Abstract:
A coating deposition apparatus has first and second charge sources. The first charge source is chargeable to a first voltage potential and the second charge source is chargeable to a second voltage potential. The coating deposition apparatus also has a first output terminal and a deposition substance connected thereto, and a second output terminal for connection to a workpiece. The consumable deposition substance is movable relative to the workpiece. The first charge source is connected between the first and second terminals whereby the first voltage potential is established therebetween. The second charge source is connected between the terminals. The coating deposition apparatus also has discharge control circuitry connected to the first and second charge sources to inhibit discharge of the second charge source through the terminals prior to commencement of discharge of the first charge source through the terminals.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/231,776, entitled ELECTRODE COATING APPARATUS AND METHOD, filed Aug. 6, 2009, the entire contents and disclosure of which is incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the field of coating technologies, and more particularly, to an electrospark deposition coating apparatus and method. 
       BACKGROUND OF THE INVENTION 
       [0003]    Electrospark deposition (ESD) is a pulsed-arc micro-welding process that uses a short duration, high-current electrical pulse to melt and deposit a portion of a consumable electrode onto a workpiece. The deposited material alloys with the workpiece to form a metallurgical bond. 
         [0004]    In the ESD process, the consumable electrode and the workpiece are connected to opposite terminals of a source of power or charge. When the consumable electrode and the workpiece are brought close together, the electric potential between the consumable electrode and the workpiece cause an electric spark. The spark generates an amount of heat, which melts a portion of the consumable electrode. The melted portion of the consumable electrode is then transferred from the consumable electrode and deposited locally on the workpiece in the region of the electric arc when the consumable electrode and the workpiece come into contact. The process may be repeated to form a coating on the workpiece. 
         [0005]    One of the main advantages of the ESD process is that the consumable electrode material is fused to the workpiece at such low heat input that the workpiece remains at or neat ambient temperature. Specifically, by controlling the spark duration to a few microseconds and the spark frequency to around 1000 Hz, for example, the welding heat is generated during less than 1% of an ESD cycle, while the heat is dissipated during 99% or more of the cycle. Furthermore, the workpiece is constantly moving relative to the consumable electrode. Thus, the location of the electric arc, and the highly localized region of the workpiece subject to the heating, changes rapidly and, at the scale of interest, substantially randomly so a different region is being heated with each spark cycle. Therefore, unless the workpiece is particularly thin or the sparking time is unusually prolonged, the workpiece will remain near ambient temperature. In addition, since the deposited material is metallurgically bonded, it is inherently more resistant than the mechanically bonded coatings produced by other low-heat input processes, such as electro-chemical plating. 
       SUMMARY OF THE INVENTION 
       [0006]    In an aspect of the invention there is a coating deposition apparatus. It has first and second charge sources. The first charge source is chargeable to a first voltage potential and the second charge source is chargeable to a second voltage potential. The coating deposition apparatus also has a first output terminal and a deposition substance connected thereto, and a second output terminal for connection to a workpiece. The consumable deposition substance is movable relative to the workpiece. The first charge source is connected between the first and second terminals whereby the first voltage potential is established therebetween. The second charge source is connected between the terminals. The coating deposition apparatus also has discharge control circuitry connected to the first and second charge sources to inhibit discharge of the second charge source through the terminals prior to commencement of discharge of the first charge source through the terminals. 
         [0007]    In another feature of that aspect of the invention, the discharge control circuitry includes at least one isolation element operable to prevent charge from flowing from the first charge source to the second charge source. In another feature of that aspect of the invention, the coating deposition apparatus has charging circuitry by which to charge the first and second charge sources. The discharge control circuitry is operable to inhibit discharge of at least one of the first and second charge sources during charging thereof. In another feature of that aspect of the invention, the coating deposition apparatus has voltage potential monitoring circuitry connected to sense voltage potential across the first charge source and across the second charge source. The discharge control circuitry is operable to inhibit discharge of the first and second charge sources until the first charge source reaches at least a first charging threshold voltage potential and the second charge source reaches at least a second threshold voltage potential. 
         [0008]    In another feature of that aspect of the invention, at least one of the first and second charge sources is one of a) a capacitor; and b) a capacitor bank. In another feature, at least one of the first and second charge sources has a variable capacitance. In another feature of that aspect of the invention, the consumable deposition substance is composed at least in part of titanium, titanium carbide, titanium diboride, nickel, molybdenum, and tungsten. In another feature, the consumable deposition substance is predominantly titanium. In another feature, the workpiece is predominantly copper. In another feature of that aspect of the invention, the coating deposition apparatus has a vibrator mounted to act on at least one of (a) the workpiece; and (b) the consumable deposition substance. In another feature of that aspect of the invention, the coating deposition apparatus has a drive to spin at least one of (a) the workpiece; and (b) the consumable deposition substance, to present a different portion of the workpiece to the consumable deposition substance as a function of time. 
         [0009]    In another aspect of the invention, there is a process of depositing a coating on an electrically conductive workpiece using a coating deposition apparatus. The coating deposition apparatus has first and second charge sources coupled between first and second output terminals. The process includes connecting a consumable deposition substance of coating material to the first terminal; connecting a workpiece to the second terminal; establishing a first voltage potential on the first charge source; establishing a second voltage potential on the second charge source; establishing the consumable deposition substance and the workpiece in close proximity; discharging charge from the first charge source between the consumable deposition substance and the workpiece, thereby melting some of the consumable deposition substance and depositing it on the workpiece; and after commencement of discharge of the first charge source, discharging charge from the second charge source between the consumable deposition substance and the workpiece, during arcing of current between the consumable deposition substance and the workpiece, thereby melting more of the consumable deposition substance and welding the melted consumable deposition substance to the workpiece. 
         [0010]    In a feature of that aspect of the invention, the process includes monitoring the first voltage potential, and commencing discharge of the second charge source when the first voltage potential falls below a first threshold value. In another feature, the process includes recharging the first and second charge sources following respective discharge thereof, and inhibiting the recharging of at least the first charge source until the first voltage potential falls below a discharge threshold value. In another feature, the process includes re-charging the first and second charge sources, and during re-charging, inhibiting discharge of the first and second charge sources. In another feature, the first charge source has a first charging threshold voltage potential, the second charge source has a second charging threshold voltage potential, and during re-charging, the discharge is inhibited until the first charge source reaches at least the first charging threshold voltage potential and the second charge source reaches at least the second charging threshold voltage potential. 
         [0011]    In a further feature, the first charging source provides a first total energy to the output terminals during discharge thereof. The second charging source provides a second total energy to the output terminals during discharge thereof. The first total energy is related to the first charging threshold voltage potential and the second total energy is related to the second charging threshold voltage potential. At least one of the first and second charging threshold voltage potentials is adjustable. In this feature the process includes adjusting the at least one charging threshold voltage potential. 
         [0012]    In another feature, the first charge source is associated with a first capacitance and the second charge source is associated with a second capacitance. The first total energy is related to the first capacitance. The second total energy is related to the second capacitance. At least one of the first and second capacitances is adjustable. In this feature, the process includes adjusting the at least one capacitance. In another feature, the process includes waiting for a predetermined period of time following discharge and inhibiting discharge during the predetermined period of time. In another feature of that aspect of the invention, the process includes selecting the consumable deposition substance from amongst substances that are at least partially one of titanium, titanium carbide, titanium diboride, nickel, molybdenum, and tungsten. In another feature, the process includes selecting a substantially titanium substance as the consumable deposition substance. In another feature, the process includes selecting a copper substance as the workpiece. In another feature of that aspect of the invention, the process includes vibrating at least one of the consumable deposition substance and the workpiece. In another feature of that aspect of the invention, the process includes rotating at least one of the workpiece and the consumable deposition substance to present a different portion of the workpiece to the consumable deposition substance as a function of time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    These and other aspects of the invention may be more readily understood with the aid of the illustrative Figures included herein, and showing an example, or examples, embodying the various aspects and features of the invention which examples are provided by way of illustration, but not of limitation of the present invention, and in which: 
           [0014]      FIGS. 1   a  and  1   b  are conceptual illustrations of the variation of the voltage and current during an ESD process; 
           [0015]      FIG. 1   c  shows a graphical representation of overall output voltage as a function of time in an ESD process as described herein; 
           [0016]      FIG. 1   d  is a graphical representation of voltage as a function of time for a first charge source of the ESD process of  FIG. 1   c;    
           [0017]      FIG. 1   e  is a graphical representation of voltage as a function of time for a second charge source of the ESD process of  FIG. 1   c;    
           [0018]      FIG. 2  is a block diagram of a system for transferring material from a consumable deposition substance to a workpiece in accordance with an aspect of the invention; 
           [0019]      FIG. 3  is a block diagram of a coating deposition apparatus of the system of  FIG. 2 ; 
           [0020]      FIG. 4  is a circuit diagram of a first charge source of the coating deposition apparatus of  FIG. 3 ; 
           [0021]      FIG. 5  is a circuit diagram of a second charge source of the coating deposition apparatus of  FIG. 3 ; 
           [0022]      FIG. 6  is a circuit diagram of first charging circuitry of the coating deposition apparatus of  FIG. 3 ; 
           [0023]      FIG. 7  is a circuit diagram of second charging circuitry of the coating deposition apparatus of  FIG. 3 ; 
           [0024]      FIG. 8  is a circuit diagram of discharge circuitry of the coating deposition apparatus of  FIG. 3 ; 
           [0025]      FIG. 9  is a block diagram of a main control circuit of the coating deposition apparatus of  FIG. 3 ; 
           [0026]      FIG. 10  is a circuit diagram of a first input conditioning circuit of the coating deposition apparatus of  FIG. 3 ; 
           [0027]      FIG. 11  is a circuit diagram of a second input conditioning circuit of the coating deposition apparatus of  FIG. 3 ; 
           [0028]      FIG. 12  is a circuit diagram of a third input conditioning circuit of the coating deposition apparatus of  FIG. 3 ; 
           [0029]      FIG. 13  is a circuit diagram of an output signal conditioning circuit of the coating deposition apparatus of  FIG. 3 ; 
           [0030]      FIG. 14  is a circuit diagram of a first voltage comparison circuit of the main control circuit  FIG. 9 ; 
           [0031]      FIG. 15  is a circuit diagram of a second voltage comparison circuit of the main control circuit of  FIG. 9 ; 
           [0032]      FIG. 16  is a circuit diagram of a charging control circuit of the main control circuit  FIG. 9 ; 
           [0033]      FIG. 17  is a circuit diagram of a digital output isolation circuit of the main control circuit of  FIG. 9 ; and 
           [0034]      FIG. 18  is a block diagram of an alternative coating deposition apparatus of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0035]    The description that follows, and the embodiments described therein, are provided by way of illustration of an example or examples, of particular embodiments of the principles of the present invention. These examples are provided for the purpose of explanation, and not limitation, of those principles and of the invention. In the description, like parts are marked throughout the specification and the drawings with the same respective reference numerals. The drawings are not necessarily to scale and in some instances proportions may have been exaggerated in order more clearly to depict certain features of the invention. 
         [0036]    Typically in electrospark deposition (ESD) processes, one terminal of a power source, or charge source, however it may be called, is connected to a consumable supply of deposition substance. The other terminal of the power source is connected to a workpiece on which an accretion of the deposition substance is desired. The consumable supply may have the form of an electrically conductive or semi-conductive rod of the deposition material. While the term “power source” may be used by persons of skill in the art, the “power source” may tend not to be a power source in the sense of a generator or supply of line power from electrical mains, but may rather tend to be a reservoir of electrical charge raised to some electrical potential that may then be permitted selectively to discharge through the various circuit elements. While this may be a power source, it is a transient source. This power source or charge source reservoir may itself be charged and recharged, as may be appropriate, by a “power source” in the sense, of a generator or a connection to a mains supply, whether direct or rectified, as may be. In the particular context of a releasable source of charge at an electrical potential, that charge source may often be a capacitor or a capacitor bank. In this document, unless otherwise noted or clear from a different context, the term “charge source” will be used to mean a power source that can be charged and discharged in a manner similar to a capacitor. For many purposes the terms power source and charge source may be used interchangeably herein. In some cases the predetermined threshold voltage to which the capacitor bank is to be charged is user adjustable. To add an additional measure of flexibility, the number of capacitors in the capacitor bank may also be varied. 
         [0037]    Once the charge source or power source has been charged to a predetermined voltage, the consumable deposition substance and the workpiece are brought into close proximity. Eventually an electric spark jumps the gap. Material from the consumable deposition substance melts and is transferred to the workpiece as current is drawn from the charge source. Up to now an assumption, or common understanding in the field, was that this transfer occurred in a single step. Specifically, it was thought that the current was drawn from the charge source in a single step (i.e. one current pulse). The inventors have observed, however, that the transfer tends to occur in two steps or phases which may have the form of two separate and distinct current pulses. Generally, the first phase is defined as the time period during which the first current pulse occurs, and the second phase is defined as the time period in which the second current pulse occurs. The first step, or phase, may be referred to as the sparking phase and the second step or phase may be referred to as the welding or arcing phase. The two phases and the associated current pulses are shown in  FIG. 1   a.    
         [0038]    To begin the ESD process the charge source is charged to a predetermined voltage  106 . The consumable deposition substance is then brought near the workpiece, triggering the first or sparking phase  102  at time t TD1 . That is, the difference in electric potential, V H , between the consumable deposition substance and the workpiece causes an electric spark between the consumable deposition substance and the workpiece. A first electric current pulse  108  of relatively high amplitude then flows between the consumable deposition substance and the workpiece. As can be seen in  FIGS. 1   a ,  1   c  and  1   d , this causes a rapid drop in the charge source voltage to some level or plateau, V P , that may be a modest value, and may approach zero volts. The heat generated in this sparking phase melts, and is thought partially to vaporize, a portion of the consumable deposition substance or a local portion of the workpiece, or both. This is believed to create a new narrow gap between the consumable deposition substance and the workpiece, and a consequent reduction in current flow. 
         [0039]    As the consumable deposition substance continues to move towards the workpiece, the heated portion of the consumable deposition substance makes contact with the workpiece. It is at this point, identified as time t DC2 , that the second or arcing phase  104  occurs. A second electric current pulse  110  flows between the consumable deposition substance and the workpiece. Typically, the current flows until the charge source is substantially completely drained and a low threshold residual voltage level, V L , is reached. The current flow produces additional heat that melts and fuses a portion of the consumable deposition substance to the workpiece, leaving an incremental accretion. The repeated additions eventually yield a coating covering, or substantially covering, the entire surface. 
         [0040]    Generally, as shown in  FIG. 1   a , the respective commencements of the two discharge phases DC 1  and DC 2  (and thus the two current pulses) are separated in time. For example, in  FIG. 1   a , there is illustrated roughly a 2.5 ms delay between the first or sparking phase  102  and the second or arcing phase  104 . Typically it is thought that the time between the two phases (and thus the time between the two current pulses) depends on the speed at which the consumable deposition substance and the workpiece are moved toward each other. If the consumable deposition substance and the workpiece are moved slowly toward each other, the two phases (and thus the two current pulses) may be quite clearly separate and may be separated by a substantial nil current plateau as shown in  FIG. 1   a . If however, the consumable deposition substance and the workpiece are moved together quickly, the two phases (and thus the two current pulses) may occur in quick succession, making it more difficult to distinguish the two phases (and thus the two pulses). An example of this is shown in  FIG. 1   b.    
         [0041]    Known earlier ESD systems typically included only a single charge source. Embodiments herein, however, relate to ESD systems that have two independent charge sources connected to provide energy to the two distinct phases of the ESD process. That is, a first, higher voltage potential charge source may be used to provide energy in the form of a discharging electrical current to the sparking phase and a second, lower voltage potential charge source may be used to provide predominantly or entirely an additional charge, or boost, of energy also in the form of an electrical current, to the arcing phase. This may tend to allow at least a measure of independent control, or controlled variation, of the two phases of the ESD process, and thus an alteration or bias in the relative proportion of energy, and thus of heating, in the first and second pulses or pulse portions. In the view of the inventors this may tend to improve consistency of deposition. In the view of the inventors, a varied or independent control of this nature may tend to permit improvement of the coating quality, improvement of the energy efficiency of the coating process, or a decrease in the processing time to apply the coating, as compared to exiting single charge source systems (e.g. single capacitor or single capacitor bank systems). Of course, as may be determined by testing for a particular geometry or combination of materials, it may be desirable not to exceed a particular level of local heating in a single pulse cycle, as may be reflect the ability to cool the workpiece. To the extent that there is relative motion between the source of material and the surface to be coated, and there is a certain randomness in the location of the next deposition point due to that motion, such that the next burst or pulse of local heating will occur in a different location, and so on. 
         [0042]    The systems described herein may be used, for example, for coating copper (Cu) or copper-based electrodes with titanium (Ti), titanium carbide (TiC), titanium diboride (TiB 2 ), nickel (Ni), tungsten (W), or molybdenum (Mb). However, the systems described herein may be used to coat other conductive workpieces with other suitable conductive materials. 
         [0043]      FIG. 2  shows an ESD system  120  for depositing material from a consumable deposition substance  122  on a workpiece  124 . System  120  may include a coating deposition apparatus  126  and an ESD applicator assembly  128  operatively coupled to coating deposition apparatus  126 . 
         [0044]    Consumable deposition substance  122  and workpiece  124  are made of electrically conductive material, such as metals, alloys, conductive ceramics and cement. When consumable deposition substance  122  and workpiece  124  are set to different electric potentials an electric spark is generated between the two components as they are brought into sufficiently close proximity. As described above, the spark functions to melt a portion of consumable deposition substance  122  and to cause the transfer of the melted portion to workpiece  124 . Consumable deposition substance  122  may, for example, be in the form of a consumable electrode or rod. Workpiece  124  may be in the form of an electrode, such as a copper or copper based welding electrode or cap or other substrate. 
         [0045]    Coating deposition apparatus  126  may include a power source operable to provide the current needed for the ESD process; and a control circuit for controlling ESD applicator assembly  128 . In some embodiments, the power source of coating deposition apparatus  126  may have first and second output terminals  130  and  132 . For example, first and second output terminals  130  and  132  may be positive and negative terminals respectively. Coating deposition apparatus  126  may also include an input panel  138  comprising one or more input ports  140  for receiving input signals from one or more external devices. A description of suitable input signals will be provided below. Coating deposition apparatus  126  may also include an output panel  142  comprising one or more output ports  144  for outputting one or more output signals to one or more external devices. A description of suitable output signals will be provided below. 
         [0046]    ESD applicator assembly  128  may include a consumable deposition substance holder  134 , and a workpiece holder  136 . Consumable deposition substance holder  134  may also be referred to as an applicator head or torch. Consumable deposition substance holder  134  and workpiece holder  136  are connected to opposite output terminals of the power source. For example, as shown in  FIG. 2 , consumable deposition substance holder  134  is connected to first terminal  130  and workpiece holder  136  is connected to second terminal  132 . Typically consumable deposition substance holder  134  is connected to the positive output terminal, and workpiece holder  136  is connected to the negative output terminal. Voltage potential across terminals  130 ,  132  is identified as V Output . This voltage potential may be taken as the potential between substance  122  and workpiece  124 . 
         [0047]    Consumable deposition substance holder  134  and workpiece holder  136  may each include, or be attached to, motors. The motors are typically used (i) to cause one of consumable deposition substance holder  134  (and incidentally the consumable deposition substance  122 ) and workpiece holder  136  (and incidentally workpiece  124 ) to vibrate; and (ii) to cause a least a portion of the other of the consumable deposition substance holder  134  and workpiece  136  to rotate. Typically, the consumable deposition substance holder is used to cause consumable deposition substance holder  134  (and incidentally consumable deposition substance  122 ) to vibrate; and the workpiece holder motor is used to rotate at least a portion of workpiece holder  136  (and incidentally workpiece  124 ). 
         [0048]    In some embodiments, consumable deposition substance holder  134  and workpiece holder  136  may be moved manually towards and away from each other. For example, in one embodiment, an operator moves consumable deposition substance holder  134  toward and away from workpiece holder  136  to bring consumable deposition substance  122  and workpiece  124  into and out of contact. In other embodiments, consumable deposition substance holder  134  is connected to a machine that moves consumable deposition substance holder  134  toward and away from workpiece holder  136  to bring consumable deposition substance  122  and workpiece  124  into and out of contact. 
         [0049]    Coating Deposition Apparatus 
         [0050]      FIG. 3  shows coating deposition apparatus  126  of  FIG. 2 . Coating deposition apparatus  126  may include an input power circuit  150 , a first power supply or first charge source  152 , first charging circuitry  154 , a second power supply or second charge source  156 , second charging circuitry  158 , discharge circuitry  160 , a main control circuit  162 , one or more input signal conditioning circuits  164 ,  166 ,  168  and  170 , one or more output signal conditioning units  172 , and a user interface  174 . As noted above, in context, persons of skill in the art may also refer to first charge source  152 , second charge source  156 , and their related circuitry as first and second power sources. 
         [0051]    Input power circuit  150  may receive AC (alternating current) power from a mains supply, for example, a standard 110V wall outlet, and converts it into stable DC (direct current) power suitable for ESD coating. The specific DC voltage required or selected as being suitable for the ESD coating process is based on the particular materials used for consumable deposition substance  122  and workpiece  124 , and may reflect previous experience or testing, or both. This voltage may be designated as the high level or initial high threshold voltage, V H . For example, in one embodiment a DC voltage of V H  of around 32Vdc has been found to be suitable, as, for example, for coating copper alloy electrodes with titanium carbide (TiC). Other voltage levels might be used in the range of about 24Vdc to about 50Vdc, or more narrowly about 30Vdc to about 36Vdc. 
         [0052]    Input power circuit  150  may include an input transformer  176 , a rectifier  178  and a main power supply charge source  180 . Input transformer  176  receives the input AC signal and steps it down or reduces it to a suitable AC signal. For example, input transformer  176  may receive a 110Vac signal, which it steps down to a 48Vac signal. Rectifier  178  may provide full-wave rectification of the reduced AC signal to produce a DC output signal. For example, rectifier  178  may receive a 48Vac signal and convert it to a 68Vdc signal. Rectifier  178  may be configured to convert the AC voltage signal received from input transformer  176  to any suitable DC voltage. For example, rectifier  178  may convert the AC voltage signal to a 150Vdc signal or a 250Vdc signal. The DC signal output by rectifier  178  is used to charge main charge source  180 . The energy stored in main charge source  180  is used to charge first and second charge sources  152  and  156  through first and second charging circuitry  154  and  158 . 
         [0053]    Main charge source  180  may be a capacitor bank made up of a plurality of capacitors connected in parallel. The capacitors may each have the same capacitance; however, they may also have different capacitances. In one embodiment, main charge source  180  may include a number of 1200 μF/120V capacitors connected in parallel. In one embodiment there may be eight such capacitors. The total capacitance of main charge source  180  may be in the range of 12,000 μF to 30,000 μF and in one embodiment may be about 17,600 μF. 
         [0054]    Conceptually, main charge source  180  functions as a large holding tank, or reservoir of charge for replenishing the first and second charge sources  152 ,  156  as may be. This recharging is inhibited, i.e., the charging circuit is disabled, during the discharging period of the operational or duty cycle of charge sources  152 ,  156 , and enabled during the recharging portion of the cycle. 
         [0055]    Input power circuit  150  may also include a bleeding resistor (not shown) and relay (not shown) connected in series with each other, and in parallel with main charge source  180 . The bleeding resistor is used slowly to discharge energy stored in main charge source  180  when power is removed from input power circuit  150 . The relay is typically enabled when power is applied to input transformer  176  which disconnects the resistor from the remainder of the input power circuit  150 . Conversely, the relay is typically disabled when power is removed from input power circuit  150  which connects the resistor to the input power circuit  150 . 
         [0056]    First charge source  152 , also referred to as the sparking charge source, is used to supply energy in the form of electrical current to the sparking phase of the ESD process. The voltage potential of charge source  152  is identified as V 152 . First charge source  152  may be a single capacitor, or a capacitor bank that includes a plurality of capacitors connected in parallel. First charging circuitry  154  charges first charge source  152  to establish a first voltage potential on first charge source  152 . This initial potential is the high threshold voltage, V H . First charge source  152  is subsequently discharged by discharge circuitry  160  to provide energy to the sparking phase of the ESD process. 
         [0057]    The total energy available to be supplied by first charge source  152  is represented by equation (1) where C is the capacitance of first charge source  152 , and V is the first voltage potential of first charge source  152  at the time the discharge commences. 
         [0000]    
       
         
           
             
               
                 
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                       2 
                     
                      
                     
                       CV 
                       
                         
                             
                         
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                   ( 
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         [0058]    In some embodiments, first charge source  152  is charged to a first charging threshold voltage potential V 1C  prior to being discharged by discharge circuitry  160 . Typically, V 1C =V H . The first charging threshold voltage V 1C  potential may be user adjustable. For example, user interface  174  may allow the user to input or select the first charging threshold voltage potential V 1C . In some embodiments, user interface  174  allows the user to select a first charging threshold voltage potential between 15Vdc and 50Vdc. The default value may be, for example, 30Vdc or thereabout. In other embodiments, user interface  174  allows the user to select a first charging threshold voltage potential up to 150Vdc or 250Vdc. For a given size of capacitor or capacitor bank the first charging threshold voltage potential V 1C  determines the amount of heat generated during the ESD process. Typically, the greater the first charging threshold voltage potential V 1C , the greater the heat generated during the sparking phase. 
         [0059]    In some embodiments, the capacitance of first power supply or charge source  152  is fixed. For example, first charge source  152  may have a fixed capacitance of 2000 μF, formed by two 1000 μF capacitors connected in parallel. In other embodiments, the total capacitance of first charge source  152  is adjustable. For example, user interface  174  may allow the operator to select a capacitance value. Relay circuits or selector switches may then be used to control the number of capacitors in first charge source  152 . 
         [0060]    The capacitance of first charge source  152  and the first charging threshold voltage potential required for the ESD process are based on the specific materials used for consumable deposition substance  122  and workpiece  124 . 
         [0061]    First charging circuitry  154  receives DC power from input power circuit  150  and charges first charge source  152  to establish a first voltage potential V H  on first charge source  152 . This charging or re-charging is indicated as  107  in  FIG. 1   d , for example. In some embodiments, first charging circuitry  154  is controlled by a first charging command signal generated by main control circuit  162 . For example, the first charging command signal may enable or disable first charging circuitry  154  when certain conditions are met. In some cases, first charging circuitry  154  (and incidentally charging of first charge source  152 ) may not be enabled, as a time RC 1  (or, t RC1 ) until main control circuit  162  detects that the second or arcing phase of the ESD process has occurred. Main control circuit  162 , may, for example, detect that the second or arcing phase of the ESD process has occurred when the voltage potential of one or both of first and second charge sources  152  and  156  have dropped below a discharge low threshold value, V L . In some embodiments, the discharge threshold value of V L  is zero or substantially zero. 
         [0062]    Second charge source  156 , also referred to as the arcing charge source, is used to provide a supplemental source of electrical current, or power, or energy to the arcing phase of the ESD process. Instantaneous voltage at any time t for second charge source  156  may be identified as V 156 . Second charge source  156  may be a single capacitor, or a capacitor bank that includes a plurality of capacitors connected in parallel. Second charging circuitry  158  charges second charge source  156  to establish a second voltage potential on second charge source  156 . Second charge source  156  is subsequently discharged by discharge circuitry  160  to provide a supplemental source of energy to the arcing phase of the ESD process. The total energy supplied by second charge source  156  is represented by equation (1) where C is the capacitance of second charge source  156 , and V is the second voltage potential of second charge source  156  at the time the discharge commences. 
         [0063]    In some embodiments, second charging source  156  is charged to a second charging threshold voltage potential V 2C  prior to being discharged by discharge circuitry  160 . Typically the second charging threshold voltage potential V 2C  is less than the first charging threshold voltage potential so that second charge source  156  is charged to a lower voltage potential than first charge source  152 . It may also be lower than the plateau voltage, V P . In some embodiments, the second charging threshold voltage potential is user adjustable. For example, user interface  174  may allow the operator to input or select the second charging threshold voltage potential. In some embodiments, user interface  174  allows the operator to select a second charging threshold voltage potential between 10Vdc and 50Vdc. The default value may be, for example, 10Vdc. 
         [0064]    In some embodiments, the capacitance of second charge source  156  is fixed. For example, second charge source  156  may have a fixed capacitance of 880 μF, formed by four 220 μF capacitors connected in parallel. In other embodiments, the capacitance of second charge source  156  is user adjustable. For example, user interface  174  may allow the operator to select a capacitance value for second charge source  156 . Relay circuits or selector switches may then be used to control the number of capacitors in second charge source  156 . 
         [0065]    The capacitance of second charge source  156  and the second charging threshold voltage potential required for the ESD process are based on the specific materials used for consumable deposition substance  122  and workpiece  124 . 
         [0066]    Second charging circuitry  158  receives DC power from input power circuit  150  and charges second charge source  156  to establish a second voltage potential V 156  on second charge source  156 . This recharging is identified at  109  in  FIG. 1   e . In some embodiments second charging circuitry  158  is controlled by a second charging command signal generated by main control circuit  162 . For example, the second charging command signal may only enable second charging circuitry  158  (and incidentally charging of second charge source  156 ) when certain conditions are met. In some cases, second charging circuitry  158  may only be enabled, as at time RC 2 , after main control circuit  162  detects that the second or arcing phase of the ESD process has occurred. Main control circuit  162 , may, for example, detect that the second or arcing phase of the ESD process has occurred when the voltage potentials of one or both of first and second charge sources  152  and  156  have fallen below a discharge threshold value. In some embodiments, the discharge low threshold value V L  is zero or substantially zero. 
         [0067]    Discharge circuitry  160  controls discharge of first and second charge sources  152  and  156 . When discharge circuitry  160  is enabled as at time SC such that the voltage potential of V 152  appears across the output terminals  130 ,  132 . First and second charge sources  152  and  156  may be discharged and thus provide power to the ESD process at the next following opportunity ( FIG. 1   c ). Conversely, when discharge circuitry  160  is disabled, first and second charge sources  152  and  156  are inhibited from being discharged, and thus no power may be provided to the ESD process. 
         [0068]    Discharge circuitry  160  is typically controlled by a discharge command signal generated by main control circuit  162 . In some embodiments, discharge circuitry  160  is disabled (and discharge of first and second charge sources  152  and  156  is inhibited) until the main control circuit  162  detects that first and second charge sources  152  and  156  have been charged to the first and second charging threshold voltage potentials V 1C  and V 2C , the charging being completed, or finishing, at times RC 1 F and RC 2 F respectively. When the controller senses that both V 152 =V 1C  and V 156 =V 2C , further charging may be inhibited, and the appropriate switch set, as at time SC to enable the next discharge. The full cycle between on discharge and the next discharge may typically be of the order of something less than a millisecond, such as perhaps 400-500 microseconds (+/−). 
         [0069]    When discharge circuitry  160  is enabled, first and second charge sources  152  and  156  may be discharged. During sparking phase  102  of the ESD process, the electric spark draws energy from the charge source with the higher voltage potential (typically first charge source  152 ). As charge is drawn from the higher voltage potential charge source, that voltage potential drops below a minimum sparking threshold voltage at which the spark can be maintained. The minimum sparking threshold voltage is somewhat higher than, but relatively close to V P , such that V P  may be considered a fair approximate of the minimum sparking threshold voltage. In one embodiment this minimum sparking threshold voltage, i.e. approximately V P , may be in the range of about 10 to 16 Vdc. During arcing phase  104  energy is drawn from both charge sources  152  and  156  until they are substantially drained. Accordingly, in arcing phase  104 , the lower voltage potential charge source (typically second charge source  156 ) can be described as boosting the current flow. 
         [0070]    To the extent that V P  is established on the basis of previous testing, V 2C  can be selected as a lower value. This value may be, typically, from about ¼ or ⅓ to about ⅖ or ½ of V H . The size of second charge source  156  may then be selected to alter the proportion of the total charge or energy pulse that occurs in the second stage or phase, making it larger than it might otherwise be such that a greater than normal proportion of the heating, and therefore melting and deposition occurs during the arcing or welding phase. This in turn may cause a greater amount of welded coating to be deposited during this phase. 
         [0071]    Main control circuit  162  receives one or more internal and external input signals, and produces one or more internal and external output signals based on the input signals. An internal input signal is defined as a signal generated by a component of coating deposition apparatus  126 . Conversely, an external input signal is defined a signal generated by a component external to coating deposition apparatus  126 . The external input signals may be received via input panel  138  and input ports  140 , and the external output signals may be output via output panel  142  and output ports  144 . Main control circuit  162  may receive, for example, the following analog input signals: a force or pressure feedback signal, a current sensor feedback signal, a first charge source voltage feedback signal, and a second charge source voltage feedback signal. 
         [0072]    The force or pressure feedback signal is a measure of the force or pressure applied at the contact interface between consumable deposition substance  122  and workpiece  124  when consumable deposition substance  122  and workpiece  124  come into contact during the ESD process. In some embodiments, the pressure is measured by using a load cell. However, the pressure may be measured by any other direct or indirect means. 
         [0073]    The current sensor feedback signal is a measure of the current flowing between consumable deposition substance  122  and workpiece  124 . In some embodiments, the current is measured by a hall effect sensor. However, the current may be measured by any other direct or indirect means. 
         [0074]    The first charge source voltage feedback signal is a measure of the first voltage potential V 152  of first charge source  152 , and the second charge source voltage feedback signal is a measure of the second voltage potential V 156  of second charge source  156 . 
         [0075]    Typically the input signals are “conditioned” by conditioning circuits  164 ,  166 ,  168  and  170  prior to being processed by main control circuit  162 . 
         [0076]    Main control circuit  162  may generate the following digital output signals: the first charging command signal, the second charging command signal, and the discharge command signal. As described above, the first charging command signal controls first charging circuitry  154  and thus the charging of first charge source  152 , and the second charging command signal controls second charging circuitry  158  and thus the charging of second charge source  156 . In some embodiments, the first and second charging command signals are pulse width modulation (PWM) signals that are only enabled after main control circuit  162  has determined that the ESD process is complete. Specifically, the first and second charging command signals may be triggered after both the spark and arcing phases of the ESD process are complete. Main control circuit  162  may determine, for example, that the ESD process is complete when the voltages of both first and second charge sources  152  and  156  dip below a discharge threshold level V L . In some embodiments, the discharge threshold level V L  may be zero or substantially zero. 
         [0077]    As described above, the discharge command signal enables discharge circuitry  160 , allowing first and second charge sources  152  and  156  to be discharged during the ESD process. In some embodiments, main control circuit  162  may enable discharge circuitry  160  only after first and second charge sources  152  and  156  have been charged to the first and second charging threshold voltage potentials respectively; and may disable discharge circuitry  160  only after the ESD process is complete (e.g. after the voltages of first and second charge sources  152  and  156  drop to below the discharge threshold level V L  (e.g. zero or substantially zero). 
         [0078]    Main control circuit  162  may also generate the following analog output signals: a first voltage command signal, a second voltage command signal, a motor speed command signal and a force or pressure command signal. In some embodiments, as shown in  FIG. 2 , the analog output signals are output as a single serial data signal. The serial data signal is then processed by output signal conditioning circuit  172  to generate the individual analog output signals. In other embodiments, the output signal conditioning circuitry is built into main control circuit  162  so that main control circuit  162  outputs the individual analog output signals directly. 
         [0079]    The first voltage command signal represents the first charging threshold voltage potential (i.e. the sparking voltage), and the second voltage command signal represents the second charging threshold voltage potential (i.e. the arcing voltage). As described above, in some embodiments, the first and second charging threshold voltage potentials may be set by an operator via user interface  174 . 
         [0080]    The motor speed command signal is used to control the motor speed of one or both of the consumable deposition substance holder  134  motor and the workpiece holder  136  motor. For example, the motor speed command signal may control the frequency, or frequency and amplitude of vibration of consumable deposition substance holder  134 , or the speed of rotation of workpiece holder  136 , or both. In some embodiments, the motor speed is user adjustable. For example, user interface  174  may allow the operator to set a motor speed parameter that is translated into a motor speed voltage by main control circuit  162 . In one embodiment, the operator may set the motor speed parameter to a value between 0 and 100% with 100% being translated into the maximum motor speed voltage. The default motor speed parameter may be 50%. 
         [0081]    The force or pressure command signal is used to control the pressure or force at which consumable deposition substance  122  is brought into contact with workpiece  124  when consumable deposition substance holder  134  is controlled by a machine rather than by an operator. The force or pressure command signal is designed to interface with a pressure actuator circuit of the machine. In some embodiments, the pressure or force is user adjustable. For example, user interface  174  may allow the operator to set a pressure or force parameter that is translated into a pressure or force voltage level by main control circuit  162 . In one specific embodiment, the operator may set the pressure parameter to a value between 0 and 100% with 100% being translated into a set maximum force or pressure voltage. The default pressure parameter may be 50%. 
         [0082]    There is typically one input signal conditioning circuit  164 ,  166 ,  168 ,  170  for each of the analog input signals received by main control circuit  162 . The purpose of each signal conditioning unit is to: (i) convert the input signal into a format that main control circuit  162  can process; and (ii) isolate main control circuit  162  from the internal or external source of the input signal. For example, in some embodiments, main control circuit  162  converts each analog input signal into a corresponding digital signal using a 2.5V reference voltage. Accordingly, main control circuit  162  can only accurately process analog signals with a range of 0V to 2.5V. Accordingly, the conditioning circuits must convert the input analog signals to be within a range of 0V to 2.5V. 
         [0083]    As described above, in some embodiments, main control circuit  162  receives the following four analog input signals: a pressure feedback signal, a current sensor feedback signal, a first charge source voltage feedback signal and a second charge source voltage feedback signal. Accordingly, in these embodiments, there are four conditioning circuits  164 ,  166 ,  168  and  170 . First input conditioning circuit  164  conditions the force or pressure feedback signal; second input conditioning circuit  166  conditions the current sensor feedback signal; third input conditioning circuit  168  conditions the first charge source voltage feedback signal; and fourth input conditioning circuit  170  conditions the second charge source voltage feedback signal. Fourth input conditioning circuit  170  is typically similar to, or identical to, third input conditioning circuit  168 . 
         [0084]    Output signal conditioning circuit  172  is used when main control circuit  162  outputs the first voltage command signal, the second voltage command signal, the pressure command signal and the motor command signal as a single digital serial data signal. In these cases, output signal conditioning circuit  172  converts the serial data signal into the individual analog signals and up-converts or down-converts the signals as required. In some embodiments, the output signal conditioning circuit includes a plurality of digital to analog converters to convert the serial data signal into analog signals. In one particular embodiment, the analog to digital converts will use a reference voltage of 2.5V which will produce analog output signals with a range of 0 to 2.5V. Where other formats or levels (e.g. 0-10V, 4-20 mA) are required, output signal conditioning circuit  172  may also include converter or driver circuits. 
         [0085]    User interface  174  may allow the operator to adjust certain operating parameters or to view diagnostic and operating parameter information, or both. For example, as described above, user interface  174  may allow the operator to adjust and view: the capacitance of first charge source  152 ; the first charging threshold voltage potential associated with first charge source  152 ; the capacitance of second charge source  156 ; the second charging threshold voltage potential associated with second charge source  156 ; the force or pressure; and the motor speed. User interface  174  is typically communicatively coupled to main control circuit  162  so that any operator-initiated changes to the operating parameters may be communicated to main control circuit  162 . 
         [0086]    In some embodiments, user interface  174  is a display and keypad unit comprising a display and a keypad. The display may be a basic LCD display, such as the Matrix Orbital™ LK122-25 intelligent LCD display. The LK122-25 provides two lines by twenty character alphanumeric LCD display, with a backlight. The keypad may be a basic numeric keypad, such as Grayhill&#39; S™ simple 4×4 button keypad. In these embodiments, user interface  174  may be connected to the main processor by an RS232 communications port. 
         [0087]    Charge Source Circuit 
         [0088]      FIG. 4  shows first charge source  152  of  FIG. 3 . As described above, first charge source  152  is used to supply energy to the sparking phase of the ESD process. In one embodiment, first charge source  152  is a capacitor bank with two capacitors  190  and  192  connected in parallel. Where capacitors are connected in parallel the total capacitance is the sum of the individual capacitances. In one embodiment, the capacitance of both first and second capacitors  190  and  192  is 1000 μF, thus the total capacitance of first charge source  152  is 2000 μF. The total capacitance of first charge source  152  required for the ESD process is based on the specific materials used for consumable deposition substance  122  and workpiece  124 . 
         [0089]    In some embodiments, the number of capacitors (e.g. first and second capacitors  190  and  192 ) forming first charge source  152  is adjustable. For example, first charge source  152  may include one or more switches, such as switch  194 , for selecting or deselecting certain capacitors (i.e. second capacitor  192 ). Typically each switch (i.e. switch  194 ) is in series with a single capacitor and is activated or deactivated by main control circuit  162 . In some cases the number of capacitors forming first charge source  152  is user selectable. For example, the user may be able to select the capacitance of first charge source  152  via user interface  174 . 
         [0090]    In operation, capacitors  190  and  192  are charged by first charging circuitry  154  to establish a first voltage potential in capacitors  190  and  192 . Capacitors  190  and  192  are subsequently discharged through discharge circuitry  160  to provide energy to the sparking phase of the ESD process. In some embodiments, discharge circuitry  160  is only enabled after capacitors  190  and  192  have been charged to the first charging threshold voltage potential V 1C . 
         [0091]      FIG. 5  shows second charge source  156  of  FIG. 3 . As described above, second charge source  156  is used to provide supplemental energy to the arcing phase of the ESD process. In one embodiment, second charge source  152  is a capacitor bank with four capacitors  200 ,  202 ,  204 , and  206  connected in parallel. Where capacitors are connected in parallel the total capacitance is the sum of the individual capacitances. In one embodiment, the capacitance of all four capacitors  200 ,  202 ,  204  and  206  is 220 μF, thus the total capacitance of second charge source  156  is 880 μF. The total capacitance of second charge source  156  required for the ESD process is based on the specific materials used for consumable deposition substance  122  and workpiece  124 . 
         [0092]    In some embodiments, the number of capacitors (e.g. first, second, third and fourth capacitors  200 ,  202 ,  204  and  206 ) forming second charge source  156  is adjustable. For example, second charge source  156  may include one or more switches, such as switches  207 ,  208  and  209 , for selecting or deselecting certain capacitors (i.e. second, third, or fourth capacitors  202 ,  204  and  206 ). Typically each switch (i.e. switches  207 ,  208  and  209 ) is in series with a single capacitor and is activated or deactivated by main control circuit  162 . In some cases the number of capacitors forming second charge source  156  is user selectable. For example, the user may be able to select the capacitance of second charge source  156  via user interface  174 . 
         [0093]    In operation, capacitors  200 ,  202 ,  204  and  206  are charged by second charging circuitry  158  to establish a second voltage potential in capacitors  200 ,  202 ,  204  and  206 . Capacitors  200 ,  202 ,  204  and  206  are subsequently discharged by discharge circuitry  160  to provide supplemental energy to the sparking phase of the ESD process. In some embodiments, discharge circuitry  160  is only enabled after capacitors  200 ,  202 ,  204  and  206  have been charged to the second charging threshold voltage potential V 2C . 
         [0094]    Charging Circuitry 
         [0095]      FIG. 6  shows first charging circuitry  154  of  FIG. 3 . As described above, first charging circuitry  154  receives DC power from input power circuit  150  and charges first charge source  152  to establish a first voltage potential on first charge source  152 . First charging circuitry  154  may include a level shifter circuit  210 , a gate driver circuit  212 , and a current supply circuit  214 . 
         [0096]    Level shifter circuit  210  receives the first charging command signal from main control circuit  162  and converts it to an appropriate level for gate driver circuit  212 . Level shifter circuit  210  may include two NOR gates  216  and  218  connected in series and a resistor  220  connected to the first input of first NOR gate  216 . NOR gates  216  and  218  may be 4093N NOR gates. 
         [0097]    Gate driver circuit  212  receives the control signal from level shifter circuit  210  and provides sufficient current to drive current supply circuit  214 . Gate driver circuit  212  may include a resistor  222 , a gate driver integrated circuit (IC) chip  224  and two capacitors  226  and  228 . Gate driver IC chip  224  may be a TC1234 dedicated MOSFET/IGBT gate driver. 
         [0098]    Current supply circuit  214  receives DC power from input power circuit  150  and, when enabled, produces a charging current from the DC power. The charging current is then supplied to first charge source  152  to establish a first voltage potential on first charge source  152 . Current supply circuit  214  is enabled by gate driver circuit  212 . Current supply circuit  214  may include four transistors  230 ,  232 ,  234 ,  236 , two resistors  238  and  240 , two diodes  242  and  244 , and four inductors  246 ,  248 ,  250  and  252 . Transistors  230 ,  232 ,  234  and  236  are connected in parallel to provide a large charging current to first charge source  152 . Inductors  246 ,  248 ,  250  and  252  are connected in parallel to support the charging current provided by the transistors. Transistors  230 ,  232 ,  234  and  236  may be insulated gate bipolar transistors (IGBT) on FGA180N30 chips, and diodes  242  and  244  may be RURG3440 ultra-fast soft-recovery diodes. 
         [0099]      FIG. 7  shows second charging circuitry  158  of  FIG. 3 . As described above, second charging circuitry  158  receives DC power from input power circuit  150  and charges second charge source  156  to establish a second voltage potential on second charge source  156 . Second charging circuitry  158 , similar to first charging circuitry  154 , may include a level shifter circuit  270 , a gate driver circuit  272 , and a current supply circuit  274 . 
         [0100]    Level shifter circuit  270  receives the second charging command signal from main control circuit  162  and converts it to an appropriate level for gate driver circuit  272 . Level shifter circuit  270  may include two NOR gates  276  and  278  connected in series and a resistor  280  connected to the first input of first NOR gate  276 . NOR gates  276  and  278  may be 4093N NOR gates. 
         [0101]    Gate driver circuit  272  receives the control signal from level shifter circuit  270  and provides sufficient current to drive current supply circuit  274 . Gate driver circuit  272  may include a resistor  282 , a gate driver integrated circuit (IC) chip  284  and two capacitors  286  and  288 . Gate driver IC chip  284  may be a TC1234 dedicated MOSFET/IGBT gate driver. 
         [0102]    Current supply circuit  274  receives DC power from input power circuit  150  and, when enabled, produces a charging current from the DC power. The charging current is then used to charge second charge source  156  to establish a second voltage potential on second charge source  156 . Current supply circuit  274  is enabled by gate driver circuit  272 . Current supply circuit  274  may include two transistors  290  and  292 , two resistors  294  and  528 , one diode  530 , and an inductor  532 . Transistors  290  and  292  are connected in parallel to provide a sufficient charging current to second charge source  156 . Transistors  290  and  292  may be insulated gate bipolar transistors (IGBT) on FGA180N30 chips, and diode  530  may be an RURG3440 ultra-fast soft-recovery diode. 
         [0103]    Discharge Circuitry 
         [0104]      FIG. 8  shows discharge circuitry  160  of  FIG. 3 . As described above, discharge circuitry  160  is used to connect first and second charge sources  152  and  156  to first and second output terminals  130  and  132  for discharging during the ESD process. Discharge circuitry  160  may include two isolation elements  310  and  312  and a switching element  314 . Isolation elements  310  and  312  are in series with first and second charge sources  152  and  156  respectively to bring first and second charge sources  152  and  156  together. In one embodiment, isolation elements  310  and  312  are connected to the positive terminals of first and second charge sources  152  and  156 . Isolation elements  310  and  312  also isolate first and second charge sources  152  and  156  to ensure that charge will not be transferred between the charge sources. Isolation elements  310  and  312  are typically switching diodes, such as 150EBU02 ultra-fast switching diodes. 
         [0105]    Switching element  314  is situated between first and second charge sources  152  and  156  and first and second output terminals  130  and  132 . When switching element  314  is enabled, first and second charge sources  152  and  156  are connected to first and second output terminals  130  and  132  and can be discharged during the ESD process. Conversely, when switching element  314  is disabled, first and second charge sources  152  and  156  are not connected to first and second output terminals  130  and  132  and thus cannot be discharged. Switching element  314  may be controlled by the discharge command signal generated by main control circuit  162 . As described above, the discharge command signal may be enabled by main control circuit  162  only after main control circuit  162  has determined that first and second charge sources  152  and  156  have been charged to the first and second charging threshold voltage potentials V 1c  and V 2C  respectively. Switching element  314  may be a high current thyristor module, such as the MCC95-io1b thyristor module. 
         [0106]    Main Control Circuit 
         [0107]      FIG. 9  shows main control circuit  162  of  FIG. 3 . Main control circuit  162  may include a first voltage comparator circuit  320 , a second voltage comparator circuit  322 , a main processor  324 , a charging control circuit  326 , and a digital output isolation circuit  328 . 
         [0108]    First voltage comparator circuit  320  determines whether first charge source  152  has been charged to the first charging threshold voltage potential V 1C . For example, first voltage comparator circuit  320  may receive as inputs the first charge source voltage feedback signal and the first voltage command signal, compare the input signals, and output a first voltage comparison signal. The first voltage comparison signal may be used to indicate when the first charge source voltage feedback signal is equal to or greater than the first voltage command signal. For example, the first voltage comparison signal may be logic high when the first charge source voltage feedback signal is equal to or greater than the first voltage command signal, and logic low otherwise. 
         [0109]    Second voltage comparator circuit  322  determines whether second charge source  156  has been charged to the second charging threshold voltage potential. For example, second voltage comparator circuit  322  may receive as inputs the second charge source voltage feedback signal and the second voltage command signal, compare the input signals, and output a second voltage comparison signal. The second voltage comparison signal may be used to indicate when the second charge source voltage feedback signal is equal to or greater than the second voltage command signal. For example, the second voltage comparison signal may be logic high when the second charge source voltage feedback signal is equal to or greater than the second voltage command signal, and logic low otherwise. 
         [0110]    Main processor  324  receives one or more input signals and generates one or more output signals based on the status of the one or more input signals. Main processor  324  may be a standard microprocessor, such as Microchip&#39; s™ PIC™ 16F886. In one embodiment, main processor  324  receives the following input signals: the conditioned force or pressure feedback signal from first input signal conditioning circuit  164 ; the conditioned current sensor feedback signal from second input conditioning circuit  166 ; the conditioned first charge source voltage feedback signal from third input signal conditioning circuit  168 ; the conditioned second charge source voltage feedback signal from fourth input signal conditioning circuit  170 ; the first voltage comparison signal from first voltage comparator  320 ; and the second voltage comparison signal from second voltage comparator  322 . Based on these input signals, main processor  324  may generate the following output signals: a master charging command signal; a first preliminary charging command signal; a second preliminary charging command signal; a discharge command signal; and a serial data signal. 
         [0111]    The master charging control signal is used to enable charging of first and second charge sources  152  and  156 . In some embodiments, the master charging control signal is only enabled after main processor  324  determines that the ESD process is complete. Main processor  324  may determine that the ESD process is complete when at least one of the first and second voltage potentials of first and second charge sources  152  and  156  respectively, drop below a discharge threshold level V L . For example, main processor  328  may monitor the first and second charge source voltage feedback signals and enable the master charge control signal only after both signals drop to zero or substantially zero. Other suitable methods of determining the completion of the ESD process may also be used. 
         [0112]    The first preliminary charging command signal is a preliminary version of the first charging control signal that controls first charging circuitry  154  and thus the charging of first charge source  152 . The first preliminary charging command signal is typically sent to charging control circuit  326  where it is used to generate the first charging command signal. 
         [0113]    In some embodiments, the first preliminary charge command signal is a pulse-width modulation (PWM) signal. The pulse width of the signal determines the magnitude of the charging current to be delivered to first charge source  152 . The duty cycle of the first PWM signal may be fixed or may be adjustable. For example, user interface  174  may allow the user to set the duty cycle for the first PWM signal. In one embodiment, the operator may set the duty cycle to any value from 0 to 100%. The default value may be, for example, 50%. Preferably the duty cycle of the first PWM signal can only be adjusted by an administrator or technician. 
         [0114]    The second preliminary charging command signal is a preliminary version of the second charging control signal that controls second charging circuitry  158  and thus the charging of second charge source  156 . The second preliminary charging command signal is typically sent to charging control circuit  326  where it is used to generate the second charging command signal. 
         [0115]    In some embodiments, the second preliminary charging command signal is a PWM signal. The pulse width of the signal determines the magnitude of the charging current to be delivered to second charge source  156 . The duty cycle of the second PWM signal may be fixed or may be adjustable. For example, user interface  174  may allow the operator to set the duty cycle for the second PWM signal. In one embodiment, the user can set the duty cycle to any value from 0 to 100%. The default value may be set to, for example, 50%. Preferably the duty cycle of the second PWM signal can only be adjusted by an administrator or technician. 
         [0116]    The discharge command signal enables discharge circuitry  160  and thus allows discharging of first and second charge sources  152  and  156 . Until the discharge command signal is enabled, first and second charge sources  152  and  156  cannot typically be discharged. In some embodiments, the discharge command is only enabled after main processor  324  has determined that first and second charge sources  152  and  156  have been charged to the first and second charging threshold voltage potentials V 1C  and V 2C  respectively. Main processor  328  may, for example, monitor the first and second voltage comparison signals and determine that first and second charge sources  152  and  156  have been charged to the first and second charging threshold voltage potentials V 1C  and V 2C  when the first and second voltage comparison signals are logic level high. 
         [0117]    In other embodiments, a second condition must also be met before the discharge command is enabled. For example, the discharge command may not be enabled unless a predetermined time has elapsed since the previous discharge. This time will be referred to as the discharge delay. The discharge delay may be fixed or user adjustable. For example, user interface  174  may allow the operator to set the discharge delay. In one embodiment, the user may set the discharge delay parameter to any value between 0 and 50 with a default value of 5. Main processor  324  may calculate the discharge delay time by multiplying the discharge delay parameter entered by the user by a time constant (e.g. 0.5 ms). Preferably the discharge delay can only be adjusted by an administrator or technician. 
         [0118]    In some embodiments, the system may include an input port  140  for receiving a discharge command signal generated by an external device or interface. The external discharge command signal would allow external control of the discharge of first and second charge sources  152  and  156 . Typically, the external discharge command signal is connected in parallel with the internal discharge command signal so that discharge of first and second charge sources  152  and  156  can be enabled by either discharge command signal. 
         [0119]    The serial data signal may be a combination of, or may contain the information to generate the following analog output signals: the first voltage command signal, the second voltage command signal, the motor speed command signal, and the force or pressure command signal. Typically the serial data signal is sent to output signal conditioning unit  172  which generates the individual analog output signals from the serial data signal. 
         [0120]    As described above, the first and second voltage command signals represent the first and second charging threshold voltage potentials V 1C  and V 2C  respectively. Where the first and second charging threshold voltage potentials V 1C  and V 2C  are user adjustable, the first and second voltage command signals are typically generated by main control circuit  162  based on the information received from user interface  174 . For example, where the operator inputs specific values for the first and second charging threshold voltage potentials to user interface  174 , then these values may be communicated to main control circuit  162  via the communication link between the main control circuit  162  and user interface  174 . If, however, the operator does not input specific values for the first and second charging threshold voltage potentials V 1C  and V 2C , then the default values may be used. The default values may be communicated to main control circuit  162  from user interface  174  via the communications link, or, alternatively, the default values may be programmed into main control circuit  162 . 
         [0121]    The motor speed command signal is used to control the motor speed of one or both of the consumable deposition substance holder  134  motor and the workpiece holder  136  motor. The force or pressure command signal is used to control the pressure or force at which consumable deposition substance  122  is brought into contact with workpiece  124  when consumable deposition substance holder  134  is controlled by a machine rather than by an operator. The force or pressure command signal is designed to interface with a pressure actuator circuit of the machine. The motor speed command signal or the force or pressure command signal, or both, may be generated based on the force or pressure feedback signal (discussed below) or the information received from user interface  174 , or both. For example, where the operator inputs specific values for the motor speed or the force or pressure, then these values may be communicated to main control circuit  162  via the communication link between the main control circuit  162  and user interface  174 . If, however, the operator does not input specific values for the motor speed or force or pressure, then the default values may be used. The default values may be communicated to main control circuit  162  from user interface  174  via the communications link, or, alternatively, the default values may be programmed into main control circuit  162 . 
         [0122]    Charging control circuit  326  receives all of the charging signals and generates the first and second charging command signals based on the received charging signals. As described above, the first and second charging command signals control first and second charging circuitry  154  and  158  respectively. That is, the first and second charging command signals control the charging of first and second charge sources  152  and  156  respectively. 
         [0123]    Charging control circuit  326  may receive the following signals as inputs: the first preliminary charging command signal generated by main processor  324 ; the second preliminary charging command signal generated by main processor  324 ; the master charging command signal generated by main processor  324 ; the first voltage comparison signal generated by first voltage comparator circuit  320 ; and the second voltage comparison signal generated by second voltage comparator circuit  322 . 
         [0124]    In some embodiments, charging control circuit  326  may output the first preliminary charging command signal (e.g. the first PWM signal) as the first charging command signal when the ESD process is complete (e.g. when the master charging command signal is logic high) and first charge source  152  has not been charged to the first voltage potential (e.g. when the first voltage comparison signal is logic low). In other cases, the first charging command signal may be set to a null value. 
         [0125]    Similarly, charging control circuit  326  may output the second preliminary charging command signal (e.g. the second PWM signal) as the second charging command signal when the ESD process is complete (e.g. when the master charging command signal is logic high) and second charge source  156  has not been charged to the second voltage potential (e.g. when the second voltage comparison signal is logic low). In other cases, the second charging command signal may be set to a null value. 
         [0126]    Digital output isolation circuit  328  isolates the digital outputs of main control circuit  162  from the other aspects of coating deposition apparatus  126 . Digital output isolation circuit  328  may include an optical coupler for each digital output. In one embodiment, main control circuit  162  produces the following three digital outputs: the first charge command signal produced by charging control circuit  326 ; the second change command signal produced by charging control circuit  326 ; and the discharge command signal produced by main processor  324 . In this embodiment, digital output isolation circuit  328  may include three optical couplers, one for each digital output signal. 
         [0127]    Conditioning Circuits 
         [0128]      FIG. 10  shows first input conditioning circuit  164  of  FIG. 3 . As described above, first input conditioning circuit  164  converts the force or pressure sensor feedback signal into a signal suitable for processing by main control circuit  162 . The output signal will be referred to as the conditioned force or pressure feedback signal. First input conditioning circuit  164  may include an amplification circuit  340  that reduces the amplitude of the pressure sensor feedback signal so that it falls within a predetermined range (e.g. 0 to 2.5 V). Amplification circuit  340  may include three resistors  342 ,  344 , and  346 , and an operational amplifier  348 . Where, for example, the force or pressure sensor feedback signal has a range of 0V to 4.5V, a 0.5 amplification circuit would be sufficient to convert the pressure sensor feedback signal to a 0 to 2.25V signal. This level of amplification could be achieved, for example, by setting the resistance of first and second resistors  342  and  344  to 1 KΩ. 
         [0129]      FIG. 11  shows second input conditioning circuit  166  of  FIG. 3 . As described above, second input conditioning circuit  166  converts the current sensor feedback signal into a signal suitable for processing by main control circuit  162 . The output signal will be referred to as the conditioned current sensor feedback signal. Second input conditioning circuit  166  may include an amplification circuit  360  that reduces the amplitude of the current sensor feedback signal so that it falls within a predetermined range (e.g. 0 to 2.5 V). Amplification circuit  360  may include two resistors  362  and  364  and an operational amplifier  366 . Where, for example, the current sensor feedback signal has a range of 0V to 4V, a 0.625 amplification circuit would be sufficient to convert the current sensor feedback signal to a 0 to 2.5V signal. This level of amplification could be achieved, for example, by setting the resistance of first resistor  362  to 1 KΩ and setting resistance of second resistor  364  to 0.6 KΩ. 
         [0130]    Due to the short duration of the current pulses, second input conditioning circuit  166  may also include a voltage peak detect and hold circuit  368  to detect and hold the peak value of the current pulses for processing by the main control circuit  162 . Voltage peak detect and hold circuit  368  may include a dedicated peak hold integrated circuit chip  370 , such as the PKD01, and a resistor  372 . 
         [0131]      FIG. 12  shows third input conditioning circuit  168  of  FIG. 3 . As described above, third input conditioning circuit  168  converts the first charge source voltage feedback signal into a signal suitable for processing by main control circuit  162 . The output signal will be referred to as the conditioned first charge source voltage feedback signal. Third input conditioning circuit  168  may include a differential circuit  390  that determines the difference between the two input points and amplifies the difference to produce the conditioned first charge source voltage feedback signal. The differential configuration helps to remove any common mode noise between main control circuit  162  and first charge source  152 . The differential circuit  390  may include four resistors  392 ,  394 ,  396  and  398 , a capacitor  400  and an operational amplifier  402  configured as a differential amplifier. 
         [0132]    Where, for example, the first charge source voltage feedback has a range of 0V to 48.75 V, a gain of 0.05128 would be sufficient to convert the first charge source voltage feedback signal to a 0 to 2.5V signal. This level of amplification could be achieved, for example, by setting the resistance of first and fourth resistors  392  and  398  to 20 KΩ and setting resistance of second and third resistors  394  and  396  to 390 KΩ. 
         [0133]    As described above, fourth input conditioning circuit  170  for conditioning the second charge source voltage feedback signal may be similar to, if not identical to, third input conditioning circuit  168 . 
         [0134]    Output Signal Conditioning Circuit 
         [0135]      FIG. 13  shows output signal conditioning circuit  172  of  FIG. 3 . As described above, output signal conditioning circuit  172  receives the digital serial data signal from main control circuit  162  and generates the following analog output signals from the serial data signal: the first voltage command signal, the second voltage command signal, the pressure command signal and the motor speed command signal. Output signal condition circuit  172  may include a digital to analog conversion circuit  410 , a voltage reference supply circuit  412  and a capacitor  414 . 
         [0136]    Digital to analog conversion circuit  410  receives the digital serial data signal from main control circuit  162 , a clock signal from main control circuit  162 , and a reference voltage from voltage reference supply circuit  412  and generates the four analog output signals. In some embodiments, voltage reference supply circuit  412  supplies a 2.5 V reference. This means that analog output signals will have a range of 0V to 2.5V. Where other formats or levels (e.g. 0-10V, or 4-20 mA) are required, output signal conditioning circuit  172  may also include converter or driver circuits. Alternatively, external converter or driver circuits may be used to achieve alternative formats or levels. 
         [0137]    In one embodiment, digital to analog conversion circuit  410  is a MAX5250 four channel voltage-output 10-bit digital-to-analog converter chip, and voltage reference supply circuit  412  is an AD580 precision voltage reference chip. 
         [0138]    Voltage Comparison Circuits 
         [0139]      FIG. 14  shows first voltage comparator circuit  320  of  FIG. 9 . As described above, first voltage comparator circuit  320  determines whether first charge source  152  has been charged to the first charging threshold voltage potential V 1C . As shown in  FIG. 14 , first voltage comparator circuit  320  may have four resistors  430 ,  432 ,  434  and  436 , one capacitor  438  and an operational amplifier  440  configured as a differential amplifier. First voltage comparator circuit  320  receives as inputs the conditioned first charge source voltage feedback signal and the first voltage command signal. As described above, the conditioned first charge source voltage feedback signal indicates the first voltage potential of first charge source  152 , and the first voltage command signal indicates the first charging threshold voltage potential. First voltage comparator circuit  320  compares the two input signals and outputs a first voltage comparison signal that is logic high when the first charge source voltage feedback signal is greater than or equal to the first voltage command signal, and logic low otherwise. The first voltage comparison signal may then be passed to main processor  324  and charging control circuit  326  for further processing. Accordingly, the first voltage comparison signal is logic high when first charge source  152  has been charged to the desired level, and is logic low otherwise. 
         [0140]      FIG. 15  shows second voltage comparator circuit  322  of  FIG. 9 . As described above, the second voltage comparator circuit  322  determines whether second charge source  156  has been charged to the second charging threshold voltage potential V 2C . As shown in  FIG. 15 , second voltage comparator circuit  322  may include four resistors  450 ,  452 ,  454  and  456 , two capacitors  458  and  460  and an operational amplifier  462  configured as a differential amplifier. Second voltage comparator circuit  322  receives as inputs the conditioned second charge source voltage feedback signal, and the second voltage command signal. As described above, the conditioned second charge source voltage feedback signal indicates the second voltage potential of second charge source  156 , and the second voltage command signal indicates the second charging threshold voltage potential. Second voltage comparator circuit  322  compares the two input signals and outputs a second voltage comparison signal that is logic high when the second charge source voltage feedback signal is greater than or equal to the second voltage command signal, and logic low otherwise. Accordingly, the second voltage comparison signal is logic high when second charge source  156  has been charged to the desired level, and is logic low otherwise. 
         [0141]    Charging Control Circuit 
         [0142]      FIG. 16  shows charging control circuit  326  of  FIG. 9 . As described above, charging control circuit  326  receives all of the charging signals (e.g. the first preliminary charging command signal generated by main processor  324 , the second preliminary charging command signal generated by main processor  324 , the master charging command signal generated by main processor  324 , the first voltage comparison signal generated by first voltage comparator circuit  320 , and the second voltage comparison signal generated by second voltage comparator circuit  322 ) and generates the first and second charging command signals based on the received signals. The first and second charging command signals control first and second charging circuitry  154  and  158  respectively. That is, the first and second charging command signals control the charging of first and second charge sources  152  and  156  respectively. Charging control circuit  326  may include a programmable array logic circuit  470 , such as a PAL16V8. 
         [0143]    In some embodiments, programmable array logic circuit  470  may output the first preliminary charging command signal (e.g. the first PWM signal) as the first charging command signal when the ESD process is complete (e.g. when the master charging command signal is logic high) and first charge source  152  has not been charged to the first voltage potential (e.g. when the first voltage comparison signal is logic low). In other cases, the first charging command signal may be set to a null value. 
         [0144]    Similarly, programmable array logic circuit  470  may output the second preliminary charging command signal (e.g. the second PWM signal) as the second charging command signal when the ESD process is complete (e.g. when the master charging command signal is logic high) and second charge source  156  has not been charged to the second voltage potential (e.g. when the second voltage comparison signal is logic low). In other cases, the second charging command signal may be set to a null value. 
         [0145]    Digital Output Isolation Circuit 
         [0146]      FIG. 17  shows digital output isolation circuit  328  of  FIG. 9 . As described above, digital output isolation circuit  328  isolates the digital outputs of main control circuit  162  from the other aspects of coating deposition apparatus  126 . As shown in  FIG. 17 , digital output isolation circuit  328  may include three optical couplers  480 ,  482 , and  484 —one for each of the following three outputs—the first charging command signal produced by charging control circuit  326 , the second charging command signal produced by charging control circuit  326 , and the discharge command signal produced by main processor  324 . The optical couplers associated with the charging command signals (e.g. first and third optical couplers  480  and  484 ) may be high-speed optical couplers, such as 6N135 high speed optical couplers. The optical coupler associated with the discharge command signal (e.g. second optical coupler  482 ) may be a standard optical coupler, such as a  4 N 33  optical coupler. 
         [0147]    Digital output isolation circuit  328  may also include three resistors  486 ,  488 , and  490  and a driving circuit  492  to control the current in optical couplers  480 ,  482  and  484 . 
         [0148]    Alternative Coating Deposition Apparatus 
         [0149]      FIG. 18  shows an alternative coating deposition apparatus  500 . Coating deposition apparatus  500 , similar to coating deposition apparatus  126  of  FIG. 3 , may include an input power circuit  150 , a first charge source  152 , first charging circuitry  154 , a second charge source  156 , second charging circuitry  158 , one or more input signal conditioning circuits  164 ,  166 ,  168  and  170 , one or more output signal conditioning units  172 , and a user interface  174 . However, coating deposition apparatus  500  includes two discharge circuits  502  and  504  which are controlled by first and second discharge control signals, respectively, generated by main control circuit  506 . 
         [0150]    First discharge circuit  502  controls discharge of first charge source  152 . When first discharge circuit  502  is enabled, first charge source  152  may be discharged and thus provide power to the ESD process. Conversely, when first discharge circuit  502  is disabled, first charge sources  152  is inhibited from being discharged, and thus no power may provided to the ESD process from first charge source  152 . 
         [0151]    First discharge circuit  502  is typically controlled by a first discharge command signal generated by main control circuit  506 . In some embodiments, first discharge circuit  502  is disabled (and incidentally discharge of first charge source  152  is inhibited) until the main control circuit  162  detects that first charge source  152  has been charged to the first charging threshold voltage potential V 1C . 
         [0152]    In one embodiment, first discharge circuit  502  includes a switching element connected in series with first charge source  152  and first and second output terminals  130  and  132 . When the switching element is enabled, first charge source  152  is connected to first and second output terminals  130  and  132  and may provide power to the ESD process. When the switching element is disabled, there is a break in the circuit so that first charge source  152  is not connected to first and second output terminals  130  and  132  and thus may not provide power to the ESD process. The switching element is typically enabled and disabled by the first discharge command signal generated by main control circuit  506 . The switching element is typically a thyristor, a power IGBT or a MOSFET, but the switching element may be any other suitable switching device. 
         [0153]    Second discharge circuit  504  controls discharge of second charge source  156 . When second discharge circuit  504  is enabled, second charge source  156  may be discharged and thus second charge source  156  may provide power to the ESD process. Conversely, when second discharge circuit  504  is disabled, second charge source  156  is inhibited from being discharged, thus second charge source  156  may not provide power to the ESD process. 
         [0154]    Second discharge circuit  504  is typically controlled by a second discharge command signal generated by main control circuit  506 . In some embodiments, second discharge circuit  504  is disabled (and incidentally discharge of second charge source  156  is inhibited) until (i) the main control circuit  506  detects that second charge source  156  has been charged to the second charging threshold voltage potential V 2C ; and (ii) discharge of first charge source  152  has commenced. These conditions are implemented to ensure that second charge source  156  has reached the second charging threshold voltage potential V 2C  prior to being discharged, and second charge source  156  cannot be discharged until the second or arcing phase of the ESD process. This may give the operator better control over the ESD process. 
         [0155]    In one embodiment, second discharge circuit  504  includes a switching element connected in series with second charge source  156  and first and second output terminals  130  and  132 . When the switching element is enabled, second charge source  156  is connected to first and second output terminals  130  and  132  and may provide power to the ESD process. When the switching element is disabled, there is a break in the circuit so that second charge source  156  is not connected to first and second output terminals  130  and  132  and thus may not provide power to the ESD process. The switching element is typically enabled and disabled by the second discharge command signal generated by main control circuit  506 . The switching element is typically a power IGBT or a MOSFET, but may be another other suitable switching device. 
         [0156]    Main control circuit  506  is identical to main control circuit  162  of  FIG. 3  except that it also generates the first and second discharge control signals in accordance with the above description. 
         [0157]    The principles of the present invention are not limited to these specific examples which are given by way of illustration. It is possible to make other embodiments that employ the principles of the invention and that fall within its spirit and scope of the invention. Since changes in or additions to the above-described embodiments may be made without departing from the nature, spirit or scope of the invention, the invention is not to be limited to those details.