Abstract:
A implantable pump system comprises an implantable pump motor and an external unit. An inverter comprises respective phases with redundant legs connected in parallel, and respective current sensors in series with each leg generating a respective measured current. A cable redundantly couples the inverter to the motor. The cable includes a respective conductor coupling each redundant leg to a respective phase of the motor. The controller receives the measured currents, monitors for a fault in the conductors by comparing the measured currents in the respective redundant legs. A fault in a pair of redundant conductors is detected if a ratio of the respective measured currents is not within a predetermined range.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to circulatory assist devices, and, more specifically, to enhanced reliability and fault monitoring of cabling that connects an external unit to an implantable pump unit. 
     Many types of circulatory assist devices are available for either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. One such system is the DuraHeart® LVAS system made by Terumo Heart, Inc., of Ann Arbor, Mich. One embodiment of the DuraHeart® system may employ a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. An electric motor magnetically coupled to the impeller is driven at a speed appropriate to obtain the desired blood flow through the pump. 
     A typical cardiac assist system includes a pumping unit, electrical motor (e.g., a brushless DC motor integrated into the pump), drive electronics, microprocessor control unit, and an energy source such as rechargeable batteries. The system may be implantable, either fully or partially. The goal of the control unit is to autonomously control the pump performance to satisfy the physiologic needs of the patient while maintaining safe and reliable system operation. A control system for varying pump speed to achieve a target blood flow based on physiologic conditions is shown in U.S. Pat. No. 7,160,243, issued Jan. 9, 2007, which is incorporated herein by reference in its entirety. Thus, a target blood flow rate may be established based on the patient&#39;s heart rate so that the physiologic demand is met. The control unit may establish a speed setpoint for the pump motor to achieve the target flow. Whether the control unit controls the speed setpoint in order to achieve flow on demand or whether a pump speed is merely controlled to achieve a static flow or speed as determined separately by a physician, it is essential to automatically monitor pump performance to ensure that life support functions are maintained. 
     A typical pump motor employed for a blood pump is a three-phase permanent magnet electric motor that can be driven as a brushless DC or a synchronous AC motor without any position sensor. The need for a position sensor is avoided by controlling motor operation with one of a variety of methods that use the measured stator phase currents to infer the position. Vector control is one typical method used in variable frequency drives to control the torque and speed of a three-phase electric motor by controlling the current fed to the motor phases. This control can be implemented using a fixed or variable voltage drive delivered via an inverter comprised of pulse width modulated H-bridge power switches arranged in phase legs. Reliability, fault detection, and fault tolerance are important characteristics of an electrically-powered blood pump, drive system, and cable, and it would be desirable to improve each of them. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a pump system comprises an implantable pump unit having a multiphase brushless motor and an external unit including a controller and an H-bridge inverter. The H-bridge inverter comprises a first phase with first and second redundant legs connected in parallel, a first current sensor in series with the first leg generating a first measured current, a second current sensor in series with the second leg generating a second measured current, a second phase with third and fourth redundant legs connected in parallel, a third current sensor in series with the third leg generating a third measured current, and a fourth current sensor in series with the fourth leg generating a fourth measured current. A cable redundantly couples the H-bridge inverter to the motor. The cable includes a first conductor coupling the first leg to a first respective phase of the motor, a second conductor coupling the second leg to the first respective phase of the motor, a third conductor coupling the third leg to a second respective phase of the motor, and a fourth conductor coupling the fourth leg to the second respective phase of the motor. The controller receives the measured currents, monitors for a fault in the first or second conductors by comparing the first and second measured currents, and monitors for a fault in the third or fourth conductors by comparing the third and fourth measured currents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of a circulatory assist system as one example of an implantable pump employing the present invention. 
         FIG. 2  is a schematic diagram showing a conventional ventricular assist system employing an H-bridge inverter. 
         FIG. 3  is a schematic diagram showing redundant phase legs and cable conductors employed in one embodiment of the present invention. 
         FIG. 4  is a logic diagram showing a preferred embodiment of fault monitoring of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , a patient  10  is shown in fragmentary front elevational view. Surgically implanted either into the patient&#39;s abdominal cavity or pericardium  11  is the pumping unit  12  of a ventricular assist device. An inflow conduit  13  conveys blood from the patient&#39;s left ventricle into pumping unit  12 , and an outflow conduit  14  conveys blood from pumping unit  12  to the patient&#39;s ascending thoracic aorta. A power cable  15  extends from pumping unit  12  outwardly of the patient&#39;s body via an incision to a compact controller  16 . A power source, such as a battery pack  17  worn on a belt about the patient&#39;s waist, is connected with controller  16 . 
     Cable  15  is flexible in order to allow freedom of movement of the patient. Such movement, however, causes stresses to cable  15  and to its connections with pumping unit  12  and controller  16 . To increase reliability and fault tolerance, the present invention uses redundant conductors in cable  15  to supply each of the phase currents that drive the pump motor. 
     A conventional inverter and cabling for an LVAD system is shown in  FIG. 2 . A DC motor in pump unit  12  has phases  20 - 22  connected by cable  15  to an H-bridge inverter  23 . A controller  24 , such as a general purpose microcontroller, implements a vector control or other algorithm to determine proper energization of phases  20 - 22  to obtain the desired motor operation. Controller  24  is connected to a driver  25  for generating drive signals coupled to the control inputs of individual switches (e.g., transistors) in inverter  23 . Controller  24  has an input/output  26  for sending messages or generating fault alarms directed at the user or a physician, for example. 
     Inverter  23  has an H-bridge configuration with a first phase leg  30 , a second phase leg  31 , and a third phase leg  32 . Phase leg  30  has an upper switch  33  and a lower switch  34  which are turned on and off by controller  24  via driver  25  as known in the art. A current sensor  35  in series with phase leg  30  provides a measured current to controller  24  as an input to the vector control algorithm. Similarly, phase leg  31  includes switches  36  and  37  and a current sensor  38 . Phase leg  32  includes switches  40  and  41 , but a current sensor may not be required since the vector control algorithm can infer a third current based on measured currents from sensors  35  and  38 . 
       FIG. 3  shows an improved ventricular assist system having higher reliability and fault tolerance as a result of redundant cable conductors and redundant phase legs. Thus, an inverter  45  is coupled by a redundant cable  46  to motor  47  in pump unit  12 . Inverter  45  has a first phase  50 , a second phase  51 , and a third phase  52 . First phase  50  has a first phase leg  55  and a second phase leg  56 . The upper and lower power switches in legs  55  and  56  are respectively coupled together to provide synchronous operation of the legs. Respective conductors  57  and  58  connect phase legs  55  and  56  to a connector terminal  60 . Cable  46  includes conductors  61  and  62  connected at terminal  60  to conductors  57  and  58 , respectively. Conductors  61  and  62  are coupled to motor  47  via a terminal connector  63  in pump unit  12 . Redundant conductors  61  and  62  become interconnected within pump unit  12  in order to drive a respective phase of motor  47 . 
     Legs  55  and  56  include respective current sensors  64  and  65  measuring the separate current magnitudes flowing in each of legs  55  and  56 . The measured currents are coupled to the controller for monitoring and motor control purposes as explained below. 
     Phases  51  and  52  of inverter  45  have an identical configuration. Thus, phase  51  includes redundant phase legs  66  and  67 , which are independently connected to terminal  60  by conductors  68  and  69 . Corresponding conductors  70  and  71  are provided in cable  46 . Current sensors  72  and  73  provide measured currents for phase leg  66  and  67  to the controller. Phase  52  includes legs  74  and  75  having their outputs connected to terminal  60  by conductors  76  and  77 . Cable  46  includes conductors  78  and  79  which connect conductors  76  and  77  to pump unit terminal  63 . Phase  52  includes current sensors  80  and  81  in legs  74  and  75 , respectively, which provide measured currents for legs  74  and  75  to the controller. 
     The redundancy of the cable conductors, phase leg switches, and phase leg conductors provide fault tolerance whereby damage such as loss of continuity in one conductor or failure of one switch does not prevent operation of the ventricle assist system. Upon failure of one of these, the redundant conductor or phase leg carries the full current load instead of being distributed between the redundant elements, thereby providing continuous operation of the pump. 
     Fault monitoring is performed by comparing measured currents within redundant phase legs. Specifically, if the currents are substantially equal (indicating that operation of electrical components is the same in each redundant leg) then conditions are nominal and no fault is detected. If the measured currents are substantially unequal, on the other hand, then a fault is detected. The fault occurrence may trigger an alarm to inform a user that steps should be taken to remedy the fault. However, regular pump operation is maintained by virtue of the redundant element continuing to supply the proper current to the motor. 
     In a preferred embodiment, measured currents from redundant legs of the same phase are compared by forming a ratio of the measured currents. Assuming no fault is present, then the currents are about equal and the ratio has a value near 1. Thus, the ratio may be compared to a range centered on 1 (e.g., from 0.8 to 1.2) such that no fault is present when the ratio is within the range, and a fault is detected when the ratio falls outside the range. In controlling the motor based on the phase currents, the controller sums the two measured currents from the redundant legs corresponding to each phase and uses each summed current as an input to the vector control algorithm. Thus, the present invention does not necessitate any changes in the motor control algorithm itself. However, it may be possible to simplify the algorithm since the invention provides actual measurements of the currents in all three phases instead of just two. 
     The controller may preferably perform fault monitoring using the logic shown in  FIG. 4 . Respective phase currents i A1  and i A2  from current sensors in the respective phase legs of a single phase A may be converted to digital values in analog-to-digital converters  85  and  86 . A ratio block  87  determines the ratio of the currents which is then provided to inputs of comparators  88  and  89 . Comparator  88  compares the ratio with an upper threshold T 1  and generates a high-level logic output signal when the ratio is greater than T 1 . Comparator  89  compares the ratio with a lower threshold T 2 . When the ratio is below threshold T 2 , then a low-level logic output is generated by comparator  89 . An OR-gate  90  has its inputs coupled to the respective outputs of comparators  88  and  89 , whereby when the ratio is outside the range defined by thresholds T 1  and T 2 , then a high-level logic signal is provided at output  91  of OR-gate  90 . In response to the detected fault, the controller may preferably generate an alarm to signify the need to take corrective action.