CARTRIDGE WITH RESONANT CIRCUIT FOR AN AEROSOL-GENERATING DEVICE

Abstract

A cartridge for an aerosol-generating device is provided, the cartridge including: an aerosol-forming substrate; an electric heater configured to heat the aerosol-forming substrate; and a resonant circuit connected in parallel with the electric heater, the resonant circuit being configured to resonate at a predetermined resonant frequency, and the predetermined resonant frequency being associated with an identity of the cartridge. An aerosol-generating device and an aerosol generating system are also provided.

Claims

1.-16. (canceled)

17. A cartridge for an aerosol-generating device, the cartridge comprising: an aerosol-forming substrate; an electric heater configured to heat the aerosol-forming substrate; and a resonant circuit connected in parallel with the electric heater, wherein the resonant circuit is configured to resonate at a predetermined resonant frequency, and wherein the predetermined resonant frequency is associated with an identity of the cartridge.

18. The cartridge according to claim 17, wherein the resonant circuit comprises a capacitor and an inductor.

19. The cartridge according to claim 18, wherein the capacitor and inductor are connected in series.

20. The cartridge according to claim 18, wherein the predetermined resonant frequency of the resonant circuit is determined by a capacitance of the capacitor, and wherein the predetermined resonant frequency can be changed by changing the capacitance of the capacitor.

21. The cartridge according to claim 17, wherein the predetermined resonant frequency is in a range of 10 kHz to 100 MHz.

22. The cartridge according to claim 17, wherein the predetermined resonant frequency is in a range of 1 MHz to 11 MHz.

23. The cartridge according to claim 18, wherein a capacitance of the capacitor is in a range of 0.1 nF to 10 nF.

24. The cartridge according to claim 18, wherein the resonant circuit further comprises a plurality of capacitors arranged in parallel, and wherein a combined capacitance of the plurality of capacitors is used to produce resonance.

25. The cartridge according to claim 18, wherein the resonant circuit is arranged on a printed circuit-board (PCB), and wherein the inductor is formed directly on the PCB as a conductive track.

26. The cartridge according to claim 17, wherein the resonant circuit comprises a capacitor connected in parallel with the heater, and wherein the resonant circuit is further configured to use a parasitic inductance of the resonant circuit in combination with a capacitance of the capacitor to produce resonance.

27. The cartridge according to claim 26, wherein the predetermined resonant frequency is in a range of 100 kHz to 100 MHz.

28. The cartridge according to claim 26, wherein the predetermined resonant frequency is in a range of 1 MHz to 50 MHz.

29. The cartridge according to claim 26, wherein the capacitance of the capacitor is in a range of 1 nF to 300 nF.

30. The cartridge according to claim 17, wherein the resonant circuit is arranged to be connected to an alternating signal source and is configured to resonate when the predetermined resonant frequency substantially equals a frequency of the alternating signal.

31. An aerosol-generating device, comprising: a housing configured to receive a cartridge according to claim 17, wherein the housing comprises an electrical connection configured to electrically connect to the cartridge; a power source configured to supply electrical power to the electric heater of the cartridge; an alternating signal source configured to input an alternating signal to the resonant circuit of the cartridge; and control circuitry configured to control a supply of electrical power to the electric heater and to controllably vary a frequency of the alternating signal supplied to the resonant circuit, wherein the control circuitry is arranged to receive an output signal from the resonant circuit, and wherein the control circuitry is further configured to: determine when resonance occurs in the resonant circuit by detecting when the output signal reaches a predetermined threshold value, determine a frequency at which resonance occurs, and identify the cartridge based on the determined resonant frequency.

32. The aerosol-generating device according to claim 31, wherein the control circuitry is further configured to sweep the frequency of the alternating signal over a predetermined frequency range within a predetermined time period, and wherein the predetermined time period is 5 milliseconds or less.

33. The aerosol-generating device according to claim 31, wherein a peak voltage of the alternating signal supplied to the resonant circuit is 2 V or less.

34. The aerosol-generating device according to claim 31, wherein a peak voltage of the alternating signal supplied to the resonant circuit is 1 V or less.

35. An aerosol-generating system, comprising: a cartridge comprising: an aerosol-forming substrate, an electric heater configured to heat the aerosol-forming substrate, and a resonant circuit connected in parallel with the electric heater and being configured to resonate at a predetermined resonant frequency, the predetermined resonant frequency being associated with an identity of the cartridge; and an aerosol-generating device comprising: a housing configured to receive the cartridge, wherein the housing comprises an electrical connection configured to electrically connect to the cartridge, a power source configured to supply electrical power to the electric heater of the cartridge, an alternating signal source configured to input an alternating signal to the resonant circuit of the cartridge, and control circuitry configured to control a supply of electrical power to the electric heater and to controllably vary a frequency of the alternating signal supplied to the resonant circuit, wherein the control circuitry is arranged to receive an output signal from the resonant circuit, and wherein the control circuitry is further configured to: determine when resonance occurs in the resonant circuit by detecting when the output signal reaches a predetermined threshold value, determine a frequency at which resonance occurs, and identify the cartridge based on the determined resonant frequency.

Description

[0102] Examples will now be further described with reference to the figures in which:

[0103] FIG. 1 is a schematic illustration of an example aerosol-generating system comprising a cartridge and an aerosol-generating device.

[0104] FIG. 2 is a block diagram showing the main electric and electronic components of an example aerosol-generating system.

[0105] FIG. 3A shows a schematic circuit diagram of an example cartridge comprising a resonant circuit, in which the example cartridge is connected to a DC voltage source.

[0106] FIG. 3B shows a schematic circuit diagram of the cartridge of FIG. 3A, in which the cartridge is connected to an alternating signal source.

[0107] FIG. 4 is a graph of frequency versus voltage showing the frequency response of the resonant circuit of FIG. 3B when using different capacitor values.

[0108] FIG. 5 is a schematic diagram of an example circuit for an aerosol-generating system for determining the resonant frequency of a resonant circuit of a cartridge. The cartridge is the example cartridge of FIGS. 3A and 3B.

[0109] FIGS. 6A to 6C are graphs of voltage versus time showing the detection of resonance for differing values of capacitor in a resonant circuit.

[0110] FIG. 7 is a plan view of a printed circuit board having a resonant circuit thereon.

[0111] FIG. 8 shows a schematic circuit diagram of another example cartridge comprising another resonant circuit, in which the example cartridge is connected to an alternating signal source.

[0112] FIG. 9 is a graph of frequency versus voltage showing the frequency response of the resonant circuit of FIG. 8 when using different capacitor values.

[0113] FIG. 10 shows a schematic circuit diagram of yet another example cartridge comprising a another resonant circuit, in which the example cartridge is connected to an alternating signal source.

[0114] FIG. 11 is a graph of frequency versus voltage showing the frequency response of the resonant circuit of FIG. 10 when using different capacitor values.

[0115] FIG. 12 is a schematic diagram of an example circuit for an aerosol-generating system for determining the resonant frequency of a resonant circuit of a cartridge. The cartridge is the example cartridge of FIG. 10.

[0116] FIGS. 13A to 13C are graphs of voltage versus time showing the detection of resonance for differing values of capacitor in the resonant circuit of FIG. 10.

[0117] FIG. 1 is a schematic illustration of an example aerosol-generating system 10. The aerosol-generating system 10 comprises two main components, a cartridge 100 and a main body part or aerosol-generating device 200. A connection end 115 of the cartridge 100 is removably connected to a corresponding connection end 205 of the aerosol-generating device 200. The connection end 115 of the cartridge 100 and connection end 205 of the aerosol-generating device 200 each have electrical contacts or connections (not shown) which are arranged to cooperate to provide an electrical connection between the cartridge 100 and the aerosol-generating device 200. The aerosol-generating device 200 contains a power source in the form of a battery 210, which in this example is a rechargeable lithium ion battery, and control circuitry 220. The aerosol-generating system is portable and has a size comparable to a conventional cigar or cigarette. A mouthpiece 125 is arranged at the end of the cartridge 100 opposite the connection end 115.

[0118] The cartridge 100 comprises a housing 105 containing an electric heater 120 and a liquid storage compartment having a first portion 130 and a second portion 135. A liquid aerosol-forming substrate is held in the liquid storage compartment. Although not illustrated in FIG. 1, the first portion 130 of the liquid storage compartment is connected to the second portion 135 of the liquid storage compartment so that liquid in the first portion 130 can pass to the second portion 135. The electric heater 120 receives liquid from the second portion 135 of the liquid storage compartment. In this embodiment, the electric heater 120 comprises a fluid permeable heating element, for example, a mesh heater. The cartridge 100 further comprises a resonant circuit 155 mounted on a printed circuit board (PCB) which is arranged to the side of the second portion 135 of the liquid storage compartment and is connected via conductors (not shown) in parallel with the heater 120.

[0119] An air flow passage 140, 145 extends through the cartridge 100 from an air inlet 150 formed in a side of the housing 105 past the heater 120 and from the heater 120 to a mouthpiece opening 110 formed in the housing 105 at an end of the cartridge 100 opposite to the connection end 115.

[0120] The components of the cartridge 100 are arranged so that the first portion 130 of the liquid storage compartment is between the heater 120 and the mouthpiece opening 110, and the second portion 135 of the liquid storage compartment is positioned on an opposite side of the heater 120 to the mouthpiece opening 110. In other words, the heater 120 lies between the two portions 130, 135 of the liquid storage compartment and receives liquid from the second portion 135. The first portion 130 of the liquid storage compartment is closer to the mouthpiece opening 110 than the second portion 135 of the liquid storage compartment. The air flow passage 140, 145 extends past the heater 120 and between the first 130 and second 135 portions of the liquid storage compartment.

[0121] The aerosol-generating system 10 is configured so that a user can puff or draw on the mouthpiece 125 of the cartridge to draw aerosol into their mouth through the mouthpiece opening 110. In operation, when a user puffs on the mouthpiece 125, air is drawn through the airflow passage 140, 145 from the air inlet 150, past the heater 120, to the mouthpiece opening 110. The control circuitry 220 controls the supply of electrical power from the battery 210 to the cartridge 100 when the system is activated. This in turn controls the amount and properties of the vapour produced by the heater 120. The control circuitry 220 may include an airflow sensor (not shown) and the control circuitry 220 may supply electrical power to the heater 120 when user puffs on the cartridge 100 are detected by the airflow sensor. This type of control arrangement is well established in aerosol-generating systems such as inhalers and e-cigarettes. So when a user puffs on the mouthpiece opening 110 of the cartridge 100, the heater 120 is activated and generates a vapour that is entrained in the air flow passing through the air flow passage 140. The vapour cools within the airflow in passage 145 to form an aerosol, which is then drawn into the user's mouth through the mouthpiece opening 110.

[0122] In operation, the mouthpiece opening 110 is typically the highest point of the system. The construction of the cartridge 100, and in particular the arrangement of the heater 120 between first and second portions 130, 135 of the liquid storage compartment, is advantageous because it exploits gravity to ensure that the liquid substrate is delivered to the heater 120 even as the liquid storage compartment is becoming empty, but prevents an oversupply of liquid to the heater 120 which might lead to leakage of liquid into the air flow passage 140.

[0123] FIG. 2 is a block diagram showing the main electric and electronic components of an example aerosol-generating system 10 comprising a cartridge 100 and an aerosol-generating device 200. The cartridge 100 comprises an electric heater 120 and a resonant circuit 155. The resonant circuit 155 is configured to resonate at a predetermined resonant frequency, which resonant frequency is associated with an identity of the cartridge 100 or the aerosol-forming substrate (not shown) contained within the cartridge 100. By determining the resonant frequency of the resonant circuit 155, the aerosol-generating device 200 is able to identify the cartridge 100 and its contents and apply appropriate aerosolisation conditions. For example, the aerosol-generating device 200 may apply a suitable heating profile for the particular liquid aerosol-forming substrate contained in the cartridge 100.

[0124] The resonant circuit 155 is connected in parallel across the electric heater 120. By connecting the resonant circuit 155 in parallel with the heater 120, only two electrical connections 242 are required to connect the cartridge 100 to the aerosol-generating device 200. The two electrical connections 242 can be used to supply power to the heater 120, to provide an input alternating signal to the resonant circuit 155 and to receive an output signal from the resonant circuit 155. Different example resonant circuits of the present disclosure are described in more detail below.

[0125] The aerosol-generating device 200 comprises a battery 210, which acts as a power source, and a microcontroller (MCU) 230, which forms part of the control circuitry of the aerosol-generating device 200. The microcontroller 230 is configured to control the supply of electrical power to the electric heater 120. The microcontroller 230 controls the supply of a DC voltage source 236 to the heater 120. The microcontroller 230 modulates the DC voltage source 236 through pulse width modulation (PWM) to provide power to the electric heater120 as a series of pulses. The DC voltage source 236 can be selectively connected to the electric heater 120 by a switch 240, which may be a transistor or other suitable electronic switch. No passive components which can generate heat, such as resistors or inductors, are connected in series between the DC voltage source and the electric heater 120. This helps to reduce energy losses.

[0126] The microcontroller 230 also controls the provision of an alternating signal source or AC source 234 to the cartridge 100, in particular as an input signal to the resonant circuit 155. The microcontroller 230 is able to vary or sweep the frequency of the alternating signal supplied to the resonant circuit 155 over a frequency range containing the resonant frequency of the resonant circuit 155. A resistor 238 is arranged within the aerosol-generating device so that it is connected in series between the alternating signal source 234 and the resonant circuit 155 in the cartridge 100. The resistor 238 forms part of a potential divider with the components of the resonant circuit 155 and allows a measurement voltage to be taken off the circuit at a point X between the resistor 238 and resonant circuit 155.

[0127] Although FIG. 2 shows the alternating signal source or AC source 234 and the DC voltage source 236 as separate blocks in the diagram, this has been done solely for clarity and in practice both of these sources are provided by the microcontroller 230, potentially with a few ancillary components such as transistors for sourcing larger currents. However, it will be appreciated that in other examples, separate AC and DC sources may be provided.

[0128] The aerosol-generating device 200 further comprises a peak detection circuit 232 which forms a further part of the control circuitry of the aerosol-generating device 200. The peak detection circuit 232 receives an output signal from the resonant circuit 155 and provides its own output to the microcontroller 230. To receive the output signal from the resonant circuit 155, the peak detection circuit 232 measures the voltage at point X between the resistor 238 and resonant circuit 155. The peak detection circuit 232 is able to determine or measure the peak amplitude that occurs in the output signal from the resonant circuit 155 or when the output signal reaches a predetermined threshold value. As mentioned above, when the resonant circuit is resonating at its resonant frequency, the output signal oscillates with a greater amplitude than at other frequencies. Therefore, the microcontroller 230 varies or sweeps the frequency of the alternating signal supplied to the resonant circuit 155 over a predetermined frequency range within which the resonant frequency is expected to be and monitors at what frequency the output signal has its greatest amplitude to determine the resonant frequency of the resonant circuit 155 and identify the cartridge 100.

[0129] Depending on the configuration of the resonant circuit 155 and the point at which its output signal is measured, it is possible that the output signal may have a minimum amplitude at resonance. Therefore, the peak detection circuit 232 is also able to measure the minimum amplitude that occurs in the output signal in order to determine the resonant frequency.

[0130] FIG. 3A shows a schematic diagram of an example cartridge 100 comprising a resonant circuit 155. The resonant circuit 155 comprises a capacitor C1 connected in series with an inductor L1. The resonant circuit 155 is arranged in parallel with the heater 120. The heater is a resistive heater and is therefore represented in FIG. 3A as a resistor RH. The resistance of the heater 120 is 0.69 Ohms. The cartridge 100 is connected to a DC voltage source V1 which provides a pulse width modulated DC voltage across the parallel arrangement of the heater 120 and resonant circuit 155. The pulse width modulation is controlled by a microcontroller (not shown). In the described example, the pulse width modulated DC voltage has an amplitude of 3.6 Volts, a period of 10 milliseconds and a duty cycle of 50 percent. This results in a pulsed current in the heater 120 of approximately 5.2 Amps.

[0131] When the cartridge 100 is connected to the DC voltage source V1, current flows only in the resistive heater 120. No current flows in the resonant circuit 155 because it is arranged in parallel with the heater 120 and the capacitor C1 in the resonant circuit blocks DC voltage, that is, it effectively acts as an open circuit for DC voltage. Thus, power is only dissipated in the heater 120 and not in the resonant circuit 155, which makes the arrangement energy efficient.

[0132] The DC voltage source V1 can be controlled to control the heating profile which is applied to a particular cartridge. Once the cartridge 100 has been identified in accordance with the procedures described below, a suitable heating profile for the particular liquid aerosol-forming substrate contained in the cartridge 100 can be applied. For different cartridges, the heating profile can be varied by varying the characteristics of the pulse width modulated DC voltage applied to the cartridge. For example, the duty cycle of the pulse width modulated DC voltage or the amount of time the pulse width modulated DC voltage is applied can be varied.

[0133] FIG. 3B shows a schematic circuit diagram of the cartridge 100 of FIG. 3A in which the cartridge is connected to an alternating voltage source or alternating signal source V2 that inputs an alternating signal to the resonant circuit 155. The alternating signal source V2 is controlled by a microcontroller (not shown) and has an amplitude of 1 Volt peak. The microcontroller is able to vary or sweep the frequency of the alternating signal of the alternating signal source V2 in order to detect resonance and therefore determine the identity of the cartridge. The frequency can be swept in the range 1 megahertz to 13 megahertz and due to the relatively high frequencies being used, it was found that the frequency sweep could be carried out in a relatively short period of time, that is, 240 microseconds. When the frequency of the input alternating signal equals the natural resonant frequency of the resonant circuit 155, the resonant circuit 155 resonates.

[0134] The resonant circuit 155 is configured to resonate at a predetermined resonant frequency to allow the cartridge 100 to be identified. As set out in Equation (1) above, the resonant frequency is a function of the capacitance of the capacitor C1 and the inductance of the inductor L1. In this described example, the predetermined resonant frequency of the resonant circuit 155 is determined by the capacitance of the capacitor C1. Different capacitors having different capacitance values can be used to produce different resonant frequencies for different cartridges. The inductance of the inductor L1 is fixed at 1 microhenry. To produce different resonant frequencies, ten different capacitor values taken from the E12 series of capacitor values were used for capacitor C1. The C1 capacitance values and the resulting resonant frequencies are shown in Table 1.

TABLE-US-00001 TABLE 1 Capacitance (nanofarads) 8.2 5.6 3.9 2.7 1.8 1.2 0.82 0.56 0.39 0.27 Resonant freq. 1.76 2.13 2.56 3.07 3.76 4.61 5.57 6.74 8.08 9.71 (megahertz)

[0135] As can be seen from Table 1, the inventors were able to achieve ten different resonant frequencies with sufficient spacing between the frequencies to clearly distinguish ten different cartridges. However, it will be appreciated that more resonant frequencies can be achieved by using more capacitor values.

[0136] FIG. 4 is a graph of frequency versus voltage showing the frequency response of the resonant circuit 155 of FIG. 3B. In FIG. 4, there is a frequency response curve for each of the different capacitor values of Table 1. The output signal for resonant circuit 155 was measured at point X in the circuit of FIG. 3B, that is, between resistor R1 and the resonant circuit 155. Resistor R1 forms part of a potential divider together with resonant circuit 155 so that a voltage can be measured at point X. Resonance occurs when the reactance of C1 and the reactance of L1 are equal in magnitude but opposite in phase such that the two reactances cancel each other. Therefore, at resonance, the impedance of the resonant circuit is at a minimum and hence the voltage measured at point X is a minimum at resonance. For each of the frequency response curves in FIG. 4, a voltage minimum Vmin can be seen occurring at each of the resonant frequencies in Table 1, which correspond to the different capacitor values used. The voltage minimums occur over a relatively small section of the entire length of the frequency response curves and are therefore easily discernible and detectable.

[0137] The minimum voltages can be detected by a peak detection circuit. The frequency at which the minimum voltage is detected provides an indication of the resonant frequency of the resonant circuit and the identity of the cartridge. Alternatively, the resonant frequency can be determined by configuring an aerosol-generating device to detect when the output signal drops below a threshold voltage Vth denoted by a horizontal dotted line in FIG. 4.

[0138] Referring again to FIG. 3B, when alternating signal source V2 is connected to the cartridge 100 to detect resonance and determine the identify the cartridge, the alternating signal splits between the heater 120 and the resonant circuit 155. In contrast to a DC voltage, an alternating signal is able to pass through the capacitor C1 of the resonant circuit. During cartridge identification, the peak current flowing through the heater 120 was measured as being approximately 100 milliamps. As mentioned above, the alternating signal source V2 has an amplitude of 1 Volt peak. Therefore, the power consumed by the heater 120 during cartridge identification can be determined from Equation (2) below.

P=I.sub.RMS×V.sub.RMS  (2)

[0139] where P is power, I.sub.RMS ms is the root mean square current which is equal to 0.707×peak current and V.sub.RMS is root mean square voltage which is equal to 0.707×peak voltage.

[0140] Furthermore, the energy consumed by the heater 120 during cartridge identification can be determined from Equation (3) below.

E=P×t  (3)

[0141] where E is energy, P is power and t is time or duration of operation.

[0142] Therefore, based on a peak current of 100 milliamps and a peak voltage of 1 Volt, the power consumed by the heater 120 during cartridge identification can be calculated as being 50 milliwatts. Furthermore, based on an operation time for cartridge identification of 240 microseconds, the energy consumed by the heater 120 during cartridge identification can be calculated as being 12 microjoules. Such a small amount of energy will not heat the heater to any appreciable extent and therefore the energy efficiency of cartridge identification is improved by avoiding energy losses in the heater 120.

[0143] Due to the short period of time required to perform cartridge identification, it could be performed during the voltage-off time of a pulse width modulated DC voltage used to power the heater. The pulse width modulated DC voltage of DC voltage source V1 (see FIG. 3A) has a period of 10 milliseconds and a duty cycle of 50 percent. The voltage-off time is therefore 5 milliseconds, which is ample time to perform cartridge identification because cartridge identification only takes 240 microseconds.

[0144] FIG. 5 is a schematic diagram of an example circuit for an aerosol-generating system 10 for determining the resonant frequency of a resonant circuit 155 of a cartridge 100. The cartridge 100 is the example cartridge of FIGS. 3A and 3B. The lower part of the circuit of FIG. 5 shows a cartridge comprising a resonant circuit 155 and a heater 120 connected to an alternating signal source V2 in order to identify the cartridge 100. The arrangement and operation of this part of the circuit of FIG. 5 is the same as the circuit shown in FIG. 3B and for conciseness is not repeated here.

[0145] The circuit of FIG. 5 further comprises a peak detection circuit 232 for detecting the maximum or minimum amplitude of the output signal from the resonant circuit 155. The peak detection circuit 232 is arranged within the aerosol-generating device 200 of aerosol-generating system 10. The peak detection circuit 232 comprises an operational amplifier U5 for amplifying a signal input to the peak detection circuit 232, a forward biased diode D1 for half-way rectifying an input alternating signal and a capacitor C2 for holding or storing a voltage of the signal received from the diode. The non-inverting input (+) of the operational amplifier acts as an input to the peak detection circuit 232 and an output from the peak detection circuit 232 is taken from an upstream terminal of the capacitor C2, that is, the terminal of the capacitor C2 connected to the diode D1. The peak detection circuit 232 further comprises a resistor R2 having a value of 10 ohms for discharging of the capacitor to ground.

[0146] Any suitable operational amplifier may be used. For example, the described example uses an LTC6244 operational amplifier manufactured by Analog Devices of Massachusetts, USA. The operational amplifier U5 is powered by a DC voltage source V3, which provides 3.1 volts. Two resistors, R3 and R4 which have values of 150 kilo-ohms and 10 kilo-ohms respectively, are provided as part of a negative feedback loop for the operational amplifier. The gain of the amplifier can be determined in accordance with Equation (4) below:

Gain=1+R3/R4  (4)

[0147] In use, the output signal from the resonant circuit 155 is taken at point X in the circuit, i.e. the mid-point of the voltage divider formed by resistor R1 and the resonant circuit 155 and fed as an input to the non-inverting input of the operational amplifier U5 of the peak detection circuit 232. The signal is half-wave rectified by diode D1 to form a series of positive pulses and the voltage of successive pulses is held or stored by capacitor C2. The output of the peak detection circuit 232, that is, the voltage stored by capacitor C2, is fed to an analogue to digital converter input of the microcontroller 230 of the aerosol-generating device 200, which periodically measures or samples the voltage stored by capacitor C2. Once the voltage has been sampled the capacitor C2 is discharged to electrical ground in preparation for taking the next sample.

[0148] The microcontroller 230 samples the output from the peak detection circuit over the time period taken to sweep the frequency of the alternating signal over the desired frequency range, which in this case is 1 megahertz to about 10.6 megahertz and takes about 240 microseconds. In this manner, the microcontroller 230 obtains a profile of the amplitude of the output signal from the resonant circuit over the range of swept frequencies. The samples are analysed by the microcontroller 230 to determine a minimum value. Since the microcontroller 230 also controls alternating signal source V2 and the frequency of the alternating signal being provided to the resonant circuit 155 of the cartridge 100, the microcontroller 230 is able to determine the frequency at which the minimum value of the output from the peak detection circuit 232 was detected. This frequency is the frequency at which resonance occurred in the resonant circuit 155 and indicates the identity of the cartridge.

[0149] FIGS. 6A to 6C are graphs of voltage versus time showing the detection of resonance for different values of capacitor in the resonant circuit 155 in the cartridge 100 of FIG. 5. The graphs show two curves: curve X which corresponds to the output signal from the resonant circuit 155 measured at point X in the circuit of FIG. 5; and curve Y which corresponds to the output from the peak detection circuit 232 at point Y in the circuit of FIG. 5.

[0150] FIG. 6A shows the output signals from the resonant circuit 155 (curve X) and peak detection circuit 232 (curve Y) when a capacitor value of 8.2 nanofarads is used in the resonant circuit 155. As shown in Table 1 above, this capacitance value produces a resonant frequency of 1.76 megahertz. The graph of FIG. 6A shows a voltage minimum Vmin occurring relatively early in curve Y, that is at about 20 microseconds into the frequency sweep. The voltage minimum indicates the occurrence of resonance and the detection time is consistent with the resonant frequency being 1.76 megahertz and the frequency being swept over the range 1 megahertz to about 10.6 megahertz in a time period of 240 microseconds.

[0151] FIG. 6B shows the output signals from the resonant circuit 155 (curve X) and peak detection circuit 232 (curve Y) when a capacitor value of 0.82 nanofarads is used in the resonant circuit 155. As shown in Table 1 above, this capacitance value produces a resonant frequency of 5.57 megahertz. The graph of FIG. 6B shows a voltage minimum Vmin occurring around the mid-point of curve Y, that is, at about 120 microseconds into the frequency sweep. The voltage minimum indicates the occurrence of resonance and the detection time is consistent with the resonant frequency being 5.57 megahertz and the frequency being swept over the range 1 megahertz to about 10.6 megahertz in a time period of 240 microseconds.

[0152] FIG. 6C shows the output signals from the resonant circuit 155 (curve X) and peak detection circuit 232 (curve Y) when a capacitor value of 0.27 nanofarads is used in the resonant circuit 155. As shown in Table 1 above, this capacitance value produces a resonant frequency of 9.71 megahertz. The graph of FIG. 6C shows a voltage minimum Vmin occurring relatively late in curve Y, that is, at about 220 microseconds into the frequency sweep. The voltage minimum indicates the occurrence of resonance and the detection time is consistent with the resonant frequency being 9.71 megahertz and the frequency being swept over the range 1 megahertz to about 10.6 megahertz in a time period of 240 microseconds.

[0153] FIG. 7 is a plan view of a printed circuit board 160 having a resonant circuit 155 arranged thereon. The resonant circuit 155 comprises an inductor L1 and a capacitor C2 connected in series. The inductor L1 is formed directly on the printed circuit board 160 as a conductive track. The inductor can be formed by any suitable method, for example, by printing an electrically conductive material on to the printed circuit board 160 or by etching a copper-clad board to form the pattern of the inductor L1. The inductor L1 comprises 15 turns (a number of turns are omitted from FIG. 7 for clarity) and the printed circuit board 160 is double-sided with half the turns being formed on one side of the printed circuit board 160 and the other half of the turns being formed on the other side. Conductive vias 164 connect the respective ends of the turns printed on each side of the printed circuit board 160. Electrically conductive contact pads 162 are formed at opposing ends of the printed circuit board 160 which can be used to connect the resonant circuit 155 in parallel across an electric heater and to an alternating signal source. One end of the inductor L1 is connected to one of the contact pads 162 and a terminal of the capacitor C2 is connected to the other contact pad 162. The dimensions of the printed circuit board 160 are 9×7×0.6 millimetres and therefore can easily fit within a cartridge or mouthpiece of an aerosol-generating system.

[0154] FIG. 8 shows a schematic circuit diagram of another example cartridge 300 comprising another resonant circuit 355. The cartridge 300 is connected to an alternating signal source V2 in order to identify the cartridge 300. The arrangement and operation of the circuit of FIG. 8 is the same as the circuit of FIG. 3B with the exception that, instead of using a single capacitor C1 in the resonant circuit 155, the resonant circuit 355 of FIG. 8 uses two capacitors C1 and C2 in parallel. The parallel arrangement of capacitors C1 and C2 helps to improve the frequency response of the resonant circuit 355.

[0155] As mentioned above, all real electronic components have parasitic elements, that is, unavoidable characteristics in addition to the intended characteristic of the component. For example, the capacitor C1 and inductor L1 in the circuit of FIG. 3B each have a parasitic resistance which is equivalent to a resistance of 0.1 ohms in series with the capacitor C1 and a resistance of 1 ohm in series with the inductor L1. These parasitic elements result in energy losses within the circuit and therefore need to be minimised.

[0156] The inventors have found that using two capacitors C1 and C2 in parallel reduces parasitic elements in the resonant circuit 355. In particular, the capacitor equivalent series resistance is reduced to 0.05 ohms. Furthermore, when capacitors are added in parallel their capacitances are summed. Therefore, by using two of the same capacitor in parallel, the inductance can be halved for the same resonant frequency. Consequently, a smaller inductor can be used which saves on printed circuit board area. The resonant circuit 355 uses an inductor having an inductance of 0.5 microhenries.

[0157] To produce the same resonant frequencies as were achieved for the circuit in FIG. 3B, the parallel arrangement of capacitors C1 and C2 in resonant circuit 355 of FIG. 8 uses two of each the capacitor values shown in Table 1 above so that the capacitance is double that shown in Table 1. The capacitance values and resulting resonant frequencies are shown in Table 2 below.

TABLE-US-00002 TABLE 2 Capacitance (nanofarads) 16.4 11.2 7.8 5.4 3.6 2.4 1.64 1.12 0.78 0.54 Resonant freq. 1.76 2.13 2.56 3.07 3.76 4.61 5.57 6.74 8.08 9.71 (megahertz)

[0158] As can be seen from Table 2, the parallel arrangement of two capacitors C1 and C2 achieves the same resonant frequencies as Table 1 when an inductor of 0.5 microhenries is used.

[0159] When the cartridge 300 of FIG. 8 is connected to a DC voltage source (not shown), it functions in the same way as the cartridge 100 of FIG. 3A. That is, capacitors C1 and C2 block DC voltage so that current passes only through the heater 320.

[0160] FIG. 9 is a graph of frequency versus voltage showing the frequency response of the resonant circuit 355 of FIG. 8. In FIG. 9, there is a frequency response curve for each of the different capacitance values of Table 2. The output signal for resonant circuit 355 was measured at point X in the circuit of FIG. 8, that is, between resistor R1 and the resonant circuit 355. The graph is very similar to the graph of FIG. 4 and shows voltage minimums Vmin at the same frequencies as in FIG. 4, which correspond to the resonant frequencies of the different capacitance values in Table 2.

[0161] However, FIG. 9 shows an improved frequency response for resonant circuit 355 of FIG. 8 compared to the frequency response for the resonant circuit 155 of FIG. 3B shown in FIG. 4. That is, the magnitude of the output signal from the resonant circuit 355 is improved in resonant circuit 355 compared to resonant circuit 155. As can be seen in FIG. 9, the frequency response curves have lower voltage minimums Vmin than the frequency response curves in FIG. 4 for a particular resonant frequency.

[0162] FIG. 10 shows a schematic circuit diagram of another example cartridge 400 comprising another resonant circuit 455. The cartridge 400 is connected to an alternating signal source V2 in order to identify the cartridge 400. The arrangement and operation of the circuit of FIG. 10 is the same as the circuits of FIGS. 3B and 8 with the exception that the resonant circuit 455 does not use an inductor, in particular it does not use an actual discrete inductor component. Instead, the resonant circuit 455 uses a parasitic inductance L1 of the resonant circuit in combination with a capacitor C1 to produce resonance. The parasitic inductance L1 is denoted in FIG. 10 with a dotted line to emphasise that it is not an actual component but is instead a characteristic of the resonant circuit 455.

[0163] The parasitic inductance L1 arises as a result of the geometry of the resonant circuit 455 which forms a half-loop or half-turn when arranged in parallel with the heater 420. The half-loop generates a small parasitic inductance L1. The parasitic inductance is equivalent to an inductance of 10 nanohenries arranged in series with the capacitor C1, as shown in FIG. 10.

[0164] Parasitic inductances are relatively small compared to the inductance of actual inductor components and consequently the resonant frequencies they produce are generally higher than the previous examples. The resonant frequency is in the range 1 megahertz to 50 megahertz and to produce resonant frequencies in this range higher capacitor values are used. To produce different resonant frequencies, ten different capacitor values taken from the E12 series of capacitor values were used for capacitor C1. The C1 capacitance values and the resulting resonant frequencies are shown in Table 3 below.

TABLE-US-00003 TABLE 3 Capacitance (nanofarads) 82 56 39 27 18 12 8.2 5.6 3.9 2.7 Resonant freq. 5.6 6.8 8.1 9.7 11.9 14.6 17.6 21.3 25.5 30.7 (megahertz)

[0165] As can be seen from Table 3, the inventors were able to achieve ten different resonant frequencies to distinguish ten different cartridges.

[0166] When the cartridge 400 of FIG. 10 is connected to a DC voltage source (not shown), it functions in the same way as the cartridge 100 of FIG. 3A. That is, capacitor C1 blocks DC voltage so that current passes only through the heater 420.

[0167] FIG. 11 is a graph of frequency versus voltage showing the frequency response of the resonant circuit 455 of FIG. 10. In FIG. 11, there is a frequency response curve for each of the different capacitance values of Table 3. The output signal for resonant circuit 355 was measured at point X in the circuit of FIG. 10, that is, between resistor R1 and the resonant circuit 455. For each of the frequency response curves in FIG. 11, a voltage minimum Vmin can be seen occurring at each of the resonant frequencies in Table 3, which correspond to the different capacitor values used. The voltage minimums occur over a relatively small section of the entire length of the frequency response curves and are therefore easily discernible and detectable.

[0168] The minimum voltages can be detected by a peak detection circuit. The frequency at which the minimum voltage is detected provides an indication of the resonant frequency of the resonant circuit and the identity of the cartridge. Alternatively, the resonant frequency can be determined by configuring an aerosol-generating device to detect when the output signal drops below a threshold voltage Vth denoted by a horizontal dotted line in FIG. 11.

[0169] FIG. 12 is a schematic diagram of an example circuit for an aerosol-generating system 10 for determining the resonant frequency of a resonant circuit 455 of a cartridge 400. The circuit of FIG. 12 is the same as that in FIG. 5 with the exception that the cartridge 400 is the example cartridge of FIG. 10. The lower part of the circuit of FIG. 12 shows a cartridge comprising the resonant circuit 455 and a heater 420 connected to an alternating signal source V2 in order to identify the cartridge 400. The arrangement and operation of this part of the circuit of FIG. 12 is the same as the circuit shown in FIG. 10 and, for conciseness, is not repeated here.

[0170] The circuit of FIG. 12 also comprises a peak detection circuit 232 for detecting the maximum or minimum amplitude of the output signal from the resonant circuit 455. The arrangement and operation of the peak detection circuit 232 is identical to that in FIG. 5 and therefore, for conciseness, is not repeated here.

[0171] The microcontroller 230 samples the output from the peak detection circuit over the time period taken to sweep the frequency of the alternating signal over the desired frequency range, which in this case is 1 megahertz to about 40 megahertz and takes about 1 millisecond. In this manner, the microcontroller 230 obtains a profile of the amplitude of the output signal from the resonant circuit over the range of swept frequencies. The samples are analysed by the microcontroller 230 to determine a minimum value. Since the microcontroller 230 also controls alternating signal source V2 and the frequency of the alternating signal being provided to the resonant circuit 455 of the cartridge 400, the microcontroller 230 is able to determine the frequency at which the minimum value of the output from the peak detection circuit 232 was detected. This frequency is the frequency at which resonance occurred in the resonant circuit 455 and indicates the identity of the cartridge.

[0172] FIGS. 13A to 13C are graphs of voltage versus time showing the detection of resonance for different values of capacitor in the resonant circuit 455 in the cartridge 400 of FIG. 12. The graphs show two curves: curve X which corresponds to the output signal from the resonant circuit 455 measured at point X in the circuit of FIG. 12; and curve Y which corresponds to the output from the peak detection circuit 232 at point Y in the circuit of FIG. 12.

[0173] FIG. 12A shows the output signals from the resonant circuit 455 (curve X) and peak detection circuit 232 (curve Y) when a capacitor value of 82 nanofarads is used in the resonant circuit 455. As shown in Table 3 above, this capacitance value produces a resonant frequency of 5.6 megahertz. The graph of FIG. 12A shows a voltage minimum Vmin occurring relatively early in curve Y, that is at about 0.1 milliseconds into the frequency sweep. The voltage minimum indicates the occurrence of resonance and the detection time is consistent with the resonant frequency being 5.6 megahertz and the frequency being swept over the range 1 megahertz to about 40 megahertz in a time period of 1 milliseconds.

[0174] FIG. 12B shows the output signals from the resonant circuit 455 (curve X) and peak detection circuit 232 (curve Y) when a capacitor value of 8.2 nanofarads is used in the resonant circuit 455. As shown in Table 3 above, this capacitance value produces a resonant frequency of 17.6 megahertz. The graph of FIG. 12B shows a voltage minimum Vmin occurring around the mid-point of curve Y, that is, between about 0.4 and 0.5 milliseconds into the frequency sweep. The voltage minimum indicates the occurrence of resonance and the detection time is consistent with the resonant frequency being 17.6 megahertz and the frequency being swept over the range 1 megahertz to about 40 megahertz in a time period of 1 milliseconds.

[0175] FIG. 12C shows the output signals from the resonant circuit 455 (curve X) and peak detection circuit 232 (curve Y) when a capacitor value of 2.7 nanofarads is used in the resonant circuit 455. As shown in Table 3 above, this capacitance value produces a resonant frequency of 30.7 megahertz. The graph of FIG. 12C shows a voltage minimum Vmin occurring relatively late in curve Y, that is, between 0.7 and 0.8 milliseconds into the frequency sweep. The voltage minimum indicates the occurrence of resonance and the detection time is consistent with the resonant frequency being 30.7 megahertz and the frequency being swept over the range 1 megahertz to about 40 megahertz in a time period of 1 milliseconds.

[0176] For the purpose of the present description and of the appended claims, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term “about”. Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein. In this context, therefore, a number A is understood as A±5 percent of A. Within this context, a number A may be considered to include numerical values that are within general standard error for the measurement of the property that the number A modifies. The number A, in some instances as used in the appended claims, may deviate by the percentages enumerated above provided that the amount by which A deviates does not materially affect the basic and novel characteristic(s) of the claimed invention. Also, all ranges include the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein.