Abstract:
An electrographic image forming device may use a feedback loop to determine environmental conditions and accordingly set one or more operating parameters. The device may detect a resistance/capacitance characteristic of a feedback loop comprising an interface between a first component and a second component of an image forming unit. The device may detect temperature measurements and humidity measurements that can be used to calculate wet-bulb temperature or other metrics used to characterize ambient environmental conditions. The interface may be one in which a toner image is transferred during image forming device operation. A controller may adjust the resistance/capacitance characteristic in response to wet-bulb temperature in conjunction with measured transfer feedback voltage.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    None. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to electrophotographic imaging devices and, more particularly, to a method of adjusting transfer voltage in an image forming device based on temperature and humidity in conjunction transfer feedback voltage. 
         [0004]    2. Description of the Related Art 
         [0005]    An electrophotographic imaging device uses electrostatic voltage differentials to promote the transfer of toner from component to component. In printers using an electrophotographic imaging device, toner is transferred by means of an electrostatic charge from the developer roll to the photo-conductor unit, and then from the photo-conductor unit to the paper. Paper is transported under the photo-conductor unit with a transfer belt. A metal transfer roll coated with a layer of foam sits under the transfer belt. A transfer voltage is applied to this transfer roll in order to move charged toner particles from the photo-conductor unit onto the paper. 
         [0006]    The effective transfer of toner within an image forming device is usually dependent on many variables, including environmental conditions such as temperature and humidity. Changes in the temperature and humidity in an environment affect the electrical properties of printer components, which can have a significant impact on print quality. 
         [0007]    Previous approaches to improving print quality by adjusting transfer voltage include using dedicated temperature and humidity sensors to detect environmental conditions. These devices may alter operating parameters, such as the transfer bias applied to a transfer member, in response to the detected environmental conditions. Another approach to improving print quality by adjusting transfer voltage includes using measured transfer voltage feedback loops in order to select an appropriate transfer voltage. 
         [0008]    A common drawback of these approaches is that temperature and humidity measurements alone are not sufficient to completely characterize the electrical behavior of the system. Further, measured feedback voltages alone cannot adequately distinguish between environmental conditions. 
         [0009]    Thus, there is still a need for an innovation that will use measurements from a temperature/humidity sensor in conjunction with measured feedback voltage measurements to adjust the transfer voltage. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention meets this need by providing an innovation that accounts for temperature and humidity measurements while setting operating parameters in an image forming device in response to periodic feedback loop checks. 
         [0011]    Accordingly, in an aspect of the present invention, an electrophotographic image forming device has an image forming unit that may comprise two or more components adapted to transfer a toner image therebetween. Periodically, a sensing unit may detect a resistance/capacitance characteristic of a feedback loop comprising an interface between the components. For example, the detected resistance/capacitance characteristic of the feedback loop may represent a detected voltage produced by passing a known current through the interface between the components. Alternatively, the detected resistance/capacitance characteristic of the feedback loop may represent a detected current produced by passing a known voltage through the interface between the components. A controller may adjust the detected resistance/capacitance in response to wet-bulb temperature values in conjunction with measured transfer feedback. The controller may also adjust the detected resistance/capacitance characteristic in response to the device throughput. 
         [0012]    The magnitude of the adjustment may be stored in memory as a lookup table comprising adjustment values corresponding to wet-bulb temperature measurements in conjunction with measured transfer feedback voltage. The wet-bulb temperature is calculated as a function of dry-bulb temperature and relative humidity measurements made by using a temperature sensor and a humidity sensor. Once the adjusted value for the resistance/capacitance characteristic is determined, operating parameters, such as bias voltage applied to a transfer or fuser component may be set. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein: 
           [0014]      FIG. 1  is a schematic view of an image forming device according to the present invention. 
           [0015]      FIG. 2  is a cross-sectional view of an image forming unit and associated power supply and transfer feedback circuit according to one embodiment of the present invention. 
           [0016]      FIG. 3  is a flow diagram illustrating a process by which operating parameters may be adjusted in response to a detected wet-bulb temperature and measured transfer feedback voltage. 
           [0017]      FIG. 4  is a representative lookup table (shown separated into three sections at lines X-X and Y-Y) showing transfer print adjustment values for various wet-bulb temperatures and measured transfer feedback voltages according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. Like numerals refer to like elements throughout the views. 
         [0019]    Referring now to  FIG. 1 , there is illustrated an image forming device  10 . The exemplary image forming device  10  comprises a main body  12  and a door assembly  13 . A media tray  98  with a pick mechanism  16 , and a multi-purpose feeder  32 , are conduits for introducing media sheets into the device  10 . The media tray  98  is preferably removable for refilling, and located on a lower section of the device  10 . 
         [0020]    Media sheets are moved from the input and fed into a primary media path. One or more registration rollers  99  disposed along the media path aligns the print media and precisely controls its further movement along the media path. A media transport belt  20  forms a section of the media path for moving the media sheets past a plurality of image forming units  100 . Color printers typically include four image forming units  100  for printing with cyan, magenta, yellow and black toner to produce a four-color image on the media sheet. 
         [0021]    An optical scanning device  22  forms a latent image on a photoconductive member  51  within the image forming units  100 . The media sheet with loose toner is then moved through a fuser  24  to fix the toner to the media sheet. Exit rollers  26  rotate in a forward direction to move the media sheet to an output tray  28 , or rollers  26  rotate in a reverse direction to move the media sheet to a duplex path  30 . The duplex path  30  directs the inverted media sheet back through the image formation process for forming an image on a second side of the media sheet. 
         [0022]    As illustrated in  FIGS. 1 and 2 , the image forming units  100  are comprised of a developer unit  40  and a photoconductor (PC) unit  50 . The developer unit  40  comprises an exterior housing  43  that forms a reservoir  41  for holding a supply of toner  70 . One or more agitating members  42  are positioned within the reservoir  41  for agitating and moving the toner  70  towards a toner adding roll  44  and the developer member  45 . The developer unit  40  further comprises a doctor element  38  that controls the toner  70  layer formed on the developer member  45 . In one embodiment, a cantilevered, flexible doctor blade as shown in  FIG. 2  may be used. Other types of doctor elements  38 , such as spring-loaded, ingot style doctor elements may be used. The developer unit  40  and PC unit  50  are structured so the developer member  45  is accessible for contact with the photoconductive member  51  at a nip  46 . Consequently, the developer member  45  is positioned to develop latent images formed on the photoconductive member  51 . 
         [0023]    The exemplary PC unit  50  comprises the photoconductive member  51 , a charge roller  52 , a cleaner blade  53 , and a waste toner auger  54  all disposed within a housing  62  that is separate from the developer housing unit  43 . In one embodiment, the photoconductive member  51  is an aluminum hollow-core drum with a photoconductive coating  68  comprising one or more layers of light sensitive organic photoconductive materials. The photoconductive member  51  is mounted protruding from the PC unit  50  to contact the developer member  45  at nip  46 . Charge roller  52  is electrified to a predetermined bias by a high voltage power supply (HPVS)  60  that is adjusted or turned on and off by a controller  64 . The charge roller  52  applies an electrical charge to the photoconductive coating  68 . During image creation, selected portions of the photoconductive coating  68  are exposed to optical energy, such as laser light, though aperture  48 . Exposing areas of the photoconductive coating  68  in this manner creates a discharged latent image on the photoconductive member  51 . That is, the latent image is discharged to a lower charge level than areas of the photoconductive coating  68  that are not illuminated. 
         [0024]    The developer member  45  (and hence, the toner  70  thereon) is charged to a bias level by the HVPS  60  that is advantageously set between the bias level of charge roller  52  and the discharged latent image. In one embodiment, the developer member  45  is comprised of a resilient (e.g., foam or rubber) roller disposed around a conductive axial shaft. Other compliant and rigid roller-type developer members  45  as are known in the art may be used. Charged toner  70  is carried by the developer member  45  to the latent image formed on the photoconductive coating  68 . As a result of the imposed bias differences, the toner  70  is attracted to the latent image and repelled from the remaining, higher charged portions of the photoconductive coating  68 . At this point in the image creation process, the latent image is said to be developed. 
         [0025]    The developed image is subsequently transferred to a media sheet being carried past the photoconductive member  51  by media transport belt  20 . In the exemplary embodiment, a transfer roller  34  is disposed behind the transport belt  20  in a position to impart a contact pressure at the transfer nip. In addition, the transfer roller  34  is advantageously charged, typically to a polarity that is opposite the charged toner  70  and charged photoconductive member  51  to promote the transfer of the developed image to the media sheet. 
         [0026]    The cleaner blade  53  contacts the outer surface of the photoconductive coating  68  to remove toner  70  that remains on the photoconductive member  51  following transfer of the developed image to a media sheet. The residual toner  70  is moved to a waster toner auger  54 . The auger  54  moves the waster toner  70  out of the photoconductor unit  50  and towards a waste toner container (not shown), which may be disposed of once full. 
         [0027]    In one embodiment, the charge roller  52 , the photoconductive member  51 , the developer member  45 , the doctor element  38  and the toner adding roll  44  are all negatively biased. The transfer roller  34  may be positively charged biased to promote transfer of negatively charged toner  70  particles to a media sheet. Those skilled in the art will comprehend that an image forming unit  100  may implement polarities opposite from these. 
         [0028]    A sensor capable of measuring both ambient temperature and relative humidity  101  is mounted directly on a circuit board at the rear of the machine. The controller  64  for this temperature and humidity sensor is also contained within this circuit board. 
         [0029]    Periodically, such as between print jobs or at the start of a print job, the HVPS  60 , under the control of controller  64 , implements a transfer servo routine to determine a transfer feedback voltage that varies in relation to changing operating conditions. The printer controller  64  may adjust operating parameters (e.g., bias voltage applied to the transfer roller  34  or the fuser  24  shown in  FIG. 1 ) based on the determined transfer feedback voltage and wet-bulb temperatures to compensate for changes in operating conditions such as temperature and humidity. 
         [0030]    In one embodiment, the transfer feedback voltage that produces a predetermined current through the transfer roller  34  is determined. More specifically, the HVPS  60  includes a sensing circuit  56  adapted to sense the voltage transmitted to the transfer roller  34  that produces a target current of 8 μA. This threshold circuit  56  produces a state change (i.e. low to high transition, otherwise referred to as a positive feedback) in a binary output signal that is sensed by the controller  64  when the transfer current equals or exceeds the target current of 8 μA. If the transfer current remains below the target current, the output of the sensing circuit  56  remains low. 
         [0031]    In the exemplary configuration shown and described, the applied current travels though various components, including the transfer roller  34 , the media transport belt  20 , the photoconductive member  51  and ultimately to the ground. Some of the applied current may also travel to the ground via the cleaner blade  53 , charge roller  52 , and/or developer member  45 . The voltage that produces the target current is referred to as the “transfer feedback voltage.” The value of the transfer feedback voltage is transmitted to or otherwise determined by the controller  64 . Wet-bulb temperature is transmitted to or otherwise determined by controller  64 . Both wet-bulb temperature and transfer feedback voltage are used to determine the appropriate value of the transfer print voltage, which are mapped in memory  66 . The controller  64  sets the appropriate transfer voltage for subsequent printing based on the value mapped in memory  66  based on wet-bulb temperature and transfer feedback voltage.  FIG. 1  shows that there are four image forming units  100  in the representative image forming device. Accordingly, the process of determining the transfer feedback voltage may be performed for each transfer location in the image forming device  10 . In one embodiment, the process is performed simultaneously at each image forming unit  100 . Alternatively, the process may be performed sequentially at each image forming unit  100 . 
         [0032]    Wet-bulb temperature is the temperature of a volume of air that is cooled to saturation at constant pressure by evaporating water into the air without adding or removing heat. A wet-bulb thermometer approximates wet-bulb temperature by measuring the temperature of the tip of the thermometer covered by a wet cloth. When the relative humidity is below 100%, water evaporates from the cloth and effectively cools the tip of the wet-bulb thermometer. Essentially, wet-bulb temperature is a quantity that combines temperature and humidity values into a single value that can be used to differentiate one environmental condition from another. Though temperature and humidity measurements change significantly within the first several minutes of printing, wet-bulb temperature does not change significantly for a given environment, and serves as a quantity that can be used to determine ambient environmental conditions regardless of internal machine temperature. To create a separation between environments, five different wet-bulb temperature ranges were chosen. Each wet-bulb temperature range corresponds to a different transfer table that determines the appropriate print voltage to use for a given transfer servo. Iterative numerical-methods techniques were used to fit a quadratic surface to data taken from the psychrometric chart. The quadratic surface establishes an orthogonal relationship for dry-bulb temperature, relative humidity, and wet-bulb temperature. A best fit quadratic surface to approximate wet-bulb temperature as a function of dry-bulb temperature and relative humidity can be written in the following form: 
         [0000]        Z=AXA 2+ BYA 2+ CXY+DX+EY+F    
       Where: 
       [0000]    
       
         A=−0.00079 
         B=−0.00047 
         C=0.00479 
         D=0.59473 
         E=0.10035 
         F=−6.32789 
       
     
       And: 
       [0000]    
       
         X=Dry-bulb Temperature (° C.) read from a thermistor 
         Y=Relative Humidity (% RH) 
         Z=Wet-bulb Temperature (° C.) 
       
     
         [0042]    The transfer feedback voltage routines described above have contemplated determining a voltage that results from transmitting a known current through a transfer roller  34 . In other embodiments, similar results may be obtained by using a constant current power supply and using a voltmeter to measure the resulting voltage produced when a known current is passed though the image forming unit  100 . Similarly, other systems may implement a constant voltage power supply and an ammeter to measure the resulting current produced when a known voltage is transmitted though the image forming unit  100 . These alternatives provide different approaches to determining the resistance/capacitance characteristics of the components within the image forming unit  100  that are involved in the transfer of toner particles. 
         [0043]    The flow diagram illustrated in  FIG. 3  shows one embodiment of a process by which transfer print voltage adjustment may be implemented. In step  300 , the transfer servo routine begins. In one embodiment, a sensing circuit  56  (see  FIG. 2 ) is adapted to sense the voltage transmitted to the transfer roller  34  that produces a pre-determined current. The transfer feedback voltage is determined in step  302 . Then the controller  64  reads the temperature and humidity measured by sensor  101  in step  303  and based on those readings the wet-bulb temperature value is determined in step  304 . The controller  64  (shown in  FIG. 2 ) may store a lookup table as per block  305  for adjusting the transfer print voltage based on wet-bulb temperature values determined in step  304  and transfer print voltage determined in step  302 . The controller  64  may read this value from memory  66  as necessary to perform the steps outlined in  FIG. 3 . 
         [0044]    Subsequently, the look-up table value corresponding to the wet-bulb temperature values determined in step  304  and transfer feedback voltage determined in step  302  are used in step the sequence of steps  306 - 308  to adjust the transfer print voltage. 
         [0045]    Lastly, the embodiments described above have contemplated an adjustment to the voltage or current that is measured in response to passing a known test signal though the image forming unit  100 . In other embodiments, the operating parameter maps stored in memory  66  may include additional entries reflecting other operating conditions. 
         [0046]    Those skilled in the art should also appreciate that the control circuitry associated with controller  64  shown in  FIG. 2  for implementing the present invention may comprise hardware, software or any combination thereof. For example, circuitry for initiating, performing, and adjusting the transfer feedback voltage may be a separate hardware circuit, or may be included as a part of other processing hardware. More advantageously, however, the processing circuitry in these devices is at least partially implemented via stored computer instructions for execution by one or more computer devices, such as microprocessors, Digital Signal Processors (DSPs), ASICs or other digital processing circuits included in the controller  64 . The stored program instructions may be stored in electrical, magnetic or optical memory devices, such as ROM and RAM modules, flash memory, hard disk drives, magnetic disk drives, optical disc drives and other storage media known in the art. 
         [0047]    Furthermore, the exemplary image forming device  10  described herein uses contact-development technology—a scheme that implements a physical contact between components to promote the transfer of toner. The transfer bias adjustment may also be incorporated in image forming devices that use a jump-gap-development technology—a scheme that implements a space between components that are involved in toner development of latent images on the photoconductor. The transfer bias adjustment may be incorporated in a variety of image forming devices including, for example, printers, fax machines, copiers, and multi-functional machines including vertical and horizontal architectures as are well known in the art of electrophotographic reproduction. 
         [0048]    The foregoing description of several embodiments of the invention has been presented for purposes of illustration. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be defined by the claims appended hereto.