Abstract:
A method for detecting errors in loading a lenticular material ( 10 ) on a printer ( 60 ) comprises loading the lenticular material ( 10 ) on a vacuum platen ( 20 ) and drawing a vacuum on the vacuum platen ( 20 ). An airflow is measured on the vacuum platen ( 20 ) and compared to a predetermined value.

Description:
FIELD OF THE INVENTION 
     This invention relates in general to detecting errors in orientation of media to be printed and in particular to detecting alignment errors in lenticular material prior to printing. 
     BACKGROUND OF THE INVENTION 
     When a sheet fed printer is loaded with media having different surface characteristics on both sides, such as lenticular media, there are many opportunities for error in terms of flipping the media the wrong way up before it is printed. Other errors include curling, buckling, skewed, misaligned, and missized sheets. The printing of quality images requires correct orientation of the media in the printer. 
     The slitting, cutting, packaging, and printer loading operations may be manual operations that take place in the dark and it is not trivial to verify that the media is loaded the correct way up in the printer. As the printing operation occurs unattended in the dark, particularly for photosensitive material, an error might not be noticed until the media is developed. If an entire stack of media is printed upside down, there is potential for significant economic loss. 
     U.S. Pat. No. 5,414,491, Vacuum Holder for Sheet Materials, shows a method of determining different sheet sizes based on changes in vacuum flow. This patent, however, does not provide a method and apparatus for distinguishing misalignments of sheets or other problems described above. 
     It is therefore desirable to provide a means for detecting errors in loading lenticular material on printers. 
     SUMMARY OF THE INVENTION 
     Briefly, according to one aspect of the present invention, a method for detecting errors in loading a lenticular material on a printer comprises loading the lenticular material on a vacuum platen and drawing a vacuum on the vacuum platen. An airflow is measured on the vacuum platen and compared to a predetermined value. 
     The lenticular media, which is used as an illustrative example, has a photographic emulsion on one side of a support and lenticular lenses on the other. When placed on a vacuum platen, the amount of flow in the vacuum lines can be used as an indication of errors in loading. A nominal “correct” value is stored to be used as a reference from which to make decisions. This nominal value is stored during initial system setup, when media is placed correctly on the vacuum platen and the source of the system vacuum is in a known state. After storing this nominal value, two possible classes of errors can be differentially distinguished; an error in placement or seating (not all of the vacuum holes in the platen covered with media), or an error in media loading (the media is loaded with the emulsion down rather than the lenticals down). In other words, if the media is loaded with the emulsion on the platen, it&#39;s smooth surface mates better with the vacuum platen and does not allow any appreciable flow in the vacuum lines. This results in a value for the flow that is less than the nominal value. If, however, the media is loaded correctly with the lenticules facing the platen, a small amount of flow in the vacuum lines is present due to leakage along the clefts of the lenticals, which can be measured and compared to the nominal value to verify that the media is placed on the platen properly. If the media has either not been seated properly on the platen, or is skewed, some of the vacuum holes will be left open resulting in a greater than nominal flow measurement which can be detected. 
     In addition, this kind of sensing could identify other major errors such as the wrong media size or type. The following table provides a summary of the sensing states: 
     TABLE 1 
     No (or very little flow): Media loaded upside down (error) 
     Moderate flow: Media loaded correctly 
     High flow: Media not seated on platen properly, media skewed, or wrong media size (error) 
     Setting the thresholds between these states in the software controlling the printer depends on the vacuum system used, the lenticular dimensions, the platen hole pattern, the sensor used to detect flow, and the electronics used to condition the raw sensor signal. A low pass filter is used in the implementation to reduce the effects of higher frequency signal noise that is prevalent in such signals, due to sensor characteristics as well as pneumatic system noise. A low pass filter can be implemented in hardware or firmware, such as a moving average signal processing technique. 
     Measuring flow is preferred over a simple vacuum pressure measurement due to the S/N ratio achievable. If pressure in the lines were measured, only a small change in signal would be noted, because the vacuum source can maintain the same nominal pressure in the lines regardless of flow. However, the amount of leakage in the platen, and therefore flow in the vacuum lines will produce a significant signal change from the appropriate flow sensor and so, can be used as a reliable sensing method. 
     Various commercially available instruments can be used to measure flow. The Pitot tube is a common method in which the flow is converted to a static stagnation pressure that can be easily measured. This is the method that has been demonstrated. In this configuration, there is still a relatively small pressure developed by the flow, on the order of 5 inches of water, (125 mm Hg). 
     As alternatives to the preferred Pitot tube embodiment, a traditional anemometer could also be used to measure the flow rate. Alternatively “hot wire” anemometers are available. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be better understood by reference to the following description taken in conjunction with the accompanying figures wherein: 
     FIG. 1 shows a side view of a sheet of lenticular material. 
     FIG. 2 is a diagrammatical view of the preferred embodiment of the apparatus. 
     FIG. 3 shows a side view of the first media condition detectable by the invention: Media loaded upside-down. 
     FIG. 4 shows a side view of the second media condition detectable by the invention: Media loaded correctly. 
     FIG. 5 shows a top view of the media properly and squarely loaded on the platen. 
     FIG. 6 shows a top view of a third media condition detectable by the invention: Media loaded at a skew angle. 
     FIG. 7 shows a top view of a fourth media condition detectable by the invention: Incorrect size media loaded. 
     FIG. 8 shows a side view of a fifth media condition detectable by the invention: Media not seated on platen properly. 
     FIG. 9 shows a side view of a specific seating problem that is more likely to occur with this loading method: Media not seated locally in area of pins. 
     FIG. 10 is a diagram of a specific embodiment of a flow sensor, specifically, a Pitot tube with pressure detector. 
     FIG. 11 is a schematic diagram of a circuit used to condition the signal from the flow sensor. 
     FIG. 12 is a flowchart describing the steps of the method. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention. 
     Referring now to FIG. 1, a form of lenticular media  10  comprises a clear support  11 , a photographic emulsion  12 , and lenticular lenses  13 . The scale of the lenses on FIG. 1 is to be used for reference only. The lenticular lenses are shown to have two regions, the apex  15  and cleft  16 . Currently utilized sheets of lenticular media have  20  or more lenticals per inch. It is also recognized that many kinds of printing can be used requiring various types of media. For example, for many printing techniques, there would be no photographic emulsion. The specific type of media is not critical to the invention, providing that one side has a different surface characteristic than the other. 
     Referring now to FIG. 2, a vacuum platen  20  is shown with vacuum holes  23  connected through a plenum (not shown) to the input side  24   a  of a blower  24  through a vacuum line  21 . Also shown on the platen, are alignment pins  22   a  and  22   b  that are used as positional references for the registration of sheets of lenticular media. When a sheet of the previously described media is placed with the lenticular lenses  13  face down on a vacuum platen  20 , air leaks through the vacuum holes  23  into the vacuum line  21  through the several small channels created by the clefts  16  of the lenticals  10  and the surface of said platen. This leakage can be measured by a flow sensor  30 , which can be, but is not restricted to any of the following sensors commonly known in the art: Pitot tube with associated pressure sensor, anemometers, hot-wire anemometers which measure flow based on the convective cooling effect that it has on a wire with electrically induced heating, Venturi, or orifice meters. All of these sensors convert the flow of air into a more easily measurable quantity. 
     For example, the Pitot tube  50  shown in FIG. 10 measures flow indirectly by taking a reading of the stagnation pressure with the in-flow stagnation pressure tap  51 . This pressure level is sensed by the pressure sensor  53  differentially, using the static pressure tap  52 , which due to its geometry is not in the flow, as a baseline. The pressure sensor converts the pressure difference to an output voltage which is acted upon by the filtration circuitry  41 . For the amount of flow which was experienced in the experiments performed, and the geometry shown in FIG. 10, an appropriate flow sensor is Model DCAL405GN low pressure sensor from Data Instruments. It is recognized that the pitch and shape of the lenticals as well as the amount of vacuum supplied will also effect sensor selection. 
     Referring back to FIG. 2, the flow sensor  30  provides a voltage output to the filtration circuitry  41 . FIG. 2 shows a simplified vacuum system. Not shown are valves controlling the vacuum and other elements which tend to disturb the system and create transient flow levels in the vacuum lines  21 . The effects of these transient flow level changes and high frequency electrical noise in the pressure sensor  30  are minimized through the use of the low pass filter  41 . The preferred circuitry is a second order Butterworth low pass filter as shown in FIG. 11 Any of a variety of low pass filters would be suitable for this application, thus the invention is not restricted to the type of filtration. The degree of necessary filtering would depend upon the sensor and pneumatic system in use. 
     FIG. 12 shows the method in flowchart form. The routine begins at step  100 , and the blower  24  is turned on in step  101 . The flow through the vacuum system with no media on the platen is measured at step  102 . The signal is low pass filtered by the filtration circuitry  41  and then stored by the signal processor  40  as the Empty Platen Flow Value  29  in step  104 . The system then loads media  10  onto the platen  20  in step  105 . A flow measurement is made of the loaded platen flow value  27  in step  106  by the flow sensor  30 . The output of the flow sensor  30  is sent to the low pass filtration circuitry  41  the signal is processed in step  107  to generate the filtered loaded platen flow value  28 . The Signal Processor  40  generates the computed ratio  42  in step  108 . This is the ratio between the filtered loaded platen flow value  28  and the Empty Platen Flow value  29  at step  109  the system checks whether a calibration is in process. If this is a calibration run, a visual verification that the media is loaded properly for the calibration is made in step  110 . If the media is deemed to be loaded correctly, the signal processor stores the computed ratio  42  as the Good Ratio  43  in step  112 , and the media is rejected in step  111 . The process then begins again with loading of media  10  in step  105 . If during a calibration procedure, the media is not found to be loaded correctly in step  110 , the media is rejected and the system again attempts to load media at step  105 . If there is already a stored Good Ratio  43  and a calibration is not required as determined in step  109 , then the computed ratio  42  is compared to the Good Ratio  43  in step  116 . A determination is then made by the signal processor  40  as to whether or not there has been an error in loading in step  116 . If computed ratio  42  is not within a given range of the Good Ratio  43 , then the media is not loaded properly. The signal processor  40  determines the type of loading error, which has occurred in step  114  and returns the appropriate error code to the operator in step  115 . At this point the appropriate error recovery routine is executed in step  119 . If the media was loaded properly, the cycle continues with the printing step  117 . The media is unloaded in step  118  and the process repeats as required with step  120  and with a return to the load media step ( 105 ). 
     FIGS. 3 through 9 show the various states that are detectable with the invention as described. FIG. 3 shows the lenticular media  10  properly loaded against the alignment pins  22   a  and  22   b , but with the photographic emulsion  12  facing the vacuum platen  20 , and the lenses facing the printer  60 . As the printing would generally not occur on the side with the lenticals, this condition represents an error in loading. This state is recognizable by the fact that the smooth emulsion side seals well with the vacuum platen  20  and does not allow appreciable leakage in the vacuum line  21 . This state of no (or very little) flow is registered by the pressure sensor  30 , filtration circuitry  41  and signal processor  40  as an error state and an alarm is sounded or other appropriate action is taken. 
     FIGS. 4 and 5 show a properly loaded sheet of lenticular media as viewed from the side. Unlike the state in FIG. 3, the media  10  in FIG. 4 is loaded up against the alignment pins  22   a  and  22   b  with the lenses facing the platen  20  and the photographic emulsion  12  facing the printer  60 . FIG. 5 shows this same properly loaded sheet of lenticular media  10  as viewed from above. As the media is the proper size, all of the vacuum holes  23  are covered. This arrangement will result in a nominal flow created by air leaking into the vacuum line  21  along the length of the lenticals in the normal spaces  25  between them and the platen  20 . This nominal flow is converted, via the previously described process (shown in FIG.  12 ), to a numeric value, the computed ratio  42 . The computed ratio  42  is compared to the good ratio  43  by the signal processor  40  to determine that the media has been loaded correctly. 
     FIGS. 6 and 7 show two possible error states detectable by the invention. In FIG. 6, the lenticular media  10  is loaded such that is does not contact both alignment pins  22   a  and  22   b . This skew angle to the platen results in not all of the vacuum holes  23  being covered by the lenticular media  10 . A greater flow results from this state than the nominal flow created in the case shown in FIGS. 4 and 5, and similar actions should be taken as was the case in FIG.  3 . In FIG. 7, an undersized sheet of lenticular media  14  has been loaded on the platen. Though the sheet is registered properly against the alignment pins  22   a  and  22   b , it does not cover all of the vacuum holes  23  on the vacuum platen  20  resulting again in a greater than nominal flow in the vacuum line  21 . Once again similar actions should be taken as was the case in FIG.  3 . 
     FIGS. 8 and 9 show another possible error state. Referring to FIG. 8, even if the lenticular media  10  is the correct size and is loaded properly against the alignment pins  22   a  and  22   b , there is a possibility that it is not seated properly on the vacuum platen  20 . In this case, an abnormal space  26  is created which is effectively added to the normal spaces  25  created by the lenticals previously discussed. As was the case in FIGS. 6 and 7, greater than nominal flow is experienced and an error state is recognized by the signal processor  40 . FIG. 9 illustrates a likely occurrence of an abnormal space  26  which is the media buckling locally at the alignment pins  22   a  and  22   b.    
     The errors shown in FIGS. 8 and 9 can occur, for example, if the media has a natural curl either towards or away from the lenticles. In such an instance, if the curl is severe, the vacuum to the platen may not be able to straighten the media resulting in the improper seating condition. 
     PARTS LIST 
       10 . Lenticular media 
       11 . Media support 
       12 . Photographic emulsion 
       13 . Lenses 
       14 . Undersized sheet of media 
       15 . Apex 
       16 . Cleft 
       20 . Vacuum platen 
       21 . Vacuum line 
       22 . Alignment pins 
       23 . Vacuum holes 
       24 . Blower 
       25 . Normal space 
       26 . Abnormal space 
       27 . Loaded platen flow value 
       28 . Filtered loaded platen flow value 
       29 . Empty platen flow value 
       30 . Flow sensor 
       40 . Signal processor 
       41 . Filtration circuitry 
       42 . Computed ratio 
       43 . Good ratio 
       50 . Pitot tube 
       51 . Stagnation pressure tap 
       52 . Static pressure tap 
       53 . Pressure sensor 
       54 . Voltage out 
       55 . Direction of air flow 
       60 . Printer