Abstract:
Disclosed herein is a washing machine including a balancer, which increases the speed of a rotation tub stepwise in a period in which an excessive vibration of a water tub occurs to pass the excessive vibration period without the vibration, and a method of controlling the same. The method of controlling the washing machine includes increasing the speed of the rotation tub stepwise in the period in which the excessive vibration of the water tub occurs; measuring current of a motor to detect an amount of unbalance while the speed of the rotation tub is increased stepwise; and controlling the speed of the rotation tub based on the detected amount of unbalance.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of Korean Patent Application No. 2007-54985, filed on Jun. 5, 2007 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field 
     The present invention relates to a washing machine including a balancer, and more particularly, to a washing machine capable of reducing a vibration of a water tub due to an eccentric state, that is, an unbalance, which may occur at the time of rotation of a rotation tub, and a method of controlling the same. 
     2. Description of the Related Art 
     In general, a washing machine (generally, a drum-shaped washing machine) includes a water tub which contains water (washing water or rinsing water) therein, a rotation tub which is rotatably installed in the water tub and contains laundry therein, and a motor which generates a driving force to rotate the rotation tub. The washing machine washes the laundry by an operation of rising and dropping the laundry contained therein along an inner wall of the rotation tub when the cylindrical rotation tub is rotated. 
     Such a washing machine washes the laundry using a series of operations including a washing mode to wash dirt out of the laundry using water in which a detergent is dissolved (i.e., washing water), a rinsing mode to rinse bubble or residual detergent out of the laundry using water in which the detergent is not dissolved (i.e., rinsing water), and a dehydrating mode to dehydrate the laundry at a high speed. In the dehydrating mode, if the rotation tub is rotated at a high speed in a state in which the laundry is unevenly distributed along an inner wall of the rotation tub and thus an unbalance or imbalance occurs, a force is biased toward a rotation shaft of the rotation tub to generate a large vibration. 
     In order to prevent the vibration due to such an unbalance, a washing machine including a race which is provided to be concentric with a rotation tub and a balancer having a plurality of balls seated in the race together with oil is disclosed in Japanese Unexamined Patent Application Publication No. 10-43472. 
     In the washing machine disclosed in the above Publication, when the rotation tub is rotated at a high speed, the balls are automatically moved in the race to prevent the force from being biased toward the rotation shaft such that the unbalance is removed. 
     However, in the washing machine including the balancer as described above, if the weight of the unbalance is larger than the total weight of the balls, the unbalance cannot be sufficiently removed even if the balls are located opposite the unbalance in the circumferential direction (opposite phase). Thus, the vibration occurs. 
     If the number of rotations of the rotation tub is less than an inherent number of vibrations of the rotation tub, a difference occurs between a movement speed of the balls and a movement speed of the unbalance (i.e., the rotation speed of the rotation tub), and thus a relative position between the unbalance and the balls periodically varies. 
     At this time, if the balls and the unbalance are arranged in phase with each other in the circumferential direction (in-phase), a larger force is applied to the rotation shaft to generate a larger vibration. If the balls and the unbalance are arranged in phase with each other at a time point when the number of rotations of the rotation tub coincides with the inherent number of vibrations, resonance becomes large enough to generate an excessive vibration of the water tub. 
     SUMMARY 
     Therefore, it is an aspect of the embodiment to provide a washing machine including a balancer, which increases a speed of a rotation tub stepwise in a period in which an excessive vibration of a water tub occurs to pass an excessive vibration period without the vibration, and a method of controlling the same. 
     It is another aspect of the embodiment to provide a washing machine capable of preventing rapid movement of balls to prevent an excessive vibration of a water tub by detecting an unbalance state in real time while a speed of a rotation tub is increased stepwise and by increasing the speed of the rotation tub when the amount of unbalance is less than or equal to a restriction value, and a method of controlling the same. 
     Additional aspects and/or advantages will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the invention. 
     In accordance with the invention, the above and/or other aspects can be achieved by the provision of a method of controlling a washing machine including a water tub, a rotation tub and at least one balancer, the method including: increasing a speed of a rotation tub stepwise in a period in which an excessive vibration of the water tub occurs; measuring current of a motor to detect an amount of unbalance while the speed of the rotation tub is increased stepwise; and controlling the speed of the rotation tub based on the detected amount of unbalance. 
     The speed of the rotation tub which is increased stepwise may be divided into a plurality of RPM ranges in the period in which the excessive vibration of the water tub occurs, and the speed of the rotation tub may be increased stepwise based on the plurality of RPM ranges. 
     When the speed of the rotation tub is increased stepwise to reach a predetermined RPM, the speed of the rotation tub may be maintained at the predetermined RPM, and a time in which the speed of the rotation tub is maintained at the predetermined RPM may be counted, and the speed of the rotation tub may be increased after a lapse of a predetermined amount of time. 
     The predetermined RPM may be the speed of the rotation tub in which an unbalance is able to occur. 
     The predetermined time may be a reference time necessary to remove the unbalance by the at least one balancer. 
     The speed of the rotation tub which is increased stepwise may be divided into a plurality of RPM ranges in the period in which the excessive vibration of the water tub occurs, and the speed of the rotation tub may be increased when the amount of unbalance is equal to or less than a predetermined restriction value based on the plurality of RPM ranges. 
     The rotation tub may be stopped when the amount of unbalance is greater than the restriction value. 
     The excessive vibration period of the water tub may be approximately 160 to 300 RPM. 
     The foregoing and/or other aspects are achieved by providing a method of controlling a washing machine including a rotation tub, a motor and at least one balancer, the method including: dividing a speed of the rotation tub stepwise in a predetermined period; increasing the speed of the rotation tub divided stepwise to reach a predetermined RPM and maintaining the speed of the rotation tub at the predetermined RPM, and counting a time in which the speed of the rotation tub is maintained at the predetermined RPM and increasing the speed of the rotation tub after a lapse of a predetermined amount of time. 
     Current of the motor may be measured to detect an amount of unbalance while the speed of the rotation tub is increased stepwise, and the speed of the rotation tub may be increased when the detected amount of unbalance is equal to or less than a predetermined restriction value. 
     The rotation tub may be stopped when the amount of unbalance is greater than the restriction value. 
     The foregoing and/or other aspects are achieved by providing a washing machine including at least one balancer, the washing machine including: a rotation tub to contain laundry therein; a motor rotating the rotation tub; and a control unit driving the motor to increase a speed of the rotation tub stepwise, measuring current of the motor to detect an amount of unbalance while the speed of the rotation tub is increased stepwise, and controlling the speed of the rotation tub based on the detected amount of unbalance. 
     The control unit may divide the speed of the rotation tub, which is increased stepwise, into a plurality of RPM ranges and increase the speed of the rotation tub stepwise based on the plurality of RPM ranges. 
     The washing machine may further include a speed detecting unit detecting the speed of the rotation tub which is increased stepwise, and the control unit may control the driving of the motor such that the speed of the rotation tub is maintained at a predetermined RPM when the speed of the rotation tub reaches the predetermined RPM in which an unbalance is able to occur. 
     The control unit may count a time in which the speed of the rotation tub is maintained at the predetermined RPM and control the driving of the motor such that the speed of the rotation tub is increased after a lapse of a predetermined amount of time. 
     The control unit may divide the speed of the rotation tub, which is increased stepwise, to a plurality of RPM ranges and control the driving of the motor such that the speed of the rotation tub is increased when an amount of unbalance is equal to or less than a predetermined restriction value based on the plurality of RPM ranges. 
     The rotation tub may be stopped when the amount of unbalance is greater than the restriction value. 
     The washing machine may further include a water tub, and the control unit may control the driving of the motor such that the speed of the rotation tub is increased stepwise in a period in which an excessive vibration of the water tub occurs. 
     The foregoing and/or other aspects are achieved by providing a method of controlling a washing machine including a water tub, a rotation tub and at least one balancer, the method including: detecting an amount of unbalance in an excessive vibration period of the water tub; and increasing a speed of a rotation tub stepwise when the detected amount of unbalance is less than or equal to a restriction value. 
     The speed of the rotation tub may be divided into a plurality of RPM ranges in the excessive vibration period of the water tub and the speed may be increased stepwise based on the plurality of RPM ranges. 
     The speed of the rotation tub may be maintained at a first speed within each of the RPM ranges and then increased to a second speed within each of the RPM ranges, the speed of the rotation tub being maintained at the first speed until a time in which the speed of the rotation tub is maintained at the first speed reaches a reference time necessary to remove the unbalance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and/or other aspects and advantages will become apparent and more readily appreciated from the following description of the embodiment, taken in conjunction with the accompanying drawings of which: 
         FIG. 1  is a cross-sectional view showing the configuration of a washing machine including balancers according to the present embodiment; 
         FIG. 2  is an exploded perspective view of a rotation tub according to the present embodiment; 
         FIG. 3  is a coupled perspective view of the rotation tub according to the present embodiment; 
         FIG. 4  is a block diagram showing a control configuration of the washing machine including the balancers according to the present embodiment; 
         FIG. 5  is a waveform diagram showing a generation of a vibration of a water tub due to a difference in rotation speed between the rotation tub and balls in the washing machine including the balancers according to the present embodiment; 
         FIG. 6  is a view showing a difference in rotation speed between the rotation tub and the balls in the washing machine including the balancers according to the present embodiment; 
         FIG. 7  is a graph showing a speed profile in a dehydrating mode of the washing machine including the balancers according to an embodiment of the present embodiment; 
         FIG. 8  is a table showing an excessive vibration controlling process stepwise in the washing machine including the balancers according to the present embodiment; 
         FIG. 9  is a flowchart illustrating the excessive vibration controlling process of the washing machine including the balancers according to the present embodiment; 
         FIG. 10  is a graph showing a relationship between the amount of unbalance and a speed in the excessive vibration controlling process of the washing machine including the balancers according to the present embodiment; 
         FIG. 11  is a graph showing a vibration value when the speed is increased at once in the excessive vibration period of the water tub; 
         FIG. 12  is a graph showing a vibration value when the speed is increased stepwise in the excessive vibration period of the water tub; 
         FIG. 13  is a graph showing a process capability obtained by repeatedly performing a vibration test in x and y axes 30 times and analyzing a maximum vibration value when the speed of the rotation tub is increased at once in the excessive vibration period of the water tub under a load condition of 80 percent of laundry; 
         FIG. 14  is a graph showing a process capability obtained by repeatedly performing a vibration test in the x and y axes 30 times and analyzing a maximum vibration value when the speed of the rotation tub is increased stepwise in the excessive vibration period of the water tub under the load condition of 80 percent of laundry. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Reference will now be made in detail to the embodiment, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiment is described below to explain the present invention by referring to the figures. 
       FIG. 1  is a cross-sectional view showing the configuration of a washing machine including balancers according to the present embodiment. 
     In  FIG. 1 , the washing machine including the balancers according to the present embodiment includes a water tub  20  which is installed in a housing  10  forming an external appearance of the washing machine and contains water therein, a rotation tub  30  which is rotatably installed in the water tub  20  and contains laundry therein, and a door  40  which is hinge-coupled to an open front surface of the housing  10 . 
     A water supplying valve  12  to supply water into the water tub  20  and a detergent supplying device  14  to supply a detergent into the water tub  20  are provided above the water tub  20 . A water draining pump  16  to drain water contained in the water tub  20  out of the housing  10  when an operation of washing the laundry is completed is provided below the water tub  20 . 
     A rotation shaft  51  is provided at a side of a rear surface of the rotation tub  30  to penetrate through a rear surface of the water tub  20 , and a motor  50  coupled to the rotation shaft  51  is provided outside the rear surface of the water tub  20 . Accordingly, when the motor  50  is operated, the rotation shaft  51  is rotated and thus the rotation tub  30  is rotated. 
     A plurality of dehydration holes  30   a  is formed in a circumferential surface of the rotation tub  30 . In a washing mode, water which is contained in the water tub  20  flows into the rotation tub  30  through the dehydration holes such that the laundry is washed by water containing a detergent therein. In a dehydrating mode, water is drained from the housing  10  through the water draining pump  16 . 
     A plurality of lifters  30   b  is provided in the rotation tub  30  in a vertical direction such that wet laundry is lifted up from a bottom of the rotation tub  30  and is lifted down to the bottom of the rotation tub  30  when the rotation tub  30  is rotated at a low speed in the washing mode. Thus, the laundry can be efficiently washed. 
     Accordingly, in the washing mode, the rotation tub  30  is rotated at a low speed while the rotation shaft  51  is alternately rotated forward and backward by the motor  50 , such that the laundry is washed. In the dehydrating mode, the rotation tub  30  is rotated at a high speed while the rotation shaft  51  is rotated in one direction, so that the laundry is dehydrated. 
     When the rotation tub  30  is rotated at the high speed in the dehydrating mode, if the center of gravity of the rotation tub  30  does not coincide with the center of rotation or the laundry is unevenly distributed in the rotation tub  30  such that an unbalance occurs in a specific portion, a force is biased toward the rotation shaft  51  of the rotation tub  30  and thus a dynamic balance of the rotation tub  30  is not maintained. 
     In order to prevent a dynamic unbalance such that the rotation tub  30  can be rotated at the high speed in a state in which the center of gravity of rotation tub  30  coincides with the center of rotation, balancers  60  are provided at a front end and a rear end of the rotation tub  30 . 
       FIG. 2  is an exploded perspective view of the rotation tub according to the present embodiment, and  FIG. 3  is a coupled perspective view of the rotation tub according to the present embodiment. 
     In  FIG. 2 , a front surface and a rear surface of the rotation tub  30  are opened. The rotation tub  30  includes a cylindrical main body  31  including the dehydration holes  30   a  and the lifters  30   b , a front side member  32  which is coupled to the opened front surface of the main body  31  and has an opening  34  through which the laundry is put into the main body  31  and is taken out from the main body  31 , and a rear side member  33  which is coupled to the opened rear surface of the main body  31  and receives the rotation shaft  51  to rotate the rotation tub  30 . 
     An annular recess  35  which has a substantially U-shape in a cross section and is opened toward a front side of the washing machine is formed in the circumference of the front side member  32  to contain a balancer  60  therein. An annular recess (not shown) which is opened toward a rear side of the washing machine is formed in the circumference of the rear side member  33  to contain another balancer  60  therein. 
     The front side member  32  and the rear side member  33  are respectively fitted into the front circumference and the rear circumference of the main body  31  by a screwing method or other fixing method, as shown in  FIG. 3 . 
     The balancers  60  are mounted in the recesses  35  of the front side member  32  and the rear side member  33 . Each of the balancers  60  is an annular single race and includes a plurality of balls  61  which is made of steel, for example, and has a balancing function and viscous fluid (not shown) to adjust the movement speed of the plurality of balls  61 . 
     The balls  61  are mounted to be moved in a circumferential direction. When the dynamic unbalance occurs in the rotation tub  30 , the balls  61  are moved in the circumferential direction to a position which is symmetrical to a position at which the dynamic unbalance occurs. Thus, the vibration of the rotation tub  30  can be reduced. 
       FIG. 4  is a block diagram showing the control configuration of the washing machine including the balancers according to the present embodiment. 
     In  FIG. 4 , the washing machine according to the present embodiment includes an input unit  100  to allow a user to input an operation command including the setting of the dehydrating mode, a control unit  102  controlling the whole operation of the washing machine such as a washing mode, a rinsing mode and a dehydrating mode, a motor driving unit  104  driving the motor  50  to rotate the rotation tub  30  under the control of the control unit  102 , a speed detecting unit  106  to send a motor speed signal corresponding to the rotation speed of the rotation tub  30  to the control unit  102 , and a current detecting unit  108  to send a motor current signal corresponding to the rotation speed of the rotation tub  30  to the control unit  102 . 
     The washing machine according to the present embodiment further includes a vibration detecting unit  110  to detect the vibration in X and Y axes. The vibration detecting unit  110  detects the vibration of the water tub  20  which is generated before the balls  61  reach a balancing position in the washing machine including the balancers  60 , thereby obtaining a signal waveform of a vibration frequency shown in  FIG. 5 . 
       FIG. 5  shows a signal waveform of the vibration frequency which is generated due to a modulation phenomenon due to a difference between the rotation speed (RPM 1 ) of the rotation tub  30  and the rotation speed (RPM 2 ) of the balls  61  shown in  FIG. 6 . 
     The control unit  102  performs the dehydrating mode with a speed profile shown in  FIG. 7  in order to dehydrate the laundry at a high speed without an excessive vibration of the water tub  20 . 
       FIG. 7  is a graph showing a speed profile of the washing machine including the balancers  60  according to the present embodiment at the time of the dehydrating mode. 
     In  FIG. 7 , the dehydrating mode includes a laundry amount detecting process  1  of detecting the weight of the laundry at the time of starting of the dehydrating mode, a laundry disentangling process  2  of reversing the left and the right of the rotation tub  30  to disentangle the laundry, a laundry rolling process  3  of increasing the speed of the rotation tub  30  at a predetermined speed to stick the laundry to the inner wall of the rotation tub  30 , an unbalance detecting process  4  of detecting the amount of unbalance using a control parameter such as the weight of the laundry and the current of the motor  50 , an excessive vibration controlling process  5  of increasing the speed of the rotation tub  30  stepwise when the amount of unbalance detected in an excessive vibration period of the water tub  20  is less than a restriction value, and a high-speed dehydrating process  6  rotating the rotation tub  30  at a high speed and draining water contained in the laundry by a centrifugal force after increasing the speed of the rotation tub  30  stepwise and passing the excessive vibration period of the water tub  20  without the vibration. 
     The excessive vibration period of the water tub  20  indicates a period in which the speed of the rotation tub  30  is 160 to 300 RPM, for example. When the speed of the rotation tub  30  is in a range from 160 to 300 RPM, a mechanical resonance point exists and a large amount of water contained in the laundry is drained. Thus, the unbalance may occur. In addition, a phenomenon that the balls  61  are dispersed also occurs and a probability that the excessive vibration of the water tub  20  occurs is high. However, it is difficult to expect the phenomenon and the probability. 
     Accordingly, in the excessive vibration controlling process  5  of the present embodiment, the speed of the rotation tub  30  is not increased at once in the excessive vibration period of 160 to 300 RPM. That is, as shown in  FIG. 8 , the excessive vibration controlling process  5  includes a first step  5 - 1  of maintaining a start point, that is, 160 RPM, of the excessive vibration period when the speed of the rotation tub  30  is increased after detecting the unbalance, a second step  5 - 2  of increasing the speed from 160 RPM to 210 RPM, a third step  5 - 3  of maintaining the speed of 210 RPM for approximately 10 seconds to prevent the balls  61  from being rapidly moved and positioned opposite the unbalance of the rotation tub  30 , a fourth step  5 - 4  of increasing the speed from 210 RPM to 260 RPM, a fifth step  5 - 5  of maintaining the speed of 260 RPM for approximately 10 seconds to prevent the balls from being rapidly moved and positioned opposite the unbalance of the rotation tub  30 , and a sixth step of  5 - 6  of increasing the speed from 260 RPM to 300 RPM, thereby increasing the speed of the rotation tub  30  stepwise such that the amount of unbalance is not rapidly changed. While the speed of the rotation tub  30  is increased stepwise, the amount of unbalance is detected in real time. If the amount of unbalance is less than or equal to a restriction limit, the speed of the rotation tub  30  is increased by the six steps such that the rapid movement of the balls  61  is prevented to pass the excessive vibration period of the water tub  20  without the vibration. 
     In the excessive vibration controlling process  5  of the present embodiment, the amount of unbalance is always detected in real time and the rotation tub  30  is stopped when the amount of unbalance is greater than the restriction value. 
     Hereinafter, the operation and effect of the washing machine and the method of controlling the same will be described. 
       FIG. 9  is a flowchart illustrating the excessive vibration controlling process of the washing machine including the balancers according to the present embodiment, that is, a method of dehydrating the laundry at a high speed while passing an excessive vibration period of the water tub  20  without the vibration in the dehydrating mode. 
     When a user puts the laundry W into the rotation tub  30  and inputs an operation command including the setting of the dehydrating mode through the input unit  100 , the control unit  102  performs the series of operations including the washing mode, the rinsing mode and the dehydrating mode. 
     Accordingly, the control unit  102  determines whether the mode becomes the dehydrating mode ( 200 ). If it is determined that the mode becomes the dehydrating mode, the laundry amount detecting process  1  of detecting the weight of the laundry W is performed as shown in  FIG. 7  ( 202 ), in order to use the weight of the laundry W as basic information to detect the amount of unbalance or determine an allowable amount of unbalance before the high-speed dehydrating process. 
     After the laundry amount detecting process  1 , the control unit  102  performs the laundry disentangling process  2  of controlling the driving of the motor  50  through the motor driving unit  104  and reversing to the left and the right of the rotation tub  30  to disentangle the laundry W as shown in  FIG. 7  ( 204 ). 
     After the laundry disentangling process  2 , the control unit  102  performs the laundry rolling process  3  of increasing the speed of the rotation tub  30  to the predetermined speed and sticking the laundry W to the inner wall of the rotation tub  30  as shown in  FIG. 7  ( 206 ). 
     After the laundry rolling process  3 , the control unit  102  performs the unbalance detecting process  4  of detecting the amount of unbalance using the control parameter such as the weight of the laundry W and the current of the motor  50  as shown in  FIG. 7  ( 208 ). 
     The processes from the laundry amount detecting process  1  to the unbalance detecting process  4  correspond to a general process of reducing the unbalance in order to make the balance of the laundry W uniform before the high-speed dehydrating process of the washing machine and thus the detailed description thereof will be omitted. 
     Thereafter, the control unit  102  determines whether the amount of unbalance detected in the unbalance detecting process  4  is equal to or less than a predetermined first restriction value ( 210 ). If it is determined that the detected amount of unbalance is greater than the first restriction value, the rotation tub  30  is stopped and the process returns to operation  202 . 
     If the detected amount of unbalance is equal to or less than the first restriction value in operation  210 , the excessive vibration controlling process  5  of increasing the speed of the rotation tub  30  stepwise to pass the excessive vibration period of the water tub  20  without the vibration is performed as shown in  FIGS. 7 and 8  ( 212 ). 
     Since the mechanical resonance point exists and a large amount of water contained in the laundry W is drained in the excessive vibration period of 160 to 300 RPM, a probability that the unbalance occurs is high. Accordingly, as shown in  FIG. 8 , in the excessive vibration controlling process  5  of the present embodiment, the speed of the rotation tub  30  is not increased at once. That is, the speed of the rotation tub  30  is increased stepwise by the first step  5 - 1  to the sixth step  5 - 6  such that the amount of unbalance is not rapidly changed. Thus, the balls  61  are prevented from being rapidly moved to pass the excessive vibration period of the water tub  20  without the vibration. 
     During the excessive vibration controlling process  5  of increasing the speed of the rotation tub  30  stepwise, the amount of unbalance is always detected in real time and it is determined whether the amount of unbalance is equal to or less than a predetermined second restriction value ( 214 ). It is determined that the amount of unbalance is greater than the second restriction value, the rotation tub  30  is stopped and the process returns to operation  202 . 
     If the detected amount of unbalance is equal to or less than the second restriction value in operation  214 , the high speed dehydrating process  6  of rotating the rotation tub  30  at a high speed and draining water contained in the laundry by a centrifugal force after increasing the speed of the rotation tub  30  stepwise and passing the excessive vibration period of the water tub  20  without the vibration is performed as shown in  FIGS. 7 and 8  ( 216 ). 
     The first restriction value and the second restriction value to determine the unbalance state of the laundry W are different from each other. The restriction values to determine the unbalance state in the processes  3  to  6  are different from each other because the unbalance degrees of the processes  3  to  6  which are performed according to the speed profile shown in  FIG. 7  are different from one another. 
     The present embodiment will now be described in detail with reference to  FIG. 10 . 
       FIG. 10  is a graph showing a relationship between the amount of unbalance and the speed when the excessive vibration controlling process  5  is performed in the washing machine including the balancers according to the present embodiment. 
     In  FIG. 10 , a thin solid line represents the rotation speed (RPM) of the rotation tub  30  which is increased stepwise according to the speed profile shown in  FIG. 7 , a thick solid line represents an actual amount of unbalance which occurs while the speed of the rotation tub  30  is increased stepwise according to the speed profile shown in  FIG. 7 , and a dotted line represents an unbalance restriction value to determine the unbalance state while the speed of the rotation tub  30  is increased stepwise according to the speed profile shown in  FIG. 7 . 
     As shown in  FIG. 10 , the unbalance restriction values of the processes are set to be different from one another. If the speed of the rotation tub  30  is increased stepwise in the excessive vibration period of the water tub  20  of 160 RPM to 300 RPM, the amount of unbalance is increased and is then decreased as denoted by a circle of “U”. This is because a time to remove a new unbalance is allowed if a predetermined rotation speed (RPM) is maintained. 
     In the washing machine including the balancers according to the present embodiment, the speed of the rotation tub  30  is increased stepwise in the period (about 160 to 300 RPM) in which the excessive vibration of the water tub  20  occurs, such that the balls  61  are prevented from being rapidly moved to pass the excessive vibration period of the water tub  20 . 
       FIG. 11  is a graph showing a vibration value (mm) when the speed of the rotation tub  30  is increased at once in the excessive vibration period of the water tub  20  and  FIG. 12  is a graph showing a vibration value (mm) when the speed of the rotation tub  30  is increased stepwise in the excessive vibration period of the water tub  20 . 
     As shown in  FIGS. 11 and 12 , it can be seen that a maximum vibration value is 15 mm when the speed of the rotation tub  30  is increased at once in the excessive vibration period of the water tub  20 , but is 7.7 mm when the speed of the rotation tub  30  is increased stepwise in the excessive vibration period of the water tub  20 . 
       FIG. 13  is a graph showing a process capability obtained by repeatedly performing a vibration test in the x and y axes 30 times and analyzing the maximum vibration value when the speed of the rotation tub  30  is increased at once in the excessive vibration period of the water tub  20  under the load condition of 80 percent of laundry and  FIG. 14  is a graph showing a process capability obtained by repeatedly performing a vibration test in the x and y axes 30 times and analyzing the maximum vibration value when the speed of the rotation tub  30  is increased stepwise in the excessive vibration period of the water tub  20  under the load condition of 80 percent of laundry.  FIGS. 13A and 14A  show the vibration in the x axis and  FIGS. 13B and 14B  show the vibration in the y axis. 
     As shown in  FIGS. 13 and 14 , it can be seen that an upper limit of the vibration when the speed of the rotation tub is increased stepwise is smaller than that of the vibration when the speed of the rotation tub is increased at once in the excessive vibration period of the water tub  20 . 
     As described above, according to a washing machine and a method of controlling the same according to the present embodiment, since the speed of a rotation tub is increased in an excessive vibration period of a water tub in the washing machine including balancers, it is possible to pass the excessive vibration period of the water tub without the vibration. 
     In addition, since the unbalance state is detected in real time while the speed is increased stepwise and the speed is increased when the amount of unbalance is less than a restriction value, it is possible to prevent the rapid movement of balls and to remove the excessive vibration of the water tub with certainty. 
     Although an embodiment has been shown and described, it would be appreciated by those skilled in the art that changes may be made in this embodiment without departing from the principles and spirit of the invention, the scope of which is defined in the claims and their equivalents.