Patent Publication Number: US-10308457-B2

Title: Capacitive sensing for paper tray

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Priority is claimed under USC § 119(e) to: (a) U.S. Provisional Application 61/915,036, filed 12 Dec. 2013, and (b) US Provisional Application 61/932,394, filed 28 Jan. 2014. 
    
    
     BACKGROUND 
     Technical Field 
     This Patent Document relates generally to printing systems/products that include sheet feeding from a paper tray. 
     Related Art 
     Printing systems/products that provide printed (paper) output, typically include sheet feeding apparatus. The sheet feeding apparatus feeds paper to a printing apparatus from a paper tray. 
     These printing systems/products commonly include various mechanical or electronic mechanisms to determine the condition or characteristics of paper in the tray. For example, paper tray sensing mechanism can be used to determine paper quantity and paper size. 
     BRIEF SUMMARY 
     This Brief Summary is provided as a general introduction to the Disclosure provided by the Detailed Description and Figures, summarizing some aspects and features of the disclosed invention. It is not a complete overview of the Disclosure, and should not be interpreted as identifying key elements or features of the invention, or otherwise characterizing or delimiting the scope of the invention disclosed in this Patent Document. 
     The Disclosure describes apparatus and methods for capacitive sensing for paper tray status (paper condition/characteristics), such as paper size, paper stack height, page count and paper dielectric. 
     According to aspects of the Disclosure, measuring paper characteristics for paper within a paper tray using capacitive sensing. The paper tray is configured with at least two capacitive sensors with respective capacitive electrodes CIN 1  and CIN 2 , each with a ground plane at a paper tray bottom, with CIN 1  and CIN 2  at a top of the paper tray oriented relative to a width dimension of the paper in the paper tray such that the paper covers CIN 1 , and partially covers CIN 2 , and with CIN 2  having a length a2=a2p+a2a, where a2p is a portion of the CIN 2  length a2 that is over the paper, and a2a is a portion of the CIN 2  length a that is not over the paper. The method can include: (a) with no paper in the paper tray, measuring a capacitance CA 0 =CIN 1  to the ground plane; (b) with paper in the paper tray, measuring a capacitance CA 1 =CIN 1  to ground; (c) measuring a capacitance CA 2 =CIN 2  to ground; and (d) determining CDIFF_w=CA 1 *(1−a2p/a2)−CA 0 *(aa/a); and (e) determining paper width based on (1) a2=a2p+a2a, (2) a2p=a2−a2a, and (3) a2a=(a2*CDIFF_w)/(CA 1 −CA 0 ). Paper width corresponds to a percentage (a2p/a) of CIN 2  covered by paper. 
     Other aspects and features of the invention claimed in this Patent Document will be apparent to those skilled in the art from the following Disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an example functional illustration of a capacitive sensing system based on projected self-capacitance, adaptable for use in printing systems/products to sense the condition/characteristics of paper in the paper tray, such as paper size and paper stack height (page count), and to determine paper type (paper dielectric). 
         FIGS. 2A, 2B and 2C  illustrates an example embodiment of a paper tray with capacitive sensing for measuring paper size/width, including: ( 2 A/B) top and side views illustrating a paper tray incorporating a capacitive sensor, including capacitive electrodes CIN 1  and CIN 2 , and ( 2 C) example capacitive sensing results. 
         FIGS. 3A, 3B and 3C  illustrate an example embodiment of a paper tray with capacitive sensing for measuring paper stack height/page count, including: ( 2 A/B) top and side views illustrating a paper tray incorporating a capacitive sensor, including capacitive electrode CIN 1 , and ( 3 C) example capacitive sensing results. 
         FIG. 4  illustrates and example embodiment of a paper tray with a capacitive sensing for measuring paper dielectric, including example inter-digitated (co-planar) capacitive and ground electrodes E 1  and G 1 . 
     
    
    
     DETAILED DESCRIPTION 
     This Description and the Figures disclose example embodiments and applications that illustrate various features and advantages of a capacitive system for sensing paper tray status. 
     In brief overview, a capacitive sensing system is based on projected self-capacitance. In example embodiments, the capacitive sensing system can be configured with one or more shielded capacitive sensors incorporated into the paper tray, and oriented relative to the paper according to the paper condition/characteristic sensed. 
       FIG. 1  is an example functional illustration of a capacitive sensing system  100  suitable for use in printing systems/products, and in particular, for use in sensing paper tray status. More particularly, capacitive sensing system  100  is adapted to embodiments of the invention used to sense the condition/characteristics of paper in the paper tray, such as paper size, paper stack height, page count and paper dielectric. 
     Capacitive sensing system  100  includes a capacitive sensor  110  and capacitance acquisition/conversion  130  formed by a capacitance-to-digital conversion (CDC) unit  150 , and a data processor  170 . 
     In example embodiments, capacitive sensor  110  is adapted for incorporation into a paper tray, and configured for capacitive sensing of condition/characteristics of paper  120 . The capacitive sensor  110  need not be co-located with the CDC unit  150 , but to reduce the effects of parasitic capacitance, CDC  150  is preferably located as close as possible to capacitive sensor  110 . 
     Capacitive sensing system  100  is configured for capacitive sensing based on projected self-capacitance. Capacitive sensor  110  includes a sensor electrode  111  and a driven sensor shield  113 , separately coupled to CDC  150  (Acquisition Channel input CH and Shield Excitation/Driver output SHIELD). 
     Capacitive sensor  110  includes a driven sensor shield  113 , also coupled to a shield driver in CDC  150 . Sensor shield  113  is disposed over, and insulated from, sensor electrode  111 . Shield drive can be provided synchronously with sensor excitation frequency, and can be used to focus sensing direction, and to counteract parasitic capacitance. 
     CDC  150  acquires capacitance measurements from capacitive sensor  110 , and converts these capacitance measurements to digital sensor data representative of paper condition/characteristics. The CDC sensor data can be input to data processor  170 , and processed to provide paper tray status information. 
       FIGS. 2A, 2B and 2C  illustrates an example paper tray  201  with capacitive sensing adapted for measuring paper size/width, including a capacitive sensor with example capacitive electrodes CIN 1  and CIN 2  ( 211  and  212 ). The example capacitive electrodes CIN 1  and CIN 2  are substantially identical in configuration with a length a (a1 and a2) and width b, with an area A=a*b. The capacitive sensing system includes a capacitive sensor structure with (shielded) capacitive electrodes CIN 1  and CIN 2  configured for capacitance measurements of paper size/width based on projected self-capacitance. 
       FIGS. 2A and 2B  are top and side views that illustrate an example arrangement for the elements of the capacitive sensor incorporated with a paper tray  201 , and in particular, the placement of the capacitive electrodes CIN 1  and CIN 2  relative to the paper  220 , which is aligned within the tray at  203 . Specifically, capacitive sensor/electrode CIN 1  is positioned so that it is covered by paper  220  in tray  201 , and capacitive sensor/electrode CIN 2  is positioned relative to the width dimension of the paper so that it is partially covered by paper  220  in tray  201 : CIN 2  has a length a2=a2p+a2a, where a2p is a portion of the CIN 2  length a2 that is over the paper, and a2a is a portion of the CIN 2  length a that is not over the paper. 
     Referring to  FIG. 2B , a capacitive sensor includes capacitive electrodes CIN 1  and CIN 2  ( 211  and  212 ), shield  213  and insulator  214 , integrated or mounted within tray  201 . A ground plane  219  is spaced from the capacitive sensors CIN 1  and CIN 2  in the projection direction. As illustrated ground plane  219  is on the bottom side of tray  201 —alternatively, the ground plane can be located within the tray, adjacent paper  201 . 
     For paper size/width measurement, CIN 1  measurements are used to calibrate for the type of paper, and combined CIN 1  and CIN 2  measurements are used to determine paper size/width. If CIN 1  and CIN 2  are not identical, CIN 2  can be calibrated. 
     An example methodology for determining paper size/width based on capacitive sensing involves first calibrating for tray thickness and sensor size/position based on a capacitive measurement CA 0 =CIN 1  to ground (with no paper present). 
     For paper size/width measurement operations, with paper present, CIN 1  and CIN 2  measurements are captured: 
     CA 1 =CIN 1  to ground 
     CA 2 =CIN 2  to ground
 
 C DIFF= CA 1 −CA 2
 
     Paper width can be determined from a percentage of CIN 2  covered by paper, as represented by CDIFF. 
     An example methodology for determining paper size/width is based on measuring CA 0 , CA 1 , and CA 2 , and using the following relationships: 
     
       
         
           
             
               
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             ε A  is the dielectric constant of the air, k accounts for fringing 
           
         
       
    
     
       
         
           
             
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             where ε p  is the dielectric constant of the paper, and 
           
         
       
    
     
       
         
           
             CDIFF_w 
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             a2p is the length of the portion of CIN 2  electrode covered by the paper 
             a2a is the length of the portion of CIN 2  electrode not covered by the paper
 
 a 2= a 2 p+a 2 a  
 
where  C DIFF_ w=CA 1*(1− a 2 p/a 2)− CA 0*( aa/a )
 
 a 2 p=a 2− a 2 a  
 
 a 2 a =( a 2 *C DIFF_ w )/( CA 1− CA 0).
 
           
         
       
    
     This methodology for determining paper size/width is independent of the dielectric of paper ε p . 
     Referring to  FIG. 2B , the capacitive sensor (shield  213 , insulator  214 , electrodes CIN 1 /CIN 2  is mounted above tray  201  and paper  220 , so that the sensing field projects through the paper  220  toward the ground plane  219 . In an alternate configuration, the sensor can be mounted on the tray, oriented 180 degrees to the orientation illustrated, with the sensor electrodes CIN 1 /CIN 2  adjacent paper  220 . 
     A third capacitive sensor/electrode CIN 3  can be used to measure paper length using a similar method. Capacitive sensor/electrode C IN3  can be positioned so that it is partially covered in the length dimension by paper  220  in tray  201  (substantially as illustrated for CIN 2  for the width dimension). 
       FIG. 2C  provides example measurement results based on the following parameters: 
     CIN 1  and CIN 2  electrode length is a=104 mm (a1+a2) 
     Letter paper is approximately 6 mm wider than A4 
     Cdiff_nopaper accounts for non-identical electrodes and is subtracted from CDIFF. 
     Example design modifications for the configuration of the capacitive electrodes CIN 1  and CIN 2  include, in addition to size/perimeter, different shapes/profiles, such as spiral. 
       FIGS. 3A, 3B and 3C  illustrates an example paper tray with capacitive sensing for measuring paper stack height and page count. The capacitive sensing system includes a capacitive sensor structure with a (shielded) capacitive electrode CIN 1  ( 311 ) configured for capacitance measurements of paper stack height/page count based on projected self-capacitance. 
       FIGS. 3A and 3B  are top and side views that illustrate an example arrangement for the elements of the capacitive sensor incorporated with a paper tray  301 , and in particular, the placement of the capacitive electrode C IN1  relative to the paper  320 , which is aligned within the tray at  303 . Specifically, capacitive sensor/electrode C IN1  is positioned so that it is covered by paper  220  in tray  201 . 
     Referring to  FIG. 3B , a capacitive sensor includes a capacitive electrode CIN 1  ( 311 ), shield  313  and insulator  314 , integrated or mounted within tray  301 . A ground plane  319  is spaced from the capacitive sensor CIN 1  in the projection direction. As illustrated ground plane  319  bottom side of tray  301 —alternatively, the ground plane can be located within the tray, adjacent paper  301 . 
     An example methodology for sensing paper stack height involves first calibrating for tray thickness and sensor size/position based on a capacitive measurement CA 0 =CIN 1  to ground with no paper present. 
     For stack height measurement operation, with paper present, the CIN 1  measurement is captured: CA 1 =CIN 1  to ground, which is proportional to a total thickness of paper between CIN 1  and ground, i.e., total paper stack height. 
     Page count can be determined from an initial sheet feed. An example methodology for calculating the number of pages in the paper stack includes: (a) feed one paper sheet, and determine from capacitive measurements the change in stack height, so that (b) page count=previous stack height/change in stack height. 
     An example methodology for determining page count includes two determinations from the capacitance measurement CA 0 . First, determine capacitance CA 1 , 0 : 
                     CA   ⁢           ⁢   1     ,     0   =       ⁢       (       CAIR     -   1       +     CPAPER     -   1         )       -   1                     =       ⁢       [       1     CA   ⁢           ⁢   0       +       dp     ɛ   ⁢           ⁢   0   ⁢           ⁢   A       ⁢     (       1     ɛ   ⁢           ⁢   P       -     1     ɛ   ⁢           ⁢   A         )         ]       -   1                   
where dp is the total thickness of paper between sensor CIN 1  and ground; and where:
 
               CA   ⁢           ⁢   0     =     k   *     ɛ   0     *     ɛ   A     *     A   d                   CAIR   =     k   *     ɛ   0     *     ɛ   A     *     A     d   -   dp                     CPAPER   =     k   *     ɛ   0     *     ɛ   P     *     A   dp             
and where
 
     da is the total thickness of air between the sensor and ground
 
 d=dp+da  
 
 da=d−dp  
 
     ε A  is the dielectric constant of the air 
     k accounts for fringing 
     A is sensor area. 
     Then feed one page of paper, and determine capacitance CA 1 , 1   
               CA   ⁢           ⁢   1     ,     1   =       [       1     CA   ⁢           ⁢   0       +         (     dp   +     d   ⁢           ⁢   1   ⁢   page       )       ɛ   ⁢           ⁢   0   ⁢           ⁢   A       ⁢     (       1     ɛ   ⁢           ⁢   P       -     1     ɛ   ⁢           ⁢   A         )         ]       -   1               
where d 1page  is the thickness of one sheet of paper.
 
     Capacitance CA 1 , 0  can be used to determine dp as the total thickness of paper between sensor CIN 1  and ground (total stack height), and CA 1 , 1  can be used to determine d1page is the thickness of one sheet of paper. Then page count can be determined as: page count=dp/(d1page). 
     This initial-sheet-feed methodology, which provides sheet thickness d1page does not require prior knowledge of the dielectric constant of the paper ε p . 
     The dielectric constant of the paper ε p  (paper type) can be determined from the above measurement for paper stack height and page count, including the determination of sheet thickness d 1page , which enables computation of the average dielectric ε eff  between C IN1  and GND. 
     Average dielectric ε eff  and the dielectric constant of the paper ε p  are related by: 
               ɛ   eff     =     1       nw   /     d   ⁡     (       1   /     ɛ   paper       -     1   /     ɛ   air         )         +     1   /     ɛ   air                 
where n=number of pages, w=sheet thickness (d1page), so that nw is stack height (dp), and d is the distance between the capacitive electrode CIN 1  and GND.
 
     Based on the known values: 
     Distance between CIN 1  and GND, d 
     Average dielectric between CIN 1  and GND, ε eff    
     Number of pages in the stack n (d1page/d) 
     Thickness of a single sheet of paper w 
     paper dielectric ε paper  (paper type) can be determined from: 
     
       
         
           
             
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       FIG. 3C  provides example page count measurement results, comparing expected to actual results. 
     As an alternate embodiment for determining page count using an initial sheet feed (i.e., to determine sheet thickness d1page), the dielectric of the paper ε paper  can be capacitively sensed, and page count determined if sheet thickness is known, or assumed. 
       FIG. 4  illustrates an example capacitive sensor arrangement adapted for measuring paper stack height/page count based on capacitively sensing paper dielectric. Two capacitive sensors are used. A capacitive sensor/electrode CIN 1  ( 411 ) is used as in the embodiment in  FIGS. 3A /B. An inter-digitated (co-planar) capacitive sensor with capacitive/ground electrodes E 1 /G 1  is used for measuring paper dielectric ε paper . 
     Capacitive electrode CIN 1  is operable for the capacitive measurement CA 1 , 0  as described in connection with  FIGS. 3A / 3 B—the capacitive measurement CA 1 , 1  after an initial sheet feed (to obtain d1page) need not be taken. The CIN 1  ground plane  419  can be disposed relative to CIN 1  as described above in connection with  FIGS. 3A / 3 B. 
     For capacitive sensing in connection with determining paper dielectric ε paper , the inter-digitated (co-planar) capacitive/ground electrodes E 1 /G 1  are disposed on the interior surface of a paper tray, for example at the alignment corner (in  FIG. 3A , at  303 ). The sensor/ground electrodes E 1 /G 1  are configured and oriented in an inter-digitated co-planar arrangement, preferably so that the projected fringe fields are confined to the typical thickness of one sheet of paper (approximately 100 microns). 
     Capacitance CD is measured with and without paper, to obtain the paper dielectric ε paper . 
     For this embodiment, which does not require an initial sheet feed to determine paper thickness (i.e., d1page), page count requires knowledge of paper dielectric ε paper  and paper sheet thickness. Paper thickness can be determined by, for example, separate input, or based on assumption, for example, a standard paper thickness of approximately 100 microns. 
     The Disclosure provided by this Description and the Figures sets forth example embodiments and applications, including associated operations and methods, that illustrate various aspects and features of the invention. Known circuits, functions and operations are not described in detail to avoid unnecessarily obscuring the principles and features of the invention. These example embodiments and applications can be used by those skilled in the art as a basis for design modifications, substitutions and alternatives to construct other embodiments, including adaptations for other applications. Accordingly, this Description does not limit the scope of the invention, which is defined by the Claims.