Reciprocating antirotation pump

Abstract

A pump for pumping discrete liquid volumes is provided with an anti-rotation construction to prevent a nut mated to a rotating head screw from rotating when the lead screw is rotated. The anti-rotation construction comprises a set of rails that slideably fit within a mating set of tracks. The pump also can include a variable homing position for a pump piston having at least two different homing positions.

Description

REFERENCE TO RELATED APPLICATION

This application is based on provisional application Ser. No. 60/876,413, filed Dec. 21, 2006.

BACKGROUND OF THE INVENTION

This invention relates to a reciprocating pump capable of dispensing discrete microliter or milliliter liquid reagents. More particularly, this invention relates to such a pump which converts rotational motion to linear motion while utilizing an antirotational device.

The science and economics of invitro diagnostic (IVD) testing has changed with developments in assay reagent variation and assay size miniaturization. The number of assays performed annually is increasing as population demographics change and the availability of diagnostic testing increases throughout the world. Instrumentation is constantly adapted to these changing conditions. The reagents are harsher, including increased usage of salt solutions and acid and alkaline solutions having significant pH ranges. The liquid that pumps must move is decreasing from milliliters in volume to microliters in volume while precision and accuracy requirements remain constant. Lastly, the instrumentation is required to perform more tests prior to regular or unscheduled maintenance.

Pumps are used to move fluids within IVD instrumentation and to transfer reagents from a source reservoir to one or more test sites. As the assay volume decreases, more precise pumps are required. However, more precise pumps are more expensive to manufacture. Presently, there is an acute need for a precise and accurate pump that is also economical.

Piston pumps have been used in fluid dispensing applications for many years and four principle areas have consistently contributed to manufactured cost. They are the motor, the piston, the seal, and the anti-rotation mechanism. In addition, presently available pumps having a home position for the pump piston to provide a base value for the computer processing unit (CPU) which controls piston movement. Pumps are available with the piston home position being either in the piston aspirated position or in the piston retracted position. Accordingly, it would be desirable to simplify the anti-rotation mechanism by designing each part to be multi-functional and manufacturable with multiple processes including machined and injection molded methods. The manufacturing process flexibility helps to control costs during the initial time period prior to high volume production. In addition, it would be desirable to provide the pump user with a choice for the piston home position to provide flexibility in choosing a desired CPU.

SUMMARY OF THE INVENTION

The present invention provides a pump for liquids which utilizes a rotary motor capable of rotating clockwise and counterclockwise, a reciprocation pump piston and means for converting rotary motion to reciprocating linear motion. A lead screw which mates within a nut portion of a ferrule is rotated by the motor. The nut portion of the ferrule moves linearly and drives a piston portion of the pump to deliver discrete volumes of liquid. An antirotational construction is provided which prevents rotation of the nut portion of the ferrule. The dispense resolution of a pump, commonly stated as microliters per step, is proportional to the pitch of the lead screw. Since most piston pumps are built with off the shelf linear actuators, lead screw pitch is limited to those offered by the linear actuator manufacturer. This invention incorporates a standard stepper motor with a custom machined lead screw that is pressed onto the shaft enabling an infinite range of dispense resolutions.

The antirotation construction comprises a set of rails that fit into tracks. Either the rails or tracks can be positioned on the nut portion of the ferrule and the mating rails or tracks are positioned on a stationary housing for the nut portion of the ferrule. The length of the rails and tracks permits the pump piston to reciprocate between a fully aspirated position and a fully retracted position.

Variations in pump construction allow for additional features. The nut in the ferrule can be slotted and combined with a single or multiple o-rings as springs to provide low-cost mechanical anti-backlash compensation. In addition, the stepper motor which turns the lead screw can be made with a shaft protruding on both sides to provide two pump heads that are controlled simultaneously with one motor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exterior isometric view of the pump of this invention.

FIG. 2 is an exploded isometric view of the components of the pump of FIG. 1.

FIG. 3 is a side cross sectional view of the pump of this invention.

FIG. 4 is a cross section view through the rail and track of the ferrule of this invention.

FIG. 5 is an isometric view of the anti-backlash nut mechanism.

FIG. 6 is an isometric view of a two headed pump on one stepper motor.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In accordance with this invention, a piston reciprocating pump is provided with a variable homing switch location. The homing switch location can be either the fully aspirated position of the reciprocal piston or the fully retracted position of the reciprocal piston.

Referring to FIGS. 1 and 2, the pump 10 of this invention comprises a rotary motor 12 adapted to rotate clockwise or counterclockwise. The piston 3 begins to reciprocate when the motor 12 turns the lead screw 16 within the nut portion 18 of the ferrule 20. The ferrule 20 is kept from rotating by the rails 32 and 33 which fit into tracks 28 and 30, which, in turn, causes the piston 22 to move back and forth.

The shaft 14 is attached to lead screw 16 so that when the shaft 14 is rotated, the lead screw 16 is rotated. Lead screw 16 mates with threaded nut portion 18 positioned within the ferrule 20. Piston 22 is secured to the ferrule 20 so that when the nut portion 18 moves linearly during rotation of screw 16, piston 22 also moves linearly. A wash disk seal 24 fits into pump head 26 and provides the sealing function to permit liquid to be pumped in discrete volumes.

The costly components within a pump 10 are the motor 12, the piston 22, the seal 24, and the anti-rotation mechanism described herein. This invention focuses on simplifying the anti-rotation mechanism and homing switch location by designing each part to be multi-functional and manufacturable with either machined or molded process.

The antirotation mechanism which prevents rotation of the ferrule 20 comprises tracks 28 and 30 on ferrule 20 which mate with rails 32 and 33 on inserts 34 and 35 which extend through openings 36 on fixed housing 38. The inserts 34 and 35 are fixed to housing 38 such as with screws. It is to be understood that the tracks 28 and 30 also can be located on inserts 34 and 35 while the rails can be located on ferrule 20 to achieve the same antirotational effect. The housing 38 is provided with two incubation posts 40 and 42 which mate with an indicator 44 or 46 which identifies the home position of the piston for a given CPU.

The ferrule 20 which connects the piston 22 to the lead screw 16 enabling the piston 22 to reciprocate, is now a single piece. The housing 38 also contains the tracks 28 and 30 that fit into a rail 32 or 33 on inserts 34 and 35 to prevent rotation of ferrule 20. The nut half of the lead screw 16 nut drive mechanism is also a feature on the ferrule, making the ferrule multi-functional on three levels. The ferrule 20 holds the piston 32, incorporates the tracks 28 and 30 for anti-rotation, and incorporates the nut 18 for reciprocation. This eliminates parts, simplifies construction, allows for machined or molded construction, and lowers manufacturing costs.

The housing 38 which is the most costly component to manufacture by the machining process, is also designed for injection molding. The housing 38 connects the motor 12 to the pump head 26 and incorporates the ferrule 20 and homing switch 48. The ferrule 20 which is universal for all pistons, mates to rails 32 and 33 in the universal body 38 to prevent rotation of the nut 18. The homing switch 48 mounts in a cavity on the side of the housing 38. The multi-functional housing design simplifies construction, allows for machined or molded construction and lowers manufacturing costs.

Long life is a desirable trait for a piston pump. An important component to long life is material selection. The ferrule 20 slides onto the rails 32 and 33 and the lead screw 16 rotates within the ferrule 20. These friction points can wear and contribute to short pump life.

In one embodiment of the invention, the rails 32 and 33 are made of Delrin AF available from DuPont, which is generically acetal with a polytetrafluoroethylene (PTFE) additive and the ferrule 20 is made from polyetheretherketone (PEEK). These surfaces exhibit good wear characteristics and persons skilled in the art would recognize that many combinations of materials would also perform satisfactorily. Grease is commonly applied to these surfaces.

The pitch of the lead screw 16 is typically either 20 or 40 threads per inch and persons skilled in the art would also recognize that a variety of pitches ranging from 2 through 100 threads per inch are possible.

In addition, persons skilled in the art would recognize that all the parts in the pump could be manufactured with a variety of processes including, but not limited to, machinery, plastic or metal injection molding, rolling, stamping, or extruding.

Referring to FIG. 3, the pump 10 which includes the lead screw 16 and the nut portion 18 of the ferrule 20 also includes two switch homing positions 40 and 42 also includes indicator posts 41 and 42 on home indicator 48.

Referring to FIG. 4, rails 32 and 33 fit into tracks 28 and 30 of ferrule 20.

Referring to FIG. 5, the nut 18 portion of the ferrule 20 is machined with four slots, creating four fingers 19 that can flex inward toward the lead screw 16. When combined with one or more o-rings 21 that compress the fingers 19 into the lead screw 16, backlash is eliminated between the lead screw 16 and the nut 18. Persons skilled in the art would also recognize other anti-backlash methods may be incorporated into the ferrule 20 to eliminate backlash and that other spring methods may be employed. In addition, the ferrule 20 may be manufactured by machining, molding, or other methods and may incorporate between two and twelve fingers 19.

Referring to FIG. 6, the motor 12 may be built with shafts 14 protruding from both sides of the motor 12. Some pump applications require two liquids to be pumped in different volumes at the same time. This allows one motor 12 to drive two pump heads 26 for that application. Persons skilled in the art would also recognize the lead screw 16 on either side of the motor 12 could be different, creating different dispense and aspirate profile curves. For example, one lead screw 16 could be a left hand thread while the other could be right hand thread. This would make both pump heads 26 dispense and aspirate simultaneously. In another example, both lead screws could be right hand thread. This would make one pump head 26 aspirate while the other dispenses and vice versa when the motor 12 direction is reversed.

Claims

1. A pump adapted to deliver a plurality of discrete liquid samples comprising a motor adapted to rotate a shaft including a lead screw clockwise or counter clockwise, a nut mated to said lead screw adapted to reciprocate when said lead screw is rotated, a first housing for said nut having an outside surface provided with either a first set of rails or a first set of tracks, a second set of tracks positioned on a stationary housing positioned to mate with said first set of rails or a second set of rails positioned on said stationary housing to mate with said first set of tracks, a pump piston mounted on said first housing, said first set of tracks or said second set of tracks having a length sufficient to permit said piston to travel a desired distance when said motor is rotated.

2. The pump of claim 1 wherein said first housing includes said first set of rails.

3. The pump of claim 1 wherein said first housing includes said first set of tracks.

4. The pump of claim 3 wherein said second set of tracks are positioned on removable inserts that are attachable to said stationary housing.

5. The pump of claim 2 wherein said second set of tracks is positioned on an inner surface of said stationary housing.

6. A pump adapted to deliver a plurality of discrete liquid samples and having a variable homing position for accepting a visible homing flag in one of at least two positions.

7. The pump of claim 6 having two of said positions for said homing flag.

8. The pump in claim 2 having an anti-backlash nut feature integrated into the ferrule.

9. The pump in claim 2 having two heads actuated by a dual shaft motor with shafts protruding from both sides of the motor.