US3433983A - Electromagnetic actuator - Google Patents

Description

3 1 l 5 a QRQJPQ maawaxwmm; wm mum! March 18, 1969 KEISTMAN ET AL 3,433,983

7 swc'momeumxc ACTUATOR Filed Nov. 14, 1966 Sheet of 2 l i I I i i flare/7101195 we, M

United States Patent 2 Claims ABSTRACT OF THE DISCLOSURE A linear electromagnetic actuator has a plurality of annular coils separated by magnetic gaps, the end pieces on the armature having an axial length which is at least equal to the sum of the actual length of two laminations and two gaps therebetween (add laminations), to provide a linear current versus force characteristic in either of two directions of stroke.

This invention relates generally to electromagnetic ac= tuators and more particularly to a reciprocating actuator for driving a load member, such as a pump or the like, in forward and reverse directions.

With the advent of sophisticated pulsatile heart pump systems such as that disclosed in the copending application of Chesnut et al., Ser. No. 406,722, filed Oct. 27, 1964, and assigned to the assignee of the present invention, the need has arisen for an electromagnetic actuator which will drive a reciprocating piston heart pump in forward and reverse directions; particularly one that can accurately control both the speed and stroke length of the pump in both directions of motion. While in its broader aspects the present actuator is not limited to use with a reciprocating piston heart pump and in fact finds applicability in many closed loop servo systems, the requirernents for controlled stroking in such pumps provides an excellent example for the application of the present actuator.

As described in the above copending application, the heart pump includes a reciprocating piston defining a blood pumping chamber in a surrounding cylinder. Cathe ters connected to the blood chamber are connected to the patients circulatory system to provide either arterio-arterial circulation assist. Briefly, the speed of movement of the heart pump piston is varied during perfusion to achieve coincidence between a cyclical signal (e.g. EKG signal) representing one of the patients physiological parameters and each pumping cycle, i.e. a forward and reverse stroke of the pumping piston. Further, the volume of blood pumped per pumping cycle is controlled by varying the length of stroke of the pumping piston.

Also disclosed in the above copending application is a computerized control system for commanding both the rate of stroking and the length of stroke of the pumping piston in accordance with certain circulatory concepts. It will be apparent that in such a system it is extremely important both from the standpoint of preventing hemolysis and providing adequate perfusion that the movement of the pumping piston be accurately controlled and driven in accordance with the command data from the associated computer.

It is, therefore, a primary object of the present invention to provide a new and improved reciprocating electromagnetic actuator for driving a reciprocating load.

It is another object of the present invention to provide a new and improved reciprocating actuator of the type described which produces a force on a reciprocating armature that is a linear function of armature current.

It is a further object of the present invention to provide a new and improved electromagnetic actuator of the type described generally above having an output force approximately constant over the entire stroke of the armature at constant armature current.

It is still another object of the present invention to provide a new and improved electromagnetic actuator of the type described above with a compensating coil for increasing the linearity of the armature force throughout its length of stroke.

A further object of the present invention is to provide a new and improved electromagnetic actuator of the type described above particularly adapted for use with a reciprocating heart pump.

Other objects and advantages of the invention will be apparent from the following description taken in connection with the accompanying drawings in which:

FIG. 1 is an elevation view, partly in cross-section, showing the present electromagnetic actuator driving a reciprocating heart pump;

FIGS. 2 to 4 are diagrammatic views of the electro magnetic actuator according to the present invention with the armature being shown in various positions along its length of stroke; and

FIG. 5 is a graph showing the relationship between armature current and armature force over the length of stroke of the armature.

While this invention is susceptible of embodiment in many difierent for-ms, there is shown in the drawings and will herein be described in detail an embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the k invention to the embodiment illustrated. The scope of the .of the same general type disclosed in the copending application of Chesnut, Ser. No} 388,526, filed Aug. 10, 1964, assigned to the assignee of the present invention. Referen'ce should be made to this application for a more complete description of the construction of the pump 10,but it will be briefly described herein for purposes of illustrating the desired output characteristics of the present electromagnetic actuator.

The pump 10 includes a generally cylindrical housing 11 having a cylinder 12 tlierein with a double ended piston 14 reciprocable in the cylinder. The piston 14 defines a blood pumping chamber 16 in the forward end of the cylinder 12 and an actuation chamber 36 in the other end. As shown, a catheter assembly 20 is connected to the blood pumping chamber 16 for arterio-arterial pumping (a circulation assist technique fully described in the above mentioned application of Chesnut et al., Ser. No. 406,722). Catheters 22 and 23 are adapted to be inserted into the patients femoral arteries and pushed up into the descending aorta. As the piston 14 reciprocates back and forth within the cylinder 12, blood or other fluid is forced through the catheter assembly 20 into the patients aorta causing an increase in the intra-aortic pressure in timed relation with the patients EKG waveform resulting in assisted peripheral perfusion. As the piston 14 withdraws from the forward end of the cylinder 12, blood is withdrawn from the catheter assembly into the chamber 16 reducing the arterial resistance against which the heart must pump thereby reducing the work load on the heart.

As described in the aforementioned copending application of Chesnut et al., Ser. No. 406,722, a computer is provided responsive to the patients heart cycle period, or more simply the heart rate, to compute both the speed of reciprocation of the piston 14 and the length of stroke thereof to maintain a maximum perfusion without producing hemolysis. Although in this copending application a hydraulic actuator is disclosed for driving the reciprocating piston of the heart pump, in the present system an electromagnetic actuator 25 is provided for driving the piston 14 through a fluid coupling 27 which permits the pump to be placed remotely from the actuator 25.

The fluid coupling 27 includes a cylindrical wall 29 fixed to the electromagnetic actuator and having formed therein a cylinder 30 which slidably receives a piston 32. The piston 32 defines in the cylinder 30 a fiuid chamber 35 communicating with chamber 36 in the pump 10 through a flexible hose 38. The coupling 27 is a rigid fluid coupling so that the piston 14 accurately follows movement of the piston 32. A feedback potentiometer 40 is connected to the piston 32 and provides a signal indicating the actual position of the piston 14 in any instance during its stroke to a closed loop servo system associated computing circuitry.

The electromagnetic actuator 25 includes a cylindrical non-magnetic housing member 42 having non-magnetic end plates 43 and 44 fixed thereto.

Slidable within the housing 42 is an armature 47, having enlarged cylindrical end pieces 49 and 50 with a reduced interconnecting portion 51. The armature 47 is rigidly connected to a shaft 53 formed with piston 32 so that the armature reciprocates the piston. There are no windings on the armature 47 so that it has a low inertia rapid response.

An annular permanent magnet 55 is mounted centrally in housing member 42 and provides the field excitation for the armature 47. Alternatively, an electromagnet may be used either in place of magnet 55 or in addition there to. However, the permanent magnet 55 has two advantages over electric field excitation in that (1) it requires no power input and (2) it tends to maintain constant field intensity in the magnetic circuit by virtue of its low reversible permeability. A plurality of annular stator coils 58 are provided on both sides of the permanent magnet 55. The coils 58 are arranged in two sets 65A and 65B on opposite sides of the magnet 55, and are connected so that when current flows in one direction in set 65A it flows in the other direction in set 65B. The armature coil current is supplied by suitable circuit which reverses the direction of current flow in each set of coils to reverse the armture 47, and translates the computer command data into a predetermined current level.

Separating the coils 58 and providing flux paths with the end pieces 49 and 50 are annular laminations 60 having enlarged inner portions 61 which reduce the gaps 62 between the laminations and assist in increasing the linearity of the actuator.

The linearity of the actuator 25, i.e. force versus displacement characteristic, is due mainly to the relationship between the axial length of the end pieces 49 and 50 with respect to the enlarged portions 61 of the laminations, the gaps 62 and the length of the coil sets 65A and 65B. A constant driving current in the coils 58 causes a uniform force throughout the displacement of armature 47 due to the constant number of flux linkages with the armature. That is, the reluctance of the magnetic circuit is substantially constant over the length of stroke of the armature 47 as the end pieces 49 and 50 have an axial length with respect to the end portions 61 of the laminations 60 such that a substantially constant lamina tion area is adjacent the end pieces in all Positions of the armature 47. In the construction shown in FIG. 1 each of the end pieces have an axial length substantially equal to the sum of the axial lengths of two lamination portions 61 and two gaps 62. Further, the length of each coil set 65A and 65B (including the laminations) is substantially greater than the axial length of each of the end pieces 49 and 50 to reduce the non-linearities caused by fringing effects. In the construction shown, the length 65 is over twice the axial length of coil sets of each of the end pieces. In addition, the end pieces 49 and 50 are wide relative to the gaps 62 between the laminations to reduce the effects of these gaps on linearity.

An annular compensating coil may be provided for overcoming the armature reaction by producing a magnetomotive force in a direction opposing the demagnetizing magnetomotive force of the armature reaction. The current supplied to the coil 70 should vary with stroke or displacement to compensate for armature reaction which varies with stroke.

In describing the operation of the electromagnetic ac tuator reference will be made to FIGS. 2 to 4 wherein an electromagnetic actuator 25 is shown somewhat modified from that illustrated in FIG. 1 in that no compensating coil 70 is provided and the field excitation is effected both by permanent magnet 55 and an electromagnet 75. The field magnet is supplied with current which flows continuously in the same direction regardless of the direction of stroking of armature 47, and as represented, the current flows into the black dots in FIGS. 2 and 4. The field magnets 55 and 75 cooperate to provide the magnetic field of flux density B. With no current flowing in the stator coils 58 as shown in FIG. 2, no motion of the armature 47 will be produced.

With armature current flowing into the black dots representing the coils 58 as shown in FIG. 3 the armature 47 will move to the left. Movement of the armature may be explained conceptually in somewhat oversimplified form as follows. The constant field excitation by permanent magnet 55 and electromagnet 75 cause end piece 49 to be a south pole and end piece 50 to be a north pole. The polarity of end pieces 49 and 50 do not change with direction of movement. With current flowing into the coils 58 in the direction shown in FIG. 3 into the black dots, the coils may be considered simply as current carry= ing conductors in a magnetic field produced by the field excitation. Thus, a force is produced on the armature conductors. In FIG. 3 this force would be to the right. Resulting from this force on the conductors of coils is a reaction force to the left on the field source i.e. the armature 47. Since the armature is free to move while the armature conductors are held stationary, movement of the armature results.

When current through the coil sets 65A and 65B is reversed as shown in FIG. 4, the force on the coils 58 will be to the left resulting in a reaction force on the armature 47 to the right.

As shown in FIG. 5 there is a linear relationship between the current I flowing in the coils 58 and the force applied to the armature 47 so that if the current is held constant the force on the armature remains constant throughout the length of stroke.

We claim:

1. An electromagnetic actuator, comprising:

a generally annular excitation member for providing a substantially constant magnetic field, a plurality of stator coils on each side of said annular field mem ber, laminations separating said coils and spaced from each other defining gaps, an armature reciprocable within said coils and annular field member, said armature having spaced generally cylindrical end pieces providing flux paths with said laminations, said annular member providing an exciting field through said laminations for said armature, each of said end pieces having an axial length which is at least equal to the sum of the axial lengths of two laminations and two gaps, whereby a constant current in said coils will cause a substantially constant force application to said armature.

2. An electromagnetic actuator, comprising:

two adjacent sets of annularly arranged coils con nected to carry a substantially constant current in opposite directions, generally annular laminations separating each of said coils and defining gaps between one another, an armature reciprocable within said coils and having spaced cylindrical end pieces arranged so both are completely within the axial length of coils and laminations, said armature being constantly magnetically excited, the distance between 3,099,260 7/ 1963 Birtwell 128-1 the outer ends of said end pieces being less than the 3,119,940 1/1964 Pettit et a1. 310-24 axial length of the coils and laminations, each of said 3,039,399 6/ 1962 Everett 103150 end pieces having an axial length which is at least 2,944,160 7/ 1960 Dickinson 3l027 XR equal to the 'sum of the axial lengths of two lamina- 3,022,450 1962 Chase.

tions and two gaps, whereby a constantlcurrent in 5 3,259,769 7/1966 Stott 310-153 XR said coils in one direction will produce -.a substan- 3,233,607 2/1966 Bolie 318127 XR tially constant force application to said armature in one direction and a reversal of said current will re- FOREIGN PATENTS verse the arnjlat-ure causing movement thereof in the 1,181,923 6/1959 Franceother direction. 10

MILTON O. I-i-IRSHFIELD, Primary Examiner. References Cited I B. A. R-EYNOLDS, Assistant Examiner. UNITED STATES PATENTS 3,024.374 3/1962 Stauder 31o 1s CL 3,135,880 6/1964 Olson et a1, 310-14 310 12, 17, 1'9, 128-1; 31s

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