Covidien Puritan Bennett 700 Series, Service Manual

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Theory of operation
Puritan Bennett™ 700 Series Ventilator System Service Manual 10070389 Rev. A 2-15
Because the piston does not contact the cylinder, there is a continuous, measurable leak
while the piston/cylinder is pressurized. To compensate for this “calibrated leak”, the
piston moves continuously within the cylinder. But, because the piston and cylinder have
no contact, friction between the piston and cylinder is eliminated, resulting in reduced
wear of piston/cylinder assembly parts, reduced electrical requirements, and enhanced
responsiveness.
The piston/cylinder gap is maintained at all possible piston positions. Each new piston/
cylinder assembly comes with several calibration constants specific to that particular
assembly. These constants represent the gaps at various points corresponding to different
piston positions. This data must be entered into NVRAM when a new assembly is installed.
• Optoswitches are read by the motor controller circuit for piston initialization purposes
during POST and for fault detection purposes during ventilation. An optoswitch is closed
when the rack flag breaks the infrared light beam (Figure 2-9).
• The motor/encoder is an FRU composed of an optical encoder attached to a brushless DC
motor .
The high-torque, direct-drive motor controls piston movement, under direction of the
motor controller circuit on the controller PCB and the motor drive circuit on the BBU PCB.
Three rotations of the motor shaft correspond to a 2.6 L volume displacement.
The optical encoder, on top of the motor, along with the motor controller circuit (on the
controller PCB), monitors the motor position (Figure 2-10). The optical encoder
incorporates an emitter section, two codewheels, and a detector section. Each codewheel
has a pattern photographically plated on it. As the motor shaft revolves, the codewheels
rotate with respect to the emitter and photodetector sections, causing the light beam to
be interrupted by the pattern of spaces and bars on the codewheels. The detectors are
positioned such that a light period on one photodetector corresponds to a dark period on
the other photodetector. The optical encoder was designed so the final outputs from the
two photodetectors (channels) are in quadrature with each other (90 degrees out of
phase). Using the photodetector outputs, a decoder in the motor controller circuit tallies
“quadcounts” and can determine the piston’s direction and speed.
• The cylinder inlet and outlet check valves are one-way valves located at the end of the
piston/cylinder. The inlet check valve opens to let mixed gas fill the cylinder during piston
retraction; it seals to prevent the cylinder contents from escaping through the gas inlet
system during gas delivery. The outlet check valve lets gas exit the cylinder during gas
delivery; it seals during piston retraction. The inlet check valve has a
translucent (clear)
leaf in a white housing, while the outlet check valve has a translucent (clear) leaf in an
blue housing.
• The cylinder pressure transducer (on the pressure solenoid PCB), a gauge type, measures
the cylinder pressure. The ventilator uses these cylinder pressure readings in various
calculations, including cylinder leak, compliance compensation, during oxygen mixing,
atmospheric pressure, and PEEP maintenance. The transducer is autozeroed via an
autozero solenoid (see below).
• The cylinder pressure transducer autozero solenoid is a three-way valve. It autozeroes the
transducer at power-on, once a minute for the first 10 minutes, and hourly thereafter. The
transducer is autozeroed by venting to atmosphere. A muffler reduces the noise from the
venting gas.