Patent Publication Number: US-2021186108-A1

Title: Apparatus for an aerosol generating device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a National Phase entry of PCT Application No. PCT/EP2019/073260, filed Aug. 30, 2019, which claims priority from Great Britain Patent Application No. 1814198.6 filed Aug. 31, 2018, each of which is fully incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates to apparatus for an aerosol generating device, in particular, apparatus for determining a property of a susceptor arrangement for use with the aerosol generating device. 
     BACKGROUND 
     Smoking articles such as cigarettes, cigars and the like burn tobacco during use to create tobacco smoke. Attempts have been made to provide alternatives to these articles by creating products that release compounds without combusting. Examples of such products are so-called “heat not burn” products or tobacco heating devices or products, which release compounds by heating, but not burning, material. The material may be, for example, tobacco or other non-tobacco products, which may or may not contain nicotine. 
     SUMMARY 
     According to a first aspect of the present invention, there is provided apparatus for an aerosol generating device, the apparatus comprising: a circuit comprising an inductive element for heating a susceptor arrangement to heat an aerosol generating material; and a controller configured to: determine a change in an electrical parameter of the circuit when the circuit is changed between an unloaded state wherein the susceptor arrangement is not inductively coupled to the inductive element, and a loaded state wherein the susceptor arrangement is inductively coupled to the inductive element; and determine a property of the susceptor arrangement from the change in the electrical parameter of the circuit. 
     The circuit may be changed from the unloaded state to the loaded state when the susceptor arrangement is received by the device, and the circuit may be changed from the loaded state to the unloaded state when the susceptor arrangement is removed from the device. 
     The change in the electrical parameter may be determined by comparing a value of the parameter measured when the circuit is in the loaded state to a value of the parameter measured when the circuit is in the unloaded state. 
     The change in the electrical parameter may be determined by comparing: a value of the parameter measured when the circuit is in the loaded state, to a predetermined value of the parameter corresponding to the circuit in the unloaded state. 
     Determining the property of the susceptor arrangement may comprise comparing the determined change in the value of the electrical parameter to a list of at least one stored value, wherein the property of the susceptor arrangement is indicated by determining to which value in the list the determined change corresponds. 
     The controller may be configured to allow activation of the aerosol generating device for use or not allow activation of the aerosol generating device for use depending on the determined property of the susceptor arrangement. 
     The controller may be configured to determine a property of the susceptor arrangement based on the magnitude of the change in the electrical parameter of the circuit. 
     The controller may be configured to determine a property of the susceptor arrangement based on the sign of the change in the electrical parameter of the circuit. 
     The property of the susceptor arrangement may be whether or not the susceptor arrangement is present in the device, and the controller may be configured to determine that the susceptor arrangement is present in the device based on whether a change in the electrical parameter is present. 
     The apparatus may comprise a temperature measuring device and the controller may be configured to receive a measured temperature of the susceptor arrangement from the temperature measuring device at a time when the circuit is changed between the loaded state and the unloaded state and use the measured temperature of the susceptor arrangement in determining the property of the susceptor arrangement. 
     The susceptor arrangement may be in a consumable comprising the aerosol generating material to be heated and the controller may be configured to determine a property of the consumable from the determined property of the susceptor arrangement. 
     The property of the consumable may comprise an indicator of whether the consumable is an approved consumable or not an approved consumable, and the controller may be configured to determine whether or not the consumable is an approved consumable and activate the device for use if the consumable is an approved consumable and not activate the device for use if the consumable is not an approved consumable. 
     The electrical parameter may be a resonant frequency of the circuit. 
     The electrical parameter may be an effective grouped resistance r of the inductive element and the susceptor arrangement. 
     The apparatus may further comprise a capacitive element and a switching arrangement for enabling a varying current to be generated from a DC voltage supply and flow through the inductive element; and the controller may be configured to determine the effective resistance r from a frequency of the varying current being supplied to the inductive element, a DC current from the DC voltage supply, and a DC voltage of the DC voltage supply, and wherein the effective grouped resistance r of the inductive element and susceptor arrangement is determined by the controller according to the relationship: 
     
       
         
           
             r 
             = 
             
               
                 
                   I 
                   s 
                 
                 
                   V 
                   s 
                 
               
                
               
                 1 
                 
                   
                     ( 
                     
                       2 
                        
                       π 
                        
                       
                           
                       
                        
                       
                         f 
                         0 
                       
                        
                       C 
                     
                     ) 
                   
                   2 
                 
               
             
           
         
       
     
     where V s  is the DC voltage and I s  is the DC current, C is a capacitance of the circuit, and f 0  is the frequency of the varying current being supplied to the inductive element. 
     According to a second aspect of the present invention there is provided a method of determining a property of a susceptor arrangement for an aerosol generating device, wherein the susceptor arrangement is for heating an aerosol generating material, and the aerosol generating device comprises a controller and a circuit comprising an inductive element for heating the susceptor, wherein the method comprises: determining, by the controller, a change in an electrical parameter of the circuit when the circuit is changed between an unloaded state wherein the susceptor arrangement is not inductively coupled to the inductive element, and a loaded state wherein the susceptor arrangement is inductively coupled to the inductive element; and determining, by the controller, the property of the susceptor arrangement from the change in the electrical parameter of the circuit. 
     The susceptor arrangement may be in a consumable comprising aerosol generating material to be heated and the method may comprise determining a property of the consumable from the property of the susceptor arrangement. 
     According to a third aspect of the present invention there is provided a controller for an aerosol generating device, wherein the controller is configured to perform a method according to the second aspect. 
     According to a fourth aspect of the present invention there is provided an aerosol generating device comprising apparatus according to the first aspect. 
     According to a fifth aspect of the present invention there is provided a set of machine readable instructions which when executed by a controller in an aerosol generating device cause the controller to execute a method according to the second aspect. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates schematically an aerosol generating device according to an example; 
         FIG. 2  illustrates schematically a resonant circuit according to an example. 
         FIG. 3  shows plots of resonant frequency of the resonant circuit of  FIG. 2  against time, according to an example. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Induction heating is a process of heating an electrically conducting object (or susceptor) by electromagnetic induction. An induction heater may comprise an inductive element, for example, an inductive coil and a device for passing a varying electric current, such as an alternating electric current, through the inductive element. The varying electric current in the inductive element produces a varying magnetic field. The varying magnetic field penetrates a susceptor suitably positioned with respect to the inductive element, generating eddy currents inside the susceptor. The susceptor has electrical resistance to the eddy currents, and hence the flow of the eddy currents against this resistance causes the susceptor to be heated by Joule heating. In cases where the susceptor comprises ferromagnetic material such as iron, nickel or cobalt, heat may also be generated by magnetic hysteresis losses in the susceptor, i.e. by the varying orientation of magnetic dipoles in the magnetic material as a result of their alignment with the varying magnetic field. 
     In inductive heating, as compared to heating by conduction for example, heat is generated inside the susceptor, allowing for rapid heating. Further, there need not be any physical contact between the inductive heater and the susceptor, allowing for enhanced freedom in construction and application. 
     An induction heater may comprise an LC circuit, having an inductance L provided by an induction element, for example the electromagnet which may be arranged to inductively heat a susceptor, and a capacitance C provided by a capacitor. The circuit may in some cases be represented as an RLC circuit, comprising a resistance R provided by a resistor. In some cases, resistance is provided by the ohmic resistance of parts of the circuit connecting the inductor and the capacitor, and hence the circuit need not necessarily include a resistor as such. Such a circuit may be referred to, for example as an LC circuit. Such circuits may exhibit electrical resonance, which occurs at a particular resonant frequency when the imaginary parts of impedances or admittances of circuit elements cancel each other. 
     One example of a circuit exhibiting electrical resonance is an LC circuit, comprising an inductor, a capacitor, and optionally a resistor. One example of an LC circuit is a series circuit where the inductor and capacitor are connected in series. Another example of an LC circuit is a parallel LC circuit where the inductor and capacitor are connected in parallel. Resonance occurs in an LC circuit because the collapsing magnetic field of the inductor generates an electric current in its windings that charges the capacitor, while the discharging capacitor provides an electric current that builds the magnetic field in the inductor. An example parallel LC circuit is described herein. When a parallel LC circuit is driven at the resonant frequency, the dynamic impedance of the circuit is at a maximum (as the reactance of the inductor equals the reactance of the capacitor), and circuit current is at a minimum. However, for a parallel LC circuit, the parallel inductor and capacitor loop acts as a current multiplier (effectively multiplying the current within the loop and thus the current passing through the inductor). Driving the RLC or LC circuit at or near the resonant frequency may therefore provide for effective and/or efficient inductive heating by providing for the greatest value of the magnetic field penetrating the susceptor. 
     A transistor is a semiconductor device for switching electronic signals. A transistor typically comprises at least three terminals for connection to an electronic circuit. In some prior art examples, an alternating current may be supplied to a circuit using a transistor by supplying a drive signal which causes the transistor to switch at a predetermined frequency, for example at the resonant frequency of the circuit. 
     A field effect transistor (FET) is a transistor in which the effect of an applied electric field may be used to vary the effective conductance of the transistor. The field effect transistor may comprise a body B, a source terminal S, a drain terminal D, and a gate terminal G. The field effect transistor comprises an active channel comprising a semiconductor through which charge carriers, electrons or holes, may flow between the source S and the drain D. The conductivity of the channel, i.e. the conductivity between the drain D and the source S terminals, is a function of the potential difference between the gate G and source S terminals, for example generated by a potential applied to the gate terminal G. In enhancement mode FETs, the FET may be OFF (i.e. substantially prevent current from passing therethrough) when there is substantially zero gate G to source S voltage, and may be turned ON (i.e. substantially allow current to pass therethrough) when there is a substantially non-zero gate G-source S voltage. 
     An n-channel (or n-type) field effect transistor (n-FET) is a field effect transistor whose channel comprises an n-type semiconductor, where electrons are the majority carriers and holes are the minority carriers. For example, n-type semiconductors may comprise an intrinsic semiconductor (such as silicon for example) doped with donor impurities (such as phosphorus for example). In n-channel FETs, the drain terminal D is placed at a higher potential than the source terminal S (i.e. there is a positive drain-source voltage, or in other words a negative source-drain voltage). In order to turn an n-channel FET “on” (i.e. to allow current to pass therethrough), a switching potential is applied to the gate terminal G that is higher than the potential at the source terminal S. 
     A p-channel (or p-type) field effect transistor (p-FET) is a field effect transistor whose channel comprises a p-type semiconductor, where holes are the majority carriers and electrons are the minority carriers. For example, p-type semiconductors may comprise an intrinsic semiconductor (such as silicon for example) doped with acceptor impurities (such as boron for example). In p-channel FETs, the source terminal S is placed at a higher potential than the drain terminal D (i.e. there is a negative drain-source voltage, or in other words a positive source-drain voltage). In order to turn a p-channel FET “on” (i.e. to allow current to pass therethrough), a switching potential is applied to the gate terminal G that is lower than the potential at the source terminal S (and which may for example be higher than the potential at the drain terminal D). 
     A metal-oxide-semiconductor field effect transistor (MOSFET) is a field effect transistor whose gate terminal G is electrically insulated from the semiconductor channel by an insulating layer. In some examples, the gate terminal G may be metal, and the insulating layer may be an oxide (such as silicon dioxide for example), hence “metal-oxide-semiconductor.” However, in other examples, the gate may be made from other materials than metal, such as polysilicon, and/or the insulating layer may be made from other materials than oxide, such as other dielectric materials. Such devices are nonetheless typically referred to as metal-oxide-semiconductor field effect transistors (MOSFETs), and it is to be understood that as used herein the term metal-oxide-semiconductor field effect transistors or MOSFETs is to be interpreted as including such devices. 
     A MOSFET may be an n-channel (or n-type) MOSFET where the semiconductor is n-type. The n-channel MOSFET (n-MOSFET) may be operated in the same way as described above for the n-channel FET. As another example, a MOSFET may be a p-channel (or p-type) MOSFET, where the semiconductor is p-type. The p-channel MOSFET (p-MOSFET) may be operated in the same way as described above for the p-channel FET. An n-MOSFET typically has a lower source-drain resistance than that of a p-MOSFET. Hence in an “on” state (i.e. where current is passing therethrough), n-MOSFETs generate less heat as compared to p-MOSFETs, and hence may waste less energy in operation than p-MOSFETs. Further, n-MOSFETs typically have shorter switching times (i.e. a characteristic response time from changing the switching potential provided to the gate terminal G to the MOSFET changing whether or not current passes therethrough) as compared to p-MOSFETs. This can allow for higher switching rates and improved switching control. 
       FIG. 1  illustrates schematically an aerosol generating device  100 , according to an example. The aerosol generating device  100  comprises a DC power source  104 , in this example a battery  104 , a circuit  150  comprising an inductive element  158 , a susceptor arrangement  110 , and aerosol generating material  116 . 
     In the example of  FIG. 1 , the susceptor arrangement  110  is located within a consumable  120  along with the aerosol generating material  116 . The DC power source  104  is electrically connected to the circuit  150  and is arranged to provide DC electrical power to the circuit  150 . The device  100  also comprises control circuitry  106 , also referred to herein as a controller. In this example the circuit  150  is connected to the power source  104  such as a battery via the control circuitry  106 . 
     The control circuitry  106  may comprise means for switching the device  100  on and off, for example in response to a user input. The control circuitry  106  may for example comprise a puff detector (not shown), as is known per se, and/or may take user input via at least one button or touch control (not shown). The control circuitry  106  may comprise means for monitoring the temperature of components of the device  100  or components of a consumable  120  inserted in the device. In addition to the inductive element  158 , the circuit  150  comprises other components which are described below. 
     The inductive element  158  may be, for example, a coil, which may for example be planar. The inductive element  158  may, for example, be formed from copper (which has a relatively low resistivity). The circuitry  150  is arranged to convert an input DC current from the DC power source  104  into a varying, for example alternating, current through the inductive element  158 . The circuitry  150  is arranged to drive the varying current through the inductive element  158 . 
     The susceptor arrangement  110  is arranged relative to the inductive element  158  for inductive energy transfer from the inductive element  158  to the susceptor arrangement  110 . The susceptor arrangement  110  may be formed from any suitable material that can be inductively heated, for example a metal or metal alloy, e.g., steel. In some implementations, the susceptor arrangement  110  may comprise or be entirely formed from a ferromagnetic material, which may comprise one or a combination of example metals such as iron, nickel and cobalt. In some implementations, the susceptor arrangement  110  may comprise or be formed entirely from a non-ferromagnetic material, for example aluminum. The inductive element  158 , having varying current driven therethrough, causes the susceptor arrangement  110  to heat up by Joule heating and/or by magnetic hysteresis heating, as described above. The susceptor arrangement  110  is arranged to heat the aerosol generating material  116 , for example by conduction, convection, and/or radiation heating, to generate an aerosol in use. In some examples, the susceptor arrangement  110  and the aerosol generating material  116  form an integral unit that may be inserted and/or removed from the aerosol generating device  100 , and may be disposable. In some examples, the inductive element  158  may be removable from the device  100 , for example for replacement. The aerosol generating device  100  may be hand-held. The aerosol generating device  100  may be arranged to heat the aerosol generating material  116  to generate aerosol for inhalation by a user. 
     It is noted that, as used herein, the term “aerosol generating material” includes materials that provide volatilized components upon heating, typically in the form of vapor or an aerosol. Aerosol generating material may be a non-tobacco-containing material or a tobacco-containing material. For example, the aerosol generating material may be or comprise tobacco. Aerosol generating material may, for example, include one or more of tobacco per se, tobacco derivatives, expanded tobacco, reconstituted tobacco, tobacco extract, homogenized tobacco or tobacco substitutes. The aerosol generating material can be in the form of ground tobacco, cut rag tobacco, extruded tobacco, reconstituted tobacco, reconstituted material, liquid, gel, gelled sheet, powder, or agglomerates, or the like. Aerosol generating material also may include other, non-tobacco, products, which, depending on the product, may or may not contain nicotine. 
     Aerosol generating material may comprise one or more humectants, such as glycerol or propylene glycol. 
     Returning to  FIG. 1 , the aerosol generating device  100  comprises an outer body  112  housing the DC power supply  104 , the control circuitry  106  and the circuit  150  comprising the inductive element  158 . The consumable  120  comprising the susceptor arrangement  110  and the aerosol generating material  116  in this example is also inserted into the body  112  to configure the device  100  for use. The outer body  112  comprises a mouthpiece  114  to allow aerosol generated in use to exit the device  100 . 
     In use, a user may activate, for example via a button (not shown) or a puff detector (not shown), the circuitry  106  to cause a varying, e.g. alternating, current to be driven through the inductive element  108 , thereby inductively heating the susceptor arrangement  110 , which in turn heats the aerosol generating material  116 , and causes the aerosol generating material  116  thereby to generate an aerosol. The aerosol is generated into air drawn into the device  100  from an air inlet (not shown), and is thereby carried to the mouthpiece  104 , where the aerosol exits the device  100  for inhalation by a user. 
     The circuit  150  comprising the inductive element  158 , and the susceptor arrangement  110  and/or the device  100  as a whole may be arranged to heat the aerosol generating material  116  to a range of temperatures to volatilize at least one component of the aerosol generating material  116  without combusting the aerosol generating material. For example, the temperature range may be about 50° C. to about 350° C., such as between about 50° C. and about 300° C., between about 100° C. and about 300° C., between about 150° C. and about 300° C., between about 100° C. and about 200° C., between about 200° C. and about 300° C., or between about 150° C. and about 250° C. In some examples, the temperature range is between about 170° C. and about 250° C. In some examples, the temperature range may be other than this range, and the upper limit of the temperature range may be greater than 300° C. 
     It will be appreciated that there may be a difference between the temperature of the susceptor arrangement  110  and the temperature of the aerosol generating material  116 , for example during heating up of the susceptor arrangement  110 , for example where the rate of heating is large. It will therefore be appreciated that in some examples the temperature at which the susceptor arrangement  110  is heated to may, for example, be higher than the temperature to which it is desired that the aerosol generating material  116  is heated. 
     Referring now to  FIG. 2 , there is illustrated an example circuit  150 , which is a resonant circuit, for inductive heating of the susceptor arrangement  110 . The resonant circuit  150  comprises the inductive element  158  and a capacitor  156 , connected in parallel. 
     The resonant circuit  150  comprises a switching arrangement M 1 , M 2  which, in this example, comprises a first transistor M 1  and a second transistor M 2 . The first transistor M 1  and the second transistor M 2  each comprise a first terminal G, a second terminal D and a third terminal S. The second terminals D of the first transistor M 1  and the second transistor M 2  are connected to either side of the parallel inductive element  158  and the capacitor  156  combination, as will be explained in more detail below. The third terminals S of the first transistor M 1  and the second transistor M 2  are each connected to earth  151 . In the example illustrated in  FIG. 2  the first transistor M 1  and the second transistor M 2  are both MOSFETS and the first terminals G are gate terminals, the second terminals D are drain terminals and the third terminals S are source terminals. 
     It will be appreciated that in alternative examples other types of transistors may be used in place of the MOSFETs described above. 
     The resonant circuit  150  has an inductance L and a capacitance C. The inductance L of the resonant circuit  150  is provided by the inductive element  158 , and may also be affected by an inductance of the susceptor arrangement  110  which is arranged for inductive heating by the inductive element  158 . The inductive heating of the susceptor arrangement  110  is via a varying magnetic field generated by the inductive element  158 , which, in the manner described above, induces Joule heating and/or magnetic hysteresis losses in the susceptor arrangement  110 . A portion of the inductance L of the resonant circuit  150  may be due to the magnetic permeability of the susceptor arrangement  110 . The varying magnetic field generated by the inductive element  158  is generated by a varying, for example alternating, current flowing through the inductive element  158 . 
     The inductive element  158  may, for example, be in the form of a coiled conductive element. For example, inductive element  158  may be a copper coil. The inductive element  158  may comprise, for example, multi-stranded wire, such as Litz wire, for example a wire comprising a number of individually insulated wires twisted together. The AC resistance of a multi-stranded wire is a function of frequency and the multi-stranded wire can be configured in such a way that the power absorption of the inductive element is reduced at a driving frequency. As another example, the inductive element  158  may be a coiled track on a printed circuit board, for example. Using a coiled track on a printed circuit board may be useful as it provides for a rigid and self-supporting track, with a cross section which obviates any requirement for multi-stranded wire (which may be expensive), which can be mass produced with a high reproducibility for low cost. Although one inductive element  158  is shown, it will be readily appreciated that there may be more than one inductive element  158  arranged for inductive heating of one or more susceptor arrangements  110 . 
     The capacitance C of the resonant circuit  150  is provided by the capacitor  156 . The capacitor  156  may be, for example, a Class  1  ceramic capacitor, for example a COG type capacitor. The total capacitance C may also comprise the stray capacitance of the resonant circuit  150 ; however, this is or can be made negligible compared with the capacitance provided by the capacitor  156 . 
     The resistance of the resonant circuit  150  is not shown in  FIG. 2  but it should be appreciated that a resistance of the circuit may be provided by the resistance of the track or wire connecting the components of the circuit  150 , the resistance of the inductor  158 , and/or the resistance to current flowing through the circuit  150  provided by the susceptor arrangement  110  arranged for energy transfer with the inductor  158 . In some examples, one or more dedicated resistors (not shown) may be included in the resonant circuit  150 . 
     The resonant circuit  150  is supplied with a DC supply voltage V 1  provided from the DC power source  104  (see  FIG. 1 ), e.g. from a battery. A positive terminal of the DC voltage supply V 1  is connected to the resonant circuit  150  at a first point  159  and at a second point  160 . A negative terminal (not shown) of the DC voltage supply V 1  is connected to earth  151  and hence, in this example, to the source terminals S of both the MOSFETs M 1  and M 2 . In examples, the DC supply voltage V 1  may be supplied to the resonant circuit directly from a battery or via an intermediary element. 
     The resonant circuit  150  may therefore be considered to be connected as an electrical bridge with the inductive element  158  and the capacitor  156  in parallel connected between the two arms of the bridge. The resonant circuit  150  acts to produce a switching effect, described below, which results in alternating current being drawn through the inductive element  158 , thus creating the alternating magnetic field and heating the susceptor arrangement  110 . 
     The first point  159  is connected to a first node A located at a first side of the parallel combination of the inductive element  158  and the capacitor  156 . The second point  160  is connected to a second node B, to a second side of the parallel combination of the inductive element  158  and the capacitor  156 . A first choke inductor  161  is connected in series between the first point  159  and the first node A, and a second choke inductor  162  is connected in series between the second point  160  and the second node B. The first and second chokes  161  and  162  act to filter out AC frequencies from entering the circuit from the first point  159  and the second point  160  respectively but allow DC current to be drawn into and through the inductor  158 . The chokes  161  and  162  allow the voltage at A and B to oscillate with little or no visible effects at the first point  159  or the second point  160 . 
     In this particular example, the first MOSFET M 1  and the second MOSFET M 2  are n-channel enhancement mode MOSFETs. The drain terminal of the first MOSFET M 1  is connected to the first node A via a conducting wire or the like, while the drain terminal of the second MOSFET M 2  is connected to the second node B, via a conducting wire or the like. The source terminal of each MOSFET M 1 , M 2  is connected to earth  151 . 
     The resonant circuit  150  comprises a second voltage source V 2 , gate voltage supply (or sometimes referred to herein as a control voltage), with its positive terminal connected at a third point  165  which is used for supplying a voltage to the gate terminals G of the first and second MOSFETs M 1  and M 2 . The control voltage V 2  supplied at the third point  165  in this example is independent of the voltage V 1  supplied at the first and second points  159 ,  160 , which enables variation of voltage V 1  without impacting the control voltage V 2 . A first pull-up resistor  163  is connected between the third point  165  and the gate terminal G of the first MOSFET M 1 . A second pull-up resistor  164  is connected between the third point  165  and the gate terminal G of the second MOSFET M 2 . 
     In other examples, a different type of transistor may be used, such as a different type of FET. It will be appreciated that the switching effect described below can be equally achieved for a different type of transistor which is capable of switching from an “on” state to an “off” state. The values and polarities of the supply voltages V 1  and V 2  may be chosen in conjunction with the properties of the transistor used, and the other components in the circuit. For example, the supply voltages may be chosen in dependence on whether an n-channel or p-channel transistor is used, or in dependence on the configuration in which the transistor is connected, or the difference in the potential difference applied across terminals of the transistor which results in the transistor being in either on or off. 
     The resonant circuit  150  further comprises a first diode d 1  and a second diode d 2 , which in this example are Schottky diodes, but in other examples any other suitable type of diode may be used. The gate terminal G of the first MOSFET M 1  is connected to the drain terminal D of the second MOSFET M 2  via the first diode d 1 , with the forward direction of the first diode d 1  being towards the drain D of the second MOSFET M 2 . 
     The gate terminal G of the second MOSFET M 2  is connected to the drain D of the first second MOSFET M 1  via the second diode d 2 , with the forward direction of the second diode d 2  being towards the drain D of the first MOSFET M 1 . The first and second Schottky diodes d 1  and d 2  may have a diode threshold voltage of around 0.3V. In other examples, silicon diodes may be used having a diode threshold voltage of around 0.7V. In examples, the type of diode used is selected in conjunction with the gate threshold voltage, to allow desired switching of the MOSFETs M 1  and M 2 . It will be appreciated that the type of diode and gate supply voltage V 2  may also be chosen in conjunction with the values of pull-up resistors  163  and  164 , as well as the other components of the resonant circuit  150 . 
     The resonant circuit  150  supports a current through the inductive element  158  which is a varying current due to switching of the first and second MOSFETs M 1  and M 2 . Since in this example the MOSFETs M 1  and M 2  are enhancement mode MOSFETS, when a voltage applied at the gate terminal G of one of the MOSFETs is such that a gate-source voltage is higher than a predetermined threshold for that MOSFET, the MOSFET is turned to the ON state. Current may then flow from the drain terminal D to the source terminal S which is connected to ground  151 . The series resistance of the MOSFET in this ON state is negligible for the purposes of the operation of the circuit, and the drain terminal D can be considered to be at ground potential when the MOSFET is in the ON state. The gate-source threshold for the MOSFET may be any suitable value for the resonant circuit  150  and it will be appreciated that the magnitude of the voltage V 2  and resistances of resistors  164  and  163  are chosen dependent on the gate-source threshold voltage of the MOSFETs M 1  and M 2 , essentially so that voltage V 2  is greater than the gate threshold voltage(s). 
     The switching procedure of the resonant circuit  150  which results in varying current flowing through the inductive element  158  will now be described starting from a condition where the voltage at first node A is high and the voltage at the second node B is low. 
     When the voltage at node A is high, the voltage at the drain terminal D of the first MOSFET M 1  is also high because the drain terminal of M 1  is connected, directly in this example, to the node A via a conducting wire. At the same time, the voltage at the node B is held low and the voltage at the drain terminal D of the second MOSFET M 2  is correspondingly low (the drain terminal of M 2  being, in this example, directly connected to the node B via a conducting wire). 
     Accordingly, at this time, the value of the drain voltage of M 1  is high and is greater than the gate voltage of M 2 . The second diode d 2  is therefore reverse-biased at this time. The gate voltage of M 2  at this time is greater than the source terminal voltage of M 2 , and the voltage V 2  is such that the gate-source voltage at M 2  is greater than the ON threshold for the MOSFET M 2 . M 2  is therefore ON at this time. 
     At the same time, the drain voltage of M 2  is low, and the first diode d 1  is forward biased due to the gate voltage supply V 2  to the gate terminal of M 1 . The gate terminal of M 1  is therefore connected via the forward biased first diode d 1  to the low voltage drain terminal of the second MOSFET M 2 , and the gate voltage of M 1  is therefore also low. In other words, because M 2  is on, it is acting as a ground clamp, which results in the first diode d 1  being forward biased, and the gate voltage of M 1  being low. As such, the gate-source voltage of M 1  is below the ON threshold and the first MOSFET M 1  is OFF. 
     In summary, at this point the circuit  150  is in a first state, wherein: 
     voltage at node A is high; 
     voltage at node B is low; 
     first diode d 1  is forward biased; 
     second MOSFET M 2  is ON; 
     second diode d 2  is reverse biased; and 
     first MOSFET M 1  is OFF. 
     From this point, with the second MOSFET M 2  being in the ON state, and the first MOSFET M 1  being in the OFF state, current is drawn from the supply V 1  through the first choke  161  and through the inductive element  158 . Due to the presence of inducting choke  161 , the voltage at node A is free to oscillate. Since the inductive element  158  is in parallel with the capacitor  156 , the observed voltage at node A follows that of a half sinusoidal voltage profile. The frequency of the observed voltage at node A is equal to the resonant frequency f 0  of the circuit  150 . 
     The voltage at node A reduces sinusoidally in time from its maximum value towards 0 as a result of the energy decay at node A. The voltage at node B is held low (because MOSFET M 2  is on) and the inductor L is charged from the DC supply V 1 . The MOSFET M 2  is switched off at a point in time when the voltage at node A is equal to or below the gate threshold voltage of M 2  plus the forward bias voltage of d 2 . When the voltage at node A has finally reached zero, the MOSFET M 2  will be fully off. 
     At the same time, or shortly after, the voltage at node B is taken high. This happens due to the resonant transfer of energy between the inductive element  158  and the capacitor  156 . When the voltage at node B becomes high due to this resonant transfer of energy, the situation described above with respect to the nodes A and B and the MOSFETs M 1  and M 2  is reversed. That is, as the voltage at A reduces towards zero, the drain voltage of M 1  is reduced. The drain voltage of M 1  reduces to a point where the second diode d 2  is no longer reverse biased and becomes forward biased. Similarly, the voltage at node B rises to its maximum and the first diode d 1  switches from being forward biased to being reverse biased. As this happens, the gate voltage of M 1  is no longer coupled to the drain voltage of M 2  and the gate voltage of M 1  therefore becomes high, under the application of gate supply voltage V 2 . The first MOSFET M 1  is therefore switched to the ON state, since its gate-source voltage is now above the threshold for switch-on. As the gate terminal of M 2  is now connected via the forward biased second diode d 2  to the low voltage drain terminal of M 1 , the gate voltage of M 2  is low. M 2  is therefore switched to the OFF state. 
     In summary, at this point the circuit  150  is in a second state, wherein: 
     voltage at node A is low; 
     voltage at node B is high; 
     first diode d 1  is reverse biased; 
     second MOSFET M 2  is OFF; 
     second diode d 2  is forward biased; and 
     first MOSFET M 1  is ON. 
     At this point, current is drawn through the inductive element  158  from the supply voltage V 1  through the second choke  162 . The direction of the current has therefore reversed due to the switching operation of the resonant circuit  150 . The resonant circuit  150  will continue to switch between the above-described first state in which the first MOSFET M 1  is OFF and the second MOSFET M 2  is ON, and the above-described second state in which the first MOSFET M 1  is ON and the second MOSFET M 2  is OFF. 
     In the steady state of operation, energy is transferred between the electrostatic domain (i.e., in the capacitor  156 ) and the magnetic domain (i.e., the inductor  158 ), and vice versa. 
     The net switching effect is in response to the voltage oscillations in the resonant circuit  150  where we have an energy transfer between the electrostatic domain (i.e., in the capacitor  156 ) and the magnetic domain (i.e., the inductor  158 ), thus creating a time-varying current in the parallel LC circuitry, which varies at the resonant frequency of the resonant circuit  150 . This is advantageous for energy transfer between the inductive element  158  and the susceptor arrangement  110  since the circuitry  150  operates at its optimal efficiency level and therefore achieves more efficient heating of the aerosol generating material  116  compared to circuitry operating off resonance. The described switching arrangement is advantageous as it allows the circuit  150  to drive itself at the resonant frequency under varying load conditions. What this means is that in the event that the properties of the circuitry  150  change (for example if the susceptor  110  is present or not, or if the temperature of the susceptor changes, or even physical movement of the susceptor element  110 ), the dynamic nature of the circuitry  150  continuously adapts its resonant point to transfer energy in an optimal fashion, thus meaning that the circuitry  150  is always driven at resonance. Moreover, the configuration of the circuit  150  is such that no external controller or the like is required to apply the control voltage signals to the gates of the MOSFETS to effect the switching. 
     In examples described above, with reference to  FIG. 2 , the gate terminals G are supplied with a gate voltage via a second power supply which is different to the power supply for the source voltage V 1 . However, in some examples, the gate terminals may be supplied with the same voltage supply as the source voltage V 1 . In such examples, the first point  159 , second point  160  and third point  165  in the circuit  150  may, for example, be connected to the same power rail. In such examples, it will be appreciated that the properties of the components of the circuit must be chosen to allow the described switching action to take place. For example, the gate supply voltage and diode threshold voltages should be chosen such that the oscillations of the circuit trigger switching of the MOSFETs at the appropriate level. The provision of separate voltage values for the gate supply voltage V 2  and the source voltage V 1  allows for the source voltage V 1  to be varied independently of the gate supply voltage V 2  without affecting the operation of the switching mechanism of the circuit. 
     The resonant frequency f 0  of the circuit  150  may be in the MHz range, for example in the range 0.5 MHz to 4 MHz, for example in the range 2 MHz to 3 MHz. It will be appreciated that the resonant frequency f 0  of the resonant circuit  150  is dependent on the inductance L and capacitance C of the circuit  150 , as set out above, which in turn is dependent on the inductive element  158 , capacitor  156  and additionally the susceptor arrangement  110 . As such, the resonant frequency f 0  of the circuit  150  can vary from implementation to implementation. For example, the frequency may be in the range 0.1 MHz to 4 MHz, or in the range of 0.5 MHz to 2 MHz, or in the range 0.3 MHz to 1.2 MHz. In other examples, the resonant frequency may be in a range different from those described above. Generally, the resonant frequency will depend on the characteristics of the circuitry, such as the electrical and/or physical properties of the components used, including the susceptor arrangement  110 . 
     It will also be appreciated that the properties of the resonant circuit  150  may be selected based on other factors for a given susceptor arrangement  110 . For example, in order to improve the transfer of energy from the inductive element  158  to the susceptor arrangement  110 , it may be useful to select the skin depth (i.e. the depth from the surface of the susceptor arrangement  110  within which the current density falls by a factor of 1/e, which is at least a function of frequency) based on the material properties of the susceptor arrangement  110 . The skin depth differs for different materials of susceptor arrangements  110 , and reduces with increasing drive frequency. On the other hand, for example, in order to reduce the proportion of power supplied to the resonant circuit  150  and/or driving element  102  that is lost as heat within the electronics, it may be beneficial to have a circuit which drives itself at relatively lower frequencies. Since the drive frequency is equal to the resonant frequency in this example, the considerations here with respect to drive frequency are made with respect to obtaining the appropriate resonant frequency, for example by designing a susceptor arrangement  110  and/or using a capacitor  156  with a certain capacitance and an inductive element  158  with a certain inductance. In some examples, a compromise between these factors may therefore be chosen as appropriate and/or desired. 
     The resonant circuit  150  of  FIG. 2  has a resonant frequency f 0  at which the current I is maximized and the dynamic impedance is maximized. The resonant circuit  150  drives itself at this resonant frequency and therefore the oscillating magnetic field generated by the inductor  158  is maximum, and the inductive heating of the susceptor arrangement  110  by the inductive element  158  is maximized. 
     In some examples, inductive heating of the susceptor arrangement  110  by the resonant circuit  150  may be controlled by controlling the supply voltage provided to the resonant circuit  150 , which in turn may control the current flowing in the resonant circuit  150 , and hence may control the energy transferred to the susceptor arrangement  110  by the resonant circuit  150 , and hence the degree to which the susceptor arrangement  110  is heated. In other examples, it will be appreciated that the temperature of the susceptor arrangement  110  may be monitored and controlled by, for example, changing the voltage supply (e.g., by changing the magnitude of the voltage supplied or by changing the duty cycle of a pulse width modulated voltage signal) to the inductive element  158  depending on whether the susceptor arrangement  110  is to be heated to a greater or lesser degree. 
     As mentioned above, the inductance L of the resonant circuit  150  is provided by the inductive element  158  arranged for inductive heating of the susceptor arrangement  110 . At least a portion of the inductance L of resonant circuit  150  is due to the magnetic permeability of the susceptor arrangement  110 . The inductance L, and hence resonant frequency f 0  of the resonant circuit  150  may therefore depend on the specific susceptor(s) used and its positioning relative to the inductive element(s)  158 , which may change from time to time. Further, the magnetic permeability of the susceptor arrangement  110  may vary with varying temperatures of the susceptor  110 . 
     In examples described herein the susceptor arrangement  110  is contained within a consumable and is therefore replaceable. For example, the susceptor arrangement  110  may be disposable and for example integrated with the aerosol generating material  116  that it is arranged to heat. The resonant circuit  150  allows for the circuit to be driven at the resonance frequency, automatically accounting for differences in construction and/or material type between different susceptor arrangements  110 , and/or differences in the placement of the susceptor arrangements  110  relative to the inductive element  158 , as and when the susceptor arrangement  110  is replaced. Furthermore, the resonant circuit is configured to drive itself at resonance regardless of the specific inductive element  158 , or indeed any component of the resonant circuit  150  used. This is particularly useful to accommodate for variations in manufacturing both in terms of the susceptor arrangement  110  but also with regards to the other components of the circuit  150 . For example, the resonant circuit  150  allows the circuit to remain driving itself at the resonant frequency regardless of the use of different inductive elements  158  with different values of inductance, and/or differences in the placement of the inductive element  158  relative to the susceptor arrangement  110 . The circuit  150  is also able to drive itself at resonance even if the components are replaced over the lifetime of the device. 
     Operation of the aerosol generating device  100  comprising resonant circuit  150 , will now be described, according to an example. Before the device  100  is turned on, the device  100  may be in an ‘off’ state, i.e. no current flows in the resonant circuit  150 . The device  150  is switched to an ‘on’ state, for example by a user turning the device  100  on. Upon switching on of the device  100  the resonant circuit  150  begins drawing current from the voltage supply  104 , with the current through the inductive element  158  varying at the resonant frequency f 0 . The device  100  may remain in the on state until a further input is received by the controller  106 , for example until the user no longer pushes the button (not shown), or the puff detector (not shown) is no longer activated, or until a maximum heating duration has elapsed. The resonant circuit  150  being driven at the resonant frequency f 0  causes an alternating current I to flow in the resonant circuit  150  and the inductive element  158 , and hence for the susceptor arrangement  110 , to be inductively heated. As the susceptor arrangement  110  is inductively heated, its temperature (and hence the temperature of the aerosol generating material  116 ) increases. In this example, the susceptor arrangement  110  (and aerosol generating material  116 ) is heated such that it reaches a steady temperature T MAX . The temperature T MAX  may be a temperature which is substantially at or above a temperature at which a substantial amount of aerosol is generated by the aerosol generating material  116 . The temperature Tuff may be between around 200 and around 300° C. for example (although of course may be a different temperature depending on the material  116 , susceptor arrangement  110 , the arrangement of the overall device  100 , and/or other requirements and/or conditions). The device  100  is therefore in a ‘heating’ state or mode, wherein the aerosol generating material  116  reaches a temperature at which aerosol is substantially being produced, or a substantial amount of aerosol is being produced. It should be appreciated that in most, if not all cases, as the temperature of the susceptor arrangement  110  changes, so too does the resonant frequency f 0  of the resonant circuit  150 . This is because magnetic permeability of the susceptor arrangement  110  is a function of temperature and, as described above, the magnetic permeability of the susceptor arrangement  110  influences the coupling between the inductive element  158  and the susceptor arrangement  110 , and hence the resonant frequency f 0  of the resonant circuit  150 . 
     The present disclosure predominantly describes an LC parallel circuit arrangement. As mentioned above, for an LC parallel circuit at resonance, the impedance is maximum and the current is minimum. Note that the current being minimum generally refers to the current observed outside of the parallel LC loop, e.g., to the left of choke  161  or to the right of choke  162 . Conversely, in a series LC circuit, current is at maximum and, generally speaking, a resistor is required to be inserted to limit the current to a safe value which can otherwise damage certain electrical components within the circuit. This generally reduces the efficiency of the circuit because energy is lost through the resistor. A parallel circuit operating at resonance does not require such restrictions. 
     In some examples, the susceptor arrangement  110  comprises or consists of aluminum. Aluminum is an example of a non-ferrous material and as such has a relative magnetic permeability close to one. What this means is that aluminum has a generally low degree of magnetization in response to an applied magnetic field. Hence, it has generally been considered difficult to inductively heat aluminum, particularly at low voltages such as those used in aerosol provision systems. It has also generally been found that driving circuitry at resonant frequency is advantageous as this provides optimum coupling between the inductive element  158  and susceptor arrangement  110 . For aluminum, it is observed that a slight deviation from the resonant frequency causes a noticeable reduction in the inductive coupling between the susceptor arrangement  110  and the inductive element  158 , and thus a noticeable reduction in the heating efficiency (in some cases to the extent where heating is no longer observed). As mentioned above, as the temperature of the susceptor arrangement  110  changes, so too does the resonant frequency of the circuit  150 . Therefore, in the case where the susceptor arrangement  110  comprises or consists of a non-ferrous susceptor, such as aluminum, the resonant circuit  150  of the present disclosure is advantageous in that the circuitry is always driven at the resonant frequency (independent of any external control mechanism). This means that maximum inductive coupling and thus maximum heating efficiency is achieved at all times enabling aluminum to be efficiently heated. It has been found that a consumable including an aluminum susceptor can be heated efficiently when the consumable includes an aluminum wrap forming a closed electrical circuit and/or having a thickness of less than 50 microns. 
     In examples where the susceptor arrangement  110  forms part of a consumable, the consumable may take the form of that described in PCT/EP2016/070178, the entirety of which is incorporated herein by reference. 
     The device  100  is provided with a temperature determiner for, in use, determining a temperature of the susceptor arrangement  110 . As is illustrated in  FIG. 1 , the temperature determiner may be the control circuitry  106 , for example, a processor that controls the overall operation of the device  100 . The temperature determiner  106  determines a temperature of the susceptor arrangement  110  based on a frequency that the resonant circuit  150  is being driven at, a DC current from the DC voltage supply V 1  and a DC voltage of the DC voltage supply V 1 . 
     Without wishing to be bound by theory, the following description explains the derivation of relationships between electrical and physical properties of the resonant circuit  150  which allow the temperature of the susceptor arrangement  110  in examples described herein to be determined. 
     In use, the impedance at resonance of the parallel combination of the inductive element  158  and the capacitor  156  is the dynamic impedance R dyn . 
     As explained above, the action of the switching arrangement M 1  and M 2  results in a DC current drawn from the DC voltage source V 1  being converted into an alternating current that flows through the inductive element  158  and capacitor  156 . An induced alternating voltage is also generated across the inductive element  158  and the capacitor  156 . 
     As a result of the oscillatory nature of the resonant circuit  150 , the impedance looking into the oscillatory circuit is R dyn  for a given source voltage V s  (of the voltage source V 1 ). A current Is will be drawn in response to R dyn . Therefore, the impedance of the load R dyn  of the resonant circuit  150  may be equated with the impedance of the effective voltage and current draw. This allows the impedance of the load to be determined via determination, for example measuring values, of the DC voltage V s  and the DC current Is, as per equation (1) below. 
     
       
         
           
             
               
                 
                   
                     R 
                     dyn 
                   
                   = 
                   
                     
                       V 
                       s 
                     
                     
                       I 
                       s 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     At the resonant frequency f 0 , the dynamic impedance R dyn  is 
     
       
         
           
             
               
                 
                   
                     R 
                     dyn 
                   
                   = 
                   
                     L 
                     Cr 
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     where the parameter r can be considered to represent the effective grouped resistance of the inductive element  158  and the influence of the susceptor arrangement  110  (when present), and, as described above, L is the inductance of the inductive element  158 , and C is the capacitance of the capacitor  156 . The parameter r is described herein as an effective grouped resistance. As will be appreciated from the description below, the parameter r has units of resistance (Ohms), but in certain circumstances may not be considered to represent a physical/real resistance of the circuit  150 . 
     As described above, the inductance of the inductive element  158  here takes into the account the interaction of the inductive element  158  with the susceptor arrangement  110 . As such, the inductance L depends on the properties of the susceptor arrangement  110  and position of the susceptor arrangement  110  relative to the inductive element  158 . The inductance L of the inductive element  158  and hence of the resonant circuit  150  is dependent on, amongst other factors, the magnetic permeability μ of the susceptor arrangement  110 . Magnetic permeability μ is a measure of the ability of a material to support the formation of a magnetic field within itself and expresses the degree of magnetization that a material obtains in response to an applied magnetic field. The magnetic permeability μ of a material from which the susceptor arrangement  110  is comprised may change with temperature. 
     From equations (1) and (2) the following equation (3) can be obtained 
     
       
         
           
             
               
                 
                   r 
                   = 
                   
                     
                       LI 
                       s 
                     
                     
                       CV 
                       s 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     The relation of the resonant frequency f 0  to the inductance L and capacitance C can be modelled in at least two ways, given by equations (4a and 4b) below. 
     
       
         
           
             
               
                 
                   
                     f 
                     0 
                   
                   = 
                   
                     1 
                     
                       2 
                        
                       π 
                        
                       
                         LC 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     4 
                      
                     a 
                   
                   ) 
                 
               
             
             
               
                 
                   
                     f 
                     0 
                   
                   = 
                   
                     
                       1 
                       
                         2 
                          
                         π 
                          
                         
                             
                         
                          
                         L 
                       
                     
                      
                     
                       
                         
                           L 
                           C 
                         
                         - 
                         
                           r 
                           2 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     4 
                      
                     b 
                   
                   ) 
                 
               
             
           
         
       
     
     Equation (4a) represents the resonant frequency as modelled using a parallel LC circuit comprising an inductor L and a capacitor C, whereas Equation (4b) represents the resonant frequency as modelled using a parallel LC circuit with an additional resistor r in series with the inductor L. It should be appreciated for Equation (4b) that as r tends to zero, Equation (4b) tends to Equation (4a). 
     In the following, we assume that r is small and hence we can make use of Equation (4a). As will be described below, this approximation works well as it combines the changes within the circuit  150  (e.g., in inductance and temperature) within the representation of L. From equations (3) and (4a) the following expression can be obtained 
     
       
         
           
             
               
                 
                   r 
                   = 
                   
                     
                       
                         I 
                         s 
                       
                       
                         V 
                         s 
                       
                     
                      
                     
                       1 
                       
                         
                           ( 
                           
                             2 
                              
                             π 
                              
                             
                                 
                             
                              
                             
                               f 
                               0 
                             
                              
                             C 
                           
                           ) 
                         
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     It will be appreciated that Equation (5) provides an expression for the parameter r in terms of measurable or known quantities. It should be appreciated here that the parameter r is influenced by the inductive coupling in the resonant circuit  150 . When loaded, i.e., when a susceptor arrangement is present, it may not be the case that we can consider the value of the parameter r to be small. In which case, the parameter r may no longer be an exact representation of the group resistances, but is instead a parameter which is influenced by the effective inductive coupling in the circuit  150 . The parameter r is said to be a dynamic parameter, which is dependent on the properties of the susceptor arrangement  110 , as well as the temperature T of the susceptor arrangement. The value of DC source V s  is known (e.g. a battery voltage) or may be measured by a voltmeter and the value of the DC current Is drawn from the DC voltage source V 1  may be measured by any suitable means, for example by use of a voltmeter appropriately placed to measure the source voltage V s . 
     The frequency f 0  may be measured and/or determined to allow then the parameter r to be obtained. 
     In one example, the frequency f 0  may be measured via use of a frequency-to-voltage (F/V) converter  210 . The F/V converter  210  may, for example, be coupled to a gate terminal of one of the first MOSFET M 1  or the second MOSFET M 2 . In examples where other types of transistors are used in the switching mechanism of the circuit, the F/V converter  210  may be coupled to a gate terminal, or other terminal which provides a periodic voltage signal with frequency equal to the switching frequency of one of the transistors. The F/V converter  210  therefore may receive a signal from the gate terminal of one of the MOSFET M 1 , M 2  representative of the resonance frequency f 0  of the resonant circuit  150 . The signal received by the F/V converter  210  may be approximately a square-wave representation with a period representative of the resonant frequency of the resonant circuit  210 . The F/V converter  210  may then use this period to represent the resonant frequency f 0  based on an output voltage. 
     Accordingly, as C is known from the value of the capacitance of the capacitor  156 , and V s , I s , and f 0  can be measured, for example as described above, the parameter r can be determined from these measured and known values. 
     The parameter r changes as a function of temperature, and further as a function of the inductance L. This means that the parameter r has a first value when the resonant circuit  150  is in an “unloaded” state, i.e. when the inductive element  158  is not inductively coupled to the susceptor arrangement  110 , and the value of r changes when the circuit moves into a “loaded” state, i.e. when the inductive element  158  and susceptor arrangement  110  are inductively coupled with each other. Similarly, as described above, the value of the resonant frequency f 0  changes as a function of temperature, and further as a function of the inductance L. 
     In an example, the controller  106  is configured to determine a change in an electrical parameter of the circuit when the circuit is changed between the unloaded state and the loaded state. In essence, any given electrical parameter of the circuit  150  which can be measured and shows a change between the loaded and unloaded states can be used by the controller  106 . In one example, the electrical parameter used is the resonant frequency of the circuit. In another example, the electrical parameter used is the parameter r. By determining a change in the given electrical parameter, the controller  106  may determine a property of the susceptor arrangement  110  which has been coupled to the inductive element  158 . In examples, the properties of a susceptor arrangement  110 , for example the type of material the susceptor arrangement  110  is formed from, or the size or shape of the susceptor arrangement  110 , affect the change in the electrical parameter when the susceptor arrangement  110  is coupled to the inductive element  158 . Certain properties of the susceptor arrangement  110 , and/or of a consumable containing the susceptor arrangement  110 , may therefore, in examples, be determined by determining or measuring a change in a given electrical parameter. 
     In examples, the circuit  150  may be changed from the unloaded state to the loaded state when a consumable containing the susceptor arrangement  110  is received by the device  100 , for example when the consumable is inserted into the device  100 . The circuit  150  may similarly be changed from the loaded state to the unloaded state when the consumable is removed from the device  100 . In the unloaded state, a given electrical parameter may take a first value, while in the loaded state the given electrical parameter may take a different value. As such, in an example, the change in the given electrical parameter between the unloaded state and the loaded state may indicate to the controller  106  the type of susceptor arrangement  110  present in the consumable. Hence, depending on the change in the given electrical parameter, the controller  106  is configured to determine a type of consumable which has been received by the aerosol generating device  100 . In some implementations, a range of consumables e.g., having different tobacco blends, or different flavors, may be provided with different susceptor arrangements  110  which can subsequently be used to identify the consumable. 
     In an example, the controller  106  may have access to a predetermined list or table of values of changes in the electrical parameter, wherein the list comprises at least one value of a change in the electrical parameter with each value being associated with a type of consumable. Therefore, a measurement of the change in the given electrical parameter may be associated, e.g. via a look-up table, with a particular type of consumable. The change in the electrical parameter may be a change in magnitude of the electrical parameter, for example a change in the magnitude of the resonant frequency of the circuit  150 , or of the parameter r, upon the circuit  150  being changed between the loaded and unloaded states. In some implementations, the sign of the change (i.e., a positive or negative with respect to the unloaded state) is alternatively or additionally taken into account when determining the susceptor arrangement and thus consumable type. For example, it has been found for an aluminum-containing susceptor arrangement that the frequency increases from that of an unloaded state to a loaded state. Without wishing to be bound by theory, this is thought to be due to the fact that aluminum has a relative permeability of 1 or close to 1, i.e. a low and is thus non-ferritic. Susceptor arrangements comprising other non-ferritic materials may similarly cause a resonant frequency of the circuit to increase when going from the unloaded state to the loaded state. Conversely, it has been found that for a ferritic material, e.g. iron, containing susceptor arrangement (which has a relative permeability greater than 1, for instance of several tens or several hundreds), the frequency decreases from an unloaded to a loaded state. Thus, the sign of the change in the electrical parameter may also be used to determine a property of the susceptor arrangement  110 . For example, the sign of the change of resonant frequency upon going from the unloaded to the loaded state may be used to determine if the susceptor arrangement  110  comprises a material with a low relative permeability or a material with a high relative permeability. In certain examples, the behavior of the resonant frequency or other electrical parameters of the circuit upon going between a loaded and an unloaded state may differ depending on properties of the circuit, such as the resonant frequency of the circuit in the unloaded state. For example, the magnitude or sign in the change in resonant frequency of the circuit when going between the loaded and unloaded states may differ dependent on the resonant frequency of the circuit. To give an example, a particular consumable may be of a particular size and comprise a particular type and amount of aerosol generating material, and comprise an aluminum susceptor arrangement  110  of a particular size and shape. The look-up table may hold a value for the magnitude of the change in resonant frequency of the circuit  150  which occurs when the circuit  150  is changed between the loaded and unloaded states by introduction of this consumable. This value may, for example, be stored in the look-up table in an initial setup of the circuit  150 , where the type of consumable is known and the change in electrical parameter it effects in the circuit  150  is measured. The controller  106  may therefore determine the change in parameter r when the circuit  150  has been changed to the loaded state by introduction of the consumable. By looking up the consumable type associated with the determined change in the parameter r in the look-up table, the type of consumable loaded into the device  100  is determined. It will be appreciated that the above description applies mutatis mutandis where the electrical parameter is the resonant frequency f 0  of the circuit  150 . 
     It should also be appreciated that there may be some slight variation in the change of the electrical parameter between consumables of the same type. For example, for susceptor arrangements  110  of the same type, there may be slight manufacturing discrepancies in the materials used (e.g., purities or defects), and the overall shape of the susceptor arrangement (e.g., a tube susceptor may end up with a slightly elliptical cross section) may impact on the change in the electrical parameter. These are discrepancies caused by the manufacture of the susceptor arrangement itself. Additionally, there may be discrepancies based on the alignment of the susceptor arrangement  110  with the consumable (e.g., how much the susceptor deviates from the axes of the consumable) and/or the alignment of the consumable within the device relative to the inductive element  158 , and again these discrepancies can affect the change in the electrical parameter. These discrepancies are caused by the manufacture of the consumable and/or device themselves. Hence, in some implementations, the look-up table mentioned above may account for these discrepancies, e.g., by specifying a range of values that satisfy each criterion of the look-up table. Alternatively, the controller  106  may implement an algorithm to identify the closest values from the look-up table. 
     It should also be appreciated that, in particular with circuitry  150 , the susceptor arrangement  110  is gradually heated once the susceptor arrangement  110  is in the loaded state and the circuitry is switched on. As discussed above, during heating, the resonant frequency changes depending upon temperature. Thus, depending on when the measurement of the given electrical parameter is made, there may also be some variation in the change of the electrical parameter due to heating. In this case, either each device can be calibrated to take into account the measurement time, or the look-up table can be modified to account for differences in measurement times. 
     In an example, using the determined change in the electrical parameter, the controller  106  may determine whether or not to allow activation of the aerosol generating device  100  for use with a received consumable. For example, the determined change in electrical parameter may be used to indicate whether the consumable is a consumable which is approved for use with the aerosol generating device  100 . The table may hold a list of one or more approved consumables and the controller  106  may activate the device  100  for use only if the consumable is determined to be an approved consumable. Approved susceptor-containing consumables may be manufactured with a known value for the change in electrical parameter that they cause in the circuit  150 . For instance, a known value of the change in resonant frequency, or of the change in parameter r caused by that consumable. 
     In examples, using the determined change in the electrical parameter, the controller  106  may determine a heating mode for the device  100  to use with a received consumable. For example, the determined change in electrical parameter may be used to indicate a type of the received consumable, e.g. the material and/or size of the susceptor arrangement and/or a type or amount of aerosol generating material in the consumable, and the controller  106  may select an appropriate mode of operation for heating the received consumable based on the determined change in the electrical parameter. For example, different heating profiles may be suitable for heating of different types of consumable and the controller  106  may select a suitable heating profile based on a determination of the properties of the received consumable. In a similar manner to as has been described above, a look-up table accessible by the controller  106  may hold a list of one or more types of consumable and one of more corresponding heating modes for each type of consumable. 
     In one implementation, the controller  106  may determine the change in the value of the electrical parameter by measuring the electrical parameter in the unloaded state and comparing this to a measurement of the electrical parameter in the loaded state. In other words, the controller  106  may be configured to activate the inductive element  158  (in other words, supply power to the inductive element  158 ) when the device is in the unloaded state to obtain a measure of the electrical parameter in the unloaded state, and to activate the inductive element  158  when the device is in the loaded state to obtain a measure of the electrical parameter in the loaded state. In one implementation, the controller  106  is configured to supply power to the inductive element  158  in a continuous manner (e.g., when a user switches on the device, such as through activation of a button), and is arranged to monitor the electrical parameter for a subsequent change in the electrical parameter (which can indicate that the device is now in the loaded state). The controller may monitor the electrical parameter continuously or intermittently. Alternatively, the controller  106  is arranged to intermittently supply power to the inductive element  158 , at a set intermission period, say once every second, and measure the electrical parameter at a corresponding timing. When there is a change in the electrical parameter between two measurements, this can indicate that the device is in the loaded state and the change in the electrical parameter, as described above, can be used to identify the consumable. Broadly, the controller  106  may therefore determine the change in the value of the electrical parameter by measuring the electrical parameter when the circuit  150  is in the loaded state and comparing this measured value to a value of the electrical parameter which is measured when the circuit  150  is in the unloaded state. In other words, the controller  106  may be configured to activate the inductive element  158  (in other words, supply power to the inductive element  158 ) when the device  100  is in the unloaded state to obtain a measure of the electrical parameter in the unloaded state, and to activate the inductive element  158  when the device  100  is in the loaded state to obtain a measure of the electrical parameter in the loaded state. For example, the controller  106  may measure the resonant frequency using a F/V converter, or measure the parameter r of the unloaded circuit  150  as described herein, e.g. using Equation 5, when the inductive element  158  is supplied with power. The electrical parameter may be measured again when the circuit  150  is brought into the loaded state, and the two measured values compared to determine the change, for example a change in magnitude, in the electrical parameter. The measurement of the electrical parameter in the unloaded state may, for example, be made when the device  100  is powered on but no susceptor arrangement  110  is inserted. As described herein, the controller  106  may determine whether the device  100  is in the loaded state or the unloaded state by any suitable means, such as via an optical sensor or a capacitive sensor which senses the insertion of a consumable, or alternatively the value of the electrical parameter, or a change therein, may indicate that the device  100  has switched between the loaded and unloaded states. The controller  106  may, as such, associate measurements of the electrical parameter with either the loaded or unloaded state. 
     In another example, the controller  106  may measure the electrical parameter when the circuit  150  is in the loaded state, e.g. as described above, and compare this measured value for the loaded state to a predetermined value of the electrical parameter for the unloaded state. That is, a value for the electrical parameter in the unloaded state may be predetermined and accessible to the controller  106  when determining the change in the electrical parameter. In examples, the value of the electrical parameter in the unloaded state may be a fixed value which is stored in a memory accessible by the controller  106 . For example, the value of the electrical parameter in the unloaded state may be a value determined based on the properties of circuit  150 , or a value measured for the circuit  150  during an initial configuring of the circuit  150 . In another example, a value of the electrical parameter for the unloaded state may be measured as described herein and stored for re-use in subsequent determinations of a change in the electrical parameter upon loading/unloading of a consumable containing the susceptor arrangement  110 . As such, if the device  101  is powered on with a susceptor arrangement  110  already received by the device  100 , the controller  106  may measure a value of the electrical parameter (i.e. a value of the circuit  150  in the loaded state) and compare this to a predetermined value of the electrical parameter when the circuit  150  is in the unloaded state. The controller  106  may determine that the measured value corresponds to the loaded state either via input from a sensor (not shown) that senses a susceptor arrangement  110 /consumable is received by the device  100  or in other examples may determine that the circuit  150  is in the loaded state by the magnitude of the electrical parameter itself. For example, the circuit  150  may store a known value for the circuit  150  in the unloaded state and may determine that the circuit  150  is in the loaded state is the measured value of the electrical parameter differs by a certain amount from the known value for the unloaded state. 
       FIG. 3  shows an example representation of a usage session of the aerosol generating device  100  in which the circuit  150  is changed from the unloaded state to the loaded state by a susceptor arrangement  110  being brought into interaction with the inductive element  158 .  FIG. 3  shows time along the horizontal axis and the resonant frequency of the circuit  150  along the vertical axis. 
     In  FIG. 3 , two plots A and B are shown, which correspond respectively to a first susceptor arrangement  110  in a first consumable and a second susceptor arrangement  110  is a second consumable. For each plot, before time t 1  the circuit  150  is in the unloaded state and has a resonant frequency f unloaded . As mentioned above, this resonant frequency is a property of the circuitry  150  and depends at least on the components of the circuit  150 . At time t 1  a consumable is inserted into the device  100 . The first plot A is a solid line and corresponds to the insertion at t 1  of a first consumable comprising a first susceptor arrangement  110 . The second plot B is a dashed line and corresponds to the insertion at t 1  of a second consumable comprising a second susceptor arrangement  110 . At time t 1 , the time of insertion, in the examples shown in Figure  FIG. 3 , the circuit  150  is changed to the loaded state, and the resonant frequency of the circuit  150  changes. In this example, the susceptor arrangements  110  have a relative permeability greater than 1, which means that the resonant frequency decreases from an unloaded state to a loaded state. For the first consumable, let us assume that the expected change in resonant frequency when going from the unloaded to the loaded state is Δf 1 . For the second consumable, let us assume that the expected change in resonant frequency when going from the unloaded to the loaded state is Δf 2 . In an example, therefore, the values Δf 1  and Δf 2  are stored in a look-up table accessible to the controller  106 , and these values are associated with the first consumable and the second consumable respectively. Upon loading of a consumable, the controller  106  may then determine the change in the resonant frequency, which is the difference between the unloaded resonant frequency f unloaded  and the measured loaded resonant frequency f loaded , of the circuit  150  and look up the determined change in resonant frequency in the look-up table. If the determined change in resonant frequency corresponds to Δf 1  the controller  106  determines that the consumable inserted is the first consumable. If the measured change in frequency corresponds to Δf 2  the controller determines that the consumable inserted is the second consumable. The reduction with time of the resonant frequency for each of the plots A and B after the time t 1  corresponds to a reduction in the resonant frequency with increasing temperature of the susceptor arrangement  110  and consumable. That is, in the plots A and B, the inserted consumable is heated from insertion at time t 1  and thus the resonant frequency f 0  decreases from that time, in both cases. 
     Once it is determined, or can be assumed, that the resonant circuit  150  is in the loaded state, with a susceptor arrangement  110  inductively coupled to the inductive element  158 , a change in the parameter r can be assumed to be indicative of a change in temperature of the susceptor arrangement  110 . For example, the change in r may be considered indicative of heating of the susceptor arrangement  110  by the inductive element  158 , rather than a change of the circuit between loaded and unloaded states. 
     In an example, the aerosol generating device comprises  100  a temperature sensor  140  for measuring a temperature indicative of a temperature of the susceptor arrangement  110  upon being loaded into the device  100 , i.e. at time t 1  in  FIG. 3 . The temperature sensor  140  may provide this measured temperature to the controller  106 . The controller  106  may use the temperature provided by the temperature sensor  140  to provide a correction to the change in the electrical parameter which is measured by the controller  106 . That is, the resonant frequency for the circuit  150  when loaded with a particular consumable is dependent on the temperature of the consumable at the time the measurement is made; the same applies for the parameter r. As such, in order to compare the change in the electrical parameter when the consumable is inserted into the device  100 , and thereby identify the consumable, the controller  106  may be configured to make a correction to the measured value of the electrical parameter to account for the temperature of the consumable/susceptor arrangement  110 . The correction may be made based on a calibration curve (not shown) of temperature against resonant frequency or parameter r for the circuit  150  loaded with a particular type of consumable. The calibration curve may be obtained by a calibration performed on the resonant circuit  150  itself (or on an identical test circuit used for calibration purposes) by measuring the temperature T of the susceptor arrangement  110  with a suitable temperature sensor such as a thermocouple, at multiple given values of the parameter r, and taking a plot of r against T. For example, a number of values for the change in electrical parameter may be stored in the look-up table upon setup, each corresponding to a different measured susceptor temperature (which is also stored in the table). When looking up the change in electrical parameter in the table, the controller  106  may in such examples also use the measured temperature in the look-up operation. In another example, an equation defining how the change in electrical parameter varies with susceptor arrangement  110  temperature may be determined, either experimentally or theoretically, and this equation applied by the controller  106  to correct the measured value of the change in the electrical parameter for looking-up in the table. As such, the controller  106  may make an accurate determination of the type of consumable received by the device  100 , accounting for the temperature of the susceptor arrangement  110  upon insertion. 
     In some examples a calibration curve such as has been described above may be pre-loaded on the device  100  and may be configured to take into account variances in the device  100 . For example, certain properties of the device  100  may vary between copies of the device  100  due to variations within manufacturing tolerances. A calibration curve may be loaded on each copy of the device  100  which takes into account these variances. Similarly, the calibration curve may take into account variances between different consumables of the same type. For example, certain properties such as the weight or composition of consumables of a certain type may vary slightly, e.g. due to tolerances in the manufacturing process. The calibration curve may take into account such variations. In other examples, each individual device  100  may be separately calibrated during the manufacturing process. This may allow for the variation between devices to be reflected in a calibration curve specific to the particular device to which the calibration corresponds. 
     In yet another example, a calibration curve for the device  100  may be determined when the device  100  is in use by a user. For example, the device  100  may be configured to determine values for the parameter r when the device  100  is first operated by a user and temperature values corresponding to the determined values of the parameter r to thereby obtain the calibration curve. The temperature values may be obtained, for example, using the temperature sensor  140 . In another example, a temperature value may be obtained using another indicator of a temperature of the susceptor arrangement, for example a property of the heating profile which indicates that the susceptor arrangement is at a known temperature. In one example this process could be performed only the first time the device  100  is operated by the user and the calibration curve generated by this process could be used for subsequent times the device  100  is operated. In another example, the calibration process could be performed multiple times, for example upon each use of the device  100 . 
     In one example, the temperature sensor  140  may be a sensor which is configured to detect a temperature ambient to the device  100 . The controller  106  may receive the temperature detected by the temperature sensor  140  and use this in making a correction to the measured change in the electrical parameter for comparison to a look-up table value. As such, the controller  106  may, in effect, assume that the temperature of the susceptor arrangement  110  upon being received by the device  100  is equal to the ambient temperature. In another example, the aerosol provision device  100  comprises a chamber for receiving the susceptor arrangement  110 , e.g. a consumable comprising the susceptor arrangement  110 , and the temperature sensor  140  may detect the temperature of the chamber prior to insertion the consumable and use this detected temperature in making the correction. 
       FIG. 3  above describes the situation in which the resonant frequency of the circuit  150  changes by a different amount (e.g., Δf 1  or Δf 2 ) depending on the properties of the susceptor arrangement  110 , or the relative placement of the susceptor arrangement  110 , etc. However, it should be appreciated that the change in resonant frequency between unloaded and loaded states may be affected by other aspects. For example, the voltage supplied to the circuit  150  may influence the change in resonant frequency. For instance, if 4 volts are supplied to the circuit  150 , the change in resonant frequency between unloaded and loaded states may be larger than if 3 volts is supplied to the circuit  150 . Hence, when determining a property of the susceptor arrangement  110  from a change in the electrical parameter of the circuit (e.g., resonant frequency or the parameter r), the controller may be configured to take into account other parameters of the circuit  150 , such as the voltage and/or current supplied to the circuit  150 , to determine the property of the susceptor arrangement. In an example that makes use of a look-up table, the look-up table may include entries for different susceptor arrangements  110  at different voltages. This observation also enables parameters of the circuit  150  to be calibrated; for example the change in frequency at different voltages may enable different electrical characteristics of the circuit  150  to be checked or derived, e.g., by solving simultaneous equations. 
     While it has been described above that the control circuitry makes use of Equations 4a and 5, e.g. to determine the parameter r, it should be appreciated that other equations achieving the same or similar effect may be used in accordance with the principles of the present disclosure. In one example, R dyn  can be calculated based on the AC values of the current and voltage in the circuit  150 . For example, the voltage at node A can be measured and, it has been found that this is different from V s —we call this voltage V AC . V AC  can be measured practically by any suitable means, but is the AC voltage within the parallel LC loop. Using this, one can determine an AC current, I AC , by equating the AC and DC power. That is, V AC I AC =V S I S . The parameters V s  and I s  can be substituted with their AC equivalents in Equation 5, or any other suitable equation for the parameter r. It should be appreciated that a different set of calibration curves may be realized in this case. 
     While the above description has described the operation of the temperature measurement concept in the context of the circuit  150  which is configured to self-drive at the resonant frequency, the above described concepts are also applicable to an induction heating circuit which is not configured to be driven at the resonant frequency. For example, the above described method of determining a property of the susceptor arrangement  110  from the change in an electrical parameter of the circuit  150  when the device  100  is changed between the loaded and unloaded states may be employed with an induction heating circuit which is driven at a predetermined frequency, which may not be the resonant frequency of that induction heating circuit. In one such example, the induction heating circuit may be driven via an H-Bridge, comprising a switching mechanism such as a plurality of MOSFETs. The H-Bridge may be controlled, via a microcontroller or the like to use a DC voltage to supply an alternating current to the inductor coil at a switching frequency of the H-Bridge, set by the microcontroller. In such an example, the above relations set out in equations (1) to (5) are assumed to hold and provide a valid, e.g. usable, estimate of the parameter r and susceptor temperature T for frequencies in a range of frequencies including the resonant frequency. 
     In some examples, the method may comprise assigning V s  and I s  constant values and assuming that these values do not change in calculating the parameter r. The voltage V s  and the current I s  may then need not be measured in order to estimate the temperature of the susceptor. For example, the voltage and current may be approximately known from the properties of the power source and the circuit and may be assumed to be constant over the range of temperatures used. In such examples, the temperature T may then be estimated by measuring only the frequency at which the circuit is operating and using assumed or previously measured values for the voltage and current. The invention thus may provide for a method of determining the temperature of the susceptor by measuring the frequency of operation of the circuit. In some implementations, the invention thus may provide for a method of determining the temperature of the susceptor by only measuring the frequency of operation of the circuit. 
     The above examples are to be understood as illustrative examples of the invention. It is to be understood that any feature described in relation to any one example may be used alone, or in combination with other features described, and may also be used in combination with one or more features of any other of the examples, or any combination of any other of the other examples. Furthermore, equivalents and modifications not described above may also be employed without departing from the scope of the invention, which is defined in the accompanying claims.