GE HEALTHCARE E-PRESTN, Technical Manual

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Respiratory Modules, E-sCAiOVX, E-sCAiOV, E-sCAiO, E-sCOVX, E-sCOV, E-sCO
8 - 13
2062973-004
NOTE: The sensor is assembled in the module using flexible suspension to prevent the
mechanical vibrations of the gas pump and cooling fan from disturbing the Oxygen
measurement. All gas lines to the sensor must also be carefully assembled so that they do not
pick up mechanical vibrations of the module mechanics.
Pneumatic System
The pneumatic system, together with the gas sampling system of the module creates the gas
flows and pressures needed for the oxygen measurement and protection of the microphones
from excessive pressure. About 30 ml/min flow of sampled gas comes to the In connector on
the MiniOM sensor. Room air is drawn to the Ref input of MiniOM also at 30 ml/min rate. About
75% of these flows are conducted to a pressure equalization chamber so that only about a
8 ml/min flow of the two gas streams continue into the air gap of the magnet. All the internal
gas flows finally get to a volume enclosed by the sensor board and the sensor body, and then
flow out through the
Out connection of the sensor. Some of the gas channels and flow
restrictors are integrated into the preamplifier electronics board utilizing the multi-layer
structure of the LTCC (Low Temperature Co-fired Ceramics) circuit board technology.
NOTE: It is very important to prevent dust or liquids from getting into the pneumatic circuit of
MiniOM and thus, the gas connections should always be closed with a protecting cap when the
sensor is not connected to the module pneumatics.
Amplifier Board
The amplifier board located in the sensor has two electric microphones for the differential
detection of pressure pulses generated in the magnet's air gap. The microphone signals are fed
to two identical signal conditioning channels with a band-pass filter and a digitally controlled
amplifier. The voltage gains of the amplifiers are set during factory calibration so that the
responses of the microphone channels match in spite of differences in microphone's
sensitivities. The amplifier board also has an amplifier for the thermistor measuring the
temperature of the magnet.
MiniOM Board
The MiniOM board has five functions
•
Drive the magnet coil.
•
Convert the microphone and temperature signals into digital format.
•
Filter digitally the microphone signals and perform the RMS-conversion.
•
Communicate digitally with the module CPU.
•
Store factory calibration data in permanent memory and communicate them to the
module CPU.
The module CPU provides the coil drive and communication enabling signals and also clock
signal for MiniOM board. The FPGA takes care of the coil drive and has also back-up clock in
case of CPU clock does not work. The FPGA takes care of the A/Dconversions which are
performed with a serial controlled SAR A/D-converter.
The digital band pass filtering and RMS conversion of the microphone signals is made with
FPGA circuit controlled by the VHDL code stored in the circuit. In order to filter out the
disturbances caused by acoustic noise, mechanical vibration and amplifier noise, the band
pass filters are designed to have as narrow a pass band as possible without slowing down the
filter's response to changes in the amplitude of the 164 Hz signal.
The FPGA circuit takes care of the digital communication between the miniOM sensor and the
module CPU.