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 combustion of the aerosol-forming substrate that would result in the generation of undesirable compounds as well as an unpleasant taste and aro
user.

It would be desirable to provide an efficient and fast method for calibrating an inductive heating device that provides for reliable temperature regulation in order to reduce the risk of overheating and ensures continued normal operation of the aerosol-generating device. <CIT> discloses a system for determining a characteristic of a susceptor element that may be associated with a vaporizer device. The system includes an inductor element and a control device. The control device is configured to detect a magnetic field associated with the inductor element and determine a characteristic of a susceptor element based on the magnetic field. It is the object of the present invention to provide an improved method and system that for calibrating an inductive heating device that provides for reliable temperature regulation in order to reduce the risk of overheating and ensures continued normal operation of the aerosol-generating device.

According to an embodiment of the present invention, there is provided a method for calibrating an inductive heating device for an aerosol-generating system. The method comprises obtaining a sequence of calibration values of an inductive heating device for an aerosol-generating system. The sequence of calibration values is associated with a calibration curve. The method further comprises smoothing the sequence of calibration values to obtain a sequence of smoothed calibration values, determining a first derivative of the sequence of smoothed calibration values, estimating a maximum value of the first derivative of the sequence of smoothed calibration values, determining at least one of a plateauing characteristic and a hill point value based on a first threshold associated with the estimated maximum value of the first derivative, and operating the inductive heating device in accordance with the determined at least one of a plateauing characteristic and a hill point value. The inductive heating device may be a handheld inductive heating device. Alternatively, the inductive heating device may be part of a handheld device.

By estimating a maximum value of a first derivative of a sequence of smoothed calibration values and determining at least one of a plateauing characteristic and a hill point value based on a first threshold associated with the estimated maximum value of the first derivativeand operating the inductive heating device in accordance with the determined at least one of a plateauing characteristic and a hill point value, a characteristic point or characteristic property of a calibration curve can be determined in a fast, robust and reliable manner. By operating the inductive heating device in accordance with the determined at least one of a plateauing characteristic and a hill point value, the characteristic point or characteristic property of a calibration curve may be used to prevent overheating of the device for an improved safety of the user. For example, it can be avoided that undesired components are formed when an aerosol-forming substrate is heated above a critical temperature.

The step of smoothing the sequence of calibration values and the step of determining the first derivative of the sequence of smoothed calibration values may be performed in one computation, which may save computing resources.

The step of estimating the maximum value of the first derivative of the sequence of smoothed calibration values may comprise comparing a value of the first derivative of the sequence of smoothed calibration values with a predetermined number of values of the first derivative of the sequence of smoothed calibration values. Alternatively, a value of the first derivative of the sequence of smoothed calibration values may be estimated as the maximum value, when an average of a predetermined number of previous values of the first derivative of the sequence of smoothed calibration values is higher than the value of the first derivative of the sequence of smoothed calibration values.

The first threshold may be a fraction of the maximum value of the first derivative of the sequence of smoothed calibration values. By determining the at least one of a plateauing characteristic and a hill point value based on a first threshold being a fraction of the maximum value of the first derivative of the sequence of smoothed calibration values, a robust method is provided for determining said at least one of a plateauing characteristic and a hill point value, since at least the plateauing characteristic may be determined although the calibration curve does not comprise a maximum.

The method may comprise the step of stopping heating of the inductive heating device, when the first derivative of the sequence of smoothed calibration values re-increases after the maximum value of the first derivative of the sequence of smoothed calibration values. The re-increase of the first derivative of the sequence of smoothed calibration values after the maximum value of the first derivative of the sequence of smoothed calibration values may be determined when an average of a specific number of consecutive values of the first derivative increase over time.

By stopping heating of the inductive heating device when the first derivative of the sequence of smoothed calibration values re-increases, a save method is provided that protects from overheating.

The method may further comprise determining a type of the calibration curve by classifying the calibration curve, and selecting, based on the determined type of the calibration curve, at least one parameter for the calibration. The calibration curve may be classified based on a slope of the calibration curve. Alternatively or additionally, the calibration curve may be classified based on a time corresponding to the maximum of the first derivative.

This further improves the accuracy and reliability of the calibration process because the parameters may be selected for a specific calibration curve.

The method may further comprise stopping heating of the inductive heating device in response to determining the at least one of a plateauing characteristic and a hill point value. This provides for an efficient and safe calibration method, since the method does not need to heat beyond the hill point to determine the hill point.

According to an embodiment of the present invention, an inductive heating device for an aerosol-generating system is provided, the inductive heating device comprising a controller configured to obtain a sequence of calibration values of the inductive heating device, wherein the sequence of calibration values is associated with a calibration curve; smooth the sequence of calibration values to obtain a sequence of smoothed calibration values; determine a first derivative of the sequence of smoothed calibration values; estimate a maximum value of the first derivative of the sequence of smoothed calibration values; and determine at least one of a plateauing characteristic and a hill point value based on a first threshold associated with the estimated maximum value of the first derivative. The inductive heating device is operated in accordance with the determined at least one of a plateauing characteristic and a hill point value. The inductive heating device may comprise an inductor being inductively couplable to a susceptor for heating an aerosol-forming substrate, and wherein the calibration values are associated with the susceptor. The inductive heating device may comprise a power source for providing a DC supply voltage and a DC current, and power supply electronics connected to the power source. The power supply electronics may comprise: a DC/AC converter; the inductor, wherein the inductor is connected to the DC/AC converter for the generation of an alternating magnetic field, when energized by an alternating current from the DC/AC converter; and the controller, wherein the controller is configured to control the power provided to the power supply electronics to cause an increase of a temperature of the susceptor. The obtaining the sequence of calibration values may comprise measuring a current associated with the supply electronics of the inductive heating device.

This provides an improved inductive heating device that can determine at least one of a plateauing characteristic and a hill point value in a reliable and robust manner for calibrating the inductive heating device with respect to the temperature, without continuously measuring the temperature.

According to an embodiment of the present invention, there is provided an aerosol-generating system, comprising the inductive heating device described above and an aerosol-generating article, wherein the aerosol-generating article comprises the aerosol-forming substrate and the susceptor.

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, 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.

<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 aerosol-generating device <NUM> may optionally further comprise a puff detector located within or near the cavity <NUM> (not shown) for detecting puffs. The puff detector is located within or near the cavity <NUM> such that the puff detector is placed along the path of the airflow when a user takes a puff. The puff detector may comprise one or more temperature detectors to detect a temperature change of air flow in the cavity <NUM> indicative of the user taking a puff. Additionally, or alternatively, the puff detector may comprise a pressure sensor to detect a decrease in pressure of the air flow in the cavity <NUM> indicative of a user taking a puff.

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>.

<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.

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) 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) 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 and the hill 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 (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 (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 and the hill. 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 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.

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. 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.

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 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 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>.

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 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 is reached and the current begins to increase. This first turning point 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 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 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 turning point 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 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 maximum 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, 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>.

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 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 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 beforehand.

Further, the aerosol-generating article <NUM> may be configured such that the current minimum is always reached within the predetermined duration of the pre-heating process. If the current minimum 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>.

During user operation of the aerosol-generating device <NUM> for generating an aerosol (referred to as the second heating phase), the apparent conductance (apparent resistance) values at the hill and valley shown in <FIG> drift over time. This is because, as shown in <FIG>, the apparent resistance of the susceptor is the sum of the ohmic resistance Rcoil of the inductor L2 and the ohmic resistance Rload of the susceptor <NUM>. Therefore any change to the temperature of the inductor L2 during operation of the device <NUM> will affect the apparent resistance. Therefore, the calibration values measured during the calibration process in the first heating phase will drift during operation of the aerosol-generating device <NUM>.

During normal operation when the aerosol-generating device <NUM> is generating an aerosol, the controller <NUM> will be operating in a heating mode for heating the aerosol-forming substrate. The controller <NUM> may be programmed to enter, from the heating mode, a recalibration mode for performing further iterations of at least part of the calibration process at predefined intervals during user operation of the aerosol-generating device <NUM> for generating an aerosol. The predefined intervals may be predefined time intervals or a predetermined number of puffs. Additionally, or alternatively, the controller <NUM> may be programmed to enter the recalibration mode for repeating at least part of the calibration process in response to detection of the completion of a puff. The calibration process may take between <NUM> milliseconds and <NUM> seconds to perform.

Performing the further iterations of at least part of the calibration process may comprise re-measuring both the calibration value at both turning points (illustrated as the hill and the valley in <FIG>) or re-measuring only the calibration value at one of the turning points, for example at the local maximum of current or conductance (the local minimum of resistance).

To perform a further iteration of the calibration process (in other words, to perform a recalibration), the controller <NUM> monitors the power source parameter associated with the susceptor <NUM> by measuring the current IDC drawn by the power supply and, optionally the power supply voltage VDC. Because the minimum operating temperature of the aerosol-generating device is greater than the first calibration temperature, as the susceptor <NUM> is heated during the further iterations of the calibration process, the measured current IDC increases until a turning point is reached and the current IDC begins to decrease. This turning point corresponds to the end point of the reversible phase transition of the susceptor <NUM>, observed as a local maximum conductance or current value (a local minimum resistance value). The controller <NUM> records the power source parameter value at the turning point as the re-measured second calibration value.

Once the first turning point has been reached, the controller <NUM> controls the DC/AC converter <NUM> to reduce the power provided to the inductor <NUM> in order to enable the susceptor <NUM> to cool. For example, the controller <NUM> may reduce the duty cycle of the DC/AC converter <NUM> to <NUM>%. The controller <NUM> may reduce the power provided to the inductor <NUM> until the susceptor <NUM> reaches the respective target operating temperature, at which point the controller <NUM> resumes normal operation in the heating mode.

Alternatively, the controller <NUM> may continue to reduce the power provided to the inductor <NUM> until another turning point is observed. This another turning point corresponds to the end point of the reversible phase transition of the susceptor, observed as a local minimum conductance or current value (a local maximum resistance value). The controller <NUM> records the power source parameter value at the another turning point as the re-measured first calibration value. As described above with respect to the calibration process, the process of measuring the first calibration value and the second calibration value may be repeated at least once during each further iteration of the calibration process.

<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. As described above, during the first heating phase <NUM>, the controller <NUM> operates in a calibration mode. Once calibration is complete, the controller enters a heating mode and may periodically switch to a re-calibration mode during the second heating phase <NUM>. 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 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 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 GTarget = GLower + (<NUM> × ΔG), a second temperature step 720b having a duration of <NUM> seconds and a target conductance value of GTarget = GLower + (<NUM> × ΔG), a third temperature step 720c having a duration of <NUM> seconds and a target conductance value of GTarget = GLower + (<NUM> × ΔG), a fourth temperature step 720d having a duration of <NUM> seconds and a target conductance value of GTarget = GLower + (<NUM> × ΔG) and a fifth temperature step 720e having a duration of <NUM> seconds and a target conductance value of GTarget = 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.

Thus, control of the operating temperature of the susceptor <NUM> for generating an aerosol depends on the first calibration value (corresponding to the first calibration temperature) and the second calibration value (corresponding to the second calibration temperature) measured during the calibration process. However, thedrift in the apparent conductance of the susceptor over the duration of the second heating phase <NUM>, means that, for the same susceptor temperature, the value of the apparent conductance decreases over the duration of the second heating phase <NUM>. Therefore, in order to be able to accurately control the susceptor temperature as well as to prevent overheating of the aerosol-forming substrate <NUM>, the controller <NUM> is programmed to periodically enter the re-calibration mode for repeating the at least part of the calibration process during the second heating phase <NUM>. For example, at least part of the calibration process is repeated every <NUM> seconds to <NUM> minutes. Preferably, at least part of the calibration process is repeated every <NUM> seconds. This is illustrated in <FIG>, which shows the second heating phase <NUM> in more detail, including recalibration during each of the temperature steps. Again, <FIG> is for illustration purposes and is not drawn to scale.

As described above, at least the second calibration value is re-measured during the further iterations of the calibration process, as shown in <FIG> Optionally, the first calibration value is re-measured during the further iterations of the calibration process. Target power source parameter values corresponding to each temperature step may be stored in the memory of the controller <NUM> and updated after each iteration of the calibration process. The controller <NUM> may adjust the target power source parameter value for each respective temperature step based on at least one of the re-measured calibration values, in other words based on at least the re-measured second calibration value. Additionally, or alternatively, the controller <NUM> may adjust the target power source parameter values for each respective temperature step based on the re-measured first calibration value. Additionally, or alternatively, the controller <NUM> may adjust the target power source parameter value for each respective temperature step based on a combination of the one or more calibration values measured during the first heating phase <NUM> and one or more calibration values measured during at least one further iteration of the calibration process during the second heating phase <NUM>.

Therefore, in the example above, for the first temperature step 720a, the target conductance will be based, at least initially at the start of the heating mode, on the calibration values GLower and ΔG obtained during the calibration process 710B of the first heating phase <NUM>. Assuming that the controller <NUM> is programmed to repeat the calibration process every <NUM> seconds, the calibration process will be repeated five times during the first temperature step, after <NUM> seconds, <NUM> seconds, <NUM> seconds, <NUM> seconds and <NUM> seconds. The calibration process will be repeated once during the second temperature step 720b after <NUM> seconds (<NUM> seconds after the beginning of the second temperature step). The calibration process will be repeated once during the third temperature step 720c after <NUM> seconds (<NUM> seconds after the beginning of the third temperature step) and at the end of the third temperature step 720c at <NUM> seconds. The calibration process will be repeated once during the fourth temperature step 720d after <NUM> seconds (<NUM> seconds after the beginning of the third temperature step). The calibration process will be repeated twice during the fifth temperature step 720e after <NUM> seconds (<NUM> seconds after the beginning of the fifth temperature step) and after <NUM> seconds (<NUM> seconds after the beginning of the fifth temperature step). After each further iteration of the calibration process, the controller <NUM> will adjust GTarget based at least in part on at least one of the calibration values resulting from the latest further iteration of the calibration process. For example, the target conductance after each recalibration is adjusted based at least in part on the re-measured calibration values GLower_i and ΔGi obtained during the respective recalibration process or based on the calibration value GLower and the re-measured value ΔGi obtained during the respective recalibration process, where i=start time of the second heating phase <NUM> + <NUM> seconds.

During the second heating phase <NUM>, the user will be drawing the aerosol generated by the aerosol-generating device into their body. In other words, the user will be puffing on the mouthpiece <NUM> of the aerosol-generating article that is partly received in the aerosol-generating device <NUM>. When the user puffs, cold air is drawn into the aerosol-generating device <NUM> and through the aerosol-generating article <NUM>, thereby cooling the susceptor <NUM>. Therefore, if recalibration is performed during a puff, the temporary cooling of the susceptor <NUM> has the effect of temporarily decreasing the difference between the calibration values (for example, decreasing the value of ΔG). In other words, referring again to <FIG>, there is a temporary decrease in the value of current at the hill and a temporary increase in the value of current at the valley for the duration of the puff. Thus, calibration values measured during a user puff will not be accurate. In particular, if the calibration values obtained during a puff were used to control the temperature of the susceptor <NUM>, there would be a risk of overheating the susceptor <NUM> with the consequent release of unwanted aerosol constituents. The controller <NUM> is therefore programmed such that recalibration does not overlap with a puff.

An inductive heating device may be a handheld inductive heating device. An inductive heating device may be restricted with respect to the number or size of electronic components comprised in the inductive heating device. Additionally or alternatively, the components comprised in the inductive heating device may be restricted with respect to processing power or memory.

An inductive heating device or a controller associated with the inductive heating device may be configured to obtain conductance values and process these conductance values to calibrate the inductive heating device. The relationship between the conductance values and a temperature of the susceptor may be known for at least one point. For example, the temperature for at least one of a plateauing characteristic and a hill point value of a calibration curve of the conductance values may be known. Thus, the calibrated inductive heating device can avoid going beyond a Curie temperature of the susceptor. The Curie temperature may be the temperature above which the susceptor loses its permanent magnetic properties.

The calibration may comprise determining a valley point of the calibration curve, which may be referred to as an S-curve, and at least one of a hill point and a plateauing characteristic of the calibration curve. The calibration curve may comprise a minimum, an inflection point, and at least one of a maximum and a plateauing characteristic.

A first characteristic point of the calibration curve may be a minimum in the calibration curve, which is followed by the inflection point and at least one of a maximum and a plateauing characteristic of the calibration curve. The maximum may correspond to a hill point, at which power that is supplied to the inductive heating device is gradually reduced to avoid over-heating. The detection of the minimum, which will also be referred to as a valley or valley point in this application, can be performed by different methods known in the art. However, it can be difficult to determine the maximum or plateauing characteristic of the calibration curve.

The first and second calibration values, as described above, may be determined based on the determined at least one of a plateauing characteristic and a hill point value.

Accordingly, it would be desirable to provide an improved method for reliably determining the maximum or plateauing characteristic of the calibration curve in a fast and efficient manner. In view of the small processing power of the inductive heating device, it would be desirable to keep the mathematical operations as simple as possible. Additionally, it would be desirable to detect overheating or to determine the maximum or plateauing characteristic of the calibration curve in less than <NUM>.

A calibration may refer to one of two type of curves comprising a calibration curve and a recalibration curve. A calibration curve may start with a well-defined valley and then gradually increase for a duration of one to several seconds before a plateauing characteristic of the calibration curve is reached. A recalibration curve may not comprise a valley before increasing, are shorter and their duration can be as short as <NUM> before a plateauing characteristic of the calibration curve is reached.

<FIG> is a flow diagram showing a method <NUM> of determining a plateauing characteristic or a hill point for calibrating an inductive heating device for an aerosol-generating system. The method comprises, in step <NUM>, obtaining a sequence of calibration values of an inductive heating device for an aerosol-generating system. The sequence of calibration values is associated with a calibration curve. The controller <NUM>, as described above, may be programmed to perform the method <NUM>.

In step <NUM>, the sequence of calibration values is smoothed to obtain a sequence of smoothed calibration values. The calibration curve C may be smoothed by convoluting it with a (half) Gaussian kernel G of standard deviation σ to denoise it.

At step <NUM>, a first derivative of the sequence of smoothed calibration values is determined. The first derivative may be determined using the property of the convolution that: <MAT> where g is a (symmetric) Gaussian kernel with a standard deviation of <NUM> · σ. σ may denote the standard deviation of a centered Gaussian density. * may denote the convolution product of two function or discrete series. The resulting first derivative is denoted Z(t). Using the associativity of the convolution, the first derivative may be determined by: <MAT> The first derivative of the sequence of smoothed calibration may be determined every millisecond. The first derivative of the sequence of smoothed calibration values may be determined as follows:.

The steps <NUM> to <NUM> may be repeated periodically. For example, the steps <NUM> to <NUM> may be repeated every <NUM>.

At step <NUM>, the inductive heating device is operated in accordance with the determined at least one of a plateauing characteristic and a hill point value. The operating the inductive heating device in accordance with the determined at least one of a plateauing characteristic and a hill point value may comprise maintaining a temperature associated with the aerosol-generating system below a specific temperature based on the determined at least one of a plateauing characteristic and a hill point value.

Calibration curves can have different shapes and/ or slopes. For example, the slope of a calibration curve may depend on the humidity of the substrate to be heated. A calibration for different calibration curves can thus last for different times, which may impact the level of smoothing, which is necessary for detection characteristic points of the calibration curve. Recalibration may take shorter than calibration. A recalibration curve may have a steeper slope compared to a calibration curve. For example, a recalibration may be performed in <NUM> until the at least one of a plateauing characteristic and a hill point value are determined.

Predicting of the duration of such curves could be done by machine learning using the time derivatives as inputs.

The derivatives (or approximate thereof), may serve as learning features and may be computed directly from the conductance for the first <NUM> using MA<NUM> to smooth the curve. The features may be computed as: <MAT> for <MAT>.

The method <NUM> may autoscale at least one parameter used for determining the at least one of a plateauing characteristic and a hill point value. For example, at step <NUM>, a type of the calibration curve may be determined by classifying the calibration curve. The calibration curve may be classified based on a slope of the calibration curve. The type may be associated with one of a recalibration and a calibration curve and a slope of the one of a recalibration and a calibration curve. The type of the calibration curve may be determined based on a second threshold associated with the estimated maximum value of the first derivative of the sequence of smoothed calibration values.

At step <NUM>, at least one parameter for the calibration may be selected, based on the determined type of the calibration curve. The at least one parameter may comprise at least one of a first parameter, which specifies the first threshold for the step of determining the at least one of a plateauing characteristic and a hill point value, a second parameter, which specifies the smoothing of the sequence of calibration values, a third parameter, which specifies a number of consecutive values of the first derivative of the sequence of smoothed calibration values used for the step of estimating the maximum value of the first derivative, a fourth parameter, which specifies a number of consecutive values of the first derivative of the sequence of smoothed calibration values used for the step of determining the at least one of a plateauing characteristic and a hill point value, and a fifth parameter, which specifies a number of values of the first derivative of the sequence of smoothed calibration values used for the step of determining of a re-increase of the first derivative of the sequence of smoothed calibration values after the maximum value of the first derivative of the sequence of smoothed calibration values. The second threshold may define a fraction of the maximum value of the first derivative of the sequence of smoothed calibration values.

Recalibration curves may be classified as 'short-R' or 'long-R' based on a threshold associated with an approximate time t = tmax of the maximum of the first derivative. <FIG> shows a classification tree that may be trained on test recalibration curves. The classification tree is depth <NUM> tree.

Calibration curves may be classified as 'short-C', 'medium-C' or 'long-C' based on a threshold associated with an approximate time t = tmax of the maximum of the first derivative. <FIG> shows a classification tree that may be trained on test calibration curves. The classification tree may be a depth <NUM> tree.

Alternatively, the recalibration and calibration curves may be classified based on a threshold value indicating a slope of the calibration curve. For example, an average value of the calibration curve at a specified time after the valley point may be compared to a threshold.

Based on the prediction category, knowing if the curve is a calibration one or not, the predicted category may be used to choose a pre-defined set of parameters. To reduce computation time, Z(tm) may be computed every <NUM> (m = <NUM>, <NUM>, <NUM>, <NUM>,. , <NUM>n - <NUM>,. ) and Z(t<NUM>n) ≐ Z(t<NUM>n-<NUM>) may be determined for even steps. The parameters for the calibration may be determined as follows:.

In experiments, the relative and absolute difference between the conductance at stopping time and the expected maximum conductance was computed. The relative difference of the detected maximum to the maximum of the conductance and the δS(loss) were computed as <NUM> · <MAT> for the calibration curve where a c=valley is present, or <MAT> where no valley is present.

An example of a recalibration curve <NUM> is shown in <FIG> also shows the determined hill point value <NUM> and a point <NUM>, which fulfills the second stopping criteria.

Claim 1:
A method (<NUM>) of calibrating an inductive heating device (<NUM>) for an aerosol-generating system, the method, performed by a controller (<NUM>), comprising:
obtaining (<NUM>) a sequence of calibration values of an inductive heating device (<NUM>) for an aerosol-generating system, wherein the sequence of calibration values is associated with a calibration curve (<NUM>), wherein the inductive heating device (<NUM>) comprises an inductor which is configured for being coupled to a susceptor (<NUM>) for heating an aerosol-forming substrate (<NUM>), wherein the sequence of calibration values comprises one of a sequence of conductance values and a sequence of resistance values, and wherein a value in the sequence of calibration values is associated with a calibration temperature of the susceptor (<NUM>),
characterized in that the method comprises:
smoothing (<NUM>) the sequence of calibration values to obtain a sequence of smoothed calibration values;
determining (<NUM>) a first derivative of the sequence of smoothed calibration values;
estimating (<NUM>) a maximum value of the first derivative of the sequence of smoothed calibration values;
determining (<NUM>) at least one of a plateauing characteristic and a hill point value based on a first threshold associated with the estimated maximum value of the first derivative; and
operating (<NUM>) the inductive heating device (<NUM>) in accordance with the determined at least one of a plateauing characteristic and a hill point value.