The invention relates to an ironing system, more particularly to an induction ironing system. 
     BACKGROUND OF THE INVENTION 
     Induction ironing systems consist of irons whose soleplates are heated by electromagnetic radiation from an induction coil. This heating method requires the soleplate to be in close proximity to the induction coil. The ironing systems usually include a temperature regulating sub-system that switches on/off the heating means (induction coil) when the soleplate attains a pre-set temperature, based on the input from a temperature sensor (e.g. a thermistor or a thermostat). However, placement of the sensor is often difficult and has an impact on the mechanical construction of the induction ironing system. Furthermore, the response time of the sensor (due to air-gaps, moisture, etc.) results in an inaccurate and slow functioning of the temperature regulating sub-system thereby causing a wide soleplate temperature range that decreases ironing performance. In a case where the induction coil is located inside an ironing board, further problems arise. Due to the dynamics of the iron during ironing, it is difficult to locate the sensor at any position, as it is a probabilistic event that the iron would come right above the sensor and provide sufficient time to detect the temperature of the soleplate. Besides, when a garment to be ironed is placed between the ironing board and the iron, the garment would interfere with the function of the sensor. 
     UK Patent Application GB2392171 describes an induction ironing system comprising an iron and an ironing board with multiple induction coils. In order to trigger the electromagnetic field, the ferrous base of the iron has to be in contact with the electromagnetic field produced by the electromagnetic coils. To stop the heating process of the iron, either the current passing through the ironing board has to be switched off or the iron has to be lifted upwards until it goes out of range of the electromagnetic field. However, this system does not ensure controlled temperature and hence may result in the scorching of fabric. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, an ironing system comprises an iron including a soleplate, wherein said soleplate comprises an induction heatable material; and a unit including at least one induction coil and a device, wherein said induction coil is configured for charging said iron and said device is configured for detecting a temperature of said soleplate by sensing a change in current flowing through said induction coil or by sensing a change in voltage across said induction coil, said change in current or voltage being caused by a change in self inductance of said induction coil, said change in self inductance further being caused by a change in magnetic permeability of said soleplate as a function of its temperature, said device further being configured for switching said induction coil on or off depending on the temperature. The iron receives the necessary energy for ironing from the induction coil that is placed in the unit. An alternating magnetic field generated by the induction coil induces eddy currents in the soleplate and thus the soleplate gets heated up. The iron is then said to be charged. The induction heatable materials change their magnetic permeability with temperature. The self inductance of the induction coil and hence the current passing through the induction coil vary as a function of the magnetic permeability This is used as a trigger to switch the induction coil on or off based on the pre-determined relationship between the desired soleplate temperature and the current passing through the induction coil. In this manner, the temperature of the soleplate can be determined without the need of a temperature sensor. 
     According to another embodiment of the invention, said device comprises a current/voltage sensing circuit connected to said induction coil, wherein said current/voltage sensing circuit senses a change in current through said induction coil or senses a change in voltage across said induction coil. The device further comprises a current switching circuit, wherein said current switching circuit switches said induction coil on or off. The device furthermore comprises a temperature control circuit, wherein said temperature control circuit controls said current switching circuit depending on the current/voltage sensed by said current/voltage sensing circuit. The device is thus capable of switching said induction coil on or off based on the current/voltage in the coil that it can sense (this current/voltage having a relationship to the soleplate temperature as pre-established and programmed in the system.). Therefore, the temperature of the soleplate can be regulated without the help of a conventional temperature sensor 
     According to an embodiment of the invention, the induction heatable material is a ferro-magnetic material having its Curie temperature substantially close to the ironing temperature. Phytherm alloys are examples of such commercially available materials/alloys. Phytherm 230 or Phytherm 260 may be used as induction heatable materials. The Curie temperature of the induction heatable material of the soleplate is in the range of 100 to 300° C. This soleplate with above mentioned Curie temperature is suitable for ironing at temperatures in the range of 100 to 250° C. However, in order to ensure safe ironing of delicate garments such as silk, in another embodiment, a shoe could be detachably connectable to the soleplate to enable ironing-delicate garments at temperatures in the range of 50° C.-150° C. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a schematic representation of a device of an ironing system according to an embodiment of the invention; 
         FIG. 2  is a representation of the output of a temperature control circuit; 
         FIG. 3  shows a response across an induction coil when a switch is opened for a high value of inductance; 
         FIG. 4  shows a current through a current/voltage sensing circuit for a high value of inductance; 
         FIG. 5  shows a voltage across a current/voltage sensing circuit for a high value of inductance; 
         FIG. 6  shows a response across an induction coil when a switch is opened for a low value of inductance; 
         FIG. 7  shows a current through a current/voltage sensing circuit for a low value of inductance; 
         FIG. 8  shows a voltage across a current/voltage sensing circuit for a low value of inductance; 
         FIG. 9  shows an ironing system according to an embodiment of the invention comprising an iron, an ironing board with one or more induction coils; 
         FIG. 10  shows an ironing system according to an embodiment of the invention comprising an iron, an ironing board and a charging base with one or more induction coils; and 
         FIG. 11  shows an iron with an ironing shoe according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will be described with respect to particular embodiments and with reference to certain drawing figures, but the invention is not limited thereto but only by the claims. Any reference signs in the claims shall not be construed as limiting the scope of the invention. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn to scale for illustrative purposes. Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. “a” or “an”, “the”, this includes a plural of that noun unless something else is specifically stated. 
     Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. 
     Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein. 
       FIG. 1  is an example schematically showing a first embodiment of the invention. As shown in  FIG. 1 , a device  130  is connected to an induction coil  120 . Further as shown in  FIG. 1 , the device  130  includes a current/voltage sensing circuit  150 , a current switching circuit  140  and a temperature control circuit  160 . The current/voltage sensing circuit  150  includes a switch  151 , a resistor  152  and a signal-conditioning device  153 . The current switching circuit  140  includes a bridge rectifier  141  and is connected to the supply voltage  142 . The induction coil  120  and a capacitor  121  together form a parallel resonant circuit  125 . A sole plate  180  is heated by electromagnetic radiation from the induction coil  120 . 
     While charging the iron either during ironing or during rest, the alternating magnetic field generated by the induction coil  120  induces eddy currents in the soleplate  180 . The magnetic properties of induction heatable material of the soleplate  180  are dependent on the temperature at which they are used. These properties progressively decrease and finally disappear beyond a characteristic temperature. This temperature is generally referred to as “Curie temperature” of the material. 
     The current/voltage sensing circuit  150  enables the temperature control circuit  160  to activate the current switching circuit  140  when the temperature of the soleplate  180  is below the Curie temperature. The self-inductance of the induction coil  120  changes when the temperature of the soleplate  180  exceeds the Curie temperature. This is because the magnetic permeability of the material of the soleplate  180  drops down to a low value approaching unity when the temperature is beyond the Curie temperature. This will be measured by the current/voltage sensing circuit  150  and the temperature control circuit  160  will be disabled. The current switching circuit  140  becomes inactive and the induction coil  120  gets switched off. The soleplate  180  cools down. The magnetism of the soleplate  180  is revived when the temperature of the soleplate  180  drops below the Curie temperature. 
     The current through the induction coil  120  can be calculated using the equation: 
     
       
         
           
             
               
                 
                   i 
                   = 
                   
                     
                       1 
                       L 
                     
                     · 
                     
                       ∫ 
                       
                         u 
                         · 
                         
                           ⅆ 
                           t 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     wherein i is the current through the induction coil  120 , L is the self-inductance of the induction coil  120 , u is the induced voltage and t is time. The self-inductance L is a function of magnetic permeability μ which in turn is a function of temperature T. For ferromagnetic materials, μ changes rapidly with temperature, when temperature T exceeds the Curie temperature. Hence the self-inductance L varies as magnetic permeability μ of the soleplate  180  varies as a function of its temperature T. According to equation 1, the current i through the induction coil  120  varies with changes in self-inductance L of the induction coil  120 . 
     The current i through the induction coil  120  is measured by the resistor  152 . The parallel resonant circuit  125 , formed by the induction coil  120  and the capacitor  121 , is connected to the supply voltage  142  via the switch  151 . When the switch  151  is closed, linearly increasing current flows through the parallel resonant circuit  125 , the switch  151  and the resistor  152 . This current i is transferred into a voltage u across the resistor  152  and is fed back via the signal conditioning device  153  to the temperature control circuit  160 . The signal conditioning device  153  provides signal conditioning (e.g. low pass filtering and amplification) to the voltage u across the resistor  152 . The temperature control circuit  160  provides the switch  151  with a square wave signal. The duty cycle of this control signal varies to enable power adjustment. It can be a fixed duty cycle (e.g. 50%) if no power control is necessary. Then it is simply a power on/off control. The switch  151  is controlled via a square wave as shown in  FIG. 2 . It is a representation of the output of the temperature control circuit  160 , used to control the switch  151 . At a high signal level the switch  151  is closed. As soon as the switch  151  is closed, current i flows through the induction coil  120 , the capacitor parallel to the coil  121 , the switch  151  and through the resistor  152 . During this phase, energy is stored in the induction coil  120 . When the switch  151  is opened, the energy is released resulting in a (induction) voltage response across the coil  120 . The frequency of this response is determined by the self-inductance L of the coil  120  and the capacitance of the capacitor  121 . The voltage response across the induction coil  120  for a high value of self-inductance L of the induction coil  120  is shown in  FIG. 3  when the switch  151  is opened. The response across the induction coil  120  for a low value of self-inductance L is shown in  FIG. 6 .  FIGS. 4 and 5  show the current i through and voltage u across the resistor  152  for a high value of self-inductance L of the induction coil  120 , whereas  FIGS. 7 and 8  show the current i through and voltage u across the resistor  152  for a low value of self-inductance L of the induction coil  120 . 
     As the switch  151  is closed, current i flows through the resistor  152 , resulting in a voltage u across this resistor  152 . The amplitude of this voltage u is used to switch the temperature control circuit  160  on or off. The amplitude of the current i determines the trigger point for temperature control circuit  160 . It is clear that the self-inductance L of the induction coil  120  is mainly determined by μ of the soleplate to be heated. When the soleplate is heated up to the Curie temperature, μ of the soleplate drops significantly, resulting in a lower self-inductance L. As the self-inductance L of the induction coil  120  decreases, the current i through the switch  151  and the resistor  152  increases. It further results in a higher voltage u across the resistor  152  and a higher response voltage across the induction coil  120  when the switch  151  is released. Both can be used to trigger the temperature control circuit  160 . 
     Commercial alloys like Phytherm 230 or Phytherm 260 can be used whose Curie temperature can be customized. Phytherm 230 has a composition with 50 wt % Ni, 10 wt % Cr and rest Fe. The Curie temperature is 230° C. Phytherm 260 has a composition with 50 wt % Ni, 9 wt % Cr and rest Fe. The Curie temperature is 260° C. 
       FIG. 9  shows an embodiment of an ironing system  200 . The ironing system  200  shown in  FIG. 9  includes an iron  210  and an ironing board  230 . The iron  210  is provided with a soleplate  212  comprising an induction heatable material. The ironing board  230  can be either a compact board or a full-size board. The one or more induction coils  220  positioned within the entire ironing board can charge the iron  210  continuously while ironing. 
       FIG. 10  shows an ironing system  500  comprising an iron  510 , an ironing board  530  and a charging base  520  with one or more induction coils  540 . The iron has a soleplate  512  made from a material whose Curie temperature is substantially close to the ironing temperature i.e. in the range of 100-300° C. The iron  510  has to be returned to the charging base  520  for charging. The alternating magnetic field generated by the induction coil  540  induces eddy currents in the soleplate  512  which then gets heated up. As the temperature of the soleplate exceeds the Curie temperature, the device  550  switches the induction coil  540  off and the iron  510  is ready for use. 
       FIG. 11  shows an iron  610  having an ironing shoe  620 . The soleplate  612  is equipped with a perforation into which the ironing shoe  620  is inserted. All above mentioned embodiments are suitable for single temperature ironing i.e., if a particular material is chosen for the soleplate of the iron, its Curie temperature is fixed and the temperature range at which the iron can be used is fixed. If the temperature chosen is high, then the delicate garments such as silk cannot be ironed. The ironing shoe  620  enables low temperature ironing for delicate garments.
An ironing system ( 500) comprises an iron ( 510) with a soleplate ( 512) comprising an induction heatable material. The ironing system ( 500) further comprises a unit ( 520, 530) that includes at least one induction coil ( 540) and a device ( 550). The induction coil ( 540) charges the iron ( 510) whereas the device ( 550) detects the temperature of the soleplate ( 512) by sensing a change in current flowing through the induction coil ( 540) or by sensing a change in voltage across the induction coil ( 540). The current or voltage changes as self inductance of the induction coil ( 540) changes with magnetic permeability of the soleplate ( 512) as a function of its temperature. The device ( 550) switches the induction coil ( 540) on or off depending on the detected temperature.