Abstract:
Vibration is minimized in an electro-optical reader for reading indicia by oscillating a counterweight and a scan component to move simultaneously toward each other, and to move simultaneously away from each other. A magnet is mounted for joint movement with the counterweight and the scan component. An electromagnetic coil magnetically interacts with each magnet to cause the counterweight and the scan component to move with oppositely directed moments that counterbalance each other.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention generally relates to electro-optical systems for reading indicia, for example, bar code symbols, having parts with different light reflectivities and, in particular, to an arrangement for, and a method of, reducing vibration in the system during symbol reading. 
   2. Description of the Related Art 
   Various electro-optical readers and systems have previously been developed for reading bar code symbols appearing on a label, or on a surface of a target. The bar code symbol itself is a coded pattern of indicia. Generally, the readers electro-optically transform graphic indicia of the symbols into electrical signals which are decoded into alphanumeric characters. The resulting characters describe the target and/or some characteristic of the target with which the symbol is associated. Such characters typically comprise input data to a data processing system for applications in point-of-sale processing, inventory control, article tracking and the like. 
   The specific arrangement of symbol elements, e.g., bars and spaces, in a symbol defines the characters represented according to a set of rules and definitions specified by a code or symbology. The relative size of the bars and spaces is determined by the type of code used, as is the actual size of the bars and spaces. 
   To encode a desired sequence of characters, a collection of element arrangements is concatenated to form the complete symbol, with each character being represented by its own corresponding group of elements. In some symbologies, a unique “start” and “stop” character is used to indicate where the symbol begins and ends. A number of different bar code symbologies presently exists. The symbologies include one-dimensional codes such as UPC/EAN, Code 39, Code 128, Codabar, and Interleaved 2 of 5. 
   In order to increase the amount of data that can be represented or stored on a given amount of symbol surface area, several new symbologies have been developed. One new code standard, Code 49, introduced a two-dimensional concept of stacking rows of elements vertically instead of extending elements horizontally. That is, there are several rows of bar and space patterns, instead of one long row. The structure of Code 49 is described in U.S. Pat. No. 4,794,239. Another two-dimensional code structure known as PDF417 is described in U.S. Pat. No. 5,304,786. 
   Electro-optical readers have been disclosed, for example, in U.S. Pat. No. 4,251,798; No. 4,369,361; No. 4,387,297; No. 4,409,470, No. 4,760,248 and No. 4,896,026, all of which have been assigned to the assignee of the present invention. These readers generally include a light source consisting of a gas laser or semiconductor laser for emitting a light beam. The use of semiconductor devices as the light source in readers is especially desirable because of their small size, low cost and low power requirements. The laser beam is optically modified, typically by a focusing optical assembly, to form a beam spot having a certain size at a predetermined target location. Preferably, the cross-section of the beam spot at the target location approximates the minimum width between symbol regions of different light reflectivity, i.e., the bars and spaces. 
   In conventional readers, the light beam is directed by a scan component along a light path toward a target symbol. The reader operates by repetitively scanning the light beam in a scan pattern, for example, a line or a series of lines across the target symbol by movement of the scan component such as a mirror disposed in the path of the light beam. The scan component may sweep the beam spot across the symbol, trace a scan line across and beyond the boundaries of the symbol, and/or scan a predetermined field of view. 
   Readers also include a sensor or photodetector which functions to detect light reflected or scattered from the symbol. The photodetector or sensor is positioned in the reader in an optical path so that it has a field of view which extends at least across and slightly beyond the boundaries of the symbol. A portion of the light beam reflected from the symbol is detected and converted into an analog electrical signal. A digitizer digitizes the analog signal. The digitized signal from the digitizer is then decoded, based upon the specific symbology used for the symbol, into a binary data representation of the data encoded in the symbol. The binary data may then be subsequently decoded into the alphanumeric characters represented by the symbol. 
   The scan pattern that scans the symbol can take a variety of forms, such as repeated line scan, standard raster scan, jittered raster scan, fishbone, petal, etc. These beam patterns are generated by controlled motions of the scan component in the beam path. Typically, the scan component is driven by some form of scanning motor to periodically deflect the beam through the desired beam scanning pattern. For a repeated line scan beam pattern, a polygonal mirror unidirectionally rotated by a simple motor can be utilized. For more complex beam patterns, more involved drive mechanisms are required. 
   The frequency at which the beam pattern is executed is also an important consideration. The more times a symbol can be scanned in a given time period, the chances of obtaining a valid read of the symbol are increased. This is particularly important when the symbols are borne by moving objects, such as packages traveling on a conveyor belt. 
   Many applications call for a hand-held reader where a user aims the light beam at the symbol, and the beam executes a scan pattern to read the symbol. For such applications, the arrangement of electro-optical components must be compact in order to be accommodated in a hand-held package which may be pistol-shaped. Moreover, such readers must be lightweight and structurally robust to withstand physical shock resulting from rough handling. It is also desirable that minimal power be consumed during operation to promote battery usage. 
   The repetitive movement of the scan component generates vibrations which are propagated through a support on which the scan component is mounted and, in turn, to the reader itself. When the reader is held by the user, these vibrations are transmitted to the user&#39;s hand and are objectionable, especially after long term usage of the reader. Moreover, depending on the scan rate, the vibrations can generate audible hum, which is likewise annoying after prolonged operation. 
   SUMMARY OF THE INVENTION 
   Objects of the Invention 
   One object of this invention is to provide an improved arrangement for and method of reducing vibration in a reader for reading a data-encoded symbol. 
   Another object of this invention is to provide an arrangement which is compact, lightweight, durable and efficient in construction and quiet in operation, and thus is ideally suited for portable hand-held applications. 
   FEATURES OF THE INVENTION 
   In keeping with these objects and others which will become apparent hereinafter, one feature of this invention resides, briefly stated, in an arrangement for, and a method of, reducing vibration in a reader for electro-optically reading indicia, such as one-and/or two-dimensional bar code symbols. 
   The invention provides a support, preferably a printed circuit board, a movable scan component for scanning the indicia, and a movable counterweight. Both the scan component and the counterweight are mounted on the support for oscillating movement about respective first and second axes of oscillation. 
   In accordance with this invention, a drive is operative for minimizing vibrations caused by the oscillating scan component from being transmitted to the support and, in turn, to the reader. The drive includes a first permanent magnet mounted on the scan component for joint movement therewith, a second permanent magnet mounted on the counterweight for joint movement therewith, and an energizable electromagnetic coil mounted between the magnets and operative, when energized by a periodic drive signal, to magnetically interact with the magnets and cause the scan component and the counterweight to move simultaneously toward each other and to move simultaneously away from each other. The turning effect or moment of the force used to move the scan component in one direction is counterbalanced by the moment used to move the counterweight in the opposite direction. 
   Thus, the moment that the scan mirror exerts on the support during oscillation is counterbalanced by the opposite moment that the counterweight exerts on the support. Preferably, this is achieved by making the weights of the scan component and the counterweight equal, as well as the perpendicular distances of the scan component and the counterweight to their respective axes of oscillation. A symmetrical mounting is obtained by fixing the scan component and the counterweight on respective resilient arms of a mounting element fixed to the support. In a rest position, the arms are generally parallel. During reading, the arms move to planes that intersect each other. Not only is vibration reduced, but the arrangement is quieter in operation. 
   The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a hand-held reader for reading a bar code symbol in accordance with the prior art; 
       FIG. 2  is a side elevation view of a vibration reducing arrangement in accordance with one embodiment of this invention for use in the reader of  FIG. 1 ; and 
       FIG. 3  is a top plan view of a vibration reducing arrangement in accordance with another embodiment of this invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference numeral  20  in  FIG. 1  generally identifies a hand-held reader for electro-optically reading indicia, such as bar code symbol  24 , located in a range of working distances therefrom. The reader  20  has a pistol grip handle  21  and a manually actuatable trigger  22  which, when depressed, enables a light beam  23  to be directed at the symbol  24 . The reader  20  includes a housing  25  in which a light source  26 , a light detector  27 , signal processing circuitry  28 , and a battery pack  29  are accommodated. A light-transmissive window  30  at a front of the housing enables the light beam  23  to exit the housing, and allows light  31  scattered off the symbol to enter the housing. A keyboard  32  and a display  33  may advantageously be provided on a top wall of the housing for ready access thereto. 
   In use, an operator holding the handle  21  aims the housing at the symbol and depresses the trigger. The light source  26  emits a light beam which is optically modified and focused by optics  35  to form a beam spot on the symbol  24 . The beam passes through a beam splitter  34  to a scan mirror  36  which is repetitively oscillated at a scan rate of at least  20  scans a second by a motor drive  38 . The scan mirror  36  reflects the beam incident thereon to the symbol  24  and sweeps the beam spot across the symbol in a scan pattern. The scan pattern can be a line extending lengthwise along the symbol along a scan direction, or a series of lines arranged along mutually orthogonal directions, or an omnidirectional pattern, just to name a few possibilities. 
   The reflected light  31  has a variable intensity over the scan pattern and passes through the window  30  onto the scan mirror  36  where it is reflected onto the splitter  34  and, in turn, reflected to the photodetector  27  for conversion to an analog electrical signal. As known in the art, the signal processing circuitry  28  digitizes and decodes the signal to extract the data encoded in the symbol. 
   In accordance with this invention, the drive  38  is configured as shown in  FIG. 2. A  support  40 , preferably a printed circuit board, is mounted within the housing  25 . A mounting element  42  has a base  44  fixed to the support by a fastener  46 , and a pair of upright resilient arms  50 ,  52  movable about respective first  54  and second  56  axes of oscillation. Mounting element  42  is preferably constituted of a metallic spring steel, but other resilient materials including plastic could be employed. In the illustrated solid line position, the arms  50 ,  52  are parallel. In the preferred embodiment, the arms  50 ,  52  are planar leaf springs. 
   The scan mirror  36  is exteriorly mounted on arm  52  and, as shown, reflects a light beam  48  incident thereon toward the incidia  24  to be scanned and read. A permanent magnet  58  is interiorly mounted on arm  52  and is jointly oscillatable with the mirror  36  about the axis  56  in the circumferential directions represented by double-headed arrow  60 . 
   In accordance with this invention, a counterweight  62  preferably having a weight equal to the weight of mirror  36  is exteriorly mounted on arm  50 . A permanent magnet  64  is interiorly mounted on arm  50  and is jointly oscillatable with the counterweight  62  about the axis  54  in the circumferential directions represented by the double-headed arrow  68 . 
   An electromagnetic coil  66  is mounted between the magnets  64 ,  58 . Coil  68  has a central passage  70  extending along a coil axis  72 . The magnets  64 ,  58  are mounted on, and axially spaced apart along, the coil axis  72 . The magnets  64 ,  58  are oriented so that opposite poles face each other. The magnets  64 ,  58  have respective magnetic axes coaxial with the coil axis  72 . 
   The coil  66  is energized by a periodic drive signal, for example a sinusoidal or sawtooth waveform, conducted along wires  74 ,  76  to create an alternating magnetic field which interacts with the permanent magnetic fields produced by the permanent magnets to cause the magnets  64 ,  58  and, in turn, the arms  50 ,  52 , as well as the counterweight and the mirror  36 , to simultaneously move toward each other, and thereupon to simultaneously move apart from each other. 
   It is generally known in the art for the scan mirror  36  to be oscillated by the interaction of magnetic fields produced by a coil and a magnet. However, this oscillation induces vibration in the support  40  which, in turn, is propagated to the housing  25  of the reader. A vibrating hand-held reader is uncomfortable to hold. Also, depending on the frequency of vibration, objectionable noise can be heard. 
   The motion of the counterweight tends to reduce the vibration and noise problems of the art. The turning effect or moment of the force used to move the mirror  36  in one direction is counterbalanced by the turning effect or moment of the force used to move the counterweight in the opposite direction. The combined weight of the counterweight and magnet  64  on the one hand matches the combined weight of the mirror and the magnet  58  on the other hand. Also, the perpendicular distances of the respective forces to the axes  54 ,  56  are preferably the same. 
   It will be understood that this invention achieves vibration reduction by generating countermoments that balance each other. Hence, the weight of the counterweight need not equal the weight of the mirror, but the perpendicular distance to the axis  54  will have to be adjusted so that the product of the force exerted on the mirror multiplied by its perpendicular distance to the axis  56  matches the product of the force exerted on the counterweight multiplied by the perpendicular distance to the axis  54 . 
   The energized coil  66  need not actively pull and push each magnet  64 ,  58 . It is sufficient for the energized coil to either pull or push each magnet, whereupon energy stored in each resilient arm  50 ,  52  will be released upon deenergization of the coil to enable oscillation to continue. In other words, the energized coil can actively pull the magnet  58  toward the left in FIG.  2  and is then deenergized. The flexed arm  52  is now free to return to its rest position, but in doing so, will overshoot its rest position and move toward the right in  FIG. 2 , and again overshoot its rest position by moving toward the left. The deenergized coil is now reenergized, at which time, the cycle repeats. 
   The scan component  36  need not be a flat mirror as illustrated, but could be a different optical component such as a lens or an aperture stop, or could be a differently shaped mirror, such as a concave reflector, or could be a light source such as a laser diode which directly emits a laser beam. In addition, the scan component  36  need not be located solely in the outgoing path of a light beam being directed to indicia  24 , but could be located in the return path of light scattered off the indicia, in which case, the scan component is sweeping the field of view of the photodetector  27 . Of course, the scan component  36  can be located in both the outgoing and return paths as shown in FIG.  1 . 
   The distance through which each magnet moves depends upon the scan angle through which the light beam and/or the field of view must be steered. If necessary, the magnets can enter the opposite open axial ends of the passage  70 . 
   In the side view of  FIG. 2 , each arm oscillates about axes  54 ,  56  that are parallel to the planar support  40 .  FIG. 3  depicts a top view of a different embodiment in which the scan component  36  and the counterweight  62  oscillate about axes that are perpendicular to the planar support  40 . 
   Thus, as shown in  FIG. 3 , the scan mirror  36  and the counterweight  62  are mounted at opposite end regions of the coil  66  energized via electrical cables  74 ,  76  by a periodic drive signal as described above. In contrast with  FIG. 2 , the mirror and counterweight are not mounted on arms configured as planar leaf springs, but instead are mounted on torsion elements  80 ,  82  which extend along torsion axes perpendicular to the support  40 . The torsion elements  80 ,  82  are configured as resilient wires having a circular cross-section and are twistable about their respective torsion axes. 
   Also, a first magnet  84  is jointly mounted with the counterweight on the wire  80 , and a second magnet  86  is jointly mounted with the mirror on the wire  82 . The magnetic axes of the magnets  84 ,  86  is perpendicular to the coil axis  72  in contrast to the  FIG. 2  embodiment where the magnetic axes of the magnets  64 ,  58  were colinear with the coil axis  72 . 
   In operation, energization of the coil  66  causes the magnets  84 ,  86  and, in turn, the wires  80 ,  82 , as well as the counterweight and the mirror to twist in the directions indicated by the double-headed arrows  88 ,  90 . As before, the moment produced by the twisting of the mirror is counterbalanced by the moment produced by the twisting of the counterweight in the opposite direction, thereby minimizing propagation of vibration and noise as described above. 
   The wires  80 ,  82  can be discrete elements, or can be interconnected by a base  92  that is fixed to the support  40 . Alternatively, the base  92  can be secured directly to a casing of the coil  66 . 
   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 vibration reducing arrangement in electro-optical readers, 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.