NASA CASE NO. I_ABJS063_-

PRINT FIG. 3 __

NDEICE

The invention disclosed in this document resulted from research in aeronautical

and space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the patent licensing regulations applicable to U.S.

Government-owned inventions (37 CFR 404.1 et seq.).

To encourage commercial utilization of NASA-owned inventions, it is NASA

policy to grant licenses to commercial concerns. Although NASA encourages

nonexclusive licensing to promote competition and achieve the widest possible

utilization, NASA will provide the necessary incentive to the licensee to achieve early

practical application of the invention.

Address informational requests for this invention to the Technology

Applications Team, NASA Langley Research Center, Code 200, Hampton, Virginia

23681-0001.

Address applications for license to the Patent Counsel Office, Mail Stop 212,

3 Langley Boulevard, Hampton, Virginia 23681-0001.

Serial No.: 08/619,779

Filed: 03/20/96 LaRC
NASA Case No. LAR 15061-1 

THICKNESS MEASUREMENT DEVICE FOR ICEIWATER MIXTURE 

A device and method are provided for determining the thickness of an ice and water 

mixture accumulated on the outer surface of an object. First and second total 

impedance sensors are operated at first and second frequencies over which the 

dielectric constants for water and ice are substantially the same. Corresponding first 

and second AC total impedance measuring circuits are coupled to the first and second 

sensors to produce output voltages based on the total impedance changes sensed by 

the sensors. A processor is coupled to the first and second measuring circuits to 

generate a voltage ratio using the measured output voltages. The voltage ratio is 

indicative of the thickness of the ice and water mixture. 

The novelty of the present invention is the use of two frequencies and dual geometry 

impedance sensors in order to be sensitive to the formation of ice. Thus, critical 

situations created by the presence of solid ice can be averted. The device is simple 

and is easily incorporated into current technology thereby making its realization costeffective. 

Inventor: Leonard M. Weinstein 

Address: 13 Burke Avenue 

~ . 

Employer: NASA LaRC 

Initial Evaluator: William L. Sellers, Ill 

Serial No.: 08161 9,779 

Filed: 3120196 
NASA Case No. LAR 15061-1 -1- PATENT APPLICATION

THICKNESS MEASUREMENTDEVICE FOR ICE/WATER MIXTURE

Origin of tJle Invention

The invention described herein was made by an employee of the

United States Government and may be manufactured and used by or for

the Government for governmental purposes without the payment of any

royalties thereon or therefor.

10 Background ofLthelnvention

1. Eield of IheJn_vention

15

This invention relates to ice detection. More specifically, the

invention is a thickness measurement device for determining the

thickness of ice and water mixtures on a surface.

2__ Desc dption of_3elated_Art

20 A dual-geometry, capacitance-type ice thickness measuring gauge

was disclosed by this Applicant in U.S. Patent No. 4,766,369. The

gauge disclosed therein uses two capacitance sensors having different

electrode configurations and spacing to measure the thickness of surface

ice independently of the temperature and impurities in the ice. The device

25 works well if the ice is homogeneously solid where no water or other

liquid is mixed in or is on top of the ice. However, ice formation often

occurs with some liquid present. In certain applications, e.g., aircraft

icing conditions, runway or roadway icing conditions, etc., it is critical to

know that ice is forming before it becomes homogeneously solid.
NASA Case No. LAR 15061-1 -2- PATENT APPLICATION

5

Unfortunately, when using the dual-geometry gauge in the presence of

liquids, the apparent dielectric constant and conductivity can vary

considerably from areas near the capacitance sensors to the outer

surface of the ice or ice/water mixture. The variations can cause large

errors in the readings by the dual-geometry gauge readings.

Summary of theJnvention

Accordingly, it is an object of the present invention to provide a

10 thickness measurement device and method that determines the thickness

of an ice and water mixture on a surface during the process of ice

formation.

Another object of the present invention is to provide a thickness

measurement device and method that determines the thickness of layers

15 of liquid and ice on a surface.

Other objects and advantages of the present invention will become

more obvious hereinafter in the specification and drawings.

In accordance with the present invention, a device and method are

provided for determining the thickness of an ice and water mixture

20 accumulated on the outer surface of an object. A first total impedance

sensor senses total impedance changes due to very small accumulations

of the ice and water mixture. A first AC total impedance measuring

circuit is coupled to the first sensor and produces output voltages based

on the total impedance changes sensed by the sensor. Output voltages

25 are measured when the first AC total impedance measuring circuit is

operated at a first frequency and a second frequency. The frequencies

are selected such that the dielectric constant and resistivity of the ice

and water mixture are substantially constant. A second total impedance

sensor senses total impedance changes due to the ice and water mixture.
NASA Case No. LAR 15061-1 -3- PATENT APPLICATION

5

A second AC total impedance measuring circuit is coupled to the second

sensor and produces output voltages based on the total impedance

changes sensed by the second sensor. Once again, the output voltages

are measured when the second AC total impedance measuring circuit is

operated at the first and second frequencies. A processor is coupled to

the first and second measuring circuits and generates a processed

voltage ratio using the measured output voltages. The processed voltage

ratio is indicative of the thickness of the ice and water mixture.

10 Brief Description of the_Drawings

FIG. 1 is a schematic diagram of the thickness measurement

device of the present invention showing the relative locations of the

various sensors near the surface of interest;

15 FIG. 2 is a schematic diagram of a first total impedance sensor

used in the thickness measuring device;

FIG. 3 is a schematic diagram of a second total impedance sensor

used in the thickness measuring device;

FIG. 4 is a schematic representation of the sensor's impedance

20 components and the impedance measuring circuit;

FIG. 5 is a graphical representation of the dielectric constant

variation of ice and water as a function of frequency and temperature;

and

FIG. 6 is a graphical representation of the resistivity variation of ice

25 and water as a function of frequency and temperature.

Detailed Description of_theJmLention

Referring now to the drawings and more particularly to FIG. 1, the
NASA Case No. LAR 15061-1 -4- PATENT APPLICATION

thickness measuring device of the present invention is shown and

referenced generally by numeral 11. Device 11 includes first total

impedance sensor 16, second total impedance sensor 18, and

temperature sensor 20. The term "total impedance" generally refers to

5 the combination of resistive, capacitive and inductive impedance.

However, in terms of the present invention, inductive impedance is zero

or negligible because no coils or coil geometries are present in the

components or the electrical representations thereof. Accordingly, the

term "total impedance" as used herein refers to only the combination of

10 resistive and capacitive impedance measured by each of sensors 16 and

18.

Device 11 is mounted near outer surface 12 of any surface of

interest such as a runway, a roadway, an aircraft surface, etc. By way

of example, outer surface 12 is part of an aircraft 10. This outer surface

15 12 may be on the wing, the engine intake, or any other surface of

aircraft 10 where the formation of ice is detrimental. A small section of

outer surface 12 is removed and replaced with embedding material 14,

which preferably is either plastic or an epoxy-type material.

Sensors 16, 18 and 20 are embedded in embedding material 14.

20 The surfaces of sensors 16, 18 and 20 can be exposed to material 22

residing on outer surface 12. For purpose of the following description,

material 22 is an ice and water mixture accumulated on outer surface 12

except where otherwise noted. To avoid corrosion problems, sensors

16, 18 and 20 are covered with a thin layer of embedding material 14

25 typically on the order of approximately 0.001 inches thick. Positioning of

sensors 16, 18 and 20 relative to outer surface 12 is such that each

sensor must be close enough to sense the presence of ice and water

mixture 22 on outer surface 12. Embedding material 14 is contoured and

smoothed so that it is flush with outer surface 12 to minimize any
NASA Case No. LAR 15061-1 -5- PATENT APPLICATION

disruption in the contour of outer surface 12.

First total impedance sensor 16, second total impedance sensor 18

and temperature sensor 20 are connected respectively to first AC total

impedance measuring circuit 24, second AC total impedance measuring

5 circuit 26, and temperature measuring circuit 28. The outputs of first

and second total impedance measuring circuits 24 and 26 and

temperature measuring circuit 28 are connected to processor 42. The

outputs of processor 42 is typically passed to an output device such as

display 44.

10 Temperature sensor 20 may be any one of several standard

temperature sensors, such as a resistance film sensor, a thermocouple or

an integrated circuit temperature sensor. Temperature measuring circuit

28 receives the output of temperature sensor 20 and determines the

temperature at outer surface 12 and thereby the temperature of ice and

15 water mixture 22 (or any other material accumulation) on outer surface

12. Such circuits are well known in the art for each type of temperature

sensor. The determined temperature is shown on display 44 and serves

as an indicator or warning that conditions may be suitable for ice

formation.

20 In general, total impedance sensors 16 and 18 are of different

configurations such that their response to the presence of an ice and

water mixture is different. For example, it is assumed herein that total

impedance sensor 16 is constructed using small conductors separated by

a small gap so that there is a very large change in response for small

25 thicknesses of water and ice mixtures. However, total impedance sensor

18 is constructed using larger conductors separated by a larger gap

(relative to sensor 16 and the expected thickness of ice and water

mixture 22) so that there is a nearly linear response to changes in

thickness of the ice and water mixture.
NASA Case No. LAR 15061-1 -6- PATENT APPLICATION

By way of an illustrative example, first total impedance sensor 16

is shown in FIG. 2. Opposite sides of first total impedance sensor 16 are

formed from parallel leads 30 and 32. Each of the two leads has

connected respectively thereto a series of interleaved electrodes 34 and

5 36. Electrodes 34 and 36 are connected to leads 30 and 32. Electrodes

34 and 36 do not touch each other and hence the presence of material in

the vicinity of electrodes 34 and 36 causes a change in total impedance

between electrodes 34 and 36. While the dimensions of first total

impedance sensor 16 may vary, a typical distance between the outside

10 edges of leads 30 and 32 is approximately 3/4 of an inch. Likewise, the

length of leads 30 and 32 which contain the connections to electrodes

34 and 36 is about 3/4 of an inch. Typically, electrodes 34 and 36 are

approximately 0.014 inches wide and have gaps of approximately 0.012

of an inch between them.

15 For pairing with sensor 16 of the configuration/size shown in FIG.

2, an illustrative example of second total impedance sensor 18 is shown

in FIG. 3 and consists of two leads 38 and 40 which are parallel to each

other. While the dimensions of second total impedance sensor 18 may

vary, a typical distance between the outside edges of leads 38 and 40 is

20 approximately two inches. Parallel leads 38 and 40 each are formed

from a single electrode which covers almost one-third of the distance

between the outer edges of leads 38 and 40. This distance is typically

approximately 0.67 of an inch. The length of each electrode is

approximately two inches. Both first total impedance sensor 16 and

25 second total impedance sensor 18 can be made from thin conductors

with a thickness of approximately 0.001 of an inch. The thickness will

be greater if bulk conductors, e.g., conductive rubber, are used.

Referring once again to FIG. 1 first total impedance sensor 16 is

connected to first AC total impedance measuring circuit 24 and second
NASA Case No. LAR 15061-1 -7- PATENT APPLICATION

total impedance sensor 18 is connected to second AC total impedance

measuring circuit 26. Each of circuits 24 and 26 is the same except for

selection of sensitivity setting components to match corresponding signal

levels. Each of circuits 24 and 26 is capable of operation at a variety of

5 frequencies.

FIG. 4 is a schematic representation of the sensor/measuring

circuit combination representative of either the sensor 16/measuring

circuit 24 combination or the sensor 18/measuring circuit 26

combination. An adjustable source 50 produces a sine wave output at a

10 selected frequency f. Source 50 is connected to a resistor Ro which is in

series with the corresponding sensor, i.e., sensor 16 or sensor 18. The

sensor is represented by a parallel arrangement of an external resistance

R E (due to external conductivity), a parasitic capacitance C O (due to the

sensor internal capacitance), and an external capacitance C E (due to the

15 change in external dielectric constant). Each of circuits 24 and 26

produces an output voltage indicative of a combination of resistive and

capacitive changes produced by the proximity of any material on outer

surface 12 for the particular operating frequency f.

Source 50 supplies a known AC sine wave voltage with RMS value

20 V o to the series hookup of resistor Ro and corresponding sensor. The

RMS voltage measured between the resistor Ro and the sensor at node

52 is defined as V. The level of voltage V is determined by the total

impedance of the sensor. The total impedance equation is

V XT

V{_ X T+R o

(1)

where, for the parallel components representing the sensor, X T is

XcTR E

XT- (2)

25 XcT+RE
NASA Case No. LAR 15061-1

where XCT is

XCT--

1

2nf (CE+C o)

-8- PATENT APPLICATION

(3)

5

When the sensor does not have external ice or water on it, only

the parasitic capacitor C O causes a signal. This value can be measured

and later used in the data reduction. If the "dry" sensor reading at node

52 is defined V, the parasitic capacitance C O is

V -V /

lj

C_- 2nfRr V / (4)

J

The present invention utilizes the fact that dielectric constants and

conductivity for ice and water can vary with change in imposed

frequency over some frequencies, but are nearly constant for others.

10 FIG. 5 shows how the dielectric constant varies with frequency for ice

and water for the temperature range from near freezing to well below

freezing. More specifically, curve 60 represents -20°C, curve 62

represents -10 ° C and curve 64 represents -1°C. Water can respond to

changes in electric fields very rapidly due to the mobility of the

15 molecules. In contrast, ice responds relatively slowly to the variations in

the electric field due to the constraints of the lattice structure of the ice.

Since ice and water have strong molecular dipole moments, the dielectric

constant e for both water and ice is about 80 at "low" frequencies up to

approximately 400 Hz. The water dielectric constant stays constant, as

20 represented by dashed line 66, to at least 100 MHz, but the ice dielectric

constant starts falling off at frequencies above approximately 400 Hz.

The roll off frequency is dependent on temperature, but for temperature

in the range of interest (0 ° to at least -20°), the dielectric constant
NASA Case No. LAR 15061-1 -9- PATENT APPLICATION

decreases at frequencies above 400 Hz, and flattens to a new value of

dielectric constant (of about 3.1) at frequencies above about 20,000 Hz.

Lower temperatures can roll off below 400 Hz, and the frequency at

which data is taken may be lowered if extremely low temperatures are

5 expected. The dielectric constant of 3.1 is typical of solids with no

dipole moment.

The resistivity of ice and water also changes with frequency and

temperature as shown in FIG. 6. Curve 70 represents -20°C, curve 72

represents-10 ° C, curve 74 represents-1°C, and dashed line 76

10 represents liquid water. The absolute level of resistivity depends strongly

on impurity levels in the water and ice, so the curves shown are relative

levels. The high mobility of the liquid water permits a high ion mobility

resulting in low resistivity that is also frequency independent. At

sufficiently low frequencies, even solid ice allows significant ion

15 migration but as the frequency is increased, the lower mobility due to the

lattice structure raises resistivity considerably. For 0 ° to -20 ° , the

resistivity of ice starts to increase at frequencies beyond about 400 HZ

and levels off to a higher value above frequencies about 20,000 Hz. The

location of the start of roll up is also somewhat temperature dependent.

20 In order to measure the presence of a non-homogenous layer of ice

and water and determine the thickness of the layer, the following

approach is taken. Measurements can be made for sensors 16 and 18 at

two different frequencies fl and f2 where the dielectric constant and

resistivity are not changing with frequency. The subscripts 1 and 2 refer

25 to the lower and higher frequency value of any two frequencies for the

remainder of the description. Measurements could be taken at, for

example, frequencies selected from the range of approximately 20-400

Hz or from the range of frequencies above approximately 20,000 Hz.

The lower range of frequencies, i.e., 20-400 Hz, are preferred for two
NASA Case No. LAR 15061-1

10

-10- PATENT APPLICATION

reasons. First, since the dielectric constant for ice and water is the same

at lower frequencies, the presence of water in the ice does not degrade

the accuracy of the thickness measurement even for non-homogenous

layers. Second, higher frequencies have significant energy dissipation

effects which further complicate the data reduction.

The external resistance R E can be determined as follows for a

dielectric constant c that is constant over frequencies fl and f2. For

simplicity of equation development, a frequency ratio fl/f2 of 1/2 is used.

(However, at the expense of a more complex equation development, the

present invention could be practiced with the frequency ratio fl/f2 being

greater or less than 1/2. Note that for values of this ratio that are near

1, the required precision of the apparatus increases substantially.) The

external resistance R E is

VRoXcT

RL,- (5)

[X :T ( V!:I-v) -R, V]

Equations (1) and (2) are combined and solved for R E at the two

15 frequencies fl and f2- The two equations for R E can be equated to each

other since R E is the same for both frequencies. It can be shown that

R 0

(6)

2O

where XCTS1 iS the total capacitive impedance for sensor 16 at frequency

fl, Vs_ is the RMS measured voltage at node 52 for sensor 16 at

frequency fl, and Vs2 is the RMS voltage measured at node 52 for sensor

16 at frequency f2-

Similarly, it can be shown that
NASA Case No. LAR 15061-1 -11-

R 0

PATENT APPLICATION

(?)

where XCTL1 is the total capacitive impedance for sensor 18 at frequency

fl, VL1 is the RMS measured voltage at node 52 for sensor 18 at

frequency fl, and VL2 is the RMS voltage measured at node 52 for sensor

18 at frequency f2-

5 The ratio of external capacitance's for a dual geometry system

was disclosed by this Applicant in U.S. Patent No. 4,766,369. This ratio

is adequate to characterize the relative thickness of the external material

as long as the dielectric constant is the same for both sensors. This is

thus valid for the present invention also. The use of two frequencies

10 according to the present invention allows the values of resistivity and

dielectric constant to be separated out, and thus allows the capacitance

for each sensor to be determined even for non-homogenous mixtures of

ice and water. The parasitic capacitance's in XCTSl and XCTL1 can be

subtracted out and the result ratioed. The final relationship is a ratio of

15 the external capacitance of sensor 18 or GEL tO the external capacitance

2O

of sensor 16 or CEs.

VL-;'- VLZ -I vl) - VL i

[

CEI _ VL-_ [ VL l

_;, V_-1- V _2 _ _

V_ 2 V_i

(8)

where V'L1 iS the RMS voltage measured at node 52 for sensor 18 when

sensor 18 is dry and the frequency of operation is f_, and V'sl is the

RMS voltage measured at node 52 for sensor 16 when sensor 16 is dry

and the frequency of operation is f_.
NASA Case No. LAR 15061-1 -12- PATENT APPLICATION

The ratio in equation (8) can be used as a parameter to calibrate

against thickness variation for known thicknesses. The calibration can

then be used to determine the thickness of unknown layers. The

variation of thickness with the ratio is nearly linear for a limited range,

5 but begins to roll off toward a maximum value beyond thicknesses of the

ice and water layer that are large compared to the spacing of the

conductors in sensor 18. More specifically, when the thickness of the

ice and water layer is less than 1/2 the electrode spacing of sensor 18,

the variation in thickness causes an approximately linear change in the

10 measured total impedance. When the thickness of the ice and water

layer exceeds 1/2 the spacing, the response becomes non-linear.

Sensitivity decreases substantially once the thickness of the ice and

water layer is greater than twice the electrode spacing of sensor 18.

Accordingly, electrode spacing for sensor 18 is selected to be at least

15 twice the thickness of any expected ice and water layer.

Referring again to FIG. 1, a preferred operating scenario will be

described. Since circuits 24 and 26 are sensitive to the presence of

water as well as a mixture of ice and water, temperature measuring

circuit 28 can control, via processor 42, when circuits 24 and 26 change

20 operating frequencies. For example, when temperature measuring circuit

28 measures a temperature for which ice formation is not possible,

circuits 24 and 26 are inactive. Once temperature measuring circuit 28

measures a temperature in the range of concern for ice formation,

processor 42 could control circuits 24 and 26 to toggle between the first

25 (e.g., 200 Hz) and second (e.g., 400 Hz) operating frequencies. As

described above, device 11 then produces a measurement based on

supply and measured voltages for both sensors at both frequencies. The

resultant thickness of the ice and water mixture can be displayed on

display 44. Note that processor 42 could also be used to control heaters
NASA Case No. LAR 15061-1 -13- PATENT APPLICATION

or other deicing equipment (not shown) in response to the detection of

ice build-up.

The advantages of the present invention are numerous. By being

sensitive to the mere formation of ice, the present invention can be an

5 early-warning system. Thus, critical situations created by the presence

of solid ice can be averted. The device is simple and is easily

incorporated into current technology thereby making its realization costeffective.

Although the invention has been described relative to a specific

10 embodiment thereof, there are numerous variations and modifications

that will be readily apparent to those skilled in the art in light of the

above teachings. It is therefore to be understood that, within the scope

of the appended claims, the invention may be practiced other than as

specifically described.

15 What is claimed as new and desired to be secured by Letters

Patent of the United States is:
"Page missing from available version"
NASA Case No. LAR 15061-1 -18- PATENT APPLICATION

THICKNESS MEASUREMENT DEVICE FOR ICE/WATER MIXTURE

5 A device and method are provided for determining the thickness of

an ice and water mixture accumulated on the outer surface of an object.

First and second total impedance sensors are operated at first and

second frequencies. Corresponding first and second AC total impedance

measuring circuits are coupled to the first and second sensors to produce

10 output voltages based on the total impedance changes sensed by the

sensors. A processor is coupled to the first and second measuring

circuits to generate a voltage ratio using the measured output voltages.

The voltage ratio is indicative of the thickness of the ice and water

mixture.
.

, . 16

4. _'

MEASURING

CIRCUIT

/

32,__

34

FIG.2

18

t

_,_,'_ _ _-'_

%*o_;

N

/

22

20 .f

• I0

FIG. 1

18,

38

k "" ' /.

Y

FIG.3
f ,r_l

---7--

: (

--3./

l-c-_t _. y-

_c

f

t , 40 • ,

l 1

isle. S

l

I

i (

_o0 _-_

,L-