Patent Document

BACKGROUND OF THE INVENTION 
     This invention relates to bar code reader devices, and more particularly to apparatus for generating a scanned light beam for use in reading bar code symbols. 
     Bar code readers are disclosed in U.S. Pat. Nos. 4,387,297, 4,409,470, 4,251,798, and 4,760,248, all assigned to Symbol Technologies, Inc. The bar code readers described in these patents, as well as other devices of this type that are commercially available, usually employ oscillating mirrors or similar mechanical means to generate a scanning pattern. While such devices are widely used in retail and other business establishments today and have been quite successful in accomplishing their objectives, there is nevertheless a continuing requirement to improve reliability, reduce power consumption, reduce size and weight, lower the parts cost and manufacturing cost, and increase the speed and accuracy of operation. One of the elements of the prior bar code scanners most susceptible to improvement along these lines is the mechanical scanner device. The scanner devices may consist of a mirror mounted on a stepper motor; the mirror includes a flat portion to direct the outgoing laser beam and also a concave portion to collect reflected light and focus it upon a photodetector. 
     Bar code readers employ decoding circuitry to interpret the signals produced by a photodetector receiving the reflected light from the bar code symbol. Conventional decoding schemes rely upon data collected by a single scanning spot moved linearly across the field where the bar code symbol is located. The bar code data is embedded in background noise, and the decoding circuitry is more effective if the signal can be enhanced. To this end, faster scanning rates would permit the implementation of multiple scans to increase reliability of the data collected, but the mechanical scan generators previously used constrict the speed and thus place limitations on the multiple scan approach. 
     It is the principal object of the invention to provide a bar code reader or the like that does not require mechanical devices such as oscillating mirrors to cause a light beam to scan a symbol to be read. Another object is to provide a bar code reader that is capable of faster scan, as by implementing the scan with no moving parts. A further object is to take advantage of fast scanning techniques to provide multiple scans to thereby increase the signal recovery ability, i.e., increase the likelihood of recovering a valid decode of the bar code signal. In addition, the capability of providing multiple scans using a fast scan method permits improved facility for reading two dimensional bar code symbols of the type having multiple rows of bar code patterns. Other objects include reducing the size, weight and power consumption of a laser scan type of bar code reader, as well as reducing the manufacturing cost and increasing the reliability and operating lifetime of such devices. 
     SUMMARY OF THE INVENTION 
     To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, a bar code scanner includes a light source for generating a light beam, a liquid crystal device for splitting light beams, a mirror system for directing the split light beams toward a bar code symbol, and a photodetector for summing the light reflected by the bar code symbol from each of the split light beams and generating an electrical signal for processing to decode the bar code symbol. A microprocessor signals the liquid crystal device to alter the splitting of the light beam when the bar code symbol is not successfully decoded, in an attempt to obtain a successful reading of the bar code symbol. 
     In accordance with another embodiment of the present invention, a method is provided for reading a bar code symbol, comprising the steps of generating and splitting a light beam, directing the light split beams toward a bar code symbol and detecting and summing the light reflected by the bar code symbol from each of the split light beams. An electrical signal, indicative of the summed light, is processed to decode the bar code symbol. If this decoding is not successful, the split light beams are modified for subsequent reading of the bar code symbol. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as other features and advantages thereof, will be best understood by reference to a detailed description of specific embodiments, when read in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a diagram in schematic form of a bar code scanner employing dual linear arrays of light sources instead of a single array as in FIG. 1, according to another embodiment of the invention; 
     FIG. 2 is a timing diagram showing events or voltage vs. time for certain occurrences in the system of FIG. 7 illustrating the cancellation of background noise; 
     FIGS. 3 and 4 are timing diagrams showing events or voltage vs. time for certain occurrences in the system of FIG. 1 illustrating compensation for faults in the bar code; 
     FIG. 4 is a timing diagram showing events or voltage vs. time for certain occurrences in the system of FIG. 1; 
     FIG. 5 is an enlarged view of a bar code symbol being scanned by dual light beams, illustrating permissible tilt; 
     FIG. 6 is a pictorial view of a bar code scanner system corresponding to FIG. 1, employing two scan lines, but using a single light source along with a beam splitter; 
     FIG. 7 shows a hand held housing; 
     FIG. 8 shows a stationary housing; 
     FIG. 9 is a diagram of a bar code symbol scanned with three scan lines; 
     FIG. 10 is a flow chart of the steps for scanning a bar code symbol; 
     FIG. 11 is a diagrammatic view of a bar code scanner corresponding to FIGS. 1 or  6 , employing multiple scan lines, using a single light source along with a liquid crystal device as a beam splitter; and 
     FIG. 12 is a sectional view of the liquid crystal device shown in FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a scanning that uses two linear arrays  10  and  10   b . Use of two arrays  10   a  and  10   b  provides two scan lines  13   a  and  13   b , one above the other, separated from one another by a distance corresponding to the physical separation of the arrays  10   a  and  10   b  and the magnification in the optical system  12 . This dual scan line technique may be advantageously employed in several ways. First, if the two rows of laser diodes  11  in the two arrays  10   a  and  10   b  are activated in parallel, in the same sequence, then the two scan lines  13   a  and  13   b  are likewise in sync: in this case if the two scan lines traverse the same bar code symbol  14 , the reflected light received by the photodetector  15  is also in sync from the two scans  13   a  and  13   b . The advantage of having two scan lines may be understood by reference to FIG. 2, where the background areas  53  are seen to return uncorrelated signals  54 , whereas the bar code symbol returns correlated waveforms from the two parts of the symbol  14  being scanned by the two scan lines  13   a  and  13   b . The single detector  15  collects reflected light from the two scans at the same time and sums the intensities of the reflections, so the contrast of the overall signal  55  detected from the bar code  14  is enhanced. On the other hand, the areas  53  outside the bar code symbol  14  will result in different signals, and so the overall contrast from these areas is reduced. The digitizing circuitry used to shape the analog waveform on the line  17  and recover the bar code information can more readily distinguish the transitions in the bar code region of the signal from the uncorrelated returns from the areas  53 . Referring to FIGS. 3 and 4, another advantage to the dual scans of FIG. 1 is that bar code imperfections can be compensated for. If the bar code symbol  14  has a defect in the form of a gap  57  as seen in FIG. 3, then the signal returned by the scan line  13   b  would have a corresponding false area  58  whereas the return from the scan line  13   a  would be valid. The composite signal  59  on the line  17  at the output of the photodiode  15  would still be able to be interpreted to recover valid data. Similarly, as illustrated in FIG. 4, if the defect is in the form of a black spot  60 , the light return for one scan line will have a false area  61  appearing as if there was a very wide bar in the symbol, but the composite electrical signal  62  representing the sum of both scans  13   a  and  13   b  has distinct transitions and can be decoded to produce valid bar code data. 
     When two scan lines  13   a  and  13   b  are used as illustrated in FIG. 1, the scan lines should be perpendicular to the individual bars of the symbol  14 . The permissible misalignment depends upon the bar code density and the amount of physical separation between the two scan lines  13   a  and  13   b . Referring to FIG. 5, assuming the diameter of the spot in the scan lines  13   a  or  13   b  to be larger than the minimum width D of a bar (or space), the maximum permissible tilt angle α is given by 
     
       
         tanα≈(0.5 D)/L 
       
     
     where L is the separation between the two scan lines  13   a  and  13   b.    
     In FIG. 1 an embodiment of the invention is shown having two arrays  10   a  and  10   b , producing two scan lines  13   a  and  13   b , but the number can be larger than two. Three or more scan lines provide the same types of benefits as just discussed, but to a greater degree. In addition, however, the capability of simultaneously scanning multiple bar code patterns is available. 
     Referring now to FIG. 6, a bar code scanner may produce dual scan lines  13   a  and  13   b  as in FIG. 1, but, according to another embodiment of the invention, the dual scan lines are produced by a single light source  68  emitting a beam  69  that passes through a beam splitter  70  to generate two separate beams  71  and  72 . The two beams are directed to a scanning mirror  73  driven by a motor  74 , from which the beams are directed out through a suitable lens system  75  to impinge upon the bar code symbol  14  as the two scan lines  13   a  and  13   b . The assembly of FIG. 6 may be mounted in a hand-held housing  30  shown in FIG. 7, or as scanner  10  attached to stationary housing  31  in FIG.  8 . The embodiment of FIG. 6 operates to provide improved resolution and decoding as discussed above with reference to FIGS. 2,  3  and  4 . 
     As disclosed above with reference to FIG. 1, the number of scan lines utilized in the embodiment of FIG. 6 is not limited to the two scan lines  13   a  and  13   b  illustrated, but instead may be more than two scan lines  13   a  ,  13   b  and  13   c  or, as illustrated in FIG. 9, a number of scan lines  13   a ,  13   b  and  13   c  may be advantageously used to scan a conventional bar code symbol  14  to obtain greater resolution, fault correction, etc. These three scan lines  13   a ,  13   b  and  13   c  are generated by a single light source using a beam splitter and a mechanical scan mirror as in FIG.  6 . 
     FIG. 10 is a flow chart of the steps for scanning a bar code using the scanner show in FIG. 6. A light beam is generated (step  100 ). The light beam is split using a liquid crystal device (step  110 ). The split light beams are directed toward a bar code using a moveable mirror (step  120 ). The light reflected from the bar code is detected and summed and an electrical signal is produced (step  130 ). This electrical signal is decoded (step  140 ). If the decoding is determined to be successful (step  150 ), the successful decode is output (step  160 ). Otherwise, electrical signals applied to the liquid crystal device are adjusted so that a successful reading of the bar code symbol may be obtained. 
     Referring to FIG. 11, another embodiment of the invention is shown wherein multiple scan lines  13   a ,  13   b ,  13   c  and  13   d  are generated from a single laser source  68  by a liquid crystal device  94 . As in FIG. 6, the beam  69  produced by the laser source  68  is split into multiple beams  95 , and a mechanical device such as an oscillating mirror  73  moves these beams simultaneously across the area of the bar code symbol as the multiple scan lines  13   a , etc. The liquid crystal device  94  has a voltage applied to it by lines  96  and  97 , and the magnitude and frequency of the voltage determines the number of beams  95  produced and thus the number of scan lines. In this manner, the beam  95  may be dynamically changed from a single beam to a split beam, by varying a voltage source  98 . For example, the signal produced on line  17  from the photodetector  15  may be of poor quality using one scan line, but may be decodable using multiple scan lines as discussed above with reference to FIGS. 2,  3  and  4 , so the control program executed by the microprocessor  20  may cause the voltage generator  98  to switch to a multiple scan line condition if a valid decode is not obtained with one scan line. Alternatively, the control program may cause a switch from multiple scan to one scan line if more intensity is needed, since the beam splitting would reduce the level of illumination with a constant output of source  68 . 
     Referring to FIG. 12, the liquid crystal device  94  may comprise two glass plates  99 , each coated with a conductive film, and a liquid crystal material  100  sandwiched between the conductive plates. The material  100  may be, for example, a p-methoxy-benzylidene or p-n-butyl-aniline. 
     The liquid crystal device  94  of FIG. 11 is capable of spitting the beam  69  into more than two beams, and so the voltage source  97  may apply voltages of several levels to produce a selection of the number of scan lines needed, depending upon the conditions detected. 
     Although according to various features of the invention the scan generation may use oscillating mirrors, the embodiments of bar code scanner devices as with no moving parts as described above have several advantages for some purposes, when compared to scanners that use electromechanical components. First, the scan rate can be much faster, so the amount of time the laser is on can be reduced, lowering power drain. The faster speed will also allow a large number of scans to be accomplished for one “read” operation, then the data signals correlated with one another using a suitable algorithm to increase the precision of the decode. Second, the scan pattern can be flexibly adapted, i.e., the scan can be tailored to fit the particular bar code symbol and position: e.g., after an initial scan it is determined that the location and width of the bar code symbol in the field of view is at a specific place, and so the field is re-scanned at only this location, which will use less power. Third, after an initial scan there can be a re-scan of only a small part that showed an ambiguous decode, attempting to get a valid decode of only the troublesome part. Fourth, improvements in reliability can be provided in a device with no moving parts but instead implemented with only electronic components and fixed optical devices. 
     In another aspect, the bar code scanner methods disclosed above wherein multiple scan lines are employed provide other features of importance. The reliability of the decoding process can be enhanced by producing a composite signal from multiple simultaneous scans, where the effects of noise or defects can be minimized. Or, the increased scanning speed permitted by the use of no moving parts allows the multiple scans to be sequential, one line at a time, which allows the reflected light from the multiple scan lines to be separated when using only one photodetector; this arrangement permits scanning of multiple-row bar code symbols or the like. 
     While this invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as other embodiments of the invention and variations in the character of the disclosed or other embodiments, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments, or variations therein, as fall within the true scope of the invention.

Technology Category: g