Abstract:
A sensor assembly for measuring the displacement or position of a movable member. The sensor assembly utilizes two or more smaller, less accurate sensors to perform the function of a single, more accurate larger sensor. The resolution of each of the smaller sensors is increased by configuring the sensor system such that the maximum travel of each sensor in a direction covers half or less of the travel of the moveable member being measured. Each sensor then moves in an opposite direction to return to its original position for the remainder of the travel of the movable member. Utilizing a cam follower system, two sensors generate a set of output signals at each position of the moveable member in a one-to-one correlation over the range of travel of the moveable member.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to printers and printing systems, and more particularly to a system for automatically detecting and measuring the size of print media loaded in a print media supply bin for printing. 
     BACKGROUND OF THE INVENTION 
     Present day imprinting mechanisms, for example, electrophotographic copiers, laser printers, inkjet printers and the like, are generally capable of handling more than a single size of print media. An automatic manner of sensing the size of print media which has been loaded in a supply bin for inputting to a printing mechanism is the subject to be addressed herein. The print media may be any type of substantially flat material, such as plain paper, specialty paper, card-stock, fabric, transparencies, foils, mylar and the like, but the most common type or print media is paper. For convenience, I will discuss printing on paper as a representative example of these various types of print media. The media may be supplied to the printing mechanism in a variety of different sizes. For instance, in desktop inkjet or laser printers, paper is typically supplied in a stack of cut-sheets, such as letter size, legal size, or A-4 size paper, which are loaded or placed in a supply bin or input tray for subsequent input to the printing mechanism. Smaller sized envelopes or postcards or other media sizes may also be used for printing. Typically, the media sheets, cards or envelopes are sequentially pulled from the top of the stack and printed on, after which the printed sheets are deposited in an output tray. 
     It is desirable to have a printer which can measure and communicate what size media has been loaded in the input tray to the printer controller and to a host computer, particularly when the printer is not within sight of the computer user, such as when several computer users on a networked system share a printer or printers. Even when a printer is located on the users desktop, it may be helpful to provide the user with a warning if the wrong size media is loaded for a particular print job. This would tell the user to load the proper size media, or to adjust the print job parameters to fit the size of the loaded print media. 
     Automatic media size sensing must be highly accurate to be useful. In order to avoid toner contamination in the mechanism, this is particularly true for printing mechanisms which are capable of printing to the edges or near the edges of the sheets of print media. Sensing of media length and width is a feature often found in high-end printers and plotters for business and industrial use. However, these size sensing devices are typically quite cost-sensitive and general application sensing devices of the quality required can be very costly. Typically, for the small business and home markets where there is much more price sensitivity, automated media type detection has not been economically viable. 
     In the past, various methods have been used for media size determination. For example the use of a retroflective photo diode or a capacitance sensor mounted at some point in the printer paper path or on the print carriage to detect leading and trailing or side edges of the media sheet. However, retroflective and capacitive sensors are themselves expensive with the added direct and indirect costs incurred due to where within the printer the sensors are mounted and operate. Linear variable resistive and capacitive sensors mounted within the input paper tray tend to be less expensive than other methods, but, nevertheless, sensors of the required accuracy and sensitivity are expensive. 
     For example, one method of print media size detection utilizes a linear potentiometer mounted in the input paper tray. While this method provides a continuous output (i.e., a smooth function of output versus position), it can be either expensive, using large sensor components, or have lower accuracy due to scaling down the traverse with gears, levers or the like, and using smaller potentiometers. Another method is to use absolute encoders (optical, magnetic, etc, for example) to measure the true position of a movable member. In a printing mechanism supporting sizes of A-5 ledger media (222 mm of travel), an encoder would require 8 bits of resolution for 1 mm accuracy. Again, this can be expensive. 
     Accordingly there is a need for a sensor for determining what size print media is loaded in a printer input or supply tray having sufficient accuracy to be useful yet inexpensive enough to be utilized for low-end applications such as small business and home use. 
     SUMMARY OF THE INVENTION 
     In a preferred embodiment, the present invention provides a sensor assembly that measures the displacement of one member with respect to another member with a high degree of accuracy and at a low cost. A sensor assembly embodying the present invention utilizes multiple low cost, smaller sensor components to provide a less expensive and more accurate sensor assembly that a single, larger sensor performing the identical task. 
     In one embodiment the present invention provides a sensor assembly for measuring the displacement of a movable member. The sensor assembly utilizes two or more smaller, less accurate sensors to perform the function of a single, larger sensor. Each sensor includes a movable portion and a stationary portion, each sensor generating an output signal in response to movement of the movable portion with respect to the stationary portion. In one embodiment, the sensors are attached to a movable member while the movable portion of each sensor is in contact with a contoured surface of a stationary member. Movement of the moveable member causes the moveable portion of each sensor to move along and follow the contoured surface of the stationary member, which, in turn, causes movement of the movable portion of each sensor relative to the stationary portion of the sensor. The amount of movement of the movable portion of the sensor is proportional to the displacement of the movable member with respect to the stationary member. The portion of the contoured surface each sensor movable portion is in contact with has a different profile. As a result, for a given displacement of the movable member, the movable portion of each sensor will have a different amount of travel with respect to the sensor stationary portion, and thus produce a different output signal. The contoured surface of the stationary member is profiled to reverse the direction of motion of each of the movable portions with respect to the stationary portions at least once over the range of movement of the movable member. The output signals of the two or more sensors then form a set which uniquely identifies the displacement which produced the set of output signals. 
     In a preferred embodiment, the present invention is embodied in a printer to provide an apparatus for measuring the size of print media sheets loaded in a print media supply bin. The supply bin (i.e., input paper tray) includes a movable member which serves as a print media fence or stop mounted and disposed at a variable position within the supply bin. The position of the movable member is adjustable to allow print media sheets of differing sizes to be loaded in the supply bin. At least two sensors are mounted on the movable member. Each sensor includes a movable portion and a stationary portion, the movable portion of each sensor contacting an associated contoured surface of the supply bin. In response to displacement of the movable member with respect to the supply bin, the movable portion of each sensor follows its associated contoured surface thereby producing movement of the movable portion with respect to the stationary portion. The stationary portion of each sensor includes a variable electrical element which is responsive to movement of the movable portion with respect to the stationary portion to generate an output signal indicative of the displacement of the movable member with respect to the supply bin. The output signals generated by the at least two sensors are indicative of a unique position of the movable member with respect to the supply bin. The unique position is then associated with a print media sheet size. 
     In one preferred embodiment, the sensor variable electrical element may be a variable resistance, the sensor forming a linear potentiometer, and the output signals generated by the sensors forming a unique set of values for each unique position of the movable member within the supply bin. In a preferred embodiment, each unique set of values corresponds to and forms a pointer to a print media sheet size held in a lookup table, each unique position of the movable member corresponding to a different print media sheet size. 
     Other embodiments and advantages of the present invention will be readily appreciated as the same become better understood by reference to the following detailed description, taken in conjunction with the accompanying drawings. The claims alone, not the preceding summary or the following detailed description, define the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the present invention and are incorporated in and constitute a part of this specification. The drawings illustrate the embodiments of the present invention and together with the following detailed description illustrate by way of example the principles of the present invention. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. In the drawings like reference numbers indicate identical or functionally similar elements throughout the several views thereof, and wherein: 
     FIG. 1 is a block diagram illustrating a typical printing system paper conveyance mechanism including a print media sheet size detector according to the principles of the present invention; 
     FIG. 2 is a schematic drawing illustrating a preferred embodiment of the present invention; 
     FIG. 3 is a schematic drawing illustrating the detail of a sensor element shown in FIG. 2; 
     FIG. 4 is a graph illustrating sensor motion with respect to displacement of a movable member; 
     FIG. 5 is a top plan view of a printer paper supply bin implementing the present invention; and 
     FIG. 6 is a lookup table correlating a physical characteristic of an article with sensor output according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As shown in the drawings for purposes of illustration, the present invention is preferably embodied in a sensor assembly which measures the displacement or position of a movable member with respect to stationary member. In one embodiment of the invention, two or more relatively small and inexpensive sensor elements are utilized to measure position or displacement with greater accuracy and at less expense than when using a single, larger and more accurate sensor to perform the same measurement. 
     Referring now to FIG. 1, a block diagram illustrating a typical printing system paper conveyance mechanism including a print media sheet size detector according to the principles of the present invention is shown. The printing system  10  includes one or more print media supply bins, such as paper input tray  12 , wherein sheets  14  of print media, such as paper, are stacked (i.e., loaded). The media feed portion of the print system  10  includes a media feed motor  16  mechanically coupled to the input tray  12  which conveys the media sheets  14  to a pinch roller  18  and an input or draw-in capstan roller  20 . A media conveyance means, such as step motor  22  is mechanically coupled to the capstan roller  20  draw the sheets  14  of print media into the printer and further convey the print media  14  to a print mechanism or print engine. The print engine may, for example, be the printing mechanism for a laser printer including a photoconductive drum  24 . Alternatively, the print mechanism may be an inkjet or sublimation color printer, for example. A media output portion of the print system  10  includes an output motor mechanically coupled to an output capstan roller  28  and another pinch roller  30 . As a sheet  14  of print media moves through the print engine past the photoconductive drum  24 , for example, the media sheet  14  is received by the output capstan  28  and pinch roller  30  and conveyed out of the print mechanism to an output tray or other receptacle (not shown). A central processing unit (CPU)  32  controls the overall operation of the printing system  10  from the print media feeding to the print media output. 
     In accordance with the principles of the present invention, a print media size sensor  34  is electrically and mechanically coupled to the media input tray  12 . The sensor  34  automatically detects the size of the sheets  14  of print media stacked in the input tray  12  and transmits the print media size information to the CPU  32 . The CPU  32 , then, uses the print media size information to control the feeding of sheets  14  of print media to the print mechanism and the operation of the step motor  22  and output motor  26  thus providing the conveyance of the print media through the print system  10  and allowing a continuously printing operation. 
     Referring now also to FIGS. 2 and 3, in a preferred embodiment, the sensor  34  is a sensor assembly  34  which includes two or more sensor elements  36  and  38 . Each of the sensor elements  36 ,  38  further includes a stationary portion  40  and a movable portion  42 . Each sensor element  36 ,  38  also includes a variable electrical element  44 , such as a variable electrical resistance ( as shown in FIG. 3) or a variable electrical capacitance, for example. As the moveable portion  42  moves with respect to the stationary portion  40 , the electrical value of the variable electrical element  44  changes proportionally to the movement of the moveable portion  42 . For example, as shown in FIG. 3, a slidable electrical contact  46  is displaced along the variable resistance  44  with movement of the moveable portion  42 . In this example, the variable resistance  44  is connected as a linear potentiometer, as is known in the art. The resistance of the potentiometer changes linearly with displacement of the sensor element moveable portion  42 . For any given position of the moveable portion  42  with respect to the stationary portion  0 , the potentiometer will have a specific resistance. Alternatively, the variable electrical element  44  may be a non-linear resistance, a linear or non-linear capacitance, or a linear or non-linear inductance. When connected in an electrical circuit, as is known in the art, each sensor element  36 ,  38  will generate an electrical output signal, the value of which is a function of the position of the slidable contact  46 . 
     The sensor assembly  34  is fixedly attached to a movable member  48  which is moveably attached to a stationary member  50 . In a preferred embodiment, the stationary member  50  includes at least two contoured surfaces  52  and  54 . Each of the contoured surfaces  52 ,  54  is associated with one of the sensor elements  36 ,  38 . The sensor assembly  34  is attached to the moveable member  48  in such a manner that the moveable portion  42  of each sensor element  36 ,  38  is in contact with the associated contoured surface  52 ,  54 . For example, an end  56  may be in sliding contact with the associated contoured surface  52 ,  54 , or, alternatively, the moveable portion  42  may have a wheel or roller  58  rotatably mounted at the end  56  and be in rolling contact with the associated contoured surface  52 ,  54 . Alternatively, the sensor assembly  34  may be fixedly attached to the stationary member  50  while the moveable member  48  includes at least two contoured surfaces  52  and  54 . Similarly, each of the contoured surfaces  52 ,  54  is associated with the moveable portion  42  of one of the sensor elements  36  and  38 . The contoured surfaces  52 ,  54  are disposed on a surface of the moveable member  48  and adapted in such a manner that as the moveable member  48  moves with respect to the stationary member  50 , the moveable portion  42  of each sensor element  36 ,  38  is in contact with the associated contoured surface  52 ,  54  and follows the profile of the associated contoured surface  52 ,  54 . 
     As the moveable member  48  moves or is displaced with respect to the stationary member  50  in the directions indicated by the arrow  60 , for example, the sensor elements  36 ,  38  will move along with the movement member  48  and the end  56  of the moveable portion  42  of each sensor element will follow the surface of the associated contoured surfaces  52 ,  54  (for example, in a “cam and cam-follower” relationship). In turn, as a result of the displacement of the moveable member  48  with respect to the stationary member  50 , the moveable portion  42  of each sensor element  36 ,  38  will be displaced a proportional amount with respect to the stationary portion  40  in the directions shown by the arrow  62 . The contoured surfaces  52 ,  54  are designed with sufficient rise and fall (i.e., positive and negative slope) and transitions (i.e., peaks and valleys) to insure full travel of the moveable portion  42  and to insure that the direction of travel of the moveable portion  42  with respect to the stationary portion  40  reverses at least once over the range of travel of the moveable member  48  with respect to the stationary member  50 . 
     Referring now also to FIG. 4, a set of graphs illustrating the output signals of the sensor elements  36  and  38  is shown. The output signal for each sensor element  36  and  38 , also referred to as sensor b and sensor a, respectively, is shown as a triangular waveform (idealized for clarity)  64  and  66 , respectively, where the horizontal axis represents displacement or position of the moveable member  48  with respect to the stationary member  50  and the vertical axis represents the fraction of full travel for the sensor element moveable portion  42  with respect to the stationary portion  40 . As the moveable member  48  is displaced or moved with respect to the stationary member  50 , the motion of the moveable portion  42  of sensor a,  38 , and sensor b,  36 , will follow the profile of the associated contoured surface  54  and  52 , respectively. In a linear system, the output signal of the sensor elements  36 ,  38  will follow the general profile of the associated contoured surface  52 ,  54 . The output signal for sensor a,  38 , over the range of travel for the moveable member  48  is plotted in graph  66 , while graph  64  shows the output signal for sensor b,  36 , over the same range of travel for the moveable member  48 . When considering the output signal for a single sensor element, sensor a,  38 , for example, a given output a 1  will not correspond to a unique position of the moveable member  48 . However, the output of both sensor elements, sensor a,  38 , and sensor b,  36 , form a set of values, a 1 , b 1 , which correspond to a unique position, S 1 , of the moveable member  48 . Using two or more sensor elements  36 ,  38  in combination provides a one-to-one correlation between the output of the sensors and the position of the moveable member  48  with respect to the stationary member  50  along the entire range of movement of the moveable member  48 . The profile of the contoured surface associated with a given sensor element must be different than the profiles of the contoured surface associated with each of the other sensor elements. Also, the profiles of the contoured surfaces  52 ,  54  are designed such that the reversal point for the motion of each sensor moveable portion  42  is not near the reversal point of another of the sensor elements which increases the accuracy of the system near the reversal points and insures that the unique one-to-one relationship (output to position) is maintained. 
     The output signals of the sensor assembly  34  may be utilized for many purposes. For example, in a preferred embodiment, the output signals may be used directly to calculate the displacement of the moveable member  48  with respect to stationary member  50 . Alternatively, the output signals of the sensor assembly  34  may be used either directly or indirectly to provide a measurement or dimension of an article or object associated with the moveable member  48 . 
     Referring now also to FIGS. 5 and 6, in a preferred embodiment, the sensor assembly  34  of the present invention is implemented in a printer system  10  (as shown in FIG. 1) to measure the size of sheets  14  of print media loaded or stacked in a print media input tray  12 . The print media supply bin or input paper tray  12  includes a rigid bottom member or plate  68  upon which sheets  14  of print media can be stacked. Typically, the sheets  14  of print media will also be confined by vertical walls (not shown) extending from the edges of the bottom plate  68 . The bottom plate  68  includes a channel  70  formed therein for receiving a slidably mounted support  72  therein, the position of the support  72  being adjustable with respect to the input tray bottom plate  68 . At one end of the support  72  a vertically extending wall or fence  73  is fixedly mounted to the support  72  and is moveable therewith. The fence  73  serves as a “media stop” restraining and confining the sheets  14  of print media in the proper position for feeding to the printer. The fence  73  is adjustable, sliding in the channel  70 , to provide for print media or paper of differing sizes to be stacked in the paper tray  12 . The range of movement for the fence  73  can be as great as 10 to 12 inches to accommodate the various sizes of print media typically used in a large, general purpose print system. In another embodiment, an additional fence  74  fixedly attached on a support  76  will be slidably mounted in a second channel  78  formed in the bottom plate  68 . As described above with respect to the adjustable fence  73 , the position of the fence  74  is also adjustable with respect to the bottom plate  68  thus the print media sizes receivable by the input tray  12  to vary both in length (L) and in width (W). 
     As described above with reference to FIGS. 2,  3  and  4 , the sensor assembly  34  of the present invention is fixedly attached to the media stop, fence  73 , at a convenient location. In a preferred embodiment, the sensor assembly  34  may be attached to portion of the fence  73  extending beyond a lower surface (not shown) of bottom plate  68  with the associated contoured surfaces  52 ,  54  disposed on the lower surface of the bottom plate  68  for cooperation with the sensor element moveable portions  42  (as shown in FIG.  2 ). When print media, such as paper, for example, is stacked in the paper input tray  12 , the position of the fence  73  is adjusted to confine and restrain the paper in its proper position. As the fence  73  is moved to the desired position, the moveable portion  42  of each sensor element  36 ,  38  of the sensor assembly  34  will move to a position with respect to the stationary portion  40  corresponding to the position of the fence  73  with respect to the input tray bottom plate  68 . The output signals of the sensor elements, sensor a and sensor b, for example, a 1 , b 1 , corresponding to the position of the fence  73  will be transmitted to the printer CPU  32  (as shown in FIG. 1) to control the printer operation. In the CPU  32 , the sensor assembly  34  output signals preferably are used to reference a look-up table  80  (as shown in FIG. 6) to obtain the print media size corresponding to the position of the fence  73 . For each set of sensor element output signals, a 1 , b 1 , a 2 , b 2 , a 3 , b 3  . . . a n , b n    82  the look-up table  80  will have a corresponding print media size, S 1 , S 2 , S 3  . . . S n    84 . Alternatively, the printer CPU  32  could use the sensor assembly  34  output signals directly without reference to a look-up table. 
     In addition to the foregoing, the logic of the present invention, including the lookup table shown in FIG. 5 can be implemented in hardware, software, firmware, or a combination thereof. In the preferred embodiment(s), the logic is implemented in software or firmware that is stored in a memory and that is executed by a suitable instruction execution system. If implemented in hardware, as in an alternative embodiment, the logic can be implemented with any or a combination of the following technologies, which are all well known in the art: a discrete logic circuit(s) having logic gates for implementing logic functions upon data signals, an application specific integrated circuit (ASIC) having appropriate logic gates, a programmable gate arrays(s) (PGA), a field programmable gate array (FPGA), etc. 
     While having described and illustrated the principles of the present invention with reference to various preferred embodiments and alternatives, it will be apparent to those familiar with the art that the invention can be further modified in arrangement and detail without departing from those principles. Accordingly, it is understood that the present invention includes all such modifications that come within the terms of the following claims and equivalents thereof.