Ice Detection Apparatus Employing Microwave Reflectance

Abstract

Apparatus and method for detecting the presence of ice or water on a road or other surface by means of reflected microwaves wherein microwave energy is directed through a window installed in the surface to be monitored and the condition of the surface is indicated by the level of microwave energy reflected.

Description

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention relates to a method and apparatus for determining the presence of ice, water or slush on a surface, particularly a ground surface, wherein signal energies having wavelengths in the microwave region are employed.

2. Description of Prior Art

The safety of ground transportation and air transportation is dependent to a great extent upon the condition of road and runway surfaces, and upon the driver's or pilot's awareness of that condition. Thin layers of ice present a particular hazard in that they are not readily detectable by visual means and may form and disappear quickly with fluctuations in temperature. Highway bridges present a localized problem in that they may become coated with frost or ice sooner and more often than their approach pavement.

It is essential that airport runways be kept free of ice at all times during use. Airport maintenance engineers are required to make frequency inspections of runways during cold, inclement weather to apply ice melting chemicals when ice is detected or there is a danger of ice formation. In extremely cold weather, ice melted by chemicals may refreeze as the temperature drops, thereby recreating the original hazard and requiring additional inspection and action by the maintenance crews.

The highway engineer is constantly plagued by a lack of information on road conditions since visual inspection of a vast road network is impossible, and he must accordingly rely on general warning signs to alert drivers to potentially slick bridge decks and overpasses. Studies have shown that such general warnings are ignored by the majority of drivers, and that some specific warning with a high level of confidence is needed to effectively alert the passing motorist.

The need for instrumentation to positively detect the presence of ice on a road surface and either activate a warning sign or signal maintenance crews to apply ice-melting chemicals has been generally recognized, and several approaches to the creation of such an instrument have been suggested. A common approach has been to make direct measurements of environmental conditions such as pavement temperature, surface moisture, air temperature, and air humidity, and to predict whether or not ice is or might be present on the surface. More sophisticated techniques suggested for determining surface conditions have involved direct measurement of the surface friction coefficient; change in heat transfer rate associated with phase changes of water; change in resonant frequency of a vibrating element when its mass is increased by ice formation; change of capacitance and dissipation factor when the air dielectric of a capacitor changes from air to air containing snow or water droplets; and light or gamma radiation scatter from a roadway surface by water, ice, or snow present on the surface.

Most of the devices suggested by the prior art are inherently unsuitable for general use on roads and runways because of poor performance or reliability, lack of ruggedness for maintenance free outdoor installation, or relatively high cost. For general highway or airport use, the device must be capable of installation in isolated areas, be relatively maintenance free, and not be affected by hostile environment or elements which may include all weather conditions, vibrations and stresses caused by passing vehicles, and contact with sand, dust, dirt, salt, and special ice-melting chemicals which are commonly found or used on road and runway surfaces.

It is accordingly an object of the present invention to provide device suitable for general use on roads and runways to detect the presence of ice, sluch, or moisture and to predict the formation of ice. It is a further object of this invention to provide a method for detecting the presence of ice and moisture on a road or other surface by utilizing a signal energy which has wavelengths in the microwave region. Other additional objects of this invention will be apparent from the following description of the apparatus and method.

SUMMARY

In accordance with the method of the present invention, microwaves are directed via a wave guide to the underside of a window substantially transparent to the microwaves and installed substantially flush with the surface of the road or other surface to be monitored. The presence and thickness within reasonable limits of any coating of ice or water present on the surface of the window is determined by measuring the amount of microwave energy reflected by the window. The reflected energy can be used to trigger a signal device such as a warning sign or to activate ice-melting means.

The basic detector apparatus employed in accordance with the present invention comprises a sensor constructed of conventional microwave hardware including a microwave generator and isolator, a wave guide coupler, a reflected power detector, and a window of a material substantially transparent to microwave energy, and a signal means for converting the reflected power signal generated by the sensor to a surface condition signal.

The sensor can be assembled in a small package suitable for installation directly in the road surface and is unaffected by the environment or by the passage of vehicles. The power supply and signal means may be remote from the sensor assembly.

DESCRIPTION OF DRAWING

FIG. 1 is a diagramatic representation of a preferred form of the basic detector apparatus illustrating the essential elements therein.

FIG. 2 is a diagramatic representation of one alternative form of the basic apparatus of FIG. 1 illustrating different means for measuring the power of the reflected signal.

FIG. 3 is a diagramatic representation of a further modification of the basic apparatus of FIG. 1 wherein the ratio of reflected power to incident power is used to indicate the condition of the surface.

FIG. 4 is a diagramatic representation of one alternative form of the apparatus of FIG. 3 illustrating a different means for measuring the power of the generated signal.

FIG. 5 is a diagramatic representation of a further modification of the basic apparatus wherein the temperature of the surface is measured, and means for heating the window above the freezing point of water are provided.

FIG. 6 is a diagramatic representation of a modification of the apparatus of FIG. 5 wherein two windows are used, one heated and one unheated, and a magic tee is employed to determine the difference between and the sum of the signals reflected from each window.

FIG. 7 is a diagramatic representation of the packaged sensor installed in a roadway and connected by electrical cables to a power source and output signal means remote from the sensor.

FIG. 8 is an illustration of a typical calibration curve showing the relationship between the nature and thickness of the coating on the window and the level reflected microwave energy.

DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with one preferred method of the present invention, microwaves are directed through a direction coupler to a transparent window installed substantially flush with the surface to be monitored. The microwaves are partially transmitted through the window and lost to the atmosphere, and partially reflected. The extent to which the microwave energy is reflected is dependent upon the condition of the surface of the window: if the window is clear, substantially all the energy will be transmitted, while if the window is covered by a coating of ice or water, a substantial amount of the incident energy will be reflected, the proportion of energy reflected depending upon the thickness and nature of the coating.

FIG. 1 is a diagramatic illustration of the basic elements of the apparatus which serves to define the principle of operation. Microwaves generated at 11 are coupled to a ferrite isolater 12 which functions to protect the generator from reflected waves and to keep the power level and frequency of the generated signal constant in spite of variations in reflected signal and drastic load changes. Wave guide 13 couples the ferrite isolater to window 14 which can be constructed of any dielectric material which is substantially transparent to microwave energy, as for example polymers of tetrafluoroethylene, copolymers of hexafluoropropylene and tetrafluoroethylene, polyethylene, and Rexolite, a cross-linded styrene copolymer having a dielectric constant of about 2.5 at a frequency of 10,000 GH.sub.z which is a product of American Enka Corporation, Brand-Rex Dividion, Willimantic, Connecticut and is offered for use in microwave systems. The wave guide is depicted in the Figure as a straight member but it is not necessarily so limited and may include elbows or other curved members if necessary or desirable for purposes of construction.

The microwave energy reflected at the window travels back along the hollow wave guide 13 and meets directional coupler 15 which bleeds from the main guide a small proportion of the reflected energy. A crystal rectifier 16 positioned in directional coupler 15 is actuated by the reflected energy, and the output from the crystal is amplified by amplifier 17 and actuates signal means 18 which may be an ammeter to quantitatively show the level of reflected energy, or may be a warning light, sign, or other signal means actuated only when the reflected energy surpasses a predetermined level indicating the presence of a dangerous surface condition.

The basic method and apparatus of FIG. 1 is subject to innumerable modifications, alterations, refinements and even improvements for specific applications. Examples of various mechanical and electrical assemblies embodying the basic concept of the invention as illustrated in FIG. 1 are illustrated in FIGS. 2 through 6. For a better understanding of the invention, its method of operation, and the advantages to be attained with its use, reference should be had to the illustrations and following description of FIGS. 2 through 6. Yet other variations will be readily apparent to those skilled in the art, and the invention is accordingly not to be limited by the details of this description.

A modified form of the basic apparatus as shown in FIG. 2 differs from FIG. 1 principally by using a probe and detector 19 to measure changes in standing waves as an indication of variations in reflected energy. In this variation of the apparatus, the reflected signal sets up standing waves in the hollow wave guide 13, and the phase and amplitude of these waves depends upon the extent of reflected signal energy. A microwave detector such as a probe and crystal rectifier can be installed in the wave guide or coupled to it at any point where the standing waves exist. The output from the detector is fed to an amplifier and signal means as in FIG. 1. The probe and rectifier 19 might alternatively be any of several other well-known devices for detecting microwaves in a hollow wave guide, such as a loop or hole combined with any suitable type of rectifier.

In FIG. 3 there is shown a further modification of the basic apparatus wherein the surface condition is determined as a function of the ratio of reflected energy to incident energy. The advantage of this system over those of FIGS. 1 and 2 is that any variation in the output of the microwave generator is automatically compensated for. The apparatus also provides a fial safe warning system in the even the generator malfunctions or the power supply is interrupted. In FIG. 3, microwaves generated at 11 are coupled to ferrite isolator 12 which in turn is coupled to window 14 by hollow wave guide 13. Incident microwaves passing from the ferrite isolator to the window pass directional coupler 20 where a small amount of the energy is bled off and activates crystal rectifier 21. Reflected microwave signal energy passing back down wave guide 13 passes directional coupler 15 where a small amount of energy is bled off and activates crystal rectifier 16. The outputs from crystal rectifiers 16 and 21 are fed to amplifier-integrator 22 where the respective levels of output are compared, and the ratio indicated by signal means 18.

A modified form of the apparatus is shown in FIG. 4 which differs from FIG. 3 principally in the means for detecting the levels of incident and reflected power. In FIG. 4, 3-port circulator 24 is substituted for ferrite isolator 12 of FIG. 3 and crystal detector 23 is mounted across the wave guide at a point intermediate microwave generator 11 and the circulator. Crystal detector 23 provides a direct measurement of the microwave generator output. The circulator functions as an isolator to protect crystal detector 23 and microwave generator 11 from reflected signal energy by effectively directing all the reflected energy through the side port where its intensity is measured by crystal detector 25. Output from crystal detectors 23 and 25 is fed to amplifier-integrator 22 which actuates signal means 18 as described in FIG. 3.

FIG. 5 is a further variation and refinement of FIG. 3 in that it provides means for measuring the temperature of the window, and also means for heating the window to a temperature above the normal freezing point of water. This system provides a method for determining whether the reflected signal is due to the presence of ice or water upon the surface of the window. In FIG. 5, the temperature of window 14, or of a mounting flange containing the window, is measured by temperature detector 26 which may be a thermistor or thermocouple. The signal from detector 26 is amplified by amplifier 29 and fed to signal means 30 which quantitatively indicates the level of reflected power and has the additional facility of indicating window temperature. In the event the window temperature is such that either ice or water could reasonably be present, the window is heated to a temperature of about 34.degree.F. by heating element 27 which can be adjacent to or integral with window 14 and which is actuated by a manually controlled power supply 28. Simple observation of the level of reflected power during the heating cycle is sufficient to determine whether the initially reflected energy was due to ice or water upon the window since if ice was initially present, the heating will result in a marked increase in reflected energy as the ice is melted, while if water was initially present, heating will have no appreciable effect on the level of reflected power.

The heating cycle and effect of heating on signal energy can be a manual operation or can be instrumented to function automatically and give a specific signal for ice, water or slush if such is desired. Characteristically, an automatic heating cycle can be imposed when the temperature is in a critical range, and the reflected energy level is above a minimum value which indicates that either ice or water is in fact present. Critical temperature range wherein either ice or water could be present is from about 20.degree.F. to 32.degree.F., considering the effect of ice melting chemicals on depressing the freezing point of water. The minimum reflected energy level would be that reasonably experience with a dry, ice-free window.

Heating the window is preferably accomplished by laminating fine heating wires in the window itself. The wires should be parallel to each other and cross-polarized to the microwave electric field so as not to interfere significantly with the microwave signal. Alternatively, the wires may be external and adjacent to the window. It is preferable to heat the window directly to minimize the heating power required and response time, but it is of course possible to heat the window indirectly by, for example, heating the holding flange, but heat loss and power requirements in such a case are unfavorable.

FIG. 6 is a further refinement of the apparatus of FIG. 5 wherein two windows are employed, 14a being unheated and 14b being treated by element 27 connected to power source 28 in order to maintain the temperature of the window at or above about 34.degree.F. to prevent the formation of ice thereon. The signal from microwave generator 11 passes through circulator 24 and into wave guide 13. The signal is divided into equal portions by magic tee 31 with one part directed to window 14a and the other part to window 14b. Signals reflected from each window return to the magic tee via their respective wave guides. Due to the characteristics of the magic tee, the sum of the reflected signals is directed into wave guide 13 wherein it proceeds to the three-port circulator 24 and is finally directed to crystal detector 25. The difference in reflected signals from windows 14a and 14b is formed in the third arm of the magic tee terminating in crystal detector 32. Signals from detectors 25 and 32 are fed to amplifier-integrator 22 where they actuate the signal means to indicate the presence of ice, slush, or water on the surface being monitored.

The advantage of the modified system of FIG. 6 resides in the ability of the system to distinguish between the presence of ice and water on the surface being monitored by virtue of the differences in signal reflected from the windows. By way of illustration, when the surface being monitored is either clear or wet, the reflection from each window is the same, and crystal detector 32 will register zero input. When the surface is covered with ice, window 14a will be covered with ice while heated window 14b will be wet but remain ice free. Under these circumstances, the energy reflected from window 14b will be substantially greater than that reflected from window 14a, and the difference in signal energy registered by detector 32 will indicate the difference in surface condition of the two windows. The distinction between clear and wet windows is made by detector 25 which registers total reflected signal. When both windows are clear, detector 32 registers no difference and detector 25 registers low total reflectance. When both windows are wet, detector 32 still registers no difference but now detector 25 registers a high value of total reflected energy. When ice is present, detector 32 registers a high value differential reflected energy while detector 25 registers an intermediate level of total reflected energy. The presence of slush on the surface rather than ice results in readings on both detectors intermediate those for ice and water. Thus, by comparing the absolute and relative values of the two crystal detectors, it is possible to obtain an unambiguous indication that the surface is clear, wet, or covered with ice or slush.

FIG. 7 illustrates one typical installation of the packaged sensor apparatus in a road bed. In the figure, the sensor is housed in cylindrical shell 3 with the window 14 protruding through and being flush with the surface of upper flange 34. The cylindrical shell is closed by lower flange 35, both upper and lower flanges being equipped with gaskets or o-rings to provide a hermetic seal. Power input to the unit for operation of the microwave generator and the window heating element where utilized, and the energy output from the crystal detectors and the window temperature detector where utilized is accomplished via electrical cables 37 which enter the cylindrical shell through hermetically sealed connection 36.

The details of the electrical circuits involved in transmitting the output energy to the signal means have not been included in the preceding description. Similar output systems are presently used in conjunction with microwave apparatus designed for measuring for example the thickness or dielectric constant of a given material, and this feature of the invention is accordingly largely conventional and within the knowledge of those skilled in the art of instrumentation.

The invention requires that the relationship between the reflected microwave energy and the condition of the window surface by established. Calibration of the detection device is easily accomplished by coating the window with well known thicknesses of ice and water and measuring the level of microwave reflection. A typical calibration curve is shown in FIG. 8. As illustrated by this figure, the output from the crystal detector actuated by the reflected microwave signal increased from 0 to about 160 mV as the coating of ice increased from 0 to about 0.12 inches and thicker. Likewise, the reflected power level increased from 0 to about 230-250 mV as the depth of water increased from 0 to 0.03 inches and deeper. The actual output values will vary depending on the nature of the apparatus and crystal detectors employed in the sensor, and the values given in FIG. 8 have no particular significance other than to illustrate the relative differences in signals obtained for ice and water.

It is apparent from the data of FIG. 8 that the invention is sensitive to extremely thin coatings of ice or water and that ice reflects significantly less energy than water. This difference in reflection makes it possible to distinguish between ice and water with great reliability as discussed in relation to the operation of the apparatus depicted in FIG. 5 to 7. In the most circumstances where the presence of ice or water is sought to be determined, the environmental conditions will be such that either water or ice can be present and it becomes necessary therefore to utilize the differences in reflected microwave energy to distinguish between ice and water. Under some conditions, however, the environmental conditions will determine that only ice or water but not both can exist. For example, in measuring the surface condition of an aircraft flying at high altitude any reflected energy would necessarily indicate the presence of ice and call for action accordingly, the presence of water being dismissed by the environmental conditions. Likewise, any positive reading of reflected energy when the ambient temperature is above freezing would be taken as an unambiguous indication of the presence of moisture. Under these circumstances the most basic apparatus of FIG. 1 can be employed with good results.

A preferred microwave signal generator for use in the apparatus of the present invention is a Gunn effect oscillator, operating at a frequency of about 10 GHz and a power output of about 25 mW. Input power requirements are relatively low, and typically about 450 mA at 10V. Any other microwave signal source capable of generating a signal of constant frequency and power over extended period of time may also be used, as for example a low power reflex kylstron tube.

Those elements of the above described apparatus designated as crystal rectifiers or detectors may be of any tuned wave guide type of detector, whether or not employing crystals, which is suitable for quantitatively detecting or rectifying microwaves to direct current.

Although hollow wave guides for conveying signal energy were shown for purposes of illustration in the preceding description, the guide means need not be restricted to this type of construction, but guides constructed of coaxial cable for example may also be used.

Other variations in the assembly or operation of the apparatus which employ the basic principle of the instant invention for detecting the presence of ice or moisture on a surface by means of reflected microwaves can be envisioned. For example, it is contemplated that the system may employ two or more sensors for detection at multiple points with results indicated on one or more remote signal means. In systems including a plurality of sensors, one or more windows may be heated to prevent the formation of ice thereon and to serve as a basis of comparison in determining whether reflected energy from unheated windows is due to ice or water according to the general method described in reference to FIG. 6.

The method described herein may be used in conjunction with other instruments and detectors, to determine for example air temperature and relative humidity as well as surface temperature in order to predict the incipient formation of moisture, frost, or ice on the surface. The presence and concentration of salt or other ice melting chemicals on the surface being monitored may be determined by measuring for example conductivity or dielectric constant of moisture on the surface as further aid to predicting the freezing point on the surface moisture.

Other variations and combinations of the present invention will be apparent to those skilled in the art as the invention is applied to specific fact situations, and the scope of the present invention is accordingly not to be limited except as defined in the claims.

Claims

The embodiments of the invention in which an exclusive property is claimed are defined as follows:

1. A system for detecting the presence of ice on a surface comprising in combination

A. a microwave generator adapted to produce signal energy,

B. a window in said surface substantially transparent to said signal energy,

C. heating means cooperating with said window and adapted to heat said window above about 32.degree.F.,

D. means for monitoring the temperature of said window in response to the action of said heating means,

E. guide means adapted to convey said signal energy from said generator to said window,

F. isolator means in said guide means intermediate said window and said generator, and

G. reflected power detector means adapted to detect the difference between the signal energy reflected from said window at ambient temperature and that reflected from said window when heated to above about 32.degree.F., a significant difference in reflected energy indicating that ice was initially present on said surface.

2. A method for determining the presence of ice on a surface which comprises directing microwave signal energy through a window in said surface, heating said window to above about 32.degree.F., monitoring the window temperature during the heating step, detecting the signal energy reflected from said window at ambient temperature and that reflected from said window when heated to above about 32.degree.F., a significant difference in reflected energy indicating that ice was initially present on said surface.