Abstract:
This invention relates to a media weight sensor of the type that includes a transducer consisting of a metal desk with a piezoelectric element fabricated by one side which form the back of a Helmholtz resonator cavity mounted in a printer so that the media going to the printer moves across the top of the Helmholtz resonator where an opening of the resonator is located. A soft, polymeric roller is used to press for media and is the transducer. The resonant frequency of the Helmholtz is affected by the media. The mass of the media adds to the mass of the resonator, thereby lowering the resonant frequency. Consequently, the heavier than media, the more the resonant frequency is lowered.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention relates to a media weight sensor of the type that includes a transducer consisting of a metal disk with a piezoelectric element fabricated on one side which form the back of a Helmholtz resonator cavity mounted in a printer so that the media going to the printer moves across the top of the Helmholtz resonator where an opening of the resonator is located. A soft, polymeric roller may be used to press the media against the transducer. The resonant frequency of the Helmholtz resonator is affected by the media. The mass of the media adds to the mass of the resonator, thereby lowering the resonant frequency. Consequently, the heavier the media, the more the resonant frequency is lowered.  
         DESCRIPTION OF THE RELATED ART  
         [0002]    It is known, in paperweight sensors, to employ optical sensors. Exemplary of such prior art is U.S. Pat. No. 5,138,178 (&#39;178) to L. F. Wong et. al., entitled “Photoelectric Paper Basis Weight Sensor” and U.S. Pat. No. 5,127,643 (&#39;643) to A. T. DeSanctis et. al., entitled “Automatic Copy Sheet Selection Device.” While the &#39;178 and &#39;643 references employ optical sensors, these sensors are used to measure thickness or weight of the paper. These measurements are accomplished by measuring the amount of light that passes through the paper. However, if the paper is coated, this coating can adversely affect how much light passes through the paper. Consequently, an accurate measurement may not be obtained.  
           [0003]    It is also known, in paperweight sensors, to measure the stiffness of the paper in order to determine the weight of the paper. Exemplary of such prior art is commonly assigned U.S. Pat. No. 5,962,861 (&#39;861) to P. Fowler, entitled “Sheet Media Weight Detector and Method” and commonly assigned U.S. Pat. No. 6,028,318 (&#39;318) to W. L. Cornelius, entitled “Print Media Weight Detection System.” While the &#39;861 and &#39;318 references measure the stiffness of the paper in order to ascertain the weight of the paper, these do not employ an acoustic resonator. Instead, these references measure the deflection of the paper that is related to the stiffness and, thereby the weight of the paper.  
           [0004]    Finally, it is known, in paperweight sensors, to measure paper thickness. Exemplary of such prior art is U.S. Pat. No. 5,806,992 (&#39;992) to Y. Ju, entitled “Sheet Thickness Sensing Technique and Recording Head Automatic Adjusting Technique of Ink Jet Recording Apparatus Using Same.” While the &#39;992 reference measures sheet thickness, it does so by measuring the amount of arm rotation, which can result in a complex and fragile assembly. While the apparatus of the &#39;992 reference may be able to accurately measure the thickness of the sheet of paper, in order to determine the weight of the paper, assumptions must be made as to the makeup of the sheet of paper. For example, it must be assumed that each sheet of paper has the same density. However, it is well known that the density of sheets of paper in the same stack of paper can vary by as much as a factor of two. Consequently, a weight determination cannot be accurately made.  
           [0005]    It is apparent from the above that there exists a need in the art for a media weight sensor system which is lightweight through simplicity of parts and uniqueness of structure, and which at least equals the media weight sensing characteristics of the known media weight sensors, but which at the same time employs an acoustic resonator. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.  
         SUMMARY OF THE INVENTION  
         [0006]    Generally speaking, this invention fulfills these needs by providing a media weight sensing apparatus, comprising a Helmholtz resonator means having an opening substantially located in one end of the resonator means and a media weight measuring means operatively connected to the other end of the resonator means and a media traversing means for traversing a media, whose weight is to be determined, across the opening in the resonator means.  
           [0007]    In certain preferred embodiments, the Helmholtz resonator includes a housing, a piezoelectric element, and a metal disk. Also, the media weight measuring means includes a drive circuit operatively connected to the piezoelectric element. Finally, the media traversing means includes a compliant roller.  
           [0008]    In another further preferred embodiment, the apparatus measures a media property that is a combination of both the media thickness and density. As a result, the measurement may more accurately reflect the media weight by measuring the change of the resonant frequency of the piezoelectric element with and without the media. Since it is a differencing measurement, it will be relatively insensitive to factors, such as wear and temperature.  
           [0009]    The preferred sensing apparatus, according to this invention, offers the following advantages: lightness in weight; ease of assembly and repair; excellent weight measurement characteristics; good stability; excellent durability; and good economy. In fact, in many of the preferred embodiments, these factors of lightness in weight, ease of assembly and repair, weight measurement characteristics, and durability are optimized to an extent that is considerably higher than heretofore achieved in prior, known media weight sensing apparatus.  
           [0010]    The above and other features of the present invention, which will become more apparent as a description proceeds, are best understood by considering the following detailed description in conjunction with the accompanying drawings, wherein like characters represent like parts throughout the several views and in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    [0011]FIG. 1 is a schematic illustration of a media weight sensing apparatus, according to one embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a schematic illustration of a drive circuit for the media weight sensing apparatus, according to the present invention; and  
         [0013]    [0013]FIG. 3 is a graphical illustration of net resonant frequency (in Hertz) vs. paperweight (in pounds).  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    With reference first to FIG. 1, there is illustrated one preferred embodiment for use of the concepts of this invention. In particular, media weight sensing apparatus  2  is illustrated. Apparatus  2  includes, in part, Helmholtz acoustic resonator housing  4 , disk  6 , piezoelectric element  7 , drive circuit  8 , opening  10 , conventional media  12 , and compliant roller  14 .  
         [0015]    Disk  6  is conventionally attached to Helmholtz resonator housing  4 . Disk  6  is, preferably, constructed of any suitable, metallic material and is conventionally attached to piezoelectric element  7 . Drive circuit  8  is conventionally attached to piezoelectric element  7  and illustrated in FIG. 2. Housing  4 , piezoelectric element  7 , disk  6 , and opening  10  make up a Helmholtz resonator. Piezoelectric element  7  and disk  6  make up a transducer that is used to measure resonant frequency. Media  12  can be, but is not limited to, paper, paperboard, plastic, cloth or the like. Roller  14 , preferably, is constructed of any suitable soft, polymeric material. It is to be understood that roller  14  may be replaced with any type of traversing device that is capable of moving media  12  past opening  10  while keeping media  12  in contact with opening  10 .  
         [0016]    As discussed above, disk  6  is attached to piezoelectric element  7  on one side, thereby forming the back of a Helmholtz resonator cavity. Preferably, this cavity is mounted in a printer. Media  12  moves across opening  10  in the direction of arrow B by the rotation of roller  14  along the direction of arrow A. Not only does roller  14  traverse media  12  along the direction of arrow B, but also it is used to press media  12  against opening  10 . The resonant frequency of the Helmholtz resonator is affected by media  12 , as shown in the equations below.  
         [0017]    The heavier the media  12 , the more the resonant frequency is lowered. Drive circuit  8  (FIG. 2) is used to oscillate apparatus  2  at the resonant frequency of the apparatus  2 -media  12  combination. By measuring the drop in the oscillator frequency caused by media  12 , the “weight” of media  12  can be accurately estimated.  
         [0018]    During the operation of apparatus  2 , a resonant frequency is obtained from apparatus  2  without any media  12  being located over opening  10  and calculated as shown in Equations 1-7, below:  
                          Using                 a                 Helmholtz                 acoustic                 resonator                                to                 measure                 paper                 density                   (   weight   )       :                         V   =           π        (     D   2     )       2        H     =     cavity                 volume                   (     mm   ^   3     )                 (     Eq   .              1     )                 L   ′     =       T   +     1.7      a       =     effective                 throat                 length                   (   mm   )                 (     Eq   .              2     )               S   =       π     a   2       =     area                 of                 hole                   (     mm   ^   2     )                 (     Eq   .              3     )               s   =       cavity                 stiffness     =       ρoc   2            S   2     V                     (     g   /     sec   2       )                 (     Eq   .              4     )               m   =       effective                 mass                 of                 air                 in                 neck     =       ρoSL   ′                     (   g   )                 (     Eq   .              5     )                 cavity                 is                 resonant                   when   :   ωom       =     s   ωo             (     Eq   .              6     )                 ωo   2     =       s   m     =           ρoc   2          S   2           v   ρo          SL   ′         =       c   2          S     V     L   ′                       (     Eq   .              7     )                               
 
         [0019]    A sample of media  12  is then placed over opening  10  by roller  14  by conventional techniques. A resonant frequency of apparatus  2  is calculated as shown in Equations 8-11, below:  
             now                 adding                 paper                 mass                 to                 effective                 air                   mass   :                               paper                 mass     =       S   *   paper                 d                 ensity     =       S        (     P   *   3.73   ×     10     -   6         )            (   g   )                 (     Eq                 8     )                   total                 resonant                 mass     =       m   ′     =       ρ                 o                 S                   L   ′       +       S        (     P   *   3.73   ×     10     -   6         )            (   g   )                  
          cavity                 is                 resonant                   when   :               (     Eq                 9     )                   ω                 o                   m   ′       =       ω                   o        (       ρ                   oSL   ′       +     S   *   P   *   3.73   ×     10     -   6           )         =     s     ω                 o                
          solving                 for                 ω                   o   :               (     Eq                 10     )                   ω                 o     =       c   *       S     L   *   V         *         ρ                 o                   L   ′           ρ                 o                   L   ′       +     P   *   3.73   ×     10     -   6                 =     ω                   o   ′     *   K                            (     Eq                 11     )                               
 
         [0020]    The resonant frequency, based upon media  12  being located over opening  10 , is compared with the resonant frequency of no media  12  being located over opening  10  to obtain a net resonant frequency, such as that shown in FIG. 3. The operator merely looks to a chart similar to the one in FIG. 3 to determine the weight of media  12 . It is to be understood that charts similar to FIG. 3 can be conventionally inputted into a conventional computing device (not shown) and an automatic media weight read out can be obtained from the computing device.  
         [0021]    With respect to FIG. 3, the efficacy of the present invention is illustrated. In this example, the various weights of paper samples were determined based upon net resonant frequency levels. The various dimensions and operating conditions shown in FIG. 1 and Equations 1-11 are shown at the top of FIG. 3. As can be seen in FIG. 3, one merely has to obtain the net resonant frequency level in order to determine the weight of the paper media. For example, if a net resonant frequency of 1600 Hertz was shown by apparatus  2  on a conventional display device (not shown), one would ascertain that the paper media had a paperweight of approximately 21 pounds.  
         [0022]    It is to be understood that apparatus  2  can be employed in a printer. For example, as media  12  is getting ready to be printed by the printer, media  12  is moved across opening  10  of apparatus  2  located within a housing (not shown) of the printer, as described above. In this manner, the weight of media  12  can be determined prior to printing. This weight determination will allow the printer to make conventional adjustments based on the weight of media  12 . For example, if it is determined that media  12  is heavier than the media just printed on, the printer can increase the strength of the impact, if the printer is a dot matrix printer. Also, if the printer is an electrophotographic printer, the weight of media  12  can affect the paper speed through the fuser and/or the fuser temperature.  
         [0023]    As can be seen, the present invention measures a property that is a combination of both the thickness of media  12  and the density of media  12 . As a result, the measurement should more accurately reflect the weight of media  12 , than a thickness-only measurement. Also, the present invention is inherently less expensive, more efficient, and more reliable than the thickness sensors. This is because piezoelectric element  6  is much less expensive than inductive sensors. Also, the present invention makes its measurement without touching the paper and is not subject to wear as is a thickness sensor that must touch the moving media. Finally, the present invention makes its measurement by measuring the change of the resonant frequency of piezoelectric element  6  with and without media  12 . Since it is a differencing measurement, it will be relatively insensitive to factors, such as wear and temperature.  
         [0024]    Once given the above disclosure, many other features, modifications or improvements will become apparent to the skilled artisan. Such features, modifications or improvements are, therefore, considered to be a part of this invention, the scope of which is to be determined by the following claims.