Control Arrangement For Transfer Roll Power Supply

Abstract

An arrangement for controlling the electrical bias applied to a bias transfer roller comprising a voltage source coupled to the transfer roller which is variable in response to a control signal, the control signal being generated by sensing the voltage across a sample of the roller material connected in series with a constant current source. Since the resistivity of the sample material varies with changes in humidity, the voltage and the control signal likewise vary with humidity to change the voltage applied to the transfer roller.

Description

BACKGROUND OF THE INVENTION

This invention relates to electrostatic transfer apparatus and, more particularly, to the transfer of electrostatically charged particles between supports.

In reproduction processes, such as in the process of xerography, wherein a latent image is first formed on an electrostatic member and then made visible, or developed, with a powderous material, the electrostatic member can be used repeatedly to form additional powder images in successive cycles if the developed image is transferred from it to another substrate such as a copy sheet during each cycle. When the xerographic process is employed, the electrostatic member can take the form of a photoconductive layer over a conductive backing material. The photoconductive layer is given a uniform electrostatic charge and then exposed to a light image conforming to the information to be reproduced to form a latent electrostatic image on the member. The member is then developed with a finely divided, pigmented, electroscopic, resinous powder called toner and the toner image is then electrostatically transferred to a copy sheet. After transfer is complete, the surface of the electrostatic member can be cleaned and then used for another latent image which is developed with toner and the toner image transferred in a similar manner.

If more than one copy of given information is to be made by the process described above, the latent image formation steps can be eliminated by using an electrostatic member having good latent image preservation characteristics. In this mode of operation, a latent image is formed on the electrostatic member by placing a uniform charge on it and then exposing it to a light image of the formation to be reproduced during the formation of the first copy. The latent image is then developed, the toner image transferred from the electrostatic member to a copy sheet and the surface of the eletrostatic member cleaned. Then, when the second copy and all succeeding copies are to be made, the charging and exposing steps can be eliminated since the latent image retention qualities of the electrostatic member maintain the latent image intact. As a result, the image preservation mode of operation, the second and all subsequent copy cycles include only the steps of developing the latent image, transferring the toner image from the electrostatic member to a copy sheet and cleaning the electrostatic member. It is obvious that this type of reproduction mode is faster than the more conventional xerographic process described beforehand since two steps, the steps of charging and exposing, are eliminated during making of all copies after the first copy.

When a reproducing system is used in the image preservation mode, special attention must be given to the technique and apparatus utilized in transferring the toner image from the electrostatic member to the copy sheet to assure that the latent image on the member does not deteriorate. A common apparatus used for the transfer step is a corona charging device. This device is widely used in electrostatic copiers, but has serious disadvantages when a copier functions in the image preservation mode. The non-image device tends to electrostatically tack the copy sheet to the electrostatic member during transfer since the copy sheet acquires a charge during the transfer step. In addition, when the copy sheet is stripped from the electrostatic member afer transfer has take place, charges on the copy sheet tend to be conducted to nonimage areas of the member due to air breakdown between it and the copy sheet. Both types of charging either add or substract charge from the electrostatic member leaving a distorted latent image thereon which is reflected in sub-image areas of the electrostatic member, they are consequently developed with toner in succeeding cycles and result in background in all subsequent copies. This situation is undesirable since distorted toner images and background toner cause noticeable degradation to copy quality.

Another transfer device used with better success than the corona device in the image preservation mode is a biased roller. This device consists of a rotatable conductive core having a relatively non-conductive surface layer. Although adequate transfer occurs when approximately 2,000 volts is placed on the roller, the roller does not efficiently effect transfer of the toner image at significantly lower voltages due to back transfer. At low voltages toner which is transferred to the copy sheet at the transfer area is retransferred back to the photosensitive member when the copy sheet is separated from the member. Th transfer of the toner to a transfer member by a roller electrode is illustrated in Fitch U.S. Pat. No. 2,807,233. In this reference the bias supplied to the transfer roller is indicated as a constant voltage energy source.

For maximum efficiency, therefore, the transfer field profile as a function of time generated by the transfer roller at any given point on the paper must be kept the same under varying conditions within the machine housing, for example, changes in humidity result in resistivity changes in the bias roller material resulting in undesirable changes in the fields generated thereby.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to improve apparatus for transferring toner images from an electrostatic member to a copy sheet.

It is a further object of the invention to improve transfer apparatus so that degradation of the latent image on an electrostatic member is prevented during the transfer step and the latent image remains suitable to be reused to form another toner image.

It is a further object of the invention to improve transfer apparatus so that transfer of toner images from an electrostatic member to a copy sheet can be carried out more efficiently than known previously.

These and other objects are accomplished according to the invention by an arrangement for controlling the electrical bias applied to a transfer roller in response to changes in humidity affecting the resistivity of the roller. The arrangement includes a voltage source coupled to the transfer roller, this source being variable in response to a control signal. The control signal is developed, in one embodiment, by sensing the voltage changes across a sample of the roller located remote of the roller itself but within the machine housing. The sample is placed in series with a constant current source, whereby changes in humidity result in changes in resistivity of the sample and voltage variations thereacross. These latter variations are utilized to control the magnitude of the voltage applied by the voltage source to the transfer roller.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustrative side view of one embodiment of the invention utilizing a continuous sensing of voltage across a sample material;

FIG. 2 is an illustrative side view of another embodiment using an extension of the transfer roller; and

FIG. 3 is an illustrative view of another embodiment of the invention utilizing an intermittent sensing and energizing technique.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention can be adapted to any reproduction apparatus wherein an electroscopic, pigmented powder used to develop an electrostatic latent image on an electrostatic member is transferred from the member to a copy sheet. For the purpose of this disclosure, however, the invention will be described within the environment of a xerographic reproduction apparatus.

Referring to FIG. 1, there is shown a continuous xerographic copier having a photosensitive member in the shape of drum 13 on which a latent electrostatic image of the information to be reproduced is formed. The rotatable drum 13 is driven by shaft 14 by any suitable drive means (not shown). The peripheral surface of the drum is covered by layer 12, an electrically conductive material, which, in turn, is covered on its outer surface with layer 11, a conducting material such as vitreous selenium. The drum has five processing stations located about its periphery which carry out the steps of the xerographic process. These stations include charging station A, exposing station B, developing station C, transfer station D, and cleaning station E.

A latent electrostatic image is formed on the drum by passing its surface through charging station A and exposing station B. The charging station includes any suitable means for placing a uniform charge on layer 11 such as a corona charging device. Exposing station B can include any suitable device which projects and focuses a light pattern on the drum conforming to the image to be reproduced by the xerographic system. The light image projected onto the charge conductive layer of the drum is synchronized with the movement of the drum and causes selective charge dissipation on elemental areas of layer 11 to form a latent electrostatic image thereon.

After the formation of the latent electrostatic image by passing the drum through stations A and B, the drum carrier the latent image to developing station a pigmented, resinous, electroscopic powder called toner is deposited on the drum in imagewise configuration in any suitable manner to develop, or make visible, the latent image. Following the development step, the drum carrier the toner image through a transer station D where the toner image is transferred from the drum surface to any suitable support material such as copy sheet 15 which can be made of paper. In addition to sheets, a continuous web or any other form of substrate may be used to receive the toner image.

Transfer of the toner image onto the copy sheet is carried out by roller 20. The copy sheet 15 is fed to the transfer station D by any suitable device such as feeding device 30 which may include tray 31 to hold a supply of copy sheets and feed roller 32 which feeds the sheets one at a time as needed toward the transfer station. The roller 20 is driven by a shaft 23 in the direction shown by the arrow and has an inner layer 22 of any suitable conductive material and an outer layer 21 of any suitable relatively nonconductive material. An electrical potential is placed on layer 22 via the shaft 23 which is also made of a conductive material.

A sheet is fed onto a conveyor 24 by feeding device 30 and is carried through transfer station D adjacent photosensitive drum 10. As it passes through the transfer station, a toner image on the drum surface adjacent the copy sheet is transferred to the copy sheet due to the electrical field created between the copy sheet and drum by the bias on layer 22 of the roller.

After the toner image has been transferred to the copy sheet and the copy sheet has passed through the transfer station, the copy sheet passes onto belt 40, which is supported by rollers 41 and 42 and driven by motor (not shown). Any suitable fixing device 43 makes the toner image permanent on the copy sheet. The copy sheet then moves off the belt 40 as the belt turns about roller 42 and falls into collection device 50 where it is stored in tray 51.

The final station shown in the drawing is cleaning station E which can include any suitable cleaning device such as a fur brush which contacts the photoconductive surface of the drum. The cleaning station is utilized to remove any residue toner particles from the photosensitive surface after transfer occurs and before another cycle is begun. It is intended that the various moving elements mentioned above be driven by any suitable means to allow the copier to function as described; for instance, a single motor can drive drum 10 and the other moving elements in the copier as well as the belts 24 and 40 and roller 20.

According to one embodiment of the invention, as shown in FIG. 1, an electrical bias is applied to the transfer roller 20 via the shaft 23 by a voltage source 52. As discussed above, the transfer field profile generated in the nip between the photosensitive drum and the transfer roller should remain constant over a prolonged period of machine operation. Assuming a constant voltage source is being used, if the resistivity of the transfer roller changes for any reason, since the voltage applied thereacross remains the same, a variation in the transfer field results. A prime cause of changes in the resistivity of the transfer roller is changing humidity conditions to which the roller is subjected.

One solution to counteract this type of variation is shown in FIG. 1 wherein a sample piece of the material 55 from which the transfer roller is made is sandwiched between two electrodes 56 and connected in series with a constant current source 57. This sample 55 may be located with the machine housing remote from the transfer roller 20 as long as it is subjected to the same humidity conditions as the transfer roller.

Th power supply 52 is selected to be of a type which is variable in response to a control voltage input 58 which input is connected to vary in response to changes in voltage across the sample 55.

In operation, the power supply 52 is set to provide an appropriate voltage to the roller 20. During the operation of the machine, if the humidity rises within the machine housing roller 20 decreases in resistivity tending to vary the transfer fields at the transfer station D due to increased currents drawn thereacross. Concurrently however, the resistivity of the sample 55 decreases resulting in a drop in the voltage thereacross. This drop is fed to the input 58 of the power supply 52 reducing the voltage applied to roller 20, to thereby compensate for the change in roller resistivity to maintain the transfer fields constant at the transfer station.

In a similar fashion, a decrease in humidity results in an increase in resistivity of roller 20 and this is compensated for by an increase in voltage applied to the roller 20 in response to the corresponding rise in voltage across the sample 55 which is fed to the control input of the power supply 52.

A second embodiment of the invention is shown in FIG. 2 in which an extra section 65 of the transfer roll 20 is mounted on the same shaft 23 as the roller, but insulated therefrom by means of an insulator length 68. The portion of the shaft 23 on which the extra section 65 rotates is connected via conductor 63 to one side of the constant current source 57. A roller contact 61 mounted on a shaft 62 and suitably grounded is also provided.

In operation, if humidity changes occur they are accompanied by changes in the resistivity of the extra section 65 and corresponding changes in the voltage thereacross, which voltage is detected on line 66 and used to control a compensating change in voltage applied by source 52, as explained hereinbefore in connection with FIG. 1.

A third embodiment of the invention is illustrated in FIG. 3 in which intermittent testing is used. In FIG. 3 a contact roll 79 is mounted on an arm 80, the arm 80 being pivoted on a pin 81. The contact roller 79 is suitably grounded but insulated from the pin 81 by means of an insulating section 82. The arm 80 is movable during the test period so that the contact roller 79 rides on the peripheral surface of the roller 20. The shaft of the roller 20 is connected via line 83 and contact arm 72 to one side of the power supply 52, the other side of the supply being grounded. The shaft 23 is also coupled via conductor 84 and contact arm 71 to a test unit 79 comprising in succession an integrator unit 75, an analog to digital converter unit 76, a digital storage register 77, and a digital to analog converter unit 78. The output of the D-A converter 78 is fed to the input of the variable supply 52 to control it in response to resistivity changes on the roller 20. A constant current generator 57 is connected so as to be brought into electrical series with the roller 20 and the power supply upon the appropriate movement of the roller 79 and the contact arm 72. The contact arms 72 and 71 may be controlled by any conventional means which may be a solenoid 73 actuated by a test signal generator 74.

Using the arrangement of FIG. 3, under normal conditions the contact arms 71 and 72 are in the positions shown by said lines and the contact roller is retracted away from the roller 20. Under these circumstances a preselected voltage is applied to the roller 20 from the power supply 52. However, periodically for example, after the completion of one or several copy cycles when the machine is not processing any paper, a test signal is generated in some appropriate fashion to actuate the solenoid 73 thereby moving contact arms 71 and 72 to the positions shown in the dotted lines. In addition, the roller contact 79 is moved against the roller 20. Under these conditions, current from the constant current source generates a current through the roller 20 and the voltage across the roller 20 is fed to the test arrangement 79.

The voltage across the roller 20 over the test period is averaged by the integrator 75, this average value being connected to a binary code by the A. D. converter 76 for storage in the register 77. At the end of the test period the storage register 77 is interrogated to provide an analog voltage which is fed to the power supply 52 to reset the voltage applied to the roller 20 in accordance with any changes in resistivity of the roller which may have occurred since the previous test period.

Details of the bias roll transfer system of the type used in the arrangement of this invention are outlined in detail in commonly assigned application Ser. No. 309,562, filed Nov. 24, 1972 and allowed July 11, 1973 and the disclosure of this latter application are incorporated hereinto by reference.

In addition to the apparatus outlined above, many other modifications and/or additions to this invention will be readily apparent to those skilled in the art upon reading this disclosure, and these are intended to be encompassed within the invention disclosed and claimed herein.

Claims

What is claimed is:

1. An arrangement for changing the voltage applied to a bias transfer roll in a xerographic copy mechanism comprising:

a housing in which said mechanism is located,

a source of voltage, said source coupled to said roll and variable in response to a control signal, and

a piece of material similar in construction to said bias roll, located apart from said roll but within said housing, means for generating a constant current through said material, and means responsive to the voltage across said material for controlling the magnitude of said voltage source.

2. An arrangement for compensating for changes in resistivity on a bias transfer roll due to changing humidity conditions comprising,

a xerographic copy machine including a bias transfer roll mounted for rotation on a shaft, a second smaller roll of similar construction mounted on an extension of said shaft and insulated therefrom, means for connecting a constant current source in series with said second roll, a voltage source coupled to said transfer roll and means responsive to changes in voltage across said second roll for altering the voltage applied by said source to said transfer roll.

3. The combination recited in claim 2 wherein said means for connecting includes an auxiliary conductive roller mounted for rotation in contact with the periphery of said second roll.

4. A humidity compensated apparatus for transferring charged particles from a support surface to a sheet of support material, comprising:

a transfer member, said transfer member cooperating electrically with the support surface to attract the charged particles therefrom to the sheet of support material;

bias means for electrically biasing said transfer member to a potential of sufficient magnitude to attract the charged particles from the support surface to the sheet of support material, means for adjusting the magnitude of the biasing potential applied to said transfer member to correct automatically for changes in the resistivity of said transfer member.

5. The combination recited in claim 4 wherein said means for adjusting includes a constant current source connected in series with said transfer member, and means responsive to changes in the voltage across said transfer member to vary the magnitude of the biasing potential applied by said bias means to said transfer member.

6. The combination recited in claim 4 wherein said means for adjusting is operative intermittently to vary said applied potential.

7. The combination recited in claim 4 wherein said means for adjusting comprises a piece of material similar in construction to said transfer member located remote therefrom, means for generating a constant current through said piece, and means responsive to the voltage across said piece to vary the magnitude of the bias potential applied to said transfer member, whereby changes in humidity result in voltage variations across said piece, said voltage variations operating to control said bias means.

8. The combination recited in claim 4 wherein said means for adjusting comprises contact means intermittently movable into contact with the surface of said transfer means for a preselected time, means actuated concurrently with the movement of said contact means for generating a constant current through said transfer member, current means for determining the average value of the voltage across said transfer member over said preselected time, and means for adjusting said bias means in accordance with the average voltage.