Methods for operating a magnetron and magnetron

Abstract

The invention relates to a method for operating a magnetron (1) with a cavity (3) enclosed by an anode (2), with a cathode (5) arranged in the cavity (3) which includes a heating element (4), in particular a heating wire. Furthermore, an adjustable voltage source (6) is provided, which is connected to the heating element (4), as well as a control unit (8). According to the invention, a setpoint (Usetpoint) for a heating voltage (U) applied to the heating element (4) is determined, and the control unit (8) controls the voltage source (6) such that the setpoint (Usetpoint) of the heating voltage (U) is applied to the heating element (4).

Description

The invention relates to a method for operating a magnetron having a cavity enclosed by an anode, having a cathode arranged in the cavity, which cathode has a heating device, in particular a heating wire, having a controllable voltage source which is connected to the heating device, and having a controller. The invention also relates to such a magnetron.Magnetrons are used as energy-efficient and cost-effective sources of microwave radiation. The microwave radiation generated in this way is used, for example, for energy transmission. In this case, this can be used, inter alia, within the scope of a plasma process for coupling heating power into the plasma and / or for exciting chemical and physical deposition processes from the plasma to a surface to be treated and / or coated.The basic structure of a magnetron includes an anode which surrounds a cavity and a cathode arranged within the cavity. The cathode is excited to separate electrons by the action of heat, in particular by the heating device. One also refers in particular to a so-called glow cathode.A high voltage is applied between the cathode and the anode, which accelerates the emerging electrons in the direction of the anode. The trajectory of these electrons is curved by a magnetic field. In particular, the cathode is surrounded coaxially by the anode, wherein the magnetic field lines likewise run substantially axially.The shape of the anode is selected such that electromagnetic resonances occur due to the electron movement. The microwaves generated in this way can be coupled out via an output formed in the anode and fed to a use.The manufacturing costs of a magnetron are high, in particular, due to the cathode material used. An essential limiting factor for the lifetime of a magnetron is the wear of the cathode. Due to the heating of the cathode and the electron current emerging therefrom, the cathode material is ablated and / or altered such that the functionality is reduced over time. From a certain degree of wear on, the cathode is no longer to be used and has to be replaced. At this degree of wear, electron emission is no longer achieved and the magnetron can no longer strike.Against this background, the invention is based on the object of prolonging the lifetime of a magnetron, in particular for use in a plasma process, preferably a plasma coating process. The subject matter of the invention and the solution of this object is a method according to claim 1 as well as a magnetron according to claim 17 and a container coating system according to claim 22.Proceeding from the method of the generic type, it is provided according to the invention that a setpoint value for a heating voltage present at the heating device is determined and that the controller actuates the voltage source in such a way that the setpoint value for the heating voltage is present at the heating device. The invention is based in particular on the finding that, in the case of voltage-regulated actuation of the heating device, the service life of the magnetron can be significantly improved.The heating device preferably has at least two electrical inputs which are connected to the voltage source by at least one supply line. The heating voltage dropped across the heating device is preferably measured directly at the inputs. The heating voltage can be designed as a direct voltage or an alternating voltage without limiting the invention. In the presence of alternating voltage components, the effective voltage is to be taken into account in particular.Expediently, the heating device is designed as a so-called heating wire or heating coil. This is an electrical line which connects the inputs of the heating device to one another and has a greater ohmic resistance than the supply line. When the heating voltage is applied, a current flow occurs which, due to the resistance conditions, is predominantly converted into heat in the heating device.In the case of a positive temperature coefficient of the heating device, the voltage-controlled regulation according to the invention additionally has the advantage compared to a current-guided control that the resistance increases at a higher temperature and, as a result, the current flow and the electrical heating power are reduced at a constant voltage. This simplifies the control of the glow cathode and contributes to an increased service life.It is preferably provided that the controller has a unit for detecting the operating time T of the magnetron and / or of the heating device and that the setpoint value is determined as a function of the operating time. It has been found within the scope of the invention that different strategies are advantageous for optimum utilization of the life of the magnetron at different times of the life cycle. Whereas the unused cathode is capable of reliable electron emission even with a low level of heat stimulation by the heating device at the beginning of the service life, a higher heating voltage sometimes has to be applied with increasing wear (at a later point in time of the service cycle), that is to say the setpoint value has to be increased. Although the magnetron could also be supplied with a permanently increased heating voltage, this would result in increased wear at the beginning of the service life. Therefore, a setpoint voltage varying over the service life, in particular determined as a function of the service life, is advantageous.It is particularly preferably provided that the setpoint value is defined starting from a minimum value U min( minimum heating voltage). This minimum value is preferably set such that, in the case of an unused, new-factory cathode, the magnetron can be ignited. This is preferably about 1 V. Ignition is referred to in particular when the (high) voltage present between the anode and the cathode-at the heating power produced by the heating device during operation with the heating voltage-is sufficient to enable stable (glow) emission and oscillation of electrons, so that the magnetron can produce microwaves in the intended manner. In a commercially available magnetron, for example type 2M 121A from Hitachi, the minimum value is preferably in a range between 1 V and 10 V.According to a preferred embodiment of the invention, it is provided that the setpoint value is determined at least in sections as a linear function of the operating time T. In particular, the linear profile lies in a time interval between a first time T 1 and a second time T 2. The first time T 1 is preferably between zero operating hours and 1,000 operating hours, while the second time T 2 is preferably between 2,000 operating hours and 5,000 operating hours. In this case, the setpoint value U Soll rises from the first time T 1 from a first voltage value U 1 to a second voltage value U 2 at the second time T 2. The voltage value U 2 is greater than the voltage value U 1. The first voltage value U 1 can correspond in particular to the minimum value U min in this case. The instantaneous setpoint value U(T) is calculated here in particular as a linear interpolation according to the following formula:Alternatively or additionally, it can be provided that the setpoint value is either increased in a previously defined manner after the occurrence of a triggering event (trigger) or alternatively checked and increased as required.In the first case, it can be provided in particular that the setpoint value is incremented by a fixed value, in particular between 1 V and 10 V, e.g. 0.1 V. It is likewise conceivable for the setpoint value to be established according to a previously determined formula. This results in a step profile of the setpoint value over the operating duration.The trigger can be, in particular, an external trigger, in particular a user input and / or an external control. The control of the magnetron is configured to make an external request to increase the setpoint value for the heating voltage. Such a request can be made in particular when certain operating parameters and / or working results do not meet the requirements.Alternatively or additionally, the trigger can also be the reaching of a threshold value of the operating time (of the magnetron and / or of the heating device). As a result, the threshold value is raised and / or a check of the threshold value is carried out at previously determined time intervals in order to enable an increase according to requirements. It is likewise conceivable for the trigger to be the reaching of a time interval of operating time since the last check or the last increase of the setpoint value.Particularly preferably, a current flow between the anode and the cathode (anode current) is detected. The trigger can in particular be the undershooting of a threshold value in the anode current. A drop or a complete breakdown of the anode current indicates an inadequate ignition of the magnetron. In this case, functioning is not guaranteed. As a result, the heating voltage (the target value of the heating voltage) needs to be increased to enable stable glow emission of the cathode.It is particularly preferably provided within the scope of the check that the setpoint value is increased successively until the anode current reaches or exceeds a threshold value. In particular, an increment between 0.01 V and 0.1 V is provided in this case. The successive incrementing of the setpoint value ensures that it is increased only to the extent required to ensure reliable magnetron ignition. Unnecessary wear is thereby avoided. It is also provided in particular within the scope of the invention that an increase of the setpoint value only occurs if at least one failed ignition of the magnetron was previously detected. It is thus expediently possible to dispense with excessive or unnecessary wear-enhancing elevations.In particular, it is provided that the setpoint value is stored in the controller and used between two successive checks. In this time interval, the setpoint value is thus constant.It is likewise conceivable, after a trigger has occurred, in particular checks triggered by the lapse of time, to initially reduce the setpoint value. This can be effected in particular by an amount between 0.1 V and 5 V, in particular approximately 1 V. It is also conceivable to successively decrement the setpoint value until an ignition error of the magnetron is detected. The setpoint value is then increased again until the ignition takes place. This makes it possible to eliminate the maximum utilization of the life of a magnetron.Preferably, it is provided that the setpoint value is not reduced below the minimum voltage. It is likewise preferably provided that the setpoint value is not increased above a maximum voltage U max( maximum heating voltage). This maximum heating voltage marks in particular the limit of the economic and / or process-safe operating parameters. The maximum heating voltage is in particular between 1 V and 10 V.It is particularly preferably provided that the magnetron is switched off by the setpoint value when the maximum value is reached. This is the case in particular when the setpoint value for the heating voltage is already set to the maximum value U max and the threshold value for the anode current is undershot-that is to say an ignition fault of the magnetron occurs. The same applies to the case where the setpoint value has not yet reached the maximum value U max a (provided) further increment government would exceed the maximum value U max. In these cases, the magnetron is classified as defective and is to be replaced. For this purpose, the magnetron is switched off, an error signal is transmitted and / or production is interrupted.The method can be used particularly preferably in the context of plasma coating. For this purpose, it is expediently provided that the magnetron emits microwave radiation to a plasma via an output, in particular for a plasma coating method. The plasma coating method is very particularly preferably a plasma coating of containers, such as bottles, for example. In other words, the invention also relates to a plasma coating method of containers.The invention also relates to a magnetron having a cavity enclosed by an anode, having a cathode arranged in the cavity and having a heating device, having a controllable voltage source which is connected to the heating device and having a controller. According to the invention, the controller is configured to determine a setpoint value for the heating voltage and to actuate the voltage supply in such a way that the setpoint value of the heating voltage is present at the heating device. This magnetron according to the invention has a clearly increased life expectancy compared to market-available copies from the prior art.Expediently, the magnetron has a voltage measuring device coupled to the controller, which is connected to the heating device. The voltage measuring device serves to detect the heating voltage applied to the heating device and to make it usable for the control.Most preferably, the voltage source is connected to the heating device via supply lines. In order to enable voltage measurement which is as precise and uncorrupted as possible, the voltage measuring device is furthermore connected via measurement lines which are formed separately from the supply lines.The voltage measuring device is particularly preferably integrated into the controller. This can be in particular a so-called analog-digital converter (AD converter). This can be embedded in particular in a controller embodied as an integrated circuit.The invention also relates to a container coating plant for plasma coating containers. This is characterized by a magnetron described above. Its advantages according to the invention make possible a particularly long and therefore cost-effective use.The invention is explained below with reference to figures illustrating only exemplary embodiments. They show schematically: FIG. 1 shows a schematic illustration of a magnetron according to the invention for carrying out the method according to the invention, FIG. 2 shows an exemplary profile of the setpoint value within the scope of a method according to the invention according to a first embodiment variant, FIG. 3A shows an exemplary profile of the setpoint value and of the anode current in the method according to the invention according to a second variant, and FIG. 3B shows an exemplary profile of the setpoint value and of the anode current in the method according to the invention according to a third variant.FIG. 1 shows a magnetron 1 according to the invention with a cavity 3 surrounded by an anode 2, in which a cathode 5 having a heating device 4 is arranged. The heating device 4 is designed as a heating coil embedded in the material of the cathode 5 and is not explicitly shown in the schematic figure. The magnetron 1 also has a controllable voltage source 6, which is connected to the heating device 4. For this purpose, the heating device 4 is formed with two inputs 4A, 4B, which are electrically connected to the voltage source 6 by supply lines 7A, 7B. The ohmic internal resistance of the heating device 4 and the line resistances of the supply lines 7A, 7B are matched to one another in such a way that during operation the ohmic power loss drops substantially completely within the heating device 4.Furthermore, a controller 8 is provided, which is connected in a controlling manner to the variable voltage source 6. According to the invention, it is provided that a setpoint value for a heating voltage present at the heating device 4 between the inputs 4A, 4B is determined, and that the controller 8 controls the voltage source 6 in such a way that the setpoint value of the heating voltage is present at the heating device 4.In the exemplary embodiment shown, a voltage measuring device 9 coupled to the controller 8 is also provided, which is connected to the heating device 4. In this case, the variable voltage supply 6, the controller 8 and the voltage measuring device 9 are connected to one another in a common control unit 10.While the voltage source is connected to the inputs 4A, 4B of the heating device 4 via supply lines 7A, 7B, the voltage measuring device 9 is connected via measuring lines 11A, 11B formed separately from the supply lines 7A, 7B.In FIG. 1, it is also indicated that an output 12 is formed at the anode 2 of the magnetron 1 in order to couple out the microwaves generated within the magnetron 1. The output 12 is connected to a waveguide 13 for supplying the microwaves to a use. It is merely schematically indicated that the microwaves are coupled into a plasma 14, which is used for coating a packaging container 15.Various variants of the method according to the invention are explained below. Since the aging process is a time process as seen with regard to operation, the setpoint value is determined according to a first variant as a function of the operating time T of the magnetron 1 and / or of the heating device 4.In the exemplary embodiment shown in FIG. 2, the setpoint value U soll is initially set to a minimum value U min when a new magnetron 1 (T=0) is put into operation. This minimum value U min is calculated in such a way that the heating power produced thereby is sufficient to enable reliable ignition in the case of a new magnetron 1. The exemplary embodiment according to FIG. 2 provides that the setpoint value U soll initially remains constant until a first time T 1. From this point in time, the setpoint value U soll is linearly interpolating and increased up to a maximum voltage U max which is reached at a second point in time T 2. After that, the set value U soll remains the same as the value of the maximum voltage U max, until a time T max the maximum life of the magnetron has been reached. At this time, reliable ignition of the magnetron 1 is no longer possible. In this case, the magnetron 1 is switched off and, in particular, the setpoint value U soll for the heating voltage U is set to zero.FIGS. 3A and 3B show variants in which the setpoint value U soll is not increased in a predetermined manner, but rather is checked and merely increased as required. For this purpose, FIG. 3A shows that the anode current I A flowing between the anode 2 and the cathode 5 is additionally detected. The curves for the setpoint voltage U soll and the anode current I A are shown in a greatly simplified manner.At the beginning of the lifetime of the magnetron 1, the setpoint value U soll is initially set to the minimum voltage U min and the anode current I A is detected in parallel during operation. As a result of increasing wear, the emission behavior of the cathode 5 deteriorates as the operating time T progresses. In the example shown, at a first point in time T a the wear is so great that the anode current I A collapses and falls below a threshold value I thres. A so-called ignition error of the magnetron occurs.In order to restore operability, a check is made in a subsequent period. The setpoint value U soll is increased stepwise-with an increment of 0.1 V-until the anode current I A exceeds a threshold value I thres again at a second time T b. In particular, the curves are not shown to scale on the time axis T. In particular, it is provided that the time period between two successive checks or ignition errors is several 100 hours. The time interval between individual incremental voltage increases of the setpoint value U soll is in the range of a few seconds, so that a check is ideally completed in a time period of less than one minute. As a result, a low reject rate due to ignition errors can be achieved.It is also shown in the figure that an ignition fault occurs again at a third time T c. After this, the setpoint value U soll is incremented again in the same way until the magnetron 1 is fired again at a fourth point in time T d. In the same way, several checks can be carried out over the lifetime of a magnetron 1. In these heating devices, the setpoint value U soll of the heating voltage U is always increased only to the necessary extent. This allows the life of the magnetron 1 to be maximized overall.The figures also show that an ignition fault occurs again at a fifth point in time T e. In the course of the subsequent check, the setpoint value U soll reaches or exceeds the provided maximum voltage U max, during the incrementation without it being possible for the ignition to be produced again. It is thus established that the magnetron 1 has reached its service life and has to be switched off. As a result, the setpoint value U soll and preferably also the anode current I A are switched off or set to zero.FIG. 3B shows a further possibility for checking the setpoint value. This can be provided alternatively or additionally to the checks triggered by an ignition fault in FIG. 3A. In the variant shown in FIG. 3B, the checking of the setpoint value U soll is triggered in a time-controlled manner, i.e. by reaching a time period of the operating duration T at the time T g. At this time, the anode current I A is still above the threshold value I thres. As can be seen from the figure, the setpoint value U soll is initially reduced by a value at the beginning of the check, so that the anode current I A collapses. The setpoint value is then increased again incrementally until the magnetron is ignited h at the time T. In this special variant, it is possible that the setpoint value U soll has a lower value after the check than before.List of reference numbers:1 Magnetron 2 Anode 3 Cavity 4 Heating device 5 Cathode 6 Voltage source 7A, 7B Supply lines 8 Controller 9 Voltage measuring device 10 Control unit 11A, 11 bmeasuring lines 12 output 13 waveguide 14 plasma 15 packaging container U soll setpoint value U min minimum value U max maximum voltage I thres threshold value IA anoden current T operating time T a first point in time T b second point in time T c third point in time T d fourth point in time T e fifth point in time T g point in time Th point in time

Claims

Method for operating a magnetron (1) having a cavity (3) enclosed by an anode (2), having a cathode (5) which is arranged in the cavity (3) and has a heating device (4), in particular a heating wire, having a controllable voltage source (6) which is connected to the heating device (4) and having a controller (8), characterized in that a setpoint value for a heating voltage (U) present at the heating device (4) is determined, and in that the controller (8) controls the voltage source (6) in such a way that the setpoint value (U soll) for the heating voltage (U) is present at the heating device (4).Method according to one of the preceding claims, characterized in that the controller (8) has a unit for detecting the operating time (T) of the magnetron (1) and / or of the heating device (4) and in that the setpoint value (U soll) is determined as a function of the operating time (T).Method according to one of the preceding claims, characterized in that the setpoint value (U soll) is defined starting from a minimum value (U min).Method according to one of the preceding claims, characterized in that the setpoint value (U soll) is determined at least in sections as a linear function of the operating time (T).Method according to one of the preceding claims, characterized in that the setpoint value (U soll) is increased at defined time intervals.Method according to one of the preceding claims, characterized in that the setpoint value (U soll) is increased and / or checked and increased as required after the occurrence of a triggering event.Method according to Claim 6, characterized in that the triggering event is realized by an external event, in particular by a user input and / or an external control.Method according to Claim 6 or 7, characterized in that the triggering event is realized by reaching a threshold value for the operating time (T).Method according to one of the preceding claims 6 to 8, characterized in that the triggering event is realized by reaching a time interval of operating time (T) since the last check and / or the last increase of the setpoint value (U soll).Method according to one of the preceding claims 6 to 9, characterized in that a current flow between the anode (2) and the cathode (5), the anode current (I A) is detected, and in that the trigger is the fall below a threshold value (I thres) in the case of the anode current (I A).Method according to one of the preceding claims, characterized in that the setpoint value (U soll) is increased until the anode current (I A) reaches or exceeds the threshold value (I thres).Method according to one of the preceding claims, characterized in that the setpoint value (U soll) is stored in the controller and used between two successive checksMethod according to one of the preceding claims, characterized in that, after a release, the setpoint value (U soll), is first reduced, in particular by an amount between 0.1 V and 5 V.Method according to one of the preceding claims, characterized in that the setpoint value (U soll) does not fall below a minimum value U min, in particular approximately 1 V.Method according to one of the preceding claims, characterized in that the setpoint value (U soll) is not increased above a maximum voltage (U max).Method according to one of the preceding claims, characterized in that, when a threshold value (I thres) for the anode current (I A) is undershot and the setpoint value (U soll) reaches the maximum value (U max) the magnetron (1) is switched off.Method according to one of the preceding claims, characterized in that the magnetron (1) emits microwave radiation via an output (12) to a plasma (14), in particular for a plasma coating method, very particularly preferably for the plasma coating of containers (15).Magnetron (1) having a cavity (3) enclosed by an anode (2), having a cathode (5) which is arranged in the cavity (3) and has a heating device (4), having a controllable voltage source (6) which is connected to the heating device (4) and having a controller (8), characterized in that the controller (8) is designed to determine a setpoint value (U soll) for a heating voltage (U) and to actuate the voltage supply (6) in such a way that the setpoint value (U soll) of the heating voltage (U) is present at the heating device (4).Magnetron (1) according to claim 18, characterised bya voltage measuring device (9) coupled to the control (8), which is connected to the heating device (4).Magnetron (1) according to one of the preceding claims 18 or 19, characterised in that the voltage source (6) is connected to the heating device (4) via supply lines (7A, 7B) and in that the voltage measuring device (9) is connected via measuring lines (11A, 11B) formed separately from the supply lines (7A, 7B).Magnetron (1) according to one of the preceding claims 18 to 20, characterised in that the voltage measuring device (9) is integrated into the controller (8).Container coating plant for plasma coating containers (15), characterized bya magnetron (1) according to one of claims 18 to 21.

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