Abstract:
A nebulizer has an aperture plate, a mounting, an actuator, and an aperture plate drive circuit ( 2 - 4 ). A controller measures an electrical drive parameter at each of a plurality of measuring points, each measuring point having a drive frequency; and based on the values of the parameter at the measuring points makes a determination of optimum drive frequency and also an end-of-dose prediction. The controller performs a short scan at regular sub-second intervals at which drive current is measured at two measuring points with different drive frequencies. According to drive parameter measurements at these points the controller determines if a full scan sweeping across a larger number of measuring points should be performed. The full scan provides the optimum drive frequency for the device and also an end of dose indication.

Description:
INTRODUCTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention relates to aerosol generators or nebulizers, having vibrating aperture plates to aerosolize medicaments. 
         [0003]    2. Prior Art Discussion 
         [0004]    Examples of nebulizers are described in U.S. Pat. No. 6,546,927 (Litherland) and EP1558315 (PARI). These disclosures include techniques to predict the end of a liquid medicament dose on the aperture plate. These techniques are based on monitoring a parameter such as drive current at different drive frequencies. 
         [0005]    The approach taught in the Litherland document involves detecting the present desired frequency, and the desired frequency may for example be the resonant frequency. Frequency is the variable that is swept, and a parameter is tracked. The tracking provides an indication of the desired drive frequency. This may change as the dose decreases in volume. For example, the resonant frequency may drift from 135 kHz to 140 kHz from a loaded state to an unloaded state. In Litherland the device can reduce power supplied to the aperture plate and/or provide a user indication as the dose is ending. 
         [0006]    The present invention is directed towards providing improved frequency control. Specifically, it is desired to improve any one or all of faster frequency control response to changes in conditions, less impact on the nebulisation process, and improved ability to accurately predict end-of-dose when used with patients who inhale medication at different rates. 
       SUMMARY OF THE INVENTION  
       [0007]    According to the invention, there is provided a nebulizer comprising a vibrating aperture plate, a mounting, an actuator, and an aperture plate drive circuit having a controller, wherein the controller is configured to:
       measure an electrical drive parameter at each of a plurality of measuring points each measuring point having a drive frequency; and   based on the values of the parameter at the measuring points make a determination of optimum drive frequency and also an end-of-dose prediction,
 
characterized in that,
   the controller is configured to perform a short scan at which a parameter is measured at each of two or more measuring points with different drive frequencies, and according to drive parameter measurements at these points determine if a full scan sweeping across a larger number of measuring points should be performed.       
 
         [0011]    In one embodiment, the short scan has fewer than five, and preferably two measuring points (CPt#1. CPt#2). 
         [0012]    In one embodiment, the controller is configured to compare measurements with tolerance ranges, and if a measurement falls outside its associated range a full scan is initiated. In one embodiment, the tolerance ranges are pre-defined. 
         [0013]    In one embodiment, the short scan is performed at regular intervals. Preferably, the intervals are sub-second. 
         [0014]    In one embodiment, the full scan has in the range of 5 to 300 measuring points. In one embodiment, the full scan has in the range of 100 to 300 measuring points. 
         [0015]    In one embodiment, the controller is configured to dynamically determine from the full scan a frequency for at least one of the short scan measuring points. 
         [0016]    In one embodiment, in the controller is configured to select a frequency value corresponding to lowest drive current as a frequency for a short scan measuring point. Preferably, said lowest drive current is determined to correspond to a resonant frequency, and a frequency value for the full scan measuring point with lowest drive current is stored for use as a short scan measuring point frequency. 
         [0017]    In one embodiment, the parameter is aperture plate drive current. 
         [0018]    In one embodiment, the controller is configured to, during a full scan:
       dynamically perform a plurality of iterations, and in each iteration compare the measured parameter against a measurement at a previous measuring point to determine end of dose and/or optimum drive frequency.       
 
         [0020]    In one embodiment, a slope in parameter measurements is analysed to determine said indication in each iteration. In one embodiment, a slope value above a threshold indicates end of dose. 
         [0021]    In another aspect, the invention provides a method of operation of a nebulizer comprising a vibrating aperture plate, a mounting, an actuator, and an aperture plate drive circuit having a controller, wherein the method comprise the steps of the controller:
       measuring an electrical drive parameter at each of a plurality of measuring points each measuring point having a drive frequency; and   based on the values of the parameter at the measuring points make a determination of optimum drive frequency and also an end-of-dose prediction,   wherein the controller performs a short scan at which a parameter is measured at each of two or more measuring points with different drive frequencies, and according to drive parameter measurements at these points determine if a full scan sweeping across a larger number of measuring points should be performed.       
 
         [0025]    In one embodiment, the short scan has fewer than five, and preferably two measuring points (CPt#1. CPt#2). 
         [0026]    In one embodiment, the controller compares measurements with tolerance ranges, and if a measurement falls outside its associated range a full scan is initiated. 
         [0027]    In one embodiment, the short scan is performed at regular intervals, preferably sub-second. Preferably, the full scan has in the range of 5 to 300 measuring points, preferably 100 to 300 measuring points. 
         [0028]    In one embodiment, the controller dynamically determines from the full scan a frequency for at least one of the short scan measuring points, and selects a frequency value corresponding to lowest drive current as a frequency for a short scan measuring point. 
         [0029]    In one embodiment, said lowest drive current is determined to correspond to a resonant frequency, and a frequency value for the full scan measuring point with lowest drive current is stored for use as a short scan measuring point frequency. 
         [0030]    In one embodiment, the parameter is aperture plate drive current. 
         [0031]    In one embodiment, the controller during a full scan:
       dynamically performs a plurality of iterations, and   in each iteration compares the measured parameter against a measurement at a previous measuring point to determine end of dose and/or optimum drive frequency.       
 
         [0034]    In one embodiment, a slope in parameter measurements is analysed to determine said indication in each iteration Preferably, a slope value above a threshold indicates end of dose. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0000]      
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The invention will be more clearly understood from the following description of some embodiments thereof, given by way of example only with reference to the accompanying drawings in which:— 
           [0036]      FIG. 1  is a diagram of a nebulizer drive circuit of the invention; 
           [0037]      FIG. 2  is a flow diagram for operation of the controller to determine when a full end-of-dose scan should be performed; 
           [0038]      FIG. 3  is a flow diagram showing the full end-of-dose scan; 
           [0039]      FIGS. 4 to 6  are plots showing aspects of change of drive current with frequency; and 
           [0040]      FIG. 7  is a plot showing variation of both flow rate and current with drive frequency. 
       
    
    
       [0041]    Referring to  FIG. 1  a drive circuit  1  for a nebulizer comprises an output drive stage  2  which provides the power required for transfer to the nebulizer load  5 . A filtering stage  3  removes undesirable electrical noise from the drive signal, ensuring compliance with EMC requirements. 
         [0042]    A high voltage resonant circuit (stage  4 ) conditions the drive signal to ensure efficient coupling of power to the nebulizer load  5 . 
         [0043]    The mechanical arrangement of aperture plate, actuator and mounting may be of the type which is known for example from our prior specification numbers WO2010035252 and US2012111970. 
         [0044]    The circuit of  FIG. 1  dynamically monitors drive current in order to determine in real time the resonant frequencies (where resonant frequencies refer to points of resonance and anti-resonance) to be used in the determination of the optimum operating frequency of the nebulizer, and to also determine end-of-dose in an integrated fashion. 
       Short Scan 
       [0045]    Referring to  FIG. 2  the controller performs a short scan multiple times each second. This involves measuring drive current at the normal drive frequency (CheckPoint #1, “CPt#1”), and measuring value of the drive current to the aperture plate. The current tolerance is predefined. The current typically only changes when a change of state of the aperture plate from wet to dry occurs Once a change is identified, it prompts a full scan. 
         [0046]    If CPt#1 drive current is outside the current tolerance a flag is set for a full scan. If not, the frequency is changed to a second reference frequency. The latter is determined according to a full scan. The drive current value for the CPt#2 frequency is determined. Likewise, if it is outside the tolerance the full scan flag is set. This loop is repeated at regular intervals, multiple times per second. 
         [0047]    The short scan is run at intervals in the order of every second or less. It only moves away from the normal operating frequency for less than 1/1000th of a second while it measures the current at this second point of measurement (CPt#2). This ensures that there is no discernible interruption of the nebulization process. 
         [0048]    In summary, the purpose of the short scan is to determine if a full scan should be performed. If the drive current is outside of tolerance then the full scan is activated in order to determine the optimum operating frequency and also to detect end of dose. The short scan does not interfere with operation of the nebulizer because it measures at only two frequencies. One of these is the resonant frequency. CPt#2 is a resonant frequency. In this embodiment the output drive stage  2  drives at CPt#1 with a frequency of 128 KHz, which has been found to provide a good performance across a range of liquid viscosities. 
         [0049]    In other embodiments it is possible that once the optimum drive frequency is determined it will be used for CPt#1. 
       Full Scan 
       [0050]    Referring to  FIG. 3  the flow to implement a full scan is illustrated. The frequency is incremented in steps between 140 kHz and 160 kHz. The step size will change depending on the resolution required. For example, in one application only one type of nebulizer may be used, therefore a simple scan using only 10 steps may be sufficient. However in an application using multiple types of nebulizer, a much larger number of steps will be required to give higher resolution, in order to distinguish between wet and dry of the different devices. 
         [0051]    At each point in the full scan the aperture plate drive current is determined. This provides a plot of drive current vs. frequency as shown in  FIG. 4 . A section with a steep slope is shown in a box in FIG.  4  and in more detail in  FIGS. 5 and 6 . In each iteration the current maximum differential (i.e. slope) is registered. Hence, in the first phase of  FIG. 3  the maximum differential is recorded in real time during the full scan. 
         [0052]    The next phase of the logic of  FIG. 3  is to dynamically record the minimum drive current value (point of anti-resonance). At the end of each loop if the drive current is lower than that recorded for the short scan CPt#2 the value is recorded, as is the corresponding frequency. The current value is recorded as the short scan CPt#2 drive current value. 
         [0053]    As illustrated by the final steps the slope recorded in the first phase of this scan provides an indication of end of dose. 
         [0054]    Importantly, this scan can be used to determine the optimum drive frequency, as described above. Importantly, real time determination of the resonant frequencies gives configurable options for changing operation of the nebulizer to the optimum drive frequency. The resonant frequency may be used for either of the short scan measuring points. 
         [0055]    This visual interruption in the plume due to the full scan lasts for only 0.3 of a second. This has negligible effect on operation of the device. 
         [0056]    The energy requirement of the aperture plate is directly proportional to the impedance offered by the plate during aerosolization. The impedance of the plate is measured by monitoring one or more electrical characteristics such as voltage, current, or phase difference, and in the above embodiment, drive current. 
         [0057]    The impedance of the plate to aerosolization is monitored at the initial drive frequency (CPt#1) and the energy requirements can be determined to be within required limits for correct operation of the vibrating mesh nebulizer, at which point the drive may continue to operate at this frequency. 
         [0058]    Alternatively, the frequency of operation of the drive circuit can be adjusted to determine the optimum operating point for the piezo-element. This can be the anti-resonance point (CPt #2 frequency), where the minimum energy consumption occurs (or an offset of this). 
         [0059]    A higher frequency, beyond the point of anti-resonance, exists where the energy consumption maximizes and this point is termed the point of resonance. 
         [0060]    These points can be used in determination of the optimum operating frequency for the creation of desired flow of aerosolized liquid. There are mechanical reasons why it is preferable not to drive at the resonant point, for example it may not result in the desired flow rate of the nebulizer or the desired particle size or may result in excessive mechanical stress on the aperture plate 
         [0061]    Referring to  FIG. 7 , the optimum drive frequency can be determined utilizing details of the electrical parameters captured by the full scan method. The determination of the point of anti-resonance (where current is minimized) and the point of resonance (where the current is maximized) can be used in choosing the optimum drive frequency. 
         [0062]    The point of resonance may provide the maximum flow rate, however this frequency may result in undesirable stresses on the mechanical structure. Runtime variation in the resonance point may also result in undesirable fluctuations in flow rate at this resonance point. Equally, the use of the anti-resonance point may not be desirable as the flow rate may not be sufficient at this point. The optimum frequency may be between these two points and the final frequency can be a fixed offset of one or more of these two points. 
         [0063]    In  FIG. 7  is might typically be the case that the operating frequency is chosen from the linear part between about 141 kHz and 150 kHz. 
         [0064]    It will be appreciated that the state of aerosolization (wet/dry) can be determined by monitoring the rate of change of plate impedance between the anti-resonance and the resonance points (or an offset thereof). This is implemented by determining the maximum positive rate of change of impedance between the two resonant frequencies. A steep or abrupt rate of change indicates that no liquid is on the plate (DRY). A flat/gentle rate of change indicates the presence of liquid (WET). 
         [0065]    Alternatively, a sudden change in the impedance of the plate at the initial frequency and/or the resonant (CPt#2) frequency can used as a quick method to indicate a possible change in aerosolization state (in the short scan). This is what triggers the full scan to actually determine the end of dose. 
         [0066]    It will also be appreciated that the short scan provides much useful information for real time control, but does not cause a visible interruption in aerosolization. The short scan is run in advance of a full scan, which can cause a visible interruption in the aerosolization of liquid. 
         [0067]    In summary, the software procedure implemented by the controller for determining the status of the nebulizer is as follows:
       Multiple times each second, the software will run a short scan ( FIG. 2 ) to determine if a change of drive current has occurred.   This change in drive current can indicate that the anti-resonance frequency and/or a change of state has occurred; therefore a full scan ( FIG. 3 ) is requested.   Upon completion of the full scan the status of the nebulizer is updated. (Wet or Dry).       
 
         [0071]    In some embodiments, the controller may initiate a full scan after a predetermined time (such as a further 5 seconds). The purpose of this additional scan is to record the maximum slope after the device has completed 5 seconds in the new state. When a device changes state, the change in the current/slope profile is almost instantaneous. However, after an additional few seconds the new profile will have changed again slightly, due to changes in the mechanical structure of the plate. to a value that the nebulizer will maintain over a longer period of time. It is important to determine this stable value to ensure that the short scan has details of the correct frequency to monitor. When prompted by the short scan (or at predefined periods e.g. 5 seconds) the purpose of the predefined full scan may be to update the check points for the quick scan, as these two points may drift slightly during normal run mode. 
         [0072]    The short scan checks the current consumption at the normal operating frequency and at one other point every interval (every second or less). If a change is found at either of these two frequencies the controller will flag a possible change of state (i.e. the device may have changed from wet to dry or from dry to wet). This change will result in a call of the full scan. 
         [0073]    In one embodiment, for the full scan the algorithm calculates a slope/differential of the drive current at each frequency step. As the algorithm steps through the frequencies, it measures the current and then subtracts the current measurement taken 16 frequency steps previously using a rolling shift register. 
         [0074]    The invention is not limited to the embodiments described but may be varied in construction and detail.