A Ground-free Static Charge Removal Device"

Background of the invention

Field of the invention

This invention relates to a ground-free device for removing static elec¬ trical charge from a conductive or semi-conductive body, and particularly a device of such type which may be mounted on transportation vehicles.

Description of the related art

While the present invention is broadly useful for removing static electrical charge from conductive and semi-conductive charged bodies, the application of particular concern is the discharge of static electricity from transportation vehicles (cars and aircrafts). Other applications include static discharge from the human body.

It is known that static electricity build-up during the movement of ground transportation vehicles is responsible for pains, fatigue, and drowsiness of passengers, as well as damage of electronic equipment in the vehicle. Recent medical studies showed that drowsiness is a result of strong stimulation of the nervous system, which, among other effects such as fatigue, occur to in¬ dividuals who become exposed to high electrostatic fields. In the case of aircrafts, static build-up results in abrubt discharges from sharp points on the body of the aircraft which generate RF (Radio Frequency) noise and interfere with the navigation systems of the aircraft. Static build-up on helicopters is further undesirable, as a hovering helicopter can reach a potential of 200KV with a stored energy of more than 30 joules, which constitutes a serious hazard to ground personnel during winching and rescue operations.

Historically, chains and conductive rods, attached to the bodies of these vehicles, have been used to drain static charges by means of friction with the surrounding medium (in the case of automobiles) or by means of Corona discharge (in the case of aircrafts).

These approaches, apart from being dangerous when used in automobiles (friction with asphalt produce sparks and may result in a fire), are also in¬ effective because the materials used for highway construction (asphalt or

cement) .are excellent insulators and require a long period of time for leaking the charge to earth.

In the case of aircrafts, Corona discharge is also ineffective since a consider¬ ably high potential on the body of the aircraft must be reached before the Corona effect is initiated.

Apart from transportation-related applications, a number of systems have been devised in the art to preclude static electricity build-up. U.S. Patent 3,634,726 issued Jan. 11, 1972 to Pierre Jay describes an appara¬ tus for removing static electricity from plastic films. The apparatus requires a ground connection.

U.S. Patent 4,523,252 issued Jun. 11, 1985 to J.O. Wallen describes a device for eliminating static electricity on machines and charged materials. The device depends on the operation of a tunnel diode, and must be connected between charging media and the machine part.

U.S. Patent 4,180,698 issued Dec. 25, 1979 to R.B. Carpenter describes a system for protection of objects located on the surface of the earth from the effects of atmospherics.

U.S. Patent 4,766,903 issued Aug. 30, 1988 to Herbert Esper describes a device for detecting and removing static charges from the human body. The device, however, requires a ground connection.

U.S. Patents 4,849,851 and 4,862,315 issued Jul. 18, 1989 and Aug. 29, 1989, respectively, to R.J. Cubbison, Jr. describe a static discharge device which may be contained in a wrist-mountable unit. The device uses a either a high electric field or a radioactive source to ionize the air molecules. The Cubbison, Jr. patents suffer various deficiencies in use, and will not achieve their intended purpose of effectively removing charges from the body, mainly due to the following reason: it is claimed that the electric field of the body can separate the closely-spaced positive and negative ions of the air. In fact, the electric field at any particular point on the skin is negligibly small, because the charge is distributed all over the body, and is usually bound by a strong electric field to earth. For that reason, the claimed effect is very slow in nature.

The "Supersonic Nozzle", which was invented in 1971 by B.R. Whewell in the U.K., has been described as achieving levels of efficiency of up to 90% in

static charge removal.

(The Supersonic Nozzle is fully described in an article titled A study of charged condensation droplets from an aircraft discharger, by B.R. Whewell and B. Makin, Proceedings of the third conference on static electrification,

London, 1971).

Despite this high efficiency, the Nozzle suffered two serious defects:

1. High efficiency can be obtained only if the air is flowing through the Nozzle at a velocity of Mach 2 (twice the speed of sound). This deficiency rendered the device non-practical for aircrafts flying at subsonic speeds.

2. Since the Supersonic Nozzle was based on Corona discharge between a pair of oppositely charged electrodes; the Nozzle, in fact, tends to charge the body of the aircraft either positively or negatively (depending on the needle's polarity), even if the body is completely neutral. Such effect is undesirable.

Accordingly, it would be a significant advance in the art to provide a ground- free device for removing static electrical charge from transportation vehicles, and from other conductive or semi-conductive bodies, including the human body, which is characterized by a high static charge removal efficiency at rel¬ atively low speeds of airflow, and which overcomes the deficiencies existent in the prior art. This is the object of the present invention.

Summary of the invention

In a broad aspect, the present invention relates to a ground-free device for removing static electrical charge from conductive and semi-conductive bod¬ ies, comprising:

a storage capacitor comprising first and second terminals;

a conductive body contact means for establishing electrical contact with the body, and connected to the storage capacitor at a first terminal thereof;

a needle electrode for partial static discharge, connected to the second terminal of the storage capacitor;

a voltage limiting means, mounted across the storage capacitor;

a Voltage-controlled Oscillator, comprising input and output termi¬ nals, with the input terminals being connected to the terminals of the storage capacitor;

an air ionization mechanism, comprising a pre-ionizer and needle dis¬ chargers, and featuring first and second terminals, with the first terminal being connected to the body contact means of the device;

a circuit for imposing on the air ionization mechanism a voltage which is sufficient for effecting ionization of the air therein but is below the break¬ down voltage of the air, and featuring input and output terminals, with the input terminals being connected to the output terminals of the Voltage- controlled Oscillator, and with the output terminals being connected to the terminals of the air ionization mechanism.

Other aspects and features of the invention will be more fully apparent from the ensuing disclosure and appended claims.

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Brief Description of the Drawings

Figure 1 is a schematic representation illustrating the general principle of capacitive removal of charges from a conductive or semi-conductive body, as employed in the present invention.

Figure 2 is a schematic representation of a complete device for removal of charges with an efficiency of 95%-100%, according to the pre.sent invention.

Figure 3(a) is an end elevation view, and Figure 3(b) is a corresponding side elevation cross-sectional view, of an air ionization mechanism, comprising a pre-ionizer and needle discharges arranged to inject charges into the stream of ionized air.

Figure 4 is a perspective view showing a helicopter-mountable static charge removal device, according to one embodiment of the invention.

SUBSTITUTE SHEET

Detailed description of the invention, and preferred embodiments thereof

The present invention is based on the fundamental concept that a con¬ ductive or semi-conductive body, such as a human body or a transportation vehicle, can be discharged by touching one side (i.e., plate or terminal) of a capacitor, and that such capacitor may be relatively small in size. (This concept, although fundamental, has not been previously recognized by those in the field, or applied in any useful device for removing static charges from a body).

In most instances where friction occurs during bodily movements, the body is charged positively because the human body is more conductive than most other materials (clothing, upholstery, carpet, etc). When the body is travel¬ ing in a car, however, the tires can be charged either positively or negatively, depending on the road conditions. For aircrafts, the charge is mostly nega¬ tive because air precedes aluminum in the triboelectric series.

In many cases, the potential on the human body is on the order of 20KV volts. For helicopters, the potential can reach 200KV, as mentioned previ¬ ously.

Despite this high potential, the amount of charge on the human body and on transportation vehicles is generally small. This is due to the capacitance between the body and the earth being very small. Typically, the capacitance of the human body is 150pF (150 x 10 ^-12 Farads). A helicopter hovering a few feet above the earth has a capacitance of about lnf (10 ^~9 ) Farads).

The voltage (Vj, the charge (Q) and the capacitance (C) are related by the following equation:

Q = CV

For a potential of 200KV on a helicopter's body, and a capacitance of lnf, the charge is

Q = 10 ^"9 x 200, 000 = 2 x 10 ^-4 Coulombs.

To appreciate the magnitude of this charge, viz., as being a small value, a capacitor holding the same charge, at a potential of 4 volts, would have a capacitance of

2 x 10 ^~4 C = = 50 x 10 ^-6 Farads = 50 μF.

which is a relatively small capacitor.

Figure 1 illustrates the basic concept underlying the present invention. This figure shows a static charge removal device 100 comprising a capacitor 102 which is connected to a body contact member 104. The body contact mem¬ ber in turn is in contact with the body 106.

Static discharge occurs as a result of the regenerative induction inside the capacitor: assuming that the body is initially positively charged; few posi¬ tive charges from the body create an electric field which ionizes some of the air molecules and attracts several electrons to the negative electrode of the capacitor (as mentioned earlier, this effect is negligibly small at the begin¬ ning). Now, by opposite induction, many more positive charges are collected at the positive electrode. The process is self-amplified rapidly, and the air is positively charged.

The role of the regenerative induction is seen by understanding that the static charge is initially distributed all over the body. At any smooth point on the human body or the outer surface of an aircraft's body, the electric field is negligibly small. As the induction process multiplies the charges on both sides of the capacitor, the positive/negative field of the body grows and ionizes an increasing number of air molecules. It is worth mentioning, how¬ ever, that induction does not allow the voltage on the capacitor to grow in an unlimited manner. As the rate of flow of charges from the body decreases, the process slows, and finally stops when the body becomes neutral.

Unfortunately, the discharge efficiency of a capacitor was found to be de¬ pendent on the strength of the electric field between the body and the earth. The electric field strength, in turn, depends on the distance between the body and the earth.

Experiments showed that a discharge efficiency of almost 100% can be reached for isolated bodies (such as the body of an aircraft flying at a high altitude), with a capacitor being mounted on the body, and with the discharge terminal of the capacitor being equipped with a needle electrode (a needle electrode activates the Corona effect, which enhances the discharge efficiency of the ca¬ pacitor). For bodies relatively close to earth, however, such as human bodies, vehicles, or a hovering helicopter, the discharge efficiency of a capacitor is poor. (Typically, a residual voltage of about 3000 volts on the human body was observed in the experiments).

Nevertheless, the fundamental concept of capacitor discharge serves as the basis for the present invention.

Figure 2 shows a complete device 200 for electrostatic discharge. The device is mountable on a body 202 which is susceptible to static charge build-up, such body may be relatively close to earth.

In the device 200, a capacitor 204 serves as an accurate means for estimat¬ ing the charge on the body 202. As shown in the figure, partial discharge occurs by means of the capacitor 204, and through a needle electrode 206 exposed to free air. The voltage build-up across the terminals of capacitor 204 is fed to a Voltage-controlled Oscillator (VCO) 208, such voltage being limited by two Zener diodes 210, as shown. The output of the VCO triggers a high-voltage generating circuit, comprising two transistors 212 and 214, and a pulse transformer 216. The high-voltage generating circuit is of a known type (a transistor blocking oscillator), being described more fully in Pulse, digital and switching waveforms, by Millman and Taub, McGraw Hill, 1984, page 605. The high-voltage circuit, in turn, activates an air ionization mech¬ anism 218, comprising a hollow cylindrical electrode 220, a solid electrode 222, and needle electrodes 224.

A detailed, cross-sectional view of the air ionization mechanism is shown in Fig.3.

The principle of operation of the device 200 is as follows: as partial dis¬ charge from body 202 occurs through the capacitor 204, a voltage builds up on the capacitor that is proportional to the amount of charge on the body (the capacitor 204 is suitably in the range of O.lnF to O.lμF). Such voltage difference, when fed to the Voltage-controlled Oscillator 208 (or eqivalently, a Voltage-to-Frequency Converter) results in pulses at the output of the os-

cillator, the frequency of such pulses being dependent on the input voltage difference, and hence on the amount of static charge on body 202 (the VCO must preferably be capable of generating pulses of a frequency in the range of 0-1 KHz). As a result, high voltage pulses are applied to the air ionization mechanism 218, with a frequency being proportional to the amount of static charge on the body 202.

The device 200, then, performs the function of ionizing the air to a degree that is proportional to the amount of charge on the body.

The air ionization mechanism 218 is the key part of the device 200. The mechanism 218, more fully apparent in Fig. 3, comprises a smooth inner electrode 322 and a hollow cylindrical electrode 320, from which a group of needle electrodes 324 emerge and surround the narrow end of the inner electrode, as shown.

Such an arrangement serves the purpose of overcoming the first deficiency of the Supersonic Nozzle, this deficiency being the requirement for a speed of airflow of about Mach 2. As shown in Figs. 3(a) and 3(b), the arrangement practically consists of two different components: the first component, 326, is a "pre-ionizer", which is effectively an air capacitor for performing the function of pre-ionizing the air (in the direction of flow indicated by arrow A in Fig. 3(b) prior to its flow into the second component 328. The component 328 is a "needle discharger" for injecting charges of single polarity into the stream of airflow.

The device 200, therefore, can only withdraw a single type of charge from the body 202 (this can be further seen by observing that the Voltage-controlled Oscillator 208 can only respond to an input voltage of single polarity). As a solution to this limitation, the body 202 must be equipped with two units of the type shown in Fig. 2; one for positive charge removal and the other for negative charge removal.

The pre-ionizer 326 serves the purpose of achieving a high efficiency of discharge at relatively low speeds of airflow; as experiments showed that a high discharge efficiency can be obtained from a needle electrode when air is pre-ionized, thus providing a higher space-charge density at a lower speed of airflow.

For aircraft applications, it is desirable to obtain a high discharge current,

SUBSTITUTE SHEET

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e.g., on the order of 1 mA (10" Amperes). Such discharge current can be calculated form the following equation:

I = p x Flow Rate

where p is the space-charge density at the outlet of the air capacitor 326 (in Coulombs/m-^) and here the flow rate is measured in m^/sec. The amount of space-charge density inside a uniform-field air capacitor can be simply obtained by applying Gauss' law inside the capacitor, Le.,

Flux density (D) x Area(A) = Q = p x Volume(V)

where Q is the total amount of unipolar charge inside the air capacitor, V is the volume of air inside the capacitor, and A is the area normal to the electric flux lines. Therefore,

D A e _a E

V gap width(d)

where € ₀ is the permittivity of free space, E is the electric field intensity, and d is the width of the air gap inside the capacitor.

Given that E is close to 3000 volte/mm and taking an air gap of 0.5 millimeters, the space-charge density can be calculated to be about 0.0531 Coulombs/m*- (it should be noted that the breakdown electric field intensity for air is 3000 volts/mm. The device must be operated with a voltage that is below the breakdown point of the air. For example, given an air gap of 0.5 mm, the high voltage circuit must supply a voltage that is less than 1500 volts).

The flow rate is equal to the product of the inlet area of the capacitor in Figure 3(b) and the speed of the aircraft. Therefore, with the space-charge density being constant, higher discharge current can be obtained by incre.asing either the inlet area of the air capacitor or the cruising speed of the aircraft. As an example, if a discharge current of 1mA is desired for an aircraft flying at a speed of 300 miles/hour, and given that p = 0.0531 Coulombs/m**- ¹ , the inlet area, given by

A \

Inlet Area = ^• p x Speed

can be bound to be about 140 mm^, i.e., for a cylindrical air capacitor such as the capacitor 326 shown in Figure 3(b), the inside diameter of the outer cylinder 320 should be approximately 0.5 inches, which allows for a compact device to be constructed and mounted on the body of the aircraft.

For helicopter applications, it should be noted that the speed of airflow is the highest at the tips of the main rotor. This location is suitable for mounting the device 200, since the device is very light in weight and very compact in form. Such an arrangement is shown in Fig. 4.

A helicopter 402 having a main rotor composed of two blades 404 and 406 carries a device for positive static discharge 408 and a device for negative static discharge 410, such devices being mounted on the rotor tips, as shown. The detailed view of the device 408 shows a housing 410 for the electronic components and the power source of the device, connected to the air ionization mechanism 418 comprising air capacitor and needle dischargers.

The devices 408 and 410 .are constructed and arranged to be continuously supplied from an independent power source. As an example, a typical 500 mAh battery has a lifetime of one year inside such a device. However, operation from the main power source of the vehicle is possible with proper isolation techniques.

A point of interest, when the application is considered for helicopters, is that rotor blades are usually constructed of fiberglass that is cast around a steel member. Since fiberglass is a good insulator, the device must be connected directly to the steel member. Alternatively, the rotor blades can be painted with a layer of conductive paint to establish a connection of low resistance between the device and the helicopter's body. As a still further alternative, the devices 408 and 410 can be mounted on the tail to receive a stream of air from the tail rotor, or elsewhere on the body of the helicopter to be exposed to the downwash of the main rotor.

It will be apparent from the foregoing that the static charge removal device

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of the present invention is a ground-free device which requires no connections or couplings to true earth or to large metallic charge-dissipation structures. Further, it will be recognized that the device of the invention may be compactly configured in any of various conformations so as to be body-mountable in character.

The device of the invention achieves a high degree of removal of charges from conductive and semi-conductive bodies (and in particular, transported vehicles) at relatively low speeds of airflow.

To overcome the deficiencies of prior systems, the device depends in its operation on a

Voltage-controlled Oscillator which activates the device only when static charge is detected on the body, and in a manner that is proportional to the amount of such charge.

One embodiment of the invention, which comprises a discharge capacitor equipped with a needle electrode, is particul.arry use for the human body to reduce the static build-up to voltage levels below 3000 volts on the body. Such an application is important for individuals who become in contact with explosive gases or materials, as for example, medical personnel, workers in munition plants, etc.

While the devices of Figures 1, 2, 3(a), 3(b), and 4 have been illustratively described hereinabove with reference to specific voltage, capacitance and frequency values, as well as specific components, it will be recognized that the device may be variously configured, utilizing other parametric values of voltage, capacitance, frequency, etc., within the skill of the art, as well as other components, as for example a Current-to-Frequency converter may be used instead of a Voltage-to-Frequency converter; in plus, different types of high voltage generating circuits can be used in the device.

Accordingly, while the invention has been described with reference to specific aspects, features, and embodiments, it will be appreciated that various modifications, alternatives, and other embodiments are possible within the broad scope of the invention, .and the invention therefore is intended to encompass all such modifications, alternatives, and other embodiments, within its scope.

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Best Mode for Carrying out the Invention

The best mode for carrying out the invention is to construct and ar¬ range the device as an independent unit, powered from an independent power source (battery, solar cells, etc.), and mounted outside the body of a vehicle to allow air circulation therethrough.

The body contact means of the device should serve as a mechanical fix¬ ture for mounting the device on the body, and, in the same time, as a means for opposite static charges to flow into the body.

Industrial Applicability of the Invention

Although the invention has been described hereinabove with reference to the removal of static charge build-up on transportation vehicles, it will be recognized that the invention is broadly useful for the removal of charges from any conductive or semi-conductive body in general, including, but not limited to: the human body, industrial machines, semi-conductive plastics, etc.

One embodiment of the invention, which comprises a discharge capacitor equipped with a needle electrode, is particularly useful for the human body to reduce the static build-up to voltage levels below 3000 volts on the body. Such an application is important for individuals who become in contact with explosive gases or materials, as for example, medical personnel, workers in munition plants, etc.