Abstract:
A steam iron ( 10 ) for clothing has direct measurement of sole plate temperature by a suitable sensor ( 19 ). A control device ( 20 ) such as a programmable logic controller, holds a characteristic relating sole plate temperature to steam production, and controls opening of an electrically actuated release valve ( 16 ) for water from a tank ( 15 ) to the sole plate ( 13 ). The sensor ( 19 ) may also control electrical energy for heating the sole plate ( 13 ).

Description:
TECHNICAL FIELD 
   This invention relates to improvements in steam irons for clothing. 
   BACKGROUND TO THE INVENTION 
   Irons for clothing have a heated sole plate with which a user may smooth out unwanted clothing creases, and reinforce or create desired creases. Heating energy is usually provided by a thermostatically controlled electric element, and the temperature of the sole plate is typically selected by the user to suit the nature of the clothing to be ironed. The user may lightly damp the clothing, in order to better eliminate unwanted creases. 
   In a refinement an iron may be provided with a water tank, and a valve assembly whereby water can be admitted from the tank to outlets of the heated sole plate where it vaporises, and issues as steam. This arrangement avoids the need for damping of clothing and can give a superior and faster ironing action. 
   In one known arrangement a progressive slide valve is provided whereby water is continually passed through the sole plate to give a substantially constant steaming effect. The slide valve typically has a variable opening so as to permit the volume of stream to be selected. In another arrangement a manual pump is provided whereby the use may force some water through the sole plate to give a single burst of steam. Both of these arrangements are often provided in clothing irons. 
   One known problem of steam irons is the phenomenon of dripping, in which unvaporised water escapes from the sole plate onto the clothing. Such dripping is usually caused by an inappropriate relationship between the temperature of the sole plate and the volume of water admitted via the slide valve, so that there is insufficient heat capacity to vaporise all of the water passing through the soleplate. Dripping may also be caused by inevitable hysteresis exhibited by the sole plate thermostat, so that as the temperature of the sole plate falls to a minimum, some water droplets escape. At higher sole plate temperature, such droplets are vaporised. 
   In order to solve the dripping problem it has been proposed to incorporate a closure valve actuated by a bi-metallic control element in the water supply passage to the sole plate. As is well known, the degree of curvature of such a control element can change appreciably on heating, and can be used to move the closure valve so as to block and unblock water flow. Thus water flow may be inhibited below a predetermined minimum sole plate temperature, so that dripping is largely prevented. 
   However, the prior art solutions do not entirely solve the problem of dripping. Furthermore such bi-metallic control elements may exhibit an undesirable clicking sound during operation. 
   What is required is an improved means of relating water flow and hence potential steam generation, to sole plate temperature. The elimination of bi-metallic control elements would be advantageous in order to avoid unwanted hysteresis, and variability due to manufacturing and assembly inconsistences. 
   SUMMARY OF THE INVENTION 
   According to a first aspect of the invention there is provided a steam iron for clothing, said iron comprising a body, a water tank in said body, a heatable sole plate having an interior side and an exterior side, through passages from said interior side to said exterior side, and a control device operable to release water from source tank to said interior side, wherein a temperature sensor having an electrical output is provided on said sole plate, and said control device is electrically operable in response to said output. 
   Such an iron provides for admission of water to the sole plate in a highly controllable manner, since the electrically operable control device is operable directly in accordance with sole plate temperature The sensor provides continuous temperature monitoring and a proportionate electrical output which varies in real time according to sole plate temperature. 
   In one preferred embodiment, the control device comprises a digital (on/off) device, and in another embodiment the volume throughput of the control device is proportioned to the temperature of the sole plate. 
   The control device may for example be an electrical solenoid valve having an armature movable in response to an electrical control signal to open a valve element. 
   In a further embodiment, the iron may include a programmable device adapted to retain a predetermined characteristic relating volume throughput of the control device to temperature of the sole plate. Such a characteristic may be non-linear, and incorporate a temperature threshold below which the control device is closed. The programmable device may comprise a programmable read only memory (ROM). 
   Since the invention gives close control of water volume in accordance with sole plate temperature, it will be appreciated that steam volume can be maximised for any particular sole plate temperature. Ironing is thus more effective without the risk of dripping. Other features of the apparatus will be apparent from the following description and the claims appended hereto. 
   According to a second aspect of the invention there is provided a method of controlling release of water from the water tank to the sole plate of a steam iron for clothing, said method comprising the steps of:
         determining the temperature of said sole plate by means of a sensor having a proportionate electrical output;   controlling release of water via an electrically actuable device operable in accordance with said output. The method permits accurate release of water in accordance with actual sole plate temperature, and thus avoids dripping. Other features of the method will be apparent from the following description and the claims appended thereto.       

   
     BRIEF DESCRIPTION OF DRAWINGS 
     Other features of the invention will be apparent from the following description of a preferred embodiment shown by way of example only in the accompanying drawings in which: 
       FIG. 1  is a partial sectional view of the nose of a steaming iron incorporating the present invention; 
       FIG. 2  illustrates in axial section a solenoid valve of the invention in the inactive (closed) condition; 
       FIG. 3  corresponds to  FIG. 2  and shows the solenoid valve in the active (working) condition. 
       FIG. 4  shows a typical temperature/volume control coefficient. 
   

   DESCRIPTION OF PREFERRED EMBODIMENT 
   With reference to  FIG. 1 , a domestic steam iron  10  comprises a main body  11  having a handle  12  above and a metal sole plate  13  below. The sole plate  13  incorporates an electric heating element for which power is supplied by a flexible cable  14 . Within the main body  11  is provided a water tank  15  having a suitable filling aperture (not shown). A valve  16 , to be described in greater detail below, is operable to admit water by gravity from the tank  15 , to the interior side of the sole plate  13 , which has an array of through apertures  17  therein. 
   As is well known, the sole plate is in use heated, and water admitted via the valve  16  is vaporised so as to issue from the apertures  17  as steam. 
   The sole plate  13  is generally triangular in plan and has a pointed nose to facilitate ironing. The main body may be of any suitable material, and is typically moulded of plastics. An adjustable thermostat (not shown) is provided whereby the user can select an appropriate sole plate temperature. A pump button  18  is provided whereby thumb actuation causes a small volume of water to be admitted to the sole plate to give a burst of steam on demand. 
   The steam iron of the invention incorporates a temperature sensor  19  fixed to the interior side of the sole plate. In the preferred embodiment a negative temperature coefficient (NTC) sensor is used, though other kinds of sensor may be suitable. Furthermore, the handle  12  incorporates a printed circuit board  20  (such as a programmable logic controller—PLC) which is pre-programmed to provide in use control of steaming in the manner now described. 
     FIGS. 2 and 3  illustrate a solenoid operated valve for controlling admission of water from the water tank  15  to the interior of the sole plate  13 . 
   The solenoid valve  16  is rotationally symmetrical about the upright axis thereof, and comprises a tubular body  21  which has an axial opening  22  at the lower end, and defines just above the opening a frusto-conical internal seat  23 . A cylindrical valve member  24  within the body  21  includes an annulus adapted to seal against the seat  23  and thereby close the opening  22 , as illustrated in enlarged portion A. The valve member is preferably of relatively soft material, such as a silicon rubber, in order to ensure effective sealing at the seat  23 . 
   The member  24  is urged downwardly to the closed condition by a light coil spring  25  which reacts against an annular abutment  26  fixed within the body  21  at mid-height. An annular flexible boot  27  seals the valve member  24  to the abutment  26  and body  21 , as illustrated, so as to permit axial movement of the member  24  whilst preventing escape of water in the upward direction. 
   As will be appreciated, the annular volume above the valve seat  23  and below the boot  27  is in communication with the interior of the water tank, for example via one or more apertures in the wall of the body  21 . 
   A solenoid armature  31  is connected to the valve member  24 , and extends generally vertically through a fixed solenoid coil  32  housed within the upper region of the tubular body  21 . The coil  32  is connected to a source of electrical energy via a cable  33 . A plug  34  closes the upper opening in the coil  32  and permits limited upward movement of the armature  31  from the inactive condition illustrated in  FIG. 2 . 
   The valve  16  is housed within a chamber  35  of the water tank, and in use is inserted from above and sealed at the bottom by an annular grommet  36 . The valve is retained in place by any suitable fastening means. 
   On activation of the solenoid coil  32 , the armature is attracted upwardly by the electrical field into contact with the plug  34 , and the valve member opens the seat  23  as illustrated in  FIG. 3  (and enlarged portion B). The size of the opening is strictly controlled by positioning of the plug  34 , which may be settable by e.g. screw-threading. When inactivated, the electrical field generated by the coil  32  ceases, and the spring  25  returns the valve member  24  to the seated condition ( FIG. 2 ). 
   The temperature sensor  19  and solenoid coil  32  are connected to the circuit board  20 , which is itself powered via the cable  14 . The sensor  19  directly and continuously monitors actual temperature of the sole plate, for example by outputting a voltage output proportional to temperature. Hysteresis of an NTC sensor, if any, is minimal. The circuit board  20  is pre-programmed to activate the solenoid coil to release water via the opening  22  according to sole plate temperature, thus ensuring that the optimum volume of steam is produced without risk of water droplets. 
   A typical control coefficient for the circuit board  20  is illustrated in  FIG. 4 . Thus no water is admitted via the solenoid valve  16  until the sole plate reaches a minimum temperature. At the minimum temperature, a predetermined volumetric flow is permitted by opening of the valve  16 , which increases progressively as sole plate temperature further increases. 
   Other control coefficients are of course possible, and water supply may be switchable on or off according to a separate control switch on the iron body. Opening of the solenoid valve may be progressive, or the solenoid valve may reciprocate at a variable rate determined at the circuit board  20 . In the latter case the volume of steam depends on the frequency and period of solenoid activation. 
   In this embodiment solenoid control of the valve  16  is the means of admitting the correct flow rate of water to the sole plate. In the alternative, any suitable electrically actuated device may be used, such as another form of electrically operated valve, or an electrically driven pump. 
   A further embodiment of the direct temperature sensor  19  is that the output may be used to directly replace the usual bi-metallic thermostat of the sole plate heating element. Thus a more accurate control of actual sole plate temperature can be assured, typically within a small hysteresis range determined by pre-programming features of the PCB  20 .