Hill-Rom Isolette C2000 - Scale Interface; Fan Control;Feedback Circuit; Temperature Measurement

Theory of Operation
Chapter 3: Theory of Operation
Seale Interface
Connector J3 provides the interface and power for the scale module. The scale
module supports serial data communication. The signal SCCLK is used for
scale communication only and is normally high when the scale is disconnected.
The signal CDATA is bi-directional and is normally in the low state when the
scale is not connected.
Fan Control/Feedback Circuit
To drive the DC fan on the sensor board, O1 is pulsed on its gate by signal
FANON at a 50% duty cycle at approximately 48 Hz to maintain proper speed
and to increase fan life. Every 4 seconds the microcontroller asserts FANON
for 42 msec, and the fan pulse detection begins.
The pulse detection is handled by U1A, which through resistors R3 and R7
sample the current spikes from the fan produced across R1. The amplifier
operates as a differentiator, providing high gain for the current spikes. This
signal is then rectified and filtered by D1 and C16 and then fed to the A/D
converter as signal FANPUL. This effectively provides a semi-DC level as a
function of the fan rotation.
Temperature Measurement
The temperature data acquisition circuit starts with analog multiplexer U6 and
U5, each
allowing an 8 to 1 signal switching. The multiplexer channel
selection is accomplished from the microcontroller by the signals MSELO,
MSELI, and MSEL2. Each multiplexer output can be inhibited by either the
signal TMISEL or TM2SEL, depending on the multiplexer (only one
multiplexer is active at a time). With each multiplexer output into the
amplification under control, this data acquisition can then be viewed as 16 to 1
analog temperature selector. The microcontroller selects a new temperature
channel every 21 milliseconds.
Resistor R14 provides the constant voltage drive required for each thermistor
as it gets selected from the appropriate multiplexer. This voltage is amplified
by a factor of 2.1083, and then sent to the A/D converter as signal TCOM.
To provide automatic calibration of the circuit (and eliminate temperature and
aging drifts), precise calibration values are introduced into each multiplexer.
These values are provided by R44 and R45, which are read through the
multiplexers and are used in software processing to eliminate the gain and
offset errors of each multiplexer/amplifier combination. These values equate to
120.87°F (49.37°C) and 72.72°F (22.62°C), which allows for precise circuit
calibration.
lsolette® Infant incubator Service Manual (man223rf) Page 3- 13