to permit passage therethrough of only the interior central portion of the Lissajous pattern, but to block the exterior por­tions thereof.

In order to generate truly multidirectional scan patterns, the use of holographic scanners was pro­posed in an article entitled "Multidirectional Holo­graphic Scanner for Point-Of-Sale Bar-Code Symbol Reader", published in Optical Engineering, Nov.-Dec. 1984, Vol. 23, No. 6, p. 784ff. A holographic disc was rotated at one speed, and a laser beam incident on the disc was reversely rotated at another speed. The re­sultant outgoing beam had multidirectional scan lines. However, in the context of designing bar code symbol readers which are hand-held, or mounted on a support surface, where the available space is at a premium, and where it is desired to keep the overall size, weight and volume of the reader to a minimum, holographic discs are not practical. The holographic disc of said article is 200 mm in diameter -- too big to provide the compact­ness desired in many applications for portable scanners; has an optical path of 350 mm -- too long for many ap­plications; and has a scan angle of ± 10° -- too inade­quate to achieve the compactness and the length of the scan desired in many applications.

In infrared search systems, complex scan pat­terns were used very widely in the past. In those systems, for example, see U.S. 4,039,246, the field of view of a detector was accurately scanned across space at high speeds in a rosette-type pattern across the detector in a gyro-optical system mounted within a radiation-seeking missile. Convex and inclined planar reflectors were ro­tated to generate the scan pattern. However, such mis­sile-mounted gyro-optical systems for detecting intruder aircraft in large areas of space were too big and complex to be used in bar code reader systems where the symbol is typically in a working range several inches, or, in some cases, several feet, from the system.

Other types of complex scan patterns were gen­erated by prism scanners comprising a pair of optical wedge-shaped prisms positioned in front of a detector, each prism rotated relative to the other. A light beam passing through each prism was refracted at each air-­ prism interface. However, chromatic and other optical aberrations were often encountered with refractive pris­matic elements and were largely uncorrectable. Also, the prismatic elements were only useful in certain ef­fective wavelength regions. It was difficult to produce a wide angle of deflection. The mounting of each prism on a rotating element, and the drive for each rotating element were complex. These drawbacks rendered the prism scanners particularly unsuitable for bar code symbol reading applications.

Other optical code readers of which the ap­plicants are aware are U.S. Pat. Nos. 3,663,800; 4,282,431; 3,718,761; 3,684,867; 4,493,989.

In all of the above-described scan pattern generators, the size and configuration of the scan pat­tern were not readily changeable, and certainly not during scanning. Applications exist, particularly in bar code symbol readers, where it would be desirable to either manually or automatically change the size and/or configuration of the scan pattern either prior to, or during, use.

It is a general object of this invention to overcome the drawbacks, and advance the state of the art, of optical scanners, particularly bar code symbol readers.

It is another object of this invention to change the zize and/or configuration of a multidirec­tional scan pattern.

Another object of this invention is to readily change the size and/or configuration of the scan pattern either prior to, or during, use of the scanner.

A further object of this invention is to op­timize the size and/or configuration of the scan pattern.

Still another object of this invention is to enable either the user and/or the manufacturer of the scanner to readily change the size and/or configuration of the scan pattern.

Yet another object of this invention is to provide a scanner of the moving light beam type, wherein a scanning light beam spot is always in the scan pattern and travels at a non-zero velocity during scanning of a symbol.

Another object of this invention is to provide a scanner of the moving light beam type, wherein the scanning light beam spot has a velocity variation during scanning which is less than that normally encountered in known scan patterns.

Still a further object of this invention is to generate a scan pattern having an N-fold symmetry and a 100% scanning efficiency, the pattern being equally and highly dense over the field of view.

Yet a further object of this invention is to reliably read height-truncated or stationary symbols by precessing a scan pattern about an axis generally perpendicular to the plane in which the symbol to be read lies.

A further object of this invention is to pro­vide a multidirectional scan pattern generator of simple, miniature, lightweight, rugged construction.

In keeping with these objects, and others which will become apparent hereinafter, one feature of this invention resides, briefly stated, in a multidi­rectional optical scanner for, and a scanning method of, multidirectionally scanning either a light-reflecting target located in the vicinity of a reference plane and/or a field of view of light reflected from the tar­get, by forming a multidirectional scan pattern in the reference plane. The size and configuration of the scan pattern are separately or simultaneously changeable.

In a preferred embodiment wherein the target is a bar code symbol, a source of light, e.g. a laser or a light emitting diode, or even a non-laser light source, generates a light beam which is directed along an optical path with first and second optical elements, e.g. light-­reflecting mirrors, preferably front surface planar mirrors, or prisms, to the symbol for reflection there­from. The light scattered and reflected off the symbol is directed in countercurrent direction along the optical path and is sensed by a detector, e.g. a photodiode, over a field of view.

In accordance with this invention, first ro­tator means are provided for mounting the first element at a first tilt angle of inclination relative to a first axis, and for rotating the first element about the first axis at a first angular speed. Second rotator means are also provided for mounting the second element at a second tilt angle of inclination relative to a second axis, and for rotating the second element about the second axis at a second angular speed. As the first and second inclined elements rotate, the light beam extending along the path between the elements describes a generally conical sur­face, also called a light cone. The light deflected by the first element describes a circle in a plane perpen­dicular to the optical path. Every point of deflected light which impinges on the second element serves as the point of origination of another light cone. The net motion of a light beam directed onto two such rotating elements generates a multidirectional scan pattern in the reference plane whose size and configuration are a function of the velocities of each angular speed, the magnitudes of each tilt angle, and the direction of ro­tation of each element. By changing said velocities and/or said magnitudes and/or said rotation directions, the size and configuration of the pattern can be ad­justed as desired.

Each rotator means can be implemented by a reversible or unidirectional electrical motor having an output drive shaft on which a respective optical element, e.g. a planar mirror, is transversely mounted relartive to the axis along which the shaft extends. One motor with a transmission to rotate both mirrors could also be employed. A brushless DC motor and a hysteresis synchron­ous motor are examples of suitable rotator means.

Instead of the second rotator means, a recip­rocating scanner element having a linear scan mirror of the type described and claimed in U.S. Pat. No. 4,496,831, the entire contents of which are incorporated herein by reference, can be substituted to move the light cone impinging on the scan mirror along a scan line. In this case, if the locus on the scan mirror is a circle, then the scan pattern on the reference plane will be a plurality of circles arranged in an overlapping rela­tionship along a linear row.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:- <ul id="ul0001" list-style="none"><li>FIG. 1 is a diagrammatic view of a preferred embodiment of the invention;</li><li>FIGs. 2 - 11 are multidirectional scan pattern examples generated by the embodiment of FIG. 1;</li><li>FIG. 12 is an alternative tilt angle adjusting arrangement for use with the embodiment of FIG. 1; and</li><li>FIG. 13 is a diagrammatic view of another preferred embodiment of the invention.</li></ul>

Referring to FIG. 1, reference numeral 10 generally identifies a multidirectional optical scan­ner for generating a multidirectional scan pattern 12 in a reference plane. Other multidirectional scan patterns are shown and discussed below in connection with FIGs. 2-11.

In a first embodiment, scanner 10 includes a light source, e.g. a gas laser, a semiconductor laser diode, a light emitting diode, etc. for generating light. The light is directed along an optical path to a first planar front surface mirror 16 from which the light is reflected along a first path portion of length Z1 to a second planar front surface mirror 18 from which the light is again reflected along a second path por­tion of length Z2 to a reference plane located away from the second mirror 18. The reference plane lies in a plane generally perpendicular to the optical path. However, for ease of illustration, the reference plane has been shown to lie in the plane of FIG. 1, whereas, in actuality, the reference plane lies in a plane nor­mal to the plane of FIG. 1.

Also, in the preferred embodiment, the path portion Z2 is many times longer than path portion Z1 such that the inter-mirror path portion Z1 is, as a practical matter, negligible. In FIG. 1, the path portion Z1 is shown to have a non-negligible length, but this was done merely to better illustrate the invention.

In analogous manner, rather than sweeping the symbol 20, the field of view of a detector is swept in the scan pattern. Hence, in another embodi­ment, the reference numeral 14 identifies a detector, e.g. a photodiode or analogous light sensor operative for sensing light of variable intensity reflected off the symbol 20, and for generating an electrical signal indicative of the sensed light. In this case, reflec­ted light is collected along path portion Z2 (in a direction opposite to the illustrated arrows) for im­pingement on second mirror 18 and, thereupon, the im­pinging light is reflected along path portion Z1 (again in a direction opposite to the illustrated arrows) for impingement on first mirror 16 and, thereupon, the light is reflected toward the detector 14.

Hence, either the symbol itself, or the field of view of the detector, is scanned with the scan pat­tern 12. In still another embodiment, both the symbol and the field of view are simultaneously scanned, and this simultaneous scanning is employed in retroreflec­tive scanners.

In order to generate the scan pattern, ac­cording to one embodiment, the first 16 and second 18 mirrors are mounted on first and second rotator means at respective tilt angles A, B, and are rotated about respective axes C, D at respective angular speeds ω₁, ω₂.

Specifically, a reversible or unidirectional electrical motor 22 under the control of a speed con­troller 24 is operative to rotate output shaft 26 in either circumferential direction at angular speed ω₁ about axis C. A support 28 is mounted at one end of shaft 26 for joint rotation therewith. The support has an outer inclined end on which the first mirror 16 is mounted, e.g. by adhesion, for joint rotation. The first mirror 16 is inclined at a first tilt angle of inclination A.

Similarly, a reversible or unidirectional electrical motor 30 under the control of a speed con­troller 32 is operative to rotate output shaft 34 in either circumferential direction at angular speed ω₂ about axis D. A support 36 is mounted at one end of shaft 34 for joint rotation therewith. The support has an outer inclined end on which the second mirror 18 is mounted, e.g. by adhesion, for joint rotation. The second mirror 18 is inclined at a second tilt angle of inclination B.

In operation, the light beam reflected by the rotating first tilted mirror 16 describes a coni­cal surface in space. Since the mirror 16 is tilted at angle A, the half-angle of the light cone extending between the mirrors is 2A. If the second mirror 18 were oriented normal to the optical axis of path por­tion Z1, then the locus of the light cone on the second mirror would be a circle. However, since the second mirror 18 is oriented at an angle to the optical axis of path portion Z2, the locus on the second mirror is an ellipse. Each point of the ellipse on the second mirror now serves as the point of origination of an­other cone. The net motion of both rotating mirrors produces multidirectional scan patterns, representa­tive ones of which are shown in FIG. 2-11. As de­scribed below, the pattern itself is a function of the angular speeds, the directions of rotation, and the magnitudes of the inclination angles, as well as the length of the optical path.

For ease of description, the letter N denotes the ratio of the angular speed of the second mirror to that of the first mirror; the letter K denotes the ratio of the magnitude of the inclination angle of the second mirror to that of the first mirror; a + sign in front of the letter N indicates that both mirrors rotate the beam in the same direction; and a - sign in front of the let­ter N indicates that both mirrors rotate the beam in oppo­site directions; and the letter Z indicates the total distance along the optical path from the first mirror to the reference plane (Z=Z1+Z2; Z2&gt; &gt;Z1).

If N is positive and an integer, then the scan pattern is closed on itself and is characterized by interior loops, the number of loops being (N+1). The longest dimension of the pattern P=(4A+4B)Z. At­tention is drawn to FIGs. 2 and 3 for the cases where N=+2 and +5, and where K=1.

If N is negative and an integer, then the scan pattern is still closed, but is characterized by exterior petals, the number of petals being (N+1). The longest dimension of the pattern P=(4A+4B)Z. Attention is drawn to FIGs. 3 and 4 for the cases where N=-2 and -5, and where K=1.

Changing the magnitude of the tilt angles, in turn, changes the overall size and longest dimension of the pattern. Attention is drawn to FIGs. 6 and 7 for the cases where N=-2 and -5, and where K=2. Hence, by doubling the tilt angle ratio, as compared to FIGs. 4 and 5, the overall longest dimension of the scan pattern is correspondingly increased.

If N is not an integer, then the pattern is not closed, but is open.

FIG. 8 shows the case where N=-2.25 and K=2. The scan is open, i.e. does not close upon itself as in FIGs. 2-7. Note that points P1 and P2 representing the beginning and end points of the pattern are spaced apart. During the first revolution of the slower mirror 16 or 18, the scan starts at point P1, traces the pattern shown in FIG. 8 and ends at point P2. During the sec­ond revolution of the slower mirror, the same scan pattern is traced, this time the entire scan pattern being precessed about an axis normal to the reference plane by an angular dis­tance equal to the fractional part of N multiplied by 360°. Thus, in this case where the fractional part = .25, then the angular distance of the precessing equals .25 x 360° = 90°. Hence, it will take four revolutions of the slower mirror to close the pattern, and this is shown in FIG. 11. FIG. 10 shows the pre­cessed pattern after the third revolution of the slower mirror, and FIG. 11 shows the precessed pattern after the fourth revolution of the slower mirror.

As described earlier, the size of the scan pattern is determined by the tilt angles of the mirrors and the distance between the second mirror and the symbol. For small tilt angles, the size of the scan pattern equals (4A+4B)Z. For example, if A=B=4° (0.07 radians) and if Z=7", the size of the scan pattern is 4.4" which is slightly longer than the longest bar code symbol.

The tilt angles can be fixed or adjustable. For example, a set of supports similar to supports 28, 36 can be provided, with each support having an inclined end pre-set to a different angle. A user would then select the appropriate support for a particular appli­cation.

Otherwise, as shown in FIG. 12, mirror 16 is adjustably set to any desired tilt angle by turning adjusting screw 40 whose free end bears against one side of a rear surface of mirror 16. The screw is threaded in a radial flange 42 mounted on a support 28′. The opposite side of the rear surface of the mirror 16 is connected to one leg 44 of a hinge whose other leg 46 is mounted on the cylinder 28′.

The speed controllers 24, 32 are conventional speed control systems. If motors 22, 30 are DC motors, then their speeds can be very accurately varied by the application of a variable control voltage. With con­stant excitation or energization of the field windings of a shunt motor, the speed is largely dependent on the voltage of the field current supplied to it. By means of a field rheostat, the voltage of the current supplied to the motor can be adjusted to obtain "infinitely vari­able" speed control and also to reverse the direction of rotation, this being done by reversing the direction of the field current.

Other than such analog-type controls, digi­tal-type controllers can also be employed to cause the motors to run at discrete speeds, each speed determined by the magnitude of a control voltage or current. Such discrete speeds can be switched in and out of op­eration by one or more switches advantageously mounted on the system and within ready access of a user. For example, the manual actuation of one momentary action switch on a hand-held head in a laser scanning system can cause the speeds and rotation directions to be set to generate the precessing scan pattern depicted in FIGs. 8-11, and the manual actuation of another such switch on the head can cause the generation of the scan pattern of FIG. 7. The choice of the pattern depends on the application. Of course, rather than the manual actuation of switches, the scan patterns can be auto­matically generated in a predetermined sequence. For example, a microprocessor can be programmed to automat­ically change the speeds and rotation directions in a certain sequence at the onset of symbol reading. Thus, the patterns of FIGs. 2-7 could be successively gener­ated in sequence, or in any order, or with any combina­tion of individual patterns repeated until eventu­ally the symbol has been successfully scanned and read.

The linear speed of a beam spot traveling along any of the above scan patterns is an important system parameter in symbol reading applications because it determines the response time of the signal process­ing and decoding electronic circuitry operative for processing the electrical signals generated by the de­tector into data descriptive of the bar code symbol. The spot speed is a complex function of the angular speeds ω₁, ω₂, the tilt angles A,B and the distance Z. For the above-described multidirectional patterns, the spot speed changes sinusoidally as the spot travels along a pattern. The linear spot speed due to each mirror is determined by: V(inches/sec) = 2 π ω (rev/sec) R (inches) where R is the radius of a circle produced by either mirror.

If V₁ and V₂ are the linear spot speeds pro­vided by first mirror 16 and second mirror 18, respec­tively, then the maximum speed V_MAX within the pattern is (V₁+V₂) and the minimum speed V_MIN within the pattern is (V₂-V₁). Assuming that V₂&gt; &gt;V₁, the average spot speed is V₂.

As a numerical example, assume that ω₂=60 rev/sec and ω₁=15 rev/sec. Further, assume that each mirror describes a circle of radius equal to 1.5 inches, and that the mirrors counterrotate. The scan pattern will have (N+1) =5 petals. The size of the scan pattern will be about 6 inches. The spot speed V₁=141 in/sec. The spot speed V₂=565 in/sec. The average spot speed is V₂=565 in/sec. V_MAX is V₁+V₂=706 in/sec. V_MIN is V₂-V₁=424 in/sec.

The above numerical example represents a preferred laser scanning application for reading bar code symbols using relatively low speed analog signal processing and decoding circuitry. The above spot speed variation between V_MAX and V_MIN is less than that normally encountered in linear scan or Lissajous scan patterns.

Typically, the higher the spot speed, the more complex and expensive will be the signal process­ind and decoding circuitry since the circuitry must respond to the maximum spot speed. The lower spot speed achieved by this invention compared to the known art permits simpler, less expensive electrical circuitry to be employed. Also, the average spot speed for a given number of frames per second is less according to this invention than in the known designs.

Also, the two-mirror scanner of this inven­tion has no "dead" time. The spot is always in the scan pattern. There are no locations at which the beam spot speed equals zero, as is the case for scan­ners of the type described in U.S. Pat. Nos. 4,387,297 or 4,409,470 or 4,369,361, for example.

In principle, all the portions of the scan pattern can be used for decoding. As shown in FIG. 7, for example, there are portions of the pattern with relatively straight and gently curved lines useful for decoding provided the radius of curvature of the line is such that the line covers at least one-half of the symbol.

In addition, as shown in FIG. 7, the scan pattern of this invention has an N-fold symmetry, and a 100% scanning efficiency. The pattern is equally and highly dense over the field of view. There are fewer non-useful, tightly curved pattern portions, as was the case for Lissajous patterns. There are more useful, gently curved pattern portions having large radii of curvature, each capable of scanning a symbol and/or the field of view of a detector.

The two-mirror scanner shown in FIG. 1 is a non-complicated, lightweight, miniature arrangement for generating fairly complex multidirectional scan patterns. It can be packaged in a hand-held head for reading bar code symbols (as shown, for example, in U.S. 4,387,297 or 4,409,470), or in a desk-top-work­station (as shown, for example, in U.S. 4,369,361). It can be easily packaged underneath a countertop in a retail, point-of-sale application. It can be easily packaged in a workstation whose head is supported on an adjustable gooseneck-type support above a work surface in a scan-above or side-scan mode.

In a modification of the FIG. 1 embodiment, FIG. 13 shown another embodiment 50 for generating a scan pattern 52, which is identical to embodiment 10, except that the second rotator has been replaced by a scanner element of the type described in U.S. 4,496,831, the entire contents of which are incorporated by ref­erence herein. It will be noted that a second planar, front surface, mirror 54 is mounted on, and in a plane parallel to, an output shaft 56 of a reciprocating motor 58. The mirror 54 is respectively oscillated in the directions of the double-headed arrow. The ellipse formed on mirror 54 is then moved linearly across the symbol 20 between the two end traces shown.

The scan patterns generated by this invention can be used not only for bar code reading applications, but also in search systems, visual entertainment systems, precision pointing, etc.

Rather than mirrors, prisms or analogous optical elements arranged to direct the light beam along the optical path may be utilized.

The precessing pattern, which rotates in space about an axis generally perpendicular to the plane in which the symbol lies (see FIGs. 8-11), is of particular advantage in those applications where the symbol is stationary, or where the height of the symbol is truncated. The space-rotating pattern mini­mizes the tendency for the symbol not to be scanned, since during the rotation of the pattern, the likeli­hood increases that at least one of the scan lines of the pattern will cross the symbol. This is to be con­trasted with Lissajous patterns which have a trans­lating wave characteristic.

It will be understood that each of the ele­ments described above, or two or more together, also may find a useful application in other types of con­structions differing from the types described above.

While the invention has been illustrated and described as embodied in a multidirectional optical scanner, it is not intended to be limited to the de­tails shown, since various modifications and structur­al changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly consti­tute essential characteristics of the generic or speci­fic aspects of this invention and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be pro­tected by Letters Patent is set forth in the appended claims.