Patent Publication Number: US-2020289749-A1

Title: Droplet measurementsystem, droplet measurement method and droplet measurement program

Description:
TECHNICAL FIELD 
     The present invention relates to a droplet measurement system that measures the volume of droplets dripping down from a nozzle, a droplet measurement method and a droplet measurement program. 
     BACKGROUND ART 
     It is important to maintain a predetermined flow rate when delivering liquid (infusion liquid), such as chemicals, nutritional supplements, or the like, intravenously. Conventionally, the infusion flow rate control has been performed by determining the number of drips per unit time by counting the number of droplets dripped in a drip tube, calculating the flow rate based on an assumption that the droplet volume is constant, and adjusting the dripping cycle of the droplets (i.e. the time interval of dripping). 
     However, in practice, the surface tension of the infusion liquid varies depending on the conditions, such as viscosity, ambient temperature, or the like, and thus, the volume per droplet is not constant. Moreover, in medical settings, patients may change their positions during infusion, and in such case, the head difference of the infusion liquid may change, and the volume of droplets may vary. Accordingly, the conventional method that controls the flow rate solely based on the dripping cycle of the droplets has been prone to errors in flow rate, and high-precision flow rate control has been difficult. 
     To handle such problems, a technique is known in which the volume of the dripping droplets is measured and used in the flow rate control. For example, PTL 1 discloses a droplet detection device provided with: a transparent drip tube; a light emitting part arranged on one side of the exterior of the drip tube; and a two-dimensional image sensor arranged at a position opposite to the light emitting part with the drip tube sandwiched therebetween, wherein a field of view of the two-dimensional image sensor is set such that a tip of a dripping nozzle in the drip tube and a predetermined droppage distance of droplets dropping from the dripping nozzle are included. 
     PRIOR ART DOCUMENTS 
     Patent Literature 
     PTL 1: JP2011-62371 A 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In PTL 1, the state in which a droplet separates and drops from the dripping nozzle tip is imaged at predetermined time intervals, an image that is one before the imaged image is investigated when the state of the droplet in such imaged image is determined to be the “droplet-dropped-state,” and the volume of the droplet is calculated from such image determined to be in the “droplet-dropped-state”when the state of the droplet in such image that is one before the imaged image is determined to be the “state before droplet droppage.” 
     In the case of PTL 1, the processing is cumbersome, and since image processing for detecting the droplet state needs to be performed on all imaged images and thus, the operational load is heavy. In order to cope with this, a high-performance processor is necessary and, as a result, the miniaturization or simplification of the entire droplet measurement system becomes difficult. 
     The present invention has been made in view of the above-described problems, and an object thereof is to provide a droplet measurement system, a droplet measurement method and a droplet measurement program, which are capable of measuring the droplet volume with high precision by means of image processing with a lighter arithmetic load than before. 
     Means for Solving the Problems 
     In order to solve the above-described problems, a droplet measurement system according to an aspect of the present invention is a droplet measurement system that measures the volume of a droplet dripping from a nozzle being provided with: an imaging unit that images an object to be imaged and outputs image data, wherein the imaging unit is placed such that a field of view is directed to a tip of the nozzle and a region extending vertically downward from the tip; an imaging control unit that sets the field of view of the imaging unit to a rectangular region long in a vertical direction and that causes the imaging unit to execute imaging; and an image processing unit that: acquires a plurality of images, in a temporal sequence order, that correspond to the rectangular region and that have captured therein the tip of the nozzle and the region extending vertically downward from the tip based on the image data output from the imaging unit; and that calculates the volume of the droplet based on the images acquired in a temporal sequence order. 
     In the above-described droplet measurement system, the image processing unit may include: a liquid discontinuity detection unit that detects a liquid discontinuity image that is an image that has captured therein the state immediately after the droplet separated from the tip, from the images in a temporal sequence order; and a volume calculation unit that: acquires the liquid discontinuity image and a predetermined number of images following the liquid discontinuity image from the images in a temporal sequence order; and that calculates the volume using the liquid discontinuity image and the predetermined number of images. 
     The above-described droplet measurement system may be further provided with a sensor that detects a tilt of the nozzle with respect to an axis in the vertical direction, and the image processing unit may further include a correction unit that performs a correction operation for the volume calculated by the volume calculation unit based on the tilt detection result by the sensor. 
     In the above-described droplet measurement system, the image processing unit may further include a correction unit that retains a reference value of dimensions of an image of the nozzle in an image corresponding to the rectangular region and that performs a correction operation for the volume calculated by the volume calculation unit based on the reference value and dimensions of an image of the nozzle captured in the liquid discontinuity image or the predetermined number of images. 
     In the above-described droplet measurement system, an imaging frame rate of the imaging unit may be 100 frames per second or more. 
     In the above-described droplet measurement system, a ratio of the length in the vertical direction in the rectangular region to the length in a horizontal direction thereof may be from 1.5 to 4.5, inclusive. 
     In the above-described droplet measurement system, the nozzle may be provided to an infusion device that performs infusion of liquid filled in a container via a drip tube and the nozzle drips the droplet consisting of the liquid in the drip tube, and the droplet measurement system may further be provided with: an actuator that varies a flow rate of the liquid by varying a pressing force with respect to a tube that causes the liquid accumulated in the drip tube to flow therethrough by driving a clamp that is provided on the tube in a pressable manner; and a flow rate control unit that controls the actuator such that the flow rate of the liquid falls within a predetermined range based on the volume calculation result. 
     In the above-described droplet measurement system, the nozzle may be provided to an infusion device that performs infusion of liquid filled in a container via a drip tube and the nozzle may drip the droplet consisting of the liquid in the drip tube, and the droplet measurement system may further be provided with: an actuator that varies a flow rate of the liquid by varying a pressing force with respect to a tube that causes the liquid accumulated in the drip tube to flow therethrough by driving a clamp that is provided on the tube in a pressable manner; a storage unit that stores therein information that is acquired in advance and that represents correlation between the volume of the droplet and a dripping cycle of the droplet; and a flow rate control unit that controls the actuator such that the flow rate of the liquid falls within a predetermined range based on the information. 
     In the above-described droplet measurement system, the storage unit may store therein multiple types of the information according to liquid types, and the flow rate control unit may acquire information corresponding to the liquid filled in the container from the multiple types of the information stored in the storage unit and control the actuator based on the acquired information. 
     The above-described droplet measurement system may further be provided with: a light source that is provided opposite to the imaging unit and that illuminates at least the tip of the nozzle and the region extending vertically downward from the tip; and a filter that controls light distribution of light output from the light source. 
     In the above-described droplet measurement system, the imaging unit may further include a telecentric lens. 
     A droplet measurement method according to an aspect of the present invention is a droplet measurement method of measuring the volume of a droplet dripping from a nozzle, including: an imaging control step of setting a field of view of an imaging unit to a rectangular region long in a vertical direction and causing the imaging unit to execute imaging, wherein the imaging unit images an object to be imaged and outputs image data, and wherein the imaging unit is placed such that the field of view is directed to a tip of the nozzle and a region extending vertically downward from the tip; and an image processing step of acquiring a plurality of images, in a temporal sequence order, that correspond to the rectangular region and that have captured therein the tip of the nozzle and the region extending vertically downward from the tip, based on the image data output from the imaging unit, and calculating the volume of the droplet based on the images acquired in a temporal sequence order. 
     A droplet measurement program according to an aspect of the present invention is a droplet measurement program for measuring the volume of a droplet dripping from a nozzle, causing a computer to execute: an imaging control step of setting a field of view of an imaging unit to a rectangular region long in a vertical direction and causing the imaging unit to execute imaging, wherein the imaging unit images an object to be imaged and outputs image data, and wherein the imaging unit is placed such that the field of view is directed to a tip of the nozzle and a region extending vertically downward from the tip; and an image processing step of acquiring a plurality of images, in a temporal sequence order, that correspond to the rectangular region and that have captured therein the tip of the nozzle and the region extending vertically downward from the tip, based on the image data output from the imaging unit, and calculating the volume of the droplet based on the images acquired in a temporal sequence order. 
     Effect of the Invention 
     Since the present invention sets the field of view of the imaging unit to a rectangular region long in the vertical direction and calculates the droplet volume based on the images, in a temporal sequence order, that have imaged therein the tip of the nozzle and the region extending vertically downward from the tip by means of the imaging unit, the droplet volume can be measured with high precision by means of image processing with a lighter arithmetic load than before. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a schematic configuration of a droplet measurement system according to a first embodiment of the present invention. 
         FIG. 2  is a block diagram showing a schematic configuration of an arithmetic device shown in  FIG. 1 . 
         FIG. 3  is a schematic diagram for describing a field of view of an imaging element shown in  FIG. 1 . 
         FIG. 4  is a schematic diagram showing an example of a screen displayed on a display device during the operation of the droplet measurement system shown in  FIG. 1 . 
         FIG. 5  is a flowchart illustrating the operation of the droplet measurement system shown in  FIG. 1 . 
         FIG. 6  is a flowchart illustrating the details of the droplet volume calculation shown in  FIG. 5 . 
         FIG. 7  is a schematic diagram showing an example of an image generated by an image processing unit shown in  FIG. 2 . 
         FIG. 8  is a diagram showing a schematic configuration of a droplet measurement system according to a second embodiment of the present invention. 
         FIG. 9  is a block diagram showing a schematic configuration of an arithmetic device shown in  FIG. 8 . 
         FIG. 10  is a schematic diagram for describing a correction operation in the second embodiment of the present invention. 
         FIG. 11A  is a schematic diagram for describing a correction operation in a third embodiment of the present invention. 
         FIG. 11B  is a schematic diagram for describing a correction operation in the third embodiment of the present invention. 
         FIG. 12A  is a schematic diagram for describing a correction operation in the third embodiment of the present invention. 
         FIG. 12B  is a schematic diagram for describing a correction operation in the third embodiment of the present invention. 
         FIG. 13  is a graph showing an example of correlation between a dripping cycle of droplets and the volume per droplet in the infusion. 
         FIG. 14  is a block diagram showing a schematic configuration of an arithmetic device provided to a droplet measurement system according to a fourth embodiment of the present invention. 
         FIG. 15  is a flowchart illustrating the operation of the droplet measurement system according to the fourth embodiment of the present invention. 
     
    
    
     EMBODIMENTS OF THE INVENTION 
     The droplet measurement system, the droplet measurement method and the droplet measurement program according to embodiments of the present invention will be described hereinafter, with reference to the drawings. It should be noted that the present invention is not limited by these embodiments. In addition, in the descriptions of the respective drawings, identical parts are denoted by identical reference numbers. 
     First Embodiment 
       FIG. 1  is a diagram showing a schematic configuration of a droplet measurement system according to a first embodiment of the present invention. As shown in  FIG. 1 , the droplet measurement system  10  according to the present embodiment is a system that measures the volume of droplets  7  dripping down from a tip  6   a  of a nozzle  6  (hereinafter also referred to as a nozzle tip) provided inside a drip tube  4  and that controls the flow rate of the infusion based on the measured volume, with respect to an infusion device  1  that supplies liquid (infusion liquid) filled in an infusion bag  2  via an intermediate tube  3 , a drip tube  4  and an infusion tube  5 . 
     The infusion bag  2  is a container filled with infusion liquid, such as chemicals, nutritional supplements, or the like, and is held in a suspended manner from a support, etc. during infusion. The intermediate tube  3  is connected, at one end thereof, to a drainage port  2   a  of the infusion bag  2  and is connected, at the other end thereof, to one end of the nozzle  6  that is attached to an upper lid  4   a  of the drip tube  4 . The other end of this nozzle  6  is provided such as to project into the drip tube  4 . 
     The infusion tube  5  is made of an elastic material. A clamp  8  that is capable of pressing the infusion tube  5  in a radial direction and an actuator  9  that drives the clamp  8  are provided in the midway of the infusion tube  5 . 
     The actuator  9  varies the pressing force exerted by the clamp  8  with respect to the infusion tube  5  by driving the clamp  8  under the electrical control. By means of which the inner diameter of the infusion tube  5  changes (opens and closes), and then the flow rate of the infusion liquid that flows in the infusion tube  5  can be adjusted. Along with this, the internal pressure of the drip tube  4  changes and thus, the dripping cycle of the droplets  7  dripping down from the nozzle  6  changes. 
     The droplet measurement system  10  is further provided with a light source  11  that illuminates the drip tube  4 , an imaging unit  12  that images the interior of the drip tube  4  and generates image data, an arithmetic device  13  that calculates the volume of the droplets based on the image data generated by the imaging unit  12 , and a display device  14  that displays the result of the droplet volume calculations, or the like. 
     The light source  11  is provided with a light emitting element, for example, a light emitting diode (LED) or the like, and an optical system, such as a filter, a lens or the like, that controls the light distribution such that the light output from the light emitting element becomes parallel light. The light source  11  is placed opposite to the field of view of the imaging unit  12  and illuminates at least the nozzle tip  6   a , from which the droplets drip down, and the region including the vertically lower side of the nozzle tip  6   a , from behind the droplets  7 . 
     The imaging unit  12  is a camera capable of high-speed imaging and is provided with an imaging element  12   a  consisting of a charge coupled device (CCD) image sensor, a complementary metal oxide semiconductor (CMOS) image sensor or the like, and the imaging unit  12  outputs image data after imaging an object to be imaged. It is preferable for the imaging frame rate to be set to 100 frames per second (fps) or higher, and it is set to 120 fps in the present first embodiment. 
     The specifications of the imaging unit  12  can be configured as appropriate, depending on the infusion device  1  being the target of measurement. As an example, when the infusion device  1  is a device generally used in the medical field, a compact camera may be used with the size of the outer diameter of a camera module ranging from approximately a few millimeters to several dozens of millimeters and with the focus distance thereof ranging from a few millimeters to a few tens of millimeters, preferably approximately several dozens of millimeters, such that the drip tube  4  can be imaged at close range and also such that the infusion operation by a user is not hindered. 
     Preferably, a telecentric lens may be provided on the imaging unit  12 , by means of which, variations in the image size of the droplet  7  caused by the change in the distance between the droplet  7  and the imaging element  12   a  can be suppressed and thus, the error in volume calculation of the droplet  7  can be suppressed. 
     As for the imaging element  12   a , an imaging element having a general geometry with an aspect ratio of a light-receiving surface being 1 (long): 2 (wide) or 2:3 may be used. However, as described below, at the orientation at which the imaging unit  12  is placed during, at least, the operation of the droplet measurement system  10 , the imaging unit  12  is controlled such that the rectangular region of the light-receiving surface of the imaging element  12   a , which is long in the vertical direction, becomes the effective imaging region that actually takes in the images. Namely, image signals are acquired only from the pixels arranged in the effective imaging region. Here, the vertical direction refers to the direction in which the droplets  7  drip down (i.e. the z direction), namely, the direction of gravitational force. Needless to say, a rectangular element having a specific aspect ratio may be used as the imaging element  12   a , the imaging unit  12  may be placed such that the long side of the imaging element  12   a  becomes the vertical direction, and the image signals may be acquired from pixels across the light-receiving surface of the imaging element  12   a . By controlling the imaging region of the imaging element  12   a  in this manner, the field of view of the imaging unit  12  is set to the rectangular region that is long in the vertical direction. 
     Such imaging unit  12  is placed such that the nozzle tip  6   a  and the region extending vertically downward from the nozzle tip  6   a  coincide with the field of view. The aspect ratio of the field of view set for the imaging unit  12  (i.e. the imaging region of the imaging element  12   a ) will be described below. 
     As for the arithmetic device  13 , in addition to devices configured specifically for the present droplet measurement system  10 , a general-purpose arithmetic device, such as a personal computer or the like, may be used.  FIG. 2  is a block diagram showing the schematic configuration of the arithmetic device  13 . As shown in  FIG. 2 , the arithmetic device  13  is provided with an input and output unit  131 , a storage unit  132 , a manipulation input unit  133  and a processor  134 . 
     The input and output unit  131  is an external interface that performs input and output of image data or various signals among various external devices, such as the imaging unit  12 , the display device  14 , and the like. 
     The storage unit  132  is configured by a disk drive, a semiconductor memory, such as ROM or RAM or the like. The storage unit  132  includes a program storage unit  132   a  that stores therein control programs for controlling various units of the arithmetic device  13  and programs that causes the arithmetic device  13  to execute predetermined operation. More specifically, the program storage unit  132   a  stores therein an image processing program for calculating the volume of the droplets from a plurality of images capturing the droplet based on the image data input from the imaging unit  12 . In addition, the storage unit  132  stores therein various parameters to be used for execution of the image processing program, the image data input from the imaging unit  12  and the result of droplet volume calculation, or the like. 
     The manipulation input unit  133  is configured by an input device, such as an input button, a switch, a keyboard, a mouse, a touch panel or the like, and inputs signals according to the manipulations made by a user into the processor  134 . 
     The processor  134  is configured by an arithmetic and logic unit, such as a CPU or the like, and various registers, and it performs data transfer and provides instructions to the respective units of the arithmetic device  13  by reading and executing the various programs stored in the program storage unit  132   a , and integrally controls the operation of the arithmetic device  13 . 
     More specifically, the processor  134  includes: an imaging control unit  135  that controls the operation of the imaging unit  12 ; a flow rate control unit  136  that controls the flow rate of the infusion liquid in the infusion device  1 ; and an image processing unit  137  that performs image processing, such as volume calculation or the like, of the droplet  7  based on the image data input from the imaging unit  12 . 
     The imaging control unit  135  causes the imaging unit  12  to operate at a predetermined imaging frame rate and also performs the control to limit the imaging region of the imaging element  12   a . More specifically, as shown in  FIG. 3 , the imaging control unit  135  sets the rectangular region of the light-receiving surface of the imaging element  12   a , which is long in the vertical direction (the z-direction), as the effective imaging region  12   b  and controls the imaging element  12   a  such that image signals are acquired only from the pixels arranged in this imaging region  12   b.    
     The aspect ratio of the imaging region  12   b  may be determined such that a few shots (three or more shots) of the state in which the droplets  7  are separating and falling from the nozzle tip  6   a  can be imaged, based on the relationship between the size of the droplet  7 , which is determined based on the diameter of the nozzle  6  or the type of the infusion liquid (viscosity, etc.), and the imaging frame rate. 
     More specifically, the width in the horizontal direction of the image corresponding to the imaging region  12   b  may preferably be set to approximately 1.5 to 2 times the size of the image of droplet  7 , in consideration of the disturbance when the droplet  7  is falling down. The length in the perpendicular direction of the image may preferably be set to approximately 3 to 9 times the size of the image of the droplet  7 , in consideration of the falling velocity of the droplet  7 . The reason for this is that, when the length in the perpendicular direction is too short, the falling droplet  7  will go out of sight immediately, and when it is too long, the image of the accelerated droplet  7  will be blurred at the lower part of the image, and thus, in any of the cases, such imaging region is not appropriate for use in the image processing. 
     Accordingly, the aspect ratio of the imaging region  12   b , i.e. the ratio of the longitudinal (long side) to latitudinal (short side) may be within a range of approximately 1.5 to 4.5, inclusive. The specific size examples of the imaging region  12   b  include 1936 pixels long×1096 pixels wide, 1936 pixels long×496 pixels wide, and the like. 
     The flow rate control unit  136  controls the operation of the actuator  9  based on the volume of the droplet  7  calculated by the image processing unit  137 . Here, the flow rate of the infusion liquid is obtained by dividing the volume of the droplet  7  by the dripping cycle of the droplets  7 . The flow rate control unit  136  retains therein a target flow rate predetermined by a user and performs control such that the actual flow rate approaches this target flow rate. 
     The image processing unit  137  generates, in a temporal sequence order, vertically long rectangular images that have captured therein the nozzle tip  6   a  and the region extending vertically downward from the nozzle tip  6   a , based on the image data input from the imaging unit  12 , and performs processing for calculating the volume of the droplet  7  based on these images. More specifically, the image processing unit  137  is provided with a liquid discontinuity detection unit  137   a  and a volume calculation unit  137   b.    
     The liquid discontinuity detection unit  137   a  detects an image that has captured therein the state immediately after the droplet  7  has separated from the nozzle tip  6   a  (i.e. a liquid discontinuity image) from the plurality of images generated in a temporal sequence order. The volume calculation unit  137   b  acquires the liquid discontinuity image and a predetermined number of images that follow the liquid discontinuity image in a temporal sequence order, and calculates the volume of the droplet that is captured in the acquired images by performing predetermined image processing on these images. 
     The display device  14  is configured by a liquid crystal display, an organic EL display, or the like, and displays a predetermined screen based on control signals output from the arithmetic device  13  and image data for display, under the control of the arithmetic device  13 . 
       FIG. 4  is a schematic diagram showing an example of a screen displayed on the display device  14  during the operation of the droplet measurement system  10 . As shown in  FIG. 4 , the screen M 1  includes: an instruction button m 10  for inputting an instruction for executing the volume calculation processing of the droplet  7 ; a plurality of images m 11  to m 15  generated based on the image data input from the imaging unit  12 ; an image m 16  that shows the two dimensional shape of the droplet  7  detected by the image processing on these images m 11  to m 15 ; and a display field m 17  where the volume calculation result of the droplet  7  is displayed. 
     It should be noted that, in the droplet measurement system  10 , the display device  14  is not required and the images m 11  to m 16  shown in  FIG. 4  also need not be displayed on the screen. In addition, instead of the display device  14 , a compact display device that only shows the volume calculation result of the droplet  7  in numerical values may be provided, a compact display device that only shows the actual flow rate, in numerical values, that is controlled based on the volume calculation result may be provided, or a display device that displays an alarm based on the volume calculation result may be provided. Moreover, an audio generation device, such as a speaker or the like, that informs the alarm through voice or a particular sound may also be added. 
     The imaging unit  12 , the arithmetic device  13  and the display device  14  may be provided as separate devices that are connected to each other by a cable, or these devices may be accommodated in a single housing. In the latter case, since the droplet measurement system  10  can be configured by: the housing in which the imaging unit  12 , the arithmetic device  13  and the display device  14  are accommodated; and the light source  11 , the entire droplet measurement system  10  can be miniaturized and it is also easy to carry around. 
     Next, the operation of the droplet measurement system  10  will be described.  FIG. 5  is a flowchart illustrating the operation of the droplet measurement system  10 . 
     Before starting the infusion, a user places the light source  11  and the imaging unit  12  in the neighborhood of the drip tube  4  (see  FIG. 1 ). At this time, the positional relationship among the light source  11 , the drip tube  4  and the imaging unit  12  may be adjusted, while looking at the screen displayed on the display device  14  (see  FIG. 4 ), such that the nozzle tip  6   a  and the region extending vertically downward from the nozzle tip  6   a  are located within the field of view of the imaging element  12   a.    
     In step S 10 , the infusion operation in the infusion device  1  starts by driving the actuator  9 , under the control of the flow rate control unit  136 , to cause the clamp  8  to open the infusion tube  5 . 
     In the subsequent step S 11 , the imaging control unit  135  causes the imaging unit  12  to start imaging at a predetermined imaging frame rate at the predetermined imaging region  12   b  (see  FIG. 3 ). 
     In the subsequent step S 12 , the image processing unit  137  calculates the volume of a droplet  7 .  FIG. 6  is a flowchart illustrating the details of the volume calculation of the droplet  7 .  FIG. 7  is a schematic diagram showing an example of an image generated by the image processing unit  137 . The image m 20  shown in  FIG. 7  captures therein the respective images m 21 , m 22 , m 23  of the nozzle  6 , the nozzle tip  6   a  and the droplet  7  shown in  FIG. 1 . 
     First, in step S 121 , the image processing unit  137  generates images (luminance images) in a temporal sequence order by sequentially acquiring the image data output from the imaging unit  12  and by applying predetermined processing thereon. 
     In step S 122 , the image processing unit  137  executes liquid discontinuity detection processing on the generated images. More specifically, the liquid discontinuity detection unit  137   a  determines, with respect to the image (see, for example, image m 20  in  FIG. 7 ) generated in step S 121 , whether the luminance of each pixel contained a line within the predetermined range Δz directly below the image m 22  of the nozzle tip  6   a  is equal to or larger than a threshold. When the luminance of all pixels contained in the line within the range Δz is equal to or larger than the threshold, such range Δz is determined to be the background. The liquid discontinuity detection unit  137   a  sequentially performs this determination on the generated images and detects an image in which the range Δz changed from the non-background state to the background state as the liquid discontinuity image. 
     When the liquid discontinuity image is not detected (step S 123 : No), the image processing unit  137  continues to execute the liquid discontinuity detection processing on the sequentially generated images (step S 122 ). In this case, the images used for the previous liquid discontinuity detection processing may be deleted. 
     On the other hand, when the liquid discontinuity image is detected (step S 123 : Yes), the image processing unit  137  saves the liquid discontinuity image (see image m 11  in  FIG. 4 ) and a predetermined number of images following this liquid discontinuity image (see images m 12  to m 15  in  FIG. 4 ) on a memory (step S 124 ). At this time, the saved images m 11  to m 15  may be displayed on the display device  14  as shown in  FIG. 4 . 
     At this time, the image processing unit  137  calculates the image interval between the currently-detected liquid discontinuity image and the previously-detected liquid discontinuity image, and calculates the dripping cycle of the droplets  7  by multiplying the image interval and the imaging frame rate. 
     In the subsequent step S 125 , the image processing unit  137  executes the volume calculation processing of the droplet  7 . More specifically, the volume calculation unit  137   b  performs an operation by a predetermined algorithm using the images m 11  to m 15  saved in step S 124 . As for the algorithm, various publicly-known approaches may be used. As an example, noise m 18  may be removed based on the saved images, only the image of the moving (falling) droplet  7  may be extracted as a target to be measured, and the two-dimensional shape may be detected by threshold processing. The volume of the droplet  7  is calculated based on this two-dimensional shape. 
     It should be noted that five images m 11  to m 15  are shown in  FIG. 4  as images saved on a memory; however, the number of images to be saved is not limited to five, and the number necessary depending on the volume calculation processing algorithm of the droplet  7  may be saved as needed. However, the longer the time elapses from the beginning of the falling of the droplet  7 , the blurrier the image of the droplet  7  gets by being accelerated, and thus, the image becomes inappropriate for image processing. Therefore, regarding the upper limit of the number of images, when the imaging frame rate is 120 fps, eight images to be saved is sufficient. 
     The image processing unit  137  also outputs the calculated droplet volume and causes the display device  14  to display the same. Thereafter, the processing returns to the main routine. 
     With reference to  FIG. 5  again, in step S 13  following step S 12 , the flow rate control unit  136  calculates the current flow rate of the droplet  7  (infusion liquid) by dividing the volume of the droplet  7  calculated in step S 12  by the dripping cycle. 
     In step S 14 , the flow rate control unit  136  determines whether the error between the current flow rate calculated in step S 13  and the predetermined target flow rate is equal to or less than a threshold. This threshold may be predetermined according to the purpose of infusion. It should be noted that, at this point, a determination may be made as to whether the current flow rate and the target flow rate are equal. 
     When the error is determined to be equal to or less than the threshold (step S 14 : Yes), the flow rate control unit  136  integrates the current flow rate in a memory (step S 15 ). Thereby, the integrated quantity of the flow rate will be updated. 
     On the other hand, when the error between the current flow rate and the target flow rate is determined to be larger than the threshold (step S 14 : No), the flow rate control unit  136  performs opening/closing control of the clamp  8  via the actuator  9  (step S 16 ). In particular, when the current flow rate is larger than the target flow rate, control for closing the clamp  8  is performed and when the current flow rate is smaller than the target flow rate, control for opening the clamp  8  is performed. Thereafter, the processing proceeds to step S 15 . 
     In step S 17  following step S 15 , the flow rate control unit  136  determines whether the integrated quantity of the flow rate is equal to or larger than a predetermined flow rate set value. When the integrated quantity of the flow rate is determined to fail to satisfy the set value (step S 17 : No), the processing returns to step S 12 . 
     On the other hand, when the integrated quantity of the flow rate is determined to be equal to or larger than the set value (step S 17 : Yes), the flow rate control unit  136  terminates the infusion (step S 18 ) by causing the clamp  8  to block the infusion tube  5  via the actuator  9 . Thereafter, the imaging control unit  135  causes the imaging unit  12  to shut off the imaging operation. Accordingly, the operation of the droplet measurement system  10  is terminated. 
     As described above, since the first embodiment of the present invention performs the imaging exclusively with respect to the nozzle tip  6   a  and the region extending vertically downward from the nozzle tip  6   a , the load required: for image data transmission from the imaging unit  12  to the arithmetic device  13 ; for image data transfer within the arithmetic device  13 ; and even for image processing executed by the image processing unit  137 , can be reduced more than before. In addition, in the image processing unit  137 , the liquid discontinuity detection processing is performed on the images generated in a temporal sequence order and the volume of the droplet  7  is calculated by means of the image processing using only the detected liquid discontinuity image and a predetermined number of images following the detected liquid discontinuity image and thus, high-precision volume calculation processing can be performed in real time with a lighter load than before. Accordingly, a high-performance processor can be dispensed with, and the miniaturization, simplification and even cost reduction of the configuration of the entire droplet measurement system can be achieved. 
     In addition, since the first embodiment of the present invention feed-back controls the flow rate via the actuator  9  based on the volume calculation result of the droplet  7 , precise infusion can be performed. 
     Second Embodiment 
       FIG. 8  is a diagram showing the schematic configuration of the droplet measurement system according to a second embodiment of the present invention. As shown in  FIG. 8 , the droplet measurement system  20  according to the second embodiment is obtained by providing an arithmetic device  21 , instead of the arithmetic device  13 , to the droplet measurement system  10  shown in  FIG. 1  and it is further provided with a tilt sensor  22  attached to the drip tube  4 . The configuration of the respective units of the droplet measurement system  20  other than the arithmetic device  21  and the tilt sensor  22  is similar to that of the first embodiment. 
     The tilt sensor  22  is configured by, for example, a gyroscope sensor or an acceleration sensor and detects the tilt of the drip tube  4  with respect to the axis in the vertical direction. Here, the nozzle  6  is fixed to the drip tube  4  and thus, the tilt of the drip tube  4  is substantially equal to the tilt of the nozzle  6 . 
       FIG. 9  is a block diagram showing the schematic configuration of the arithmetic device  21 . As with the arithmetic device  13  shown in  FIG. 2 , the arithmetic device  21  is provided with an input and output unit  131 , a storage unit  132  and a manipulation input unit  133 , and only the configuration of a processor  211  differs from that of the processor  134  shown in  FIG. 2 . The processor  211  is provided with the imaging control unit  135 , the flow rate control unit  136  and an image processing unit  212  which includes a correction unit  212   a , in addition to the liquid discontinuity detection unit  137   a  and the volume calculation unit  137   b . The operation of the imaging control unit  135 , the flow rate control unit  136 , the liquid discontinuity detection unit  137   a  and the volume calculation unit  137   b  is similar to that of the first embodiment. 
     The correction unit  212   a  performs a correction operation for the droplet volume calculation processing by the volume calculation unit  137   b  based on the tilt of the drip tube  4  (i.e. the tilt of the nozzle  6 ) detected by the tilt sensor  22 . 
       FIG. 10  is a schematic diagram for describing the correction operation in the second embodiment of the present invention and shows the state in which the center axis C of the drip tube  4  is tilted by an angle θ with respect to the axis G in the vertical direction. Here, if it is assumed that the drip tube  4  is not tilted, when the distance between the droplet  7 , which is the object to be imaged, and the imaging element  12   a  is set to d 1 , the distance between the droplet  7  and the imaging element  12   a  will change to d 2  due to the tilt of the drip tube  4 . In other words, due to the variations in distances d 1 , d 2  arising from the tilt of an angle θ, the image of the droplet  7  becomes enlarged or reduced, and thus, the precision of volume measurement is decreased. 
     Then, the correction unit  212   a  acquires the tilt angle θ detected by the tilt sensor  22  and corrects, after the liquid discontinuity detection processing (see step S 122  in  FIG. 6 ), the two-dimensional shape of the droplet (see image m 16  in  FIG. 4 ) detected by the volume calculation unit  137   b  in the volume calculation processing (see step S 125  in  FIG. 6 ) by way of enlargement or reduction in accordance with the angle θ. The volume calculation unit  137   b  calculates the volume of the droplet based on this corrected two-dimensional shape. 
     As described above, since the second embodiment of the present invention performs the correction operation based on the tilt of the drip tube  4  detected by the tilt sensor  22 , higher precision volume calculation and flow rate control can be achieved. 
     Third Embodiment 
     The configurations of the droplet measurement system according to the third embodiment of the present invention and the arithmetic device provided to such droplet measurement system are, on the whole, similar to those shown in  FIGS. 8 and 9 . In the present third embodiment, the correction unit  212   a  shown in  FIG. 9  corrects the droplet volume calculation error arising from the focus deviation of the imaging unit  12  in the volume calculation processing (see step S 125  in  FIG. 6 ). In this case, the correction unit  212   a  may perform both the correction operation based on the tilt of the drip tube  4 , as described in the second embodiment, and a correction operation described below, or the correction unit  212   a  may perform only the correction operation described below. In the latter case, the tilt sensor  22  shown in  FIG. 8  may be omitted. 
       FIGS. 11A and 12B  are schematic diagrams for describing the correction operation in the third embodiment of the present invention. For example, when performing a typical infusion of approximately 20 droplets/mL, the drip tube  4  having the nozzle  6  fixed to the upper lid  4   a  is used as shown in  FIG. 11A . In this case, when measuring the volume of the droplet  7 , imaging is performed by the imaging unit  12  focusing on the central axis of the nozzle  6 . Typically, the nozzle  6  is designed such that its central axis coincides with the central axis of the drip tube  4  and thus, the relative position of the imaging unit  12  and the drip tube  4  is determined, using the position of the drip tube  4  as a guide, such that the imaging unit  12  focuses on the central axis of the drip tube  4 . However, in practice, due to the component tolerance or the like, the position or tilt of the central axis of the nozzle  6  may deviate from the central axis of the drip tube  4 . In such case, the focus of the imaging unit  12  deviates from the central axis of the nozzle  6  and the image m 31  of the droplet  7  will be blurred in the image m 30  shown in  FIG. 11B . As a result, an error may be generated in the volume of the droplet  7  calculated based on such image m 31 . 
     For example, when performing a low flow rate infusion of approximately 60 droplets/mL, a separate needle (i.e. a needle-shaped nozzle)  6   b  is attached to and used with the drip tube  4  as shown in  FIG. 12A . In this case, in addition to due to the component tolerance, the focus of the imaging unit  12  may deviate from the central axis of the needle  6   b  depending on how the needle  6   b  is attached. Accordingly, in the image m 40  shown in  FIG. 12B , the blurring of the image m 41  of the droplet  7  may be generated. 
     Then, the correction unit  212   a  acquires, after the liquid discontinuity detection processing (see step S 122  of  FIG. 6 ), any of the images saved on the memory (see step S 124  of  FIG. 6 ) and corrects the droplet volume calculation error arising from the focus deviation of the imaging unit  12  based on the acquired image. 
     More specifically, the width of the image of the nozzle  6  or the image of the needle  6   b  on the image when the imaging unit  12  is focused on the central axis of the nozzle  6  or the needle  6   b  is calculated based on the relationship among the designed widths (diameters) w 1 , w 3  of the nozzle  6  or the needle  6   b , the focus length of the imaging unit  12  and the size of the imaging element  12   a , and the calculated width is pre-retained in the correction unit  212   a  as a reference value. 
     When performing an infusion using the nozzle  6 , the correction unit  212   a  calculates the ratio of the pre-retained reference value of the width of the image of the nozzle  6  and the width w 2  of the image m 32  of the nozzle  6  in the image m 30  shown in, for example,  FIG. 11B . Then, based on this ratio, the correction is made by way of enlargement or reduction of the two-dimensional shape of the droplet  7  (see image m 16  of  FIG. 4 ) detected by the volume calculation unit  137   b . The volume calculation unit  137   b  calculates the volume of the droplet  7  based on this corrected two-dimensional shape. 
     When performing an infusion using the needle  6   b , the correction unit  212   a  similarly calculates the ratio of the pre-retained reference value of the width of the image of the needle  6   b  and the width w 4  of the image m 42  of the needle  6   b  in the image m 40  shown in, for example,  FIG. 12B . Then, the two-dimensional shape of the droplet  7  is corrected using this ratio. 
     As described above, since the third embodiment of the present invention corrects the volume calculation error of the droplet  7  arising from the focus deviation of the imaging unit  12 , higher precision volume calculation and flow rate control can be achieved. 
     Fourth Embodiment 
     Next, the droplet measurement system according to a fourth embodiment of the present invention will be described.  FIG. 13  is a graph showing an example of correlation between the dripping cycle (in seconds) of the droplets and the volume per droplet in the infusion, and is obtained through an experiment using the droplet measurement system  10  shown in  FIG. 1 . 
     Conventionally, the infusion flow rate control has been performed by adjusting the dripping cycle of the droplets based on the assumption that the volume of the droplets dripped down in the drip tube is constant. However, even when the dripping cycle was precisely controlled, in practice, the infusion still had not yet finished by the expected infusion termination time or, on the contrary, the infusion had already terminated, and thus, there existed a problem to the effect that high precision flow rate control was difficult. 
     Therefore, the inventors of the present application performed experiments of measuring the volume of the droplet  7  while changing the dripping cycle of the droplets  7  using the droplet measurement system  10  shown in  FIG. 1 . As a result, knowledge was obtained to the effect that the volume of the droplet  7  varied not only by the conditions, such as the viscosity of the infusion liquid, the environmental temperature or the like, or by the unexpected change in the situation, such as the movement of the patient, but also by the dripping cycle of the droplets  7 . More specifically, as shown in  FIG. 13 , it was found that the volume per droplet tended to increase when the dripping cycle of the droplets was made shorter. 
     The present fourth embodiment utilizes the above-described knowledge and enables high precision flow rate control by acquiring in advance and accumulating, by means of the droplet measurement system  10  (or the droplet measurement system  20  shown in  FIG. 8 ), the information representing the correlation between the dripping cycle of the droplets and the volume per droplet and by utilizing the accumulated information. The present fourth embodiment acquires the information representing the above-described correlation on an infusion liquid-type basis and enables the flow rate control according to the type of the infusion liquid. 
     Various methods are conceivable for the flow rate control based on the information representing the correlation between the dripping cycle of the droplets and the volume per droplet. In the present fourth embodiment, as an example, a method will be described in which the flow rate according to the dripping cycle is calculated based on the dripping cycle of the droplets and the volume per droplet, and a correlation table in which this flow rate and the dripping cycle are associated with each other is created and used in the flow rate control of the droplets. The flow rate according to the dripping cycle is calculated by dividing the droplet volume corresponding to the dripping cycle by the dripping cycle. 
       FIG. 14  is a block diagram showing the schematic configurations of the arithmetic device provided to the droplet measurement system according to the fourth embodiment of the present invention. It should be noted that the configuration of the droplet measurement system according to the fourth embodiment is, on the whole, common to that of  FIG. 8 . 
     As shown in  FIG. 14 , the arithmetic device  31  is provided with the input and output unit  131 , a storage unit  311 , the manipulation input unit  133  and a processor  312 . Among these, the operation of the input and output unit  131  and the manipulation input unit  133  is similar to that of the first embodiment. 
     The storage unit  311  is provided with, in addition to the program storage unit  132   a , a correlation table storage unit  311   a  that stores therein information representing the correlation between the dripping cycle of the droplets and the volume thereof in the form of a table. The correlation table storage unit  311   a  stores therein a plurality of such tables (correlation tables) on an infusion liquid-type basis. It should be noted that, as the information representing the correlation, a function may be used instead of the correlation table. 
     The processor  312  is provided with the imaging control unit  135 , a flow rate control unit  313  and the image processing unit  212 . Among these, the configuration and the operation of the imaging control unit  135  and the image processing unit  212  are similar to those of the first to third embodiments. It should be noted that, in the fourth embodiment, the tilt sensor  22  (see  FIG. 8 ) and the correction unit  212   a  are not required and may therefore be omitted. 
     With reference to the correlation tables stored in the correlation table storage unit  311   a , the flow rate control unit  313  acquires the dripping cycle adjustment amount necessary for making the current flow rate approach the target flow rate, and controls the actuator  9  based on this adjustment amount. 
       FIG. 15  is a flowchart illustrating the operation of the droplet measurement system according to the fourth embodiment. 
     Before starting the infusion, a user inputs information on the infusion liquid types into the arithmetic device  31  via the manipulation input unit  133 . As with the first embodiment, the light source  11  and the imaging unit  12  are placed in the neighborhood of the drip tube  4 . 
     In step S 30 , the flow rate control unit  313  acquires a correlation table associated with the infusion liquid type from the correlation table storage unit  311   a  based on the information input by the user in advance. 
     The processing in the subsequent steps S 10 , S 11  is similar to that of the first embodiment. 
     In step S 31  following step S 11 , the image processing unit  212  acquires the dripping cycle of the droplets  7 . More specifically, the image processing unit  212  generates images in a temporal sequence order by sequentially acquiring the image data output from imaging unit  12  and by applying predetermined processing thereon, and executes liquid discontinuity detection processing on these images. It should be noted that the details of the liquid discontinuity detection processing are similar to those in step S 122  in  FIG. 6 . After a liquid discontinuity image is detected, the image processing unit  212  calculates the image interval from the previously-detected liquid discontinuity image and calculates the dripping cycle by multiplying the image interval and the imaging frame rate. 
     In step S 32 , the flow rate control unit  313  acquires the flow rate (current flow rate) corresponding to the dripping cycle acquired in step S 31  by referring to the correlation table. 
     In step S 33 , the flow rate control unit  136  determines whether the error between the current flow rate and the target flow rate is equal to or less than a threshold. It should be noted that, at this time, a determination may alternatively be made whether the current flow rate and the target flow rate are equal. 
     When the error is determined to be equal to or less than the threshold (step S 33 : Yes), the processing proceeds to step S 15 . The processing in step S 15  and steps S 17 , S 18 , subsequent to step S 15 , is similar to that of the first embodiment. 
     On the other hand, when the error is determined to be larger than the threshold (step S 33 : No), the flow rate control unit  313  acquires the dripping cycle adjustment amount necessary for causing the current flow rate to transition to the target flow rate. More specifically, the flow rate control unit  313  acquires the dripping cycle corresponding to the target flow rate by referring to the correlation table and calculates the difference between such dripping cycle and the current dripping cycle. This difference corresponds to the dripping cycle adjustment amount. 
     In step S 35 , the flow rate control unit  136  performs opening and closing control of the clamp  8  via the actuator  9  by aiming at the dripping cycle adjustment amount acquired in step S 34 . For example, when the dripping cycle is made short, the volume of the droplet  7  becomes large and the flow rate increases suddenly and thus, the actuator  9  is controlled such that the clamp  8  is opened on a modest scale. Thereafter, the processing proceeds to step S 15 . 
     As described above, since the fourth embodiment of the present invention performs flow rate control by adjusting the dripping cycle based on the information representing the correlation between the pre-acquired dripping cycle of the droplets  7  and the volume thereof, the infusion liquid flow rate can be made to quickly approach the target flow rate. 
     The current dripping cycle is acquired by means of image processing in the above-described fourth embodiment; however, the current dripping cycle may be acquired by means of a conventional method of directly counting the number of dripping droplets by a sensor or the like. Namely, by combining the flow rate control based on the information representing the correlation as described in the present fourth embodiment with the conventional-type infusion system, desired flow rate control can be performed within a shorter time period and with higher precision than before. 
     The present invention is not limited to the above-described first to fourth embodiments and various inventions can be made by appropriately combining a plurality of components disclosed in the respective embodiments. For example, inventions can be made by removing certain components from the entirety of the components shown in the respective embodiments, or by appropriately combining the components shown in different embodiments. 
     DESCRIPTION OF REFERENCE NUMBERS 
     
         
           1  Infusion device 
           2  Infusion bag 
           2   a  Drainage port 
           3  Intermediate tube 
           4  Drip tube 
           4   a  Upper lid 
           5  Infusion tube 
           6  Nozzle 
           6   a  Tip (nozzle tip) 
           6   b  Needle 
           7  Droplet(s) 
           8  Clamp 
           9  Actuator 
           10 ,  20  Droplet measurement system 
           11  Light source 
           12  Imaging unit 
           12   a  Imaging element 
           12   b  Imaging region 
           13 ,  21 ,  31  Arithmetic device 
           14  Display device 
           22  Tilt sensor 
           131  Input and output unit 
           132 ,  311  Storage unit 
           132   a  Program storage unit 
           133  Manipulation input unit 
           134 ,  211 ,  312  Processor 
           135  Imaging control unit 
           136 ,  313  Flow rate control unit 
           137 ,  212  Image processing unit 
           137   a  Liquid discontinuity detection unit 
           137   b  Volume calculation unit 
           212   a  Correction unit 
           311   a  Correlation table storage unit