Patent Publication Number: US-2022228586-A1

Title: Diaphragm pump and blood purification apparatus using same

Description:
TECHNICAL FIELD 
     The present invention relates to a diaphragm pump and a blood purification apparatus using the same. 
     BACKGROUND ART 
     Diaphragm pumps perform pump operation by displacing a diaphragm, which is a flexible membrane, in a case and thereby changing a volume of a space in the case (referred to as a first space) through which a liquid to be fed passes. Drive sources to displace the diaphragm include a mechanically-driven type (cam type, solenoid type, etc.), an electrically-driven type (piezoelectric element, etc.), a hydraulic drive, and a pneumatic drive. 
     In, e.g., hydraulically-driven or pneumatically-driven diaphragm pumps, a diaphragm is displaced by compressing or decompressing a driving fluid filling a space separated from the first space by the diaphragm in the case (referred to as a second space) (see, e.g., Patent Literature 1). 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese published examined application No. 61-4998 
     SUMMARY OF INVENTION 
     Technical Problem 
     The present inventors conceived of using a diaphragm pump as a liquid feed pump of a blood purification apparatus. The diaphragm pump is desired to be as compact as possible to avoid an increase in size of the blood purification apparatus. 
     Therefore, it is an object of the invention to provide a compact diaphragm pump and a blood purification apparatus using the same. 
     Solution to Problem 
     To solve the problem mentioned above, the invention provides a diaphragm pump, comprising:
         a case;   a diaphragm dividing a space in the case into a first space and a second space;   a liquid feed flow path comprising an inflow path to introduce a liquid to be fed into the first space and a outflow path to discharge the liquid to be fed from the first space;   a drive unit comprising a compression/decompression device that repeatedly causes displacement of the diaphragm by repeating compression and decompression of a driving fluid filling the second space; and   a valve mechanism to open and close the inflow path and the outflow path,   wherein the drive unit comprises a pressure release mechanism to release pressure of the driving fluid after the driving fluid is compressed or decompressed by the compression/decompression device.       

     To solve the problem mentioned above, the invention also provides a blood purification apparatus, comprising:
         a blood circuit to extracorporeally circulate blood of a patient;   a liquid supply flow path to supply a supply liquid to the blood circuit or to a blood purifier provided on the blood circuit, and   a waste liquid flow path to discharge a waste liquid from the blood purifier,   wherein at least one of liquid feed pumps provided on the blood circuit, the liquid supply flow path and the waste liquid flow path comprises the diaphragm pump described above.       

     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to the invention, it is possible to provide a compact diaphragm pump and a blood purification apparatus using the same. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram illustrating a blood purification apparatus in an embodiment of the present invention. 
         FIG. 2A  is a schematic configuration diagram illustrating a diaphragm pump in the embodiment of the invention. 
         FIG. 2B  is a cross-sectional view showing a case. 
         FIG. 2C  is a schematic configuration diagram illustrating a reciprocating pump. 
         FIG. 3  is a flowchart showing a control flow executed by a control unit. 
         FIG. 4A  is an explanatory diagram illustrating a drive range of a plunger of the invention. 
         FIG. 4B  is an explanatory diagram illustrating the drive range of the plunger in a conventional technique. 
         FIG. 5  is a flowchart showing a modification of the control flow executed by the control unit. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiment 
     An embodiment of the invention will be described below in conjunction with the appended drawings. 
     (Blood Purification Apparatus) 
     Firstly, a blood purification apparatus in which a diaphragm pump of the present embodiment is used will be described.  FIG. 1  is a schematic configuration diagram illustrating a blood purification apparatus in the present embodiment. 
     As shown in  FIG. 1 , a blood purification apparatus  10  includes a liquid supply flow path  13  to supply a supply liquid to a blood circuit  11  extracorporeally circulating blood of a patient or to a blood purifier  12  provided on the blood circuit  11 , and a waste liquid flow path  14  to discharge a waste liquid from the blood purifier  12 .  FIG. 1  shows an example in which the liquid supply flow path  13  is a dialysate flow path  13   a  to supply a dialysate to the blood purifier  12 . However, it is not limited thereto, and the liquid supply flow path  13  may be a replenishing liquid flow path to supply a replenishing liquid directly to the blood circuit  11 , or may include both the dialysate flow path  13   a  and the replenishing liquid flow path. 
     The blood circuit  11  is composed of, e.g., a flexible tube, etc. A blood pump  111 , the blood purifier  12  and a gas-liquid separator  112  are sequentially provided on the blood circuit  11  from the upstream to the downstream of a blood flow. The blood pump  111  is a liquid feed pump to send blood. The gas-liquid separator  112  is a device to remove air bubbles from the blood. 
     From a RO (Reverse Osmosis) device (not shown) which produces clean water (referred to as dialysis water) using a reverse osmosis membrane (RO membrane), dialysis water is supplied to the dialysate flow path  13   a . Two types of undiluted dialysate fluids, an undiluted fluid A and an undiluted fluid B, are also supplied to the dialysate flow path  13   a . The two undiluted fluids are respectively stored in undiluted fluid storage tanks  151 , and the undiluted fluid A and the undiluted fluid B are supplied to the dialysate flow path  13   a  respectively from the undiluted fluid storage tanks  151  via undiluted fluid flow paths  152 . Undiluted fluid injection pumps  153 , which are liquid feed pumps pumping out the undiluted fluid A or the undiluted fluid B, are respectively provided on the two undiluted fluid flow paths  152 . The undiluted fluid A and the undiluted fluid B are mixed with the dialysis water in the dialysate flow path  13   a  and the dialysate is thereby prepared. The prepared dialysate is introduced into the blood purifier  12  via a duplex pump  16 . 
     The waste liquid from the blood purifier  12  is discharged through the waste liquid flow path  14 . The duplex pump  16  is provided over the dialysate flow path  13   a  and the waste liquid flow path  14  and performs pump operation so that an amount of the dialysate introduced into the blood purifier  12  is equal to an amount of the waste liquid discharged from the blood purifier  12 . In addition, a water removal flow path  14   a  is provided on the waste liquid flow path  14  so as to bypass the dual pump  16 , and a water removal pump  17  is provided on the water removal flow path  14   a . When the water removal pump  17  is driven, the amount of the waste liquid discharged from the blood purifier  12  becomes larger than the amount of the dialysate introduced into the blood purifier  12  and water is removed from the blood. It is possible to adjust the amount of water removed from the blood by adjusting the amount of liquid sent by the water removal pump  17 . 
     In the blood purification apparatus  10  of the present embodiment, a diaphragm pump  1  of the invention is used as at least one of liquid feed pumps provided on the blood circuit  11 , the liquid supply flow path  13  (the dialysate flow path  13   a  in this example) and the waste liquid flow path  14 . Although  FIG. 1  shows an example in which the diaphragm pump  1  is used as the water removal pump  17 , it is not limited thereto. The diaphragm pump  1  can be used as the other liquid feed pump such as the blood pump  111 , the undiluted fluid injection pump  153  or the duplex pump  16 . 
     The configuration of  FIG. 1  is only an example and a specific configuration of the blood purification apparatus  10  can be changed appropriately. 
     (Diaphragm pump  1 ) 
       FIG. 2A  is a schematic configuration diagram illustrating the diaphragm pump  1  in the present embodiment,  FIG. 2B  is a cross-sectional view showing a case, and  FIG. 2C  is a schematic configuration diagram illustrating a reciprocating pump. As shown in  FIGS. 2A to 2C , the diaphragm pump  1  includes a case  2 , a diaphragm  3  dividing a space in the case  2  into a first space  2   a  and a second space  2   b , a liquid feed flow path  4  having an inflow path  4   a  to introduce a liquid to be fed (the waste liquid in case of the water removal pump  17 ) into the first space and a outflow path  4   b  to discharge the liquid to be fed from the first space  2   a , a drive unit  5  to repeatedly cause displacement of the diaphragm  3  by repeating compression and decompression of a driving fluid filling the second space  2   b , a valve mechanism  6  capable of opening and closing the inflow path  4   a  and the outflow path  4   b , and a control unit  7 . 
     The case  2  is composed of a hard resin molded article, etc. The case  2  integrally includes a first connection part  21  being in communication with the first space  2   a  and connected to the inflow path  4   a  and a second connection part  22  being in communication with the first space  2   a  and connected to the outflow path  4   b . The case  2  also integrally includes a protruding part  23  that is in communication with the second space  2   b  and protrudes outward. The pressure sensing device  1  also includes a socket part  24  which is provided separately from the case  2  and into which the protruding part  23  is inserted and connected. Although it is not shown, a driving flow path  53  (described later) is connected to the socket part  24 , and the driving flow path  53  is communicated with the second space  2   b  by inserting and connecting the protruding part  23  to the socket part  24 . The case  2  is configured to be removable from the socket part  24  by detaching the protruding part  23  from the socket part  24 , which allows the case  2  to be disposable. In this regard, the case  2  does not need to be entirely disposable and may be configured to be splittable on, e.g., the second space  2   b  side relative to the diaphragm  3  so that only a portion of the case  2  including the first space  2   a  can be disposable. It is not necessary to separately form the case  2  and the socket part  24 , and the case  2  may be integrally formed with the socket part  24 . 
     The diaphragm  3  is a flexible membrane and is provided in the case  2  so as to divide an internal space of the case  2  into two spaces, the first space  2   a  and the second space  2   b . Materials of the case  2  and the diaphragm  3  are not specifically limited. 
     The inflow path  4   a  and the outflow path  4   b  are in communication with the first space  2   a  of the case  2 . However, it is not limited thereto. It may be configured such that the inflow path  4   a  and the outflow path  4   b  are connected and a connection flow path extending from the connected portion therebetween is in communication with the first space  2   a  of the case  2 . In this case, the case  2  needs to have only one connection port to the first space  2   a  and it is thus possible to reduce the number of components such as sealing member and to reduce the cost. 
     An inflow-side solenoid valve  6   a  capable of opening and closing the inflow path  4   a  is provided on the inflow path  4   a . An outflow-side solenoid valve  6   b  is provided on the outflow path  4   b . The inflow-side solenoid valve  6   a  and the outflow-side solenoid valve  6   b  constitute the valve mechanism  6  and are controlled to be opened and closed by the control unit  7 . 
     The valve mechanism  6  is controlled by the control unit  7  to open and close the inflow path  4   a  and the outflow path  4   b  according to displacement of the diaphragm  3 . In more detail, by opening the inflow-side solenoid valve  6   a  and closing the outflow-side solenoid valve  6   b  in a decompression step and a compression relief step described later, the liquid to be fed is introduced (sucked) into the first space  2   a  from the inflow path  4   a . On the other hand, by closing the inflow-side solenoid valve  6   a  and opening the outflow-side solenoid valve  6   b  in a compression step and a decompression relief step described later, the liquid to be fed is sent out from the first space  2   a  to the outflow path  4   b . The liquid is fed by repeating this operation. 
     The drive unit  5  has a compression/decompression device  51  and a pressure release mechanism  52 . The compression/decompression device  51  repeatedly causes displacement of the diaphragm  3  by repeating compression and decompression of the driving fluid filling the second space  2   b . In the present embodiment, a reciprocating pump  51   a  is used as the compression/decompression device  51 . 
     The reciprocating pump  51   a  is also called a plunger pump or a piston pump, and has a cylinder  511  in communication with the second space  2   b  via the driving flow path  53 , a plunger  512  (or a piston) provided so as to be able to advance and retract within the cylinder  511 , and a plunger driving part  513  to advance and retract the plunger  512 . 
     A positive and negative pressure source such as compressor or vacuum generator can be used as the compression/decompression device  51 , but in this case, a regulator to control the pressure or a flow path switching mechanism needs to be provided and the system configuration thus becomes complicated and large and operating sound is also loud. Meanwhile, a peristaltic pump configured to squeeze a tube can be also used as the compression/decompression device  51 , but as compared to the reciprocating pump  51   a  which is used as the compression/decompression device  51  in the present embodiment, a worn part needs to be replaced since the squeezed tube deteriorates over time or operating sound from the squeezed part is loud. By using the reciprocating pump  51   a  as the compression/decompression device  51 , the size can be small with a simple system configuration and the operating noise can also be reduced. 
     The second space  2   b , the driving flow path  53  and the cylinder  511  are filled with the driving fluid and it is possible to compress/decompress the driving fluid by advancing/retracting the plunger  512  within the cylinder  511  by the plunger driving part  513  of the reciprocating pump  51   a . In the present embodiment, air is used as the driving fluid. As the plunger driving part  513 , it is possible to use, e.g., a stepping motor. 
     The pressure release mechanism  52  is to release pressure of the driving fluid (to bring the pressure closer to the atmospheric pressure) after the driving fluid is compressed or decompressed by the reciprocating pump  51   a  as the compression/decompression device  51 . The pressure release mechanism  52  has a pressure release flow path  521  with one end in communication with the second space  2   b  and the other end opened to the atmosphere, and a pressure release valve  522  provided on the pressure release flow path  521  to open/close the pressure release flow path  521 . In the example shown in the drawing, the pressure release flow path  521  is connected at one end to the driving flow path  53  and is in communication with the second space  2   b  via the driving flow path  53 . 
     In the present embodiment, the other end of the pressure release flow path  521  is opened to the atmosphere since air is used as the driving fluid. In this regard, when a liquid is used as the driving fluid, the other end of the pressure release flow path  521  should be connected to a container for storing this liquid. The container for storing the liquid may be opened to the atmosphere or may be, e.g., expandable and contractable in a balloon like manner. However, since use of a liquid as the driving fluid makes handling difficult, it is more desirable to use the air as the driving fluid. In addition, the pressure to which the driving fluid is released is the atmospheric pressure in the present embodiment, but it is not limited thereto. The pressure to which the driving fluid is released may not be the atmospheric pressure. 
     An air filter  531  is provided in the driving flow path  53 . The air filter  531  is a so-called hydrophobic filter, and is configured to allow gases to pass therethrough but to not allow liquids to pass therethrough (very high resistance to the passage of liquids). Furthermore, a pressure sensor  532  to measure the pressure of the driving fluid is provided on the driving flow path  53  on the reciprocating pump  51   a  side relative to the air filter  531 . In the event that, e.g., the liquid to be fed leaks to the second space  2   b  due to damage on the diaphragm  3 , etc., the air filter  531  gets wet and pressure of the driving fluid when performing compression by the reciprocating pump  51   a  becomes higher. Thus, the control unit  7  can detect failure of the diaphragm pump  1  by determining whether an output value of the pressure sensor  532  is more than a predetermined threshold. 
     In addition to the failure detection mentioned above, the control unit  7  controls opening and closing of the valve mechanism  6 , driving of the reciprocating pump  51   a  as the compression/decompression device  51 , and opening and closing of the pressure release valve  522  of the pressure release mechanism  52 . The control unit  7  is realized by appropriately combining an arithmetic element such as CPU, a storage device such as memory, a software, and an interface, etc. 
     Next, the drive control of the reciprocating pump  51   a  and the opening/closing control of the pressure release valve  522  by the control unit  7  will be described. As shown in  FIG. 3 , the control unit  7  sequentially and repeatedly performs a decompression step (Step S 1 ), a decompression relief step (Step S 2 ), a compression step (Step S 3 ), a compression relief step (Step S 4 ), and a plunger position adjustment step (Step S 5 ). 
     In the decompression step of Step S 1 , the plunger  512  is firstly retracted from a reference position in Step S 11 , and whether a predetermined time has elapsed is determined in Step S 12 . When the determination made in Step S 12  is No, the process returns to Step S 11 . When the determination made in Step S 12  is Yes, the plunger  512  is stopped in Step S 13 . The reference position in the present embodiment is a position of the plunger  512  when fully pushed (the most advanced position). By performing the decompression step, the driving fluid is decompressed and the diaphragm  3  is displaced such that the volume of the first space  2   a  increases. At this time, the control unit  7  opens the inflow-side solenoid valve  6   a  and closes the outflow-side solenoid valve  6   b , and the liquid to be fed is thereby introduced (sucked) into the first space  2   a  from the inflow path  4   a.    
     In the decompression relief step of Step S 2 , after the driving fluid is released to the atmospheric pressure by opening the pressure release valve  522  in Step S 21 , the pressure release valve  522  is closed in Step S 22 . Since pressure of the driving fluid becomes the atmospheric pressure by performing the decompression relief step, the diaphragm  3  returns to a no-load position. Before opening the pressure release valve  522  in Step S 21 , the control unit  7  closes the inflow-side solenoid valve  6   a  and opens the outflow-side solenoid valve  6   b  so that the liquid to be fed does not flow back. In this regard, the outflow-side solenoid valve  6   b  may be closed. 
     In the compression step of Step S 3 , the plunger  512  is advanced in Step S 31 , and whether a predetermined time has elapsed is determined in Step S 32 . When the determination made in Step S 32  is No, the process returns to Step S 31 . When the determination made in Step S 32  is Yes, the plunger  512  is stopped in Step S 33 . By performing the compression step, the driving fluid is compressed and the diaphragm  3  is displaced such that the volume of the first space  2   a  decreases. At this time, the control unit  7  maintains the state in which the inflow-side solenoid valve  6   a  is close and the outflow-side solenoid valve  6   b  is open, and the liquid to be fed is thereby sent out from the first space  2   a  to the outflow path  4   b.    
     Since the air is sucked from the outside by the decompression relief step of Step S 2 , the volume of the driving fluid filling the second space  2   b , the driving flow path  53  and the cylinder  511  is larger at the time of the compression step than at the time of the decompression step. Thus, if, e.g., the plunger  512  is advanced to the reference position in the compression step, the driving fluid may be excessively compressed, causing problems such as damage on the diaphragm  3 . Therefore, in case that pressure (absolute value) at the time of compression and pressure (absolute value) at the time of decompression are set to approximately the same, a travel distance of the plunger  512  needs to be smaller at the time of the compression step than at the time of the decompression step. In the present embodiment, the travel distance of the plunger  512  at the time of the compression step is smaller than that at the time of the decompression step. 
     In the compression relief step of Step S 4 , the driving fluid is released to the atmospheric pressure by opening the pressure release valve  522  in Step S 41 . Since pressure of the driving fluid becomes the atmospheric pressure by performing the compression relief step, the diaphragm  3  returns to the no-load position. At this time, the control unit  7  opens the inflow-side solenoid valve  6   a  and closes the outflow-side solenoid valve  6   b  so that the liquid to be fed does not flow back. In this regard, the inflow-side solenoid valve  6   a  may be closed. Then, the inflow-side solenoid valve  6   a  may be opened after driving the plunger  512  in the plunger position adjustment step of Step S 5  in a closed state of the both solenoid valves  6   a  and  6   b  which are closed in the compression relief step of Step S 4 , or in case of repeating Step S 1  again after Step S 5 , the inflow-side solenoid valve  6   a  may be opened after driving the plunger  512  in Step S 1 . 
     In the plunger position adjustment step of Step S 5 , the plunger  512  is advanced to the reference position (in this example, the position of the plunger  512  when fully pushed) in Step S 51  and the plunger  512  is stopped in Step S 52 , and after that, the pressure release valve  522  is closed in Step S 53 . In the present embodiment, since the travel distance of the plunger  512  at the time of the compression step is smaller than that at the time of the decompression step as described above, it is necessary to return the plunger  512  to the reference position by the plunger position adjustment step. In case that pressure (absolute value) at the time of compression is set to higher than pressure (absolute value) at the time of decompression, the plunger  512  can be moved to the reference position in the compression step and the plunger position adjustment step of Step S 5  can be omitted. In case that the plunger  512  is advanced beyond the reference position in the compression step, the plunger  512  should be retracted and returned to the reference position in the plunger position adjustment step. 
     After that, in Step S 6 , the control unit  7  determines whether to end the pump operation. When the determination made in Step S 6  is No, the process returns to the decompression process of Step S 1 . When the determination made in Step S 6  is Yes, the process ends. The pump operation is ended when, e.g., the output value of the pressure sensor  532  exceeds a predetermined threshold or when a termination command by an operation of a user or a program, etc., is input to the control unit  7 . 
     In the diaphragm pump  1  of the present embodiment, after the plunger  512  is retracted in the decompression step, it is opened to atmosphere and the plunger  512  is then advanced in the compression step, hence, a travel region of the plunger  512  in the cylinder  511  when decompressing and that when compressing can be the same, as shown in  FIG. 4A . Therefore, the travel distance (stroke distance) of the plunger  512  can be reduced, which contributes to size reduction of the reciprocating pump  51   a  and size reduction of the entire diaphragm pump  1 . 
     In contrast to this, in a conventional diaphragm pump which does not include the pressure release mechanism  52 , after retracting the plunger  512  in the decompression step, pressure of the fluid to be fed and pressure of the driving fluid are balanced by advancing the plunger  512  in the compression step, and after that, the plunger  512  needs to be further advanced to compress the driving fluid, hence, the travel region of the plunger  512  in the cylinder  511  when decompressing is different from that when compressing, as shown in  FIG. 4B . Therefore, in the conventional structure, the travel distance (stroke distance) of the plunger  512  is long and the size of the reciprocating pump  51   a  is thus large. In this regard, it is conceivable to increase a cross-sectional area of the cylinder  511  to reduce the travel distance (stroke distance) of the plunger  512 , but in this case, a pressure-receiving area of the plunger  512  increases and the plunger driving part  513  which drives the plunger  512  is thus increased in size.  FIGS. 4A and 4B  show that the diaphragm  3  at the time of decompression moves toward a broken line and how the diaphragm  3  at the time of compression moves toward a dash-dot line. 
     When the diaphragm pump  1  is used as, e.g., a blood pump, blood flows through the liquid feed flow path  4  and the first space  2   a , hence, it is desirable that the liquid feed flow path  4  and the case  2  be removable from the driving flow path  53  and be disposable. Even when the diaphragm pump  1  is used as another liquid feed pump, configuring the liquid feed flow path  4  and the case  2  to be removable from the driving flow path  53  and to be disposable eliminates time and effort for cleaning and improves the convenience. Furthermore, by configuring the liquid feed flow path  4  and the case  2  to be disposable, it is also possible to suppress a decrease in discharge accuracy due to deterioration over time, precipitation of calcium carbonate in the dialysate or adhesion of proteins contained in the waste dialysate. In this regard, the case  2  does not need to be entirely disposable and may be configured to be splittable on, e.g., the second space  2   b  side relative to the diaphragm  3  so that only a portion of the case  2  including the first space  2   a  can be disposable. 
     (Modification) Although the position of the plunger  512  when fully pushed is set as the reference position in the present embodiment, the reference position can be appropriately set. In this case, however, to return the plunger  512  to the reference position, it is necessary to provide a plunger position detection unit capable of detecting that the plunger  512  is located at the reference position. As the plunger position detection unit, it is possible to use, e.g., an encoder that detects a rotational speed of a motor used for the plunger driving part  513 , etc., or a linear potentiometer that directly detects the position of the plunger  512 , etc. In addition, when using a stepping motor (pulse motor) as the plunger driving part  513 , it is possible to detect the position of the plunger  512  also based on a driving amount (a number of output pulses) of the stepping motor. Furthermore, as the plunger position detection unit, it also possible to use a contact type sensor such as limit switch, strain gauge or piezoelectric element sensor and it is also possible to use a non-contact sensor such as photoelectric sensor or pressure sensor. 
     A diaphragm position detection unit to detect a position of the diaphragm  3  may be further provided. The diaphragm position detection unit may be configured to detect the position of the diaphragm  3  using, e.g., a sensor or the like such as photoelectric sensor. It is also possible to estimate the position of the diaphragm  3  by using the detection result of the encoder or linear potentiometer mentioned above, or the driving amount of the stepping motor, or an output of the pressure sensor  532 . In case that the diaphragm position detection unit is provided, the control unit  7  ends the decompression step when the position of the diaphragm  3  reaches a predetermined decompression position in the decompression step, and ends the compression step when the position of the diaphragm  3  reaches a predetermined compression position in the compression step. 
       FIG. 5  shows a control flow when the plunger position detection unit and the diaphragm position detection unit are provided.  FIG. 5  is a modification of the control flow of  FIG. 3  in which Step S 12  in the decompression step of Step S 1 , Step S 32  in the compression step of Step S 3  and Step S 51  of the plunger position adjustment step of Step S 5  in are changed. 
     In case that the diaphragm position detection unit is provided, in the decompression step of Step S 1 , the plunger  512  is retracted from the reference position in step S 11 , and whether the diaphragm  3  has reached a predetermined decompression position is determined in Step S 12  based on the detection result or estimation result of the diaphragm position detection unit. When the determination made in Step S 12  is No, the process returns to Step S 11 . When the determination made in Step S 12  is Yes, the plunger  512  is stopped in Step S 13 . 
     Meanwhile, in the compression step of Step S 3 , the plunger  512  is advanced in Step S 31 , and whether the diaphragm  3  has reached a predetermined compression position is determined in Step S 32  based on the detection result or estimation result of the diaphragm position detection unit. When the determination made in Step S 32  is No, the process returns to Step S 31 . When the determination made in Step S 32  is Yes, the plunger  512  is stopped in Step S 33 . 
     Furthermore, in the plunger position adjustment step of Step S 5 , the plunger  512  is advanced in Step S 511 , and whether the plunger  512  is located at the reference position is determined in Step S 512 . When the determination made in Step S 512  is No, the process returns to Step S 511 . When the determination made in Step S 512  is Yes, the plunger  512  is stopped in Step S 52  and the pressure release valve  522  is then closed in Step S  53 . 
     Since the travel distance of the plunger  512  can be controlled more accurately by having the plunger position detection unit, the discharge amount of the reciprocating pump  51   a  can be controlled with high repetition accuracy. In addition, by using the stepping motor for the plunger driving part  513 , it is possible to realize the plunger position detection unit without separately adding a sensor, etc., thereby contributing to the reduction in the number of components, cost reduction, and size reduction. Furthermore, when stopping the reciprocating pump  51   a , the stopping process can be performed without via a sensor or the like, hence, it is possible to accurately stop the plunger  512  at a desired position without being affected by a time lag, and the discharge amount and the introduction amount of the reciprocating pump  51   a  can be arbitrarily set with higher accuracy. 
     In addition, by having the diaphragm position detection unit and configuring to switch between compression and decompression according to the position of the diaphragm  3 , it is possible to suppress excessive load on the diaphragm  3 , to further increase safety by suppressing problems such as damage on the diaphragm  3 , and to suppress deterioration of the diaphragm  3 . 
     Furthermore, the diaphragm  3  may be configured to be naturally displaced such that the volume of the first space  2   a  is increased by liquid pressure of the liquid to be fed. In this case, by using, e.g., a predetermined liquid feed drive source provided on the upstream side of the inflow-side solenoid valve  6   a  in a state in which the pressure release valve  522  is opened, the diaphragm  3  is displaced such that the volume of the first space  2   a  is increased by the liquid pressure of the liquid to be fed, and the liquid to be fed thereby flows into the first space  2   a . Then, by driving the compression/decompression device  51  (the reciprocating pump  51   a ) in a state in which the pressure release valve  522  is closed, the driving fluid is compressed and this causes the diaphragm  3  to be displaced such that the volume of the second space  2   b  increases, and the liquid to be fed thereby flows out of the first space  2   a.    
     Functions and Effects of the Embodiment 
     As described above, in the diaphragm pump  1  of the present embodiment, the drive unit  5  has the pressure release mechanism  52  to release pressure of the driving fluid after the driving fluid is compressed or decompressed by the compression/decompression device  51  (the reciprocating pump  51   a ). 
     By having the pressure release mechanism  52 , it is possible to release pressure of the driving fluid after compressing (or decompressing) the driving fluid and then quickly compress (or decompress) the driving fluid. As a result, the cycle time to decompress and compress the driving fluid, i.e., a cycle of displacement of the diaphragm  3  can be shortened, and the diaphragm pump  1  having a high flow rate can be realized. 
     When the pressure release mechanism  52  is not provided, the diaphragm  3  gradually returns to the no-load position with the movement of the plunger  512  after decompression or compression of the driving fluid, and the length of time that a load (tension) is applied to the diaphragm  3  is longer. By having the pressure release mechanism  52  as in the present embodiment and releasing the pressure after decompression or compression of the driving fluid, it is possible to reduce the length of time that a load (tension) is applied to the diaphragm  3  since the diaphragm  3  instantaneously returns to the no-load position. That is, according to the present embodiment, it is possible to reduce damage on the diaphragm  3  and possible to realize a long-life diaphragm pump  1 . 
     Furthermore, when the pressure release mechanism  52  is not provided, the driving fluid is sealed in. Therefore, in case that air is used as the driving fluid, the air expands or contracts due to an influence of temperature, and intended displacement of the diaphragm  3  may not be obtained even if the plunger  512  is operated with the same stroke, resulting in variation in the liquid feed amount. In addition, when the driving fluid becomes hot and expands, a large load may be applied to the diaphragm  3 , causing damage on the diaphragm  3 . According to the present embodiment, by releasing the pressure every time the driving fluid is compressed or decompressed, it is possible to eliminate the influence of temperature of the driving fluid, to suppress problems such as damage on the diaphragm  3 , and to accurately control the liquid feed amount. 
     Furthermore, in the present embodiment, when the reciprocating pump  51   a  is used as the compression/decompression device  51 , the compression/decompression device  51  can be reduced in size by reducing the travel distance (stroke distance) of the plunger  512 . In addition, by using the reciprocating pump  51   a  as the compression/decompression device  51 , it is possible to realize the diaphragm pump  1  with less vibration and less operating sound than general compressors, etc. 
     Summary of the Embodiment 
     Technical ideas understood from the embodiment will be described below citing the reference numerals, etc., used for the embodiment. However, each reference numeral, etc., described below is not intended to limit the constituent elements in the claims to the members, etc., specifically described in the embodiment. 
     [1] A diaphragm pump ( 1 ) comprising: a case ( 2 ); a diaphragm ( 3 ) dividing a space in the case ( 2 ) into a first space ( 2   a ) and a second space ( 2   b ); a liquid feed flow path ( 4 ) comprising an inflow path ( 4   a ) to introduce a liquid to be fed into the first space ( 2   a ) and an outflow path ( 4   b ) to discharge the liquid to be fed from the first space ( 2   a ); a drive unit ( 5 ) comprising a compression/decompression device ( 51 ) that repeatedly causes displacement of the diaphragm ( 3 ) by repeating compression and decompression of a driving fluid filling the second space ( 2   b ); and a valve mechanism ( 6 ) to open and close the inflow path ( 4   a ) and the outflow path ( 4   b ), wherein the drive unit ( 5 ) comprises a pressure release mechanism ( 52 ) to release pressure of the driving fluid after the driving fluid is compressed or decompressed by the compression/decompression device ( 51 ). 
     [2] The diaphragm pump ( 1 ) described in [1], comprising: a control unit ( 7 ) that controls the compression/decompression device ( 51 ) and the pressure release mechanism ( 52 ), wherein the control unit ( 7 ) repeatedly performs a decompression step of decompressing the driving fluid by the compression/decompression device ( 51 ), a decompression relief step of releasing the pressure of the driving fluid by the pressure release mechanism ( 52 ), a compression step of compressing the driving fluid by the compression/decompression device ( 51 ), and a compression relief step of releasing the pressure of the driving fluid by the pressure release mechanism ( 52 ). 
     [3] The diaphragm pump ( 1 ) described in [2], wherein the compression/decompression device ( 51 ) comprises a reciprocating pump ( 51   a ) that comprises a cylinder ( 511 ) in communication with the second space ( 2   b ), a plunger ( 512 ) provided so as to be able to advance and retract within the cylinder ( 511 ), and a plunger driving part ( 513 ) to advance and retract the plunger ( 512 ), and compresses and decompresses the driving fluid by advancing and retracting the plunger ( 512 ) within the cylinder ( 511 ) by the plunger driving part ( 513 ). 
     [4] The diaphragm pump ( 1 ) described in [3], wherein the control unit ( 7 ) is configured to decompress the driving fluid by retracting the plunger ( 512 ) from a reference position in the decompression step, and performs a plunger position adjustment step of moving the plunger ( 512 ) to the reference position after the compression relief step. 
     [5] The diaphragm pump ( 1 ) described in [4], comprising: a plunger position detection unit capable of detecting that the plunger ( 512 ) is located at the reference position. 
     [6] The diaphragm pump ( 1 ) described in any one of [2] to [5], comprising: a diaphragm position detection unit to detect a position of the diaphragm ( 3 ), wherein the control unit ( 7 ) ends the decompression step when the position of the diaphragm ( 3 ) reaches a predetermined decompression position in the decompression step, and ends the compression step when the position of the diaphragm ( 3 ) reaches a predetermined compression position in the compression step. 
     [7] The diaphragm pump ( 1 ) described in any one of [1] to [6], wherein the driving fluid comprises air, and wherein the pressure release mechanism ( 52 ) comprises a pressure release flow path ( 521 ) with one end in communication with the second space ( 2   b ) and the other end opened to the atmosphere, and a pressure release valve ( 522 ) provided on the pressure release flow path ( 521 ) to open/close the pressure release flow path ( 521 ). 
     [8] The diaphragm pump ( 1 ) described in any one of [1] to [7], comprising: a socket part ( 24 ) to which the drive unit ( 5 ) is connected, wherein the case ( 2 ) and the diaphragm ( 3 ) are provided so as to be removable from the socket part ( 24 ). 
     [9] A blood purification apparatus ( 10 ), comprising: the diaphragm pump ( 1 ) described in any one of [1] to [7] as at least one of liquid feed pumps provided on a blood circuit ( 11 ) to extracorporeally circulate blood of a patient, a liquid supply flow path ( 13 ) to supply a supply liquid to the blood circuit ( 11 ) or to a blood purifier ( 12 ) provided on the blood circuit ( 11 ), and a waste liquid flow path ( 14 ) to discharge a waste liquid from the blood purifier ( 12 ). 
     Although the embodiment of the invention has been described, the invention according to claims is not to be limited to the embodiment described above. In addition, not all combinations of the features described in the embodiment are necessary to solve the problem of the invention. 
     The invention can be appropriately modified and implemented without departing from the gist thereof. For example, although the example in which the valve mechanism  6  is composed of a solenoid valve has been described in the embodiment, the valve mechanism  6  may be composed of a check valve. 
     REFERENCE SIGNS LIST 
     
         
           1  diaphragm pump 
           2  case 
           2   a  first space 
           2   b  second space 
           3  diaphragm 
           4  liquid feed flow path 
           4   a  inflow path 
           4   b  outflow path 
           5  drive unit 
           51  compression/decompression device 
           51   a  reciprocating pump 
           511  cylinder 
           512  plunger 
           513  plunger driving part 
           52  pressure release mechanism 
           521  pressure release flow path 
           522  pressure release valve 
           53  driving flow path 
           531  air filter 
           532  pressure sensor 
           6  valve mechanism 
           6   a  inflow-side solenoid valve 
           6   b  outflow-side solenoid valve 
           7  control unit 
           10  blood purification apparatus 
           11  blood circuit 
           111  blood pump 
           112  gas-liquid separator 
           12  blood purifier 
           13  liquid supply flow path 
           13   a  dialysate flow path 
           14  waste liquid flow path 
           14   a  water removal flow path 
           151  undiluted fluid storage tank 
           152  undiluted fluid flow path 
           153  undiluted fluid injection pump 
           16  duplex pump 
           17  water removal pump