Electrostatic coater and electrostatic coating method

Abstract

A charge remaining in an electrostatic coater when power supply to the electrostatic coater is stopped is neutralized at an early stage. A rotary atomizing head 102 receives a high voltage of negative polarity from a cascade 104. An electrostatic coater 100 further includes a second high-voltage generator 110 that generates a high voltage of positive polarity. The second high-voltage generator 110 is composed of a Cockcroft-Walton circuit. The Cockcroft-Walton circuit is composed of diodes and capacitors. A high voltage of the electrostatic coater 100 is controlled by a controller 10. Immediately after running of the electrostatic coater 100 is stopped by stopping power supply to the cascade 104, power is supplied to the second high-voltage generator 110. The high voltage of positive polarity generated by the second high-voltage generator 110 is supplied to the rotary atomizing head 102 for a predetermined time period.

Claims

1. An electrostatic coater that charges an atomized paint to cause the atomized paint to electrostatically attach to a workpiece, the electrostatic coater comprising: a first high-voltage generator configured to generate a first high voltage for charging the atomized paint during operation in which the workpiece is coated by using the electrostatic coater; a second high-voltage generator configured to generate a second high voltage of reverse polarity of the first high voltage generated by the first high-voltage generator; a controller coupled to the first high-voltage generator and the second high-voltage generator wherein the controller is configured to monitor current flow between the workpiece and the controller to detect an abnormal current value, and wherein the controller is configured to control the second high-voltage generator to generate the second high voltage to reduce a charge on the electrostatic coater upon receiving power immediately after power to the first high-voltage generator is stopped in response to detection of the abnormal current value, wherein the charge is reduced by neutralizing the first high voltage with the second high voltage; and a rectifying device that prevents a current from flowing through the second high-voltage generator during electrostatic coating as the first high voltage generated by the first high-voltage generator is supplied to the electrostatic coater, wherein the rectifying device and the second high-voltage generator are connected in series.

2. The electrostatic coater of claim 1, wherein the controller includes a safety circuit that forcibly stops power to the first high-voltage generator when detecting the abnormal current value, and when the safety circuit is operated, power is supplied to the second high-voltage generator for a predetermined time period.

3. The electrostatic coater of claim 2, further comprising: a resistance interposed in a conducting wire configured to carry the second high voltage generated by the second high-voltage generator to a charged portion of the electrostatic coater.

4. The electrostatic coater of claim 3, wherein the resistance is composed of a semiconductor.

5. The electrostatic coater of claim 1, further comprising: a resistance interposed in a conducting wire configured to carry the second high voltage generated by the second high-voltage generator to a charged portion of the electrostatic coater.

6. The electrostatic coater of claim 5, wherein the resistance is composed of a semiconductor.

7. The electrostatic coater of claim 1, wherein the second high-voltage generator is configured to reduce the charge as a distance between the electrostatic coater and the workpiece is decreased.

8. The electrostatic coater of claim 1, wherein the controller supplies power to the second high-voltage generator for a predetermined time period and then stops power to the second high-voltage generator.

9. The electrostatic coater of claim 1, wherein the first high-voltage generator or the second high-voltage generator is a high tension generator.

10. The electrostatic coater of claim 1, comprising a first electrical pathway between the controller and the first high-voltage generator, wherein the first electrical pathway comprises the first high-voltage generator and a bleeder resistor, wherein the first high-voltage generator and the bleeder resistor are connected in series, and wherein the bleeder resistor is upstream from the first-high voltage generator.

11. The electrostatic coater of claim 10, comprising a second electrical pathway between the controller and the rectifying device, wherein the second electrical pathway comprises the rectifying device and the second high-voltage generator, and wherein the rectifying device is downstream from the second high-voltage generator.

12. An electrostatic coater, comprising: a first generator configured to provide a charge to a spray; and an ion generator, wherein the ion generator is configured to generate ions of reverse polarity from that of a high voltage generated by the first generator, the ion generator is arranged in an air passage configured to supply air to the electrostatic coater; and a controller coupled to the first generator and the ion generator, wherein the controller is configured to monitor current flow between a workpiece and the controller to detect an abnormal current value, and wherein the controller controls the ion generator to neutralize a charged state of the electrostatic coater upon receiving power in response to detection of the abnormal current value, and wherein the air ionized by the ion generator is supplied to the electrostatic coater immediately after power supply to the first generator is stopped.

13. The electrostatic coater of claim 12, wherein the first generator comprises a first high-voltage generator.

14. An electrostatic coater configured to charge an atomized paint to cause the atomized paint to electrostatically attach to a workpiece, the electrostatic coater comprising: a first high-voltage generator configured to generate a high voltage to charge the atomized paint during operation in which the workpiece is coated by using the electrostatic coater; a second high-voltage generator configured to generate a high voltage of reverse polarity of the high voltage generated by the first high-voltage generator; a controller coupled to the first high-voltage generator and the second high-voltage generator, wherein the controller is configured to monitor current flow between the workpiece and the controller to detect an abnormal current value, and wherein the controller is configured to control the second high-voltage generator to generate the high voltage for neutralizing a charged state of the electrostatic coater upon receiving power immediately after power to the first high-voltage generator is stopped in response to detection of the abnormal current value; a rectifying device configured to prevent a current from flowing through the second high-voltage generator during electrostatic coating as the first high-voltage generated by the first high-voltage generator is supplied to the electrostatic coater, wherein the rectifying device and the second high-voltage generator are connected in series; and a first electrical pathway between the controller and the first high-voltage generator, wherein the first electrical pathway comprises the first high-voltage generator and a bleeder resistor, wherein the first high-voltage generator and the bleeder resistor are connected in series, and wherein the bleeder resistor is upstream from the first-high voltage generator.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a diagram for explaining an outline of a coating robot to which an electrostatic coater of an embodiment is mounted, and an automobile coating booth in which the coating robot is installed.

(2) FIG. 2 shows a diagram for explaining an outline of an electrostatic coater of a first embodiment.

(3) FIG. 3 shows a flowchart for explaining one example of control of the electrostatic coater of the first embodiment.

(4) FIG. 4 shows a diagram for explaining an outline of an electrostatic coater of a modification of the first embodiment.

(5) FIG. 5 shows a diagram for explaining an outline of an electrostatic coater of a second embodiment.

(6) FIG. 6 shows a flowchart for explaining one example of control of the electrostatic coater of the second embodiment.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

(7) In the following, preferred embodiments of the present invention will be described based on the accompanying drawings. FIG. 1 shows a diagram for explaining a general outline of a coating system 2 as one example. The coating system 2 in the drawing is applied to coating of automobiles.

(8) First Embodiment (FIGS. 1 to 3):

(9) By reference to FIG. 1, reference numeral 4 denotes a coating booth. An explosion-proof space is formed by the coating booth 4. A plurality of coating robots 6 are installed in the coating booth 4. An electrostatic coater 100 of a first embodiment is mounted to a distal end of an arm of each of the coating robots 6. Electrostatic coating is given to an automobile W by the coating robots 6. The automobile W is an object to be coated (a workpiece) fed into the coating booth 4.

(10) A controller 10 is installed outside the coating booth 4. The controller 10 and the electrostatic coater 100 are connected via a low-voltage (LV) cable 12. A high voltage of the electrostatic coater 100 is controlled by the controller 10. The controller 10 includes a safety circuit, which stops running of the electrostatic coater 100 when detecting that the electrostatic coater 100 is in a dangerous state. Since the above configuration including the safety circuit is conventionally well known, a detailed description thereof is omitted.

(11) FIG. 2 shows a diagram for explaining an outline of an internal structure of the electrostatic coater 100 of the first embodiment. By reference to FIG. 2, the electrostatic coater 100 is a rotary-atomizing coater. The rotary-atomizing electrostatic coater 100 includes a rotary atomizing head 102 at its distal end. The rotary atomizing head 102 is called a bell cup in the industry. The rotary atomizing head 102 is driven by an air motor (not shown). A high-voltage generator 104 that supplies a high voltage to the rotary atomizing head 102 is incorporated in the electrostatic coater 100. In the following description, the high-voltage generator 104 is referred to as an operation high-voltage generator. The operation high-voltage generator 104 is called a cascade in the industry. The cascade includes a bleeder resistance 106.

(12) The operation high-voltage generator 104 is generally composed of a Cockcroft-Walton circuit. As is well known, the Cockcroft-Walton circuit is composed of diodes and capacitors. Since the Cockcroft-Walton circuit and the bleeder resistance 106 are described in detail in Patent Literature 1, the disclosure in Patent Literature 1 is incorporated herein, so that a detailed description thereof is omitted.

(13) Note that the operation high-voltage generator 104 may be incorporated in the electrostatic coater 100, or may be incorporated outside the electrostatic coater 100, e.g., in the coating robot 6.

(14) The operation high-voltage generator 104 generates a high voltage of negative polarity, and supplies the high voltage to the rotary atomizing head 102. Note that the automobile W fed into the coating booth 4 is maintained in a grounded state. Fine paint particles discharged from the rotary atomizing head 102 of the electrostatic coater 100 are in a negatively-charged state, and the paint particles charged with a negative potential are electrostatically attracted to the grounded automobile W, and electrostatically attach to the automobile W. This is a principle of electrostatic coating.

(15) The electrostatic coater 100 of the first embodiment further includes a second high-voltage generator 110. The second high-voltage generator 110 generates a high voltage of reverse polarity to that of the above first operation high-voltage generator 104. A conductor portion (a charged portion) of the electrostatic coater 100 is indicated by oblique lines in FIG. 2. The second high-voltage generator 110 is connected to the conductor portion (the charged portion) of the electrostatic coater 100. That is, the second high-voltage generator 110 can generate a high voltage of positive polarity to supply the high voltage to the rotary atomizing head 102.

(16) In addition to the second high-voltage generator 110, the electrostatic coater 100 may include a device (typically, a diode) 112 having a rectifying function to cause a current to flow only in one direction. As described above, the charged portion of the electrostatic coater 100 is indicated by oblique lines in FIG. 2. It is preferable to arrange the rectifying device 112 adjacent to the charged portion. Most preferably, the second high-voltage generator 110 is composed of a Cockcroft-Walton circuit. Since the Cockcroft-Walton circuit includes a diode as described above, the diode in the Cockcroft-Walton circuit can be caused to function as the above rectifying device 112.

(17) By providing the above rectifying device 112 in the electrostatic coater 100, it is possible to prevent the high voltage generated by the operation high-voltage generator 104 from leaking outside through the second high-voltage generator 110 during operation of the electrostatic coater 100.

(18) One example of control of the second high-voltage generator 110 will be described based on a flowchart in FIG. 3. First, in step S1, a current i flowing between the electrostatic coater 100 and the workpiece W is monitored, and it is determined whether or not the current i has a value within a normal range. When the monitored current i indicates an abnormal value, the control proceeds to step S2. In step S2, power supply to the operation high-voltage generator 104 included in the electrostatic coater 100 is forcibly stopped by assuming that the electrostatic coater 100 abnormally approaches the workpiece W.

(19) By stopping the power supply to the operation high-voltage generator 104, the operation high-voltage generator 104 (the cascade) loses its function to generate the high voltage of negative polarity, and resultantly cannot supply the high voltage of negative polarity to the rotary atomizing head 102. The rotary atomizing head 102 and the air motor or the like, to which the high voltage of negative polarity has been supplied until just before the supply stop, remain in a state of being charged with negative polarity, while the electrification charge is discharged outside through the bleeder resistance 106 included in the cascade.

(20) In step S3 subsequent to step S2 described above, power supply to the second high-voltage generator 110 is started. The second high-voltage generator 110 generates the high voltage of positive polarity to supply the high voltage to the rotary atomizing head 102. Subsequently, in step S4, the power supply to the second high-voltage generator 110 is stopped after passage of a predetermined time period from the start of the power supply to the second high-voltage generator 110.

(21) The forced running stop of the operation high-voltage generator 104 is performed not only when the monitored current i is abnormal as described above, but also when the safety circuit of the controller 10 detects abnormality. Items in which the safety circuit detects abnormality are exemplified as follows.

(22) (1) Absolute sensitivity abnormality (COL): An IM amount is sampled at predetermined intervals, and the sampled IM amount is compared with a COL sensitivity threshold. When a plurality of the IM amounts in succession are larger than the COL sensitivity threshold, it is determined as COL abnormality.

(23) (2) SLP (DiDt sensitivity abnormality): The IM amount sampled at predetermined intervals is compared with an SLP sensitivity threshold. When a plurality of the IM amounts in succession are larger than the SLP sensitivity threshold, it is determined as SLP abnormality.

(24) (3) TCL (transformer primary current excessive abnormality): A CT transformer current is sampled at predetermined intervals, and the sampled current value is compared with a TCL sensitivity threshold. When a plurality of the current values in succession are larger than the TCL sensitivity threshold, it is determined as TCL abnormality.

(25) (4) VO (Abnormal high voltage): A KV amount is sampled at predetermined intervals, and the sampled KV amount is compared with a VO sensitivity threshold. When a plurality of the KV amounts in succession are larger than the VO sensitivity threshold, it is determined as VOL abnormality.

(26) (5) VU (Abnormal low voltage): The sampled KV amount is compared with a VU sensitivity threshold. When a plurality of the KV amounts in succession are smaller than the VU sensitivity threshold, it is determined as VOL abnormality.

(27) (6) WT1 (AB-phase current difference): When a state in which a current difference between an A phase and a B phase is 0.5 A or more continues for a predetermined time period, it is determined as abnormality.

(28) (7) WT2 (CT disconnection detection): If a transformer current continues to be 0.1 A or less for a predetermined time period when a high voltage value is 30 kV or more, it is determined as WT2 abnormality.

(29) (8) WT3 (Detection of IM line short): If an average high-voltage current value (HEIIM) continues to be 5 A or less for a predetermined time period when a high voltage monitor value (KVM) is 30 kV or more, it is determined as WT3 abnormality.

(30) When the safety circuit detects the above abnormality during the operation of the electrostatic coater 100, and forcibly stops the running of the above operation high-voltage generator 104, the control may proceed to step S3 described above to perform the power supply to the second high-voltage generator 110.

(31) In the electrostatic coater 100 of the first embodiment, a value of the high voltage of negative polarity generated by the operation high-voltage generator 104 (the cascade) is, for example, 120 kV to 30 kV, and typically, 90 kV to 60 kV. In contrast, a value of the high voltage of positive polarity generated by the second high-voltage generator 110 is +20 kV to +30 kV. The value of +20 kV to +30 kV is merely an example, and an optimum value may be set by an experiment.

(32) Even when the running of the operation high-voltage generator 104 is forcibly stopped in order to avoid danger, the front end portion of the electrostatic coater 100 including the rotary atomizing head 102, the air motor and the like is in the state of being charged with negative polarity. Immediately after the forced stop of the main high-voltage generator 104, the high voltage of reverse polarity is supplied to the rotary atomizing head 102 and the air motor from the second high-voltage generator 110 for a predetermined time period, so that the charged state with negative polarity of the charged portion (the oblique-line portion in FIG. 2) including the rotary atomizing head 102 of the electrostatic coater 100 can be immediately neutralized by the high voltage of reverse polarity.

(33) The voltage value of the high voltage of reverse polarity may be changed according to magnitude of the value of the high voltage supplied to the rotary atomizing head 102 during the operation of the electrostatic coater 100. To be more specific, when the electrostatic coater 100 is operated by supplying a negative-polarity voltage of 90 kV to the rotary atomizing head 102, a voltage having a voltage value of 30 kV, as the voltage value of the high voltage of positive polarity as reverse polarity thereto, is supplied to the rotary atomizing head 102. On the other hand, when the electrostatic coater 100 is operated by supplying a negative-polarity voltage of 60 kV to the rotary atomizing head 102, a voltage having a voltage value of 20 kV, as the voltage value of the high voltage of positive polarity as reverse polarity thereto, is supplied to the rotary atomizing head 102.

(34) To confirm the effect of the electrostatic coater 100 of the first embodiment, a case in which the second high-voltage generator 110 was not run (Comparative Example), and a case in which the second high-voltage generator 110 was run (the effect of the embodiment) were compared. In the case in which the second high-voltage generator 110 was not run as the Comparative Example, it required two seconds to discharge the electrification charge through the bleeder resistance 106. In contrast, in the case in which the second high-voltage generator 110 was run, the electrification charge was neutralized only by 0.5 seconds. Note that an operation voltage of the electrostatic coater 100 was 90 kV, and a time period required for neutralization (the above 0.5 seconds) was measured by determining that the electrification charge was neutralized when the value of the high voltage was reduced to 1 kV. The voltage value, that is, 1 kV is a value where no spark discharge possibly occurs. Of course, the second high-voltage generator 110 may be run until complete neutralization, that is, until the voltage value is reduced to 0.

(35) Modification (FIG. 4) of the First Embodiment

(36) FIG. 4 shows a modification 120 of the electrostatic coater 100 of the first embodiment. In an electrostatic coater 120 shown in FIG. 4, the second high-voltage generator 110 is arranged outside the electrostatic coater 120 (for example, in the coating robot 6). The high voltage of positive polarity generated by the second high-voltage generator 110 is supplied to the conductor portion (the charged portion) of the electrostatic coater 120 through a conducting wire 122.

(37) The electrostatic coater 120 internally includes resistance 124, and the resistance 124 is connected to the conducting wire 122. By interposing the resistance 124 in the conducting wire 122, apparent capacitance of the conducting wire 122 can be reduced. In other words, the conducting wire 122 for supplying the high voltage to the electrostatic coater 120 is a charged body of the electrostatic coater 120. By interposing the resistance 124 in the conducting wire 122, the capacitance of the conducting wire 122 can be practically lowered. As a modification of the electrostatic coater 120 shown in FIG. 4, a whole or a portion of the conducting wire 122 may be composed of a wire of a semiconductor instead of the above resistance 124.

(38) Regarding the configuration in which the resistance 124 is interposed in the conducting wire 122 or the conducting wire 122 is composed of the wire of the semiconductor, it goes without saying that the configuration may be incorporated in the electrostatic coater 100 of the first embodiment described above.

(39) Second Embodiment (FIGS. 5 and 6)

(40) FIG. 5 shows a diagram for explaining an outline of an electrostatic coater 200 of a second embodiment. Although the electrostatic coater 100 of the first embodiment employs the configuration in which the charge retained in the distal end portion of the electrostatic coater 100 is neutralized by supplying the voltage of reverse polarity (positive polarity) to the rotary atomizing head 102 as described above, the electrostatic coater 200 of the second embodiment (FIG. 5) employs a configuration in which the charge remaining in the distal end portion of the electrostatic coater 200 is neutralized by supplying air charged with reverse polarity (positive polarity) to the electrostatic coater 200.

(41) In a description of the electrostatic coater 200 of the second embodiment, the same elements as those of the electrostatic coater 100 of the above first embodiment are assigned the same reference numerals, and a description thereof is omitted.

(42) The electrostatic coater 200 of the second embodiment externally includes an ion generator 202 that generates plus ions, and the ion generator 202 is installed in an ionized air pipe 204. The ionized air pipe 204 leads to an air source (not shown). The electrostatic coater 200 includes a passage switching valve 208 that is interposed in an air-system pipe 206 such as a shaping air passage and the air motor, and the above ionized air pipe 204 is connected to the passage switching valve 208.

(43) One example of control of the electrostatic coater 200 of the second embodiment will be described based on a flowchart in FIG. 6. When the safety circuit of the controller 10 detects abnormality in step S21, the control proceeds to step S22. In the step S22, a safety signal is output from the controller 10, and power supply to the operation high-voltage generator 104 (the cascade) included in the electrostatic coater 200 is forcibly stopped. In next step S23, power is supplied to the ion generator 202, and the passage switching valve 208 is switched based on an instruction from the controller 10. Accordingly, air ionized in positive polarity generated by the ion generator 202 is introduced into the electrostatic coater 200, and the air ionized in positive polarity is supplied to the shaping air passage and the air motor of the electrostatic coater 200. After the ionized air is continuously supplied for a predetermined time period, the air supply to the electrostatic coater 200 is stopped, so that the electrostatic coater 200 is brought into a suspended state (S24).

(44) As a time period in which the air ionized in positive polarity is supplied to the electrostatic coater 200, a fixed time period may be set regardless of magnitude of an absolute value of the operation voltage of the electrostatic coater 200, or the time period in which the air ionized in positive polarity is supplied may be made different according to the magnitude of the absolute value of the operation voltage. For example, when the operation voltage of the electrostatic coater 200 is 90 kV, the time period in which the ionized air is supplied may be set to a relatively long time period. For example, when the operation voltage of the electrostatic coater 200 is 60 kV, the time period in which the ionized air is supplied may be set to a relatively short time period.

(45) The time period in which the air ionized in positive polarity is supplied to the electrostatic coater 200 may be set to a time period in which the charged state with negative polarity of the front end portion of the electrostatic coater 200 can be neutralized by the reverse-polarity ionized air when the supply of the operation voltage (the high voltage of negative polarity) to the electrostatic coater 200 is forcibly stopped. While the time period may be determined by an experiment, the time period may be set to a time period required for completely neutralizing the charged state with negative polarity of the front end portion of the electrostatic coater 200, or a time period required until the charged state reaches a practically neutralized point by considering a point where the charged state is reduced to a level at which safety can be ensured (e.g., a point where a potential of the rotary atomizing head 102 is reduced to 1 kV) as the practically neutralized point.

(46) The control of actively neutralizing the charged state of the charged portions of the electrostatic coaters 100 and 200 when the controller 10 detects abnormality and stops the power supply to the operation high-voltage generator 104 that generates the high voltage of negative polarity has been described above. The present invention is not limited thereto, and even when the running of the first and second electrostatic coaters 100 and 200 is stopped in normal control during the operation of the first and second electrostatic coaters 100 and 200, the control of actively neutralizing the charged state of the charged portions of the first and second electrostatic coaters 100 and 200 that have stopped running may be performed.

(47) In accordance with the electrostatic coaters 100 and 200 of the first and second embodiments, a danger level of the charged state of the charged portions of the electrostatic coaters 100 and 200 can be immediately lowered, so that an occurrence risk of a spark discharge along with the approach of the electrostatic coaters 100 and 200 and the workpiece W can be significantly reduced. For example, even when the controller 10 detects abnormality and stops running of the coating robot 6, the robot 6 approaches the workpiece W by inertia, though only by about a few cm. Even in this situation, the electrostatic coaters 100 and 200 of the first and second embodiments can effectively suppress the occurrence of the spark discharge.

(48) As described above, even when the electrostatic coaters 100 and 200 of the embodiments approach the workpiece W, the occurrence of the spark discharge can be avoided. In other words, a coating work can be executed in a state in which the electrostatic coaters 100 and 200 are located closer to the workpiece W than that in a conventional case, so that coating efficiency can be improved. While a distance (a coating distance) between the workpiece W and a coater is set to about 30 cm to ensure safety in conventional electrostatic coating, the electrostatic coaters 100 and 200 of the embodiments can perform coating by setting the coating distance to a distance smaller than 30 cm. When the coating distance is decreased, the coating efficiency can be improved.

(49) The present invention can be widely applied to the electrostatic coating. To be more specific, although the rotary-atomizing coater has been described in the embodiments, the present invention can be also applied to an air-atomizing electrostatic coater (including a handgun), and a hydraulically-atomizing electrostatic coater (including a handgun). Also, although the embodiments have been described by using the coating robot as an example, the present invention can be effectively applied to a reciprocator as well as the coating robot.

(50) W Automobile (Object to be coated: Workpiece)

(51) 2 Coating system

(52) 4 Coating booth

(53) 6 Coating robot

(54) 10 Controller

(55) 100 Electrostatic coater of the first embodiment

(56) 102 Rotary atomizing head (Bell cup)

(57) 104 Operation high-voltage generator (Cockcroft-Walton circuit)

(58) 106 Bleeder resistance

(59) 110 Second high-voltage generator

(60) 122 Conducting wire

(61) 124 Resistance

(62) 200 Electrostatic coater of the second embodiment

(63) 202 Ion generator that generates plus ions

(64) 204 External pipe (Air supply pipe)

(65) 206 Air-system pipe

(66) 208 Passage switching valve