Aerosol-generating device with feedback control of transducer

Abstract

An aerosol-generating device is provided. The aerosol-generating device can include a piezoelectric transducer (10). The aerosol-generating device can include a drive circuit (12) connected to the piezoelectric transducer and configured to apply an oscillating current to the transducer. The aerosol-generating device can include a control circuit (14) connected to the drive circuit and configured to monitor a resonant behavior of the piezoelectric transducer, the control circuit configured to control operation of the drive circuit based on the resonant behavior of the piezoelectric transducer.

Description

TECHNICAL FIELD

[0001] The present invention relates to aerosol-generating devices using a liquid aerosol-forming substrate. In particular, the present disclosure relates to aerosol-generating devices using a vibrating transducer to move or atomise a liquid aerosol-forming substrate. BACKGROUND

[0002] One example of an aerosol-generating device is an electronic cigarette. Typically, in an electronic cigarette, a liquid aerosol-forming substrate is heated to generate a vapour to generate an aerosol. However, alternative designs have been proposed to generate droplets from a liquid aerosol-generating device using a vibrating transducer, where the droplets form an aerosol.

[0003] Vibrating transducers can also be used as the basis for small liquid pumps. Small liquid pumps can be used in aerosol-generating devices, for example electronic cigarettes, to deliver a liquid aerosol-forming substrate from a reservoir to an atomisation element, for example a heater or a vibrating transducer. SUMMARY

[0004] It is desirable to optimise the efficiency of any vibrating transducer used in an aerosol-generating device to generate an aerosol or to move a liquid within the device as much as possible. This is particularly important in handheld aerosol-generating devices, for example electronic cigarettes, which are typically powered by a battery and are desired to be as small as possible, but need to generate a large amount of aerosol according to the user’s needs.

[0005] It is also desirable to be able to quickly and simply detect any changing operating conditions that affect the output and efficiency of an aerosol-generating device.

[0006] According to one aspect of the present invention, there is provided an aerosol-generating device. The aerosol-generating device can comprise a transducer. The transducer can be a piezoelectric transducer. The aerosol-generating device can comprise a drive circuit connected to the transducer and configured to apply an oscillating current to the transducer. The aerosol-generating device can comprise a control circuit connected to the drive circuit and configured to monitor a resonant behaviour of the transducer, the control circuit being configured to control operation of the drive circuit based on the resonant behaviour of the transducer.

[0007] As used herein, “resonant behaviour” means any measurable aspect of the response of a piezoelectric transducer to an oscillating input signal. For example, the resonant behaviour can be one or several resonant frequencies, or a change in one or several resonant frequencies, a maximum or minimum response amplitude, a maximum or minimum input impedance, a phase response, or a change in a phase response.

[0008] The resonant behaviour of a transducer can be represented by one or more measurable parameters, for example an input impedance or one or more maximum amplitude response frequencies.

[0009] The piezoelectric transducer can be part of a transducer assembly. The transducer assembly can be configured to interact with a liquid aerosol-forming substrate within the device. The transducer assembly can be configured to generate an aerosol from the liquid aerosol-forming substrate. The transducer assembly can be configured to move the liquid aerosol-forming substrate.

[0010] In some embodiments, the transducer assembly comprises a perforated membrane or mesh. The transducer can be configured to drive the perforated membrane or mesh to vibrate at one or more frequencies. Vibration of the perforated membrane or mesh can force liquid aerosol-forming substrate through the perforated membrane or mesh, which can result in the formation of an aerosol comprising droplets of liquid aerosol-forming substrate.

[0011] In some embodiments, the transducer assembly comprises a membrane or surface configured to contact a liquid aerosol-forming substrate. The transducer can be configured to drive the membrane or surface to vibrate at one or more frequencies. Vibration of the membrane or surface can force liquid through an adjacent mesh or perforated membrane, which can result in the formation of an aerosol comprising droplets of liquid aerosol-forming substrate.

[0012] In some embodiments, the transducer assembly comprises an atomising surface configured to contact a liquid aerosol-forming substrate and an electrode on the transducer configured to generate a surface acoustic wave (SAW) on the atomising surface. The SAW generates droplets of liquid aerosol-forming substrate, which form an aerosol.

[0013] In some embodiments, the transducer assembly forms part of a liquid pump. The transducer can comprise a membrane or surface configured to contact a liquid aerosol-forming substrate. The transducer can be configured to drive the membrane or surface to vibrate at one or more frequencies. Vibration of the membrane or surface can force liquid through an adjacent liquid valve.

[0014] In all of these embodiments, it can be beneficial to control the operating frequency of the transducer or the operating power of the transducer (or both the operating frequency and the operating power of the transducer) in response to changes in the resonant behaviour of the transducer. In particular, it can be beneficial to control the operating frequency in order to improve the efficiency of the system. Controlling the operating frequency can allow aerosol generation to be maximised.

[0015] The control circuitry can be configured to control operation of the drive circuitry when the device is first activated. The control circuitry can be configured to control operation of the drive circuitry periodically or intermittently during operation of the device.

[0016] The resonant behaviour of a piezoelectric transducer can change during operation of the device for a number of reasons. One parameter that can affect the resonant behaviour of a transducer is temperature. The resonant frequency of a transducer assembly can change as the temperature changes, causing dimensional changes and changes in residual stresses within the transducer assembly, as the material of the transducer assembly expands or contracts. The temperature of the transducer assembly can change due to changes in the ambient temperature. The temperature of the sensor assembly can change due to heating of the sensor assembly due to energy dissipation within the device during operation. Typically, the transducer assembly will heat up during operation of the device. Heating of the transducer assembly can cause the resonant frequency of the transducer to decrease.

[0017] Other changes in environmental conditions can affect the resonant behaviour of the transducer. For example, changes in atmospheric pressure or humidity can affect the resonant behaviour of the transducer.

[0018] Changes in the material in contact with the transducer assembly can change the resonant behaviour of the transducer. In particular, changes in the load on the transducer can change the resonant behaviour of the transducer. For example, changes in the volume of liquid aerosol-forming substrate in contact with the transducer assembly can change the load on the transducer. Changes in the composition of the liquid aerosol-forming substrate in contact with the transducer assembly can change the load on the transducer.

[0019] The resonant behaviour of the transducer can change due to ageing of one or more components of the transducer assembly.

[0020] It can therefore be seen that changes in the resonant behaviour of the transducer assembly can be rapid, or can include longer term drift. It is beneficial for the device to be able to cope with both rapid changes and long term drift.

[0021] The control circuitry can be configured to control operation of the drive circuitry so that the oscillating current has a frequency equal to the resonant frequency of the piezoelectric transducer. Operating at the resonant frequency can allow the maximum amount of power to be delivered to the transducer. Operating at the resonant frequency can result in the maximum vibration amplitude and the maximum vibration velocity. This can be beneficial for generating an aerosol with the required properties.

[0022] The control circuitry can be configured to control operation of the drive circuitry so that the oscillating current has a frequency offset from the resonant frequency of the transducer. This can be advantageous in some circumstances. For example, when the impedance of the transducer at its resonant frequency does not match the output impedance of the drive circuitry, a small offset in frequency can be used to operate the system at the point of impedance matching where the highest power is delivered to the transducer. This frequency can be some frequency between the resonant frequency and the anti-resonant frequency. In this region, the impedance changes significantly with frequency, allowing for precise tuning.

[0023] The control circuit can be configured to monitor the resonant behavior of the transducer at a plurality of resonant frequencies corresponding to different vibration modes. The transducer can be driven at a plurality of different frequencies in order to generate aerosol droplets with different characteristics.

[0024] The control circuit can be configured to monitor the resonant behavior of the piezoelectric transducer by measuring the power delivered to the transducer or the input impedance of the transducer. At the resonant frequency, the delivered power is maximized and the input impedance is minimized.

[0025] The control circuit can be configured to monitor the resonant behavior of the piezoelectric transducer by determining a zero crossing point of the output signal from the transducer or an inflection point of the output signal from the transducer. The zero crossing point or the inflection point can be used to determine the operating frequency.

[0026] The control circuit can comprise a phase-locked loop (PLL). The phase-locked loop can comprise a phase comparator and can determine a phase shift in the response from the transducer. Using a phase-locked loop can be advantageous because it does not require a microprocessor. It can be a low cost and high reliability solution.

[0027] In some embodiments, the drive and control circuit is configured to:

[0028] a) apply a current having a drive frequency to the transducer;

[0029] b) during periodically spaced time slots, apply a current having a second frequency, the second frequency being a higher or lower frequency than the drive frequency;

[0030] c) determine whether the power delivered to the transducer is increased at the second frequency when compared to the first frequency; and

[0031] d) if the power delivered at the second frequency is increased, use the second frequency as the drive frequency, otherwise maintain the existing drive frequency; and

[0032] e) repeat steps a) to d).

[0033] In alternative time slots, the drive circuit can use a second frequency that is higher than the drive frequency and in alternative time slots, a second frequency that is lower than the drive frequency can be used.

[0034] This process enables the device to automatically track the resonant frequency of the transducer.

[0035] The second frequency can be higher or lower than the drive frequency by a predetermined amount.

[0036] Other options for detecting changes in the resonant frequency include the use of dedicated MEMS sensors. For example, a low-inertia MEMS cantilever can be placed in contact with the vibrating element of the transducer assembly, such that the cantilever synchronizes with the oscillation of the transducer assembly and provides a corresponding electrical signal. Another option is to use real-time impedance measurements by monitoring the voltage and current in the transducer. In this case, the series and parallel modes of the transducer can be addressed separately. The transducer behavior can be described in terms of an electrical equivalent circuit consisting of a series of a capacitance CI, an inductance LI and a resistance Rl for the mechanical part, and another capacitance CO and a resistance RO for the parallel electrical part of the transducer. Depending on the frequency, this circuit can operate in a state where the series branch (CI, LI, Rl) is self-resonant (series resonance mode) or where this LCR series is in resonance with the parallel CO (parallel resonance mode). The series resonance frequency is close to the resonance frequency of the transducer, and the parallel resonance frequency is close to the anti-resonance frequency of the transducer. The series mode resonance provides a low impedance, and for the same power, a higher current and a lower voltage. Mechanically, this provides a large displacement amplitude. The parallel mode resonance provides a high impedance, and for the same power, a lower current and a higher voltage. The losses are lower mechanically. In this case, the tuning series inductance can be increased, and the electroacoustic energy transfer efficiency can be higher at the anti-resonance.

[0037] The device can comprise means for tuning the resonant frequency of the transducer. For example, a membrane coupled to a piezoelectric transducer can be pre-stressed by applying a DC bias voltage, which will change its resonant behavior. It can be beneficial to tune the resonant response of the device to match a particular required frequency or frequencies associated with the aerosol-forming substrate as components of the device age.

[0038] The control circuit can comprise a microprocessor. The control circuit can comprise a field programmable gate array (FPGA). The drive circuit and the control circuit can be integrated into a single circuit.

[0039] The control circuit can be configured to control the carrier frequency, the duty cycle, the power, the modulation frequency, or the amplitude of the oscillating current from the drive circuit.

[0040] As described, the resonant behavior of the transducer can be affected by the amount of liquid in contact with parts of the transducer assembly. The control circuit can be configured to detect a reduction in the amount of liquid in contact with the transducer assembly based on a change in the resonant behavior of the transducer. This can be based on a sudden change in the resonant frequency that is greater than a threshold amount. The control circuit can be configured to stop operation of the drive circuit in response to detecting a significant reduction in the liquid delivered to the transducer assembly. The control circuit can be configured to stop or modify operation of the drive circuit based on any faults of the device determined based on the resonant behavior of the transducer.

[0041] The piezoelectric transducer can comprise a single crystal material. The piezoelectric transducer can comprise quartz. The piezoelectric transducer can comprise a ceramic. The ceramic can comprise barium titanate (BaTi03). The ceramic can comprise lead zirconate titanate (PZT). The ceramic can comprise a dopant material, for example, Ni, Bi, La, Nd or Nb ions. The piezoelectric transducer can be poled. The piezoelectric transducer can be unpoled. The piezoelectric transducer can comprise both poled and unpoled piezoelectric materials.

[0042] The drive circuit can be configured to apply an oscillating current at a frequency between 20 kHz and about 1500 kHz, or between about 50 kHz and about 1000 kHz, or between about 100 kHz and about 500 kHz. This can provide a desired aerosol output rate and a desired droplet size.

[0043] The aerosol-generating device can be configured to generate an aerosol for inhalation by a user. The aerosol-generating device can be an electrically operated smoking device.

[0044] The aerosol-generating device can comprise a liquid reservoir comprising a liquid aerosol-forming substrate. In use, the piezoelectric transducer can be in contact with liquid from the liquid reservoir.

[0045] The aerosol-generating device can comprise a liquid storage portion comprising a liquid aerosol-forming substrate reservoir. The liquid storage portion can form part of a cartridge that can be separable from the remainder of the device. The liquid storage portion of the aerosol-generating system can comprise a generally cylindrical housing with an opening at one end of the cylinder. The housing of the liquid storage portion can have a generally circular cross-section. The housing can be a rigid housing. As used herein, the term “rigid housing” is used to mean a self-supporting housing. The rigid housing of the liquid storage portion can provide mechanical support to the heating means.

[0046] The liquid storage portion can further comprise a carrier material within the housing for holding the aerosol-forming substrate.

[0047] The liquid aerosol-forming substrate can be adsorbed or otherwise loaded onto a carrier or support. The carrier material can be made from any suitable absorbent plug or absorbent body, for example, a foamed metal or plastic material, polypropylene, dacron, nylon fibres or ceramic. The liquid aerosol-forming substrate can be retained in the carrier material prior to use of the aerosol-generating system. The liquid aerosol-forming substrate can be released into the carrier material during use. The liquid aerosol-forming substrate can be released into the carrier material immediately prior to use.

[0048] In one example, a liquid aerosol-forming substrate is held in a capillary material. A capillary material is a material that actively transports liquid from one end of the material to the other end. The capillary material can be advantageously orientated in the housing to transport the liquid aerosol-forming substrate to the transducer assembly. The capillary material can have a fibrous structure. The capillary material can have a sponge-like structure. The capillary material can comprise a bundle of capillaries. The capillary material can comprise a plurality of fibres. The capillary material can comprise a plurality of threads. The capillary material can comprise a fine bore tube. The capillary material can comprise a combination of fibres, threads and fine bore tubes. The fibres, threads and fine bore tubes can be generally aligned to transport liquid to the vibratable element. The capillary material can comprise a sponge-like material. The capillary material can comprise a foam-like material. The structure of the capillary material can form a plurality of pores or tubes through which liquid can be transported by capillary action.

[0049] The capillary material can comprise any suitable material or combination of materials. Examples of suitable materials are sponge or foam materials, ceramic or graphite-based materials in the form of fibres or sintered powders, foam metals or plastics materials, for example fibrous materials made from spun or extruded fibres such as cellulose acetate, polyester or bonded polyolefin, polyethylene, terylene or polypropylene fibres, nylon fibres or ceramics. The capillary material can have any suitable capillarity and porosity for use with different liquid physical properties. The liquid aerosol-forming substrate has physical properties including (but not limited to) viscosity, surface tension, density, thermal conductivity, boiling point and atomic pressure, which allow the liquid to be transported through the capillary material by capillary action. The capillary material can be configured to transport the aerosol-forming substrate to the transducer assembly.

[0050] The carrier material can abut the transducer assembly. The liquid aerosol-forming substrate can be transported from the liquid storage portion to the transducer assembly by capillary action.

[0051] Alternatively or additionally, the device can comprise a pump. The liquid aerosol-forming substrate can be delivered from the reservoir to the transducer assembly by the pump.

[0052] The aerosol-generating device can comprise a liquid aerosol-forming substrate in the housing of the liquid storage portion. The liquid aerosol-forming substrate is a substrate capable of releasing volatile compounds that can form an aerosol. The volatile compounds can be released by moving the liquid aerosol-forming substrate through the passageway of the vibratable element.

[0053] The liquid aerosol-forming substrate can comprise nicotine. The nicotine comprising liquid aerosol-forming substrate can be a nicotine salt substrate. The liquid aerosol-forming substrate can comprise a plant-based substrate material. The liquid aerosol-forming substrate can comprise tobacco. The liquid aerosol-forming substrate can comprise a tobacco-containing material containing volatile tobacco flavour compounds which are released from the aerosol-forming substrate upon heating. The liquid aerosol-forming substrate can comprise homogenised tobacco material. The liquid aerosol-forming substrate can comprise a tobacco-free material. The liquid aerosol-forming substrate can comprise a homogenised plant-based material.

[0054] The liquid aerosol-forming substrate can comprise at least one aerosol-former. An aerosol-former is any suitable known compound or mixture of compounds which, in use, is beneficial in forming a dense and stable aerosol and which is substantially resistant to thermal degradation at the operating temperature of the system. Suitable aerosol-formers are well known in the art and include, but are not limited to: polyhydric alcohols such as triethylene glycol, 1,3-butanediol and glycerol; esters of polyhydric alcohols such as glycerol mono-, di- or triacetate; and fatty acid esters of mono-, di- or poly-carboxylic acids such as dimethyl dodecanedioate and dimethyl tetradecanedioate. The aerosol-former can be a polyhydric alcohol or a mixture thereof, for example triethylene glycol, 1,3-butanediol and glycerol. The liquid aerosol-forming substrate can include other additives and ingredients, for example flavourants.

[0055] The aerosol-forming substrate can comprise nicotine and at least one aerosol-former. The aerosol-former can be glycerol. The aerosol-former can be propylene glycol. The aerosol-former can comprise both glycerol and propylene glycol. The aerosol-forming substrate can have a nicotine concentration of between about 2% and about 10%.

[0056] The aerosol-forming substrate can have a dynamic viscosity (μ) at a temperature of 20 °C of between about 0.4 mPa.S (0.4 mPl, 0.4 cP) and about 1000 mPa.S (1000 mPl, 1000 cP) or between about 1 mPa.S and 100 mPa.S or about 1.5 mPa.S and about 10 mPa.S.

[0057] The aerosol-generating device can comprise a power source. The power source can be a battery. The battery can be a lithium-based battery, such as a lithium-cobalt, lithium-iron-phosphate, lithium-titanate, or lithium-polymer battery. The battery can be a nickel-metal-hydride battery or a nickel-cadmium battery. The power source can be another form of charge storage device, such as a capacitor. The power source can require recharging and be configured for a number of charge-discharge cycles. The power source can have a capacity that allows sufficient energy to be stored for one or more smoking experiences; for example, the power source can have sufficient capacity to allow aerosol to be generated continuously for a period of about six minutes, corresponding to the typical time taken to smoke a conventional cigarette, or for multiples of six minutes. In another example, the power source can have sufficient capacity to allow a predetermined number of puffs or discrete actuations of the heating means and actuator.

[0058] The aerosol-generating device can be portable. The aerosol-generating device can have a size comparable to a conventional cigar or cigarette. The aerosol-generating system can have an overall length of between about 30 mm and about 150 mm. The aerosol-generating device can have an outer diameter of between about 5 mm and about 30 mm.

[0059] The aerosol-generating device can comprise a housing. The housing can be elongate. The housing can comprise any suitable material or combination of materials. Examples of suitable materials include metals, alloys, plastics, or composites containing one or more of those materials, or thermoplastics suitable for food or pharmaceutical applications, such as polypropylene, polyether ether ketone (PEEK), and polyethylene. The material can be lightweight and non-brittle.

[0060] The housing can comprise a cavity for receiving a power source. The housing can comprise a mouthpiece. The mouthpiece can comprise at least one air inlet and at least one air outlet. The mouthpiece can comprise more than one air inlet.

[0061] The action of the sensor assembly on the liquid aerosol-forming substrate can heat the aerosol-forming substrate. This can be desirable when it is desired to deliver a warm aerosol to a user. Alternatively or additionally, the device can comprise a heater. The heater can heat the liquid aerosol-forming substrate before it reaches the transducer assembly, at the transducer assembly, or after an aerosol has been formed.

[0062] In another aspect of the application, there is provided a method of operating an aerosol-generating device. The device can comprise a piezoelectric transducer. The device can comprise a drive circuit connected to the piezoelectric transducer. The device can comprise a control circuit configured to monitor a parameter of the piezoelectric transducer and connected to the drive circuit. The method can comprise applying an oscillating current to the transducer using the drive circuit. The method can further comprise monitoring the resonant behaviour of the piezoelectric transducer using the control circuit. The method can further comprise controlling the operation of the drive circuit based on the monitored resonant behaviour of the piezoelectric transducer.

[0063] The piezoelectric transducer can be part of a transducer assembly. The transducer assembly can be in a liquid pump. The transducer assembly can comprise a membrane or surface configured to contact a liquid aerosol-forming substrate. The piezoelectric transducer can be configured to drive the membrane or surface to vibrate. The vibration of the membrane or surface can force the liquid through an adjacent liquid valve in the liquid pump.

[0064] The method can comprise stopping operation of the drive circuit based on the monitored resonant behaviour of the piezoelectric transducer. The method can comprise controlling a carrier frequency, a duty cycle, a power, a modulation frequency or an amplitude of the oscillating current from the drive circuit.

[0065] The step of monitoring the resonant behaviour can comprise applying oscillating currents having different frequencies and determining the resonant behaviour of the transducer at the different frequencies. The method can comprise applying an oscillating current comprising a plurality of sinusoidal frequencies.

[0066] The present invention can provide the advantage of efficient operation throughout operation regardless of changes in load on the transducer and changes in environmental or device conditions. The present invention can also provide a means for detecting faulty and abnormal operating conditions, for example a reduced supply of liquid aerosol-forming substrate.

[0067] The present invention is defined in the claims. However, a non-exhaustive list of non-limiting examples is provided below. Any one or more features of these examples can be combined with any one or more features of another example, embodiment or aspect described herein.

[0068] Example Ex1 : An aerosol-generating device comprising: a piezoelectric transducer; a drive circuit connected to the piezoelectric transducer and configured to apply an oscillating current to the transducer; and a control circuit connected to the drive circuit and configured to monitor a resonant behaviour of the piezoelectric transducer, the control circuit being configured to control operation of the drive circuit based on the resonant behaviour of the piezoelectric transducer.

[0069] Example Ex2: The aerosol-generating device according to example Ex1, wherein the control circuit is configured to control operation of the drive circuit such that the oscillating current has a frequency equal to a resonant frequency of the piezoelectric transducer.

[0070] Example Ex3: The aerosol-generating device according to example Ex1, wherein the control circuit is configured to control operation of the drive circuit such that the oscillating current has a frequency offset from a resonant frequency of the piezoelectric transducer.

[0071] Example Ex4: An aerosol-generating device according to any one of the preceding examples, wherein the control circuitry is configured to monitor the resonant behaviour of the piezoelectric transducer at a plurality of resonant frequencies of the piezoelectric transducer corresponding to different vibration modes.

[0072] Example Ex5: An aerosol-generating device according to any one of the preceding examples, wherein the control circuitry is configured to monitor the resonant behaviour of the piezoelectric transducer by measuring a power delivered to the piezoelectric transducer or an impedance of the piezoelectric transducer.

[0073] Example Ex6: An aerosol-generating device according to any one of the preceding examples, wherein the drive circuitry and control circuitry comprise a phase-locked loop (PLL).

[0074] Example Ex7: An aerosol-generating device according to any one of the preceding examples, wherein the piezoelectric transducer is an aerosol-generating element configured to generate an aerosol from a liquid aerosol-forming substrate.

[0075] Example Ex8: An aerosol-generating device according to Example Ex7, wherein the piezoelectric transducer comprises a perforated plate.

[0076] Example Ex9: An aerosol-generating device according to any one of Examples Ex1 to Ex6, wherein the piezoelectric transducer is part of a liquid pump.

[0077] Example Ex10: An aerosol-generating device according to any one of the preceding examples, comprising a liquid reservoir containing a liquid aerosol-forming substrate, wherein in use the piezoelectric transducer is in contact with liquid from the liquid reservoir.

[0078] Example Ex11 : An aerosol-generating device according to Example Ex10, wherein the liquid comprises a mixture of different compounds.

[0079] Example Ex12: An aerosol-generating device according to Example Ex10 or Ex11, wherein the control circuitry is configured to detect a reduction in the amount of liquid in contact with the piezoelectric transducer based on a change in the resonant behaviour of the piezoelectric transducer.

[0080] Example Ex13: An aerosol-generating device according to any one of the preceding examples, wherein the aerosol-generating device is an electronic cigarette.

[0081] Example Ex14: An aerosol-generating device according to any one of the preceding examples, wherein the oscillating current comprises a first frequency modulated with at least one other frequency.

[0082] Example Ex15: A method of operating an aerosol-generating device, the device comprising: a transducer assembly; a drive circuit connected to the piezoelectric transducer; and a control circuit configured to monitor a parameter of the piezoelectric transducer and connected to the drive circuit; the method comprising:

[0083] applying an oscillating current to the transducer using the drive circuit; and

[0084] monitoring the resonant behaviour of the piezoelectric transducer using the control circuit, and

[0085] controlling the operation of the drive circuit based on the monitored resonant behaviour of the piezoelectric transducer. BRIEF DESCRIPTION OF DRAWINGS

[0086] Several examples will now be described further with reference to the drawings, in which:

[0087] Figure 1 a feedback control system according to the application is shown;

[0088] Figure 2 is a schematic diagram of an aerosol-generating device according to the application;

[0089] Figure 3 a transducer assembly in a system for Figure 2 is shown;

[0090] Figure 4 is a schematic graph showing the response of a transducer over time;

[0091] Figure 5 an example of a drive and control circuit implementing feedback control is shown; and

[0092] Figure 6 is a schematic diagram of an aerosol-generating device according to the application comprising a piezoelectric pump. DETAILED DESCRIPTION

[0093] Figure 1 is a schematic diagram of a feedback control loop according to the application. The feedback loop comprises a transducer 12, a drive circuit 14 and a control circuit 14. The transducer in this example is a piezoelectric transducer. The transducer is coupled to a membrane and vibrates the membrane for generating an aerosol from a liquid supply. The transducer 12 is driven by the drive circuit 12 at a certain drive frequency. The drive circuit 12 supplies an oscillating current to the transducer which causes it to expand and contract. This in turn causes the membrane to vibrate.

[0094] The transducer has one or more resonant frequencies. The resonant frequency depends on several factors, including the load on the transducer. The load on the transducer depends on the properties of the membrane and any load on the membrane. For example, the resonant frequency also depends on the temperature.

[0095] To ensure that the transducer is driven at the resonant frequency by the drive circuit, the control circuit 14 completes a feedback loop. The control circuit receives a feedback parameter, such as a phase shift or amplitude of oscillation, from the transducer. The value of the feedback parameter varies depending on how close the drive frequency is to the resonant frequency of the transducer. The drive circuit 12 adjusts the drive frequency of the oscillating current applied to the transducer 10, and the effect of this change in drive frequency on the feedback parameter is monitored by the control circuit. The control circuit then sends a control signal to the drive circuit, and the drive circuit adjusts the frequency of the applied oscillating current based on the control signal in order to achieve a particular effect. In many cases, it is desirable to have the transducer as close to the resonant frequency as possible. But in some cases, it can be desirable to drive the transducer at a particular offset from the resonant frequency or at a frequency between the resonant frequency and the anti-resonant frequency. The control circuit can include a filter, a microcontroller, or any analog or digital device to process the feedback parameter in order to generate the control signal.

[0096] Figure 2 is a schematic view of a first embodiment of an aerosol-generating device according to the application, the aerosol-generating device comprising Figure 1 feedback control shown in Figure 2 is schematic in nature. In particular, the components shown are not necessarily to scale, independently or relative to each other. The aerosol-generating device comprises a reusable device portion 100 which cooperates with a cartridge 200, which is preferably disposable. In Figure 2 the device is an electrically operated smoking system.

[0097] The device portion 100 comprises a main body having a housing 101. The housing 101 is generally circular cylindrical and has a longitudinal length of about 100 mm and an outer diameter of about 20 mm, which is comparable to a conventional cigar. In the device, a power supply in the form of a battery 102 and an electrical control circuit 104 are provided. The electrical control circuit 104 comprises a drive circuit and a control circuit for a transducer, as described with reference to Figure 1 The main body housing 101 also defines a cavity 112 into which the cartridge 200 is received.

[0098] The cartridge 200 Figure 2 comprises a rigid housing defining a liquid storage portion 201. The liquid storage portion 201 holds a liquid aerosol-forming substrate (not shown). The cartridge 200 has a fluid-impermeable outer shell, but has an open end (not shown) which can be covered by a removable cap (not shown) when the cartridge is removed from the device 100. The cap can be removed from the cartridge 200 before the cartridge is inserted into the device. The cartridge 200 includes a keying feature (not shown) to ensure that the cartridge 200 cannot be inserted into the device upside down.

[0099] The device portion 100 also includes a mouthpiece portion 120. In this example, the mouthpiece portion 120 is connected to the main body housing 101 by a hinged connection, but any kind of connection can be used, for example, a snap fit or a screw fit. The mouthpiece portion 120 includes a plurality of air inlets 122, an air outlet 124, and an aerosol-forming chamber 125 and an atomizer 300 (shown schematically in Figure 2 Figure 1 1 ) mounted therein. When the mouthpiece portion is in the closed position, the air inlets 122 are defined between the mouthpiece portion 120 and the main body housing 101 of the device 100, as shown in Figure 2 Figure 1 1. The airflow path 127 is formed from the air inlets 122, through the aerosol-forming chamber 125 and the atomizer 300 to the air outlet 124, as shown by the arrows in Figure 2 Figure 1 1.

[0100] As shown in Figure 3 Figure 1 1, the atomizer 300 includes a vibratable element 301 and a transducer 302 housed inside an atomizer housing 304. The atomizer housing 304 comprises a hollow cylindrical box having an inlet opening 305 and an outlet opening 306 arranged coaxially aligned on opposite sides of the housing 304. The housing 304 is removably connected to the mouthpiece 120 of the device portion 100 by a threaded connection (not shown). An external thread (not shown) is provided at an outer surface of the atomizer housing 304, which is complementary to an internal thread (not shown) on an inner surface of the mouthpiece 120. The atomizer 300 can be removed from the mouthpiece portion 120 of the device portion for disposal or for cleaning.

[0101] The vibratable element 301 comprises a generally circular aluminium disc having a thickness of about 2 mm and a diameter of about 15 mm.

[0102] A plurality of passages 303 extend from an inlet side 308 to an opposite outlet side 309 of the vibratable element. The plurality of passages form an array having a generally circular shape. The generally circular array has a diameter of about 7 mm and is generally centrally arranged in the element 301.

[0103] The passages (not shown) have a generally circular cross-section and taper from the inlet side 308 to the outlet side 309 of the vibratable element 301. The passages have a diameter of about 8 μιη at the inlet side and a diameter of about 6 μιη at the outlet side. The passages are typically formed by high speed laser drilling. The plurality of passages consists of about 4000 passages arranged at equal spacing across the array.

[0104] The transducer 302 comprises a piezoelectric transducer. The piezoelectric transducer is a generally circular annular disc of piezoelectric material, typically zirconium titanate. The piezoelectric transducer has a thickness of about 2 mm, an outer diameter of about 17 mm and an inner diameter of about 8 mm.

[0105] AsFigure 3 As shown in FIG. 3, the transducer 302 is in direct contact with the vibratable element 301 at the outlet side 309 of the vibratable element. The inner diameter of the piezoelectric transducer 302 encircles the array of passages 303 of the vibratable element 301 such that the open ends of the passages at the outlet side are not covered by the piezoelectric transducer 302. In other embodiments (not shown), it is contemplated that the piezoelectric transducer 302 can be in direct contact with the vibratable element 301 at the inlet side 308.

[0106] The vibratable element 301 and the piezoelectric transducer 302 are supported within the atomiser housing 304 by a pair of elastomeric O-rings 311, which allows the vibratable element 301 and the piezoelectric transducer 302 to vibrate within the housing 304. The vibratable element 301 and the piezoelectric transducer 302 are held together by pressure from the opposing O-rings 311. However, in other embodiments (not shown), the vibratable element 301 and the piezoelectric transducer 302 can be bonded by any suitable means, for example, an adhesive layer.

[0107] The vibratable element 301 and the piezoelectric transducer 302 are arranged within the atomiser housing 304 such that the array of passages 303 is coaxially aligned with the inlet opening 305 and the outlet opening 306 of the housing 304.

[0108] One or more spring pins 310 extend through an opening 312 in the atomiser housing 304 to provide an electrical connection of the piezoelectric transducer 302 to the control circuitry 104 and the battery 102 of the device 100. The one or more spring pins 310 are held in contact with the piezoelectric transducer 302 by pressure rather than by a mechanical connection, such that good electrical contact is maintained during vibration of the piezoelectric transducer 302.

[0109] In use, when the atomiser 300 is removably connected to the mouthpiece portion 120 of the device portion 100 and the cartridge 200 is received in the cavity 112 of the device, the elongate capillary body 204 (not shown in FIG. 3) extends from the liquid storage portion 201 of the cartridge 200 to the atomiser 300 to fluidly connect the cartridge 200 to the atomiser 300. As shown in FIG. 4, the capillary body 204 is shown extending into the atomiser housing 304 and abutting the inlet side 308 of the vibratable element 301 at the array of passages 303. Figure 2 In use, when the atomiser 300 is removably connected to the mouthpiece portion 120 of the device portion 100 and the cartridge 200 is received in the cavity 112 of the device, the elongate capillary body 204 (not shown in FIG. 3) extends from the liquid storage portion 201 of the cartridge 200 to the atomiser 300 to fluidly connect the cartridge 200 to the atomiser 300. As shown in FIG. 4, the capillary body 204 is shown extending into the atomiser housing 304 and abutting the inlet side 308 of the vibratable element 301 at the array of passages 303. Figure 3 In use, when the atomiser 300 is removably connected to the mouthpiece portion 120 of the device portion 100 and the cartridge 200 is received in the cavity 112 of the device, the elongate capillary body 204 (not shown in FIG. 3) extends from the liquid storage portion 201 of the cartridge 200 to the atomiser 300 to fluidly connect the cartridge 200 to the atomiser 300. As shown in FIG. 4, the capillary body 204 is shown extending into the atomiser housing 304 and abutting the inlet side 308 of the vibratable element 301 at the array of passages 303. Figure 3 In use, when the atomiser 300 is removably connected to the mouthpiece portion 120 of the device portion 100 and the cartridge 200 is received in the cavity 112 of the device, the elongate capillary body 204 (not shown in FIG. 3) extends from the liquid storage portion 201 of the cartridge 200 to the atomiser 300 to fluidly connect the cartridge 200 to the atomiser 300. As shown in FIG. 4, the capillary body 204 is shown extending into the atomiser housing 304 and abutting the inlet side 308 of the vibratable element 301 at the array of passages 303. Figure 2 or Figure 3 In use, when the atomiser 300 is removably connected to the mouthpiece portion 120 of the device portion 100 and the cartridge 200 is received in the cavity 112 of the device, the elongate capillary body 204 (not shown in FIG. 3) extends from the liquid storage portion 201 of the cartridge 200 to the atomiser 300 to fluidly connect the cartridge 200 to the atomiser 300. As shown in FIG. 4, the capillary body 204 is shown extending into the atomiser housing 304 and abutting the inlet side 308 of the vibratable element 301 at the array of passages 303.

[0110] In use, liquid aerosol-forming substrate (not shown) is transported from the liquid storage portion 201 by capillary action from the end of the capillary body 204 extending into the liquid storage portion 201, through the heater coil 205, and to the other end of the capillary body 204 which extends into the atomiser housing 304 and abuts the vibratable element 301 at the inlet side 308 of the array of passages 303.

[0111] When a user draws on the air outlet 124 of the mouthpiece portion 120, ambient air is drawn through the air inlet 122. In Figure 2 In embodiments, the puff detection device 106, also in the form of a microphone, is also provided as part of the control electronics 104. A small air flow is drawn through a sensor inlet 121 in the main body housing 101, past the microphone 106, and up into the mouthpiece portion 120. When the circuit 104 detects a puff, the circuit 104 activates the heater coil 205 and the piezoelectric transducer 302. The battery 102 supplies electrical energy to the coil heater 205 to heat the capillary body 204 surrounded by the coil heater.

[0112] The battery 102 further supplies electrical energy to the piezoelectric transducer 302 under the control of the drive and control circuit, which vibrates, deforming in the thickness direction. The piezoelectric transducer 302 typically vibrates at approximately 150 kHz. The drive current supplied to the transducer has an initial frequency and waveform based on parameters stored in memory. During manufacture of the device, the frequency response of the transducer assembly, including the vibratable element 301, can be characterised and a set of initial frequencies and waveforms. The piezoelectric transducer 302 transmits the vibrations to the vibratable element 301, which also deforms in the thickness direction. The LED 108 is also activated to indicate that the device is activated. As will be described, during operation, a feedback control loop is used to adjust the drive current supplied to the transducer in response to detected changes in resonant behaviour.

[0113] The coil heater 205 heats the liquid aerosol-forming substrate being transported along the capillary body, through the coil heater 205, to a predetermined temperature of approximately 45°C.

[0114] The vibration of the vibratable element deforms a plurality of passageways 303 which draw heated liquid aerosol-forming substrate from the capillary body 204, through the plurality of passageways 303 at the inlet side 308 of the vibratable element 301, and expel the atomised droplets of liquid aerosol-forming substrate from the passageways at the outlet side 309 of the vibratable element 301, thereby forming an aerosol. At the same time, the heated liquid being atomised is replaced by further liquid moving along the capillary body 204 by capillary action. (This is sometimes referred to as 'wicking action'). The aerosol droplets expelled from the vibratable element 301 mix with and are carried in the airflow 127 from the inlet 122 in the aerosol-forming chamber 125, and are carried towards the air outlet 124 of the mouthpiece 120 for inhalation by a user.

[0115] As previously described, during operation the resonant response of the transducer can vary. Figure 4 is a schematic of a sensed parameter from the transducer, showing the frequency over time. The time distance between the zero crossings is a measure of the frequency, which can be used to synchronize the drive signal with the operating frequency of the transducer, in this example with its resonance frequency. The signal can be, for example, the current measured by a sense resistor in series with the transducer. In this case, the amplitude can have units of amperes for the current, or the amplitude can be normalized, for example, by its maximum value, in which case the amplitude has units of 1. The time can have units of, for example, milliseconds or microseconds, depending on the characteristic frequency operating range covered by the transducer.

[0116] One specific example of a possible implementation of this feedback loop is shown in Figure 5 The transducer 500 connected to the vibratable perforated plate in the embodiment of Figure 3 is driven by a half bridge 505, which consists of two power MOSFETs 510, 515. An optional series inductor 520, for example a 10 microhenry inductor, can be used between the half bridge and the transducer to tune the impedance. A current sense resistor 525, for example 1 ohm, can be placed at the low voltage end of the transducer 500. The voltage measured across the current sense resistor is proportional to the current through the transducer. This voltage signal can be filtered and amplified by a filter and gain stage 530. The filter and gain stage 530 can include, for example, a low pass filter to cut off high frequency harmonics, and a FET amplifier, for example an AD823, to amplify the signal. A comparator 540 produces a feedback signal, in this example a square wave signal, as the appropriate input waveform for the gate driver 550. For example, the gate driver 550 can be an IC of the type LT1162, and drives the half bridge 505. When a frequency change is detected and sent back to the driver, the transducer 500 will always be driven at its operating frequency, for example its resonance frequency. Figure 5 The drive and control circuit shown in Figure 2the control circuit 104 shown in the middle.

[0117] Figure 6 schematic view of an aerosol-generating device according to another embodiment of the application. Figure 6 are schematic in nature. In particular, the components shown are not necessarily to scale, independently or relative to each other. Figure 6 The device generates aerosol by heating a liquid aerosol-forming substrate using a heater. However, the device includes a pump that uses a piezoelectric transducer to deliver the liquid aerosol-forming substrate to the heater.

[0118] The device is a hand-held electrically operated smoking device 600 and includes a housing 610. Within the housing 610 there is a power supply in the form of a battery 612 and control circuitry 614. There is also within the housing a liquid reservoir 620 containing a liquid aerosol-forming substrate that is vaporised in order to form an aerosol that is inhaled by a user. Within the housing there is an atomiser assembly 630 coupled to the liquid reservoir 620. The atomiser assembly includes a vaporiser 634, in this example an electric heater, and a pump 632 positioned to pump liquid from the liquid reservoir 620 to the vaporiser 634. Both the pump 632 and the electric heater 634 are provided with power from the battery 612 under the control of the control circuitry 614 as will be described.

[0119] The housing 610 includes an air inlet 618 and an air outlet 616. The air outlet 616 is provided at the mouth end of the housing. In use, a user sucks on the mouth end of the housing. This draws air into the housing through the air inlet 618, past the vaporiser 634 and out through the outlet 616 into the user's mouth. The air drawn past the vaporiser entrains the vaporised aerosol-forming substrate. The vaporised aerosol-forming substrate cools as it moves through the device and into the user's mouth to form an aerosol.

[0120] Activation of the heater can be controlled directly by the user pressing a button on the housing 610. Alternatively, the system can include an air flow sensor, such as a microphone 615, that detects air flow through the system, and the heater can be activated based on a signal from the air flow sensor. When a user draws air through the system, which is referred to herein as a puff, air flows past the air flow sensor 615. If the air flow detected by the air flow sensor exceeds a threshold, then the control circuit can activate the heater by supplying power to the heater. The control circuit can supply power to the heater for a predetermined period of time, or can supply power to the heater for as long as the detected air flow exceeds the threshold. The control circuit can include a temperature sensing means, such as a dedicated temperature sensor, or by monitoring the resistance of the heater. The control circuit can then supply power to the heater to raise the temperature of the heater to a desired temperature range. The temperature should be sufficient to vaporize the aerosol-forming substrate, but not so high that there is a significant risk of combustion.

[0121] The liquid in this example comprises a mixture of water, glycerol, propylene glycol, nicotine, and a flavourant. The liquid is held within a liquid reservoir 620. The liquid reservoir is provided as a cartridge, which can be replaced when the liquid has been used up. To prevent leakage of the liquid before and during use, the liquid reservoir has a housing formed from a rigid plastics material, and is liquid-tight. As used herein, "rigid" means that the housing is self-supporting. In this example, the reservoir is formed by 3D printing using an acrylic-based photopolymer. The cartridge must be robust and able to withstand significant loading during shipping and storage. However, because the liquid reservoir housing is sealed and rigid, the liquid reservoir has a fixed internal volume. Reduction in the internal pressure inside the liquid reservoir as liquid is removed can adversely affect the ability to pump liquid out of the reservoir. To prevent a significant pressure drop, the liquid reservoir has a breather air inlet valve 622. The breather valve 622 allows air into the liquid reservoir when the pressure difference between the inside of the reservoir and the outside of the reservoir exceeds a threshold pressure difference.

[0122] The pump can be activated in the same way as the heater. For example, the control circuit can supply power to the pump for the same period of time as power is supplied to the heater. Alternatively, the control circuit can supply power to the pump for a period immediately after the heater is activated.

[0123] The control circuit 614 includes a microcontroller 640 for controlling the pump 632 as well as the heater 630. The microcontroller 640 is connected to the heater 630 via a heater driver 642, and to the pump 632 via a pump driver 644. The microcontroller 640 is also connected to the microphone 615, and to a user-operable switch 646. The microcontroller 640 is also connected to a power supply 648, which in this example is a rechargeable battery. Figure 1The feedback loop shown. Pump 632 comprises a piezoelectric transducer that drives vibrations of a flexible diaphragm. As the diaphragm vibrates, it reduces the volume of the chamber, pushing liquid aerosol-forming substrate out of the pump chamber through the outlet valve, and increases the volume of the chamber, drawing liquid aerosol-forming substrate into the pump chamber through the inlet valve. To maximise the efficiency of pumping, it is advantageous to operate pump 632 at or close to the resonant frequency of the transducer. However, as described previously, the resonant frequency of the transducer can change for a variety of reasons.

[0124] Changes in the resonant frequency of the transducer caused by temperature changes, other environmental changes, or ageing can be monitored, and the drive signal modified accordingly using one of the feedback mechanisms described above.

[0125] Changes in the resonant frequency of the transducer due to a lack of liquid being drawn into the pump chamber can be detected as a sudden change in resonant behaviour, for example a change between two measurement periods that is greater than a predetermined threshold. If a sudden change in resonant behaviour is detected, operation of the pump and heater can be stopped until a new reservoir of liquid is placed in the device.

Claims

1. An aerosol-generating device comprising: a piezoelectric transducer; a drive circuit connected to the piezoelectric transducer and configured to apply an oscillating current to the transducer; and a control circuit connected to the drive circuit and configured to monitor the resonant behaviour of the piezoelectric transducer, the control circuit being configured to control the operation of the drive circuit based on the resonant behaviour of the piezoelectric transducer; wherein the piezoelectric transducer forms part of a transducer assembly in a liquid pump, and the transducer assembly comprises a membrane or surface configured to contact a liquid aerosol-forming substrate, the transducer assembly being configured to drive the membrane or surface to vibrate, the vibration of the membrane or surface forcing liquid through an adjacent liquid valve in the liquid pump.

2. An aerosol-generating device according to claim 1, wherein the control circuit is configured to control the operation of the drive circuit such that the oscillating current has a frequency equal to the resonant frequency of the piezoelectric transducer.

3. An aerosol-generating device according to claim 1, wherein the control circuit is configured to control the operation of the drive circuit such that the oscillating current has a frequency offset from the resonant frequency of the piezoelectric transducer.

4. An aerosol-generating device according to any one of claims 1 to 3, wherein the control circuit is configured to monitor the resonant behaviour of the piezoelectric transducer at a plurality of resonant frequencies of the piezoelectric transducer corresponding to different vibration modes.

5. An aerosol-generating device according to any one of claims 1 to 3, wherein the control circuit is configured to monitor the resonant behaviour of the piezoelectric transducer by measuring the power delivered to the piezoelectric transducer or the impedance of the piezoelectric transducer.

6. An aerosol-generating device according to any one of claims 1 to 3, wherein the drive circuit and the control circuit comprise a phase-locked loop.

7. An aerosol-generating device according to any one of claims 1 to 3, wherein the piezoelectric transducer is an aerosol-generating element configured to generate an aerosol from a liquid aerosol-forming substrate.

8. An aerosol-generating device according to claim 7, wherein the piezoelectric transducer comprises a perforated plate.

9. An aerosol-generating device according to any one of claims 1 to 3, comprising a liquid reservoir containing a liquid aerosol-forming substrate, wherein in use the piezoelectric transducer is in contact with liquid from the liquid reservoir.

10. An aerosol-generating device according to claim 9, wherein the liquid comprises a mixture of different compounds.

11. An aerosol-generating device according to claim 9, wherein the control circuit is configured to detect a reduction in the amount of liquid in contact with the piezoelectric transducer based on a change in the resonant behaviour of the piezoelectric transducer.

12. An aerosol-generating device according to any one of claims 1 to 3, wherein the aerosol-generating device is an electronic cigarette.

13. An aerosol-generating device according to any one of claims 1 to 3, wherein the oscillating current comprises a first frequency modulated with at least one other frequency. ​ 14. A method of operating an aerosol-generating device, the device comprising: A transducer assembly in a liquid pump, wherein the transducer assembly comprises a piezoelectric transducer and a membrane or surface configured to contact a liquid aerosol-forming substrate, the piezoelectric transducer being configured to drive the membrane or surface to vibrate, the vibration of the membrane or surface forcing liquid through an adjacent liquid valve in the liquid pump; a drive circuit connected to the piezoelectric transducer; and a control circuit configured to monitor a parameter of the piezoelectric transducer and connected to the drive circuit; the method comprising: applying an oscillating current to the transducer using the drive circuit; and monitoring the resonant behaviour of the piezoelectric transducer using the control circuit, and controlling the operation of the drive circuit based on the monitored resonant behaviour of the piezoelectric transducer.

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