Abstract:
The present invention relates to a method of determining a liquid level in a boiler ( 12 ) of an apparatus ( 10 ) having a temperature sensor ( 24 ) for sensing a temperature (T) which is indicative of a temperature of the liquid inside the boiler ( 12 ), comprising the steps of: inducing a temporary deflection of the temperature (T) sensed by the temperature sensor ( 24 ); determining an evaluation temperature (T ev ) sensed by the temperature sensor ( 24 ) after inducing the deflection; and determining the liquid level from at least the evaluation temperature (T ev ). Advantageously, the invention provides that the evaluation temperature (T ev ) is determined by monitoring the deflection and by defining an extreme value (T max , T min ) of the deflection as the evaluation temperature (T ev ). Further, the present invention relates to an steam generating apparatus adapted for executing such a method, a domestic appliance having such a steam generating apparatus.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to a method of determining a liquid level in a boiler of an apparatus having a temperature sensor for sensing a temperature which is indicative of a temperature of the liquid inside the boiler, comprising the steps of: inducing a temporary deflection of the temperature sensed by the temperature sensor; determining an evaluation temperature sensed by the temperature sensor after inducing the deflection; and determining the liquid level from at least the evaluation temperature. 
       BACKGROUND OF THE INVENTION 
       [0002]    Steam generating apparatuses are used for heating water in order to generate steam which can be used for example for ironing cloth. In these apparatuses the steam is generated within a boiler in which the temperature of the water can be controlled within a certain temperature range by means of a heating device and a temperature sensor as follows: When the temperature signal of the temperature sensor indicates, that the temperature of the water falls below a certain level, the heating device is activated and the water is heated. If the temperature signal indicates, that the water temperature rises above a certain level, the heating device is deactivated. 
         [0003]    Generally, in such steam generating apparatuses, the boiler is refilled with water from a larger water tank automatically. As a prerequisite for realizing a smooth refilling operation, the water level inside the boiler has to be detected, such that water is refilled into the boiler when the water level drops below a certain water level. A water level sensor which would be arranged inside the boiler would very soon provide inaccurate results due to calcification of the inside of the boiler or the usage of de-mineralized water which can cause inaccurate sensing. 
         [0004]    EP 0 843 039 B1 describes a steam generator which comprises an evaporation boiler associated with heater elements and fed with water by a pump, a solenoid valve for expelling steam, and regulator means including a temperature sensor or a pressure sensor for monitoring the quantity of water introduced into the boiler and for controlling the pump, wherein the regulator means are associated with an electronic system for taking successive measurements for actuating the pump as a function of the value of the slope of a curve representing variation in temperature or pressure, said slope being analyzed by the electronic system. 
         [0005]    However, actuating the pump according to the value of the slope of the curve representing variation in temperature requires a very accurate and expensive temperature sensor since this slope method requires a very direct heat transfer through a thin wall such that slightest variations of the slope of the curve are detected. Also the mounting of such an temperature sensor costs much effort because the required sensing accuracy requires a very good heat conductivity between the temperature sensor and the water inside the boiler. The required sensing accuracy requires to arrange the temperature sensor at a flat portion for a proper mounting of the sensor, which in turn complicates the forming of the shell. A heat conductive paste may be applied between the temperature sensor and the boiler shell. However, this makes additional mounting processes necessary. Arranging the temperature sensor somewhere else is not possible since the sensing accuracy would not be sufficient for determining the water level with the method described in EP 0 843 039 B1. 
         [0006]    It is therefore an object of the invention to provide an alternative method of determining the liquid level in a boiler of a steam generating apparatus providing a comparable accuracy in determining the liquid level and providing more flexibility in arranging a required temperature sensor. 
       SUMMARY OF THE INVENTION 
       [0007]    This object is solved by the features of the independent claims. Further developments and preferred embodiments of the invention are outlined in the dependent claims. In accordance with the invention, there is provided a method of determining a liquid level in a boiler of an apparatus having a temperature sensor for sensing a temperature which is indicative of a temperature of the liquid inside the boiler, comprising the steps of: inducing a temporary deflection of the temperature sensed by the temperature sensor; determining an evaluation temperature sensed by the temperature sensor after inducing the deflection; and determining the liquid level from at least the evaluation temperature, wherein the evaluation temperature is determined by monitoring the deflection and by defining an extreme value of the deflection as the evaluation temperature. This method provides the advantage that the liquid level inside the boiler can be sensed very accurate without the need of using a highly accurate and expensive temperature sensor. This method even offers the possibility to arrange the temperature sensor not directly to the wall of the boiler, but to arrange it at any place at which the temperature it senses is indicative of the temperature of the liquid. The main advantage of the invention is that with the method of the present invention, the design constraints compared to the slope method can be overcome. By determining the liquid level using a extreme value, the determination is not that delicate anymore since compared to the slope method there exists no time aspect (the slope is the first derivative of the temperature-time curve). Further, it is possible to use an already available temperature sensor which is in contact with a heating plate of the heating device. In the context as used herein, temporary deflection means that the temperature course (or curve) rises after a certain action in form of a positive curve (overshot) and then falls again leaving a maximum value. Also, temporary deflection means that the temperature course (or curve) falls after a certain action in form of a negative curve (undershot) and then rises again leaving a minimum value. The above mentioned determination of the evaluation temperature consists of two steps which are: monitoring the deflection and defining an extreme value of the deflection as the evaluation temperature. Monitoring the deflection means that the temperature course is observed by sensing the temperature continuously. The extreme value can be determined by comparing two consecutive temperature values such that it can be determined if the extreme value of the deflection is reached. The present invention is suitable to analyze the following phenomena. When during the operation of the steam generating apparatus liquid is refilled such that the refilled liquid lands on the liquid already inside the boiler and such that it is directed to a spot in the vicinity of where the temperature sensor senses the temperature, the refilled cold liquid causes a temperature drop down to a minimum temperature value. Then the already present hot liquid will mix up with the refilled cold liquid and raise the temperature sensed by the temperature sensor again. This downward peak of the temperature course is referred to as temperature undershot, the magnitude of which is dependent on the liquid level inside the boiler. When the steam generating system is powered on, the temperature rises due to operating a heating device. Even after powering off the heating device, the temperature will still rise up to a maximum value, because of the heat accumulated in the heating device. Then the temperature sensed by the temperature sensor will decrease again after reaching a certain maximum value which depends on the liquid level inside the boiler—the higher the liquid level the more the accumulated heat is drawn off the heating device after it is turned off. This upward peak of the temperature course is referred to a temperature overshot. The method of the present invention is suitable for analyzing the liquid level in both situations accurately by sensing a temperature undershot or a temperature overshot by searching for extreme values of the temperature. 
         [0008]    According to a specific embodiment it is preferred that the temporary deflection of the temperature is induced by turning on a heating device for heating the liquid. This embodiment refers to the above described case in which the steam generating system is powered on such that the temperature rises due to operating a heating device. In this embodiment the temporary deflection is a temperature overshot. 
         [0009]    Advantageously, the liquid level is determined as low, if the evaluation temperature is greater than or equal to a first low level threshold. Having an additional reference value, for example the temperature at the start of the heating or the temperature at which the heating device is turned off, only the evaluation temperature is necessary to determine the liquid level inside the boiler. 
         [0010]    In accordance with an embodiment of the invention, the heating device stays turned on until the temperature is greater than or equal to a first threshold temperature. In this embodiment, this is the relevant time for starting the monitoring for determining an extreme temperature. After passing the threshold temperature and turning off the heating device, the temperature still rises up to a peak or a maximum value before it lowers again such that the temperature course describes the overshot. 
         [0011]    In a specific embodiment, the liquid level is determined as low, if the difference between the evaluation temperature and the first threshold temperature is greater than or equal to a second low level threshold. The second low level threshold is a predetermined temperature difference. This embodiment is intended to sense the liquid level during the start-up of the steam generating apparatus. 
         [0012]    According to another embodiment, the temporary deflection of the temperature is induced by filling liquid into the boiler. This embodiment refers to the above described case in which liquid is filled into the boiler on the basis of a regularly repeating procedure for maintaining the liquid level within the boiler after extracting steam from the boiler. In this embodiment the temporary deflection is a temperature undershot. 
         [0013]    In this regard, it is advantageous that the liquid level is determined based on a difference between a comparison temperature and the evaluation temperature, wherein the comparison temperature is sensed when filling liquid into the boiler is started. Thus, when the comparison temperature is sensed at the beginning or right before the filling operation starts, a reference value for the determination of the liquid level can be obtained which clearly shows the temperature drop caused by filling in a predetermined amount of liquid. Due to the predetermined amount, the method is able to compare an expected temperature drop with an actual temperature drop in order to determine the liquid level inside the boiler. 
         [0014]    According to a preferred embodiment of the present invention, the comparison temperature is sensed after an accumulated time of extracting steam from the boiler is greater than or equal to a steaming threshold. After extracting steam for a certain time period, a fixed volume of liquid has to be refilled into the boiler anyway in order to maintain a sufficient liquid level inside the boiler. The invention enables to combine this automatically repeated refilling operation with the method for detecting the liquid level according to the present invention. 
         [0015]    If the refill is not sufficient, the steaming threshold is varied depending on the liquid level as determined before. Thus, if despite of the refill the liquid level is determined as being low, the interval until the next fixed volume of liquid is refilled is shortened by setting the steaming threshold to a predetermined value. For the case that a high liquid level is determined, the refill interval is enlarged by increasing the steaming threshold. 
         [0016]    In another advantageous embodiment, it is concluded that a liquid tank from which a pump supplies the boiler with liquid is empty, if the liquid level within the boiler does not rise despite of operating the pump. This way the present invention can be also used for detecting and indicating that the liquid tank from which the boiler is supplied is empty. Therefore, the method of the present invention is suitable for a overall liquid management and only requires a single temperature sensor. 
         [0017]    In accordance with the invention, there is also provided a steam generating apparatus and a domestic appliance which provide the same advantages as described above. 
         [0018]    These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  shows a schematic set up of a steam generating device according to the present invention. 
           [0020]      FIG. 2  shows a flow diagram of an initial water level sensing. 
           [0021]      FIG. 3  shows a flow diagram of water level sensing during steaming. 
           [0022]      FIG. 4  shows a flow diagram of a water tank empty detection. 
           [0023]      FIG. 5  shows a flow diagram of another water tank empty detection. 
           [0024]      FIG. 6  shows a flow diagram of a water tank empty routine. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0025]      FIG. 1  shows a schematic set up of a steam generating device  10  according to the present invention. The steam generating device  10  comprises a water boiler  12  being manufactured by connecting at least two formed metal shells of stainless steal. The boiler  12  has a flat bottom portion  16  and is mounted in a plastic enclosure in a horizontal arrangement. Other orientations like a non horizontal arrangement are also possible. The flat bottom portion  16  of the boiler  12  is attached to a heating device  14  comprising a heating plate  15  and a heating element  22  which is attached to the heating plate  15  by forming an intermetallic layer or by casting to improve the heat transfer. The heating plate is made of aluminum—an aluminum alloy or other materials with excellent heat conductivity can also be used. The heating plate  15  comprises a flat upper portion  18  and is attached with its flat upper portion  18  to the flat bottom portion  16  of the body  12  by formation of an intermetallic layer  20 . The intermetallic layer  20  may be formed by welding, brazing, soldering, and the like. The heating element  22  is attached to the heating plate  15  also by forming an intermetallic layer by welding, brazing, soldering, a similar joining method or by casting-in, to ensure good heat transfer ability. Further, the heating device  14  comprises a temperature sensor  24  for sensing a temperature T which is indicative of a water temperature inside the boiler  12 . The boiler  12  of the steam generating device  10  is further equipped with a safety valve  32 , an electrical steam output valve  34  and a water inlet  36 . The water inlet  36  of the boiler  12  is connected with an electrical water pump  38  connected with a water tank  40  which holds preferably water but which can also store other liquids like water with certain additives. Between the water pump  38  and the water inlet  36 , a de-airing valve  42  is provided, enabling a connection of the boiler  12  with the water tank  40  being open to the atmosphere. Furthermore, the boiler  12  is connected via an electrical steam output valve  34  and a steam delivery hose  44  with a steam iron  46 . The steam iron comprises a steam trigger  48 . An electronic control unit  26  is connected with the water pump  38 , the heating element  22 , the temperature sensor  24 , the electrical steam output valve  34 , and with the steam trigger  48  of the steam iron  48 . This electronic control unit  26  controls the user interface with steam rate selection buttons and LED lights for system status indication. 
         [0026]    The steam generating device  10  is suitable for use in a domestic appliance comprising, besides the steam ironing device shown as a preferred embodiment, a steamer, a steam cleaner, an active ironing board, a facial sauna, a steam cooking device, a coffee making machine and the like. The temperature sensor  24  is used to detect changes in the water level of the boiler  12 . When the water level is lower than a certain level or the boiler  12  is empty, the electronic control unit  26  activates the pump  38  for a certain time period to pump water into the boiler  12  for raising the water level. The de-airing valve  42  provides a connection of the boiler  12  with the atmosphere to prevent the boiler  12  from being overfilled with water, if during cooling down after use, a vacuum is formed inside the boiler  12 . The temperature sensor  24  may be mounted on the heating plate  15  (as shown), in this way, the temperature sensor  24  is located adjacent to an area being in good thermal contact with the water inside the boiler  12  in order to properly sense the water temperature. Preferably, the temperature sensor  24  is located at a position underneath the water inlet  36  (as shown) such that the incoming water is guided to the sensing area of the temperature sensor  24 . Alternatively, the temperature sensor  24  may be mounted on the side walls of the boiler  12 , wherein water supplied via the water inlet  36  should be guided to flow down the inner surface of the wall to the sensing area of the temperature sensor  24 . If the sensed temperature is lower than a preset temperature value, the pressure is also lower than the required level. In this case, the electronic control unit  26  activates the heating element  12 . If the temperature sensor  24  signals a water temperature reaching or exceeding the preset temperature value, the heating element  22  is turned off by the electronic control unit  26 . This is a simple way of controlling the steam pressure inside the boiler  12 . After activating the steam trigger  48 , the air will be released together with the steam. 
         [0027]      FIG. 2  shows a flow diagram of an initial water level sensing. This routine is executed by the electronic control unit  26  when the steam generating apparatus  10  is started. In step S 100 , the routine starts when the steam generating device  10  is powered on. In the next step S 102 , the water within the boiler  12  is heated by turning on the heating device  14 . Due to turning on the heating device  14  in step S 102 , the temperature of the water within the boiler  12  and thus the temperature T sensed by the temperature sensor  24  increases, i.e. the temperature course is deflected upwards. In step S 104 , the routine holds until the temperature T sensed by the temperature sensor  24  reaches or exceeds a first threshold temperature T th1  ( FIG. 2  only shows a “&gt;” sign but a “≧” sign has the same effect—this is applicable for the entire disclosure herein where a “&gt;” or a “≧” sign is used). In order to check whether the first threshold temperature T th1  is reached, the temperature is monitored accordingly. In this regard, monitoring means that the temperature T is sensed continuously and it is checked if the individual sensed temperature values fulfill the respective condition. As soon as this first threshold temperature T th1  is reached or exceeded, the routine proceeds to step S 106 , where the heating device  14  is turned off. Turning off the heating device causes the temperature T to rise further up to a maximum temperature T max , because of the heat accumulated in the heating plate  15 . After reaching the maximum, the temperature falls again making the just mentioned temperature deflection to a temporary deflection. In step S 108 , the temperature T sensed by the temperature sensor  24  is again monitored as soon as the first threshold temperature T th1  is reached. As soon as the temperature decreases, the highest temperature T max , i.e. the maximum value, is stored in a memory of the electronic control unit  26  as an evaluation temperature T ev . In step S 110 , the water level is considered as being low, if the evaluation temperature T ev  is equal to or greater than a first low level threshold  T   low1 , wherein the first low level threshold T low1  is a value of X degrees. Alternatively, in step S 110 , the water level can also be considered as being low, if the evaluation temperature T ev  minus the first threshold temperature T th1  is equal to or greater than a second low level threshold T low2  which is a Δ-value of X degrees. For the case that the water level is determined as being low, step S 112  directs the routine to step S 114  where a predetermined amount of water is pumped into the boiler  12  by operating the water pump  38  for a predetermined fixed time period of X seconds. Otherwise, the routine is by-passed step S 114 . In any case, the routine reaches step S 116  where the initial water level sensing routine ends. Summarizing the above routine, during the start-up of the steam generating device  10 , the power of the heating device  14  is turned on such that the water temperature and temperature T sensed by the temperature sensor  24  rises to a certain value. Then the power of the heating device  14  is turned off. After turning off the power of the heating device  14 , there is always a temperature overshot, the magnitude of which depends on the water level in the boiler  12 . The higher the water level, the lower the temperature overshot. Based on the overshot magnitude, the initial water level can be sensed in order to start pumping water into the boiler  12 , if the water level is determined as being low. 
         [0028]      FIG. 3  shows a flow diagram of a water level sensing during steaming. This routine is executed repeatedly during the operation of the steam generating apparatus  10 . During the normal ironing process, a predetermined fixed amount of water will be pumped into the boiler  12  after steaming for a certain accumulated time. This refilling operation has to be executed anyway in order to ensure that water is refilled into the boiler  12  after steam was extracted for a certain accumulated time period. The refilling of water will cause a negative temporary temperature deflection which is in this case a temperature undershot at the temperature sensor  24 , because the relatively cold water is directed to a spot in the vicinity of the temperature sensor  24 . Within short the already present hot water in the boiler  12  mixes with the refilled cold water such that the temperature T rises again. The undershot magnitude, i.e. the minimum temperature after each pumping, is dependent on the water level within the boiler  12 . The higher the water level inside, the less the undershot. This water level sensing during steaming is now described referring to  FIG. 3  in more detail. The water level sensing routine is started when steaming is performed by pushing the steam trigger  48 . In step S 202  the time period for which the steam trigger  48  is pressed down is accumulated in the memory of the electronic control unit  26 . Step S 204  ensures that the routine only proceeds to step S 206  after the accumulated steaming time is greater than or equal to a steaming threshold which is a value of X seconds. In step S 206 , the water pump  38  is operated for a predetermined fixed time period of X seconds. The predetermined time period can be chosen based on the flow rate of the pump, in order to pump a fixed volume of water from the water tank  40  into the boiler  12 . Also in step S 206 , the temperature T sensed by the temperature sensor  24  is saved directly before or simultaneously with starting pumping. This temperature at the start of the pumping is saved as a comparison temperature T 1  in the memory of the electronic control unit  26 . The pumping time can also be adjusted depending on different temperatures and pressures, and the triggering time of the steam trigger  48  in order to ensure that each time the same amount of water will be pumped into the boiler  12 . After pumping the water into the boiler  12 , in step S 208 , the temperature of the temperature sensor  24  is monitored while it drops until it starts to rise again. In this regard monitoring means that the temperature is sensed continuously and it is checked if the individual sensed temperature values fulfill the respective condition, i.e. reached a minimum value. Then, the lowest temperature T min , i.e. the minimum value, is saved as the evaluation temperature T ev  in the memory of the electronic control unit  26 . Due to waiting until the minimum value is reached, the temperature sensor  24  can cool down completely, in order to sense the water level more accurately. Thereafter, in step S 210 , the water level is determined as being low, if the comparison temperature T 1  minus the evaluation temperature T, is equal to or greater than a third low level threshold T low3 , wherein the third low level threshold is a Δ-value of X degrees. In step S 212 , the routine is directed to step S 216 , if the water level is determined as being low, and the routine is directed to step S 214 , if the water level is determined as being high. In step S 214 , the steaming threshold is increased, in order to enlarge the interval from one pumping to the next pumping of the water pump  38 . In step S 216 , the steaming threshold is set to a predetermined value, thus defining the interval from one pumping to the next pumping of water into the boiler  12 . After step S 216 , the routine proceeds to step S 218  where a predetermined amount of water is pumped into the boiler  12  by operating the water pump  38  for a predetermined time period. After step S 214  and step S 218 , the routine returns to step S 200  where the routine is restarted. Two actions will cause the temperature to rise or to drop during the normal operation, namely these are steaming and pumping water into the boiler  12 . The temperature drop during water being pumped into the boiler  12  is larger than the temperature drop due to steaming when the volume of water pumped in is large enough. Alternatively to the above description, the water level can be checked in step S 210  based on T ev  only. In this alternative, the water level is determined as being low, if the evaluation temperature T ev  is below or equal to a fourth low level threshold T low4 . 
         [0029]    Similar to the boiler water detection, the following two routines can be used to determine if water is coming from the water tank  40  into the boiler  12 . 
         [0030]      FIG. 4  shows a flow diagram of a water tank empty detection. This routine is executed by the electronic control unit  26  in certain intervals or after the routine of  FIG. 3  determined several times that the water level is low. This routine is started in step S 300  and right thereafter a flag is set to 0 in step S 302 . The steps S 304  and S 306  are identical to the previously described steps S 206  and S 208 , respectively, such that their description is not repeated. Thereafter, when the routine reaches step S 308 , it is determined, if the comparison temperature T 1  minus the evaluation temperature T ev  is smaller than a first tank empty threshold T em1  which is a Δ-value of X degrees ( FIG. 4  only shows a “&lt;” sign but a “≦” sign has the same effect—this is applicable for the entire disclosure herein where a “&lt;” or a “≦” sign is used). If it is determined in step S 308  that this is not the case, the routine proceeds to step S 320  where it is determined that the water tank is not empty and the routine ends at step S 322 . If in step S 308  it is determined that the difference between the comparison temperature T 1  and the evaluation temperature T ev  is smaller than the first tank empty threshold T em1 , the routine proceeds to step S 310  where it is determined, if the flag is set to 1. If this is not the case, in step S 312  one longer pumping will be done when the steam trigger  48  is released and the steaming stops. After this pumping, the flag is set to 1 in step S 314  and the routine returns to step S 304 . If in step S 310  the flag is set to 1, the water tank is determined being empty in step S 316  and a water tank empty routine is started in step S 318 , which is shown in  FIG. 6 . Summarizing the above, the difference between the comparison temperature T 1  and the evaluation temperature T ev  is compared with the first tank empty threshold T em1 , and if the difference does not drop below this first tank empty threshold T em1 , one longer pumping will be done when the steam trigger  48  is released and steaming stops. If the difference does not drop below the first tank empty threshold T em1  again, the water tank is determined as being empty. 
         [0031]      FIG. 5  shows a flow diagram of another water tank empty detection. This routine can be executed by the electronic control unit  26  alternatively to the routine shown in  FIG. 4 . The routine is started with step S 400 . The two steps thereafter, namely steps S 402  and S 404  are identical to the above described steps S 206  and S 208 , respectively. Thereafter, in step S 406  it is determined if the comparison temperature T 1  minus the evaluation temperature T ev  is smaller than a second tank empty threshold T em2  wherein the second tank empty threshold T em2  is Δ-value of X degrees. If this is not the case, it is determined in step S 412  that the water tank  40  is not empty and the routine ends at step S 414 . If step S 406  is positive, the water tank  40  is determined as being empty in step S 408 . Step S 408  is followed by step S 410 , where the water tank empty routine of  FIG. 6  is executed. In the just described routine of  FIG. 5 , the second tank empty threshold T em2  is predetermined based mainly on the volume of water filled into the boiler  12  each time the water pump  38  is operated. Therefore the second tank empty threshold T em2  can be adjusted based on the volume of a water, if the volume changes. 
         [0032]      FIG. 6  shows a flow diagram of a water tank empty routine. In the water tank empty routine the following steps will be executed. First, in step S 500 , it is indicated to consumers that the water tank  40  is empty by turning on a water tank empty light. In the following step S 502 , the output valve  34  is blocked to stop steaming which leads to another indication to consumers that the water tank  40  is empty. Thereafter, in steps S 504  and S 506 , the steam trigger  48  is disabled for a certain time period of X seconds. This avoids a dry pumping of the water pump  38  when the water tank  40  is empty and therefore protects the water pump  38  from any damage. After the waiting period of step S 506 , in step S 508  the steam trigger  48  is released such that the consumer is allowed to press the steam trigger  48  to restart the system. When the steam trigger  48  is pressed, step S 510  directs the routine to step S 512  where the routine is directed such that the water tank empty detection routine shown in  FIG. 4  or  FIG. 5  will be started again, in which water will be pumped from the water tank  40  to the boiler  12  first. 
         [0033]    Equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.