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Purpose
This document provides detailed information about the
Rosemount Aerospace model 0871LH1 Freezing Rain Sensor for
use in ground-based meteorological applications. Topics covered
include requirements, qualification categories and methodology,
and detailed design information.
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General
The Rosemount Aerospace 0871LH1 Freezing Rain Sensor is a
one-piece unit that detects the presence of icing condition.
Twenty-four volts DC input power is provided to the freezing
rain sensor. The freezing rain sensor outputs include ice detection
indication and fault status indication. These outputs are provided
through an RS-422 interface and discrete outputs. One freezing
rain sensor is used on each station and provides the primary
means of ice detection. The ice signal is used to indicate to the
operator that an icing condition exists so that appropriate actions
can be taken.
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Detailed Principle of Operation
The freezing rain sensor uses an ultrasonically axially vibrating
probe to detect the presence of icing conditions. The sensing
probe is a nickel alloy tube mounted in the strut at its midpoint
(node) with one inch exposed to the elements. This tube exhibits
magnetostrictive properties: it expands and contracts under the
influence of a variable magnetic field. A magnet mounted inside
the strut and modulated by a drive coil surrounding the lower half
of the tube provides the magnetic field.
A magnetostrictive oscillator (MSO) circuit is created with the
above components and the addition of a pickup coil and an
electronic comparator. The ultrasonic axial movement of the tube
resulting from the activation of the drive coil causes a current to
be induced in the pickup coil. The current from the pickup coil
drives the comparator that, in turn, provides the signal for the
drive coil.
The oscillation frequency of the circuit is determined by the
natural resonant frequency of the sensor tube, which is tuned to
40 kHz. With the start of an icing event, ice collects on the
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