GB2078402A

GB2078402A - Position adjustment system

Info

Publication number: GB2078402A
Application number: GB8115691A
Authority: GB
Original assignee: Ransburg Corp
Current assignee: Ransburg Corp
Priority date: 1980-05-29
Filing date: 1981-05-21
Publication date: 1982-01-06
Also published as: BR8102781A; AU7006581A; IT8167713A0; JPS5712849A; FR2483647A1

Abstract

A system for adjusting the "head-target" spacing between a device 12 for dispensing charged particulate coating material and an article 14, to be coated to account for variations in the profile or contour of the article or movement of the article transversely to the direction of motion of a conveyor 15 upon which the target is being conveyed past the device 12 continuously monitors the head-target spacing by monitoring the currant resulting from the flow of electrostatically charged atomised coating material particles to the target. The system feeds back this head-target spacing-related signal to a head positioning servomechanism including an hydraulic piston and cylinder 706 which controls protraction and retraction of the dispensing device 12. <IMAGE>

Description

SPECIFICATION Position adjustment system This invention relates to a position adjustment system for a coating material dispensing device, and also to a control system for an electrostatic coating apparatus for maintaining substantially constant head-target spacing between an atomising device or head and a target to be coated by atomised coating material particles dispensed from the head. Usually, articles to be coated by an electrostatic coating apparatus are conveyed past the coating apparatus on a conveyor. Such articles are subject to motion, not only past the apparatus, but also oscil!atory motion, e.g. swinging motion toward and away from the high voltage electrode of the coating apparatus.

Additionally, such articles usually have profiles or contours which are not perfectly flat in planes parallel to their direction of motion along the conveyor. Therefore it is frequently desirable to maintain substantially constant head-target spacing, to move the head slightly towards or away from the conveyor, as the contour of the target article dictates. Since the head is usually mounted for reciproaction toward and away from the target article, e.g. on the piston of a piston-and-cylinder fluid motor, the control system of the invention can be incorporated into the fluid control of such fluid motor.

In industrial electrostatic coating systems, high voltage direct current power supplies are used which produce across a pair of terminals potentials having high magnitudes, for example, 140 KV, DC. One of the terminals is at ground or approximately ground potential while the other is held at a high (frequently negative) potential. This high potential terminal is connected to a charging device which charges particles of the coating material. The atomised and charged particulate material moves through the electric field between the charging device and the article in the direction of the article, strikes the latter, and sticks to it. Generally the article is maintained at a low potential, e.g. approximately ground, just as is the low potential terminal of the high voltage supply.

In an automatic electrostatic coating installation, articles to be coated are usually carried by a conveyor and are free to swing back and forth in the direction of the charging device.

As an article moves towards the charging device, the potential gradient between the charging device and the article can increase quite rapidly. The rapidity of the increase depends in part upon how rapidly the article is swinging. The maximum and minimum values of the potential gradient depend upon the amplitude of the swing. The current between the charging device and the article which results in large part from the flow of charged particles of coating material across the space therebetween varies as the potential gradient between the article and the charging device varies, the current increasing as the spacing between the article and charging device decreases toward a minimum, and decreasing as the spacing between the article and the charging device increases to a maximum. Appreciation of these characteristics has been demonstrated in United States Patents Nos.

3,851,618, 3,875,892, 3,894,272, 4,075,677 and 4,187,527.

As can be appreciated, a considerable hazard presented by movement of articles to be coated with respect to the charging device, is the possibility of spark discharge across the space between the charging device and the freely moving articles. The need for a system which can prevent or reduce such spark discharge is apparent. Operators of electrostatic coating apparatus occasionally occupy work stations quite close to the charging device, the articles being coated, or to both. Additionally, some materials used in the coating process or in operations related to the coating process, are volatile, and the vapours of such a material may be present in the atmosphere near the apparatus. Many such materials are flammable. Further, fine particles of coating material are frequently suspended in the atmosphere surrounding the apparatus.

The safety hazards presented by the possibility of a high voltage spark between the charging device and the article to be coated evinces the need for a system which can predict with reasonable accuracy conditions conducive to arcing and which can act to prevent arcing. Additionally, high voltage arcing can be detrimental to parts of the electrostatic coating apparatus itself, e.g. the high voltage supply.

Additional safety hazards are presented by the everchanging contours of non-planar articles which are conveyed on a conveyor past the atomising device for coating. it will be appreciated that as the contour of such an article passes the atomising device, the headtarget spacing changes. If such changes in the head-target spacing are not taken into consideration, the head-target spacing may be reduced to such an extent that an ionised path can readily develop between the head and target, promoting a spark.

According to this invention, a position adjustment system for a coating material dispensing device comprises:-- means for selectively movably positioning the device relative to a target to be coated by coating material dispensed from the device; means for sensing the spacing between the dispensing device and that portion of the target directly adjacent the dispensing device and for generating a signal indicative of such spacing; and means for coupling the sensing means to the positioning means such that, as the spacing tends to decrease, the dispensing device tends to move away from the target, and as the spacing tends to increase, the dispensing device tends to move toward the target.

The system may include means for conveying targets serially past the dispensing device.

The targets may be mounted on the conveying means so that they are free to move in directions perpendicular to the general direction of motion of the conveyor means such that the dispensing device-target spacing is variable substantially independently of the relative position of the target on the conveying means. Further such targets can be of shapes which vary the spacing between the normal position of the dispensing device and that portion of the target which lies directly adjacent the dispensing device.

The means for sensing the spacing between the dispensing device and the target may comprise a high voltage electrostatic supply, a first terminal adjacent the path of motion of the targets past the dispensing device, means for coupling the supply to the first terminal and the targets to establish a field between the first terminal and the targets, and means for sensing current flow between the targets and the first terminal and for generating a control signal in response to such current flow. The first terminal may be provided directly on or by the dispensing device itself.

The means for sensing current flow between the targets and the first terminal may comprise a filter for removing substantially all AC components above a selected frequency from the current flow to provide the control signal. Thus minute spacing variations, noise spikes due to the action of the high voltage electrostatic supply and like high-frequency perturbations, do not enter into the control of the head-target spacing. This prevents the positioning means from "hunting" or oscillating about a desired position establishing a required head-target spacing.

The invention will now be described by way of example, with reference to the drawings, in which: Figure 1 is a simplified block diagram of a coating apparatus incorporating the position adjustment system of the invention; Figure 2 is a partly block and partly schematic diagram of a portion of the system illustrated in Fig. 1, showing the position adjustment system in somewhat greater detail; and Figure 3 is a partly block and partly schematic diagram of a portion of the system illustrated in Fig. 1, showing the position adjustment system in somewhat greater detail.

Turning now to Fig. 1, an automatic system 10 is for electrostatic, high voltage deposition of coating material from an atomising and charging head 1 2 upon articles 14, for example car bodies, as they move past the atomising and charging head 1 2 on a grounded conveyor 1 5. The system includes a main power supply 1 6 for producing direct current at an intermediate voltage, e.g. 28 voits. In addition, an auxiliary power supply 1 8 produces direct current at one or more relatively low voltages, e.g. plus or minus 1 5 volts.

The system 10 further includes a control and indicator panel 20 from which the operating status of the system is continuously displayed. To produce the large magnitude voltage necessary for electrostatic deposition, e.g.

negative 1 40 kilovolts (KV), a switching and regulating circuit 22 and a high voltage transformer 24 are provided. High voltage transformer 24 includes a primary winding 26 and a secondary winding 28.

A high voltage rectifier and multiplier 30 is coupled to the secondary winding 28 of transformer 24. Articles 14 are maintained at or near the potential of one of a pair of high voltage output terminals 32, 34. High voltage rectifier and multiplier 30 produces across terminals 32, 34 sufficient potential so that atomized particles of coating material, e.g., paint, will be attracted toward and deposited upon articles 14.

A clock circuit 38 drives switching and regulating circuit 22 to switch the main power supply 1 6 voltage across primary winding 26 and produce high voltage in secondary winding 28.

Articles 14 are typically conveyed past atomizing and charging head 1 2 on conveyor 15. Thus, articles 14 are movable with respect to atomizing and charging head 1 2 and it is desirable to control the potential across output terminals 32, 34 such that, as the contours of the articles 14, or the transverse movement of the articles 14 on conveyor 15, tend to reduce the head 12-target 14 spacing, a position adjustment control system 50 tends to move the head 1 2 away from the articles 14, thereby maintaining the head 12-target 14 spacing substantially constant.

Similarly, as movement of the articles 14 transversely of their direction of motion on conveyor 15, or the contours of articles 14 themselves, tend to move articles 14 away from head 12, thereby increasing the head 12-target 14 spacing, the control system 50 tends to move head 1 2 toward conveyor 15, again maintaining the head 12-target 14 spacing substantially constant.

Turning now to the details of the control system 50 for adjustment of the position of head 12, reference will be made to Figs. 2-3.

Fig. 2 illustrates in greater detail the high voltage rectifier and multiplier 30 and its associated circuitry.

High voltage rectifier and multiplier 30 generates a high-magnitude negative voltage, e.g., -140 KV DC. To generate this high voltage, the voltage variations induced in high voltage transformer 24 secondry winding 28 are rectified and multiplied, illustratively by a factor of six, in circuit 30. Twelve high voltage rectifying diodes 522-544 are coupled in series between terminal 546 of secondary winding 28 and the negative high voltage terminal 548.Six pairs of series-coupled storage capacitors 550, 552; 554, 556; 558, 560; 562, 564; 566, 568; and 570, 572 are coupled, respectively, between the anode of diode 522 and the anode of diode 530; the cathode of diode 524 and the cathode of diode 532; the anode of diode 530 and the anode of diode 538; the cathode of diode 532 and the cathode of diode 540; the anode of diode 538 and the anode of a Zener diode 580, the cathode of which is coupled to terminal 546; and the cathode of diode 540 and the other terminal 582 of secondary winding 28.

A large-value series resistor 584 is coupled between negative high voltage terminal 548 and output terminal 32. A series combination of a resistor 586 and terminals 588, 590 of a shorting device 36 are coupled between terminal 32 and ground. Terminals 588, 590 are the normally closed terminals of a solenoid-actuated relay. The control solenoid 592 of this relay is serially coupled between terminal 160 of the control panel 20 (Fig. 1) and ground. A bidirectional Zener diode 598 is also coupled between terminal 1 60 and ground to protect against excessive voltage across solenoid 592. When winding 592 is actuated, high voltage is supplied from terminal 548 through resistor 584 and device 36 to terminal 32.Any interruption of current flow through winding 592 returns device 36 to its position illustrated in Fig. 2, shorting output terminal 32 through resistor 586 to ground.

High voltage circuit 30 additionally includes some sensing circuits. One terminal of a very large-value resistor 600 is coupled to terminal 548. The remaining terminal of resistor 600 is coupled to the parallel combination of a kilovolt meter 602 and a meter-scale controlling resistor 604. The parallel combination of a large-value resistor 606 and a capacitor 608 is coupled between terminal 398 and ground.

In the circuit including resistors 600. 606, the resistance value of the parallel combination of KV meter 602 and scale resistor 604 is negligible compared to the values of resistors 600 and 606. Thus, resistors 600, 606 constitute an extremely high resistance voltage divider between negative high potential terminal 548 and ground. As was previously mentioned, a voltage signal directly related to the high voltage at terminal 548 is available at terminal 398.

One terminal of a parallel combination of a microammeter 610 and a scale resistor 612 is coupled to terminal 546 of secondary winding 28. A parallel combination of a capacitor 614 and a current-sensing resistor 61 6 is coupled between the other terminal 61 8 of the microammeter-scale resistor circuit and ground.

Since the junction of high voltage capacitor 568 and Zener diode 580 is at ground, it can be seen that terminal 61 8 will be maintained at a slightly positive potential (less than or equal to the reverse breakdown voltage of Zener diode 580). Since the microammeter 610 circuit is coupled between terminal 546 of secondary winding 28 and ground, the current through the circuit will be equal to the current flowing between terminals 32, 34 of high voltage circuit 30. The voltage at terminal 61 8 will always be directly proportional to the current flowing between terminals 32, 34.

Turning now to Fig. 3, the manner in which the signal generated by the current-sensing circuit is used will be discussed. The signal representative of current flow between high voltage circuit 30 terminals 32, 34 is coupled from terminal 61 8 to a three-pole active filter 620. Filter 620 is a Butterworth filter which includes three series resistors 622, 624, 626 coupled between terminal 61 8 and the noninverting input terminal, pin 3, of an amplifier 628. The output terminal, pin 6, of amplifier 628 is coupled through a feedback resistor 630 to the inverting input terminal, pin 2, of amplifier 628. A capacitor 632 is coupled between the junction of resistors 622, 624 and ground, as is a Zener diode 634, the anode of which is coupled to ground. A capacitor 636 is coupled between pin 3 and ground.A capacitor 638 is coupled between pin 6 and the junction of resistors 624, 626.

Pin 2 is coupled to ground through a resistor 640. An indicator circuit 642 including a transistor-controlled LED provides a visual indication of the presence of signal at the output terminal of amplifier 628 of filter 620.

The signal at the output terminal of amplifier 628 of filter 620 is a signal containing substantially no alternating current components above the corner frequency of the filter 620. This signal is a DC and low-frequency signal related to current flow between the head 1 2 and target 14.Therefore, this signal is related to the coating material transfer rate between head 1 2 and target 1 4. As will be appreciated, a signal containing information relative to the transfer rate of coating material between head 1 2 and target 1 4 contains information relative to the spacing between head 1 2 and target 1 4. This can be appreciated by remembering that the spacing between head 1 2 and target 14 will be interpreted by the high voltage rectifier and multiplier 30, and by terminals 32, 34 as a variable load resistance, with the resistance value being essentially related to the head 12-target 1 4 spacing.The greater the spacing, the greater the resistance, and therefore the lower the current and coating material transfer rate. Conversely, the less the spacing, the less the resistance, and the greater the current and coating material transfer rate. Of course, other factors contribute to the current flow between the head 1 2 and target 14.

Typically, however, within the range of current values with which the present invention is concerned, these other factors can generally be ignored.

Referring now to Figs. 1 and 3, the control system 50 includes a servodriver circuit 52 which has inputs 54, 56. Input 54 is coupled to a head 1 2 position feedback potentiometer 58. A normal head 12 position adjust circuit 472 operates to establish a so-called "nor- mal" spacing between the head 1 2 and target 14. It will be understood that this position is established based upon the mean or average position of the article 1 4 surface adjacent head 1 2. That is, while the article 14 surface adjacent head 1 2 will vary basically continuously due to the changing contour or profile of the article 14, the contour assumes some mean or average position.It is upon the basis of this average position that the positionadjust circuit 472 operates.

Circuit 472 includes an amplifier 474, the inverting input terminal (-) of which is coupled through a series resistor 476 to the output terminal, pin 6, of amplifier 628. A position-adjust potentiometer 478 is coupled between a supply + V and ground. The wiper of potentiometer 478 is coupled to the noninverting input terminal (+) of amplifier 474.

The output terminal of amplifier 474 is coupled through a feedback resistor 488 to the inverting input terminal (-) thereof.

The output terminal of amplifier 474 is coupled through two series resistors 490, 492 to the pole 493 of an FET switch 494.

One throw 495 of switch 494 is coupled to input terminal 56 of the servodriver circuit 52. The other throw 497, the "enable" terminal, of switch 494 is coupled te the collector of a start-up delay transistor 788. The emitter of transistor 788 is grounded. Throw 497 is also coupled through a resistor 500 to a supply + V.

The junction of resistors 490, 492 is coupled through series Zener diodes 502, 504 to ground.

The output signal from amplifier 474 and the head 1 2 position signal on the wiper 662 of potentiometer 58 are summed at a summing point 654. The combined signal at summing point 654 is coupled to the inverting input terminal (-) of a comparator 668 in servodriver 52. The output terminal of comparator 668 is coupled through a series resistor 674 to the inverting input terminal (-) of a servodriver amplifier 692. Servodriver 52, which is illustrated somewhat diagrammatically in Fig. 3, can be of any of a number of known types.

The outputs 700 of servodriver 52 control the flow of hydraulic fluid through a two-way servovalve 702 from a hydraulic fluid supply 704 to a fluid motor 706, such as a hydraulic piston and cylinder, which controls the protraction and retraction of head 12, and consequently, the head 1 2target 14 spacing.

When the high voltage across terminals 32, 34 (Figs. 1-2) is off or rising to its steady state, the start-up delay transistor 788 is conductive with its collector at approximately ground potential. This ground potential at the enable terminal 497 of FET switch 494 opens the switch 494 and removes the output of amplifier 474 from input terminal 56. However, when the high voltage across terminals 32, 34 reaches a steady state in preparation for the beginning of a coating operation, transistor 788 becomes non-conductive, and the FET switch 494 switches to the position illustrated in Fig. 3. The output signal from amplifier 474 is thereby supplied to terminal 56 of servodriver 52.Amplifier 474 is connected as a comparator whose output is positive when the pack return current-related signal at the output of amplifier 628 is below a reference value established at the non-inverting input terminal (+) of amplifier 474 by potentiometer 478. When the head 1 2 is in its neutral position and the potentiometer 58 is balanced with substantially zero volts on wiper 662, a positive voltage at the output of amplifier 474, and consequently at the input 56 of servodriver 52, causes the servovalve 702 to actuate the fluid motor 706 to move atomiser 1 2 toward the grounded target 14, thereby increasing the current flow between head 1 2 and target 14. When the currentrelated signal at the output terminal of amplifier 628 exceeds the reference value established by potentiometer 478, the output signal from amplifier 474 goes negative, causing the servovalve 702 to actuate the fluid motor 706 to move head 12 away from the grounded article 14, reducing current flow between head 12 and target 14. The amount of head 1 2 movement from the so-called "normal" spacing is limited by the breakdown voltages of Zener diodes 502, 504. Thus, at very low head 12-target 14 current flow, which may be interpreted as, for example, an ungrounded article 14, Zener diode 504 breaks down, preventing further movement o the head 1 2 toward the target 14.

Claims

1. A position adjustment system for a coating material dispensing device, the systerr' comprising means for selectively movably positioning the device relative to a target to be coated by coating material dispensed from th device; means for controlling the positioning means; means for sensing the spacing between the dispensing device and that portion of the target directly adjacent the dispensing device and for generating a signal indicative of such spacing; and means for coupling the sensing means to the positioning means such that, as the spacing tends to decrease, the dispensing device tends to move away from the target, and as the spacing tends to increase, the dispensing device tends to move toward the target.

2. A system according to Claim 1, comprising means for conveying targets serially past the dispensing device.

3. A system according to Claim 2, wherein the targets are mounted on the conveying means so that they are free to move in directions perpendicular to the direction of motion of the conveyor means such that the dispensing device-target spacing is variable independently of the relative position of the target on the conveyor means.

4. A system according to Claim 2 or Claim 3, wherein the targets contain profiles or contours which vary the spacing between the dispensing device and the target.

5. A system according to any of Claims 2 to 4, wherein the means for sensing the spacing between the dispensing device and the target includes a voltage supply, a first terminal adjacent the path of motion of the targets past the dispensing device, means for coupling the supply to the first terminal and the targets to establish a field between the first terminal and targets, and means for sensing current flow between the targets and the first terminal and for generating a control signal in response to such current flow.

6. A system according to Claim 5, wherein the first terminal is provided on the dispensing device.

7. A system according to Claim 5 or Claim 6, wherein means for sensing current flow between the targets and the first terminal comprises a filter for removing substantially all alternating current components above a selected frequency from the current flow to provide the control signal.

8. A system according to Claim 1, wherein the means for sensing the spacing between the dispensing device and the target includes a voltage supply, a first terminal, means for mounting the first terminal for motion with the dispensing device, means for coupling the supply to the first terminal and the target to establish a field between the first terminal and target, and means for sensing current flow between the first terminal and target and for generating a control signal in response to such flow.

9. A system according to Claim 8, wherein the first terminal is provided on the dispensing device.

10. A system according to Claim 9, wherein the means for sensing current flow between the targets and the first terminal comprises a filter for removing substantially all alternating current components above a selected frequency from the current flow to provide the control signal.

11. A method for adjusting the spacing between a target to be coated by coating material and a selectively movable dispensing device for the coating material, comprising the steps of sensing the spacing between the dispensing device and that portion of the target adjacent the dispensing device, and moving the dispensing device in response to the sensed spacing.

1 2. A method according to Claim 11, wherein the step of moving the dispensing device in response to the sensed spacing comprises moving the device such that, as the spacing tends to decrease, the dispensing device tends to move away from the target, as the spacing tends to increase, the dispensing device tends to move toward the target, and as the spacing tends to remain substantially constant, the dispensing device tends to remain substantialy stationary with respect to the target.

1 3. Apparatus for adjusting the spacing between a target to be coated by coating material and a dispensing device for the coating material, comprising means for selectively movably mounting the dispensing device, means for sensing the spacing between the dispensing device and that portion of the target adjacent the dispensing device and for generating a signal in response to such sensing, and means for coupling the signal to the selectively movable mounting means.

14. A position adjustment system constructed and arranged substantially as hereinbefore described and shown in the drawings.