Abstract:
Systems and methods in accordance with the invention cause substantially the entire area of a fabric article to be laundered is efficiently and completely exposed to focused ultrasound. In this way, the benefits of cavitation are applied to the article as a whole rather than on a “spot” basis.

Description:
RELATED APPLICATION 
       [0001]    The present application claims priority to, and the benefits of, U.S. Ser. No. 61/116,832, filed on Nov. 21, 2008, the entire disclosure of which is hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to cleaning of fabrics and textile materials, and in particular to ultrasound-based cleaning. 
       BACKGROUND 
       [0003]    Fabrics and textiles are typically cleaned in washing machines that soak the fabric in generally hot, detergent-laden water with mechanical agitation. In essence, the washing machine applies mechanical energy, thermal energy, and chemical action to the soiled articles. Because chemical cleaning agents can be both expensive and environmentally unfriendly, substantial effort has been directed toward cleaning systems that use no additives—just plain water, which ideally might be reused after the washing cycle, e.g., for agriculture or, with filtration, in subsequent cleaning cycles. 
         [0004]    Ultrasound energy offers a viable alternative to traditional detergent-based cleaning approaches, since it is capable of dislodging soils without chemical assistance. Although various deployments of ultrasound in fabric-washing equipment have been attempted, none has attained commercial acceptance. A key limitation of systems thus far proposed is the inability to ensure efficient and complete exposure of the article to adequate levels of ultrasound energy. If the ultrasound is applied with insufficient focus, the energy fluence through the fabric will be inadequate to dislodge soil. On the other hand, highly focused ultrasound may not encounter all portions of a fabric article to be cleaned, or else may require excessive washing times. 
       SUMMARY 
       [0005]    In accordance with some embodiments of the present invention, substantially the entire area of a fabric article is efficiently and completely exposed to focused ultrasound. As used herein, the term “substantially” means within 10%, and ideally within 5%. In this way, the benefits of cavitation are applied to the article as a whole rather than on a “spot” basis. 
         [0006]    Cavitation is a threshold phenomenon triggered by oscillating pressure waves. In the present context, it is caused by the interaction of the acoustic beam with micro-bubbles in the fluid. Cavitation involves two mechanisms: streaming cavitation, in which gas micro-bubbles stream as a result of the acoustic beam generating high shear forces, and inertial cavitation, in which micro-bubbles implode and generate extremely high temperatures and pressures at the micron level. Initiating cavitation requires the existence of micro-bubbles in the fluid. Generating micro-bubbles typically requires a very high cavitation threshold. It is, however, possible to significantly reduce the generation threshold (also called nucleation threshold) for micro-bubbles by actively nucleating the fluid with micro-bubbles. The average diameter of the micro-bubbles desirably is smaller than the resonance radius, which depends on parameters such as the acoustic frequency, fluid parameters, temperature, pressure, etc. The following simplified equation describes the relationship among the resonance radius R 0 , the resonance frequency f 0 , the ambient pressure P 0 , the polytropic exponent of gas κ, and the density ρ of the liquid: 
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                       κ 
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                         P 
                         0 
                       
                     
                     ρ 
                   
                 
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                   1 
                   
                     R 
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         [0007]    A typical desired radius is ˜1 μm within a 1 MHz ultrasound field or and 10 μm in a 0.1 MHz ultrasound field. 
         [0008]    Streaming and inertial cavitation can be used to clean fabrics. Sheer forces generated by the streaming cavitation and localized high pressures and temperatures generated by the inertial cavitation remove soil without the need for chemical additives (e.g., detergents), although it should be emphasized that systems in accordance herewith may be used with detergents in a manner that reduces their environmental impact—e.g., enabling the use of smaller amounts of traditional detergents, or enhancing the action of more environmentally friendly but less efficacious detergents so they become more acceptable to consumers or reducing the power consumption used to heat the water. 
         [0009]    Accordingly, in one aspect, an apparatus for laundering fabric articles in accordance with the invention may include a chamber for receiving fabric articles to be laundered; a source of ultrasound energy focusing to one or more foci, each of which may be, for example, point-shaped or linear, within the chamber; and a handling system for ensuring that substantially the entireties of the fabric articles pass at least once through at least one of the foci during a cleaning cycle. The apparatus may direct an acoustic beam in the form of a pressure wave within the cavity so that it interacts with soiled fabrics in various ways, e.g., via propagation, reflection, absorption, scatter and/or cavitation. 
         [0010]    The handling system may include a feeding mechanism (based, for example, in an Archimedes screw) to draw the articles into the chamber. In various embodiments, the apparatus further comprising means for enforcing a standing-wave condition in the cavitation chamber, e.g., by adaptively changing the frequency or phasing, or the water level. Means for introducing micro-bubbles into the cavitation chamber may also be included, so that the fabric articles are exposed to streaming and inertial cavitation. 
         [0011]    Various other features may be included. For example, the apparatus may include comprising a sensing module to monitor the extent of cleaning. A controller, responsive to the sensing module, may cause water in the cavitation chamber to be filtered or replaced with a new or recycled volume of water. A sensing module may be employed to monitor cavitation and the controller may responsively alter acoustic power, a temporal transmission regime and/or frequency of the ultrasound energy. 
         [0012]    In some embodiments, the cavitation chamber is cylindrical with a first portion containing an acoustic transducer with a line focus extending axially along the center of the chamber and a second portion opposed to the first portion forming a reflector. In other embodiments, the ultrasound energy is focused to multiple foci distributed within the cavitation chamber. 
         [0013]    The apparatus may have a separate cleaning chamber, in which case the handling system transfers fabric articles from the cleaning chamber to the cavitation chamber. The cavitation chamber, in turn, may take the form of a drum having, disposed along an inner wall thereof, a series of acoustic-wave emitting plates having axial foci each at different focal depths. In some embodiments, the emitting plates have the same focal depth and a reflector with a different focal depth is set in opposition to each emitting plate. 
         [0014]    In another aspect, an apparatus for laundering fabric articles comprises a rotatable chamber for receiving the fabric articles to be laundered, at least a portion of the chamber being substantially transparent to ultrasound energy; at least one stationary ultrasound source, surrounding the rotatable chamber, for directing ultrasound energy to different foci within the chamber; and a controller for rotating the chamber and selectively activating the at least one ultrasound source during the rotation. The apparatus may further comprise a water-handling system for introducing water into and withdrawing water from the rotatable chamber during a cleaning cycle. The controller may, for example, ensure a minimum water level during activation of the ultrasound sources. 
         [0015]    In some embodiments, at least a portion of the rotatable chamber is substantially transparent to ultrasound energy. For example, the apparatus may comprise a plurality of circumferentially spaced-apart ultrasound sources, with the rotatable chamber equipped with a plurality of circumferentially spaced-apart windows transparent to ultrasound energy and, between the windows, segments of a material that reflects ultrasound energy. The ultrasound sources may have foci at different focal depths, or the reflective segments may each focus ultrasound to a focus different from that of the other segments. 
         [0016]    In still another aspect, the invention relates to an apparatus for laundering fabric articles. The apparatus comprises a chamber for receiving the fabric articles to be laundered; means for directing ultrasound energy into the chamber; a handling system for drawing fabric articles through the chamber; and means for introducing micro-bubbles into the chamber, whereby the fabric articles are exposed to streaming and inertial cavitation. The micro-bubbles have sizes optimized to enhance cavitation. 
         [0017]    In yet another aspect, the invention pertains to a method of laundering fabric articles. The method comprises the steps of receiving, in a chamber, the fabric articles to be laundered; directing ultrasound energy to one or more foci within the chamber; and handling the fabrics such that substantially the entire areas of the fabric articles pass at least once through a cavitation region during a cleaning cycle. 
         [0018]    Still another aspect of the invention relates to a method of laundering fabric articles that involves receiving, in a chamber, the fabric articles to be laundered so the articles are submerged in a liquid; directing ultrasound energy into the chamber; drawing fabric articles through the chamber; and introducing micro-bubbles into the chamber, whereby the fabric articles are exposed to streaming and inertial cavitation. 
         [0019]    In still another aspect of the invention, a method of laundering fabric articles comprises the steps of rotating a chamber in which the fabric articles are submerged in a liquid; and during the rotation, selectively activating a plurality of circumferentially disposed, stationary ultrasound sources around the chamber to direct ultrasound energy to different foci within the chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    In the drawings, like reference characters generally refer to the same parts throughout the different views. Also, the drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the following description, various embodiments of the present invention are described with reference to the following drawings, in which: 
           [0021]      FIG. 1  schematically illustrates a representative embodiment of the invention; 
           [0022]      FIG. 2  schematically illustrates a representative mechanical configuration of a focused-ultrasound washing machine in accordance with the invention; 
           [0023]      FIG. 3  is a cross-section through a representative cavitation chamber; 
           [0024]      FIG. 4A  illustrates a segment of a cylindrical cavitation chamber in accordance with another embodiment of the invention; 
           [0025]      FIG. 4B  is a cross-section through the embodiment shown in  FIG. 4A ; and 
           [0026]      FIG. 4C  is a cross-section through an alternative embodiment in which the transducers remain fixed. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    An exemplary system  100  in accordance with the present invention is illustrated in  FIG. 1 . The system includes a cleaning chamber  105  that receives fabric articles to be cleaned and an amount of water sufficient to immerse the articles. A cavitation chamber  108 , which includes an ultrasound transducer  112 , is mechanically coupled to the cleaning chamber  105  such that fabric articles may be passed between the chambers  105 ,  108  by a mechanical handling system (described below). In general, chambers  105 ,  108  are metal, particularly where ultrasound reflections are produced as discussed below, although it is possible to coat the interior surface with a thin layer of plastic that does not interfere with energy transmission. Cleaning takes place within the cavitation chamber  108 . A conventional acoustic driver circuit  115 , under the control of a system controller  120 , operates the transducer  112 . 
         [0028]    As further described below, the ultrasound beam is focused within the cavitation volume to trigger cavitation effects, and a source  122  of micro-bubbles, also operated by system controller  120 , saturates the fluid in the cavitation chamber with micro-bubbles having sizes (e.g., a radius smaller than the resonance radius) optimized to enhance cavitation. A sensing device  125  monitors the level of cavitation in the in chamber  108 , e.g., by means of a conventional acoustic sensor. In some embodiments, sensing device  125  also monitors the cleanliness level of the water and/or the fabrics. For example, the device  125  may measure the clarity of the water to assess whether cleaning has been completed; alternatively, the device may measure the reflectance of the fabrics. In other embodiments, a separate cleaning sensor  130  is disposed within cleaning chamber  105 , and operates by measuring water clarity or fabric reflectance (or both). 
         [0029]    Sensing device(s)  125 ,  130  are operated by conventional circuitry  133 , which supplies power to the device(s), receives sensor signals, and communicates with system controller  120 . In some embodiments, circuitry  133  receives signals (e.g., digital signals) from controller  120  periodically during a cleaning cycle and, in response, obtains readings from device(s)  125 ,  130 . These readings may, for example, be in analog form, in which case circuitry  133  includes an analog-to-digital converter, which outputs a pulse train indicative of the sensed reading to controller  120 . Alternatively, the sensor(s) may be operated continuously. 
         [0030]    Articles within cleaning chamber  105  may be subjected to mechanical agitation in order to further the cleaning process in the manner of a conventional clothes washer. A central, finned agitation post, for example, may be operated by a mechanical motion module  137  under the control of system controller  120 . Water fills cleaning chamber  105  and is drained therefrom by conventional plumbing and valves (not shown). Instead of being drained during a cleaning cycle, however, water in the cleaning chamber  105  may be filtered and recycled back into the chamber  105  by means of a recycling module  140 . The recycling module  140  is valved to the drain plumbing and contains one or more particle and/or other filters for removing soils from the water. Modules  137 ,  140  are operated by system controller  120  over the course of a cleaning cycle, for example, based at least in part on feedback from the sensing device(s)  125 ,  130 . 
         [0031]    In operation, fabric articles are loaded into cleaning chamber  105 , where system controller  120  causes water to be introduced so as to fully immerse the articles. Controller  120  may thereupon direct mechanical motion module  137  to impart an initial interval of agitation, followed by water filtration and re-introduction by means of the recycling module  140 . Articles then pass into the cavitation chamber  108 , where they are subjected to focused ultrasound and subsequently discharged back into cleaning chamber  105 . During ultrasound treatment, controller  120 , via sensing device  125 , determines the level of cavitation. Controller  120  changes—or alerts the user to change—the acoustic power, temporal transmission regime and/or frequency of the energy emitted by transducer  112  to achieve the desired cleaning effect. Based on the sensed level of water cleanliness, controller  120  may, for example, cause the water to be filtered or replaced with new volume of water via recycling module  140 , and/or cause the fabrics to undergo another sonication in chamber  108 , and/or adjust the operation of transducer  112 . Finally, controller  120  causes the fabric articles in chamber  105  to undergo a conventional drain/wash/rinse cycle. 
         [0032]    More generally, of course, it is possible to use “open-loop” approaches that do not involve feedback, based, for example, on a timer governing the stages of a cleaning cycle in terms of fixed intervals, or on visual inspection. 
         [0033]      FIG. 2  illustrates a representative implementation of chamber  108  and its disposition within chamber  105 . The chamber  108  takes the form of a cylindrical pipe with a flared receiving end  150 . The transducer  112  (see  FIG. 1 ) extends over a cleaning zone Z having a volume of, for example, 10 to 60 liters. A conical Archimedes screw  155  captures soiled fabrics within chamber  105  and feeds them into the cavitation chamber  108 . The rate at which the fabrics are fed is determined by controller  120  and depends on the level of cleaning required: for light cleaning the feed rate will be fast, while for dirty fabrics the feed rate will be low. For example, the rate may be set by controller  120  based on an initial reflectance reading from sensing device  130 . Archimedes screw  155  forces articles through the length of chamber  108  as it receives new articles from chamber  105 , and finally forces the last articles through chamber  108  by simple conveyance of water. 
         [0034]    In the illustrated embodiment, chamber  108  is canted with respect to chamber  105  to facilitate the flow of fabrics therethrough while keeping them below the water line. Chamber  108  may be incorporated within a central agitation post for compactness of construction. 
         [0035]    A representative cavitation chamber  108 , shown sectionally in  FIG. 3 , takes the form of a short (e.g., 20 to 60 cm) cylindrical pipe divided into two portions: the upper half-cylinder portion  160  comprises an acoustic transducer with a line focus extending axially along the center of the pipe, while the lower half-cylinder portion is metallic and acts as refocusing reflector. For example, the interior surface of half-cylinder  160  may be the output surface of transducer  112  (see  FIG. 1 ) which, as shown in  FIG. 3 , emits ultrasound toward the center C (so that along the length of the transducer  112 , ultrasound is focused along the central axial line extending through cylinder  108 ). 
         [0036]    Bubble-generation module  122  (see  FIG. 1 ) may be used to nucleate the cavitation volume with micro-bubbles. Exposure of the fabric surface area to the ultrasound focus or, more preferably, foci is achieved by utilizing a chamber having a size and shape optimized to generate cavitation throughout its volume (or at least a large fraction of the volume). In  FIG. 3 , the single line focus means that fabrics must be agitated for a sufficient time and with adequate movement in the chamber to ensure that all points pass through the linear focus. Alternatively, the reflector segment  165  may be shaped by deviating from the cylindrical surface or by tilting the cylindrical surface to create multiple focal lines through the chamber; the greater the number of ultrasound foci, the less time and agitation will be needed to ensure complete exposure of the fabric to cavitation. Alternatively or in addition, the upper half-cylinder  160  (i.e., the transducer) may be designed with multiple foci by deviating from cylindrical surface or by building it as a phased array capable of steering the beam and the focus elctronically. 
         [0037]    In still another implementation, illustrated in  FIGS. 4A and 4B , sonication occurs within the cleaning chamber  108 . One or more cylindrical sectors of the interior drum wall contain or are configured as acoustic-wave emitting plates, two of which are representatively indicated at  112   1 ,  112   2 . For example, each plate  112   n  may extend over the entire cylindrical height of the chamber  108  as illustrated, or instead, circumferentially adjacent plates may extend over partial but overlapping (or adjacent) portions of the cylindrical height. The plates have different axial foci, each at a different focal depth, as shown in  FIG. 4B . This can be accomplished, for example, by pre-shaping the transmitting surface to focus at a point or a line or by using lenses  180  associated with each of the plates  112   1  . . .  112   n . The lenses  180  may be, for example, plastic or other suitable material. 
         [0038]    Alternatively, on the opposite side of the drum from each emitting plate, a reflector with a different focal depth may be disposed. In still another alternative, the semicylindrical transducer  112  shown in  FIG. 1  may be employed as a stationary fixture around half of the rotating chamber  108 . These arrangements can accommodate top-loading or side-loading configurations. 
         [0039]    To avoid the need to power rotating arrays, the drum  108  can be made from an acoustically transparent material (e.g., MYLAR) or include windows  192   1  . . .  192   n , (collectively  192 ) of such material as shown in  FIG. 4C . The transducers  112   1  . . .  112   n , (collectively  112 ) are arranged around a stationary fixture  195  that surrounds the drum  108 . In this way, operation of the stationary transducer segments  112  is synchronized to the rotation and orientation of the drum  108  by a conventional motor  198 , such that the segments  112  are active only when facing an acoustic window  192  of the rotating drum. Because motor  198  is operated by controller  198 , the controller can readily track the instantaneous angular positions of the windows  192 . Once again, the transducer segments  112  may have different foci or, instead, the unwindowed portions of drum  108 , which act as reflectors for ultrasound passing through opposed windows  192 , can be focused along different interior line foci. 
         [0040]    In a representative implementation, the invention takes the form of a traditional front-loading washing machine having a static, horizontally oriented drum of radius R in which the transducer segments are mounted and, concentrically within the static drum, a smaller-diameter rotating drum for containing fabric articles to be cleaned. The interior drum has a depth L and, after loading with soiled fabric articles, the interior drum is filled with water to a height of R/2. The rotating drum has N acoustically transparent windows around its circumference (between N+1 ribs or reflective segments). But the transducer segments are disposed only around the lower semicylindrical half of the static drum. 
         [0041]    In particular, the lower half of the external static drum surface has M&lt;N/2 transducer segments of size L×W, each of which can be switched on independently of one another. Each of these segments has a preset focal area within the rotating interior drum. The transducer width W&lt;2π/N, and each transducer segment is pre-focused at a predefined distance D&lt;R/2. Preferably, one or more standing waves is induced and maintained during operation; this minimizes the input energy necessary to sustain the cleaning process. A standing wave can be created and maintained by adaptively changing the frequency or phasing, or the water level. In the representative implementation, roughly up to ⅓ of the drum surface radiates at any given time, and a given transducer segment is active for roughly ⅓ of a full rotation. Assuming a drum speed of 60 RPM, the duty cycle is 33% at most, with a burst pulse repetition rate of 1 sec. Controller  120  monitors the water level and causes water to be added as necessary, disabling the transducer segments if the water level becomes insufficient, and may also control the frequency and/or phasing to enforce a standing-wave condition. 
         [0042]    In operation, the interior drum is rotated at a normal speed in both directions in order to cause the fabric articles to mix and change relative location within the drum. As the drum rotates, controller  120  monitors the instanteous angular position of the drum relative to the fixed transducers, and as a window begins to pass in front of a transducer segment, controller  120  activates that segment via an associated driver  115 , causing the transducer to emit an energy burst that sustains cavitation in the water above it. Controller  120  deactivates the segment when the window rotates out of alignment therewith. In general, the transducer segments are distributed symmetrically around the circumference of and, as a result, will be simultaneously active or inactive. Controller  120  integrates sonication cycles within the overall cleaning cycle for maximum effectiveness, subsequently initiating a standard drain/wash/spin cycle. 
         [0043]    The terms and expressions employed herein are used as terms and expressions of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof. In addition, having described certain embodiments of the invention, it will be apparent to those of ordinary skill in the art that other embodiments incorporating the concepts disclosed herein may be used without departing from the spirit and scope of the invention. Accordingly, the described embodiments are to be considered in all respects as only illustrative and not restrictive.