Abstract:
A primary scotch yoke and a secondary scotch yoke are coupled in opposing arrangement and rigidly attached to a linearly reciprocating element. Each scotch yoke includes a rotary actuator with a cam follower and a yoke attached to the reciprocating element. Each yoke has a cam with an open gap through which the follower may be disengaged from the cam and stopped to an idle position. The follower of the secondary scotch yoke is kept idle in a position similarly disengaged from its cam. A controller detects a failure in the primary scotch yoke, disengages the cam follower of the primary yoke from its cam, and energizes the secondary scotch yoke, thereby causing its cam follower to engage its yoke and continue to provide uninterrupted motion to the reciprocating element, in particular the portable pneumatic pump driving an artificial heart.

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part application of Ser. No. 12/454,440, filed May 18, 2009, entitled “Actuating Mechanism for Pneumatically-Driven Artificial Heart.” 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a rotary-to-linear motion conversion device suitable for use where redundant rotary actuation is required. In particular, the invention relates to a device for actuating a pneumatic pump driving an artificial heart. 
     2. Description of the Prior Art 
     The heart is the muscle that drives the cardiovascular system in living beings. Acting as a pump, the heart moves blood throughout the body to provide oxygen, nutrients, hormones, and to remove waste products. The blood follows two separate pathways in the human body, the so-called pulmonary and systemic circulatory circuits. In the pulmonary circuit, the heart pumps blood first to the lungs to release carbon dioxide and bind oxygen, and then back to the heart. Thus, oxygenated blood is constantly being supplied to the heart. In the systemic circuit, the longer of the two, the heart pumps oxygenated blood through the rest of the body to supply oxygen and remove carbon dioxide, the byproduct of metabolic functions carried out throughout the body. The heart supplies blood to the two circuits with pulses generated by the orderly muscular contraction of its walls. 
     In order to keep blood moving through these two separate circulatory circuits, the human heart has four distinct chambers that work in pairs. As illustrated in  FIG. 1 , the heart  10  includes a right atrium  12 , a right ventricle  14 , a left atrium  16 , and a left ventricle  18 . One pair of chambers, the right ventricle and left atrium, is connected directly to the pulmonary circuit. In it, de-oxygenated blood from the body is pumped from the right ventricle  14  to the lungs, where it is oxygenated, and then back to the left atrium  16 . 
     In the systemic circuit, the other pair of chambers pumps the oxygenated blood through body organs, tissues and bones. The blood moves from the left atrium  16 , where it flows from the lungs, to the left ventricle  18 , which in turn pumps the blood throughout the body and all the way back to the right atrium  12 . The blood then moves to the right ventricle  14  where the cycle is repeated. In each circuit, the blood enters the heart through an atrium and leaves the heart through a ventricle. 
     Thus, the ventricles  14 , 18  are essentially two separate pumps that work together to move the blood through the two circulatory circuits. Four check valves control the flow of blood within the heart and prevent flow in the wrong direction. A tricuspid valve  20  controls the blood flowing from the right atrium  12  into the right ventricle  14 . Similarly, a bicuspid valve  22  controls the blood flowing from the left atrium  16  into the left ventricle  18 . Two semilunar valves (pulmonary  24  and aortic  26 ) control the blood flow leaving the heart toward the pulmonary and systemic circuits, respectively. Thus, in each complete cycle, the blood is pumped by the right ventricle  14  through the pulmonary semilunar valve  24  to the lungs and back to the left atrium  16 . The blood then flows through the bicuspid valve  22  to the left ventricle  18 , which in turn pumps it through the aortic semilunar valve  26  throughout the body and back to the right atrium  12 . Finally, the blood flows back to the right ventricle  14  through the tricuspid valve  20  and the cycle is repeated. 
     When the heart muscle squeezes each ventricle, it acts as a pump that exerts pressure on the blood, thereby pushing it out of the heart and through the body. The blood pressure, an indicator of heart function, is measured when the heart muscle contracts as well as when it relaxes. The so-called systolic pressure is the maximum pressure exerted by the blood on the arterial walls when the left ventricle of the heart contracts forcing blood through the arteries in the systemic circulatory circuit. The so-called diastolic pressure is the lowest pressure on the blood vessel walls when the left ventricle relaxes and refills with blood. Healthy blood pressure is considered to be about 120 millimeters of mercury systolic and 80 millimeters of mercury diastolic (usually presented as 120/80). 
     Inasmuch as the function of the circulatory system is to service the biological needs of all body tissues (i.e., to transport nutrients to the tissues, transport waste products away, distribute hormones from one part of the body to another, and, in general, to maintain an appropriate environment for optimal function and survival of tissue cells), the rate at which blood is circulated by the heart is a critical aspect of its function. The heart has a built-in mechanism (the so-called Frank-Starling mechanism) that allows it to pump automatically whatever amount of blood flows into it. Such cardiac output in a healthy human body may vary from about 4 to about 15 liters per minute (LPM), according to the activity being undertaken by the person, at a heart rate that can vary from about 50 to about 180 beats per minute. 
     Several artificial devices have been developed over the years to supplement or replace the function of a failing heart in patients. These include devices developed by companies as well as research institutions such as the Berlin Heart Institute, the Pennsylvania State University, the University of Utah, the Cleveland Clinic Foundation, the University of Perkinje (in Bruno, Czechoslovakia), the University of Tokyo, the Thoratec Corporation, Abiomed Inc., Novacor, and Symbion Inc. Typically, these artificial devices consist of pumps that aim at duplicating the required pumping functions of the left and right human ventricles. One method of actuation for these pumps has been through the pneumatic action of an external mechanism. See, for example, U.S. Pat. Nos. 4,611,578 and 5,766,207. Periodic pulses of compressed air drive the pumps at the desired pressure and rate of cardiac output. A moderate vacuum may be applied between pulses to allow more rapid refilling of the ventricles with blood flowing from the respective atrium. 
     One notable artificial heart currently in use as an implant for patients waiting for a heart transplant is the Total Artificial Heart manufactured by SynCardia Systems, Inc., of Tucson, Ariz. Designed to operate much the same way as a human heart, this artificial heart replaces the two active chambers (i.e., the ventricles) of the human heart with corresponding artificial components. As illustrated in  FIG. 2 , such artificial heart  30  includes two separate chambers or ventricles  32  and  34  that replace the right and left ventricles of the human heart, respectively. Each chamber is equipped with a respective diaphragm ( 36  and  38  in the right and left chamber, respectively) that has an air contact side and a blood contact side. Each diaphragm is designed as a spherical hemisphere. As shown in  FIG. 3 , the artificial heart  30  is implanted by connecting the top of the right chamber  32  to the right atrium  12  and the top of the left chamber  34  to the left atrium  16 . The bottom of each chamber is provided with an air line ( 40  and  42  in the right and left chamber, respectively) that is embedded in the patient&#39;s body but extends outside for connection to a pneumatic driver. 
     When driven by a supply of pressurized air from the pneumatic driver, each diaphragm  36 , 38  discharges blood from the respective chamber  32 , 34  simulating the function of a ventricle. This phase is referred to in the art as systole or equivalently as the ejection phase. When the pressurized air is removed from the diaphragm, known as diastole or the filling phase, blood can enter the ventricle from the connected atrium. The rate at which blood enters the ventricle depends on the difference between the atrial pressure and the pressure on the air-side of the diaphragm. To increase this filling rate, a slight vacuum of about 10 mm Hg is normally applied to the air-side of the diaphragm during diastole. Artificial valves  44   a  (tricuspid),  46   a  (bicuspid) and  44   b  (pulmonary),  46   b  (aortic) control the flow from the respective atrium into each artificial ventricle and out to the circulatory systems, respectively. 
     The pneumatic drivers used to date for driving all artificial hearts have been cumbersome and inadequate for affording patients any degree of independent mobility. They employ compressors, vacuum pumps, and air tanks coupled to electrically actuated valves, all of which amounts to a large and heavy apparatus that can only be transported on wheels and with considerable effort. Therefore, many attempts have been made during the last two decades to produce a portable driver for these devices. However, because of the complexity of the required functionality and the hardware necessary to produce it, pneumatic heart drivers continue to be bulky, require frequent maintenance, and often provide air pulses that do not match the performance of the larger drivers they are meant to replace. Even at the approximate weight of 20 pounds and size of about 0.7 cubic feet achieved so far, pneumatic drivers remain unwieldy and substantially not portable for a patient who is kept alive by an artificial heart. 
     In essence, a portable driver needs to be reliable, durable, easy to use, and sufficiently simple in design to be affordable. Unfortunately, each of these requirements contributes to the complexity of the design, which in turn has produced devices that are not sufficiently small and light-weight to be manageable in the hands of a patient. Furthermore, it is essential that the pneumatic driver be able to provide the correct pressure balance between the left and right ventricles of the artificial heart to ensure the proper operating pressure to the pulmonary and systemic circuits regardless of the speed of operation. Typically, this requires that the driver be able to operate so as maintain, on average, a right atrial pressure of about 9 mmHg, a mean pulmonary artery pressure of about 35 mmHg, a left atrial pressure of about 10 mmHg, and a mean aortic pressure of about 95 mmHg. 
     This need to provide different operating pressures to the right and left chambers (ventricles) of the artificial-heart device has not been met heretofore with a simple design suitable for a portable driver. For example, the blood pump described in U.S. Pat. No. 4,611,578 includes a configuration wherein two reciprocating pistons in a common cylinder may be operated alternatively to provide redundancy or independently to actuate two separate pneumatically driven blood pumps. This issue is not addressed in the patent, but it describes a sophisticated control system that arguably could be used to provide the correct operating pressure to each chamber of the artificial heart. However, the complex and multi-component structure of the device necessarily requires a relatively heavy and large apparatus, though described as portable. The commercially available module weighs about 25 pounds and is approximately 0.6 cubic feet in volume. 
     U.S. Pat. No. 5,766,207 describes another portable pneumatic driver for ventricular assist devices that could also be adapted for an artificial heart. The single pump of the invention could be used to drive both ventricles of an artificial heart, but only at the same pressure and volume rate. Thus, this device, even if modified to meet the other requirements of a portable artificial-heart driver, would not be suitable as an alternative to the stationary modules currently in use. 
     Copending U.S. application Ser. No. 12/454,440, hereby incorporated by reference, describes a portable driver configured to optimize size, weight, reliability, durability, extended battery life, ease of use, and simplicity of design. The driver consists of a pneumatic pump that provides coordinated and independent periodic actuation pressure to each ventricle of the artificial heart, limiting peak pressures and peak vacuums to provide a safe and efficient cycle of operation, allowing only partial filling of each ventricle of the cardiac device to ensure redundancy of capacity, providing sufficient pneumatic stroke to completely eject the blood from the ventricles at each beat, readily adjusting the rate at which the artificial heart is actuated, and minimizing overall size and weight to enable portability. 
     In the preferred embodiment, the pneumatic pump  50  comprises two coaxial cylindrical pumping chambers ( 60  and  62 ) defined by a common housing  52 , each enclosing a disk-shaped piston ( 54  and  56 ) incorporating seals  66  to eliminate leakage, as illustrated schematically in  FIG. 4 . The pistons are connected to one another through a partition  58  by a tube  64 , thereby forming a monolithic piston assembly that is driven axially by a common electrical actuator  68  providing reciprocating motion through a rod connected to the top piston. The tube travels through a seal in the partition that separates the two chambers and, by defining the boundary between the pistons, also acts as a bulkhead for the top chamber. 
     The volume in the bottom chamber is selected as needed to provide the desired pressure in the left ventricle of the artificial heart driven by the pump. According to one aspect of the invention, the diameter of the tube connecting the pistons is selected such that the stroke volume (i.e., the displacement) of the top chamber is reduced with respect to that of the bottom chamber as needed to match the reduced pressure requirements of the right ventricle of the artificial heart. Namely, the maximum pressure achieved at the respective output port ( 70  and  72 ) in each chamber should be as needed to fully eject blood from each ventricle of the artificial heart substantially at the operating pressures of the human pulmonary and systemic circulatory circuits. A limit check valve ( 74  and  76 ) is preferably used in each chamber to ensure venting of excess pressure during the compression stroke. A limit check valve ( 78  and  80 ) is also preferably used in each chamber to limit the vacuum generated during the reverse, aspiration stroke. 
       FIG. 5  illustrates in sectioned view the actual pneumatic driver that incorporates the concepts of the invention disclosed in Ser. No. 12/454,440. As clearly illustrated by  FIGS. 4 and 5 , the pneumatic pump is actuated by a reciprocating mechanism that drives the pistons. Many such mechanisms are known in the art that could be used to operate the pump. The device commonly referred to as a scotch yoke is preferred because of its simplicity, reliability and suitability for implementation in a small volume, all of which are critical for a portable artificial heart driver. 
     However, an additional requirement for such an application is redundancy, which is not readily available from a conventional application of scotch-yoke technology wherein a motor  68  ( FIG. 5 ) with a rotating output shaft is the driving source for actuating the reciprocating pistons of the pump. The present invention provides an ingenious solution to this problem in a configuration designed particularly to meet the critical redundancy requirements of the SynCardia artificial heart driver. 
     SUMMARY OF THE INVENTION 
     The invention lies in the recognition that a conventional scotch yoke may be modified advantageously by creating a gap in the yoke&#39;s structure that allows the engagement and disengagement of the yoke from the cam follower that drives it. As a result, two scotch yokes may be combined to provide redundant operation. In particular, because of its simplicity of design and structural compactness, the invention enables redundant operation of a drive for a portable artificial heart. 
     Accordingly, in the preferred embodiment of the invention a primary scotch yoke and a secondary scotch yoke are coupled in side-by-side arrangement and rigidly attached to a linearly reciprocating element. Each scotch yoke includes a rotary actuator with a cam follower and a yoke attached to the reciprocating element. The yokes have a cam surface incorporating an open gap wherein the follower is disengaged. Under normal operation, the follower of the secondary scotch yoke idles within this gap. A controller is utilized to detect a failure of the primary drive system and, in such event, it disables the primary drive and energizes the secondary drive. Upon activation of the secondary drive system, the secondary follower/yoke pair forces the primary follower into the idling gap of the primary yoke where it remains as the secondary yoke continues to provide uninterrupted motion to the reciprocating element. 
     Because of these functional features, the scotch-yoke assembly of the invention is most suitable to provide the critical redundancy required to drive a portable artificial heart such as the SynCardia device described above. Additional features and advantages of the invention will be forthcoming from the following detailed description of certain specific embodiments when read in conjunction with the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a representation of a human heart. 
         FIG. 2  is a schematic view of the SynCardia artificial heart for which the present invention has been developed. 
         FIG. 3  is a representation of the artificial heart of  FIG. 2  connected to the heart atria of a human body. 
         FIG. 4  is a schematic view of a pneumatic driver according to the preferred embodiment of the invention disclosed in Ser. No. 12/454,440. 
         FIG. 5  is a sectioned view of the actual pneumatic driver that incorporates the concepts of the invention disclosed in Ser. No. 12/454,440. 
         FIG. 6  is a schematic illustration of a conventional scotch yoke. 
         FIGS. 7A-7F  show schematically the modified scotch yoke of the invention in several rotational positions to illustrate the sequence of engagement of the yoke cam by the cam follower during a cycle of operation. 
         FIGS. 8A and 8B  illustrate schematically the disengagement of the yoke from the cam follower afforded by the scotch-yoke configuration of the invention. 
         FIG. 9  illustrates the secondary scotch yoke coupled in side-by-side configuration with the primary yoke of  FIG. 7  and to a common linearly reciprocating element according to the invention. 
         FIGS. 10A and 10B  illustrate schematically, using the scotch yoke of  FIG. 9  as example, the required idle positions of the cam follower in order to effect its engagement of, or disengagement from, the yoke of the invention without interference with the continued linear motion of the yoke. 
         FIG. 11  is a schematic top view of the scotch-yoke assembly of the invention. 
         FIG. 12  is a sectioned view of an actual scotch-yoke assembly that incorporates the concepts of the invention. 
         FIG. 13  is another sectioned view of the assembly showing the cam of the primary yoke and the idle follower of the secondary yoke 
         FIG. 14  is an illustration of a portable driver actuated by the scotch-yoke assembly of the invention and connected to an artificial heart implanted in a human body. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 6  is a schematic illustration of a conventional scotch yoke, a device considered to be the most compact rotary-to-linear conversion mechanism (and vice versa). The typical scotch yoke  82  comprises a yoke  84  with a slot defining a cam  86  connected through some rigid means, such as a connecting rod or other element  88 , to a target device requiring reciprocating linear translation (not shown). The scotch yoke is constrained so as to cause the rod  88  to follow the linear path meeting the target requirement. A rotary actuator  90  interfaces with the yoke via a cam follower  92 . As the rotary actuator rotates, the cam follower pushes the yoke in one direction and then the other in a reciprocating, periodic manner. The motion of the reciprocating element  88  is a pure sine wave over time for a constant rotational speed of the actuator  90 . 
     In some critical applications, such as for a pump driving an artificial heart, it is necessary to provide a backup for the drive system. If the primary drive is determined not to meet some critical operational characteristic, then it will be deactivated and a backup drive will be activated, and the switching must occur without loss of operation. In such a critically redundant system for an artificial heart, it is particularly desirable to use the scotch-yoke mechanism because of its relative simplicity, reliability and suitability for implementation in a small volume. 
     Still with reference to  FIG. 6 , it is apparent that only half of the surface of the cam  86  is actively engaged by the cam follower  92  during a cycle of rotation of the rotary actuator  90 . That is, if the actuator rotates in the direction indicated by the arrow R in the figure, the left half of the bottom cam surface is engaged while the follower pushes the rod  88  down (toward the right side of the figure), and the right half of the top cam surface is engaged while the follower pushes the rod up) toward the left side of the figure). The balance of the cam surface remains disengaged at all times. Therefore, according to the invention, such unused portions of the cam are removed and the gaps so created are exploited to enable the automatic switching between two scotch yokes coupled to the same reciprocating element in the system. Accordingly, the system provides the type of redundancy critically needed for a portable artificial heart drive, such as the SynCardia device described above. 
       FIGS. 7A-7E  illustrate a scotch yoke  100  modified according to the invention. As mentioned, the conventional yoke  84  of  FIG. 6  is modified so as to provide two gaps G in the cam, retaining as cam surfaces only most of the active segments required for operation in a predetermined direction. For example, for an actuator rotating  90  counterclockwise with respect to a side viewer, as seen in  FIG. 7B , the yoke  100  of the invention would comprise two yoke sections  102  and  104  with respective cams  106  and  108  separated by the gaps G, as illustrated in  FIG. 7A  with the rotary actuator  90  and the cam follower  92  removed from the cam structure. The cams  106 , 108  are preferably formed in a block  110  that is connected to the reciprocating rod  88 . As shown in the elevational side view of  FIG. 7B , the scotch yoke  100  also includes the rotary actuator  90  that propels it in the usual manner through a follower  92  (shown in phantom line because obscured to a viewer by the actuator  90 ). Differently from conventional ones, the scotch yoke  100  is capable of operation only with the rotary actuator rotating in a single direction, as illustrated by the arrow R in the figure. However, this apparent drawback is the advance that makes it possible to implement the desired automatic switching between coupled scotch yokes that is required when driving an artificial heart. 
       FIGS. 7B-7F  illustrate the function of a primary scotch yoke  100  driven by a rotary actuator  90  rotating counterclockwise, as indicated by the arrow R. The actuator  90  rotates around a fixed axis A and the cam follower  92  is positioned radially from the axis A so as to follow a circular path as the actuator rotates. The follower  92  is preferably also capable of rotation around its own axis  112  in order to minimize the friction generated by its engagement of the cam surfaces. As a result of the rotation of the actuator  90 , the cam follower  92  pushes the yoke in one linear direction or the other, as indicated by the arrows D in the figures, depending on the position of the follower during the cycle of operation of the actuator. 
     For example, when in the position of  FIG. 7B , the follower  92  will push the yoke section  104  upward, in the direction indicated by the arrow F. Therefore, the block  110  and the reciprocating rod  88  attached to it will also move in the upward direction, as indicated by the arrow D. This will continue for  90  degrees of rotation of the cam follower  92 , until the follower reaches the top of the circular path defined by the rotation of the actuator  90  around its fixed axis A, as illustrated in  FIG. 7C . (Note that the actuator  90  is no longer shown in the remaining figures for clarity of illustration, its presence and relative position being illustrated by the placement of its axis of rotation A.) At the precise top position shown in  FIG. 7C , the vertical direction of motion of the follower  92  switches and the force applied on the scotch yoke will similarly switch from the cam  108  of yoke section  104  to the cam  106  of yoke section  102  within a few degrees of rotation of the actuator  90 , thereby causing the motion of the yoke, and therefore also of the rod  88 , to switch to the opposite (downward) direction, as indicated by arrow D. Note that the follower  92  will disengage the cam  108  and immediately engage the cam  106 . Therefore, the action of the follower  92  on the yoke  100  is essentially continuous in spite of the gap G in the cam. The direction of the force applied to the cam and of the motion of the yoke/rod assembly remains downward for the next  180  degrees of rotation of the actuator  90 , as illustrated in  FIGS. 7D and 7E . When the follower  92  reaches the bottom of its circular path ( FIG. 7E ), the force applied by the follower  92  on the yoke switches (again within a few degrees or rotation of the actuator  90 ) from the cam  106  back to the cam  108 , thereby causing the motion of the yoke and rod to switch back to the upward direction, as indicated by arrow D, where it remains for the next 180 degrees of rotation.  FIG. 7F  illustrates, halfway through the upward motion of the yoke  100 , the completion of a cycle of rotation of the actuator  90  (the same position shown in  FIG. 7B ). 
     It is apparent that the gap sections G of the scotch yoke of the invention make it possible to disengage the follower  92  from the yoke when the motion of the rotary actuator  90  is interrupted, such as when it fails. For example, should the actuator  90  break down immediately prior to reaching the position illustrated in  FIG. 7E , the follower  92  could be disengaged simply by braking the inertial rotary motion of the actuator  90  so as to cause the follower  92  to stop in a position aligned with the bottom gap G between the yoke sections  102 , 104 . As illustrated in  FIG. 8A , this would allow the reciprocating motion of the yoke block  110  to continue without obstruction by the idled follower  92 . In such a case, assuming the yoke block  100  was driven in the same manner by a separate actuator, it could proceed with its upward motion while the follower  92  remained in its idle position clearing the yoke through the slot  114  defined by the bottom gap G. The same could be achieved at the opposite end of the rotation cycle if the breakdown occurred prior to reaching the position illustrated in  FIG. 7C  near the top gap G. In such case, as shown in  FIG. 8B , the yoke block  100  could continue reciprocating with its downward motion while the idle follower  92  cleared the yoke through the slot  116  defined by the top gap G. Therefore, differently from prior-art embodiments, the scotch yoke of the invention is suitable for use in a system that requires its disengagement, so that the reciprocal motion of the yoke can continue even when its rotary drive fails. 
     A further critical advantage of this feature of the invention lies in the fact that it can be similarly exploited to provide the automatic switching for engagement by a secondary, back-up scotch yoke as required for continued redundant operation of a linearly reciprocating drive. Thus, according to the invention, a secondary scotch yoke  100 ′ configured exactly the same way as the primary scotch yoke  100  shown in  FIGS. 7A-7F , but coupled side-by-side in opposing fashion to the primary scotch yoke, is coupled to the same rod  88  (or other reciprocating element requiring redundancy of operation with automatic switching between two independent actuating mechanisms). Preferably, the cams of both yokes are formed in a single block  110  with the same exact geometry; that is, such that a 90-degree rotation of the block  110  would show exactly the same cam configuration (that of  FIG. 7A ).  FIG. 9  illustrates such secondary, alternatively engaged, scotch yoke  100 ′ in the same position as the primary yoke  100  of  FIG. 7B . A secondary cam follower  92 ′ is rotatably coupled to a secondary actuator  90 ′ for engagement of the cams  106 ′ and  108 ′ of the secondary yoke during the cycle of operation of the actuator  90 ′. The secondary rotary actuator  90 ′ is aligned with the primary actuator  90  along the common axis of rotation A and it is similarly able to rotate only in one direction R′, counterclockwise. However, while still counterclockwise with respect to a side viewer, as shown in the figure, the direction R′ in fact is opposite to the direction R of the primary actuator with reference to the common axis of rotation A. That is, if the direction of rotation R of the primary actuator is counterclockwise relative to the axis A, the direction of rotation R′ of the secondary actuator is clockwise relative to the same axis A viewed from the same vantage point. Thus, it is clear that in absolute spatial terms the actuators rotate in opposite directions. 
     In practice only one of the scotch yokes is energized at any given time and the system is configured so that the secondary yoke is automatically engaged when the primary yoke fails and becomes disengaged (or vice versa). Each actuator  90 , 90 ′ is engineered to stop only at positions wherein the cam followers  92 , 92 ′ are aligned with the gaps ( 114 , 116  and  114 ′, 116 ′, respectively), so that the block  110  incorporating both yokes can continue its linear movement in both directions, propelled by the functioning cam follower, without interference from the idle follower. This condition is achieved if the cam follower stops immediately upon entering the gap encountered after the failure of the rotary actuator, which can be accomplished by friction and/or a braking mechanism. Thus, for example, in its idle condition the secondary cam follower  92 ′ would be positioned either in alignment with the gap  114 ′ as shown in  FIG. 10A , or with gap  116 ′ as shown in  FIG. 10B , and the yoke block  110  could continue to move freely in its reciprocating linear motion (indicated by arrow AA). Similarly, in its idle condition the primary cam follower  92  would be positioned either in alignment with the gap  114  or the gap  116  as shown in  FIGS. 8A  and B, respectively. 
     Note that, because of the configuration of each cam, the secondary drive system will drive the primary follower to the idle gap automatically when the primary actuator fails and the secondary one is energized. The only requirement is that the primary system have sufficient friction or electronic brake to keep it from over-rotating when it reaches the gap. That is, it is not necessary for the primary follower to stop in a gap. The primary system can fail anywhere and, as soon as the secondary system begins driving the reciprocating element, it also drives the primary yoke that in turn drives the primary follower to the primary idle gap. Once the primary follower reaches its idle gap, it will no longer contact the yoke and the secondary will no longer need to drive it. It is clear that the functions of the primary and secondary systems, as described, are interchangeable. 
     Assuming a failure of the primary actuator  90  while operating in the quadrant of  FIG. 7B  according to the cycle illustrated in  FIGS. 7B-7F , the actuator would stop in the position of  FIG. 7C . The immediate subsequent actuation of the secondary rotary actuator  90 ′ would cause the secondary cam follower  92 ′ to bear on its cam  106 ′ and continue the reciprocating motion of the rod  88  through the action of the yoke sections  102 ′ and  104 ′. If, for example, the secondary cam follower  92 ′ had been idle in the position shown in  FIG. 10A , it would be able to rotate unhindered through the gap  114 ′ to engage the cam  106 ′ and push the yoke down (as illustrated by arrow F in  FIG. 7D  with reference to the primary yoke). If, on the other hand, the secondary cam follower  92 ′ had been idle in the position shown in  FIG. 10B , it would immediately engage the cam  106 ′. The follower  92 ′ would similarly engage the cam  108 ′ and push the yoke upward if the primary rotary actuator failed with its follower v 92  in the down position of its circular motion. 
     Thus, by coupling the reciprocating element  88  to both primary and secondary scotch yokes as shown herein, the continuity of its motion is assured by the ability to automatically switch from the primary to the secondary actuator in case of failure. The two scotch yokes are preferably combined facing one another, as taught, coupled to the reciprocating element  88  to provide the exact same linearly reciprocating motion. Sensors and controllers are utilized in conventional manner to detect at all times the motion of the yokes and, upon failure of the primary rotary actuator, to stop its rotation and at the same time energize the secondary actuator. The stopped position of the idled cam follower is controlled with friction or a conventional braking mechanism. 
       FIG. 11  is a schematic top view of a scotch-yoke assembly according to the invention to illustrate the side-by-side opposing arrangement of the scotch yokes described in  FIGS. 7 and 9 . The primary scotch yoke  100  is shown on the left-hand side of the figure with the cam follower  92  driving the cam surface  106  during rotation of the primary rotary actuator  90 . The block  110  incorporating both yoke sections is attached to the linearly reciprocating element  88  (not shown in this top view). The secondary scotch yoke  100 ′ is coupled to the primary one by having its cam incorporated into the same block  110  opposite to the cam of the primary yoke. The secondary rotary actuator  90 ′ is shown idle at the top of its rotation cycle with its cam follower  92 ′ positioned in the gap G of the secondary yoke. A controller  120  is provided to detect a failure in the primary scotch yoke  100  (such as, for example, a break down in the actuator  90 , or a failure in the follower&#39;s bearing); in such event, to de-energize the primary actuator and stop its motion so that the cam follower becomes idle at a position in the gap G that will not interfere with the continued reciprocating motion of the yoke block  110 ; and then to energize the secondary rotary actuator  100 ′ so as to cause its cam follower  92 ′ to immediately engage the cam section  106 ′ and continue operating the assembly.  FIG. 12  is a sectioned view of an actual scotch-yoke assembly that incorporates the concepts of the invention.  FIG. 13  is another sectioned view of the assembly showing the cam of the primary yoke and the idle follower of the secondary yoke. Finally,  FIG. 13  is an illustration of a portable driver actuated by the scotch-yoke assembly of the invention and connected to an artificial heart implanted in a human body. 
     While the invention has been shown and described herein with reference to what is believed to be the most practical embodiment, it is recognized that departures can be made within the scope of the invention. For example, the two scotch yokes have been illustrated in opposing side-by-side configuration, but they could be arranged in any other manner allowing the disengagement of the primary one upon its failure and the subsequent engagement of the secondary one. For example, a mirror-image configuration would work with both actuators rotating in the same spatial direction. In fact, the two yokes could be coupled to the reciprocating element in any arbitrary manner, so long as they were both configured to produce the same reciprocating motion and to enable the deactivation/activation procedure described herein. Therefore, the invention is not to be limited to the disclosed details, but is intended to embrace all equivalent structures and methods.