Apparatus for a resonance circuit

Disclosed is a method and apparatus for use with an RLC resonance circuit for inductive heating of a susceptor of an aerosol generating device. The apparatus is arranged to determine a resonant frequency of the RLC resonance circuit; and determine, based on the determined resonant frequency, a first frequency for the RLC resonance circuit for causing the susceptor to be inductively heated, the first frequency being above or below the determined resonant frequency. The apparatus may be arranged to control a drive frequency of the RLC resonance circuit to be at the determined first frequency in order to heat the susceptor. Also disclosed is an aerosol generating device including the apparatus.

PRIORITY CLAIM

The present application is a National Phase entry of PCT Application No. PCT/EP2018/057835, filed Mar. 27, 2018, which claims priority from GB Application No. 1705206.9, filed Mar. 31, 2017, each of which is hereby fully incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to apparatus for use with an RLC resonance circuit, more specifically an RLC resonance circuit for inductive heating of a susceptor of an 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 disclosure, there is provided apparatus for use with an RLC resonance circuit for inductive heating of a susceptor of an aerosol generating device, the apparatus being arranged to: determine a resonant frequency of the RLC resonance circuit; and determine, based on the determined resonant frequency, a first frequency for the RLC resonance circuit for causing the susceptor to be inductively heated, the first frequency being above or below the determined resonant frequency.

The first frequency may be for causing the susceptor to be inductively heated to a first degree at a given supply voltage, the first degree being less than a second degree, the second degree being that to which the susceptor is caused to be inductively heated, at the given supply voltage, when the RLC circuit is driven at the resonant frequency.

The apparatus may be arranged to control a drive frequency of the RLC resonance circuit to be at the determined first frequency in order to heat the susceptor.

The apparatus may be arranged to control the drive frequency to be held at the first frequency for a first period of time.

The apparatus may be arranged to control the drive frequency to be at one of a plurality of first frequencies each different from one another.

The apparatus may be arranged to control the drive frequency through the plurality of first frequencies in accordance with a sequence.

The apparatus is arranged to select the sequence from one of a plurality of predefined sequences.

The apparatus may be arranged to control the drive frequency such that each of the first frequencies in the sequence is closer to the resonant frequency than the previous first frequency in the sequence, or control the drive frequency such that each of the first frequencies in the sequence is further from the resonant frequency than the previous first frequency in the sequence.

The apparatus may be arranged to control the drive frequency to be held at one or more of the plurality of first frequencies for a respective one or more time periods.

The apparatus may be arranged to measure an electrical property of the RLC circuit as a function of the drive frequency; and determine the resonant frequency of the RLC circuit based on the measurement.

The apparatus may be arranged to determine the first frequency based on the measured electrical property of the RLC circuit as a function of the drive frequency at which the RLC circuit is driven.

The electrical property may be a voltage measured across an inductor of the RLC circuit, the inductor being for energy transfer to the susceptor.

The measurement of the electrical property may be a passive measurement.

The electrical property may be indicative of a current induced in a sense coil, the sense coil being for energy transfer from an inductor of the RLC circuit, the inductor being for energy transfer to the susceptor.

The electrical property may be indicative of a current induced in a pick-up coil, the pick-up coil being for energy transfer from a supply voltage element, the supply voltage element being for supplying voltage to a driving element, the driving element being for driving the RLC circuit.

The apparatus may be arranged to determine the resonant frequency of the RLC circuit and/or the first frequency substantially on start-up of the aerosol generating device and/or substantially on installation of a new and/or replacement susceptor into the aerosol generating device and/or substantially on installation of a new and/or replacement inductor into the aerosol generating device.

The apparatus may be arranged to determine a characteristic indicative of a bandwidth of a peak of a response of the RLC circuit, the peak corresponding to the resonant frequency; and determine the first frequency based on the determined characteristic.

The apparatus may comprise a driving element arranged to drive the RLC resonance circuit at one or more of a plurality of frequencies; wherein the apparatus is arranged to control the driving element to drive the RLC resonant circuit at the determined first frequency.

The driving element may comprise an H-Bridge driver.

The apparatus may further comprise the RLC resonance circuit.

According to a second aspect of the present disclosure, there is provided an aerosol generating device comprising: a susceptor arranged to heat an aerosol generating material thereby to generate an aerosol in use, the susceptor being arranged for inductive heating by an RLC resonance circuit; and the apparatus according to the first aspect.

The susceptor may comprise one or more of nickel and steel.

The susceptor may comprise a body having a nickel coating.

The nickel coating may have a thickness less than substantially 5 μm, or substantially in the range 2 μm to 3 μm.

The nickel coating may be electroplated on to the body.

The susceptor may be or comprise a sheet of mild steel.

The sheet of mild steel may have a thickness in the range of substantially 10 μm to substantially 50 μm, or may have a thickness of substantially 25 μm.

According to a third aspect of the present disclosure, there is provided a method for use with an RLC resonance circuit for inductive heating of a susceptor of an aerosol generating device, the method comprising: determining a resonant frequency of the RLC circuit; and determining a first frequency for the RLC resonance circuit for causing the susceptor to be inductively heated, the first frequency being above or below the determined resonant frequency.

The method may comprise controlling a drive frequency of the RLC resonance circuit to be at the determined first frequency in order to heat the susceptor.

According to a fourth aspect of the present disclosure, there is provided a computer program which, when executed on a processing system, causes the processing system to perform the method of according to the third aspect.

Further features and advantages of the disclosure will become apparent from the following description of preferred embodiments of the disclosure, given by way of example only, which is made with reference to the accompanying drawings.

DETAILED DESCRIPTION

Induction heating is a process of heating an electrically conducting object (or susceptor) by electromagnetic induction. An induction heater may comprise an electromagnet and a device for passing a varying electric current, such as an alternating electric current, through the electromagnet. The varying electric current in the electromagnet produces a varying magnetic field. The varying magnetic field penetrates a susceptor suitably positioned with respect to the electromagnet, 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 whether 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.

Electrical resonance occurs in an electric circuit 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 a RLC circuit, comprising a resistance (R) provided by a resistor, an inductance (L) provided by an inductor, and a capacitance (C) provided by a capacitor, connected in series. Resonance occurs in an RLC 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. When the circuit is driven at the resonant frequency, the series impedance of the inductor and the capacitor is at a minimum, and circuit current is at a maximum.

FIG.1illustrates schematically an example aerosol generating device150comprising an RLC resonance circuit100for inductive heating of an aerosol generating material164via a susceptor116. In some examples, the susceptor116and the aerosol generating material164form an integral unit that may be inserted and/or removed from the aerosol generating device150, and may be disposable. The aerosol generating device150is hand-held. The aerosol generating device150is arranged to heat the aerosol generating material164to 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. 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 toFIG.1, the aerosol generating device150comprises an outer body151housing the RLC resonance circuit100, the susceptor116, the aerosol generating material164, a controller114, and a battery162. The battery is arranged to power the RLC resonance circuit100. The controller114is arranged to control the RLC resonance circuit100, for example control the voltage delivered to the RLC resonance circuit100from the battery162, and the frequency fat which the RLC resonance circuit100is driven. The RLC resonance circuit100is arranged for inductive heating of the susceptor116. The susceptor116is arranged to heat the aerosol generating material364to generate an aerosol in use. The outer body151comprises a mouthpiece160to allow aerosol generated in use to exit the device150.

In use, a user may activate, for example via a button (not shown) or a puff detector (not shown) which is known per se, the controller114to cause the RLC resonance circuit100to be driven, for example at the resonant frequency frof the RLC resonance circuit100. The resonance circuit100thereby inductively heats the susceptor116, which in turn heats the aerosol generating material164, and causes the aerosol generating material164thereby to generate an aerosol. The aerosol is generated into air drawn into the device150from an air inlet (not shown), and is thereby carried to the mouthpiece160, where the aerosol exits the device150.

The controller114and the device150as a whole may be arranged to heat the aerosol generating material to a range of temperatures to volatilize at least one component of the aerosol generating material 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 250° C., between about 50° C. and about 150° C., between about 50° C. and about 120° C., between about 50° C. and about 100° C., between about 50° C. and about 80° C., or between about 60° C. and about 70° C. In some examples, the temperature range is between about 170° C. and about 220° 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 is desirable to control the degree to which the susceptor116is inductively heated, and hence the degree to which the susceptor116heats the aerosol generating material164. For example, it may be useful to control the rate at which the susceptor116is heated and/or the extent to which the susceptor116is heated. For example, it may be useful to control heating of the aerosol generating material164(via the susceptor116) according to a particular heating profile, for example in order to alter or enhance the characteristics of the aerosol generated, such as the nature, flavor and/or temperature, of the aerosol generated. As another example, it may be useful to control heating of the aerosol generating material164(via the susceptor116) between different states, for example a ‘holding’ state where the aerosol generating medium is heated to a relatively low temperature which may be below the temperature at which the aerosol generating medium produces aerosol, and a ‘heating’ state where the aerosol generating material164is heated to a relatively high temperature at which the aerosol generating material164produces aerosol. This control may help reduce the time within which the aerosol generating device150can generate aerosol from a given activation signal. As a further example, it may be useful to control heating of the aerosol generating material164(via the susceptor116) such that it does not exceed a certain extent for example to ensure that it is not heated beyond a certain temperature, for example so that it does not burn or char. For example, it may be desirable that the temperature of the susceptor116does not exceed 400° C., in order to ensure that the susceptor116does not cause the aerosol generating material164to burn or char. It will be appreciated that there may be a difference between the temperature of the susceptor116and the temperature of the aerosol generating material164as a whole, for example during heating up of the susceptor116, for example where the rate of heating is large. It will therefore be appreciated that in some examples the temperature at which the susceptor116is controlled to be or which it should not exceed may be higher than the temperature to which the aerosol generating material164is desired to be heated to or which it should not exceed, for example.

One possible way of controlling the inductive heating of the susceptor116by the RLC resonance circuit100is to control a supply voltage that is provided to the circuit, which in turn may control the current flowing in the circuit100, and hence may control the energy transferred to the susceptor116by the RLC resonance circuit100, and hence the degree to which the susceptor116is heated. However, regulating the supply voltage would lead to increased cost, increased space requirements, and reduced efficiency due to losses in voltage regulating components.

According to examples of the present invention, an apparatus (for example the controller114), is arranged to control the degree to which the susceptor116is heated by controlling a drive frequency f of the RLC resonance circuit100. In broad overview, and as described in more detail below, the controller114is arranged to determine a resonant frequency frof the RLC resonance circuit100, for example by looking up the resonant frequency of the circuit100, or by measuring it, for example. The controller114is arranged to then determine, based on the determined resonant frequency fr, a first frequency for causing the susceptor to be inductively heated, the first frequency being above or below the determined resonant frequency fr. The controller114is arranged to then control a drive frequency f of the RLC resonance circuit100to be at the determined first frequency in order to heat the susceptor116. Since the first frequency is above or below the resonance frequency frof the RLC resonance circuit100(i.e. is ‘off resonance’), then driving the RLC circuit100at the first frequency will result in less current I flowing in the circuit100as compared to when driven at the resonant frequency frfor a given voltage, and hence the susceptor116will be inductively heated to a lesser degree as compared to when driven the circuit100is driven at the resonant frequency frfor the given voltage. Controlling the drive frequency of the resonant circuit to be at the first frequency therefore allows a control of the degree to which the susceptor116is heated without needing to control the voltage supplied to the circuit, and hence allows for a cheaper, more space and power efficient device150.

Referring now toFIG.2a, there is illustrated an example RLC resonance circuit100for inductive heating of the susceptor116. The resonance circuit100comprises a resistor104, a capacitor106, and an inductor108connected in series. The resonance circuit100has a resistance R, an inductance L and a capacitance C.

The inductance L of the circuit100is provided by the inductor108arranged for inductive heating of the susceptor116. The inductive heating of the susceptor116is via an alternating magnetic field generated by the inductor108, which as mentioned above induces Joule heating and/or magnetic hysteresis losses in the susceptor116. A portion of the inductance L of circuit100may be due to the magnetic permeability of the susceptor116. The alternating magnetic field generated by the inductor108is generated by an alternating current flowing through the inductor108. The alternating current flowing through the inductor108is an alternating current flowing through RLC resonance circuit100. The inductor108may, for example, be in the form of a coiled wire, for example a copper coil. The inductor108may comprise, for example, a Litz wire, for example a wire comprising a number of individually insulated wires twisted together. Litz wires may be particularly useful when drive frequencies f in the MHz range are used, as this may reduce power loss due to the skin effect, as is known per se. At these relatively high frequencies, lower values of inductance are required. As another example, the inductor108may 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 Litz wire (which may be expensive), which can be mass produced with a high reproducibility for low cost. Although one inductor108is shown, it will be readily appreciated that there may be more than one inductor arranged for inductive heating of one or more susceptors116.

The capacitance C of the circuit100is provided by the capacitor106. The capacitor106may be, for example, a Class 1 ceramic capacitor, for example a COG capacitor. The capacitance C may also comprise the stray capacitance of the circuit100; however, this is or can be made negligible compared with the capacitance C provided by the capacitor106.

The resistance R of the circuit100is provided by the resistor104, the resistance of the track or wire connecting the components of the resonance circuit100, the resistance of the inductor108, and the resistance to current flowing the resonance circuit100provided by the susceptor116arranged for energy transfer with the inductor108. It will be appreciated that the circuit100need not necessarily comprise a resistor104, and that the resistance R in the circuit100may be provided by the resistance of the connecting track or wire, the inductor108and the susceptor116.

The circuit100is driven by H-Bridge driver102. The H-Bridge driver102is a driving element for providing an alternating current in the resonance circuit100. The H-Bridge driver102is connected to a DC voltage supply VSUPP110, and to an electrical ground GND112. The DC voltage supply VSUPP110may be, for example, from the battery162. The H-Bridge102may be an integrated circuit, or may comprise discrete switching components (not shown), which may be solid-state or mechanical. The H-bridge driver102may be, for example, a High-efficiency Bridge Rectifier. As is known per se, the H-Bridge driver102may provide an alternating current in the circuit100from the DC voltage supply VSUPP110by reversing (and then restoring) the voltage across the circuit via switching components (not shown). This may be useful as it allows the RLC resonance circuit to be powered by a DC battery, and allows the frequency of the alternating current to be controlled.

The H-Bridge driver104is connected to a controller114. The controller114controls the H-Bridge102or components thereof (not shown) to provide an alternating current I in the RLC resonance circuit100at a given drive frequency f. For example, the drive frequency f 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 other frequencies f or frequency ranges may be used, for example depending on the particular resonance circuit100(and/or components thereof), controller114, susceptor116, and/or driving element102used. For example, it will be appreciated that the resonant frequency frof the RLC circuit100is dependent on the inductance L and capacitance C of the circuit100, which in turn is dependent on the inductor108, capacitor106and susceptor116. The range of drive frequencies f may be around the resonant frequency frof the particular RLC circuit100and/or susceptor116used, for example. It will also be appreciated that resonance circuit100and/or drive frequency or range of drive frequencies f used may be selected based on other factors for a given susceptor116. For example, in order to improve the transfer of energy from the inductor108to the susceptor116, it may be useful to provide that the skin depth (i.e. the depth from the surface of the susceptor116within which the alternating magnetic field from the inductor108is absorbed) is less, for example a factor of two to three times less, than the thickness of the susceptor116material. The skin depth differs for different materials and construction of susceptors116, and reduces with increasing drive frequency f. In some examples, therefore, it may be beneficial to use relatively high drive frequencies f. On the other hand, for example, in order to reduce the proportion of power supplied to the resonance circuit100and/or driving element102that is lost as heat within the electronics, it may be beneficial to use lower drive frequencies f. In some examples, a compromise between these factors may therefore be chose as appropriate and/or desired.

As mentioned above, the controller114is arranged to determine a resonant frequency frof the RLC resonance circuit100, and then determine the first frequency f at which the RLC resonance circuit100is to be controlled to be driven based on the determined resonant frequency fr.

FIG.3aillustrates schematically a frequency response300of the resonance circuit100. In the example ofFIG.3a, the frequency response300of the resonance circuit100is illustrated by a schematic plot of the current I flowing in the circuit100as a function of the drive frequency f at which the circuit is driven by the H-Bridge driver104.

The resonance circuit100ofFIG.2ahas a resonant frequency frat which the series impedance Z of the inductor108and the capacitor106is at a minimum, and hence the circuit current I is maximum. Hence, as illustrated inFIG.3a, when the H-Bridge driver104drives the circuit100at the resonant frequency fr, the alternating current I in the circuit100, and hence in the inductor108, will be maximum Imax. The oscillating magnetic field generated by the inductor106will therefore be maximum, and hence the inductive heating of the susceptor116by the inductor106will be maximum. When the H-Bridge driver104drives the circuit100at a frequency f that is off-resonance, i.e. above or below the resonant frequency fr, the alternating current I in the circuit100, and hence the inductor108, will be less than maximum, and hence the oscillating magnetic field generated by the inductor106will be less than maximum, and hence the inductive heating of the susceptor116by the inductor106will be less than maximum (for a given supply voltage VSUPP110). As can be seen inFIG.3atherefore, the frequency response300of the resonance circuit100has a peak, centered on the resonant frequency fr, and tailing off at frequencies above and below the resonant frequency fr.

As mentioned above, the controller114is arranged to determine the resonant frequency frof the circuit100.

In one example, the controller114is arranged to determine the resonant frequency frof the circuit100, by looking up the resonant frequency fr, for example from a memory (not shown). For example, the resonant frequency frof the circuit100may be calculated or measured or otherwise determined in advance and pre-stored in the memory (not shown), for example on manufacture of the device150. In another example, the resonant frequency frof the circuit100may be communicated to controller114, for example from a user input (not shown), or from another device or input, for example. Using a pre-stored resonant frequency as the resonant frequency frof the circuit100on the basis of which the circuit is to be controlled allows for a simple control of the circuit100. Even if the pre-stored resonant frequency is not exactly the same as the actual resonant frequency of the circuit100, useful control on the basis of the pre-stored resonant frequency100may still be provided.

The resonant frequency frof the circuit100(series RLC circuit) is dependent on the capacitance C and inductance L of the circuit100, and is given by:

As mentioned above, the inductance L of the circuit100is provided by the inductor108arranged for inductive heating of the susceptor116. At least portion of the inductance L of circuit100is due to the magnetic permeability of the susceptor116. The inductance L, and hence resonant frequency frof the circuit100may therefore depend on the specific susceptor(s) used and its positioning relative to the inductor(s)108, which may change from time to time. Further, the magnetic permeability of the susceptor116may vary with varying temperatures of the susceptor116. In some examples therefore, in order to determine the resonant frequency of the circuit100more accurately, it may be useful to measure the resonant frequency of the circuit100.

In some examples, in order to determine the resonant frequency of the circuit100, the controller114is arranged to measure a frequency response300of the RLC resonance circuit100. For example, the controller may be arranged to measure an electrical property of the RLC circuit100as a function of the driving frequency f at which the RLC circuit is driven. The controller114may comprise a clock generator (not shown) to determine the absolute frequency at which the RLC circuit100is to be driven. The controller114may be arranged to control the H-bridge104to scan through a range of drive frequencies f over a period of time. The electrical property of the RLC circuit100may be measured during the scan of drive frequencies, and hence the frequency response300of the RLC circuit100as a function of the driving frequency f may be determined.

The measurement of the electrical property may be a passive measurement i.e. a measurement not involving any direct electrical contact with the resonance circuit100.

For example, referring again to the example shown inFIG.2a, the electrical property may be indicative of a current induced into a sense coil120aby the inductor108of the RLC circuit100. As illustrated inFIG.2a, the sense coil120ais positioned for energy transfer from the inductor108, and is arranged to detect the current I flowing in the circuit100. The sense coil120amay be, for example, a coil of wire, or a track on a printed circuit board. For example, in the case the inductor108is a track on a printed circuit board, the sense coil120amay be a track on a printed circuit board and positioned above or below the inductor108, for example in a plane parallel to the plane of the inductor108. As another example, in the example where there is more than one inductor108, the sense coil120amay be placed between the inductors108, for energy transfer from both of the inductors. For example in the case of the inductors108being tracks on a printed circuit board and lying in a plane parallel to one another, the sense coil120amay be a track on a printed circuit board in-between the two inductors, and in a plane parallel to the inductors108. In any case, the alternating current I flowing in the circuit100and hence the inductor108causes the inductor108to generate an alternating magnetic field. The alternating magnetic field induces a current into the sense coil120a. The current induced into the sense coil120aproduces a voltage VINDacross the sense coil120a. The voltage VINDacross the sense coil120acan be measured, and is proportional to the current I flowing in RLC circuit100. The voltage VINDacross the sense coil120amay be recorded as a function of the drive frequency f at which the H-Bridge driver104is driving the resonance circuit100, and hence a frequency response300of the circuit100determined. For example, the controller114may record a measurement of the voltage VINDacross the sense coil120aas a function of the frequency f at which it is controlling the H-Bridge driver104to drive the alternating current in the resonance circuit100. The controller may then analyze the frequency response300to determine the resonant frequency frabout which the peak is centered, and hence the resonant frequency of the circuit100.

FIG.2billustrates another example passive measurement of an electrical property of the RLC circuit100.FIG.2bis the same asFIG.2aexcept in that the sense coil120aofFIG.2ais replaced by a pick-up coil120b. As illustrated inFIG.2b, the pick-up coil120bis placed so as to intercept a portion of a magnetic field produced by the DC supply voltage wire or track110when the DC current flowing therethrough changes due to changing demands of the RLC circuit. The magnetic field produced by the changes in current flowing in the DC supply voltage wire or track110induces a current in the pick-up coil120b, which produces a voltage VINDacross the pick-up coil120b. For example, although in an ideal case the current flowing in the DC supply voltage wire or track110would be direct current only, in practice the current flowing in the DC supply voltage wire or track110may be modulated to some extent by the H-Bridge driver104, for example due to imperfections in the switching in the H-Bridge driver104. These current modulations accordingly induce a current into the pick-up coil, which are detected via the voltage VINDacross the pick-up coil120b.

The voltage VINDacross the pick-up coil120bcan be measured and recorded as a function of the drive frequency f at which the H-Bridge driver104is driving the resonance circuit100, and hence a frequency response300of the circuit100determined. For example, the controller114may record a measurement of the voltage VINDacross the pick-up coil120aas a function of the frequency f at which it is controlling the H-Bridge driver104to drive the alternating current in the resonance circuit100. The controller may then analyze the frequency response300to determine the resonant frequency frabout which the peak is centered and hence the resonant frequency of the circuit100.

It is noted that in some examples it may be desirable to reduce or remove the modulated component of the current in the DC supply voltage wire or track110that may be caused by imperfections in the H-Bridge driver104. This may be achieved, for example, by implementing a bypass capacitor (not shown) across the H-bridge driver104. It will be appreciated that in this case, the electrical property of the RLC circuit100used to determine the frequency response300of the circuit100may be measured by means other than the pick-up coil120b.

FIG.2cillustrates an example of an active measurement of an electrical property of the RLC circuit.FIG.2cis the same asFIG.2aexcept in that the sense coil120aofFIG.2ais replaced by an element120c, for example a passive differential circuit120c, arranged to measure the voltage VLacross the inductor108. As the current I in the resonance circuit100changes, the voltage VLacross the inductor108will change. The voltage VLacross the inductor108can be measured and recorded as a function of the drive frequency f at which the H-Bridge driver104drives the resonance circuit100, and hence a frequency response300of the circuit100determined. For example, the controller114may record a measurement of the voltage VLacross the inductor108as a function of the frequency f at which it is controlling the H-Bridge driver104to drive the alternating current in the resonance circuit100. The controller114may then analyze the frequency response300to determine the resonant frequency frabout which the peak is centered, and hence the resonant frequency of the circuit100.

In each of the examples illustrated inFIGS.2ato2c, or otherwise, the controller114may analyze the frequency response300to determine the resonant frequency frabout which the peak is centered. For example, the controller114may use known data analysis techniques to determine the resonant frequency from the frequency response. For example, the controller may infer the resonant frequency frdirectly from the frequency response data. For example, the controller114may determine the frequency f at which the largest response was recorded as the resonant frequency fr, or may determine the frequencies f for which the two largest responses were recorded and determine the average of these two frequencies f as the resonant frequency fr. As yet another example, the controller114may fit a function describing current I (or another response such as impedance etc.) as a function of frequency f for an RLC circuit to the frequency response data, and infer or calculate from the fitted function the resonant frequency fr.

Determining the resonant frequency frbased on a measurement of the frequency response of the RLC circuit100removes the need to rely on an assumed value of the resonant frequency for a given circuit100, susceptor1116, or susceptor temperature, and hence provides for a more accurate determination of the resonant frequency of the circuit100, and hence for more accurate control of the frequency at which the resonance circuit100is to be driven. Further, the control is more robust to changes of the susceptor116, or the resonance circuit100, or the device as a whole350. For example, changes in the resonant frequency of the resonance circuit100due to a change in temperature of the susceptor116(for example due to changes in the susceptor's magnetic permeability, and hence inductance L of the resonance circuit100, with changing temperature of the susceptor116), may be accounted for in the measurement.

In some examples, the susceptor116may be replaceable. For example, the susceptor116may be disposable and for example integrated with the aerosol generating material164that it is arranged to heat. The determination of the resonant frequency by measurement may therefore account for differences between different susceptors116, and/or differences in the placement of the susceptor116relative to the inductor108, as and when the susceptor116is replaced. Furthermore, the inductor108, or indeed any component of the resonance circuit100, may be replaceable, for example after a certain use, or after damage. Similarly, the determination of the resonant frequency may therefore account for differences between different inductors108, and/or differences in the placement of the inductor108relative to the susceptor116, as an when the inductor108is replaced.

Accordingly, the controller may be arranged to determine the resonant frequency of the RLC circuit100substantially on start-up of the aerosol generating device150and/or substantially on installation of a new and/or replacement susceptor116into the aerosol generating device150and/or substantially on installation of a new and/or replacement inductor108into the aerosol generating device150.

As mentioned above, the controller114is arranged to determine, based on the determined resonant frequency, a first frequency f for causing the susceptor116to be inductively heated, the first frequency being above or below the determined resonant frequency (i.e. off resonance).

FIG.3billustrates schematically a frequency response300of the RLC resonance circuit100, according to an example, with specific points (black circles) marked on the response300corresponding to different drive frequencies fA, fB, fc, f′A. In the example ofFIG.3b, the frequency response300of the resonance circuit100is illustrated by a schematic plot of the current I flowing in the circuit100as a function of the drive frequency f at which the circuit100is driven. The response300may correspond, for example, to the current I (or alternatively another electrical property) of the circuit100measured, for example by the controller114, as a function of the drive frequency f at which the circuit100is driven. As illustrated inFIG.3b, and as described above, the response300forms a peak centered around the resonant frequency fr. When the resonance circuit100is driven at the resonant frequency fr, the current I flowing in the resonance circuit100is maximum Imaxfor a given supply voltage. When the resonance circuit is driven at a frequency f′Athat is above (e.g. higher than) the resonant frequency fr, the current IAflowing in the resonance circuit100is less than the maximum fmaxfor a given supply voltage. Similarly when the resonance circuit is driven at a frequency fA, fB, fcthat is below (e.g. lower than) the resonant frequency fr, the current IA, IB, ICflowing in the resonance circuit100is less than the maximum Imaxfor a given supply voltage. Since there is less current I flowing in the resonance circuit when it is driven at one of the first frequencies fA, fB, fc, f′Aas compared to when the circuit is driven at the resonant frequency fr, for a given supply voltage, then the energy transfer from the inductor108of the resonance circuit110to the susceptor116will be less, and hence the degree to which the susceptor116is inductively heated will be less, as compared to the degree to which the susceptor116is inductively heated when the circuit is driven at the resonant frequency fr, for a given supply voltage. By controlling the resonance circuit100to be driven at one of the first frequencies fA, fB, fc, f′Atherefore, the controller can control the degree to which the susceptor116is heated.

As will be appreciated, the further away (above or below) the frequency at which the resonance circuit100is controlled to be driven is from the resonant frequency fr, the less the degree to which susceptor116will be inductively heated. Nonetheless, at each of the first frequencies fA, fB, fc, f′A, energy is transferred from the inductor108of the circuit100to the susceptor116, and the susceptor116is inductively heated.

In some examples, the controller114may determine one or more of the first frequencies fA, fB, fc, f′Aby adding or subtracting a pre-determined amount to or from the determined resonant frequency or by multiplying or dividing the resonant frequency frby a pre-determined factor, or by any other operation, and control the resonance circuit100to be driven at this first frequency. The predetermined amount or factor or other operation may be set such that the susceptor116is still inductively heated when the resonance circuit100is driven at the first frequency fA, fB, fc, f′A, i.e. such that the first frequency fA, fB, fc, f′Ais not so far off resonance that the susceptor116is substantially not inductively heated. The pre-determined amount or factor or operation may be determined or calculated in advance, for example during manufacture, and stored in a memory (not shown) accessible by the controller114, for example. For example, the response300of the circuit100may be measured in advance, and the operations resulting in first frequencies fA, fB, fc, f′Awhich correspond to different current flow IA, IB, Icin the circuit100and hence different degrees of inductive heating of the susceptor116, determined, and stored in a memory (not shown) accessible by the controller114. The controller may then select an appropriate operation, and hence first frequency fA, fB, fc, f′A, in order to control the degree to which the susceptor116is inductively heated.

In other examples, as mentioned above, the controller114may determine the response300of the resonant circuit100as a function of the drive frequency f, for example by measuring and recording an electrical property of the circuit100as a function of the drive frequency f at which the circuit100is driven. As described above, this may be conducted on start-up of the device150or on replacement of component parts of the circuit100, for example. This may alternatively or additionally be conducted during operation of the device. The controller114may then determine the first frequency fA, fB, fc, f′Arelative to the resonant frequency fr, by analyzing the measured response300, for example using techniques as described above. The controller114may then select the appropriate first frequency fA, fB, fc, f′A, in order to control the degree to which the susceptor116is inductively heated. Similarly to as described above, determining the first frequency based on a measured response of the resonant circuit100may allow a control that is more accurate and robust against changes within the device150, such as replacement of component parts of the resonant circuit100or relative positioning thereof, as well as changes in the response300itself for example due to different temperatures or other conditions of the susceptor116, resonance circuit100, or device150.

In some examples, the controller114may determine a characteristic indicative of a bandwidth of the peak of the response300, and determine the first frequency fA, fB, fc, f′Abased on the determined characteristic. For example, the controller may determine the first frequency fA, fB, fc, f′Abased on a bandwidth B of the peak of the response300. As illustrated inFIG.3a, the bandwidth B of the peak is the full width of the peak in Hz at Imax/√{square root over (2)}. The characteristic indicative of the bandwidth B of the peak of the response300of the resonance circuit100may be determined in advance, for example during manufacture of the device, and pre-stored in a memory (not shown) accessible by the controller114. The characteristic is indicative of the width of the peak of the response300. Accordingly, use of this characteristic may provide a simple way for the controller114to determine a first frequency that will result in a given degree of inductive heating relative to the maximum at the resonant frequency without analyzing the response300. For example, the controller114may determine the first frequency for example by adding or subtracting from the determined resonant frequency fra proportion or multiple of the characteristic indicative of the bandwidth B. For example, the controller114may determine the first frequency by taking the determined resonant frequency frand adding or subtracting from the determined resonant frequency fra frequency that is half of the bandwidth B. As can be seen fromFIG.3a, this would result in a current I flowing in the circuit of Imax/√{square root over (2)} and hence a reduction of the degree to which the susceptor116is heated as compared to when the circuit100is driven at the resonant frequency, for a given voltage.

It will be appreciated that in other examples, the controller114may determine the characteristic indicative of the bandwidth B from analyzing the response300of the circuit100, for example from a measurement of an electrical property of the circuit100as a function of the drive frequency f at which the circuit100is driven, as described above.

The determined first frequency fA, fB, fc, f′Aat which the circuit100is controlled to be driven is above or below the resonant frequency fr(i.e. off-resonance), and hence the degree to which the susceptor116is inductively heated by the resonance circuit100is less than as compared to when driven at the resonant frequency fr, for a given supply voltage. Control of the degree to which the susceptor116is inductively heated is thereby achieved.

As mentioned above, it may be useful to control the rate at which the susceptor116is heated and/or the extent to which the susceptor116is heated. To achieve this, the controller114may control the drive frequency f of the resonant circuit100to be at one of a plurality of first frequencies fA, fB, fc, f′Aeach different from one another. For example, the plurality of first frequencies fA, fB, fc, fAmay each be determined by the controller114, and then an appropriate one of the plurality of first frequencies fA, fB, fc, f′Aselected, according to the desired degree to which the susceptor116(and hence aerosol generating material164) is to be heated.

As mentioned above, it may be useful to control heating of the aerosol generating material164(via the susceptor116) according to a particular heating profile for example in order to alter or enhance the characteristics of the aerosol generated, such as the nature, flavour and/or temperature, of the aerosol generated. To achieve this, the controller114may control the drive frequency f of the resonance circuit100sequentially through the plurality of first frequencies in accordance with a sequence. For example, the sequence may correspond to a heating sequence, where the degree to which the susceptor116is inductively heated is increased through the sequence. For example, the controller114may control the drive frequency f at which the resonant circuit100is driven such that each of the first frequencies in the sequence is closer to the resonant frequency than the previous first frequency in the sequence. For example, referring toFIG.3b, the sequence may be first frequency fcfollowed by first frequency fBfollowed by first frequency fA, where fAis closer to the resonant frequency frthan is fB, and fBis closer to the resonant frequency frthan is fC. In this case, the current I flowing in the resonant circuit100will accordingly be ICfollowed by IBfollowed by IA, where ICis less than IBwhich is in turn less than IA. As a result, the degree to which the susceptor116is inductively heated increases as a function of time. This may be useful to control and hence tailor the temporal heating profile of the aerosol generating material164, and hence tailor the aerosol delivery, for example. The device150is therefore more flexible. For example, the sequence may correspond to a heating sequence, where the degree to which the susceptor116is inductively heated is increased through the sequence. As another example, the controller114may control the drive frequency fat which the resonant circuit100is driven such that each of the first frequencies in the sequence is further from the resonant frequency than the previous first frequency in the sequence. For example, referring toFIG.3b, the sequence may be first frequency fAfollowed by first frequency fBfollowed by first frequency fC, and hence the current I flowing in the resonant circuit100will accordingly be IAfollowed by IBfollowed by IC, where ICis less than IBwhich is in turn less than IA. As a result, the degree to which the susceptor116is inductively heated decreases as a function of time. This may be useful to reduce the temperature of the susceptor116or aerosol generating medium164in a more controlled manner, for example. Although in the sequences mentioned above, each frequency in the sequence was closer (or further) from the resonant frequency than the last, it will be appreciated that this need not necessarily be the case, and other sequences may be followed comprising any order of a plurality of first frequencies as desired.

In some examples, the controller114may select a sequence of a plurality of first frequencies fA, fB, fc, f′Afrom a plurality of predefined sequences, for example stored on a memory (not shown) accessible by the controller114. The sequence may be, for example, the heating sequence or the cooling sequence mentioned above, or any other predefined sequence. The controller114may determine which of the plurality of sequences to select based on, for example, user input such as a heating or cooling mode selection, the type of susceptor116or aerosol generating medium164being used (as identified by user input or from another identification means, for example), operational inputs from the overall device150such as a temperature of the susceptor116or aerosol generating medium164etc. This may be useful to control and hence tailor the temporal heating profile of the aerosol generating material164according to user desire or operational circumstance, and allows for a more flexible device150.

In some examples, the controller114may control the drive frequency f to be held at a first frequency fA, fB, fc, f′Afor a first period of time. In some examples, the controller114may control the first frequency f to be held at one or more of the plurality of first frequencies fA, fB, fc, fAfor a respective one or more time periods. This allows for further tailoring and flexibility of the heating profile of the susceptor116and aerosol generating material164.

As a specific example, it may be useful to control heating of the aerosol generating material164(via the susceptor116) between different states or modes, for example a ‘holding’ state where the aerosol generating material164is heated to a relatively low ‘holding’ or ‘pre-heating’ degree for a period of time, and a ‘heating’ state where the aerosol generating material164is heated to a relatively high degree for a period of time. As explained below, control between such states may help reduce the time within which the aerosol generating device150can generate a substantial amount of aerosol from a given activation signal.

A specific example is illustrated schematically inFIG.3b, which illustrates schematically a plot of temperature T of the susceptor116(or aerosol generating material164) as a function of time t, according to an example. Before a time t1, the device150may be in an ‘off’ state, i.e. no current flows in the resonance circuit100. The temperature of the susceptor116may therefore be an ambient temperature TG, for example 21° C. At the time t1, the device150is switched to an ‘on’ state, for example by a user turning the device150on. The controller114controls the circuit100to be driven at a first frequency fB. The controller114holds the drive frequency f at the first frequency fBfor a time period P12. The time period P12may be an open-ended period in the sense that it endures until a further input is received by the controller114at a time t2, as described below. The circuit100being driven at the first frequency fBcauses an alternating current IBto flow in the circuit100, and hence the inductor108, and hence for the susceptor116to be inductively heated. As the susceptor116is inductively heated, its temperature (and hence the temperature of the aerosol generating material164) increases over the time period P12. In this example, the susceptor116(and aerosol generating material164) is heated in the period P12such that it reaches a steady temperature TB. The temperature TBmay be a temperature which is above the ambient temperature TG, but below a temperature at which a substantial amount of aerosol is generated by the aerosol generating material164. The temperature TBmay be 100° C. for example. The device150is therefore in a ‘pre-heating’ or ‘holding’ state or mode, wherein the aerosol generating material164is heated, but aerosol is substantially not being produced, or a substantial amount of aerosol is not being produced. At a time t2, the controller114receives an input, such as an activation signal. The activation signal may result from a user pushing a button (not shown) of the device150or from a puff detector (not shown), which is known per se. On receipt of the activation signal, the controller114may control the circuit100to be driven at the resonant frequency fr. The controller114holds the drive frequency f at the resonant frequency frfor a time period P23. The time period P23may be an open-ended period in the sense that it endures until a further input is received by the controller114at a time t3, 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 circuit100being driven at the resonant frequency frcauses an alternating current IMAXto flow in the circuit100and the inductor108, and hence for the susceptor116to be inductively heated to a maximum degree, for a given voltage. As the susceptor116is inductively heated to the maximum degree, its temperature (and hence the temperature of the aerosol generating material164) increases over the time period P23. In this example, the susceptor116(and aerosol generating material164) is heated in the period P23such that it reaches a steady temperature TMAX. The temperature TMAXmay be a temperature which is above the ‘pre-heating’ temperature TB, and substantially at or above a temperature at which a substantial amount of aerosol is generated by the aerosol generating material164. The temperature TMAXmay be 300° C. for example (although of course may be a different temperature depending on the material164, susceptor116, the arrangement of the overall device105, and/or other requirements and/or conditions). The device150is therefore in a ‘heating’ state or mode, wherein the aerosol generating material164reaches a temperature at which aerosol is substantially being produced, or a substantial amount of aerosol is being produced. Since the aerosol generating material164is already pre-heated, the time taken from the activation signal for the device150to produce a substantial amount of aerosol is therefore reduced as compared to the case where no ‘pre-heating’ or ‘holding’ state is applied. The device150is therefore more responsive.

Although in the above example the controller114controlled the resonance circuit100to be driven at the resonance frequency on fron receipt of the activation signal, in other examples the controller114may control the resonance circuit100to be driven at first frequency fA, fc, closer to the resonance frequency frthan the first frequency fBof the ‘pre-heating’ mode or state.

In some examples, the susceptor116may comprise nickel. For example the susceptor116may comprise a body or substrate having a thin nickel coating. For example, the body may be a sheet of mild steel with a thickness of about 25 μm. In other examples, the sheet may be made of a different material such as aluminum or plastic or stainless steel or other non-magnetic materials and/or may have a different thickness, such as a thickness of between 10 μm and 50 μm. The body may be coated or electroplated with nickel. The nickel may for example have a thickness of less than 5 μm, such as between 2 μm and 3 μm. The coating or electroplating may be of another material. Providing the susceptor116with only a relatively small thickness may help to reduce the time required to heat the susceptor116in use. A sheet form of the susceptor116may allow a high degree of efficiency of heat coupling from the susceptor116to the aerosol generating material164. The susceptor116may be integrated into a consumable comprising the aerosol generating material164. A thin sheet of susceptor116material may be particularly useful for this purpose. The susceptor116may be disposable. Such a susceptor116may be cost effective. In one example, the nickel coated or plated susceptor116may be heated to temperatures in the range of about 200° C. to about 300° C., which may be the working range of the aerosol generating device150.

In some examples, the susceptor116may be or comprise steel. The susceptor116may be a sheet of mild steel with a thickness of between about 10 μm and about 50 μm, for example a thickness of about 25 μm. Providing the susceptor116with only a relatively small thickness may help to reduce the time required to heat the susceptor in use. The susceptor116may be integrated into the apparatus105, for example as opposed to being integrated with the aerosol generating material164, which aerosol generating material164may be disposable. Nonetheless, the susceptor116may be removable from the apparatus115, for example to enable replacement of the susceptor116after use, for example after degradation due to thermal and oxidation stress over use. The susceptor116may therefore be “semi-permanent”, in that it is to be replaced infrequently. Mild steel sheets or foils or nickel coated steel sheets or foils as susceptors116may be particularly suited to this purpose as they are durable and hence, for example, may resist damage over multiple uses and/or multiple contact with aerosol generating material164, for example. A sheet form of the susceptor116may allow a high degree of efficiency of heat coupling from the susceptor116to the aerosol generating material164.

The Curie temperature Tcof iron is 770° C. The Curie temperature Tcof mild steel may be around 770° C. The Curie temperature Tcof cobalt is 1127° C. In one example, the mild steel susceptor116may be heated to temperatures in the range of about 200° C. to about 300° C., which may be the working range of the aerosol generating device150. The susceptor116having a Curie temperature Tcthat is remote from the working range of temperatures of the susceptor116in the device150may be useful as in this case changes to the response300of the circuit100may be relatively small over the working range of temperatures of the susceptor116. For example, the change in saturation magnetization of a susceptor material such as mild steel at 250° C. may be relatively small, for example less than 10% relative to the value at ambient temperatures, and hence the resulting change in inductance L, and hence resonant frequency fr, of the circuit100at different temperatures in the example working range may be relatively small. This may allow for the determined resonant frequency frto be accurately based on a pre-determined value, and hence for simpler control.

FIG.4is a flow diagram schematically illustrating a method400of controlling the RLC resonance circuit100for inductive heating of the susceptor116of the aerosol generating device150. In402, the method400comprises determining a resonant frequency frof the RLC circuit100, for example by looking it up from a memory, or by measuring it. In404, the method400comprises determining a first frequency fA, fB, fc, f′Afor causing the susceptor116to be inductively heated, the first frequency being above or below the determined resonant frequency fr. For example, the determination may be by adding or subtracting a pre-stored amount from the resonant frequency fr, or based on a measurement of the frequency response of the circuit100. In406, the method400comprises controlling a drive frequency f of the RLC resonance circuit100to be at the determined first frequency fA, fB, fc, f′Ain order to heat the susceptor116. For example, the controller114may send a control signal to the H-Bridge driver114to drive the RLC circuit100at the first frequency fA, fB, fc, f′A.

The controller114may comprise a processor and a memory (not shown). The memory may store instructions executable by the processor. For example, the memory may store instructions which, when executed on the processor, may cause the processor to perform the method400described above, and/or to perform the functionality of any one or combination of the examples described above. The instructions may be stored on any suitable storage medium, for example, on a non-transitory storage medium.

Although some of the above examples referred to the frequency response300of the RLC resonance circuit100in terms of a current I flowing in the RLC resonance circuit100as a function of the frequency f at which the circuit is driven, it will be appreciated that this need not necessarily be the case, and in other examples the frequency response300of the RLC circuit100may be any measure relatable to the current I flowing in the RLC resonance circuit as a function of the frequency f at which the circuit is driven. For example the frequency response300may be a response of an impedance of the circuit to frequency f, or as described above may be a voltage measured across the inductor, or a voltage or current resulting from the induction of current into a pick-up coil by a change in current flowing in a supply voltage line or track to the resonance circuit, or a voltage or current resulting from the induction of current into a sense coil by the inductor108of the RLC resonance circuit, or a signal from a non-inductive pick up coil or non-inductive filed sensor such a s a Hall effect device, as a function of the frequency f at which the circuit is driven. In each case, a frequency characteristic of a peak of the frequency response300may be determined.

Although in some of the above examples the Bandwidth B of the peak of the response300was referred to, it will be appreciated that any other indicator of the width of the peak of the response300may be used instead. For example, the full width or half-width of the peak at an arbitrary predetermined response amplitude, or fraction of a maximum response amplitude, may be used. It will also be appreciated that in other examples, the so called “Q” or “Quality” factor or value of the resonance circuit100, which may be related to the bandwidth B and the resonant frequency frof the resonance circuit100via Q=fr/B, may be determined and/or or measured and used in place of the bandwidth B and/or resonant frequency fr, similarly to as described in the examples above with appropriate factors applied. It will therefore be appreciated that in some examples the Q factor of the circuit100may be measured or determined, and the resonant frequency frof the circuit100, bandwidth B of the circuit100, and/or the first frequency at which the circuit100is driven may be determined based on the determined Q factor accordingly.

Although the above examples referred to a peak as associated with a maximum, it will be readily appreciated the this need not necessarily be the case and that, depending on the frequency response300determined and the way in which it is measured, the peak may be associated with a minimum. For example, at resonance, the impedance of the RLC circuit100is minimum, and hence in cases where the impedance as a function of drive frequency f is used as a frequency response300for example, the peak of the frequency response300of the RLC circuit will be associated with a minimum.

Although in some of the above examples it is described that the controller114is arranged to measure a frequency response300of the RLC resonance circuit100, it will be appreciated that in other examples the controller114may determine the resonant frequency or first frequency by analyzing frequency response data communicated to it by a separate measurement or control system (not shown), or may determine the resonant frequency or first frequency directly by being communicated them by a separate control or measurement system, for example. The controller114may then control the frequency at which the RLC circuit100is driven to the first frequency so determined.

Although in some of the above examples, it is described that the controller114is arranged to determine the first frequency and control the frequency at which the resonance circuit is driven, it will be appreciated that this need not necessarily be the case, and in other examples an apparatus that need not necessarily be or comprise the controller114may be arranged to determine the first frequency and control the frequency at which the resonance circuit is driven. The apparatus may be arranged to determine the first frequency, for example by the methods described above. The apparatus may be arranged to send a control signal, for example to the H-Bridge driver102, to control the resonance circuit100to be driven at the first frequency so determined. It will be appreciated that this apparatus or the controller114need not necessarily be an integral part of the aerosol generating device150, and may, for example, be a separate apparatus or controller114for use with the aerosol generating device150. Further, it will be appreciated that the apparatus or controller114need not necessarily be for controlling the resonance circuit, and/or need not necessarily be arranged to control the frequency at which the resonance circuit is driven, and that in other examples the apparatus or controller114may be arranged to determine the first frequency but not itself control the resonance circuit. For example, having determined the first frequency, the apparatus or controller114may send this information or information indicating the determined first frequency to a separate controller (not shown), or the separate controller (not shown) may obtain the information or indication from the apparatus or controller114, which separate controller (not shown) may then control the frequency at which the resonance circuit is driven based on this information or indication, for example control the frequency at which the resonance circuit is driven to be at the first frequency, for example control the H-Bridge driver102to drive the resonance circuit at the first frequency.

Although in the above examples it is described that the apparatus or controller114is for use with an RLC resonance circuit for inductive heating of a susceptor of an aerosol generating device, this need not necessarily be the case and in other examples the apparatus or controller114may be for use with an RLC resonance circuit for inductive heating of a susceptor of any device, for example any inductive heating device.

Although in the above examples it is described that the RLC resonance circuit100is driven by the H-Bridge driver102, this need not necessarily be the case, and in other examples the RLC resonance circuit100may be driven by any suitable driving element for providing an alternating current in the resonance circuit100, such as an oscillator or the like.