JPS5820268A - Electrostatic coating method - Google Patents

Abstract

PURPOSE:To drive protecting function before spark discharge is generated, by reducing a reference value, to be compared with a detected value of an output current, to a prearranged level by a paint spray-starting signal, and raising the reference value to another prearranged level by a paint spray-stopping signal. CONSTITUTION:A resistance value of a volume VL2 in a variable reference voltage circuit 8 rotating being interlocked with the volume VL1 of an output voltage setting changing part 5a is selected in a manner such that it offers a reference value VS, in proportion to the value of a curve C or D, to an output current-output voltage characteristic curve A or B representing the existence or absence of paint spray, respectively. Accordingly, a desired output voltage VO is preset by adjusting the volume VL1, so that the reference value VS is automatically set on the basis of the curve C or D. When paint is sprayed to a workpiece 4, to be coated, by opening a valve 13, the output voltage is lowered, and a voltage switching circuit 9 is changed from high-level voltage to low-level one by a signal from a control circuit 10.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrostatic coating method, and particularly to an electrostatic coating method that has a protective function for safely and efficiently performing coating operations.

Generally speaking, in electrostatic coating equipment, which uses a high-voltage charged coating machine to apply atomized paint to an article to be coated that is separated from the coating machine, the coated article or other conductive object is charged at a high voltage. They may come abnormally close to the paint sprayer. In this case, there is a risk of inducing spark discharge and igniting the solvent, so it is necessary to detect a sign of the occurrence of spark discharge and promptly activate the protection function before spark discharge occurs. Determination・
A typical conventional protection method is to detect the magnitude of the load current, and if this detected value exceeds a predetermined fixed reference value, the operation of the low-voltage power supply including the inverter is stopped or cut off. Alternatively, there are methods that cut off the high-voltage side power supply path. The predetermined fixed reference value in this conventional method is
It is set to a substantially constant value based on the operation data of the coating line, regardless of whether paint is being ejected from the coating machine or not, and regardless of the charge voltage.

However, although it varies depending on the structure of the paint sprayer and the characteristics of the paint, it has been found that in general, the output current is significantly different when the paint sprayer is spraying paint and when it is not spraying paint. Figure 1 is a diagram showing this. At time t, a paint spray start signal is issued and paint is sprayed from the sprayer while the distance between the sprayer and the object to be coated and the output voltage are held constant. Then, the output current decreases rapidly, and at time t2
It can be seen that when the spraying of paint from the paint sprayer is stopped by the paint jetting stop signal for stopping the spraying of paint, the output current Io rises rapidly.

Curves A and B in Figure 2 are the output currents when the paint is not sprayed (curve A) and when the paint is sprayed (curve B), respectively, with the distance between the paint sprayer and the object to be coated kept constant. The Io-output voltage Vo characteristics are shown, and these curves A,
The output current is I when paint is being ejected from B and when it is not being ejected.

It can be seen that there is a significant difference. Although it is true that the output current Io changes significantly depending on whether or not paint is being ejected from the paint sprayer, in the case of the conventional method, the reference value is set to a predetermined constant value. Because there are
When the paint is being sprayed, the standard value has a margin of several hundred centimeters or more, resulting in the disadvantage that a sufficient protective function cannot be achieved.

To explain this point in more detail, in the conventional case, as shown by the straight line X in Fig. 1, the standard value is several tens of so , for example, is fixed at a value corresponding to a 60% larger level. This value of 30 inches is a value that is determined depending on various painting conditions, taking into account noise margins, etc., but when paint is ejected, the standard value becomes a large value with a margin of about 800 inches. I end up. This means that when the paint is being sprayed from the paint sprayer, in order for the output current 0 to increase to around the level In other words, the output current Io increases to near level X. This indicates that the article to be coated is very close to the coating machine. According to experiments, in the section where paint is sprayed from the paint sprayer,
When the output current Io increases by a slightly smaller amount than level Since it is in a region where the output current increases with a higher-order function than the function, even a slight approach of the article to be coated causes the output current Io to increase significantly, inducing spark discharge before the protective function is activated. 100 ways to prevent this drawback
The protective function can be activated by using a short-circuit switch that can ground high-voltage charged electrodes at an extremely high speed of less than a microsecond, but currently it is possible to activate the protection function within the DC output voltage range used in normal electrostatic painting. It is extremely difficult to obtain a high voltage switch with a withstand voltage of 60KV to 120KV that has an operating speed of about 100 microseconds or less.
Conventional protection is therefore very unreliable, which makes electrostatic painting operations not only inefficient but also dangerous.

In order to eliminate the drawbacks of the conventional method, the present invention reduces the reference value to which the detected value of the output current is compared to a predetermined value by means of a paint jet start signal to indicate the start of paint jet. In addition, by increasing the reference value to a predetermined value by the paint jet stop signal to indicate the stop of paint jetting, the protective function is activated before spark discharge occurs regardless of the condition of the paint sprayer. It is characterized by being able to operate quickly and reliably, improving the efficiency of painting work and eliminating danger.

An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 6, 1 is an AC power supply device consisting of a step-up transformer (not shown) and an oscillator connected to its primary winding, and 2 is a secondary winding (not shown) of the step-up transformer.
a DC high voltage generator, such as a Kotscroft-Walton circuit, connected to a paint sprayer, 3 being shown as a high voltage charged electrode charged to a high voltage by the DC high voltage generator 2;
4 is a grounded article to be painted; 5 is a control circuit of the AC power supply 1; 6 is a current detection circuit connected between the positive output terminal of the DC high voltage generating circuit 2 and the ground terminal; 7 is a coating machine A short-circuit device including a short-circuit switch element connected between a high-voltage charged electrode 6 and a low-voltage terminal such as a ground terminal; 8 a polyimide VLI for output voltage setting in the output voltage setting change section 5a of the control circuit 5; Volume VL for standard value setting that can be linked with
2, a variable reference voltage circuit 9, a pulp control picture frame 1o;
Different voltages VH and V depending on the pulp opening/closing signals from
I.

A voltage switching circuit that switches between high voltage VU
There is a predetermined delay time when switching from VL to lower voltage VL. Incidentally, it is preferable that these voltages VH1VL can be varied depending on coating conditions and the like. Furthermore, 11 is a comparison circuit that compares the detection signal VD from the current detection circuit 6 with the reference signal Vs from the variable reference voltage circuit 8, 12 is a paint supply device, and 16 is a paint supplied from the paint supply device 11 to the coating machine 6. This is the pulp that opens and closes the paint.

Next, the method according to the present invention will be explained while explaining the operation of the apparatus having such a configuration.

First, the resistance value of the volume VL2 of the variable reference voltage circuit 8, which rotates in conjunction with the volume VL1 of the output voltage setting change section 5a, is determined by changing the setting of the volume VLI, depending on whether or not paint is ejected from the paint sprayer 3. Curves C and D for curves A and B in Figure 2 showing the output current-output voltage characteristics of
is selected so as to provide a reference value proportional to the value indicated by . Therefore, the volume VL1 of the output voltage setting change section 5a
By adjusting VO to set the desired output voltage VO,
The reference value Vs is also automatically set based on the curves C and D. Here, a curve C1D showing the proportional value of the reference value v8
Although it varies depending on various coating conditions such as the shape of the article to be coated, for example, it is set to a value several tens of times larger than the output current in consideration of tolerance.

First, in FIG. 2, it is assumed that electrostatic coating work is being performed with an output voltage setting in a range where the output current of curve A is larger than that of curve B. As shown in FIG. 1, in the section before time tl, no paint is being ejected from the coating machine 6, and therefore the output current IO is at a high level. In this state, the voltage switching circuit 9 is switched to the high level voltage VH side, so that the reference value 'Vs becomes a value corresponding to the high level portion Y1 of the curve Y in FIG. Next, at time t1, when a paint jetting start signal for jetting paint is issued from the pulp control circuit 1'0, the pulp 15 is opened and paint is jetted from the atomizer 6 toward the article 4 to be coated. As a result, the output current I as shown in FIG.

decreases relatively rapidly, for example, to a low level with a falling time of several tens to hundreds of milliseconds, while the voltage switching circuit 9 changes to a high level voltage vH in response to the paint ejection start signal from the pulp control circuit 10. The voltage is switched from VL to a low level voltage VL. This switching requires an output current I. This is done gradually over a period of time equal to or longer than the falling time of , and this point will be explained in more detail with reference to FIG. 4, which shows a specific circuit configuration of a portion of the voltage switching circuit 9α.

In Figure 4, Sl and S2 are normally closed contacts, S3
The contacts 81 to S3 are normally open contacts, and in a state where paint is not being jetted from the atomizer 6, that is, after a paint jetting stop signal is applied from the pulp control circuit 10, the contacts 81 to S3 are at the positions shown. In the interval at time t1i when paint is not being ejected, the electrolytic capacitor C is charged to a voltage of +V via the resistor R1 and the contact S1. +V is applied to the inverting input terminal of operational amplifier OA via contact S2 and resistor R2.
A voltage of +■ is applied, and this voltage +■ corresponds to the high level voltage VH. Therefore, the output voltage of operational amplifier oA vs
' is at a high level. Next, at time t1, a paint jetting start signal is issued from the valve control circuit 10, and when this signal energizes a relay coil (not shown), its contacts S1 and 82 open, and at the same time, the contact S3 closes. As a result, the charging voltage (+■) of capacitor C is
3 and resistor R2 to the inverting terminal of operational amplifier oA, and this photoelectric voltage is discharged through resistor R3 at a predetermined discharge time constant. This discharge time constant approximately corresponds to the slope of the descending portion Y2 of the curve Y in FIG.

Then, as the capacitor C discharges, the voltage at point a drops to the value (VVZl) obtained by subtracting the Zener voltage Vz1t of the Zener diode Dz1 from the control power supply voltage V, and the Zener diode C1 breaks down and the voltage at point a
The voltage of is held at (VVZl). This voltage (V-V
ZI) corresponds to the low level voltage vL, and in this case, as described above, the output of the operational amplifier OA, ie, the output voltage vs' of the voltage switching circuit 9, is at a low level. This voltage vs' is adjusted by adjusting the value of the feedback variable resistor Rv. Note that R4 is an input resistor connected to the non-inverting terminal of the operational amplifier oA, and DZ2 is a constant voltage element for limiting the input of the operational amplifier oA to a certain value or less.

In this way, the reference value Vs changes according to the curve Y in FIG.
It then drops to a low level Y3. and time t2
When a paint ejection stop signal is applied from the valve control circuit 10 to the voltage switching circuit 9, the voltage switching circuit 9 rapidly switches from the low level voltage vL side to the high level voltage vH side, thus changing the reference value. The curve Y shown changes again to level Y4, which is equal to level Y1. To explain this with reference to FIG. 4, a relay coil (not shown) is energized by the paint ejection stop signal, that is, the relay coil is deenergized, and as a result, the normally closed contacts S1 and S2 are closed again. done,
Normally open contact S3 is opened. Therefore, the voltage at the inverting terminal of operational amplifier OA rapidly rises to +V voltage, and its output voltage vs' also rises to a predetermined value.

As described above, in this embodiment, the reference signal Vs changes from low level to high level depending on the paint jetting start and stop signals, and is also adjusted in relation to the output voltage setting. , this reference signal VS is transmitted from the positive output terminal of the DC high voltage generator 2 to the current detection circuit 6, to the ground terminal, to the article to be coated 4, and to the high voltage charged electrodes! 1 through which the output current flows to its negative output terminal. detection signal VD
and is compared in the comparison circuit 11. This comparison circuit 11
does not output an actuation signal when the reference signal Vs is larger than the detection signal VD, but outputs an actuation signal when the detection signal vD increases to more than the reference signal v8. In response to this activation signal, the shorting device 7 short-circuits the high voltage charged electrode '5 to the low voltage terminal. On the other hand, the activation signal from the comparison circuit 11 is applied to the control circuit 5, and the control circuit 5 responds to this activation signal to shut off the AC power supply device 1, and as a result, the power supply is stopped.

In the above explanation, the paint jetting start or stop signal is described as the opening/closing control signal for the valve 16, but these paint jetting start or stop signals can be obtained by operating the trigger of the paint sprayer. , and a signal resulting from detecting paint supplied to the paint sprayer from a paint tank. However, the fall time of the reference value at the start of paint ejection is selected to be equal to or greater than the fall time of the output current, and the rise υ of the reference value at the time of stop of paint ejection is 5.
It is desirable to set the current so that the rise of the current is steep.

As described above, according to the present invention, the reference value to be compared with the detected value of the output current is lowered and raised by 1θ to the value scheduled by the paint jetting start signal and the jetting stop signal, thereby causing spark discharge. Because we can quickly and reliably detect predictions of occurrence,
Regardless of the condition of the coating machine, the protection function can be operated reliably and quickly before spark discharge occurs, thereby improving the efficiency of coating work and preventing ignition and electric shock to the human body.

[Brief explanation of the drawing]

FIG. 1 and FIG. 2 are diagrams showing the current one-hour characteristic and current-output voltage four characteristics, respectively, to explain the present invention, and FIG. 6 is a diagram showing the z0 lock configuration of the device for implementing the present invention. FIG. 4 is a diagram showing a specific example of a circuit of a part of the apparatus shown in FIG. 6. 1... AC power supply 2... DC high voltage generator 6... Painting machine 4... Article to be coated 5...
Control circuit 5a... Output voltage setting change section 6...
Current detection circuit 7... Short circuit device 8... Variable reference voltage equalization 9... Voltage switching circuit 10... Valve control circuit 11... Comparison circuit 12... Paint supply device 16... parv

Claims (1)

[Claims] detecting the current flowing from the positive output terminal of the DC high voltage generator to its negative output terminal via the article and the high voltage charged electrode, and cutting off the high voltage output when the detected value of the current exceeds a predetermined reference value; Alternatively, in an electrostatic coating system having a protective function that short-circuits the high-voltage charged electrode to the low-voltage terminal, or performs both of these functions, the predetermined reference value υ is determined by a paint jetting start signal indicating the start of paint jetting. An electrostatic coating method characterized in that the predetermined reference value is raised in response to a paint jetting stop signal indicating the stop of paint jetting.