Abstract:
A force sensor assembly for use in peristaltic pumps includes a housing and a load cell at least partially disposed within the housing. A plunger, pivotable about an axis, has an upper surface and an underside surface distal from the upper surface. The underside surface cooperates with the load cell. The force sensor further includes a mechanism to reduce the load cell&#39;s sensitivity to the positioning of an applied force on the upper surface.

Description:
TECHNICAL FIELD 
     The present invention relates generally to a force sensor assembly for use in infusion pumps. More particularly, the present invention relates to an assembly that reduces a load cell&#39;s sensitivity to the placement of an intravenous tube on the force sensor assembly in an infusion pump. 
     BACKGROUND OF THE INVENTION 
     Various devices have been developed to administer intravenous (IV) fluids to patients. One such device, a peristaltic infusion pump, operates a series of fingers or rollers which deform and occlude a resiliently deformable IV drip tube at multiple points sequentially along the tube&#39;s length. These occlusions form a wave like motion which forces the IV fluid under positive pressure along the tube. After each successive occlusion, the tube resiliently rebounds to its original diameter. The repetitive deformation of the tube may, however, ultimately weaken the resilience of the tube material. After prolonged use, a tube may not fully rebound to its former shape, thereby partly or fully occluding the tube at a point along its path. In addition, IV drip sets often contain clamps, which can be inadvertently be left closed, thus partly or fully occluding the tube. 
     In order to effectively control IV fluid delivery, it is essential that the infusion system constantly determine whether fluid is in fact being delivered to the patient. Interruptions of the fluid flow may occur for number of reasons, such as for example, occlusion of the tube or a blocked catheter. If the pump mechanism does not stop when the tube is occluded, either the pump will stall, the pump will continue to run with no fluid delivered, or the fluid pressure in the tube will increase until the obstruction catastrophically clears, possibly injuring the patient. 
     Accordingly, many infusion pump systems include a force or pressure sensor to determine whether there is an increase or loss of pressure within the tube. The sensor determines whether the fluid flow in the tube has been interrupted, and the pumping mechanism may be stopped and/or medical personnel notified. Because of the potentially harmful consequences of such interruptions, it is important that these sensors be as accurate and reliable as possible. Also, due to an infusion pump&#39;s portability and arduous operating conditions, it is desirable for these sensors to be small and rugged. 
     Force or pressure sensors used in infusion pumps typically contain a plunger that is either constrained in some way, such as with a pin in a hole, or of a free floating type. A constrained plunger type force sensor assembly could be comprised of an actuation plunger connected to a pin positioned and guided within a hole in a sensor housing. A transducer or load cell is positioned along the central axis of the plunger, remote from an IV drip tube. When the IV drip tube is positioned directly over the central axis of the plunger, a force created by the internal pressure of the IV drip tube is applied via the plunger and pin to the load cell, which measures the applied force. In this scenario, the measured force would be comparatively accurate as there is typically only a small loss of transferred force due to friction. However, when the IV drip tube is positioned off-center to the plunger&#39;s central axis, the plunger tends to rotate causing side loading on the plunger pin by the sensor housing which may bind the plunger pin in the hole. This side loading creates a friction force between the plunger pin and the sensor housing which results in a loss of force being applied to the load cell. This friction force ultimately leads to inaccurate and unreliable force measurement results. The above scenario may occur for example where an IV tube is misplaced on the force sensor assembly or where the tube drifts along the plunger surface during use. 
     A free floating plunger type force sensor could be comprised of an actuation plunger positioned within a hole in a sensor housing providing clearance allowing the plunger to float freely. The plunger is positioned over a well of force transmitting gel. The plunger transfers force applied by the IV tubing to the gel, which in turn transfers the force to a transducer or load cell situated within the gel. An example of a gel-type sensor is disclosed in U.S. Pat. No. 5,661,245. The plunger is typically allowed to angulate when a force is applied by the IV tubing to the plunger along an off-center axis. The angled plunger transfers force to the gel with less efficiency, which in turn transfers less force to the transducer or load cell situated within the gel. This reduced force ultimately leads to inaccuracies in the force measurement results. Additional inaccuracies may also be experienced due to frictional reaction forces between the plunger edges and the sensor housing. 
     Accordingly, there is a need for a small and inexpensive force sensor assembly which provides accurate and reliable results regardless of the placement of the IV tubing on the force sensor assembly. 
     SUMMARY OF THE INVENTION 
     According to the invention there is provided a force sensor assembly which is adapted to reduce a load cell&#39;s sensitivity to the positioning of an IV tube on the load cell&#39;s actuation plunger. The force sensor assembly comprises a housing, a load cell at least partially disposed within the housing and a plunger, which is pivotable about an axis. The plunger may be hinged to the housing or to another fixed point proximate to the housing, and may for example be a living hinge or a small pin pivot hinge. The plunger further comprises an upper surface which may be shaped to compensate for variations in measured force-caused by possible misalignment of the IV tubing on the plunger&#39;s upper surface. The plunger also comprises an underside surface distal from the upper surface. 
     In use an IV tube is placed on the plunger&#39;s upper surface, pressure within the IV tube applies a force to the upper surface of the plunger, causing the plunger to pivot about the axis. The pivoting plunger&#39;s underside makes contact with the load cell and thereby wholly transfers the applied force to the sensor for measurement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the nature and objects of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 is a cross sectional diagrammatic view of an IV tube enclosed within an infusion pump; 
     FIG. 2 is an exploded isometric view of an embodiment of the present invention; 
     FIG. 3 is an isometric view of the present invention with an IV tube in a first orientation, 
     FIG. 4 is a diagrammatic side view of the sensor assembly of FIG. 3; 
     FIG. 5 is an isometric view of an alternative embodiment of the invention; 
     FIG. 6 is an isometric view of yet another alternative embodiment of the invention; 
     FIG. 7 is an isometric view of an embodiment of the present invention with an IV tube in a second orientation; 
     FIG. 8 is a diagrammatic side view of the sensor assembly of FIG. 7; 
     FIG. 9 is a diagrammatic side view similar to that of FIG. 8; 
     FIG. 10 is an isometric view of an alternative embodiment of the present invention; 
     FIG;  11  is an exploded isometric view of a further alternative embodiment of the present invention; 
     FIG. 12 is an isometric view of yet another alternative embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates a cross sectional view of an IV tube enclosed within an infusion pump, at the load cell. The IV tube  100  is compressed between a clamping member  102  and a force sensor assembly  104 . Ideally the IV tube  100  is positioned directly over the force sensor assembly  104 , centered on centerline  106 , such that the measured force at the force sensor assembly and a force created by the internal pressure of the IV tube  100  lie along the same plane. While this ideal condition ensures high force measurement accuracy, it is often difficult to guarantee under normal operating conditions. 
     FIG. 2 shows an exploded isometric view of an embodiment of the present invention. The sensor assembly  200  comprises a housing structure  202  defining a hole therethrough  204 . A pressure or load cell  206  is disposed within the housing  202 . The load cell  206  is preferably located directly under the aforementioned hole  204  and is of any typical construction (i.e. gel or oil filled with micro-machined silicon die, direct die contact, strain gage etc.). The load cell is selected to have a particular construction that provides little or no mechanical friction in its force transfer mechanism (not shown) to its sensing element (not shown) and is also relatively insensitive to the location of the measured applied force. A suitable sensor may be similar to the sensor disclosed in U.S. Pat. No. 5,760,313 which is hereby incorporated by reference. An actuation plunger  216  is fastened, by means of a screw  210  or the like, to a low mechanical friction hinge  208  that provides support in all directions, but allows rotation in an actuation direction. Such a hinge may, for example, be a living hinge or a small pin pivot hinge as depicted by reference numeral  1102  in FIG.  11 . The living hinge may for example be a resilient metal or plastic strip, as is known in the art. Hinge  208  is in turn fastened to the housing  202  by means of another screw  212 . It should be appreciated that one could use other fastening means instead of screws  210  and  212 , such as for example glue or rivets. Both the shape of the plunger&#39;s upper surface  214  and the shape of the plunger&#39;s underside  218 , may be varied to provide optimum results. This embodiment utilizes a circular shaped upper surface  214  and a chamfered underside  218 . The underside  218  of the hinged plunger  216  is shaped so that it preferably contacts with the load cell  206  at a single contact point. Other shapes, such as for example a semicircular shaped underside, may also be utilized. A base  220  seals the housing  202  on the side distal from the hole  204 . The base  220  furthermore fastens the sensor  206  in the housing  202  and includes contacts  222  disposed thereon, which connect outputs  224  from the sensor  206  to other measurement circuitry (not shown). 
     FIG. 3 shows an isometric view of the present invention with an IV tube in a first orientation. In this preferred embodiment, an IV drip tube  302  is placed across the sensor assembly  300 , perpendicular to the hinge axis. This orientation is preferred as the force sensor assembly  300  is not subject to a moment arm effect discussed infra. FIG. 4 illustrates a diagrammatic side view of the sensor assembly of FIG.  3 . If the IV tubing crosses the plunger  402  off-center applying a force  400  to the plunger  402 , the hinge, rotatable about hinge axis  404 , provides a reaction force minimizing or preventing angulation of the plunger  402 . This embodiment of the sensor assembly thus gives a more accurate reading irrespective of whether or not the tube is centered above the load cell  408  or not. Therefore, little or no reduction in force is transmitted to the load cell through the plunger from the IV tube if it is positioned off-center to the load cell. 
     FIG. 5 shows an isometric view of an embodiment of the invention. Sensor assembly  300  includes a modified upper surface  500  of the plunger. Square upper surface  500  is preferably utilized in conjunction with the embodiment described in relation to FIGS. 3 and 4, where the tube is oriented perpendicular to the hinge axis. The square shaped upper surface  500  maintains a constant area along the hinge axis. 
     FIG. 6 shows an isometric view of another embodiment of upper surface  600 . The upper surface of the plunger may be shaped so that the tubing contact area changes with the distance of the IV tubing from the location of the center of the load cell. A change in the tubing contact area produces a change in the force transmitted to the plunger due to tubing internal pressure, and therefore a change in the force relationship with the load cell. Hourglass shaped upper surface  600  is also preferably utilized in conjunction with the embodiment described in relation to FIGS. 3 and 4. The hourglass shape, when implemented in the appropriate orientation to the hinge axis, can provide an increase in tubing contact area with an increase in the distance from the centerline of the load cell. This can counteract any loss in force to the load cell due to side loading of the hinge components, thus minimizing any effects of the IV tubing being off center. 
     FIG. 7 shows an isometric view of an alternative embodiment of the invention with an IV tube in a second orientation. In this embodiment, IV drip tube  700  is placed across the sensor assembly  702 , parallel to the hinge axis. 
     FIG. 8 illustrates a diagrammatic side view of the sensor assembly shown in FIG. 7. A plunger  802  is pivotably hinged about a line  800 , allowing the plunger to make single point contact with a load cell  804 . When a force  806 , caused by pressure within the IV tube is applied directly above the load cell  804 , the load cell measures a reaction force  808  which is substantially the same as the applied force  806 . However, as illustrated in FIG. 9, when a force  900  is applied off-center to the load cell  804 , reaction force  902  measured at load cell  804  will be larger than the applied force  900 , due to a moment arm effect. As there is no way to accurately determine the distance of the applied force  900  from hinge line  800 , this orientation of the IV tube on the sensor assembly is not preferred. To overcome this problem, the shape of the upper surface of the plunger may once again be varied to compensate for the misalignment of the IV tube. Instead of a round upper surface of the plunger as shown in FIGS. 2,  3  and  7 , or an hourglass shaped upper surface as shown in FIG. 6, the upper surface of the plunger may be shaped so that the tubing contact area changes with the distance of the IV tubing from the location the hinge axis  800 . A change in the tubing contact area produces a change in transmitted force due to the tubing internal pressure and thus a change in the force relationship with the load cell. Variations in measured force caused by the misplacement of the tube on the upper surface of the plunger may therefore be counteracted by tailoring the shape of the upper surface of the plunger. As shown in FIG. 10, a triangular or tear drop shaped upper surface  1000 , narrowing away from the hinge axis may preferably be utilized in conjunction with the embodiment described above in relation to FIGS. 7-9, where the tube is oriented parallel to the hinge axis. 
     FIG. 11 illustrates an exploded isometric view of another embodiment of the present invention. The living hinge  208  of FIG. 2 has been replaced with a plunger  1100  that is itself hinged to the housing  202  by means of a small hinge pin  1102 . The separately hinged actuation plunger  1100  provides stability and low mechanical friction, resulting in low sensitivity to the positioning of an off-centered IV tube. 
     Other embodiments of the present invention may include a force assembly where the hinge and actuation plunger are formed integral with the hinge support housing (i.e. all molded together as one piece) instead of separate parts. FIG. 12 shows an embodiment of the present invention with a plastic living hinge  1200  integrated into the housing of the IV mechanism. The actuation plunger may also be held by flexible supports at several points around its circumference to minimize moment arm changes with tubing off-center positioning. Furthermore, the hinge could be attached to some other structure of the IV pump proximate to the sensor assembly housing. 
     The foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description only. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.