Electrostatic Holddown

Abstract

An interdigitated grid, powered by a high-voltage generator, and covered by a bed of material having a bulk resistivity which will allow an electrostatic charge to be built up quickly, but decay in holding power in a very short time.

Description

BACKGROUND OF THE INVENTION

Electrostatic holddown devices are many and varied in nature, according to the intended use. The basic premise upon which all act is to induce electrostatic charges between a support surface and the sheet to be supported, which charges are in the nature of static electricity and produce holding action by induced opposite electrical charges.

U.S. Pat. No. 2,834,132 issued to J. O. Taylor et al. in 1958, illustrates the basic premise upon which substantially all known electrostatic devices operate. This electrostatic sheet applying and holding device comprises a sheet-holding means including a relatively smooth sheet-supporting surface of electrical insulating material. A flat electrode is supported on the back of the sheet-supporting surface. An instrument which the inventor terms an extended current-carrying surface, provides a second electrode. The instrument is in the form of a roller platen. A source of high-voltage current is connected with the flat electrode whereby when a sheet is placed upon the sheet-supporting surface, the sheet is charged when the roller instrument is moved thereover. The sheet is sustained upon the supporting surface by the electrostatic charge induced on the sheet.

Another prior art device, U.S. Pat. No. 3,359,469, produces a charged surface by means of a high-voltage generator, and then causes a sheet to adhere electrostatically by supplying an opposite charge to the sheet. Such opposite supply is available from the human body of the operator, or by an ionization wand device. When oppositely charged in this manner, a paper sheet will adhere tenaciously for a long period of time.

Finally, a U.S. application, Ser. No. 302,544, corresponding to British Pat. specification No. 1,043,298 teaches the use of two individual sets of conductors alternately interspersed with a bed of insulating material such as fiberglass covering them. This teaching indicates that the holding power will persist for some period of time after the voltage source is disconnected. Each of the known prior art teachings is of residual persistance after holding power is established.

SUMMARY OF THE INVENTION

This invention differs from prior art in that there is created an ability to attract and hold sheets of paper, for example, only during the application of electrical potential to a grid.

It is an object of this invention to provide an instantaneous holding power attraction even for highly insulating paper sheets, which holding power is available upon the application of a high electrical potential, and is dissipated substantially at the instant the power is terminated.

ADVANTAGES OF THE INVENTION

The principal advantage of this invention is that paper can be attracted and held without use of ionization wands, grounded rollers, or the human body as a ground.

Further, it is an advantage and object of this invention to allow paper sheets to release from the board surface as soon as the electrical power is removed. No substantial residual holding power is experienced, and even very lightweight paper is removable without undue delay or effort. This is referred to as the relaxation time.

DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic cross section of a piece of paper held electrostatically to a board surface;

FIG. 2 is a conventional circuit diagram representation of the structure shown in FIG. 1;

FIG. 3 is a schematic illustration of a process camera having a holddown board for original material, which board incorporates the principles of this invention;

FIG. 4 is an exploded schematic illustration of the preferred relationship of essential structural elements in a holddown board;

FIG. 5 is a plan view of the board top surface with interdigitated electrodes positioning within the board shown in phantom outline;

FIG. 6 is a section taken along line 6--6 of FIG. 5;

FIGS. 7 and 8 are alternative grid patterns.

THEORY OF OPERATION

The actual construction of the holddown device is relatively straightforward, and is in fact quite simple to manufacture and use, provided the perimeters of construction and operation are observed. Following the examples and suggestions given, an operative device may be constructed by anyone skilled in the art and will operate as indicated. A degree of certainty exists as to the phenomena taking place, but is here postulated only as a theory because of the susceptibility of other possible interpretations.

One of the most important discoveries which made possible the present invention is the role of bulk resistivity of the covering bed over interdigitated electrodes, coupled with an understanding of the function of the cover.

First, a series of calculations will indicate the role of the bulk resistivity in producing a workable holddown action. Refer to FIG. 1. If the electrodes in fact induce a current flow through the cover plate and through the paper sheet being held, then the arrow diagrams indicate electrical circuits. These electrical circuits are set forth in conventional configuration in FIG. 2.

To get a satisfactory holddown, it is postulated that I.sub.1 must be greater than or equal to I.sub.2. Then, the resistances through the circuit including R.sub.1 plus R.sub.2 must be less than or equal to the resistance R.sub.3.

The resistances may be calculated by using the bulk resistivity of the cover material multiplied by the length of the path the current must travel, and divided by the cross-sectional areas. Knowing the selected width of the interdigital fingers of the electrodes, and the thickness of the cover plate chosen, a working mathematical formula may be established to solve for the value of bulk resistivity which will be acceptable for the cover plate in order to achieve the desired and necessary current flow values.

Using conventional electrical mathematics, well known to those skilled in art, the relaxation time is equal to the resistance of the material multiplied by capacitance, or, T= R.times. C. C is established as the permittivity of free space (Eo) multiplied by a constant (K) and the ratio of the area to thickness of the holddown cover desired. The constant (K) is a mathematical convenience and is derived as the ratio of the permittivity of the material as compared to free space

(Eo). Hence, C= Eo.times. KA/t Where K= Em/Eo

Thus C= Eo.times. Em/Eo.times. A/t= EA/t

Resistance (R) is a factor of the bulk resistivity of a material (Pv) multiplied by the ratio of the thickness to area of the holddown cover desired. Thus R= Pv t/A . Hence T= Pv t/A.times. Em A/t or T= Pv= Em.

From a material standpoint, rather than electrical mathematics, the relaxation time is a factor of the resistivity (P) multiplied by permittivity (E). The result is the same.

Simple observation of available materials will reveal that any possibly suitable plastic material will have a dielectric constant which will vary by a factor of less than 5, but a bulk resistivity which may vary by a factor of 10,000 . Hence, bulk resistivity is the major factor of influence in material selection. By arbitrarily selecting a maximum time in which the holding power of the board must be reduced to a level which will allow paper to separate easily, then the material may be selected from a manufacturer's tables of characteristics.

In FIG. 3, a practical environment for the present invention is illustrated in the form of a schematic representation of a commercially available process camera. The process camera is designed for making electrostatic masters from original material placed on a copyboard 10. The usual method of holding original material for process camera work is to place a clear glass cover over the copyboard to hold the material in a flat plane. It is the object of this invention to avoid the use of the holding glass in order to avoid the need to keep a glass plate spotlessly clean and also to avoid the annoyance of having small lightweight original sheets moved by windage created by closing of a glass cover.

In the FIG. 3, the board 10 is held at a tabletop height and attitude in order that original material may be placed on the board either as a single piece or by a composition of several pieces. The board is then moved to a vertical position as shown in dotted outline by a system of links operated by a handle 12.

In the vertical position, the original material is placed on an object plane and is moved into an opening of a light chamber 15. Illuminating bulb 16 floods the chamber 15 and the surface of the board with the proper intensity of light for exposure.

A bellows 18 and a lens 19 complete the enclosure portion of the process camera. A mirror 20 is included in the system in order to project the image upwardly to a horizontal object plane. An autofocus linkage 21 causes the lens 19 and mirror 20 to move in proper relationship for focusing the image. FIG. 3 illustrates one side of the autofocus link structure and a similar structure is placed on the opposite side of the bellows with lens 19 on a carrier therebetween.

A source of unexposed electrostatic master paper is shown in a stack 22 ready to be delivered through a charging station 23 to the bottom surface of a vacuum conveyor 24. The charged master is conveyed until it touches a switch 25 carried with the mirror 20. The switch 25 causes the conveyor to stop until exposure is made. Thus, regardless of the extended position of the mirror 20, the sheet of master material is in proper location upon the image plane.

After exposure, the master is conducted through a toning station 26 to a conveyor 27 which then passes the toned master into a fusing station 29, and ultimately deposits the fused master on a reception table 30.

The physical construction of a holddown board as illustrated in the drawings consists essentially of three parts:

1. the base 32;

2. the grid 34;

3. the top coating 36.

In the preferred embodiment, the grid 34 is mounted between the base and top coating, although there are other configurations for special circumstances. See the prior art referred to.

The base is the foundation upon which the entire board is built. In FIG. 3 the reference character 10 indicates the entire structure of the holddown board, which includes a frame and a removable board insert 40. It is the removable insert 40 that is illustrated in FIG. 4 and will be referred to generally hereafter as the board. It must have a suitable strength in the illustrated embodiment to stand by itself. The reason for this is that it is used as a removable unit on the machine and it must be capable of being taken out and set aside without any special handling. Also, the board 40 must have some flexibility in order that it will yield when it is used in the unit 10 with a thick magazine in place, and a glass cover plate over the magazine. The board is capable of holding paper sheets, but not heavy items, and accordingly in some instances a cover plate is required as in conventional structures.

Because the board is a removable unit that merely plugs into the structure 10, standard printed circuit connectors are employed to make electrical connection to the board. The base may be selected from any one of a variety of conventional materials, and its only essential property is that its bulk resistivity must be greater than 10.sup.15 ohm-centimeters, or at least greater than or equal to the resistivity of the top coating 36. The best materials for the illustrated embodiment which have been found, have about 10.sup.16 ohm-centimeters or greater bulk resistivities. Some of the base materials employed are polyethylene, rigid polyvinylchloride, glass reinforced epoxy and phenolic resins.

The top coating 36 is a white opaque plastic. It must be opaque to hide the grid 34, and white because it is used as a photographic copyboard. This is an application limitation and not necessary to the functioning of the board. If the board were used as a wall hanging bulletin board, then any color, or even a clear plastic covering would suffice. Another practical limitation is that the top coating be at least 10 mils thick. This limitation is to keep any operator who might come in contact with the surface of the board while it is in operation from getting a shock. If there is no problem of personal contact, then the top coating may be made thinner and will permit greater holding power. Conversely, a thicker top coating will give less holding power. The most important property of the top coating 36 is that its bulk resistivity be 15 ohm-centimeters or less. For this top coating 36, such plastics as flexible polyvinylchloride, polyvinylfluoride, and glass reinforced melamine have been used. Top coatings can be of multilayer design also, to obtain certain surface properties. For example, color, opaqueness, cleanability, wear resistance, and to protect indicia lines are reasons for making composite surfaces. Certain materials, such as nylons and acetates will operate well at normal humidities, but cease to function at low humidities.

The grid 34, which is normally between the two layers of material constituting the base and top coating, consists of two electrodes 45 and 46. These electrodes are spaced so that an electric potential may be applied between them, generating an electrostatic field. The electrodes are in the form of interdigitated fingers about three-fourths inch wide, connected by header bars with the normal spacing between the fingers of the electrodes about three-sixteenths of an inch. The grid, or electrode pattern, may be mounted on the base in many ways. The standard printed circuit technique of etching conductors in copper-clad material, or silk screening a conductive ink onto a base material, or hot stamping, are other standard methods that can be used. The electrodes may also be formed from metal foil, wires, conductive inks, or even pencil lines. They may be applied by printing, silk screening lithography, etching, electroplating, or drawing. Those versed in the arts of lithography, silk screening, printed circuits, and printing will immediately see many ways of producing the electrodes. The term conductor or conductive ink or paint used in this description includes resistive conductors, inks or paints. This is true because the current needed to cause the holddown board to operate satisfactorily is so very small that materials normally considered as being resistive or even insulating will actually carry enough current to make the board operate satisfactorily.

The top coating and the base material may be joined in many ways. Examples are by laminating, heat sealing, ultrasonic welding, gluing or adhesives. The top coating may also be merely laid on top of the base material and will still operate as a holddown. If an adhesive or glue is used, this material must be about the same bulk resistivity as the top coating. If it has a high resistivity and gets between the electrodes and the top coating, then no current will flow through the top coating, and no holddown will occur.

The electrostatic holding action is created by applying an electrical potential between the electrodes of from 1,000 to 4,000 volts. A voltage multiplier 50 is shown as a source of such potential. Voltage multipliers are well known. The fact that a very small amount of current is consumed, enables the practical use of a voltage multiplier of inexpensive proportions. The exact voltage potential will be determined by electrode spacing and the top coating resistivity.

The theoretical predictions have been conferred in experimental study. It has been proven that thick art materials may be held. The holding power increases with the increase in finger width, because the flux pattern is provided in a high arc over the gap from the extreme edges of the fingers. So, for composing original work by assembly of sheet material overlapping one another, wide electrodes are preferred. But very small pieces which do not bridge over two or more fingers and a gap, may not develop enough holding power. Therefore, the fingers and spacing should not be greater than the smaller possible pieces to be held. For example, one electrode as in the prior art will not hold a paper sheet without the use of a movable electrode. This invention object is directed to the elimination of the need for separate electrodes.

Claims

What is claimed is:

1. In an electrostatic sheet holding apparatus having a base supporting a plurality of electrodes, the electrodes being respectively commonly connected in two individual sets, and means for applying an electrical potential to said electrodes of the two sets so as to develop an electrostatic field adjacent to the surface of the base and electrostatically attract a sheet to said surface;

the improvement comprising a cover plate over said electrodes, said cover plate being a thin sheet of a semiconductor material stable under light in its conductive characteristics, having a bulk resistivity of not more than 10.sup.15 ohm-centimeters, and characterized in that a charge induced by said electrodes will not persist and will dissipate substantially immediately upon removal of any applied potential to said electrodes and thereby release a sheet from said surface.

2. In a sheet-holding apparatus as defined in claim 1, said means for applying an electrical potential being a direct current source of high voltage and low amperage, and said cover plate having a thickness related to the applied voltage such that a field generated by said electrodes will penetrate said cover plate and induce a charge in any electrostatically chargeable object in contact with said plate.

3. In a sheet-holding apparatus as defined in claim 1, said electrodes being interdigital.

4. In a sheet-holding apparatus as defined in claim 1,

means for transporting said sheet-holding apparatus between a horizontal attitude loading position and a vertical position;

control means for applying the potential when said holddown device is moved in a direction away from said loading position toward said vertical position, said control means disconnecting said potential source when said device is returned to said loading position to turn off the power to said electrodes whereby the electrostatic influence is substantially instantaneously terminated.

5. The exposure device as claimed in claim 4 wherein the third electrode is a polyvinylchloride.