Apparatus for avoiding sparks in an electrostatic coating system

Claim 2

In an electrostatic coating system including a high voltage source having a high voltage output and a ground return; a conveyor for transporting articles to be coated; means to spray a coating material, said high voltage output and ground return of the high voltage source being connected to create electrostatic deposition of sprayed coating material on the articles to be coated; and means connected to the ground return of the high voltage source to sense the electric current between the high voltage source and ground and to turn off the high voltage source, the improvement comprising means to sense the electrical current returning from ground to the high voltage source and to develop a signal with a component related to an incipient grounding condition; means to split the developed signal into two channels; means forming two separate and independent signal channels and performing independent operations in each channel to develop different signal components at each channel output; means to combine the different signal components to develop the component relating to an incipient grounding condition; means to detect the level of the signal component corresponding to incipient grounding; means including a hermetically sealed enclosure housing a normally-closed high voltage switch connected between the high voltage output of the high voltage source and ground, and an electromagnetic coil to open said switch; and a control to operate said coil and high voltage source during electrostatic coating operations and to disconnect said coil and high voltage source in response to said means to detect the level of signal component corresponding to incipient grounding, said switch rapidly closing in the hermetically sealed space defined by the enclosure to ground the high voltage system.

Claim 3

The apparatus of claim 1, wherein the means to sense the electrical circuit returning from ground to the high voltage source and to develop the signal includes an operational amplifier connected to transform the signal developed across resistance connected from ground to the ground return of the high voltage source by the transfer function -HWo.sup.2 /(S.sup.2 +.alpha.WoS+Wo.sup.2), the means to split the developed signal into two channels includes a clock circuit to create a series of successive time intervals and two gated sampling circuits triggered alternatively in successive time intervals thereby, the means forming two separate and independent signal channels includes a holding circuit to retain the developed signal from one time interval and an independent inverting circuit to invert the developed signal in the successive time intervals, and the means to combine the retained signal and the inverted signal includes a summing circuit.

Claim 4

The apparatus of claim 1, wherein the means including a hermetically sealed enclosure housing the switch is insulated by oil at its outer surface and the high voltage source includes a multimegohm resistor.

Claim 5

In an electrostatic coating system including a high voltage supply having two terminals; a conveyor for transporting grounded articles to be coated; an electrostatic coating device connected with the one terminal of the high voltage supply to create electrostatic deposition of atomized coating material; and means connected between the other terminal of the high voltage supply and ground to sense the electric current between the high voltage supply and ground, and to disconnect the high voltage supply from its energizing source, the improvement in which the means connected between the above terminal and ground includes an operational network to provide a signal with attenuation of all electrical current components significantly above direct current, while amplifying direct currents; a clock to generate a sampling gate for the signal; two gated sampling circuits to detect the direct current level of the signal when gated by the clock, one of the gated sampling circuits being connected to a holding circuit for the detected signal for one cycle of the gating signal, and the other gated sampling circuit being connected to an inverting circuit to invert the signal from the other gated sampling circuit; a summer to determine the difference of the signal from the holding circuit and the inverted signal from the other gated sampling circuit; a level detector to generate an operative signal in the event the level exceeds a predetermined level which represents incipient grounding of the high voltage electrode; and a grounding device having a normally-closed switch connected between the one terminal of the high voltage supply and ground, and being held open by a control during normal operation of the electrostatic system and closed by the level-detector in the event of incipient grounding.

Claim 6

In an electrostatic coating system including a high voltage supply having two terminals; a conveyor for transporting grounded articles to be coated; an electrostatic coating device connected with the one terminal of the high voltage supply to create electrostatic deposition of atomized coating material; and means connected between the other terminal of the high voltage supply and ground to sense the electric current between the high voltage supply and ground, and to disconnect the high voltage supply from its energizing source, the improvement in which the means connected between the above terminal and ground includes an operational network to provide a signal with attenuation of all electrical current components significantly above direct current, while amplifying direct currents; a clock to generate a sampling gate for the signal; two gated sampling circuits to detect the direct current level of the signal when gated by the clock, one of the gated sampling circuits being connected to a holding circuit for the detected signal for one cycle of the gating signal, and the other gated sampling circuit being connected to an inverting circuit to invert the signal from the other gated sampling circuit; a summer to determine the difference of the signal from the holding circuit and the inverted signal from the other gated sampling circuit; a level detector to generate an operative signal in the event the level exceeds a predetermined level which represents incipient grounding of the high voltage electrode; and a grounding device having a normally-closed switch connected between the one terminal of the high voltage supply and ground, and being held open by a control during normal operation of the electrostatic system and closed by the level-detector in the event of incipient grounding.

Claim 7

A system of claim 4, wherein the low pass operational network includes an active filter designed to transform the voltage input into an output by the transfer function -HWo.sup.2 /S.sup.2 +.alpha.WoS+Wo.sup.2 in which H equals 10, Wo equals 10 cycles per second, and .alpha. equals .sqroot.2 for maximally flat butterworth response.

Claim 8

A system of claim 5 wherein the clock circuit provides gating pulses with a time interval of 120th of a second to apply to the gated sampling circuits.

Claim 9

A system of claim 4 wherein the first holding circuit includes means to subtract a signal from its output proportional to the magnitude of the output signal during the time interval generated by the clock.

Claim 10

In an electrostatic coating system including a source with a high voltage output, an electrostatic coating device including means to spray a coating material, a high voltage electrode connected with the source of the high voltage output, said high voltage source being connected with ground to create electrostatic deposition of sprayed material on articles to be coated, a grounding device connected between the high voltage output and ground, and an electrical circuit actuated by the electric current flowing from ground to the high voltage source and adapted to disconnect the source, the improvement comprising a normally-closed spring switch housed within a hermetically sealed nonconductive enclosure forming said grounding device, said enclosure defining a space being adapted to insulate said contacts when open and carrying connections to the switch, a holding coil to open said switch, a high voltage control to actuate the holding coil and to open the normally-closed switch, said circuit including means to attenuate all electrical components of the electrical current flowing from ground to the high voltage source above substantially pure direct current, and to provide a substantially pure DC signal portion that corresponds to incipient grounding of the high voltage electrode and to detect such incipient grounding, and means to deactivate the holding coil of the grounding means and thereby ground the high voltage output upon detection of incipient grounding.

Claim 11

In an electrostatic coating system including a source with a high voltage output, an electrostatic coating device including means to spray a coating material, a high voltage electrode connected with the source of the high voltage output, said high voltage source being connected with ground to create electrostatic deposition of sprayed material on articles to be coated, a grounding device connected between the high voltage output and ground, and an electrical circuit actuated by the electric current flowing from ground to the high voltage source and adapted to disconnect the source, the improvement comprising a normally-closed spring switch housed within a hermetically sealed nonconductive enclosure forming said grounding device, said enclosure defining a space being adapted to insulate said contacts when open and carrying connections to the switch, a holding coil to open said switch, a high voltage control to actuate the holding coil and to open the normally-closed switch, said circuit including means to attenuate all electrical components of the electrical current flowing from ground to the high voltage source above substantially pure direct current, and to provide a substantially pure DC signal portion that corresponds to incipient grounding of the high voltage electrode and to detect such incipient grounding, and means to deactivate the holding coil of the grounding means and thereby ground the high voltage output upon detection of incipient grounding.

Claim 12

A high voltage electrostatic coating system comprising means to electrostatically deposit coating material on an article to be coated including means to spray a coating material, a charged high voltage electrode and a grounded electrode, said electrodes being free to move toward each other; means to supply high voltage to the charged high voltage electrode, said high voltage means being connected with ground to create electrostatic deposition of sprayed material on articles to be coated; means to ground the charged high voltage electrode; means to generate a signal including the current between the charged high voltage electrode and the grounded electrode; means to attenuate all electrical current components of said signal above substantially pure direct current and to provide the amplified DC portion of said signal; means to sample the amplified DC portion of said signal to provide samples from successive time intervals faster than the anticipated rate of approach of the grounded electrode to the high voltage electrode; means to compare the levels of the signals from successive time intervals; means to determine if the comparison of the signals from the successive time intervals indicate incipient grounding of the high voltage electrode; and means to actuate said means to ground the charged high voltage upon incipient grounding.

Claim 13

A high voltage electrostatic coating system comprising means to electrostatically deposit coating material on an article to be coated including means to spray a coating material, a charged high voltage electrode and a grounded electrode, said electrodes being free to move toward each other; means to supply high voltage to the charged high voltage electrode, said high voltage means being connected with ground to create electrostatic deposition of sprayed material on articles to be coated; means to ground the charged high voltage electrode; means to generate a signal including the current between the charged high voltage electrode and the grounded electrode; means to attenuate all electrical current components of said signal above substantially pure direct current and to provide the amplified DC portion of said signal; means to sample the amplified DC portion of said signal to provide samples from successive time intervals faster than the anticipated rate of approach of the grounded electrode to the high voltage electrode; means to compare the levels of the signals from successive time intervals; means to determine if the comparison of the signals from the successive time intervals indicate incipient grounding of the high voltage electrode; and means to actuate said means to ground the charged high voltage upon incipient grounding.