Biomedical Engineering Reference
In-Depth Information
FIGURE 8-73
Pump flow as a
function of time.
Once this stroke is complete, the pump starts to reduce the volume of the pump cavity.
This results in an increase in pressure, which closes the inlet valve and opens the outlet
valve to allow the liquid to escape.
During the delivery stroke, flow increases from zero up to a maximum and then
decreases back to zero during the intake stroke, when flow is zero. The pressure inside
the pump changes in the same way as flow, going from zero to a maximum value before
reverting to zero during the intake stroke, as shown in Figure 8-73.
For some applications these pressure pulses are undesirable, but because the cardiovas-
cular system is designed to accommodate such variations pulsatile pumps are commonly
used for artificial hearts and LVADs.
There are many ways to generate a more uniform flow, if that is required. One approach
is to keep the single-piston design but to vary the shape of the cam or the speed of the
motor. The shape of the cam leads to a flatter flow curve at the middle of the delivery
stroke. In addition, the motor can be made to speed up during the intake stroke and slow
down during the delivery stroke. Some pulsation still remains, however, and these pumps
often use some form of pulse dampening to further reduce the flow fluctuations if required.
Another common approach combines the output flow from two heads operating 180
degrees out of phase, such that the intake stroke from one head coincides with the delivery
stroke from the other. This means that while one cylinder is filling the pump cylinder the
second cylinder is delivering. Then, when the second refilling, the first cylinder delivers.
These two flows can be combined by feeding each pump output into a tee that connects a
common outlet. The inlet line from the reservoir likewise is fed to a tee, which branches
to feed both cylinders of the pump.
8.7.3.2 Diaphragm Pumps
The operating principle of a diaphragm pump and its construction are very simple, as can
be seen in Figure 8-74. The diaphragm is clamped at its circumference between the pump
housing and the pump head. An eccentric cam displaces the connecting rod, which in turn
moves the diaphragm back and forth. This produces a periodic change in volume of the
FIGURE 8-74
Schematic diagram
of a diaphragm
pump. (a) Inlet
stroke. (b) Exhaust
stroke.
