Electrostatic coating machine

Abstract

According to the electrostatic coating machine disclosed by the embodiment of the invention, the risk of sparks can be reduced, even if the set linear speed is higher than the linear speed of coating according to a close-range painting method before. In a coater that performs electrostatic coating using a high voltage generated by a cascade, when a time constant [tau] is defined as a product of a total capacitance Co of the electrostatic coater and a bleeder resistor R1, the product of the total capacitance Co and the bleeder resistor R1 is set such that the time constant [tau] is 0.005 to 0.050.

Description

Technical Field

[0001] Embodiments of the present invention may relate to an electrostatic coating machine. More specifically, embodiments of the present invention may relate to an electrostatic coating machine suitable for use in a close-range coating method. Conventionally, the coating distance, that is, the distance between the electrostatic coating machine and the object to be coated (hereinafter referred to as the "workpiece"), is typically set to about 150mm to 300mm. A close-range coating method is one in which the electrostatic coating machine is operated at a distance closer than 150mm. Conventional coating methods in which the distance between the workpiece and the electrostatic coating machine (i.e., the coating distance) is set to about 150mm to 300mm are referred to as "long-range coating methods", while coating methods operated at a coating distance shorter than 150mm are referred to as "close-range coating methods". According to embodiments of the present invention, the risk of ignitable discharges (hereinafter referred to as "sparks") that are problematic in close-range coating methods can be reduced. The term "spark" refers to a spark discharge with a discharge energy (0.24mJ) or higher in which solvent vapor ignites. Background Art

[0002] Electrostatic coating machines coat by using high voltage. The electrostatic coating machine charges the paint by applying a high voltage. The charged paint is electrostatically adsorbed on the workpiece. Therefore, the electrostatic coating machine includes a high voltage application path for applying the high voltage generated by a high voltage generator to the end of the coating machine (for example, a rotary atomizing head or an external electrode). The coating machine including the electrostatic coating machine has a high voltage controller for controlling the electrostatic coating machine. To ensure safety during operation, a high voltage control function is incorporated into the high voltage controller to control the high voltage applied to the electrostatic coating machine. In addition, the high voltage controller is constantly fed with a high voltage current flowing through the high voltage application path to perform high voltage safety control.

[0003] There are various methods for generating high voltage, such as Cockcroft-Walton circuit (multi-stage rectifier circuit), AC voltage rectifier type, electrostatic generator and pulse type etc. Cockcroft-Walton circuit (multi-stage rectifier circuit) is a circuit that generates high voltage by charging multiple capacitors to generate high voltage. The high voltage generator of Cockcroft-Walton circuit has been miniaturized and is referred to as "cascade". Cascade is now usually incorporated into electrostatic coating machine. Electrostatic coating machine with built-in cascade is referred to as "pack-in type electrostatic coating machine". Electrostatic coating machine that receives high voltage supply from external high voltage generator through high voltage cable (HV cable) is referred to as "pack-out type electrostatic coating machine". Compared with "pack-out type electrostatic coating machine", pack-in type electrostatic coating machine has the advantage of not requiring HV cable.

[0004] There are three solutions that have been implemented in remote coating methods to address the sparking problem during coating operations. The first is an overcurrent safety control function, the second is an output high voltage control function, and the third is a minimum high voltage protection control function. Patent Document 1 discloses these three functions.

[0005] The overcurrent safety control function includes a first function for detecting high voltage and current anomalies based on absolute sensitivity (current limit (CL) control), and a second function for detecting an abnormal increase in high voltage and current over a certain period of time based on differential sensitivity (slope sensitivity (SLP)). When an anomaly is detected, the power supply to the cascade is cut off, and the output of the high voltage generator is stopped.

[0006] In current buffer control, the output high voltage control function reduces the absolute value of the high voltage output of the high voltage generator when the high voltage current reaches the current limit (CB), without stopping the output of the high voltage generator. In other words, the output high voltage control function reduces the absolute value of the high voltage output of the high voltage generator so that the high voltage current does not exceed the current limit.

[0007] The minimum high voltage protection control function operates by operating the output high voltage control function to reduce the output voltage of the high voltage generator, and when the absolute value drops below the high voltage lower limit sensitivity (UV) in undervoltage control, the minimum high voltage protection control function performs an output stop operation of the high voltage generator.

[0008] [Patent Document 1] Japanese Patent Publication No. H02(1990)-298374A Summary of the Invention

[0009] In conventional remote coating methods, when the electrostatic coating machine approaches abnormally close to the workpiece, causing an abnormal increase in high voltage and current, the overcurrent safety control function detects this and immediately activates. Based on the operation of the overcurrent safety control function, the cascade stops its output.

[0010] To improve coating efficiency, it is best to bring the electrostatic coating machine closer to the workpiece. From this perspective, a close-range coating method (Japanese Patent Publication No. 2017-13009A) has been considered.

[0011] The close-range coating method uses a short coating distance. If the spark prevention measures implemented in the long-range coating method are directly applied to the close-range coating method, the spark prevention measures may not be able to keep up with the shortened coating distance. As a result, the close-range coating method increases the risk of sparks.

[0012] In recent years, there has been a trend toward increasing the coating speed (hereinafter referred to as "line speed") of coating machines to improve production efficiency. Electrostatic coating machines have long been used for automotive coating. Conventional line speeds for automotive coating range from 300 mm / sec to 500 mm / sec, but there is a trend toward increasing line speeds to 500 mm / sec to 1200 mm / sec.

[0013] Coating robots are widely used not only for automobile coating but also for general coating of various products. In general coating, close-range coating methods and faster line speeds have begun to be considered for higher efficiency.

[0014] Although the spark prevention measures used in the above-mentioned remote coating method are applied to the close-range coating method, when the line speed is increased, the spark prevention measures are increasingly difficult to comply with, with the result that the risk of spark generation is naturally increasing.

[0015] An object of an embodiment of the present invention may be to provide an electrostatic coating method that can reduce the risk of spark generation even when the line speed is set faster than conventional when coating according to a proximity coating method.

[0016] The embodiment of the present invention can be understood by focusing on the relationship between the line speed of the electrostatic coating machine and the time constant τ, so as to quickly decay the residual charge in the high voltage supply system of the electrostatic coating machine to a safe level after the high voltage safety function is enabled, thereby reducing the spark caused by the residual voltage in the coating machine. The feature of the embodiment of the present invention may be that the product of the total capacitance C0 of the electrostatic coating machine and the bleeder resistor R1 that decays the residual voltage is set so that the time constant τ is 0.005 to 0.050. The bleeder resistor can be a single resistor, or multiple resistors arranged in series, parallel, or a combination thereof to achieve the desired time constant.

[0017] In the embodiment of the present invention, a time constant τ of 0.015 to 0.033 may be preferable.

[0018] Here, the time constant τ is defined by the following Equation 1.

[0019] (Equation 1) τ = C0 × R1

[0020] Here, as explained later, C0 is the total capacitance, and R1 is the bleeder resistor.

[0021] Generally speaking, the "time constant" refers to the relaxation time in a circuit and is defined as the time required to reach 63.2% of equilibrium. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1is a general overview of an electrostatic coating machine to which embodiments of the present invention may be applied.

[0023] Figure 2 Demonstrated ideal high voltage control for preventing sparking in electrostatic coating machines.

[0024] Figure 3 Shows the overall configuration of the high voltage safety control unit of the electrostatic coating machine.

[0025] Figure 4 Demonstrates the decay of residual voltage in a conventional electrostatic coating machine to avoid sparking when approaching a workpiece at a line speed of 300 mm / sec.

[0026] Figure 5 Demonstrates the decay of residual voltage in a conventional electrostatic coating machine to avoid sparking when approaching a workpiece at a line speed of 500 mm / sec.

[0027] Figure 6 It was demonstrated that conventional methods cannot avoid sparking when approaching a workpiece at a line speed of 1200 mm / sec in a conventional electrostatic coating machine.

[0028] Figure 7 is a table showing the relationship between linear speed, braking distance, and braking time.

[0029] Figure 8 The effect of designing an electrostatic coating machine to have a time constant according to an embodiment at a line speed of 1200 mm / sec is demonstrated.

[0030] Figure 9 The relationship between the high voltage value on the ideal line of high voltage operation and the workpiece current at each coating distance is shown.

[0031] Figure 10 The relationship between the coating distance and the workpiece current for each line speed when -60 kV is applied is shown.

[0032] Figure 11 The relationship between the coating distance and the workpiece current for each line speed when -30 kV is applied is shown.

[0033] Figure 12 is a diagram for explaining the changing characteristics of the set values ​​of CB and CL when control is performed to change the set values ​​of CB and CL. DETAILED DESCRIPTION

[0034] Figure 1 FIG. 2 shows the overall configuration of an electrostatic coating machine to which an embodiment of the present invention can be applied. Reference numeral 2 denotes an electrostatic coating machine, and reference numeral 4 denotes a high voltage controller. Figure 1 The reference numerals in represent the following elements.

[0035] R1: Bleeder resistor

[0036] R2: Series resistor

[0037] V: High voltage at the end of the coating machine

[0038] I1: Total current (high voltage generator current)

[0039] I2: Leakage current

[0040] I3: Bleed resistor current (I3 = V / R1)

[0041] I4: output current (I4 = I1 - I3)

[0042] I5: Workpiece current (I5 = I1-I2-I3)

[0043] C1: Capacitor of cascade 6

[0044] C2: Capacitance from cascade 6 to the end of electrostatic coating machine 2, not included in cascade 6

[0045] C0: total capacitance

[0046] De: discharge energy

[0047] L: coating distance (the distance between the coating machine and the workpiece)

[0048] W: workpiece

[0049] Figure 1 The illustrated electrostatic coater 2 is a rotary atomizing coater equipped with a rotary atomizing head 2a, but the present invention is not necessarily limited to rotary atomizing coaters but may also be applied to air atomizing electrostatic coaters or hydraulic atomizing electrostatic coaters.

[0050] refer to Figure 1 , the discharge energy De when spark occurs due to the energy possessed by the entire electrostatic coating machine 2 (including the cascade 6 generating the high voltage) can be expressed by the following equation 2.

[0051] (Equation 2) De = (1 / 2) × C0 × V 2

[0052] Here, the total capacitance of the electrostatic coating machine 2 (ie, the total capacitance C0) may be defined by the following Equation 3.

[0053] (Equation 3) C0 = C1 + C2

[0054] As is well known, the cascade 6 consisting of a Cockcroft-Walton circuit (multi-stage rectifier circuit) charges the capacitor from the internal transformer side with an AC high voltage, and stacks it while rectifying it with a diode to generate a negative DC high voltage. Here, the diode prevents the boosted negative high voltage from returning to the internal transformer side. The high voltage current flows from the output terminal within the cascade 6 to the workpiece W and the bleeder resistor R1. Therefore, only the bleeder resistor R1 substantially removes the charging capacitance or residual charge of the capacitor of the cascade 6. Based on this analysis, the time required to discharge the residual charge (i.e., the time constant τ) can be defined by the above equation 1 (τ=C0×R1). The capacitor C0 can be a single capacitor, a series capacitor, a parallel capacitor, or a combination thereof.

[0055] In reference Figure 1 In the above-described electrostatic coating machine 2, Figure 2 The spark generation area SPar based on experimental results is shown. The dashed line SL represents a safety line, which serves as a high-voltage safety guide to prevent sparks during the coating process. If the coating distance L is interpreted in units of 10 mm, the safety line SL shows that when the electrostatic coater 2 approaches the workpiece W by 10 mm, sparks can be prevented by reducing the absolute value of the high voltage V by approximately 6 kV.

[0056] Taking into account the safety line SL, Figure 2 Solid line 200 in FIG. 1 illustrates ideal safe high voltage control in a close-range coating method when electrostatic coater 2, applying a negative high voltage of -60 kV, approaches workpiece W from L = 200 mm (a safe distance in conventional remote coating methods). As electrostatic coater 2 approaches workpiece W from coating distance L = 100 mm, the high voltage is controlled along safety line SL. When electrostatic coater 2 approaches workpiece W by 10 mm, control is executed to reduce the absolute value of high voltage V by approximately 6 kV, preventing sparks from occurring.

[0057] Currently, in coating operations performed by coating robots, large workpieces, such as automobile bodies, are coated while being placed on a carriage. The workpiece on the carriage undergoes only minor positional shifts due to being placed on the carriage during the coating process. This allows the coating machine to perform the coating process while maintaining a desired relative position relative to the workpiece under the control of the coating robot.

[0058] Relatively small workpieces (such as door mirror covers) are hung on a hanger and suspended from an overhead trolley-type conveyor. That is, the conveyor is placed on the ceiling, and the hanger on which the workpiece is suspended is transported by the conveyor. Electrostatic coating is then performed by using a coating robot. The workpiece is placed on the hanger, and then the hanger is suspended from the conveyor by a worker. It is not easy to accurately place the workpiece on the hanger. In addition, the hanger suspended from the conveyor swings during transportation by the conveyor. Therefore, the position of the coating machine relative to the workpiece is not constant. In order to adopt a close-range coating method in this case, it is necessary to develop a method to reduce the risk of sparks. That is, it is necessary to develop a method to ensure a high level of safety under proper operation of the close-range coating method.

[0059] The high voltage controller 4 has a memory M ( Figure 1 ), and the high voltage controller 4 includes a circuit that performs an overcurrent safety control function, an output high voltage control function, and a minimum high voltage protection control function, such as Figure 3 shown.

[0060] The high voltage safety control unit 30 includes a high voltage current value monitoring unit 310 that constantly monitors the current high voltage output current I4 and high voltage monitoring units 312(A) and 312(B). The high voltage current value monitoring unit constantly monitors the current high voltage V. The high voltage current value monitoring unit 310 captures the high voltage current that is not affected by the ripple component ΔV due to the Cockcroft-Walton circuit.

[0061] The current value of the high-voltage output current I4 is supplied from the high-voltage current value monitoring unit 310 to the output high-voltage control unit 302 and the overcurrent safety control unit 304. When the rising high-voltage output current I4 reaches the current limit value CB, the output high-voltage control unit 302 generates an output high-voltage control signal that reduces the value of the output high voltage V of the cascade connection 6. Based on this output voltage control signal, the output of the cascade connection 6 is controlled (an output high-voltage control function (CB) is executed).

[0062] As in the past, the overcurrent safety control unit 304 generates an output stop signal when the high voltage output current I4 abnormally rises to a value exceeding the absolute value sensitivity CL. Based on this output stop signal, the output of the cascade 6 is stopped (executing the overcurrent safety control function (CL)).

[0063] exist Figure 3, reference numeral 320 denotes a CB setting change unit, and reference numeral 322 denotes a CL setting change unit. Although the CB setting change unit 320 and the CL setting change unit 322 are not necessarily essential components, they are preferably provided to reliably prevent the occurrence of sparks. The current value of the high voltage V is input from the high voltage monitoring units 312 (A) and 312 (B). In the CB setting change unit 320, the registered value of the current limit value CB read from the memory M is input to the CB setting change unit 320, and the set value of the current limit value CB is changed based on the current value of the high voltage V so as to correspond to the current value. In the CL setting change unit 322, the registered value of the absolute sensitivity CL read from the memory M is input, and the set value of the absolute sensitivity CL is changed based on the current value of the high voltage V so as to correspond to the current value.

[0064] Output high voltage control unit 302 performs output high voltage control (CB) based on the current limit CB setting value received from CB setting change unit 320 . Overcurrent safety control unit 304 performs overcurrent safety control (CL) based on the absolute sensitivity CL setting value received from CL setting change unit 322 .

[0065] The value of the high voltage V generated by the output high voltage control unit 302 is supplied as the current high voltage value to the minimum high voltage protection control unit 306 via the high voltage monitoring unit 314. The minimum high voltage protection control unit 306 receives the registered value of the high voltage lower limit sensitivity UV read from the memory M. The minimum high voltage protection control unit 306 performs minimum high voltage protection control based on the set value of the high voltage lower limit sensitivity UV. In other words, when the absolute value of the output voltage of the cascade connection 6 becomes less than or equal to the high voltage lower limit sensitivity UV, the output of the cascade connection 6 is stopped.

[0066] During electrostatic coating by operating a coating robot (not shown), if the electrostatic coating machine 2 approaches the workpiece W abnormally, causing an abnormal increase in output current I4 (output current (I4 = I1 - I3)), the overcurrent safety control function immediately activates upon detecting this. Upon activation of the overcurrent safety control function (hereinafter referred to as CL control and SPL control), the output of the cascade 6 is stopped. Furthermore, the output high voltage control function (CB control) requires rapid reduction of the absolute value of the high voltage V at the end of the electrostatic coating machine, corresponding to the line speed.

[0067] SLP control is well known. High-voltage current values ​​are read at regular intervals (sampling times) and added to the registered value of differential sensitivity SLP read from memory M to set as a threshold. As described above, SLP control serves as a safety control function when the high-voltage output current I4 rises abnormally. It should be noted that for safe electrostatic coating operation, it is necessary to prepare safety control functions in duplicate or triplicate. Incidentally, the optimal sampling time for the higher line speeds used in proximity coating is 10 to 100 msec.

[0068] In the close-range coating method, if the output of the cascade 6 is stopped due to abnormal proximity or the absolute value of the output high voltage is reduced through CB control, a new problem arises, different from conventional remote coating methods. This new problem is the residual charge on the electrostatic coating machine. This problem will be explained below.

[0069] CL control and SLP control start working in response to an emergency stop signal based on the detection of an abnormal increase in high voltage current. After the high voltage controller 4 cuts off the power supply to the cascade 6, a charge remains in the electrostatic coating machine 2. The amount of residual charge depends on the size of the total capacitance C0 of the high voltage application path including the cascade 6 (for example, the high voltage electrode (such as the rotary atomizing head 2a) or the pneumatic motor). If the total capacitance C0 is large, the amount of residual charge increases, and it takes time for the residual voltage to decay.

[0070] Next, the coating robot takes time to come to a complete stop after the emergency stop signal is generated. This time is called the "braking time." Furthermore, the coating robot continues moving from the time the emergency stop signal is generated until its inertia completely stops. The amount of movement the coating robot undergoes due to this continued inertia is called the "braking distance."

[0071] In the close-range coating method, coating is performed with a coating machine that is close to the workpiece. Therefore, even if an abnormality is detected and an emergency stop signal is generated, and control to stop the movement of the coating robot is executed based on this emergency stop signal, the movement of the coating robot will not stop immediately, and the above-mentioned braking time and braking distance problems will always occur. That is, during the braking time, the coating robot moves, and the coating machine moves together with the coating robot. In close-range coating, depending on the braking distance, the electrostatic coating machine 2 that constitutes a part of the painting robot may intrude into the spark generation area SPar. As a result, there is a risk of sparks being generated due to residual voltage.

[0072] The CB control reduces the absolute value of the high voltage output by the high voltage generator when the high voltage current reaches the current limit value (CB) without stopping the output of the high voltage generator. At this time, even if the high voltage controller 4 performs control to reduce the absolute value of the high voltage at this time, the high voltage cannot be reduced faster than the time it takes for the residual voltage of the electrostatic coating machine 2 to decay.

[0073] Figure 4 is a diagram related to a conventional electrostatic coating machine, and is a diagram showing the corresponding Figure 2 FIG. 2 is a graph showing the residual voltage decay of the high voltage operation ideal line (ie, the high voltage safety ideal line 200 ). The time constant τ of the conventional electrostatic coating machine is calculated to be τ=0.132. Figure 4 This figure assumes the worst case scenario when approaching the workpiece at a linear speed of 300 mm / sec.

[0074] Figure 4 The double-dotted line 100 is the residual voltage decay curve when the output high voltage of cascade 6 stops or the voltage drop control amount of CB control is maximized, and the terminal voltage V drops to 63.2% every 0.132 seconds starting from the point where the coating distance L of the electrostatic coating machine is 100 mm and the terminal voltage V is -60 kV.

[0075] refer to Figure 4 As shown by the dashed line 100 in the diagram, when the electrostatic coating machine is operating at a linear speed of 300 mm / sec, it travels 100 mm in 0.33 seconds. The distance into the spark-generating area (SPar) is approximately 13 mm. Based on a linear speed of 300 mm / sec and a braking distance of approximately 22 mm, reverse calculations indicate that if an emergency stop signal is sent to the coating robot approximately 35 mm before the spark-generating area (SPar), sparks can be avoided. Since the dashed line 100 approximates the ideal high-voltage operation line (solid line), CB control is also possible using the output high-voltage control function.

[0076] Figure 5 This figure is based on the assumption that the coating machine approaches the workpiece at a speed of 500 mm / sec. Figure 5 As shown by the double-dashed line 102 in the figure, at a linear speed of 500 mm / sec, the coating machine travels 100 mm in 0.20 seconds. The distance into the spark-generating area SPar is approximately 23 mm. Based on a linear speed of 500 mm / sec and a braking distance of approximately 50 mm, reverse calculations indicate that if an emergency stop signal is sent to the coating robot approximately 73 mm before the spark-generating area SPar, sparks can be avoided.

[0077] Figure 6 The situation when operating at a line speed of 1200 mm / sec is shown. Figure 6In the double-dotted line 104, when the linear speed is 1200mm / sec, the coating machine travels 100mm in 0.083sec. The distance to enter the spark generation area SPar is about 44mm. Therefore, the braking distance of 1200mm / sec is 280mm. It is necessary to send an emergency stop signal to the coating robot about 324mm before the spark generation area Spar. This means that the close-range coating method itself cannot be established. Even if CB control is performed by using the output high voltage control function, CB control is impossible because it does not approximate the high voltage operation ideal line 200 (i.e., the high voltage safety ideal line).

[0078] That is, even if the high voltage controller 4 controls with the high voltage operation ideal line 200 as the target, the residual voltage V of the entire electrostatic coating machine 2 is much higher than the high voltage operation ideal line 200. Therefore, the CB control cannot follow the high voltage operation ideal line 200.

[0079] Figure 7 The relationship between the line speed, braking distance, and braking time of a typical coating robot is shown (actual measured values).

[0080] In order to achieve a line speed of 1200 mm / sec in a proximity coating process, it is necessary to attenuate the residual voltage at high speed. Figure 8 A specific example of how this can be achieved is shown in . Figure 8 , Figure 8 The single-dot dash line 110a and the double-dot dash line 110b in the figure show the descending curves at the time constant τ=0.005, which are less than the ideal line for high voltage operation at a line speed of 1200mm / sec. Here, the single-dot dash line 110a shows the fastest decay at -60kV from the coating distance L=100mm. In the coating distance L=100mm to 50mm, the high voltage V at the end of the coating machine is lower than the ideal line for high voltage operation 200, so CB control is used to operate according to the ideal line for high voltage operation 200, and at the coating distance L=50mm, the high voltage drops to -30kV. When UV is detected at the point of coating distance L=50mm and drops to -30kV, the output high voltage stops. The double-dot dash line 110b shows the decay from -30kV and coating distance L=50mm. As mentioned above, even if the coating machine approaches at a line speed of 1200mm / sec, the residual voltage drops below the safety voltage, so it can be seen that the coating machine does not enter the region SPar. The safety voltage is the voltage at which De does not reach the ignition energy (0.24mJ) from the discharge energy (Equation 2). The safety voltage is the product of the high voltage V at the end of the electrostatic sprayer and the total capacitance C. o Specifically, -3.5kV is used as a guide example for the safety voltage.

[0081] When the time constant τ = 0.005 is small, the size of the cascade 6 becomes larger due to the power characteristics of the bleeder resistor R1 and the boost characteristics of the Cockcroft-Walton circuit, and the cascade 6 cannot be installed in the electrostatic coating machine 2. The cascade 6 can be arranged outside. As a modification, a semiconductor film known from Japanese Patent Laid-Open No. H08 (1996)-187453, or a resistor having a function equivalent to that of the bleeder resistor R1, can be provided on the periphery or inside between the high-voltage body and the ground point of the electrostatic coating machine.

[0082] When the time constant τ=0.005, the total capacitance C0 and the bleeder resistor R1 are C0=10 pF and R1=500 MΩ. Here, the optimal range of the time constant τ in the specific example is 0.015 to 0.033.

[0083] As described above, in a close-range coating method where the coating distance L is 150 mm or less, control is required in units of at least 10 mm. When the line speed is 1200 mm / sec, the travel time of 10 mm is 8.3 msec. Therefore, Figure 1 The high voltage controller 4 in reads the output high voltage and high voltage current fed back from the cascade 6 at a higher speed, with a sampling time of 0.1msec to 2msec, and processes them at high speed to perform high voltage operation targeting the high voltage operation ideal line 200.

[0084] Figure 9 : is a graph with the following relationship added to show the above relationship between the coating distance L and the high voltage. That is, Figure 9 In the figure, at each coating distance L = 100 mm or less, the coating current I5 at the high voltage value V on the high voltage operation ideal line 200 is increased. The mark "△" represents the current value of the plate-shaped workpiece W. The mark "○" represents the current value of the spherical workpiece W. Based on the spherical workpiece W in which high voltage current does not flow easily, high safety can be maintained by performing CB control on the workpiece current I5 according to the current output high voltage. In addition, the difference in workpiece current caused by the difference in the shape of the workpiece becomes smaller from about -30 kV, about the coating distance L = 50 mm. That is, at about -30 kV or less, only the relationship between the output high voltage value and the high voltage current value makes it possible to read it as the coating distance L. At the same time, at most the coating distance L = 50 mm, the excessively high voltage current to the edge-shaped workpiece can be suppressed.

[0085] Figure 10 The figure shows the relationship between the coating distance L and the coating current I5 when the high voltage at the coating machine end is -60kV. Figure 11Shown are different line speeds of 300mm / sec (mark: x), 600mm / sec (mark: △), 900mm / sec (mark: ◇) and 1200mm / sec (mark: □) at -30kV. The solid line represents the average value, and the dot mark (·) represents the standard deviation at each coating distance L. These show that regardless of the line speed, the standard deviation is less than 5, indicating that the relationship between the coating distance L and the workpiece current I5 is consistent. This confirms that the coating distance L can be inferred from the relationship between the high voltage V at the end of the electrostatic coating machine and the workpiece current I5.

[0086] Considering the portable mass of the coating robot wrist and the braking distance of the coating robot wrist, the electrostatic coating machine located at the end of the wrist is preferably light in weight. The volume of the cascade 6 built into the coating machine is limited to about 180 cm 3 (36mm × 180mm). Here, attention is focused on the nominal power and volume of the bleeder resistor R1. For example, when 60kV is applied to 100MΩ, 600μA always flows to the bleeder resistor. Since the power dissipation at this time is 36W, a large resistor for high power is required to reduce heat generation. The volume of the bleeder resistor R is approximately 780cm 3 (46mm×470mm). Therefore, it is very difficult to pack the cascade into the coating machine. If the bleeder resistor R1 is 500MΩ, the power consumption will be as low as 7.2W. Therefore, a typical resistor can be used. The volume is about 9cm 3 (φ8.5mm x 162mm). If this is the case, the bleeder resistor R1 can be included.

[0087] In the case of an ultra-close coating method with a coating distance L = 100 mm or less, in order to ensure a high safety level in which the output high voltage V drops accurately and quickly according to the coating distance L, it is preferable to use a mechanism for detecting high voltage leakage disclosed in Japanese Patent No. 4678858. In this mechanism, for the coating current I5 obtained by subtracting the bleeder resistor current I3 and the leakage current I2 from the total current I1, control of changing the CB control value according to the current output high voltage V of the coater tip is preferably performed.

[0088] Figure 12 This is used to explain the CB setting changer 320 and the CL setting changer 322 (see Figure 3 ) is a diagram showing changes in the CB setting value associated with the current limit value (CB). The CB setting changer has a function of changing the setting value of the current limit value (CB), and the CL setting changer has a function of changing the setting value of the absolute sensitivity (CL). Figure 12In this embodiment, the high voltage change region (voltage drop due to output high voltage control (CB)) in the operation of the close-range coating method is 60 kV or less of the output voltage V. The area above the change region of the output voltage V is referred to as the "relatively high voltage region Har". On the other hand, the area where the absolute value of the output voltage V is lower than the relatively high voltage region Har is referred to as the "relatively low voltage region Lar". The relatively low voltage region Lar corresponds to a coating distance L of 20 mm to 200 mm. The minimum high voltage protection area (MV) indicates an area where UV is blocked.

[0089] In the relatively high voltage region Har, the current limit value (CB) is a constant value relative to the output voltage V (CB is about 50 μA). On the other hand, in the relatively low voltage region Lar, the set value of the current limit value (CB) is changed. Specifically, in the relatively low voltage region Lar, when the absolute value of the output voltage V is small compared to when the absolute value of the output voltage V is large, the set value of the current limit value (CB) is changed to a small value. Preferably, in the relatively low voltage region Lar, as the absolute value of the output voltage V decreases, the set value of the current limit value (CB) is set to a smaller value (about 30 μA or more and about 50 μA or less). More preferably, as can be seen from Figure 12 It can be seen that in the relatively low voltage region Lar, as the absolute value of the output voltage V decreases, the setting value of the current limit value (CB) can be gradually changed to a smaller value. The setting of the current limit value (CB) is determined by the CB setting change unit 320 ( Figure 3 Preferably, each value of the output voltage V and the corresponding set value of the current limit value (CB) are registered in the memory M.

[0090] During operation, the CB setting change section 320 reads the registered value of the current limit (CB) corresponding to the current value of the high voltage V from the memory M and supplies the setting value of the current limit (CB) based on this read registered value of the current limit (CB) to the output high voltage limit control section 302 ( Figure 3 The output high voltage control performed in the output high voltage limit control section 302 is executed based on the setting value of the current limit value (CB) received from the CB setting change section 320 .

[0091] By changing the set value of the current limit (CB), the set value of the current limit (CB) changes sensitively in response to the decrease in the absolute value of the high voltage V at the end of the coating machine. Then, based on this changed current limit (CB), the absolute value of the high voltage V at the end of the machine is reduced (output high voltage control function). This causes the high voltage V at the end of the coating machine 2 to drop sharply. In addition, the high voltage total current I1 also quickly becomes a small value.

[0092] By setting the product of the total electrostatic capacitance C0 of the electrostatic coater 2 and the discharge resistor R1 to achieve a time constant τ between 0.005 and 0.050, and by adding control to change the setting of the above current limit (CB), the risk of spark generation can be reliably reduced.

[0093] Similarly, the CL setting change unit 322 can also be set such that as the absolute value of the high voltage V at the coater tip decreases, the sensitivity value (CL) is preferably set to a smaller value, as Figure 12 shown.

[0094] In the relatively low voltage region Lar, when the absolute value of the high voltage V is large compared to the absolute value of the low voltage V, the set value of the absolute value sensitivity (CL) is set to a smaller value when the absolute value of the high voltage V is small. Preferably, in the relatively low voltage region Lar, as the absolute value of the high voltage V becomes lower, the set value of the absolute value sensitivity (CL) changes to a smaller value. Even more preferably, as can be seen in Figure 12 it is that in the relatively low voltage region Lar, as the absolute value of the high voltage V becomes lower, the set value of the absolute sensitivity (CL) gradually changes to a smaller value. Then, when comparing at the same high voltage, the absolute value sensitivity (CL) is set to a value greater than the current limit (CB) (CB < CL). Preferably, the smaller the absolute value of the high voltage V, the greater the difference between the absolute value sensitivity (CL) and the current limit value (CB) should be set.

[0095] For example, when a spark is about to occur due to breakdown or accident, that is, when the output high voltage control (CB) cannot keep up in time, the overcurrent safety control operates based on the absolute sensitivity (CL) set to a small value as the absolute value of the high voltage V decreases, and the output of the cascade 6 is interrupted. This overcurrent safety control can be used as a backup for the safety performance function.

[0096] In some embodiments, an electrostatic coating machine for electrostatic coating using a high voltage generated by a cascade connection is characterized in that, when the time constant τ is defined as the product of the total capacitance Co of the electrostatic coating machine and the bleeder resistor R1, the product of the total capacitance Co and the bleeder resistor R1 is set so that the time constant τ is 0.005 to 0.050. In certain embodiments, the product of the total capacitance Co and the bleeder resistor R1 is set so that the time constant τ is 0.015 to 0.033. In certain embodiments, the electrostatic coating machine includes a high voltage safety control unit that controls the high voltage applied to the electrostatic coating machine. When the high voltage current reaches the current limit, the high voltage safety control unit controls to reduce the absolute value of the high voltage output by the high voltage generator without stopping the output of the high voltage generator. In some embodiments, the high voltage safety control unit further includes a CB setting change unit that changes the current limit setting value based on the current value of the high voltage in the CB setting change unit in a relatively low voltage region (wherein the absolute value of the high voltage is less than a predetermined threshold). When the absolute value of the high voltage is large and when the absolute value of the high voltage is small, the current limit is set to a smaller value when the absolute value of the high voltage is smaller. In some embodiments, the electrostatic coating machine includes a high voltage safety control unit that controls the high voltage applied to the electrostatic coating machine. When the high voltage current reaches the current limit, the output high voltage control unit reduces the absolute value of the high voltage output by the high voltage generator without stopping the output of the high voltage generator. In some embodiments, the high voltage safety control unit further includes a CB setting change unit that changes the set value of the current limit based on the current value of the high voltage in the CB setting change unit in a relatively low voltage region (wherein the absolute value of the high voltage is less than a predetermined threshold value), and when the absolute value of the high voltage is large and when the absolute value of the high voltage is small, the current limit is set to a smaller value. In some embodiments, the electrostatic coating machine includes a cascade. In some embodiments, the electrostatic coating machine is a rotary atomizing type electrostatic coating machine having a rotary atomizing head.

[0097] Description of Reference Numerals

[0098] 2: Electrostatic coating machine

[0099] 4: High voltage controller

[0100] 6: Cascade

[0101] R1: Bleeder resistor

[0102] W: workpiece

[0103] L: Coating distance (separation distance between the coating machine and the workpiece)

[0104] C0: Capacitance of the entire electrostatic coating machine (total capacitance)

[0105] I1: total current

[0106] 30: High voltage safety control department

[0107] 320: CB setting change department

Claims

1. An electrostatic coating machine, the electrostatic coating machine being configured to perform electrostatic coating by charging the coating with a voltage generated by a cascade and directing the coating toward a grounded workpiece, wherein: The cascade comprises: Ground resistance; A grounding capacitor, wherein the grounding resistor and the grounding capacitor are connected in parallel; and A time constant, wherein the time constant is the arithmetic product of the ground resistance in ohms and the ground capacitance in farads, wherein the time constant is between 0.005 seconds and 0.050 seconds.

2. The electrostatic coating machine according to claim 1, wherein: The time constant is between 0.015 seconds and 0.033 seconds.

3. The electrostatic coating machine according to claim 2, wherein: The time constant is between 0.018 seconds and 0.030 seconds.

4. The electrostatic coating machine according to claim 1, wherein: The grounding resistor includes a bleeder resistor connected in parallel with the DC power supply, wherein the bleeder resistor includes a first bleeder resistor and a second bleeder resistor, the first bleeder resistor being connected between a terminal of the DC power supply and the grounded second bleeder resistor.

5. The electrostatic coating machine according to claim 4, wherein: The bleeder resistor includes a resistance of 1000 megohm to 2000 megohm.

6. The electrostatic coating machine according to claim 4, wherein: The grounded capacitor is composed of a first capacitor and a second capacitor, wherein the first capacitor includes a primary first capacitor and a secondary first capacitor, and the primary first capacitor is connected between a terminal of the DC power supply and a grounded secondary first capacitor.

7. The electrostatic coating machine according to claim 6, wherein: The ground capacitance is between 20 picofarads and 40 picofarads, and the value of the first capacitance is the ground capacitance minus the second capacitance.

8. The electrostatic coating machine according to claim 7, wherein: The second capacitor includes a primary second capacitor connected between the series resistor and the applicator and a secondary second capacitor connected to the ground.

9. The electrostatic coating machine according to claim 8, wherein: The second capacitor is connected in parallel with the bleeder resistor and the first capacitor.

10. The electrostatic coating machine according to claim 1, further comprising: An output voltage control unit is configured to reduce an absolute value of a voltage generated by the cascade without stopping the output of the high voltage generator when the output current reaches a current limit value.

11. The electrostatic coating machine according to claim 10, wherein: The current limit value is based on a dynamic value of a voltage generated by the cascade.

12. The electrostatic coating machine according to claim 11, wherein: The current limit value increases as the absolute value of the voltage increases within the first voltage domain.

13. The electrostatic coating machine according to claim 12, wherein: The current limit value is static as the absolute value of the voltage increases within a second voltage domain, wherein the second voltage domain is higher than the first voltage domain.

14. The electrostatic coating machine according to claim 13, wherein: The output voltage control unit is configured to stop the voltage generated by the cascade when the output current reaches an absolute sensitivity value, wherein the absolute sensitivity value is based on a dynamic value of the voltage generated by the cascade.

15. The electrostatic coating machine according to claim 14, wherein: The absolute sensitivity value increases as the absolute value of the voltage increases within the first voltage domain, and wherein the absolute sensitivity value is static as the absolute value of the voltage increases within the second voltage domain.

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