Paper property sensing system

Abstract: A sensor system for measuring physical properties of paper. The paper property sensor system includes a surface and a diaphragm (202) opposed to each other. The small diaphragm includes a first pair (A 1 , A 2 ) and a second pair (B 1 , B 2 ) p-type piezoresistors. Each piezoresistor (A 1 , A 2 ) of the first pair is located perpendicular to and very close to one of the long edges (Fig. 5) of the diaphragm (202). Each piezoresistor (B 1 , B 2 ) of the second pair is located between and parallel to first pair (A 1 , A 2 ) of piezoresistors, and away from the short edges of the diaphragm (202). Balancing the first pair of piezoresistors against the second pair in a first Wheatstone bridge (Fig. 7) produces a voltage generated proportional to the thickness of a sheet, while balancing the piezoresistors of the first pair against one another of the piezoresistors of the second pair produces a voltage representative of the coefficient of friction of the sheet.

Description: The present invention relates to sensor systems for sheet handling devices. In particular, the present invention relates to a sensor system for determining the coefficient of friction and thickness of paper. Reproduction Machines The quality of text or images written to or read from a markable medium such as paper by devices such as laser printers, xerographic printers, scanners, or ink jet printers is highly dependent on physical characteristics of the medium. The thickness, curl, mass, and stiffness of the medium being marked all affect the speed and accuracy with which a printer can transport a sheet of a the medium, as well as affecting the accuracy with which text or images can be transferred to the medium. Generally, printers and copiers work well only with a limited range of paper types, with a sheet transport mechanism and an image transfer mechanism optimized for that range. In extreme cases, reasonable print quality is only possible with specially developed paper supplied by the manufacturer and maintained in pristine condition, unbent and at certain humidity levels to limit curling. A sheet that is too thick, too thin, or even slightly curled may increase the risk jamming or blocking the sheet transport mechanism. One solution might be to provide various sheet type settings for printers, copiers, and scanners to prevent errors in sheet transport, and increase image quality without necessarily requiring specific manufacturer supplied papers. For example, a user could manually select a "thick paper" setting if thicker papers or thin cardboard stock is to be fed through the sheet transport mechanism. Spacing of pinch rollers and speed of transport would then be automatically adjusted to compensate for the increased paper thickness. Unfortunately, this solution requires extra effort from a user to identify the correct grade or type of markable medium being supplied to the printer. Further, this system is somewhat unwieldy if multiple paper types are intermixed, since the "thick paper" setting must be regularly enabled and disabled by the user as various paper types are fed through the sheet transport mechanism. Thus, a need exists for an inexpensive sheet handling system that automatically detects sheet properties, and automatically adjusts settings of a sheet transport mechanism to optimize sheet handling speed, spacing, or other paper transport characteristics based on the detected paper properties. Such a system would require minimal input from a user, and would automatically attempt to optimize its sheet handling characteristics to support use of a wide range of markable mediums. Such a sheet handling system would allow for greater use of recyclable papers of differing quality and consistency, and could limit paper wastage by permitting use of lower quality or even slightly damaged papers, while still providing transport results comparable to those of pristine, newly manufactured papers. Additionally, a need exists for a sheet handling system that provides information concerning sheet properties to allow for optimizing adjustments in an image transfer mechanism. If sheet properties such as heat capacity, thermal conductivity, dielectric constants, or resistance are determined prior to image transfer, the image transfer mechanism can be suitably optimized to ensure the best possible text or image transfer. Silicon Pressure Transducers The drive toward miniaturization led to the development of silicon pressure transducers. Figure 1 illustrates one typical prior silicon pressure transducer 100. Transducer 100 is a square silicon (100) diaphragm 102 aligned with the <110> crystal orientation, 1 mm square. Diaphragm 102 includes four piezoresistors 124 and 126, 128, and 130. Piezoresistors 124 and 128 are located perpendicular to their associated edges 104 and 108 and so close to their associated edges as to be in the region of maximum compressive stress. Piezoresistors 124 and 128 are stressed longitudinally when force is applied to diaphragm 102. In contrast, piezoresistors 126 and 130 are parallel to their associated edges and are located so close to their associated edges 106 and 110 as to be in the region of maximum compressive stress. Piezoresistors 126 and 130 are stressed transversely to their longitudinal axes when force is applied to diaphragm 102. These piezoresistors are coupled together in a Wheatstone bridge so that the longitudinal stress (+ΔR) of piezoresistors 124 and 128 balances the transversal stress (-ΔR) of piezoresistors 126 and 130. The output voltage of the Wheatstone bridge is proportional to the pressure applied to diaphragm 102. Figure 2A illustrates contours of constant deflection over a quarter of diaphragm 102 when uniformly pressurized. Figure 2B illustrates contours of constant y-component of stress for a quarter of diaphragm 102 when uniformly pressurized. Figure 3 is a dark field optical micrograph of diaphragm 102 when uniformly pressurized. An object of the present invention is to enable automatic adjustment of sheet handling in response to physical properties of a sheet of a markable medium. Another object of the present invention is to enable optimization of sheet transport mechanisms to permit use of a wide range of markable media. A still further object of the present invention is to reduce jams in sheet transport mechanisms. These and other objects are'satisfied by a paper property sensor system according to claim 1. From EP 0467 117 a device is known with a plate spring with strainganges for measuring the thickness of a sheet of material. Other objects, features, and advantages of the present invention will be apparent from the following detailed description, in conjunction with the accompanying drawings, in which: Figure 1 illustrates a prior silicon pressure transducer; Figures 2A and 2B illustrate contours of constant deflection for the prior pressure transducer; Figure 3 is a dark field micrograph for the prior pressure transducer; Figure 4 illustrates a reproductive machine incorporating a paper property sensor system according to an embodiment of the present invention; Figure 5 illustrates a tactile sensor of the present invention; Figures 6A and 6B illustrate contours of constant deflection for the tactile sensor; Figure 7A illustrates a Wheatstone Bridge configuration for measuring normal force exerted on a taxel; Figure 7B illustrates a Wheatstone Bridge configuration for measuring shear force exerted on a taxel; Figures 8A, 8B, and 8C are cross-sectional views of the tactile sensor; Figure 9 Is a plan view of the paper property sensor system of the present invention; Figure 10 is a plot of taxel output voltage versus location with the tactile array; Figure 11 is a cross-sectional view of a first embodiment of the paper property sensor system of the present invention; and Figure 12 is a cross-sectional view a second embodiment of the paper property sensor system of the present invention. Figure 4 illustrates a reproductive machine 9 incorporating paper property sensor system 200 according to an embodiment of the present invention. Paper property sensor system 200 enables reproductive machine 9 to alter its handling of sheets in response to the thickness and coefficient of friction of the sheet currently within machine 9. Briefly described, each embodiment of paper property sensor system 200 includes a surface and a diaphragm opposed to each other and in contact with the sheet of paper. The small, rectangular diaphragm includes a first pair and a second pair of piezoresistors. Each piezoresistor of the first pair is located perpendicular to and very close to one of the long edges of the diaphragm. Each piezoresistor of the second pair is located between and parallel to first pair of piezoresistors, and away from the short edges of the diaphragm. Balancing the first pair of piezoresistors against the second pair in a Wheatstone bridge, produces a voltage dependent on the thickness of a sheet Balancing the resistors of the first pair against each other, and the resistors of the second pair against each other generates a voltage dependent on the coefficient of friction of the sheet. Use of paper property system 200 is not limited to reproductive machine 9; system 200 can be used as part of any machine with a sheet transport mechanism. A. The Reproduction Machine Prior to a more detailed discussion of the paper property sensor system 200 according to an embodiment of the present invention, consider reproduction device 9, illustrated in Figure 4. Reproduction machine 9 includes a belt 10 having a photoconductive surface. After development of an electrostatic latent image thereon, belt 10 advances to the transfer station. At the transfer station a copy sheet is moved into contact with the toner powder image on belt 10. The copy sheet is fed from either tray 34 or 36 to the transfer station. After transfer, conveyor 32 advances the sheet to a fusing station. Paper property sensor system 200 is disposed between copy paper trays 34 and 36 and conveyor 32 at any convenient location within the copy paper transport path. Information provided by paper property sensor system 200 allows controller 38 to prevent jams by adjusting the speed of conveyors 32, and 46 and a further conveyer, and the spacing between a nip and a further nip and between rollers 42 and 44. Controller 38 includes a processor and memory. The processor controls and coordinates the operations of reproductive machine 9 by executing instructions stored electronically in memory, including instructions for controlling paper property sensor system 200. Instructions representing the methods discussed herein may be realized in any appropriate machine language. Semiconductor logic devices that can be used to realize memory include read only memories (ROM), random access memories (RAM), dynamic random access memories (DRAM), programmable read only memories (PROM), erasable programmable read only memories (EPROM), and electrically erasable programmable read only memories (EEPROM), such as flash memories. The elements 12, 14, 15, 18, 17, 18, 20, 22, 24, 26, 28, 30, 46, 48, 50, 52, 54, 56, 58 and 60 of Figure 4 are not described in more detail. B. The Tactile Sensor Paper property sensor system 200 includes at least one tactile sensor 202, enabling system 200 to measure both the thickness and coefficient of friction of any sheet of material in contact with and moving past tactile sensor 202. Figure 5 is a plan view of one tactile sensor 202. Tactile sensor 202 is a rectangular silicon (100) diaphragm aligned with the <110> crystal orientations, into which several pairs of piezoresistors have been diffused. Preferably, diaphragm 202 is n-type silicon while both pairs of piezoresistors are p-type. Parallel, opposed sides 204 and 206 range between 1-2 cm in length, while parallel, opposed sides 208 and 210 range between 1-3 mm in length. Other dimensions are possible; however, the aspect ratio should be greater than 1; i.e., diaphragm 202 should not be square for performance reasons. The depth of silicon diaphragm 202 is small compared to its width, preferably, less than a tenth of the width. Diaphragm 202 is clamped along edges 204, 206, 208, and 210, above a cavity not illustrated in this figure. Diaphragm 202 includes several taxels 220, each of which includes two pairs of piezoresistors. Piezoresistors 222 and 224, also designated A2 and A1, form one pair, and piezoresistors 226 and 228, also designated B1 and B2, form another pair. Both piezoresistors 222 and 224 extend perpendicularly to their associated edges 204 and 206. Piezoresistors 222 and 224 reside so close to their associated edges 204, 206 that they experience the highest compressive stress (-ΔR). When pressure is applied to diaphragm 202, piezoresistors 222 and 224 experience force primarily along their longitudinal axes. Piezoresistors 226 and 228 are located between and extend parallel to piezoresistors 222. They are also spaced apart from and parallel to each other. Piezoresistors 226 and 228 each reside at least one half the length of edge 208 away from both edges 208 and 210. As a result, piezoresistors 226 and 228 experience force primarily along their longitudinal axes when force is applied to diaphragm 202. Piezoresistors 202 reside within the region of maximum tensile stress (+ΔR). Figure 6A illustrates contours of constant z-deflection for diaphragm 202. Figure 6B illustrates contours of constant y-component of stress for diaphragm 202. Within Figure 6B center black region 250 indicates the area of highest tensile stress within diaphragm 202. Piezoresistors B1 and B2 reside here. Outer black region 252 of Figure 6B indicates the areas of highest compressive stress within diaphragm 202; this is where piezoresistors A1 and A2 reside. Figure 7A illustrates Wheatstone Bridge 260a, which generates an output voltage representative of the normal force exerted against a taxel 220. This normal force is proportional to the thickness of a sheet (not shown) passing over diaphragm 202. Figure 7B illustrates Wheatstone Bridge 260b, which generates an output voltage representative of the shear force exerted against a taxel 220. Junctions between piezoresistors (A 1 , B 1 , A 2 , B 2 ) in both circuits 260a and 260b are coupled to bond pads 214. Figure 8A is a cross-section of diaphragm 202 taken along line 216 of Figure 5, as are Figures 8B and 8C. These cross-sections reveal cavity 203 underneath diaphragm 202. In the absence of pressure, diaphragm 202 is level. Figure 8B illustrates the deformation of diaphragm 202 when subjected to a normal force, N. Normal force, N, flexes diaphragm 202 symmetrically relative to the z-axis, inducing equal stress in piezoresistors A1 and A2. Using outputs from Wheatstone bridge 260a normal force, N, is calculated according to the formula: N ∝ [(A1-B1) + (A2-B2)]/2. Normal force, N, is a measure of the thickness of a sheet (not shown) of material passing over diaphragm 202. Figure 8C illustrates the deformation of diaphragm 202 when subjected to both a normal force, N, and a shear force, S. Shear force, S, causes asymmetrical deformation, inducing unequal stress in piezoresistors A1 and A2. Using the outputs of Wheatstone bridge 260b, shear force, S, can be calculated according to the formula: S ∝ [A1-A2]. The ratio of shear force, S, to normal force, N, is a measure of the coefficient of friction of the sheet of material (not shown) passing over diaphragm 202. Tactile sensor 202, like many other micromachined devices, can be produced using standard semiconductor batch fabrication and wafer processing. C. The Paper Property Sensor System Figure 9 is a plan view of paper property sensor 200. Paper property sensor 200 is a nip, only one surface of which is illustrated, surface 270. Both surfaces of nip 200 contact sheet of a markable medium 272 as it moves between them in the direction of arrows 274. Tactile arrays 202 (see Fig. 5) of surface 270 are preferably in contact with the opposite surface when sheet 272 is not present. This approach eliminates the need for tight tolerances required in embodiments of paper property sensor system 200 with gaps between surface 270 and its opposite surface. Permanent contact between tactile array 202 and the opposite surface is achieved via an overpressure, ΔP, of diaphragms 202. Constant overpressure ΔP can be achieved be sealing cavities 203 underneath diaphragms 202. Alternatively, overpressure ΔP can be modulated using a heater resistance to measure the pressure change required to keep the reaction force constant. Tactile arrays 202 are preferably located along the longitudinal axis of surface 270 such that some of the taxels 220 (see Fig. 5) contact paper 272, and some taxels do not, as sheet 272 travels through nip 202. In combination with the constant contact, this permits differential measurements to be taken in space and in time. Differential measurements in space result from taking the output of taxels in contact with sheet 272 versus the output of taxels not in contact with paper 272. Figure 10 graphs taxel output voltage versus taxel location along the length of the tactile array (diaphragm) 202. Contact with sheet 272 leads to an increase, ΔV, in output voltage, which can be used to identify the edge of paper 272. Differential measurements in time result from taking the output voltages of taxels while sheet 272 is within nip 200 versus the output of taxels without sheet 272 in nip 200. Differential measurements in time and space cancel out short and long term drift in the response of tactile arrays 202 to pressure, which may result from temperature changes and wearing of surface 270. Figure 11 illustrates in cross section one embodiment of paper property sensor system 200. Roller 280 opposes surface 270 and its tactile arrays 202. Arrows 274 indicate the direction of sheet 272 through nip 200. Skin 290 protects diaphragm 202 from abrasion and distributes forces uniformly over tactile array 202. Figure 12 illustrates in cross section another embodiment of paper property sensor system 200, in which two surfaces 270 and 300 are opposed to each other. Each surface includes tactile arrays in opposition to those of the other surface.

Claims: A sensor system for performing at least one of the measurements of thickness and coefficient of friction of a sheet of material, the sensor system comprising: a) a surface in contact with the sheet; and b) a first diaphragm (202) located opposite the surface and in contact with the sheet, the diaphragm being rectangular and having a pair of short edges (208,210) and a pair of long edges (204,206), the long edges being less than a predetermined length, the short edges having a first length, the diaphragm including a first pair of piezoresistors (222,224) and second pair of piezoresistors (226,228), each piezoresistor (222,224) of the first pair being located adjacent and perpendicular to a one of the long edges (204,206), each piezoresistor (226,228) of the second pair being located between the piezoresistors (224,222) of the first pair and perpendicular to a one of the long edges (204,206), each piezoresistor (226,228) of the second pair being located at least a half of the first length away from each of the short edges (208,210); the first pair (222,224) and the second pair (226,228) of piezoresistors being coupled together electrically to produce a first electrical signal representative of the thickness of the sheet of material and a second electrical signal representative of the coefficient of friction of the sheet of material. The sensor system of claim 1, wherein the first diaphragm (202) is formed of a semiconductor material, preferably silicon, and/or the first (222,224) and second (226,228) pair of piezoresistors comprise piezoresistors fabricated by diffusion. The sensor system of claim 1 or 2, wherein the diaphragm includes at least another pair of the first pair of piezoresistors and at least another pair of the second pair of piezoresistors. The sensor system of claim 3, wherein a one of the first pair (222,224) and a one of the second pair (226,228) of piezoresistors are located to experience a force exerted by the sheet of material and an other of the first pair and an other of the second pair of piezoresistors are located to not experience the force exerted by the sheet of material. The sensor system of any of the preceding claims wherein the surface comprises the surface of a roller (280). The sensor system of any of the preceding claims, wherein the surface comprises a second diaphragm, the second diaphragm being identical to the first diaphragm (202). The sensor system of any of the preceding claims, wherein the sheet of material comprises a sheet of paper, or a transparency. The sensor system of any of the preceding claims, wherein the first (222,224) and second (226,228) pair of piezoresistors are coupled together to form a Wheatstone Bridge (260). The sensor system of any of claims 6 to 9, wherein the first diaphragm and the second diaphragm have a depth being substantially less than the first length.

Inventors: HUBBLE FRED F III, KUBBY JOEL A, PEETERS ERIC, VITURRO ENRIQUE R, WALLACE STANLEY J, WERNER ALAN J JR, HUBBLE, FRED F., III, KUBBY, JOEL A., PEETERS, ERIC, VITURRO, ENRIQUE, R., WALLACE, STANLEY J., WERNER, ALAN J., JR.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1997
Main classification: G01B7
IPC codes: G01L5, G01N19, G01N33, G01B7, B81B3