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the frequency decreases, and it is this frequency change that the
microcontroller annunciates in the form of Ice Signal #1.
1.5
Heater Control
The heater control turns the probe heater on and off as
commanded by the microcontroller and monitors the actual heater
state (ON or OFF) for verification by the microcontroller. Two
outputs are required from the microcontroller to turn on the
heater. This minimizes the possibility of an unintended heater ON
condition. The heater control also monitors the state of the heater
and provides feedback to the microcontroller so that it can be
determined whether the heater is on or off.
1.6
Drive Coil
The drive coil modulates the magnetic field of the
magnetostrictive oscillator and causes an ultrasonic axial
movement of the probe.
1.7
Feedback Coil
The feedback coil senses the movement of the probe and when
employed in the probe oscillator circuit, completes the feedback
portion of the MSO.
1.8
Heater
The probe heater de-ices the probe. It is activated when the
nominal icing trip point of 0.020” is reached and is turned off
five seconds after the MSO has returned to at least 39,970 Hz
(the additional five seconds allows the strut probe time to shed
the de-bonded ice). The maximum heater ON time is 25 seconds.
If the probe frequency has not returned at least 39,970 Hz by that
time, a de-ice failure is declared and the heaters are turned off.
An open circuit of the heater is detected by the microcontroller.
1.9
DC Power Supply
The DC power supply provides 24 VDC for the heater circuitry.
Internal circuitry converts the 24 VDC input power to 5 VDC for
use by the microcontroller and associated circuits. It employs a
large input capacitor to provide enough time between detection of
input power loss and actual loss of DC power, for the
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