Inflight Icing Detection Mechanism
Magnetostrictive oscillation – magnetic and mechanical energy coupling
The inflight icing detection 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 sensing probe. The added mass of accreted ice causes the frequency of the sensing probe to decrease by the laws of classical mechanics. A 0.5mm (0.020’’) thickness of ice on the probe causes the operating frequency of the probe to decrease by approximately 130 Hz. The freezing rain sensor onboard software monitors the probe frequency, detects and annunciates any frequency decrease. At the same time, the internal probe heater power is applied until the frequency rises to a predetermined set-point plus an additional delay factor to assure complete de-icing.
Once de-iced, the sensing probe cools within a few seconds and is ready to sense ice formation again. When ice forms on the sensing probe again to the point where the MSO frequency decreases by 130 Hz, the sensor de-ices itself again. This cyclic process is repeated as long as the freezing rain sensor remains in an icing environment. The ice signal activates at 0.5mm ice accretion and stays on for 60 seconds after the end of the icing encounter. Specifically, when the output is activated, a 60-second counter is reset. In effect, the output stays on for 60 seconds after the beginning of the “last” icing encounter. The Status output indicates whether the freezing rain sensor is correctly functioning using tests that are described in more detail in the following sections of this document.