US 3885393 A
The invention consists of a device for use in an artificial heart which efficiently matches the force/displacement characteristics of a thermodynamic engine to the force/displacement characteristics of the blood. The matching is accomplished hydraulically and allows for interruption of the engine cycle periodically for control purposes.
Description (OCR text may contain errors)
United States Patent Hanson 1 May 27, 1975  HYDRAULIC LOAD MATCHING DEVICE 3,080,820 3/1963 Browne 417/383 3,257,8[0 6 1966 H 60 547  Inventor: John P. Hanson, McKeesport, Pa. mm  Assignee: The United States of America as FOREIGN PATENTS 0R APPLICATIONS represented he United Sates Franc: A t r r t i .4 Energy Research and Development Administration, Washington, DC. Primary Examiner-Martin P. Schwadron  Filed: Sem 7 1973 Assistant Examiner-H. Burks, Sr.
Appl. No: 395,219
Attorney. Agent, or Firm-Dean E. Carlson; Robert J. Marchick [S2] U.S. Cl .3 60/583; 4l7/383  ABSTRACT  Int. Cl. ..F15b 7/00 Th t f d f 581 Field of Search 3. 60/537, 547, 583, 593, e F 3 1a] heart which efficiently matches the forceldisplace- 60/533, 585, 417/383 ment charactensncs of a thermodynamic engine to the  Rererences Cited force/displacement characteristics of the blood. The matchlng is accomplished hydrauhcally and allows for UNITED STATES PATENTS interruption of the engine cycle periodically for con- 777,008 l2/l904 Defies et al 60/593 [r0] purposes. 2,490,118 l2/l949 Dickinson 4l7/383 3,044,267 7/1962 Hicks 60/533 3 Claims, 2 Drawing Figures v 20 12 15 17 1s 4 l 14 as F f 2 22 r V 11 '7 i 25 PATENTEDMAY 2 7 I975 SHEET N MI: 228% ust m5 8 Q0 3 to 8 N6 3 o I I o SEZE MSBG m o5 5 T BEmS mxomhm 4 5m mo .zmom um HYDRAULIC LOAD MATCHING DEVICE BACKGROUND OF THE INVENTION DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a thermodynamic engine cylin- The imention relates to a load matching device use' 5 der ll contains a piston I2 which drives a piston I3,
ful in a heart pump which may be implanted in a human body for replacement of the natural heart. The need in artificial hearts is for a simple, compact, lightweight, reliable, low energy requirement pump having the capability of closely approximating the pumping ability of the normal human heart. The pump must correspond functionally as nearly as possible with the natural heart.
The load matching device of the present invention is designed to couple the output of a thermodynamic engine to a load such as a blood pump. For best efficiency, the working fluid of a thermodynamic engine must be allowed to expand during a portion of the power stroke. As this occurs, the pressure within the cylinder drops. For this reason, the force developed by the piston varies through the power stroke. For most applications, the load does not match this force variation, and some means must be found of storing the en ergy during the first portion of the stroke and releasing it during the later low pressure portions of the stroke. For most application, where the speed is not expected to vary significantly from stroke to stroke, this is accomplished by converting the reciprocating motion of the piston to rotary motion, thus allowing the excess energy to be stored as a small increase in the angular velocity of a flywheel. However, there are applications where the engine must come to a complete stop at the end of each stroke, and the use ofa flywheel to match the engine characteristics to the load characteristics would not be convenient.
One exampie of an application where the use of a flywheel for force matching between the prime mover and the load is inconvenient is the so called heart rate artificial heart. In such a device, the blood pump is allowed to fill at a rate determined by the venous blood pressure. When the blood pump fills completely, the engine is triggered, emptying the accumulated blood into the aorta. As the pump discharge pressure is essentially constant over the power discharge stroke, a mis-match exists between the prime mover force/displacement characteristics and the force/displacement characteristics of the blood.
SUMMARY OF THE INVENTION Accordingly. it is an object of the present invention to provide an improved fluid flow apparatus which smooths a varying fluid stream.
It is a further object ofthe invention to provide an apparatus for hydraulically matching the force/displacement characteristics of the blood. without the use of valves or accumulators, and without the need for rotating masses or bearings. The device consists of a liquid filled tube joining a master and slave piston and cylinder.
The invention will be best understood with reference to the following specification taken in conjunction with the accompanying drawings, of which:
FIG. 1 is a view showing the structure of the hydraulic load matching device.
FIG. 2 shows a typical displacement/time curve for hydraulic load matching device as applied to a blood pump.
the pistons being connected by a shaft 14. A return spring 15 is located within engine cylinder II around shaft [4 in order to improve piston return timing. Piston I3 is enclosed within an master cylinder 16, which is the first stage in the load matching device of the present invention.
In operation, the force transmitted from the engine to the master cylinder I6 forces fluid through a nozzle 17 and then into a transmission tube [8. Part of the input force is transmitted hydrostatically through a diffuser 19 to slave cylinder 20, causing the piston 2]. contained within the slave cylinder 20, to move linearly. Any excess force provided at the master cylinder appears as a positive pressure difference between the master and slave cylinder. The pressure difference accelerates the liquid in the transmission tube. providing energy storage in the liquid in the form of kinetic energy. When the force on the master cylinder falls below that needed to move the load at a steady rate, the liquid decelerates. converting the kinetic energy of the moving fluid into a negative pressure difference, and allow ing the load to keep moving.
FIG. 2 presents a typical displacement/time curve for a system using the load matching device of the subject invention. The timing has been arbitrarily set at 0.2 seconds for the systole (pumping) part of the stroke and 0.3 seconds for the diastole (return) part ofthe stroke.
In operation, the load could be a blood pump 22. In this case, the inlet valve 23 of the pump is closed while the output valve 24 is open and the pressure in the slave cylinder 20 is proportional to the arterial pressure during systole. This pressure varies from approximately mm Hg to approximately I20 mm Hg during the stroke. The actual pressure would depend on the area ratio between the slave cylinder piston 21 and that of the blood pump 22. During diastole, the pump body is connected to the veins, and the pressure is essentially zero. The pump body could be fitted with a flexible dome 25 to prevent the return spring from collapsing the veins during diastole.
The actual size of the transmission tube 18 will depend on the area ratio between the slave cylinder 20 and the pump diaphragm, as well as the working fluid selected. In addition, the engine expansion ratio and the desired timing will affect the dimensions of the device. Using the timing shown in FIG. 2, with a lzl area ratio between slave cylinder and pump diaphragm, the displacement volume should be approximately cc to provide 12 liters/minute of blood flow. Using water as the transmission fluid, the length of the tube should be about 3 feet with a diameter of about inch in order to obtain the performance shown in FIG. 2. This is perhaps longer than needed to connect a typical engine and pump, but the excess length could be coiled around the engine housing with little loss in performance. The dimensions of the master and slave cylinders can be varied to suit the installation as long as the displaced volume is kept constant.
What is claimed is:
l. A device for matching load characteristics to engine characteristics comprising:
an input shaft from an associated engine cylinder;
first piston at the end of said shaft;
master cylinder containing said shaft and said piston:
nozzle at one end of said master cylinder; cylindrical transmission tube at the converging end of said nozzle;
conical diffuser at the opposite end of said transmission tube;
slave cylinder connected to said diffuser at the divergent end of said diffuser;
second piston contained within said slave cylinder;