WO2014128477A1

WO2014128477A1 - Method and apparatus for controlling a powder coater

Info

Publication number: WO2014128477A1
Application number: PCT/GB2014/050504
Authority: WO
Inventors: David Hugh Campbell; Man Kin Mickey KO
Original assignee: Yu Tung Investment Holdings Limited; MANLEY, Nicholas, Michael
Priority date: 2013-02-21
Filing date: 2014-02-20
Publication date: 2014-08-28
Also published as: GB201303083D0

Abstract

A method for controlling a powder coating apparatus; the powder coating apparatus comprising an electrostatic spray coating gun (10) for supplying an airborne powder cloud and means (50) for electrostatically charging the powder cloud with respect to a grounded workpiece (20), the method comprising the steps of sensing at least one signal corresponding to an electrostatic discharge voltage and/or discharge current;comparing the magnitude of the sensed signal in relation to a predetermined threshold level; and applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current if the sensed signal exceeds the predetermined threshold level as the spray coating gun (10) approaches the workpiece (20).

Description

METHOD AMD APPARATUS FOR CONTROLLING A POWDER COATER

This invention relates to a method and apparatus for controlling a powder coater. i particular, this invention relates to a method and apparatus for safely and efficiently controlling an electrostatic powder coating process which yields a high-quality durable surface finish.

Electrostatic powder coating is a well-estabiishad painting process which uses a free-flowing dry paint which is supplied as an airborne powder cloud from a spray gun. The paint powder is electrostatically charged to enable it to adhere to a grounded workpiece being sprayed. This is normally achieved via a discharge electrode located on the spray gun, which is maintained at an elevated electrical potential with respect to the grounded workpiece. in this configuration, an electrostatic field is created between the discharge electrode and the workpiece. The powder cloud becomes charged as it passes through the electromagnetic field, so that it Is attracted and adheres to the grounded workpiece. When heated, the powder meifs and bonds to the surface of the workpiece as a uniform film, and subsequently cools to form a protective and decorative surface.

Capital equipment and operating costs for a powder coating line are generally less than for a corresponding conventional painting process line, and the dry nature of the coating process allows the majority of any powder coating overspray to be easily recycled. This, coupled with the durability of the final surface finish, has led to powder coating becoming increasingly popular for coating many types of industrial and domestic products.

In the majority of powder coating applications the electrostatic charge is provided by means of high voltage direct current corona discharge at the nozzle of the spray gun.. As the spray gun approaches the grounded workpiece, the discharge current will normally rise following Ohms Law as the resistance of the air reduces. Without any form of regulation this can lead to undesirable coating effects and also creates the risk of sparks and possible ignition of the paint powder cloud. The corona discharge is usually regulated by a control system to prevent excess current which could cause hazardous sparking and possible ignition. The spray operator may also have control of the maximum discharge voltage level (typically between 0 to 100kV) and also the maximum discharge current level (typica!ly between 5 to 100μΑ) in order to select the optimum charging characteristics for the powder, the workpiece and the environmental conditions. However, even with an experienced spray operator, various coating problems can sometimes occur.

The charging mechanism reiias upon charging the airspace between the spray gun and the workplace and this charged air then transfers charge to the powder passing therethrough. The amount of charge required to ionise the air will depend largely upon the distance of the spray gun from the workpiece. In si npla terms, the closer the spray gun is to the workpiece, the less high voltage discharge is required to saturate the air with electrostatic charge (ionise the air). If however too much charge is applied to a given airspace, the excess charge tends to be neutralised by the earthed (grounded) workpiece.

This does not necessarily cause a problem until the powder starts to be deposited onto the surface of the workpiece, whereby it tends to insulate the workpiece so that the excess charge cannot be neutralised. This excess charge then builds up on the surface of the powder layer where it repels further powder deposition and can cause an unwanted "orange peel" type uneven surface finish.

The excess charge can also build up within the powder layer itself causing the powder layer to erupt in a back ionisation process creating holes or pitting in the final powder layer. Additionally, if the charge is too strong, the electrostatic field which is established between the spray gun and the closest part of the workpiece can actually prevent successful coating of recesses and contoured parts. This is because the powder will fend to follow the electrostatic field lines of force to the closest parts of the workpiece and will not penetrate into any recesses in the workpiece. These recesses are referred to in the art as Faraday Cages. In order to minimise this coating problem if is necessary to reduce the charge significantly. The powder will still become charged and the lines of electrostatic force will become much weaker, enabling the charged powder to successfully penetrate the recesses and fully coat the workplace.

Over the years, various techniques have been proposed to alleviate these specific coating problems, Many prior art electrostatic powder coating apparatus are operated in what is termed "constant current" control, whereby the discharge current is observed to rise exponentially as the spray gun approaches the grounded workpiece, but only up to a preset value selected by an operator. When the spray gun is close enough to the workpiece that this current level is reached, the discharge currant will not rise any higher. The discharge voltage will automatically reduce in a linear manner as the spray gun continues to move towards to the workpiece due to the characteristics of the voltage regulator circuit. Figure 1 is illustrative of a "constant current" control method known in the art, whereby the input current to the spray gun has been set by the operator and limited to 800mA.

Although this technique helps to mitigate some of the coating problems observed in the art it does little to address the "orange peeP and pitting effects observed on the surface of the workpiece that occur as the spray gun approaches the workpiece. The discharge current drives the transfer of charge to the powder and this does not reduce as the spray gun distance reduces.

One control system which addresses this further is a type of charge control whereby the discharge current, as well as the discharge voltage, is reduced as the spray gun approaches the workpiece. Figure 2 illustrates such a further prior art technique and it can he seen that when the input gun current of 800mA is exceeded as the spray gun approaches the workpiece, both the discharge voltage and discharge current are reduced by the control system in a linear manner.

Whilst these two known control techniques do realise some improvements in terms of alleviating the coating problems described above. They all work however by progressively reducing the level of charge in a linear manner, whereas the optimum level of charge reduction that is required as the spray gun approaches the workplace should instead vary in a non-linear or stepped manner. it is the object of the present invention to provide a method and apparatus for controlling a powder coater which overcomes the drawbacks associated with known modes of operation. The method and apparatus enabling safe and efficient control of an electrostatic powder coating process which produces a coated workplace with a high-quaiity durable surface finish.

According to the present invention there is provided a method for controlling a powder coating apparatus, the powder coating apparatus comprising an electrostatic spray coating gun for supplying an airborne powder cloud and means for electrostatically charging the powder cloud with respect to a grounded workptece, the method comprising the steps of;

sensing at least one signal corresponding to an electrostatic discharge voltage and/or discharge current;

comparing the magnitude of the sensed signal in relation to a predetermined threshold level; and

applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current if the sensed signal exceeds the predetermined threshold level as the spray coating gun approaches the workplace.

An advantage of using the present invention for controlling a powder coating apparatus is that when the sensed signal exceeds the predetermined threshold level, the active control starts to operate which provides a rapid step down in discharge energy which is proportional to the reduction in air volume between the spray gun and the grounded workplace. Use of the invention achieves a desired energy reduction characteristic that overcomes the coating problems described above.

Preferably, the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current is reversible as the electrostatic spray coating gun moves away from the workplace. Further preferably, the at least one signal Is selected from the group consisting of step of spray gun input current, spray gun control voltage, electrostatic discharge current and electrostatic discharge voltage. In use, the predetermined threshold level may be selected in dependence of the properties of the powder cloud and/or the workplace shape and/or type of spray nozzle and/or atmospheric humidify and/or other environmental conditions.

Preferably, the step of applying at least one stepped reduction of the eiectrostafic discharge voltage and/or discharge currant is repeated until a predetermined minimum value of the electrostatic discharge voltage and discharge current is reached.

Further preferably, the predetermined minimum value corresponds to a full foldback condition where the magnitude of the electrostatic discharge voltage and discharge current approaches zero or near zero levels.

In use, the predetermined minimum value of the eiectrostafic discharge voltage and discharge current may be reached after the application of 3 to 7 successive stepped reductions.

Preferably, the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current further comprises reducing the spray gun control voltage by between 1 to 3V.

Further preferably, the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current further comprises reducing the spray gun control voltage by around 2V. In use, the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current may further comprise reducing the electrostatic discharge voltage by between 5kV and 25kV. Preferably, the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current further comprises reducing the electrostatic discharge current by between 20mA to 150mA, Further preferably, the successive stepped reductions of the electrostatic discharge voltage and/or discharge current are of equal magnitude and/or are graduated and/or comprise alternative large and small step reductions In order to achieve a desired energy reduction characteristic, in use, a large step reduction may comprise reducing the spray gun control voltage by between 7 to 21 % and a small step reduction comprises reducing the spray gun control voltage by between 2 to 4%.

Also according to the present invention there Is provided a method of controlling the discharge energy between a high potential electrode and a grounded workpiece, comprising the steps of:

sampling a current representative of a discharge current and when the sampled current reaches a predetermined threshold level reducing the discharge energy to the electrode corresponding to the reduction in air volume between the electrode and the grounded workpiece.

Preferably, the step of sampling a current representative of a discharge current has a sampling rate of between 5 to 50ms, Further preferably, the reduction in air volume between the electrode and the grounded workpiece follows an inverse cube law.

Further according to the present invention there is provided a computer program product for controlling a powder coating apparatus, the powder coating apparatus comprising an electrostatic spray coating gun for supplying an airborne powder cloud and means for electrostatically charging the powder cloud with respect to a grounded workpiece, comprising:

computer program product means for sensing at least one signal corresponding to an electrostatic discharge voltage and/or discharge current; ø computer program product means for comparing the magnitude of the sensed signal in reiation to a predetermined threshoid level] and

computer program product means for applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current if the sensed signal exceeds the predetermined threshold level as the spray coating gun approaches the workplace.

Also further according to the present invention there is provided a computer program product for controlling the discharge energy between a high potential electrode and a grounded workplace, comprising:

computer program product means for sampling a current representative of a discharge current and when the sampled current reaches a predetermined threshold level reducing the discharge energy to the electrode corresponding to the reduction in air volume between the electrode and the grounded workpiece,

Likewise according to the present invention there is provided a powder coating apparatus for coating a grounded workpiece, comprising:

an electrostatic spray coating gun having a gun body and means for connecting to a supply of coating powder;

means for supplying a voltage for connection to a discharge electrode; means for monitoring the discharge current and/or gun supply current; and

means for modifying the discharge voltage and/or discharge current based on the monitored discharge current and/or gun supply current, wherein the means for modifying is adapted to adjust the discharge voltage and/or discharge current in a stepped manner as the spray coating gun approaches the workpiece.

Preferably, the means for supplying, the means for monitoring and the means for modifying are implemented in a microprocessor or digital signal processor.

Further preferably, the microprocessor or digital signal processor also Includes additional programmable functionality, which Is selected from the group consisting, but not limited to, any one of the following: the capabliity to notify the user of the various process conditions, or to warn and/or act upon overcurrent and/or overvoStage conditions, mode of operation.

St is believed that a method and apparatus for controlling a powder coater in accordance with the present invention at least addresses the problems outlined above. The advantages of the present invention are that a method and apparatus for safely and efficiently controlling an electrostatic powder coating process are provided. The method and apparatus enabling production of a coated workplace with a high-quality durable surface finish. if will be obvious to those skilled in the art that variations of the present invention are possible and if is intended that the present invention may be used other than as specifically described herein.

A specific non-limiting embodiment of the invention will now be described by way of example and with reference to the accompany drawings, in which;

Figures 1 and 2 are respectively graphs illustrating constant current control and combined voltage and current control modes of operation of an electrostatic powder coating process;

Figure 3 is a schematic circuit diagram for controlling an electrostatic powder coating process in accordance with the present invention;

Figure 4 illustrates how the volume of air defined between the nozzle of a spray gun and a workplace can he determined;

Figures 5 to 7 show how the discharge current and voltage can be reduced in steps as the spray gun approaches the workplace in accordance with the present invention, and vice-versa as the spray gun is moved away from the workpiece; Figures 8 and 10 illustrate an alternative embodiment of the present invention wherein the magnitude of the spray gun input voltage is held constant between steps and the spray gun input supply current is permitted to increase normally as the spray gun approaches the workpiece to control the desired charge reduction; and s Figures 9 and 11 show the corresponding spray gun output voltage and current obtained from Figures 8 and 10, respectively.

Referring now to the drawings, a method and apparatus for controlling a powder coating process is shown in Figure 3. Figure 3 is a schematic diagram showing how the automatic powder coating process can be controlled using a processing unit 30, The coating process is achieved using a conventional powder coating spray gun 10 which comprises a body 12, a grip 14 and a nozzle 18, Towards the bottom of the grip 14, there is a connector 52 for connecting the spray gun 10 to a hose (not shown) that supplies dry powder thereto. The skilied person will appreciate that the processing unit 30 controls many aspects of the spray gun's 10 operation, such as the powder throughput which Is supplied from a pump (not shown) The processing unit 30 Is, accordingly, operatively connected to control the pump so thai the delivery of powder to the nozzle 18 of the gun 10 can be conveyed through an internai conduit 48.

The upper part of the body 12 of the gun 10 also houses a variabie high voltage power supply which typically comprises an oscillator 40, a high voltage transformer 42 and a high voltage multi-stage multiplier 44. The output of the high voltage multi-stage multiplier 44 is connected to a high voltage discharge electrode 50 situated at the nozzle 18. As shown, the discharge electrode 50, which can be in the form of a wire, passes through the nozzle 16 and protrudes beyond the front face of the body 12 of the gun 10.

The variable high voltage power supply is controlled by the processing unit 30, and converts a 12V DC input signal 38 to a ± 100 kV DC output voltage. The power supplied to the discharge electrode 18 is controlled by adjusting the variabie high voltage power supply,

A power sensing circuit 38 and control circuit 34 is also provided to monitor and modify the discharge voltage and current. The power sensing circuit 36 monitors the load drawn by the transformer 42, which is assumed to vary as a function of the load at the discharge electrode 50. The measured discharge voltage and current are fed to the processing unit 30 (via appropriate analogue to digital converters) which monitors and logs the respective readings, if the measured voltage or current moves outside specified ranges, then the processing unit 30 outputs a low voltage signal 38 to adjust the variable high voltage power supply to bring the voltage and/or current at the discharge electrode 50 back within the specified range.

The control circuit 34 is supplied via a low voltage power supply 32 which is connected to an external power source 100.

The skilled person will appreciate that the processing unit 30 may he provided as a microprocessor or digital signal processor. In use, the microprocessor or digital signal processor includes a set of instructions or algorithm written in software. The set of instructions or algorithm written in software controls the power sensing 36 and control circuit 34. The microprocessor or digital signal processor is adapted to reduce the voltage and current in steps as the spray gun 10 approaches the grounded workplace 20. The position and level of each step is selected by the microprocessor or digital signal processor to give the required reduction in discharge energy relative to the maximum settings (kV and pA) of the control unit.

The processing unit 30 also includes various programmable primary and secondary functionality, for example the capability to notify the user of the various process conditions, or to warn and act upon, if necessary, dangerous overcurrent and/or overvoltage conditions. The processing unit 30 also includes a user display and interface {not shown) so that an operator can specify the mode of operation and/or the process parameters, e.g. the maximum predetermined discharge current and voltage and stepped control voltages,

A trigger 54 is provided on the grip 14 of the manual spray gun 10 so that an operator can start or stop the spray coating process as desired. The product or workpiece 20 that is to be spray coated is positioned in front of the spray gun 10. In use, the workpiece 20 can be grounded using a number of different techniques, for example, the embodiment shown in Figure 3 shows the product 20 being mounted on a conductive product hanger 22 which is then connected a grounded product support 24. The present invention can also be implemented in an automatic gun, in which the coating process may be started or stopped manually under the control of the processing unit 30 or automatically by other devices, such as light curtains or other workpiece detectors, connected to the processing unit 30. The present invention uses an active automatic control system to reduce the discharge voltage and current in a non-linear or stepped manner as the spray gun 10 approaches the grounded workpiece 20, To more accurately determine the level of charge reduction required it is useful to refer to Figure 4 which illustrates an effective volume of air that is formed between the nozzle of a spray gun 10 and a grounded workpiece 20, By equating this as a hemisphere, the volume of air V is given by the equation V™2/3nr³ _s where r is the distance between the no zle of the spray gun 10 and the grounded workpiece 20,

If the distance r is reduced by a factor of 2, the hemispherical air volume is reduced by a factor of 8, Utilising the "constant current" control method, as illustrated in Figure 1 if the distance between the nozzle of the spray gun 10 and the grounded workpiece 20 is reduced by a factor 2, the reduction in the discharge voltage only would have the effect of reducing the discharge energy by a factor of 2, Reducing the discharge voltage and discharge current, as illustrated in the prior art technique of Figure 2, is slightly more effective since it reduces discharge energy by a factor of 4, if the nozzle of the spra gun 10 is moved the same distance. Clearly the linear response of both of these prior art approaches means that the charge levels are still too high, which can lead to one or more of the coating problems identified above occurring.

The present invention relates to an activ automatic control system to reduce the discharge voltage and current in a non-linear or stepped manner as the spray gun 10 approaches the grounded workpiece 20, as shown in Figure 5. The present invention uses a combination of constant current control combined with an additional step down or step up of the control voltage at predetermined levels of control voltage, in one embodiment, the digital constant current system can operate by sampling the input gun current or the discharge current at regular intervals, for instance, every 5 to 10ms. If the spray gun's 10 proximity to earth 24 or the workpiece 20 is close enough that the sensed current level reaches a predetermined threshold value (in the example shown in Figure 5 the predetermined threshold value is set at 300mA) the active control circuitry is then initiated whereby the control voltage is continuously adjusted in order to maintain the input current at that constant current setting. Each time the current is sampled the control voltage is adjusted up or down if the current is lower or higher than the previous measurement respectively in order to maintain the current at that value. If the current is unchanged, or below the constant current value, then the voltage wi!l not be changed,

The example shown in Figure 5 illustrates the spray gun input current and control voltage as a function of the distance between the spray gun and the workpiece. The skilled person will understand that whilst the signals are shown as continuous lines in the left hand side of Figure 5. since the processing unit 30 is configured in the preferred embodiment using a microprocessor or digital signal processor, the voltages and currents are in fact digital representations thereof. The right hand side of Figure 5 depicts expanded representations of the spray gun input current A' and spray gun control voltage B^! at points A and B. As described above_: when the sampled input current exceeds the predetermined threshold value of 300mA in Figure 5 it is then held at that constant current by reducing the gun control voltage. As the spray gun is moved closer towards the grounded workpiece, for the reasons described above, the gun input current tends to rise as the air resistance falls, if the gun current then exceeds the predetermined threshold value, the control voltage is further reduced. Therefore the obtained "constant current" signal, when implemented digitally, is reminiscent of a sawtooth waveform as depicted at point A". Equally, the digital control voltage is reduced in a series of discrete steps, as shown at point B^J.

The skilled person will also appreciate that the invention could be implemented using conventional electronics and operational amplifiers to sense the reduction in resistance that occurs as the distance between the spray gun and the workpiece is reduced and then applying negative feedback to reduce the control voltage to the variable high voltage power supply, in this case, the obtained responses shown in Figure 5 would be true continuous-time signals, as opposed digital representations thereof. Gve -and-above the continuous reduction in control voltage in order to maintain the gun current at a near constant current setting, as the spray gun 10 further approaches the grounded orkpiece 20 one or more preset stepped control voltages can be set at which the voltage adjustment may be a large adjustment to force down the discharge voltage and current in a non-linear manner. This will give a step down in discharge energy which can he made proportional to the reduction in air volume between the spray gun 10 and the grounded workpiece 20 being sprayed. In the embodiment shown in Figure 5, the first preset stepped voltage reduction occurs when the control voltage is reduced to 12V. After each step, the new value of current which is measured becomes the new value for maintaining the constant current setting.

The number of steps can be determined by the number of preset stepped control voltages at which the steps are instigated. The number of steps between the instigation of the active charge control and the full fo!dback of discharge voltage and current (where the discharge voltage and current are reduced to a minimum value which corresponds to zero or near zero levels) could be achieved in 1 step or many steps. It has been found that 3 to 7 steps are sufficient to give good control of the charging characteristics within the normal spraying distance between the spray gun 10 and the workpiece 20, In the example shown in Figure 5, the skilled person will appreciate that 3 preset stepped control voltages at 12V, 8V and 4V cause a significant reduction in the discharge energy to optimise the charge in proximity of the spra gun 10 to the earthed workpiece 20 being sprayed. In the example shown in Figure 5, the magnitude of the stepped control voltage reduction is around 2V. The process is also reversible as the spray gun 10 is moved away from the workpiece 20 as shown in Figure 6.

The current at which the active control circuit starts to operate may be adjusted by the operator in order to control the intensify of the electrostatic discharge to suit the workpiece 20, the powder and/or the environmental conditions and/or the spray nozzle of the gun 10, Figure 7 shows how the discharge current and voltage can be reduced in steps as the spray gun 10 approaches the workpiece 20. The predetermined threshold value where the active control circuitry is initiated is set at 600mA. Whilst Figures 5 to ? refer to gun input or supply currents, the relationship between the gun supply current and the gun discharge current is proportional with a ratio of approximately 10,000 : 1. In this configuration a supply current of 1A would give a discharge current of approximately 100μΑ. The present invention can also be configured whereby the discharge current, as well as the discharge voltage, is reduced as the spray gun 10 approaches the workplace 20. The current is monitored in the same way as with the "constant current" system, but when the active control is operating (above the preset current limit), the control voltage adjustment is greater so that the discharge current is not held constant but is reduced or increased accordingly. The same principles of preset control voltages may be used to establish the iarge steps in output voltage or current in a non-linear manner. Again, the process is reversible as the spray gun 10 is moved away from the workpiece 20. The current at which the active control circuit starts to operate may be adjusted by the operator in order to control the intensity of the electrostatic discharge.

The voltage adjustment which is made at each measurement of current can be by a fixed voltage of around 0.25V, or it could be by a percentage of around 3% or it could be by a factor to achieve the desired effect of either constant current, or voltage plus current foidback, The voltages at which the Iarge adjustment steps are instigated could be of equal value or graduated, in order to achieve a desired energy reduction characteristic.

The desired foidback characteristics could in an alternative embodiment be achieved by simply controlling the magnitude of the control voltage steps and the threshold current at which the foidback is initiated. The speed of response being controlled by the time intervals at which the gun current is sampled. For example, if the maximum control voltage is say 12V and the voltage steps are say 3V, then if the gun current is sampled every 30ms, full foidback from the threshold current being sensed can be achieved in 90ms in 4 steps and also making the electrostatic discharge safe from the possibility of creating incendive sparks. In this method the gun suppiy voltage is held constant between steps and the gun supply current Is permitted to Increase normally as the gun approaches the earthed workplece 20 to control the desired charge reduction.

Figure 8 shows the input currents and voltages to a spray gun 10 where the active control has been set to 300m.A, The corresponding typical gun discharge voltage and current are then shown in Figure 9, Equally the inputs and outputs for a stepped control occurring at 800mA are shown in Figures 10 and 11, respectively. Again, the process is reversible and the control vol age will step up if a reduction of the gun supply current is sensed as the spray gun 10 is moved away from the earthed workplace 20. Various alterations and modifications may be made to the present invention without departing from the scope of the invention.

Claims

CLAIM

1 , A method for controlling a powder coating apparatus, the powder coating apparatus comprising an electrostatic spray coating gun for supplying an airborne powder cloud and means for electrostatically charging the powder cloud with respect to a grounded workplace, the method comprising the steps of:

sensing at least one signa corresponding to an electrostatic discharge voltage and/or discharge current;

applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current if the sensed signal exceeds the predetermined threshold level as the spray coating gun approaches the workplace. 2, The method as claimed in claim 1 , wherein the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current is reversible as the electrostatic spray coating gun moves away from the workpiece, 3, The method as claimed in claims 1 or 2, wherein the at least one signal is selected from the group consisting of step of spray gun input current, spray gun control voltage, electrostatic discharge current and electrostatic discharge voltage. 4, The method as claimed in any preceding claim, wherein the predetermined threshold level is selected in dependence of the properties of the powder cloud and/or the workpiece shape and/or type of spray nozzle and/or atmospheric humidify and/or other environmental conditions. 5. The method as claimed in any preceding claim, wherein the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current is repeated until a predetermined minimum value of the electrostatic discharge voltage and discharge current is reached.

8. The method as claimed in claim 5, wherein the predetermined minimum value corresponds to a full foldback condition where the magnitude of the electrostatic discharge voltage and discharge currant approaches zero or near zero levels,

7. The method as claimed in claim 5, wherein the predetermined minimum value of the electrostatic discharge voltage and discharge current is reached after the application of 3 to 7 successive stepped reductions. 8. The method as claimed as claimed in claim 3, wherein the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current further comprises reducing the spray gun control voltage by between 1 to 3V. 9. The method as claimed as claimed in claim 3_S wherein the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current further comprises reducing the spray gun control voltage by around 2V, 10. The method as claimed as claimed in claim 3, wherein the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge currant further comprises reducing the electrostatic discharge voltage by between SkV and 25kV. 11. The method as claimed as claimed in claim 3, wherein the step of applying at least one stepped reduction of the electrostatic discharge voltage and/or discharge current further comprises reducing the electrostatic discharge current by between 20mA to 150mA. 12. The method as claimed as claimed in any of claims 5 to 7, wherein the successive stepped reductions of the electrostatic discharge voltage and/or discharge current are of equal magnitude and/or are graduated and/or comprise alternative large and small step reductions in order to achieve a desired energy reduction characteristic.

13. The method as claimed as claimed in claim 12, wherein a large step reduction comprises reducing the spray gun control voltage by between 7 to 21% and a small step reduction comprises reducing the spray gun control voltage by between 2 to 4%.

5

14. A method of controlling the discharge energy between a high potential eiectrode and a grounded orkpiece, comprising the steps of:

sampling a current representative of a discharge current and when the sampled current reaches a predetermined threshold level reducing the discharge 10 energy to the electrode corresponding to the reduction in air volume between the eiectrode and the grounded workpiece.

15, The method as claimed as claimed in claim 14, wherein the step of sampling a current representative of a discharge current has a sampling rate of :i.S between 5 to 50ms.

18. The method as claimed as claimed in claim 14, wherein the reduction in air volume between the electrode and the grounded workpiece foliows an inverse cube law.

20

17. A computer program product for controlling a powder coating apparatus, the powder coating apparatus comprising an electrostatic spray coating gun for supplying an airborne powder cloud and means for electrostatically charging the powder cloud with respect to a grounded workpiece, comprising:

25 computer program product means for sensing at least one signal corresponding to an electrostatic discharge voltage and/or discharge current; computer program product means for comparing the magnitude of the sensed signal in relation to a predetermined threshold level; and

computer program product means for applying at least one stepped

30 reduction or^" the electrostatic discharge voltage and/or discharge current if the sensed signal exceeds the predetermined threshold level as the spray coating gun approaches the workpiece.

18. A computer program product for controlling the discharge energy between 35 a high potential electrode and a grounded workpiece, comprising: computer program product means for sampling a current representative of a discharge current and when the sampled current reaches a predetermined threshold level reducing the discharge energy to the electrode ccrresponding to the reduction in air voiume between the electrode and the grounded workpiece,

19. A powder coating apparatus for coating a grounded workpiece, comprising:

20. The powder coating apparatus as claimed in claim 19, wherein the means for supplying, the means for monitoring and the means for modifying are implemented In a microprocessor or digital signal processor. 21. The powder coating apparatus as claimed in claim 20, wherein the microprocessor or digital signal processor also includes additional programmable functionality, which is selected from the group consisting, but not limited to, any one of the following: the capability to notify the user of the various process conditions, or to warn and/or act upon overcurrent and/or overvolfage conditions, mode of operation,

22. A method for controlling a powder coating apparatus as hereinbefore described.

23. A method of controlling the discharge energy between a high potential electrode and a grounded workpiece as hereinbefore described.

24. A computer program product for controlling a powder coating apparatus as described herein with reference to Figures 3 to 11 of the accompanying drawings.

25. A computer program product for controlling the discharge energy between a high potential electrode and a grounded workpiece as described herein with reference to Figures 3 to 11 of the accompanying drawings.

26. A powder coating apparatus for coating a grounded workpiece as described herein with reference to Figures 3 to 11 of the accompanying