Patent Publication Number: US-2021187209-A1

Title: End-of-dose detection in nebulizers

Description:
INTRODUCTION 
     Field of the Invention 
     The invention relates to automatic detection of end of dose during operation of a nebulizer. 
     Prior Art Discussion 
     It has been known for many years that drive current of a nebuliser plate falling below a threshold, or reaching a minimum is indicative of an aperture plate becoming dry, WO93/09881 (Medix) and WO2011/018777 Aerosurgical). 
     Our prior PCT specification number WO2015/010809 describes an approach in which slope of a plot of aperture plate drive current vs. drive frequency indicates end of dose (conversion of aperture plate from a wet state to a dry state). 
     The invention is directed towards improving accuracy of prediction of end of dose. 
     SUMMARY OF THE INVENTION 
     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 aperture plate drive current at each of a plurality of measuring points in a scan, each measuring point having a drive frequency;   determine a minimum value of the drive current during the scan;   determine a value for maximum rate of change of drive current during the scan; and to   use said minimum drive current value in combination with said maximum rate of change value to execute an algorithm to calculate an indicator value for end-of-dose.       

     In one embodiment, the controller is configured to determine a ratio of said maximum slope value and said minimum value to provide the indicator. In one embodiment, the controller is configured to scale said indicator by a constant value. 
     In one embodiment, the controller is configured to perform the scan across a frequency range of 128 kHz to 165 kHz. 
     In one embodiment, the controller is configured to initiate the scan in response to a trigger of possible end of dose. In one embodiment, the trigger is a short scan with a smaller number of measuring points and which detects a change of drive current above a threshold. In one embodiment, the drive current threshold change is above 5 mA, and preferably about 8 mA. 
     In one embodiment, the actuator is encapsulated. 
     In one embodiment, a minimum value of drive current which is approximately less than 30% of a minimum value of drive current of a scan for a wet state of the aperture plate is determined as being indicative of end of dose. This may for example contribute to the trigger from the short scan of possible end of dose. 
     In another aspect, the invention provides a method of operation of a controller of a nebulizer comprising a vibrating aperture plate, a mounting, an actuator, and an aperture plate drive circuit, wherein the method comprises:
         measuring aperture plate drive current at each of a plurality of measuring points in a scan, each measuring point having a drive frequency;   determining a minimum value of the drive current in said scan;   determining a value for maximum rate of change of drive current during the scan; and   using said minimum value in combination with said maximum rate of change value to execute an algorithm to calculate an indicator value for end-of-dose.       

     In one embodiment, the controller utilizes the ratio of maximum slope value and the minimum parameter value to provide said indicator. 
     In one embodiment, the controller multiplies said ratio or a value derived from the ratio by a constant value to provide the indicator. In one embodiment, the controller performs the scan across a frequency range of 128 kHz to 165 kHz. Preferably, the controller initiates the scan in response to a trigger of possible end of dose. 
     In one embodiment, the trigger is a short scan with a smaller number of measuring points and which detects a change of drive current above a threshold. In one embodiment, the drive current threshold change is above 5 mA, and preferably about 8 mA. 
     In one embodiment, a minimum value of drive current which is approximately less than 30% of a minimum value of drive current of a scan for a wet state of the aperture plate is determined as being indicative of end of dose. This may for example contribute to the trigger from the short scan of possible end of dose, and it may be the sole trigger event. 
     In another embodiment, the method comprises the step of the controller automatically stopping operation of the actuator upon detection of end of dose. 
     In a further aspect, the invention provides a non-transitory computer readable medium comprising software code to perform a method as defined in any embodiment when executing on a digital processor. 
     Additional Statements 
     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 in a scan, each measuring point having a drive frequency; and   based on the values of the parameter at the measuring points, make a determination of end of dose;   wherein the controller is configured to determine a minimum value or a maximum value of the drive parameter in said scan and to execute an algorithm using said minimum or maximum value to calculate an indicator value for end-of-dose.       

     In one embodiment, the parameter is drive current and a minimum value of drive current during the scan is used for said algorithm. 
     In one embodiment, a minimum value of drive current which is approximately less than 30% of a minimum value of drive current of a scan for a wet state of the aperture plate is determined as being indicative of end of dose. 
     In one embodiment, the controller is configured to use said minimum value in combination with a value for maximum rate of change of said parameter during the scan. 
     In one embodiment, the controller is configured to utilize the ratio of maximum slope value and the minimum parameter value to provide an indicator. 
     In one embodiment, the controller is configured to multiply said indicator by a constant value. 
     In one embodiment, the controller is configured to perform the scan across a frequency range of 128 kHz and 165 kHz. 
     In one embodiment, the controller is configured to initiate the scan in response to a trigger of possible end of dose. 
     In one embodiment, the trigger is a short scan with a smaller number of measuring points and which detects a change of drive current above a threshold. 
     In one embodiment, the drive current threshold change is above 5 mA, and preferably about 8 mA. 
     According to another aspect, the invention provides a method of operation of a controller of a nebulizer comprising a vibrating aperture plate, a mounting, an actuator, and an aperture plate drive circuit, wherein the method comprises:
         measuring an electrical drive parameter at each of a plurality of measuring points in a scan, each measuring point having a drive frequency;   based on the values of the parameter at the measuring points, making a determination of end of dose;   determining a minimum value or a maximum value of the drive parameter in said scan and executing an algorithm using said minimum or maximum value to calculate an indicator value for end-of-dose.       

     In one embodiment, the parameter is drive current and a minimum value of drive current during the scan is used for said algorithm. 
     In one embodiment, a minimum value of drive current which is approximately less than 30% of a minimum value of drive current of a scan for a wet state of the aperture plate is determined as being indicative of end of dose. 
     In one embodiment, the controller uses said minimum value in combination with a value for maximum rate of change of said parameter during the scan. 
     In one embodiment, the controller utilizes the ratio of maximum slope value and the minimum parameter value to provide an indicator. 
     In one embodiment, the controller multiplies said indicator by a constant value. 
     In one embodiment, the controller performs the scan across a frequency range of 128 kHz and 165 kHz. 
     In one embodiment, the controller initiates the scan in response to a trigger of possible end of dose. 
     In one embodiment, the trigger is a short scan with a smaller number of measuring points and which detects a change of drive current above a threshold. 
     In one embodiment, the drive current threshold change is above 5 mA, and preferably about 8 mA. 
     In another aspect, the invention provides a non-transitory computer readable medium comprising software code to perform a method as defined above in any embodiment when executing on a digital processor. 
    
    
     
       DETAILED DESCRIPTION OF THE INVENTION 
       Brief Description of the Drawings 
       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:— 
         FIG. 1  is a plot of drive current versus frequency for wet and dry aperture plate states, 
         FIG. 2  is a diagram illustrating improvement of the invention in identification of a dry state; 
         FIG. 3  is a set of plots of values of a dry plate indicator value, a cut off or threshold fixed value, and wet plate values, versus sample number; and 
         FIGS. 4 and 5  are flow diagrams for operation of a nebulizer controller to predict end of dose,  FIG. 4  showing a change of state predictor short scan and  FIG. 5  showing a full scan for full analysis of potential end of dose. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     A nebulizer of the invention has a vibrating aperture plate, a mounting for the plate, an actuator such as a piezoelectric element, and an aperture plate drive circuit having a controller with a digital processor. The digital processor is programmed to drive the aperture plate so that liquid medication is aerosolized. It predicts when the liquid medication has been consumed, with the plate becoming dry. This is referred to as “end-of-dose (EOD)”. 
     Referring to  FIG. 1  plots for change of drive current with drive frequency for wet (dotted line) and dry (full line) states are illustrated for full scans which are triggered by change of state short scans in a manner as described in WO2015/010809. The full scan has many measuring points each with a specific drive frequency in the range of 137 kHz and 156 kHz in this embodiment. This is triggered in response to a short scan identifying a change in drive current. However, in other embodiments there may not be a preceding short scan. For example, the full scan may be initiated in response to any other configured trigger, such as elapse of a pre-set time or a user input. 
     For aperture plate drive current measurement, the processor measures a voltage across a fixed shunt resistor. This provides enough information to determine the impedance of the nebulizer. This is essentially a current measurement. 
     When operating close to anti-resonance more current is consumed nebulizing in the wet state than in the dry state. During the scan of drive current values across multiple frequencies the minimum drive current is identified and the current consumption for this point is recorded. 
     The slope differential is also determined during the scan. The drive current (or “current consumption”) at each measuring point is compared to the current drawn at the previous measuring point. The software routine executed by the processor records the maximum differential slope (“MDS”) of drive current measurements found across the full frequency range. 
     As more power is necessary to drive the aperture plate in its wet state, the presence of liquid on the plate results in a higher current than the dry state. In addition to this, the presence of liquid has a dampening effect, resulting in a small rate of change of current across the frequencies in the wet state. On the other hand, in the dry state the MDS differential is a large value. These attributes are illustrated in the plots of  FIG. 1 . 
     Following the wet state plot of  FIG. 1  it is seen that the maximum positive slope (“Wet MDS”) is small and current at the anti-resonant point in the wet state is much higher than when in the dry state. Typically, the MDS for the wet state is in the range of 10° to 54°. 
     On the other hand, for the dry state plot the current dips to a minimum which is only about 30% of that for the wet state. Also, the maximum slope (“Dry MDS”) is much higher, by a factor of about 4 when compared to the wet state maximum slope. 
     It has been found that a particularly reliable and accurate end-of-dose determination can be made by employing both the scan&#39;s minimum current and MDS during the scan. In one embodiment the algorithm is: 
       EOD_Value=(max.slope/min. I )*1000         where,   max.slope is the scan&#39;s maximum slope value,   1000 is a fixed number to scale the value and being selected for ease of computations, and to allow use of low-cost processor, and   min.I is drive current at the anti-resonant point, which is the preceding minimum.       
     As shown in  FIG. 2  there is a 75% improvement in divergence between dry and wet, as compared to the prior art approach in which only MDS is employed. This arises from employing the preceding current minimum value according to this algorithm. 
       FIG. 3  shows examples of the values. As is clear from this plot, the values for the dry state are very well above the threshold, and so there is very accurate and prompt identification of the dry state. 
     The full EOD algorithm ( FIG. 5 ) is executed by a digital processor of the nebulizer controller upon a trigger provided by a short scan, shown in  FIG. 4 . The short scan is a predictor of potential end of dose. It identifies changes of current at the measuring points, and if the current at either of these two points has changed by greater than 8 mA this routine will flag a possible change of state of the aperture plate and call for a full scan to confirm the change of state. The short scan method monitors the current consumption at two different frequencies of operation. 
     1. 137 KHz/128 KHz (Operating Frequency) 
     2. 140 KHz (2 nd  frequency where nebulization is not interrupted) 
     Referring to  FIG. 5 :
         The minimum current point relates to the minimum point preceding the MDS (as shown in  FIG. 1 ) and is referred to as the anti-resonant current.   The EOD_value formula contains a multiplier of 1000, this is an arbitrary value chosen to simplify the microprocessor calculations.   When a wet state is identified, the EOD routine sets the operating voltage and then records the current consumption at the two check points which is then used by short scan ( FIG. 4 ) method as the reference current.       

     Configuration Parameters 
       
     
       
         
           
               
               
             
               
                   
               
             
            
               
                 SCAN_FREQ (frequency values for each 
                 137 KHz to 156 KHz 
               
               
                 measuring point) 
               
            
           
           
               
               
               
            
               
                 OPERATING_FREQ 
                 137 
                 KHz 
               
               
                 SCAN_VOLTS 
                 10 
                 V 
               
               
                 RUN_VOLTS 
                 12 
                 V 
               
            
           
           
               
               
            
               
                 EOD_CUTOFF 
                 174 
               
               
                   
               
            
           
         
       
     
     The EOD_CUTOFF value was determined from a sample size of 175 pieces, it was chosen as the mid-point between the max wet and the min dry values. Please refer to  FIG. 3 . 
     The addition of utilizing the minimum currents in the calculation of an EOD_value provides a more robust method of determining the wet/dry state of the nebulizer. These minimum currents correspond to the anti-resonant frequency of the aperture plate, and this anti-resonant frequency may differ from the wet to dry state, due to the presence of liquid on the aperture plate 
     Also, the anti-resonance current when the aperture plate is wet will be higher than the anti-resonance current when it is dry as additional energy is required to displace the liquid in the wet state, which in turn translates to an additional electrical power requirement. 
     In summary, the main function of full end-of-dose scan is to determine the status of the plate. The scan finds the maximum rate of change in current found between successive frequencies (maximum slope). Another function is to determine the minimum current, that drawn at anti-resonance. The ratio of the max.slope/min.current is used to determine the status of the plate. This value is much greater when the plate is dry than when it is wet, with a much greater differential than when only using the slope value. 
     When a direct comparison to the prior end-of-dose method described in WO2015/010809 is made, it can be seen that the method of the invention has greater than 75% improved divergence between the wet and dry groupings, as shown in  FIG. 2 . 
     By providing significantly more accurate and reliable indications of end of dose, the invention helps to achieve a longer useful lifetime of an aperture plate. 
     Also, the invention helps to provide a nebulizer which caters much better with variation of parameters associated within large scale manufacture. Despite excellent manufacturing practice, small variations in parameters exist within all the stages of manufacture, and when these variations are combined, they can lead to considerable differences in the operating parameters of a nebulizer. The extent of divergence in the calculated values between wet and dry states, ensures that EOD detection accuracy will be achieved every time. 
     Also, in the case, of over-moulded nebulizers, the complete vibratory actuator, including the piezo-electric element, is encapsulated to allow easier cleaning of the nebulizer after use. This however causes a restriction of movement, which significantly reduces the capability of the processor to identify the presence of liquid on the aperture plate. The invention circumvents this restraint due to its extent of sensitivity to the aperture plate becoming dry. 
     Nebulizers of the invention also provide several therapeutic benefits for patients such as those suffering from Cystic Fibrosis (“CF”), who need to spend long periods taking medication, often every day. Nebulizers for this application are typically over-moulded, and because the processor according to the invention provides instantaneous feedback on dose completion which, in the case of children, means less dosing time and also much less monitoring/assistance requirements from parents. This is especially so if the controller is configured to shut down the nebulizer and so extend the life of the nebulizer aperture plate. Also, by shutting off at the end of the delivery of medication, as identified by the end of dose, the nebulizer will dramatically improve the capability of battery powered devices to deliver longer periods on a single charge/set of batteries. 
     The invention is not limited to the embodiments described but may be varied in construction and detail. As an alternative to directly measuring drive current at each of the measuring points, it is envisaged that it may be indirectly measured by for example measuring impedance of the aperture plate. In this specification, where it is stated that drive current is measured, this may be either directly or indirectly. The frequency range may be within a different range, but is preferably within 128 kHz and 165 kHz, and more preferably about 137 kHz and 156 kHz.