Electricity: electrical systems and devices – Discharging or preventing accumulation of electric charge
Reexamination Certificate
2000-06-08
2001-07-10
Fleming, Fritz (Department: 2836)
Electricity: electrical systems and devices
Discharging or preventing accumulation of electric charge
C361S213000, C361S229000, C361S235000
Reexamination Certificate
active
06259591
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to controlling static charge on work pieces. More particularly, this invention relates to air ionizers for controlling static charge on moving webs of non-conductive material.
Background of the Invention
Many industrial operations are confronted by the build up of static charge on work pieces which then contribute to undesirable particulate contamination, unwanted movement, or other undesirable physical parameters associated with the work pieces. In the preparation of continuous films of sheet plastic materials, extended lengths of non-conductive plastic films pass rapidly over one or more rollers and accumulate substantial electrostatic charge that then attracts surface contaminants, and inhibits tight compaction in take-up rolls, impedes surface coating processes, and otherwise interferes with safe processing of the films.
Air ionizers, designed in a shape of a rod or a bar, are commonly positioned in close proximity to such moving webs to supply positive and negative ions for substantially neutralizing static charge on the web material. These air ionizers commonly contain pointed ionizing electrodes and operate at voltages of several kilovolts supplied to the ionizer via cables from remote generators positioned away from the ionizer. In large industrial applications, such webs may be several feet wide, operate at high linear speeds, and exhibit wide variations in the amount of static charge requiring neutralization at any given time or location along the moving web.
Typically, ionizing currents of about 1 to 5 microamperes per linear inch of the moving web are required for neutralization. The webs may vary in widths from several inches to 20 feet. This requires that the generators which supply such ionizers be capable of sustaining the output current of about 1-5 milliamperes at voltage levels of about 3-15 kilovolts.
There is a common problem with all air ionizer. This problem is dirt and residue accumulation on the tips of ionizing electrodes that limits their ionizing efficiency.
A problem with conventional ionizers that there is no economical and practical way to measure and monitor the ionizing efficiency of the electrodes without employing complex sensors and circuitry. For air ionizers with generators that produce high voltage output of the alternating current power at the power line frequency(AC) the difficulty of measuring the ionizing efficiency arises from the fact that the alternating potential applied to the electrodes couples capacitively to the electrically grounded components of the ionizer and the generator to produce a significant capacitive current that has a different phase and can substantially exceed the ionizing current.
For instance, in U.S. Pat. No. 5,017,876 the monitoring of the ion current from discharge electrodes of an AC ionizer is accomplished with a use of one or more sensors adjacently spaced from discharge electrodes. In one example of that device, one sensor picks up a capacitive current signal, while a second sensor picks up the total signal which represents the sum of the capacitive and corona (ion) currents. The outputs of the sensors are coupled to electronic circuitry, such as differential amplifier, to separate capacitive current from the total current signal. The problem with this approach, is that it requires adding sensors to the ionizer's construction. That increases the cost and manufacturing complexity of the equipment.
European Patent Application No. 97116167.4 (EP 0 844 726 A2) describes a different approach to detection of contamination on the discharge electrodes of an AC ionizer. In this application a complex electronic circuit with a microprocessor is employed to monitor and process a signal representing the output current of a high voltage AC transformer.
In another European Patent Application No. 97112236.1 (EP 0 850 759 A1) describes a system which includes an ionizer bar and circuitry for detection of contamination on ionizer electrodes. In order to achieve that the ionizer bar contains, in addition to ionizer electrodes, multiple contamination detecting sensors imbedded into the bar's body. That increases the cost and manufacturing complexity of the equipment.
SUMMARY OF THE INVENTION
In accordance with the method of the present invention, the ionizer measures and monitors its ionizing efficiency without employing dedicated sensors or a complex circuitry. In accordance with the present invention, two high voltage generators are operated to produce positive or negative voltages of about 3-15 kilovolts. The positive high voltage and negative high voltage are supplied to separate respective electrodes that are positioned in close proximity to the work piece (e.g., a moving web) to be neutralized with air ions. The positive generator output voltage can be made higher than the output voltage of the negative generator due to lower negative ionization onset level and higher mobility of negative ions. This is done in order to avoid unintentional application of charges on to a web.
The generators which apply high voltages of predetermined polarities to the respective electrodes include ground return electrical paths through which electrical charges are conducted away from the generators at rates corresponding to the rates of ion currents conducted by the respective electrodes into the air in their vicinities. Associated metering circuitry is placed in each of the ground return electrical paths.
In accordance with the illustrated embodiment of the present invention, the ionizing electrode of one polarity is positioned in close proximity to an electrode of the opposite polarity, and the sufficient potential difference is established between the electrodes. As a result, the positive electrodes act as the electrical potential reference for the negative electrodes positioned in close proximity thereto, and the negative electrodes act as the electrical potential reference for the positive electrode, to produce the desirable intense electrical field required for generation of air ions.
With the sufficient electric field at the ionizing electrodes, that is due to their close proximity to the electrodes of the opposite polarity and the potential difference between the electrodes, a certain ionizing current from positive electrodes flows to the negative electrodes, a certain ionizing current from negative electrodes flows to the positive electrodes. In the absence of an external electrostatic field from a surface, such as moving web, in the vicinity of the ionizer electrodes, substantially all ion currents flow between the electrodes of opposite polarities, and the currents in the ground return paths of each generator will be close to the maximum possible current. Measuring the magnitude and changes in these currents makes it possible to ascertain the changes in the ionizing efficiency of the ionizer.
If the web carries surface charge, the associated external electrostatic field causes ions of the polarity opposite to the polarity of the surface charge on the web to leave the ionizer electrodes and flow to the charged surface. For example, when the moving web carries a negative electrostatic charge, its electrostatic field attracts the ions from positive electrodes. As a result, some positive ion current flows to the moving web to neutralize its surface charge, while IS the rest of positive ion continue flowing to the negative electrodes. At the same time the ion current from the negative electrodes significantly flows to the positive electrodes.
The outcome of this redistribution of the destinations for various ion flows is that substantially the same positive ion current, as under the no-external electrostatic field conditions, leaves the positive electrode, and substantially the same negative ion current arrives to the positive electrode, and therefore the current in the ground return path of the positive generator is substantially the same as before the introduction of the external electrostatic field. On the other hand, while the same negative ion current, as under the no-e
Blitshteyn Mark
Pitel Ira J.
Fenwick & West LLP
Fleming Fritz
Ion Systems, Inc.
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