Toner Usage Sensing System

Abstract

A toner usage sensing system for an electrostatic reproduction apparatus which utilizes multicomponent developer material including electrostatically charged toner to develop electrostatic latent images on a photosensitive plate directly senses the amount of toner consumed by the reproduction process. Since the average toner body charge (coulomb/gram) is constant for a given desired toner concentration, the weight of the electrostatically charged toner consumed in sensed by measuring the charge removed from the developer material during the reproduction process. The developer unit containing the developer material is electrically isolated from the remainder of the reproduction apparatus and grounded through a single connection. The current in the ground connection returns charge to the developer unit sufficient to replace the charge removed and is sensed to obtain a measurement of the toner consumed. A measured amount of toner is added to the developer material when a preselected amount of charge has been sensed as being removed. In one embodiment, a DC motor drives the toner replenisher and rotates an amount proportional to the sensed current thereby providing a mechanical storage of the incremental amounts of toner removed on widely spaced reproduction runs.

Description

CROSS-REFERENCE TO RELATED APPLICATION

The following application is assigned to the same assignee as the present application:

U.S. Pat. application Ser. No. 110,725, entitled "Coated Carrier Particles with Polarity and/or Magnitude of Triboelectric Charge Controlled, Method of Making Same, and Improved Electrophotographic Process," William J. Kukla and Howard Everett Munzel, inventors, filed Jan. 28, 1971.

BRIEF BACKGROUND OF INVENTION

1. Field

This invention relates to a toner usage sensing system for an electrostatic reproduction apparatus and, more particularly, to sensing apparatus for directly measuring the amount of toner consumed in order to maintain constant image density of an electrostatic printer which utilizes a developer material including toner.

2. Description of the Prior Art

In well-known electrostatic printing processes, a surface bearing a latent electrostatic image is developed by applying a developer material comprising toner and a carrier material to the image bearing surface. The toner and carrier material have differing triboelectric characteristics and are agitated together prior to being applied to the image-bearing surface, thereby creating electrostatic charges of opposite polarity on the toner particles and carrier material. The electrostatically charged surface preferentially attracts a portion of the applied toner to the image area thereof, and the remainder of the developer material is removed and allowed to recirculate to develop subsequent images. In order to insure that there is a sufficient and proper amount of toner in the developer material, it is necessary to add quantities of toner material to the developer material to replace that which has been used for developing images.

The most rudimentary prior art system for controlling the toner concentration of a developer material included a manually settable device for controlling the amount of toner metered to the developer material during each reproduction cycle of the apparatus. Since the density of the image area and, hence, the amount of toner material utilized varies widely in accordance with the material being reproduced, this method of control is dependent upon the operator who must judge the quality of the output and set the device to dispense more or less toner as the output usage changed or output quality degrades. Since there is a time lag between the time that the operator takes corrective action and the time that an overly lean mixture has sufficient toner properly mixed with the developer material or conversely, an overly rich mixture expels excess toner by development of images, the operator's presence must be maintained in order to assure that the corrective action resulted in the desired print quality. Thus, such systems necessarily depend upon the expert judgement of the operator and further require the constant presence of the operator.

Various prior art systems have been proposed for automatically controlling the toner concentration of the developer material within a desired range of concentration, thereby eliminating the need for operator intervention. All of these prior art systems rely on measuring a physical characteristic of a sample of the developer material and for effecting the automatic addition of toner to the developer material when the measured physical characteristic differs from a preset standard. One such prior art system relies on the differing resistivity characteristics of the toner and carrier to produce a given resistivity characteristic when mixed together in a desired proportion. Thus, once the resistance of a sample of the developer material differs from a preset standard, additional toner is added to the developer material until the resistance characteristic equals the standard. A further prior art technique teaches passing a sample of the developer material through a coil connected to an oscillator circuit to measure the inductance of the developer material which comprises steel carrier. When the inductance of the material reaches a preset level, additional toner is added. A further prior art technique teaches directing a portion of the developer material over a conductive plate for a preset period of time, the plate being charged by the toner particles in the developer material. The charge induced on the plate is then sensed and, if below a preset level, initiates the replenishment of additional toner particles to the developer material. A still further prior art technique comprises biasing a probe to a predetermined bias level and thereafter contacting the probe with the developer material. The amount of toner adhering to the probe is then optically sensed to determine the toner concentration in the developer material. Thus, each of the prior art techniques determine the percentage of toner concentration in the developer material by measuring a physical parameter of a sample of developer material. Toner is automatically added to the developer material when the physical parameter differs from a preset standard.

Since a large quantity of developer material must be utilized to assure uniform reproduction runs, there is a long lag time from the time additional toner is introduced into the developer material and the time that it becomes uniformly distributed throughout the developer material. Due to this long lag time, it becomes difficult to exactly correct for the amount of toner removed since the sample measured may not represent the developer material after uniformity is obtained. This problem becomes acute when dark and light masters are copied in adjacent reproduction runs.

A further shortcoming of the prior devices is that they must utilize extremely sensitive measuring units in order to detect slight variations from the given standard. This is because a small change in resistivity, optical density, inductance, and charge level results in wide disparity in the quality of the reproduced document. Accordingly, sensitive circuits are required to sense minute changes and to insure that the reference does not vary.

SUMMARY

In order to overcome the above-noted shortcomings of the prior art and to provide a toner usage sensing system which consistently and accurately detects the amount of toner which must be added to the developer material to maintain the desired image density, the present invention directly measures the amount of toner consumed during each reproduction cycle and automatically adds a corresponding amount of toner back into the developer material. The amount of toner which is consumed is measured by measuring the charge removed from the developer material due to the removal of the charged toner during development. The developer material is located within a developer unit which is electrically isolated from the remainder of the reproduction apparatus and is connected to a current source. Accordingly, a charge equal to the charge removed during development returns to the developer unit through the connection to the current source. This current is measured to determine the charge removed. Since for a specific type of toner and carrier the average toner charge (coulomb/gram) is approximately constant for a given toner concentration, the weight of the toner removed by development is proportional to the charge removed. Accordingly, replenishing is effected by adding a fixed weight of toner each time a predetermined amount of charge has been removed by development. Thus, the amount of toner added to the developer material is constant and determined by the detected charge removed and the optimum average body charge of the toner for a desired image density. By directly sensing the amount of toner consumed, there is no delay in obtaining a properly mixed sample of developer material as was inherent in the aforenoted prior art devices.

A mechanical storage element is utilized to store a representation of the amount of charge removed during widely spaced reproduction runs thereby maintaining accurate measurement of all toner removed during development and effecting the automatic addition of fresh toner when a predetermined amount of toner has been removed.

Accordingly, it is the principle object of the invention to automatically and precisely control toner dispensing in an electrostatic reproduction system. A further object of the invention is to maintain consistency in image quality of an electrostatic printing device. Such consistency is obtained by maintaining the average toner body charge of the developing material constant.

It is a further object of the invention to directly sense the amount of toner consumed during the reproduction process and to add a corresponding amount of toner back into the developing unit to maintain constant image density.

The foregoing objects, features, and advantages of the invention will be apparent from the following more particular description of the preferred embodiments of the invention as illustrated in the accompanying drawings.

In the drawings:

FIG. 1 is a schematic illustration of a conventional electrostatic reproduction apparatus incorporating the toner usage sensing system of the present invention.

FIG. 2 is a circuit diagram of a portion of the toner usage sensing system depicted in FIG. 1.

FIG. 3 is a schematic block diagram of an alternate embodiment of the toner usage sensing system of the present invention.

FIG. 4 is a schematic block diagram of a second alternate embodiment of the toner usage sensing system of the present invention.

DESCRIPTION

Referring now to the drawings, and more particularly to FIG. 1 thereof, a schematic illustration of a conventional electrostatic reproduction apparatus incorporating the toner usage sensing system of the present invention is depicted.

The reproduction apparatus comprises a plurality off processing stations located about a cylindrically shaped electrostatic photosensitive plate 11. The cylindrical plate comprises a layer of photoconductive material superimposed over a conductive backing. A suitable photoconductive material is disclosed in U.S. Pat. No. 3,484,237 issued Dec. 16, 1969. The photoconductive material is sensitized by a corona generating device 13 as the plate rotates therepast in the direction of arrow 15. A light image of the copy 17 to be reproduced is projected onto the sensitized surface of the electrostatic photosensitive plate 11 rotating thereunder to form an electrostatic latent image thereon. The rotating plate thereafter passes a developer station 19 where multicomponent developer material including electrostatically charged toner is applied to the surface of the electrostatic photosensitive plate 11 containing the electrostatic image thereon. The charged toner particles are preferentially attracted to the latent image on the plate 11 and are subsequently transferred to a support surface 21 by conventional electrostatic transfer or by heat transfer. The plate continues to rotate past a cleaning station 25 which removes residual toner from the surface of the plate.

The developer station 19 comprises a sump portion 31 containing multicomponent developer material 33. The principle components of the developer material are electroscopic toner and a carrier material. Suitable materials for use as toners are well known in the art and generally comprise finely divided resinous materials capable of being attracted and held by electrical charges. Examples of materials which can be utilized for toners are listed in the aforereferenced copending application of William J. Kukla, et al. Examples of toners which can be employed are commercially marketed by the International Business Machines Corporation as IBM part number 1162057 and IBM part number 1162051. The IBM part number 1162057 toner comprises a copolymer of styrene/n-butyl methacrylate resin, maleic anhydride modified polyester, polyvinyl stearate plasticizer and carbon black pigment. The IBM part number 1162051 toner comprises a copolymer of n-butyl methacrylate/methymethacrylate resin, maleic anhydride modified polyester, polyvinyl butyral plasticizer, carbon black pigment, and a fumed silica physically mixed in the toner after compounding. Many well-known suitable carrier materials can be utilized, the carrier particles generally being between 50 and 1,000 microns in size. Often, the carrier particles are manufactured by coating sand, glass beads, or metallic beads with a material which triboelectrically interacts with the selected toner to produce a desired charge on the toner in order to provide good imaging quality. An example of such a carrier and the method by which it can be made to obtain a desired triboelectric characteristic for any selected toner is disclosed in the aforereferenced copening application of William J. Kukla, et al.

A toner dispensing unit 35 is provided to dispense toner particles 37 into the multicomponent developer material 33 located in the sump portion 31 of the developer unit 19. Counter-rotating augers 39 and 40 stir the freshly added toner with the developer material to assure complete mixing thereof.

A bucket conveyor 41 rotates through the sump portion 31 of the developer station 19 and scoops up quantities of developer material 33 for delivery to the magnetic brush unit 43. The magnetic brush unit comprises a conductive rotatable cylindrical member 45 having located therein a magnetic field producing means 47. The core material of the carrier particles consists of a ferromagnetic material which causes the carrier particles to be magnetically attracted to the surface of the cylindrical member 45 and held thereon by magnetic forces produced by the magnetic field producing means 47. The cylindrical member 45 rotates in the direction of the arrow 49 under a doctor blade 51 which governs the amount of developer material located on the surface of the cylindrical member 45 as it rotates to a position adjacent to the electrostatic photosensitive plate 11. The magnetic field producing means 47 creates a normal magnetic field at approximately the 9 o'clock position of the cylindrical member 45 causing the magnetic carrier particles in the developer material 33 to form in bristle-like arrays emanating from the surface of the cylindrical member 45.

The small toner particles of the developer material 33 are held onto the surfaces of the relatively large carrier particles by electrostatic forces, which develop from the contact between the toner and the outer surface of the carrier particles which produces triboelectric charging of the toner and carrier material to opposite polarities. In the system depicted, the toner particles are triboelectrically charged positively with respect to the negative triboelectric charge on the carrier particles. The total triboelectric charge on the carrier material particles is equal and opposite to the total triboelectric charge on the toner particles.

A potential source 52 is connected to the cylindrical member 45 thereby biasing the cylindrical member to a fixed potential. As the magnetically formed bristles of carrier material containing toner triboelectrically attracted thereto rotate past and in contact with the electrostatic photosensitive plate 11, the triboelectrically charged toner particles are attracted to the electrostatic latent image on the plate 11 and adhere thereto. The potential on the cylindrical member correctly orientates the electrical field in which the charged toner particles move to produce a uniformly developed image on the surface of the plate 11. The plate 11 containing the toner particles continues to rotate and leaves the developer station 19.

The carrier material particles continue to be held to the surface of the rotating cylindrical member 45 until they reach approximately the 6 o'clock position of the cylindrical member. The magnetic field produced by the magnetic field producing means 47 is shunted so that no magnetic field is produced at the 6 o'clock position of the rotating cylindrical member 45. Thus, the carrier particles and those toner particles which were not attracted to the electrostatic photosensitive plate are released from the cylindrical member 45 into the sump portion 31 of the developer unit 19. The thus returning carrier particles contain a negative charge equal and opposite to the charge of the toner particles which were electrostatically attracted to the electrostatic photosensitive plate 11.

In order to assure high quality images on the support surface 21, it is necessary that the proper toner body charge be maintained in the developer material since this charge plays a major role in determining the density and quality of the developed image. By varying this charge, for example, by having an excessive amount of toner, the toner body charge will be low resulting in dense, blurred, overdeveloped images on the plate 11 and, hence, on the support surface 21. When too little toner is present in the developer material, the toner body charge will be high and light underdeveloped images are obtained. In order to automatically control the toner body charge in the developer material 33 and maintain it at a desired level, the amount of toner consumed during the reproduction process is determined and, an amount of toner corresponding to that consumed is dispensed from the toner dispensing unit 35 to thereby replenish the developer material 33. The amount of toner consumed is determined by measuring the charge removed from the developer material during development.

The developer station 19 is electrically isolated from the remainder of the reproduction apparatus including the electrostatic photosensitive plate 11 and is connected to ground 55 through the lead 57. As charge is removed from the developer material 33 with the removal of the triboelectrically charged toner, an equal charge returns to the developer material through the developer station 19 and lead 57. The current in the lead 57 is therefore representative of the time rate of charge removal from the developer material. This current is integrated by the operational amplifier 59 to obtain the total charge removed by development of the electrostatic latent image on the plate 11. Thus, if Q.sub.t represents the total charge removed with the toner and i.sub.d represents the current through lead 57, ##SPC1##

For a specific type of toner and carrier material with a constant mixing time, the average toner charge (coulomb/gram) is a constant for a given toner concentration. As the toner concentration increases, the average body charge of the toner will drop, and as the toner concentration decreases, the average body charge of the toner increases. If the quantity q/w represents the average body charge for the toner when the toner concentration is at the desired level, then the weight of toner removed through development will be: ##SPC2##

This then will be the weight of toner to be replenished when a charge Q.sub.t has been removed from the developer during development. The replenishing is effected by adding a fixed weight, W, each time a fixed charge Q.sub.t has been detected as being removed from the developer material 33.

The current, i.sub.d, flowing from ground 55 through the operational amplifier 59 effects the buildup of a charge on capacitor 61. When the charge reaches a predetermined level, corresponding to the charge Q.sub.t, a level detector 63 initiates the timer 65 which provides a gating signal to drive the motor 67 for a preset time period. Rotation of the motor 67 effects corresponding rotation of the dispenser member 69 which meters a measured amount of toner 37 through the opening 71 each time a tooth 73 rotates past the opening 71. Such a toner dispensing apparatus is described in U.S. Pat. No. 3,572,555 issued Mar. 30, 1971, assigned to the assignee of this invention. Thus, the toner dispensing unit 35 effects the dispensing of a measured volume of toner for each rotation of the dispenser member 69. Since the toner has a uniform weight per unit of volume, a fixed weight of toner is dispensed for each revolution of the dispenser member 69.

By adding a fixed weight of toner each time a fixed charge has been measured as being removed from the developer material, the density of the image transferred to the support surface 21 is maintained approximately constant, and the toner concentration is allowed to vary to offset the variations in mixing time that occur due to high toner usage ratio with dense masters and low toner usage rates with light masters. This is because the system operates by adjusting the toner concentration around an optimum value to maintain a constant toner body charge, which is the parameter that determines the toner usage and, hence, the copy density. It has been observed, for example, that variations in toner concentration of between 0.7 to 1.0 percent by weight of toner to coated steel carrier produced a constant toner body charge when the master developed was varied from very light to very dense. This is due to the variation in mixing time of the toner as seen between very light and dense copies. Thus, when the body charge of the toner decreases, more toner will be used before the preset charge level Q.sub.t is reached. The replenishing rate of toner is fixed, however, at W grams. The toner concentration in the developer station 19 will thus be reduced since the rate of toner being introduced into this system from the toner dispenser 35 is lower than the rate of toner being expended through development. As the toner concentration is reduced, the average body charge of the toner will rise, thereby reducing the usage rate of the toner and, hence, the density of the developed image. The exact opposite occurrence of events takes place when the body charge increases above the average body charge sought to be maintained due to lower toner concentrations than desired. That is, as the average body charge increases, less toner (containing a higher charge) is consumed by development than that replenished from the toner replenisher 37 when the charge Q.sub.t is reached. The additional toner results in the lowering of the average body charge, thereby increasing the usage rate and, hence, the developed image density. Accordingly, by adding a fixed weight, W, of toner each time a fixed charge, Q.sub.t, is detected as being lost by the developer station 19 due to development, optimum developed image density is maintained.

Once the fixed charge, Q.sub.t is sensed, the level detector 63 actuates the toner dispenser as has been described and further causes relay 75 to transfer thereby providing a discharge path for capacitor 61. The capacitor 61 is discharged thereby effectively resetting it to zero whereby it can again measure the charge, Q.sub.t.

Referring now to FIG. 2 of the drawings, a circuit diagram of a portion of the toner usage sensing system depicted in FIG. 1 is shown. As developer current, i.sub.d is drawn through the lead 57 and the operational amplifier 59 from ground 55, a voltage builds up across the capacitor 61. When this voltage exceeds the voltage at terminal 101 of the level detector 63 as determined by the setting of the variable resistor 103, transistor 105 turns on and transistor 107 turns off. This action creates a positive going signal at the base electrode of transistor 109 which, in turn, provides a negative going signal at its collector electrode 111. The negative going signal is applied to the single shot circuit 113 which provides a positive going pulse at the collector electrode 115 of transistor 117. This pulse is applied both to a timer circuit and to a reset circuit.

The pulse applied to the timer circuit is applied to the base electrode of transistor 119 which, in turn, drives transistor 121 with a positive going pulse at its base electrode thereby providing a connection to ground terminal 122 through the transistor 121. Current then flows through the relay coil 123 from the +12 volt terminal 124 causing the operating straps 125 and 127 to transfer. Current continues to flow through the relay coil 123 through the connection provided by the operating strap 127 and the switch 129 to the ground terminal 131. The motor 67 is actuated through the contact provided by the operating strap 125. Additionally, the timer motor 133 is turned on by the same connection. As the timer motor rotates by a fixed amount, a cam (not shown) attached to the output shaft of the motor opens switch 129 thereby removing the ground connection to the ground terminal 131. With the ground connection thus removed, current stops flowing through the relay coil 123 thereby retransferring the operation straps 125 and 127 to their normal positions as shown. At this time, the motor 67 and the timer motor 133 are no longer energized. Thus, the pulse supplied to the base electrode of transistor 119 causes the motor 67 to be energized for a fixed time interval. The switch 129 is reset at the start of the timer motor cycle.

As discussed heretofore, the pulse output of the level detector 63 is also provided to a reset circuit. This pulse is applied to the base electrode of the transistor 135 causing its collector electrode to go negative thereby drawing current through the relay coil 137. Current through the relay coil 137 effects the transfer of the relay 75 thereby providing a discharge path for the capacitor 61 through the resistor 139. Thus, the capacitor 61 is reset to a zero voltage condition whereby it can be again utilized to measure the charge replenished by the current, i.sub.d, through the lead 57.

It should be noted at this point that the voltage across the capacitor 61 initiates the level detector 63 when this voltage exceeds the voltage at terminal 101 as determined by the setting of the variable resistor 103. Since the motor 67 is actuated for the same period of time regardless of the setting of the variable resistor 103, a fixed weight of toner is dispensed for any charge Q.sub.t as selected by the setting of the variable resistor 103. Thus, the setting of the variable resistor 103 controls the average body charge of the toner in the multicomponent developer material 33 of FIG. 1 and, accordingly, controls the ultimate image density. Additionally, it should be noted that the capacitor 61 and operational amplifier 59 are selected to have low leakage. It has been found that the charge level can be maintained in excess of eight hours without appreciable error when the combined leakage of these two units is in the order of 10.sup.12 ohms.

Referring now to FIG. 3 of the drawings, a schematic block diagram of an alternate embodiment of the toner usage sensing system of the present invention is depicted. In this embodiment, a DC motor 151 is utilized to drive the dispenser member 69. Further, the power supply 153 which biases the cylindrical member 45 to provide a development electrode effect is connected to the sump portion 31 of the developer unit as well as to the cylindrical member 45. As with the device depicted in FIG. 1, the current, i.sub.d, flowing through lead 57 from the ground terminal 55 flows through an operational amplifier 155. The operational amplifier can be of the same type as the operational amplifier 59 of FIG. 1 which, by way of example, can be an AD 503K. The voltage, V.sub.o, at the terminal 157 of the operational amplifier is equal to the current, i.sub.d, times the value, R, of the resistor 159. This voltage, V.sub.o is applied to the DC motor 151.

The speed of rotation of the DC motor 151 is directly proportional to the input applied voltage. Since the angle, .theta., through which the DC motor rotates in a given time period is proportional to its speed of rotation and, hence, to its applied voltage, the angle, .theta., is thereby proportional to the integral of the current, i.sub.d, flowing through the lead 57. Rotation of the DC motor 151 effects corresponding rotation of the fluted barrel-type dispenser member 69 which periodically adds toner to the developer material located in the sump portion 31 of the developer unit 19. The total angle of rotation, .theta., of the dispenser member 69 is thereby proportional to the integral of the developer current, i.sub.d. The dispenser member 69 thus provides a mechanical "storage" which "remembers" the incremental amount of toner removed on previous reproduction runs, even though the time lapse between reproduction runs is great. That is, if a small amount of toner is removed by development on a first reproduction run, the DC motor 151 effects rotation of the dispenser member 69 through a small angle which may not be sufficient to cause the dispensing of additional toner particles. Although the next reproduction run could take place days later, the fact that some toner has been removed from the system is "stored" by the positional relationship of the dispenser member 69 with respect to the opening 71 of the toner dispenser unit 35. Accordingly, the DC motor 151 performs the function of integrating the current i.sub.d in the lead 57.

It is to be noted that a cleaning station 25 can be employed which removes residual toner from the plate 11 after it passes the transfer station 160 and effects the recirculation of the removed toner back into the developer station 19. That is, as toner is removed from the plate 11 by the brush 161, it falls into the sump portion 162 of the cleaning station 25. An air pump 163 circulates the removed toner from the sump portion 162 to the developer unit 19 where it is mixed with developer material 33. The toner which is thus introduced into the developer unit 19 contains a charge which is approximately equal to the average toner body charge (less losses and/or gains imparted by electrostatic coaction with the cleaning station 25). This toner charge tends to neutralize the charge of opposite polarity imparted by the carrier returning from the cylindrical member 45, thereby reducing the amount of current, i.sub.d, and the voltage V.sub.o. The reduction in voltage V.sub.o results in less toner being dispensed from the toner dispensing unit 35. If more recirculated toner returns to the developer unit 19 than is expended during a given reproduction run, the polarity of the voltage V.sub.o reverses thereby reverse driving the dispenser. Such a reverse drive over a portion of a toner dispensing revolution has the effect of decreasing the amount of toner dispensed from the toner dispensing unit 35.

Referring now to FIG. 4 of the drawings, a schematic block diagram of a second alternative embodiment of the toner usage sensing system of the present invention is depicted. This system is basically similar to that described with respect to FIG. 1 with the exception of the sensing circuit utilized to detect the current, i.sub.d, flowing in lead 57 and the utilization of a cascade development system in lieu of a magnetic brush developing system. Thus, the current, i.sub.d, flowing in the lead 57 from ground 55 effects the buildup of a charge on capacitor 171. When this charge reaches a predetermined level as detected by the level detector 63, the timer 65 is actuated which effects the energization of motor 67 for a set time period. Energization of the motor 67 for a preset time period effects the metering out of a fixed quantity of toner from the toner dispenser 37 into the sump portion 31 of the developer station 19 in the same manner as described with respect to FIG. 1. Further, the level detector 63 provides a pulse to the relay 75 which causes the capacitor 171 to discharge and, hence, reset to a zero condition.

A bucket conveyor 175 is utilized to convey developer material 33 from the sump portion 31 of the developer station 19 to the surface of the rotating electrostatic photosensitive plate 11. The developer material cascades over the surface of the rotating plate 11 with the toner being preferentially attracted to the latent electrostatic image thereon in a conventional manner. The remaining portion of the developer material including the carrier material and unused toner returns to the sump portion 31 of the developer station 19 and carries with them a charge equal and opposite to the charge of the consumed toner. It is this charge which is neutralized through the flow of the current, i.sub.d, through the lead 57.

Operation

Referring once again to FIG. 1 of the drawings, a latent electrostatic image is formed on the electrostatic photosensitive plate 11 which rotates in the direction of arrow 15 past the developer station 19. Developer material 33 containing triboelectrically charged toner and carrier is applied to the surface of the cylindrical member 45 of the magnetic brush unit 43 which rotates in the direction of arrow 49 to a position closely adjacent to the electrostatic photosensitive plate 11. The magnetic carrier material of the developer material is attracted to and held onto the surface of the cylindrical member 45 by a magnetic field produced by the magnetic field producing means 47. The triboelectrically charged toner particles, triboelectrically attracted to the carrier material, are attracted to the latent electrostatic image on the electrostatic photosensitive plate 11 by the field set up between that image and the bias applied to the cylindrical member 45. The carrier particles and those toner particles not adhering to the plate 11 continue to rotate with the rotation of the cylindrical member 45 and are subsequently dropped into the sump portion 31 of the developer unit 19. These carrier and toner particles which are dropped from the cylindrical member 45 contain an electrostatic charge equal and opposite to the charge of the toner particles which were consumed during development.

The developer station 19 is electrically isolated from the remainder of the reproduction apparatus and is connected to ground 55 by lead 57. The current flowing from ground through lead 57 neutralizes the charge of the returning developer material. The integral of this current thus represents the total charge removed by the developing process. This current is integrated by the operational amplifier 59 which effects the buildup of a voltage on capacitor 61 which is proportional to the integral of the current flowing through the lead 57. When the voltage across the capacitor 61 reaches a preset level, the level detector 63 provides an output pulse to the timer 65 which, in turn, effects the energization of the motor 67 for a predetermined time period. The level detector 63 also provides an output signal causing relay 75 to transfer thereby discharging the capacitor 61 and resetting it to zero.

Rotation of the motor 67 for a predetermined time interval effects the metering out of a specific volume of toner 37 in the toner dispensing unit 35. The thus replenishing toner is supplied to the sump portion 31 of the developer station 19 and is mixed with the remainder of the developer material by the counter-rotating augers 39 and 40. Thus, each time a predetermined charge has been removed from the developer material 33 through development, the toner dispenser 35 meters a fixed quantity of weight of toner into the developer station 19 to be mixed with the multicomponent developer material 33.

Since a fixed quantity of toner is added each time a predetermined charge has been removed from the developer material by developing the latent image on the plate 11, the average body charge of the toner will rapidly seek a preset level depending upon the weight-charge ratio which, in turn, is set by operator control. Once the desired average body charge is reached, the density of the developed images on the electrostatic photosensitive plate 11 remains constant. Since the amount of toner transferring to the electrostatic latent image is governed by the charge level of the toner which, in turn, depends in part upon the toner concentration, the present system which maintains a constant charge level on the toner results in far more uniform copy density than those prior systems which measure a physical parameter of the developer material in order to maintain a predetermined ratio of toner to carrier. Further, since the amount of toner transferred during development from the developer unit 19 is continuously monitored and continuously corrected through operation of the toner replenisher, there are no time lags in obtaining a properly mixed developer material sample for measurement. Accordingly, copy density can be maintained within a close tolerance.

While the description has related to the utilization of a device for measuring the charge of the transferred toner and thereby calculating the weight of the toner which must be replenished to replace the removed toner, it is recognized by those skilled in the art that any system which directly measures the toner consumed on a continuing basis and which effects the replenishing of an amount of toner equal to the consumed toner would have the same advantages ascribed to the present system. Thus, for example, if the amount of toner transferring to the plate 11 were weighed and an equal weight of toner was applied from the replenisher, the system would have the same rapid response time as does the system described with respect to FIG. 1. The toner adhering to the plate could also be optically sensed to determine the weight of the toner to be replenished. Further, the invention can be readily incorporated in devices which utilize a single component developer material (toner) which is charged prior to development. In such a system, the total charge applied to the toner and the charge on nonconsumed toner could be measured to determine the charge of the toner consumed during development.

As is further recognized by those skilled in the art, various forms of developing units, for example, screen developing units, could be utilized in lieu of the magnetic brush and cascade developer units described. Further, such units could be connected through multiple leads to the same or differing current sources, it being necessary only to measure the current flowing to the unit which neutralizes a substantial portion of the charge carried by the carrier material and/or toner returning from the development zone.

Additionally, it has been described that the toner body charge can be varied by varying the amount of charge which is detected and utilized to effect the addition of a fixed weight of toner to the developer material. As is recognized by those skilled in the art, the same charge could be detected and variation of the toner body charge can be effected by varying the weight of the toner added.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it should be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A toner usage sensing system for use in an electrostatic reproduction apparatus having an electrophotographic plate and a developer unit electrically isolated from the electrophotographic plate for applying developer material including electrostatically charged toner to electrostatic latent images on the electrophotographic plate thereby producing developed images thereon, including:

toner supply means;

actuable means for dispensing a measurable quantity of toner from the toner supply means into the developer unit for replenishing the developer material with toner;

at least one current source;

connecting means for connecting said developer unit to said current source;

sensing means for sensing the quantity of toner adhering to the plate and thereby removed from the developer unit by development of the image, said sensing means comprising integrating means for integrating the current in said connecting means and providing an output signal representative of the charge of toner particles removed by development of the electrostatic latent image;

control means responsive to the sensing means for actuating said actuable means to dispense an amount of toner corresponding to the sensed quantity of toner.

2. The toner usage sensing system set forth in claim 1 wherein said control means is responsive to said output signal for actuating said actuable means when said charge reaches a predetermined level.

3. The toner usage sensing system set forth in claim 1 wherein said sensing means comprises a mechanical integrator, the output of which is mechanically connected to said control means.

4. A toner usage sensing system for use in electrostatic reproduction apparatus having an electrophotographic plate comprising:

an electrically isolated developer unit for applying multicomponent developer material including electrostatically charged toner to electrostatic latent images on the electrophotographic plate to produce developed images thereon;

said developer unit including a sump portion for maintaining a quantity of developer material therein and means for removing developer material therefrom to said electrophotographic plate;

a toner container;

actuable dispensing means for dispensing toner into the developer unit;

a current source;

connecting means for connecting said current source to the developer unit;

sensing means for sensing the current in the connecting means, the amount of current supplied being proportional to the charge of the toner particles adhering to the electrophotographic plate and thereby removed from the developer unit by development of the image;

control means responsive to the sensing means for actuating said actuable means to dispense an amount of toner corresponding to the amount of toner adhering to the electrophotographic plate.

5. The toner usage sensing system set forth in claim 4 wherein said sensing means comprises integrating means for integrating the current in the connecting means and for providing an output signal representative of the charge of toner particles removed by development of the electrostatic latent image.

6. The toner usage sensing system set forth in claim 5 wherein said control means is responsive to said output signal for actuating said actuable dispensing means when said charge reaches a predetermined level.

7. The toner usage sensing system set forth in claim 6 wherein said actuable dispensing means dispenses a preset amount of toner for each actuation thereof.

8. The toner usage sensing system set forth in claim 6 wherein said control means further comprises settable means for defining a plurality of predetermined level s and means responsive to said settable means and to said output signal for actuating said actuable means;

said actuable means dispensing a preset amount of toner for each actuation thereof.

9. The toner usage sensing system set forth in claim 4 wherein said sensing means comprises a mechanical integrator, the output mechanical position of which is mechanically connected to said control means;

said control means being responsive to the output mechanical position of the mechanical integrator for actuating said actuable dispensing means;

said actuable dispensing means thereby dispensing an amount of toner in accordance with the mechanical position of the mechanical integrator.

10. The toner usage sensing system set forth in claim 6 wherein said control means further comprises settable means for defining one of a plurality of charge-amount ratio settings and means responsive to the settable means and the output signal for actuating aid actuable means;

said actuable means dispensing an amount of toner proportional to the charge of toner particles removed in accordance with a ratio defined by said settable means.

11. A toner usage sensing system for use in an electrostatic reproduction apparatus having an electrophotographic plate and a developer unit electrically isolated from the electrophotographic plate for applying multicomponent developer material including electrostatically charged toner to electrostatic latent images on the electrophotographic plate thereby producing developed images thereon, including:

first toner supply means;

actuable means for dispensing a fixed quantity of toner from the first toner supply means into the developer unit for each actuation thereof for replenishing the developer material with toner;

second toner supply means for dispensing toner having an electrostatic charge to the developer unit;

at least one current source;

connecting means for connecting the developer unit to the current source;

sensing means for sensing the net charge of the toner removed from the developer unit by development of the image and the toner added to the developer unit by said second toner supply means and for providing an output signal representative of the sensed charge, said sensing means comprising integrating means for integrating the current in the connecting means;

control means responsive to the output signal of the sensing means for actuating said actuable means when the sensed charge reaches a predetermined level.