Improvements in or relating to acoustically operated ice detectors

1,117,664. Ice detectors on aircraft. GLASS DEVELOPMENTS Ltd. 6 June, 1966 [8 June, 1965], No. 24225/65. Heading B7W. [Also in Division H4] Apparatus for indicating the presence, thickness and rate of formation of an ice layer on, for example, an aircraft comprises an ultrasonic transmitter and a receiver spaced from each other and acoustically coupled to an ultrasonic-wave conductor, the presence of an ice layer on the conductor causing a change to occur in ultrasonic signals reaching the receiver. In the embodiment of Fig. 1, ultrasonic waves, preferably pulsed shear waves, pass from transmitter 12 to receiver 16 via the path 30-32-34 in a brass plate 10 faired into an aircraft wing. When an ice layer 36 is present, the waves also traverse path 30-32-38-40-42. The difference in path length introduces a phase difference between waves received via the two paths and the signal output level from receiver 16 may be calibrated in terms of ice thickness. Reduction of the output level below a preset value may cause energization of heaters for de-icing the aircraft and plate 10, and the frequency at which the de-icing cycle is repeated provides a quantitative indication of the icing rate. Control circuitry, which may be coupled cyclically to a plurality of ice sensors (transmitter/receiver pairs) and which also provides time-gating of the receiver output signal to eliminate spurious signals, is described (Fig. 3, not shown). In a second embodiment the path length in plate 10 is such that the ultrasonic waves undergo multiple reflections at the plate boundaries (Fig. 2). Transducer details.-The transmitter 12 and receiver 16 comprise lead zirconate or barium titanate plates on methyl methacrylate coupling blocks 14, 18 shaped so that wave mode conversion, between longitudinal mode in the blocks and shear mode in plate 10, occurs at the interfaces between plate 10 and the blocks. The blocks are further shaped, as at 24, and/or may be coated on their free surfaces with acoustically absorbent rubber to reduce internal reflections. The transducer plates may be backed with absorbent acoustic loads 26, 28 to reduce the length of transmitted or received acoustic pulses.