Abstract:
A method for controlling operation of a pump unit, where the pump unit includes a primary piston pump having a primary piston and a secondary piston pump having a secondary piston. The primary piston pump is fluidically connected with the secondary piston pump. The primary piston pump includes an inlet valve and an outlet valve, and the pump unit operates periodically according to a pump cycle. The method includes determining a fluid pressure of fluid dispensed by the pump unit, and performing a closed loop control of a position of the primary piston in dependence on the fluid pressure of the fluid dispensed by the pump unit during a first time interval of the pump cycle.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a continuation of application Ser. No. 12/431,972, filed on Apr. 29, 2009, the disclosure of which is incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The disclosed embodiments relate to a method for controlling operation of a pump unit. The present invention further relates to a pump unit, and to a fluid separation system for separating compounds of a sample fluid in a mobile phase. 
         [0003]    U.S. patent application 2006/0219618 A1 relates to solvent supply with a correction of piston movement. 
         [0004]    International patent application WO 2006017121 describes a feedback control loop for a high pressure pump that modifies the accumulator velocity and pressure during solvent transfer. The accumulator velocity is adjusted to maintain the system pressure equal to the expected pressure to thereby eliminate the effect of the flow deficit created by a thermal effect. 
       SUMMARY 
       [0005]    It is an object of the invention to provide an improved operation of a pump unit comprising a primary piston pump fluidically connected with a secondary piston pump. The object is solved by the independent claim(s). Further embodiments are shown by the dependent claim(s). 
         [0006]    According to embodiments of the present invention, a method for controlling operation of a pump unit is proposed, the pump unit comprising a primary piston pump having a primary piston and a secondary piston pump having a secondary piston. The primary piston pump is fluidically connected with the secondary piston pump. The primary piston pump comprises an inlet valve and an outlet valve, and the pump unit operates periodically according to a pump cycle comprising a deliver-and-fill phase for delivering fluid from the primary piston pump to the secondary piston pump and to a fluidic system located downstream of the pump unit. The method comprises determining a fluid pressure of fluid dispensed by the pump unit, and—during a first time interval of the pump cycle—performing a closed loop control of a position of the primary piston in dependence on the fluid pressure of the fluid dispensed by the pump unit, while a predefined position-versus-time curve is performed by the secondary piston. The first time interval corresponds to the deliver-and-fill phase. 
         [0007]    During the first time interval, the closed loop control is applied to the primary piston pump. Accordingly, during the first time interval, corrective movements are superimposed onto the primary piston&#39;s movement. 
         [0008]    Superposing corrective movements onto the primary piston&#39;s movement leads to a number of advantages. First of all, by applying the closed loop control to the primary piston pump, various errors are compensated at the location where they occur, i.e. at the primary piston pump. 
         [0009]    For example, a compression stroke may be performed by the primary piston, According to embodiments of the present invention, in case the compression stroke is too short, an additional downward movement is applied to the primary piston. By applying the additional downward movement to the primary piston, the error is counteracted at the location where it has occurred, i.e. at the primary piston pump. As a result, a continuous prolonged downward movement of the primary piston is obtained. 
         [0010]    Also with regard to thermal fluctuations that may lead to volumetric errors, it is proposed to counteract these thermal effects at the location where they occur, i.e. at the primary piston pump. Thus, undesired effects related to temperature fluctuations are kept as small as possible. For this reason, during the first time interval, the closed loop control is applied to the primary piston pump. 
         [0011]    Furthermore, when imposing a corrective movement onto the primary piston pump, any discontinuity of flow related to this correction is dampened when passing the fluid from the primary piston pump to the pump system&#39;s outlet. The primary piston pump is located upstream of the secondary piston pump, and therefore, the fluid has to pass the additional flow path between the primary piston pump and the secondary piston pump before reaching the pump system&#39;s outlet. This additional hydraulic capacitance is sufficiently large to dampen any discontinuity of flow and pressure that occurs when applying a correction onto the primary piston pump&#39;s piston movement. 
         [0012]    As another advantage, when the closed loop control is applied to the primary piston pump during the first time interval, and not to the secondary piston pump, it is much easier to maintain synchronization between the piston movement of the primary piston pump and the piston movement of the secondary piston pump. The secondary piston pump is permanently exposed to system pressure, and therefore, any correction applied to the secondary piston&#39;s movement may affect the pump cycle of the secondary piston pump. In contrast, the primary piston pump is alternatingly coupled to and decoupled from system pressure. In case a corrective movement is superimposed onto the primary piston&#39;s movement when the primary piston pump is coupled to system pressure, the corrective movement may also effect the period of time needed for a respective phase of the primary piston pump&#39;s operation. However, during the periods of time when the primary piston pump is decoupled from system pressure, the effects imposed onto the timing of the primary piston pump&#39;s operation may be compensated for. For this reason, it is possible to maintain a constant pump cycle even in case corrective movements are applied to the primary piston&#39;s movement. In this regard, the corrections applied to the piston movement of the primary piston pump do not disturb the synchronization between the primary piston pump and the secondary piston pump. A correction applied to the secondary piston pump would be much more critical in terms of disturbing the synchronization between the two piston pumps. 
         [0013]    During the first time interval, the closed loop control of the position of the primary piston is performed while a predefined position-versus-time curve is performed by the secondary piston. Hence, during the first time interval, the closed loop control is solely applied to the primary piston pump. 
         [0014]    According to a preferred embodiment, the primary piston pump is configured for delivering, during the first time interval, fluid to the secondary piston pump and to a fluidic system located downstream of the pump unit. 
         [0015]    According to a preferred embodiment, the first time interval is said deliver-and-fill phase. As long as the primary piston pump delivers fluid to the secondary piston pump and to the fluidic system located downstream of the pump unit, the corrective movements are superimposed onto the primary piston&#39;s movement. Thus, any pressure discontinuities are counteracted at the location where they occur. 
         [0016]    According to a preferred embodiment, performing the closed loop control of the primary piston&#39;s position comprises determining a first position correction signal for imposing, during the first time interval, a corrective movement onto a regular piston movement of the primary piston. 
         [0017]    According to a preferred embodiment, performing the closed loop control of the primary piston&#39;s position comprises determining a variance between the fluid pressure of the fluid dispensed by and the pump unit and a predetermined target pressure, and deriving, from said variance, a first position correction signal for imposing, during the first time interval, a corrective movement onto a regular piston movement of the primary piston. As an outcome of the closed loop control, the fluid pressure of the fluid dispensed by the pump unit is driven towards the predetermined target pressure. 
         [0018]    According to a preferred embodiment, performing the closed loop control of the primary piston&#39;s position comprises determining a first position correction signal for imposing, during the first time interval, a corrective movement onto a regular piston movement of the primary piston, and applying, during the first time interval, the first position correction signal to the primary piston pump. 
         [0019]    According to a preferred embodiment, during the first time interval, in case the fluid pressure of the fluid dispensed by the pump unit is too small, movement of the primary piston is corrected by superimposing a forward movement onto the primary piston, and in case the fluid pressure of the fluid dispensed by the pump unit is too large, movement of the primary piston is corrected by superimposing a backward movement onto the primary piston. 
         [0020]    According to a preferred embodiment, the pump cycle further comprises at least one of: an intake phase for drawing fluid into the primary piston pump; an inlet valve settle phase for closing the inlet valve of the primary piston pump; a compression phase for bringing a fluid contained in the primary piston pump to system pressure; an outlet valve settle phase for closing the outlet valve of the primary piston pump; a decompression phase for bringing a fluid remaining in the primary piston pump from system pressure to an initial pressure. 
         [0021]    According to a preferred embodiment, during at least one second time interval of the pump cycle, which is different from the first time interval, a closed loop control of the secondary piston&#39;s position is performed in dependence on the fluid pressure of the fluid dispensed by the pump unit. 
         [0022]    According to a preferred embodiment, during the at least one second time interval, the closed loop control of the position of the secondary piston is performed while a predefined position-versus-time curve is performed by the primary piston. Preferably, the at least one second time interval does not overlap substantially with the first time interval. 
         [0023]    According to a preferred embodiment, the at least one second time interval does not substantially overlap with the deliver-and-fill phase. During the deliver-and-fill phase, the primary piston pump is responsible for supplying fluid at system pressure, and therefore, the closed loop control may e.g. be applied to the primary piston pump. In contrast, during the at least one second time interval, the primary piston pump&#39;s outlet valve may e.g. be closed, and the closed loop control may be applied to the secondary piston pump. 
         [0024]    According to a preferred embodiment, the at least one second time interval includes at least one of: an intake phase for drawing fluid into the primary piston pump, an inlet valve settle phase for closing the inlet valve of the primary piston pump, a compression phase for bringing a fluid contained in the primary piston pump to system pressure, an outlet valve settle phase for closing the outlet valve of the primary piston pump, a decompression phase for bringing a fluid remaining in the primary piston pump from system pressure to an initial pressure. Any pressure discontinuity that occurs during any one of the above-mentioned phases may be counteracted by superposing a corresponding corrective movement onto the secondary piston&#39;s movement. 
         [0025]    According to a preferred embodiment, during a subinterval of the pump cycle, said closed loop control is alternatingly applied to the primary piston pump and to the secondary piston pump. According to this embodiment, during the subinterval of the pump cycle, the closed loop control is switched between the primary and the secondary piston pump, in order to yield the best possible results with regard to stabilizing fluid pressure. 
         [0026]    According to a preferred embodiment, during a subinterval of the pump cycle, said closed loop control is alternatingly applied to the primary piston pump and to the secondary piston pump, and during a remaining part of the pump cycle, said closed loop control is inactive. For example, during the secondary piston pump&#39;s delivery phase, it may not be required to perform any closed loop control of the piston movements. 
         [0027]    According to a preferred embodiment, during the pump cycle, said closed loop control is alternatingly applied to the primary piston pump and to the secondary piston pump. 
         [0028]    According to a preferred embodiment, at the beginning of the first time interval, the closed loop control is switched from controlling the secondary piston pump to controlling the primary piston pump. According to a further preferred embodiment, at the end of the first time interval, the closed loop control is switched from controlling the primary piston pump to controlling the secondary piston pump. 
         [0029]    According to a preferred embodiment, during the first time interval, said closed loop control of the position of the primary piston is performed in a way that the fluid pressure of the fluid dispensed by the pump unit continues to follow its former trend. Thus, the fluid pressure of the fluid dispensed by the pump unit is stabilized. 
         [0030]    According to a preferred embodiment, during the first time interval, said closed loop control of the primary piston&#39;s position is performed in a way that the fluid pressure of the fluid dispensed by the pump unit is driven towards or substantially kept at a predetermined target pressure. 
         [0031]    Preferably, the target pressure is determined by performing an extrapolation of former values of the fluid pressure of the fluid dispensed by the pump unit. 
         [0032]    Further preferably, the target pressure is determined by performing an extrapolation of former values of the fluid pressure of the fluid dispensed by the pump unit in a way that the fluid pressure of the fluid dispensed by the pump unit shows a continuous progression. 
         [0033]    According to a preferred embodiment, the method further comprises deriving, from the closed loop control of the primary piston&#39;s position performed during the first time interval, at least one correction to be applied to a regular piston movement of at least one of the primary piston pump and the secondary piston pump. The corrections imposed onto the piston movements contain information about how the predefined regular piston movement should be modified to accomplish a stable pressure of the fluid dispensed by the pump unit. Therefore, the corrections can be used for modifying and improving the regular piston movement of at least one of the primary and the secondary piston pump. 
         [0034]    According to a preferred embodiment, the method further comprises deriving, from the closed loop control of the primary piston&#39;s position performed during the first time interval, at least one of the following: a correction of the compression jump and a correction of the fluid&#39;s thermal expansion. 
         [0035]    According to a preferred embodiment, the method further comprises deriving, from the closed loop control the primary piston&#39;s position performed during the first time interval, at least one correction to be applied to a regular piston movement of at least one of the primary piston pump and the secondary piston pump, with said at least one correction being used for modifying a regular piston movement of at least one of the primary piston pump and the secondary piston pump during subsequent pump cycles. 
         [0036]    A pump unit according to embodiments of the present invention comprises: a primary piston pump with a primary piston and a secondary piston pump with a secondary piston, the primary piston pump being fluidically connected with the secondary piston pump, the primary piston pump comprising an inlet valve and an outlet valve, and the pump unit operating periodically according to a pump cycle comprising a deliver-and-fill phase for delivering fluid from the primary piston pump to the secondary piston pump and to a fluidic system located downstream of the pump unit. The pump unit comprises a pressure detection unit configured for determining a fluid pressure of fluid dispensed by the pump unit. Further, the pump unit comprises a control unit configured for performing, during a first time interval of the pump cycle, a closed loop control of a position of the primary piston in dependence on the fluid pressure of the fluid dispensed by the pump unit, while a predefined position-versus-time curve is performed by the secondary piston. The first time interval corresponds to the deliver-and-fill phase. 
         [0037]    A fluid separation system for separating compounds of a sample fluid in a mobile phase according to embodiments of the present invention comprises a pump unit as described above, the pump unit being configured for driving the mobile phase through the fluid separation system, and a separation unit, preferably a chromatographic column, configured for separating compounds of the sample fluid in the mobile phase. 
         [0038]    According to a preferred embodiment, the fluid separation system further comprises at least one of: a sample injector configured for introducing the sample fluid into the mobile phase; a detector configured for detecting separated compounds of the sample fluid; a collection unit configured for collecting separated compounds of the sample fluid; a data processing unit configured for processing data received from the fluid separation system; a degassing apparatus configured for degassing the mobile phase. 
         [0039]    Embodiments of the invention can be partly or entirely embodied or supported by one or more suitable software programs or program code, which can be stored on or otherwise provided by any kind of computer readable storage medium, and which might be executed in or by any suitable data processing system. The computer readable storage medium may utilize optical, magnetic, chemical, electrical, or any other suitable properties for receiving, storing, or delivering instructions and commands, and may include magnetic media, such as a diskette, disk, memory stick or computer hard drive, which is readable and executable by a computer. In other embodiments, the computer readable storage medium may include optical disks, read-only-memory (“ROM”) floppy disks and semiconductor materials and chips, or any suitable technology for implementing the embodiments disclosed herein. Software programs or routines can be preferably applied for controlling respective movements of at least one of the primary piston and the secondary piston. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0040]    Other objects and many of the attendant advantages of embodiments of the present invention will be readily appreciated and become better understood by reference to the following more detailed description of embodiments in connection with the accompanied drawing(s). Features that are substantially or functionally equal or similar will be referred to by the same reference sign(s). 
           [0041]      FIG. 1  shows a dual piston serial type pump comprising a primary piston pump and a secondary piston pump; 
           [0042]      FIG. 2  shows the piston movements of the primary piston and the secondary piston as a function of time; 
           [0043]      FIG. 3  shows a set up of a pump system according to embodiments of the present invention; 
           [0044]      FIG. 4  indicates when the first position correction signal is active and when the second position correction signal is active; 
           [0045]      FIG. 5  shows position-versus-time curves for the primary and the secondary piston; 
           [0046]      FIG. 6  depicts both the first position correction signal and the second position correction signal as a function of time; and 
           [0047]      FIG. 7  shows the first position correction signal and the second position correction signal after a modification of the regular piston movements. 
       
    
    
     DETAILED DESCRIPTION 
       [0048]      FIG. 1  shows a dual piston serial-type pump comprising a primary piston pump  100  that is fluidically connected in series with a secondary piston pump  101 . The primary piston pump  100  comprises an inlet  102  with an inlet valve  103 , a piston  104  that reciprocates in the primary piston pump  100 , and an outlet  105  with an outlet valve  106 . The outlet  105  is fluidically connected with an inlet  107  of the secondary piston pump  101 . A piston  108  reciprocates in the secondary piston pump  101 . The secondary piston pump  101  further comprises an outlet  109  for delivering a flow of fluid. 
         [0049]    In the upper portion of  FIG. 2 , the primary piston&#39;s position p 1  is depicted as a function of time, and in the lower portion of  FIG. 2 , right below the primary piston&#39;s position p 1 , the secondary piston&#39;s position p 2  is shown as a function of time. During an intake phase  200  of the primary piston pump  100 , the primary piston  104  performs an upward stroke, as indicated by arrow  110 . The inlet valve  103  is opened, and fluid at atmospheric pressure is drawn into the primary piston pump  100 . 
         [0050]    During an inlet valve settle phase  201 , the inlet valve  103  is closed. Then, starting at the point of time  202 , the primary piston  104  performs a compression stroke  203  in the downward direction, as indicated by arrow  112 , and the fluid contained in the primary piston pump  100  is compressed to a system pressure of several hundred or even more than thousand bar. During the compression phase  203 , both the inlet valve  103  and the outlet valve  106  are closed. 
         [0051]    At a point of time  204 , the fluid contained in the primary piston pump  100  has reached system pressure, and the outlet valve  106  opens. In a subsequent delivery phase  205  of the primary piston pump  100 , the primary piston  104  continues its downward movement, and a flow of fluid is dispensed at the outlet  105  of the primary piston pump  100 . Accordingly, during a deliver-and-fill phase  206  indicated in the lower portion of  FIG. 2 , the flow of fluid provided by the primary piston pump  100  is supplied to the secondary piston pump  101  and to the fluidic system located downstream of the pump unit, and the secondary piston pump&#39;s pump chamber is filled up. 
         [0052]    During the deliver-and-fill phase  206 , fluid may e.g. be supplied to the secondary piston pump  101  at a flow rate of about 5 to 20 ml/min. As a consequence of this large resupply rate, the deliver-and-fill phase  206  can be quite short. In the examples shown in  FIG. 2 , the deliver-and-fill phase  206  only extends over a small portion of a pump cycle  211 . For example, the deliver-and-fill phase may extend over less than 10% of the pump cycle. 
         [0053]    At the point of time  207 , the downward stroke of the primary piston  104  is finished, and during an outlet valve settle phase  208 , the outlet valve  106  is closed. At the end of the primary piston&#39;s downward stroke, a certain dead volume of fluid remains in the pump chamber of the primary piston pump  100 , said dead volume of fluid being at system pressure. To decompress this dead volume of fluid, the primary piston  104  performs a decompression stroke  209 , which is a fast movement in the upward direction. At the point of time  210 , the dead volume of fluid is approximately at atmospheric pressure, and the inlet valve  103  opens. Now, the pump cycle  211  is finished, and a new pump cycle  212  starts. During an intake phase  213  of the primary piston pump  100 , the primary piston  104  performs an upward stroke, as indicated by arrow  110 , and fluid at atmospheric pressure is drawn into the primary piston pump  100 . 
         [0054]    The lower portion of  FIG. 2  shows the position p 2  of the secondary piston pump&#39;s piston as a function of time. During a delivery phase  214  of the secondary piston pump  101 , the secondary piston  108  performs a downward movement, as indicated by arrow  111 , and dispenses a continuous flow of fluid at the outlet  109  of the secondary piston pump  101 . 
         [0055]    Then, at the point of time  204 , the outlet valve  106  is opened. During an intake phase  215  of the secondary piston pump  101 , the secondary piston  108  performs an upward stroke, as indicated by arrow  113 , and draws in fluid supplied by the primary piston pump  100 . During the intake phase  215 , the flow of fluid supplied by the primary piston pump  100  is partly used for filling up the fluid chamber of the secondary piston pump  101  and partly used for maintaining a continuous flow of fluid at the outlet  109 . At the point of time  207 , the pump chamber of the secondary piston pump  101  is filled with fluid. Then, during a subsequent delivery phase  216 , the secondary piston  108  performs a downward stroke, as indicated by arrow  111 , and a flow of fluid is dispensed at the outlet  109 . 
         [0056]    The primary piston pump  100  and the secondary piston pump  101  may e.g. perform predefined regular piston movements as shown in  FIG. 2 . The pump system may e.g. comprise an actuation mechanism for operating the primary and the secondary piston in accordance with these predefined piston movements. However, especially in the time interval around the deliver-and-fill phase  206 , the flow of fluid dispensed by the pump system may fluctuate, and accordingly, the pressure at the outlet may be subjected to fluctuations as well. 
         [0057]    To counteract these fluctuations observed at the pump system&#39;s outlet and to stabilize pressure and flow of the dispensed fluid, corrective movements are superimposed onto at least one of the predefined regular piston movements shown in  FIG. 2 . According to embodiments of the present invention, a closed loop control is set up for controlling at least one of the piston movements in accordance with a fluid pressure detected at the pump system&#39;s outlet. The pressure at the outlet may e.g. be compared with a predefined setpoint value indicating a target pressure. In case the actual pressure is too small, an additional forward displacement may be imposed onto at least one of the primary and the secondary piston&#39;s movement. In case the pressure detected at the outlet is too large, an additional backward displacement may be imposed onto at least one of the primary and the secondary piston&#39;s movement. By adaptively controlling the piston positions in accordance with a closed loop control, fluid pressure at the outlet of the pump system is stabilized, and fluctuations of fluid pressure are reduced. 
         [0058]      FIG. 3  shows a pump system according to embodiments of the present invention. The pump system comprises a pump unit  300  and a control unit  301  adapted for performing a closed loop control of the pump unit&#39;s operation. The pump unit  300  comprises a primary piston pump  302  that is fluidically connected in series with a secondary piston pump  303 . The primary piston pump comprises a primary piston  304 , the primary piston  304  being driven by a first actuator mechanism  305 . The primary piston pump  302  further comprises an inlet valve  306  and an outlet valve  307 . The secondary piston pump  303  comprises a secondary piston  308 , the secondary piston  308  being driven by a second actuator mechanism  309 . 
         [0059]    The pressure of the fluid dispensed by the pump unit  300  is determined by a pressure detection unit  310  located downstream of the pump unit  300 . The actual pressure value  311  determined by the pressure detection unit  310  is forwarded to the control unit  301 . There, the actual pressure value  311  is compared with a setpoint value  312  that indicates a desired target pressure. The setpoint value  312  may for example be obtained by extrapolating a plurality of former pressure values. The control unit  301  may further receive phase information  313  indicating a phase of operation of the first actuator mechanism  305  and/or of the second actuator mechanism  309 . 
         [0060]    The control unit  301  is configured to determine, based on the variance between the actual pressure value  311  and the setpoint value  312 , at least one position correction signal for imposing a corrective movement onto a regular piston movement of at least one of the pistons  304  and  308 . In the embodiment shown in  FIG. 3 , two position correction signals  314 ,  315  are generated, the first position correction signal  314  being provided to the first actuator mechanism  305 , and the second position correction signal  315  being provided to the second actuator mechanism  309 . The corrective movements imposed onto the regular piston movements are chosen such that the fluid pressure at the outlet of the pump system is driven towards the target pressure indicated by the setpoint value  312 . Thus, the fluid pressure at the outlet of the pump system is stabilized. 
         [0061]    The closed loop control shown in  FIG. 3  does not have to be active during the entire pump cycle. For example, during the intake phases  200 ,  213  of the primary piston pump, the secondary piston pump dispenses a steady flow of fluid. During the intake phases  200 ,  213  of the primary piston pump, the flow of fluid obtained at the pump system&#39;s outlet is quite stable. Therefore, during these intervals of the pump cycle, it is not necessary to perform a closed loop control of output pressure. 
         [0062]    According to preferred embodiments of the present invention, during a pump cycle, position correction signals are alternatingly applied to the piston movement of the primary piston  304  and to the piston movement of the secondary piston  308 . For example, during the compression phase  203  shown in the upper portion of  FIG. 2 , the second position correction signal  315  may be active. Hence, during the compression phase  203 , a corrective movement is imposed onto the movement of the secondary piston  308 , whereas the primary piston  304  performs a predefined regular piston movement. 
         [0063]    At the point of time  204 , the outlet valve  307  of the primary piston pump is opened, the primary piston pump  302  starts dispensing fluid, and the closed loop control is switched from the secondary piston  308  to the primary piston  304 . During the deliver-and-fill phase  206 , corrective piston movements are solely applied to the primary piston  304 , while the secondary piston  308  performs a predefined regular movement. 
         [0064]    At the point of time  207 , the deliver-and-fill phase  206  is terminated, and the closed loop control is switched back from the primary piston  304  to the secondary piston  308 . During the outlet valve settle phase  208  and the decompression phase  209 , the closed loop control is solely applied to the secondary piston  308 , while the primary piston  304  performs a predefined regular movement. 
         [0065]    At the point of time  210 , the decompression phase  209  is finished, and the primary piston&#39;s intake phase  213  is started. During the primary piston&#39;s intake phase  213 , a steady flow of fluid is dispensed by the secondary piston pump, and hence, no closed loop control of the piston movement is necessary. Therefore, according to a preferred embodiment of the present invention, the closed loop control of the secondary piston&#39;s movement is switched off at the point of time  210 , or right after the point of time  210 . 
         [0066]    Hence, according to a preferred embodiment of the present invention, the closed loop control is switched back and forth between the primary piston pump  302  and the secondary piston pump  303 . According to a further preferred embodiment, the closed loop control is only active during a subinterval of a pump cycle. 
         [0067]    In  FIG. 4 , which is located right below  FIG. 2 , it is indicated when the first position correction signal  314  is active, and when the second position correction signal  315  is active. During the compression phase  203 , the second position correction signal  315  is active, which is indicated by a hatched segment  400 . Then, at the point of time  204 , the closed loop control is switched from the secondary piston pump  303  to the primary piston pump  302 . During the deliver-and-fill phase  206 , the second position correction signal  315  is inactive, and the first position correction signal  314  is active, which is indicated by the hatched segment  401 . Then, at the point of time  207 , the closed loop control is switched back from the primary piston pump  302  to the secondary piston pump  303 . Hence, the first position correction signal  314  becomes inactive, whereas the second position correction signal  315  is activated, as indicated by the hatched segment  402 . Hence, during the outlet valve settle phase  208  and the decompression phase  209  of the primary piston pump, the closed loop control is applied to the secondary piston pump. Then, during the intake phase  213  of the primary piston pump, both the first position correction signal  314  and the second position correction signal  315  are inactive, and no corrective movements are superimposed onto the regular piston movements of the primary piston  304  and the secondary piston  308 . 
         [0068]      FIG. 5  depicts both the position vs. time curve  500  of the primary piston pump and the position vs. time curve  501  of the secondary piston pump for a subinterval of the pump cycle in which the closed loop control of the pump system is active. 
         [0069]    During the inlet valve settle phase  502 , the closed loop control is not active yet. At the point of time  503 , the closed loop control of the secondary piston pump is started. During the compression phase  504 , the outlet valve of the primary piston pump is still closed, and the volume of fluid contained in the primary piston pump is compressed to system pressure. During the compression phase  504 , the closed loop control is applied to the secondary piston. 
         [0070]    Then, at the point of time  505 , the outlet valve of the primary piston pump opens, and the closed loop control is switched from the secondary piston pump to the primary piston pump. During the deliver-and-fill phase  506 , the closed loop control is applied to the primary piston pump. During the deliver-and-fill phase  506 , the primary piston pump supplies a flow of fluid to the secondary piston pump and to the fluidic system located downstream of the pump unit, and the volume of fluid is taken in by the secondary piston pump. 
         [0071]    At the point of time  507 , the deliver-and-fill phase  506  is finished, and the closed loop control is transferred from the primary piston pump back to the secondary piston pump. During the outlet valve settle phase  508  and the decompression phase  509 , the pressure at the outlet of the pump system is stabilized by imposing corrective movements onto the secondary piston&#39;s movement. At the end of the decompression phase  509 , the closed loop control is switched off, and during the primary piston&#39;s intake phase, the closed loop control remains inactive. 
         [0072]      FIG. 6  shows both the first position correction signal  600  for the primary piston pump and the second position correction signal  601  for the secondary piston pump as a function of time, whereby the pump phases indicated in  FIG. 6  correspond exactly to the pump phases shown in  FIG. 5 . 
         [0073]    Before the point of time  602 , none of the position correction signals  600 ,  601  is active. Then, during the compression phase  603 , the second position correction signal  601  is active. At the point of time  604 , the primary piston pump&#39;s outlet valve is opened, the second position correction signal  601  becomes inactive, and the first position correction signal  600  becomes active. Then, during the deliver-and-fill phase  605 , the closed loop control of the fluid pressure is solely performed by the first position correction signal  600 . Hence, during the deliver-and-fill phase  605 , the closed loop control is solely applied to the primary piston pump. 
         [0074]    In the example shown in  FIG. 6 , the compression stroke performed during the compression phase  603  has been too short. Therefore, the fluid pressure determined by the pressure detection unit at the point of time  604  is below the desired target value. To drive the fluid pressure towards the desired target pressure, the first position correction signal  600  imposes an additional downward displacement  606  onto the primary piston&#39;s regular movement. This additional downward displacement  606  may be seen as an extension of the compression stroke performed during the compression phase  603 . 
         [0075]    Both the compression stroke performed during the compression phase  603  and the additional downward displacement  606  cause a temperature increase of the fluid contained in the primary piston pump. Hence, after the fluid has been compressed, the fluid&#39;s temperature is increased. Now, temperature relaxation processes take place, and the fluid slowly cools down, which leads to a corresponding volumetric contraction of the volume of fluid in the primary piston pump. 
         [0076]    To compensate for the thermal contraction of the volume of fluid, the first position correction signal  600  shows a slow decline, which is indicated by the dashed line  607 . The slow decline of the first position correction signal  600  superimposes an additional downward movement onto the primary piston&#39;s regular movement. The additional downward movement compensates for the slow thermal contraction of the volume of fluid and stabilizes the fluid pressure at the outlet of the pump system. 
         [0077]    At the point of time  608 , the deliver-and-fill phase  605  is finished, and the closed loop control is switched from the primary piston pump back to the secondary piston pump. Accordingly, at the point of time  608 , the first position correction signal  600  becomes inactive, as indicated by the straight line  609 , and the second position correction signal  601  is activated. During the outlet valve settle phase  610  and the decompression phase  611 , the closed loop control of the fluid pressure at the pump system&#39;s outlet is solely performed by the second position correction signal  601 . For example, corrective movements  612  of the secondary piston may compensate for thermal effects or for errors that occur when closing the outlet valve. After the decompression phase  611 , the second position correction signal  601  becomes inactive. 
         [0078]    According to a preferred embodiment of the invention, the information contained in the first position correction signal  600  and the second position correction signal  601  may be used for modifying the regular piston movements of the primary and the secondary piston in subsequent pump cycles. The first position correction signal  600  and the second position correction signal  601  contain information about the deviation between the required piston movements and the regular piston movements. From the first position correction signal  600  and the second position correction signal  601 , information about the errors of the regular piston movements may be derived, and said information may be used for modifying the regular piston movements. As a result, in subsequent pump cycles, the extent of required corrections is reduced. 
         [0079]    For example, in the example shown in  FIG. 6 , the compression stroke has been too small, and therefore, it has been necessary to impose an additional downward movement  606  onto the primary piston&#39;s movement. The additional downward movement  606  indicates that the compression stroke defined in the regular piston movement is too small. In fact, the additional downward movement  606  may be seen as an extension of the regular compression stroke performed during the compression phase  603 . Therefore, the additional downward movement  606  can be used as an indication showing how to adapt the regular piston movement in a way that during the following pump cycles, the magnitude of the correction signals will be reduced. In particular, during the next pump cycle, the regular compression stroke may be extended, which means that the additional downward movement is added to the regular compression stroke. As a consequence, during the next and all the following pump cycles, the magnitude of the correction signals will be reduced. 
         [0080]    In addition to the additional downward movement  606  shown in  FIG. 6 , also the slow additional downward movement that is employed for counteracting the thermal contraction of the volume of fluid in the primary pump chamber may be used for modifying the regular piston movements. In particular, by including the additional slow downward movement into the regular piston movement of the primary piston, the magnitude of the corrective movements imposed during the next pump cycles can be further reduced. 
         [0081]    By modifying the regular piston movements of the primary and the secondary piston, the corrections imposed by the position correction signals can be reduced during the next and all the following pump cycles. This is shown in  FIG. 7 .  FIG. 7  shows both the first position correction signal  700  and the second position correction signal  701  during a subsequent pump cycle, which occurs after the regular piston movements of the primary and the secondary piston pump have been modified. 
         [0082]    During a compression phase  702 , a compression stroke is performed, with the closed loop control being applied to the secondary piston pump. At the point of time  703 , the compression phase  702  is finished, and the closed loop control is transferred from the secondary piston pump to the primary piston pump. The first position correction signal  700  is activated, whereas the second position correction signal  701  becomes inactive. 
         [0083]    At the point of time  703 , the pressure measured at the pump system&#39;s outlet is still smaller than system pressure, and therefore, an additional downward movement  704  is imposed onto the primary piston&#39;s movement. 
         [0084]    However, as shown in  FIG. 7 , the additional downward movement  704  is significantly smaller in magnitude than the corresponding additional downward movement  606  shown in  FIG. 6 , because in the meantime, the magnitude of the compression stroke of the primary piston pump&#39;s regular piston movement has been modified. In particular, the compression stroke of the primary piston pump&#39;s regular piston movement has been increased, and therefore, the magnitude of the additional downward movement  704  is decreased. 
         [0085]    Furthermore, also the slow decline of the first position correction signal  600 , which is indicated by the dashed line  607  in  FIG. 6 , has been used for modifying the primary piston pump&#39;s regular piston movement. As a result, in the first position correction signal  700  shown in  FIG. 7 , the slow decline is no longer present. Instead, during the deliver-and-fill phase  705 , the first position correction signal  700  is substantially kept constant, as indicated by the dashed line  706 . The reason is that the thermal contraction has already been considered in the primary piston pump&#39;s regular piston movement. 
         [0086]    At the point of time  707 , the deliver-and-fill phase  705  is finished, and the closed loop control is handed over to the secondary piston pump. During the outlet valve settle phase  708  and the decompression phase  709 , the second position correction signal  701  is activated, and the closed loop control is applied to the secondary piston pump. 
         [0087]    Hence, as a result of transferring corrective movements to the regular piston movements, the extent of the corrections applied according to  FIG. 7  is considerably smaller than the extent of the corrections shown in  FIG. 6 .