Patent Publication Number: US-11028517-B2

Title: Washing machine appliance and methods for out-of-balance detection and mitigation

Description:
FIELD OF THE INVENTION 
     The present subject matter relates generally to washing machine appliances, such as vertical axis washing machine appliances, and methods for detecting and mitigating out-of-balance conditions in such washing machine appliances. 
     BACKGROUND OF THE INVENTION 
     Washing machine appliances generally include a cabinet which receives a wash tub for containing water or wash fluid (e.g., water and detergent, bleach, or other wash additives). The wash tub may be suspended within the cabinet by a suspension system to allow some movement relative to the cabinet during operation. A wash basket is rotatably mounted within the wash tub and defines a wash chamber for receipt of articles for washing. A drive assembly is coupled to the wash tub and configured to selectively rotate the wash basket within the wash tub. 
     Washing machine appliances are typically equipped to operate in one or more modes or cycles, such as wash, rinse, and spin cycles. For example, during a wash or rinse cycle, the wash fluid is directed into the wash tub in order to wash and/or rinse articles within the wash chamber. In addition, the wash basket and/or an agitation element can rotate at various speeds to agitate or impart motion to articles within the wash chamber. During a spin cycle, the wash basket may be rotated at high speeds, e.g., to wring wash fluid from articles within the wash chamber. 
     A significant concern during operation of washing machine appliances is out-of-balance conditions within the wash tub. For example, articles and water loaded within a wash basket may not be equally weighted about a central axis of the wash basket and wash tub. Accordingly, when the wash basket rotates, in particular during a spin cycle, the imbalance in clothing weight may cause the wash basket to be out-of-balance within the wash tub, such that the axis of rotation does not align with the axis of the cylindrical wash basket or wash tub. Such out-of-balance issues can cause the wash basket to contact the wash tub during rotation and can further cause movement of the wash tub within the cabinet. Significant movement of the wash tub can, in turn, generate increased noise, vibrations, washer “walking,” and/or cause excessive wear and premature failure of appliance components. 
     Various methods are known for monitoring load balances and preventing out-of-balance scenarios within washing machine appliances. Such monitoring and prevention may be especially important, for instance, during the high-speed rotation of the wash basket, e.g., during a spin cycle. However, such methods typically monitor load balance and detect out-of-balance states during the spin cycle, when the wash basket is already spinning at a high rate of speed. Accordingly, noise, vibration, movement, or damage may occur despite the out-of-balance detection. 
     Accordingly, improved methods and apparatus for monitoring load balance in washing machine appliances are desired. In particular, methods and apparatus which provide accurate monitoring and detection at earlier times during the wash cycle would be advantageous. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be apparent from the description, or may be learned through practice of the invention. 
     In one exemplary embodiment, a method for operating a washing machine appliance is provided. The washing machine appliance includes a wash tub, a wash basket rotatably mounted within the wash tub for receiving a load of articles, an agitation element rotatably mounted within the wash basket, a measurement device mounted to the wash tub, and a motor mechanically coupled to the wash basket and the agitation element. The method includes flowing a volume of water into the wash tub, obtaining a subwasher load weight, and obtaining a displacement threshold based at least in part on the subwasher load weight. The method further includes operating the motor to spin the wash basket at a first speed, measuring a displacement amplitude of the wash tub using the measurement device, and determining that the displacement amplitude exceeds the displacement threshold. The method then includes operating the motor to rotate the agitation element to agitate or redistribute the load of articles in response to determining that the displacement amplitude exceeds the displacement threshold. 
     In another exemplary embodiment, a washing machine appliance is provided. The washing machine appliance includes a wash tub positioned within a cabinet, a wash basket rotatably mounted within the wash tub, the wash basket defining a wash chamber for receipt of a load of articles for washing, and an agitation element positioned in the wash basket. A motor is in mechanical communication with the wash basket and the agitation element, the motor being configured for selectively rotating the wash basket and the agitation element within the wash tub. A measurement device is mounted to the wash tub and a water control valve regulates a flow of water from a water supply into the wash tub. A controller is in operative communication with the motor, the measurement device, and the water control valve, and is configured for regulating the water control valve to flow a volume of water into the wash tub, obtaining a subwasher load weight, and obtaining a displacement threshold based at least in part on the subwasher load weight. The controller then operates the motor to spin the wash basket at a first speed, measures a displacement amplitude of the wash tub using the measurement device, and determines that the displacement amplitude exceeds the displacement threshold. The controller then operates the motor to rotate the agitation element and agitate or redistribute the load of articles in response to determining that the displacement amplitude exceeds the displacement threshold. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures. 
         FIG. 1  provides a perspective view of a washing machine appliance according to an exemplary embodiment of the present subject matter with a door of the exemplary washing machine appliance shown in a closed position. 
         FIG. 2  provides a perspective view of the exemplary washing machine appliance of  FIG. 1  with the door of the exemplary washing machine appliance shown in an open position. 
         FIG. 3  provides a side, cross sectional view of the exemplary washing machine appliance of  FIG. 1  according to an exemplary embodiment of the present subject matter. 
         FIG. 4  provides a schematic, front view of the exemplary washing machine appliance of  FIG. 1  according to example embodiments of the present subject matter. 
         FIG. 5  depicts certain components of a controller according to example embodiments of the present subject matter. 
         FIG. 6  illustrates a method for controlling a washing machine appliance in accordance with one embodiment of the present disclosure. 
         FIG. 7  provides a plot illustrating the relationship between a tub displacement, a subwasher weight, and a displacement threshold for the exemplary washing machine appliance of  FIG. 1 . 
         FIG. 8  illustrates an exemplary decision tree or flow diagram of an operating method of the washing machine appliance of  FIG. 1  according to an exemplary embodiment of the present subject matter. 
     
    
    
     Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present invention. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, 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 scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
       FIGS. 1 through 4  illustrate an exemplary embodiment of a vertical axis washing machine appliance  100 . Specifically,  FIGS. 1 and 2  illustrate perspective views of washing machine appliance  100  in a closed and an open position, respectively.  FIGS. 3 and 4  provide side and front cross-sectional views of washing machine appliance  100 , respectively. Washing machine appliance  100  generally defines a vertical direction V, a lateral direction L, and a transverse direction T, each of which is mutually perpendicular, such that an orthogonal coordinate system is generally defined. 
     While described in the context of a specific embodiment of vertical axis washing machine appliance  100 , it should be appreciated that vertical axis washing machine appliance  100  is provided by way of example only. It will be understood that aspects of the present subject matter may be used in any other suitable washing machine appliance, such as a horizontal axis washing machine appliance. Indeed, modifications and variations may be made to washing machine appliance  100 , including different configurations, different appearances, and/or different features while remaining within the scope of the present subject matter. 
     Washing machine appliance  100  has a cabinet  102  that extends between a top portion  104  and a bottom portion  106  along the vertical direction V. As best shown in  FIG. 3 , a wash tub  108  is positioned within cabinet  102  and is generally configured for retaining wash fluids during an operating cycle. Washing machine appliance  100  further includes a primary dispenser  110  ( FIG. 2 ) for dispensing wash fluid into wash tub  108 . The term “wash fluid” refers to a liquid used for washing and/or rinsing articles during an operating cycle and may include any combination of water, detergent, fabric softener, bleach, and other wash additives or treatments. 
     In addition, washing machine appliance  100  includes a wash basket  112  that is positioned within wash tub  108  and generally defines a wash chamber  114  including an opening  116  for receipt of articles for washing. More specifically, wash basket  112  is rotatably mounted within wash tub  108  such that it is rotatable about an axis of rotation A. According to the illustrated embodiment, the axis of rotation A is substantially parallel to the vertical direction V. In this regard, washing machine appliance  100  is generally referred to as a “vertical axis” or “top load” washing machine appliance  100 . However, it should be appreciated that aspects of the present subject matter may be used within the context of a horizontal axis or front load washing machine appliance as well. As used herein, terms of approximation, such as “approximately,” “substantially,” or “about,” refer to being within a ten percent margin of error. 
     As illustrated, cabinet  102  of washing machine appliance  100  has a top panel  118 . Top panel  118  defines an opening ( FIG. 2 ) that coincides with opening  116  of wash basket  112  to permit a user access to wash basket  112 . Washing machine appliance  100  further includes a door  120  which is rotatably mounted to top panel  118  to permit selective access to opening  116 . In particular, door  120  selectively rotates between the closed position (as shown in  FIGS. 1 and 3 ) and the open position (as shown in  FIG. 2 ). In the closed position, door  120  inhibits access to wash basket  112 . Conversely, in the open position, a user can access wash basket  112 . A window  122  in door  120  permits viewing of wash basket  112  when door  120  is in the closed position, e.g., during operation of washing machine appliance  100 . Door  120  also includes a handle  124  that, e.g., a user may pull and/or lift when opening and closing door  120 . Further, although door  120  is illustrated as mounted to top panel  118 , door  120  may alternatively be mounted to cabinet  102  or any other suitable support. 
     As best shown in  FIGS. 2 and 3 , wash basket  112  further defines a plurality of perforations  126  to facilitate fluid communication between an interior of wash basket  112  and wash tub  108 . In this regard, wash basket  112  is spaced apart from wash tub  108  to define a space for wash fluid to escape wash chamber  114 . During a spin cycle, wash fluid within articles of clothing and within wash chamber  114  is urged through perforations  126  wherein it may collect in a sump  128  defined by wash tub  108 . Washing machine appliance  100  further includes a pump assembly  130  ( FIG. 3 ) that is located beneath wash tub  108  and wash basket  112  for gravity assisted flow when draining wash tub  108 . 
     An impeller or agitation element  132  ( FIG. 3 ), such as a vane agitator, impeller, auger, oscillatory basket mechanism, or some combination thereof is disposed in wash basket  112  to impart an oscillatory motion to articles and liquid in wash basket  112 . More specifically, agitation element  132  extends into wash basket and assists agitation of articles disposed within wash basket  112  during operation of washing machine appliance  100 , e.g., to facilitate improved cleaning. In different embodiments, agitation element  132  includes a single action element (i.e., oscillatory only), a double action element (oscillatory movement at one end, single direction rotation at the other end) or a triple action element (oscillatory movement plus single direction rotation at one end, single direction rotation at the other end). As illustrated in  FIG. 3 , agitation element  132  and wash basket  112  are oriented to rotate about axis of rotation A (which is substantially parallel to vertical direction V). 
     As best illustrated in  FIGS. 3 and 4 , washing machine appliance  100  includes a drive assembly  138  in mechanical communication with wash basket  112  to selectively rotate wash basket  112  (e.g., during an agitation or a rinse cycle of washing machine appliance  100 ). In addition, drive assembly  138  may also be in mechanical communication with agitation element  132 . In this manner, drive assembly  138  may be configured for selectively rotating or oscillating wash basket  112  and/or agitation element  132  during various operating cycles of washing machine appliance  100 . 
     More specifically, drive assembly  138  may generally include one or more of a drive motor  140  and a transmission assembly  142 , e.g., such as a clutch assembly, for engaging and disengaging wash basket  112  and/or agitation element  132 . According to the illustrated embodiment, drive motor  140  is a brushless DC electric motor, e.g., a pancake motor. However, according to alternative embodiments, drive motor  140  may be any other suitable type or configuration of motor. For example, drive motor  140  may be an AC motor, an induction motor, a permanent magnet synchronous motor, or any other suitable type of motor. In addition, drive assembly  138  may include any other suitable number, types, and configurations of support bearings or drive mechanisms. 
     Turning briefly to  FIG. 4 , washing machine appliance  100  may include a vibration damping system or suspension system  144  which generally operates to damp or reduce dynamic motion and absorb vibrations of a subwasher  146 . As used herein, the term “subwasher” is used generally to refer to those components of a washing machine appliance suspended within the appliance cabinet by a suspension system or assembly. For example, according to the illustrated embodiment, a subwasher  146  is suspended within cabinet  102  by suspension system  144  and includes wash tub  108 , wash basket  112 , agitation element  132 , drive assembly  138 , and other components. 
     Suspension system  144  can include one or more suspension springs  148  for supporting subwasher  146  and absorbing the forces resulting from the movement of wash basket  112  within the tub  108 . Specifically, according to an exemplary embodiment, suspension system  144  includes four suspension springs  148  which are spaced apart about the wash tub  108 . For example, each suspension springs  148  may be connected at one end proximate a corner of cabinet  102  and at an opposite end to wash tub  108 . 
     According to alternative embodiments, washing machine appliance  100  may further include other vibration dampening elements, such as balance rings positioned at around the upper and/or lower circumferential surfaces of the wash basket  112 . Balance rings may be used to counterbalance an out-of-balance condition for washing machine appliance  100  as wash basket  112  rotates within wash tub  108 . 
     Referring still to  FIGS. 1 through 4 , a control panel  150  with at least one input selector  152  ( FIG. 1 ) extends from top panel  118 . Control panel  150  and input selector  152  collectively form a user interface input for operator selection of machine cycles and features. A display  154  of control panel  150  indicates selected features, operation mode, a countdown timer, and/or other items of interest to appliance users regarding operation. 
     Operation of washing machine appliance  100  is controlled by a controller or processing device  156  that is operatively coupled to control panel  150  for user manipulation to select washing machine cycles and features. In response to user manipulation of control panel  150 , controller  156  operates the various components of washing machine appliance  100  to execute selected machine cycles and features. As described in more detail below with respect to  FIG. 5 , controller  156  may include a memory and microprocessor, such as a general or special purpose microprocessor operable to execute programming instructions or micro-control code associated with methods described herein. Alternatively, controller  156  may be constructed without using a microprocessor, e.g., using a combination of discrete analog and/or digital logic circuitry (such as switches, amplifiers, integrators, comparators, flip-flops, AND gates, and the like) to perform control functionality instead of relying upon software. Control panel  150  and other components of washing machine appliance  100  may be in communication with controller  156  via one or more signal lines or shared communication busses. 
       FIG. 5  depicts certain components of controller  156  according to example embodiments of the present disclosure. Controller  156  can include one or more computing device(s)  156 A which may be used to implement methods as described herein. Computing device(s)  156 A can include one or more processor(s)  156 B and one or more memory device(s)  156 C. The one or more processor(s)  156 B can include any suitable processing device, such as a microprocessor, microcontroller, integrated circuit, an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field-programmable gate array (FPGA), logic device, one or more central processing units (CPUs), graphics processing units (GPUs) (e.g., dedicated to efficiently rendering images), processing units performing other specialized calculations, etc. The memory device(s)  156 C can include one or more non-transitory computer-readable storage medium(s), such as RAM, ROM, EEPROM, EPROM, flash memory devices, magnetic disks, etc., and/or combinations thereof. 
     The memory device(s)  156 C can include one or more computer-readable media and can store information accessible by the one or more processor(s)  156 B, including instructions  156 D that can be executed by the one or more processor(s)  156 B. For instance, the memory device(s)  156 C can store instructions  156 D for running one or more software applications, displaying a user interface, receiving user input, processing user input, etc. In some implementations, the instructions  156 D can be executed by the one or more processor(s)  156 B to cause the one or more processor(s)  156 B to perform operations, e.g., such as one or more portions of methods described herein. The instructions  156 D can be software written in any suitable programming language or can be implemented in hardware. Additionally, and/or alternatively, the instructions  156 D can be executed in logically and/or virtually separate threads on processor(s)  156 B. 
     The one or more memory device(s)  156 C can also store data  156 E that can be retrieved, manipulated, created, or stored by the one or more processor(s)  156 B. The data  156 E can include, for instance, data to facilitate performance of methods described herein. The data  156 E can be stored in one or more database(s). The one or more database(s) can be connected to controller  156  by a high bandwidth LAN or WAN, or can also be connected to controller through network(s) (not shown). The one or more database(s) can be split up so that they are located in multiple locales. In some implementations, the data  156 E can be received from another device. 
     The computing device(s)  156 A can also include a communication module or interface  156 F used to communicate with one or more other component(s) of controller  156  or washing machine appliance  100  over the network(s). The communication interface  156 F can include any suitable components for interfacing with one or more network(s), including for example, transmitters, receivers, ports, controllers, antennas, or other suitable components. 
     During operation of washing machine appliance  100 , laundry items are loaded into wash basket  112  through opening  116 , and washing operation is initiated through operator manipulation of input selectors  152 . Wash basket  112  is filled with water and detergent and/or other fluid additives via primary dispenser  110 . One or more valves can be controlled by washing machine appliance  100  to provide for filling wash tub  108  and wash basket  112  to the appropriate level for the amount of articles being washed and/or rinsed. By way of example for a wash mode, once wash basket  112  is properly filled with fluid, the contents of wash basket  112  can be agitated (e.g., with agitation element  132  as discussed previously) for washing of laundry items in wash basket  112 . 
     More specifically, referring again to  FIGS. 3 and 4 , a water fill process will be described according to an exemplary embodiment. As illustrated, washing machine appliance  100  includes a water supply conduit  160  that provides fluid communication between a water supply source  162  (such as a municipal water supply) and a discharge nozzle  164  for directing a flow of water into wash chamber  114 . In addition, washing machine appliance  100  includes a water fill valve or water control valve  166  which is operably coupled to water supply conduit  160  and communicatively coupled to controller  156 . In this manner, controller  156  may regulate the operation of water control valve  166  to regulate the amount of water within wash tub  108 . In addition, washing machine appliance  100  may include one or more pressure sensors  170  for detecting the amount of water and or clothes within wash tub  108 . For example, pressure sensor  170  may be operably coupled to a side of tub  108  for detecting the weight of wash tub  108 , which controller  156  may use to determine a volume of water in wash chamber  114  and a subwasher load weight, as described below. 
     After wash tub  108  is filled and the agitation phase of the wash cycle is completed, wash basket  112  can be drained, e.g., by drain pump assembly  130 . Laundry articles can then be rinsed by again adding fluid to wash basket  112  depending on the specifics of the cleaning cycle selected by a user. The impeller or agitation element  132  may again provide agitation within wash basket  112 . One or more spin cycles may also be used as part of the cleaning process. In particular, a spin cycle may be applied after the wash cycle and/or after the rinse cycle in order to wring wash fluid from the articles being washed. During a spin cycle, wash basket  112  is rotated at relatively high speeds to help wring fluid from the laundry articles through perforations  126 . After articles disposed in wash basket  112  are cleaned and/or washed, the user can remove the articles from wash basket  112 , e.g., by reaching into wash basket  112  through opening  116 . 
     Referring still to  FIGS. 3 and 4 , one or more measurement devices  172  may be provided in the washing machine appliance  100  for measuring movement of wash tub  108 , in particular during rotation of wash basket  112  prior to the spin cycle. Specifically, for example, movement of wash tub  108  may be measured as one or more displacement readings, e.g., certain displacement amplitudes measured at the center of gravity of wash tub  108 . Measurement devices  172  may measure a variety of suitable variables that can be correlated to movement of wash tub  108 . The movement measured by such devices  172  can be utilized to, e.g., monitor the load balance state of wash tub  108 , determine the displacement amplitudes of wash tub  108  at the center of gravity or other locations, and to adjust operation of washing machine appliance  100  to facilitate agitation in a particular manner and/or for particular time periods to adjust the load balance state, e.g., to attempt to balance articles within wash basket  112 . 
     A measurement device  172  in accordance with the present disclosure may include an accelerometer which measures translational motion, such as acceleration along one or more directions. Additionally or alternatively, a measurement device  172  may include a gyroscope, which measures rotational motion, such as rotational velocity about an axis. Moreover, according to exemplary embodiments, a measurement device  172  may include more than one gyroscope and/or more than one accelerometer. 
     Control panel  150  and other components of washing machine appliance  100 , such as drive assembly  138  and measurement device  172 , may be in communication with controller  156  via one or more signal lines or shared communication busses. Optionally, measurement device  172  may be included with controller  156  or may alternatively be a printed circuit board that includes the gyroscope and accelerometer thereon. According to exemplary embodiments, measurement devices  172  may include a dedicated microprocessor that performs the calculations specific to the measurement of motion with the calculation results being used by controller  156 . 
     According to the illustrated embodiment, measurement device  172  is mounted to wash tub  108  to sense movement of wash tub  108  relative to the cabinet  102 , e.g., by measuring uniform periodic motion, non-uniform periodic motion, or excursions of the tub  108  during appliance  100  operation. For instance, movement may be measured as discrete identifiable components (e.g., in a predetermined direction). More specifically, according to the illustrated embodiment, measurement device  172  is mounted to a bottom wall of wash tub  108 , though other suitable positions on subwasher  146  are possible. Controller  156  may use measurement device  172  to determine the movement of any other position on wash tub  108 , such as the center of gravity of wash tub  108 . However, it should be appreciated that according to alternative embodiments, any suitable number, type, and position of measurement devices may be used. 
     The measurement device  172  may be mounted to wash tub  108  (e.g., via a suitable mechanical fastener, adhesive, etc.) and may be oriented such that the various sub-components (e.g., the gyroscope and accelerometer) are oriented to measure movement along or about particular directions as discussed herein. Notably, the gyroscope and accelerometer in exemplary embodiments are advantageously mounted to wash tub  108  at a single location (e.g., the location of the printed circuit board or other component of the measurement device  172  on which the gyroscope and accelerometer are grouped). Such positioning at a single location advantageously reduces the costs and complexity (e.g., due to additional wiring, etc.) of out-of-balance detection, while still providing relatively accurate out-of-balance detection as discussed herein. Alternatively, however, the gyroscope and accelerometer need not be mounted at a single location. For example, a gyroscope located at one location on wash tub  108  can measure the rotation of a gyroscope located at a different location on tub  108 , because rotation about a given axis is the same everywhere on a solid object such as wash tub  108 . 
     Now that the construction of washing machine appliance  100  and the configuration of controller  156  according to exemplary embodiments have been presented, an exemplary method  200  of operating a washing machine appliance will be described. Although the discussion below refers to the exemplary method  200  of operating washing machine appliance  100 , one skilled in the art will appreciate that the exemplary method  200  is applicable to the operation of a variety of other washing machine appliances, such as horizontal axis washing machine appliances. In exemplary embodiments, the various method steps as disclosed herein may be performed by controller  156  or a separate, dedicated controller. 
     Referring now to  FIG. 6 , method  200  includes, at step  210 , flowing a volume of water into a wash tub. In this regard, for example, controller  156  may regulate water control valve  166  to dispense a predetermined amount of water from water supply source  162  into wash tub  108  through discharge nozzle  164 . According to an exemplary embodiment, step  210  is performed after wash tub  108  has been drained, but prior to a spin cycle of washing machine appliance  100 . Specifically, for example, water from an initial agitation or wash cycle could be used to perform the out-of-balance detection, e.g., to prevent the need for filling up wash tub  108  again. In this manner, an out of balance detection procedure may be performed prior to spinning wash basket  112  at high speeds during the spin cycle, thereby reducing the likelihood of excessive tub displacement, vibrations, noise, and impact. 
     Step  220  includes determining a subwasher load weight. As explained above, subwasher  146  is used generally herein to refer to the load suspended by suspension system  144  within cabinet  102 . Thus, the subwasher load weight measured at step  220  generally includes the weight of the load of dry clothes and the water added to wash tub  108 . In determining the subwasher load weight, the weight of wash tub  108 , drive assembly  138 , wash basket  112 , agitation element  132 , and other fixed component weights are not included in the subwasher load weight. 
     According to an exemplary embodiment, obtaining the subwasher load weight includes determining a dry weight of the load of articles within wash basket  112  before flowing water into wash tub  108 . The dry weight may be determined in any suitable manner, e.g., by accelerating wash basket  112  and measuring a motor current, a motor voltage, and/or a time to reach a specific speed. Other methods for determining the dry weight of the load of articles in wash basket  112  are known and contemplated as within the scope of the present subject matter. 
     After determining the dry weight of the load of articles the predetermined volume of water is added to the wash tub (step  210 ) and its weight is determined. More specifically, according to an exemplary embodiment, the known volume (e.g., in gallons) of water is multiplied by conversion factor (e.g., in pounds per gallon) to determine the weight of water, e.g., in pounds. The total subwasher load weight, referred to herein as W SW , is the sum of the dry weight of the load of articles and the weight of the added water. 
     Step  230  includes obtaining a displacement threshold (δ LMT ) based at least in part on the subwasher load weight (W SW ). In general, the displacement threshold (δ LMT ) may represent a limit in the amount of displacement permissible at the center of gravity of wash tub  108  to prevent undesirable vibrations, noise, or tub contact during a spin cycle of washing machine appliance  100 . Although the actual displacements and the displacement threshold (δ LMT ) of wash tub  108  are described herein in reference to a center of gravity of wash tub  108 , it should be appreciated that according to alternative embodiments the displacement and thresholds could be measured at any other suitable location. 
     According to an exemplary embodiment, the displacement threshold (δ LMT ) may be determined as a function of an out of balance weight threshold (W OOB ) and a sub washer weight (W SW , e.g., as determined at step  220 ). Notably, the out of balance weight threshold (W OOB ) may generally be appliance specific. For example, some washing machine appliances may be configured for withstanding a 2 pound out of balance, while others may be capable of withstanding up to a 10 pound out of balance. 
     Specifically, according to an exemplary embodiment, the displacement threshold (δ LMT ) may be calculated using the following equation:
 
δ LMT   =W   OOB ( C   1   +C   2   ·W   SW )
         where:
           δ LMT =the displacement threshold;   W OOB =the out-of-balance weight threshold;   C 1 =a subwasher weight intercept coefficient;   C 2 =a subwasher weight slope coefficient; and   W SW =the subwasher weight.   
               

     Referring now briefly to  FIG. 7 , a plot illustrating the relationship set forth in the above equation is provided. For purposes of explanation, this plot is associated with the operation of washing machine appliance  100  with a subwasher weight of between 50 and 75 pounds rotated at 40 RPM. As shown, the displacement threshold (δ LMT ) is equivalent to the line of best fit associated with the subwasher load weight (e.g., 50, 65, or 75 pounds) and the out-of-balance weight threshold (W OOB ) for the appliance. Notably, linear interpolation may be used to determine the displacement threshold (δ LMT ) for loads having a weight in between those shown. It should also be appreciated that the relationship illustrated is only exemplary and not intended to limit the scope of the present subject matter. 
     In addition, constants C 1  and C 2  may be known or empirically determined constants that are a function of the washing machine configuration and may depend, for example, on the out-of-balance weight threshold (W OOB ) of a washing machine appliance and/or the basket speed at which the out of balance detection is performed (e.g., the first speed as described below). Furthermore, it should be appreciated that the constants described below are only exemplary and may be scaled, e.g., by 1000 or any other suitable value. According to an exemplary embodiment, constants C 1  and C 2  may be determined using the table below for washing machine appliance  100  having an out of balance threshold of approximately 4 pounds. 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Basket Speed (RPM) 
                 C 1   
                 C 2   
               
               
                   
               
             
            
               
                 40 
                 229.2 
                 −1.975 
               
               
                 45 
                 272.2 
                 −1.901 
               
               
                   
               
            
           
         
       
     
     After obtaining a displacement threshold (δ LMT ) at step  230  using the equation above, step  240  includes operating a motor (e.g. drive motor  140 ) to spin wash basket  112  at a first speed. According to exemplary embodiments, the first speed is a predetermined speed that is lower than a spin cycle basket speed. In addition, the first speed may depend on the measured subwasher weight (W SW ). In this regard, because the displacement at the center of gravity of wash tub  108  decays linearly with increasing subwasher weight (W SW ), the first speed may be selected based on subwasher weight (W SW ) to provide the largest displacement amplitude without causing wash tub  108  to strike cabinet  102 . For example, as shown in the table above, if the subwasher weight (W SW ) is less than 75 pounds, the first speed should be 40 RPM, whereas if the subwasher weight (W SW ) is greater than 75 pounds the basket speed should be 45 RPM. It should be appreciated that these subwasher weight values and basket speeds are only used for the purpose of explanation. Other appliances and embodiments may include different values for the first speeds, the subwasher weights, the system constants, the out of balance thresholds, etc. 
     Method  200  further includes, at step  250 , measuring a displacement amplitude (δ CG ) of the wash tub using a measurement device. Continuing the example from above, measurement device  172  may be used to measure the displacement amplitude (δ CG ) at the center of gravity of wash tub  108 . As may be appreciated by one skilled in the art, by placing measurement device  172  on the bottom of wash tub  108  (e.g., a rigid body), measurements obtained such as displacement and rotation at the mounting location may be used to determine the displacement and rotation at the center of gravity using a transfer function based on the geometry of wash tub  108 . 
     Step  260  includes determining that the displacement amplitude (δ CG ) exceeds the displacement threshold (δ LMT ). For example, referring again briefly to  FIG. 7 , the displacement amplitude (δ CG ) is illustrated above or exceeding the displacement threshold (δ LMT ) for a four pound out-of-balance weight threshold (W OOB ). In this regard, a comparison is made between the actual measured displacement at the center of gravity (δ CG ) and the maximum desired displacement threshold (δ LMT , e.g., determined at step  230 ). Step  270  includes operating the motor to rotate an agitation element in response to determining that the displacement amplitude (δ CG ) exceeds the displacement threshold (δ LMT ). In this manner, the agitation element may act to agitate or redistribute the load of articles in wash tub  108  prior to entering the spin cycle, thereby avoiding issues associated with operating an at high speeds with an out of balance load. 
     After the load of articles is redistributed within wash tub  108 , washing machine appliance  100  may repeat the out of balance detection process by starting at step  240  and operating the motor to spin wash basket  112  at the first speed again. This process may be repeated until the clothes are redistributed sufficiently to reduce the measured displacement amplitude (δ CG ) below the displacement threshold (δ LMT ). According to an exemplary embodiment, this process may be limited to a certain predetermined number of attempts. If the displacement amplitude (δ CG ) still exceeds the displacement threshold (δ LMT ) after the predetermined number of attempts, the cycle may be terminated or the spin cycle may be entered despite the out-of-balance condition. 
     At this point, method  200  may further include, at step  280 , determining that the displacement amplitude (δ CG ) is below the displacement threshold (δ LMT ). Step  290  further includes initiating a spin cycle of washing machine appliance in response to determining that the displacement amplitude (δ CG ) is below the displacement threshold (δ LMT ). In general, initiating the spin cycle may include using a drain pump assembly (e.g., drain pump assembly  130 ) to drain the water from wash tub  108 . The spin cycle may then include operating the motor to spin the wash basket at a second speed that is higher than the first speed. 
     Although the discussion herein refers to method  200  for operating washing machine appliance  100 , one skilled in the art will appreciate that the features and configurations described may be used for operating other washing machine appliances in other manners. For example, method  200  may be used to operate a horizontal axis washing machine appliance, the out-of-balance weight thresholds may vary, system constants (C 1 , C 2 ) may be different, measurement devices  172  may be positioned at different locations, etc. Other variations and modifications of the exemplary embodiment described below are possible, and such variations are contemplated as within the scope of the present subject matter. 
       FIG. 6  depicts steps performed in a particular order for purposes of illustration and discussion. Those of ordinary skill in the art, using the disclosures provided herein, will understand that the steps of any of the methods discussed herein can be adapted, rearranged, expanded, omitted, or modified in various ways without deviating from the scope of the present disclosure. Moreover, although aspects of method  200  are explained using washing machine appliance  100  as an example, it should be appreciated that these methods may be applied to the operation of any suitable washing machine appliance. 
     Referring now to  FIG. 8 , an exemplary illustration of the decision making process or control method implemented by controller  156  to perform method  200  is illustrated. It should be appreciated that the flow diagram  300  is intended only to provide a simple illustration of an exemplary control method. The flow diagram  300  is not intended to limit the scope of the present subject matter in any manner. 
     As shown, after the cycle is initiated, the controller performs a dry load sense procedure ( 302 ) to determine a size or dry weight of a wash load, e.g., in pounds. For example, controller may use a pressure sensor to detect the load weight after a wash load is added but prior to adding water. The controller may then add some amount of water and calculate the subwasher weight ( 304 ). 
     After the subwasher weight is determined, drive motor may rotate the agitation element to agitate articles within the wash tub for a specific period of time ( 306 ), e.g., to distribute the load about the axis of rotation. After the load is distributed, steps  308  and  310  define an optional calibration process for the measurement devices for measuring displacement amplitude. For example, washing machine appliance may stop agitation and allow the agitation element and wash basket to stop moving in order to zero out the accelerometer and/or gyroscope. 
     Once calibration is complete, drive motor may rotate wash basket at a constant, low-speed ( 312 ), e.g. to facilitate the out of balance detection or sensing procedure. Steps  314  through  318  define a tub displacement measurement procedure. For example, step  314  includes measuring the displacement amplitude of the tub. According to an exemplary embodiment, step  316  may include waiting for the tub displacement to reach steady state. For example, according to one embodiment, step ( 316 ) may include measuring multiple displacement amplitudes within a given time and comparing the measured values with each other to make sure that they are within the threshold percentage of each other, e.g., to avoid erroneous measurements or statistical errors. According to an exemplary embodiment, step  318  may include averaging the last ten measured displacement amplitudes to obtain a true and accurate measure of the displacement of the center of gravity of the wash tub. According to alternative embodiments, a single displacement amplitude may be measured once steady state is reached. 
     Step  320  includes comparing the displacement amplitude (or an averaged displacement amplitude) to a displacement threshold to determine whether the out of balance condition exceeds an appliance threshold. Specifically, the displacement threshold may be calculated in a manner similar to that described above as a function of the out of balance threshold for a given appliance and the sub washer weight (e.g., determined at step  304 ). If the displacement value measured at step  318  is less than the displacement threshold, the controller may determine that the out of balance within the wash basket does not exceed the appliance threshold. At this point, controller may then complete the operating cycle, e.g., by performing a spin cycle ( 322 ). By contrast, if the displacement amplitude exceeds the displacement threshold, the controller may reenter the agitation process (e.g., step  306 ) in an attempt to redistribute the load again. This procedure may continue until the out of balance in wash basket is below the desired threshold out of balance for the washing machine. It should be appreciated that the procedure illustrated by diagram  300  may omit certain steps for simplicity of discussion. Variations and modifications to this control method are possible and within the scope of the present subject matter. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.