Abstract:
Disclosed herein is a stable solvent delivery device capable of delivering solvent both at high pressure and at constant flow rate. A solvent delivery device comprises a plurality of plungers which reciprocate in the respective pump chambers including an eluent charge side pump chamber and an eluent discharge side pump chamber, a motor to reciprocate these plungers, a control unit to control the operation of the motor, valves which are respectively set at the eluent inlet and outlet of the eluent charge side pump chamber, a first sensor to measure the quantity of load received by the plunger in the eluent charge side pump chamber, and a second sensor to measure the pressure of the eluent discharged from the eluent discharge side pump chamber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a liquid delivery device which delivers liquid. More particularly, it relates to a solvent delivery device which delivers eluent in an analytical system such as a liquid chromatograph. 
     2. Description of the Related Art 
     In liquid chromatography, it is ideal that a solvent delivery device always deliver eluent at a constant flow rate. If the flow rate is not constant, the analysis accuracy is lowered. However, it is common that the rate of flow from a solvent delivery device periodically changes (hereinafter this phenomenon being termed pulsation). 
     An example of the prior art techniques intended to reduce pulsations is described in Japanese Patent No. 3709409. However, this technique might face a problem if applied to a solvent delivery device in such an analytical system as a liquid chromatograph where solvent must be delivered at very high pressure. Specifically, this is because providing a pressure sensor increases the volume of eluent to be compressed. If this volume is so large that the eluent cannot fully be compressed, it may be impossible to deliver the eluent at a constant flow rate. Here, “very high pressure” means such a high pressure that the eluent is considerably compressed at the pressure. For example, if the eluent is methanol and delivered at 60 MPa, the methanol reduces 6.4% in volume as compared with its volume before compressed. 
     In a solvent delivery device disclosed in JP-A-2001-254683, a load sensor is used to reduce the volume of eluent to be compressed and the volume of eluent to be compressed is small. However, this does not contribute to reducing pulsations since the quantity of load measured by the load sensor is not related to the pressure of the eluent. 
     SUMMARY OF THE INVENTION 
     The present invention provides a stable solvent delivery device capable of delivering solvent both at high pressure and at a constant flow rate. 
     According to one aspect of the present invention, there is provided a solvent delivery device comprising: a plurality of plungers which reciprocate in respective pump chambers; a motor which is used to reciprocate the plungers; a control unit which controls the operation of the motor; check valves which are respectively disposed at an eluent inlet and an eluent outlet of the eluent charge side pump chamber; a first sensor which measures the quantity of load received by the plunger; and a second sensor which measures the pressure of the eluent discharged from the eluent discharge side pump chamber. 
     According to another aspect of the present invention, the eluent discharge flow rate is controlled based on the output signals respectively of the first sensor and second sensor. 
     According to another aspect of the present invention, the motor is controlled based on the output signals respectively of the first sensor and second sensor. 
     The above-mentioned and other aspects of the present invention will be described by the present specification and the drawings. 
     According to an embodiment of the present invention, it is possible to provide a stable solvent delivery device capable of delivering solvent both at very high pressure and at a constant flow rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects and advantages of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which: 
         FIG. 1  schematically shows a solvent delivery device where the present invention is embodied; 
         FIG. 2  schematically shows a liquid chromatograph where the solvent delivery device of the present invention is applied; and 
         FIG. 3  is an exemplary graph showing relations among the opening/closing of check valves, pump flow rates, overall discharge flow rate, and motor rotation speed during each delivery cycle of the solvent delivery device of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows an example of a liquid chromatograph used where the present invention is implemented. Eluent  41  is delivered to a separation column  43  by a solvent delivery device  42 . A sample is introduced into the separation column  43  by a sample introduction device  44 . As the sample passes through the separation column  43 , components of the sample separate from each other. The separated sample components are detected by a detector  45 . Signals concerning the detected sample components are sent to a data processor  46  where necessary processing is performed. After adding to the eluent a certain pressure which depends on the analysis, the solvent delivery device  42  discharges and delivers the eluent. If the eluent discharge pressure is constant, the flow rate is also constant. It is ideal for the solvent delivery device  42  to deliver the eluent always at the same flow rate. If the flow rate is not constant, the analysis accuracy lowers. However, it is common that such a solvent delivery device periodically changes the flow rate (hereinafter this phenomenon being termed pulsation). A solvent delivery device which solves the pulsation problem is described below. 
       FIG. 1  shows one embodiment of a solvent delivery device of the present invention. A rotary shaft  16  has cams  17   a  and  17   b  set thereon. The rotary shaft  16  has also a pulley  12  set thereon at an end thereof. From a pulley  12  fixed to the motor  10 , the rotary motion of the motor  10  is transmitted to the rotary shaft  16  via a belt  13  and the pulley  12 . In addition, a disk member  14  having a slit  15  formed therethrough is fixed to the rotary shaft  16 . The cam positions of the cams  17   a  and  17   b  are detected by detecting the slit  15  through a cam position detecting sensor  18 . 
     With its base end in contact with the cam  17   a , a slider  6   a  reciprocates. A load sensor  9  is disposed at the other end of the slider  6   a . The non-wetted end of a plunger  2   a  is disposed perpendicular to the load-sensing surface of the load sensor  9 . A slider  6   b  also reciprocates with its base end in contact with the cam  17   b . Opposite to the cam  17   b , a plunger  2   b  is disposed in association with the slider  6   b.    
     The pump head  3   a  of a plunger pump  1   a  has check valves  4  and  5  provided respectively in the inlet and outlet thereof. The plunger  2   a  is provided within the pump chamber  7   a  of the pump head  3   a . The plungers  2   a  and  2   b  are respectively provided with plunger seals  30   a  and  30   b  for preventing leakage. When the wetted end of the reciprocating plunger  2   a  is moved to the cam  17   a  side, eluent  22  is charged into the pump chamber  7   a  from the check valve  4 . Then, when the wetted end of the plunger  2   a  is moved to the pump head  3   a  side, the eluent  22  is compressed in the pump chamber  7   a  and discharged from the check valve  5 . Since the check valves  4  and  5  do not open until the inlet eluent pressure becomes equal to the outlet eluent pressure, the compressed eluent  22  can be delivered always in a given direction. 
     The pump head  3   b  of a plunger pump  1   b  has the plunger  2   b  provided in the internal pump chamber  7   b  thereof. The plunger  2   b  reciprocates. When the wetted end of the plunger  2   b  is moved to the cam  17   b  side, eluent  22  is charged into the pump chamber  7   b  from the pump head  3   a  side. Then, when the wetted end of the plunger  2   b  is moved to the pump head  3   b  side, the eluent  22  is compressed in the pump chamber  7   b  and discharged to a pressure sensor  8  side. By the check valve  5 , the eluent  22  can be delivered always in a given direction. 
     The pressure sensor  8  constantly measures the pressure of the eluent  22  discharged from the pump chamber  7   b . The measured pressure is converted to an electrical signal and reported to a control circuit  21  which controls the motor drive. The load sensor  9  constantly measures the quantity of load which acts on the plunger  2   a . The load quantity is converted to an electrical signal and reported to the control circuit  21 . The control circuit  21  is connected to an input unit  20  for entering operating commands and necessary information. 
       FIG. 3  shows exemplary reciprocating motions of the plungers.  FIG. 3  indicates relations among the opening and closing of the check valves  4  and  5 , the charge and discharge flow rates of the plunger pumps  1   a  and  1   b , and the overall discharge flow rate of the solvent delivery device. One reciprocating cycle of the plungers for eluent delivery is divided into three phases. 
     In phase  1 , the check valve  4  is opened and the check valve  5  is closed so that the plunger pump  1   a  is charged with the eluent  22  and only the plunger pump  1   b  discharges the eluent  22 . 
     In the compression period R of phase  2 , the check valves  4  and  5  are closed so that the eluent  22  charged into the plunger pump  1   a  is compressed and only the plunger pump  1   b  discharges the eluent  22  during the compression. If the eluent  22  charged into the plunger pump  1   a  is compressed to the discharge pressure, the check valve  5  is opened with the check valve  4  closed and consequently both plunger pumps  1   a  and  1   b  discharge the eluent  22 . 
     In phase  3 , the check valve  4  is closed and the check valve  5  is opened so that only the plunger pump  1   a  discharges the eluent  22  and the plunger pump  1   b  is charged with the eluent  22  discharged from the plunger pump  1   a.    
     Depending on the discharge pressure from the solvent delivery device, the length of the eluent  22  compression period R is changed. For delivery at high pressure, the compression period R is set long. For delivery at low pressure, the compression period R is set short. 
     On the assumption that the quantity of eluent  22  to be delivered per unit time is set to Q, the following describes the compression period R. In the compression period R, the check valve  5  is closed so that only the plunger pump  1   b  delivers at a discharge flow rate of Q while the eluent charged into the plunger pump  1   a  is compressed. The cam profile of the plunger pump  1   b  for the compression period R is designed so that the discharge flow rate is reduced from Q to ½Q. However, the discharge flow rate from the plunger pump  1   b  is set to Q by temporally doubling the rotation speed N of the motor  10 . This intends to keep the overflow flow rate at Q by doubling the rotation speed N of the motor  10  during the compression period. Otherwise, in the compression period R, the overall flow rate would fall to Q/2 since only the plunger pump  1   b  discharges and the plunger pump  1   a  does not discharge. At the end of the compression period R, the rotation speed of the motor  10  is returned to the regular delivery rotation speed N from 2N. 
     The quantity of load on the load sensor  9  while the check valve  5  is open after the end of the compression R is stored in the control circuit  21 . In the subsequent reciprocating cycle of the plungers, if the quantity of load on the load sensor  9  reaches the load quantity stored in the control circuit  21 , the control circuit  21  terminates the compression period R by returning the rotation speed of the motor  10  to N from 2N. Since the load sensor is calibrated by the pressure sensor, it is possible to accurately terminate the compression period R immediately after the check valve  5  is opened, resulting in pulsation-free delivery. 
     If a pulsation occurs in the first delivery cycle, the rotation speed of the motor  10  is compensated for the difference between the pressure measured by the pressure sensor  8  and the target pressure in order to keep the pressure constant. The quantity of load acting while the check valve  5  is open is measured by the load sensor  9  and stored in the control circuit  21  as the load value to end the compression period R. Even in this case, since the load sensor is calibrated by the pressure sensor, it is possible to accurately terminate the compression period R immediately after the check valve  5  is opened, resulting in pulsation-free delivery. 
     The following provides a description of a gradient solvent delivery application where the target flow rate is changed with time. If the target flow rate changes, the control circuit  21  changes the rotation speed of the motor  10  based on the pressure which is constantly measured by the pressure sensor  8 . Changing the rotation speed of the motor  10  changes the quantity of load on the load sensor  9  while the check valve  5  is open, with the result that a new load quantity is stored in the control circuit  21 . In the subsequent delivery cycles, since the compression period R is terminated when the quantity of load on the load sensor  9  reaches the new load quantity stored in the control circuit  21 , pulsation-free delivery can be done by terminating the compression period R immediately after the check valve  5  is opened. The length of the compression period R is calculated and determined based on the flow rate and discharge pressure as the case may be. Gradient solvent delivery is performed in this manner. 
     In phase  1  of the example in  FIG. 3 , with the check valve  4  opened and the check valve  5  closed, the plunger pump  1   a  is charged with the eluent  22  at a flow rate of 3Q and only the plunger pump  1   b  discharges the eluent  22  at a flow rate of Q. 
     In the compression period R of phase  2 , with the check valves  4  and  5  closed, while the eluent  22  charged into the plunger pump  1   a  is compressed, only the plunger pump  1   b  discharges the eluent  22  at a flow rate of Q during the compression. When the eluent charged into the plunger pump  1   a  is compressed to the discharge pressure (end of the compression period R), the check valve  5  is opened with the check valve  4  kept closed so that both plunger pumps  1   a  and  1   b  discharge respectively at a flow rate of Q/2. 
     In phase  3 , with the check valve  4  closed and the check valve  5  opened, only the plunger pump  1   a  discharges the eluent  22  at a flow rate of 3Q/2+Q and the plunger pump  1   b  is charged at a flow rate of 3Q/2 with the eluent  22  discharged from the plunger pump  1   a.    
     In the embodiment of the present invention, since no pressure sensor is needed between the plunger pump  1   a  and the plunger  1   b  thanks to the use of a load sensor, the volume of eluent to be compressed can be reduced and consequently it is possible to deliver eluent at very high pressure. 
     In addition, although the signal from the load sensor generally includes errors due to various frictions, accurate eluent delivery is possible since calibration is performed based on the pressure sensor during each reciprocating cycle of the plungers. 
     As mentioned so far, the present invention attains the object of providing a stable solvent delivery device capable of delivering eluent both at high pressure and at a constant flow rate through the following means which uses a pressure sensor and a load sensor. The pressure of eluent discharged from the discharge side pump chamber is constantly measured by the pressure sensor and reported to a control circuit. Based on the measured pressure, the reciprocating speed of the plungers is controlled by the control circuit so that the discharge pressure of eluent is kept constant. The quantity of load acting on the plunger in the charge side pump chamber is constantly measured by the load sensor and reported to the control circuit. To keep the discharge pressure constant, the motor rotation is controlled so as to secure that the eluent charged into the charge side pump chamber is compressed to the discharge pressure. While the outlet check valve is open, the internal pressure of the charge side pump chamber is equal to the discharge pressure. The signal obtained from the load sensor is calibrated based on the discharge pressure signal from the pressure sensor. The motor rotation is controlled by using the calibrated signal. Note that various modifications and applications are possible within the scope of the technical concept of the present invention. For example, it is possible to improve the analysis accuracy of liquid chromatographs and spectrophotometers by incorporating the above-mentioned solvent delivery device of the present invention in such analytical systems.