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9
Design and Theory of Operation
SIGMA Manual Number 41019
Revision AA
mechanisms eliminate the possibility of incorrect sensing motor velocity after a detectable single
fault condition.
Motor and Gear Drive
SIGMA Spectrum employs a 16 mm DC motor, encoder and planetary gearbox. The output shaft of
the gearbox is coupled to the pump drive camshaft by two identical gears, thereby creating a 1:1
drive ratio between the camshaft and output of the gearbox. Motor speed is controlled by a closed-
loop pulse width modulation (PWM) drive system that derives its power directly from the battery or
9VDC source when the AC power adapter is connected.
Fluid Temperature
Physical properties of administration set tubing are subject to the effects of fluid temperature. In gen-
eral, colder temperatures can cause negative flow rate errors in an uncompensated system. Similarly,
hotter temperatures can cause positive flow rate errors in an uncompensated system. The SIGMA
Spectrum employs a temperature compensation system to maintain flow rate accuracy over the oper-
ating temperature range of the device. Approximate fluid temperature is measured by two negative
temperature coefficient thermistors, one proximal to the pumping mechanism and one distal to the
pumping mechanism. The proximal sensor is housed by the upstream sensor assembly. The distal
sensor is located in the lower tubing guide, just below the distal occlusion sensor. Both sensors are
calibrated at the factory, and are used to check each other for single fault failure conditions. The out-
put of both thermistors are filtered and fed to A/D converters for processing by Spectrum processors
for compensating pump motor speed to maintain flow rate accuracy.
Upstream Sensors
Optical Tube Sensor
The optical tube sensor consists of a light emitting diode (LED) emitting infrared light and a photo-
transistor. The LED and phototransistor are located at the bottom of the ultrasonic sensor housings,
facing opposite to each other. This position is marked as load point 2 and is indicated in red color
next to the “direction of flow” label inside the pump door. The optical tube sensor detects that an IV
set is loaded at load point 2 to assist in proper loading of the IV set into the pump. It must recognize
the presence or absence of the IV tube. The sensor is activated only when the door is open. If the IV
tube is properly placed in the space between the LED and phototransistor, the infrared light is dif-
fracted and the phototransistor outputs a high voltage. If there is no IV tube present, the phototran-
sistor receives continuous infrared light and a low voltage is output.
The software monitors the output voltage and displays a green bar with a check mark to indicate a
properly loaded tube or a red bar with loading instructions when there is no tube or if the tube is
improperly placed in the sensor. This sensor is calibrated through a biomed option.
Proximal Occlusion Detection
The proximal occlusion sensor consists of two pairs of ultrasonic transmitters and receivers. A
“pusher” connected to the door assembly assures that a constant force is exerted to push the admin-
istration set tubing against the sensor housing. The transmitted ultrasonic signal is a constant wave