Abstract:
The invention relates to a method for cleaning filters of a vacuum cleaner having a dirt collecting container, which has a suction inlet and is in flow connection with at least one suction unit via at least one filter and at least one suction extraction line, and at least one external air inlet which opens into the suction extraction line downstream of the filter and can be closed by means of a closing valve. The closing valve has a movable valve body acted upon by a closing spring with a closing force and, in the closed position, additionally by a magnetic holder with a magnetic holding force. In order to clean the filter, at least one closing valve is opened and the side of the filter oriented away from the dirt collecting container is impinged upon by external air. A vacuum cleaner for carrying out the method is also provided.

Description:
This application is a continuation of international application number PCT/EP2006/007544 filed on Jul. 29, 2006. 
    
    
     The present disclosure relates to the subject matter disclosed in international application number PCT/EP2006/007544 of Jul. 29, 2006, which is incorporated herein by reference in its entirety and for all purposes. 
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for cleaning the filters of a vacuum cleaner comprising a dirt collecting container, which has a suction inlet and is in flow connection with at least one suction unit via at least one filter and at least one suction extraction line, and comprising at least one external air inlet which opens into the suction extraction line downstream of the at least one filter and is adapted to be closed by means of at least one closing valve, wherein the at least one closing valve has a valve body which is movable back and forth between a closed position, in which it lies against a valve seat, and an open position, in which it is at a distance from the valve seat, wherein it is acted upon by a closing spring with a closing force and, in the closed position, additionally by a magnetic holder with a magnetic holding force, wherein, in order to clean the at least one filter, at least one closing valve is opened and the side of the at least one filter that is oriented away from the dirt collecting container is impinged upon by external air. 
     The invention also relates to a vacuum cleaner for carrying out the method. 
     By means of the vacuum cleaners in question here, dirt and preferably also liquid can be sucked up using at least one suction unit to apply negative pressure to the dirt collecting container, so that a suction flow forms and dirt and liquid can be sucked into the dirt collecting container. The vacuum cleaners have one or more filters, which are disposed in the flow path between the dirt collecting container and the at least one suction unit and serve for separating solid substances from the suction flow. During suction operation, dirt particles are increasingly deposited on the side of the at least one filter that is oriented toward the dirt collecting container, so that after some time the filter or filters have to be cleaned. For cleaning, the side of the filters that is oriented away from the dirt collecting container can be impinged upon by external air, by opening at least one closing valve, so that external air can flow into the at least one suction extraction line via the external air inlet and impinge upon the side of the at least one filter that is orientated away from the dirt collecting container. 
     In utility model DE 298 23 411 U1, to clean a filter it is proposed to close for a short time a suction hose that is connected to the suction inlet of the dirt collecting container, so that a strong negative pressure forms in the dirt collecting container, and a closing valve is subsequently to be opened for a short time. The filter is then flowed through by the external air in the direction of counterflow, that is to say counter to the direction of the suction flow that prevails during normal suction operation, so that dirt particles adhering to the filter become detached. 
     In DE 199 49 095 A1, it is proposed to use two filters, which are cleaned in turn, it being possible during the cleaning of one filter for the suction operation to be continued to a restricted extent by way of the other filter. 
     JP-A-2002028107 discloses a method for cleaning a filter in which the filter can be flowed through by external air counter to the direction of flow prevailing during normal suction operation. Provided for this purpose is an external air inlet, which is adapted to be closed by a valve. When the valve is opened, external air is sucked in on account of the pressure difference between the pressure in the dirt collecting container and the pressure in the outside ambience. This difference in pressure is also present when the suction operation is stopped. 
     It is an object of the present invention to develop a method of the generic type in such a way that particularly effective cleaning of the at least one filter is made possible. 
     SUMMARY OF THE INVENTION 
     This object is achieved according to the invention in the case of a method of the type mentioned at the beginning by providing that, after the at least one suction unit is switched off, at least one closing valve is opened at least once and then closed again and the side of the at least one filter that is oriented away from the dirt collecting container is impinged upon at least once by external air, the magnetic holder being equipped with an electromagnet, to which current is applied to close the closing valve, and the current supply to the electromagnet also being switched off at the same time as the suction unit is switched off. 
     In the case of the vacuum cleaner according to the invention, the suction air is filtered by means of the filters present, all the filters being used at the same time during normal suction operation. This may involve a single filter or a number of filters that are flowed through simultaneously. During normal suction operation, the at least one closing valve is closed, its valve body lying against the valve seat in a sealing manner and being acted upon by the closing spring with a closing force. The entering of external air into the suction extraction line is consequently prevented. To clean the at least one filter, the closing valve is opened, so that the entering external air can impinge upon the side of the at least one filter that is oriented away from the dirt collecting container. Such cleaning can be repeated during the suction operation in dependence on the degree of soiling of the at least one filter or else time-dependently at periodic intervals. If suction operation is ended, the user switches off the at least one suction unit. Here it is provided that, after the at least one suction unit is switched off, the closing valve is opened and closed again at least one further time. This has the consequence that the negative pressure still present in the dirt collecting container immediately after the suction unit is switched off is used for the purpose of sucking in external air via the re-opened closing valve, so that the filters present are cleaned once again after the at least one suction unit is switched off. Therefore, when the vacuum cleaner is switched off, filter cleaning is once again carried out in a structurally simple manner. Here, the invention makes use of the realization that, when the suction unit is switched off, there is still negative pressure in the dirt collecting container and in the suction extraction line, so that the at least one closing valve is subject to a pressure difference, since it is exposed on one side to the pressure of the external air and is exposed on the other side to the negative pressure within the suction extraction line. This pressure difference makes it possible in a structurally simple manner for the spring-loaded closing valve to be opened once again and subsequently closed again after the suction unit is switched off, so that the side of the at least one filter that is oriented away from the dirt collecting container is once again impinged upon by external air for a short time after normal suction operation has ended. 
     According to the invention, the magnetic holder is equipped with an electromagnet, to which current is applied to close the closing valve, and the current supply to the electromagnet is also switched off at the same time as the suction unit is switched off. During normal suction operation, current may be applied to the electromagnet in order to hold the valve body reliably on the valve seat. If the current supply to the electromagnet is interrupted, the magnetic holding force of the electromagnet is no longer present, and the valve body can lift off from the valve seat counter to the closing force of the closing spring on account of the pressure difference prevailing at the closing valve. External air can consequently enter the suction extraction line and can impinge upon the side of the at least one filter that is oriented away from the dirt collecting container to clean said filter. The valve body is subsequently returned again to the valve seat by the closing spring, and renewed application of current to the electromagnet allows the valve body to be held again on the valve seat for the continuation of normal suction operation. If suction operation is ended, the suction unit is switched off, and the current supply to the electromagnet is also interrupted at the same time. This has the consequence, as explained above, that the closing valve once again opens and closes again, the valve body subsequently no longer being held reliably on the valve seat on account of the switched-off electromagnet, but rather it can repeatedly perform a movement back and forth on account of the loading by the closing spring and the running-down suction unit, so that particularly effective filter cleaning can be achieved. 
     It is of particular advantage if, after the suction unit is switched off, at least one closing valve is repeatedly opened and closed again, since this allows the final cleaning operation to be intensified. The repeated opening and closing of the closing valve after the suction unit is switched off can be achieved in a manner that is structurally simple by the entering external air obtained when the closing valve is first opened after the suction unit is switched off being at least partly extracted again with the aid of the gradually running-down suction unit, so that a certain negative pressure is once again established in the suction extraction line after the closing valve is closed for the first time. The automatic closing of the closing valve is achieved by the valve body being acted upon by the closing spring with a closing force. If the suction unit is switched off after normal suction operation has ended, the closing valve can be opened automatically on account of the pressure difference acting, so that external air can flow into the suction extraction line. Under the effect of the closing spring, the valve body lifting off from the valve seat can be returned again to the valve seat, and the external air that has in the meantime flowed in can be extracted at least partly from the suction extraction channel by the suction unit that is still rotating, so that a negative pressure is once again formed in the suction extraction channel, and consequently there is also a pressure difference at the closing valve, so that the latter opens once again for a short time. 
     In the case of a preferred embodiment of the procedure according to the invention, the final cleaning operation may be intensified by making a closing force of the closing spring act permanently on the valve body, irrespective of its position, and additionally making a repulsion force of a resilient stop element act upon said valve body in a position at a distance from the valve seat. The resilient stop element that is used in addition to the closing spring ensures that the at least one filter can be effectively cleaned within a very short time, in that it acts with a repulsion force on the valve body lifting off from the valve seat, so that said valve body is returned to the valve seat within a short time. The quick return of the valve body into its closed position has the advantage that, after the suction unit is switched off, the closing valve can be opened and immediately closed again repeatedly within a very short time, so that filter cleaning can be repeatedly carried out for a short time and within the running-down time of the suction unit. A further advantage of the use of a resilient stop element is that cleaning of the at least one filter during normal suction operation can be carried out within a very short time, preferably within a time of less than 0.5 of a second, for example within a time of less than 400 milliseconds, in particular less than 200 milliseconds. This has the consequence that there is no noticeable interruption in suction operation for the user, but rather the suction operation proceeds virtually continuously in spite of periodic filter cleaning. The brief opening of the at least one closing valve allows the at least one filter to be impinged upon by the abruptly entering external air with a pressure surge, which leads to a shaking effect, and consequently mechanical cleaning of the filter. Furthermore, the at least one filter is flowed through by external air in the direction of counterflow for a short time. The suction unit is in flow connection with the filter even during the cleaning of the at least one filter, that is to say during the opening of the closing valve. The external air flowing into the suction extraction line for a short time is consequently sucked away again immediately, so that, after the closing valve is closed, the negative pressure prevailing during normal suction operation is established again in the dirt collecting container already within fractions of a second. Accordingly, during normal suction operation, the at least one filter is flowed through within fractions of a second first by the original suction flow then for a short time by an external air flow and subsequently again by the suction flow. This results in alternating mechanical loading of the at least one filter, whereby the filter cleaning is assisted. The brief opening of the closing valve takes place under the action of the repulsion force of the resilient stop element, which acts with a force upon the valve body only when the latter is disposed at a distance from the valve seat. If the valve body is lying against the valve seat in a sealing manner, the resilient stop element does not exert any force on the valve body. The stop element forms a kind of shock absorber for the valve body, which absorbs the kinetic energy of the valve body and accelerates the valve body back in the direction of the valve seat. 
     In the case of a preferred refinement of the method according to the invention, the vacuum cleaner is equipped with just a single filter, which is impinged upon by external air for a short time to clean it. 
     The vacuum cleaner may have a number of external air inlets, which can in each case be closed by means of a closing valve. By simultaneous opening of all the closing valves, a large amount of external air can be fed to the at least one filter within a short time. However, this requires careful coordination of the opening movements of the valve bodies of the closing valves that are used. It is more advantageous if the entire surface area of the at least one filter that is oriented away from the dirt collecting container is impinged upon by external air by opening only a single closing valve. 
     The at least one closing valve is preferably actuated electronically. As already explained, it is possible for example to use an electromagnet, with the aid of which the movable valve body can be held on the valve seat in a closed position by applying current to the electromagnet. In order to open the closing valve, the current supply to the electromagnet is interrupted. Here it is of advantage if the valve body is equipped with a magnetizable element, which in the closed position of the valve body lies against the end face of the electromagnet and forms a closed magnetic circuit with the electromagnet. The magnetizable element, for example a plate containing iron, concentrates the field lines of the electromagnet in the valve body, so that the latter is reliably held in its closed position under the action of the magnetic force. If, however, the valve body assumes a distance from the valve seat, the magnetic circuit is interrupted, since the magnetizable element also assumes a distance from the electromagnet. This has the consequence that the magnetic force exerted by the electromagnet has only a very short range. In the case of an opening movement, the valve body consequently only experiences a magnetic force in the direct vicinity of the valve seat, while even at a distance of approximately 2 mm between the electromagnet and the magnetizable element the magnetic force is so small that it cannot return the valve body into its closed position. Rather, the closing spring is used to return the valve body. 
     It is advantageous if the at least one closing valve is actuated in a time-controlled manner. For example, it may be provided that the closing valve is actuated periodically, preferably at time intervals of less than one minute, in particular time intervals of 10 to 30 seconds, the closing valve being opened for less than one second, in particular for less than 0.5 of a second. 
     The actuation of the at least one closing valve may take place at different time intervals during normal suction operation. For example, the closing valve may initially be kept closed for 10 to 30 seconds, to then be opened for a short time, for example, for approximately one tenth of a second, after a number of shorter time intervals. 
     As mentioned at the beginning, the invention also relates to a vacuum cleaner for carrying out the aforementioned method. The vacuum cleaner comprises a dirt collecting container, which has a suction inlet and is in flow connection with at least one suction unit via at least one filter and at least one suction extraction line. The vacuum cleaner also comprises at least one external air inlet which opens into the suction extraction line downstream of the at least one filter and is adapted to be closed by means of at least one closing valve, wherein the at least one closing valve has a valve body which is movable back and forth between a closed position, in which it lies against a valve seat, and an open position, in which it is at a distance from the valve seat, wherein it is acted upon by a closing spring with a closing force and, in the closed position, additionally by a magnetic holder with a magnetic holding force. 
     In order to develop such a vacuum cleaner in such a way that it makes particularly effective cleaning of the at least one filter possible, it is proposed according to the invention that, after the suction unit is switched off, the at least one closing valve is adapted to be automatically opened at least once and then closed again and the side of the at least one filter that is oriented away from the dirt collecting container can be impinged upon at least once by external air, the magnetic holder having an electromagnet to which current can be applied to close the closing valve, and it being possible for the current supply to the electromagnet also to be switched off at the same time as the suction unit is switched off. The vacuum cleaner according to the invention consequently makes a final cleaning of the filters used possible after normal suction operation has ended. For this purpose, the at least one closing valve is adapted to be automatically opened and automatically closed after the suction unit is switched off, so that, with the suction unit already switched off, external air can once again impinge upon the side of the at least one filter that is oriented away from the dirt collecting container and, as a result, can clean said filter. After the suction unit is switched off, the at least one filter consequently experiences a further pressure surge and is once again flowed through in the direction of counterflow. 
     The magnetic holder of the vacuum cleaner according to the invention has an electromagnet, to which current can be applied to close the closing valve, and the current supply to the electromagnet can also be switched off at the same time as the suction unit is switched off. As already explained, the valve body may be held on the valve seat in a sealing manner during normal suction operation by means of the electromagnet to which current has been applied. For brief filter cleaning, the current supply to the electromagnet may be interrupted, so that the valve body lifts off from the valve seat under the action of the pressure difference prevailing at the closing valve. Under the action of the closing spring and the resilient stop element that is additionally used in the case of a preferred embodiment, the valve body is returned again to the valve seat within a very short time. At the latest at this point in time, current can again be applied to the electromagnet in order to hold the valve body reliably on the valve seat again after a short cleaning operation. If the user ends normal suction operation, said user switches off the at least one suction unit, and the current supply to the electromagnet is interrupted at the same time, so that the valve body is no longer acted upon by a holding force in its closed position. As a result, as already mentioned, a repeated opening and closing movement of the spring-loaded valve body can be achieved in a simple manner after the suction unit is switched off. 
     It is advantageous if, after the vacuum cleaner is switched off, the closing valve can be automatically opened and then closed again repeatedly. The final cleaning operation can be intensified as a result, as already explained above. 
     In the case of a particularly preferred configuration of the vacuum cleaner according to the invention, associated with the valve body is a resilient stop element, which acts with a repulsion force upon the valve body in a position at a distance from the valve body. The resilient stop element allows a short opening movement of the valve body to be achieved in a structurally simple manner, wherein initially, starting from its closed position, it is only acted upon by the closing force of the closing spring. Only when the valve body assumes a certain distance from the valve seat does the resilient stop element come into effect, acting upon the valve body with a repulsion force. The resilient stop element absorbs the kinetic energy of the valve body and accelerates it back in the direction of the valve seat. With the aid of the resilient stop element, the closing valve can be closed again within a very short time, in particular after a time of less than one second, preferably less than 0.5 of a second. The normal suction operation of the vacuum cleaner can proceed virtually continuously, and effective filter cleaning nevertheless achieved. External air only enters the dirt collecting container for a very short period of time, so that the suction flow in the region of the suction inlet of the dirt collecting container is not noticeably interrupted. The vacuum cleaner is consequently distinguished by a simple construction, it being possible for all the filters present to be flowed through by suction air at the same time in suction operation and for the entire side of the at least one filter that is oriented away from the dirt collecting container to be impinged upon by external air by the at least one closing valve being opened for a short time. The external air is suddenly fed to the filter, the at least one suction unit being flow-connected to the filter permanently, that is to say also during the time during which it is being cleaned. If the suction unit is switched off after normal suction operation has ended, it still exerts a suction effect for a certain time, for example for a few seconds. Since the closing valve of the vacuum cleaner according to the invention is adapted to be opened and closed again within a very short time, the relatively short running-down time of the switched-off suction unit is sufficient to allow an effective final filter cleaning to be carried out within this time. 
     The resilient stop element may be configured in various forms. It is preferably formed as a stop spring. In the case of a preferred configuration, it has a greater spring constant than the closing spring. The stop spring is consequently harder than the closing spring, that is to say a higher force is required to compress the stop spring than is the case with the closing spring. In a way similar to the closing spring, the stop spring may have a linear characteristic or a non-linear characteristic. For example, it may be provided that the stop spring and/or the closing spring become harder with increasing spring deflection. 
     In the case of a preferred configuration of the vacuum cleaner according to the invention, the closing spring and the stop spring are configured as helical springs of different diameters, one of the two helical springs circumferentially surrounding the other helical spring. This makes it possible for the closing spring and the stop spring to be disposed in a space-saving manner and additionally makes simple assembly possible. 
     Preferably, the closing spring circumferentially surrounds the stop spring. This has the advantage that the valve body is guided into its closed position by a relatively large closing spring. This improves the tilting stability of the valve body. 
     In the case of a structurally simple configuration, the suction unit and the electromagnet are connected via a common switching element to voltage supply terminals. The switching element may form the main switch of the vacuum cleaner, which can be operated manually by the user. If the main switch is closed, the suction unit is put into operation and current is applied to the electromagnet. Normal suction operation consequently begins. If the main switch is opened, the current supply, both to the suction unit and to the electromagnet, is interrupted. As already explained, the suction unit then still performs a running-down movement and the at least one filter is subjected to a final cleaning operation. 
     The electromagnet is preferably connected to a control unit for the time-dependent control of the electromagnet. By means of the time-dependent control, periodic filter cleaning can be achieved in a structurally simple manner. 
     It is of particular advantage if the electromagnet is disposed on a valve holder of the closing valve that forms the valve seat, and if the valve body has a magnetizable element which is associated with the electromagnet and forms a closed magnetic circuit with the electromagnet in the closed position of the valve body. In the closed position of the valve body, the magnetizable element concentrates the field lines of the electromagnet. If, however, the valve body assumes a distance from the valve seat, the magnetic circuit between the electromagnet and the magnetizable element is interrupted. The magnetizable element may be formed for example as a plate and be produced from a material containing iron. In the closed position of the valve body, it may lie against the end face of the electromagnet. 
     The combined use of an electromagnet, the closing spring and a resilient stop element is of particular advantage, since this allows the valve body to be reliably held in a sealing manner on the valve seat in its closed position. If the closing valve is to be opened, the current supply to the electromagnet is interrupted. With the closing valve closed, the valve body is impinged upon on its side that is oriented toward the external air inlet by the pressure of the external air, that is to say for example by atmospheric pressure, whereas negative pressure is present on its side that is oriented away from the external air inlet. If the current supply to the electromagnet is interrupted, the pressure difference acting on the valve body has the effect that it lifts off from the valve seat counter to the action of the closing spring, so that the closing valve is opened. At a distance from the valve seat, the valve body butts against the resilient stop element, which acts upon the valve body with a repulsion force in the direction of the valve seat. Under the action of the repulsion force and the closing force exerted by the closing spring, the valve body reaches the valve seat again within a very short time. The closing spring has the function here of returning the valve body into the range of the magnetic field of the electromagnet, so that the valve body can be held on the valve seat during normal suction operation by the electromagnet to which current is again applied. If, however, the current supply to the electromagnet is interrupted at the same time as the at least one suction unit is switched off after suction operation is ended, the magnetic holding force is no longer present, and the valve body can perform a repeated movement back and forth for a final filter cleaning operation during the running-down time of the suction unit, the travel of the valve body from its closed position being gradually reduced. 
     The vacuum cleaner may have a number of filters; it proves to be particularly advantageous if the vacuum cleaner comprises a single filter. In particular, it may be provided that the filter is adapted to be impinged upon by external air over its entire surface area by all the closing valves being opened at the same time. In the case of a structurally particularly simple configuration of the vacuum cleaner according to the invention, it has merely a single closing valve, which is positioned on the side of a filter holder having flow passages that is oriented away from the single filter. The single filter is impinged upon by external air over its entire surface area by opening of the closing valve. 
     The configuration according to the invention of the vacuum cleaner makes it possible during normal suction operation for the side of the at least one filter that is oriented away from the dirt collecting container to be impinged upon by external air for a short time and for this air to be extracted within a short time by means of the suction unit, which is in flow connection with the filter even when the closing valve is open. It is advantageous if, during the cleaning of the filter, the valve body performs a continuous movement from its closed position via its open position back into its closed position. In the case of such a configuration, when the closing valve is opened, the valve body is first accelerated strongly in the direction away from the valve seat and then strongly decelerated with the aid of the closing spring and preferably with the aid of the resilient stop element, and reversed in its direction of movement, to then be accelerated back in the direction of the valve seat. The entire movement of the valve body from its closed position via the open position back into the closed position can take place within fractions of a second. 
     In the case of an advantageous embodiment, the at least one filter is adapted to be acted upon by means of the closing valve for less than 400 milliseconds, preferably less than 200 milliseconds, in particular for less than 100 milliseconds. Being acted upon in this way means that there is no noticeable interruption in suction operation for the user, but it results in effective cleaning of the filter during normal suction operation. 
     The at least one filter is preferably adapted to be impinged upon by external air by means of the at least one closing valve during normal suction operation while maintaining a negative pressure in the region of the opening of a suction tube opening into the suction inlet. If the at least one closing valve is opened, the pressure on the side of the filter that is oriented away from the dirt collecting container abruptly increases and is then reduced again. The impact-like pressure increase brings about effective cleaning of the filter, since the pressure increase is however reduced again right away by the at least one suction unit and does not lead to a complete interruption in the negative pressure in the region of the opening of the suction hose opening into the suction inlet. Rather, virtually continuous suction operation can be maintained. 
     The following description of a preferred embodiment of the invention serves for further explanation in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic sectional view of a vacuum cleaner according to the invention; 
         FIG. 2  shows an enlarged representation of detail A from  FIG. 1 ; and 
         FIG. 3  shows a block diagram of a supply circuit for an electromagnet and a suction unit of the vacuum cleaner. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Schematically represented in the drawing is a vacuum cleaner  10 , with a lower part, which forms a dirt collecting container  12 , and with an upper part  14 , which is mounted on the lower part and accommodates a suction unit  16 . The dirt collecting container  12  comprises a suction inlet  18 , to which a suction hose  20  can be connected. At the free end of the suction hose  20  (not represented in the drawing to achieve a better overview), a suction nozzle may be connected. Alternatively, it may be provided that the suction hose  20  is connected to a working tool, for example a drilling unit or a milling unit, so that dust produced during the operation of the working tool can be sucked away. 
     The upper part  14  forms a suction outlet  22  for the dirt collecting container  12 . Held on the suction outlet  22  is a folded filter  24 , which is followed by a suction extraction line in the form of a suction channel  26 . By way of the suction channel  26 , the folded filter  24  is permanently flow-connected to the suction unit  16 . The dirt collecting container  12  can be acted upon permanently by negative pressure from the suction unit  16  via the suction channel  26  and the folded filter  24 , so that there forms a suction flow, symbolized in  FIG. 1  by the arrows  28 , under the action of which dirt can be sucked into the dirt collecting container  12 . By means of the folded filter  24 , the dirt particles can be separated from the suction flow  28 . 
     Disposed above the folded filter  24  in the upper part  14  is a closing valve  30 , which is shown enlarged in  FIG. 2 . It comprises a valve holder  32 , which is fixedly disposed in the upper part  14 , forms a valve seat and interacts with a valve body in the form of a circular valve disk  34 . The valve disk  34  is acted upon by a closing force in the direction of the valve holder  32  by means of a closing spring  36 . The closing spring  36  has a linear characteristic and is restrained between a plate-like filter holder  38 , having a multiplicity of flow passages and fixedly disposed in the upper part  14 , and the valve disk  34 . The filter holder  38  has on its upper side that is oriented toward the closing valve  30  an outer annular collar  40 , which circumferentially surrounds the neighboring end region of the closing spring  36  formed as a helical spring. The valve disk  34  has on its underside that is oriented toward the filter holder  38  an annular bead  41 , against the outside of which the closing spring  36  lies. 
     In addition to the closing spring  36 , the filter holder  38  carries a resilient stop element in the form of a stop spring  43 , which in a way similar to the closing spring  36  is formed as a helical spring and has a linear characteristic. For holding the stop spring  43 , the filter holder  38  has on its upper side that is oriented toward the closing valve  30  an inner annular collar  44 , which is disposed concentrically in relation to the outer collar  40  and is entered by an end portion of the stop spring  43 . Formed on the underside of the valve disk  34 , aligned with the inner annular collar  44 , is a guiding pin  46 , which in the closed position of the valve disk  34  that is represented in  FIG. 2  is surrounded by an end region of the stop spring  43 . By contrast with the closing spring, the stop spring  43  is not biased in the closed position of the valve disk. Only when the valve disk  34  lifts off from the valve seat of the valve holder  32  does the stop spring  43  come up against the underside of the valve disk  34  and become a little compressed when there is further movement of the valve disk  34 . 
     The valve holder  32  has a multiplicity of through-openings (not represented in the drawing), which open into the valve seat against which the valve disk  34  lies in a sealing manner when it assumes its closed position. Level with the valve holder  32 , the upper part  14  has a lateral opening  48 . External air can flow into the through-openings of the valve holder  32  via the lateral opening  48 . If the valve disk  34  assumes a position at a distance from the valve holder  32 , and consequently also from the valve seat, the lateral opening  48  is in flow connection with the suction channel  26  via the through-openings of the valve holder  32  and external air can impinge upon the side of the filter  24  that is oriented away from the dirt collecting container  12 . If the valve disk  34  assumes its closed position, the flow connection between the suction channel  26  and the lateral opening  48  is interrupted. 
     In a central region, the valve holder  32  carries a magnetic holder in the form of an electromagnet  50  with a magnetic core  51 , which is surrounded by a magnetic coil  52 . The outer termination of the electromagnet  50  is formed by a cylindrical shell  53 , which in the same way as the magnetic core  51  is produced from a magnetizable material. The shell  53  is circumferentially surrounded by a guiding receptacle in the form of an annular space  55 , which is entered by a guiding sleeve  56  formed on the upper side of the valve disk  34 . The annular space  55  and the guiding sleeve  56  form guiding elements for the displaceable mounting of the valve disk  34 . The guiding sleeve  56  receives a magnetizable element in the form of an iron plate  58 , which in the closed position of the valve disk  34  lies against the free end face of the electromagnet  50  and in combination with the magnetic core  51  and the sleeve  53  forms a closed magnetic circuit. The closed magnetic circuit concentrates the magnetic field lines of the electromagnet  50 . 
     The current supply to the electromagnet  50  and the suction unit  16  is diagrammatically represented in  FIG. 3 . The suction unit  16  has an electric motor  60 , which is connected via a first supply line  61  and a second supply line  62  to voltage supply terminals  64  and  65 , respectively. The voltage supply terminals  64  and  65  may be configured, for example, as mains power terminals, to which a mains power cable of the vacuum cleaner  10  can be connected. 
     An electrical switching element in the form of a main switch  67  of the vacuum cleaner  10  is connected into the supply lines  61  and  62 . The main switch  67  can be actuated manually by the user. If the main switch  67  is closed, the electric motor  60  of the suction unit  16  is connected to the voltage supply terminals  64 ,  65  and can be supplied with the supply voltage. If the main switch  67  is open, the connection between the electric motor  60  and the voltage supply terminals  64 ,  65  is interrupted. 
     Connected parallel to the electric motor  60  is a control unit  69 , which supplies the electromagnet  50  with current if the main switch  67  is closed. If the main switch  67  is open, the current supply to the electromagnet  50  is interrupted. 
     If the user switches the vacuum cleaner  10  on, by closing the main switch  67 , the supply voltage is applied to the electric motor  60  and the suction unit  16  is put into operation. At the same time, the electromagnet  50  is supplied with current via the control unit  69  and the valve disk  34  is acted on by a magnetic holding force from the electromagnet  50 , which reliably keeps it in its closed position. The suction unit  16  acts upon the dirt collecting container  12  and the suction channel  26  with a negative pressure, so that dirt particles and similarly liquid droplets can be sucked into the dirt collecting container  12 . Dirt particles are deposited on the filter  24 , so that it gradually becomes clogged during normal suction operation. At time intervals of 10 to 30 seconds, for example, in particular time intervals of approximately 15 seconds, the current supply to the electromagnet  50  is therefore interrupted for a short time by means of the control unit  69 , for a time period of approximately 0.1 of a second. This has the consequence that the magnetic field of the electromagnet  50  breaks down, and consequently the magnetic holding force for the valve disk  34  is no longer present. This in turn has the effect that the valve disk  34  lifts off from the valve seat counter to the action of the closing spring  36 , on account of the pressure difference acting on it, which is produced by the outside pressure of the external air present in the region of the valve holder  32  and the inside pressure within the suction channel  26 . External air can then suddenly flow into the suction channel  26  through the through-opening of the valve holder. The filter  24  is consequently impinged upon by external air in an impact-like manner on its side that is oriented away from the dirt collecting container  12 . This leads to the filter  24  undergoing a mechanical shaking effect. Furthermore, the filter is flowed through by the external air in the direction of counterflow. This has the overall consequence of effective cleaning of the filter  24 . 
     After a short lifting movement, the valve disk  34  lifting off from the valve seat comes with its underside up against the stop spring  43 , which acts upon the valve disk  34  with a repulsion force in the direction of the valve holder  32 . The stop spring  43  absorbs the kinetic energy of the valve disk  34 . The latter is accelerated by the stop spring  43  in the direction of the valve seat. When the valve disk  34  approaches the valve seat, the stop spring  43  releases the valve disk  34 . The latter is returned to the valve seat by the closing spring  36 , so that the iron plate  58  comes to lie against the end face of the electromagnet  50  again. At the latest at this point in time, current is once again applied to the electromagnet  50  by the control unit  69 , so that the valve disk  34  is again held in a sealing manner against the valve seat by the electromagnet  50 . The interruption of the current supply for the electromagnet  50  takes-place only over a time period of at most 100 milliseconds, so that the closing valve  30  opens only for a very short time and external air can get to the filter  24  only for a very short time. Subsequently, current is again applied to the electromagnet  50 , to be precise for a time period of approximately 10 to about 30 seconds, in particular for a time period of approximately 15 seconds. Consequently, during suction operation of the vacuum cleaner  10 , brief filter cleaning takes place every 15 seconds. On account of the short opening of the closing valve  30 , a negative pressure is maintained in the region of the opening of the suction hose  20  flowing into the suction inlet  18  even during the filter cleaning. This has the consequence that virtually continuous suction operation is possible for the user and reliable filter cleaning is nevertheless ensured. 
     If the user ends suction operation, he opens the main switch  67 . This has the consequence that the electric motor  60  of the suction unit  16  is disconnected from the voltage supply terminals  64 ,  65  at the same time as the electromagnet  50 . With the switching off of the suction unit  16 , the magnetic field of the electromagnet  50  therefore also breaks down. Consequently, the magnetic holding force acting on the valve disk  34  no longer occurs. At this point in time, the dirt collecting container  12  and the suction channel  26  are still under negative pressure, so that the valve disk  34  is subjected to a pressure difference and lifts off from the associated valve seat counter to the action of the closing spring  36 . In a way corresponding to the cleaning operation explained above, the valve disk subsequently butts against the stop spring  43 , which acts upon the valve disk  34  with a repulsion force in the direction of the valve seat, so that the valve disk  34  comes to lie against the valve holder  32  again after a very short opening movement. During the opening movement, external air flows against the side of the filter  24  that is oriented away from the dirt collecting container  12  and leads to further filter cleaning. The suction unit  16  is set in rotation by the electric motor  60  during normal suction operation. After the electric motor  60  is switched off, the suction unit  16  gradually runs down. This has the consequence that at least part of the external air flowing in during the brief opening movement of the valve disk  34  is sucked away, so that, after renewed closing of the closing valve  30 , a negative pressure once again forms in the suction channel  26  and also in the dirt collecting container  12 . This has the effect that the valve disk  34  lifts off once again from the valve seat, and consequently external air can once again reach the side of the filter  24  that is oriented away from the dirt collecting container  12 . Consequently, in combination with the switching off of the electromagnet  50  and the spring-loaded valve disk  34 , the running-down suction unit  16  results in the filter  24  being subjected to a final cleaning operation when the suction unit  16  is switched off, the valve disk  34  repeatedly performing a movement back and forth and it being possible for the filter  24  to undergo a number of pressure surges. Consequently, not only is the filter  24  subjected to cleaning at time intervals during normal suction operation, but a final filter cleaning additionally takes place after the suction unit  16  is switched off. When the vacuum cleaner  10  is put into operation again, it consequently has a cleaned filter  24 .