Electricity: measuring and testing – Electrostatic field – Using modulation-type electrometer
Reexamination Certificate
2002-08-21
2004-10-19
Nguyen, Vincent Q. (Department: 2858)
Electricity: measuring and testing
Electrostatic field
Using modulation-type electrometer
C324S663000, C324S665000, C324S713000
Reexamination Certificate
active
06806717
ABSTRACT:
BACKGROUND AND SUMMARY
The basic reprographic process used in an electrostatographic printing machine generally involves an initial step of charging a photoconductive member to a substantially uniform potential. The charged surface of the photoconductive member is thereafter exposed to a light image of an original document to selectively dissipate the charge thereon in selected areas irradiated by the light image. This procedure records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. The latent image is then developed by bringing a developer material, including toner particles adhering triboelectrically to carrier granules into contact with the latent image. The toner particles are attracted away from the carrier granules to the latent image, forming a toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet having the toner image thereon is then advanced to a fusing station for permanently affixing the toner image to the copy sheet in image configuration. In electrostatographic machines using a drum-type or an endless belt-type photoconductive member, the photosensitive surface thereof can contain more than one image at one time as it moves through various processing stations.
The portions of the photosensitive surface containing the projected images, so-called “image areas”, are usually separated by a segment of the photosensitive surface called the inter-document space. After charging the photosensitive surface to a suitable charge level, the inter-document space segment of the photosensitive surface is generally discharged by a suitable lamp to avoid attracting toner particles at the development stations. Various areas on the photosensitive surface, therefore, will be charged to different voltage levels. For example, there will be the high voltage level of the initial charge on the photosensitive surface, a selectively discharged image area of the photosensitive surface, and a fully discharged portion of the photosensitive surface between the image areas.
The approach utilized for multicolor electrostatographic printing is substantially identical to the process described above. However, rather than forming a single latent image on the photoconductive surface in order to reproduce an original document, as in the case of black and white printing, multiple latent images corresponding to color separations are sequentially recorded on the photoconductive surface. Each single color electrostatic latent image is developed with toner of a color complimentary thereto and the process is repeated for differently colored images with the respective toner of complimentary color. Thereafter, each single color toner image can be transferred to the copy sheet in superimposed registration with the prior toner image, creating a multi-layered toner image on the copy sheet. Finally, this multi-layered toner image is permanently affixed to the copy sheet in a substantially conventional manner to form a finished color copy.
As described, the surface of the photoconductive member must be charged by a suitable device prior to exposing the photoconductive member to a light image. This operation is typically performed by a corona charging device. One type of corona charging device comprises a current carrying electrode enclosed by a shield on three sides and a wire grid or control screen positioned thereover, and spaced apart from the open side of the shield. Biasing potentials are applied to both the electrode and the wire grid to create electrostatic fields between the charged electrode and the shield, between the charged electrode and the wire grid, and between the charged electrode and the (grounded) photoconductive member. These fields repel electrons from the electrode and the shield resulting in an electrical charge at the surface of the photoconductive member roughly equivalent to the grid voltage. The wire grid is located between the electrode and the photoconductive member for controlling the charge strength and charge uniformity on the photoconductive member as caused by the aforementioned fields. Control of the field strength and the uniformity of the charge on the photoconductive member are very important because consistently high quality reproductions are best produced when a uniform charge having a predetermined magnitude is obtained on the photoconductive member.
A useful tool for measuring voltage levels on the photosensitive surface is an electrostatic voltmeter (ESV) or electrometer. The electrometer is generally rigidly secured to the reproduction machine adjacent the moving photosensitive surface and measures the voltage level of the photosensitive surface as it traverses an ESV probe. The surface voltage is a measure of the density of the charge on the photoreceptor, which is related to the quality of the print output. In order to achieve high quality printing, the surface potential on the photoreceptor at the developing zone should be within a precise range. In a typical xerographic charging system, the amount of voltage obtained at the point of electrostatic voltage measurement of the photoconductive member, namely at the ESV, is less than the amount of voltage applied at the wire grid of the point of charge application.
A fundamental challenge in designing an ESV is measuring a voltage in the 1 KV range without touching the surface being measured. Commercially available devices generally work in the 30 to 50 volt range. All commercially available ESVs including the Xerox designed units (such as disclosed in U.S. Pat. No. 5,489,850, entitled “Balance Beam Electrostatic Voltmeter Modulator Employing A Shielded Electrode and Carbon Fiber Conductors” are based on a null-balance feedback system.
It is the object of the present invention to achieve, a “non-floating” i.e. “connected to ground” ESV. Generally, a circuit powered by a “floating” power supply, as shown in
FIG. 4
is used to sense and process the modulated signal generated by a variable capacitance “modulator” or “probe”. This modulator interrupts the electrostatic field generated between the surface being tested and the sense electrode, thus converting the DC voltage difference between that surface and the sensing electrode into an AC signal that is proportional to the voltage difference and the capacitance coupling. The capacitance is dependent on the spacing between the electrode and the surface under test. The result is an AC signal that is both voltage and spacing dependent. This signal is then processed by additional circuitry and converted to a DC voltage which drives a “high voltage stage” which is connected between ground and the floating circuit and which “drives” the floating circuit to the same voltage as that being sensed. Usually this is done by an integrating circuit in basic classical control form, as shown in FIG.
5
.
The system has been “null balanced”. The speed and accuracy of this processing is dependent on the “gain” of the system which is a function of the spacing and the modulation frequency. The common practice is to include an electronic “gain” adjustment to optimize the performance at the operating spacing.
Referring to
FIG. 5
, the system is dependent on having a high voltage output device
1
; a high voltage power supply
2
; a low voltage power supply
3
that floats at the voltage being measured and a low voltage power supply
4
referenced to earth ground. A cost analysis shows that a significant portion of the cost of the ESV is related to high voltage components, i.e. items
1
,
2
, and
3
. Also, from classical control theory, the integral feedback system limits the overall speed of response to about 10 times the period of the modulation frequency. It is an object of the present invention to eliminate the need for these items.
It is also an object of the invention to utilize reliable, low cost, and potentially high precision micro sensors. For example, Polysilicon microbridges have been driven vertically and laterally as r
Hubble, III Fred F.
Kubby Joel A.
Peeters Eric
Viturro R. Enrique
Wallace Stanley J.
Bean II Lloyd F.
Nguyen Vincent Q.
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