Patent Application
20050191194
This invention relates to the field of low power electromagnetic pumps that, for example, can be used in implantable medical device applications, and more particularly to a new improved low power electromagnetic pump having an internal compliant element.
An example of a low power electromagnetic pump provided with an accumulator is found in U.S. Pat. No. 5,797,733 issued Aug. 25, 1988, the disclosure of which is hereby incorporated by reference. This patent shows an outlet tube extending from the pump, with an accumulator attached to the outlet tube and a catheter extending from the accumulator. It has been found to be desirable to incorporate an accumulator in the flow path of a low power electromagnetic pump for several reasons. Such placement of the accumulator allows for both the rapid actuation of the pump and the relative slow delivery of the pumped fluid.
The need for an accumulator in a pump flow system arise out of two somewhat different causes, namely, viscous pressure drops which may severely limit flow, and inertial effects which may cause the flow to continue after the pump stroke is complete. The design of a low power electromagnetic pump of the type shown in U.S. Pat. No. 5,797,733 is such that the pump operates more and more efficiently as the rate of the plunger strokes increases. The magnetic force required to move the plunger needs to be maintained for a short time and the electrical energy required to energize the electromagnetic coil can be minimized. However, it is usually not possible to move fluid rapidly through the entire flow path, that is, from the fluid reservoir to the outlet of the flow system. In implantable drug delivery systems it is generally required that the drug be delivered through a small diameter catheter. High flow rates through a small diameter catheter lead to high viscous pressure drops and impose significant performance limitations on the pumping device. This problem is typically alleviated by installing an accumulator at some point between the pump outlet and the catheter to accept the rapid pump outflow and deliver it slowly to the catheter.
If the catheter or the associated tubing are of larger diameter so that the viscous pressure or orifice drops no longer predominate, then the inertial effects may come into play. The inertia of the flow through the rigid inlet and tubes can be an important factor tending to degrade the accuracy of low power electromagnetic pumps. One way this flow inertia is controlled is by the combination of suitable located orifices and properly chosen catheter and accumulator designs.
The inertial flow problem is aggravated in a pump of the type shown in U.S. Pat. No. 5,797,733, due to the long length of the rigid outlet tube located between the pump and the accumulator. The length of this outlet tube was determined not by the performance requirements of the flow system, but rather by the need to bend the tubing as it was being installed in a particular device. Reducing the diameter of the particular tubing would have allowed the bending requirement to be met with a shorter length of tubing, but this would have been at the expense of increased inertial and perhaps viscous effects degrading the pump accuracy.
The benefits of reduction of inertial flow and rapid pull-in can be retained even with a long outlet tube of small inner diameter (for flexibility) if the accumulator can be placed between the pump and the outlet tube. This normally requires that the accumulator be hermetically isolated from the environment. Most accumulator designs used in hydraulic systems meet that requirement or can be modified to do so. However, simple accumulators used for testing the pump, for example a short length of silicone rubber tubing between the rigid outlet tube and catheter, do not satisfy the hermetic requirement.
Published International Patent Application PCT/US99/13902 discloses a solenoid pump including a titanium aneroid accumulator element installed within a sideport assembly. The sideport assembly is located on an external surface of the pump housing between an exit port of the pump mechanism an a catheter. With the compliant element being installed between the pump and the outlet tube, the flow through the outlet tube can be decoupled from the flow through the pump, thus reducing or eliminating the inertial flow. The outlet tube-may therefore be longer and of smaller diameter as required, thus providing the flexibility desired by the customer without degrading the pump accuracy.
One aspect of this invention involves the benefits that are derived from an alternative approach which comprises moving the accumulator from the end of the outlet tube to the interior of the pump body. These benefits comprise: a more compact pump assembly; facilitated installation of the pump in implantable medical devices; decreased pump housing size; and a decrease in the number of external parts and components. Additionally, there is the added benefit that the accumulator is protected from the outside environment by the pump body.
Another aspect of the invention involves a low porosity filter at the inlet side of the pump. In particular, on the inlet side of the pump the connecting tubing is usually short enough and of large enough diameter so that the viscous pressure drop through the tubing is not a problem, and a conventional accumulator is not required. However, in some applications it may be desirable to install a low porosity filter on the inlet side, but this filter may be incompatible with high flow rates. In such cases the filter itself, or the structure which supports the filter in the pump housing, are designed so that they flex during the pump stroke so that flow may be delivered rapidly to the pump inlet without passing through the filter. Flow may then pass through the filter more slowly driven by the spring constant of the deformed filter or its supporting structure during the interval between pumping strokes. This is a special type of accumulator in which the total internal volume of the flow system is not changed as the accumulator is emptied and refilled, but the volume change downstream exactly compensates for the volume change upstream.
Thus, the invention encompasses a low power electromagnetic pump having an internal compliant element. The pump has a pump body or housing defining an interior fluid containing region comprising a inlet port and an outlet port that are in fluid communication with one another. Check valve means are operatively associated with the fluid containing region for allowing fluid flow in a direction from the inlet port through the outlet port and blocking fluid flow in a direction from the outlet port through the inlet port. An accumulator is located inside the pump body in the interior fluid containing region and is fluid communication with the inlet port and outlet port. The electromagnet means are carried by the pump body and located external to the interior fluid containing region defined in the pump body. An armature is positioned in the interior fluid containing region of the housing and comprises a pole portion for attraction to the electromagnet means. The armature is movably supported in the housing for movement from a rest position through a forward pumping stroke when the pole portion is attracted by the electromagnet to force fluid out the outlet port and for movement in an opposite direction through a return stroke back to the rest position. There are means defining a magnetic circuit including the electromagnet means and the armature and a gap between the pole portion of the armature and the electromagnet means for moving the armature toward the electromagnet means to close the gap in response to electrical energization of the electromagnet means. The accumulator may comprise a bellows-shape or a diaphragm shape. The inlet filter may also serve as an accumulator.
As shown in
As shown in
The pump body 32 defines a plurality of chambers in the pump 10. These chambers comprise an armature shaft chamber 124, a main spring retainer chamber 126, a bypass chamber 136, and the accumulator recess 320 in fluid communication with one another. The inlet port 18 is in fluid communication with and leads to the armature shaft chamber 124 which is sized to accommodate the pump armature 45 therein. The armature shaft chamber 124 leads to and is in fluid communication with the main spring retainer chamber 126 the width or cross-section dimension of which is greater than the width or cross-section dimension of the armature shaft chamber 124. The main spring retainer chamber 126 is in fluid communication with and leads to output chamber 16, the width or cross-section dimension of the output chamber 16 being greater than the width or cross-section dimension of the main spring retainer chamber 126. The output chamber 16 leads to outlet tube 130 defined in the pump body 32, and the outlet tube 130 is in fluid communication with the compliant element recess 320 defined in the pump body 32. The compliant element recess 320 is sized to hold the internal compliant element 300 therein, thus allowing the internal compliant element 300 to be in fluid communication with the fluid output chamber 16, outlet tube 130, and outlet port 20. The outlet tube 130 is short. As an illustrative example the length of the outlet tube 130, designated L in
A passage or orifice 44 is defined in the housing 32 and leads from the armature shaft chamber 124 to a plug chamber 134. The orifice 44, which may be of small diameter, provides for fluid communication between the armature shaft chamber 124 and the plug chamber 134. The plug chamber 134 leads to and is in fluid communication with a bypass chamber 136. The bypass chamber 136 is in fluid communication with the output chamber 16. These chambers thus provide for a bypass passage in the pump 10
The pump armature 45 comprises a pole portion 48 joined to a plunger portion 59 by inner weld ring 75. The armature 45 plunger portion 59 comprises a first shaft portion 60, a second shaft portion 62 of slightly greater diameter than the first shaft portion 60, a third shaft portion 64 of greater diameter than the second shaft portion 62, and a head portion 66 comprising a diameter many time greater than the diameter of the third shaft portion 64. The plunger portion 59, which might be titanium and/or its alloys and/or biocompatible materials, may machined and/or formed from a piece of plunger stock. The inner weld ring 75 thus joins the head portion 66 of the armature 45 with the pole portion 48 and a shell 108. The shell 108 holds a magnetic body 109, and a vacuum hole 70 is provided in armature head portion 66. During assembly of the pump armature 45 a vacuum is created through the vacuum hole 70 and in the shell 108 to draw the magnetic body 109 against the head portion 66 whereupon the hole is sealed by a plug 71. The body 109 is thus held tightly inside the shell 108 as the armature 45 cycles. The pole portion 48 is thus encased to protect the body 109 against potentially corrosive effects of the fluid being pumped.
The main check valve means 24 shown in
An electromagnet means 100 is isolated from the fluid being pumped by a plate or diaphragm 110. The plate 110 serves as a barrier to prevent the fluids being pumped from contacting the electromagnet 100 and its parts and components. The electromagnet means 100 is activated cyclically to generate an electromagnetic field to pull the armature 45 towards which draws fluid into the pump 10. When the electromagnet means 100 is deactivated, the armature 45 is returned to its at rest state (
A retainer element 52 having an annular body 54 and a lip portion 55 is provided for the main spring 90 to act against. During assembly, the first and second shaft portions 60, 62 of the armature 45 are fitted through the bore of the retainer element 52, until the retainer element 52 contacts a shoulder 68 formed on the armature 45. The retainer element 52 is joined to the second shaft portion 62 by welding/laser welding and/or friction fitting.
A retainer plate 80 is provided, having a bypass fluid chamber opening 82, an outlet opening 84, and a central opening 86. The central opening 86 is sized to receive the third plunger shaft portion 64 therein, as shown in
An outer weld ring 94 comprises an annular support protrusion or lip 95. The retainer plate 80 is positioned between the pump body 32 and the support protrusion 95, and becomes trapped therebetween upon welding the outer weld ring 94. This prevents the movement of the retainer plate 80 as the pump 10 cycles.
The electromagnet means 100 is carried by the pump body 32 and is external to the fluid containing region of the pump body 32. The electromagnet 100 may comprise a core wrapped in a coil and is capable of rapidly energizing and de-energizing to create a magnetic field. This magnetic field then attracts the pole portion 48 of the armature 45. When the pole portion 48 is attracted, the armature 45 compresses the main spring 90 as it moves towards the electromagnet 100. At substantially the same time fluid is drawn into the pump 10. When the electromagnet 100 de-energizes the main spring 90 expands and applies force on the retainer element 52 which moves the armature 45 back to its at rest position in the pump 10 (
A means for bypass check valving 74 (bypass check valve means) 74 is positioned internal to the pump body 32, between the orifice 44 and a bypass chamber 136. Spring 76 is located between check valve element 78 and a plug 42 mounted to the housing 32 in a plug chamber 134. The bypass check valve means 74 controls fluid communication between the orifice 44 and bypass fluid chamber 136. During the return stroke when the armature 45 returns to its rest position, the bypass check valve means 74 opens. Fluid from the armature shaft chamber 124 flows through the orifice 44 and forces element 78 to open the bypass check valve means 74. The fluid then flows into the bypass chamber 136.
Assembly and Movement of the Armature
During assembly of the armature, the first shaft portion 60, second shaft portion 62, and third shaft portion 64, are moved through the central opening 86 in the spring retainer plate 80 and through the main spring 90. Then, the first and second shaft portions 60,62, respectively, are moved through the retainer element 52 until the retainer element 52 contacts shoulder 68. The retainer element 52 is joined, welded/laser welded, or pressure fitted and welded/laser welded to the second shaft portion 62. The armature 45 is then inserted into the armature shaft chamber 124.
The outer weld ring 94 is moved into the pump body 32 around the retainer plate 80, until the outer weld ring 94 support protrusion 95 and retainer plate 80 contact. The outer weld ring 94 is welded/laser welded to the pump body 32, trapping the retainer plate 80 between the pump body 32 and the support protrusion 95.
For a further and/or more detailed description of the structure of pump 10 and the assembly of the parts thereof, reference may be made to pending U.S. patent application Ser. No. 10/291,130 filed Nov. 8, 2002 and entitled “Low Power Electromagnetic Pump,” now U.S. patent application Publication No. 20030086799 published May 8, 2003, the disclosure of which is hereby incorporated by reference.
Reference is made to the diagrammatic views shown in
The above-described pumping cycle can be repeated at predetermined intervals. It is noted that the closed check valve 24 during the return stroke 149 prevents fluid from exiting the inlet port 18. Also, the following structure for the pump is for illustrative purposes, and the installation of the internal compliant 300 will work with pumps embodied with different internal structure.
Calculated Results
Compliance may be related to how a fluid path, as defined by the structural body forming the path or part of the path, expands, contracts or deflects under an environmental input, such as, for example, a pressure load from a pump mechanism that is intended to deliver an amount of fluid to an output component, for example a catheter.
One of the purposes of the internal compliant element 300 is to allow for rapid pumping of the pump 10, and the subsequent slow delivery of the fluid pumped to the outlet port 20, and then to, for example, a catheter. Another of the purposes of the internal compliant element 300 is to reduce inertial effects of the fluid being pumped, as such inertial effects can interfere with the smooth operation of the pump 10 and may cause delivery problems.
The following results compare the calculated fluid volume delivered by a pump 10 having an internal compliant element with a pump where the compliant element is located external to the pump body at the end of an outlet tube. A primary basis for the comparison is the percentage reduction in the pulse volume when the back pressure is increased from 0 (zero) pounds per square inch (hereinafter psi) to 10 (ten) psi. A typical performance specification for a pump of the type shown in the above-referenced U.S. Pat. No. 5,797,733 allows for about a 10% decrease in pulse volume when the outlet pressure is increased by 10 psi.
The two basic configurations compared are:
An orifice located downstream of the compliant element would not limit the speed of the pump armature 45 or the volume of internal flow. It is further noted that both configurations are also assumed to incorporate a bypass orifice 44 to help control the inertial flow at the end of the armature 45 stroke.
The 3.46 inch effective length outlet tube used in this calculation is intended to represent an actual outlet tube 2.57 inches long terminated by an outlet fitting. Since the outlet fitting is assumed to be of smaller inner diameter than the outlet tube, the inertial effective length of the outlet tube is increased by the fitting by an amount greater than the physical length of the outlet fitting.
The results are believed accurate enough to provide a useful estimate of the effects.
Table 1 shows the calculated results for a representative configuration of a low power electromagnetic pump with an external accumulator similar to that shown in U.S. Pat. No. 5,9979,733. Although values for leakage through the armature pump body clearance and for the inertial flow were calculated they are omitted from the tabulated results.
The results shown incorporate the calculated magnetic force on the armature, the drag of the pole button as it moves through the pump body and approaches the diaphragm face, the inertia of the plunger, the inertia of the flow upstream of the pump and downstream of the pump outlet, pressure drops in the main flow, bypass circuit and outlet tube caused by check valves, viscosity and orifice restrictions, leakage of fluid through the between the pump body and armature, the increase in pressure in the accumulator during the pumping pulse.
For the external accumulator, the outlet tube has a length of 2.6 inches. The effective outlet tube length with outlet fitting is 3.46 inches. Table 1 shows the calculated results for an external accumulator with the effective length of outlet tube being 3.46 inches. It is noted that in the table the phrase “pounds per square inch” has been abbreviated to PSI in the tables.
As shown in Table 1, the results for the 0.03 μL/psi accumulator and the 0.005 inch outlet orifice correspond to a pump of the type shown in U.S. Pat. No. 5,797,733 if it is driven by the lower excitation normally used to drive a pump shown in U.S. Pat. No. 6,264,439. The results indicate that under these conditions the armature 45 is not drawn in fully against 10 psi before the capacitor is discharged and the pulse volume against 10 psi is 10% lower than the pulse volume with no pressure increase across the pump 10, a value just meeting the specified performance of the pump. The pulse volume ratio (pulse volume against 10.0 psi divided by the pulse volume against 0 psi) can be improved to 0.956 by increasing the accumulator compliance to 0.1 μL/psi or to 0.937 by increasing the diameter of the outlet orifice to 0.009 inches. If both changes are made the pulse volume ratio is degraded to 0.91 by increased inertial flow.
The third value of compliance listed represents the compliance which would exist if a 50 μL bubble occupied the volume of the pump body 32. It is assumed that there is no air in the pump chamber (between the check valve means 24 and armature 45), because even a small bubble could cause a reduction in pump volume and a decrease in pump accuracy. The use of the 0.03 orifice in this calculation reflects the fact that there is no satisfactory location for an orifice between the armature 45 and the accumulator (bubble) and therefore the effective orifice is large.
Table 1 also shows that pulse volume is reduced from that calculated for the 0.1 μL/psi example. This occurs because the inertia of the fluid in the long outlet tube no longer affects the flow during the pumping pulse. The calculated PV ratio at 10 psi of 0.952 is well above the specified lower limit of 0.9. However, the PV ratio is less meaningful than the ratio of the fluid delivered with the bubble present to the delivery with the bubble in normal operation with no bubble. Against 10 psi with a 0.005 orifice and a 0.1 μL/psi accumulator the ratio is 0.5182/0.4828=1.0735. Thus the pump 10 would deliver greater volume with the bubble within the pump body 32 than it would with no bubble present.
Table 2 shows the results of calculations in which the accumulator 300 is assumed to be internal to the pump body 32. This placement of the accumulator 300 shortens the effective length of the outlet tube and reduces the inertial effect. If necessary an orifice can also be accommodated within the pump body 32 upstream of the accumulator. Results are therefore shown for all three orifice sizes. Since the outlet orifice serves to control the inertia effects of both the inlet and outlet tubes, there may be a benefit from including an orifice even though there is effectively no outlet tube. As in Table 1, a 0.005 inch outlet orifice would slow the armature pull-in to a point where the pull-in would be incomplete at 10 psi with a less compliant accumulator and the calculated PV ratio is an unacceptable 0.894. Increasing the orifice size to 0.03 inch improves the PV ratio at 10 psi to 0.94 with the 0.03 μL/psi accumulator. Note that all three of the orifice sizes and both the accumulator compliances meet the PV ratio accuracy criterion, except for the combination of the smallest (0.005 inch) orifice and the least compliant accumulator (0.03 μL/psi).
Table 2 shows the results of locating a bellows-type accumulator 304 internal to the pump body 32, between the outlet tube 130 and pump outlet port 20, as shown in
There still remains some inertial effect even with an internal accumulator installed in the pump 10. The source of this inertial effect is the inlet tube, which has not been varied in these calculations. Thus, in the example of the 0.1 μL/psi compliant element, which does not control inertial flow as well as the stiffer compliance, a 0.009 inch orifice, as compared with the 0.03 inch orifice, improves the PV ratio from 0.953 to 0.971. However, with the 0.03 μL/psi compliance the pump 10 is more accurate with the larger 0.03 inch orifice.
The effect of a bubble in the body of the pump 10 on the volume delivered against 10 psi is less when the accumulator is internal. As shown in Table 2, the volume delivered with the bubble present is 0.5183 μL. If the accumulator compliance without the bubble is 0.1 μL/psi, then the normal delivered volume is 0.5081 μL so that the effect of the bubble in the pump body is to increase the delivered volume by the ratio 0.5183/0.5081=1.02, that is, by about two percent (2%).
An advantage of placing the accumulator (compliant element) within the pump body 32 is that it reduces or eliminates the effect of the inertial flow and orifice 132 in the outlet tube 130 on the delivered pulse volume, thereby improving the accuracy of the pump 10. If a bubble (not shown) should be trapped within the pump body 32 it also acts as an internal compliant element and changes (while the bubble is present) the delivered volume. Placing the compliant element 300 within the pump body 32 has the effect of reducing the magnitude of the change due to the bubble and assists in maintaining the accuracy of the low power electromagnetic pump 10. Other advantages of placing the compliant internal to the pump body include a more compact pump.
In another embodiment, shown in
Applications arise in which it is desirable to install a low porosity filter on the inlet side of the pump 10, but such filters are incompatible with high flow rates. In such cases the filter itself or the structure which or means for support 305 that supports the filter in the pump housing are designed so that they flex during the pump stroke so that flow may be delivered rapidly to the pump inlet without passing through the filter. O-rings can be used as the means for support along with other suitable structures. Flow may then pass through the filter more slowly driven by the spring constant of the deformed filter or its supporting structure during the interval between pumping strokes. This is a special type of accumulator in which the total internal volume of the flow system is not changed as the accumulator is emptied and refilled, but the volume change downstream exactly compensates for the volume change upstream.
It is to be understood that the diameters of the orifice 132 presented in the Table 2 (0.005, 0.009, and 0.03 inches) are not the only sized orifices available for use in the present invention. The orifice diameter of the outlet tube 130 may be in the range of 0.004 inches to 0.04 inches, and the present invention encompasses all outlet orifices sized in this range. Also, the length of the outlet tube 130 may be about 0.1 inches.
In another embodiment shown in
Another embodiment of the low power electromagnetic pump with internal compliant element 10 is shown in
Thus, it has been shown that the internal compliant element 300 may be variously embodied, all of these within the scope of the present low power electromagnetic pump having an internal compliant element. Also, the performance benefits obtainable by installing an accumulator within the pump body 32 of a low power electromagnetic pump 10 rather than at the end of an external outlet tubing have been calculated, and suitable configurations of the accumulator are shown and described. In addition, an accumulator which does not cause a change in the volume of the flow path has been shown and described. This accumulator is particularly suitable for use in combination with a low porosity filter, since it permits slow flow through the filter and rapid flow through the electromagnetic pump.
It will be appreciated by those skilled in the art that while the low power electromagnetic pump having an internal compliant element has been described in connection with particular embodiments and examples, the low power electromagnetic pump having an internal compliant element is not necessarily so limited and that other examples, uses, modifications, and departures from the embodiments, examples, and uses may be made without departing from the low power electromagnetic pump having an internal compliant element. All these embodiments are intended to be within the scope and spirit of the appended claims.
