Abstract:
A system for reading a coded marker of an ink stick has been developed. The system includes an optical detector that detects light reflected from different areas of the generates signals in response to detecting light having at least two signal strengths that is reflected from different areas of the coded marker. A controller processes the signals from the optical detector to identify a code word encoded into the coded marker of the ink stick.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of co-pending application Ser. No. 11/472,977, entitled “Solid Ink Stick with Coded Markings and Method and Apparatus for Reading Markings,” and filed on Jun. 22, 2006, which issued as U.S. Pat. No. 7,874,661 on Jan. 25, 2011. Reference is made to commonly-assigned copending U.S. patent application Ser. No. 11/473,632, entitled “Solid Ink Stick with Interface Element,” and filed on Jun. 23, 2006, which issued as U.S. Pat. No. 7,857,439 on Dec. 28, 2010, the disclosure of which is incorporated herein. 
    
    
     TECHNICAL FIELD 
     This disclosure relates generally to phase change ink jet printers, the solid ink sticks used in such ink jet printers, and the load and feed apparatus for feeding the solid ink sticks within such ink jet printers. 
     BACKGROUND 
     Solid ink or phase change ink printers conventionally use ink in a solid form, either as pellets or as ink sticks of colored cyan, yellow, magenta and black ink fed into shape coded openings. These openings fed generally vertically into the heater assembly of the printer where they were melted into a liquid state for jetting onto the receiving medium. The pellets were fed generally vertically downwardly, using gravity feed, into the ink loader. These pellets were elongated with separate multisided shapes each corresponding to a particular color. 
     Solid ink sticks have been typically either gravity fed or spring loaded into a feed channel and pressed against a heater plate to melt the solid ink into its liquid form. These ink sticks were shape coded and of a generally small size. One system used an ink stick loading system that initially fed the ink sticks into a preload chamber and then loaded the sticks into a load chamber by the action of a transfer lever. Earlier solid or hot melt ink systems used either a flexible web of hot melt ink that was incrementally unwound and advanced to a heater location or particulate hot melt ink that was delivered by vibrating the particulate into the melt chamber. 
     In previously known phase change ink jet printing systems, the interface between a control system for a phase change ink jet printer and a solid ink stick provided little information about the solid ink sticks loaded in the printer. For instance, control systems are not able to determine if the correct color of ink stick is loaded in a particular feed channel or if the ink that is loaded is compatible with that particular printer. Provisions have been made to ensure that an ink stick is correctly loaded into the intended feed channel and to ensure that the ink stick is compatible with that printer. These provisions, however, are generally directed toward excluding wrong colored or incompatible ink sticks from being inserted into the feed channels of the printer. For example, the correct loading of ink sticks has been accomplished by incorporating keying, alignment and orientation features into the exterior surface of an ink stick. These features are protuberances or indentations that are located in different positions on an ink stick. Corresponding keys or guide elements on the perimeters of the openings through which the ink sticks are inserted or fed exclude ink sticks which do not have the appropriate perimeter key elements while ensuring that the ink stick is properly aligned and oriented in the feed channel. 
     While this method is effective in ensuring correct loading of ink sticks in most situations, there are still situations when an ink stick may be incorrectly loaded into a feed channel of a printer. For example, world markets with various pricing and color table preferences have created a situation where multiple ink types may exist in the market simultaneously with nearly identical size/shape ink and/or ink packaging. Thus, ink sticks may appear to be substantially the same but, in fact, may be intended for different phase change printing systems due to factors such as, for example, market pricing or color table. In addition, due to the soft, waxy nature of an ink stick body, an ink stick may be “forced” through an opening into a feed channel. The printer control system, having no information regarding the configuration of the ink stick, may then conduct normal printing operations with an incorrectly loaded ink stick. If the loaded ink stick is the wrong color for a particular feed channel or if the ink stick is incompatible with the phase change ink jet printer in which it is being used, considerable errors and malfunctions may occur. 
     SUMMARY 
     An ink stick for use in a phase change ink imaging device is provided. The ink stick comprises a three dimensional ink stick body having an exterior surface. The ink stick includes one or more coded markers formed in the exterior surface from a leading end to a trailing end of the ink stick body parallel to a feed direction of the ink loader, each coded marker including a coded pattern of indicia for being optically read as the coded marker passes a sensor in the feed channel. The coded pattern of indicia may include areas of varying widths and/or varying reflective properties for generating a coded signal pattern indicating variable control/attribute information to a control system of an imaging device. 
     In another embodiment, a method of feeding ink sticks in an ink loader of a phase change imaging device is provided. The method comprises inserting an ink stick into a feed channel of a phase change imaging device, the ink stick including a coded marker formed in an exterior surface of the ink stick from a leading end to a trailing end of the ink stick parallel to a feed direction of the ink loader, the coded marker including a coded pattern of indicia. The ink stick is urged along the feed channel toward the melt end of the feed channel. A beam of light may be directed onto the coded pattern of indicia of the coded marker as the ink stick is being urged along the feed channel. A signal strength of the light reflected from the coded pattern of indicia is detected, and a signal pattern is generated that corresponds to the detected signal strength of the light reflected from the coded pattern of indicia. The signal pattern may then be decoded to determine variable control/attribute information encoded into the coded pattern of indicia. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a phase change printer with the printer top cover closed. 
         FIG. 2  is an enlarged partial top perspective view of the phase change printer with the ink access cover open, showing a solid ink stick in position to be loaded into a feed channel. 
         FIG. 3  is a side sectional view of a feed channel of a solid ink feed system taken along line  3 - 3  of  FIG. 2 . 
         FIG. 4  is a perspective view of one embodiment of a solid ink stick with a coded marker. 
         FIG. 5  is a schematic view of a sensor system for reading a coded marker of the ink stick of  FIG. 4 . 
         FIG. 6  is a perspective view of another embodiment of a solid ink stick with a coded marker. 
         FIG. 7  is a schematic side view of a sensor system for reading a coded marker of the ink stick of  FIG. 6 . 
         FIG. 8  is another schematic side view of a sensor system for reading a coded marker of the ink stick of  FIG. 6 . 
         FIG. 9  is another schematic side view of a sensor system for reading a coded marker of the ink stick of  FIG. 6 . 
         FIG. 10  is another schematic side view of a sensor system for reading a coded marker of the ink stick of  FIG. 6 . 
         FIG. 11  is a perspective view of an embodiment of a solid ink stick with two coded markers. 
         FIG. 12  is a schematic side view of a sensor system for reading the coded markers of the ink stick of  FIG. 11 . 
     
    
    
     DETAILED DESCRIPTION 
     For a general understanding of the present embodiments, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate like elements. 
       FIG. 1  shows a solid ink, or phase change, ink printer  10  that includes an outer housing having a top surface  12  and side surfaces  14 . A user interface, such as a front panel display screen  16 , displays information concerning the status of the printer, and user instructions. Buttons  18  or other control elements for controlling operation of the printer are adjacent the front panel display screen, or may be at other locations on the printer. An ink jet printing mechanism (not shown) is contained inside the housing. An example of the printing mechanism is described in U.S. Pat. No. 5,805,191, entitled Surface Application System, to Jones et al., and U.S. Pat. No. 5,455,604, entitled Ink Jet Printer Architecture and Method, to Adams et al. An ink loader  100  delivers ink to the printing mechanism. The ink loader  100  is contained under the top surface of the printer housing. The top surface of the housing includes a hinged ink access cover  20  that opens as shown in  FIG. 2 , to provide the operator access to the ink loader  100 . 
       FIG. 2  illustrates the printer  10  with its ink access cover  20  raised revealing an ink load linkage element  22  and an ink stick feed assembly or ink loader  100 . In the particular printer shown, the ink access cover  20  is attached to an ink load linkage element  22  so that when the printer ink access cover  20  is raised, the ink load linkage  22  slides and pivots to an ink load position. The interaction of the ink access cover and the ink load linkage element is described in U.S. Pat. No. 5,861,903 for an Ink Feed System, issued Jan. 19, 1999 to Crawford et al. As seen in  FIG. 2 , the ink loader includes a key plate  26  having keyed openings  24 . Each keyed opening  24 A,  24 B,  24 C,  24 D provides access to an insertion end of one of several individual feed channels  28 A,  28 B,  28 C,  28 D of the ink loader (see  FIG. 3 ). 
     Each longitudinal feed channel  28  of the ink loader  100  delivers ink sticks  30  of one particular color to a corresponding melt plate  32 . Each feed channel has a longitudinal feed direction from the insertion end of the feed channel to the melt end of the feed channel. The melt end of the feed channel is adjacent the melt plate. The melt plate melts the solid ink stick into a liquid form. The melted ink drips through a gap  33  between the melt end of the feed channel and the melt plate, and into a liquid ink reservoir (not shown). The feed channels  28 A,  28 B,  28 C,  28 D (see  FIG. 3 ) have a longitudinal dimension from the insertion end to the melt end, and a lateral dimension, substantially perpendicular to the longitudinal dimension. 
     Each feed channel  28  in the particular embodiment illustrated includes a push block  34  driven by a driving force or element, such as a constant force spring  36  to push the individual ink sticks along the length of the longitudinal feed channel toward the melt plates  32  that are at the melt end of each feed channel. The tension of the constant force spring  36  drives the push block  34  toward the melt end of the feed channel. In a manner similar to that described in U.S. Pat. No. 5,861,903, the ink load linkage  22  is coupled to a yoke  38 , which is attached to the constant force spring mounted in the push block. The attachment to the ink load linkage  22  pulls the push block  34  toward the insertion end of the feed channel when the ink access cover is raised to reveal the key plate  26 . In the implementation illustrated, the constant force spring  36  can be a flat spring with its face oriented along a substantially vertical axis. 
     A color printer typically uses four colors of ink (yellow, cyan, magenta, and black). Ink sticks  30  of each color are delivered through a corresponding individual one of the feed channels  28 A,  28 B,  28 C,  28 D. The operator of the printer exercises care to avoid inserting ink sticks of one color into a feed channel for a different color. Ink sticks may be so saturated with color dye that it may be difficult for a printer operator to tell by the apparent color alone which color is which. Cyan, magenta, and black ink sticks in particular can be difficult to distinguish visually based on color appearance. The key plate  26  has keyed openings  24 A,  24 B,  24 C,  24 D to aid the printer operator in ensuring that only ink sticks of the proper color are inserted into each feed channel. Each keyed opening  24 A,  24 B,  24 C,  24 D of the key plate has a unique shape. The ink sticks  30  of the color for that feed channel have a shape corresponding to the shape of the keyed opening. The keyed openings and corresponding ink stick shapes exclude from each ink feed channel ink sticks of all colors except the ink sticks of the proper color for that feed channel. 
     An exemplary solid ink stick  30  for use in the ink loader is illustrated in  FIG. 4 . The ink stick is formed of a three dimensional ink stick body. The ink stick body illustrated has a bottom exemplified by a generally bottom surface  52  and a top exemplified by a generally top surface  54 . The particular bottom surface  52  and top surface  54  illustrated are substantially parallel one another, although they can take on other contours and relative relationships. Moreover, the surfaces of the ink stick body need not be flat, nor need they be parallel or perpendicular one another. 
     The ink stick body also has a plurality of side extremities, such as side surfaces  56  and end surfaces  61 ,  62 . The illustrated embodiment includes four side surfaces, including two end surfaces  61 ,  62  and two lateral, side surfaces  56 . The basic elements of the lateral side surfaces  56  are substantially parallel one another, and are substantially perpendicular to the top and bottom surfaces  52 ,  54 . The end surfaces  61 ,  62  are also basically substantially parallel one another, and substantially perpendicular to the top and bottom surfaces, and to the lateral side surfaces. One of the end surfaces  61  is a leading end surface, and the other end surface  62  is a trailing end surface. The ink stick body may be formed by pour molding, injection molding, compression molding, or other known techniques. 
     Referring again to  FIG. 4 , the ink stick may include one or more coded markers  70  for encoding variable control information or attribute information into the ink stick  30 . Each coded marker  70  may be configured to interface with a sensor system in a feed channel of an ink loader to generate a coded signal pattern that corresponds to the variable control and/or attribute information. The coded signal pattern may take any form that is suitable to convey information to an imaging device control system such as, for example, a waveform, pulse-width modulated signal, etc. Each coded marker  70  includes a coded pattern of indicia for generating the coded signal pattern. In one embodiment, coded marker  70  comprises a generally linear array of indicia that forms a path substantially parallel to the feed direction that may be read as the ink stick is urged along a feed channel by the push block or by gravity. The pattern of indicia of a coded marker, however, may have any suitable arrangement, pattern, or the like, including arrays perpendicular to the feed direction, concentric rings, etc. 
     A coded marker  70  may be located in a predetermined position corresponding to a sensor location in a feed channel. In the embodiment of  FIG. 4 , shown on the top surface  54  of the ink stick  30  although coded markers may be formed on any surface or more than one surface of the ink stick depending on sensor placement. The number and positioning of coded markers that may be placed on an ink stick is limited only by the geometry of the ink sticks and sensor placement options in an ink loader. A coded marker may be beneficially placed in a location on the exterior surface of an ink stick where handling damage cannot easily influence sensor interface with the ink loader such as, for example, a recess or inset portion in the exterior surface of the ink stick. 
     In one embodiment, information may be encoded into a coded marker  70  by selecting a unique identifier, or code word, to be indicated by a coded marker  70  and implementing an encoding scheme in the coded marker such that coded pattern of signals generated corresponds to the code word. A code word may comprise one or more values, alphanumeric characters, symbols, etc. that may be associated with a meaning by an imaging device control system. The code word may be assigned to indicate control and/or attribute information that pertains to an ink stick. The code word may be read by an imaging device control system and translated into the control and/or attribute information pertaining to the ink stick that may be used in a number of ways by the control system. The control system may use the code word as a lookup key for accessing data stored in a data structure, such as, for example, a database or table. The data stored in the data structure may comprise a plurality of possible code words with associated information corresponding to each code word. 
     The coded signal pattern indicating a code word may correspond to an optical characteristic of the coded pattern of indicia. For instance, the coded signal pattern generated may correspond to the reflectivity of the coded pattern of indicia. In this embodiment, the coded marker includes a plurality of areas  74   a  treated to modify the reflectance characteristics of the areas relative to the untreated areas  74   b  of the ink stick  30 . The coded marker  70  of the ink stick of  FIG. 4  shows a plurality of textured areas  74   a  of varying widths although non-textured areas may have varying widths as well. The texture of the areas  74   a  may be any texture that may cause a light beam to be scattered in a manner sufficient to cause the beam to be detected at a different signal level than the untreated or non-textured areas  74   b  of the ink stick  30 . The textured areas  74   a  may be formed by high-pressure injection molding. Thus, the textured areas may be incorporated into an ink stick during initial manufacturing of an ink stick. Alternatively, the textured areas  74   a  may be formed by subsequent treatment of the ink stick such as by stamping or embossing. Treating the surface of an ink stick to form a coded marker  70  eliminates handling issues that may be caused by tagging the ink with foreign material. Foreign tagging material on an ink stick may interfere with the melting process, block print jets, contaminate the ink, etc. 
     Thus, in one embodiment, the coded marker  70  of the ink stick of  FIG. 4  may be read by serially illuminating the textured  74   a  and non-textured areas  74   b  of the coded marker  70  and detecting the signal strength of the light reflected from the areas (explained in more detail below). The reflected light from the non-textured areas may produce a “high” signal output and reflected light from the textured areas may produce a “low” signal output. In addition to “high” and “low” signals, encoding information may comprise varying the width of the textured  74   a  and non-textured areas  74   b  such that the duration of the “high” and “low” signals detected may be varied. In embodiments in which the width of the areas may be varied, the varying widths may be integer multiples of a standard width. For example, the coded marker  70  of the ink stick of  FIG. 4  includes “wide” textured areas that correspond to approximately twice the width of the “narrow” textured areas. Textured and non-textured areas may be provided that are two or more times wider than a standard “narrow” element. 
     Thus, a variety of encoding schemes may be implemented by providing coded markers  70  with various patterns of textured and non-textured areas of varying widths. For example, the coded marker  70  of the ink stick of  FIG. 4  may be used to implement a binary encoding scheme. Because the textured  74   a  and non-textured areas  74   b  of the coded marker  70  are configured to generate “low” and “high” signal values respectively, a signal or waveform having two amplitude values may be generated. Using a binary encoding scheme, a code word encoded into a coded marker may comprise an n-bit binary code word with the high and low bit values corresponding to the high and low signal values. The widths of the textured and non-textured areas of the coded marker may be varied to provide additional opportunities for encoding bit information. For instance, “wide” textured areas may be configured to generate a signal indicating a “high” or “low” signal value with the “narrow” areas generating the opposite value. Additionally, the width of the textured and non-textured areas may be varied to indicate a bit sequence of the code word having bits of the same value, i.e. 111 or 000. Thus, two or more consecutive high bits (ones) may be indicated by providing a non-textured area of the coded marker that is an integer n wider than a non-textured area for indicating a single high value where n corresponds to the number of consecutive high bits. 
     With an n-bit binary code word, there are 2 n  possible bit combinations, or code words, which may be generated. Thus, a code of twenty bits can generate over 1 million possible code words and thirty bits over 1 billion code words. Binary representations of data may be less complex to implement than other encoding schemes and may have a high signal-to-noise ratio because there are only two possible signal values to be detected. Any suitable encoding scheme, however, may be implemented. Standard barcode encoding and reading techniques may be implemented. Additionally, by treating areas of a coded marker to generate three or more possible signal values, base three and higher level encodings may be implemented. 
     The number of bits that may be encoded into a coded marker  70  may depend on the size of the ink stick as well as the resolution of the sensor system for detecting the pattern of indicia of the coded marker  70 . Referring now to  FIG. 5 , a schematic of a code reader  80  for a feed channel of an ink loader is shown. The code reader  80  comprises an optical source  84  for directing a light beam  88  onto a coded marker as an ink stick  30  is transported along a feed channel in the feed direction F, and an optical sensor  90  for detecting a signal strength of the beam  92  that is reflected from the coded marker and generating a signal pattern, or waveform, that corresponds to the reflected signal strength. 
     In the embodiment of  FIG. 5 , the optical source  84  and the optical sensor  90  are fixedly mounted in a feed channel (not shown) of an ink loader in a position for the optical source to direct the light beam  88  onto a coded marker  70  of an ink stick. The optical source  84  and the optical sensor  90  are situated so that light emitted from the source  84  is directed at a coded marker of an ink stick  30  and is reflected by the patterned indicia of the coded marker onto the optical sensor  90  as the ink stick  30  is transported along the feed channel. In one embodiment, the optical source  84  and optical sensor  90  are placed such that the angle of illumination D relative to a center line  94  is approximately the same as the angle at which the sensor is placed relative to center line  94 . The optical source  84  and the optical sensor  90  may be located at any point along the path of movement of the ink stick  30 . Coded markers may be read during insertion or as the ink stick moves forward in the feed channel. Code reading in the channel may occur one or more times at one or more positions along the path of travel of the ink stick. 
     In the embodiment of  FIG. 5 , the optical source  84  comprises a line generator for projecting an optical line  104  (see  FIG. 4 ) onto the coded marker  70  as the coded marker  70  passes the line generator  84  in the feed channel. The line  104  comprises a projection of light with a high aspect ratio (width over thickness). A line  104  having a high aspect ratio may prevent surface regularities inherent in the molding of the ink stick from interfering with the intended signal received at the optical sensor. In addition, the resolution and accuracy of the code reader  80  may depend on the focus or width of the line  104  relative to the widths of the encoded areas  74   a  of a coded marker  70 . Thus, the width of the line  104  generated by the line generator  84  of the code reader of  FIG. 5  is less than the width of the encoded areas  74   a  of the coded marker  70 . 
     The optical sensor  90  may comprise a photodiode which converts detected light to electrical signals. The optical sensor  90  may include an amplifier (not shown) for amplifying the detected signal and an optical filter (not shown) tuned to the wavelength of light emitted by the line generator for eliminating stray light. While the optical sensor  90  described comprises a photodiode, other types of light sensors, such as photo-conductors, may be employed as the optical sensor  90  within the spirit and scope of the disclosure. 
     As the ink stick  30  proceeds along a feed channel in the feed direction F, the optical line  104  generated by line generator  84  scans over the textured  74   a  and non-textured areas  74   b  of the coded marker  70  causing the optical sensor  90  to vary in its electrical stimulation due to the scattering or absorbing effects of the areas. The optical sensor  90  outputs an analog signal that corresponds to the electrical stimulation caused by the coded marker  70  which may then be amplified and input to an analog-to-digital (A/D) converter  108  where the analog signal may be subjected to a threshold level for converting the output signal of the optical sensor  90  to a binary signal suitable for input to the controller  110 . 
     The analog signal may be sampled at any suitable rate for conversion to the binary signal. For example, in one embodiment, the analog signal may be sampled at a rate that corresponds to the feed rate of the ink sticks along the feed channel to ensure that portions of the coded marker are not read more than once while an ink stick is not moving in the feed channel. Feed rate may be determined by calculating ink mass consumption using any suitable method such as, for instance, counting pulses of the print head or by determining position of the push block in the feed channel. As an alternative to sampling the analog signal at a sampling rate corresponding to the feed rate, the sensor system may be configured to read the coded markers in a manner independent of the feed rate. For example, the sensor system may be configured to scan over the coded marker by moving the optical source and sensor in relation to an ink stick. 
     In one embodiment, the bit pattern, or code word, of the binary signal may then be determined by the controller  110 . The code word may be translated by the controller  110  into information that may be used in a number of ways by the control system of a printer. For example, the controller  110  may compare the reference signal to the data stored in the data structure, or table, stored in memory. The data stored in the data structure may comprise a plurality of possible code words with associated information corresponding to code word. The associated information may comprise control and/or attribute information that pertains to an ink stick such as, for example, ink stick color, printer compatibility, ink stick composition information, or may comprise printer calibration information pertaining to the ink stick, such as, for example, suitable color table, thermal settings, etc. that may be used with an ink stick. The control and/or attribute information may be used by a controller  110  in a suitably equipped phase change ink jet printer to control imaging operations. For example, the control system  110  may enable or disable operations, optimize operations or influence or set operation parameters based on the “associated information” that corresponds to the code word encoded in a coded marker. 
     Referring now to  FIG. 6 , there is shown another embodiment of a coded marker  70  for encoding information into an ink stick. In this embodiment, the coded marker  70  includes indicia having variable heights/depths comprising recessed areas  174   a  and raised areas  174   b . The coded signal pattern generated by this embodiment of a coded marker  70  corresponds to the signal strength of light reflected from the variable heights of the recessed  174   a  and raised areas  174   b . The recessed  174   a  and/or raised areas  174   b  of the coded marker  70  may be formed by injection molding, stamping or any suitable method. 
     In a manner similar to that described above, the coded marker  70  may be read by serially illuminating the recessed  174   a  and raised areas  174   b  of the coded marker  70  and detecting the signal strength of the light reflected from the areas  174   a ,  174   b . In one embodiment, the reflected light from the raised areas  174   b  may produce a “high” signal output, and the reflected light from the recessed areas  174   a  may produce a “low” signal output. Similar to above, encoding information may include varying the width of the recessed and raised areas such that the duration of the “high” and “low” signals detected may be varied. In embodiments in which the width of the areas may be varied, the varying widths may be integer multiples of a standard unit width. 
       FIG. 7  shows an embodiment of a code reader  120  for reading the coded marker  70  of  FIG. 6 . In this embodiment, the code reader  120  includes an optical source  124  and an optical sensor  128 . The optical source  124  may comprise a light emitting diode (LED) or laser diode and a collimating lens which collimates the beam  130  emitted from the LED or laser diode toward a focus point in which the beam impinges on the coded marker of the ink stick. The optical sensor  128  may comprise a photodiode which converts detected light to electrical signals. The optical sensor  128  may include an amplifier (not shown) for amplifying the detected signal and an optical filter (not shown) tuned to the wavelength of light emitted by the optical source  124  for eliminating stray light. While the optical sensor  128  described comprises a photodiode, other types of light sensors, such as photo-conductors, may be employed. 
     Referring to  FIG. 7 , the optical source  124  is oriented at an angle A relative to the optical sensor  128  such that the source  124  is focused at a point  130  directly below the optical sensor  128  when sensing a raised area  174   b  of a coded marker  70 . This provides for the optical sensor  128  to be stimulated by light being scattered by the surface of the raised areas of the coded marker. Referring to  FIG. 8 , as the ink stick is fed along the feed channel and a recessed area is moved under the sensor  128 , the focus point  134  of source  124  is shifted such that it is no longer beneath the optical sensor  128  thereby decreasing the stimulation of the optical sensor by the light beam. 
     In order to increase the signal-to-noise ratio of the coded signal pattern indicated by the coded marker, the code reader may include an opaque wall  138  for increasing the contrast between the signals generated by the raised and recessed areas of the coded marker as shown in  FIG. 9 . The wall  138  may be provided as part of a housing for enclosing the optical source  124  and sensor  128  or, alternatively, may be provided as a separate element. The opaque wall  138  may be composed of an opaque material such as an opaque plastic, or may be comprised of any suitable material having an opaque coating. 
     The wall  138  is positioned such that the end of the wall is adjacent the focus point  130  of the light beam when the optical sensor  128  is positioned above a raised area  174   b  of the coded marker  70 . Thus, referring to  FIG. 9 , the optical source is focused at a point directly below the optical sensor  128  and directly ahead of wall  138 . Referring now to  FIG. 10 , as the ink stick  30  is fed along the feed channel and a recessed area  174   a  is moved into an area below the optical sensor  128 , the focus point  134  of source is shifted such that it falls beneath wall  138  causing it to be shaded from sensor  128 . The sensor  128  is therefore no longer stimulated providing for an indication of the recessed area  174   a.    
     As mentioned above, the number and positioning of coded markers that may be formed into an ink stick is limited only by the geometry of the ink sticks and sensor placement options in an ink loader. Referring to  FIG. 11 , there is shown an embodiment of an ink stick having two coded markers  70  formed on the top surface  54  of an ink stick. In this embodiment, each coded marker  70  may be configured to indicate a separate code word to a control system, or the coded markers may be configured to indicate a single “long” code word. 
       FIG. 12  shows an embodiment of a code reader for reading multiple tracks. In this embodiment, the code reader  140  includes a single optical source  144  for illuminating each coded marker  70  of the ink stick and a pair of optical sensors  148 ,  150 , one for detecting the signal strength of the light scattered by the coded patterns of indicia of the coded markers  70 . The optical source  144  and pair of optical sensors  148 ,  150  are provided in a housing  154  with sections divided by opaque walls  160  for preventing stray light from one coded marker being detected by an optical sensor of the other coded marker. In addition, the housing may include a contrast wall  164  for increasing the contrast of the sensed light between the encoded areas of a coded marker in a manner similar to the wall  138  of  FIGS. 9 and 10 . 
     Those skilled in the art will recognize that numerous modifications can be made to the specific implementations described above. Therefore, the following claims are not to be limited to the specific embodiments illustrated and described above. The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.