Patent GB-575257-A (GB)
Title: Electrical apparatus for measuring the thickness of ice on a surface
IPC: B64D15/20
Filed: 1943-07-07
Abstract: 575,257. Capacitance measurements ; determining physical qualities of materials. HONEYWELL-BROWN, Ltd. July 7, 1943, No. 11069. Convention date, Oct. 24, 1942. [Class 37] [Also in Groups XXXII and XXXIII] A device for responding to the formation of ice comprises a condenser plate and insulating means mounting the plate and formed to constitute a continuation of the surface thereof, and means for making an electrical connection to the plate. Moulded in the centre of a curved disc 10, Fig. 2, of insulating material is a metal disc 11 provided with hexagonal perforations over its entire surface. The insulating disc 10 is bent to the shape of, and mounted flush with, the leading edge of the wing 13 of an airplane. The disc 10 is mounted within an opening in the wing 13 by a mounting ring 14, which may be moulded directly in the insulating ring 10, and by a spacer 15 and backing ring 16 all of these being fastened to the wing by screws 17, 18. The metal disc 11 forms one plate of the condenser of the pick-up unit and is insulated from any ice which may form on the outer surface of the disc 10. A connector 19 connected to the plate 11 and moulded into the member 10 is connected to an amplifier unit by a cable. The wing 13 and mounting ring 14 are earthed to the plane and serve as the other plate of the condenser, the capacity of which is determined by the dielectric properties of the insulating disc 10 and the thickness of the ice which forms on the outer surface of the disc 10 and wing 13. The pick-up unit 23, Fig. 3, may be mounted in the wing of the plane directly behind the de-icing boot 22 and is connected by a cable 25 to an amplifying unit 24, which is mounted on the floor of the wing 13 by U-shaped rubber shock absorbers 26. A modified form of pick-up unit is shown in Figs. 4 and 5. A curved disc 30 of insulating material is pre-formed to the shape of the leading edge of the wing 31 of an airplane. Mounted behind the disc 30 is a smaller metal disc 32 forming one plate of the condenser, the other plate being formed by the wing of the plane and a metal mounting ring 33 into which the disc 30 fits. The ring 33 is connected to a backing ring 34 and the entire unit is mounted in an opening in the wing 31 and flush therewith by screws 35. A thin sheet 36 of insulating material is placed over the rear face of the insulating disc 30 for insulating and protecting the condenser plate 32. A second condenser plate 38 is mounted adjacent to the ring 33 by pins 39, spacers 40, and insulating sleeves 41. Between the plate 38 and the ring 33 is a member 42 composed of the same material as the disc 30. The member 42 has a central opening to receive a piece of material from which the de-icing boot is made. This second condenser forms a temperature compensator for the first condenser. The detecting and amplifying unit 45 is mounted on a base 46 which is supported on the ring 33 by means of brackets 47 and U-shaped rubber shock absorbers 48. Another form of pick-up unit is shown in Figs. 8 and 9. The body 60 is of rubber and is moulded with a flat surface 61 and a raised surface 62. Condenser plates 63, 65 of wire screening or braided wire are moulded into the body 60, as also is a third condenser plate 68. The plate 65 is connected to the central conductor 66 of a coaxial cable 67 and the plate 68 is connected to the central conductor 70 of a coaxial cable 71. The plate 63 is connected to the outer conductor of the cable 71, which is earthed at 73. The flat surface 61 is cemented to the leading edge of the wing of an airplane, the plates 63, 65 forming the ice-responsive condenser and the plates 68, 63 the temperature compensating condenser. Another construction is shown in Figs. 12 and 13 and is mounted on an external metallic surface 80. An insulating spacer 81 and metal backing plate 82 are fastened by bolts 83 which also electrically connect the plates 80, 82. In the plate 80 is an oval opening 85 and a similar but smaller concentric opening 86 is formed in the spacer 81. This opening is provided with a shoulder on which is seated an oval metal plate 87 forming one plate of a condenser of which the other plate is the plate 80. A sheet of mica, celluloid &c. is placed within the opening 85 in the plate 80 and lies flush therewith to protect the plate 87 from moisture or ice forming on the surface. The backing plate 82 is provided with an oval opening 89 and the spacer 81 has a smaller opening 90 concentric with the opening 89. The opening 90 is formed with a shoulder in which a metal plate 92 is seated. The openings 86, 90 are connected by a groove 93 to accommodate conductors 94, 95 connected to the plates 87, 92. The condenser formed by the plates 87, 80 varies in capacity in accordance with the thickness of ice formed on the plate 80, whilst the condenser formed by the plates 92, 82 compensates for temperature changes. A circuit arrangement for detecting and amplifying changes in capacity of the pick-up condenser for controlling an indicating meter, signal light or buzzer, or control mechanism for a de-icer unit is shown in Fig. 14. The ice-responsive condenser 100 and the temperature compensating condenser 104 are connected in a bridge circuit and have trimmer condensers 106, 107 in parallel with them. The bridge circuit also includes an inductance coil 108 earthed at its centre point 109 through a resistance 110. Power is supplied to the bridge circuit through a coil 116 coupled with the coil 108. The coil 116 is connected to the grid 119 of a pentagrid valve 105, this grid acting as a plate. The grid 120 is connected to the lower end of the coil 108 to form a feedback so that the system operates as an oscillator. The control grid 124 is connected to the centre tap 109 of the inductance coil 108 and the plate 128 is connected in circuit with a milliammeter 130 and relay winding 132. The bridge may be adjusted so that when there is no ice formation the valve passes a small amount of current, which causes the needle of the instrument 130 to move from an " off " position to a zero position. The relay 132 is adjusted so that it operates when a predetermined thickness of ice has been formed and closes the circuit of a signal light or buzzer 137. A manual switch 138 may also be closed to energize a control device 139 for the de-icer. The effect of temperature on the resistivity of the ice causes a component in the voltage wave impressed on the grid 124 which is in quadrature with that caused by changes in the dielectric. This effect is partly eliminated owing to the valve 105 only passing current when both grids 120, 124 are positive with respect to the cathode. The coil 108 may be divided into halves, the coil 116 being placed between them and coupled more closely to one half than to the other and the junction of the half-coils being earthed directly. The control grid 124 is then connected to the upper end of the upper half of the coil. A thermostat may control the power supply, so that power is only supplied when the temperature is within the range of icing conditions.