Method and apparatus for determining the height of a stack of sheets

Abstract: An apparatus for determining the height of a stack of sheet media (160) in a supply bin is provided. The apparatus includes a sensor (202) and a substantially linearly extending member (200). The member (200) is operatively associated with the optical sensor (202) to permit relative motion between the sensor (202) and the member (200). At least one of the sensor (202) and the member (200) moves with the supply bin. The sensor (202) generates a signal indicative of the relative motion between the sensor (202) and the member (200). The relative motion provides an indication of the height of the stack of the sheet media (160).

Description: This invention relates generally to a method and apparatus for determining the height of a stack of sheets and more particularly, but not exclusively, to a paper tray gauge for a printing machine such as a multifunction paper tray gauge for an electrophotographic printing machine. The productivity of electrophotographic copy machines and printers has increased greatly over the years. Many of these machines are capable of producing 60 to 100 copies per minute and several machines are capable of even higher productivities. These faster, high productive machines require large quantities of paper. In order to maintain the productivity of these machines, the number of machine interruptions to add paper are preferably minimized. Solutions to this paper change problem include the use of multiple paper trays, paper trays which may be accessed during the operating of the machine, and higher capacity paper feeders. The use of higher capacity paper feeders is particularly popular in recent years. The use of copy machines and printers with cut 8 1/2 x 11 (21.6 x 27.9cm) sheets of copy paper and particularly those copy machines that have high capacity output require accurate placement of the sheets to be fed through the copy machine. Typically, these sheets are stacked on trays. The sheets are fed from the trays with the top sheet fed first. This means that the trays are raised and lowered to align the sheet to be fed with a paper feed mechanism located within the machine. For low capacity feed trays, the feed trays may be spring biased with a spring force pushing the top sheet of the tray upward against a stop. The sheet can then be fed from the top. For higher capacity feeders, for example, for those feeders having a capacity of one thousand sheets or more, the use of spring biased feed trays is increasingly difficult. The greater spring force which is required to urge the trays upwardly interferes with the pulling of the top sheet from the tray. A solution to this problem is to utilize a motor to position the top sheet of the tray in a constant position in alignment with the feed mechanism. This motor is typically an electrical motor. This motor must position the paper in a fairly accurate alignment with the feed mechanism of the machine. In prior art high capacity feed trays for copiers and printers, the use of a positioning motor with a rotary encoder has been used. Such a system is shown in US-A 4.960.272 to Wierszewski et al. The rotary encoder is expensive, unreliable and inaccurate. Furthermore, the use of a rotary encoder requires a tray down and a tray up switch to indicate when the tray is full or empty. A further problem is encountered in that the failure of either the tray down or up switch will indicate to the machine control logic that these switches have not been reached. Therefore, the feed mechanism will continue to operate until it has over traveled and perhaps, caused significant damage to the feed mechanism of the machine. A typical solution to this problem is to add a second or redundant switch to the tray down switch and the tray up switch. The redundant switches are likewise connected to the logic and like the original switches, if the redundant switches become defective the logic would indicate that they have not been actuated either. The redundant switches are somewhat effective in that the concurrent failure of both the tray switches and the redundant switches is somewhat unlikely. The use of these switches is expensive, increases the complexity of the machine, and reduces the reliability of the machine. Further, the use of the switches even including the redundant switches provides only a redundant system and not a fail-safe system. If both the switch and the redundant switch are to fail at the same time, the feed mechanism would still over-travel and wreck. Further, the positioning ability of the rotary encoder is limited. This limited accuracy results in only a very rough approximation of the amount of paper in the machine. This lack of accuracy in the number of sheets in the tray causes an uncertainty to a machine operator which results in greater attention needed to be given to the machine. In high production print shops and printing departments, the printing operators may be required to operate more than one machine and uncertainty of the amount of paper in the machine requires the operator to pay undue attention to the machine. U.S.-A-5,573,236 discloses an apparatus for determining the dimensions of a stack of sheets in the tray. An optical sensor is arranged so that movement of the sheet guides in a paper tray causes a variably graduated scale to be moved past the sensor. The sensor may either be a transmissive or reflective type analog sensor in which the strength of the signal generated by the sensor is converted into a position of the side guides and as the analog scale is continuously variable, there is no need for separate discrete sensors or switches and sheet sizes of any dimension can be accommodated. A recalibration process is used to prevent contamination of the gauge and the associated change in signal strength by this sensor from causing the size determinations to be inaccurate. Alternatively, a digital sensor in cooperation with a digital bar code or other digital scale can be utilized to determine the variable sheet size, which digital sensor and variable scale are insensitive to contamination by dirt or paper particles, etc. The sensor system herein is very robust and provides a simple device for determining the size of a stack of sheets in a paper tray U.S.-A-5,152,515 discloses a stacking system for sequentially feeding flimsy sheets to be stacked in a generally horizontal stack in a stacking tray. The sheets are ejected sequentially out over the stack with a preset sheet ejection trajectory angle to fall by gravity and settle onto the top of the stack. The height of the stack is first estimated to provide a stack height control signal proportional to the height of the stack. The trajectory angle is then changed in response to the control signal. U.S.-A-5,078,378 discloses a system for detecting the approximate size of a set or stack of document sheets. A separate system provides rough initial stack height. A counter for counting the sheets fed from the tray is used to compare the sheets fed with the rough initial stack height to improve the accuracy of the rough initial stack height. U.S.-A-4,970,544 discloses paper tray control system which includes a tray control separated from the main controller and a communication line between the tray controller and the main controller. Instructions from the main controller is fed by the communication line to the tray controller. U.S.-A-4,960,272 discloses feeder stack height detection calibration system. A high resolution rotary encoder and sensor beam is connected to a stack height arm at a pivot point. As the arm is flipped pulses are counted and this information is used to represent one sheet. Software stores this information to create a table of stack heights. U.S.-A-4,835,573 discloses data processing elements such as a micro-code which counts the number of sheets feed from a bin and the amount of bin travel. The number of sheet and bin travel yield data include the actual sheet thickness and weights. The amount of bin travel may be controlled by a DC motor. A tachometer is used to monitor the amount of bin travel. Other types of detector are also mentioned. U.S.-A-4,469,320 discloses a sheet stack sensor which operates in two modes. The finger member of the sensor controls a feeding mechanism by sensing the height of the stack. The finger also determines when the stack is empty by a second switch when the finger in a second position. In accordance with one aspect of the present invention, a method for determining the height of a stack of sheet media in a supply bin comprises providing a substantially linearly extending member operatively associated with an optical sensor, one of the sensor and member being movable with the supply bin; permitting relative motion between the member and the sensor; generating a signal indicative of the relative motion therebetween; and providing an indication of the height of the stack of the sheet media corresponding to the signal, and is characterised in that the member comprises a strip including a pattern of transparent segments and opaque segments, wherein the height of at least one of said segments is substantially greater than the height of the remaining segments so as to enable a home position to be detected. In accordance with a second aspect of the invention, apparatus for determining the height of a stack of sheet media in a supply bin, comprises a substantially linearly extending member operatively associated with an optical sensor to permit relative motion therebetween, one of said sensor and said member being movable with the supply bin whereby said sensor generates a signal indicative of the relative motion between said sensor and said member to provide an indication of the height of the stack of sheet media, and is characterised in that the member comprises a strip including a pattern of transparent segments and opaque segments, wherein the height of at least one of said segments is substantially greater than the height of the remaining segments so as to enable a home position to be detected. An electrophotographic printing machine having an apparatus according to the second aspect of the present invention may be provided. The present invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an elevational view of a paper tray in the full position using the linear encoder for combined use as paper tray gauge and traydown switch according to the present invention; Figure 2 is a plan view of an optical sensor and scale for the Figure 1 paper tray; Figure 3 is a partial plan view of the Figure 2 optical sensor and scale; Figure 4 is a partial plan view of the Figure 2 scale showing regular and long transparent segments; and Figure 5 is an elevational view of the Figure 1 paper tray in the empty position. Referring now to Figure 1, a high capacity sheet feeder 150 is shown. The high capacity sheet feeder 150 is shown in position in a xerographic copy machine (not shown). The high capacity sheet feeder 150 includes a drive assembly 152 which is used to position feeder assembly 154. The position of the feeder assembly 154 is measured by height detection system 156. The feeder assembly 154 contains a large quantity of sheet media 160. Top sheet 162 of the sheet media 160 is fed from the high capacity sheet feeder 150 by any suitable means, for example, as shown in Figure 1 by a feed roll 164 positioned on the top sheet 162. The feed roll 164 is driven by any suitable means, for example, by motors (not shown). The feed roll 164 advances sheet media 162 through support rolls 166 and then through chute 170 and further into the paper path. Referring again to Figure 1, the feeder assembly 154 may have any structure capable of positioning the sheet media 160 in order that the top sheet 162 is placed adjacent the feed roll 164 to permit the proper feeding of the sheet media 160. For example, as shown in Figure 1, the feeder assembly 154 may include a platform 172 on which the sheet media 160 are carried. The platform 172 is supported in any suitable manner in order that it may be raised and lowered as the sheet media 160 are removed. For example, the platform 172 may be supported by rails 174 which are affixed to machine frame 176. The platform 172 is guided by rails 174 as it moves upward and downward. The platform 172 may be guided by the rails 174 in any suitable fashion, for example, as shown in Figure 1, the platform 172 may include bearings 180 which are secured to the platform 172. The bearings 180 are slidably attached to the rails 174 and permit the accurate motion of the platform 172 relative to the rails 174. When rails 174 are used, the feeder assembly 154 may include as few as two rails 174 or preferably, to reduce skew and binding of the platform 172, three or four rails 174 equally spaced along the periphery of the platform 172 are preferred. Drive assembly 152 is used to properly position the platform 172 of the feeder assembly 154. The drive assembly 152 includes a motor 182 to which mechanism 184 is attached. The motor 182 may be any suitable motor capable of raising and lowering the platform 172 of the feeder assembly 154. For example, the motor 182 may be a positioning motor 182, for example, a DC stepping motor. The drive mechanism 184 may be any suitable drive mechanism. For example, the drive mechanism may be a chain and sprocket, a series of gears, or as shown in Figure 1, include a pinion gear 186 which is engaged with and meshes with rack 190. To obtain the relative motion between the pinion 186 and the rack 190, the rack may be translatable in the vertical direction with the pinion gear 186 being fixed, or as shown in Figure 1, the pinion gear 186 may be movable in the vertical direction with the rack 190 being secured to the machine frame 176. The pinion gear 186 as shown in Figure 1 moves vertically with the platform 172 of the feeder assembly 154. For example, the pinion gear 186 may be secured to the platform 172 and move vertically therewith. The pinion gear 186 includes teeth 192 which match with teeth 194 on the rack 190. A flexible electric cable 196 permits the motor 182 to travel vertically with the pinion gear 186. The motor 182 may be secured to the pinion gear 186 by any suitable method and for example, the pinion gear 186 may be directly connected to the shaft (not shown) of the motor 182. Referring again to Figure 1, according to one embodiment of the present invention, the height detection system 156 is shown. The height detection system 156 includes a linearly extending member 200 which cooperates with a sensor 202. As relative motion between the linearly extending member 200 and the sensor 202 is required, either the linearly extending member 200 or the sensor 202 may be fixed while the other of these two items moves with the feeder assembly 154. Since the sensor 202 is smaller in size than the member 200, preferably, the sensor 202 moves with the platform 172. While the invention may be practiced with a linearly extending member 200 of any suitable configuration, preferably, the member 200 is in the form of a scale. For example, the scale 200 may be in the form of a fixed encoder strip. The strip 200 may be used either to absorb a signal 204 from the sensor 202, or as shown in Figure 1, includes transparent segments 206 through which the signal 204 may pass. The sensor 202 may be any optical sensor capable of operation with the linearly extending member 200. The optical sensor 202 may be in the form of an analog optical sensor or a digital optical sensor. As shown in Figure 1, preferably, the optical sensor 202 is in the form of an analog optical sensor 202. Referring now to Figure 2, the strip 200 and the sensor 202 are shown in greater detail. The optical sensor 202 is preferably in the form of a U-channel optical sensor. The signal 204 passes across the sensor 202 from a first tine 210 to a second tine 212 of the sensor. The U-channel sensor 202 may be any standard commercially available U-channel sensor, for example, sensor model no. TR11 2995 from Temics Corporation. The signal 204 contacts encoder strip 200 at either transparent segments 206 or opaque segments 214. When the signal 204 is aligned with transparent segment 206, the signal is transmitted through the segment 206, while when the signal 204 is aligned with opaque segment 214, the signal 204 does not pass through the opaque segment 214. Referring now to Figure 3, the U-channel sensor 202 and the encoder strip 200 are shown in greater detail. The encoder strip 200 may have any configuration capable of passing and not passing the signal from sensor 202 in spaced apart intervals in the direction of travel of the paper tray. The U-channel sensor 202 includes an optical transmitter 216 located in first tine 210 which sends out the signal 204. When the signal 204 passes through transparent segment 206, the signal 204 may reach optical receiver 218 in the second tine 212 of the sensor 202. If, on the other hand, the optical transmitter 216 sends out signal 204 which impinges on the opaque segment 214, the signal 204 does not reach the optical receiver 218 and a signal can thus not be transmitted. The encoder strip 200 includes the opaque segments 214 and the transparent segments 206. Preferably, the opaque segments 214 and the transparent segments 206 are alternatively positioned next to each other. The transparent segments 206 permit the transfer of the signal from the sensor 202, while the opaque segments prohibit the transfer of the signal from the sensor 202. Referring now to Figure 4, the opaque segments 214 and the transparent segments 206 are shown in greater detail. The strip 200 may be made of any suitable durable material. For example the strip 200 may be made from stamped sheet metal or may be molded from a plastic material. The opaque segments 214 may be solid areas of the plastic or sheet metal strip 200, while the transparent segments 206 may be apertures, or slots in the strip 200. The strip 200 preferably includes void in the form of holes or slots, the holes representing the transparent segments 206. Slots as shown in Figure 5, are preferred to accommodate positioning errors in the sensor 202. The solid areas represent the opaque segments 214. The transparent segments 206 may be centrally located slots in the strip 200, or as shown in Figure 4, be in the from of rectangular notches in one side of the strip 200. Typically, the opaque segments 214 have a width W O approximately equal to a width W T of the transparent segments 206. The width of the transparent segment W T and the width W O of the opaque sections combine to form a distance W P between adjacent transparent segments 206. The applicants have found that a distance W P of approximately 2mm is effective to practice the invention. With an equal width for both width W T and width W O and a distance W P of 2mm, the width W T is 1mm and the width W O is 1mm. As the sensor 202 (see Figure 4) travels from a first transparent segment 220 to a second transparent segment 222, the sensor 202 has moved a distance of 2mm. The sensor 202 sends a first signal as it passes by the first transparent segment 220 and a second signal as it passes by the second transparent segment 222. The number of these signals may be added. The summed number of the signals multiplied by the distance W P per signal will indicate the travel of the sensor 202 in the direction 224 along axis 226 of the strip 200. Referring again to Figure 1, signal 204'from sensor 202 is transported via conduit 230 to controller 232. Controller 232 may be capable of receiving signal 204'from sensor 202 and emitting signal 234 to motor 182 through conduit 236 to control the motor 182. The controller 232 adds the signals 204 from the sensor 202 to get a total number 238 of signals 204 which through the controller will be converted into the distance traveled by the sensor and, correspondingly, the position or height H of the platform 172. Since adjoining transparent segments 206 are separated by a distance of approximately 2mm, the height H of the platform 172 may be determined within 2mm of its actual position. Preferably, the controller 232 includes a timer 240 which in conjunction with the signal 204 from the sensor 202 may operate within the controller 232 to determine the time between consecutive signals 204. If the time between consecutive signals 204 exceeds a predetermined amount or an amount calculated from the two prior consecutive signals 204, the controller 232 may determine that the transparent segments 206 and the opaque segments 214 are no longer of equal width. Since the controller 232 is capable of determining when the opaque segment 214 has an exceedingly large width, this feature may be used to detect such occurrences. This feature may be used to detect tray full home position 240 or tray empty home position 242. Referring again to Figure 2, the encoder strip 200 may include tray empty opaque segment 244 which has a width W E which is significantly larger than width W T of the remaining transparent segments 206. Likewise, the strip 200 may include tray full opaque segment 246 which has a width W F which is significantly greater than width W T of the transparent segment 206. When the sensor 202 reaches either segment 244 of segment 246, the signal 204 remains off for a period of time which is greater than that preselected from the controller 232. Thus, the controller 232 may be able to determine when the sensor 202 has reached home position 242 or tray full home position 240. Because the height detection system 156 may determine when home positions 242 and 240 have been met, the use of the height detection system 156 may obviate the need for separate home switches. Also, since the signal 204 is not transmitted through the sections 244 and 246 for an extended period of time to signal the home positions, the home position sensed by the detection system 156 is fail-safe, in that if the sensor 202 is to fail, the signal 204 will not remain in effect for a period of time greater than that preselected. Referring again to Figure 1, the high capacity sheet feeder 150 with height H at its maximum or with the feeder 150 completely full of sheet media 160. The sensor 202 is thus located at position 240 in alignment with tray full transparent segment 246. Referring now to Figure 5, the high capacity sheet feeder 150 is shown with sheet media 160 in an empty situation with the height H of the sheet media being approximately equal to zero. This height H may be equal to approximately 2mm for the encoder strip 200 heretofore described. The sensor 202 is at tray empty position 242 and in line with tray empty segment 244. The height detection system of the present invention provides for an inexpensive and highly reliable method of determining fairly accurately the number of sheets in a high capacity sheet feeder. The height detection system 156 of the present invention provides for significantly improved accuracy in determining the height of the stack of sheet media than that of prior art height detection measurement systems for high capacity sheet feeders. The use of a height detection,system including a U-shaped sensor in conjunction with transparent linearly extending encoder strip, the strip and sensor may also be used as a limit switch by having transparent segments of extended width which in correlation with a controller having a timer may be used to indicate home positions. By utilizing the height detection system of the present invention including a scale having wide transparent sections, the height detection system may serve as a fail-safe limit switch to protect the motor and drive assembly of the high capacity sheet feeder. By utilizing the height detection system of the present invention, additional home positions along the encoder strip may be added to indicate other positions of the feeder 150 at no additional cost. The use of a height detection system of the present invention provides for home position switching at no additional cost and with no additional components which may fail and increase the unreliability of the machine. In recapitulation, there is provided an apparatus for determining the dimensions of a stack of sheets in the tray. An optical sensor is arranged so that movement of the sheet guides in a paper tray causes a continuously variably graduated scale to be moved past the sensor. The sensor may either be a transmissive or reflective type sensor and the strength of the signal generated by the sensor is converted into a position of the side guides as the scale is continuously variable, there is no need for discrete sensors or switches and sheet sizes of any dimension can be accommodated. A recalibration process is used to prevent contamination of the gauge and the associated change in signal strength by this sensor from causing the size determinations to be inaccurate. Alternatively, a digital optical sensor in cooperation with a digital bar code or other digital pattern can be utilized to determine the variable sheet size, which digital sensor and variable scale are substantially insensitive to contamination by dirt or paper particles, etc. The sensor system herein is very robust and provides a simple device for determining the size of a stack of sheets in a paper tray.

Claims: A method for determining the height of a stack of sheet media (160) in a supply bin comprising: providing a substantially linearly extending member (200) operatively associated with an optical sensor (202), one of the sensor and member being movable with the supply bin; permitting relative motion between the member and the sensor; generating a signal indicative of the relative motion therebetween; and providing an indication of the height of the stack of the sheet media corresponding to the signal, characterised in that the member (200) comprises a strip including a pattern of transparent segments and opaque segments (206,214), wherein the height of at least one of said segments is substantially greater than the height of the remaining segments so as to enable a home position to be detected. A method as claimed in claim 1, wherein the generating step comprises: transmitting an optical signal through the strip (200); receiving a substantial portion of the signal when the sensor (202) and the transparent segments (206) are aligned; and failing to receive a significant portion of the signal when the opaque segments (214) and the sensor (202) are aligned, whereby the alignment of the transparent segments and the sensor define high signals and whereby the alignment of the opaque segments and the sensor define low signals. A method as claimed in claim 2, wherein the providing an indication step comprises the steps of: adding the number of transitions from low signals to high signals and the number of transitions from high signals to low signals; storing information indicative of the length of the transparent segments (206) and the opaque segments (214) ; and calculating the height of the stack based on the number of transitions and the length of the segments. An apparatus for determining the height of a stack of sheet media (160) in a supply bin, comprising a substantially linearly extending member (200) operatively associated with an optical sensor (202) to permit relative motion therebetween, one of said sensor (202) and said member (200) being movable with the supply bin whereby said sensor (202) generates a signal indicative of the relative motion between said sensor (202) and said member (200) to provide an indication of the height of the stack of sheet media (160), characterised in that the member (200) comprises a strip including a pattern of transparent segments and opaque segments (206,214), wherein the height of at least one of said segments is substantially greater than the height of the remaining segments so as to enable a home position to be detected. An apparatus according to claim 4, wherein said sensor (202) comprises a U channel optical sensor (202). An apparatus according to claim 4 or claim 5, wherein said sensor (202) transmits an optical signal through said strip, said sensor receiving a substantial portion of said optical signal when said sensor (202) and said transparent segments (206) are aligned and said sensor (202) failing to receive a significant portion of said signal when said opaque segments (214) and said sensor (202) are aligned, whereby the alignment of said transparent segments and said sensor define high signals and whereby the alignment of said opaque segments and said sensor define low signals. An apparatus according to claim 6, further comprising a controller (232) for adding the number of transitions from low signals to high signals and the number of transitions from high signals to low signals, for storing information indicative of the length of the transparent segments and the opaque segments and for calculating the height of the stack based on the number of transitions and the length of this segments. An apparatus according to claim 7, wherein said controller (232) further comprises a timer for determining the time between the transitions. An apparatus according to claim 8: wherein said segment with the substantially greater height has a greater time between transitions; and wherein said controller (232) distinguishes said segment with the substantially greater height. An electrophotographic printing machine having an apparatus for determining the height of a stack of sheet media in a supply bin, characterised in that the apparatus is as defined in any one of claims 4 to 9.

Inventors: AZZOPARDI JOSEPH, FAGUY GARY A, JOYCE DAVID G, AZZOPARDI, JOSEPH, FAGUY, GARY A., JOYCE, DAVID G.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1997
Main classification: B65H1
IPC codes: B65H1, B65H7, B65H31