A medical device known as an implantable therapeutic substance delivery
device is configured for implanting in humans to deliver a therapeutic
substance such as pharmaceutical compositions, genetic materials, and
biologics to treat a variety of medical conditions such as pain,
spasticity, cancer, and many other conditions. The infusion device
incorporates a stepper motor that controls the infusion flow rate during
the service life of the device. The stepper motor is controlled by
continuously varying electrical pulse parameters based on the
continuously decreasing power source voltage during the service life of
the substance delivery device. In particular the stepper motor electrical
pulse parameters, especially duty cycle, are selected to efficiently
compensate for decreasing battery voltage thereby optimizing the motor
performance while maximizing the power source service life. The infusion
device has a housing, a power source, a therapeutic substance reservoir,
a therapeutic substance pump, and electronics. Many embodiments of the
therapeutic substance delivery device with optimized pump motor drive and
its methods of operation are possible.