Patent Abstract:
An electrical linear actuator employs a reversible motor driving a helical wire spring. The coils of the spring engage a follower that moves along the axis of the spring with rotation of the motor to provide linear motion. This actuator may be used as a linear drive in an appliance lock.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
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     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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     BACKGROUND OF THE INVENTION 
     Low-cost, electric, linear actuators are used in a variety of consumer products, including home appliances and automobiles, to move various components, including lock bolts, valve plates and the like, on the occurrence of an electrical signal. 
     Common linear actuators include solenoids, wax motors, and DC motors driving gear trains or screw threads. In a solenoid, a metal plunger loosely surrounded by a coil of wire is moved under the influence of a magnetic field produced by an electrical current in the coil. A wax motor employs an electrical current to heat wax contained in a closed volume so that the expanding wax drives a piston out of the volume. 
     Conventional solenoids and wax motors use a return spring to return the plunger or piston to its unactuated state, and thus require continued power to retain their actuated state. In contrast, small DC (direct current) motors, driving a rack-and-pinion gear or screw and nut, can be reversed by changing the polarity of the driving current, avoiding the need for a return spring and allowing the actuator to retain its actuated state after power is withdrawn. 
     One problem with DC motor linear actuators is friction in the gear train or screw and nut, particularly when the latter become contaminated during use. The high mechanical advantage typically present in a screw and nut design can cause jamming of the screw and nut at the end of travel under the momentum of the motor. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved DC motor linear actuator in which a screw and nut are replaced by a helical wire spring and a follower. The wire helix may be given a large pitch to prevent excessive force on the follower that might lead to jamming. Further, the flexibility of the wire of the helix can cushion the shock at the end of travel. The open construction of the wire helix resists the build up of contamination that can cause excessive friction. The wire helix further lends itself to simple fabrication and attachment to a motor. 
     Specifically then, the present invention provides an electrical actuator having an electric motor with a motor shaft rotating about an axis. A wire helix is attached to the shaft to rotate therewith and a helix follower interfits with the wire helix to translate along a path with rotation of the wire helix. 
     Thus, it is an object of the invention to provide for a simple and cost-effective mechanism for converting the rotary motion of a small DC electric motor into linear motion. 
     The wire helix may have a lead angle of between 5 and 55 degrees. 
     Thus, it is an object of the invention to permit relatively large helix lead angles that reduce jamming forces while providing rapid actuation. 
     The wire of the helix may be sized to flex under a force of the motor when the helix follower is restrained. 
     It is thus another object of the invention to provide a mechanism that naturally absorbs shocks, for example, when the helix follower reaches stop points, and that readily accommodates axial misalignment. 
     The wire helix may provide a first portion having a first diameter engaging the helix follower, and a second portion having a second diameter conforming to the diameter of the motor shaft. 
     Thus, it is an object of the invention to provide a simple means of attaching the helix to the shaft by using helical coils of the wire. 
     The wire helix may provide a first portion with a lead angle and a second portion with a second lead angle, the first and second portions at different times engaging the helix follower. 
     Thus it is an object of the invention to provide a simple method of changing the lead angle of the helix, and thus the relative mechanical advantage between the helix and the follower over the length of the helix, such as may be used to change the actuation force, for example, near the ends of motion of the helix follower to prevent jamming. 
     The second portion may be between the motor shaft and the first portion, and the second lead angle may be larger than the first lead angle. 
     Thus, it is an object of the invention to provide for a decrease in actuation force when the helix follower is closest to the motor where the helix itself cannot serve, through its elasticity, to cushion the forces generated when the helix follower confronts a stop. 
     The helix follower may be a bar fitting within the coils of the helix. 
     Thus it is an object of the invention to provide a simple follower suitable for a wire helix and resistant to jamming. 
     The helix follower may contact only one side of the helix. 
     It is thus another object of the invention to provide a helix follower that can decouple from the helix, upon direction reversal, to decrease the load on the motor during its startup. 
     The helix follower may contact the helix at only a single point. 
     It is thus another object of the invention to provide a small contact area between the helix follower and the helix that resists capture of contamination. 
     The helix may be a non-magnetic stainless steel. 
     It is thus another object of the invention to provide an actuator that is corrosion resistant, durable and which does not divert magnetic flux. 
     The motor may be a permanent magnet DC motor. 
     It is thus another object of the invention to provide a simple actuation mechanism that may be used with small motors. 
     The helix follower may be attached to a switch throw, which may, for example, be a sliding conductive element moving along an axis of the wire helix with the rotation of the helical wire, and pressing outward perpendicularly to the axis of the helical wire against opposed poles. 
     It is thus an object of the invention to provide a signal indicating the motion of the actuator and to provide a switch compatible with the present system that does not exert a torque on the follower, such as would require friction-increasing stabilization of the helical coil or follower. 
     The switch throw may be a V-shaped metal spring contacting the poles at the ends of the V. 
     It is thus another object of the invention to provide a simple throw mechanism that provides balanced outward forces. 
     The linear electrical actuator may be employed in an appliance latch where the helix follower attaches to a bolt that may extend from one of the housing or a door of the appliance to engage a strike placed on the other of the housing or door. 
     Thus, it is an object of the invention to provide a low cost latch mechanism suitable for use in appliances that provides for rapid engagement and disengagement and which is stable in engagement and disengagement without the application of electrical power (to reduce electrical consumption), and yet may be readily reversed simply by reversal of power to the motor. These particular objects and advantages may apply to only some embodiments falling within the claims, and thus do not define the scope of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary perspective view of a washing machine showing the positioning of a latch employing the present invention, such as may extend a bolt to engage a strike in the edge of a door; 
         FIG. 2  is a front elevational view of a bezel that may serve to attach the latch of  FIG. 1  to the housing of the washing machine; 
         FIG. 3  is a cross-sectional view taken along line  3 - 3  of  FIG. 1  showing the latch of  FIG. 1  as held by the bezel, and showing tipping of the latch prior to a final installation using screws, such as causes blocking of the bolt that may be detected to signal incomplete installation of the latch; 
         FIG. 4  is an exploded view of an electrical actuator used in the latch of  FIGS. 1-3  showing a DC motor that may turn a helical wire spring engaged by a helix follower bar held below the bolt of the latch; 
         FIG. 5  is a top plan view of the wire helix and shaft of the motor of  FIG. 4  showing changes in pitch and diameter of the wire helix such as changes the lead angle; 
         FIG. 6  is a cross-section along line  6 - 6  of  FIG. 4  showing the orientation of the bar of the helix follower as it engages the helix at a single point on a single side of the helix; 
         FIG. 7  is a top plan view of a switch having a V-shaped throw compressed between opposing poles of the switch and attached to the bolt of  FIG. 4 ; 
         FIG. 8  is a detailed fragmentary perspective view of one arm of the V-shaped throw showing a bifurcation of the contact surface and a supporting slider tip; 
         FIG. 9  is a fragmentary cross-section taken along line  3 - 3  of  FIG. 1  when the washing machine door is closed showing engagement of the bolt in a strike hole of the door to receive an upwardly extending tooth in the door locking the bolt when the door is lifted during engagement. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to  FIG. 1 , an appliance  10 , such as a washing machine, may have a housing  12  having an opening over which a hinged door  14  may close, for example, to cover a wash basket  16 . The door  14  may be locked when closed to prevent injury to a user during the spin cycle of the washing machine. For this purpose, a front edge of the door  14  may include a strike aperture  18 , which may receive a bolt  20  when the door  14  is in the closed position. The bolt  20  may extend from a latch mechanism  22  positioned within the housing  12  under the control of an electrical signal. As used herein, the term “bolt” may embrace any similar locking element such as a hook, pin, latch bar, shaft or the like. 
     Referring now to  FIGS. 2 and 3 , the latch mechanism  22  may be positioned within the housing  12  behind an aperture  21  through which the bolt  20  (not shown in  FIG. 3 ) may extend. The latch mechanism  22  may be held in position by means of a bezel  24  having a central aperture  26  aligning with aperture  21  and a pair of rearwardly extending posts  28 . The posts  28  that may pass through corresponding apertures (not shown) in the housing  12  to be received by sockets  30  molded in the side of the latch housing  23 . 
     The rearwardly extending posts  28  include upwardly extending teeth  34  that may engage a lip  36  of the socket  30  holding the bezel  24  and housing  23  loosely engaged so as to prevent the housing  23  from dropping downward free of the bezel  24  during assembly. When the posts  28  are received by the socket  30 , screws  38  may be inserted through bases  40  of the sockets  30  to engage threadable portions of the posts  28 . 
     Tightening of the screws  38  draws the bezel  24  tightly down against the housing  12  and to pull the latch housing  23  upward against the inner surface of the housing  12 . When so tightened, the bolt within the latch housing  23  will extend along a bolt axis  42  that is generally horizontal to be received by the strike aperture  18  of the door  14  when the door  14  is closed. Prior to this tightening, however, gravity will pull the latch housing  23  downward, as shown by a dashed outline of latch housing  23 ′, causing the bolt axis  42 ′ to tip upward. This misalignment will prevent the bolt from fitting into the strike aperture  18 . Blockage of the bolt can be detected by a switch attached to the bolt, as will be described below, providing an error signal to a controller within the appliance  10  indicating a problem with the assembly of the latch housing  23 . 
     Aperture  26  of the bezel  24  is surrounded by a rearwardly concave and flexible skirt  32  having a curvature with a radius slightly smaller than the radius of curvature of the housing  12  beneath the bezel  24 . Thus, when the bezel  24  is pulled tightly against the housing  12  with the screws  38 , the skirt  32  flexes outward forming a tight seal with the surface of the housing  12 . The housing  23  and bezel  24  are constructed of a flexible thermoplastic material that also provides for electrical insulation and that freely passes magnetic flux. 
     Referring now to  FIG. 4 , the bolt  20  may be driven by and form part of a linear actuator  44  comprising a permanent magnet DC motor  46  having a shaft  48  that may rotate in one of two directions according to the polarity of electrical voltage applied to the motor  46  over motor leads  50 . Attached to the shaft  48  and axially aligned therewith is a wire helix  52 , both of which are generally parallel to the bolt axis  42 . 
     Paddles  54 , extending downward from the bolt  20 , flank the left and right side of the wire helix  52  and receive a transversely extending metal bar  56  passing through corresponding holes  58  in each of the paddles  54  to intersect the wire helix  52  and to be held captive by its coils. The paddles  54  and bar  56  provide a helix follower that moves along the axis  42  with rotation of the wire helix  52 . 
     The wire helix  52  is preferably a spiral of spring-tempered stainless steel wire following a three-dimensional curve that lies on a cylinder of a defined diameter and having a central axis parallel to axis  42 . The wire of the wire helix  52  will have a defined angle with respect to a plane perpendicular to the axis  42  termed its lead angle. The lead angle may be controlled simply by spacing between wire coils along the axis of the wire helix  52 . 
     Referring now to  FIG. 5 , the wire helix  52  provides a number of different pitches and diameters and thus different lead angles, where lead angle  65 , as described above, is the angle between a plane orthogonal to the axis  42  and the wire of the helix  52 . For a given helix diameter, the lead angle will increase as the pitch increases. In a first region  60 , near where the wire helix  52  is attached to the motor shaft  48 , the wire helix  52  is given a small diameter  62  so that it may be press fit and welded directly to the shaft  48 . The pitch  64  in this first region  60  is such that the windings of the wire helix  52  abut each other and thus is approximately equal to the diameter of the wire of the wire helix  52 . Here the lead angle may be relatively low. 
     In a second region  66 , displaced from the motor  46  by region  66 , the diameter  61  of the wire helix  52  increases, while the pitch  68  is retained at pitch  64  for the purpose of stable transition. 
     In a next region  70  proceeding outward from the motor  46 , the pitch is abruptly increased to an expanded pitch  72  (increasing the lead angle) and then, at succeeding region  74  encompassing the remainder of the wire helix  52 , the pitch decreases slightly to a reduced pitch  76  (and reduced lead angle), both lead angles being typically greater than five degrees and less than fifty-five degrees. These regions  70  and  74  provide drive surfaces for the helix follower of the bar  56  and create a relatively large opening between coils of the wire helix  52  such as to resist entrapment of contaminants. 
     Referring also to  FIG. 4 , when the bolt  20  is fully extended and the bar  56  is in the region  74 , the bolt  20  may hit a stop  78 . A PTC thermister (not shown) may be placed in series with the motor to prevent over-current of the motor  46  when the motor  46  stalls, but even with current limiting, the interaction of the bolt  20  with the stop  78  can produce a relatively high instantaneous torque (and resulting actuation force) caused by the rapid deceleration of rotating mass of the motor  46 . However, any jamming of the bar  56  and wire helix  52 , such as might prevent reversal of the wire helix  52 , is forestalled by the natural compliance of the wire helix  52 , which compresses slightly to slow the deceleration of the motor  46  decreasing the peak torque. 
     When the motor  46  is reversed and the bolt  20  is drawn inward against a second stop  80  adjacent to the motor  46 , there is less length of the wire helix  52  to act as a spring to slow the deceleration of the motor  46 . In this case, the increased lead angle of the wire helix  52  in region  70 , serves to reduce the axial force and to prevent jamming. 
     Referring now to  FIG. 6 , the bar  56  of the helix follower may be installed at an angle with respect to the axis  42  to contact the coils of the wire helix  52  at a single point only, thus reducing potential entrapment of contaminants. Further, the angle of the bar  56  is such that the bar  56 , at any time, contacts only one side of the wire helix  52 . This allows the load of the bolt  20  to be decoupled from the wire helix  52  upon change in direction of the motor  46 , preventing stalling of the starting motor  46  in a position of low torque. This decoupling also allows the motor to start up in a reversed direction with reduced load to gain speed before the bar  52  recontacts the side of the wire helix  52 . The bar  56  may be molded into paddles  54  or may be a metal bar held by the paddles providing improved wear resistance. In one embodiment, shown in  FIG. 4 , the bar  56  may be surrounded with a sleeve  57  (for example a self-lubricating plastic material) that provides a lower-friction contact between the bar  56  and the helix  54  by action of the sleeve  57  rolling about the bar  57 . 
     Referring now to  FIGS. 4 and 7 , extending axially rearward from the bolt  20 , is a metallic V-shaped throw  84 . The throw  84  has outwardly diverging arms  88  that are flexible and compressed between opposed surfaces of pole  90  on one side, and pole  92  or  94  on the opposite side as the bolt  20  and throw  84  move axially throughout the length of travel of the bolt  20 . The pole  90  is continuous while pole  92  and  94  occupy opposite axial ends of a track  96 . Electrical continuity exists from the pole  90  through spring throw  84  to pole  92  when the bolt  20  is fully retracted and from the pole  90  through spring throw  84  to pole  94  when the bolt  20  is fully extended. Electrical continuity is broken when the bolt  20  is neither fully retracted nor fully extended. In this way, three distinct signals may be generated, one each for when the bolt is fully extended, fully retracted and in transition. Referring now also to  FIG. 8 , an outwardly convex dimple  102  may be placed at the ends of the arms  88  where they ride against the poles  90 ,  92 , or  94  (only pole  90  is shown), to provide a contact surface. The dimple  102  may include an axial groove,  103  bifurcating the surface of the contact where it connects with one of the poles  90 ,  92 , or  94  to provide improved contact reliability. 
     The vertex of the V-shaped throw  84  is pivotally attached to a downwardly extending pivot pin  86  on the bolt  20  so that the throw  84  is self-aligning between pole  90  and pole  92  and  94  on track  96 . Referring now also to  FIG. 8 , inwardly extending tabs  98  are formed on the ends of the arms  88  to ride on tracks  100  positioned between the ends of the arms  88 . The tabs  98  help stably locate the ends of the arms  88  against rotational movement. It will be understood from this description that there is no rotational torque exerted by the V-shaped throw  84  on the bolt during switching action such as might tend to cam the bolt  20  or divert the wire helix  52  off axis. 
     Referring now to  FIGS. 1 and 9 , when the bolt  20  is inserted through the strike aperture  18  in the door  14  and the door  14  is lifted upward, as indicated by arrow  104 , a tooth  106  formed in the door  14  behind the strike aperture  18  may engage a corresponding socket  108  formed in the lower side of the bolt  20 . The interengagement of the tooth  106  and socket  108  prevents force on the door  14  possibly sufficient to bend the bolt  20 , or from disengaging the bolt  20  from the strike aperture  18 . 
     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments, including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.

Technology Classification (CPC): 3