US3787707A

US3787707A - Spark detector apparatus and method

Info

Publication number: US3787707A
Application number: US00360896A
Authority: US
Inventors: P Gregg
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1973-05-16
Filing date: 1973-05-16
Publication date: 1974-01-22
Anticipated expiration: 1991-01-22

Abstract

A spark detector for electrostatic coating apparatus which distinguishes disruptive discharges in the electrostatic system from a corona discharge or random sparking of other electrical equipment. A frequency selective radio wave receiver is connected with an envelope detector, the output of which is analyzed to detect the presence of repetitive discharges.

Description

United. States Patent 1 1 1111 3,787,707

Gregg 7 Jan. 22, 1974 [5 SPARK DETECTOR APPARATUS AND 2,767,359 10/1956 Larsen et a1. 317/3 METHOD 3,217,207 11/1965 Webb 317/51 3,586,908 6/1971 Vosteen 317/4 Paul S. Gregg, Indianapolis, Ind.

Ransburg Electro-Coacting Corp., Indianapolis, Ind.

Filed: May 16, 1973 Appl. NO.: 360,896

Related U.S. Application Data Continuation of Ser. No. 77,299, Oct. 1, abandoned.

Inventor:

Assignee:

U.S. Cl 317/9 R, 317/3, 317/51 Int. Cl. 02h 3/14 Field of Search 317/3, 4, 51, 262 A, 9 R

. References Cited UNITED STATES PATENTS 5/1950 Ransburg et a1. 317/3 X HIGH VOLTAGE Primary Examiner-James D. Trammell Attorney, Agent, or Firm-Hofgren, Wegner, Allen, Stellman & McCord [57] ABSTRACT A spark detector for electrostatic coating apparatus which distinguishes disruptive discharges in the electrostatic system from a corona discharge or random sparking of other electrical equipment. A frequency selective radio wave receiver is connected with an envelope detector, the output of which is analyzed to detect the presence of repetitive discharges.

7 Claims, 5 Drawing Figures PATENTED JAN 2 21974 saw 2 BF 3 PAIENTEB JAN 2 2 I974 sum 3 III 3 IIIIIIIIIIIII womnom I M .I I I I I I9: mo m zou IIIII IIIII 6523 :2: I I I T I II I 2%;

H VEEW '1 SPARK DETECTOR APPARATUSAND METHOD I This is a continuation of application Ser. No. 77,299 filed Oct. l, 1970 now abandoned.

This invention relates to a detector for sparks which may occur in connection with the operation of electrostatic apparatus and particularly an electrostatic'coab ing apparatus. In electrostatic coating it is common to operate a device for dispersing and charging the coating material at a potential of the order of 40,000 volts or higher. Where the coating material is a paint having a volatile solvent, the danger of an explosion or fire from sparking is quite serious. Electrostatic coating apparatus has been designed which, as a result of its physical and electrical characteristics, is incapable of causing objectionable sparking even whenmoved very close to a grounded conductor. Details of the design of such an apparatus are disclosed in Juvinall et al. US. Pat. No. 3,048,498. Such devices have greatly reduced, but not completely eliminated, the danger from sparking in an electrostatic coating operation.

For example, in a typical coating system articles to be coated are transported through a coating zone by a mechanical conveyor. The conveyor is operated at ground potential and the articles being coated are carried from the conveyor on hangers or supports of conductive material so that thearticles themselves are at ground potential. The coating device includes an electrically charged electrode, preferably at a negative potential with respect to ground. If the article support or hanger should become so coated with paint that the article is electrically insulated from the hanger, an electrostatic charge can accumulate on the article. If the charge reaches sufficient magnitude, a spark can occur from the article to the hanger, providing a path for discharge of the energy stored on the article. Such sparking may occur repetitively at a rate depending on the rate of accumulation of charge on the article, the capacitance of the articlewith respect to ground and the breakdown voltage of the electrical insulation between the article and the hanger. A charge can accumulate on other isolated objects in the coating zone and sparks can occur from such objects to nearby grounded objects.

The sparking may cause a fire or explosion if it occurs in a flammable atmosphere. Furthermore, many of the benefits of electrostatic coating are lost if the article is not grounded to terminate the electrostatic field.

The terms disruptive discharge and spark are used synonymously herein.

One feature of the invention is the provision in an electrostatic coating system of a means for detecting the occurrence of a disruptive discharge in the electrostatic field and a means responsive to the occurrence of a discharge for disabling the high voltage source which energizes the coating device. More specifically, the detecting means includes means for distinguishing the disruptive discharge from either a corona discharge or from a random discharge within the sensing field.

Another feature of the invention is that the detector includes an antenna, a tuned radio wave receiver having a pass band below the frequency of corona discharge and a detector. A further feature is that the receiver includes an envelope detector having a time constant longer than the period of the corona discharge.

! Yet anotherfeature of the invention is that the detector has a means connected therewith for responding to plural successive discharges, to distinguish from raniidhi discharges which may occur within the sensing field, generally from sources other than the electrostatic coating apparatus.

Another feature of the invention is a method of inspecting a conductive article to insure proper grounding, prior to the coating operation, without contacting the article. The article is placed on a grounded support which is subject to the unintended accumulation of coating material which will undesirably isolate the article from ground. Prior to moving the support and article into a coating zone, the article is subjected to an electrostatic ionizing field. An ungrounded article will acquire a charge which may spark to a grounded object. Detection of the spark indicates an unsatisfactory ground and actuates an alarm or interrupts the electrostatic coating operation.

Further features and advantages of the invention will readily be apparent from the following specification and from the drawings, in which:

FIG. 1 is a diagrammatic illustration of an electrostatic coating system;

FIG. 2 is a schematic diagram of an equivalent circuit illustrating the problem;

FIG. 3 is a schematic diagram of a preferred embodiment of the detector circuit;

.FIG. 4 is a representative wave shape of the output of the envelope detector of the radio'wave receiver resulting from the occurrence of successive disruptive discharges; and

FIG. 5 is a diagrammatic illustration of an electrostatic coating system in which articles are moved by a conveyor through the coating zone, with a spark detector ahead of the coating zone.

The invention is illustrated and described as incorporated in an electrostatic printing system. It is particularly suited for use in this environment as the problem of isolation of the articles being coated from ground is particularly prevalent in a painting system where the article supports may become covered with paint and interrupt the desired ground connection. Furthermore, the danger of an explosion or fire as a result of a spark discharge is particularly great in a painting system which frequently utilizes flammable and sometimes very volatile coating materials. However, certain aspects of the invention have utility in detecting the occurrence of disruptive discharges or sparking in other electrostatic systems, as for coating with powder or flock material or for providing an electrostatic treatment, for example.

In FIG. 1 of the drawings an apparatus 10 is provided for coating an article 11 transported through a coating zone by a conveyor 12 connected with a reference potential or ground 13. The article 11 is supported from the conveyor by a conductive article support or hanger 14.

The coating apparatus 10 is here shown as a gun 15 having a handle 16 for manipulation by an operator. The gun is connected with a source of coating material through a flexible hose 1'] and to a high voltage power supply 18 through a cable 19. The gun 15 has a rotating bell 20 which is electrically connected with the high voltage source R8 and to which the coating material is delivered. As the bell rotates the coating material is distributed across its surface and is electrostatically atomized from its periphery. The particles of coating material are charged and are attracted to the grounded article II by the electrostatic field. Further details of the construction and operation'of discharge device may be found in Juvinall et al. U.S. Pat. No. 3,048,498. The system may also utilize other types of coating apparatus, as, for example, a gun having air atomization (Juviriall et al. U.S. Pat. No. 3,367,578) or a hydrostatic electrostatic device (Juvinall U.S. Pat. No. 3,169,883).

While article 11 remains electrically grounded, and so long as coating apparatus 10 is of a design in accordance with the Juvinall et al.- U.S. Pat. No. 3,048,498, dangerous sparking is not a problem. However, if article 11 should become isolated from ground 13, as by a coating of paint on hanger 14 which insulates the article from the grounded conveyor, the system has an equivalent circuit of the character illustrated in FIG. 2. The high voltagepower supply 18 is represented by a battery having itspositive terminal grounded and its negative terminal connected through impedance Z1 with article] 1. Impedance Z1 is made up of a combination of the series resistance in the discharge device 10 and the leakage resistance" of the space between the discharge device and the article. The leakage resistance of the space is determined by the rate at which charged atmospheric and paint particles carry charge to the article. This impedance may have a value as low as 300 megohms, for example, depending on the resistance present in .the discharge device, the physical spacing between the discharge device and the article, and other physical constants of the system. The article ll appears as one plate of a capacitor, with the other plate being grounded objects such as the hanger 14 or object 23, in its vicinity. Article 11 accumulates an electric charge due to the flow of current through ZR from high voltage power supply 18; and the voltage between the article and ground 13 begins to approach the terminal voltage of the power supply at a rate determined by Z! and the capacitance of the articles. When the voltage on the article reaches the breakdown voltage of the electrical insulation on the article support or hanger 14, there will be a disruptive discharge between plate 11 and the grounded article support or hanger l4. Ionization of the surrounding atmosphere will provide a low impedance Z2 indicated by broken lines, through which the energy stored on article 11 is dissipated to ground.

The voltage builds up on article 11 to the point of breakdown and discharges therefrom at rates determined by the breakdown voltage and the physical and electrical characteristics of the system as set forth above. This causes a series of disruptive discharges or sparks. The repetitive nature of the discharge provides a basis for distinguishing sparks due to the high voltage from random sparks which may be caused by switches, for example.

The circuit of the radio wave receiver and the spark discriminator is shown in FIG. 3. A tuned antenna30 is connected through a coaxial cable 31 with the tuned input circuit 32 of a first amplifier stage 33. Second and third tuned amplifier stages 34 and 35 are connected in cascade. Each stage of RF amplification has essentially the same frequency characteristics with a center frequency of the order of 100 Khz and a pass band of the order of 30 Khz, well below the portion of the spectrum (above 1 Mhz) where the corona energy is concen trated. The specific circuitry of the tuned amplifier stages is not critical and will not be discussed in detail.

A suitable B+ source is provided.

Antenna 30 can be mounted on the barrel of the I coating apparatus 110, or otherwise mounted in the spraying area to detect the unwanted sparks.

5 The output of the third amplifier stage 35 is connected through coupling capacitor 36, 0.01 pf, and across resistor 37, I00 Kohms, to a diode 38 which, together with filter 39, including resistor 40, 250 Kohms, shunted by a capacitor 41, 0.0002 pf, forms an envelope detector having an RC time constantof the order of 50 microseconds, substantially greater than the period of the corona discharge. The detected signal has a form illustrated in FIG. 4 with a negative spike 44 representingthe initial spark discharge followed by a damped oscillatory wave 45 caused by ringing of the tuned amplifier circuits. The detector output is inverted by amplifier 46 so that the pulses representing the'spark are positive. These pulses are utilized to actuate a spark discriminator circuit 47, which responds to repetitive positive signals but not to signals which occur at random.

Diode 48 clips the output of amplifier 44 so that only positive signal components appear at the input of discriminator 47. Furthermore, the positive pulses which occur across cathode resistor 49 are differentiated by series capacitor 50 and shunt resistor 51 and applied to discriminator input 52.

The spark discriminator includes a monostable multivibrator 55 having a first triode section 56 which is normally nonconducting and a second triode section 57 which normally conducts. Connected with the plate of section 57 is the coil of a relay 58 which has a normally closed contact 58a connected in series with the cath- 5 ode of control gate triode 59. The term normally closed is usedwith reference to the position of the contacts in the unenergized condition of relay 58. As tube 57 conducts in the absence of a spark when the circuit is operative, relay 58 is energized. and contact 58a is open, preventing conduction of gate tube 59. A control tube 60 has a positive bias applied to its control grid through resistor 61. Connected in its cathode circuit are normally open contacts 62a, associated with relay 62 in the plate circuit of the control tube. A momentary contact reset switch 63 is connected across relay contacts 62a. A second set of normally open contacts 62b is connected in series with the energizing circuit of high voltage power supply 18.

When the system is first turned on, the operator closes switch 63, completing the cathode circuit of control tube 60. The tube conducts by virtue of the positive bias potential applied to the control grid, energizing relay 62, closing contacts 62a and 62b. Contacts 62a serve to complete a holding circuit for the control tube while contacts 62b energize the high voltage power supply. 118.

On occurrence of a spark, the positive pulse at the input 52 of spark discriminator 47 drives the first section 56 of multivibrator 55 into conduction, cutting off the second section 57. This deenergizes relay 58 and contacts 58a in the cathode of control tube gate 59 close. The deenergization of relay S8 is delayed by shunt connected capacitor 65 so that control gate 59 is not enabled during the occurrence of the spark which triggers the multivibrator. A delay of 0.03 seconds has been found satisfactory. After this delay, relay 58 is deenergized, closing contacts 58a and permitting conduction of gate tube 59.

The control grid of tube 59 is returned to ground through a resistive network while the cathode is at a positive potential by virtue of the current of control tube 60 flowing through common cathode resistor 66. Accordingly, gate 59 does not conduct. This condition of the circuit is maintained fora period of the order of one second by the timing relationship of monostable multivibrator 55 which maintains tube 47 in a nonconducting state. If during this period a second spark occurs, the positive pulse at the input 52 of the spark discriminator S7 is applied to the grid of gate tube 59 through the resistive network and causes gate tube 59 to conduct, in turn cutting off control tube 60 and deenergizing relay 62. This opens

contacts

620 and 62b, deenergizing the high voltage power supply and shutting down the electrostatic system. A warning signal can also be energized.

The operator canfthen rectify the problem, as by cleaning the article hanger which is at fault. The high voltage system is returned to operation by closing reset switch 63.

Preferably, the detector is used as part of an automatic inspection system to detect article supports that have collected a hazardous accumulation of material. In accordance with this aspect of the invention, articles to be coated are placed on the article supports outside of the coating zone where flammable vapors can exist. The articles on their supports are conveyed past a high voltage ionizing electrode and are subjected to an ionizing electrostatic field. If an article or a support is not adequately grounded, the accumulated charge will reach sufficient voltage to spark over to ground. This sparking is sensed by the detector and a warning is given.

automatic coating zone 74. Following the coating operation, the conveyor may move the articles through a drying oven (not shown). In the coating zone, automatic electrostatic spraying units are provided, arranged and controlled to apply a coating on the article being processed. They may, for example, incorporate plural rotating bell discharge units 75, such as those shown in .luvinall U.S. Pat. No. 2,839,425, with which a liquid coating material is electrostatically atomized, or may utilize guns for pneumatically .or hydrostatically atomizing the coating material. Each of the discharge devices has an electrode means connected with high voltage source 77 to establish the high intensity electrostatic field which charges the coating material particles and causes them to deposit on the grounded articles. In case of apparatus utilizing rotating bells or discs, the electrostatic field also affects the dispersion of the coating material into particles. The coating zone 74 may be enclosed within a booth 78, if desired.

Each of the articles 70 is tested as it passes through the inspection zone to determine whether it is adequately grounded to travel through the electrostatic coating zone without acquiring a static charge which could cause sparking. Many of the coating materials used in industrialcoating operations include volatile Y solvents which have flammable vapors. Detection of an ungrounded article which might cause sparking, before subjecting the article to the coating operation, reduces the danger of a fire or explosion.

In. the inspection zone, before entry into the coating zone, each of the articles is subjected to an ionizing electrostatic field. A high voltage source is connected with a charging electrode 81 mounted on a stand 82 located adjacent the path of travel of the articles. The field from electrode 8l is preferably of sufficient intensity to induce a charge on an ungrounded article which is great enough, if discharged disruptively (e.g., a spark) with sufficient energy to ignite the vapors present in the coating zone. The field should be at least as strong as that to which the articles are subjected in the coating zone 74; and in many causes

high voltages sources

77 and 80 may be combined. Also mounted on stand 82 is an antenna 83 connected with spark detector 84. The antenna and spark detector are preferably of the character illustrated in FIG. 3. Antenna 83 is preferably located in a position closely adjacent the interengaging surfaces of the article 70 and hanger 71 to be as close as possible to the point at which sparking is most likely to occur.

When a spark is detected, an alarm 85 is actuated which may provide either a visual or audible indication to an operator. The output of spark detector 84 is also connected with spray unit control 87 which acts to disable the automatic spray units and high voltage source 77. In addition, detection of a spark may be utilized to stop conveyor drive 88.

Where the coating material build-up on hanger 71 is not completely dry at the time of inspection, the sparking which can occur as a result of the test procedure may cause a fire. A fire extinguisher 90 located adjacent the inspection zone is actuated by the output of spark detector 84 to direct the fire extinguishing material toward the interengaging surfaces of the hanger and article.

I claim:

1. A detector for a disruptive discharge which may occur in a zone in which a high intensity electric field is established between a charged electrode and an electrical reference, said electrode being connected with one terminal of ahigh voltage DC power supply which has a reference terminal connected with said electrical reference, said zone being subject to corona discharge from said electrode, said detector comprising:

an antenna having a circuit returned to said electrical reference and characterized by an absence of a connection with the path of current flow through said high voltage power supply, said antenna receiving electromagnetic energy radiated from a disruptive discharge and from said corona;

a tuned amplifier connected with said antenna circuit and having a passband excluding the frequencies of corona discharge;

an envelope detector connected with said amplifier for detecting the occurrence of a disruptive discharge in said zone; and

utilization circuit means connected with said detector and responsive to detection of a disruptive discharge.

2. The detector of claim 1 in which said envelope detector has a time constant of the order of 50 microseconds.

3. The detector of claim 1 in which the passband of said tuned amplifier has a center frequency of the order of 100 kilohertz.- I

4. The detector of claim 1 in which said utilization circuit means includes a timing circuit for establishing a timing period, means responsive to detection of a first discharge for actuating said timing circuit and further circuit means responsive to detection of a second disruptive discharge occurring during said timing period for causing response of said utilization circuit.

5. The detector of claim 4 in which said timing circuit means includes a monostable multivibrator having a charge to said reference potential.

Claims

1. A detector for a disruptive discharge which may occur in a zone in which a high intensity electric field is established between a charged electrode and an electrical reference, said electrode being connected with one terminal of a high voltage DC power supply which has a reference terminal connected with said electrical reference, said zone being subject to corona discharge from said electrode, said detector comprising: an antenna having a circuit returned to said electrical reference and characterized by an absence of a connection with the path of current flow through said high voltage power supply, said antenna receiving electromagnetic energy radiated from a disruptive discharge and from said corona; a tuned amplifier connected with said antenna circuit and having a passband excluding the frequencies of corona discharge; an envelope detector connected with said amplifier for detecting the occurrence of a disruptive discharge in said zone; and utilization circuit means connected with said detector and responsive to detection of a disruptive discharge.

3. The detector of claim 1 in which the passband of said tuned amplifier has a center frequency of the order of 100 kilohertz.

5. The detector of claim 4 in which said timing circuit means includes a monostable multivibrator having a first condition and a second condition and a gate circuit responsive to said multivibrator.

6. The detector of claim 5 in which said multivibrator includes a relay having contacts in said gate circuit.

7. The detector of claim 1 wherein there is a conductive article in said zone, nominally maintained at said reference potential, but subject to undesired isolation from the reference potential and, when so isolated, acquiring an induced electrostatic charge which may discharge to said reference potential.