Abstract:
Electrostatic discharge can damage electronic equipment. It has been known to start a fire when it occurs when a motor vehicle&#39;s gasoline tank is being filled. A device for detecting, alarming, and/or neutralizing an electrostatic charge can help avoid these problems. A variable capacitor, the capacitance of which is made to vary periodically, is used to detect electrostatic charge. A first electrode of the capacitor is the object for which electrostatic charge is important. The capacitor&#39;s electrode is one by which the capacitance is varied. An amplitude of a signal from the electrode can be shown to be proportional to the electrostatic charge on the object.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an electrical device. More particularly the present invention relates to a device for detecting an electrostatic charge on an object and alarming on the potential for electrostatic discharge. An additional action taken on sensing high potential for electrostatic discharge is the use of a positive or negative ion generator to remove the built-up charge. 
     2. Background Art 
     Electrostatic discharge between a body or object and an electronic device, such as a circuit board, can damage or destroy the electronic device. Electrostatic discharge can be very dangerous when it occurs during the process of filling a motor vehicle with gasoline, in the vicinity of the gasoline vapors. At other times, electrostatic discharge may cause discomfort. Various devices are used to dissipate an electrostatic electric charge from a body, such as a wrist strap connected to a wire going to ground. Some devices initiate an alarm in the event of an electrostatic discharge. Other devices measure and alarm on static electric charge accumulation. Wilson, in U.S. Pat. No. 6,150,945 reveals a capacitance device for measuring a charge on a body. The device is not grounded and uses a capacitor with a fixed capacitance. A charge accumulates on the fixed capacitance capacitor due to triboelectric processes. The signal, proportional to the charge on the device of U.S. Pat. No. 6,150,945, will tend to decay in magnitude over time due to the use of a high-impedance differential amplifier. This differential amplifier has a finite input resistance, and consequently, discharges the voltage on the very small capacitance produced by a triboelectric charge event. 
     There is, therefore, a need for a method and device for measuring the electrostatic charge accumulation on a body or object and its potential for electrostatic discharge, all without the inconvenience of a connection to ground. There is a further need for a device for measuring static charge accumulation without causing the signal representing the electrostatic charge to artificially dissipate. A further need is for a method and device for removing an accumulated electrostatic charge from a body or object. 
     BRIEF SUMMARY OF THE INVENTION 
     A purpose of this invention is to provide a method and apparatus for accurately measuring an electrostatic charge accumulated on a body or object; and, therefore, the potential for an electrostatic discharge between that body or object and another body or object having a different electrostatic charge and the capacity to change its electrostatic charge. Another purpose of the present invention is to provide a device for measuring electrostatic charge without dissipating the measured charge during the measurement. A further objective of the invention is to provide a method and apparatus for removing an electrostatic charge based on the electrostatic charge measurement results. 
     To measure a magnitude of electric potential with respect to ground, a capacitance of a conductor having a capacitance to ground is modulated in a periodic fashion. When the capacitance is modulated, the magnitude of the charges on this conductor also changes periodically. The amplitude of this periodic charge is proportional to the electric potential compared to ground. 
     To effect the above, a rotor is used, having a shape like two identical segments of a circular disk, symmetrical about a pivot point. The rotor oscillates at a fixed frequency about the pivot point with the aid of a torsion spring. One section of this rotor is one plate, or electrode, in the capacitor and oscillates behind a faraday shield in which a hole, or “window,” is provided. During its travel, this rotor section will pass the window, at one point exposing the greatest surface area of the rotor to the window. It will also pass out of the window such that no part of this rotor section is exposed by the window. Any portion of this rotor section exposed by the window is exposed to the ambient. The electric charge detector experiences its lowest capacitance when this rotor section is completely hidden behind the faraday shield. When the maximum area of this rotor section is exposed to the window, the electric charge detector realizes its greatest capacitance. A second rotor section is an electrically separate conductor used as the movable part, or armature, of an electrostatic motor. 
     The result of the oscillation and variable capacitance is an Alternating Current (AC) signal, the magnitude of which is proportional to the electric potential applied to the electric charge detector. This signal can also be shown to be proportional to the magnitude of the electrostatic charge potential between the body or object of interest and ground. The polarity of the charge may also be determined by this invention. 
     When a body or object reaches a threshold charge magnitude, the electric charge detector may be configured to provide an alarm. Such an alarm would be perceptible by one or more of the senses, thus audible, visible, tactile, etc. Another, optional reaction to the detection of an electrostatic charge greater than a predetermined threshold is the generation of positive or negative ions, depending on the sign of the accumulated electrostatic charge, to neutralize the electrostatic charge on the body or object. 
     The novel features which are believed to be characteristic of this invention, both as to its organization and method of operation together with further objectives and advantages thereto, will be better understood from the following description considered in connection with the accompanying drawings in which a presently preferred embodiment of the invention is illustrated by way of example. It is to be expressly understood however, that the drawings are for the purpose of illustration and description only and not intended as a definition of the limits of the invention. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     FIG. 1 is a plan view of an electric charge detector of the present invention along with a schematic of associated electronics; 
     FIG. 2 a  is the same plan view as seen in FIG. 1, showing an orientation for FIG. 2 b;    
     FIG. 2 b  is a side elevation view of the electric charge detector; 
     FIG. 3 is a time trace of five (5) AC signals; 
     FIG. 4 is the same plan view as seen in FIG. 1, showing a wire neutralization grid inside a faraday shield window; 
     FIG. 5 is a functional diagram of the electric charge detector with an optional ion generator; 
     FIG. 6 is a functional diagram of the electric charge detector without an ion generator; 
     FIG. 7 is a perspective view of the electric charge detector of the present invention mounted on a headband; and 
     FIG. 8 is a perspective view of the electric charge detector of the present invention mounted on a wrist band. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the diagram of FIG. 1, the top edge view of a multiple-plate air capacitor is shown along with electronic function blocks and electrical connections; all comprising the electric charge detector  100 . A moveable plate in the capacitor which, the first embodiment is a rotor  105 , is comprised of a first rotor section  105   a  and a second rotor section  105   b ,both approximately disk sections. The rotor  105  is suspended by a torsion spring (not shown) at its center of gravity  110  so the rotor can oscillate at a fixed, resonate frequency. As the rotor oscillates, it assumes a position behind a faraday shield  200  (FIG. 2) behind which the rotor  105 , and additional plates  115 ,  120  reside (for clarity, the faraday shield is not seen in FIG.  1 ). The first rotor section  105   a  is the conductor having variable capacitance to ground. It is connected to the charge detector circuitry. The second rotor section  105   b  is an electrically separate conductor which moves the rotor using electrostatic forces. When the first rotor section  105   a  is completely hidden by the faraday shield  200 , the capacitance is at a minimum. As the rotor  105  continues to oscillate, the first rotor section  105   a  assumes a position exposed to the ambient via a window  125  in the faraday shield  200 . When the rotor is in this position, the capacitance is a maximum. Only the portion of the surface of the first rotor section  105   a  exposed to the ambient via the window  125  in the faraday shield  200  participates in the capacitance of the first rotor section  105   a . Varying the exposure of the first rotor section  105   a  to ambient is analogous to varying the surface area of the first rotor section  105   a . The rotor  105  is shown halfway between these two extreme positions in FIG. 1, exposing a portion of a first rotor section  105   a  to the ambient. 
     Five (5) AC signals: S 1 , S 2 , S 3 , S 4 , and S 5  are depicted in FIG.  3 . The abscissa is time. Each signal has an ordinate that is voltage. The first four (4) signals: S 1 , S 2 , S 3 , and S 4  are generated in an AC signal generator  130 . Two signals, S 1  and S 2 , are applied to a pair of additional plates, one lower plate  120   a  and one upper plate  115   a  respectively, while two more signals, S 3  and S 4 , are applied to another pair of additional plates, one lower plate  120   b  and one upper plate  115   b , respectively. The relative positions of the additional plates  115 ,  120 , as well as the rotor  105  are depicted in FIG. 2 b : additional plates  115   a ,  115   b  are above the rotor  105 , while additional plates  120   a , 120   b  are below the rotor  105 . The AC signals, S 1 , S 2 , S 3 , and S 4 , induce resonate mechanical oscillations in the rotor  105  by electrostatic attraction to the second rotor section  105   b . The induction of a drive signal in the first section  105   a  of the rotor, furthest from the additional plates  115 ,  120 , via the second rotor section  105   b  is minimized by charge balancing, as well as frequency doubling. The charge balance results from an upper additional plate  115   a ,  115   b  and its associated lower additional plate  120   a , 120   b  having signals of equal amplitude and opposite polarity. Thus, the induced charges on the top and bottom surfaces of the second rotor section  105   b  between these additional plates  115 ,  120  are equal. Note that signals S 1 , S 2 , S 3 , and S 4  have twice the frequency of the input signal, S 5 , to the phase detector  135 . The frequency doubling exploits the polarity insensitivity of electrostatic attraction between oppositely charged conductors. Electrically insulating the second rotor section  105   b  from the first rotor section  105   a , and connecting the second rotor section  105   b  to a constant voltage reference  140 , also contributes to signal isolation. These strategies minimize the corruption of the signal to be detected on the first rotor section  105   a  by the signals S 1 , S 2 , S 3 , and S 4 , that sustain the mechanical oscillations of the rotor  105 . When an electric charge is applied to the device, an AC signal at the frequency shown for signal S 5  in FIG. 3 will be induced in the first rotor section  105   a , because this rotor section has a variable capacitance to the ambient. This AC signal, S 5 , is proportional to the electric charge on the apparatus and its phase with respect to signal S 5  indicates the polarity of the electrostatic charge on the device. 
     To maximize the sensitivity of the electric charge detector  100 , the surface of the first rotor section  105   a  can be covered with an array of sharp points to maximize the capacitance to the ambient. Sensitivity can also be improved by signal amplification  145  and filtering  150 . 
     Other arrangements of the movable plate electrode  105  and fixed plates  115 ,  120  than those proposed, are possible. In a second embodiment of the invention, the variable capacitor is implemented by varying the surface texture of one electrode instead of variable area. This embodiment may be easier to implement using Micro Electro Mechanical on Silicon (MEMS) technology. In this second embodiment, an array of ribbons, which form a smooth surface, yield a minimum capacitance in one position, and in another position, an array of sharp edges, yields a maximum capacitance. With this embodiment, only small movements are required. 
     In a third embodiment, a capacitance divider arrangement consisting of capacitance to ambient in series with a variable capacitance to a floating ground of the device, also implies a small movement, since distance between conductors (tens of nanometers) rather than area can be modulated. A fourth embodiment is based on measuring the force of repulsion of like charges on two conductors. However, this embodiment has the disadvantage of its inability to yield polarity information of the electrostatic charge on the object or body. 
     A fifth embodiment represents an alternative to signal amplitude measurement. In this embodiment, the electric field is neutralized to the ambient from the first rotor section  105   a  using a wire grid  400  at the window in the faraday shield  200  as shown in FIG. 4. A voltage applied to this wire grid  400  with a voltage source  410  can be adjusted to cancel the electric field to ambient resulting from an electrostatic charge applied to the electric charge detector. The neutralization voltage applied to the wire grid  400  is proportional to the electrostatic charge applied to the electric charge detector  100 . The output signal from the phase detector  135  can be used as an error signal to control the voltage used to null this error signal. This embodiment is more expensive in MEMS technology as a result of the higher mechanical complexity. It has the advantage of greater precision, because the signal amplitude measurement is temperature sensitive due to parasitic capacitance, which is a significant fraction of the small capacitance (tens of nanofarads) to the ambient. This parasitic capacitance is temperature sensitive. 
     A typical application is illustrated in FIG.  5 . The electric charge detector  100  could be mounted on a headband  700 , as shown in FIG. 7, to be worn by a person concerned about static charge buildup. Two ion generation wicks  710  are mounted on opposite sides of the headband (only one shown) to prevent ions from one wick from being collected on the other. 
     Another implementation of the present invention is shown in FIG. 6, wherein an ion generator is not used. The wristband  800  of FIG. 8 would be a suitable fastener for this application. 
     In either FIG. 5 or FIG. 6, the threshold generator  500  converts the analog output of the electric charge detector  100  to three (3) digital control signals: “&gt;|THRESHOLD A |”, “&gt;|THRESHOLD I |”, and “POLARITY”. The first, &gt;|THRESHOLD A |, is asserted when the electrostatic potential is dangerous to sensitive devices. The second, &gt;|THRESHOLD I |, is asserted when the electrostatic potential exceeds the measurement resolution threshold. This second threshold is chosen to prevent excessive cycling on and off of the ion generators  51 O a ,  51 O b , resulting in excessive power consumption. The third digital control signal, POLARITY, is asserted when a positive polarity of electric potential is present on the electric charge detector  100 . These three signals can be used to activate a sensory alarm  520  and/or to activate the appropriate polarity ion generator  510   a ,  510   b.    
     The above embodiment is the preferred embodiment, but this invention is not limited thereto. It is, therefore, apparent that many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.