Abstract:
A method and apparatus for replenishing toner based on the electric current used over time by the exposure subsystem. Toner take-out for each image is estimated by measuring the current used by the exposure system, subtracting the quiescent current, integrating over a page or frame, and multiplying by a predetermined value that indicates the amount of toner required by the image, based on the average current used for the exposure and other process parameters. These calculations are done either in hardware or in software. The replenishment system is used to add the correct amount of toner to the developer station to maintain the toner concentration at an approximately constant aim value.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims the benefit of the priority date of Provisional Patent Application Ser. No. 60/302,209 filed Jun. 29, 2001. 

   FIELD OF INVENTION 
   This invention relates to electrographic recording apparatus such as that used in document copiers and printers, and more specifically to control of toner replenishment and monitoring of toner usage in an electrophotographic recording apparatus. 
   Definitions 
   The following terms well known in the art are defined here:
     I exp —Writer current used during exposure.   V exp —Writer voltage used during exposure.   E 0 —Light produced by the print head.   E—Actual exposure of photoconductor.   V 0 —Primary voltage (relative to ground) on the photoconductor just after the charger.   This is sometimes referred to as the “initial” voltage.   V B —Development station electrode bias.   

   The light E 0  produced by the print head illuminates the photoconductor and causes a particular level of exposure E of the photoconductor. 
   In general contrast and density control are achieved by the choice of the levels of V 0 , E 0 , and V B  as is well known and described in the published literature. 
   DISCUSSION OF PRIOR ART 
   Two-component development systems for electrography or electrophotography use a toner and a magnetic carrier. Other ingredients are frequently included as flow aids or charge aids for these two principal components. During normal operation of a printing system, fresh toner is added periodically to the developer mixture to replace toner that leaves the toning system as images are developed. To indicate when more toner is required, a toner concentration monitor or process control patch is frequently used, as is well known in the art. From toner concentration, the amount of toner takeout can be determined. 
   There are direct and indirect methods of monitoring toner concentration in multicomponent systems. See U.S. Pat. No. 5,729,787 (Resch), incorporated herein by reference, for a short summary and further references. One measurement method indirectly measures toner concentration by measuring the toner laid down on the photoconductor. Direct methods use measurements made at the development stations. In one known approach, an infrared source is directed through a window in the development sump and the reflections back are measured and used to infer toner concentration. In another approach, a planar electric coil is disposed at a suitable position in the developer container surrounded by a stream of developer. The coil inductance increases as toner concentration decreases. In yet another approach, magnetic detectors are provided at a position in a container that holds the magnetic carrier and a color toner. A coupling coefficient of the magnetic circuit changes with concentration of the toner. Still another approach sends electromagnetic energy along a probe and into the development (toner/carrier) mixture. The difference in impedance between the mixture and the probe is a measure of concentration and is used to initiate adjustment of the composition content of the development mixture. 
   For toner replenishment system calibration, see U.S. Pat. No. 5,649,266 (Rushing) incorporated herein by reference. For closed loop control of toner concentration for use in controlling replenishment of toner to the development station, see U.S. Pat. No. 5,678,131 (Alexandrovich, et al.), incorporated herein by reference. For a detailed explanation of the overall process control methods used in support of toner replenishment, see U.S. Pat. No. 6,121,986 (Regelsberger, et al.), incorporated herein by reference. For a detailed explanation of the toner replenishment process itself, see U.S. Pat. No. 6,181,886 (Hockey, et al.), incorporated herein by reference. 
   Reflectivity of image or test areas has been used to manage toner replenishment. U.S. Pat. No. 4,502,778 (Dodge, et al.) uses a sensor and a comparator for producing an output signal indicative of the reflectivity of the photoconductor using test patches on that photoconductor. U.S. Pat. No. 4,377,338 (Ernst) uses light reflectance of a maximum toned area and a minimum toned area, again using text patches on the photoconductor. 
   Grid and development bias voltages have also been used. U.S. Pat. No. 5,262,825 (Nordeen, et al.) shows an image density process control system for a full color electrophotographic proofing system. The system uses grid and development bias voltages combined with density measurements to create a model set of parameter values for image density control, but does not use exposure current or power. The method is defined for laser printers and copiers. 
   Further methods use pixel count and pixel type to manage toner replenishment. U.S. Pat. No. 5,724,627 (Okuno, et al.) uses a correction coefficient determined on the basis of the pixel frequency at each density level of a document read by image scanning, combined with tone curves and tone expression patterns (set by the emission duty ratio and emission cycle of the laser which exposes the photosensitive member) selected by an operator. The method is defined for laser printers and copiers. U.S. Pat. No. 4,847,659 (Resch) uses a toner depletion signal proportional to the number of character print signals applied to a print head, the characters preferably being pixels to be toned. 
   For digital printers, pixel counting provides an advance estimate of toner use before any change is observed in the image density or toner concentration. However, it has the offsetting disadvantage of requiring special electronics to be added to a standard raster image processor. Additionally, for gray-scale printing the density of each pixel also needs to be taken into account, which complicates the use of the pixel count, a second disadvantage. 
   Other approaches use test or reference image toner density measurements, and many further methods use direct toner density/concentration measurement, to manage toner replenishment. Each of the families of methods outlined above has its own associated cost and complexity. 
   SUMMARY 
   The invention is a system and process for measuring toner consumption and replenishing consumed toner in an electrographic printing system. In electrophotographic engines, the invention uses a photoconductor traveling along a path for receiving and developing a latent image. The path passes a plurality of processing stations including a charging station for charging the photoconductor to a desired charge level, an exposure station for exposing the photoconductor to a document to selectively discharge the photoconductor and form a latent image of the document, a toning station for applying toner to the photoconductor to develop the latent image, and a transfer station for transferring the developed latent image to a receiver sheet. At the exposure station, the invention mounts a current sensor. It calibrates the sensor by measuring quiescent current of the exposure device before exposure and storing that measurement. It uses the sensor throughout imaging by measuring the image exposure current of the exposure device. The invention compares the calibration current level with the averaged image current level using a differential amplifier and an integrator, and uses a logic and control unit for generating a toner replenishment signal proportional to the difference between the two averaged quantity signals. The invention may obtain its estimate of toner takeout by measuring currents, voltages, light intensities, power consumption, photoconductor toner densities, receiver sheet toner densities, or percent area coverage, any of which can be translated into a proportionate toner takeout measurement over time. The invention generates the replenishment signal by multiplying its measurements by a predetermined value that indicates the amount of toner required by each image. The invention then sends the replenishment signal to the toner replenishment subsystem. 
   Those skilled in the art understand that electrical power is the product of voltage and current. In conventional electrographic and electrophotographic machines, the voltage for the writer is usually held at a constant value and the current varies. As such, measuring only the current is sufficient to measure power. In a more general sense, one could measure both the applied voltage and the applied current, derive a product of the two over time, and then integrate the product over time to measure the total energy used to write an image. Power integrated over time is energy. 
   Other transducers can measure power in different ways. For example, a photocell in a densitometer measures power by converting the intensity of incident light into a current or a voltage. A portion of the incident light from the exposure system can be measured by a photodetector and is proportional to the power consumed in production of a latent image and ultimately the amount of toner used for that image. Light transmitted through a photoconductor, reflected from a photoconductor, or stray light from lenses can be used. Light reflected or transmitted from a toned photoreceptor or a copy sheet that carries the toned image can also be used to estimate the amount of toner required for replenishment of the toning system 
   Another way of measuring power and energy is monitoring laser current and laser shutter current to generate signals representative of energy used to form an image in a laser print engine. 
   In its most general form, the invention consists of estimating toner takeout by monitoring the energy required to produce a latent image and replenishing the development system with a proportional amount of toner. The measurement of energy per image can be made during the process of creating the image or estimated afterwards from characteristics of the image such as average voltage of a latent image, area coverage of a toned image, or average density. 

   
     DESCRIPTION OF DRAWINGS 
       FIG. 1  shows the invention as installed in a typical electrophotographic printing system. 
       FIG. 2  shows the invention&#39;s connections between the current sensor, the integrator, and the LCU. 
   

   DETAILED DESCRIPTION OF INVENTION 
   The machine  10  shown in  FIG. 1 , an electrophotographic printer, is typical of devices containing the invention. In machine  10 , a moving recording member such as photoconductive belt  18  is driven by a motor  20  past a series of work stations of the printer. A logic and control unit (LCU)  24  has a digital computer that operates a stored program for sequentially actuating the workstations. 
   A charging station  28  sensitizes belt  18  by applying a uniform electrostatic charge of predetermined primary voltage V 0  to the surface of the belt  18 . The output of the charger  28  is regulated by a programmable controller  30 , which is in turn controlled by LCU  24  to adjust primary voltage V 0  in accordance with a grid control signal, V grid  that controls movement of charges from charging wires to the surface of the recording member, as is well known. 
   At an exposure station  34 , light projected from a write head dissipates the electrostatic charge on the photoconductive belt  18  to form a latent image of a document to be copied or printed. The write head preferably has an array of light-emitting diodes (LEDs) or some other light source such as lasers for exposing the photoconductive belt picture element (pixel) by picture element with an intensity regulated by a data source programmable controller  36  as determined by LCU  24 . Alternatively, the exposure may be by optical projection of an image of a document onto the photoconductor. A still further alternative is creating electrostatic latent images on an electrographic recording medium using needle-like electrodes or other known means for forming such latent images. 
   Where an LED or other electro-optical exposure source is used, image data for recording is provided by a data source  36  such as a computer, a document scanner, a memory, a data network, etc. Signals from the data source  36  and/or LCU  24  may also provide control signals to a writer network, etc. Signals from the data source  36  and/or LCU  24  may also provide control signals to a writer interface  32  for identifying and selecting exposure correction parameters for use in controlling image density. The output of the writer interface  32  contains data on line  70  for the exposure station  34  and controls the writer power supply on line  72  that generates the current for the LEDs in the exposure station  34 . In order to form calibration patches with density, the LCU  24  may be provided with ROM memory representing data for creation of a patch that is input into the data source  36 . Travel of belt  18  brings the areas bearing the latent charge images into a development station  38 . Development station  38  has magnetic brushes in juxtaposition to the travel path of the belt. Magnetic brush development stations are well known. 
   LCU  24  selectively activates the development station  38  in relation to the passage of the image areas containing latent images to selectively bring the magnetic brush into engagement with or a small spacing from the belt. The charged toner particles of the engaged magnetic brush are attracted imagewise to the latent image pattern to develop the pattern. 
   As is well understood in the art, conductive portions of the development station  38 , such as conductive applicator cylinders, act as electrodes. The electrodes are connected to a variable supply of D.C. or A.C.+D.C. potential V B  regulated by a programmable controller  40 . Details regarding the development station  38  are provided as an example, but are not essential to the invention. 
   As is also well known, a transfer station  46  is provided for moving a receiver sheet S into engagement with the photoconductor on belt  18 , in register with the image, for transferring the image to receiver S. Alternatively, the image may be transferred to an intermediate member, and then from the intermediate member to receiver S. A cleaning station  48  is downstream from transfer station  46  and removes toner from belt  18  to allow reuse of the surface for forming additional images. A belt  18 , a drum photoconductor or other structure may be used for supporting an image. After transfer of the unfixed toner images to receiver sheet S, sheet S is transported to a fuser station  49  where the image is fixed. 
   LCU  24  provides overall control of the apparatus and its various subsystems as is well known. Programming commercially available microprocessors is a conventional skill well understood in the art. LCU  24  maintains and stores parametric values necessary for the operation of both the invention and the overall electrophotographic apparatus  10 . Among these parameters is the aim value for toner concentration, which determines how much stored toner must be supplied to the mixture to maintain image quality. 
   The invention uses a current sensor  80  to measure the current I exp  used by the writer at exposure station  34  to estimate the amount of toner to be used for an image. The writer interface has two output lines,  70  and  72 . Line  70  carries the data for switching the LEDs in the writer on and off as well as conventional communication control signals. Line  72  carries power control signals for operating the writer power supply that supplies current to the LEDs of the writer  34 . In its simplest form, a current sense signal is the voltage across a resistor in series with the writer power supply. In the form shown in detail in  FIG. 2 , the current sensor  80  is a combination of an offset control differential amplifier  88  and a shunt resistor  89 . The shunt resistor  89  has a very low resistance on the order of 0.001 ohms. The differential amplifier has a high input impedance. It senses the voltage drop across the shunt resistor  89  and provides an output signal I exp  representative of the exposure current. The current sensor  89  receives a control signal from the LCU  24  to zero the current sensor or optionally to provide automatic offset adjustment and null out standby current of the writer. Estimating toner takeout from I exp  is based on the fact that the total exposure energy E 0  is proportional to V exp , the writer voltage, times I exp . For a constant voltage power supply for the writer, therefore, exposure energy E 0  is proportional to I exp  alone. The exposure energy E 0 , initial voltage V 0  of the photoconductor, and the intrinsic photoconductor properties determine the voltage of the exposed image used for development. 
   The invention uses writer current, as just described, for systems using LEDs as writing devices. For systems that write with lasers, the lasers may be switched on and off, or they may be gated by some means of interrupting the flow of light energy to the photoconductor. In systems where the lasers are switched on and off, the invention uses the writer current to the lasers. In systems where the lasers are gated, the invention uses the controlling voltages or currents to the gating components in place of the writer current, in such a way as to calculate the total energy used in the writing of the image. With any exposure means, the system can use the intensity of light transmitted through the photoconductor or reflected from the photoconductor to calculate the total energy used in writing the image. 
   Calibration 
   In the preferred embodiment, the invention calibrates toner replenishment rate as follows. A first current measurement is made using current sensor  80  when the writer is in a quiescent or “standby” state. Other measurements are made for exposure of a process control patch. Image density measurements are likewise made and the LCU  24  determines TU, the amount of toner used per unit energy of exposure or unit current used for exposure I unit-exp . For many applications, the amount of toner used per unit of exposure is approximately constant and can be pre-determined. For extremely precise control, the toner take-out per unit of exposure can be recalculated periodically. It can also depend upon the initial photoconductor voltage, V 0 , the state of the toning station, toner charge-to-mass ratio, and aim image density. The rate of toner use per unit of exposure, TU, as determined by LCU  24  during calibration, is stored in LCU  24  for use during normal operation. LCU  24  also stores writer quiescent current level I qui  and writer unit current used for exposure I unit-exp  as measured at current sensor  80 . It should be noted that voltages or other signals capable of being combined arithmetically, as discussed here, may represent current levels. 
   Normal Operation 
   In normal operation, while images are being exposed onto the photoconductor, LCU  24  receives toner usage signals by monitoring the current to exposure station  34 . Refer to  FIG. 2 , which shows a more detailed view of the connections between current sensor  80 , integrator  84 , and LCU  24 . When writer  34  is in normal operation, integrator  84  receives current measurement signals representing I exp  from current sensor  80  for exposure of an image, and current level signals representing I qui  from LCU  24  for writer current during quiescence. Integrator  84  calculates the difference representing I exp −I qui  using a differential amplifier  84   a , and integrates the difference over time using an amplifier  84   b  with a capacitor  84   c  to determine total current consumption for the entire image, I image . Integrator  84  transmits a signal representing I image  to LCU  24 . Between images, LCU  24  sends integrator  84  a reset signal to prepare for the integration of current signals for the next image. The reset signal is applied to the integrator  84   b ,  84   c  via a zeroing switch  84   d , which is here shown as a JFET. Zeroing switch  84   d  may also be a MOSFET or other switching device with similarly acceptable characteristics. 
   LCU  24  uses the calibrated TU with the measured I image  to determine the amount of toner TI used for the image exposure. The calculation is, essentially, TI=TU×(I image /I unit-exp ). Based on the calculated value of TI, the supplied value of toner concentration TC, and the aim value for toner concentration, LCU  24  sends to the replenishment subsystem a toner replenishment signal TR, which triggers the replenisher to add toner from the toner bottle to the toning station so that toner concentration is maintained well within useful limits. 
   LCU  24  may initiate a calibration cycle between images in order to adjust and store any previously calibrated values. The methods of scheduling and carrying out such calibrations are numerous and well known in the art. 
   The use of an analog integration process to determine the amount of toner takeout is fast, simple, and inexpensive. By contrast, prior-art methods relying on pixel counts require an investment in raster image processing software and hardware for the system, to count the pixels and calculate the energy required for each pixel. The invention eliminates this investment and complexity. An image already stored on a computer as a bitmap would require pixel-by-pixel processing using these prior-art methods, but measurement of the writer current eliminates such a process entirely. Such an image can be printed directly. 
   In the preferred embodiment, the invention&#39;s method of toner replenishment is supplemented by algorithms based on estimates of toner concentration TC in the toning station that are activated when toner concentration deviates far from the aim value. A magnetic toner monitor in the development station usually determines toner concentration. Methods of determining toner concentration are numerous and well known in the art, as are the algorithms for their use in toner replenishment. The present invention considers their use as supplementary to the invention&#39;s own method as described above, and necessary only in exceptional cases. Such cases may occur when the toning station&#39;s concentration of toner deviates sharply from the invention&#39;s basic projections as described here. 
   This means of toner replenishment can be used with process control schemes for maintaining image density that, for example, adjust V 0  and exposure. The aim value of toner concentration can change depending on conditions such as toner charge or developer life, photoconductor or image voltage, and exposure. In particular, if initial photoconductor voltage or exposure intensities are near maximum values, the aim toner concentration can be increased. 
   The invention&#39;s method is a means of determining toner replenishment rates based on estimates of toner takeout for the actual images that are printed. Similar methods for estimating toner takeout per image include the following. 
   One alternative method is to estimate the actual exposure and the corresponding toner usage by measuring the intensity of light transmitted or reflected from the photoconductor adjacent to the exposure device, using a light pipe or large area photodetector. By translating the light intensity level into a voltage or current signal, and by calibrating light intensity versus toner consumption, the light intensity over time is integrated and applied using the invention&#39;s method as described above. 
   A second alternative method is to measure the density of the toned image with a densitometer having the width of the image. This densitometer replaces the existing densitometer, or else is situated adjacent to the post-development erase lamp(s). Again, by translating the measured image density into a voltage or current signal, and by calibrating density versus toner consumption, the image density over time is integrated and applied using the invention&#39;s method as described above. 
   A third alternative method is to measure the density of the toned image on the receiver. This differs from the second alternative method only in the location of measurement. 
   Any of these means of estimating toner takeout per image can also be used for replenishment algorithms that supplement or replace replenishment methods based on measurements of average toner concentration. 
   Overall, the invention uses a simple analog integration technique to produce a fast, accurate, and useful measure of toner consumption. This technique obviates the need for digital calculation and its supporting hardware, and may be used to replace other more-complex replenishment processes. The invention&#39;s simplicity and effectiveness make it less costly to build, install, and maintain. This advantage consequently renders the electrophotographic systems in which the invention operates more robust and less costly, which translates into a commercial advantage for the makers of such products. 
   CONCLUSION, RAMIFICATIONS, AND SCOPE OF INVENTION 
   From the above descriptions, figures and narratives, the invention&#39;s advantages in providing accurate and inexpensive toner replenishment should be clear. 
   Although the description, operation and illustrative material above contain many specificities, these specificities should not be construed as limiting the scope of the invention but as merely providing illustrations and examples of some of the preferred embodiments of this invention. 
   Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given above. 
   For example, the invention may be applied to an electrographic printer or so-called direct printer. Those printers use ion beams or toner streams to directly apply toner to a copy sheet. As mentioned above, when the applied voltage of the writer is held constant, the applied current is representative of power. However, the invention can be used with variable voltage and variable currents. A signal representative of power can be derived by sampling the variable voltage and variable current, storing the sampled values, multiplying the stored values together to derive a power value and then integrating the power values over the measured time period to derive an energy signal. 
   The invention also contemplates variables in the electrographic or electrophotographic machine. It is possible that a user will vary the TU constant in accordance with V O , toning station state, image aim (target) density or toner charge-to-mass ratio. Those skilled in the art will recognize that corresponding changes must be made in the energy consumption estimate of toner consumption.