Implantable medical devices such as ventricular assist devices are being developed for long term treatment of chronic heart failure. Such devices require a pumping mechanism to move blood. Due to the nature of the application, the pumping mechanism must be highly reliable. Patient comfort is also a significant consideration.
Electrically powered pumping mechanisms typically rely on a motor such as a brushless DC motor. Brushless DC motors offer maintenance advantages in implant applications due to the lack of wear-prone brushes and slip rings. Due to the lack of these mechanical commutation components, however, commutation must be provided electrically by the drive electronics. In order to provide proper commutation, the mechanical angle of the motor's rotor must be determined. Typically, speed control is also desired.
One method of motor drive control for three-phase motors is referred to as a six step drive. The six step drive provides a square wave as the drive voltage for each motor phase. One type of six step drive uses a phase-locked loop to generate an error between the rotation speed indicated by the back emf zero crossing frequency and a commanded rotational speed. This error signal is then used to control the motor drive voltage.
Another type of six step drive uses the back emf zero crossing to supply an appropriate delay to a commutation sequencer circuit. This approach typically requires a center tap for each motor winding. The center tap is undesirable in medical implant applications because it introduces additional lead wires that must be routed from the pump to the controller. Both six step drive back emf sensing approaches typically require a phase of the motor to be open-circuited for a large portion of the commutation period and are susceptible to false triggering due to electrical switching noise.
Another disadvantage of six step drive controls is the “on-off” nature of the drive voltage. In a three-phase motor application, for example, the six step drive powers only two phases at a time. The stepping nature of the driving voltage waveform introduces harmonics and electromagnetic noise. Additionally, the stepping nature of the drive voltage results in increased torque-ripple. These effects generate acoustical noise and vibration which are undesirable for medical implant applications.
An alternative motor drive system uses sinusoidal drive voltages for the motor phases. The sinusoidal drive voltage significantly reduces torque ripple resulting in improved acoustical and vibration characteristics. Typically, information about the angular position of the rotor is needed for adequate motor control.
The rotor position information can be indicated by sensors, such as Hall-effect sensors, or through the use of encoders or resolvers. The use of additional sensors in medical implant applications, however, is undesirable as introducing additional cost, complexity, and points of failure for the device. An alternative method samples the state of the motor and infers the position of the rotor from a mathematical model. Disadvantages of this approach include susceptibility to errors in the model, variations in the model due to manufacturing tolerances, and system electrical noise.
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