US3787706A - Apparatus for the control of charge on a moving web - Google Patents

Abstract

A method for controlling electrostatic charges on a web, wherein the web is pulled over a metal roller which is connected to a D.C. voltage supply, the electrostatic field of the web is measured, and the voltage supply is controlled as a function of the measured field to produce a voltage of a polarity whereby the electrostatic field produced by the web is greatly neutralized.

Description

United States Patent [1 1 De Geest Jan. 22, 1974 [54] APPARATUS FOR THE CONTROL OF 3,521,126 7/1970 Granzow et a1 317/262 A X CHARGE ON A MOVING WEB 2,980,834 4/1961 Tregay et al 317/262 A 2,826,166 3/1958 Davis 317/2 [75] Inventor: Wilfried Florent De Geest, Berchem,

Belgium Prima Examiner--J. D. Miller A i tN.V., M t l, [73] sslgnee gg g fg or Se Assistant Examiner1-1arry E. Moose, Jr.

Attorney, Agent, or Firm-William J. Daniel [22] Filed: May 3, 1971 [21] Appl. No.: 139,745

7 [57] ABSTRACT [30] Foreign Application Priority Data May 4, 1970 Great Britain 21,365/70 A method for controlling electrostatic charges on a web, wherein the web is pulled over a metal roller {52] US. Cl. 317/2 R, 317/262 A which is connected to a DC. voltage supply, the elec- [51 Int. Cl. 1105f 3/00 trostatic field of the web is measured, and the voltage [58] Field of Search 317/2, 262 A supply is controlled as a function of the measured field to produce a voltage of a polarity whereby the electro- [56] References Cited static field produced by the web is greatly neutralized.

UNITED STATES PATENTS 3,671,806 6/1972 Whitmore 317/2 R 2 Claims, 2 Drawing Figures D.C. SOURCE it A ,4

CONTROLLER 72 ELECTROMETER PAIENIEDJAN 2'? I914 3. 787, 706

. D.C. SOURCE 12 I6 17 1a ELECTROMETER CONTROLLER Fig. I

20 -o.c. SOURCE 1 ELECTROMETER 23 V 2 Fig. 2

APPARATUS FOR THE CONTROL or CHARGE N A MOVING WEB The present invention relates to an apparatus for the control of charge on a moving web, in particular on a moving web which is being wound or unwound.

It is known that insulators, and conductors when not electrically connected to the ground, becomes easily charged. This comes about as a consequence of rubbing against other surfaces. Friction may be involved, or sometimes only contact followed by separation. In the treatment of a weblike material, the electrostatic charging thereof frequently forms a serious problem because, since a web is generally transported over a plurality of rollers, its electrostatic charge may become important.

A common method for discharging or neutralizing a web comprises moving the web adjacent a plurality of grounded points. When the field produced by the charged web near the tips of the points attains a critical value, the air in the small region near the tip is ionized so that ions are projected onto the web in such a way as to neutralize the excess field atevery instant. Said method shows the disadvantage that for wide webs a great number of pointsis required. The points are liable to deformation when a discharge unit is manipulated and they may damage a swinging web. In addition, the critical value of afield which is capable of producing ionization is about 500. V/cm so that a complete discharge of a web by means of said method is not possible.

Another method for rendering the surface of a moving. web electrically neutral comprises removing charges from the web by intimately contacting a grounded conductor to the opposed surfaces of the web. Further, this method is not capable of completely discharging a web because, mainly as a consequence of contact potential effects, it is not possible to reduce the charge on a web below 200 V/cm.

In the treatment of webs, and in particular in the winding of webs, it has been found that the electrostatic field strength ofa roll of web which is being wound may easily amount to kV/cm when the web is not sufficiently discharged, that is below 200 V/cm, prior to its winding. The same effect may occur with a roll of web which is being unwound, with this particularity that the roll field may be zero or practically so when the unwinding is started and that it may gradually increase as the web is unwound. As a consequence of all this uncontrolled discharges may occur as a roll is approached by an operator or a grounded object, and said discharge may have deleterious effects, particularly in the case of light-sensitive photographic material.

It is an object of the present invention to provide an arrangement for the control of charge on a moving web, whereby the mentioned adverse effects may be avoided.

According to the present invention, the charge on a moving web is controlled by:

a. moving the web with at least one side in intimate contact over a control electrode which is electrically insulated from the ground and which is connected to a source capable of producing a variable D.C. potential,

b. measuring the electrostatic field produced by the web at a location close to the said electrode, and

c. controlling the said source as a function of the measured field so as to produce a D.C. potential of i a magnitude and a polarity which is capable of substantially reducing the field produced by the web.

According to a preferred embodiment of the invention, the measurement of the electrostatic field is done at a location which is between the said control electrode and the winding station, or between the said electrode and the unwinding station, if the charge on the web is to be controlled in connection with a winding, or an unwinding, respectively.

If the measurement of the field is done at a location which is downstream of the control electrode according to the web advance, the source of D.C. potential may be controlled in such a way as to obtain a zero field measurement.

The invention will be described hereinafter with ref erence to the drawings, wherein:

FIG. 1 is a diagrammatic view which illustrates the apparatus according to the invention for the charge control of a web which is being wound up, and

FIG. 2 is a diagrammatic view which illustrates the apparatus according to the invention for the charge control of a web which is being unwound.

In FIG. 1 a web 10 is passed over a grounded metal roller 11 before it is wound onto a roll 12. The web 10 may be a photographic support such as a triacetate or polyethylene terephthalate film which has been passed over a plurality of rollers during its coating and drying and which has become charged as a consequence of its passage on said rollers. The roller 11 may be the last of said plurality of rollers.

- A common value of the field strength which may be measured at the locus indicated by the arrow 13 amounts to 200 V/cm. The field strength measured at the roll 12, on the contrary, may be comprised between 1 and 5 kV/cm. This latter field strength may also be measured at a locus comprised between the roll 12 and the grounded roller 11, for instance at the locus indicated by the arrow 14. It should be understood that the charge on the wound web gradually increases and that a field strength of l to 5 kV/cm is only attained near the termination of the winding. It is precisely at this moment that the roll of wound web must be manipulated in order to cut the trailing end of the web and to remove the roll from the winder.

The charge on the wound web is considerably reduced if the charge is controlled by an arrangement which comprises the measuring electrode 15, the electrometer 16, the controller 17, the course 18 which is capable of producing a D.C. output voltage which is a function of the output signal of the controller 17, and a control electrode 1.9 in the form of an electrically conductive roller which is electrically insulated from the ground. In the drawing, the roller 19 has been drawn in tangential contact with the web 10 because hereinbefore the linear web path has been considered between the roller 11 and the roll 12, but it should be understood that the roller 19 will be placed so that the web contacts the roller 19 over an angular part of its periphery, so that an intimate contact occurs between the web and the said roller. In addition, means should be provided for controlling the web path from the roller 19 to the electrode 15, so that the distance between the web and the electrode 15 remains constant as the diameter of the roll 12 increases during winding.

The measuring electrode 15 is an elongated conductor which extends transversally of the webpath and is separated therefrom, and which is connected to an electrometer 16. Actually, the electrode 15 forms with the electrometer. 16 a field strength meter.

The electrometer 16 produces a D.C. output signal which corresponds in magnitude and polarity with the measured charge on the web. A controller 17 amplifies the output signal of the electrometer l6 and controls the source 18 in such a way that a D.C. voltage is applied at the control electrode 19 which is capable of reducing the field produced by the web;

it should be understood that the charge on the web which determines the field at the electrode 15, is determined by the charge which is present on the web after its passing over the roller 11, and also by the charge which is being accumulated or produced on the roll 12.

The control of the charge onthe web occurs by the injection of electrons on the lower web surface by the roller 19 if the field measured at the electrode 15 is positive, or by the corresponding withdrawal of electrons if the measured field is negative. Said field measured at the electrode 15 may be caused by a determined surplus or lack of charge carriers on only one surface of the web, but said field may also result from different charge conditions on the different surfaces of the web. The measuring electrode actually measures the resulting field at the locus of said electrode, and it controls the control electrode 19 in order to modify the charge on the web in such a way that at the locus of the measuring electrode the field is considerably reduced, or even becomes zero. Thus, it will be understood that during the winding of the web the charge on each side of the web is not necessarily removed or neutralized, but that the overall charge is controlled in such a way that an uncontrolled charge on the web which may cause excessive field strengths at the winding station, is

. avoided.

Whereas formerly field strengths of kV/cm and even higher could be measured on the roll 12 of the wound web, it has been shown that the described apparatus permits to control the charge on the web in such a way that the field at the roll 12 remains within acceptable limits, that is lower than 500 V/cm.

ln the arrangement according to FIG. 2, a web 25 is unwound from a roll 20 and is passed over two electrically conductive rollers 21 and 22, which are insulated from the ground.

The electric insulation of the roller 21 in respect of the ground amounts to ohms and the roller is connected to the input of a vacuum tube electrometer 23. The output current of the electrometer controls a source 24 which is capable of producing a D.C. output varying between 0 and 1,000 volts. In the present case, the polarity of the D.C. output is fixed because it is admitted that the arrangement is used for the control of charge on webs of the same type, and in such case the magnitude of the field increases as the web moves, but the polarity thereof does not vary. The described arrangement permits to control effectively the charge on the web, and it has been found that at the unwinding of the web any difficulty with respect to excess field strengths is avoided, and this in particular for webs which have been previously wound without any control of the charge thereon.

The present invention is not limited to the described embodiments.

The apparatus according to the invention may also be used for the control of charge on moving webs at other locations than near a winding or unwinding station. Thus, the web charge may be controlled near coating stations, laminating stations, drying stations, etc.

The charge on a web may be controlled by measuring the electrostatic field at both the web sides and by controlling independent control electrodes at either side of the web. Both said measurements may occur at locations which are close to each other, but it is also possible to measure the field on one side of the web, for instance at an unwinding station for the web, and to measure the field on the other web side at the winding station.

We claim:

1. Apparatus for reducing the electrostatic charge on a web moving along a path which comprises:

a. a conductive electrode roller having a smooth rigid surface and disposed along said web path with the surface thereof in direct contact with one surface of said web, said electrode roller being electrically insulated from ground,

b. a source of variable D.C. potential connected to said electrode roller,

c. means for measuring the electrostatic field produced by the web at a locus adjacent said electrode roller and producing a signal corresponding to the magnitude of an electrostatic charge on said web, and

d. a controller for said D.C. source, said controller receiving said signal from said field measuring means and controlling said source to apply to said electrode roller a D.C. potential of a polarity opposite to the measured field and a magnitude sufficient to substantially reduce the charge on the web,

said web having the opposite surface thereof free of contact with an electrode roller in the region of said conductive roller.

2. Apparatus according to claim 6 wherein said field measuring means is disposed downstream of said electrode roller.

Claims (2)

1. Apparatus for reducing the electrostatic charge on a web moving along a path which comprises: a. a conductive electrode roller having a smooth rigid surface and disposed along said web path with the surface theReof in direct contact with one surface of said web, said electrode roller being electrically insulated from ground, b. a source of variable D.C. potential connected to said electrode roller, c. means for measuring the electrostatic field produced by the web at a locus adjacent said electrode roller and producing a signal corresponding to the magnitude of an electrostatic charge on said web, and d. a controller for said D.C. source, said controller receiving said signal from said field measuring means and controlling said source to apply to said electrode roller a D.C. potential of a polarity opposite to the measured field and a magnitude sufficient to substantially reduce the charge on the web, said web having the opposite surface thereof free of contact with an electrode roller in the region of said conductive roller.

2. Apparatus according to claim 6 wherein said field measuring means is disposed downstream of said electrode roller.

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