GB2212913A - Ice warning detector - Google Patents

Abstract

A refractive ice detector comprises a block 11 through which a beam 15 passes from LED1 to photodiode D1 with total internal reflection at surfaces 12 and 13. Any ice or mush builds up at 16 and/or 17, allowing refraction of light out of the beam 15. The reduced intensity is detected by D1, and photodiode D2 detects some of the scattered light. The ratio of the signals from D1 and D2 is determined by ratio circuit 48. LED1 is pulsed by pulse generator 41, and diodes D1 and D2 feed chopper amplifiers. Diode D1 also detects the ambient light level while LED1 is off, allowing adjustment for moderate ambient light and disabling the system for high ambient light. The electronics may be separated from the block 11 by a fibre optic cable. Further sensors may be provided to detect the coefficient of refraction of de-icing fluid and hence the dilution of antifreeze in it, and a temperature sensor T1 may be provided. From the concentration of antifreeze, its rate of dilution, and the temperature and its rate of fall, to time left for an aircraft to reach its icing point may be estimated. <IMAGE>

Description

Ice Warning Detector The present invention relates to detection systems for warning of icy conditions, particularly though not exclusively for use on aircraft on the ground.

The presence of ice on an aircraft on the ground and waiting to take off is clearly hazardous, and means for detecting ice or allied conditions are therefore desirable. Such means are subject to a number of requirements. The detection system should have high reliability, and hence it should be simple and have a minimum of moving parts. It should be small and easily fitted into the wing of an aircraft. Preferably also it should be relatively easy to fit the device into existing aircraft.

Accordingly the present invention provides a refractive ice detector comprising a light source and a light detector coupled to a block of transparent material, the block having a surface on which ice mdy form and being shaped such that light passes from the light source to the light detector along a light path which includes a total internal reflection at said surface in the absence of ice, but the light is scattered and/or transmitted through that surface in the presence of ice.

The basic refractive ice detection system, although designed for aircraft, can be used to detect a build up. of ice on a surface in dnf situation. The scope of this invention should not be limited to aviation.

In a development of the invention for use with aircraft, there are also provided further sensor means for sensing the coefficient of refraction of an antifreeze-contAining liquid on the detector, temperature detecting means for detecting the temperature of such liquid, and processing means for determining from the coefficient of refraction and temperature their rates of change and estimating, from the coefficient of refraction and temperature and their rates of change, the time left before the icing point is reached.

A refractive ice detection system in accordance with the invention, and various modifications and å development thereof, will now be described, by way of example, with reference to the drawings, in which; Fig; 1 is a general diagram of the basic detection. system; Figs. 2 and 3 show alternative shapes for the detector block of the system; Fig. 4 shows 8 modified detector using fibre optics; Fig. 5 is a general diagram of the development; Fig. 6 shows a detail of a further sensor of the development; and Figs 7 and 7A show a modification of the development.

Referring to Fig. 1, the detection system comprises a sensor unit 10 and usoociated electronic circuitry 20. The sensor unit 10 consists of a block of transparent material 11 having attached to it a light emitting diode LED1 and two photo diodes D1 and D2. This block 11 is mounted in the upper surface 13 of an aircraft wing; a suitable location is where the wing surface is roughly horizontal, roughly a third of the width of the wing back from the leading edge.

A plurality of such detectors may be mounted at suitable intervals along the length of each wing.

The block 11 is rectangular with two corners cut as shown to provide two optical surfaces 12 and 13; suitable dimensions are in the region of 35 x 25 mm, and 6 to 12 mm thick. LED1 (which may be a laser LED) is attached to it as shown and arranged to produce a collimated beam 15 which passes along the path shown to diode D1. The orientations of the surfaces 12 and 13 are selected in relation to the refractive index of the material of the block 11 so that the beam 15 undergoes total internal reflection at those surfaces, but so that the total internal reflection condition is destroyed by the presence of watery material on those surfaces.

The material of block 11 is chosen to be substantially transparent to the light emitted by LED1, and the diode D1 is chosen to be sensitive to this light (as also is diode D2). The material of block 11 may be chosen to be absorbent of light of different wavelengths from that of the beam 15. The colour of the block 15, i.e. the waveband for which it is transparent, may be chosen to minimize interference from external light sources. (The term "light" here includes infrared and ultraviolet light as well as visible light.) Such interference can be reduced by two mechanisms; if the transparent waveband of the block 11 is narrow, then ambient light outside that waveband is excluded; and the waveband can be chosen in a low intensity region of the ambient light spectrum.

In icing conditions, a build up 16 and 17 of watery material - ice or mush - will occur on the two surfaces 12 and 13 as indicated. This material will alter the optical properties of the surfaces 12 and 13 so that, instead of total internal reflection occurring at these surfaces, a considerable amount of light from the beam 15 passes through these surfaces into the watery material. The intensity of the beam 15 as detected by the diode D1 is therefore greatly reduced.

The second diode D2 is attached to the block 11 at a point where it receives substantially no light from the beam 15. However, it receives ambient light from outside the device, and also receives light scattered from the beam 15 by any build up of ice or mush 16 and 17.

The block can have any convenient shape providing one or more surfaces at which totdl internal reflection occurs when the block is dry but not when the surface has watery material against it. Figs. 2 and 3 show two further possible shapes. It is preferable for the surface(s) at which total internal reflection occurs to form an oblique angle with the wing surface. This is because a smooth thin layer of water on the surface will not have a substantial effect on the occurrence of total internal reflection. (Some of the light in the beam will pass into the water layer, but will be refracted to a' more oblique angle, and will therefore be totally internally reflected from the water-air interface.) If this layer freezes to a thin sheet of ice, the same effect will occur. It is therefore desirable for the surface to form a slight valley with the wing surface, so that a build-up of watery material in the valley can occur as shown at 16 and 17 in Fig. 1. Light can readily escape from such a build-up.

However, even with a totally internally reflecting surface parallel to the wing surface, the freezing of a water layer on that surface will normally result in irregularities due to non-uniform freezing and crystallization, and these irregularities will cause interference between light penetrating the layer and that reflected from the surface of the block, so that the beam will be changed by the presence of ice.

The device is provided with a semiconductor temperature detector T1.

Rain falling on the detector could cause a-false result, but this is prevented by sensing the temperature at the detector. If this is above 0'' C then the circuit is shut down.

Fig. 4 shows a modified form of the device, in which the electrical connections are remote from the sensor unit. The sensor unit is coupled to the diodes LED1, D1, and D2 by means of optical fibres, in a fibre cable 30, which transmit the light from LED1 to the block 11 and back from the block 11 to the diodes D1 and D2. The fibre optic cable 30 can thus be the only connection to the detector. This allows the safe installation of the sensor unit in or near fuel tanks. The electronics for a number of detectors can then be sited at a common location away from danger.

If it is desired to sense the temperature of a sensor unit using fibre optic coupling, a further light emitting diode LED2 and a further photodiode D3 can be provided adjacent to the other diodes and coupled to the block 11 through the cable 30. The two fibres from these diodes LED2 and D3 are connected to the sides of the block 11, with a light path 31 between them. A layer 32 of temperature-sensitive liquid crystal material is provided between the block and one of these fibres, the liquid crystal material being chosen to change state at O" C. Thus the coupling between LED2 and D3 changes at 0" C, and the system can thus determine whether or not the temperature is low enough for ice to form.

h plurality of diodes D3A, D3B, etc (not shown) can be provided if desired, with their optical fibres being coupled to the block 11 through liquid crystal materials which change state at different temperatures, to allow a more accurate temperature determination.

Returning to Fig. 1, the electronic circuitry 20 will now be described.

LED1 is driven 'by a pulse generator 41, which continuously generates square or rectangular pulses. Diode D1 feeds a buffer circuit 42 which is driven by the pulses from the pulse generator 41. Circuit 42 is an amplifier operating on well-known chopper amplifier principles and producing a smoothed output. Diode D2 feeds two buffer circuits 43 and 44, similar to circuit 42. Circuit 43 is fed with the same signal from pulse generator 41 as is circuit 42; circuit 44 is fed with the complement of that signal, produced by an inverter 45.

Circuit 43 produces a signal representing the intensity of light detected by diode D1 when LED1 is on. The outputs of circuits 42 and 43 are fed, via difference circuits 46 and 47 which will be discussed shortly, to a ratio circuit 48, which subtracts the signal from diode D2 from that from diode D1. The output of circuit 48 represents the strength of the light passing along beam 15 to Di, i.e. undergoing total internal reflection in the block 11, in relation to the strength of the light scattered from the surfaces 12 and 13.

Circuit 44 produces a signal representing the intensity of the ambient light falling on the aircraft (and the sensor in particular), since it responds to the output of diode D2 when LED1 is off. (The output of diode D1 could be used instead of, or as well as, the output of diode D2 during the off periods of LED1 for this purpose.) This signal is fed to the subtract inputs of the difference cir-iuitb 4G and 47, so that the signals fed to the ratio circuit 48 have the components due to ambient light removed from them before their ratio is determined. The ratio circuit 48 feeds a difference circuit 49 which is also fed with a reference signal REF.Hence the output of this circuit indicates whether or not the internal reflection in the block 11 at the surfaces 12 and 13 is substantially total, i.e. whether or not there is ice on these surfaces.

Circuit 44 also feeds a fast response difference circuit 50, which is also fed with a reference signal AMBMAX representing a high level of ambient light.

Circuit 50 feeds a gating circuit 52 via an inverter 51, gating circuit 51 also being fed from circuit 49. If the level of ambient light exceeds that set by the signal AMBMAX, e.g. because of the occurrence of a strobe light flash, then the signals from diodes D1 and D2 will be swamped by the strobe light and accurate comparison of them will not be possible. Ratio circuit 48 feeds a gating circuit 52 which is controlled by inverter 51. The output of circuit 52 is fed to an audible or visual alarm 53, which is therefore operated if the ratio circuit 48 output is high (not substantially total internal reflection in block 11) and the ambient light level is not too high.

The temperature sensor Tl feeds a circuit 52 which produces an output if the temperature is below oc C. Circuit 52 feeds a third input to gate 52.

It will be understood that appropriate scaling of the signals is performed in the circuitry described. Also,.the functions of the circuitry described can be performed in various other ways. For example, circuit 48 can be a simple difference circuit rather than a ratio detector; and the compensation of the signals from diodes D1 and D2 for ambient light can be performed by feeding the signals from circuits 42 to 44 into a single 3-input adder circuit, replacing the circuits 46 to 48. Many of the circuits may comprise operational amplifiers (op amps). Also, the signals from the diodes D1 and D2 can be digitized and the signal processing performed by digital circuitry, which may be simple logic circuitry, an application specific integrated circuit, or a suitably programmed microcomputer.

A number of detectors can be connected to a common electronic unit which will light. the required signals and sound alarms as required, to alert the operator to the presence of ice.

In more detail, the problem of icing of aircraft which this invention is concerned with arises when an aircraft is standing on the ground and awaiting the opportunity to take off. In practice, de-icing means are often provided, which will wash an aircraft, using a liquid which contains a de-icing (antifreeze) constituent and is also warm. The antifreeze and the warmth of the liquid both assist in removing ice.

However, such de-icing is generally performed at a location remote from the runway, and an aircraft may have to wait for some considerable time after deicing before being cleared for take-off. It will therefore be in danger of icing up again. The present invention, as described above, will detect such icing up. However, it is also desirable to have advance warning of the likelihood ot such icing up, so that the aircraft can be de-iced again early, rather than the need for further de-icing being discovered perhaps only shortly before the aircraft reaches the head of a queue for take-off.

The time for which a de-icing lasts depends on a variety of factors. The icing temperature depends on how much antifreeze remains on the aircraft from the de-icing; this depends on such factors as the rate at which the de-icing liquid is washed and/or blown off the aircraft and the rate at which it is diluted by falling snow, rainy or fog. The time taken for the aircraft to reach the icing point depends on the temperature to which it was warmed by the de icing and the rate of cooling, which in turn depends on such factors as the wind speed and temperature. It is therefore difficult to estimate this time.

A development of the present invention provides means for estimating or predicting more effectively the closeness of the aircraft to the icing point.

This is achieved by providing means for monitoring the temperature of the aircraft and the concentration of antifreeze in any liquid layer on the aircraft.

From this information, the time which the aircraft will take to reach the icing point can be estimated.

Fig. 5 is a block diagram of this embodiment. A sensor array 60 consists of a central sensor 10, as shown in Fig. 1, flanked by a plurality of auxiliary sensors 10a to 10d as shown. These sensors feed, via a cable 61, circuitry 62 comprising a set of individual sensor circuits 63, one for each sensor, and a processing circuit 64. The sensor array 60 also includes a temperature sensor T1 feeding the circuit 65. (This temperature sensor may alternatively be the Fig.. 2 arrangement including light path 31 and liquid crystal layer 32.) The circuit 64 also monitors the temperature of the aircraft at the sensor array, and determines the rate of change of temperature. Circuit 64 feeds an indicator pones 65.

The sensors iQa to 10d are arranged to detect the coefficient of refraction of any liquid layer which may lie over them. Their operation is generally similar to that of sensor 10, but they are arranged so that the loss of total internal reflection occurs not when any liquid lies on the internally reflecting surfaces, but when the coefficient of refraction of such liquid has a predetermined value. The sensors are therefore preferably made of denser material that that of sensor 10 (i.e. with a higher coefficient of refraction than that of sensor 10), and/or are made with the angle at which the light beam strikes the internally reflecting surfaces reduced as indicated in Fig. 6.The sensors are preferably also somewhat smaller than the sensor 10, to improve retention of deicing liquid in their valleys. (Sensor 10 has only to retain ice or mush.) The sensors are designed so that their predetermined values form a suitable sequence. The signal from each sensor will change at a particular value of the coefficient of refraction of the de-icing liquid on it, and by a suitable choice of values, the actual coefficient of refraction can be determined reasonably accurately from the outputs of the sensors ila to ild. The circuit 64 can measure the times at which it equals that set bygone of the sensors ila to lid, and obtain an estimate of it at other times by extrapolation.It can therefore determine the coefficient of refraction of any de-icing liquid on the aircraft and the rate at which the coefficient of refraction is changing. These quantities are equivalent to the concentration of antifreeze in any de-icing liquid on the aircraft and the rate at which it is being diluted in the liquid.

From the concentration of antifreeze in any liquid on the aircraft and the rate at which it is being diluted, and the temperature of the aircraft and that rate at which it is falling, the time left before the aircraft reaches the icing point can be calculated. Circuit 64 is arranged to compute this time, from stored tabular information, stored equations, or otherwise. The time so computed is displayed on the indicator panel 66.

Fig. 7 shows an alternative sensor for measuring the refractive index of liquid on the sensor. This sensor would be located adjacent to the ice sensor shown in Fig. 1. The sensor comprises a transparent block 70 (seen from above in the upper part of the Figure) with a prismatic hole 71 formed in it. A light emitting diode LEDP is attached to one side of the block to provide a narrow beam of light 72 as shown, and a plurality of photodiodes DPa, DPb, etc are attached to the other side of the block as shown.The apex angle of the prismatic hole 71, the positions of the light emitting diode and the photodiodes, and the coefficient of refraction of the material of the block 70 are chosen such that when the hole 71 is filled with liquid containing antifreeze, which one of the photodiodes the beam 72 reaches depends on the coefficient of refraction of the liquid, i.e. on the concentration of antifreeze. in the liquid.

The lower part of the Figure shows a section through the sensor, along the line 7-7. The hole 71 is closed by a piston 73, which is carried on one end of an arm 74 pivoted at its centre 75. The arm 73 has two armatures 76 and 77 one at each end, and two coils 78 and 79 are supported on a fixed support 80 one adjacent to each of the armatures 76 and 77 as shown. By energizing coil 77, arm 74 is rocked clockwise, pushing the piston 73 to the top of the hole 71 and so emptying it of any liquid. By energizing coil 78, the arm 73 is rocked anticlockwise, pulling the piston 73 down the hole and drawing into the hole a fresh sample of any liquid lying over the device.The clois 78 and 79 are driven by the circuitry 62.' It will be realized that a variety of alternative means may be used to monitor the coefficient of refraction of any liquid on the aircraft. For example, samples of liquid may be drawn into and expelled from a prismatic chamber by the use of a vacuum pump, pulsed heating, or ultrasonics. Also, mechanical scanning may be used to detect the position of the beam 72 after it has passed through the prismatic hole 71 (or the light emitting diode LEDP may be mechanically scanned).

The detector system is provided with a calibrate/reset button or switch 67 (Fig. 5 > that is operated when the wings of the aircraft are washed down with de-icing fluid. Operating the switch 67 will initiate a calibration process which will test all the circuits and warning lights and then store the information received as a reference. If the aircraft has been washed down with deicing liquid, the system operates as discussed above to calculate the time left before the de-icing fluid has no more effect.

The reference levels may be set at the time of manufacture. The calibratioii system may be either automatic or manually operated.

Claims (15)

C la im s

1 A refractive ice detector comprising a light source and a light detector coupled to a block of transparent material, the block having a surface on which ice may form and being shaped such that light passes from the light source to the light detector along a light path which includes a total internal reflection at said surface in the absence of ice, but the light is scattered and/or transminted through that surface in the presence of ice.

2 A refractive ice detector according to claim 1 wherein the block is in the form of a prism with one corner cut off to provide said surface and with the other two corners cut off and having the light emitter and light detector attached to them and facing said surface.

3 A refractive ice detector according to claim 1 wherein the block is in the form of a rectangle with two adjacent corners cut off and the light emitter and light detector attached to the side opposite the cut corners, so providing two surfaces at which the light undergoes said total internal reflection.

4 A refractive ice detector according to any previous claim including a further light detector attached to the block out of the light path, and means for determining the ratio between the signals produced by the two light detectors.

5 A refractive ice detector according to any previous claim wherein the light emitter and light detector(s) are a light emitting diode and photodiode(s) attached directly to the block.

6 A refractive ice detector according to any previous claim wherein the light emitter and light detector(s) are a light emitting . diode and photodiode(s) coupled to the block via a fibre optic cable.

7 A refractive ice detector according to claim 6 including a further light emitter and detector coupled to each other through the block via the fibre optic cable, with a temperature-sensitive liquid crystal material being included in the coupling adjacent to the block.

8 A refractive ice detector according to any previous claim wherein the light emitter is driven by a pulse generator and the light detector(s) feed chopper amplifiers also driven by the pulse generator.

9 A refractive ice detector according to claim 8 including a further chopper amplifier fed by a light detector and driven in antiphase to the light emitter, its output being subtracted from the outputs of the other amplifier(s).

10 A refractive ice detector according to any previous claim including means for inhibiting the system if the output of a light detector exceeds a predetermined limit.

11 A refractive ice detector according to any previous claim including further sensor means for sensing the 'coefficient of refraction of an antifreeze-contain- ing liquid on the detector, temperature detecting means for detecting the temperature of such liquid, and processing means for determining from the coefficient of refraction and temperature their rates of change and estimating, from the coefficient of refraction and temperature and their rates ot change, the time left before the icing point is reached.

12 A refractive ice detector according to claim 11, wherein tre further sensor means comprise a plurality of sensors each similar to the sensor of any previous claim but arranged so that total internal reflection is lost for different coefficients of refraction of the liquid on each further sensor.

13 A refractive ice detector according to either of claims 11 and 1?., wherein the further sensor means comprise a prismatic chamber having a light source on one side and means on the other side for detecting the angle of emergence of light from the light source passing through the chamber.

14 A refractive ice detector according to claim 13, including means for drawing liquid into and expelling it from the chamber.

15 A refractive ice detector substantially as herein described and illustrated.