Siemens SC 7000 Service Manual page 43

SC 7000 and SC 9000XL Patient Monitors
9.3 Transducers
9.4 Pneumatic Controls
9.5 Safety timer
9.6 Logic gate array
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thermal transients to settle. Either V1 or V2 is now modulated to control the
deflation rate. The choice of V1 or V2 and the initial pulse width is made
based on the inflation cycle. The chosen valve is modulated and the pulse
width (open time) is continuously adjusted to provide a constant deflation
rate. If initial deflation was started with V1 the software may determine that
it needs to switch to V2 to maintain proper deflation. In any case when the
measurement cycle is complete, V2 is opened fully (de-energized) to allow
for rapid deflation.
The measurement pressure transducer is DC coupled to a 16 bit A/D
converter so that cuff pressure is measured with adequate resolution to
detect blood pressure pulses.
The overpressure transducer has two threshold settings. The adult setting
is 300 ±30 mmHg and the nominal neonatal setting is 158 ±7 mmHg. Both
transducers share a common manifold and are mounted on the main PC
board.
The P1 control provides 3 functions.
• It limits current to the pump when the pump starts to prevent power
supply overload.
• It rapidly decelerates the pump when the pump is shut off, by applying
a low resistance across the motor.
• It provides a closed loop speed control for low speed neonatal operation.
A relatively high pulse voltage is used to drive V1 and V2 to get quick
response. This pulse lasts for approximately 2 milliseconds after which time
the valve voltage is lowered to a holding value. At the end of the valve "on"
time period, the valve voltage is allowed to reverse and the energy stored
in the solenoid inductance is rapidly released into a relatively high voltage
clamp circuit.
P1 and V2 are supplied by a redundant power switch so that, under fault
conditions, they can be de-energized.
The software limits measurement time to 119 secs for adult mode, 89 secs
for neonatal mode and 59 secs for French neonatal mode. A safety timer
circuit monitors current in P1 and V2, and if due to some failure (hardware
or software), P1 or V2 remain activated for more than 120±1 seconds in
adult mode, 90±1 seconds for neonatal mode or 60±1 seconds in French
neonatal mode, the circuit latches on, causing the redundant power switch
to P1 and V2 to switch off. When the safety timer latch has been set, V1 is
opened as an additional safety feature. Only recycling the monitor resets
the safety timer latch. The safety timer circuit is functionally independent of
the logic gate array.
When the unit is powered up, the safety timer is de-activated until the pump
is started the first time. This feature allows service calibration without
triggering the safety timer. Once the pump has been activated the timer
circuit becomes functional.
The main FPGA provides the following control functions for the pneumatics
and the communications.
• Clock generation for safety timer
• 12 bit 20 Hz PWM and pulse control for V1 and V2
• Pump control
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Service Manual
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