Theory of operation
2-14
10070389 Rev. A
Puritan Bennett™ 700 Series Ventilator System Service Manual
2.2.2 Piston/cylinder system
The piston/cylinder system, shown in Figure 2-6 and Figure 2-7, draws gas from the gas inlet
system for delivery to the patient. See Section 2.5 for an integrated description of how the
piston/cylinder works in conjunction with the electronics.
Figure 2-6. Piston/cylinder system pneumatic diagram
2.2.2.1 Component descriptions
The piston/cylinder system includes the components described below; these are shown in
Figure 2-7 and Figure 2-8.
• Gas is delivered to the patient by an aluminum
piston/cylinder
. The piston, inside a
cylinder, moves backward and forward to draw in and expel gas. The piston/cylinder is
connected via a rack and pinion to a
motor/encoder
. The motor moves the piston as
directed by the electronics, while feedback from the encoder is used to monitor the
piston’s position.
The piston is mounted on a shaft, which rides on precision linear bearings at each end of
the cylinder. The shaft has a rack gear profile. It mates with a pinion gear attached to the
motor shaft (Figure 2-9). When the motor speed and direction change, the piston also
changes speed and direction. An interrupter flag located on the rack helps confirm piston
position limits, in conjunction with two optoswitches. The piston/cylinder can deliver up to
2 L gas per breath.
An important element of this piston-based pneumatic system is that no contact occurs
between the piston and cylinder wall. The piston/cylinder has no seal; instead, there is a
minute gap of 50
±
5
μ
m (0.002 in.) between the two. This gap is approximately the
thickness of a thin sheet of paper.
7-00032
Motor/
encoder
To
cylinder pressure
transducer
Cylinder pressure
transducer
autozero solenoid
Inlet
check valve
From mixing
manifold
Outlet
check valve
Piston/cylinder
assembly
Exhaust
port
To air intake
manifold
To inspiration
manifold
Summary of Contents for Puritan Bennett 700 Series
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