Patent Publication Number: US-2018051410-A1

Title: An ironing appliance with means for controlling the heating power

Description:
FIELD OF THE INVENTION 
     The present invention relates to an ironing appliance and a method for controlling an ironing appliance. 
     BACKGROUND OF THE INVENTION 
     A typical conventional ironing appliance uses a soleplate that comes in contact with the garment during ironing. The soleplate is typically heated by a heating element associated with a device such as a thermostat or thermistor, with the aim of heating the soleplate to a set temperature. Conventionally the heating element is controlled by switching it to be at maximum power or off with reference to the set temperature. Typically, the heating element is controlled to be at maximum power until the temperature of the soleplate reaches the set temperature, at which point it is turned off. 
     The limitation of such conventional methods of temperature control is that there will be considerable temperature overshoot as the soleplate is heated due to “heat mass inertia” of the soleplate. In other words, if the soleplate is heated by the heating element until the set temperature is reached, the temperature of the soleplate will keep rising for a period of time, even with the heating element being off. This problem of overshoot is made worse when using higher heating powers and lighter masses of the soleplate. 
     Also, after the temperature overshoot, the temperature of the soleplate will eventually fall back towards the set temperature. Once the temperature of the soleplate reaches the set temperature again, the heating element is controlled to have maximum power again. However, it will be appreciated that there will be temperature undershoot as the temperature of the soleplate keeps falling past the set temperature, before the temperature of the soleplate rises again. 
     As a result, the temperature of the sole plate will be continually cycled, going from below the set temperature to above, and back below. As a result, controlling the heating element to be either at maximum power (when below the set temperature) or off (when above the set temperature) results in non-precise temperature control. 
     It is to be noted that Japanese patent publication JP2966505B discloses an ironing appliance having a heated soleplate in which the heating means for heating the soleplate are controlled based on an observation of change in use state of the appliance. By measuring a sudden change rate of the temperature gradient a transfer between the appliance being in a standby condition and a use condition (or vice versa) is observed. In response to a use state change the so-called control temperature is changed, e.g. to a temperature higher than the fixed (or set) temperature. 
     It is therefore an aim of the present invention to provide an improved ironing appliance that overcomes these problems. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to propose an improved ironing appliance that avoids or mitigates above-mentioned problems. 
     The invention is defined by the independent claims. The dependent claims define advantageous embodiments. 
     According to the present invention, there is provided an ironing appliance comprising: a soleplate; a heating element for heating the soleplate; a temperature sensor for sensing a temperature of the soleplate; a controller for: a) determining a temperature gradient of the soleplate as a rate of change of the temperature of the soleplate over time, and a temperature difference between said temperature of the soleplate and a first predetermined temperature, b) controlling the heating power of the heating element based on said temperature gradient and said temperature difference. 
     By determining a temperature gradient of the soleplate and a temperature difference between a temperature of the soleplate and the first predetermined temperature, and by controlling the heating power of the heating element based on the temperature gradient and the temperature difference, the soleplate can be heated in a way that minimises overshoot and undershoot of temperature. 
     In some embodiments, the controller is arranged to control the heating power of the heating element by either varying the time duration the heating element is turned on in a given time period with a constant electrical power supplied to the heating element, or by varying the electrical power supplied to the heating element. 
     In some embodiments, the controller is arranged to control the heating element to use a first heating power when temperature of the soleplate is less than a second predetermined temperature that is lower than the first predetermined temperature, and to control the heating power of the heating element based on said temperature gradient and said temperature difference when the temperature of the soleplate is between the second predetermined temperature and the first predetermined temperature. Hence, below the second predetermined temperature, the controller can control the heating element at a set power (e.g. maximum). This simplifies the control method. 
     In some embodiments, the first heating power is a maximum heating power. 
     This maximises the speed of the heating of the soleplate. 
     In some embodiments, if said temperature gradient is positive, the controller is arranged to control the heating power of the heating element so that the heating power decreases for increasing values of the temperature of the soleplate from the second predetermined temperature to the first predetermined temperature, wherein this decrease in heating power is larger for larger positive temperature gradients. 
     In some embodiments, if said temperature gradient is negative, the controller is arranged to control the heating power of the heating element so that the heating power decreases for increasing values of the temperature of the soleplate from the second predetermined temperature to the first predetermined temperature, wherein this decrease in heating power is larger for smaller negative temperature gradients. 
     In some embodiments, the controller is arranged to control the heating power of the heating element based on said temperature gradient and said temperature difference when the temperature of the soleplate is between the first predetermined temperature and a third predetermined temperature that is higher than the first predetermined temperature. 
     In some embodiments, if said temperature gradient is positive, the controller is arranged to control the heating power of the heating element so that the heating power is off as the temperature of the soleplate decreases from the third predetermined temperature to the first predetermined temperature. 
     In some embodiments, if said temperature gradient is negative, the controller is arranged to control the heating power of the heating element so that the heating power increases for decreasing values of the temperature of the soleplate from the third predetermined temperature to the first predetermined temperature, wherein this increase in heating power is larger for larger negative temperature gradients. 
     In some embodiments, the controller is arranged to control the heating element so that the heating power is off if the temperature of the soleplate is above the third predetermined temperature. 
     In some embodiments, the range of temperatures between the second predetermined temperature and the third predetermined temperature forms a band for controlling the heating element based on the temperature gradient and the temperature difference between a temperature of the soleplate and the first predetermined temperature T 1 . Below the band (i.e. below the second predetermined temperature), the heating element may be a full power (or some other appropriate power). Above the band (i.e. above the third predetermined temperature T 3 ), the heating element may be off. 
     In some embodiments, ironing appliance further comprises a memory to store a look up table containing values of heating powers at different combinations of temperature differences between the sole plate and the first predetermined temperate and the temperature gradient of the sole plate, the controller being arranged to use said values in the look up table to control the heating power of the heating element. 
     According to another aspect of the invention, there is provided a method of controlling an ironing appliance comprising a soleplate and a heating element for heating the soleplate, the method comprising: determining a temperature gradient of the soleplate and a temperature difference between a temperature of the soleplate and a first predetermined temperature; and controlling the heating power of the heating element based on said temperature gradient and said temperature difference. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of an ironing apparatus  1  according to an embodiment of the invention; 
         FIG. 2  is an illustration of an ironing apparatus  1  according to an embodiment of the invention; 
         FIG. 3  is graph showing the variation of the temperature of a soleplate along the time; and 
         FIG. 4  is a flow chart according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       FIG. 1  shows a schematic illustration of an ironing apparatus  1  according to an embodiment of the invention. The ironing apparatus includes a soleplate  10 , a heating element  20  for heating the soleplate  10 , a controller  30  for controlling the heating power of the heating element  10 , and a temperature sensor  40  for determining the temperature of the soleplate  10 . 
     The heating element  20  in this embodiment is for example an electric heating element and the soleplate  10  is heated by the heating element  20 . The temperature of the soleplate  10  is measured by the temperature sensor  40 . Embodiments of the invention could use any suitable temperature sensor that is thermally coupled to the soleplate  40 . For example, a positive temperature coefficient (PTC) resistor, a negative temperature coefficient (NTC) resistor, and a thermocouple element could be used. 
     The heating power of the heating element  20  is controlled by the controller  30 . 
     In this embodiment, the heating element  20  is controlled by the controller  30  to be turned on or off, with different duty cycles of the on/off time being used to deliver different heating powers. In this embodiment, a triac (not shown) is for example used to deliver intended power to the heating element  20 , though in other embodiments other suitable components (e.g. a solid state switch) could be used. In this embodiment, the triac is controlled by the controller  30  (e.g. including a logic device or MCU) to vary the duty cycle of its conduction on/off time. 
     In other embodiments, the power control can be done by “chopping” the AC waveform. This may, in some embodiments, give better resolution to the power but may result in EMC harmonics noise. The later takes more time to complete one full cycle of power. 
     Hence, in some embodiments, the controller  30  is arranged to control the heating power of the heating element  20  by varying the time duration the heating element  20  is turned on in a given time period (e.g. with a constant electrical power supplied to the heating element  20 ). In other embodiments, the controller  30  is arranged to control the heating power of the heating element  20  by varying the electrical power supplied to the heating element  20 . 
     The desired temperature of the soleplate during ironing can be set by the user by means of a temperature selector or temperature control dial (not shown in  FIG. 1 ), but alternatively any other known control means such as push-buttons or touch controls can be used. 
     In use, the controller  30  compares the instantaneous temperature of the soleplate  10  with the desired temperature and controls the heat production of the heating element  20 . As will be discussed in more detail below, the controller  30  is adapted to control the heating power of the heating element  20  based the temperature gradient of the soleplate  10  and the temperature different between the soleplate  10  and the desired temperature. 
     In the following, the “temperature gradient of the soleplate” refers to the rate of change over time of the temperature of the soleplate. In a more mathematical phrasing the temperature of the sole plate can be understood as the first derivative of the soleplate temperature as a function of time. It will be appreciated that the temperature gradient of the soleplate can be positive (for a rising temperature of the soleplate) or negative (for a falling temperature of the soleplate). 
     The gradient is measured at temperature sensing point on the soleplate  10  where the sensor  40  (e.g. a thermistor) is mounted on the soleplate  10  so as to sense actual temperature and control the heat input. 
     In this embodiment, the controller  30  periodically determines the temperature of the soleplate  10  via the sensor  40 . From this periodic measurement, the controller  30  can determine the temperature gap at a certain time and the temperature gradient at that time. For example, the controller  30  may determine the temperature of the soleplate  10  every 20 ms, though embodiments are not limited thereto. 
     In this embodiment, the controller  30  for example comprises a memory (not shown) to store a look up table containing values of heating powers at different combinations of temperature differences between the soleplate  10  and the desired temperature and the temperature gradient of the sole plate  10 . In such embodiments, the controller  30  is arranged to use values in the look up table to control the heating power of the heating element  20 . 
       FIG. 2  shows schematically shows locations of the soleplate  10 , the heating element  20 , the controller  30 , and the sensor  40  in an ironing appliance  1  according to the first embodiment. The heating element  20  and the sensor  40  are within the body of the soleplate  10 . However, it will be appreciated that embodiments of the invention could be used with any ironing appliance design, and the locations of the various components in  FIG. 2  should not be construed as limiting. For example, the controller  30  could be located in any suitable location in the ironing appliance  1 . 
     The ironing apparatus  1  of  FIG. 2  comprises the soleplate  10  which is heated by the electric heating element  20 . The instantaneous temperature of the soleplate  10  is measured by means of the temperature sensor  40 , for example a PTC resistor, an NTC resistor or a thermocouple element, which is thermally coupled to the soleplate  10 . The desired soleplate temperature can be set by the user by means of a temperature selector or temperature control dial  8 , but alternatively any other known control means such as push-buttons or touch controls can be used. The controller  30  compares the instantaneous temperature of the soleplate  10  with the desired temperature and controls the heat production of the heating element  20 , for example by means of a triac in series with the heating element  20 , in such a manner that the instantaneous temperature becomes equal to the desired temperature. Instead of the shown control using a temperature sensor  40  and a triac it is possible to use other methods such as using a thermostat to control the temperature of the soleplate  10 . 
     The ironing apparatus  1  of this embodiment further comprises a steam generator  12  having a water reservoir  14 , a water pump  16  and a steam chamber  18  which is heated by the soleplate  10 . The water pump  16  pumps water from the water reservoir  14  to the steam chamber  18  via a tube  21 . The water evaporates in the steam chamber  18  and escapes via steam ports  22  formed in the soleplate  10 . The supply of steam is controlled by means of an activation signal AS supplied by the controller  30  in response to a control signal from a control knob or control dial  26  by means of which the amount of steam to be produced can be set. 
     The ironing apparatus  1  of this embodiment further comprises an hand sensor  24  arranged in the handle of the steam iron. The hand sensor can be of any known type, for example a capacitive sensor. The hand sensor  24  informs the controller  30  whether or not the steam iron is in use. 
     It will, however, be appreciated that the above description of  FIG. 2  is merely for illustrative purposes, and that embodiments of the invention could be applied to any type of ironing apparatus with a soleplate  10 , heating element  20 , temperature sensor  40  and a controller  30 . 
     As discussed, in conventional arrangements, the soleplate of an ironing appliance would be heated until the desired temperature is reached, at which point the heating element would be switched off, leading to problems of temperature overshoot and undershoot as discussed above. 
       FIG. 3  shows a graph of temperature against time during the heating of the soleplate  10 . In this graph, it is assumed that the desired (e.g. via user control) temperature of the soleplate  10  is a first predetermined temperature T 1 . 
       FIG. 4  shows a flow chart according to an embodiment of the invention. At step S 1 , the temperature gradient of the soleplate  10  and at step S 20 , the temperature difference between the temperature of the soleplate  10  and the first predetermined temperature T 1  is determined. Steps S 10  and S 20  may be done in either order, or simultaneously. A step S 30  the heating power of the heating element  20  is controlled based on the temperature gradient and the temperature difference. 
     The control logic of the controller  30  of this embodiment aims reduce the temperature overshoot by varying the heating power of the heating element  20  with respect to the temperature gradient of the soleplate  10  and the temperature difference between the temperature of the soleplate  10  and the first predetermined temperature T 1 . 
     In the following description the temperature difference between the temperature of the soleplate  10  and the first predetermined temperature T 1  will be referred to as a “temperature gap”, that is either positive of negative depending on whether the temperature of the soleplate  10  is below the first predetermined temperature T 1  (negative temperature gap) or whether the temperature of the soleplate  10  is above the first predetermined temperature T 1  (positive temperature gap). 
     In this embodiment, the controller  30  controls the heating power of the heating element  20  based on the temperature gradient and the temperature gap when the temperature of the soleplate is between a second predetermined temperature T 2  that is lower than the first predetermined temperature T 1  and a third predetermined temperature T 3  that is higher than the first predetermined temperature T 1 . As shown in  FIG. 3 , in this embodiment, the first predetermined temperature T 1  is midway between the second and third predetermined temperatures T 2 , T 3 , but embodiments of the invention are not limited in this way. 
     In this embodiment, when the temperature of the soleplate  10  is less than the second predetermined temperature T 2 , the controller  30  is arranged to control the heating element  20  to use maximum heating power. Hence, below the second predetermined temperature T 2 , the heating element uses maximum heating power to heat the soleplate  10  as quickly as possible. However, in other embodiments, it may be desired to use a different heating power, for example if the user set first predetermined temperature T 1  is a low setting. 
     In this embodiment, the controller  30  is arranged to control the heating power of the heating element  20  based on the temperature gradient and the temperature gap when the temperature of the soleplate  10  is between the second predetermined temperature T 2  and the third predetermined temperature T 3 . In other words, the second predetermined temperature T 2  and the third predetermined temperature T 3  define a temperature window for the heating power of the heating element  20  to be based on the temperature gradient and the temperature gap. 
     In this embodiment, the second predetermined temperature T 2  is 10° C. less than the first predetermined temperature T 1 , and the third predetermined temperature T 3  is 10° C. greater than the first predetermined temperature T 1 . In some embodiments, the differences between the second predetermined temperature T 2  and the first predetermined temperature T 1  and between the first predetermined temperature T 1  and the third predetermined temperature T 3  may be varied depending on the temperature setting of the ironing appliance. For example, a smaller set of differences may be selected if the user set first predetermined temperature T 1  to have a low setting. 
     In other embodiments, a different gap between the second predetermined temperature T 1  and the first predetermined temperature T 1  may be used when compared to the gap between the first predetermined temperature and the third predetermined temperature T 3 . For example, the absolute value of the temperature gap between the third predetermined temperature T 3  and the first predetermined temperature T 1  could be less than the absolute value of the temperature gap between the second predetermined temperature T 2  and the first predetermined temperature T 1 . 
       FIG. 3  will be explained in more detail with reference to Table 1. In Table 1, there can be considered to be four zones of operation. Zone A is characterised by a negative temperature gap and a negative temperature gradient. Zone B is characterised by a negative temperature gap and a positive temperature gradient. Zone C is characterised by a positive temperature gap and a positive temperature gradient. Zone D is characterised by a positive temperature gap and a negative temperature gradient. 
     In Zone A, the controller  30  is arranged to control the heating power of the heating element  20  so that the heating power decreases for increasing values of the temperature of the soleplate  10  from the second predetermined temperature T 2  to the first predetermined temperature T 1 , with this decrease in heating power being larger for smaller negative temperature gradients. 
     In Zone B, the controller  30  is arranged to control the heating power of the heating element  20  so that the heating power decreases for increasing values of the temperature of the soleplate  10  from the second predetermined temperature T 2  to the first predetermined temperature T 1 , with this decrease in heating power being larger for larger positive temperature gradients. 
     In Zone C, the controller  30  is arranged to control the heating power of the heating element  20  so that the heating power is off as the temperature of the soleplate  10  decreases from the third predetermined temperature T 3  to the first predetermined temperature T 1 . 
     In Zone D, the controller  30  is arranged to control the heating power of the heating element  20  so that the heating power increases for decreasing values of the temperature of the soleplate  10  from the third predetermined temperature T 3  to the first predetermined temperature T 1 , with this increase in heating power being larger for larger negative temperature gradients. 
     Table 1 shows different heating powers (as % s, with the maximum heating power being 100%) applied to the soleplate  10  at different temperature gaps and temperature gradients. As discussed, in this embodiment, the controller  30  comprises a memory (not shown) to store a look up table containing values of heating powers at different combinations of temperature differences between the soleplate  10  and the desired temperature  10  and the temperature gradient of the sole plate  10 . The values of Table 1 could form the basis for such a look up table. 
     In  FIG. 3 , points p 1  to p 10  are shown, with these labels shown at appropriate boxes in Table 1. Points p 1  to p 10  represent different points in time as the soleplate  10  during operation. The arrows associated with points p 1  to p 10  in  FIG. 3  represent relative heating powers. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                 Negative Temperature Gap 
                   
                 Positive Temperature Gap 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 −10° C. 
                 −5° C.  
                   
                   0° C.  
                  +5° C.  
                   
               
               
                   
                   
                   
                 to  
                 to 
                   
                 to 
                 to 
                   
               
               
                   
                   
                 &lt;−10° C. 
                  −5° C. 
                   0° C. 
                 0° C. 
                 +5° C. 
                 +10° C. 
                 &gt;+10° C. 
               
               
                   
               
               
                 Negative 
                 &lt;−2° C./sec 
                 100% 
                 90% 
                 70% 
                   
                 50% 
                 30% 
                 0% 
               
               
                 Temperature 
                   −2° C./sec 
                 100% 
                 70% 
                 50% 
                   
                 30% 
                 10% 
                 0% 
               
               
                 Gradient 
                   
                   
                   
                 (p5) 
                   
                 (p4)  
                   
                   
               
               
                   
                   −1° C./sec 
                 100% 
                 50% 
                 30% 
                   
                 10% 
                  0% 
                 0% 
               
               
                   
                   
                   
                 (p6) 
                 (p9) 
                   
                 (p8)  
                   
                   
               
               
                   
                   
                   
                   
                   
                   
                 (p10) 
                   
                   
               
               
                   
                     0° C./sec 
                   
                   
                   
                   
                   
                   
                   
               
               
                 Positive 
                   +1° C./sec 
                 100% 
                 70% 
                 50% 
                   
                  0% 
                  0% 
                 0% 
               
               
                 Temperature 
                   
                 (p1) 
                   
                   
                   
                   
                   
                   
               
               
                 Gradient 
                   +2° C./sec 
                 100% 
                 50% 
                 30% 
                   
                  0% 
                  0% 
                 0% 
               
               
                   
                   
                   
                 (p2) 
                 (p7) 
                   
                   
                   
                   
               
               
                   
                 &gt;+2° C./sec 
                 100% 
                 30% 
                 10% 
                   
                  0% 
                  0% 
                 0% 
               
               
                   
                   
                   
                   
                 (p3) 
               
               
                   
               
            
           
         
       
     
     In this embodiment, temperatures more than 10° C. under the first predetermined temperature T 1  (i.e. a negative temperature gap of &lt;−10° C.) or more than 10° C. over the first predetermined temperature T 1  (i.e. a positive temperature gap of &gt;+10° C.) fall outside of the range of the second predetermined temperature T 2  to the third predetermined temperature T 3 . In this embodiment, for temperatures lower than the second predetermined temperature T 2  (i.e. a negative temperature gap of &lt;−10° C.), full heating power (i.e. 100%) is used regardless of the temperature gradient. Similarly, in this embodiment, for temperatures greater than the third predetermined temperature T 5  (i.e. a positive temperature gap of &gt;+10° C.), zero heating power (i.e. 0%) is used regardless of the temperature gradient. 
     Points p 1  to p 10  represent an example of temperature gaps and temperature gradients during the operation of an ironing appliance according to an embodiment of the invention. At each point p 1  to p 10 , the controller  30  will determine the temperature gradient of the soleplate  10  and the temperature gap using the sensor  40 . 
     At point p 1  (region of +1° C./sec positive temperature gradient and a negative temperature gap of more than −10° C.), the temperature of the soleplate  10  is lower than the second predetermined temperature T 2  and the temperature of the soleplate  10  is relatively slowly rising. 
     At point p 1  in this embodiment, the controller  30  controls the heating element  20  to heat the soleplate  10  at 100% heating power. This is because, in this embodiment, at point p 1 , the temperature of the soleplate is outside the operating window of control based on the temperature gradient and temperature gap. It will be appreciated, however, that in some embodiments, the controller  30  may control the heating element  20  based on the temperature gradient and temperature gap for all detected temperatures of the soleplate  10 . 
     At point p 2  (region of +2° C./sec positive temperature gradient and a negative temperature gap of between −10° C. and −5° C.) and at point p 3  (region of greater than +2° C./sec positive temperature gradient and a negative temperature gap of between −5° C. and −0° C.), the temperature of the soleplate  10  is above the second predetermined temperature T 2  but lower than the second predetermined temperature T 2 . 
     From p 1  to p 3 , the temperature of the soleplate  10  is increasing, and so the controller  30  reduces the power of the heating element  20  so as to slow the heating of the soleplate  10  as the temperature of the soleplate  10  approaches the first predetermined temperature T 1  with a positive temperature gradient. This slowing of the heating of the soleplate  10  as the soleplate  10  rises towards the first predetermined temperature T 1  helps to minimise temperature overshoot. 
     As shown in  FIG. 3 , following p 3 , the temperature of the soleplate  10  will overshoot the first predetermined temperature T 1 , with the amount of overshoot being smaller than it would have been if the heating power had been 100% until the first predetermined temperature T 1  was reached. This smaller overshoot is because of the reduction in the heating power of the heating element  20  as the temperature of the soleplate  10  approached the first predetermined temperature T 1 . 
     The temperature of the soleplate  10  will then peak and then fall. At point p 4  (region of −2° C./sec negative temperature gradient and a positive temperature gap of between 0° C. and +5° C.) the temperature of the soleplate  10  is falling and a small (30%) heating power is provided so as to attempt to minimise the undershoot of the first predetermined temperature T 1 . 
     At point p 5  (region of −2° C./sec negative temperature gradient and a negative temperature gap of between 0° C. and −5° C.) the temperature of the soleplate  10  is still falling and a 50% heating power is provided so as to raise the soleplate  10  back towards the first predetermined temperature T 1 . At point p 6  (region of −1° C./sec negative temperature gradient and a negative temperature gap of between −5° C. and −10° C.), the 50% heating power is maintained. 
     Hence, following p 4 , the temperature of the soleplate  10  will undershoot the first predetermined temperature T 1  (points p 5  and p 6 ), with the undershoot amount being smaller than it would have been if no heating power had been used as the temperature of the soleplate  10  approached the first predetermined temperature T 1  with a negative temperature gradient. It will be appreciated that at points p 5  and p 6 , the control method is providing an under-damped response. 
     The temperature of the soleplate  10  will then reach maximum undershoot and start rising again. At point p 7  (region of +2° C./sec positive temperature gradient and a negative temperature gap of between 0° C. and −5° C.), the temperature of the soleplate  10  is rising, and so the heating power is lowered (as compared to point p 6 ) to 30%. 
     By point p 8  (region of −1° C./sec negative temperature gradient and a positive temperature gap of between 0° C. and 5° C.), the temperature of the soleplate  10  has passed the first predetermined temperature T 1 , peaked again, and is now falling again. Hence, a small 10% heating power is used so as to minimum undershoot. 
     At point p 9  (region of −1° C./sec negative temperature gradient and a negative temperature gap of between 0° C. and −5° C.) the temperature of the soleplate  10  is falling below the first predetermined temperature T 1 , and therefore a 30% heating power (i.e. larger than that at point p 8 ) is used. 
     Point p 10  is in the same region of Table 1 as point p 8  (i.e. 1° C./sec negative temperature gradient and a positive temperature gap of between 0° C. and 5° C.). The controller  30  will continue to regulate the heating of the soleplate  10  in this way. 
     Although this embodiment has been discussed in the context of a look up table in the form of Table 1, it will be appreciated that other embodiments could use other ways of storing the relationship between the temperature gradient and the temperature gap. Furthermore, in embodiments that use a look up table, the values in the look up table may be scaled in some embodiments depending on the first predetermined temperature T 1  (i.e. the set temperature) or other factors. 
     As discussed, in embodiments of the invention, there is provided an ironing appliance comprising a soleplate  10 , a heating element  20  for heating the soleplate  20 , a controller  30  for controlling the heating power of the heating element  20 , and a temperature sensor  40 . The controller  30  determines a temperature gradient of the soleplate  10  and a temperature difference between a temperature of the soleplate  10  and a first predetermined temperature T 1 , and the controller  30  is adapted to control the heating power of the heating element ( 20 ) based on said temperature gradient and said temperature difference. 
     By determining a temperature gradient of the soleplate  10  and a temperature difference between a temperature of the soleplate  10  and the first predetermined temperature, and by controlling the heating power of the heating element  20  based on the temperature gradient and the temperature difference, the soleplate  10  can be heated in a way that minimises overshoot and undershoot. 
     In some embodiments, the range of temperatures between the second predetermined temperature T 2  and the third predetermined temperature T 3  forms a band for controlling the heating element  20  based on the temperature gradient and the temperature difference between a temperature of the soleplate  10  and the first predetermined temperature T 1 . Below the band (i.e. below the second predetermined temperature T 2 ), the heating element  20  may be a full power (or some other appropriate power). Above the band (i.e. above the third predetermined temperature T 3 ), the heating element  20  may be off. 
     As a result, the inputs of the control method of embodiments of the present invention as the temperature gradient and the gap between the actual temperature and the set point (i.e. first predetermined temperature T 1 ). The control method of the present invention aims to reduce the temperature range within which the soleplate fluctuates around the set point (i.e. first predetermined temperature T 1 ). The control method of some embodiments reduces the temperature gradient when the temperature gap is small. Some embodiments can provide an under-damped or a critically damped temperature response. 
     It will be appreciated that there are several external factors that may affect the temperature of the sole plate, such as whether the ironing apparatus is in contact with clothes, whether a steam function of the ironing apparatus is on, whether the ironing apparatus is resting on its heal or resting horizontally on an ironing board. The present invention can take these factors into account by controlling the heating element based on the temperature gap and the temperature gradient. 
     The above embodiments as described are only illustrative, and not intended to limit the technique approaches of the present invention. Although the present invention is described in details referring to the preferable embodiments, those skilled in the art will understand that the technique approaches of the present invention can be modified or equally displaced without departing from the spirit and scope of the technique approaches of the present invention, which will also fall into the protective scope of the claims of the present invention. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. Any reference signs in the claims should not be construed as limiting the scope.