Abstract:
A washing machine and a washing method thereof are disclosed, in which waste of washing water is reduced and washing efficiency is improved. The washing method includes the steps of determining the volume of laundry in a tub, setting a first water level for a preliminary washing stroke and a second water level for a main washing stroke based on the determined volume of laundry, carrying out the preliminary washing stroke using washing water of the first water level and steam, and converting the preliminary washing stroke into the main washing stroke based on washing water of the second water level depending on an inner temperature of the tub.

Description:
[0001]     This application claims the benefit of the Korean Patent Application No. P2005-15829, filed on Feb. 25, 2005, which is hereby incorporated by reference as if fully set forth herein.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a washing machine, and more particularly, to a washing machine using steam and a washing method thereof.  
         [0004]     2. Discussion of the Related Art  
         [0005]     A washing machine is a representative electric home appliance that washes the laundry using detergent and water. The washing machine is classified into a top loading washing machine and a front loading washing machine depending on the position where the laundry is loaded into the washing machine.  
         [0006]     Generally, the top loading washing machine includes a tub vertically stood to receive the laundry, a pulsator rotating in the tub to wash the laundry, and a lid provided on the top of the washing machine to open and close the tub. The top loading washing machine washes the laundry using friction between the laundry and a water stream generated by rotation of the pulsator. The top loading washing machine has advantages in view of short washing duration, large capacity, and low cost. However, the top loading washing machine provided with the pulsator has drawbacks in that tangling of the laundry occurs and damage of the laundry is relatively high.  
         [0007]     The front loading washing machine generally includes a drum and a tub provided in parallel with each other to receive the laundry, a plurality of lifters provided in the drum to lift and drop the laundry when the drum is rotated, and a door provided on a front surface of the washing machine to open and close the drum. The front loading washing machine washes the laundry by rotating the drum at low speed after putting water, detergent, and the laundry into the drum. The front loading washing machine has advantages in that damage of the laundry is small and tangling of the laundry does not occur.  
         [0008]     However, the above washing machines require a great volume of washing water to carry out washing processes along with a long time taken to supply/drain the washing water to/from the washing machine.  
       SUMMARY OF THE INVENTION  
       [0009]     Accordingly, the present invention is directed to a washing machine and a washing method thereof, which substantially obviate one or more problems due to limitations and disadvantages of the related art.  
         [0010]     An object of the present invention is to provide a washing machine and a washing method thereof, in which waste of washing water is reduced and washing efficiency is improved.  
         [0011]     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.  
         [0012]     To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a washing method according to the present invention includes the steps of determining the volume of laundry in a tub, setting a first water level for a preliminary washing stroke and a second water level for a main washing stroke based on the determined volume of laundry, carrying out the preliminary washing stroke using washing water of the first water level and steam, and converting the preliminary washing stroke into the main washing stroke based on washing water of the second water level depending on an inner temperature of the tub. The second water level for the main washing stroke is higher than the first water level for the preliminary washing stroke.  
         [0013]     The step of carrying out the preliminary washing stroke based on washing water of the first water level and the steam includes the steps of determining whether the washing water of the first water level decreases, and resupplying the washing water into the tub if the washing water of the first water level decreases.  
         [0014]     The step of converting the preliminary washing stroke into the main washing stroke depending on the inner temperature of the tub includes the step of stopping a steam generator from steaming if the inner temperature of the tub is higher than a reference temperature. The reference temperature is set based on the volume of laundry.  
         [0015]     In another aspect of the present invention, a washing machine includes a temperature sensor sensing an inner temperature of a tub, a steam generator supplying steam into the tub, and a controller setting a first water level for a preliminary washing stroke and a second water level for a main washing stroke based on the volume of laundry in the tub, and converting the preliminary washing stroke based on washing water of the first water level and the steam into the main washing stroke based on washing water of the second water level depending on the inner temperature of the tub.  
         [0016]     In still another aspect of the present invention, a washing method includes the steps of determining the volume of laundry in a tub, setting a first water level for a preliminary washing stroke, a second water level for a main washing stroke, a preliminary washing duration, and a main washing duration based on the determined volume of laundry, carrying out the preliminary washing stroke using washing water of the first water level and steam during the preliminary washing duration, and converting the preliminary washing stroke into the main washing stroke based on washing water of the second water level during the main washing duration.  
         [0017]     In further still another aspect of the present invention, a washing machine includes a water level sensor sensing a water level in a tub, a steam generator supplying steam into the tub, and a controller setting a first water level for a preliminary washing stroke, a second water level for a main washing stroke, a preliminary washing duration and a main washing duration based on the volume of laundry in the tub, and sequentially carrying out the preliminary washing stroke based on washing water of the first water level and the steam during the preliminary washing duration and the main washing stroke based on washing water of the second water level during the main washing duration.  
         [0018]     It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:  
         [0020]      FIG. 1  is a perspective view illustrating a washing machine according to the present invention;  
         [0021]      FIG. 2  is a sectional view illustrating the washing machine shown in  FIG. 1 ;  
         [0022]      FIG. 3A  is a cutaway perspective view illustrating a steam generator of the washing machine shown in  FIG. 1 ;  
         [0023]      FIG. 3B  is a cutaway perspective view illustrating another steam generator of the washing machine shown in  FIG. 1 ;  
         [0024]      FIG. 4  illustrates a nozzle assembly connected with a water supply hose and a supply hose;  
         [0025]      FIG. 5  is a block diagram illustrating elements required for a washing stroke according to the present invention;  
         [0026]      FIG. 6  is a flow chart illustrating a washing method according to the first embodiment of the present invention; and  
         [0027]      FIG. 7  is a flow chart illustrating a washing method according to the second embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.  
         [0029]      FIG. 1  is a perspective view illustrating a washing machine according to the present invention, and  FIG. 2  is a sectional view illustrating the washing machine shown in  FIG. 1 . Referring to  FIG. 1  and  FIG. 2 , a front loading drum washing machine is exemplarily shown. The front loading drum washing machine includes a tub  200  provided in a case  100  to receive washing water, and a drum  300  rotatably provided in the tub  200  to receive the laundry. The tub  200  and the drum  300  are provided in a horizontal direction. The present invention is not limited to the front loading drum washing machine shown in  FIG. 1  and  FIG. 2 . The present invention may be applied to a top loading pulsator washing machine in which the tub and the drum are provided in a vertical direction.  
         [0030]     Hereinafter, the washing machine according to the present invention will be described in more detail with reference to  FIG. 1  and  FIG. 2 . Referring to  FIG. 1  and  FIG. 2 , the case  100  of the washing machine includes a base  110 , a wall  120 , and a top plate  130 . The base  110  constitute the bottom of the case  100 . A damper  20  is provided on the base  110  to support the tub  200  that will be described later. The wall  120  is stood above the base  110  to form a space therein, where the tub  200  is to be provided. The wall  120  constitutes a front surface, a rear surface and both sides of the case  100 . The top plate  130  is provided at an opened upper portion of the wall  120  to seal the inner space of the case  100  formed by the wall  120  and the base  110 .  
         [0031]     A control panel  80  is provided on a top of the front surface of the wall  120  or a top surface of the top plate  130  to allow a user to manipulate the washing machine. Springs  10  are connected with the wall  120  or an inner surface of the top plate  130  to allow the tub  200  to be hung thereon. A loading hole  125  is formed on any one of the wall, for example, the front surface of the wall  120  to take the laundry ‘m’ in and out. The loading hole  125  is opened and closed by a door  150  hinged on the front surface of the wall  120 . The door  150  includes a door frame  151  and a door glass  155 . The door glass  155 , as shown in  FIG. 2 , is provided in a hole formed at the center of the door frame  151 . Therefore, the user can view the inner portion of the washing machine, more specifically, the inner portion of the drum  300 , through the door glass  150 . The drum  300  will be described later.  
         [0032]     The tub  200  is provided in the inner space of the case  100 . The tub  200  is supported by the springs  10  and the damper  20  in a state that it floats at the center of the inner space. The tub  200  is provided such that its opened front surface faces the loading hole  125  of the wall  120 . The washing water is supplied into the tub  200  and stored therein.  
         [0033]     The drum  300  is rotatably provided in the inner space of the tub  200 . To this end, a motor  250  is provided in the case  100  to rotate the drum  300 . An example of a shaft of the motor  250  directly fixed to the drum  300  is shown in  FIG. 2 . In this case, the shaft is fixed to the rear surface of the drum  300  by passing through the tub  200 . Meanwhile, although not shown, the motor  250  may be provided to indirectly rotate the drum  300 . In this case, the drum  300  and the motor  250  may be connected with each other by a power transmission member such as a belt.  
         [0034]     A plurality of through holes  310  are formed along the circumference of the drum  300  as shown in  FIG. 2 . The washing water stored in the tub  200  can enter the inner space of the drum  300  through the through holes  310 . As shown in  FIG. 1  and  FIG. 2 , a plurality of lifters  320  are projected from the inner circumference of the drum  300 . The lifters  320  lift and then drop the laundry ‘m’ when the drum  300  is rotated.  
         [0035]     As shown in  FIG. 2 , a gasket  25  is provided between the tub  200  and the front surface of the wall  120 . The gasket  25  prevents the washing water and the laundry ‘m’ in the tub  200  from leaking out of the tub  200  and entering the inner space of the case  100 . Meanwhile, a nozzle assembly  60  is provided at an upper portion of the gasket  25  to pass through the gasket  25 . The nozzle assembly  60  will be described later.  
         [0036]     A water supply valve  400  is provided at one side of the case  100 , for example, the rear surface of the wall  120 , as shown in  FIG. 1  and  FIG. 2 . The water supply valve  400  is connected with an outer water supply source, for example, a tap, and controls the washing water supplied from the water supply source. In the washing machine according to the present invention, the water supply valve  400  includes at least two valves, i.e., a first valve  410  and a second valve  420 .  
         [0037]     The first valve  410  is connected with the tub  200  through a first hose, for example, a water supply hose  510 . The second valve  420  is connected with the tub  200  through a second hose, for example, a supply hose  520 . As shown in  FIG. 1 , the second hose, i.e., the supply hose  520  communicates the water supply valve  400  with the tub  200  through a path different from that of the first hose, i.e., the water supply hose  510 .  
         [0038]     The water supply valve  400  supplies water into the tub  200  through two hoses having paths different from each other as above, i.e., the water supply hose  510  and the supply hose  520 . The water supply valve  400  can simultaneously or separately control the first valve  410  and the second valve  420 . Thus, the water supply to the tub  200  through the water supply hose  510  and the water supply to the tub  200  through the supply hose  520  can be carried out simultaneously or separately.  
         [0039]     The water supply hose  510  that communicates the first valve  410  with the tub  200  passes through a detergent box  50  as shown in  FIG. 1 . The water supply hose  510  may be provided to directly pass through the detergent box  50 . Alternatively, the water supply hose  510  may communicate with a part of the detergent box  50  so that it is supplied with the detergent from the detergent box  50 . Therefore, the washing water flown to the water supply hose  510  through the first valve  410  is supplied into the tub  200  after always passing through the detergent box  50 . The washing water supplied into the tub  200  through the water supply hose  510  flows down along the inner surface of the tub  200  and is collected in the bottom of the tub  200 . Meanwhile, the first valve  410  and the water supply hose  510  communicating with the detergent box  50  may be provided in large numbers as shown in  FIG. 1 . In such case, the detergent for washing and the detergent for rinsing can respectively be supplied into the tub  200  at a timing interval.  
         [0040]     The supply hose  520  that communicates the second valve  420  with the tub  200  detours the detergent box  50  as shown in  FIG. 1 . Instead, the supply hose  520  passes through a tank  610 , for example. The tank  610  stores a predetermined volume of water or overflows the water stored therein to supply the water into the tub  200 . Further, the tank  610  may supply the predetermined volume of water stored therein into the tub  200  at one time. The supply hose  520  passing through the tank  610 , as shown in  FIG. 2 , is connected with the nozzle assembly  60  provided to pass through the gasket  25 . Therefore, the washing water flown to the supply hose  520  through the second valve  420  is supplied into the tub  200  after always passing through the tank  610 .  
         [0041]     Meanwhile, it is possible to obtain higher washing effect when the laundry is washed using heated water than washing effect obtained when the laundry is washed using cold water. Therefore, in the washing machine according to the present invention, a steam generator  600  is provided as shown in  FIG. 1  and  FIG. 2  so that hot steam is supplied into the tub  200  to enhance the washing effect. To allow the steam generator  600  to supply the steam into the tub  200 , a water tank storing water supplied from the water supply source, a heater heating the water stored in the water tank, and a path connecting the water supply source, the water tank and the tub  200  with one another are required.  
         [0042]     The washing machine according to the present invention are provided with the second hose, i.e., the supply hose  520  connecting the water supply valve  400  with the tub  200  and the tank  610  provided at a middle portion of the supply hose  520  to store the water therein. Therefore, to efficiently use the inner space of the washing machine and reduce the number of parts, the tank  610  and the supply hose  520  are used as the parts of the steam generator  600 .  
         [0043]     In the present invention, the steam generator  600  that uses the tank  610  and the supply hose  520  as its parts can supply the washing water supplied from the second valve  420  into the tub  200  through the nozzle assembly  60  in a liquid or steam state.  FIG. 3A  and  FIG. 3B  illustrate the steam generator  600  in detail, and  FIG. 5  illustrates elements related to water supply. Hereinafter, the steam generator  600  will be described in more detail.  
         [0044]     The steam generator  600 , as shown in  FIG. 3A , includes the tank  610  having an inlet  520  and an outlet  630 , a heater  640  provided on the bottom inside the tank  610 , a sensor assembly  650  sensing a water level in the tank  610 , and at least one temperature sensor  617  sensing a temperature inside the tank  610 .  
         [0045]     The tank  610  has a space therein, which can receive a predetermined volume of water. A flange  611  and an extension  612  are formed on an outer surface of the tank  610  to fix the tank  610  to the inner surface of the case  100 . The inlet  620  and the outlet  630  are formed at an upper portion of the tank  610 . This is to prevent the water in the tank  610  from flowing backward toward the second valve  420  through the inlet  620  and effectively drain out the steam generated in the tank  610  through the outlet  630 . Portions of the tank  610  where the inlet  620  and the outlet  630  are formed are locally projected as shown in  FIG. 1  and  FIG. 3A .  
         [0046]     The inlet  620  communicates with the second valve  420  through the supply hose  520  while the outlet  630  communicates with the nozzle assembly  60  through supply hose  520 . Meanwhile, the inlet  620  and the outlet  630  are not provided with a separate on/off valve. Therefore, the inlet  620  can counteract the outlet  630  and vice versa. For example, the inlet  620  may be used as the outlet while the outlet  630  may be used as the inlet. In such case, when the position of the tank  610  should be changed in the case  100 , the inlet  620  and the outlet  630  are used to be compatible with each other. Thus, the tank  610  can be used for different models. However, the outlet  630  and the inlet  620  may respectively be provided with an on/off valve as occasion demands.  
         [0047]     The heater  640  includes a radiator  641  and a terminal  645 . The radiator  641  of the heater  640 , as shown in  FIG. 3A , is uniformly provided on the bottom inside the tank  610 . The terminal  645  of the heater  640  is exposed to the outside after passing through the side of the tank  610 . Meanwhile, one end of the radiator  641  is supported in a state that it is spaced apart from the bottom of the tank  610  at a predetermined distance by a clamp  615  provided on the bottom of the tank  610 .  
         [0048]     The sensor assembly  650  includes a plurality of electrodes that sense a minimum water level and a full water level in the tank  610 . The minimum water level is to prevent the radiator  641  of the heater  640  from being overheated. The minimum water level is determined at the position a little higher than the top of the radiator  641  to prevent the radiator  641  from being exposed. The full water level is to prevent the water supplied into the tank  610  from being overflown through the outlet  630 . The full water level is determined at the position a little lower than the outlet  630 .  
         [0049]     The sensor assembly  650  that senses the minimum water level and the full water level includes a common electrode  651 , a first electrode  653 , and a second electrode  655 , as shown in  FIG. 3A . The common electrode  651 , the first electrode  653 , and the second electrode  655  are vertically arranged in a state that they are spaced apart from one another at a predetermined distance. Top ends of the electrodes are provided to pass through the top surface of the tank  610 . Terminals are respectively formed on the top ends of the electrodes exposed by passing through the tank  610 .  
         [0050]     The common electrode  651  and the first electrode  653 , as shown in  FIG. 3A , are long and their heights are substantially the same as each other. Therefore, the common electrode  651  and the first electrode  653  are simultaneously soaked in the water or exposed from the water. If the common electrode  651  and the first electrode  653  are simultaneously soaked in the water, the common electrode  651  and the first electrode  653  are electrically connected with each other. Thus, a controller  700  such as a microprocessor determines that the water level in the tank  610  is higher than the minimum water level.  
         [0051]     By contrast, if the common electrode  651  and the first electrode  653  are simultaneously exposed from the water as the water level in the tank  610  decreases, the common electrode  651  and the first electrode  653  are electrically disconnected from each other. Thus, the controller  700  determines that the water level in the tank  610  is lower than the minimum water level. If the water level in the tank  610  is lower than the minimum water level, the controller  700  stops the operation of the heater  640  to prevent the heater  640  from being damaged by overheat.  
         [0052]     The second electrode  655  is shorter than the common electrode  651  and the first electrode  653 . Therefore, a lower end of the second electrode  655  is positioned to be higher than lower ends of the common electrode  651  and the first electrode  653 . If the second electrode  655  is not soaked in the water as the water level in the tank  610  is low, the common electrode  651  and the second electrode  655  are electrically disconnected from each other. Thus, the controller  700  determines that the water level in the tank  610  does not reach the full water level.  
         [0053]     By contrast, if the second electrode  655  is soaked in the water as the water level in the tank  610  increases, the common electrode  651  and the second electrode  655  are electrically connected with each other. Thus, the controller  700  determines that the water level in the tank  610  reaches the full water level. If the water level in the tank  610  reaches the full water level, the controller  700  closes the second valve  420  to stop the water from being supplied into the tank  610  when the steam generator  600  generates steam. However, the controller  700  does not close the second valve  420  even if the sensor assembly  650  senses the full water level when the water is supplied into the tub  200  through the steam generator  600 . Thus, the water continues to be supplied into the tank  610 . As a result, the water supplied into the tank  610  is overflown from the tank  610  so that the water can be supplied into the tub  200  through the outlet  630 .  
         [0054]     Hereinafter, the procedure of generating steam through the aforementioned steam generator  600  will be described in brief. First, the controller  700  measures the water level in the tank  610  using the sensor assembly  650 . If the water level in the tank  610  is low, the controller  700  opens the second valve  420  to supply the washing water into the tank  610 . However, if the sensor assembly  650  senses the full water level, the controller  700  closes the second valve  420  to stop the washing water from being supplied into the tank  610 .  
         [0055]     If the tank  610  is filled with the washing water, the heater  640  is operated to heat the washing water in the tank  610 . If the washing water is heated, steam is generated. The generated steam is sprayed into the tub  200  through the outlet  630 . The water level in the tank  610  is gradually lowered as the steam continues to be supplied into the tub  200 . If the washing water in the tank  610  is evaporated to allow the water level in the tank  610  to reach the minimum water level, the controller  700  turns the heater  640  off. If necessary, the controller  700  turns again the heater  640  on to supply the steam into the tub  200  after supplying the washing water into the tank  610 .  
         [0056]     The case where the water in the tank  610  is supplied into the tub  200  through the outlet  630  as the water in the tank  610  overflows has been described as above. However, the present invention is not limited to such case. As another example, the water stored in the tank  610  may be supplied into the tub  200  at one time if the water in the tank  610  reaches the full water level. To this end, as shown in  FIG. 3B , a second outlet  660  that can be turned on/off is provided at a lower portion of the tank  610 . The second outlet  660  is connected with the supply hose  520  connected with the tub  200 . The second outlet  660  constructed as above is usually closed. The second outlet  660  is selectively opened to supply the washing water stored in the tank  610  into the tub  200  at one time only when the user intends to supply the washing water into the tub  200  while measuring the volume of the washing water.  
         [0057]     Meanwhile, as shown in  FIG. 2 , a drain  210  is formed at the lower portion of the tub  200 . The drain  210  is connected with a bellows tube  33 . The bellows tube  33  is connected with a pump unit that pumps the water supplied into the tub  200  through the drain  210  and the bellows tube  33  to drain the water out or circulates the water in the drum  300 .  
         [0058]     The pump unit, as shown in  FIG. 1 , includes a pump housing  45 , a circulating pump  30 , and a drain pump  40 . The pump housing  45  is supplied with the water passing through the drain  210  and the bellows tube  33 . The drain pump  40  is connected with a drain hose  37  that communicates with the outside. The drain pump  40  drains the washing water supplied into the pump housing  50  out through the drain hose  37  during a drain stroke of the washing machine.  
         [0059]     The circulating pump  30  is connected with a circulating hose  35 . The circulating hose  35  is connected with the nozzle assembly  60  of which one end is provided to pass through the gasket  25  as shown in  FIG. 2 . The circulating pump  30  pumps the washing water supplied into the pump housing  45  toward the circulating hose  35  during washing and rinsing strokes of the washing machine. The pumped washing water is sprayed into the tub  200  through the nozzle assembly  60 .  
         [0060]     As described above, the nozzle assembly  60  provided to pass through the gasket  25  is connected with the circulating hose  35  and the supply hose  520 , respectively. The nozzle assembly  60 , as shown in  FIG. 4 , includes a first nozzle  61  connected with the circulating hose  35  to spray the washing water pumped by the circulating pump  30  into the tub  200 , and a second nozzle  62  connected with the supply hose  520  to spray the steam generated by the steam generator  600  or the washing water passing through the steam generator  60  into the tub  200 . The first nozzle  61  and the second nozzle  62 , as shown in  FIG. 4 , are arranged in parallel with each other and formed in a single body to facilitate their manufacture and arrangement.  
         [0061]     Meanwhile, in the present invention, the washing water is supplied into the tub  200  through the supply hose  520  that detours the detergent box  50  as well as the water supply hose  510  that passes through the detergent box  50 . Therefore, a greater volume of the washing water can be supplied into the tub  200  within a shorter time in comparison with the related art washing machine that supplies the water only through the water supply hose  510 .  
         [0062]     Hereinafter, embodiments of a water supply method according to the present invention will be described in detail.  
       FIRST EMBODIMENT  
       [0063]      FIG. 6  is a flow chart illustrating a washing method according to the first embodiment of the present invention. Referring to  FIG. 6 , if a washing stroke starts, the controller  700  senses the volume of the laundry (S 10 ). The controller  700  repeatedly rotates the drum  300  in forward and reverse directions to sense the volume of the laundry. The controller  700  determines the volume of the laundry based on load and rotational speed of the drum  300  detected when the drum  300  is rotated. Another methods of determining the volume of the laundry may be applied to the present invention.  
         [0064]     If the volume of the laundry is determined, the controller  700  selects various washing options depending on the volume of the laundry (S 20 ). For example, the controller  700  can determine the volume of the washing water (first water level) to be used for a preliminary washing stroke, the volume of the washing water (second water level) to be used for a main washing stroke, duration of the washing stroke, a number of times/duration of a rinsing stroke, a number of times/duration of a dehydrating stroke, and so on based on the volume of the laundry. The preliminary washing stroke is to sufficiently soak the laundry using a small volume of washing water and increase an inner temperature of the tub  200  and a temperature of the washing water using steam. The main washing stroke is to normally wash the laundry. Since the first water level is lower than the second water level, the washing water in the main washing stroke should be resupplied into the tub  200 . The controller  700  can determine the volume (second water level minus first water level) of the washing water to be resupplied in the main washing stroke. Optionally, the controller  700  can set a reference temperature T used for the preliminary washing stroke, duration of the preliminary washing stroke, duration of the main washing stroke, the volume of the washing water supplied to the steam generator  600 , driving duration of the heater  640 , and so on depending on the volume of the laundry.  
         [0065]     If various options are selected, the washing water is supplied into the tub  200  by the first water level. To this end, the controller  700  opens the first valve  410  to supply the washing water of the water supply source into the tub  200  through the water supply hose  510 . The water supply hose  510  passes through the detergent box  50  as mentioned above. Therefore, if there is the detergent in the detergent box  50 , the detergent is supplied into the tub  200  along with the washing water flown into the water supply hose  510 . The washing water supplied into the tub  200  through the water supply hose  510  flows down along the inner surface of the tub  200  and is collected in the bottom of the tub  200 . With the lapse of time, the water level in the tub  200  gradually increases. At the same time, the controller  700  opens the second valve  420  to supply the washing water to the steam generator  600 . The washing water flown into the supply hose  520  is supplied into the tank  610  of the steam generator  600 . With the lapse of time, the tank  610  is filled with the washing water.  
         [0066]     While the washing water is supplied into the tank  610 , the sensor assembly  650  of the steam generator  600  senses the water level in the tank  610 . If the water level in the tank  610  reaches the full water level, the controller  700  closes the second valve  420 . The controller  700  turns the heater  600  on for a set time, for example, several seconds or several tens of seconds to heat the washing water in the tank  610 . If the washing water in the tank  610  starts to boil, the steam is generated and the pressure in the tank  610  increases. The controller  700  turns the heater  640  off after the lapse of the set time.  
         [0067]     While the washing water is supplied into the tub  200 , a main water level sensor  330  repeatedly senses the water level of the washing water flowing into the tub  200 . The main water level sensor  330  converts the weight or pressure of the washing water in the tub  200  into a frequency signal and transmits the frequency signal to the controller  700 . The controller  700  determines the water level of the washing water based on the frequency signal transmitted from the main water level sensor  330 . The controller  700  closes the first valve  410  if the sensed water level reaches the first water level. For example, supposing that the water level frequency corresponding to the first water level is in the range of 1000 kHz to 1200 kHz, the controller  700  closes the first valve  410  when the frequency signal transmitted from the main water level sensor  330  reaches 1000 kHz to 1200 kHz.  
         [0068]     If the washing water in the tank  610  is boiled and the water level in the tub  200  reaches the first water level, the controller  700  supplies the steam into the tub  200  and at the same time rotates the drum  300 . In other words, the controller  700  carries out the preliminary washing stroke using the steam and the washing water of the first water level (S 30 ).  
         [0069]     In addition, since the laundry absorbs the water during the preliminary washing stroke, the water level in the tub  200  may be lowered. If the water level in the tub  200  becomes lower than the first water level, the controller  700  resupplies the washing water into the tub  200 . For example, if the frequency signal transmitted from the main water level sensor  330  is lower than 1000 kHz, the controller  700  resupplies the washing water into the tub  200  at a certain volume.  
         [0070]     While the preliminary washing stroke is carried out, a temperature sensor  800  provided in the tub  200  repeatedly senses the inner temperature of the tub  200 . Then, the controller  700  compares the sensed inner temperature ‘t’ with a previously set temperature ‘T’ (S 40 ). If the inner temperature ‘t’ sensed by the temperature sensor  800  is lower than the previously set temperature ‘T,’ the controller  700  continues to drive the heater  640  and maintains the preliminary washing stroke. In other words, the preliminary washing stroke using the steam is maintained until the sensed inner temperature ‘t’ reaches the previously set temperature ‘T.’ The heater  640  is set to be driven for a previously set time. However, the heater  640  is driven again if the sensed inner temperature ‘t’ is lower than the previously set temperature ‘T.’ 
         [0071]     Afterwards, if the sensed inner temperature ‘t’ reaches the previously set temperature ‘T,’ the controller  700  stops driving of the steam generator  600  to prevent the steam from being supplied into the tub  200  (S 50 ). The controller  700  converts the preliminary washing stroke into the main washing stroke. To carry out the main washing stroke, the controller  700  simultaneously opens the first valve  410  and the second valve  420  and supplies the washing water into the tub  200  through the water supply hoses  510  and  520  until the water level in the tub  200  reaches the second water level (S 60 ).  
         [0072]     If the water level in the tub  200  reaches the second water level, the controller  700  carries out the main washing stroke for a set duration of the main washing stroke (S 70 ˜S 80 ).  
       SECOND EMBODIMENT  
       [0073]      FIG. 7  is a flow chart illustrating a washing method according to the second embodiment of the present invention. Referring to  FIG. 7 , if a washing stroke starts, the controller  700  determines the volume of the laundry (S 110 ) and selects various washing options depending on the volume of the laundry (S 120 ). For example, the controller  700  can set the volume of the washing water (first water level) to be used for the preliminary washing stroke, the volume of the washing water (second water level) to be used for the main washing stroke, number of times/duration of the rinsing stroke, number of times/duration of the dehydrating stroke, duration of the preliminary washing stroke (first set time), duration of the main washing stroke (second set time), and so on depending on the volume of the laundry.  
         [0074]     Then, the controller  700  carries out the preliminary washing stroke using the steam and the washing water of the first water level during the preliminary washing stroke duration (first set time) (S 130 ˜S 140 ). If the preliminary washing stroke duration passes, the controller  700  converts the preliminary washing stroke into the main washing stroke.  
         [0075]     To convert the preliminary washing stroke into the main washing stroke, the controller  700  stops the steam from being supplied into the tub  200  (S 150 ) and supplies the washing water into the tub  200  through the water supply hoses  510  and  520  until the water level in the tub  200  reaches the second water level (S 160 ).  
         [0076]     If the water level in the tub  200  reaches the second water level, the controller  700  carries out the washing stroke during the set duration of the main washing stroke (second set time) (S 170 ˜S 180 ).  
         [0077]     In the present invention, the washing stroke has been described. The present invention may be applied to a soaking stroke and a rinsing stroke. For example, the rinsing stroke of the present invention may include a preliminary rinsing stroke based on a small volume of washing water and steam and a main rinsing stroke based on a large volume of washing water.  
         [0078]     As described above, since the small volume of the washing water is used for the preliminary washing stroke, the concentration of the detergent contained in the washing water is high. Since the temperature in the tub and the temperature of the washing water are increased using the steam, washing efficiency is improved. Moreover, since the large volume of the washing water is used for the main washing stroke, it is possible to easily separate the laundry from contaminants.  
         [0079]     It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the inventions. Thus, it is intended that the present invention covers the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.