Patent Description:
Aerosol-generating devices may comprise an electrically-operated heat source that is configured to heat an aerosol-forming substrate to produce an aerosol. It is important for aerosol-generating devices to accurately monitor and control the temperature of the electrically operated heat source to ensure optimum generation and delivery of an aerosol to a user. In particular, it is important to ensure that the electrically-operated heat source does not overheat the aerosol-forming substrate as this may lead to the generation of undesirable compounds as well as an unpleasant taste and aroma for the user. To this end, aerosol-generating devices may comprise safety mechanisms in response to detection of overheating, such as generating an alarm and switching off the electrically-operated heat source. <CIT> describes an inductive heating device configured to receive an aerosol-generating article including an aerosol-forming substrate and a susceptor, and to heat the susceptor when the article is received by the device, the device including a DC power supply to provide a DC supply voltage and a DC current; and power supply electronics including a DC/AC converter connected to the power supply, and an inductor connected to the converter to inductively couple to the susceptor when the article is received by the device, the electronics being configured to supply power to the inductor from the power supply, via the converter, for heating the susceptor when the article is received by the device, the supplied power being provided in a plurality of pulses separated by time intervals, and to control a duration of the time intervals between successive pulses based on measurements of the current provided by the power supply. <CIT> relates to an inductive heating configured to receive an aerosol-generating article including an aerosol-forming substrate and a susceptor, and to heat the susceptor when the article is received by the device, the device including a DC power supply; and power supply electronics configured to supply power to the inductor in a plurality of pulses separated by time intervals, for heating the susceptor when the article is received by the device, the pulses including two or more heating pulses and one or more probing pulses between successive heating pulses, and to control a duration of a time interval between the successive heating pulses based on one or more measurements of the current supplied from the DC power supply in one or more of the one or more probing pulses. <CIT> describes an inductive heating device for heating an aerosol-forming substrate comprising a susceptor comprises: a device housing, a DC power source for providing a DC supply voltage (VDC) and a DC current (IDC), a power supply electronics comprising a DC/AC converter comprising an LC load network comprising a series connection of a capacitor and an inductor having an ohmic resistance, a cavity in the device housing for accommodating a portion of the aerosol-forming substrate to inductively couple the inductor to the susceptor.

It would be desirable to provide temperature monitoring and control of an inductive heating device that provides for reliable temperature regulation in order to reduce the risk of overheating and ensure continued normal operation of the aerosol-generating device.

According to an embodiment of the present invention, there is provided a method for controlling aerosol production in an aerosol-generating device. The aerosol-generating device comprises an inductive heating arrangement for heating a susceptor. The inductive heating arrangement comprises power supply electronics and a power source for providing power to the power supply electronics. The method comprises controlling the power provided to the power supply electronics to cause the susceptor to have a target temperature; measuring a temperature associated with the power supply electronics during operation of the aerosol-generating device for generating an aerosol; and adjusting the power provided to the power supply electronics based on a change of the measured temperature associated with the power supply electronics.

Adjusting the power provided to the power supply electronics based on a change of the measured temperature associated with the power supply electronics provides enables the temperature of the susceptor to be more accurately and reliably regulated, while reducing the need for recalibration during operation of the aerosol-generating device, which may affect the user experience.

Controlling the power provided to the power supply electronics to cause the susceptor to have a target temperature may comprise controlling the power provided to the power supply electronics to maintain a conductance value or a current value associated with the susceptor at a target value that corresponds to the target temperature.

Adjusting the power provided to the power supply electronics based at least in part on a change of the measured temperature associated with the power supply electronics may comprise controlling the power provided to the power supply electronics to decrease the conductance value or the current value associated with the susceptor as the measured temperature increases.

This prevents overheating for improved safety of the device when the aerosol-generating device is operating at or close to a maximum temperature. Further, overheating of the aerosol-forming substrate may result in the formation of undesired components of the aerosol-forming substrate. Thus, the more accurate and reliable regulation of the temperature of the susceptor improves safety for the user.

Decreasing the conductance value or the current value associated with the susceptor as the measured temperature increases may comprise decreasing the target conductance or current value by an amount based on a value of the change of the measured temperature such that the amount by which the target conductance or current value is decreased increases as the value of the change of the measured temperature increases.

The amount by which the target conductance or current value is decreased may be based on the amount of change of the measured temperature multiplied by a drift compensation value.

Controlling the power provided to the power supply electronics to cause the susceptor to have a target temperature may comprise controlling the power provided to the power supply electronics to maintain a resistance value associated with the susceptor at a target resistance value that corresponds to the target temperature.

Adjusting the power provided to the power supply electronics based at least in part on a change of the measured temperature associated with the power supply electronics may comprise controlling the power provided to the power supply electronics to increase the resistance value associated with the susceptor as the measured temperature increases.

Increasing the resistance value associated with the susceptor as the measured temperature increases may comprise increasing the target resistance value by an amount based on a value of the change of the measured temperature such that the amount by which the target resistance value is increased increases as the value of the change of the measured temperature increases.

The amount by which the target resistance value is decreased may be based on the amount of change of the measured temperature multiplied by a drift compensation value.

The drift compensation value may be a constant.

The drift compensation value may increase as the measured temperature associated with the power supply electronics increases.

This further reduces the risk of overheating the aerosol-forming substrate by further reducing the target conductance value or further increasing the target resistance value as the measured temperature increases.

The drift compensation value may increase according to a piecewise linear function, wherein the piecewise linear function comprises a first degree polynomial having a positive gradient and a first degree polynomial having a gradient of zero.

The drift compensation value may increase according to a square root function.

The method may further comprise storing at least one drift compensation value in a memory of the aerosol-generating device.

The method may further comprise storing a plurality of drift compensation values and respective corresponding temperature values in a memory of the aerosol-generating device.

The drift compensation value may be between <NUM> and <NUM>.

The method may further comprise determining the drift compensation value. Determining the drift compensation value may comprise the steps of: i) controlling the power provided to the power supply electronics to cause the susceptor to have a first known temperature; when the susceptor is at the first known temperature: ii) determining a conductance value, a current value or a resistance value associated with the susceptor; iii) determining a temperature associated with the power supply electronics; and repeating steps i) to iii) at least twice.

The target conductance value, target current value, or target resistance value may be determined based on a first calibration value corresponding to a first known temperature of the susceptor and a second calibration value corresponding to a second known temperature of the susceptor. The second known temperature of the susceptor may be greater than the first known temperature of the susceptor.

The target conductance value, target current value, or target resistance value may be defined according to a heating profile as a predetermined percentage of a difference between the first calibration value and the second calibration value.

The heating profile may define a stepwise increase of temperature from a first operating temperature to a second operating temperature.

The first operating temperature may be sufficient for the aerosol-forming substrate to form an aerosol.

The second operating temperature may be below the second known temperature.

The heating profile may define at least three consecutive temperature steps, each temperature step having a respective duration.

Controlling the power provided to the inductive heating arrangement to cause the step-wise increase of a temperature of the susceptor enables generation of an aerosol over a sustained period encompassing the full user experience of a number of puffs, for example <NUM> puffs, or a predetermined time interval, such as <NUM> minutes, where the deliveries (nicotine, flavors, aerosol volume and so on) are substantially constant for each puff throughout the user experience. Specifically, the stepwise increase if the temperature of the susceptor prevents the reduction of aerosol delivery due to substrate depletion in the vicinity of the susceptor and reduced thermodiffusion over time. Furthermore, the step-wise increase in temperature allows for the heat to spread within the substrate at each step.

The method may further comprise calibrating the aerosol-generating device to measure the first calibration value and the second calibration value. Calibrating the aerosol-generating device may comprise: controlling the power provided to the inductive heating arrangement to cause heating and cooling of the susceptor through a predetermined temperature range; and monitoring a power source parameter to identify a start point and an end point of a reversible phase transition of the susceptor, wherein the power source parameter is one of a current, a conductance or a resistance. The first calibration value may be a power source parameter value corresponding to the start point of the reversible phase transition of the susceptor. The second calibration value may be a power source parameter value corresponding to the end point of the reversible phase transition of the susceptor.

The calibrating the aerosol-generating device to measure the first calibration value and the second calibration value before operation of the heating arrangement for generating an aerosol.

The method may further comprise calibrating the aerosol-generating device to measure the first calibration value and the second calibration value during operation of the heating arrangement for generating an aerosol.

Accordingly, the calibration values used to control the heating process are more accurate and reliable than if the calibration process were performed at manufacturing. This is especially important if the susceptor forms part of a separate aerosol-generating article that does not form part of the aerosol generating device. In such circumstances calibration at manufacturing is not possible.

Measuring a temperature associated with the power supply electronics during operation of the aerosol-generating device for generating an aerosol may comprise measuring the temperature of a first portion of the power supply electronics using a first temperature sensor.

The first temperature sensor may be one of a thermocouple, a negative temperature coefficient resistive temperature sensor, and a positive temperature coefficient resistive temperature sensor.

Measuring a temperature of at least one portion of the power supply electronics during operation of the aerosol-generating device further comprises measuring the temperature of a second portion of the power supply electronics using a second temperature sensor.

The second temperature sensor may be one of a thermocouple, a negative temperature coefficient resistive temperature sensor, and a positive temperature coefficient resistive temperature sensor.

The method may further comprise measuring a DC current drawn the power source, wherein the conductance value or the resistance value is determined based on a DC supply voltage of the power source and the DC current drawn from the power source.

The method may further comprise measuring the DC supply voltage of the power source.

According to another embodiment of the present invention, there is provided an aerosol-generating device. The aerosol-generating device comprises an inductive heating arrangement for heating a susceptor and a controller. The inductive heating arrangement comprises power supply electronics and a power source for providing power to the power supply electronics. The controller comprises at least one temperature sensor arranged to measure a temperature associated with the power supply electronics during operation of the aerosol-generating device for generating an aerosol. The controller is configured to: control the power provided to the power supply electronics to cause the susceptor to have a target temperature; and adjust the power provided to the power supply electronics based on a change of the measured temperature associated with the power supply electronics.

The controller may be configured to decrease the conductance value or the current value associated with the susceptor as the measured temperature increases by decreasing the target conductance or current value by an amount based on a value of the change of the measured temperature such that the amount by which the target conductance or current value is decreased increases as the value of the change of the measured temperature increases.

Controlling the power provided to the power supply electronics to cause the susceptor to have a target temperature may comprise controlling the power provided to the power supply electronics to maintain a resistance value associated with the susceptor at a target value that corresponds to the target temperature.

The aerosol-generating device may further comprise a memory configured to store at least one drift compensation value.

The aerosol-generating device may further comprise a memory configured to store a plurality of drift compensation values and respective corresponding temperature values.

The controller may configured to determine the drift compensation value by performing steps comprising: i) controlling the power provided to the power supply electronics to cause the susceptor to have a first known temperature; when the susceptor is at the first known temperature: ii) determining a conductance value, a current value or a resistance value associated with the susceptor; iii) determining a temperature of associated with the power supply electronics; and repeating steps i) to iii) at least twice.

The target conductance value, current value or resistance value may be determined based on a first calibration value corresponding to a first known temperature of the susceptor and a second calibration value corresponding to a second known temperature of the susceptor. The second known temperature of the susceptor may be greater than the first known temperature of the susceptor.

The target conductance value, current value or resistance value may be defined according to a heating profile as a predetermined percentage of a difference between the first calibration value and the second calibration value.

The aerosol-generating device, wherein the controller may be further configured to calibrate the aerosol-generating device to measure the first calibration value and the second calibration value. Calibrating the aerosol-generating device may comprise: controlling the power provided to the inductive heating arrangement to cause heating and cooling of the susceptor through a predetermined temperature range; and monitoring a power source parameter to identify a start point and an end point of a reversible phase transition of the susceptor. The power source parameter may be one of a current, a conductance or a resistance. The first calibration value may be a power source parameter value corresponding to the start point of the reversible phase transition of the susceptor. The second calibration value may be a power source parameter value corresponding to the end point of the reversible phase transition of the susceptor.

The controller may be further configured to perform a calibration of the aerosol-generating device to measure the first calibration value and the second calibration value before operation of the heating arrangement for generating an aerosol.

The controller may be further configured to calibrate the aerosol-generating device to measure the first calibration value and the second calibration value during operation of the heating arrangement for generating an aerosol.

The at least one temperature sensor may be one of a thermocouple, a negative temperature coefficient resistive temperature sensor, and a positive temperature coefficient resistive temperature sensor.

The at least one temperature sensor may comprise a first temperature sensor and a second temperature sensor.

The first temperature sensor may be one of a thermocouple, a negative temperature coefficient resistive temperature sensor, and a positive temperature coefficient resistive temperature sensor and the second temperature sensor may be one of a thermocouple, a negative temperature coefficient resistive temperature sensor, and a positive temperature coefficient resistive temperature sensor.

The aerosol-generating device may further comprise a current sensor configured to measure a DC current drawn from the power source, wherein the conductance value or the resistance value is determined based on a DC supply voltage of the power source and the DC current drawn from the power source.

The aerosol-generating device may further comprise a voltage sensor configured to measure the DC supply voltage of the power source.

According to another embodiment of the present invention, there is provided an aerosol-generating system comprising: the aerosol-generating device described above and an aerosol-generating article. The aerosol-generating article may comprise an aerosol-forming substrate and the susceptor in thermal contact with the aerosol-forming substrate.

As used herein, the term "aerosol-generating device" refers to a device that interacts with an aerosol-forming substrate to generate an aerosol. An aerosol-generating device may interact with one or both of an aerosol-generating article comprising an aerosol-forming substrate, and a cartridge comprising an aerosol-forming substrate. In some examples, the aerosol-generating device may heat the aerosol-forming substrate to facilitate release of volatile compounds from the substrate. An electrically operated aerosol-generating device may comprise an atomizer, such as an electric heater, to heat the aerosol-forming substrate to form an aerosol.

As used herein, the term "aerosol-generating system" refers to the combination of an aerosol-generating device with an aerosol-forming substrate. When the aerosol-forming substrate forms part of an aerosol-generating article, the aerosol-generating system refers to the combination of the aerosol-generating device with the aerosol-generating article. In the aerosol-generating system, the aerosol-forming substrate and the aerosol-generating device cooperate to generate an aerosol.

As used herein, the term "aerosol-forming substrate" refers to a substrate capable of releasing volatile compounds that can form an aerosol. The volatile compounds may be released by heating or combusting the aerosol-forming substrate. As an alternative to heating or combustion, in some cases, volatile compounds may be released by a chemical reaction or by a mechanical stimulus, such as ultrasound. The aerosol-forming substrate may be solid or may comprise both solid and liquid components. An aerosol-forming substrate may be part of an aerosol-generating article.

As used herein, the term "aerosol-generating article" refers to an article comprising an aerosol-forming substrate that is capable of releasing volatile compounds that can form an aerosol. An aerosol-generating article may be disposable. An aerosol-generating article comprising an aerosol-forming substrate comprising tobacco may be referred to herein as a tobacco stick.

An aerosol-forming substrate may comprise nicotine. An aerosol-forming substrate may comprise tobacco, for example may comprise a tobacco-containing material containing volatile tobacco flavor compounds, which are released from the aerosol-forming substrate upon heating. In preferred embodiments an aerosol-forming substrate may comprise homogenized tobacco material, for example cast leaf tobacco. The aerosol-forming substrate may comprise both solid and liquid components. The aerosol-forming substrate may comprise a tobacco-containing material containing volatile tobacco flavor compounds, which are released from the substrate upon heating. The aerosol-forming substrate may comprise a non-tobacco material. The aerosol-forming substrate may further comprise an aerosol former. Examples of suitable aerosol formers are glycerin and propylene glycol.

As used herein, "aerosol-cooling element" refers to a component of an aerosol-generating article located downstream of the aerosol-forming substrate such that, in use, an aerosol formed by volatile compounds released from the aerosol-forming substrate passes through and is cooled by the aerosol cooling element before being inhaled by a user. An aerosol cooling element has a large surface area, but causes a low pressure drop. Filters and other mouthpieces that produce a high pressure drop, for example filters formed from bundles of fibers, are not considered to be aerosol-cooling elements. Chambers and cavities within an aerosol-generating article are not considered to be aerosol cooling elements.

As used herein, the term "mouthpiece" refers to a portion of an aerosol-generating article, an aerosol-generating device or an aerosol-generating system that is placed into a user's mouth in order to directly inhale an aerosol.

As used herein, the term "susceptor" refers to an element comprising a material that is capable of converting the energy of a magnetic field into heat. When a susceptor is located in an alternating magnetic field, the susceptor is heated. Heating of the susceptor may be the result of at least one of hysteresis losses and eddy currents induced in the susceptor, depending on the electrical and magnetic properties of the susceptor material.

As used herein when referring to an aerosol-generating device, the terms "upstream" and "front", and "downstream" and "rear", are used to describe the relative positions of components, or portions of components, of the aerosol-generating device in relation to the direction in which air flows through the aerosol-generating device during use thereof. Aerosol-generating devices according to the invention comprise a proximal end through which, in use, an aerosol exits the device. The proximal end of the aerosol-generating device may also be referred to as the mouth end or the downstream end. The mouth end is downstream of the distal end. The distal end of the aerosol-generating article may also be referred to as the upstream end. Components, or portions of components, of the aerosol-generating device may be described as being upstream or downstream of one another based on their relative positions with respect to the airflow path of the aerosol-generating device.

As used herein when referring to an aerosol-generating article, the terms "upstream" and "front", and "downstream" and "rear", are used to describe the relative positions of components, or portions of components, of the aerosol-generating article in relation to the direction in which air flows through the aerosol-generating article during use thereof. Aerosol-generating articles according to the invention comprise a proximal end through which, in use, an aerosol exits the article. The proximal end of the aerosol-generating article may also be referred to as the mouth end or the downstream end. The mouth end is downstream of the distal end. The distal end of the aerosol-generating article may also be referred to as the upstream end. Components, or portions of components, of the aerosol-generating article may be described as being upstream or downstream of one another based on their relative positions between the proximal end of the aerosol-generating article and the distal end of the aerosol-generating article. The front of a component, or portion of a component, of the aerosol-generating article is the portion at the end closest to the upstream end of the aerosol-generating article. The rear of a component, or portion of a component, of the aerosol-generating article is the portion at the end closest to the downstream end of the aerosol-generating article.

As used herein, the term "inductively couple" refers to the heating of a susceptor when penetrated by an alternating magnetic field. The heating may be caused by the generation of eddy currents in the susceptor. The heating may be caused by magnetic hysteresis losses.

As used herein, the term "puff" means the action of a user drawing an aerosol into their body through their mouth or nose.

As used herein, the term "temperature sensor" refers to a thermocouple, a negative temperature coefficient resistive temperature sensor or a positive temperature coefficient resistive temperature sensor.

Embodiments will now be further described with reference to the figures in which:.

<FIG> illustrates a schematic side sectional view of an aerosol-generating article <NUM>. The aerosol-generating article <NUM> comprises a rod of aerosol-forming substrate <NUM> and a downstream section <NUM> at a location downstream of the rod of aerosol-forming substrate <NUM>. The aerosol-generating article <NUM> comprises an upstream section <NUM> at a location upstream of the rod of aerosol-forming substrate. Thus, the aerosol-generating article <NUM> extends from an upstream or distal end <NUM> to a downstream or mouth end <NUM>. In use, air is drawn through the aerosol-generating article <NUM> by a user from the distal end <NUM> to the mouth end <NUM>.

The downstream section <NUM> comprises a support element <NUM> located immediately downstream of the rod of aerosol-forming substrate, the support element <NUM> being in longitudinal alignment with the rod <NUM>. The upstream end of the support element <NUM> abuts the downstream end of the rod of aerosol-forming substrate <NUM>. In addition, the downstream section <NUM> comprises an aerosol-cooling element <NUM> located immediately downstream of the support element <NUM>, the aerosol-cooling element <NUM> being in longitudinal alignment with the rod <NUM> and the support element <NUM>. The upstream end of the aerosol-cooling element <NUM> abuts the downstream end of the support element <NUM>. In use, volatile substances released from the aerosol-forming substrate <NUM> pass along the aerosol-cooling element <NUM> towards the mouth end <NUM> of the aerosol-generating article <NUM>. The volatile substances may cool within the aerosol-cooling element <NUM> to form an aerosol that is inhaled by the user.

The support element <NUM> comprises a first hollow tubular segment <NUM>. The first hollow tubular segment <NUM> is provided in the form of a hollow cylindrical tube made of cellulose acetate. The first hollow tubular segment <NUM> defines an internal cavity <NUM> that extends all the way from an upstream end <NUM> of the first hollow tubular segment <NUM> to a downstream end <NUM> of the first hollow tubular segment <NUM>.

The aerosol-cooling element <NUM> comprises a second hollow tubular segment <NUM>. The second hollow tubular segment <NUM> is provided in the form of a hollow cylindrical tube made of cellulose acetate. The second hollow tubular segment <NUM> defines an internal cavity <NUM> that extends all the way from an upstream end <NUM> of the second hollow tubular segment <NUM> to a downstream end <NUM> of the second hollow tubular segment <NUM>. In addition, a ventilation zone (not shown) is provided at a location along the second hollow tubular segment <NUM>. A ventilation level of the aerosol-generating article <NUM> is about <NUM> percent.

The downstream section <NUM> further comprises a mouthpiece <NUM> positioned immediately downstream of the aerosol-cooling element <NUM>. As shown in the drawing of <FIG>, an upstream end of the mouthpiece <NUM> abuts the downstream end <NUM> of the aerosol-cooling element <NUM>. The mouthpiece <NUM> is provided in the form of a cylindrical plug of low-density cellulose acetate.

The aerosol-generating article <NUM> further comprises an elongate susceptor <NUM> within the rod of aerosol-generating substrate <NUM>. In more detail, the susceptor <NUM> is arranged substantially longitudinally within the aerosol-forming substrate <NUM>, such as to be approximately parallel to the longitudinal direction of the rod <NUM>. As shown in the drawing of <FIG>, the susceptor <NUM> is positioned in a radially central position within the rod and extends effectively along the longitudinal axis of the rod <NUM>.

The susceptor <NUM> extends all the way from an upstream end to a downstream end of the rod of aerosol-forming substrate <NUM>. In effect, the susceptor <NUM> has substantially the same length as the rod of aerosol-forming substrate <NUM>. The susceptor <NUM> is located in thermal contact with the aerosol-forming substrate <NUM>, such that the aerosol-forming substrate <NUM> is heated by the susceptor <NUM> when the susceptor <NUM> is heated.

The upstream section <NUM> comprises an upstream element <NUM> located immediately upstream of the rod of aerosol-forming substrate <NUM>, the upstream element <NUM> being in longitudinal alignment with the rod <NUM>. The downstream end of the upstream element <NUM> abuts the upstream end of the rod of aerosol-forming substrate. This advantageously prevents the susceptor <NUM> from being dislodged. Further, this ensures that the consumer cannot accidentally contact the heated susceptor <NUM> after use. The upstream element <NUM> is provided in the form of a cylindrical plug of cellulose acetate circumscribed by a stiff wrapper.

The susceptor <NUM> comprises at least two different materials. The susceptor <NUM> comprises at least two layers: a first layer of a first susceptor material disposed in physical contact with a second layer of a second susceptor material. The first susceptor material and the second susceptor material may each have a Curie temperature. In this case, the Curie temperature of the second susceptor material is lower than the Curie temperature of the first susceptor material. The first material may not have a Curie temperature. The first susceptor material may be aluminum, iron or stainless steel. The second susceptor material may be nickel or a nickel alloy.

The susceptor <NUM> may be formed by electroplating at least one patch of the second susceptor material onto a strip of the first susceptor material. The susceptor may be formed by cladding a strip of the second susceptor material to a strip of the first susceptor material.

The aerosol-generating article <NUM> illustrated in <FIG> is designed to engage with an aerosol-generating device, such as the aerosol-generating device <NUM> illustrated in <FIG>, for producing an aerosol. The aerosol-generating device <NUM> comprises a housing <NUM> having a cavity <NUM> configured to receive the aerosol-generating article <NUM> and an inductive heating device <NUM> configured to heat an aerosol-generating article <NUM> for producing an aerosol. <FIG> illustrates the aerosol-generating device <NUM> when the aerosol-generating article <NUM> is inserted into the cavity <NUM>.

The inductive heating device <NUM> is illustrated as a block diagram in <FIG>. The inductive heating device <NUM> comprises a DC power source <NUM> and a heating arrangement <NUM> (also referred to as power supply electronics). The heating arrangement comprises a controller <NUM>, a DC/AC converter <NUM>, a matching network <NUM> and an inductor <NUM>.

The DC power source <NUM> is configured to provide DC power to the heating arrangement <NUM>. Specifically, the DC power source <NUM> is configured to provide a DC supply voltage (VDC) and a DC current (IDC) to the DC/AC converter <NUM>. Preferably, the power source <NUM> is a battery, such as a lithium ion battery. As an alternative, the power source <NUM> may be another form of charge storage device such as a capacitor. The power source <NUM> may require recharging. For example, the power source <NUM> may have sufficient capacity to allow for the continuous generation of aerosol for a period of around six minutes or for a period that is a multiple of six minutes. In another example, the power source <NUM> may have sufficient capacity to allow for a predetermined number of puffs or discrete activations of the heating arrangement.

The DC/AC converter <NUM> is configured to supply the inductor <NUM> with a high frequency alternating current. As used herein, the term "high frequency alternating current" means an alternating current having a frequency of between about <NUM> kilohertz and about <NUM> megahertz. The high frequency alternating current may have a frequency of between about <NUM> megahertz and about <NUM> megahertz, such as between about <NUM> megahertz and about <NUM> megahertz, or such as between about <NUM> megahertz and about <NUM> megahertz.

<FIG> schematically illustrates the electrical components of the inductive heating device <NUM>, in particular the DC/AC converter <NUM>. The DC/AC converter <NUM> preferably comprises a Class-E power amplifier. The Class-E power amplifier comprises a transistor switch <NUM> comprising a Field Effect Transistor <NUM>, for example a Metal-Oxide-Semiconductor Field Effect Transistor, a transistor switch supply circuit indicated by the arrow <NUM> for supplying a switching signal (gate-source voltage) to the Field Effect Transistor <NUM>, and an LC load network <NUM> comprising a shunt capacitor C1 and a series connection of a capacitor C2 and inductor L2, corresponding to inductor <NUM>. In addition, the DC power source <NUM>, comprising a choke L1, is shown for supplying the DC supply voltage VDC, with a DC current IDC being drawn from the DC power source <NUM> during operation. The ohmic resistance R representing the total ohmic load <NUM>, which is the sum of the ohmic resistance Rcoil of the inductor L2 and the ohmic resistance Rload of the susceptor <NUM>, is shown in more detail in <FIG>.

Although the DC/AC converter <NUM> is illustrated as comprising a Class-E power amplifier, it is to be understood that the DC/AC converter <NUM> may use any suitable circuitry that converts DC current to AC current. For example, the DC/AC converter <NUM> may comprise a class-D power amplifier comprising two transistor switches. As another example, the DC/AC converter <NUM> may comprise a full bridge power inverter with four switching transistors acting in pairs.

Turning back to <FIG>, the inductor <NUM> may receive the alternating current from the DC/AC converter <NUM> via a matching network <NUM> for optimum adaptation to the load, but the matching network <NUM> is not essential. The matching network <NUM> may comprise a small matching transformer. The matching network <NUM> may improve power transfer efficiency between the DC/AC converter <NUM> and the inductor <NUM>.

As illustrated in <FIG>, the inductor <NUM> is located adjacent to the distal portion <NUM> of the cavity <NUM> of the aerosol-generating device <NUM>. Accordingly, the high frequency alternating current supplied to the inductor <NUM> during operation of the aerosol-generating device <NUM> causes the inductor <NUM> to generate a high frequency alternating magnetic field within the distal portion <NUM> of the aerosol-generating device <NUM>. The alternating magnetic field preferably has a frequency of between <NUM> and <NUM> megahertz, preferably between <NUM> and <NUM> megahertz, for example between <NUM> and <NUM> megahertz. As can be seen from <FIG>, when an aerosol-generating article <NUM> is inserted into the cavity <NUM>, the aerosol-forming substrate <NUM> of the aerosol-generating article <NUM> is located adjacent to the inductor <NUM> so that the susceptor <NUM> of the aerosol-generating article <NUM> is located within this alternating magnetic field. When the alternating magnetic field penetrates the susceptor <NUM>, the alternating magnetic field causes heating of the susceptor <NUM>. For example, eddy currents are generated in the susceptor <NUM> which is heated as a result. Further heating is provided by magnetic hysteresis losses within the susceptor <NUM>. The heated susceptor <NUM> heats the aerosol-forming substrate <NUM> of the aerosol-generating article <NUM> to a sufficient temperature to form an aerosol. The aerosol is drawn downstream through the aerosol-generating article <NUM> and inhaled by the user.

The controller <NUM> may be a microcontroller, preferably a programmable microcontroller. The controller <NUM> is programmed to regulate the supply of power from the DC power source <NUM> to the inductive heating arrangement <NUM> in order to control the temperature of the susceptor <NUM>.

The power supply electronics <NUM> may comprise one or more temperature sensors (not shown) to measure a temperature of the power supply electronics <NUM>. The controller <NUM> is configured to read the output of the one or more temperature sensors. At least one temperature sensor of the one or more temperature sensors may be located on the printed circuit board of the power supply electronics <NUM>. The controller <NUM> may comprise at least one temperature sensor. Preferably, at least one temperature sensor is configured to measure at least the temperature of the printed circuit board of the power supply electronics <NUM>. The at least one temperature sensor may be located so as to measure the temperature of the inductor L2. The at least one temperature sensor may comprise one or more of a thermocouple, a negative temperature coefficient resistive temperature sensor or a positive temperature coefficient resistive temperature sensor.

<FIG> illustrates the relationship between the DC current IDC drawn from the power source <NUM> over time as the temperature of the susceptor <NUM> (indicated by the dashed line) increases. More specifically, <FIG> illustrates the remotely-detectable DC current changes that occur when a susceptor material undergoes a phase transition associated with its Curie point. The DC current IDC drawn from the power source <NUM> is measured at an input side of the DC/AC converter <NUM>. For the purpose of this illustration, it may be assumed that the voltage VDC of the power source <NUM> remains approximately constant. It is to be noted that because the behaviour of DC current drawn from the power source IDC over the temperature range of the phase transition is used to calibrate the aerosol-generating device <NUM>, as described in detail below, the characteristic shape of the relationship between the DC current IDC drawn from the power source <NUM> over time as the temperature of the susceptor <NUM> increases may be referred to as the calibration curve <NUM>.

As the susceptor <NUM> is inductively heated, the apparent resistance of the susceptor <NUM> increases. This increase in resistance is observed as a decrease in the DC current IDC drawn from the power source <NUM>, which at constant voltage decreases as the temperature of the susceptor <NUM> increases. The high frequency alternating magnetic field provided by the inductor <NUM> induces eddy currents in close proximity to the susceptor surface, an effect that is known as the skin effect. The resistance in the susceptor <NUM> depends in part on the electrical resistivity of the first susceptor material, the resistivity of the second susceptor material and in part on the depth of the skin layer in each material available for induced eddy currents, and the resistivity is in turn temperature dependent.

As the second susceptor material reaches its Curie temperature, it loses its magnetic properties. This causes an increase in the skin layer available for eddy currents in the second susceptor material, which causes a decrease in the apparent resistance of the susceptor <NUM>. The result is a temporary increase in the detected DC current IDC. Then, when the skin depth of the second susceptor material begins to increase, the resistance begins to fall. This is seen as the valley (the local minimum) <NUM> in <FIG>.

As heating continues, the current continues to increase until the maximum skin depth is reached, which coincides with the point where the second susceptor material has lost its spontaneous magnetic properties. This point is called the Curie temperature and is seen as the hill (the local maximum) <NUM> in <FIG>. At this point the second susceptor material has undergone a phase change from a ferro-magnetic or ferri-magnetic state to a paramagnetic state. At this point, the susceptor <NUM> is at a known temperature (the Curie temperature, which is an intrinsic material-specific temperature).

If the inductor <NUM> continues to generate an alternating magnetic field (i.e. power to the DC/AC converter <NUM> is not interrupted) after the Curie temperature has been reached, the eddy currents generated in the susceptor <NUM> will run against the resistance of the susceptor <NUM>, whereby Joule heating in the susceptor <NUM> will continue, and thereby the resistance will increase again (the resistance will have a polynomial dependence of the temperature, which for most metallic susceptor materials can be approximated to a third degree polynomial dependence for our purposes) and current will start falling again as long as the inductor <NUM> continues to provide power to the susceptor <NUM>.

Therefore, the second susceptor material undergoes a reversible phase transition when heated through the (known) temperature range between the valley <NUM> and the hill <NUM> shown in <FIG>. As can be seen from <FIG>, the apparent resistance of the susceptor <NUM>, and hence the start and end of the phase transition, can be remotely detected by monitoring the DC current IDC drawn from the power source <NUM>. Alternatively, the apparent resistance of the susceptor <NUM>, and hence the start and end of the phase transition, can be remotely detected by monitoring a conductance value (where conductance is defined as the ratio of the DC current IDC to the DC supply voltage VDC) or a resistance value (where resistance is defined as the ratio of the DC supply voltage VDC to the DC current IDC). At least the DC current IDC drawn from the power source <NUM> is monitored by the controller <NUM>. Although the DC supply voltage VDC is known, preferably both the DC current IDC drawn from the power source <NUM> and the DC supply voltage VDC are monitored. The DC current IDC, the conductance value and the resistance value may be referred to as power source parameters.

As the susceptor <NUM> is heated, a first turning point <NUM> (corresponding to a local minimum for current and a local maximum for resistance) corresponds to the start of the phase transition. Then, as the susceptor continues to be heated, a second turning point <NUM> (corresponding to a local maximum for current and a local minimum for resistance) corresponds to the end of the phase transition.

Furthermore as can be seen from <FIG>, the apparent resistance of the susceptor <NUM> (and correspondingly the current IDC drawn from the power source <NUM>) may vary with the temperature of the susceptor <NUM> in a strictly monotonic relationship over certain ranges of temperature of the susceptor <NUM>, such as between the valley <NUM> and the hill <NUM>. The strictly monotonic relationship allows for an unambiguous determination of the temperature of the susceptor <NUM> from a determination of the apparent resistance (R) or apparent conductance (<NUM>/R). This is because each determined value of the apparent resistance is representative of only one single value of the temperature, so that there is no ambiguity in the relationship. The monotonic relationship of the temperature of the susceptor <NUM> and the apparent resistance in the temperature range in which the second susceptor material undergoes the reversible phase transition allows for the determination and control of the temperature of the susceptor <NUM> and thus for the determination and control of the temperature of the aerosol-forming substrate <NUM>.

The controller <NUM> regulates the supply of power provided to the heating arrangement <NUM> based on a power supply parameter. The heating arrangement <NUM> may comprise a current sensor (not shown) to measure the DC current IDC. The heating arrangement <NUM> may optionally comprise a voltage sensor (not shown) to measure the DC supply voltage VDC. The current sensor and the voltage sensor are located at an input side of the DC/AC converter <NUM>. The DC current IDC and optionally the DC supply voltage VDC are provided by feedback channels to the controller <NUM> to control the further supply of AC power PAC to the inductor <NUM>.

The controller <NUM> may control the temperature of the susceptor <NUM> by maintaining the measured power supply parameter value at a target value corresponding to a target operating temperature of the susceptor <NUM>. The controller <NUM> may use any suitable control loop to maintain the measured power supply parameter at the target value, for example by using a proportional-integral-derivative control loop.

In order to take advantage of the strictly monotonic relationship between the apparent resistance (or apparent conductance) of the susceptor <NUM> and the temperature of the susceptor <NUM>, during user operation for producing an aerosol, the power supply parameter measured at the input side of the DC/AC converter <NUM> is maintained between a first calibration value corresponding to a first calibration temperature and a second calibration value corresponding to a second calibration temperature. The second calibration temperature is the Curie temperature of the second susceptor material (the hill <NUM> in the current plot in <FIG>). The first calibration temperature is a temperature greater than or equal to the temperature of the susceptor at which the skin depth of the second susceptor material begins to increase, leading to a temporary lowering of the resistance (the valley <NUM> in the current plot in <FIG>). Thus, the first calibration temperature is a temperature greater than or equal to the temperature at maximum permeability of the second susceptor material. The first calibration temperature is at least <NUM> degrees Celsius lower than the second calibration temperature. At least the second calibration value may be determined by calibration of the susceptor <NUM>, as will be described in more detail below. The first calibration value and the second calibration value may be stored as calibration values in a memory of the controller <NUM>.

Further, the controller <NUM> may maintain the temperature of the susceptor <NUM> below a predetermined threshold temperature by maintaining the measured conductance or current value below a predetermined threshold conductance value or by maintaining the measured resistance value above a predetermined threshold resistance value. The predetermined threshold temperature is chosen to prevent overheating of the aerosol-forming substrate. The predetermined threshold temperature may be the same as the second calibration temperature. If the measured power supply parameter indicates that the temperature of the susceptor is above the predetermined threshold temperature, the controller <NUM> is programmed to enter a safety mode. In the safety mode, the controller <NUM> is configured to perform one or more actions such as generating an alarm that (visually and additionally or alternatively audibly) provides an overheating warning to the user, switching off the aerosol-generating device and preventing further use if the aerosol-generating device for a predefined period of time.

Since the power supply parameter will have a polynomial dependence on the temperature, the power supply parameter will behave in a nonlinear manner as a function of temperature. However, the first and the second calibration values are chosen so that this dependence may be approximated as being linear between the first calibration value and the second calibration value because the difference between the first and the second calibration values is small, and the first and the second calibration values are in the upper part of the operational temperature range. Therefore, to adjust the temperature to a target operating temperature, the power supply parameter is regulated according to the first calibration value and the second calibration value, through linear equations.

For example, if the first and the second calibration values are conductance values, the target conductance value, GR, corresponding to the target operating temperature may be given by: <MAT> where ΔG is the difference between the first conductance value and the second conductance value and x is a percentage of ΔG.

The controller <NUM> may control the provision of power to the heating arrangement <NUM> by adjusting the duty cycle of the switching transistor <NUM> of the DC/AC converter <NUM>. For example, during heating, the DC/AC converter <NUM> continuously generates alternating current that heats the susceptor <NUM>, and simultaneously the DC current IDC and optionally the DC supply voltage VDC may be measured, preferably every millisecond for a period of <NUM> milliseconds.

For example, if the conductance or current is monitored by the controller <NUM> for adjusting the susceptor temperature, when the conductance or current reaches or exceeds a value corresponding to the target operating temperature for adjusting the susceptor temperature, the duty cycle of the switching transistor <NUM> is reduced. If the resistance is monitored by the controller <NUM> for adjusting the susceptor temperature, when the resistance reaches or goes below a value corresponding to the target operating temperature, the duty cycle of the switching transistor <NUM> is reduced. For example, the duty cycle of the switching transistor <NUM> may be reduced to about <NUM>%. In other words, the switching transistor <NUM> may be switched to a mode in which it generates pulses only every <NUM> milliseconds for a duration of <NUM> millisecond. During this <NUM> millisecond on-state (conductive state) of the switching transistor <NUM>, the values of the DC supply voltage VDC and of the DC current IDC are measured and the conductance is determined. As the conductance decreases (or the resistance increases) to indicate that the temperature of the susceptor <NUM> is below the target operating temperature, the gate of the transistor <NUM> is again supplied with the train of pulses at the chosen drive frequency for the system.

The power may be supplied by the controller <NUM> to the inductor <NUM> in the form of a series of successive pulses of electrical current. In particular, power may be supplied to the inductor <NUM> in a series of pulses, each separated by a time interval. The series of successive pulses may comprise two or more heating pulses and one or more probing pulses between successive heating pulses. The heating pulses have an intensity such as to heat the susceptor <NUM>. The probing pulses are isolated power pulses having an intensity such not to heat the susceptor <NUM> but rather to obtain a feedback on the power supply parameter and then on the evolution (decreasing) of the susceptor temperature. The controller <NUM> may control the power by controlling the duration of the time interval between successive heating pulses of power supplied by the DC power supply to the inductor <NUM>. Additionally or alternatively, the controller <NUM> may control the power by controlling the length (in other words, the duration) of each of the successive heating pulses of power supplied by the DC power supply to the inductor <NUM>.

The controller <NUM> is programmed to perform a calibration process in order to obtain the calibration values at which the power supply parameter is measured at known temperatures of the susceptor <NUM>. The known temperatures of the susceptor may be the first calibration temperature corresponding to the first calibration value and the second calibration temperature corresponding to the second calibration value. The calibration process is performed each time the user operates the aerosol-generating device <NUM>. For example, the controller <NUM> may be configured to enter a calibration mode for performing the calibration process when the user switches on the aerosol-generating device. The controller <NUM> may be programmed to enter the calibration mode each time the user inserts an aerosol-generating article <NUM> into an aerosol-generating device <NUM>. Thus, the calibration process is performed during a first heating phase of the aerosol-generating device, before user operation of the aerosol-generating device <NUM> for generating an aerosol.

During the calibration process, the controller <NUM> controls the DC/AC converter <NUM> to continuously or continually supply power to the inductor <NUM> in order to heat the susceptor <NUM>. The controller <NUM> monitors the power supply parameter by measuring the current IDC drawn by the power supply and, optionally the power supply voltage VDC. As discussed above in relation to <FIG>, as the susceptor <NUM> is heated, the measured current decreases until a first turning point <NUM> is reached and the current begins to increase. This first turning point <NUM> corresponds to a local minimum conductance or current value (a local maximum resistance value). The controller <NUM> may record the power supply parameter at the first turning point <NUM> as the first calibration value.

The conductance or resistance values may be determined based on the measured current IDC and the measured voltage VDC. Alternatively, it may be assumed that the power supply voltage VDC, which is a known property of the power source <NUM>, is approximately constant. The temperature of the susceptor <NUM> at the first calibration value is referred to as the first calibration temperature. Preferably, the first calibration temperature is between <NUM> degrees Celsius and <NUM> degrees Celsius. More preferably, when the aerosol-forming substrate <NUM> comprises tobacco, the first calibration temperature is <NUM> degrees Celsius. The first calibration temperature is at least <NUM> degrees Celsius lower than the second calibration temperature.

As the controller <NUM> continues to control the power provided by the DC/AC converter <NUM> to the inductor <NUM>, the controller <NUM> continues to monitor the power supply parameter until a second turning point <NUM> is reached. The second turning point corresponds to a maximum current (corresponding to the Curie temperature of the second susceptor material) before the measured current begins to decrease. This second turning point <NUM> corresponds to a local maximum conductance or current value (a local minimum resistance value). The controller <NUM> records the power supply parameter value at the second turning point <NUM> as the second calibration value. The temperature of the susceptor <NUM> at the second calibration value is referred to as the second calibration temperature. Preferably, the second calibration temperature is between <NUM> degrees Celsius and <NUM> degrees Celsius. When the second turning point <NUM> is detected, the controller <NUM> controls the DC/AC converter <NUM> to interrupt provision of power to the inductor <NUM>, resulting in a decrease in susceptor <NUM> temperature and a corresponding decrease in measured current.

Due to the shape of the graph <NUM>, this process of continuously heating the susceptor <NUM> to obtain the first calibration value and the second calibration value may be repeated at least once during the calibration mode. After interrupting provision of power to the inductor <NUM>, the controller <NUM> continues to monitor the power supply parameter until a third turning point is observed. The third turning point corresponds to a second minimum conductance or current value (a second maximum resistance value). When the third turning point is detected, the controller <NUM> controls the DC/AC converter <NUM> to continuously provide power to the inductor <NUM> until a fourth turning point in the monitored power supply parameter is observed. The fourth turning point corresponds to a second maximum conductance or current value (a second minimum resistance value). The controller <NUM> stores the power supply parameter value that is measured at the third turning point as the first calibration value and the power supply parameter value measured the fourth turning point as the second calibration value. The repetition of the measurement of the turning points corresponding to minimum and maximum measured current significantly improves the subsequent temperature regulation during user operation of the device for producing an aerosol. Preferably, controller <NUM> regulates the power based on the power supply parameter values obtained from the second maximum and the second minimum, this being more reliable because the heat will have had more time to distribute within the aerosol-forming substrate <NUM> and the susceptor <NUM>.

The controller <NUM> is configured to detect the turning points <NUM> and <NUM> by measuring a sequence of power source parameter values. With reference to <FIG>, the sequence of measured power source parameter values will form a curve, with each value being greater than or less than the previous value. The controller <NUM> is configured to measure the calibration value at the point where the curve begins to flatten. In other words, the controller <NUM> records the calibration values when the difference between consecutive power supply parameter values is below a predetermined threshold value.

Further, during the first heating phase, in order to further improve the reliability of the calibration process, the controller <NUM> may be optionally programmed to perform a pre-heating process before the calibration process. For example, if the aerosol-forming substrate <NUM> is particularly dry or in similar conditions, the calibration may be performed before heat has spread within the aerosol-forming substrate <NUM>, reducing the reliability of the calibration values. If the aerosol-forming substrate <NUM> were humid, the susceptor <NUM> takes more time to reach the valley temperature (due to water content in the substrate <NUM>).

To perform the pre-heating process, the controller <NUM> is configured to continuously provide power to the inductor <NUM>. As described above with respect to <FIG>, the measured current starts decreasing with increasing susceptor <NUM> temperature until a turning point <NUM> corresponding to minimum measured current is reached. At this stage, the controller <NUM> is configured to wait for a predetermined period of time to allow the susceptor <NUM> to cool before continuing heating. The controller <NUM> therefore controls the DC/AC converter <NUM> to interrupt provision of power to the inductor <NUM>. After the predetermined period of time, the controller <NUM> controls the DC/AC converter <NUM> to provide power until the turning point <NUM> corresponding to the minimum measured current is reached again. At this point, the controller controls the DC/AC converter <NUM> to interrupt provision of power to the inductor <NUM> again. The controller <NUM> again waits for the same predetermined period of time to allow the susceptor <NUM> to cool before continuing heating. This heating and cooling of the susceptor <NUM> is repeated for the predetermined duration of time of the pre-heating process. The predetermined duration of the pre-heating process is preferably <NUM> seconds. The predetermined combined durations of the pre-heating process followed by the calibration process is preferably <NUM> seconds.

If the aerosol-forming substrate <NUM> is dry, the first current minimum of the pre-heating process is reached within the pre-determined period of time and the interruption of power will be repeated until the end of the predetermined time period. If the aerosol-forming substrate <NUM> is humid, the first current minimum of the pre-heating process will be reached towards the end of the pre-determined time period. Therefore, performing the pre-heating process for a predetermined duration ensures that, whatever the physical condition of the substrate <NUM>, the time is sufficient for the substrate <NUM> to reach the minimum operating temperature, in order to be ready to feed continuous power and reach the first maximum. This allows a calibration as early as possible, but still without risking that the substrate <NUM> would not have reached the valley <NUM> beforehand.

Further, the aerosol-generating article <NUM> may be configured such that the current minimum <NUM> is always reached within the predetermined duration of the pre-heating process. If the current minimum <NUM> is not reached within the pre-determined duration of the pre-heating process, this may indicate that the aerosol-generating article <NUM> comprising the aerosol-forming substrate <NUM> is not suitable for use with the aerosol-generating device <NUM>. For example, the aerosol-generating article <NUM> may comprise a different or lower-quality aerosol-forming substrate <NUM> than the aerosol-forming substrate <NUM> intended for use with the aerosol-generating device <NUM>. As another example, the aerosol-generating article <NUM> may not be configured for use with the heating arrangement <NUM>, for example if the aerosol-generating article <NUM> and the aerosol-generating device <NUM> are manufactured by different manufacturers. Thus, the controller <NUM> may be configured to generate a control signal to cease operation of the aerosol-generating device <NUM>.

As mentioned above, as the first stage of the calibration process, the pre-heating process may be performed in response to receiving a user input, for example user activation of the aerosol-generating device <NUM>. Additionally or alternatively, the controller <NUM> may be configured to detect the presence of an aerosol-generating article <NUM> in the aerosol-generating device <NUM> and the pre-heating process may be performed in response to detecting the presence of the aerosol-generating article <NUM> within the cavity <NUM> of the aerosol-generating device <NUM>.

<FIG> is a graph of conductance against time showing a heating profile of the susceptor <NUM>. The graph illustrates two consecutive phases of heating: a first heating phase <NUM> comprising the pre-heating process 710A and the calibration process 710B described above, and a second heating phase <NUM> corresponding to user operation of the aerosol-generating device <NUM> to produce an aerosol. It is to be understood that <FIG> is not shown to scale. Specifically, the first heating phase <NUM> has a shorter duration that the second heating phase <NUM>. For example, the first heating phase <NUM> may have a duration of between <NUM> seconds and <NUM> seconds, preferably between <NUM> and <NUM> seconds. The second heating phase <NUM> may have a duration of between <NUM> and <NUM> seconds.

Further, although <FIG> is illustrated as a graph of conductance against time, it is to be understood that the controller <NUM> may be configured to control the heating of the susceptor <NUM> during the first heating phase <NUM> and the second heating phase <NUM> based on measured resistance or current as described above. Indeed, although the techniques to control of the heating of the susceptor during the first heating phase <NUM> and the second heating phase <NUM> have been described above based on a determined conductance value or a determined resistance value associated with the susceptor, it is to be understood that the techniques described above could be performed based on a value of current measured at the input of the DC/AC converter <NUM>.

As can be seen from <FIG>, the second heating phase <NUM> comprises a plurality of conductance steps, corresponding to a plurality of temperature steps from a first operating temperature of the susceptor <NUM> to a second operating temperature of the susceptor <NUM>. The first operating temperature of the susceptor is a temperature at which the aerosol-forming substrate <NUM> forms an aerosol so that an aerosol is formed during each temperature step. Preferably, the first operating temperature of the susceptor is a minimum temperature at which the aerosol-forming substrate will form an aerosol in a sufficient volume and quantity for a satisfactory experience when inhaled a user. The second operating temperature of the susceptor is the temperature at maximum temperature at which it is desirable for the aerosol-forming substrate to be heated for the user to inhale the aerosol.

The first operating temperature of the susceptor <NUM> is greater than or equal to the first calibration temperature of the susceptor <NUM>, corresponding to the first calibration value (the valley <NUM> of the current plot shown in <FIG>). The first operating temperature may be between <NUM> degrees Celsius and <NUM> degrees Celsius. The second operating temperature of the susceptor <NUM> is less than or equal to the second calibration temperature of the susceptor <NUM>, corresponding to the second calibration value at the Curie temperature of the second susceptor material (the hill <NUM> of the current plot shown in <FIG>). The second operating temperature may be between <NUM> degrees Celsius and <NUM> degrees Celsius. The difference between the first operating temperature and the second operating temperature is at least <NUM> degree Celsius.

It is to be understood that the number of temperature steps illustrated in <FIG> is exemplary and that second heating phase <NUM> comprises at least three consecutive temperature steps, preferably between two and fourteen temperature steps, most preferably between three and eight temperature steps. Each temperature step may have a predetermined duration. Preferably the duration of the first temperature step is longer than the duration of subsequent temperature steps. The duration of each temperature step is preferably longer than <NUM> seconds, preferably between <NUM> seconds and <NUM> seconds, more preferably between <NUM> seconds and <NUM> seconds. The duration of each temperature step may correspond to a predetermined number of user puffs. Preferably, the first temperature step corresponds to four user puffs and each subsequent temperature step corresponds to one user puff.

For the duration of each temperature step, the temperature of the susceptor <NUM> is maintained at a target operating temperature corresponding to the respective temperature step. Thus, for the duration of each temperature step, the controller <NUM> controls the provision of power to the heating arrangement <NUM> such that the measured power source parameter is maintained at a target value corresponding to the target operating temperature of the respective temperature step, where the target value is determined with reference to the first calibration value and the second calibration value as described above.

As an example, the second heating phase <NUM> may comprise five temperature steps: a first temperature step 720a having a duration of <NUM> seconds and a target conductance value of GR = GLower + (<NUM> × ΔG), a second temperature step 720b having a duration of <NUM> seconds and a target conductance value of GR = GLower + (<NUM> × ΔG), a third temperature step 720c having a duration of <NUM> seconds and a target conductance value of GR = GLower + (<NUM> × ΔG), a fourth temperature step 720d having a duration of <NUM> seconds and a target conductance value of GR = GLower + (<NUM> × ΔG) and a fifth temperature step 720e having a duration of <NUM> seconds and a target conductance value of GR = GLower + (<NUM> × ΔG). These temperature steps may correspond to temperatures of <NUM> degrees Celsius, <NUM> degrees Celsius, <NUM> degrees Celsius, <NUM> degrees Celsius and <NUM> degrees Celsius.

However, the first calibration value and the second calibration value used to determine the target power source parameter value for each temperature step will drift over the duration of the second heating phase <NUM> due to the fact that the temperature of the power supply electronics <NUM> increases during operation of the aerosol-generating device <NUM>. Specifically, as shown in <FIG>, the apparent resistance of the susceptor <NUM> is the sum of the ohmic resistance Rcoil of the inductor L2 and the ohmic resistance Rload of the susceptor <NUM>, meaning that any change to the temperature of the inductor L2 during operation of the device <NUM> will affect the apparent resistance. This is illustrated in <FIG>, which is a graph of conductance over time showing the downward drift of the calibration curve over time as the power supply electronics are heated.

<FIG> illustrates a first calibration curve 800A obtained during calibration during the first heating phase <NUM> as a solid line. The (increasing) temperature of the power supply electronics <NUM> is illustrated as a dashed line. The calibration curves 800B-F having a dashed line represent exemplary calibration curves that would be obtained if calibration were to be performed at a later time while the temperature of the power supply electronics <NUM> increases. As can be seen from <FIG>, the value of conductance at the turning points of the calibration curves drift downwards. In particular, the value of conductance at the hill <NUM> drifts downwards as the temperature of the power supply electronics <NUM> increases, indicated by the dotted line. Further, as can be seen from <FIG>, the temperature of the power supply electronics <NUM> increases more rapidly at the beginning due to there being a larger temperature gradient before levelling off. Accordingly, the downward drift of the calibration curves 800A-F is more rapid at the beginning when the temperature is changing more rapidly.

<FIG> illustrates the downward drift in conductance in more detail. Calibration curve S<NUM> represents the calibration curve measured during the calibration process during the first heating phase <NUM>. As described above, the heating of the susceptor <NUM> is then regulated at a value of target conductance GR defined by a predetermined percentage of ΔG<NUM>. In this example, the heating of the susceptor <NUM> is initially regulated at <NUM>% of ΔG<NUM> such that GR<NUM> = Glower + (<NUM>x ΔG<NUM>).

Calibration curve S<NUM> represents a calibration measured at a later time, when the temperature of the power supply electronics <NUM> is higher than during the calibration process to obtain curve S<NUM>. As can be seen from <FIG>, the difference between the conductance values at the first and second turning points remains unchanged (ΔG<NUM> = ΔG<NUM>), but the values of conductance at the first turning point <NUM> and the second turning point <NUM> have decreased by a value ΔD. However, the temperatures of the susceptor <NUM> at the turning points <NUM> and <NUM> remain the same as this is a property of the susceptor materials. Thus, for a given susceptor temperature, the target conductance or current value will drift downwards during operation of the aerosol-generating device <NUM>. (In terms of resistance, for a given susceptor temperature, the target resistance value will drift upwards during operation of the aerosol-generating device <NUM>).

Accordingly, if the measured conductance were to be maintained at the target value of GR<NUM> throughout the second heating phase, the temperature of the susceptor <NUM> would increase over time. In particular, as can be seen from <FIG>, GR<NUM> is not at <NUM>% of ΔG<NUM>, but is closer to the hill of calibration curve S<NUM>. It must be ensured that the temperature regulation always occurs between the first and the second calibration values in order to avoid overheating of the aerosol-generating substrate <NUM>. Thus, at the time at which the second calibration curve S<NUM> is measured, the target conductance GR<NUM> must be GR<NUM> - ΔGR to maintain the same susceptor temperature. In other words, GR<NUM> = Glower + (<NUM>xΔG<NUM>) - ΔGR<NUM>.

During the second heating phase <NUM>, the temperature of the power supply electronics <NUM> will be continuously monitored using the temperature sensor of the controller <NUM> and the power provided to the power supply electronics <NUM> will be adjusted based on a change of the measured temperature. Specifically, the target conductance or current value for each temperature step will decrease over the duration of the respective temperature step based on the measured temperature. The target resistance value for each temperature step will increase over the duration of the respective temperature step depending on the measured temperature. This is illustrated in <FIG>, which shows the heating profile of <FIG> adjusted to compensate for the drift of the calibration values. It is to be understood that <FIG> is for illustrative purposes and not drawn to scale.

The amount of decrease of the current or conductance (the amount of increase of the resistance) is proportional to the change of the measured temperature of the power supply electronics <NUM>. This ensures that the target power source parameter value remains between the hill <NUM> and the valley <NUM> of the calibration curve, thereby preventing overheating. The slope of each temperature step will progressively decrease until reaching a substantially flat shape towards the end. More specifically, the amount by which conductance is reduced is defined as: <MAT> where k is a drift compensation value and ΔT is a change of the measured temperature of the power supply electronics. The drift compensation value may be a constant. The drift compensation value may increase as the change of the measured temperature of the power supply electronics increases. Accordingly, ΔGR may be determined based on a drift compensation value of a plurality of drift compensation values. This provides for more precise temperature regulation and in particular further ensures that overheating is prevented because the value of ΔGR is further increased with larger increases in temperature.

One or more drift compensation values may be determined by performing the calibration process at least twice while heating the susceptor <NUM>. The determination of the drift compensation values may be performed during manufacturing of the aerosol-generating device <NUM>. Additionally or alternatively, the determination of the drift compensation values may be performed prior to the first heating stage <NUM>, for example during configuration of the aerosol-generating device <NUM> when the user switches on the aerosol-generating device <NUM> for the first time. The calibration values obtained from each repetition of the calibration process are then used to determine one or more drift compensation values. The one or more drift compensation values may be stored in a memory of the aerosol-generating device <NUM>, such as a memory of the controller <NUM>. Thus, for each of a plurality of predefined changes in temperature of the power supply electronics <NUM>, a drift compensation value may be stored.

In addition, during the second heating phase <NUM>, the controller <NUM> may be configured to enter a recalibration mode to recalibrate the aerosol-generating device <NUM> by repeating at least part of the calibration process described above. By recalibrating the aerosol-generating device <NUM>, the controller <NUM> re-measures at least one of the calibration values. The target power source parameter value for each temperature step will be determined using the last-measured at least one calibration value. The re-calibration may be performed periodically during the second heating phase <NUM>, for example at one or more of predetermined time intervals or after a predetermined number of puffs. The first target power source parameter value after re-calibration will therefore initially be determined based on the re-measured calibration values. The drift compensation described above will be applied in response to detection of a temperature change of the power supply electronics <NUM> following the re-calibration. Accordingly, adjusting the target power source parameter values based on the temperature change of the power supply electronics provides the advantage of reducing the frequency of recalibrations needed during the second heating phase <NUM>.

<FIG> is a flow diagram of a method <NUM> for controlling aerosol-production in an aerosol-generating device <NUM>. As described above, the controller <NUM> may be programmed to perform the method <NUM>.

The method begins at step <NUM>, where the controller <NUM> detects user operation of the aerosol-generating device <NUM> for producing an aerosol. Detecting user operation of the aerosol-generating device <NUM> may comprise detecting a user input, for example user activation of the aerosol-generating device <NUM>. Additionally or alternatively, detecting user operation of the aerosol-generating device <NUM> may comprise detecting that an aerosol-generating article <NUM> has been inserted into the aerosol-generating device <NUM>.

In response to detecting the user operation at step <NUM>, the controller <NUM> enters a calibration mode. During the calibration mode, the controller <NUM> may be configured to perform the optional pre-heating process described above (step <NUM>). At the end of the predetermined duration of the pre-heating process, the controller <NUM> is configured to perform the calibration process (step <NUM>) as described above. Alternatively, during the calibration mode, the controller <NUM> may be configured to proceed to step <NUM> without performing the pre-heating process. Following completion of the calibration process, the controller <NUM> enters the heating mode of the second heating phase in which the aerosol is produced at step <NUM>.

Claim 1:
A method for controlling aerosol production in an aerosol-generating device (<NUM>), the aerosol-generating device comprising an inductive heating arrangement for heating a susceptor (<NUM>), the inductive heating arrangement comprising power supply electronics (<NUM>) and a power source (<NUM>) for providing power to the power supply electronics, the method comprising:
controlling the power provided to the power supply electronics to cause the susceptor to have a target temperature;
measuring, using at least one temperature sensor, a temperature of the power supply electronics during operation of the aerosol-generating device for generating an aerosol; and
adjusting the power provided to the power supply electronics based on a change of the measured temperature of the power supply electronics.