Abstract:
An apparatus and method for minimizing contamination buildup on corona emitters that are employed in an ionizer. Contamination buildup control is accomplished with solely electronic means. High voltage is applied to the emitters with waveforms that serve to push contaminants away from the emitter, rather than attracting contaminants toward the emitters. The results are fewer cleaning cycles, more time between cleaning cycles, and more stable ionizer operation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application 60/918,512 entitled “Method and Apparatus for Control Contamination of Ion Emitters” filed Mar. 17, 2007 by Lawrence Levit and Peter Gefter. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to AC powered ionizers for that are used for static charge control. More specifically, the invention is targeted at the problem of ion emitter contamination in the AC ionizers, while the ionizer performs useful neutralization. 
     With AC ionizers, each emitter receives a positive voltage during one time period and a negative voltage during another time period. Hence, each emitter generates both positive and negative ions. 
     Both positive and negative ions are directed toward a charged target for the purpose of neutralizing the charge. 
     2. Description of Related Art 
     Ion emitters generate both positive and negative ions into the surrounding air or gas media. To generate ions, the amplitude of applied AC voltage must be high enough to produce a corona discharge between at least two electrodes, where at least one of them is an ion emitter. 
     The minimum voltage for the establishment of corona discharge is called corona onset voltage or the corona threshold voltage. According to theoretical and experimental studies of corona discharge this voltage mainly depends upon the ion emitter geometry, polarity of applied voltage, gas composition and pressure [F. W. Peek, “Dielectric Phenomena in High Voltage Engineering” McGraw Hill, New York, 1929 and J. M. Meek and J. D. Craggs “Electrical Breakdown of Gases” John Wiley &amp; Sons, Chichester, 1978]. 
     For wire or filament-type ion emitters, the corona onset voltage is typically in the range of positive 5 to 6 kV for positive ionizing voltage and in the range of negative 4.5 to 5.5 kV for negative ionizing voltage. For point-type ion emitters, the absolute values of onset voltage are typically 1-1.5 kV lower. These stated corona onset voltages apply to clean emitters. If the emitters are not clean, corona onset voltages change. 
     It is known in art that airborne particles from the surrounding air or gas accumulate on the emitters. Effectively, the emitters are functioning as electrostatic precipitators. Emitter contamination is an expected consequence of corona discharge in open air. Contamination buildup changes the emitter&#39;s geometry and raises onset voltage. 
     Once contaminated, real time ion production decreases, and the efficiency of the AC ionizer decreases significantly. This buildup must be removed to restore proper operation of the ionizer. In large facilities, thousands of emitters are present. Contamination removal becomes a large and objectionable use of resources. 
     Prior art contamination removal methods include manual brush abrasion and automatic brush abrasion. These methods of mechanical cleaning are effective, but require additional mechanical parts or operator time. In some cases, abrasive cleaning transfers contamination accumulated by ion emitters to the product, which must be kept clean. 
     A new method is needed to reduce the contamination deposition rate on the ion emitters. Ideally, the method would arise from basic physics or electronics, and operate without taking the ionizer out of service. 
     Further, the contamination prevention method should apply to a variety of emitter configurations: points, wires, filaments, or loops. 
     BRIEF SUMMARY OF THE INVENTION 
     Particles or large molecules, which are convertible into particles, exist in the atmosphere of a cleanroom. When a prior art ionizer is operated within the cleanroom, particles accumulate on the emitters because the particles are drawn toward the emitter by the electric field emanating from the emitter. 
     This instant invention reduces contamination buildup on emitters within AC ionizers. The novel principle lies in the application of voltage waveforms onto the emitters through programmed power supplies. These electrical waveforms, when applied to the emitter points, drive particles away from the emitter electrode(s) rather than attract particles to the emitter electrode(s). 
     The instant invention is solely an electronic method of preventing contamination buildup on the emitters. The invention does not require air flow or mechanical components to function. However, this invention may be combined with air flow or mechanical components. 
     There are two dominant mechanisms of particle attraction to emitters: (1) Coulombic attraction and (2) dielectrophoretic attraction. Both attraction mechanisms can be understood in relation to fundamental physical forces. 
     Coulombic forces can be attractive or repulsive. Coulombic particle attraction occurs when a particle is positive and the emitter is negative. Alternately, a particle is negative and the emitter is positive. Invented waveforms are designed to minimize attractive Coulombic forces and maximize repulsive Coulombic forces. 
     The second force is the dielectrophoretic attraction. This force operates whenever an asymmetric electric field is present, but ceases operation when the asymmetric electric field ceases. Asymmetric electric fields exist near ionizer emitters, regardless of whether the emitter is a pointed shaft, a wire filament, a loop, or alternate shape. 
     Dielectrophoretic force has two unique properties. First, the dielectrophoretic force on a particle is always attractive in air, nitrogen, or inert gas. Second, the dielectrophoretic force operates on neutral particles. 
     The invented electronic waveforms, which are delivered to the emitters through one or more high voltage power supplies, are combinations of some or all of the following components:
         ion generation signal amplified to an ion generation voltage such that peak voltages exceed the corona onset voltage,   positive cleaner signal amplified to a positive cleaner voltage that repels positive particles,   negative cleaner signal amplified to a negative cleaner voltage that repels negative particles,   positive ion driver signal amplified to a positive ion driver voltage that drives positive ions toward the target,   negative ion driver signal amplified to a negative ion driver voltage that drives negative ions toward the target, and   an OFF period.       

    
    
     
       BRIEF SUMMARY OF THE FIGURES 
         FIG. 1  shows the electronics and ionizing waveform for an ionizer designed for discharging targets that are close to the ionizer. 
         FIG. 2  shows a corona emitter surrounded by balanced ions and a neutral particle. This condition exists when the ionizing waveform incorporates only a balanced ion generating signal. 
         FIG. 3  shows a corona emitter when the ionizing waveform incorporates both a balanced ion generating signal and a positive cleaner signal. A nearby particle acquires a positive charge, and is repelled by Coulombic force. 
         FIG. 4  shows a corona emitter when the ionizing waveform incorporates both a balanced ion generating signal and a negative cleaner signal. A nearby particle acquires a negative charge, and is repelled by Coulombic force. 
         FIG. 5  shows the electronics and ionizing waveform for an ionizer embodiment, where the ionizing waveform incorporates both cleaner signals and ion driver signals. 
         FIG. 6  shows the electronics and ionizing waveform for an ionizer embodiment, where the ionizing waveform incorporates cleaner signals and a period when ions are not generated. 
         FIG. 7  shows the electronics and ionizing waveform for an ionizer embodiment, where the ionizing waveform incorporates cleaner signals, ion driver signals, and a period when ions are not generated. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention applies to all ionizers with corona emitters, and is particularly useful for ionizing bars. The invention is an electronic method to prevent contamination buildup on corona emitters. 
     Electronic waveforms are applied to an ionizer&#39;s corona emitters through the high voltage power supplies. The waveforms are designed to accomplish two goals. The first goal is to generate ions and deliver them to a charged target. The second goal is to reduce contamination buildup on the corona emitters. 
       FIG. 1  diagrams a first embodiment of the electronics for an ionizer with reduced contamination of corona emitters. The system shown in  FIG. 1  is appropriate for charged targets  13  which are within 6 inches of the ionizer. 
     A high frequency signal generator  1  produces an ion generation signal  2  that is fed to the input of a high-frequency power supply  3  that produces a high voltage output. The high frequency power supply  3  amplifies the ion generation signal  2  to create an ion generating voltage  4 . 
     Simultaneously, a low frequency signal generator  5  produces a positive cleaner signal  6 A and a negative cleaner signal  6 B, which are fed to the input of a low frequency power supply  7  that produces a high voltage output. The low frequency power supply  7  amplifies the positive cleaner signal  6 A and negative cleaner signal  6 B to create a positive cleaner voltage  8 A and negative cleaner voltage  8 B. 
     The ion generating voltage  4 , the positive cleaner voltage  8 A, and negative cleaner voltage  8 B combine in a summing block  11  to create the ionizing waveform  9 . The ionizing waveform  9  is connected to the emitter  10 . Reference electrode  12  provides a ground reference. 
       FIG. 1  shows two signal generators and two power supplies, but more or fewer signal generators and power supplies may be used. 
     During time periods where only the ion generation signal  2  is applied and no charged target  13  is nearby, a steady state density of balanced ions is created in the vicinity of the emitter  10 . The reason is that the frequency of the ion generation signal  2  is roughly 1,000 to 100,000 Hertz, with a typical frequency of 20,000 Hertz. 
     At 20,000 Hertz, ions do not have sufficient time to escape before the polarity of the emitter reverses. Hence, the created ions oscillate in a volume of space near the emitter  10 . A particle that approaches the emitter  10  will be quickly neutralized, and experience neither Coulombic attraction or Coulombic repulsion. 
       FIG. 2  describes the volume of space near an emitter  20  when only the ion generation signal is applied. The ions  21  near the emitter are balanced because the ion generation signal has a mean voltage of zero. A particle  22  near the emitter  20  is neutral because neither the emitter  20  nor the ions  21  have a net charge. Hence, there is no Coulombic force that attracts the particle  22  toward the emitter  20 . Only a dielectrophoretic force  23  acts to move the particle  22  toward the emitter  20 . 
     Refer to  FIG. 3 . This situation changes when a positive cleaner signal is applied. The emitter  30  now acquires a positive voltage, relative to a ground reference. The positive charged emitter  30  imbalances the ions  31 . More positive ions than negative ions are present. A particle  32  equilibrates with the positive distribution of ions  31 , and becomes positive itself. The positive particle  32  now experiences Coulombic repulsion, and moves away from the positive emitter  30  along repulsion direction  33 . Movement of 0.1 centimeter is sufficient to prevent recapture. The probability of this particle  32  contaminating the emitter  30  has been minimized by the application of the positive cleaner signal. 
     Refer to  FIG. 4 . When a negative cleaner signal is applied, a particle  42  is repelled for the same reasons. Only the polarity is different. The emitter  40  now acquires a negative voltage, relative to a ground reference. The negative charged emitter  40  imbalances the ions  41 . More negative ions than positive ions are present. The particle  42  equilibrates with the negative distribution of ions  41 , and becomes negative itself. The negative particle  42  now experiences Coulombic repulsion, and moves away from the negative emitter along repulsion direction  43 . Again, the chance of the particle  42  contaminating the emitter  40  is minimal. 
     The reason for using both positive cleaner signals and negative cleaner signals is to maintain overall ionizer balance. Cleaner signals typically have a frequency of 0.1 to 200 Hertz. The ion generation signal is typically run by itself after a positive cleaner signal or a negative cleaner signal to achieve neutralization of the particles. 
     When the ionizer is disposed further from a charged target, positive ion driver signals and negative ion driver signals may be incorporated into an ionizing waveform. The purpose is to push ions toward the target. 
       FIG. 5  shows another embodiment of the electronics for an ionizer with reduced contamination of corona emitters. This embodiment is appropriate for a charged target more than 6 inches away from the ionizer. 
     In  FIG. 5 , a high frequency signal generator  51  produces an ion generation signal  52  that is fed to the input of a high-frequency power supply  53  that produces a high voltage output. The high frequency power supply  53  amplifies the ion generation signal  52  to create an ion generating voltage  54 . 
     Simultaneously, a low frequency signal generator  55  produces a positive cleaner signal  56 A, a negative cleaner signal  56 B, a positive ion driver signal  56 C, and a negative ion driver signal  56 D, which are fed to the input of a low frequency power supply  57  that produces a high voltage output. The low frequency power supply  57  amplifies the positive cleaner signal  56 A, the negative cleaner signal  56 B, the positive ion driver signal  56 C, and the negative ion driver signal  56 D to create a positive cleaner voltage  58 A, a negative cleaner voltage  58 B, a positive ion driver voltage  58 C, and a negative ion driver voltage  58 D. 
     The ion generating voltage  54 , the positive cleaner voltage  58 A, the negative cleaner voltage  58 B, the positive ion driver voltage  58 C, and the negative ion driver voltage  58 D combine in a summing block  61  to create the ionizing waveform  59 . The ionizing waveform  59  is connected to the emitter  60  which operates in relation to a reference electrode  62 . 
     The positive cleaner signal  56 A is designed to move particles from the vicinity of the emitter via Coulombic repulsion. The positive ion driver signal  56 C is designed to move positive ions toward the charged target  63 . The positive cleaner signal  56 A and the positive ion driver signal  56 C have the same polarity, but magnitudes and durations may be different. Normally, the amplitude of the positive ion driver signal  56 C is less than the amplitude of the positive cleaner signal  56 A because ions are more mobile than particles. However, this is not a requirement. 
       FIG. 6  shows the introduction of periods where the emitters generate no ions. The introduction of non-generating periods has very minor effect on the ionizer&#39;s performance. However, there are several benefits. First, power consumption is reduced. Second, ozone generation is reduced. Third, emitter erosion is reduced. Fourth, a reduced duty cycle further reduces the particle generation. 
     Fifth, dielectrophoretic attraction of neutral particles toward the emitter is reduced, which further reduces contaminant buildup on the emitters. The equation which describes dielectrophoretic attraction is—
 
 F   d =4π R   3 ∈ 1 {(∈ 2 −∈ 1 )/(∈ 2 +2∈ 1 )} E∇·E  
 
where
 
     ∈ 1 —permittivity of air or gas surrounding a particle, 
     ∈ 2 —particle permittivity, 
     R—radius of the particle and 
     ∇·E is the field intensity gradient. 
     Since particles always have higher permittivity than air or gas, the equation shows that, the dielectrophoretic force, F d , is attractive. That is, particles are moved toward the emitter whenever the emitter is charged. Turning the power off interrupts the attractive dielectrophoretic force, and provides time for the particles to be moved away from the emitter by Coulombic repulsion. 
     For the embodiment in  FIG. 6 , a high frequency signal generator  71  produces an ion generation signal  72 A that is fed to the input of a high-frequency power supply  73  that produces a high voltage output. The high frequency power supply  73  amplifies the ion generation signal  72 A to create an ion generating voltage  74 . As shown, the ion generation signal  72 A is not continuous, and includes an OFF period signal  72 B. No ions are generated during the OFF period signal  72 B. 
     Simultaneously in  FIG. 6 , a low frequency signal generator  75  produces a positive cleaner signal  76 A and a negative cleaner signal  76 B, which are fed to the input of a low frequency power supply  77  that produces a high voltage output. The low frequency power supply  77  amplifies the positive cleaner signal  76 A and negative cleaner signal  76 B to create a positive cleaner voltage  78 A and negative cleaner voltage  78 B. 
     The ion generating voltage  74 , the positive cleaner voltage  78 A, and negative cleaner voltage  78 B combine in a summing block  81  to create the ionizing waveform  79 . The ionizing waveform  79  is delivered to the emitter  80 . Note that the ionizing waveform  79  includes a time period in which no ionization occurs, corresponding to OFF period signal  72 B. 
       FIG. 7  shows an another embodiment using an OFF period  92 B which is contained within an ion generation signal  92 A. In  FIG. 7 , a high frequency signal generator  91  produces an ion generation signal  92 A that is fed to the input of a high-frequency power supply  93  that produces a high voltage output. The high frequency power supply  93  amplifies the ion generation signal  92  to create an ion generating voltage  94 . 
     Simultaneously, a low frequency signal generator  95  produces a positive cleaner signal  96 A, a negative cleaner signal  96 B, a positive ion driver signal  96 C, and a negative ion driver signal  96 D, which are fed to the input of a low frequency power supply  97  that produces a high voltage output. The low frequency power supply  97  amplifies the positive cleaner signal  96 A, the negative cleaner signal  96 B, the positive ion driver signal  96 C, and the negative ion driver signal  96 D to create a positive cleaner voltage  98 A, a negative cleaner voltage  98 B, a positive ion driver voltage  98 C, and a negative ion driver voltage  98 D. 
     The ion generating voltage  94 , the positive cleaner voltage  98 A, the negative cleaner voltage  98 B, the positive ion driver voltage  98 C, and the negative ion driver voltage  98 D combine in a summing block  101  to create the ionizing waveform  99 . The ionizing waveform  99  is connected to the emitter  100 . 
     The positive cleaner signal  96 A is designed to move particles from the vicinity of the emitter via Coulombic repulsion. The positive ion driver signal  96 C is designed to move positive ions toward the charged target. The positive cleaner signal  96 A and the positive ion driver signal  96 C have the same polarity, but magnitudes and durations may be different. Normally, the amplitude of the positive ion driver signal  96 C is less than the amplitude of the positive cleaner signal  96 A because ions are more mobile than particles. However, this is not a requirement. 
     The negative cleaner signal  96 B and the negative ion driver signal  96 D perform the same functions as the positive cleaner signal  96 A and the positive ion driver signal  96 C, but use a negative polarity. 
     The ion generation signal is typically run by itself after a positive ion driver signal  96 C or a negative ion driver signal  96 D. 
     The ionizing waveform  99  shows a period where no ions are generated. 
     For cost and space considerations, it is desirable to reduce the number of signal generators and power supplies. This can be done by combining the low frequency signals with one low frequency signal generator, and forwarding the combined signal to one low frequency power supply. Similarly, high frequency signals can be processed by one high frequency signal generator, and forwarded to one high frequency power supply. 
     Signal time period durations, sequence orders, and voltage amplitudes are variable, depending on the type and concentration of airborne contaminants near the ionizer. Furthermore, signals may have shapes beyond square waves. Rounded, trapezoidal, triangular, or asymmetric are applicable. Such variation is within the scope of this invention.