11
Display
The display is a custom 3½ digit low power LCD. The seven segments of
three digits are driven directly by ports A, B and C of the microcontroller
with port PA0 driving the back plane. The decimal point, “1” digit, and the
other legends are driven by the 8 bit shift register, IC2, which is controlled
by the microcontroller via the SPI interface. The back plane is driven by a
square wave of nominally 60 Hz. The individual segments are driven by a
similar square wave, which is in phase with the backplane when the
segment is off and 180 degrees out of phase when the segment is on.
Serial Interface (Micro Plus unit only)
Serial communications are established from the microprocessor to the
external RS232 port using its serial communications interface (IC6)
Transducer interface
The rotation of the vane inside the transducer is sensed by the interruption
of an infrared beam produced by the LED and sensed by the
phototransistor. The LED is controlled by the emitter follower (TR1) and is
only energised during a spirometry test when the BLOW legend on the
display is showing.
The light beam is detected by the phototransistor, which is in common
emitter configuration. The load resistor is factory adjusted using VR1 to
give the largest collector swing when the turbine is subjected to a flow or
air at 37 degrees Celsius saturated with water vapour. VR1 is factory set
and should not be adjusted by the user. The signal at the collector is
conditioned by the action of the schmitt inverter (IC3) and applied to the
pulse capture input of the microcontroller (Pin 42 of IC1). The
microcontroller calculates the expired volume and flow from the number
and rate of received pulses.
Calibration
The sensitivity of the Micro Medical digital volume transducer depends
only upon the fixed geometry of the stator and is inherently stable. The
calibration will be unaffected by any dirt which may build up on the stator
due to poor cleaning procedures. However, physical damage to stator
may adversely affect calibration and in this instance the unit should be
returned to Micro Medical for transducer replacement and re-calibration.
At Micro Medical calibration is performed with a computer controlled
waveform generator, approved by the American Thoracic Society.
