Mallinckrodt NELLCOR NPB-4000 Service Manual page 112

Section 12: NIBP - Theory of Operation
12.3.1 Pressure Transducers
12.3.2 Filtering the Oscillatory Signal
12.3.3 Power Supplies
12.3.4 Over-Pressure Detector
12.3.5 Pump and Valve Drivers
12-4
Pressure transducers PS1 and PS2 convert pneumatic pressure levels to voltages
in the range from 0.33 volts at 0 mmHg to 4 volts at 300 mmHg. Transducer
PS1 is used to sense the main cuff pressure. Transducer PS2 is used as a backup
cuff pressure sensor, from which over-pressure warning signals are obtained.
Both transducers outputs are smoothed to remove the effects of pumping
pulsation. The output of PS1 is identified as signal NIBPPRSR1, while that of
PS2 is NIBPPRSR2.
Amplifiers U16-1 and U16-14, in combination with the lumped resistance and
capacitance values of the passive components, form a band pass filter that
separates and amplifies the oscillatory signals OSC from the deflating cuff
transducer signal NIBPRSR1. Amplifier U16-8 provides hysteresis.
Battery power, VBATTP, connected to the board at J3, pins 13 and 14, is
regulated in U15 when that component is enabled by the NPANPWR analog
power On/Off signal input to U10-4. The regulator accepts a battery voltage of 6
to 8 volts and delivers regulated +5 volts to power the pressure transducers and
biasing circuitry for the operational amplifiers.
Pump and valve drivers are controlled by the NPPVEN pump and valve enable
signal that is generated in U16-7 level-detector amplifier circuit. One input to
that amplifier is the NIBPRSR2 signal from transducer P2. The other is a value
determined by voltage divider R68 and R64, and feedback resistor R61.
The pump and valve drivers are enabled only when cuff pressure is below
nominal 330 mmHg for adult mode, and below 165 mmHg for neonatal
measurements.
Three-way valve V1 coil is powered by VPS that is switched in series with the
coil through n-channel MOSFET Q10-6. Switching action of Q10-6 is
controlled by the input signal NP3WYV, that is gated by the enabling signal
NPPVEN at switch U23. The VPS signal is developed from VBATTP through
Q11-6 that is also controlled by NPVEN gating transistor Q6. Diode D5 acts to
suppress voltage spikes. Resistor R101 ensures that Q10-6 will be OFF in case
of an open to the input line. The valve is either fully closed, or fully open.
Control valve V2 coil is powered by VBATTP and controlled by switching n-
channel MOSFET Q10-7 in series with the coil. Q10-7 is controlled by input
signal NIBPCNTLVLV. Pulse width modulation (PWM) of a five-volt level is
used to operate the control valve. No-voltage results in a fully closed valve. As
the duty cycle of the PWM is varied from 0 to 100 percent, the valve changes
smoothly from fully closed to fully open. Repetition rate for the PWM is at least
9 kHz. Diode D4 suppresses voltage spikes, and resistor R100 assures that
Q10-7 will be OFF if the input line should open.