WO1991019903A1

WO1991019903A1 - Fluid pumps

Info

Publication number: WO1991019903A1
Application number: PCT/GB1991/000888
Authority: WO
Inventors: Richard Graham Fletcher
Original assignee: Pwm Drives Limited; Neo-Medical Consultants Limited
Priority date: 1990-06-16
Filing date: 1991-06-04
Publication date: 1991-12-26
Also published as: GB9013499D0

Abstract

An intravenous fluid infusion pump (1) has a housing (2) having a pump chamber (15) provided with a fluid inlet (16) and a fluid outlet (18), and a magnetisable plunger (20) reciprocable within the chamber by an external electromagnet (8) and dividing the chamber into an input compartment (21) and an output compartment (22). The plunger (20) has a transfer passage (24) extending therethrough provided for transfer of fluid from the input compartment (21) to the output compartment (22) during pumping. A first float (27) is arranged to float in the fluid supplied to the input compartment (21) to close off the inlet (16) during transfer of fluid along the transfer passage (24) from the input compartment (21) to the output compartment (22) on displacement of the plunger (20) in one direction, and a second float (28) is arranged to float in the fluid supplied to the output compartment (22) to close off the transfer passage (24) during discharge of fluid from the output compartment (22) through the outlet (18) and during supply of fluid to the input compartment (21) through the inlet (16) on displacement of the plunger (20) in the opposite direction. The pumping action of such a pump is advantageous in that pumping cannot continue when the pump is empty and the pump will not pump any substantial quantities of air. The pump is also compact and can be produced economically.

Description

"Fluid Pumps" This invention relates to fluid pumps and is more particularly, but not exclusively, concerned with intravenous fluid infusion pumps. An infusion pump for intravenous administration of fluid to a patient is known (U.S. Specification No. 3985133) which includes a disposable volumetric cassette for receiving the fluid to be pumped from input tubing and for supplying measured quantities of fluid to output tubing for infusion into the patient. The cassette includes a pump chamber alternately connectable to the input and output tubing by a common inlet/outlet passage and a rotary valve, and a piston reciprocable within the chamber to draw fluid from the input tubing and discharge fluid to the output tubing. The valve and the piston are driven in synchronism by an external motor so as to provide the required pumping action.

However, such pumps are subject to a number of drawbacks in use. Their reliability is prejudiced by the provision of a piston seal which may fail in use. Furthermore, in order to attempt to eliminate air from the fluid supplied, they incorporate a bubble detector which is prone to set-up and sensitivity problems. In addition such pumps can be relatively costly to produce. It is an object of the invention to provide a novel form of fluid pump which can be used for intravenous administration of fluid to a patient and in other applications requiring high reliability. According to the present invention there is provided a fluid pump comprising a housing having a pump chamber provided with a fluid inlet and a fluid outlet at opposite ends of the chamber, a agnetisable plunger reciprocable within the chamber and dividing the chamber into an input compartment adjoining the fluid inlet and an output compartment adjoining the fluid outlet, the plunger having a transfer passage extending therethrough provided for transfer of fluid from the input compartment ■ to the output compartment during a predetermined part of the pumping cycle, a first valve member arranged to close off the fluid inlet during transfer of fluid along the transfer passage from the input compartment to the output compartment on displacement of the plunger in one direction, and a second valve member arranged to close off the transfer passage during discharge of fluid from the output compartment through the fluid outlet and during supply of fluid to the input compartment through the fluid inlet on displacement of the plunger in the opposite direction, the plunger being reciprocable by suitable electromagnet means to effect pumping by magnetic action.

Since the pumping action of such a pump relies on the fluid pressures exerted on the valve members by the fluid being pumped, it will be appreciated that an advantage of such a pump is that pumping cannot continue when the pump is empty of fluid. Furthermore the pump will not pump any substantial quantities of air. The closing off of the fluid inlet by the first valve member during transfer of fluid along the transfer passage also ensures that siphoning cannot occur through. the pump. A further advantage of such a pump is that the pump and the mechanism for reciprocating the pump may be integrated, the pump plunger itself forming the moving part of the mechanism. This enables the pump housing, which is preferably formed as a replaceable cassette, to be made more compact and at lower cost than in conventional pumps provided for the same purpose. The pump can also be made to be extremely reliable in use, particularly since it does not require use of a bellows-type seal, and to give precise delivery of volume. Furthermore the controller for operating the pump, which will generally be adapted to. receive a cassette which is replaceable after each use, may be made to be particularly reliable in long-term use since it will generally have no moving parts.

The pump preferably includes electromagnet means which is adapted to displace the plunger in one direction on application of an energising signal, and spring means, such as a compression spring, which acts to return the plunger in the opposite direction on removal of the energising signal. Such an arrangement can be used to ensure that the pump inlet is closed off when no energising signal is supplied to the electromagnet means. The electromagnet means preferably incorporates two opposite magnetisable poles spaced apart in the direction of reciprocating movement of the plunger and positioned such that the plunger is asymmetrically disposed with respect to the poles when in at least one of its limiting positions.

The electromagnet means may comprise at least one coil and preferably at least one substantially C- shaped core of magnetisable material having arms defining the poles at their free ends. Additionally a further substantially C-shaped core of magnetisable material may be provided at the ends of each of said arms, the two further cores extending along substantially parallel planes transverse to the direction of reciprocating movement of the plunger and receiving the pump housing between their arms. The housing may be in the form of a replaceable cassette which is easily insertable between the arms.^• Furthermore it is preferred that the first valve member is adapted to float in the fluid supplied to the input compartment to close off the fluid inlet, and the second valve member is adapted to float in the fluid supplied to the output compartment to close off the transfer passage. The valve members are conveniently ball floats.

In a development of the invention a detector, such as an optical detector, is provided to indicate when the pump is empty in dependence on the position of one of the valve members during a predetermined part of the pumping cycle.

The invention also provides a fluid pump comprising a housing having a pump chamber, a magnetisable plunger reciprocable within the chamber to effect fluid pumping, and electromagnet means for reciprocation of the plunger by magnetic action, wherein the electromagnet means includes a substantially C-shaped core of magnetisable material having two arms lying on a common plane, and the housing is in the form of a replaceable cassette which is insertable between the arms such that the direction of reciprocating movement of the plunger is transverse to said common plane. In order that the invention may be more fully understood, a preferred fluid pump in accordance with the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figures 1 and 2 show the pump in axial section in two operating positions;

Figure 3 is a perspective view on a reduced scale of the pump electromagnet;

Figure 4 is a schematic circuit diagram of a current source for the pump electromagnet; Figure 5 is a perspective view on an enlarged scale of an alternative pump electromagnet; and

Figures 6 and 7 show two variants of the pump in axial section.

Referring to Figures 1 and 2, the pump 1 has a two-part housing 2 made _of transparent plastics material and comprising an upper part 3 having a female connector 4 for connection to input tubing (not shown) and a lower part 5 having a male connector 6 for connection to output tubing (not shown) . The housing 2 is in the form of a replaceable cassette which may be inserted in the central bore 7 of an electromagnet 8 (see Figure 3) so as to extend along an axis 9. The electromagnet 8 ^'comprises a substantially C- shaped core 10 and a coil 11 which may be energised by a pulse of unipolar current i. The C-shaped core 10 forms opposite poles 12 and 13 which are spaced apart axially of the housing 2. A further substantially C-shaped core 1 . may be provided to balance laterally the magnetic forces on the housing 2 and to increase the magnetic flux, and both cores 10 and 14 are preferably formed from laminated grain-orientated silicon steel.

Within the housing 2 is an elongate cylindrical pump chamber 15 having a fluid inlet 16 at its upper end provided with a valve seat 17 and a fluid outlet 18 at its lower end provided with a valve seat 19. A cylindrical plunger 20 made of magnetisable material, such as mild steel suitably plated in order to prevent rusting or magnetic stainless steel, is reciprocable within the chamber 15 and forms the moving part of a solenoid formed by the electromagnet 8 and the plunger 20 . Furthermore the plunger 20 divides the chamber 15 into an input compartment 21 adjoining the inlet 16 and an output compartment 22 adjoining the outlet 18, and is provided with an O-ring 23 to prevent fluid leakage along the sides of the plunger. The plunger 20 has a transfer passage 24 extending axially therethrough provided for transfer of fluid from the input compartment 21 to the output compartment 22 during pumping. A valve seat 25 is provided where the transfer passage 24 opens into the input compartment 21, and a valve seat 26 is provided where the transfer passage 24 opens into the output compartment 22.

A first ball float 27 is adapted to float in the fluid in the input compartment 21 and to close off the valve seat 17 and/or the valve seat 25, and a second ball float 28 is adapted to float in the fluid in the output compartment 22 and to close off the valve seat 19 and/or the valve seat 26. The two ball floats 27 and 28 are made of polypropylene or hollow metal. A return compression spring 29 made of stainless steel acts between the lower housing part 5 and the plunger 20 so as to provide return upward movement of the plunger 20 following a downstroke of the plunger 20 effected by energisation of the electromagnet 8 through magnetic action.

The manner of operation of the pump 1 to pump a fluid will now be described with reference to Figures 1 and 2 in which Figure 1 shows the pump with the plunger 20 in its upper limiting position, and Figure 2 shows the pump with the plunger 20 in its lower limiting position. Initially, with the electromagnet 8 deactivated, the plunger 20 is forced- upwardly into its upper limiting position by the spring 29 so as to seat the first ball float 27 against the valve seats 17 and 25, as shown in Figure 1, thus closing off both the fluid inlet 16 and the inlet opening of the transfer passage 24 and preventing siphoning through the pump. During a normal pumping cycle the output compartment 22 will be filled with the fluid being pumped and this will cause the second ball float 28 to float and to seat against the valve seat 26 thus closing off the outlet opening of the transfer passage 24. In this position the fluid outlet 18 is not closed off by the second ball float 28. Furthermore the plunger 20 is asymmetrically located between the poles 12 and 13 of the electromagnet 8.

When a pulse of unipolar current is supplied to the coil 11 of the electromagnet 8 (in either sense) , the plunger 20 is subjected to a downward magnetic force which causes the plunger 20 to move downwards against the action of the spring 29 in order to attempt to align itself symmetrically between the poles 12 and 13. This causes the fluid in the output compartment 22 to be discharged through the fluid outlet 18, and simultaneously results in movement of the first ball float 27 away from the valve seat 17 thus permitting supply of fluid through the fluid inlet 16 to the input compartment 21. Furthermore the second ball float 28 seats against the valve seat 19 thus closing off the fluid outlet 18, as shown in Figure 2. During this downstroke of the plunger 20, the fixed plunger displacement results in a predetermined volume of fluid being pumped.

When the coil 11 of the electromagnet 8 is subsequently deactivated the spring 29 forces the plunger 20 upwardly thus lifting the valve seat 26 at the outlet opening of the transfer passage 24 away from the second ball float 28 and permitting supply of fluid along the transfer passage 24 to the output compartment 22. As the output compartment 22 fills with fluid the second ball float 28 is lifted by floating in the fluid until it again engages the valve seat 26 thus closing off the transfer passage 24 whilst at the same time opening the fluid outlet 18. Simultaneously the valve seat 25 on the plunger 20 is lifted to engage the second ball float 27 thus closing off the inlet opening of the transfer passage 24. During this upstroke the first ball float 27 seats against the valve seat 17 thus closing off the fluid inlet 16 and preventing fluid from flowing from the input compartment 21 out of the fluid inlet 16. Thus, during the upstroke, the fluid in the input compartment 21 is forced into the output compartment 22 by way of the transfer passage 24 with the second ball float 28 being forced away from the valve seat 26 by the fluid flow during such fluid transfer.

It will be appreciated that the pump 1 is prevented from pumping air since, when the output compartment 22 is empty, the second ball float 28 will seat against the valve seat 19 thus closing off the fluid outlet 18. Because th*& floating properties of the ball floats 27 and 28 form an integral part of the pump operation, the pump must generally be orientated in a near vertical position. Furthermore, as the electromagnet coil ^• l O -

ll heats up due to power losses, its resistance will increase. If the power supply to the coil 11 is a voltage source, such as a battery, the coil current would vary with temperature and the pumping action would thus be affected by temperature variations due to the dependence of the pumping force on current. It is therefore preferred that the coil 11 is supplied from a controlled current source, as shown in Figure 4, comprising an operational amplifier 30, a transistor 30A, a current sense resistor 31, a resistor 32 and a diode 33 to dissipate current. Such an arrangement not only eliminates temperature effects but can also be employed to control the plunger motion such that fluid pumping pulsation^' is reduced. In many applications the necessity to load the pump housing 2 on replacement of the cassette through the electromagnet bore 7 may be undesirable, and in such applications it is preferred that an alternative form of electromagnet 40 is used as shown in Figure 5. In this electromagnet 40 two substantially C-shaped cores 41 and 42 extend along substantially parallel planes perpendicular to the pump axis 43, that is the direction of reciprocating movement of the plunger. The cores 41 and 42 define the poles of the electromagnet and are each positioned with their -arms embracing the housing 2. The cores 41 and 42 are electromagnetically arranged in conjunction with a further substantially C-shaped core 44 around which the coil 45 is wound. Such an arrangement enables output tubing connected to the pump housing 2 to be inserted between the arms of the cores 41 and 42 from the side in the direction of the arrow 46 and the housing 2 to be pushed down between the poles, whilst at the same time balancing the lateral forces exerted by the electromagnet 40 on the plunger 20. Such an arrangement is shown from above on a smaller scale in Figure 5(a).

The state of the pump being empty of fluid may be detected in two ways. If the volume of fluid to be supplied by the pump is known, and since the pump has a fixed stroke, the number of strokes may be counted until the calculated volume to be supplied is reached. When this is indicated by a count output signal, pumping is stopped by deactivation of the electromagnet and an empty condition is signalled.

Alternatively or additionally an optical detector may be used to indicate when the pump is empty in dependence on the position of the second ball float 28, as shown schematically in Figure 6. The optical detector comprises a light source 50 and a light receiver 51 both of which are external to the pump and form part of a main controller used to control the pump electrically. During normal operation of the pump, the second ball float 28 should float in the fluid in the output compartment 22 immediately before each downstroke of the plunger 20 thus interrupting the light beam between the source 50 and receiver 51. However, if the pump is empty, the second ball float 28 will no longer float but will be in a position 28' indicated by broken lines in Figure 6. In this position the second ball float 28 will no longer interrupt the light beam at this point in the pump cycle and detection of the light beam by the receiver.51 will cause a pump empty condition to be signalled. If the electromagnet arrangement of Figure 3 is used, the source 50 and- receiver 51 would need to be embedded in the coil 11, or alternatively an extra float chamber would have to be used requiring use of additional pump components. By suitable design of the pump it is possible to allow for use of the pump at angles greater than say 15° from the vertical. The shape of the input compartment 21 in conjunction with the size of the first ball float 27 permits operation with large angles from the vertical. However, a modification of the second ball float 28, as shown in Figure 7, may be necessary to permit use over a wide operating angle. In this modification a stem 60 is provided on the second ball float 28 which protrudes into the ^"transfer passage 24 so as to ensure correct location of the second ball float 28 over a wide angle range. As shown in the view of the second ball float 28 from above shown in Figure 7(a), the cross-section of the stem 60 is chosen such that fluid flow in the transfer passage is not significantly effected. A similar stem arrangement may also be provided on the first ball float 27 if required.

Furthermore, in the modification of Figure 7, a plastics retaining washer 61 having a serrated inner profile, as shown in Figure 7(b), is positioned in the input compartment- 21 so that the ball float 27 seats on the washer 61 rather than on a valve seat 25 at the top of the transfer passage 24 on supply of fluid through the fluid inlet 16. Because of the shape of the inner profile of the washer 61, fluid can pass beyond the ball float 27 when it is seated on the washer 61. The use of such a washer not only permits operation of the pump at an increased angle from the vertical, but also decreases the settling time of the ball float 27 in operation. Alternatively the function of the washer 61 could be performed by a check valve spring.

A microprocessor controller can be used to perform all calculations, interpret conditions and handle user inputs and outputs. The above-described pump arrangements are particularly advantageous for use in intravenous fluid infusion since they incorporate a replaceable cassette providing a sealed, sterile and non-pyrogenic fluid path, and since the pump is self-priming, non-siphoning and dry- running proof and operates by positive pressure with low fluid pulsation and no back pressure and provides precise volume delivery. Furthermore the pump is compact, requires no maintenance or alignment and can be loaded with the removable cassette quickly and without difficulty. If requiaed the volume supply per stroke can be varied by varying the plunger diameter.

In a non-illustrated variant of the invention the pump is of broadly similar construction to the pump of Figures 1 and 2 except that it is arranged the other way up and both floats are replaced by non-magnetic steel balls. In this case the fluid is sucked upwardly through the pump, and the balls are acted upon by gravity to close off the inlet and the transfer passage respectively at appropriate times during the pumping cycle.

Claims

CLAIMS 1. A fluid pump comprising a housing (2) having a pump chamber (15) provided with a fluid inlet (16) and a fluid outlet (18) at opposite ends of the chamber, a magnetisable plunger (20) reciprocable within the chamber (15) and dividing the chamber into an input compartment

(21) adjoining the fluid inlet (16) and an output compartment (22) adjoining the fluid outlet (18) , the plunger (20) having a transfer passage (24) extending therethrough provided for transfer of fluid from the input compartment (21) to the output compartment (22) during a predetermined part of- the pumping cycle, a first valve member (27) arranged to close off the fluid inlet (16) during transfer of fluid along the transfer passage (24) from the input compartment (21) to the output compartment

(22) on displacement of the plunger in one direction, and a second valve member (28) arranged to close off the transfer passage (24) during discharge of fluid from the output compartment (22) through the fluid outlet (18) and during supply of fluid to the input compartment (21) through the fluid inlet (16) on displacement of the plunger in the opposite direction, the plunger being reciprocable by suitable electromagnet means (8;40) to effect pumping by magnetic action. 2. A pump according to claim 1, wherein it includes the electromagnet means (8;40) .

3. A pump according to claim 2, wherein the electromagnet means (8;40) is adapted to displace the plunger (20) in one direction on application of an energising signal, and spring means (29) acts to return the plunger (20) in the opposite direction on removal of the energising signal. 4. A pump according to claim 2 or 3, wherein the electromagnet means (8;40) incorporates two opposite magnetisable poles (12,13) spaced apart in the direction of reciprocating movement of the plunger (20) and positioned such that the plunger (20) is asymmetrically disposed with respect to the poles (12,13) when in at least one of its limiting positions.

5. A pump according to claim 4, wherein the electromagnet means (8;40) comprises at least one coil (11;45) and at least one substantially C-shaped core (10;44) of magnetisable material having arms defining the poles (12,13) at their free ends.

6. A pump according to claim 5, wherein a further substantially C-shaped core (41,42) of magnetisable material is provided at the ends of each of said arms, the two further cores (41,42) extending along substantially parallel planes transverse to the direction of reciprocating movement of the plunger (20) and receiving'the pump housing (2) between their arms.

7. A pump according to any preceding claim, wherein the first valve member (27) is adapted to float in the fluid supplied to the input compartment (21) to close off the fluid inlet (16) , and the second valve member (28) is adapted to float in the fluid supplied to the output compartment (22) to close off the transfer passage (24) .

8. A pump according to any one of claims 1 to 6, wherein the first valve member (27) is adapted to close off the fluid inlet (16) under the action of gravity, and the second valve member (28) is adapted to close off the transfer passage (24) under the action of gravity.

9. A pump according to any preceding claim, wherein a detector (50,51) is provided to indicate when the pump is empty in dependence on the position of one of the valve members (27,28) during a predetermined part of the pumping cycle.

10. A fluid pump comprising a housing (2) having a pump chamber (15) , a magnetisable plunger (20) reciprocable within the chamber (15) to effect fluid pumping, and electromagnet means (8;40) for reciprocation of the plunger (20) by magnetic action, wherein the electromagnet means (8;40) includes a substantially C-shaped core (41,42) of magnetisable material having two arms lying on a common plane, and the housing (2) is in the form of a replaceable cassette which is insertable between the arms such that the direction of reciprocating movement of the plunger (20) is transverse to said common plane.