Abstract:
Systems and methods for setting up grid voltage for a tandem pin charging device for charging a photoreceptor in a xerographic printing machine. A charge-generating emitter ratio of a first charging unit is determined and a first grid voltage is set based on the charge-generating emitter ratio of the first charging unit. A charge-generating emitter ratio of a offset voltage is then determined and a second grid voltage is set, based on the determined charge-generating emitter ratio of the offset voltage. A final voltage of a photoreceptor is then compared with a final target voltage.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to systems and methods for controlling process parameters for a xerographic printing machine. 
     2. Description of Related Art 
     The use of charging devices for photoreceptors in xerographic printing is well known in the art. Typically, such charging devices may be one or more of a corotron, a dicorotron, a pin corotron, a scorotron, a discorotron, and/or a pin scorotron. Such charging devices may include a chamber arranged with one or more charge-generating emitters such as, for example, a wire, a dielectric wire, or a pin array. Some charging devices may also include a control grid to regulate and control the charge provided to the photosensitive member. In this way, the photosensitive member may receive a uniform charge at a desired potential. 
     As known in the art, a key characteristic of a charging device is the di/dV ratio or charge generating emitter ratio of the charge-generating emitter of the charging device. The di/dV ratio is also known as the “slope” of the emitter, and is generally expressed in units of Amperes per volt-meter. Typically, charging devices having a high slope have high overshoot output voltage, i.e., generate a voltage on the photoreceptor that is above the grid voltage, and have poor charging uniformity. In a pin scorotron, the ions generated from the coronode are accelerated by the field force past the screen or grid to reach the photoreceptor surface, thus increasing the surface potential beyond the grid voltage. 
     When the surface potential reaches the same voltage as the voltage on the screen or grid, there is no electrostatic field between the screen and the photoreceptor. However, since the ions have high residual momentum as the ions approach the grid from the coronode side, the ions will continue to penetrate the grid and build up a space charge. This extra space charge drives some ions to the photoreceptor surface, increasing the surface potential further, until the repulsion field force is large enough to prevent further ion transport. The overshoot voltage may be defined as the extra difference in voltage, above the grid voltage, that the photoreceptor potential needs to reach to prevent further ion transport. 
     As the current flowing from each pin differs from pin to pin, the time to reach the final overshoot voltage also varies from pin to pin. The time required for charging a surface under the charging device is determined by the width of the charging device and the process speed of the photoreceptor surface being charged past the charging device. This time may be limited by practical considerations, and not all pins may reach the ultimate overshoot voltage. All of this tends to limit the voltage uniformity of practical pin devices. 
     However, uniform photoreceptor charging is required to achieve high-quality xerographic results. As such, various ways to achieve desired levels of uniform charging are known. For example, U.S. Pat. No. 6,459,873 to Song et al. discloses a DC pin scorotron charging apparatus for charging a photoreceptor to a desired voltage. In this charging device, a first DC pin scorotron charging device initially charges the photoreceptor to an intermediate overshoot voltage. A second DC pin scorotron charging device thereafter uniformly charges the photoreceptor to the final voltage. The first charging device provides a generally high percent open control grid area, a generally high emitter slope, and a generally high emitter pin current. The second charging device provides a generally low percent open control grid area, a generally low emitter slope, and a generally low emitter pin current. 
     The goal of the first charging device is to provide the majority of the charging ions to the photoreceptor. The first charging device is designed as a high slope device with high screen open area, high coronode voltage (current) and close pin-to-screen spacing. This design tends to result in high overshoot voltage. Therefore, the screen voltage is purposely set lower than the required charging voltage. An offset voltage is defined as the grid voltage difference between the first charging device and the second charging device. The offset voltage is important and should be greater than the overshoot voltage of the first charging device. 
     The second charging device provides “uniform” charge leveling with little charge-up needed to bring the entire voltage of the surface being charged to the desired photoreceptor potential as uniformly as possible. Because the first charging device has provided most of the charging ions to the photoreceptor, the first charging device significantly reduces the required charging capability of the second charging device. Thus, the second charging device may be a low slope, low overshoot device. This may be accomplished by decreasing the screen open area, for example, to less than 50–60 percent, lowering the coronode voltage (current) and/or increasing the pin-grid spacing in the second charging device relative to the first charging device. Because each of these changes may improve the charging uniformity of the second charging device relative to the first charging device, the final photoreceptor potential should be close to the applied screen voltage on the second charging device with little overshoot. 
     SUMMARY OF THE INVENTION 
     In an image forming device that uses multiple charging devices to charge the photoreceptor to a final voltage, it is desirable that the offset voltage, that is, the voltage difference between the grid voltages of the different charging devices, be set appropriately. If the offset voltage is too small, the overshoot voltage of the first device results in the photoreceptor charging potential being higher than the grid voltage of the second device. In this case, the second device is effectively shut off. Consequently, the charging uniformity on the photoreceptor will be poor, because the system fails to benefit from the charge uniformity the second charging device is able to create. If the offset voltage is too large, a high charge-up requirement is imposed on the second device. Because the second charging device is a lower slope device, it may be not be capable of achieving the ultimate desired charge potential on the photoreceptor from all of the pin emitters, thus reducing the charging uniformity of the charge on the photoreceptor. 
     Static set points generally cannot be used to set the grid voltage of the first and second charging devices because such static set points cannot meet the performance requirements for charging photoreceptors. Variations in charging performance may occur, for example, because of deviations in mechanical tolerances, variations in environmental conditions, such as, for example, temperature, pressure, and/or humidity, and the like. In addition, mechanical tolerance deviations, as well as environmental variations, vary over time. Thus, ultimate copy quality is affected over time, as a result of mechanical and environmental variations. This results in increased service calls related to copy quality problems. As such, there is a need for a low cost solution for controlling the offset voltage. 
     This invention provides systems and methods for automatically setting up the grid voltages for a dual charger charging system. 
     This invention separately provides methods that automatically compensate for mechanical and environmental effects to ensure the appropriate set points for a dual-charger charging system. 
     This invention separately provides systems and methods for substantially improving charging uniformity by exploiting the maximum benefits of the second charging unit of a dual-charger charging system. 
     This invention separately provides systems and methods for enabling the use of low cost devices such as pin scorotrons to replace higher cost devices to enhance copy quality and reduce service calls related to copy quality problems. 
     In various exemplary embodiments according to this invention, the final ideal photoreceptor potential and the preferred grid voltage of the second unit are set to the same voltage. The interim ideal photoreceptor potential after passing the first charging device, may be, for example, 50–70 volts lower than the final ideal photoreceptor voltage. Due to the high overshoot capability of the first charging unit, the grid voltage of the first charging unit should be set even lower. 
     In various exemplary embodiments, systems and methods according to this invention may be used to compensate for variations in charging conditions due to mechanical tolerances and/or environmental conditions. For example, during machine warm-up, the first charging unit of the dual pin scorotron system is enabled, while the second charging unit is disabled. Then, the grid voltage of the first charging unit is started at a low setting and is increased in desired increments. An electrostatic voltage meter (ESV) may be used to measure the interim photoreceptor potential after the photoreceptor is charged by the first charging device. The measured interim photoreceptor potential from the ESV is used in a closed-loop feed-back system to adjust the first voltage of the first charging unit. In various exemplary embodiments, these closed loop feed-back adjustment systems and methods enable the interim photoreceptor potential, after passing the first charging device, to be about 40 volts less than the second grid voltage and final photoreceptor potential. Thus, the interim photoreceptor potential, after passing the first charging device, is at a desirable point for the second charging device to add additional charge to the photoreceptor and to reduce voltage nonuniformity. 
     These and other features and advantages of this invention are described in, or are apparent from, the following detailed description of various exemplary embodiments of the systems and methods according to this invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Various exemplary embodiments of the invention will be described with reference to the accompanying drawings, in which like elements are labeled with like numbers, and in which: 
         FIG. 1  shows one exemplary embodiment of a dual pin scorotron system usable with this invention and a corresponding graph illustrating the resulting photoreceptor potential; 
         FIG. 2  is a flowchart outlining one exemplary embodiment of a method for automatically setting up the grid voltages of a dual-charger charging system; 
         FIG. 3  is a flowchart outlining in greater detail one exemplary embodiment of the method for determining the slope of the first grid of  FIG. 2 ; 
         FIG. 4  is a flowchart outlining in greater detail one exemplary embodiment of the method for determining the offset voltage slope of  FIG. 2 ; 
         FIG. 5  is a flowchart outlining in greater detail one exemplary embodiment of the method for setting up the voltages on the grids of the dual-charger charging system of  FIG. 2 ; and 
         FIG. 6  is a block diagram outlining one exemplary embodiment of a charging system control system usable to determine charging grid voltages for a multiple charging device charging system. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  illustrates one exemplary embodiment of a dual pin scorotron system  100  and a graph  150  that illustrates the measured voltage on a photoreceptor  130  as the photoreceptor  130  passes a first charging unit  110  and a second charging unit  120  of the dual pin scorotron system  100 . In a typical dual pin scorotron system  100 , ions  116  generated from a pin scorotron  112  of the first charging unit  110  are accelerated by a field force past a first grid  114  to reach the photoreceptor  130 , thus increasing the surface potential of the photoreceptor  130 . When the surface potential V 1C  of the photoreceptor  130  reaches the same voltage V grid1  as the voltage on the first grid  114 , there is no electrostatic field between the first grid  114  and the photoreceptor  130 . However, since the ions  116  have high residual momentum as they approach the first grid  114  from the first charging unit  110 , the ions  116  will continue to penetrate the first grid  114  and build up a space charge. This extra space charge drives some ions  116  to the surface of the photoreceptor  130 . This further increases the surface potential V 1C  of the photoreceptor  130  until the repulsion field force is large enough to prevent further transport of the ions  116  through the first grid  114 . As stated earlier, the overshoot voltage V 1O  is defined as the extra difference in voltage that the surface potential V 1C  of the photoreceptor  130  can reach above the voltage V grid1  of the first grid  114 . 
     The first charging unit  110  provides a majority of ions  116  to the photoreceptor  130  and is typically a high slope device with a high screen open area, higher voltage and close pin-to-grid spacing. As such, the first charging unit  110  tends to cause high overshoot voltage. Thus, the grid voltage V grid1  of the first grid  114  is purposely set lower than the required charging voltage V 1t  for the first charging unit  110 . As stated earlier, the offset voltage V O  is defined as the difference in grid voltage between the first charging unit  110  and the second charging unit  120 . 
     A curve  158  of the graph  150  illustrates the change in voltage as the photoreceptor  130  passes both the first charging unit  110  and the second charging unit  120 . A graph line  156  represents the target voltage of the photoreceptor  130  before passing either of the first charging unit  110  and the second charging unit  120 . As illustrated, a graph line  152  represents the target surface potential V 1t  of the photoreceptor  130  after passing the first charging unit  110 . This first or interim target surface potential V 1t  is, for example, 500 volts. As illustrated by the curve  158 , the voltage V C  of the photoreceptor  130  is not uniform, and is, in fact highly varied, after passing the first charging unit  110 . However, the voltage V C  becomes quite uniform after the photoreceptor  130  passes the second charging unit  120 . A graph line  154  represents the final target voltage V Ft  after the photoreceptor  130  passes the second charging unit  120 . This final target voltage V Ft  is, for example, 650 volts. 
     The second charging unit  120  may be a low slope, low overshoot device having a decreased screen open area with lowered voltage and increased pin grid spacing relative to the first charging unit  110 . Thus, the second charging device  120  has an improved charging uniformity relative to the first charging unit  110 . In this embodiment, the final photoreceptor potential V 2C  may be close to the applied voltage V grid2  on a second grid  124  of the second charge device with very little overshoot. Ions  126  may be generated from a pin scorotron  122 . 
     In one exemplary embodiment, the first charging unit  110  may have a large grid open area (70 percent) and a high pin current (9.9 uA/pin). The first charging unit  110  may have a slope of 1.8 uA/(m-V) or more. The overshoot voltage for such a first charging unit  110  may be typically about 100 to 170 volts. The second charging unit  120  may have a small grid open area (50 percent) with a low pin current (7.5 uA/pin). The final target charging potential V Ft  of the photoreceptor may be, for example, 650 volts. Because the overshoot V 1O  of the first charging unit  110  may be about 100 to 120 volts, the voltage V grid1  of the first grid  114  may be set at about 500 volts. Thus, the photoreceptor potential after passing the first charging unit  110 , i.e. the voltage V 1C  on the photoreceptor, may be about 600 to 620 volts. 
     As stated previously, various factors, such as coronode surface conditions and differences in photoreceptor initial voltage across the surface at the entrance to the device, may affect performance and cause poor charging uniformity after passing the first charging unit  110 . Because the first charging unit  110  delivers the majority of charging current and brings the potential close to the desired voltage V Ft  (650 volts), the required charging range for the second device need only be, for example, about 100 volts to 30 volts. Thus, the second charging unit  120  may be a low slope and low overshoot device. With low overshoot, the photoreceptor potential V 2C  may stay close to the final target voltage V Ft  of 650 volts. The actual final potential V 2C  depends on the voltage V grid2  of the second grid  124  and the photoreceptor potential V 1C  after passing the first charging unit  110  and may be insensitive to other factors. The required minimum current per pin scorotron  122  for the second charging unit  120  will depend on the process speed of the photoreceptor  130 . 
     With the dual pin scorotron system  100  of this embodiment, traditional low-cost pin scorotrons may be used as the first and second charging devices  110  and  120 . As a result, the dual pin scorotron system  100  may be used to achieve a much higher charging uniformity than a traditional single charging unit device. As such, when using a dual-charger charging system according to this invention, the difference between the photoreceptor initial voltage and the intercept voltage of the second charging unit is small. Thus, excellent uniformity can be achieved even though the slope of the second charging unit  120  is relatively low. 
       FIG. 2  illustrates an exemplary embodiment of a method for automatically setting up the grid voltages V grid1  and V grid2  of a dual pin scorotron system according to this invention. As shown in  FIG. 2 , operation of the method begins in step S 100 , and proceeds to step S 200 , where the slope of V grid1  to the obtained charge V 1C  on the charge retentive surface due to the first grid charging device is determined. Then, in step S 400 , the desired target offset voltage ΔV grid  between the target voltage V 1t  on the grid of the first charging unit and the combined target voltage V Ft  of the charge retentive surface is determined based on the slope of V grid2  to the charge obtained on the charge retentive surface due to the second charging device. Next, in step S 600 , the final voltage V 2C  (or V F ) is adjusted, if necessary to come within the desired tolerance of the target voltage V Ft . However, it should be appreciate that it may not be necessary to set or adjust the final voltage V 2C  in the event that the final voltage V 2C  is already within a desired tolerance of the target voltage V Ft . Operation then continues to step S 800  where operation of the method ends. 
       FIG. 3  is a flowchart outlining in greater detail one exemplary embodiment of the method for determining the slope of V grid1  to V 1C  for the first charging grid of step S 200  according to this invention. As shown in  FIG. 3 , operation begins in step S 200 , and proceeds to step S 210 , where the target voltage V 1t  to be imparted on the charge retentive surface by the first grid is determined, selected or input. Then, in step S 220 , the combined target voltage V Ft  to be imparted to the charge retentive surface by the first charging device and second charging device together is determined. In various exemplary embodiments, the target voltage V 1t  on the charge retentive surface may be 500 volts, while the target voltage V 2t  on the charge retentive surface may be 650 volts, i.e. that the first and second charginge devices together impart a total charge V 2C  of 650 volts, on the charge retentive surface. Operation then continues to step S 230 . 
     In step S 230 , environmental sensor data is input or read. In various exemplary embodiments, the input or read environmental data is stored in memory. In various exemplary embodiments, memory comprises a non-volatile memory. However, it should be appreciated that data may be stored in any type of known or later-developed memory device. This environmental data may include such information such as, for example, temperature and/or humidity. Next, in step S 240 , the first grid is set to a first test voltage V grid1a . In various exemplary embodiments, this first test voltage level is 100 volts below the target voltage V 1t  of the first grid. Then, in step S 250 , the second grid voltage is set to a minimum value such as 0 volts. Operation then continues to step S 260 . 
     In step S 260 , the selected first test voltage V grid1a  is applied to first grid as charges are applied to the charge retentive surface by the first charging device. Then, in step S 270 , the charge imparted to the charge retentive surface V 1Ca  with the first grid voltage set to the first test voltage V grid1a  is read and stored in memory. In various exemplary embodiments, the charge is read using an electronic voltage meter or electrostatic voltage meter (ESV). Next, in step S 280 , the first grid voltage is set to a second test voltage V grid1b . In various exemplary embodiments, this second test voltage V grid1b  is 100 volts above the target voltage V 1t  of the first grid. Operation then continues to step S 290 . 
     In step S 290 , the grid voltage V grid2  of the second charging unit is again set to a minimum value, such as 0 volts. Next, in step S 300 , the selected second test grid voltage V grid1b  is applied to the first grid as the grid charges are applied to the charge retentive surface by the first charging device. Then, in step S 310 , the charge imparted to the charge retentive surface V 1Cb , with the voltage on the first grid set to the second test voltage V grid1b  is read and stored in memory. Operation then continues to step S 320 . 
     In step S 320 , the slope of V grid1  to V 1C  is determined using the stored charge values V 1Ca  and V 1Cb  obtained by applying the first and second test voltages V grid1a  and V grid1b  to the first grid. As described earlier, the slope of V grid1  to V 1C  is expressed in units of Amperes per volt-meter (A/v·m). Based on the response curve for the first charging grid, the voltage level V grid1  on the control grid of the first charging unit that will charge the charge retentive surface to the desired target potential voltage V 1t  can be determined. Operation then continues to step S 330 , where operation of the method ends. 
       FIG. 4  is a flowchart outlining in greater detail one exemplary embodiment of the method for determining the response curve of the second charging grid according to this invention. As shown in  FIG. 4 , operation of the method begins in step S 400  and proceeds to step S 410 , where, based on the slope of V grid1  to V 1C , the grid voltage on the control grid of the first charging unit is set to a voltage level that will achieve a charge of V 1t  on the charge retentive surface. Then, in step S 420 , the offset voltage ΔV grid  is set to a first test voltage ΔV grida . In various exemplary embodiments, the first offset test voltage ΔV grida  is 100 volts “more” than the intermediate target voltage V 1t . Typically, the charge retentive surface is regularly charged. In this case ΔV grida  is −100V (i.e., 100 volts below V grid1 ). Next, in step S 430 , the charge retentive surface is charged with the ΔV grida . Then, in step S 440 , the charge level imparted to the charge retentive surface V 2Ca  is sensed and stored in memory. Operation then continues to step S 450 . 
     In step S 450 , the offset voltage ΔV grid  is set to a second test voltage ΔV gridb . In various exemplary embodiments, the second test voltage ΔV gridb  is 200 volts “more” than the intermediate target voltage V 1t . Thus, when the charge retentive surface is negatively charged, ΔVgridb is −200V. Then, in step S 460 , the charge retentive surface is charged with the first charging grid set to achieve the intermediate target voltage of V 1t  and the second charging grid is set to achieve an offset voltage ΔV grid  of ΔV gridb . Next, in step S 470 , the charge imparted to the charge retentive surface V 2Cb  is sensed and stored in memory. Operation then proceeds to step S 480 . 
     In step S 480 , based on the stored charge levels V 2Ca  and V 2Cb  corresponding to ΔV grida  and ΔV gridb , the slope of the offset voltage ΔV grid  to V 2C  is determined. Operation then continues to step S 490 , where operation of the method returns to step S 600 . 
       FIG. 5  is a flowchart outlining in greater detail one exemplary embodiment of the method for determining whether the final photoreceptor voltage V 2C  is within an acceptable range or tolerance of the target final voltage V Ft  of  FIG. 2  according to this invention. Operation of the method begins in step S 600 , and proceeds to step S 610 , where the voltages V grid1  and V grid2  on the control grid of the first and second charging devices are set based on the determined slopes for V grid1  and ΔV grid , to the achieve the target voltage V Ft . Then, in step S 620 , the charge retentive surface is charged using the first and second charging devices having the control grids set based on V grid1  and ΔV grid . Next, in step S 630 , the actual final voltage V Fa  on the charge retentive surface, caused by first and second control grids being set as described is read and stored. Operation then continues to step S 640 . 
     In step S 640 , a determination is made whether the actual final voltage V Fa  is within a predetermined tolerance of the target voltage V Ft . In various exemplary embodiments, the tolerance for the actual final voltage V Fa  can be ±10 volts of the target voltage V Ft . If, in step S 640 , a determination is made that the final actual voltage V Fa  is within an acceptable tolerance of the target voltage V Ft , operation jumps to step S 710 . Otherwise, processing proceeds to step S 650 . 
     In step S 650 , the offset voltage ΔV grid  is adjusted by altering the offset voltage ΔV grid  by a determined increment. For example, if the actual voltage V Fa  is too high, the offset voltage ΔV grid  is adjusted up by the determined increment. If the actual voltage V Fa  is too low, the offset voltage ΔV grid  is adjusted down by the determined increment. It should be appreciated that the determined increment can be predetermined or can be dynamically determined or determined on the fly. For example, the determined increment can be determined based on the difference between the actual and target final voltages V Fa  and V Ft . In various exemplary embodiments, a reasonable predetermined increment is 5 volts. Next in step S 660 , the charge retentive surface is charged using the first and second charging devices having the control grids set based on V grid1  and ΔV grid . Operation then continues to step S 670 . 
     In step S 670 , the voltage value V Fa  imparted to the charge retentive surface based on the new value for ΔV grid  is again sensed and stored. Then, in step S 680  a loop counter, representing the number of adjustments that have been made to the offset voltage ΔV grid  is incremented. Then, in step S 690 , a determination is made whether the value of the loop counter is equal to the maximum allowable number of iterations. If the maximum allowable number adjustments has been made, operation proceeds to step S 700 . Otherwise, operation returns to step S 640 . In step S 700 , a fault indication is output. Operation then continues to step S 710 , where operation of the method returns to step S 800 . Thus, once either a fault indication has been output or the measured voltage on the charge retentive surface V Fa  is determined to be within the acceptable tolerance of the target voltage V Ft , operation of the method returns to step S 800 . 
       FIG. 6  is a block diagram outlining one exemplary embodiment of a charging system control system  200  according to this invention. As shown in  FIG. 6 , the charging system control system  200  has an input/output interface  210  that is linked to an electronic volt meter  300  (or any other appropriate charging sensing device) by a link  310 . The input/output interface  210  is also linked to an environmental data source  400  by a link  410 , a first charging unit voltage setting device  500  by a link  510 , and a second charging unit setting device  600  by a link  610 . The charging system control system  200  also includes a controller  220 , a memory  230 , a first charging unit target voltage determining circuit, routine or application  240 , a second charging unit target voltage determining circuit, routine or application  260 , a slope determining circuit, routine or application  250 , and a final voltage comparing circuit, routine or application  270 . 
     Each of the links  310 – 610  can be any known or later developed connection system or structure usable to connect the respected devices to the charging system control system  200 . It should also be understood that the links  310 – 610  do not need to be of the same type. 
     The memory  230  can be implemented using any appropriate combination of alterable volatile or non-volatile memory, or non-alterable or fixed memory. The alterable memory whether volatile or non-volatile can be implemented using any one or more of static or dynamic RAM, a floppy disk and disk drive, a writable or rewritable optical disk and disk drive, a hard drive, flash memory or the like. Similarly, the non-alterable or fixed memory can be implemented using any one or more of ROM, PROM, EPROM, EEPROM, and gaps in an optical ROM disk, such as a CD ROM or DVD ROM disk and disk drive, or the like. 
     In one exemplary embodiment of the operation of the charging system control system  200  according to this invention, environmental data is read by the environmental data source  400 . The read environmental data is forwarded from the environmental data source  400  over the link  410  to the charging system control system  200 . The received environmental data is output through the input/output interface  210  and stored into the memory  230 . A first charging unit target voltage is determined by the first charging unit target voltage determining circuit, routine or application  240 . Next, the second charging unit target voltage is determined by the second charging unit target voltage determining circuit, routine or application  260 . The first charging unit voltage setting device  500 , based on control signals from the controller  220 , sets the control grid voltage for the first charging device to a first value below the determined first charging unit target voltage. 
     The first charging device is then used to charge a photoreceptor or other charge-retentive surface. The charge applied to the charge-retentive surface by the first charging device is then read by the electrostatic volt meter  300 . The read charge is input by the electronic volt meter  300  over the link  310  to the charging system control system  200 . The received data is input through the input/output interface  210  and stored in the memory  230 . 
     The first charging unit voltage setting device  500 , based on control signals from the controller  220 , sets the control grid voltage for the first charging device to a second value above the determined first charging unit target voltage. The first charging device is then again used to charge a photoreceptor or other charge-retentive surface. The charge applied to the charge-retentive surface by the first charging device is then again read by the electronic volt meter  300 . The read charge is input by the electronic volt meter  300  over the link  310  to the charging system control system  200 . The received data is input through the input/output interface  210  and stored in the memory  230 . The charge-generating emitter ratio (slope) of the first charging unit is then determined by the charge-generating emitter ratio determining circuit, routine or application based on the first and second voltages the control grid of the first charging device was set to and the high and low voltages read by the volt meter  300  and stored in memory  230 . 
     Based on the charge-generating emitter ratio of the first grid as determined by the charge-generating emitter ratio determining circuit, routine or application  250 , the first charging unit target voltage determining circuit, routine or application  240  determines a setback target voltage and stores this data in the memory  230 . The first grid voltage is then set to the setback target voltage by the first charging unit voltage setting device  500  based on control signals from the controller  220 . The second charging unit voltage setting device  600  then, based on control signals from the controller  220 , sets the control grid of the second charging device to 100 volts below the final target voltage. The first and second charging devices are then used to charge the charge retentive surface. The electronic volt meter  300  then reads the voltage on the charge retentive surface and stores the voltage in the memory  230 . The second charging unit voltage setting device  600  then sets, based on control signals from the controller  220 , the control grid of the second charging device to 100 volts above the final target voltage. The first and second charging devices are then used to charge the charge retentive surface. The electrostatic volt meter  300  then reads the voltage on the charge retentive surface and stores the voltage in the memory  230 . The charge-generating emitter ratio determining circuit, routine or application  250  then determines the slope of the second charging unit based on the high and low voltages read by the volt meter  300  and stored in the memory  230 . The second charging unit target voltage determining circuit, routine or application  260  determines the second target voltage of the second charging unit. 
     The final voltage comparing circuit, routine or application  270  determines whether the final actual photoreceptive voltage on the charge retentive surface is within an acceptable range of the target final voltage, and thus whether the setup process is complete. The grid voltage of the second charging device is set to the determined second target voltage by the second charging unit voltage setting device  600 . The first and second charging devices are then used to charge the charge retentive surface. The voltage on the charge retentive surface is then read by the electrostatic volt meter  300  and stored in the memory  230 . The final voltage comparing circuit, routine or application  270  then determines whether the actual final voltage is within a determined tolerance of the target final voltage. If the actual final voltage is within the determined tolerance of the final target voltage, the setup process is complete. If the actual final voltage is not within a determined tolerance of the final target voltage, the second charging unit target voltage setting device  260  adjusts the second target voltage in determined increments and this process is repeated until the actual final voltage is within the determined tolerance of the target final voltage, or after some predetermined number of increments have been performed. In that case, a fault indication is output by the final voltage comparing circuit, routine or application  270 . 
     It should also be understood that each of the circuits, routines and/or applications shown in  FIG. 6  can be implemented as portions of a suitably programmed general purpose computer. Alternatively, each of the circuits, routines and/or applications shown in  FIG. 6  can be implemented as physically distinct hardware circuits using a digital signal processor or using discrete logic elements or discrete circuit elements. The particular form each of the circuits, routines and/or applications in  FIG. 6  will take is a design choice and will be obvious and predictable to those skilled in the art. It should also be appreciated that the circuits, routines and/or applications shown in  FIG. 6  do not need to be of the same design. 
     While this invention has been described in conjunction with the specific embodiments outlined above, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the preferred embodiments of the invention, as set forth above, are intended to be illustrative, not limiting. Various changes may be made without departing from the spirit and scope of the invention.