Patent Publication Number: US-2013243631-A1

Title: Uniform flow displacement pump

Description:
This application is a continuation of U.S. Pat. No. 7,150,607, filed on Oct. 29, 2003, which claims the benefit of U.S. Provisional Application No. 60/427,468, filed Nov. 18, 2002. 
    
    
     FIELD OF THE INVENTION  
     The present invention relates to methods and systems for analyzing particles in a dilute fluid sample, and more particularly to pumps utilized by such systems to manipulate the fluid samples. 
     BACKGROUND OF THE INVENTION  
     Methods and systems for analyzing particles and particularly sediments are well known in the art, as disclosed in U.S. Pat. Nos. 4,338,024 and 4,393,466, which are incorporated herein by reference. Such systems utilize a flow cell though which fluid samples are passed, and a particle analyzer for capturing still frame images of the fluid passing through the flow cell. Thus, the flow cell positions and presents the sample fluid containing particles of interest for analysis. The more accurately that the sample fluid is positioned by flow cell, the better the analysis of the particles therein that can be made. 
     Typical flow cells cause the sample fluid, and a sheath fluid that buffers the sample fluid, to flow together from a large entry chamber into a small cross sectional examination area or region. The transition from the inlet or entry chambers to the examination region forms a hydrodynamic lens that squeezes both the sample fluid and the sheath fluid proportionally into the smaller space. Where the particles of interest are microscopic particles, the resulting cross-sectional space occupied by the sample fluid must be positioned within the depth of field of the analyzer, such as an optical system or a laser system, to obtain the best analytical information. For the best hydrodynamic focus, a large area of sheath flow must envelop the small area of sample fluid without any swirling or vortices. Thus, uniform flow of sample and sheath fluids through the flow cell is essential for optimal operation of particle analyzers. 
     Displacement pumps, (e.g. tubing or peristaltic pumps), are well known in the art and have been used to pump fluid samples and sheath fluids through flow cells. Conventional peristaltic pumps include multiple rollers that roll along flexible tubing containing fluid. The rollers push the fluid along the length of the tubing, drawing fluid into an input end of the tubing and forcing fluid out an output end of the tubing. A common configuration includes a rotating hub with rollers on its periphery, and an annularly shaped housing against which the tubing is pressed. With each rotation of the hub, each roller engages with, rolls along the length of, and disengages from, the tubing. At least one of the rollers is in contact with the tubing at all times so that fluid cannot flow backwards through the tubing. 
     Conventional peristaltic pumps have several drawbacks. For example, multiple rollers engaging with and disengaging from the flexible tube cause pulsations in the fluid flow through the pump, which can be problematic for proper operation of flow cells. Moreover, the amount of fluid delivered by the pump for n degrees of rotation is dependent on the starting angle of the rollers. Most pump designs only retain the tube at its ends, relying on the multiple rollers engaged with tubing to hold it in its circular path along the housing. Thus, the tube can stretch and contract as the rollers move across its length, which again can cause varying flow and uncertainty in the volume moved by rollers. Lastly, when the pump is shut down, rollers are left in contact with the tube, causing compression setting (flat spotting) of the tube, which adversely affects the uniform flow of the fluid after the pump is activated again. 
     There is a need for a displacement pump that provides uniform fluid flow of known and repeatable quantities, and which does not produce flat spots on the tube during non use. 
     SUMMARY OF THE INVENTION  
     The present invention is a pump that includes a compression surface, a hollow compression tube secured to the compression surface, and compression means for incrementally compressing the compression tube against the compression surface to create a moving occlusion of the compression tube that uniformly pushes fluid through the compression tube, wherein the compression means has at least one rest position in which the compression means does not compress the compression tube. 
     In another aspect of the present invention, a pump includes a pump assembly and a cassette assembly. The pump assembly includes a pump housing that defines a cavity, a roller disposed in the cavity, and a motor for moving the roller relative to the housing. The cassette assembly is removably disposed in the cavity and includes a cassette housing having a compression surface, and a hollow compression tube secured to the compression surface. As the motor moves the roller, the roller presses the compression tube against the compression surface to create a moving occlusion of the compression tube for pushing fluid through the compression tube. 
     Other objects and features of the present invention will become apparent by a review of the specification, claims and appended figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1A  is an exploded view of the pump assembly of the present invention. 
         FIG. 1B  is a perspective view of the pump assembly of the present invention. 
         FIG. 2A  is an exploded view of the cassette assembly of the present invention. 
         FIG. 2B  is a perspective view of the cassette assembly (without compression tube) of the present invention. 
         FIG. 2C  is a perspective view of the cassette assembly of the present invention. 
         FIG. 3  is a top view of an alternate embodiment of the present invention. 
         FIG. 4  is a top view of a second alternate embodiment of the present invention. 
         FIG. 5  is a side view of a third alternate embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     The uniform displacement pump of the present invention is illustrated in  FIGS. 1A-1B  and  2 A- 2 C, and includes a pump assembly  10  and a cassette assembly  12 . 
       FIGS. 1A-1B  illustrate the pump assembly  10 , which includes a housing having upper and lower housing portions  20   a / 20   b  respectively, that are hingedly attached to each other by a hinge  22  and hinge bracket  24 . When upper housing  20   a  is closed over lower housing  20   b,  an annular cavity  26  is defined thereby. A roller arm  28 , which is preferably spring loaded, is disposed in the cavity  26 . Roller arm  28  has a proximal end at the center of the cavity  26 , and a distal end with an outwardly facing compression roller  29  mounted thereon. A motor  30  has a drive shaft  32  that extends into the cavity  26  and is attached to the proximal end of the roller arm  28 , for rotating the roller  29  around the periphery of the cavity  26 . A sensor assembly  34  is mounted to the lower housing  20   b  and includes a sensor switch  36  for detecting a closure pin  38  from the upper housing  20   a,  indicating that the upper housing  20   a  is in a closed position over lower housing  20   b.  Sensor assembly  34  also includes a sensor switch  37  that detects the presence of the cassette assembly  12  in cavity  26 , and a sensor  40  that detects and verifies the position of the roller arm  28 . 
       FIGS. 2A-2C  illustrate the cassette assembly  12 , which includes a housing having upper and lower cassette housing portions  46   a / 46   b  respectively, that snap together via engagement tabs  48  that extend from the upper cassette housing  46   a  and engage with lower cassette housing  46   b.  Lower cassette housing  46   b  includes an annular sidewall  50  with a shoulder  52  extending from an inner surface of the sidewall  50 . Upper cassette housing  46   a  includes an annular sidewall  54 . When upper/lower cassette housings  46   a / 46   b  are snapped together, upper cassette sidewall  54  fits inside lower cassette sidewall  50 , where sidewall  54  and the shoulder portion of sidewall  50  together define an inwardly facing annular compression surface  56 . Upper cassette sidewall  54  is positioned a fixed distance away from shoulder  52  to define a channel  58  in the annular compression surface  56 . 
     A hollow compression tube  60  is removably disposed along the compression surface  56 . The compression tube  60  includes a flange  62  adhered thereto or integrally formed therewith. The flange  62  snuggly inserts into channel  58  with a friction fit that evenly secures compression tube  60  against compression surface  56 . Preferably, flange  62  is a solid cylindrically-shaped member that is integrally formed as part of the compression tube  60 , and that has a thickness corresponding to the width of channel  58 . The compression tube  60  has an input end  60   a  and an output end  60   b.    
     To assemble pump  1 , upper and lower cassette housings  46   a / 46   b  are snapped together, with a compression tube  60  secured against compression surface  56  via flange  62  (held in channel  58 ). The upper pump housing  20   a  is rotated open (away from lower pump housing  20   b ), and the cassette assembly  14  is inserted in lower pump housing  20   b.  The upper pump housing  20   a  is then closed, securely holding cassette assembly  12  in cavity  26 . 
     When motor  30  is activated, roller arm  28  rotates within the cavity  26 , so that roller  29  engages with compression tube  60  and compresses it against compression surface  56 . The spring loaded roller arm  28  ensures that roller  29  is compressed against compression tube  60  with the desired amount of force, so that roller  29  creates an occlusion in the compression tube  60  which moves along the length of tube  60  as roller arm  28  makes a single revolution within cavity  26 . The moving tube occlusion pushes a known quantity of fluid through the compression tube  60  in a uniform manner. By the time the roller arm  28  completes its single revolution, the roller  29  has moved along the entire length of the compression tube portion that is disposed on compression surface  56 , and has disengaged from compression tube  60 . The pump shown in the figures occludes the compression tube during (or for) 285 degrees of the rotation of roller arm  28 , leaving 75 degrees of rotation where the roller  29  does not compress tube  60 . 
     Ideally, the diameter of the compression tube  60  is selected so that the desired amount of fluid for a single process step (e.g. collection of images via a flow cell) can be produced by a single revolution of the roller arm  28 , thus avoiding any pulsations caused by the repeated engagement and disengagement of the roller  29  with compression tube  60 . By continuously anchoring the compression tube  60  against the compression surface (i.e. using the continuous flange  62  engaged in the continuous channel  58 ), tube squirm and fluid flow variations caused therefrom are avoided. A uniform delivery of fluid volume results from each incremental degree of rotation of roller arm  28 . When the pump is inactive, the roller  29  is preferably parked in a default or rest position shown in  FIG. 1A , where the roller  29  does not contact the compression tube  60 , thus preventing premature tube failure due to the formation of flat spots therein. However, roller  29  can be temporarily parked on compression tube  60  so that the (stalled) tube occlusion acts as a temporary pinch-valve for the fluid inside compression tube  60 . 
     The removable cassette  12  allows for easy replacement of the compression tubing  60  by the user. Insertion of the flange  62  into channel  58  is convenient and provides a repeatable positioning of the tubing  60  against compression surface  56 . The tubing  60 , and/or the cassette assembly  12  in its entirety, can be replaced by the user as tube  60  ages, ideally without the use of any tools. Closing upper housing  20   a  onto lower housing  20   b  compresses the cassette assembly  12  to secure compression tubing  60  and compression surface  56  in place (relative to pump assembly  10  and in particular roller  29 ). The clamping features of both the cassette assembly  12  and pump assembly  10  provide repeatable and convenient assembly and performance of the pump. The pump preferably uses tubing  60  having a symmetrical cross-section, which permits more uniform fabrication of the tubing and more repeatable pump performance, and is ideal for clamping features of the cassette assembly  12 . 
     It is to be understood that the present invention is not limited to the embodiment(s) described above and illustrated herein, but encompasses any and all variations falling within the scope of the appended claims. For example, while pump housing portions  20   a / 20   b  are shown hingedly attached, they could instead snap together in the manner shown for cassette housing portions  46   a / 46   b,  and vice versa. Arm  28  need not necessarily be spring loaded. Compression surface  56  need not be circular, so long as the spring loaded roller arm  28  can maintain a desired minimal force for compressing compression tube  60 . For example, the compression surface could be elliptical, where the rotating spring loaded roller arm has enough longitudinal travel (along the length of arm  28 ) to maintain contact with the compression tube  60  with sufficient force during the arm&#39;s revolution, as illustrated in  FIG. 3 . Alternately, the amount of longitudinal travel of the rotating arm could be more limited, where the roller  29  ceases compression of, and even possibly loses contact with, the compression tube at multiple points through its revolution, as illustrated in  FIG. 4 . In this case, the roller  29  twice loses contact with the compression tube  60 , so that the pump produces two separate pulses of fluid flow per full revolution of the arm  28 . In fact, roller  29  need not rotate about a fixed point, but can include translational movement, as shown in  FIG. 5 . In this embodiment, spring loaded arm  28  is connected to a moving conveyor belt or track  64  that moves roller  29  along a planar compression surface  56 . One or more additional roller arms  28  (with rollers  29 ) can be added to belt/track  64 , so long as only one roller is engaged with compression tube  60  at any given time.