DEVICE AND METHOD FOR MONITORING CONTAMINATION ON DISCHARGE ELECTRODES

Abstract

Method for monitoring contamination on discharge electrodes (3, 4, 5, 6, 7, 8) which serve for the contactless discharge of electrically charged surfaces, wherein the discharge electrodes (3, 4, 5, 6, 7, 8) have at least one emission tip (32, 62), and wherein the method comprises the following: - detecting a partial current (i1(t), i2(t), i3(t)) which is coupled from the at least one emission tip (32, 62) to the earth (21) connected to the discharge electrode (3, 4, 5, 6, 7, 8); - determining an active power (208) based on the partial current (i1(t), i2(t), i3(t)); - Generating a status signal based on the calculated active power (208).

Description

The present invention relates to a device for monitoring contamination at discharge electrodes. According to a further aspect, the present invention relates to a method for monitoring contamination at discharge electrodes.Passive and active discharge electrodes or active charge electrodes are known in a variety of embodiments and variations. Frequently, such electrodes have a plurality of emission tips which are arranged in a single row, double row or also as a planar emission tip array in different grid widths in such a way that they resemble a fakir board, for example. Very often, such emission tips are embedded together with a current-limiting resistor in elongated U-profiles by means of insulating casting resin. The electrical resistance is associated with either each individual peak or also n-peaks. Passive discharge electrodes are often used in practice even without current-limiting resistors.In the presence of an electric field, the highest possible electric field strength should be effective in the case of an arrangement of emission tips of active and passive high-voltage electrodes at the tips. In addition to complying with further boundary conditions, the respective tip would have to protrude sufficiently far from the insulating embedding for this purpose. This is quite comparable to the necessarily free-standing end of a lightning arrester above the object to be protected.By applying a high voltage and the associated high electric field strength, a corona discharge occurs at the emission tips, which is an electric discharge in a non-conductive medium, for example air. Such corona discharges can be used in a variety of applications. By way of example, mention may be made here of the electrical surface treatment of plastic films, paper and aluminum foils, in which an insulator surface is uniformly electrically charged. On the other hand, electrostatically charged surfaces of insulating materials can be discharged in a contactless manner by the emission tips of the discharge electrodes emitting bipolar gas ions and electrons by corona discharge.An example of a discharge electrode with emission tips known from the prior art can be taken from EP 3 248 254 B1. According to the discharge electrode illustrated herein, a plurality of emission tips are insulatedly arranged in a metal support profile, the metal profile being connected to a ground terminal. In alternative embodiments, the emission tips are arranged in a plastic profile which has an embedded metallic conductor which is in turn connected to the ground connection.In practice, when such discharge electrodes are used, one or more emission tips of the discharge electrode inevitably become contaminated. The contamination can be, for example, dry particles such as dust, which are deposited on the emission tips. These can be referred to as insulating contamination, for example, since they lead to a reduction in the amount of released bipolar ions required for the function, up to complete loss of function. This also applies to the passively acting portion of such discharge electrodes.Conductive contamination of the surface of the discharge electrode is, on the other hand, referred to as leakage current paths from the emission tip operated with high voltage to the nearest machine ground. This can be caused, for example, by moisture deposits which connect the emission tips to a conductive part of the housing of the discharge electrode. Such leakage current paths lead on the one hand to significant power losses and on the other hand to destruction of the entire discharge electrode, with often serious consequences for the respective production process in which the electrode is used. The low-resistance connection already occurring via the leakage current path of an emission tip to the ground potential of the machine ground means, if appropriate, the functional failure of the entire discharge electrode. In extreme cases, the discharge electrode may burn. If the carrier profile consists of conductive material provided with a high-resistance surface coating, destruction of the carrier profile caused by leakage current can be avoided, but the temporary local failure of affected emission peaks is nevertheless present until the required cleaning.On the basis of the problem mentioned above, the industry requires that both the occurrence and the type of contamination be gradually detected with the highest possible sensitivity. Industry 4.0 and the associated intelligent networking of machines and processes in the industry are increasingly benefiting from such fault detection systems that enable automatic fault recovery. For example, the respective discharge electrode can be temporarily switched off in order to avoid a fire or total failure of the system. A method and device known from the prior art for monitoring unloading devices can be taken from DE 10 2009 053 788 A1. DE 10 2011 007 136 A1 relates to an antistatic device for reducing electrostatic charges on moving material webs. EP 3 248 254 B1 discloses an emission tip arrangement on high voltage electrodes for charging and discharging substrates. EP 0 968 628 B1 relates to the use of an active electrode.It is an object of the present invention to provide a device for monitoring contamination at discharge electrodes, by means of which necessary maintenance can be detected in good time, that is to say in particular before an expected failure of the discharge electrode.According to the invention, the above object is achieved by a method according to independent claim 1. A corresponding device is specified in the subordinate patent claim 10. Advantageous further developments of the solution according to the invention are disclosed in the dependent claims.Accordingly, the present invention relates to a method for monitoring contamination of discharge electrodes according to claim 1.The invention is based on the idea that the active power consumed by the individual discharge electrodes is variable due to contamination. Accordingly, on the basis of the change in the active power, conclusions can be drawn about the contamination of the discharge electrodes or the associated emission peaks. However, there are some problems that make such monitoring of the active power appear unattractive. In particular, the active performance is only minimally impaired by the insulating contamination, for example by stowage of the emission tips. This is under the noise level of the currents arising from the passive and active discharge and is therefore substantially not recognizable or only recognizable too late. In addition, in order to detect the total active power of a plurality of discharge electrodes of an ionization installation, it would be necessary to provide a separate transformer per discharge electrode in order to be able to determine the active power of the individual discharge electrodes.The present invention avoids the above-described problem in that only partial currents which flow between the emission tips of the discharge electrode and the ground terminal of the device are measured. In normal operation, i.e. when no conductive contamination or no excessively large insulating contamination is present, such partial currents are caused by a capacitive coupling between the emission tips and the metal profile carrier or between the emission tips and the insulatedly embedded conductor, as will be explained in more detail with reference to the drawings. The partial currents generated by the capacitive coupling are based only on the alternating currents caused by the alternating voltage, since a passive discharge via the emission peaks leads to a direct current which cannot have any influence during the capacitive coupling. Accordingly, the partial current is also free of the relatively high current components which are generated by the passive discharge. The signal for the partial current obtained in this way is highly resolving and can thus be used for detecting even very slight soiling, that is to say an active power drop of approximately 1% of the maximum possible useful signal or less.It is known to the person skilled in the art that in the case of insulating contamination of the emission peaks, a slow (often over weeks and months) drop in the active power occurs. As will be explained in more detail below, a drop in the effective power can be used on the basis thereof to draw conclusions about the contamination.It should be noted that the partial currents flowing between the emission tips and the ground terminal of the device may also arise in some situations independently of the capacitive coupling. For example, these may be leakage currents which occur when there is conductive contamination between the emission tips and the ground connection. For example, moisture deposits between the tips and the metal profile carrier can lead to leakage currents occurring between the tips and the carrier, which lead to the detected partial currents and the associated active power abruptly increasing. Thus, on the basis of the above-mentioned partial currents, not only a conclusion can be drawn about insulating contaminants, but also conductive contaminants, such as moisture.For detecting the partial stream, the control device can be designed to receive / request different data / parameters. For example, this data may include a voltage drop across a reference resistor disposed between the emission tip and the ground terminal. It should be noted that the invention is not limited to the manner in which the sub-stream is detected. Rather, the invention is based on the idea of using such a partial current as a parameter for the contamination of the discharge electrode.For calculating the active power, the control device is designed according to one embodiment to determine the mean value of the product of the instantaneous values of a partial current and of the supply voltage, measured on the secondary side of the transformer, that is to say the mean value of the instantaneous powers. The control device can be designed, on the one hand, to determine the product of the secondary voltage and partial current in an analogous manner. On the other hand, it is of course also possible for the control device to digitally determine the product of voltage and current.According to a further embodiment, the control device is designed for:• comparing the calculated active power with a first active power reference value;• Generating the status signal on the basis of the comparison between the calculated active power and the first active power reference value.The active power reference value may be an expected active power achieved when the discharge electrode is clean, i.e. without significant contamination. Of course, this value is a parameter which depends on many factors, such as the number of emission peaks and also the materials used and the state of the discharge electrode. Accordingly, the active power reference value of each discharge electrode can be calibrated at the factory or also at the user's site.In a further embodiment, the active power reference value can also be changed automatically or manually in order to take account of the wear of the discharge electrode over time. In other words, over time, the current flowing between the emission tips and the ground may change (e.g., decrease) without being caused by contamination of the discharge electrode. In order to take this change over time in the expected active power value into account, the user can, for example after each cleaning of the electrode system, carry out a calibration measurement of the active power and define the value thus generated as a new active power reference value. Of course, this can also be done automatically.According to a further embodiment, the control device is designed for:• generating a first status signal representative of insulating contamination if the calculated active power is below the first active power reference value; and / or• Generating a second status signal representative of conductive contamination if the calculated active power is above the first active power reference value.As already indicated above, a reduction in the active power compared to the active power reference value can indicate an insulating contamination, as a result of which the capacitive coupling between the emission peaks and the parts of the discharge electrode connected to the ground also decreases. Accordingly, the partial flow also changes, which is detected by the control device and ultimately leads to a reduction in the active power determined. In this case, i.e. when the determined active power is below the first active power reference value, the control device can output a status signal which indicates an insulating contamination. This status signal can be output to the user, for example, via an optical or acoustic warning signal. On the other hand, the status signal may additionally or alternatively be used to automatically clean the discharge electrode. The situation is similar if the determined active power lies above the first active power reference value. This can indicate a conductive contamination, such as too high a humidity at the discharge electrode. The second status signal can be used to inform the user accordingly or to take countermeasures automatically.In a further embodiment, the first status signal can be output if a first active power reference value is undershot and the second status signal can be output if a second active power reference value is exceeded. The second active power reference value can be significantly higher than the first active power reference value. This is due in particular to the fact that, in the case of conductive contamination, the active power determined is expected to increase greatly. By using two active power reference values, it is possible to prevent insignificant changes in the active power with respect to the active power reference value from leading to an unwanted fault message.According to a further embodiment, a plurality of discharge electrodes is monitored simultaneously, wherein a partial current is detected for each discharge electrode, which partial current is coupled to the ground connection by the at least one emission tip of the respective discharge electrode. In other words, the method is designed to detect a number of different partial currents, which corresponds to the number of discharge electrodes to be monitored. On the hardware side, only one discharge electrode resistor per discharge electrode is necessary in order to set the device to more or less discharge electrodes to be monitored.According to a further embodiment, the partial current is detected, which is capacitively coupled from the emission tip to the metal profile or the insulatedly embedded conductor and consequently to the ground connection.According to a further embodiment, the detection of the partial current and thus the signal generation and evaluation for detecting dirt can be carried out selectively and independently of one another for any desired number of discharge electrodes.According to a further embodiment, a contamination-dependent active resistance is determined by squaring the secondary voltage of a transformer divided by the contamination-dependent active power. The contamination-dependent resistance determined in this way is in particular independent of possible mains voltage fluctuations.According to a further embodiment, the detected partial current for generating the status signal is independent of a direct current component which, during the discharge of highly charged surfaces, has to flow off to ground via the emission tip and the secondary winding of a transformer.According to a further embodiment, the status signal comprises separate status data for each discharge electrode.According to a further aspect, the present invention relates to a device according to claim 10.According to a further embodiment, the device has at least one discharge electrode resistor, via which a metal profile or an insulatedly embedded conductor is connected to the ground terminal, wherein the device is designed to measure a voltage drop across the discharge electrode resistor, wherein the control device is furthermore designed to determine the partial current on the basis of the voltage drop across the discharge electrode resistor. If the device is to be used to monitor a plurality of discharge electrodes, the device has at least one discharge electrode resistance per discharge electrode. The discharge electrode resistors are each connected to the metal profile or embedded conductor of the discharge electrode on the one hand and the ground terminal on the other hand. Accordingly, the partial current which flows through the capacitive coupling or the leakage currents between the emission tips and the ground terminal can be determined in a simple manner by the discharge electrode resistors.According to a further embodiment, the device has a transformer which can be connected on the secondary side to the discharge electrode, wherein the control device is connected on the secondary side, in particular via a voltage divider, to the transformer and is designed to determine the secondary voltage of the transformer. In this case, the voltage divider can be designed such that the voltage values output to the control device are in the preferred operating range of the control device. The control device can determine the secondary-side voltage of the transformer on the basis of the voltage output via the voltage divider and multiply this voltage by the determined partial current, as already mentioned above, in order to determine the active power. By connecting the control device to the secondary side of the transformer, the voltage which varies over time can also be taken into account by the control device. Fluctuations in the mains voltage can therefore not falsify the measurement result, that is to say the ascertainment of the active power.According to a further embodiment, the device has a display for analog or digital display of the status signal. Thus, for example, if the above-mentioned active power reference value is undershot, the user can be informed via the display that there is possibly insulating contamination at the discharge electrode. As already mentioned above, the device can also inform the user which discharge electrode appears to be soiled by the independent evaluation of the individual partial currents. The device can also output further information via the display, for example in order to prompt the user to clean a specific discharge electrode.The present invention is explained in more detail below with reference to the embodiments shown in the drawings. The following are shown: FIG. 1 shows a schematic circuit diagram of a device for monitoring contamination at discharge electrodes according to the present invention for discharge electrodes with metal profiles; FIG. 2 shows a schematic, idealised representation of the current, voltage and instantaneous power curve and the calculated active power as the mean value of the instantaneous powers over a period; and FIG. 3 shows a schematic circuit diagram of a device for monitoring contamination at discharge electrodes according to a further embodiment for monitoring discharge electrodes with insulated embedded conductors.FIG. 1 shows a schematic circuit diagram of a device according to the invention for detecting soiling at discharge electrodes. In the example of FIG. 1, the device 100 is connected to three discharge electrodes 3, 4, 5. However, it should already be mentioned at this point that the device 100 according to the invention can be adapted to any desired number of discharge electrodes 3, 4, 5.The device comprises a high-voltage transformer 10 and a measurement or evaluation module 20, the primary winding 11, 12 of the high-voltage transformer 10 being connected to the supply network or to a variable-frequency and voltage supply unit (not shown). A first end of the secondary winding 13 is connected to the three discharge electrodes 3, 4, 5 via a high voltage cable 2. The discharge electrodes 3, 4, 5 are generally AC high-voltage discharge electrodes or high-voltage ionizers.The second end of the secondary winding 14 is connected to a ground terminal 21. The alternating high voltage delivered to the high-voltage cable 2 via the secondary winding is typically in the voltage range from 4 to 10 kV. Such high voltages are required in order to produce a high electric field strength at the emission tips of the discharge electrodes 3, 4, 5 with an accompanying corona discharge. This corona discharge shows bipolar gas ions and free electrons by impact ionization. Such free charge carriers are capable of neutralizing positively or negatively charged surfaces or of discharging their surface.A portion of the alternating high voltage of the transformer 10 provided on the secondary side is provided as an input signal of a control device 27 via a voltage divider 22, 23. Accordingly, the voltage signal u(t) provided to the control device 27 is proportional to the alternating high voltage which is present in the high-voltage cable ( 2). As will be explained in more detail later, the voltage signal u(t) can be used by the control device 27 to determine the active power.The control device 27 of the evaluation module 20 is furthermore connected to a metallic part of each of the discharge electrodes 3, 4, 5. In other words, the control device 27 serves for detecting parameters or data which provide information about a partial current of the discharge electrodes 3, 4, 5 flowing via a direction of the ground connection 21. The partial current of the discharge electrode 3 is schematically shown here as "i1(t)". The partial current of the second discharge electrode 4 is schematically shown as "i2(t)". Finally, the partial current of the third discharge electrode 5 is schematically shown as "i3(t)". However, it should be mentioned at this point that the control device 27 does not have to be supplied with a direct measurement of the partial currents i1(t), i2(t), i3(t). Rather, any parameters or data which provide information about the partial currents flowing to the ground connection 21 can be passed on to the control device 27.As is shown schematically in FIG. 1, for this purpose the control device can respectively multiply the voltage signal u(t) by the partial currents i1(t), i2(t), i3(t) and form an average value in order to determine the active power of the partial currents i1(t), i2(t), i3(t) which is caused by a coupling of the emission peaks to the ground connection 21. Expressed mathematically, the control unit can calculate the active power caused by the capacitive coupling or by the leakage currents, in each case as follows:The capacitive coupling indicated above will be explained in more detail below. The discharge electrodes 3, 4, 5 shown in FIG. 1 are substantially identical. However, it should be noted that the arrangement and number of emission peaks may be significantly different between the discharge electrodes. The outlined arrangements according to FIG. 1 are accordingly to be regarded only as schematic. For example, only the discharge electrode 3 of FIG. 1 will therefore be described below.The discharge electrode 3 has one or more emission tips 32, each of which is assigned a current-limiting resistor 31. The emission tips 32 and current-limiting resistors are embedded by means of insulating potting compound in a metal profile 33, which serves as a carrier profile for the emission tips. This metal profile 33 preferably has an insulating, voltage-resistant surface. The current-limiting resistors 31 are connected on the one hand to the high-voltage cable 2 of the device 100 and on the other hand to its corresponding emission tip 32. Thus, the alternating high voltage provided by the transformer 10 can generate a high electric field strength with an accompanying corona discharge at the emission tips 32. This corona discharge produces, by impact ionization, bipolar gas ions and free electrodes. A portion of these charge carriers generated by the ionization current and thus a portion of the active electrical power emitted by the emission tips 32 is transmitted from the emission tips 32 to the metal profile 33 via a capacitive coupling 34, shown schematically in dashed lines.The metal profile 33 of the discharge electrode 3 is connected to the ground terminal 21 of the device 100 via an electrical terminal 35. Between the electric terminal 35 and the ground terminal 21, i.e., the ground, a first discharge electrode resistor 24 is disposed. The same applies to the second and third discharge electrode resistors 25, 26, which are respectively arranged between the second and third discharge electrodes 4, 5 and the ground terminal 21.The voltage drop at the first discharge electrode resistor 24 is proportional to the ion-generating active current i1(t) of the first discharge electrode 3; this also applies analogously to the discharge electrodes 4, 5 and to the current measurement via the voltage drop at the discharge electrode resistors 25, 26. In summary, it remains to be stated that the discharge electrode resistors 24, 25, 26 (for example by determining the voltage drop) serve to determine the respective partial current which is coupled via the capacitive coupling 34 from the emission tips 32 to the metal profile 33 and thus to the ground connection 21.It has been found that the capacitively coupled currents of the discharge electrodes 3, 4, 5 are dependent on the contamination of the discharge electrodes. Thus, in the case of insulating contamination, that is to say, for example, in the case of stowage of the emission peaks, a change in the partial currents i1(t), i2(t), i3(t) or in the active power associated therewith can be detected. By the separately provided discharge electrode resistors 24, 25, 26, it is accordingly possible to separately detect insulating contaminants at each individual discharge electrode 3, 4, 5.However, the device 100 is not only capable of detecting partial currents which are generated via the abovementioned capacitive coupling 34. Rather, leakage currents between the emission tips 32 and the metal profile 33 can also be determined. Such leakage currents are produced, for example, by undesired low-ohmic connections between the emission tips and the metal profile 33, as can be produced, in particular, by moisture deposits on the emission tips. Such leakage currents are generally many times higher than the partial currents generated by the capacitive coupling 34. In other words, in the case of conductive contamination (for example, too high humidity at the emission peaks), partial currents i 1( t), i 2( t), i 3(t) occur in the direction of the ground terminal 21, the active current portions of which are significantly higher than the partial currents i 1( t), i 2( t), i 3(t) generated by the capacitive coupling 34. As will be explained in more detail later, the control device 27 can accordingly infer a conductive contamination of one or more of the discharge electrodes 3, 4, 5 in the event of a sudden change in one or more of the partial currents i1(t), i2(t), i3(t).The high sensitivity of the partial currents allows early detection and indication of the contamination state even with a slight change in the emission power of the discharge electrodes. Thus, by the status signal based on the partial streams i1(t), i2(t), i3(t), which is generated by the control device 27, a safe planning for preventive maintenance can be achieved without undesired machine downtime. The signal is unambiguous and is assigned only to the electrode that is respectively soiled in an insulating or conducting manner.The apparatus 100 also has a field bus connection 28 which is connected to a bidirectionally operating connection of the control device 27. The status signal generated by the control device 27 can accordingly be output to any desired process environment for the higher-order quality assurance or the general provision of the data in the sense of industry 4.0. For this purpose, the device 100 can have an interface, which is not discussed in greater detail.In the embodiment of the device 100 illustrated in FIG. 1, a display 29 is also provided, via which data can be output by the control device 27. Thus, for example, a display or visualization of the degree of contamination can take place numerically or graphically on the display 29. For example, the status signal of the control device 27 can be used to output warning messages to the user via the display if a conductive or insulating contamination has been detected by the control device 27. The output can further inform the user which of the discharge electrodes 3, 4, 5 is / are affected. In a further embodiment, the status signal of the control device 27 can also contain information or proposals for removing the soiling. Alternatively or additionally, the status signal generated by the control device 27 can also be used to initiate automatic cleaning of the respective discharge electrode 3, 4, 5.The method carried out by the control device for generating a status signal which is representative of the contamination status of the discharge electrodes is explained in more detail below.As already mentioned above, the alternating high voltage provided in the high-voltage cable 2 results in a high electric field strength at the emission tips with an accompanying corona discharge. A portion of the emitted electronic charge carriers and thus a portion of the emitted active electrical power is transmitted from the emission tips 32 to the metal profile 33 via the capacitive coupling 34. Such charge carriers are discharged in the form of a partial current i1(t) in the direction of the ground connection 21. The current flow / partial current i1(t) arising from this can be detected via a voltage drop at the discharge electrode resistor 24.The control device 27 is designed to multiply the current intensity of the partial current i1(t) detected in this way with the voltage signal u(t) and to form an average value of the product in order to determine the part of the emitted electrical active power which is transmitted to the metal profile via capacitive coupling.In this connection, reference is also made to FIG. 2. FIG. 2 shows a schematic, idealised graph of the current, voltage and instantaneous power signal and the active power profile. In the schematic illustration according to FIG. 2, the voltage signal (u(t)) and the current signal (i(t)) are shown purely sinusoidally. In reality, only the voltage signal corresponds approximately to the sinusoidal profile illustrated here. However, the current signal of the partial current 202 has a very high harmonic component, so that no sinusoidal profile is present. Accordingly, reference is made only schematically to the illustration according to FIG. 2.The instantaneous power signal 206 resulting from the multiplication of the voltage signal 204 and the current signal 202 is likewise schematically illustrated in FIG. 2. The control unit is designed to calculate the mean value of the instantaneous powers (p) and thus the active power 208 (P). This can be done on the one hand by an analog circuit and on the other hand by a digital calculation according to the integral presented further above. The control unit compares the active power calculated in this way with an active power reference value.In the event of contamination of the discharge electrodes, a phase shift of the current signal with respect to the voltage signal 204 occurs. The phase shift is schematically indicated in FIG. 2 by arrows. Thus, in the case of conductive contamination, which can generate leakage currents, a phase shift of the current signal in the direction of the voltage signal occurs and thus frequently the abrupt increase of the active power 208. With insulating contamination, the phase difference between the voltage signal 204 and the current signal 202 of the partial current increases, so that a gradual reduction of the active power 208 occurs. It should be pointed out at this point that monitoring the active power (i.e. the change in the active power), that is to say calculating the product of the current intensity and voltage and averaging, represents a particularly fast and reliable method in order to detect a change in the phase shift between the current and voltage signals and thus to be able to draw conclusions about the contamination of the discharge electrodes. However, it is also conceivable to detect the phase shift solely on the basis of the current signal 202. This can be achieved, for example, by a Fourier analysis of the current signal, wherein in this case a calculation of the active power is not absolutely necessary. Rather, in this case, the control unit can be designed to draw conclusions directly about the insulating or conductive contamination of the discharge electrodes on the basis of the phase shift.It should also be noted with regard to FIG. 2 that, in the case of conductive or insulating contamination of the discharge electrodes, the amplitude of the current signal 202 substantially does not change, so that pure observation of the amplitude of the current signal 202 does not allow any meaningful conclusions about the contamination of the discharge electrodes; this is the case above all with less insulating contamination.The control device 27 can be equipped with an active power reference value which is determined, for example, at the factory. The active power reference value is a value for the active power generated on account of the capacitive coupling 34, which is to be expected in the case of uncontaminated discharge electrodes. This can be determined, for example, at the factory by measurements of the uncontaminated discharge electrodes and stored in a memory of the control device 27. On the other hand, the active power reference value can also be variable / adjustable by the user, as will be explained in more detail later.The control device is designed to compare the active power based on the partial current i1(t) with the active power reference value. In this case, a calculated active power which lies below the active power reference value can be an indication that there is insulating contamination, as has already been explained above. The active power reference value may be a few percent below the actually expected active power with the discharge electrode clean, in order to generate a warning signal, i.e. a change in the status signal of the control device, only when a certain degree of contamination has already occurred. This can be provided in particular for compensating natural fluctuations of the calculated active power.The control device can be designed to output a first status signal which is representative of insulating contamination if the calculated active power is below the first active power reference value. By way of example, FIG. 2 shows a first active power reference value 210, which lies below the active power 208 in the non-soiled state.The control device can generate a second status signal which is representative of a conductive contamination if the calculated active power is above the first active power reference value.Since the expected active powers in the case of conductive or insulating contamination are fundamentally different, the control device can be equipped with a second active power reference value in addition to the first active power reference value. Accordingly, the control device can compare the calculated active powers in each case continuously or in intervals with the first active power reference value and the second active power reference value. If the calculated active power is below the first active power reference value, the control device generates the first status signal, which indicates an insulating contamination. In the event that the calculated active power lies above the second active power reference value, the control device outputs the second status signal, which indicates conductive contamination. In this case, the second active power reference value is preferably many times higher than the first active power reference value. Typically, the difference may be about a factor of 100.According to a further embodiment variant, the control device can also be designed to determine a difference between the active power and the first active power reference value or the second active power reference value. On the basis of the difference between the active power reference values and the calculated active power, the control device 27 can generate the status signal. For example, in the case of an active power which lies below the first active power reference value, a negative difference results. The negative difference may be used by the controller to output the first status signal. Alternatively or additionally, the control device 27 can be designed to check whether the difference exceeds a certain threshold value. It can thus be ensured, for example, that the first status signal is only generated when the calculated active power falls below the first active power reference value by a specific value or a specific percentage proportion. This is advantageous because the calculation of the active power according to the present invention is highly sensitive and can already detect very slight soiling of the discharge electrodes. In order not to notify the user unnecessarily about very small soilings, the threshold value can accordingly be set by the user. This can be set as a percentage, for example, via the display, so that the first status signal is only generated when the calculated active power deviates significantly from the first active power reference value (e.g., above 10%).The control device can be designed accordingly to determine the difference between the calculated active power and the second active power reference value and to use this difference, as described above in connection with the first active power reference value, to generate the second status signal. An exemplary second active power reference value 212 can likewise be seen from FIG. 2 and lies above the active power P when the discharge electrode is not soiled.If there is insulating contamination and conductive contamination occurs simultaneously (e.g. since water drips onto the electrode), a clearly higher active power will always be established, so that the control unit outputs only the second status signal for the conductive contamination. Conductive contamination generally has the highest and thus first priority. Only when the discharge electrode has dried off again and the insulating contamination still existed, would the control unit output the first status signal for an insulating contamination after the conductive contamination disappearing.It should be mentioned at this point that the expected effective power, which is achieved in particular via the capacitive coupling 34, is variable over time as the discharge electrodes 3, 4, 5 wear. Accordingly, it may be useful to readjust the first active power reference value from time to time. For this purpose, the device 100 can have interfaces for the input of new reference values by the user. For example, the display 29 can be designed as a touchscreen in order to input the desired active power reference value. For this purpose, the display can have a corresponding input mask. Alternatively or additionally, the control device can be designed to automatically readjust the active power reference value at regular intervals. For this purpose, only an input by the user is necessary in order to confirm that the relevant discharge electrode is free of contamination. The control device can then carry out a test operation of the relevant discharge electrode in order to determine the corresponding effective power of the now clean discharge electrode. The mean value of this active power can then be stored as a new first active power reference value.Since the size and arrangement of the different discharge arrangements 3, 4, 5 can differ substantially from one another, it is generally provided that the control device has a corresponding first and / or second active power reference value for each discharge electrode to be monitored. The status signals generated by the control device can thus be adapted to the circumstances of the different discharge electrodes 3, 4, 5. Thus, the signal generation and evaluation for detecting dirt is possible selectively and independently of one another for any desired number of discharge electrodes.If discharge electrodes are operated with the device 100 according to the invention against an electrically highly charged moving substrate, a DC discharge current flow takes place via the emission tips 32 and the secondary winding 13, 14 of the transformer, depending on the polarity of the substrate charge. However, such a DC discharge current flow does not lead to capacitive coupling and is accordingly also not detected by the control device 27. Thus, the calculation of the active power according to the present invention is also independent of the very high DC discharge current flows, whereby the sensitivity of the proposed contamination detection can be increased even further.FIG. 3 shows a schematic circuit diagram of the device 100 according to the invention, in cooperation with a second type of discharge electrodes 6, 7, 8. In other words, the device 100 is identical to the device 100 according to FIG. 1. Only the discharge electrodes 6, 7, 8 of FIG. 3 monitored by the device 100 differ from the discharge electrodes of FIG. 1. In particular, the discharge electrodes 6, 7, 8 of FIG. 3 are discharge electrodes having a carrier profile made of insulating material 63 with at least one embedded conductor 65 which is connected via an electrical connection point 66 via a conductor to the ground connection 21 of the device 100. The function of the discharge electrodes 6, 7, 8 shown in FIG. 3 with carrier profiles made of insulating material 63 can be seen in detail in EP 0 968 628 B1.In the case of discharge electrodes with carrier profiles made of insulating material, too, a part of the charge carriers generated by the ionization current and emission tips 62, and thus a part of the emitted electrical active power, is transmitted via the capacitive coupling 64 to the conductor 65 embedded in insulating material 63 and is thus discharged in the direction of the ground connection 21. The electrical connection of the conductor 65 to the ground terminal 21 is effected via the resistor 24. the voltage drop at the resistor 24 is therefore in turn proportional to the ion-generating active current of the discharge electrode 6 and thus proportional to the active power. This also applies analogously to the discharge electrodes 7, 8 and to the current measurement via the voltage drop at the discharge electrode resistors 25, 26.The mode of operation of the control device 27 for determining the status signal is substantially identical to the mode of operation which has already been described in connection with FIG. 1. Accordingly, for the use of the device on different discharge electrodes, no further modification of the device 100 is required. Only the active power reference values of the discharge electrodes 6, 7, 8 differ substantially from the active power reference values of the discharge electrodes 3, 4, 5 according to FIG. 1.

Claims

Method for monitoring soiling at discharge electrodes (3, 4, 5, 6, 7, 8) which serve for the contactless discharge of electrically charged surfaces, wherein the discharge electrodes (3, 4, 5, 6, 7, 8) have at least one emission peak (32, 62), and wherein the method comprises: - detecting a partial current (i1(t), i2(t), i3(t)) which is coupled by the at least one emission peak (32, 62) to the earth (21) connected to the discharge electrode (3, 4, 5, 6, 7, 8); - determining an active power (208) on the basis of the partial current (i1(t), i2(t), i3(t)); - generating a status signal on the basis of the calculated active power (208).Method according to claim 1, wherein the method further comprises the steps of: - comparing the determined active power (208) with a first active power reference value (210, 212); - generating the status signal on the basis of the comparison between the determined active power (208) and the first active power reference value (210, 212).Method according to claim 2, wherein the method further comprises the steps of: - generating a first status signal representative of an insulating contamination if the calculated active power (208) is below the first active power reference value (210, 212); and / or - generating a second status signal representative of a conductive contamination if the calculated active power (208) is above the first active power reference value (210, 212).Method according to claim 2 or 3, wherein the first active power reference value (210, 212) is set at the factory and / or is settable by the user.Method according to one of Claims 1 to 4, wherein the method is used for simultaneously monitoring a multiplicity of discharge electrodes (3, 4, 5, 6, 7, 8), and wherein a partial current (i1(t), i2(t), i3(t)) which is coupled to the ground connection (21) by the at least one emission tip (32, 62) of the respective discharge electrode (3, 4, 5, 6, 7, 8) is detected for each discharge electrode (3, 4, 5, 6, 7, 8).Method according to any one of claims 1 to 5, wherein the sub-stream (i1(t), i2(t), i3(t)) is detected, which is capacitively coupled from the emission tip (32, 62) to a metal profile (33) or to an insulatedly embedded conductor (65) and thus to the ground terminal (21).Method according to one of Claims 1 to 6, wherein the detection of the partial current (i1(t), i2(t), i3(t)) and thus the signal generation and evaluation for detecting contamination take place selectively and independently of one another for any desired number of discharge electrodes (3, 4, 5, 6, 7, 8).Method according to one of Claims 1 to 7, wherein a contamination-dependent active resistance is determined by squaring a secondary voltage of the transformer (10) divided by a contamination-dependent active power (208).Method according to one of Claims 1 to 8, wherein the detected partial current (i1(t), i2(t), i3(t)) for generating the status signal is independent of a direct current component which has to flow off to ground via the emission tip (32, 62) and the secondary winding of a transformer (10) during the discharge of highly charged surfaces.Device (100) for monitoring contamination at discharge electrodes (3, 4, 5, 6, 7, 8) having at least one emission tip (32, 62) which serve for the contactless discharge of electrically charged surfaces, the device (100) having: - a ground connection (21) which is connected or can be connected to the discharge electrode (3, 4, 5, 6, 7, 8); - a control device (27), the control device (27) being designed to carry out the method according to one of Claims 1 to 9.Device (100) according to claim 10, wherein the device (100) has at least one discharge electrode resistor (24), via which a metal profile (33) or an insulatedly embedded conductor (65) is connected to the ground terminal (21), and wherein the device (100) is designed to measure a voltage drop across the discharge electrode resistor (24), wherein the control device (27) is designed to determine the partial current (i1(t), i2(t), i3(t)) on the basis of the voltage drop across the discharge electrode resistor (24).Device (100) according to Claim 10 or 11, wherein the device (100) has a transformer (10) which can be connected on the secondary side to the discharge electrodes (3, 4, 5, 6, 7, 8), and wherein the control device (27) is connected on the secondary side, in particular via a voltage divider, to the transformer (10) and is designed to determine the secondary voltage of the transformer (10).

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