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Document Ref 942175-001 Rev 3
TQM for Fuel Dispensers
Component Technical Manual
HALL EFFECT SENSORS (DIRECTION)
Two sensors (sensors A and B) are placed above the outer ring of the disc. They
are used to indicate the direction of the disc. The diagram above indicates the
output of the two sensors when the disc rotates clockwise. Each transition of
the signal coming from sensor B represents 0.5 cl.
This block checks the input and output of the output driver during the 400
microsecond pulse. If the output line is high (idle state) while the input of the
driver is activated, it will send a “not equal” signal to the error block.
The inputs of the comparator used for monitoring lines A and B are provided
with a digital filter which will remove glitches of less than 32 microseconds
from these two signals.
HALL EFFECT SENSOR (FAULT DETECTION)
A third sensor (sensor C) is placed above the inner ring of the disc. As this ring
contains 10 north and 10 south polarised magnetic parts, the output frequency
of this sensor will be the half of the frequency coming from the two other sensors.
This difference in frequency makes it possible to distinguish normal operation
from sensor defects or disc oscillations.
DIGITAL FILTER
This part will remove glitches shorter than 8 microseconds from the sensor
signals.
SEQUENCE DETECTOR
The sequence detector identifies the different movements of the disc by
evaluating the three sensor signals.
BACKWARD FLOW COUNT (CENTILITRES)
When the disc turns counter-clockwise, the sequence detector generates
backward count pulses. The backward flow count buffer keeps up with the
amount of backward count pulses. When the buffer reaches 20 cl (40 pulses), it
generates an overflow message. If the disc turns clockwise, the sequence detector
generates forward count pulses. The backward flow count buffer will decrease
with the amount of forward count pulses until it reaches
0 cl. At this point, the buffer will not decrease any more and the switch is
closed so that the timer receives forward count pulses on its trigger input.
TIMER (400 S)
A pulse on the trigger input of the timer will start the one-shot 400 microsecond
timer. During the 400 microseconds, the timer will activate the output driver
for line A (or B). Every 0.5 cl, the timer alternates between lines A and B.
Therefore if there was a pulse on output line A, then 0.5 cl later, it will be
followed by a pulse on output line B and vice versa.
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