Abstract:
A pumping device includes a working chamber and a piston provided to slide in the working chamber so as to vary the useful volume of the chamber during pumping, and anti-rotation elements for the piston. Advantageously, the anti-rotation elements include an index ( 107 ) mounted radially with respect to the axis of the piston and the device includes a longitudinal slot ( 108 ), the index being provided so as to move in the slot. Advantageously, the index ( 107 ) has two approximately parallel planar faces that extend longitudinally.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the field of piston pumps used in automatic withdrawals, in particular pumps of the syringe type used to withdraw a blood sample. 
     DESCRIPTION OF THE RELATED ART 
     A piston pump comprises a work chamber and a piston slidingly mounted in the chamber. The work chamber is connected to withdrawal means such as a line and a needle. The volume of the chamber is varied, depending on whether one wishes to suction or discharge the withdrawn sample, by moving the piston. 
     These pumps are generally used to withdraw very small sample quantities, which requires considerable precision in the volumes, and therefore the positioning and movement of the piston. Generally, the sample itself does not penetrate the chamber, which, like the line, is occupied by an intermediate liquid that acts as a liquid piston. 
     The piston is generally translated by a screw that is engaged on a threaded bush rigidly secured to the piston and situated in the extension thereof, the screw in turn being driven by a motor, for example a stepping motor. One drawback of this assembly is that the screw tends to rotate the piston around its axis if its rotation is not blocked. Another problem arises from the fact that the seal providing sealing between the piston and the work chamber does not make it possible to ensure precise enough guiding of the piston in its movement and the latter may slightly tilt in a rotational movement orthogonal to its axis. 
     SUMMARY OF THE INVENTION 
     The invention aims to resolve these problems. To that end, it proposes a pumping device, which may comprise a work chamber and a piston provided to slide in said work chamber so as to vary the working volume of the chamber during pumping, characterized in that it comprises anti-rotation means of the piston. 
     The anti-rotation means make it possible to ensure that the piston will only be driven in an axial translational movement. 
     Advantageously, the anti-rotation means may comprise an index radially mounted relative to the axis of the piston and the device may comprise a longitudinal slit, said index being provided to move in said slit. 
     The pumping device comprises an index protruding transversely relative to the piston axis and a rectilinear guide slit, parallel to the axis of the piston, for example formed in the body of the pump. The index that moves in this guide slit ensures that the piston will not be rotated by the screw. Of course, the transverse extension of the index is equal to or very slightly smaller than the width of the guide slit, such that it can slide freely. 
     Advantageously, the index may comprise two substantially parallel planar faces that extend longitudinally. 
     When the index has a small longitudinal extension, for example when it is a cylindrical pin, the friction surface between the index and the flanks of the guide slit is small, which creates rapid wear of the index and/or the flanks of the slit. To prevent this wear, it is advantageous to give a certain longitudinal extension to the index, for example to equip it with two parallel and longitudinal planar faces, those faces sliding along the flanks of the slit and reducing the friction. 
     Advantageously, the pumping device may comprise at least one end-of-travel detecting means situated near one end of the slit. 
     The pumping device is generally provided with at least one end-of-travel detection means to stop the motor and prevent any end-of-travel deterioration. This end-of-trouble detector may be situated at any carefully chosen location of the pump, but it may advantageously be situated near one end of the slit, in particular the end close to the work chamber. 
     Advantageously, the slit is extended by a cavity, the end-of-travel detector being provided to penetrate the cavity when it reaches the end of its travel. 
     In certain assemblies, the slit communicates, at that end near the working chamber, with an area with a greater width than the slit, but limited longitudinal extension, provided to receive an end-of-travel detector. 
     If the index has a small bulk, for example a pin, it may completely leave the slit, penetrating the detection area, and no longer perform its anti-rotation function of the piston. 
     Advantageously, the longitudinal extension of the index is greater than the distance between the end of the slit and the end-of-travel detection area of the index. 
     The cavity may assume any shape, but generally has a rectangular section, with a width larger than that of the slit and a limited longitudinal extension. 
     The detection area must be understood as the point, direction or surface which, when reached by the index, causes the end-of-travel signal by the detector. 
     These arrangements make it possible for the index to remain engaged on the flanks of the slit—and therefore continue to perform its anti-rotation function—when the detection occurs. 
     Furthermore, during the inverse movement of the piston, the index does not risk abutting on the narrowing constituted by the passage from the cavity to the slit. 
     Advantageously, the anti-rotation means further comprise a partition situated upstream from the work chamber and through which the piston is provided to slide. 
     The piston crosses through a partition of the body of the pump before reaching the work chamber. The thickness of this partition is sufficient to guide the piston axially, i.e., to prevent any tilting movement thereof transverse to its axis. 
     Advantageously, the piston is driven in translation by a screw engaged on a bush, and the pumping device further comprises means for canceling the play between said screw and said bush. 
     As seen above, the piston is translated by a screw that is engaged on a threaded bush. This assembly is generally a source of play and therefore imprecision in the measurement. It is therefore advantageous to provide means for canceling out that play. The invention proposes two alternatives. 
     In a first alternative, the screw is a ball screw. Ball screws are known for eliminating the play between the screw and the part unscrewed above it, but the use of such an assembly in the present pump is atypical in that the ball screw is only maintained by a single bearing, situated at the end of the screw close to the motor. 
     In the second alternative, the screw is a traditional threaded screw and the pump further comprises means for compensating play between said screw and said bush, in particular a spring. 
     A spring continuously puts the same faces of the threads of the screw and the bush in contact, canceling the play between those two elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Embodiments and alternatives will be described hereinafter, as non-limiting examples, in reference to the appended drawings, in which: 
         FIG. 1  shows a perspective view of a pump, 
         FIGS. 2A, 2B and 2C  respectively show a pump in planar top view and longitudinal cross-sectional view along A-A, 
         FIG. 3  shows the pump in planar top view without an end-of-travel detector, 
         FIG. 4  shows an end-of-travel detector in perspective view, 
         FIG. 5  shows an enlarged longitudinal cross-section at the end of the slit of the pump. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The pump illustrated in  FIGS. 1, 2A and 2B  comprises a body  100  with a substantially parallelepiped shape with cut-out panels, topped by a work compartment  120  fixed on a transverse face of the body  100 . The body  100  contains a piston  101  that is slidingly mounted within the body  100 . The piston passes through the partition  102   a , which adjoins the work compartment  120 , and emerges in a work chamber  121  formed within that compartment. A seal  103 , situated at the junction of the work chamber  121  and the body  100 , ensures sealing around the piston  101 . On the side of the work compartment  120 , at least one line (not shown) is fixed to the pump and communicates with the work chamber. 
     The movement of the piston modifies the working volume of the work chamber  121 . If this work chamber is filled with a liquid, causing the piston to penetrate it drives the liquid into the line, and withdrawing the piston suctions liquid in the line. This liquid may serve as an intermediary or “liquid piston” between the piston  121  of the pump and a sample to be withdrawn. 
     The piston  101  is rigidly secured to a support  104  situated in the body  100 , in which a cavity  105  is formed axially aligned with the piston  101 . Opposite the piston  101 , the support  104  is rigidly fastened on a threaded bush  106  coaxial with the cavity  105 . The screw  131  of the motor  102   b  is placed in the cavity  105  and in the threaded bush  106 . The screw illustrated in  FIG. 2B  is a ball screw engaged on a suitable sleeve  106 . This ball screw has the particularly, in this application, of being used without any bearing remote from the motor  102   b.    
     In another embodiment, the screw  131  is a traditional threaded screw, but the play between the screw and the sleeve  106  is canceled owing to a spring  113  (shown schematically in  FIG. 2C ) that continuously biases the sleeve in the same axial direction, so as to eliminate the play between the screw and the threaded sleeve. 
     An index  107  extends radially from the support  104  and is positioned in a slit  108  formed in one face of the body  100 , but without protruding relative to that face of the body  100 . The index has a substantially rectangular transverse section, the small side being substantially equal to the width e of the slit  108  so as to slide without friction in the slit. In this way, the index prevents the support  104  and therefore the piston  101  from rotating on its axis when the spindle  131  is actuated in rotation by the motor, only a translational movement being possible. 
     At its end near the work compartment  120 , the slit  108  is extended by a cavity  109  delimiting a wider area with a substantially rectangular shape. This cavity is intended to receive an end-of-travel detector of the piston, by means of the entry into that area of the index  107 . 
       FIG. 4  illustrates one such end-of-travel detector in the form of an optical detector  110  or optical jumper. This optical jumper  110  comprises two substantially parallel branches  111   a  and  111   b , one of the branches being provided on its inner face with an optical transmitter (not shown), for example a diode, the other branch being provided on its opposite face with an optical detector (not shown), which are aligned along an optical axis X-X′ substantially orthogonal to the branches  111   a ,  111   b . The optical detector  110  further comprises two fastening tabs  112   a  and  112   b , situated on either side of the detector in a same plane orthogonal to the branches  111   a ,  111   b . The fastening tabs each comprise a through opening for using a screw to fasten the optical jumper  110  on a longitudinal face of the body  100  of the pump. 
     The branches  111   a ,  111   b  of the detector are provided to be placed in the cavity  109  of the body  100 , on either side of the axis of the slit  108 , such that the space comprised between the branches  111   a  and  111   b  enters the extension of the slit  108 . When it reaches the end of travel, i.e., the end of the slit  108 , the index  107  therefore passes between the branches  111   a  and  111   b  of the detector and interrupts the optical beam. 
       FIG. 5  shows a cross-section of an index  107  arriving at the end of travel in the slit  108 . The front face of the index passes through the optical axis X-X′ of the optical jumper  110  between its branches  111   a  and  111   b , which causes the pump to stop and the progression of the index to end. The longitudinal extension or length L of the index  107  is greater than the distance d between the end of the slit  108  and the optical axis X-X′. In this way, the index  107  remains engaged on the flanks of the slit  108  until the end of travel is detected and therefore continues to play its anti-rotation role with respect to the piston  101 , even if the separation between the opposite faces of the branches  111   a ,  111   b  of the optical sensor is greater than the width e of the slit  108 . 
     This arrangement procures another advantage: when the index  107  moves away from the sensor  110  after reaching its end of travel, it does not risk being blocked against the face  109   a  of the cavity  109  adjacent to the end of the slit  108 , which would risk deteriorating the pump. 
     The figures illustrate the use of an optical jumper  110 , but the invention is not limited to such a detector. Many detectors are covered by the invention, for example a contact detector, provided the length L of the index is greater than the distance between the end of the slit  108  and the end-of-travel detection point. This condition will always be met if the length L of the index is greater than the depth P of the cavity  109 . 
     It is also appropriate for the piston  101  to be guided in rotation transverse to its axis, so as to prevent any tilting movement of said piston. This transverse guiding is done by the partition  102   a  passed through by the piston  101 , said partition to that end being provided with a sufficient thickness.