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Revision E
Equipment Overview: Theory of Operation
driving R312), with a 50:1 ratio, 15Vin = 300mV out, hence 15V full scale). Since
one terminal is always zero (grounded) and the other is driven with a variable duty
cycle between zero and 12V, the feedback signal is positive regardless of motor
direction. C259 filters the switching noise from feedback voltage.
NOTE
The frequency and duty cycle of the motor drive signal are random. This serves
to reduce EMI by spreading any emitted noise across a wide frequency
spectrum. An RC snubber (R273 and C260) suppresses ringing on the motor
lines.
Larry maintains precise motor speed control by comparing the frequency of the
tachometer pulse train emitted by the motor's integral encoder to an internally
generated reference frequency derived from Larry's resonator. Larry processes
motor position information on both edges of both encoder signals for a total of 64
loop correction cycles per rotation of the motor shaft. This high angular sampling
rate allows Larry to achieve accurate and smooth speed regulation down to zero
speed.
Paper Jam / Pull Detection
Larry monitors the servo error variable to determine whether the servo loop is
closed. If the error variable saturates "on" for more than a predetermined time it is
assumed that the paper drive torque has become excessive, or the motor has stalled.
This condition is reported as a Paper Jam Error.
Similarly, if the servo error variable saturates at "off" for more than a predetermined
time, it is assumed that the someone is pulling on the paper with a force that exceeds
the paper drive system torque, and as a result paper speed has been pulled out of
regulation. This condition is reported as a Paper Pull Error.
Cue Hole Sensor
Cue and out-of-paper conditions are sensed via the thermal print head's integral
optical cue sensor. Larry monitors the cue sensor's logic output.
Cue Hole Detection
Larry monitors the output of the cue sensor to detect the presence or absence of
paper under the sensor, and hence the absence or presence of cue holes.
Paper Tracking Fault Detection
Larry monitors the cue sensor for abnormally long paper travel without
encountering a cue hole. This condition is reported as a Paper Fault.
Paper Out Detection
Larry reports excessive paper travel without sensing paper as a paper out condition.
Analog Inputs
Larry digitizes four analog inputs at eight bits resolution each. Two inputs handle
external analog signals, such as those produced by ergometers or analog output
blood pressure monitors. Thermal printhead temperature is measured for use in
compensating strobe pulse width to maintain constant print density over a wide
range of thermal printhead temperatures. The output of the thermal printhead pixel
test hardware is also digitized to allow the resistance measurements on individual
print elements.
MAC 5500 resting ECG analysis system
2020299-020
2-31
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