Abstract:
Disclosed is a method and apparatus for preventing condensation from forming on the internal components of an optical scanner, allowing the scanner to be moved between hot and cold environments without sacrificing scanning performance or damaging the scanner. The optical scanner can be a laser scanner or a solid state imager. One method of preventing moisture from condensing on the inside of a scanner is to control the internal temperature of the scanner. This is accomplished by installing a thermostatically controlled heater module on the inside of the scanner and warming up the scanner&#39;s internal air when the air temperature falls below a predetermined point. Another method of preventing moisture from condensing on the inside of the scanner is to hermetically seal the scanning enclosure and fill with inert gas.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of optical scanners. More specifically, the invention relates to an optical scanner, such as a bar code scanner, which can operate in extreme thermal conditions without performance degradation due to condensation and frost. 
     BACKGROUND OF THE INVENTION 
     Electro-optical scanners, such as bar code symbol scanners, are now quite common. Typically, a bar code symbol comprises one or more rows of light and dark regions, typically in the form of rectangles. The widths of the dark regions, i.e., the bars, and/or the widths of the light regions, i.e., the spaces between the bars, when partitioned into groups, indicate encoded information to be read. 
     A bar code symbol reader illuminates the symbol and senses light reflected from the coded regions to detect the widths and spacings of the coded regions and derive the encoded information. Bar code reading type data input systems improve the efficiency and accuracy of data input for a wide variety of applications. The ease of data input in such systems facilitates more frequent and detailed data input, for example to provide efficient inventories, tracking of work in progress, etc. 
     A variety of scanning systems are known. One particularly advantageous type of reader is an optical scanner which scans a beam of light, such as a laser beam, across the symbols. Laser scanner systems and components of the type exemplified by U.S. Pat. Nos. 4,387,297 and 4,760,248—which are owned by the assignee of the instant invention and are incorporated by reference herein—have generally been designed to read indicia having parts of different light reflectivity, i.e., bar code symbols, particularly of the Universal Product Code (UPC) type, at a certain working range or reading distance from a hand-held or stationary scanner. 
     FIG. 1 illustrates an example of a prior art bar code symbol reader  10  implemented as a gun shaped device, having a pistol-grip type of handle  53 . A lightweight plastic housing  55  contains a light source  46 , a detector  58 , optics  57 , signal processing circuitry  63 , a programmed microprocessor  40 , and a power source or battery  62 . An exit window  56  at the front end of the housing  55  allows an outgoing light beam  51  to exit and an incoming reflected light  52  to enter. A user aims the reader at a bar code symbol  70  from a position in which the reader  10  is spaced from the symbol, i.e. not touching the symbol or moving across the symbol. 
     As further depicted in FIG. 1, the optics may include a suitable lens  57  (or multiple lens system) to focus the scanned beam into a scanning spot at an appropriate reference plane. The light source  46 , such as a semiconductor laser diode, introduces a light beam into an optical axis of the lens  57 , and other lenses or beam shaping structures as needed. The beam is reflected from an oscillating mirror  59  which is coupled to a scanning drive motor  60  energized when a trigger  54  is manually pulled. The oscillation of the mirror  59  causes the outgoing beam  51  to scan back and forth in a desired pattern. 
     A variety of mirror and motor configurations can be used to move the beam in a desired scanning pattern. For example, U.S. Pat. No. 4,251,798 discloses a rotating polygon having a planar mirror at each side, each mirror tracing a scan line across the symbol. U.S. Pat. Nos. 4,387,297 and 4,409,470 both employ a planar mirror which is repetitively and reciprocally driven in alternate circumferential directions about a drive shaft on which the mirror is mounted. U.S. Pat. No. 4,816,660 discloses a multi-mirror construction composed of a generally concave mirror portion and a generally planar mirror portion. The multi-mirror construction is repetitively reciprocally driven in alternative circumferential directions about a drive shaft on which the multi-mirror construction is mounted. 
     The light  52  reflected back by the symbol  70  passes back through the window  56  for transmission to the detector  58 . In the exemplary reader shown in FIG. 1, the reflected light reflects off a mirror  59 , passes through an optical filter  47  and impinges on the light sensitive detector  58 . The filter is typically designed to have a band-pass characteristic in order to pass the reflected (return) laser light and block the light coming from other optical sources. The detector  58  produces an analog signal proportional to the intensity of the reflected light  52 . 
     The signal processing circuitry includes a digitizer  63  mounted on a printed circuit board  61 . The digitizer processes the analog signal from detector  58  to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars. The digitizer serves as an edge detector or wave shaper circuit, and a threshold value set by the digitizer determines what points of the analog signal represent bar edges. The pulse signal from the digitizer  63  is applied to a decoder, typically incorporated in the programmed microprocessor  40  which will also have associated program memory and random access data memory. The microprocessor decoder  40  first determines the pulse widths and spacings of the signal from the digitizer. The decoder then analyses the widths and spacings to find and decode a legitimate bar code message. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. This may also include an initial recognition of the particular standard to which the scanned symbol conforms. This recognition of the standard is typically referred to as autodiscrimination. 
     To scan the symbol  70 , the user aims the bar code reader  10  and operates movable trigger switch  54  to activate the light source  46 , the scanning motor  60  and the signal processing circuitry. If the scanning light beam  51  is visible, the operator can see a scan pattern on the surface on which the symbol appears and adjust aiming of the reader  10  accordingly. If the light beam  51  produced by the source  46  is marginally visible, an aiming light may be included. The aiming light, if needed produces a visible-light spot which may be fixed, or scanned just like the laser beam  51 . The user employs this visible light to aim the reader at the symbol before pulling the trigger. 
     The reader  10  may also function as a portable data collection terminal. If so, the reader would include a keyboard  48  and a display  49 , such as described in the previously noted U.S. Pat. No. 4,409,470. 
     In electro-optical scanners (readers) of the type discussed above, the laser source, the optics, the mirror structure, the drive to oscillate the mirror structure, the photodetector, and the associated signal processing and decoding circuitry can all be packaged in a “scanning module”, which in turn is placed into the scanner&#39;s, or terminal&#39;s housing. 
     Another type of an optical scanner is a solid state imaging (SSI) reader. The SSI reader will typically contain a solid state detector, such as a charge coupled device (CCD) or a metal-oxide semiconductor field effect transistor (MOSFET). Imaging scanners do not use a moving spot to illuminate an indicia being read, but instead flood illuminate the target, or use flood illumination from external sources, and detect the reflected light from at least a portion of the illuminated target. 
     Both, the scanning-type readers and the SSI-type readers have found wide acceptance in the retail, wholesale and industrial applications, such as point-of-sale, warehouse and manufacturing operations. Bar code readers are typically specified to operate in non-condensing environments. Certain applications require the reader to operate over a wide temperature range, for example from −30 to 50 degrees Celsius. In certain situations, such as when an operator must use the scanner inside and outside of a freezer, moving the scanner back and forth between hot to cold locations creates condensing environment around and inside the scanner. Condensation can form on both, exterior and interior surfaces of the scanner. The reader&#39;s exit window is particularly susceptible to condensation and frost. External condensation on the exit window can disable scanning by altering the outgoing scan beam. Internal condensation on the optical elements can do the same, and the condensation on the electronic components can cause short circuits and catastrophic failure of the device. Thus, the result of operating a scanner in a condensing environment can vary from a non-read or a miss-read of a scanned item, to a damaged device. 
     The need exists for a scanning system which can be operated in a condensing environment. 
     The need also exists for a scanning system which can be operated in cold and hot environments without degradation in performance due to frost and condensation. 
     The need also exits for an indicia reader which is not susceptible to condensation and frost, and allows for an uninterrupted operation during movements between the hot and cold environments. 
     The need also exists for a bar code reader which is not susceptible to condensation and frost, and allows an uninterrupted operation during movements between the hot and cold environments. 
     The need also exists for a laser-type bar code reader which is not susceptible to condensation and frost, and allows an uninterrupted operation during movements between the hot and cold environments. 
     The need also exists for a SSI type reader which is not susceptible to condensation and frost, and allows an uninterrupted operation during movements between the hot and cold environments. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an optical scanner which is not susceptible to condensation and frost and can be moved from hot and cold environments without any down time. 
     In one preferred embodiment, the applicants invented a heater module which is used to keep the scanner&#39;s internal ambient temperature within a set range. As the scanner is moved from hot to cold environments and the scanner&#39;s internal ambient temperature starts to fall, the heating element inside the heater module gets turned on. A re-circulating fan, also located inside the heater module, forces the air inside the scanner to flow over the heating element. The heated air is blown over the scanner&#39;s internal components, warming them up and preventing condensation. As the temperature of the scanner&#39;s exit window is raised, the likelihood of frost forming on the exterior is also reduced. 
     In another embodiment, the scanning components are placed in a hermetically sealed enclosure filled with inert gas. The absence of moisture in the enclosure eliminates the problem of condensation on the internal components. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a prior art bar code reader; 
     FIG. 2 illustrates a bar code reader of the present invention; 
     FIG. 3 illustrates a block diagram of the heating module of the preferred embodiment of the invention; 
     FIG. 4 illustrates an assembly of the heater module of the present invention, absent the power supply and the control circuit; 
     FIG. 5 illustrates an alternate embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the following description discloses the use of the invention in bar code scanners, the invention is applicable to any type of optical scanners (readers) and the term “indicia” encompasses any type of machine readable medium. 
     In order to overcome the problem of condensation and frost on the scanner&#39;s components, the applicants invented a heater module which is located on the inside of the scanner&#39;s housing. FIG. 2 illustrates a bar code reader of the present invention. As can be seen from FIG. 2, the heater module  64  is located in the upper part of the housing, preferably near the exit window. The reason the heater module has been placed close to the exit window  56  is because the exit window typically has the lowest thermal isolation factor and is one of the first components that is likely to have condensation deposited on its internal surface when the scanner is transported between the extreme temperature environments. Also, because condensation on the interior surface of the exit window impacts scanner performance more quickly and adversely then elsewhere. The module is designed to keep the ambient temperature inside the scanner&#39;s housing at such a point that would prevent condensation from forming on the scanner&#39;s internal components. As long as the heater module can keep the internal temperature high enough to prevent condensation, the module can be located any place within the housing. It should be noted that the module must not obstruct the optical paths of the outgoing optical beams and the incoming light reflected off the indicia. 
     FIG. 3 is a block diagram of the heater module  64  of the present invention. The module  64  includes a power supply  66 , a temperature sensor  68 , a control circuit  70 , a heating element  72  and a fan  74 . The power supply provides regulated voltage to the individual components of the heater module  64 . The temperature sensor  68  can be a thermal zener diode which produces a voltage that is proportional to the sensed temperature inside the scanner. The sensor outputs the produced voltage to the control circuit  70 . The control circuit  70  includes a comparator  69  which compares the received zener voltage to a known reference voltage. If the sensed temperature is below the required level, the comparator  69  signals a heater driver  71  to turn on the heating element  72 . The heating element  72  and the fan  74  are turned on and the heater module  64  starts blowing heated air inside the interior of the reader. When the sensed temperature reaches required level, the heating element  72  and the fan  74  are turned off to avoid overheating. The heating element  72  is typically a high power dissipation resistor which is either a discrete component or a resistive strip laid out on a printed circuit board. The heating element can also contain a fuse that limits the maximum amount of current which is allowed to pass through the resistor. In a KS3000 scanner manufactured by Symbol Technologies, Inc., the assignee of the present invention, the resistor dissipates 16 watts of power using 1.25 amperes of current. The power supply accepts a wide range of voltages and generates regulated voltage outputs, 5 volts for the fan and 12 volts for the heating element. 
     The power supply can be designed to source more than 3 amperes, allowing for a single 5 volts voltage level to be used for driving the fan and the heating element. The control circuit  70  can also be designed to vary the fan speed and/or the power dissipated by the heating element. 
     The system described above provides a stable ambient temperature inside the scanner. In certain embodiments it may be preferable to locate the power supply and the control circuit outside the scanner. This may be done in order to reduce the volume occupied by the heating module inside the scanner. In such a case, the power supply and the control circuit would be located outside the scanner, while the temperature sensor, the heating element and the fan would be located inside the scanner. 
     FIG. 4 illustrates a heater module assembly of the present invention, having the power supply and the control circuit located outside the assembly. The heater module assembly includes a top cover  76  and a bottom cover  78 . A temperature sensor  82  is located underneath the interior surface of the top cover  76 . The sensor  82  senses the scanner&#39;s interior temperature through a pair of air holes  86   a  and  86   b  in the top cover  76 . The fan  80  and the heating element  84  are positioned between the top and bottom covers. The heating element  84  of the illustrated embodiment is a resistive strip deposited on a top surface of a circuit board. A spacer  90  separates the heating element from the top surface of the bottom cover  78 . The fan  80  is positioned over a circular hole  88  in the bottom cover. During operation the fan  80  sucks the air through the circular hole  88  and redirects the air through an opening  79  in its housing. The redirected air warms up as it blows over the heating element  84 . The heated air is forced into the scanner&#39;s interior through an opening between the front edges of the two covers. The covers are held together by three screw  93   a ,  93   b  and  93   c , which are inserted into their respective holes  92   a ,  92   b  and  92   c  in the bottom cover  78  and are threaded into the top cover  76 . The sensor wires  83 , the fan wires  80  and the heating element wires  85  are all fed outside the assembly through a cutout  94  in the top cover  76 . The wires electrically interconnect the temperature sensor  82 , the fan  80  and the heating element  84  to the power supply and the control circuit located outside the scanner. 
     The present invention is not meant to be limited in the way the elements of the heating module are packaged together or in the specific way the air is being circulated inside the heating module and the scanner. The idea is to be able to control the scanner&#39;s internal temperature by warming up the air inside the scanner. In certain applications where there exists strong airflow, the fan  80  may be eliminated. 
     In an alternate embodiment, all of the elements of FIG. 3 are located in the heater module assembly. 
     In order to reduce the power load of the heating module and to reduce heat transfer through the scanner housing, the system may include a thermal cover which can either be placed onto the scanner&#39;s exterior or lined along the scanner&#39;s interior surfaces. 
     In situations where the only existing problem is condensation on the exit window, a heating element can be deposited on the interior surface of the window in a form of a conductive strip. The heating element must be positioned on the window in such a way as to avoid interference with the outgoing and the return light passing through the exit window. 
     In another alternate embodiment, the heating element can take the form of an optically transparent conductive film. The film&#39;s optical characteristics must be such that the film would not interfere with the outgoing and the return light passing through it. 
     While the present invention is applicable to any type of scanning applications, it is most easily implemented in warehousing operations, where goods stored in freezers, or refrigerators, are being transported in and out of the freezers on forklifts. Compared with a present day batteries used in portable scanners, a typical forklift battery is capable of supplying much larger amounts of current. The forklift battery is used to feed the regulating power supply of the heater module of the present invention. Compared with other loads connected to the forklift battery, the scanner&#39;s load is relatively small. Thus, the scanner of the present invention does not introduce a significant power burden on the forklift battery and at the same time provides an operator with a non-condensing scanning apparatus allowing for an uninterrupted operation during transitions from the extreme thermal environments. 
     Another way to prevent condensation from forming on the inside of a scanner is to fill the scanner with inert gas and to hermetically seal the scanner. While it may be somewhat difficult, although not impossible, to provide an air-tight enclosure for a scanner housing having the gun-shape form of the type shown in FIG. 2, the invention is easily implemented with respect to scanning modules. A scanning module is a miniaturized self contained scanning engine, including all the optical and electronic components needed for scanning an item, which is designed to be installed into a scanner housing as a module. The scanning module can be placed in a small hermetically sealed enclosure. The embodiment is illustrated in FIG.  5 . 
     As shown in FIG. 5, a scanning module  100  is positioned in an enclosure  102 . The enclosure  102  has an opening  104  at which an optically transmissive enclosure window  106  is located. The scanner module includes an electrical cable  108  for interfacing to other parts of the scanning system. The cable  108  passes through an opening  110  in the rear of the enclosure  102 . The design may provide for a number shock mounts  112   a  and  112   b  to be installed in the enclosure in order to prevent mechanical failure the scanning module due to shock and vibration. Once the scanning module  100  is installed in the enclosure  102 , the enclosure gets filled with an inert gas and is hermetically sealed. Alternately, a silica gel cartridge may be incorporated into the enclosure to remove any internal moisture. 
     After the scanning module has been placed and sealed inside the enclosure  102 , the enclosure is placed inside a scanner housing. In order to prevent condensation on the inside surface of the scanner&#39;s exit window, the exterior surface of the enclosure&#39;s window  106  is affixed to the interior surface of the scanner&#39;s exit window, thus removing moisture&#39;s access to the inside of the scanner&#39;s exit window. The two windows can be joined together by an optical grade epoxy or adhesive. 
     In some applications where a scanner is operated in high ambient temperatures it may be advantageous to design a cooling module, instead of a heating module, to be installed into the scanner. The cooling module can help keep the temperature of the internal scanner components within their respective operating temperature range. 
     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 the 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 scope of the following claims.