Page 555 - Handbook of Biomechatronics

548 Graham Brooker

Fig. 23 Measured performance of the VentraCor VentrAssist at 2000rpm.

by the rotors and forced outwards by centrifugal force where it exits through
a pipe on the outside edge.
Some performance data from Stanfield and Selzman (2013a, b) shows the
differential pressure-flow rate curve for the VentrAssist at 2000rpm in
Fig. 23.
Motors to drive continuous flow heart pumps need to use brushless com-
mutation to avoid releasing wear products into the mechanism. These can
use the conventional Hall-effect switch-based commutation in which the
rotor angle is sensed from its magnetic field, and field effect transistors excite
the required coils to maintain the correct rotation. Alternatively, a synchro-
nous motor configuration can be used in which an external variable fre-
quency drive excites the stator coil to generate a rotating magnetic field
that causes the magnetic rotor to rotate at the correct speed.
The primary differences between centrifugal and axial-flow pumps are in
the design of their rotary elements. A centrifugal pump operates as a
“thrower” meaning that the blood is captured and thrown tangentially off
the blade tips. In contrast, axial pumps operate as “pushers” or as an auger,
screwing through the fluid (Joyce et al., 2012).
Both pump types generate a pressure differential by creating a vortex and
then converting using either flow straighteners in axial pumps or a volute in
centrifugal pumps. An intermediate configuration, known as a mixed flow
pump, combines the pusher and thrower mechanisms in a continuum. This
continuum is quantified by the specific speed, n s , of the pump type that is a
function of the rotation rate, the flow rate and the pressure head.
The specific speed determines the general shape of a centrifugal pump
impeller. As the specific speed increases, the ratio of the impeller outlet