Corona current measurement and control arrangement

Abstract

Apparatus is disclosed in accordance with the teachings of the present invention for obtaining a true measurement of charging current supplied by a corona discharge device to a charge receiving surface. According to the invention, a bridge circuit is formed which includes the corona device as one arm and a first resistor connected in the current return path from the corona device to a power supply as another arm thereof. The remaining arms of the bridge include a capacitor and a resistor connected in series with each other across the source to complete the bridge and provide the signal required to compensate for reactive current through the corona device. The bridge is first brought into balance with the power supply below corona onset voltage, then when the power supply is increased above the corona outset voltage the bridge output potential is used as a feedback signal to maintain the corona current constant. The bridge output may also be used in determining the true corona charging current.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to A.C. corona discharge devices and in particular to circuit arrangements for accurately measuring the amount of charging current delivered by an A.C. corona discharge device to a charge receiving surface.

In commercial electrophotographic reproducing devices using conventional xerographic process, a uniform layer of electrostatic charge is deposited on the surface of a photoreceptor and is selectively dissipated in accordance with modulated radiation imaged thereon to form an electrostatic latent image of an original document. The electrostatic image is then developed and transferred to a support surface to form a final copy of the original document.

The preferred device for depositing the charge on the photoreceptor surface is the corona discharge device. Such corona discharge devices may be energized by D.C. and A.C. sources. In addition to the deposition of a uniform charge on a photoreceptor, it has been found that many of the D.C. corotrons developed by the prior art are susceptible to the accumulation of dust and toner particles on and about the interior of the corotron, such that the corona current generated thereby substantially decreased as the density of particle accumulation increases. Accordingly, various A.C. corotrons which are less affected by toner accumulation have been proposed to perform the functions previously performed by the D.C. corotron.

A.C. corona discharge devices are used for a wide variety of other applications in electrophotography. Among these applications precleaning a photoreceptor by neutralizing the charge on toner particles adhering to the surface of the photoreceptor after transfer of the developed image to a support surface, pretransfer charging for altering the range of electrostatic charge on toner prior to transfer, tacking, detacking, and paper charge neutralization.

When A.C. corona devices are used in situations wherein the process speed is such that the surface collecting the corona current travels an appreciable distance during one alternation of the A.C. cycle, then a strobed charge pattern emerges which is undesirable. To avoid strobing the trend has been to use A.C. power supplies operating at higher frequencies. Commercial machines are now on the market which employ power supplies with output frequencies of 400, 600, and 800 hertz.

It has also been popular over the past few years to use constant current power supplies to minimize shifts in the electrical characteristics of corona generating devices caused by changes in ambient temperature, air pressure, and humidity. Constant current power supplies are also used to combat long term changes in the uniformity of the corona output as toner accumulates on the surface of the corona generating apparatus.

The arrangement presently used commercially to achieve constant current characteristics may best be explained by reference to FIG. 1. Charging current received by the grounded plate 2 from the corona generator 3 creates a voltage drop across a feedback resistor 4 in series with the current return path to the power supply 5. This voltage drop across the feedback resistor is sensed to generate a control signal on line 7 to regulate the power supply output voltage so that the current remains constant.

The method shown in FIG. 1, however, is not an effective way to obtain constant current characteristics from AC corotrons when the frequency of the applied high voltage source is more than a few hundred hertz. Measurements made on actual corotrons have shown that the presence of the reactive component of the current can lead to significant errors between the measured value and the true corona current. The reasons why this is so can be explained with the aid of the equivalent electrical circuit for an AC corotron shown in FIG. 2.

In this circuit zener diodes CR.sub.1 and CR.sub.2 have characteristic zener breakdown voltages that correspond to the negative and positive corona threshold voltages respectively. As is well known, these breakdown voltages are generally somewhat different. R.sub.1 is a non-linear resistance representing the corona resistance and C.sub.1 is the capacity between the corona wire and the current collecting surfaces. R.sub.2 is the series feedback resistor in series with the power supply and is selected to have a value so that R.sub.2 << R.sub.1. The current passing through R.sub.2 is the vector sum of the corona current I.sub.c (the current through R.sub.1) and displacement current, I.sub.x (the current through C.sub.1). Only the corona current I.sub.c is important in xerographic processes since it alone represents the free charge transported per unit time by ions impinging on the collecting surface.

The problem is both components I.sub.c and I.sub.x will contribute to the feedback signal appearing across R.sub.2. To illustrate how this can be a problem, consider a typical corotron which delivers a corona current 30.mu.a to the shield and photoreceptor for each inch of its length when the wire potential is 6.5 KV. The corona resistance in this instance is:

R(ohms) = 6.5 .times. 10.sup.3 volts/30 .times. 10.sup..sup.-6 amps/in.

= 2.16 .times. 10.sup.8 ohms/in.

If the capacity of the corona generating device is on the order of 1 picofarad for each inch of length then its reactance at 60 hz will be:

X.sub.c (ohms) - 1/(377 .times. 1 .times. 10.sup..sup.-12) = 2.65 .times. 10.sup.9 ohms/in.

Since the reactive current will be an order of magnitude smaller and 90.degree. out of phase with the corona current, its contribution to the voltage drop across the feedback resistor is small and can be neglected with very little error.

The situation is much different when the supply frequency is increased to 600 hz. Now the reactive current becomes comparable with the corona current and if the method in FIG. 1 is used, it is the total current (through R.sub.2) that will be kept constant and not necessarily the corona current I.sub.c.

The reactive current is linear with the applied voltage whereas the corona current is non-linear with the applied voltage. Consequently, the phase angle of the current through the feedback resistor with respect to the applied voltage is not constant as voltage changes. Furthermore, since the capacitive coupling and corona resistance vary within wide limits and in a non-linear fashion as a function of the spacing between the corona device and the collecting surface it is clear that a given voltage across the feedback resistor may not correspond to a unique corona current.

In addition, as shown in FIG. 3, the distributed capacity C.sub.3 between ground and the high tension lead connecting the corotron to the remote power supply will be in shunt with the corona resistance and will also contribute to the voltage developed across the feedback resistor R.sub.2. In practice, the high tension cable capacity may be orders magnitude larger than that of the corotron itself. Hence, the signal in the feedback resistor due to the corona current may be swamped or masked by the much larger reactive current. As a result, large excursions in the corona current may represent only a small perturbation in the total power supply current which is beyond the ability of the control circuit to resolve. Therefore, the constant corona current characteristics are not realized.

OBJECTS & SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide a method for measuring the true corona current deposited by an A.C. corotron on a charge collecting surface.

A further object is to provide a means for regulating the current output of a corona device to supply a constant charging current to a charge receiving surface.

With an understanding of the problems involved in obtaining true corona current measurements noted hereinbefore, the invention provides an arrangement for discriminating between true corona current and (the current associated with the flow of charge to the surface to be charged) and reactive current capacitatively shunting the collecting surface. Essentially, a corrective signal, equal in magnitude and 180.degree. out of phase with the reactive component of the corotron current is added to the normal signal to eliminate the reactive component from the current reading. According to the invention, a bridge circuit is formed which includes the corona device as one arm and a first resistor connected in the current return path from the corona device to a power supply as another arm thereof. The remaining arms of the bridge include a capacitor and a resistor connected in series with each other across the source to complete the bridge and provide the signal required to compensate for reactive current through the corona device. The bridge is first brought into balance with the power supply below corona onset voltage, then when the power supply is increased above the corona outset voltage the bridge output potential is used as a feedback signal to maintain the corona current constant. The bridge output may also be used in determining the true corona charging current.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical commercial circuit arrangement for obtaining constant current charging.

FIGS. 2 and 3 are equivalent circuits of the corona discharge arrangement of FIG. 1, and

FIG. 4 shows a bridge arrangement of the invention for obtaining accurate corona charging current measurements and for obtaining constant corona charging current.

DETAILED DESCRIPTION OF THE INVENTION

The practical solution of this invention to the above discussed problem of obtaining a true corona charging current measurement is provided by the bridge arrangement of FIG. 4.

In the bridge arrangement of FIG. 4, the R.sub.1, CR.sub.1, CR.sub.2 and C.sub.1 parallel combination represents the equivalent electrical circuit of the corona arrangement, discussed in connection with FIG. 2. Shown in dotted lines is the corona device and charge collecting plate to indicate the physical connections which are made to implement this circuit in a practical environment. The feedback resistor R.sub.2 which is selected to be much smaller than R.sub.1 and the impedence of C.sub.1 discussed with reference to FIGS. 1 and 2 is connected as another arm of the bridge. The reservoir R.sub.2 is connected into the current return path of the power supply in order to measure corona charging current.

The remaining arms of the bridge are formed by the capacitor C.sub.0 and the variable resistor R.sub.3. Capacitor C.sub.0 and resistor R.sub.3 are added to obtain the necessary compensating signal and to complete the bridge circuit. The A.C. power supply 5 is connected across the bridge in the conventional fashion and a high impedence voltmeter is coupled between the nodes A and B on the bridge to provide a reading of the condition thereof.

The nodes A and B of the bridge are also coupled to the input terminals of an amplifier 16 via conductors 14 and 15. The amplifier 16 generates a signal on line 17 which is feed back to the control terminal of the power supply 5 to thereby maintain a constant corona current by varying the voltage supplied by the source in response to the output of the bridge.

In order to utilize the arrangement of FIG. 4, the bridge is first balanced by setting the power supply to a magnitude below the corona onset voltage and varying R.sub.3 until a minimum or zero reading is noted on the voltmeter. The same effect may be obtained by using a fixed R.sub.3 and a variable C.sub.0. Once balanced, the bridge will remain in this condition until the power supply voltage exceeds the corona onset value. In this balanced condition, the current through the CBD side of the bridge (i.e., through C.sub.1 and R.sub.2) will be different than, but in phase with the current through the CAD side of the bridge (C.sub.0 and R.sub.3). The voltage across C.sub.1 will equal the voltage across C.sub.0 and the drop across R.sub.3 will equal to the drop across R.sub.2. Since R.sub.2 was selected to be much smaller than R.sub.1 and X.sub.c1, most of the applied voltage will be dropped across C.sub.1 and R.sub.1.

Now, when the voltage applied to the bridge by the source is increased above the critical corona onset voltage, the current in R.sub.2 is increased by the amount of the corona current and the voltage drop across R.sub.2 is greater than that across R.sub.3 by the amount of the corona current drop across R.sub.2. This voltage difference appears across the nodes A and B and may be used as a control signal to hold the corona current constant by feedback to the control terminal of the power supply. Since this voltage change is small, an amplifier 16 is used to step it up before application to the variable supply. The true corona current value may be determined by dividing the voltmeter reading by the value of resistance R.sub.2 and in this way the corona charging current may be adjusted accurately during the initial set up of the machine.

The following values for the components of FIG. 4 are typical where a 600 hz supply is used:

R.sub.2 = 1000 ohm

C.sub.1 = 15 picofarads (assuming 1 pf/in. of electrode length)

C.sub.0 = 5 picofarads

R.sub.3 = 3,000 ohm

R.sub.1 = 2 .times. 10.sup.8 ohm

While the charge collecting surface has been described hereinbefore as a grounded metal place, in a practical xerographic environment it is a photoreceptor drum or surface carried by a grounded metal substrate. The arrangement of the invention would operate equally satisfactorily in either case.

While the invention has been particularly shown with reference to a specific embodiment thereof, it will be obvious to those skilled in the art that various changes and modifications in form and details may be made, without departing from the spirit and scope of the invention. It is therefore intended that the appended claims be interpreted as including such changes and modifications.

Claims

What is claimed is:

1. A bridge circuit for measuring the corona charging current deposited by an A.C. corona discharge device on a charge collecting surface, said corona device including an electrode spaced from said surface, said bridge having power input nodes and sensing nodes comprising

an A.C. voltage source coupled to said input nodes,

a first resistor coupled in series with said device across said input nodes to form a first current path,

a capacitor and a second resistor connected in series across said input nodes to form a second current path in parallel with said first current path across said source,

means coupled to said sensing nodes for measuring the output of said bridge, and,

means for balancing said bridge when the output of said source is below the corona onset value whereby when the voltage of said source is raised above the corona onset potential, said corona charging current may be determined from the measured value of said imbalance and the value of said first resistor.

2. The combination recited in claim 1 wherein said source is a variable source and further including means coupled to said sensing nodes for providing a control signal to adjust said source in response to said bridge imbalance.

3. The combination recited in claim 1 wherein said second current path has an impedence which is much greater than said first current path.

4. A method of holding constant the corona charging current output of an A.C. corona discharge device comprising

forming an electrical bridge having input nodes and ouput nodes, said bridge including said device and a first resistor in series with each other across said input nodes, and a capacitor and second resistor in series with each other connected across said input nodes, said capacitor and corona device forming adjacent arms of the bridge,

coupling a variable A.C. voltage to said input nodes,

adjusting said source to a value below the corona onset potential,

adjusting said bridge to a balanced condition at which the voltage across said output nodes is at a minimum,

re-adjusting said source to a value above the corona onset potential to thereby generate an imbalance voltage across said output nodes,

and thereafter continuously adjusting said source to maintain said imbalance voltage constant.

5. The method recited in claim 4 wherein said device is selected to include an electrode spaced from a charge collecting surface, and said first resistance is selected to be small relative to the impedence of the inherent capacity exhibited between said electrode and said surface.