An aerosol generating device and a method of controlling heating of aerosol generating material

Abstract

A method of controlling heating of aerosol generating material is described. The heating is controlled during a pre-heating phase of an aerosol generating device. The method includes controlling heating of the aerosol generating material using a closed loop controller (42) based on the error between a power setpoint (PSET) and an estimated or determined power applied to the aerosol generating material (PAGM).

Description

[0001]AN AEROSOL GENERATING DEVICE AND A METHOD OF CONTROLLING HEATING OF AEROSOL GENERATING MATERIAL Technical Field The present disclosure relates generally to an aerosol generating device, and in particular to a device that is configured to heat aerosol generating material to generate an aerosol for inhalation by a user. The present disclosure is particularly applicable to a portable (hand-held) aerosol generating device. The aerosol generating material may be part of an aerosol generating article that may be received in the device in use. The present disclosure also relates generally to a method of controlling heating of aerosol generating material, and in particular during a pre-heating phase of an aerosol generating device. Technical Background Devices which heat, rather than burn, an aerosol generating material to produce an aerosol for inhalation have become popular with consumers in recent years. A commonly available reduced-risk or modified-risk device is the heated material aerosol generating device, or so-called heat-not-burn device. Devices of this type generate an aerosol or vapour by heating an aerosol generating material to a temperature typically in the range 150°C to 300°C. This temperature range is quite low compared to an ordinary cigarette. Heating the aerosol generating material to a temperature within this range, without burning or combusting the aerosol generating material, generates a vapour which typically cools and condenses to form an aerosol for inhalation by a user of the device. An aerosol may also be produced without heating (e.g., by using ultrasonic or chemical reaction), particularly if the device uses a liquid aerosol generating material or substrate. It is known for such aerosol generating devices to be controlled to heat the aerosol generating material using a temperature setpoint or profile. For example, a closed loop controller may control a heater of the aerosol generating device based on the error between a temperature setpoint and the temperature of the heater. The temperature P51622EP-6348 setpoint may be varied during a pre-heating or heating phase of the aerosol generating device to control the heating of the aerosol generating material. The control method of the present disclosure aims to control heating of the aerosol generating material in a different way, and in particular one that may be more effective at evaporating water from the aerosol generating material when it is first heated at the start of a pre-heating phase. A pre-heating phase may generally be intended to pre-heat the aerosol generating material to a target temperature, and a subsequent heating phase (or vaping phase) may be generally intended to heat the aerosol generating material for a longer period during which an aerosol is generated. Aerosol generating material will typically contain water, for example because common aerosol formers such as glycerine are hygroscopic and tend to absorb moisture from the air. The water in the aerosol generating material should preferably be removed (i.e., evaporated by heating) as soon as possible during the pre- heating phase because it may compromise the quality and quantity of the aerosol generated during the heating phase. More particularly, if the generated aerosol contains too much evaporated water, it may have a negative impact on the taste of the aerosol and the user may experience an unpleasant hotness or heat when vaping. In some cases, the user is prompted to take a “dummy puff” to remove the water. The control method of the present disclosure aims to achieve the same objective without the need for the user to take a “dummy puff”, for example. Summary of the Disclosure According to a first aspect of the present disclosure, there is provided a method of controlling heating of aerosol generating material during a pre-heating phase of an aerosol generating device, the method comprising controlling heating of the aerosol generating material using a closed loop controller based on the error between a power setpoint and an estimated or determined power applied to the aerosol generating material. The closed loop controller controls the heating of the aerosol generating material using a power setpoint and the estimated or determined power applied to the aerosol generating material. This is instead of using a temperature setpoint and the temperature P51622EP-6348 of the heater, for example. Such a method using a power setpoint may provide improved heating control. The aerosol generating material may form part of an aerosol generating article (or “consumable”) and may be surrounded by a paper wrapper. The aerosol generating article may be adapted to be received in a heating chamber of the aerosol generating device. When received in the heating chamber, there may be an air gap between the side wall of the heating chamber and the outer surface of the aerosol generating article (e.g., the paper wrapper). The aerosol generating article may be formed substantially in the shape of a stick, and may broadly resemble a cigarette, having a tubular region with an aerosol generating material or substrate arranged in a suitable manner. The aerosol generating article may include a filter segment, for example comprising cellulose acetate fibres, at a proximal end of the aerosol generating article. The filter segment may constitute a mouthpiece filter and may be in coaxial alignment with the aerosol generating material. One or more vapour collection regions, cooling regions, and other structures may also be included in some designs. For example, the aerosol generating article may include at least one tubular segment upstream of the filter segment. The tubular segment may act as a vapour cooling region. The vapour cooling region may advantageously allow the heated vapour generated by heating the aerosol generating material to cool and condense to form an aerosol with suitable characteristics for inhalation by a user, for example through the filter segment. The aerosol generating material may comprise any type of solid or semi-solid material. Example types of aerosol generating solids include powder, granules, pellets, shreds, strands, particles, gel, strips, loose leaves, cut filler, porous material, foam material or sheets. The aerosol generating material may comprise plant derived material and in particular, may comprise tobacco. It may advantageously comprise reconstituted tobacco, for example including tobacco and any one or more of cellulose fibres, tobacco stalk fibres and inorganic fillers. P51622EP-6348 The aerosol generating material may comprise an aerosol-former. Examples of aerosol- formers include polyhydric alcohols and mixtures thereof such as glycerine or propylene glycol. Typically, the aerosol generating material may comprise an aerosol- former content of between approximately 5% and approximately 50% on a dry weight basis. In some embodiments, the aerosol generating material may comprise an aerosol- former content of between approximately 10% and approximately 20% on a dry weight basis, and possibly approximately 15% on a dry weight basis. The aerosol generating device may be configured to heat the aerosol generating material or substrate, without burning the aerosol generating material, to volatise at least one component of the aerosol generating material and thereby generate a heated vapour which cools and condenses to form an aerosol for inhalation by a user of the aerosol generating device. The volatile compounds released from the aerosol generating material may include nicotine or flavour compounds such as tobacco flavouring. In general terms, a vapour is a substance in the gas phase at a temperature lower than its critical temperature, which means that the vapour may be condensed to a liquid by increasing its pressure without reducing the temperature, whereas an aerosol is a suspension of fine solid particles or liquid droplets, in air or another gas. It should, however, be noted that the terms ‘aerosol’ and ‘vapour’ may be used interchangeably in this specification, particularly with regard to the form of the inhalable medium that is generated for inhalation by a user. The aerosol generating device is typically a hand-held, portable, device. The method may further comprise estimating or determining a temperature of the aerosol generating device, for example a heater temperature or a temperature of the heating chamber. The estimated or determined temperature may be used to estimate or determine the power currently being applied to the aerosol generating material. The temperature of the aerosol generating device may be measured using a temperature sensor, for example. The temperature sensor may be located to measure the heater temperature or the temperature of the heating chamber – e.g., the temperature sensor P51622EP-6348 may be located adjacent a heater of the aerosol generating device or on a wall of the heating chamber. If the aerosol generating device comprises a heater (e.g., a resistive heater) adapted to heat the aerosol generating material, the temperature of the aerosol generating device may be the heater temperature. The heater temperature may be estimated or determined based on one or more electrical parameters of the heater. For example, the heater temperature ^^^^may be estimated or determined as follows: ^^^^ = ^^^^^^^^^^where ^^^^is a known positive temperature coefficient (PTC) characteristic or function and ^^^^is the heater resistance. The heater resistance ^^^^may be estimated or determined as follows: ^^^ − ^^^ ^^^^^= ^^ ^^^^^^ where a measured input voltage ^^^is taken from the mid-point of a voltage divider circuit that comprises a heater resistance ^^^^and a reference resistance ^^^^and may be estimated or determined as follows: ^ ^^^=^^^^^^^^^or as follows: ^ ^^^^^^=^ ^^^^^^^^^ ^^^ depending on the connection order of the heater and reference resistances. And where ^^^^is the voltage applied to the heater. P51622EP-6348 The current temperature of the aerosol generating material may be estimated or determined using: ^ the current heater temperature, which may be measured using a temperature sensor or estimated or determined based on one or more electrical parameters of the heater as described above, for example, ^ a thermal resistance between the heater and the aerosol generating material, which may optionally be stored in a memory of the aerosol generating device in advance, and ^ a power previously applied to the aerosol generating material (i.e., a “previously applied power”). For example, the current temperature of the aerosol generating material ^^^^^^^^, i.e., for a current time ^^, may be estimated or determined as follows: ^^^^^^^^ = ^^^^^^^^^^^^^^ − ^^^^^^^^ where ^^^is the thermal resistance between the heater and the aerosol generating material, ^^^^^^^^^^ is the previously applied power, i.e., for a previous time ^^^^, and ^^^^^^^^is the current temperature of the heater. The previously applied power may be estimated or determined during a previous iteration of the control method and may be stored in a memory of the aerosol generating device. At the start of the control method, a preset value may be used. The thermal resistance ^^^between the heater and the aerosol generating material may be determined by one or more individual thermal resistances, for example the thermal resistance of one or more insulating layers, the side wall of the heating chamber, the air gap between the side wall of the heating chamber and the aerosol generating article, the paper wrapper that surrounds the aerosol generating material etc. The thermal resistance may be determined in advance for a particular aerosol generating system – e.g., for a particular combination of aerosol generating device and aerosol generating article that contains the aerosol generating material, and may be stored in a memory of the aerosol generating device. If the aerosol generating device is adapted to be used with different P51622EP-6348 aerosol generating articles, a plurality of different thermal resistance values may be stored where each value may be used when a particular aerosol generating article is received in the device.The current temperature of the aerosol generating material ^^^^^^^^ , may also bemeasured by a temperature sensor, for example. The temperature sensor may be located adjacent or inside the aerosol generating material and may be formed as part of an aerosol generating article, for example. This may simplify the control method but may also increase the complexity of the aerosol generating system and may require an electrical connection to be provided between the temperature sensor and the aerosol generating device. The power currently applied to the aerosol generating material may be estimated or determined using: ^ the current temperature of the aerosol generating material, ^ a previous temperature of the aerosol generating material, ^ the mass of the aerosol generating material, which may optionally be stored in the memory in advance, and ^ the specific heat capacity of the aerosol generating material, which may optionally be stored in the memory in advance. If the aerosol generating device is adapted to be used with different aerosol generating articles, a plurality of different mass and specific heat capacity values may be stored where each value may be used when a particular aerosol generating article is received in the device. Heating of the aerosol generating material may be controlled based on the error between the power setpoint and the power currently applied to the aerosol generating material. For example, the power currently applied to the aerosol generating material ^^^^^^^^, i.e., for a current time ^^, may be estimated or determined as follows: P51622EP-6348^^^^^^^^ = ^^^^^^^^^ − ^^^^^^^^^^^^^^^^^^^^^ where ^^^^and ^^^^are respectively the mass and specific heat capacity of the aerosol generating material. The previous temperature of the aerosol generating material ^^^^^^^^^^ may be estimated or determined during a previous iteration of the control method and may be stored in a memory of the aerosol generating device. At the start of the control method, a preset value may be used. The preset value may an ambient temperature, for example. As mentioned above, the heating of the aerosol generating material is controlled using a closed loop controller based on the error between a power setpoint and the estimated or determined power applied to the aerosol generating device (i.e., ^^^^^^^^). The closed loop controller may comprise a controller with one or more controller constants or gains. For example, the controller may be a proportional-integral (PI) or proportional-integral-derivative (PID) controller so that the closed loop controller may continuously calculate the error between the power setpoint and the power currently applied to the aerosol generating material by the heater and apply a correction based on proportional, integral and derivative terms. The output of the controller may be used to control the supply of power to the heater – e.g., by varying a duty cycle of a suitable control algorithm such as pulse width modulation (PWM). It will be understood that the term duty cycle describes the percentage of time when the heater is electrically connected to a power source over an interval or period of time. A low duty cycle corresponds to low power and a high duty cycle corresponds to high power. The power source may be an energy storage device such as a Lithium-ion secondary battery, for example. The method may further comprise: ^ setting the power setpoint to a first power at the start of the pre-heating phase; ^ subsequently setting the power setpoint to a second power that is higher than the first power; and ^ subsequently setting the power setpoint to a third power that is lower than the second power. P51622EP-6348 The power setpoint may be set to the second power when the temperature of the aerosol generating material exceeds a threshold temperature (e.g., a temperature of between about 95^C and about 110^C) or when a rate of change of the temperature of the aerosol generating material falls below a threshold rate, for example. This may help to remove water from the aerosol generating material during the pre-heating phase because the power supplied to the heater may be increased at the appropriate time. More particularly, Figure 1 shows how at the start of a pre-heating phase that is intended to pre-heat the aerosol generating material to a target temperature, the temperature of the aerosol generating material will typically increase rapidly to a temperature of about 100^C. The temperature may then plateau (or “stall”) as the water in the aerosol generating material is evaporated. In other words, the power supplied to the aerosol generating material by the heater is used to evaporate the water rather than raising the temperature of the aerosol generating material. After the majority of the water has evaporated, the temperature of the aerosol generating material will start to increase again towards the target temperature. Figure 1 assumes that the power setpoint (or temperature setpoint) remains constant during the pre-heating phase. But setting the power setpoint to a second power that is higher than the first power when the temperature of the aerosol generating material exceeds a threshold temperature between about 95^C and about 110^C, or when a rate of change of the temperature of the aerosol generating material falls below a threshold rate indicating that the temperature may be starting to plateau or stall, for example, may avoid any significant plateauing so that the temperature of the aerosol generating material continues to increase towards the target temperature. This may be seen in Figure 5. Setting the power setpoint to a second power that is higher than the first power at the appropriate time may allow the water in the aerosol generating material to be rapidly removed. This is because the heating of the aerosol generating material is deliberately increased at the point when the majority of the water in the aerosol generating material is being evaporated and converted into water vapour. The user of the aerosol generating device may be notified that a first puff may be taken a preset time after the power setpoint is set to the second power. After a preset time P51622EP-6348 with the power setpoint at the second power, it may be assumed that substantially all of the water in the aerosol generating material has evaporated. This means that the quality of the aerosol in the first puff is not compromised if it is taken after the user has been notified. The setpoint power may remain at the second power after the user has been notified that a first puff may be taken. Eventually the power setpoint is set to the third power and the amount of power supplied to the heater is reduced. The third power is lower than the second power and it may be higher, lower or the same as the first power. The power setpoint may be set to the third power in response to a detected first puff by the user (e.g., by a puff detector of any suitable type). The first puff will normally remove any remaining water from the aerosol generating material so there is no need for the power setpoint to remain at the second power. This may provide a power saving if the power setpoint would otherwise have remained at the second power for a longer period of time – e.g., a preset time that is intended to substantially remove all of the water from the aerosol generating material. The power setpoint may be set to the third power a preset time after the power setpoint is set to the second power (or after the start of the pre-heating phase). After a preset time with the power setpoint at the second power, it may be assumed that substantially all of the water in the aerosol generating material has evaporated so there is no need for the power setpoint to remain at the second power. If not already notified, the user may be notified that a first puff may be taken when the power setpoint is set to the third power. The power setpoint may be set to the third power based on an estimated or determined amount of water in the aerosol generating material, e.g., when the estimated or determined amount of water falls below a threshold amount. If not already notified, the user may be notified that a first puff may be taken when the power setpoint is set to the third power. The amount of water may be estimated or determined by a suitable sensor or by measuring one or more electrical parameters of the aerosol generating material such as resistance, inductance or capacitance, for example. Once substantially all of the water in the aerosol generating material has evaporated, there is no need for the power P51622EP-6348 setpoint to remain at the second power. The aerosol generating device may comprises a sensor or sensing circuit such as a capacitive sensor, for example, which may output a signal indicative of the amount of water in the aerosol generating material. The output signal may be compared against a threshold to determine when substantially all of the water in the aerosol generating material has been evaporated so that the power setpoint may be set to the third power. Using a sensor or sensing circuit to estimate the amount of water in the aerosol generating material means that the power setpoint can be maintained at the second power for the minimum time necessary to substantially evaporate the water. It also means that the power setpoint may be maintained at the second power for a longer period of time if the aerosol generating material contains a higher than normal amount of water. But it also increases the complexity of the aerosol generating device. The second power may be set based on the amount of water in the aerosol generating material that is estimated or determined by the sensor or sensing circuit – e.g., when the aerosol generating article is first inserted into the heating chamber of the aerosol generating device. For example, if there is a higher amount of water than normal in the aerosol generating material, a higher second power may be used and vice versa. The heating of the aerosol generating material during the period when the power setpoint is set to the second power may therefore be controlled or adjusted based on the amount of water in the aerosol generating material. According to a second aspect of the present disclosure, there is provided an aerosol generating device comprising a heater adapted to heat aerosol generating material, and a closed loop controller adapted to control the heater during a pre-heating phase of the aerosol generating device, the closed loop controller being further adapted to control the heater based on the error between a power setpoint and an estimated or determined power applied to the aerosol generating material. The closed loop controller may be further adapted to control the heater as described above. P51622EP-6348 Brief Description of the Drawings Figure 1 is a graphical view showing how the temperature of aerosol generating material varies during a conventional pre-heating phase; Figure 2 is a diagrammatic cross-sectional view of an aerosol generating system comprising an aerosol generating device and an aerosol generating article ready to be positioned in a heating chamber of the aerosol generating device; Figure 3 is a schematic representation of an example of an electrical circuit of the aerosol generating device; Figure 4 is a diagrammatic view of an example of a closed loop controller; and Figure 5 is a graphical view showing how the temperature of aerosol generating material varies during a pre-heating phase when the heating is controlled using the closed loop controller of Figure 4. Detailed Description of Embodiments Embodiments of the present disclosure will now be described by way of example only and with reference to the accompanying drawings. Referring initially to Figure 2, there is shown diagrammatically an example of an aerosol generating system 1. The aerosol generating system 1 comprises an aerosol generating device 10 and an aerosol generating article 100 for use with the device 10. The aerosol generating device 10 comprises a main body 12 housing various components of the aerosol generating device 10. The main body 12 may have any shape that is sized to fit the components described in the various embodiments set out herein and to be comfortably held by a user unaided, in a single hand. A first end 14 of the aerosol generating device 10, shown towards the bottom of Figure 2, is described for convenience as a distal, bottom, base or lower end of the aerosol generating device 10. A second end 16 of the aerosol generating device 10, shown towards the top of Figure 2, is described as a proximal, top or upper end of the aerosol generating device 10. During use, the user typically orients the aerosol generating device 10 with the first end 14 downward and / or in a distal position with respect to the P51622EP-6348 user’s mouth and the second end 16 upward and / or in a proximate position with respect to the user’s mouth. The aerosol generating device 10 comprises a heating chamber 18 positioned in the main body 12. The heating chamber 18 defines an interior volume in the form of a cavity 20 having a substantially cylindrical cross-section for receiving an aerosol generating article 100. The heating chamber 18 has a longitudinal axis defining a longitudinal direction and is formed of a heat-resistant plastics material, such as polyether ether ketone (PEEK). The aerosol generating device 10 further comprises a power source 22, for example one or more batteries which may be rechargeable, and an electrical circuit 24. The electrical circuit 24 may comprise one or more integrated circuits and other electrical components. For example, an integrated circuit may comprise at least one of a microcontroller unit (MCU) and microprocessor unit (MPU). The heating chamber 18 is open towards the second end 16 of the aerosol generating device 10. In other words, the heating chamber 18 has an open first end 26 towards the second end 16 of the aerosol generating device 10. The heating chamber 18 is typically held spaced apart from the inner surface of the main body 12 to minimise heat transfer to the main body 12. The aerosol generating device 10 may optionally include a sliding cover 28 movable transversely between a closed position (shown in Figure 2) in which it covers the open first end 26 of the heating chamber 18 to prevent access to the heating chamber 18 and an open position (not shown) in which it exposes the open first end 26 of the heating chamber 18 to provide access to the heating chamber 18. The sliding cover 28 may be biased to the closed position in some embodiments. The heating chamber 18, and specifically the cavity 20, is arranged to receive a correspondingly shaped generally cylindrical or rod-shaped aerosol generating article 100. Typically, the aerosol generating article 100 comprises a pre-packaged aerosol generating material or substrate 102. The aerosol generating article 100 is a disposable and replaceable article (also known as a “consumable”) which may, for example, P51622EP-6348 contain tobacco as the aerosol generating material 102. The aerosol generating article 100 has a proximal end 104 (or mouth end) and a distal end 106. The aerosol generating article 100 further comprises a mouthpiece segment 108 positioned downstream of the aerosol generating material 102. The aerosol generating material 102 and the mouthpiece segment 108 are arranged in coaxial alignment inside a wrapper 110 (e.g., a paper wrapper) to hold the components in position to form the rod-shaped aerosol generating article 100. The mouthpiece segment 108 may comprise one or more of the following components (not shown in detail) arranged sequentially and in co-axial alignment in a downstream direction, in other words from the distal end 106 towards the proximal (mouth) end 104 of the aerosol generating article 100: a cooling segment, a centre hole segment and a filter segment. The cooling segment typically comprises a hollow paper tube having a thickness which is greater than the thickness of the wrapper 110. The centre hole segment may comprise a cured mixture containing cellulose acetate fibres and a plasticizer, and functions to increase the strength of the mouthpiece segment 108. The filter segment typically comprises cellulose acetate fibres and acts as a mouthpiece filter. As heated vapour flows from the aerosol generating material 102 towards the proximal (mouth) end 104 of the aerosol generating article 100, the vapour cools and condenses as it passes through the cooling segment and the centre hole segment to form an aerosol with suitable characteristics for inhalation by a user through the filter segment. The heating chamber 18 has a side wall (or chamber wall) 30 extending between a base 32, located at a second end 34 of the heating chamber 18, and the open first end 26. The side wall 30 and the base 32 are connected to each other and may be integrally formed as a single piece. In the illustrated embodiment, the side wall 30 is tubular and, more specifically, cylindrical. The side wall 30 may be formed so that the cross-section of the heating chamber 18 is a perfect circle or an ellipse. In other embodiments, the side wall 30 may have other suitable shapes, such as a tube with an elliptical or polygonal cross section. In yet further embodiments, the side wall 30 may be tapered. P51622EP-6348 In the illustrated embodiment, the base 32 of the heating chamber 18 is closed, e.g., sealed or air-tight. That is, the heating chamber 18 is cup-shaped. This may ensure that air drawn from the open first end 26 is prevented by the base 32 from flowing out of the second end 34 and is instead guided through the aerosol generating material 102. It may also ensure that a user inserts the aerosol generating article 100 into the heating chamber 18 an intended distance and no further. The device 10 includes a heater 36, which is configured to heat the aerosol generating material 102 when the aerosol generating article 100 is received in the heating chamber 18. An example of a simple electrical circuit 24 is shown in Figure 3. The electrical circuit 24 includes an MCU 38 and a semiconductor switch Q1 that is electrically connected between the positive terminal of the power source 22 and the heater 36. The electrical circuit 24 also includes a voltage sensing circuit 40. Although not shown, the electrical circuit 24 may optionally include other components such as a charging circuit, a DC / DC converter, a low-dropout (LDO) regulator etc. The MCU 38 includes a first input / output terminal (labelled “I / O”) electrically connected to the semiconductor switch Q1 for switching it on and off, and a second input / output terminal (labelled “I / O”) electrically connected to the voltage sensing circuit 40. For example, the semiconductor switch Q1 may be an n-channel MOSFET where its gate terminal is electrically connected to the first input / output terminal of the MCU 38. Although not shown, the MCU 38 may include other terminals such as a ground terminal electrically connected to ground, a voltage input terminals for receiving a regulated voltage supply, a serial data terminal and a serial clock terminal etc. When the semiconductor switch Q1 is switched on, the positive terminal of the power source 22 is electrically connected to the heater 36. When the semiconductor switch Q1 is switched off, the heater 36 is electrically isolated from the positive terminal of the power source 22. The MCU 38 may control the amount of power supplied to the heater P51622EP-6348 36 by controlling the switching of the semiconductor switch Q1 using any suitable control algorithm, e.g., pulse width modulation (PWM). The voltage sensing circuit 40 shown in Figure 3 comprises a voltage divider circuit that includes the heater 36 having a heater resistance ^^^^and a reference resistor having a reference resistance ^^^^. A mid-point of the voltage divider circuit is electrically connected to the second input / output terminal of the MCU 38. The connection order of the heater 36 and the reference resistor may be reversed. In other words, the reference resistor may instead be electrically connected between the heater 36 and the semiconductor switch Q1. Figure 4 shows a closed loop controller 42, which may be implemented using the MCU 38. The closed loop controller 42 includes a proportional-integral-derivative (PID) controller 44 and a thermal model 46. The output of the PID controller 44 is used to control a heating assembly 48 that includes the heater 36. The heating assembly 48 also includes the one or more components of the aerosol generating device 10 that control the amount of power that is supplied to the heater 36. For example, the heating assembly 48 may include the semiconductor switch Q1 that is switched on any off by the MCU 38. It may also include the voltage sensing circuit 40. The thermal model 46 receives an estimated or determined temperature of the heater 36 (i.e., ^^^^^^^^). The heater temperature may be measured by a temperature sensor (not shown) or it may be estimated or determined based on one or more electrical parameters of the heater 36 as described above. In the latter case, instead of receiving an estimated or determined heater temperature, the thermal model 46 may use the one or more electrical parameters of the heater 36 to estimate or determine a heater resistance ^^^^^^^^ and may use this to estimate or determine the current heater temperature ^^^^^^^^ as follows: ^^^^^^^^ = ^^^^^^^^^^^^^^ P51622EP-6348 where ^^^^is a known positive temperature coefficient (PTC) characteristic or function. As described above, the heater resistance ^^^^may be estimated or determined as follows: ^^^ − ^^^ ^^^^^= ^^ ^^^^^^where an input voltage ^^^is taken from the mid-point of the voltage divider circuit of the voltage sensing circuit 40 shown in Figure 3 and which includes the heater 36 having the heater resistance ^^^^and the reference resistor having the reference resistance ^^^^and may be estimated or determined as follows: ^ ^^^=^^^^ ^^^^^^ And where ^^^^is the voltage applied to the heater 36. If the connection order of the heater 36 and the reference resistor is reversed, the input voltage ^^^may be estimated or determined as follows: ^ ^^^^^^= ^^^^^^^ The thermal model 46 estimates or determines the power currently applied to the aerosol generating material 102 (i.e., ^^^^^^^^) as follows: ^^^^^^^^ = ^^^^^^^^^ − ^^^^^^^^^^^^^^^^^^^^^ where ^^^^and ^^^^are respectively the mass and specific heat capacity of the aerosol generating material 102. P51622EP-6348 The current temperature of the aerosol generating material ^^^^^^^^ may be estimated or determined as follows: ^^^^^^^^ = ^^^^^^^^^^^^^^ − ^^^^^^^^where ^^^is the thermal resistance between the heater 36 and the aerosol generating material 102, ^^^^^^^^^^ is the previously applied power, i.e., for a previous time ^^^^), and ^^^^^^^^ is the current temperature of the heater 36. The previously applied power from a previous iteration of the control method is stored in a memory (not shown) of the aerosol generating device 10. The thermal resistance ^^^between the heater 36 and the aerosol generating material 102 may be determined by one or more individual thermal resistances, for example the thermal resistance of one or more insulating layers, the side wall 30 of the heating chamber 18, the air gap between the side wall 30 of the heating chamber 18 and the aerosol generating article, the paper wrapper 110 that surrounds the aerosol generating material 102 etc. The thermal resistance ^^^is stored in a memory (not shown) of the aerosol generating device 10. The estimated or determined current power and temperature values may be stored (e.g., in a memory (not shown)) to be used in a subsequent iteration of the control method. An error between a power setpoint ^^^^^^^^ and the power currently being applied to the aerosol generating material ^^^^^^^^is determined, e.g., by a summing node 50. The error is provided to the PID controller 44. The output of the PID controller 44 is used to control the power supplied to the heater 36 in a known manner, e.g., by controlling when the semiconductor switch Q1 is switched on and off by the MCU 38 to vary the duty cycle of the control algorithm. As shown in Figure 5, the power setpoint ^^^^is variable. At the start of the pre-heating phase (labelled “PHP”), the power setpoint ^^^^is set to a first power P1 which in this example is about 8 W. P51622EP-6348 The temperature of the aerosol generating material 102 may start at ambient temperature and will increase rapidly as it is heated by the heater 36. The temperature of the aerosol generating material may be estimated or determined as described above (e.g., based on the current heater temperature, a stored thermal resistance, and the previously applied power). When the temperature of the aerosol generating material 102 reaches a temperature threshold of about 100^C, the power setpoint ^^^^is set to a second power P2 which is in this example is about 55 W. This temporary increase in the power setpoint (or “power boost”) results in a significant increase in the amount of power supplied to the heater 36. This increases the heating of the aerosol generating material 102 and rapidly removes the water from the aerosol generating material 102. Figure 5 shows how the temperature of the aerosol generating material 102 continues to increase towards a target temperature. In other words, the temperature of the aerosol generating material 102 does not plateau or stall as the water is rapidly evaporated from the aerosol generating material 102 by the “power boost” function. The user of the aerosol generating device 10 may be notified that a first puff may be taken a preset time after the power setpoint ^^^^is set to the second power P2. After a preset time, it may be assumed that substantially all of the water in the aerosol generating material 102 has evaporated. This means that the quality of the aerosol in the first puff is not compromised. The power setpoint ^^^^may be set to a third power P3 (which in this example is about 8 W) a preset time after the power setpoint is set to the second power P2. After a preset time, it may be assumed that substantially all of the water in the aerosol generating material 102 has evaporated so there is no need for the power setpoint ^^^^to remain at the second power P2. Alternatively, the power setpoint ^^^^may be set to the third power P3 in response to a detected first puff by the user. The first puff will normally remove any remaining water from the aerosol generating material 102 so there is no need for the power setpoint ^^^^to remain at the second power P2 after the first puff has been taken. Alternatively, the power setpoint ^^^^may be set to the third power P3 based on an estimated or determined amount of water in the aerosol generating material 102, e.g., P51622EP-6348 when the estimated or determined amount of water falls below a threshold amount. The amount of water may be estimated or determined by a suitable sensor or sensing circuit (not shown) or by measuring one or more electrical parameters of the aerosol generating material 102 such as resistance, inductance or capacitance, for example. Once substantially all of the water in the aerosol generating material has evaporated, there is no need for the power setpoint ^^^^to remain at the second power P2. The power setpoint ^^^^may subsequently be varied during a heating phase (labelled “HP”) as shown in Figure 5 to control the heating of the aerosol generating material 102. Although exemplary embodiments have been described in the preceding paragraphs, it should be understood that various modifications may be made to those embodiments without departing from the scope of the appended claims. Thus, the breadth and scope of the claims should not be limited to the above-described exemplary embodiments. Any combination of the above-described features in all possible variations thereof is encompassed by the present disclosure unless otherwise indicated herein or otherwise clearly contradicted by context. Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise”, “comprising”, and the like, are to be construed in an inclusive as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to”. P51622EP-6348

Claims

Claims 1. A method of controlling heating of aerosol generating material (102) during a pre-heating phase of an aerosol generating device (10), the method comprising controlling heating of the aerosol generating material (102) using a closed loop controller (42) based on the error between a power setpoint and an estimated or determined power applied to the aerosol generating material (102).

2. A method according to claim 1, further comprising estimating or determining a temperature of the aerosol generating device (10) and using the estimated or determined temperature to estimate or determine the power applied to the aerosol generating material (102).

3. A method according to claim 2, wherein the aerosol generating device (10) comprises a heater (36) adapted to heat the aerosol generating material (102), and wherein the temperature of the aerosol generating device (10) is the heater temperature.

4. A method according to claim 3, wherein the heater temperature is estimated or determined based on one or more electrical parameters of the heater (36).

5. A method according to claim 3 or claim 4, wherein the current temperature of the aerosol generating material (102) is estimated or determined using: ^ the current heater temperature, ^ the thermal resistance between the heater (36) and the aerosol generating material (102), optionally stored in a memory of the aerosol generating device in advance, and ^ a power previously applied to the aerosol generating material (102).

6. A method according to claim 5, wherein the power currently applied to the aerosol generating material (102) is estimated or determined using: ^ the current temperature of the aerosol generating material (102), ^ a previous temperature of the aerosol generating material (102), P51622EP-6348^ the mass of the aerosol generating material (102), optionally stored in the memory in advance, and ^ the specific heat capacity of the aerosol generating material (102), optionally stored in the memory in advance; and wherein heating of the aerosol generating material (102) is controlled based on the error between the power setpoint and the power currently applied to the aerosol generating material (102).

7. A method according to any preceding claim, wherein the closed loop controller (42) comprises a controller (44) with one or more controller constant or gains.

8. A method according to any preceding claim, further comprising: setting the power setpoint to a first power at the start of the pre-heating phase; subsequently setting the power setpoint to a second power that is higher than the first power; and subsequently setting the power setpoint to a third power that is lower than the second power.

9. A method according to claim 8, wherein the power setpoint is set to the second power when the temperature of the aerosol generating material (102) exceeds a threshold temperature.

10. A method according to claim 8, wherein the power setpoint is set to the second power when the rate of change of the temperature of the aerosol generating material (102) falls below a threshold rate.

11. A method according to any of claims 8 to 10, further comprising notifying the user of the aerosol generating device (10) that a first puff may be taken a preset time after the power setpoint is set to the second power.

12. A method according to any of claims 8 to 11, wherein the power setpoint is set to the third power in response to a detected first puff by the user. P51622EP-634813. A method according to any of claims 8 to 11, wherein the power setpoint is set to the third power a preset time after the power setpoint is set to the second power.

14. A method according to any of claims 8 to 11, wherein the power setpoint is set to the third power based on an estimated or determined amount of water vapour in the aerosol generating material (102).

15. An aerosol generating device (10) comprising a heater (36) adapted to heat aerosol generating material (102), and a closed loop controller (42) adapted to control the heater (36) during a pre-heating phase of the aerosol generating device (10), the closed loop controller (42) being further adapted to control the heater (36) based on the error between a power setpoint and an estimated or determined power applied to the aerosol generating material (102). P51622EP-6348

Images