Abstract:
Position sensors having an active area are assembled into an array in which the electrodes of adjacent sensors are offset and interleaved so that the electrodes are adjacent an active area, thereby avoiding dead spots. The position sensor array is position adjacent to the path of an object opposite to a source of light. As the object passes it alters the distribution of light on the position sensor array that generates signals indicative of the objects position in response to the changes in impinging light.

Description:
FIELD OF THE INVENTION 
     The present invention is directed to a system and method for automatically detecting the dimensions or edge of a document being scanned by a scanning system. More particularly this invention is directed to an overlapping sensor array which provides positioning accuracy independent of tracking distance and eliminates dead zones in large area scanning. 
     BACKGROUND OF THE INVENTION 
     Traditionally, the term copier, when used in the office equipment context, refers to a light lens xerographic copier in which paper originals are reproduced. Images of the original document are focused on an area of a photoreceptor, which is subsequently developed with toner. The developed image on the photoreceptor is then transferred to a copy sheet which in turn is used to create a permanent copy of the original. 
     Digital copiers are now available that perform the same functions as a light lens copier, except that the original image to be copied is not directly focused on a photoreceptor. Instead, with a digital copier, the original image is scanned by a device generally known as a raster input scanner (RIS) which is typically in the form of the linear array of small photosensors. 
     The original image is focused on the photosensors in the RIS. The RIS converts the various light and dark areas of the original image to a set of digital signals. These digital signals are temporarily retained in a memory and then eventually used to operate a digital printing apparatus or for other reproduction purposes. The digital printing apparatus can be any known type of printing system responsive to digital data. 
     With the migration of the copying and scanning systems to a digital base system, the systems face different problems than from the light lens or analog copying systems. More specifically, in a digital scanning system, the scanning system needs to locate the document in the operational sequence of the machine. This needs to be accomplished accurately for image processing and other functions. 
     In the past this tracking function was accomplished by an array of sensors which generate pixel signals representing the entire page. This creates a significant amount of extraneous data when all that is needed are data indicative of the edge of the document. A similar approach has been used utilizing individual or groups of position sensors place in a location in the document path at which the document is made to cast a shadow over the sensor which alters the output of the sensor and thereby generates an indication of the location of an edge of the document. 
     Position sensors of this type are available from, for example, SDT Sensors, Inc. of Hawthorne, Calif. These lateral photodiode position sensitive detectors comprise a semiconductor strip of silicon having a composition and structure that, when stimulated by the impingment of light, collects a current at each end of the strip. By a simple mathematical comparison of the relative currents at each electrode, the centroid of the light impinging on the strip can be determined. This data allows the position of an edge of a document to be determined as the object alters the distribution of light on the sensor strip. A problem with such sensors is that their accuracy falls off as the centroid of light approaches either of the electrodes. This hinders the assembly by abutment of such sensors into arrays as dead spots will occur that detract from the reliability and accuracy of the generated data. 
     It is an object of this invention to provide a simple sensing device for tracking the position of an edge of a document or other object as it proceeds through the operational cycle of a machine such as a printer or multifunction digital scanner. It is a further object of this invention to construct an array of semiconductor position sensors to accomplish edge tracking without dead spots. The invention may be used to determine the position of any object along an object transport path with a prescribed resolution throughout. It is particularly suited for large area tracking. 
     SUMMARY OF THE INVENTION 
     Semiconductor position sensors consisting of strips of light sensitive silicon composition film are constructed with electrodes at either end of each strip that collect currents generated by impinging light on the sensors. The sensor strips are strategically positioned in the path of an object whose position is to be monitored. As the object passes by the sensor the impingement of light on the sensor is altered. The currents generated are indicative of the position of the centroid of light impinging on the strip. From this data, the position of the edge of an object passing the position sensor can be monitored. According to this invention, multiple position sensors are assembled into an array in which the electrodes at the ends of individual sensors are positioned in overlapping interleaved relation to adjacent sensors. The array is assembled to form a sensing plane in which each sensor is located next to another sensor, but offset so that an edge image or shadow, passing from one electrode to the other of a particular sensor, is always positioned on the central zone of high accuracy of at least one sensor. This can be accomplished by physical placement of individual sensors or by shaping the sensor strips to allow the necessary interleaving. It also can be accomplished by a patterned installation of the electrodes to form sensing zones on a strip in which each zone can function substantially the equivalent of individual sensors. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described by way of example with reference to the accompanying drawings, wherein like reference numerals refer to like elements, and in which: 
     FIG. 1 is a block diagram of the position monitoring system of the subject invention; 
     FIG. 2 is a schematic diagram of a position sensor used in the subject invention; 
     FIG. 3 a  is a top view of a first embodiment of an array of position sensors constructed according to the subject invention; 
     FIG. 3 b  is an edge view of a first embodiment of an array of position sensors constructed according to the subject invention; 
     FIG. 4 second embodiment of an array of position sensors constructed according to the subject invention; and 
     FIG. 5 is a third embodiment of an array of position sensors constructed according to the subject invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As shown in FIG. 1, a sensor array  1  is constructed to provide an accurate indication of the position of an object, for example a document  2 , as the document  2  progresses through an image processing system. The processing system may be for use in any number of applications for example, scanning, printing, copying, and the like. It can be used to track packages, luggage, or parts on an assembly line. With the use of low powered ir beams, even the position of people moving by a fixed location can be determined. This invention is useful in any situation where objects are moving past a sensor. The operation of this invention will be described in terms of a document  2  being processed within a multifunction printing machine  8  that includes a scanner  9 , image processor  10 , and print engine  11 . The machine is controlled by a central process controller  7  which receives the document position information from the document monitoring system  6  and processes it for use in multiple functions throughout the printing machine operating cycle. 
     Monitoring system  6  consists of sensor array  1 , which is positioned to receive light from a light source  3 . Sensor array  1  generates a two channel signal from each sensor. The two channel signal is converted from analog to digital form in converter  4 . The digital signal is used to calculate the position of the centroid of the light impinging on array  1  by the centroid position processor  5 . As document  2  progresses through the operational path  12  of document handling system  14  in the general direction of arrow  15 , it will generate a shadow, as it passes between light source  3  and sensor array  1 . This will cause the centroid to move across the sensor array  2  and provide an accurate indication of the position of the edge  13  of document  2 . As an alternative, reflected light could be used as the source by positioning the light source  3  on the same side as the detector array  1 . In this instance, the light scatters off the paper into the detector. The centroid of the scattered light would also provide an accurate indication of the position of the edge  13  of document  2 . 
     An individual position sensor  20  is shown schematically in FIG.  2 . The sensor  20  is formed as a strip of silicon semiconductor material whose surface composition is stimulated by light to generate currents I 1  and I 2  flowing to its electrodes  21  and  22 . Electrodes  21  and  22  are connected to leads  24  and  25  respectively. Currents I 1  and I 2  may be compared to obtain an indication of the position X of the centroid of light  26 . As document  2  passes in the direction of arrow  15 , the centroid X will move from left to right in FIG.  2 . The centroid position, measured from electrode  21 , may be calculated by the formula 1: 
     
       
           X=L/ 2-( I   1   −I   2 ) L/ {2( I   1   +I   2 )} or  L[I   2 /( I   1   +I   2 )]; 
       
     
     where L is the distance between the electrodes  21  and  22 . 
     The position p of the edge  13 , with respect to electrode  21 , may then be determined by the formula 2: 
     
       
           p =2 X−L  if  X&gt;L/ 2 or  p =2 X  if  X&lt;L/ 2. 
       
     
     These formulas assume that the dark portions of the image are completely dark, and provide zero current. It is straightforward to extend them to the case when the dark portions have non-zero illumination by subtracting out the non-zero dark portions. It can be observed from these calculations that the accuracy of the resulting position will diminish as the edge approaches either electrode. In this case I 1 ≈I 2  so there is no accuracy for X≈L/2 and therefore p switches between 0 and L for small noise fluctuations. This results in a zone of low signal to noise ration near the electrodes and resulting inaccuracies as indicated by dashed reference lines  23  of FIG.  2 . The effective active area of each sensor is limited to the central region  26  of the sensor element. This characteristic of position sensors has limited their use in applications requiring high accuracy and large sensing areas. To avoid such limitations, an array of position sensors is constructed as described below. 
     In the embodiment illustrated in FIG. 3 a,  the position sensor array  1  is constructed of groups  30 - 33  of position sensors  34 . Each sensor in a group is spaced from its adjacent sensor by a gap  35 . Groups  30 - 33  are arranged in an interleaved manner with the position sensors  34  of adjacent groups extending into the gaps  35  so that the ends of the sensors of each group overlap. This forms a sensing plane in which the regions of inaccuracy surrounding the electrodes of each sensor are offset from adjacent sensors to provide a continuous sensing surface. The sensing surface of this sensor array will be substantially free of dead spots and will provide an accurate representation of the position of either the leading or trailing edges of document  2  or other object moving in the direction of arrow  15  in FIG. 3 a.    
     Each pair of electrodes of the sensors  34  in group  30  are  30  connected to buses  36  and  37 . As the edge of document  2  passes group  30 , currents I 36  and I 37  are generated in the respective buses. These currents are converted to digital form and are processed as described above to obtain the edge position of document  2 . Similarly bus pairs  38  and  39 ,  40  and  41 , and  42  and  43  connect the sensors  34  of groups  31 - 33  respectively. In this manner an array of position sensors is constructed and form a sensing plane which provides data from which an accurate reading of the position of the document  2  can be obtained. As shown in FIG. 3 b,  array  1  may be formed on a transparent substrate  44  to allow light radiated from below to impinge upon the sensor array. The position of the light source  3  would have to be altered if an opaque substrate is used, i.e., the light would have to enter from the top. 
     Instead of using individually formed sensors as shown in FIG. 3 a,  it may be advantageous to form the sensors in irregularly shaped strips  50 - 53  having electrodes  55 ,  56 ,  57 ,  58 ,  59 ,  60 ,  61  and  62  imbedded along the outer edges thereof, as shown in FIG.  4 . In this embodiment each row of sensors  50 ,  51 ,  52 , and  53  form a continuous surface. Projections, such as at  54  of each sensor are interleaved with the projections of neighboring sensors in order to overlap the central active regions of the sensors. This provides a continuous position signal without regard to the length of an individual sensor. The operation and signal processing in this embodiment will be comparable to the embodiment of FIG.  3 . Sensor  50  is formed with edge electrodes  55  and  56 , sensor  51  is formed with edge electrodes  57  and  58 , sensor  52  is formed with edge electrodes  59  and  60  and sensor  53  is formed with electrode pair  61  and  62 . This embodiment will provide more flexibility, and a reduction in manufacturing complexity. This embodiment provides continuous position information for an object moving in the direction of arrow  15  in FIG.  4 . 
     In a third embodiment of this invention, a strip of light sensitive material is constructed with an array of electrodes, as shown in FIG.  5 . Electrodes  70 - 79  form the boundary of sensing zones  80  defined in between. The document or object will pass as shown by arrow  15  and sequentially generate currents collected by electrodes  70 - 79  from which the centroid of impinging light can be calculated as described above. Each electrode  70 - 79  is held at a ground potential to measure the current induced by the illuminated zone of the sensor. Unlike the preceding embodiments, illumination of the sensor zones adjacent to the zone containing the edge  13  generates a current contribution to the adjoining electrodes. The centroid position may be derived from an analysis of the four currents adjoining the edge. In particular currents I 1 ′=I 7 −1/2I 6  and I 2 ′=I 8 −1/2I 9  may be compared to obtain an indication of the position X of the centroid of the light X between electrodes  77  and  78 . As document  2  passes in the direction of arrow  15 , the centroid X will move from left to right in FIG.  5 . The centroid position X measured from electrode  77  may be calculated by the formula 3: 
     
       
           X=L/ 2−( I   1   ′−I   2 ′) L/{ 2(I 1   ′+I   2 ′)}= L*I   2 ′/( I   1   ′+I   2 ′); 
       
     
     where L is the distance between the electrodes  77  and  78 . 
     The position p of the edge  13  with respect to electrode  77  may then be determined by the formula 4: 
     
       
           p= 2 X−L  if  X&gt;L/ 2 or  p= 2 X  if  X&lt;L/ 2. 
       
     
     As before these formulas assume that the dark portions of the image are completely dark, i.e. provide zero current. It is straightforward to extend them to the case when the dark portions have non-zero illumination by subtracting out the non-zero dark portions. 
     While the invention has been described with reference to specific embodiments, the description of the specific embodiments is illustrative only and is not to be construed as limiting the scope of the invention. Various other modifications and changes may occur to those skilled in the art without departing from the spirit and scope of the invention as defined by the claims.