Abstract:
A method and device for monitoring operation of a high voltage generator which converts an input voltage into an output voltage which is higher than the input voltage, by measuring at least one first physical quantity which is representative of the input voltage; deriving at least one parameter from the at least one measured first physical quantity; comparing the at least one parameter with a second measured physical quantity which is representative of the magnitude of the output voltage; and producing a response when a predetermined minimum differential exists between the at least one parameter and the second measured physical quantity.

Description:
BACKGROUND OF THE INVENTION 
     The invention concerns processes and a device for producing a high voltage, particularly for electrostatic application of a coating product in an installation for spraying a coating product in liquid or powder form. It also concerns an installation of this type equipped with such a device. 
     In such installations, which can be manual or automatic, one or several high voltage electrodes must be supplied electrically in order to place them at a high voltage which creates an electrostatic field between a coating product sprayer and the workpiece to be coated. The voltage delivered to such an electrode is of the order of several tens of kilovolts (kV). In order to supply an electrode from power mains or any other conventional voltage source such as a battery, it is necessary to substantially increase the input voltage. The current to and from the high voltage electrode is generally limited to several milliamps for safety reasons and to limit the electric power consumed by the electrode, this power being partially dissipated in the form of heat in the device. 
     The electrode is most often supplied by two subsystems, one disposed in a fixed location located in proximity to the installation and normally termed &#34;low voltage unit&#34;, and the other forming a unit with the sprayer and referred to herein as a &#34;rectifier-step-up transformer&#34;. These two units are connected together by a bundle of conductive cables. 
     The magnitude of the high voltage utilized in such an installation must vary, during use of the installation, as a function of the geometry of the surface to be coated, the rate of flow of the coating product, the relative positions of objects to be coated and product sprayers, etc. The operator of a coating installation can vary this magnitude manually or can program the desired values for the high voltage to be furnished to the, or each, electrode by controlling, for example, the voltage delivered by the low voltage unit. A measurement of the value of the resulting high voltage permits establishment of a closed measurement and control loop, and the supply voltage from the low voltage unit can be adjusted as a function of the measured value of the high voltage. 
     For a fixed high voltage output, one can apply to the rectifier-step-up transformer a periodic signal having a frequency proximate to its resonant frequency in order to obtain the desired high voltage with a minimal input voltage, which allows heating of the components to be avoided. Systems of this type are disclosed in French Patent Application A-2,618618 and U.S. Pat. No. 5,159,544. 
     It has been found that known systems of this type are not completely reliable because an error in the measurement of the voltage downstream of the transformer can be provoked by a poor connection or by the introduction of foreign material, such as water, grease, or silicone, or any other material, into the connector or into the solder which serves as a point of contact between the measuring conductor and the line supplying high voltage for the high voltage electrode. 
     Such an error creates a variation in the supply voltage from the low voltage unit, which can lead to a dangerous condition which exposes the operator to electrocution or the installation to a fire. In addition, an error in the measurement of the output voltage can disturb the implementation of the algorithm which determines the resonant frequency. 
     Thus, for example, the measured high voltage value can be of the order of 70 kV while the real value available at the electrode is of the order of 120 or 140 kV. If the nominal, or desired, value is 80 kV, the system which controls the installation will have a tendency to raise the supply voltage in order to produce a higher voltage. This being done, there is a risk of exceeding the voltage value which is acceptable for installation safety and of generating a spark which could trigger a fire or could destroy components of the high voltage unit due to heating. 
     SUMMARY OF THE INVENTION 
     It is a primary object of the present invention to overcome the above-described problems. 
     The above and other objects are achieved, according to the present invention, by a method for monitoring operation of a high voltage generator which converts an input voltage into an output voltage which is higher than the input voltage, the method comprising: measuring at least one first physical quantity which is representative of the input voltage; deriving at least one parameter from the at least one measured physical quantity; comparing the at least one parameter with a second measured physical quantity which is representative of the magnitude of the output voltage; and producing a response when a predetermined minimum differential exists between the at least one parameter and the second measured physical quantity. 
     Objects according to the invention are further achieved by a method for monitoring operation of a high voltage generator which converts an input voltage into an output voltage which is higher than the input voltage, the method comprising: selecting an assigned value for the output voltage; deriving at least one parameter from the assigned value; comparing the at least one parameter with at least one measured physical quantity which is representative of the magnitude of the output voltage; and producing a response when a predetermined minimum differential exists between the at least one parameter and the at least one physical quantity. 
     Objects according to the invention are additionally achieved by a device for generating a high voltage comprising: means for producing a high voltage in response to an input signal; means for determining a relation between at least one parameter defining the input signal and at least one measured physical quantity representative of the magnitude of the high voltage; and means connected for controlling the magnitude of the high voltage as a function of the relation determined by the determining means. 
     The processes and devices according to the present invention assure that a measurement error will be immediately detected so that appropriate corrective action can be taken. Such corrective action can include shut-down of the installation and/or triggering of an alarm. The dangers of electrocution and fire are thus avoided. 
     The physical quantity that is measured can be the output voltage of the rectifier-step-up transformer, the current in the secondary of a transformer supplying the rectifier-step-up transformer, or a combination of these two values. The parameter defining the periodic signal can be its nominal voltage, nominal current, or frequency, the voltage or current supplying the generator of the periodic signal, or a combination of these parameters. 
     According to an advantageous feature of the invention, a calculator is utilized to compare the measured value with a reference value supplied by a memory or derived according to an algorithm which performs a calculation that is a function of the value of the selected parameter. The differential between the reference value and the measured value permits detection of measurement errors and the calculator acts on a shut-off means or an alarm as a function of this differential and of a fixed threshold value for this differential. 
     The invention will be better understood and other advantages thereof will become more readily apparent from the following description of two embodiments of a device for producing a high voltage for an installation for electrostatic spraying of a coating product, given solely by way of non-limiting example and with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic diagram of a high voltage generator device according to the present invention. 
     FIG. 2 is a diagram similar to that of FIG. 1 showing a second embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The circuit shown in FIG. 1 includes a low voltage unit 1 composed essentially of a regulated power supply 2 which is connected to power mains and which delivers a direct current at an adjustable direct voltage to a generator 3 which produces a periodic electric current at an adjustable frequency, f. Regulated power supply 2 can also receive input power from a battery or any other appropriate current source. The periodic electric signal produced by generator 3 is basically a square wave signal having a constant waveform selected, in a known manner, to minimize first order harmonics, i.e., the waveform is as close as possible to a perfect sinusoid. A filter 11 permits this periodic signal to be partially smoothed. The magnitude of the periodic current signal is variable as a function of the voltage supplied to generator 3 by regulated power supply 2. Although the waveform of the periodic signal as described above is particularly advantageous and simple to implement, any other form of periodic signal can be employed in the practice of the present invention, in particular a true sinusoidal signal. 
     The periodic current signal produced by low voltage unit 1 is applied through a bundle of flexible conductive cables 4 to the input of a rectifier-step-up transformer unit 5 which comprises an input transformer 6 and a voltage multiplier 7. The output of the voltage multiplier 7 is connected to a high voltage, or live, electrode 8 of an electrostatic sprayer (not shown) for spraying a coating product. 
     The operator can decide to vary the voltage supplied to electrode 8 as a function of the desired spraying characteristics and/or as a function of the form of the workpieces to be coated. The voltage also varies as a function of high voltage leaks at the level of the electrode, which depend on its environment, for example on the distance to the nearest ground point. 
     According to the invention, a calculator 10 is connected to a part of the rectifier-step-up transformer 5 and receives two signals representative of the output voltage (U HT ) of, and the current (I HT ) in, the rectifier. The voltage U HT  is measured across a measuring resistance 15. Calculator 10 also receives input signals representative of the output voltage (U alim ) and the current (I alim ) from the regulated power supply 2. The voltage U alim  is measured across a measuring resistance 16. As a function of the values for the voltage U HT  and the current I HT  in the rectifier-step-up transformer 5, calculator 10 can obtain reference values for the voltage (U HTref ) and the current (I HTref ) which the rectifier-step-up transformer 5 should provide to electrode 8. In FIG. 1, memory 12 is shown as a component external to calculator 10; however, in practice, memory 12 can be a component part within calculator 10. 
     Calculator 10 compares at least one of the measured values (U HT , I HT ) with a corresponding reference value (U HTref , I HTref ) and determines the percentage difference between the associated values. If the percentage difference is equal to or less than a selected value which is assumed to be acceptable, for example 20%, the installation is assumed to be operating properly and its operation is maintained. If the percentage difference is greater than the selected value, calculator 10 activates a malfunction procedure which comprises operating a means 13 for shutting off operation of the low voltage unit. In the case of a regulated power supply 2 connected to power supply mains, the means 13 is a switch. Simultaneously with, or in place of, operation of shut-off means 13, calculator 10 can also trigger an alarm 14 in order to warn the operator of the occurrence of the malfunction. 
     The threshold value for initiating a malfunction procedure is determined by the characteristics of the installation. The value of 20% is given above solely by way of example and any value between 0% and 1000% can be chosen. 
     The comparison described above takes place continually at a frequency linked to the operating speed of calculator 10, of the order of 50 kHz, given that calculator 10 is permanently connected to low voltage unit 1 and rectifier-step-up transformer 5. Taking into account the transient conditions existing during start-up of power supply 2 and the substantial variations which can occur at that time, monitoring of the measured high voltage and associated current values is not effected during a predetermined period after each start-up. In practice, this period can be fixed at several tens of milliseconds, up to one second. 
     FIG. 2 shows a variant of the embodiment of FIG. 1 in which elements identical to those of FIG. 1 have the same reference numerals increased by 100. The high voltage, U HT , and the associated current, I HT , in rectifier-step-up transformer 105 are measured and transmitted to calculator 10 which adjusts the value of the direct voltage delivered by regulated power supply 102 and the value of the frequency, f, of the periodic current supplied by generator 103 until the high voltage achieves a desired value, U des  defined by the operator via a keyboard 112. The system thus comprises a regulation loop. 
     On the other hand, the voltage, U T  between the cables in bundle 104, and the current I T  being conducted by those cables are measured and furnished as input values to calculator 110. Starting from the desired value, U des , and theoretically or experimentally determined physical laws, calculator 110 is arranged to determine theoretical values, U Tcalc , I Tcalc , for the voltage U T  and the current I T  in the bundle of cables 104. 
     For example, the voltage across the secondary of transformer 106 could be expressed as 
     
         U.sub.sec =K.sub.1 ×U.sub.T 
    
     where K 1  is a numerical coefficient dependent on the characteristics of transformer 106. 
     The voltage drop in rectifier-step-up transformer 105 has the form: ##EQU1## where f is the frequency of the periodic signal provided by generator 103; C is the unitary value of the capacitance of each of the capacitors in rectifier 107, and 
     N is the number of voltage doubling stages in rectifier 107; in the illustrated embodiment, rectifier 107 contains three voltage doubling stages. 
     The following may be noted: 
     
         ΔU=K.sub.2 ×I.sub.HT 
    
     where K 2  is a constant numerical coefficient characteristic of the type of cascade employed. 
     The value for the output voltage from rectifier-step-up transformer 105 is a multiple of the voltage across the secondary of transformer 106, the multiplying coefficient, K 3 , being the voltage gain of the cascade. The high voltage at the output of the rectifier can be expressed as a function of the other values as follows: 
     
         U.sub.HT =K.sub.3 ×U.sub.sec -ΔU =K.sub.1 ×K.sub.3 ×U.sub.T -K.sub.2 ×I.sub.HT. 
    
     When the value of U HT  must be equal to the desired value, U des , and when I HT  is known, the calculated value for U T  is obtained by solving the following first order equation: 
     
         U.sub.Tcalc =(U.sub.des +K.sub.2 ×I.sub.HT)/(K.sub.1 ×K.sub.3). 
    
     The comparison between U Tcalc  and U T  permits a determination to be made of whether the generator is functioning in an abnormal manner. One can proceed as in the embodiment of FIG. 1 by detecting a malfunction when the difference between the calculated value, U Tcalc , and the measured value, U T  exceeds a given percentage and a given difference in volts, for example 5 V. In the same manner, the comparison can be made between I Tcalc  and I T . It is to be noted that, in this embodiment, a measurement is not performed directly on the high voltage produced in rectifier-step-up transformer 105 because the malfunction measurement is performed independently of the regulation loop. A poor contact within the measurement line, because of the lower level of U T , thus does not significantly influence the reliability of the malfunction detection. 
     According to another embodiment of the invention, which is not shown, the frequency, f, of the periodic signal produced by generator 103 can also be used as one of the parameters defining the periodic signal. In effect, this frequency has a relation to the other measured physical quantities because it is related to the gain of the rectifier-step-up transformer. 
     The invention can be implemented by comparing only one measured physical quantity with only one reference value, the reference value being determined as a function of one or several parameters which determine the periodic signal. In effect, for example, the output voltage depends on a combination of the supply voltage, the supply current and the frequency of the periodic signal. 
     In the embodiments of the invention described herein, the low voltage unit 1 is physically separated from the rectifier-step-up transformer 5 and is connected thereto by a bundle of flexible cables 4. However, the invention can also be applied to arrangements in which these subassemblies form part of an integrated or compact system. 
     This application relates to subject matter disclosed in Application number 95 08621, filed on Jul. 10, 1995 in France, the disclosure of which is incorporated herein by reference. 
     While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. 
     The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.