Surface dielectric measurement method and apparatus

A device implementing antennas transmitting and receiving electromagnetic waves for measuring the surface dielectric of a pre-defined surface area is disclosed herein. This area can be a small portion of a large surface, or a surface of a sample extracted from a larger volume. The sample might be cylindrical in shape. The device includes a dielectric spacer of known dielectric properties and geometries, placed between the material under test and the transmitting and receiving antennas. The dielectric value and thickness of the dielectric spacer are selected so as to control the effective area over which the dielectric is measured.

FIELD OF THE DISCLOSED TECHNOLOGY

The disclosed technology relates generally to an apparatus and method for calculating the surface dielectric of a medium, and, more specifically, to an apparatus and method for making wide-band real-time surface dielectric measurements of localized areas in situ or in samples of media extracted from a larger source. The disclosed technology is particularly suited for surface dielectric measurement of regularly sized samples of media.

BACKGROUND OF THE DISCLOSED TECHNOLOGY

The measurement of dielectric is a straight-forward process when performed using waveguide methods (Tonn, David A., U.S. Pat. No. 7,288,944) or Microwave Free-Space methods (Aziz, Md. Maniruzzaman B. A., et, al, 2010, ARPN Journal of Engineering and Applied Sciences, v. 5, no 11). These methods, however, are limited in their applicability to usage in controlled environments, such as laboratories, and require samples with very specific geometries. Therefore, such methods cannot measure the localized dielectric of a large surface having varying dielectric properties.

Another method of measuring dielectric is using an open-ended coaxial probe. Such a probe is portable, but has a different set of limitations. The coaxial probe requires good contact with the medium being tested, and the medium surface must be at least as flat as the probe face. Additionally, the sample thickness must be sufficient so that the sample appears infinite to the probe. Furthermore, there are accuracy limitations to this method, which has an accuracy of about 5% according to NASA Technical Memorandum 110147.

Another method for measuring the dielectric of a medium in situ, which is increasingly being used, is ground penetrating radar (GPR). This method has been applied for decades to accurately measure the surface dielectric of asphalt using non-contact horn antennas, which are typically mounted on vehicles.

More recently, smaller-size dipole-type antennas have been used to measure the dielectric of asphalt to a higher degree of accuracy. These dipole-type antennas have been used in a non-contact manner similar to the aforementioned horn antennas. In use, such dipole-type antennas are mounted 6 to 12 inches above the asphalt surface, thereby illuminating an area approximately 6″ in diameter, while being sensitive to edge diffractions from large dielectric discontinuities over a greater diameter.

Measurement of the dielectric of cores, varying in thickness from 0.5 inches to greater than 6 inches, and of asphalt cylinders manufactured by gyratory compactors, in near real-time, is especially important to industries that rely on calibrating dielectric measurements to variations in asphalt compaction using cores or manufactured cylinders. Currently, such measurements can only accurately be accomplished using waveguide methods, which are impractical outside of laboratory conditions.

An additional feature which would be beneficial to the industry is the ability to measure the surface dielectric of a pre-defined small area in real-time, for example, for the purpose of selecting locations to core. The waveguide method is unsuitable for this task.

Thus, there is a need in the industry for a device and/or a method for quickly and accurately measuring the dielectric of cores and manufactured asphalt specimens that may be, for example, approximately 6″ in diameter, and for measuring the in situ surface dielectric of small areas on a large surface.

SUMMARY OF THE DISCLOSED TECHNOLOGY

According to an aspect of some embodiments of the teachings herein, there is provided an apparatus for measuring a surface dielectric of a material under test (MUT) over a predefined area. The apparatus includes a ground penetrating radar (GPR) antenna, for measuring the surface dielectric of the MUT over the predefined area, and a dielectric spacer, disposed directly beneath the GPR antenna. A transmitter is functionally associated with the GPR antenna, and is useful for transmitting measurements obtained by the GPR antenna to a remote location, such as a controlling computer. The dielectric spacer may be designed and selected to slow propagation of waves through the spacer, so as to prevent interference to the surface dielectric measurement from signals arriving from edges of the predefined area.

In some embodiments, the dielectric spacer is in direct contact with the surface of the MUT. In some embodiments, the dielectric spacer includes a substantially homogenous substrate. In some embodiments, the dielectric spacer has a higher dielectric than air. For example, the dielectric spacer may have an estimated dielectric close to that of the MUT. In some embodiments, the dielectric spacer is mounted onto a plurality of wheels, so that it is movable relative to the MUT during measurements by the GPR antenna. In these cases, the dielectric spacer is elevated above the MUT. In some embodiments, a thickness of the dielectric spacer is selected based on a size of the predefined area.

In some embodiments, the predefined area is the flat circular area on one end of a cylindrical MUT. In such embodiments, the GPR antenna and the apparatus measure the dielectric of the surface of the cylindrical MUT. The cylindrical MUT may be, for example, an asphalt core. In some embodiments, the GPR antenna and the apparatus measure the surface dielectric of a sample of asphalt representing a range of compaction levels, to establish the relationship between a dielectric and a compaction level for a given asphalt mixture.

There is further provided, in accordance with another embodiment of the disclosed technology, a method for measuring a surface dielectric of a material under test (MUT) over a predefined area. The method includes placing an apparatus according to the disclosed technology, as described hereinabove, above a surface of the MUT in the predefined area, and subsequently obtaining at least one surface dielectric measurement from the GPR antenna, via the transmitter.

In some embodiments, the dielectric spacer is placed in directed contact with the surface of the MUT. In other embodiments, the dielectric spacer is elevated relative to the surface of the MUT, and obtaining at least one surface dielectric measurement occurs while the apparatus is in motion relative to the surface of the MUT. In some embodiments, the method further includes calibrating a thickness of the dielectric spacer to a size of the predefined area and to the MUT, prior to placing of the apparatus above the surface of the MUT.

In some embodiments, the MUT is a cylindrical MUT, and the predefined area includes the rounded surface of the cylindrical MUT, and the obtained measurements are substantially accurate measurements of the surface dielectric of the side of the cylindrical MUT. In some embodiments, the cylindrical MUT is an asphalt core. In some embodiments, the surface area of the MUT is in the range of 36 square inches and 600 square inches.

In some embodiments, the MUT includes a sample of asphalt representing a range of compaction levels, and the obtaining at least one surface dielectric measurement includes using at least one surface dielectric measurement, establishing a relationship between a dielectric and a compaction level for a given asphalt mixture.

In an embodiment of the disclosed technology, a device implementing antennas transmitting and receiving electromagnetic waves is used for obtaining the surface dielectric of a pre-defined surface area. This area may be a small portion of a large surface, or a surface of a sample extracted from a larger volume. The sample might be cylindrical in shape, such as an asphalt core and or a gyratory-compacted asphalt sample. The device according to the present invention utilizes a medium of known dielectric properties and geometries, referred to as a “dielectric spacer”, placed between the material being tested and the transmitting and receiving antennas. The dielectric value and the thickness of the dielectric spacer determine the effective area over which the dielectric is measured. As such, the invention can be utilized to measure the surface dielectric of 6 inch diameter asphalt cores or the surface dielectric of a small area of a large asphalt surface where a 6 inch diameter core will be obtained. Additionally, the invention can be used to measure the surface dielectric of gyratory-compacted asphalt samples to determine the relationship between asphalt dielectric and compaction level for a given asphalt design mixture.

Embodiments of the disclosed technology will become clearer in view of the following description of the drawings.

Reference is now made toFIG. 1, which is a perspective view illustration of an apparatus10for measuring a dielectric of a Material Under Test (MUT)3according to an embodiment of the disclosed technology. As seen inFIG. 1, a GPR (Ground Penetrating Radar) antenna1is disposed on top of a dielectric spacer2, which is in direct contact with the MUT3. Measurement information may be transmitted to a control unit or another remote location via a communication cable4extending from GPR antenna1.

The apparatus10measures the dielectric over the surface area of the antenna1. In some embodiments, the apparatus10is designed to minimize the surface area over which the dielectric is measured.

In some embodiments, the dielectric spacer2is a substantially homogeneous substrate. In some embodiments, the dielectric spacer2has a higher dielectric than air, and sometimes has an estimated dielectric which is close to that of the medium being measured, thus causing slower propagation of waves there-through. In the context of the present application and claims, “close to” relates to two measurements being within 10%, within 20%, within 25% and/or within 30% of each other.

FIG. 2is a perspective view of the apparatus10, placed on top of a cylindrical MUT5, according to an embodiment of the disclosed technology. The cylindrical MUT5may be of any suitable material, such as, for example, an asphalt core.

FIG. 3is a perspective view of the apparatus10ofFIG. 1, disposed above a MUT3. The apparatus ofFIG. 3is modified to include wheels7connected to a base surface or to axles6. In this embodiment, the apparatus10is elevated above the MUT3to a small degree, so as to facilitate dielectric measurements while the apparatus is moved over the surface of MUT3. In some embodiments, the distance between a lower surface of dielectric spacer2and the upper surface of MUT3is in the range of 2 to 10 mm.

In some embodiments, and particularly for application involving measurement of a dielectric from a defined surface area, such as a cylinder, the dielectric material/values of the dielectric spacer2, and thickness of the dielectric spacer2are selected such that the reflection which is the earliest to arrive at GPR antenna1from the surface area is not impacted by diffractions arriving at GPR antenna1from edges of the surface area at a later stage.

FIG. 4is a perspective view of the apparatus10ofFIG. 3, including travel paths of radiated energy that are used for calculations according to an embodiment of the disclosed technology. Specifically,FIG. 4shows the GPR antenna1and dielectric spacer2, indicating locations of centers of a transmitting antenna8and a receiving antenna9.FIG. 4additionally illustrates the surface reflection travel path10, the edge diffraction travel path11, and multiple reflection travel paths12. Knowledge of the separation distance between the transmitting and receiving antennas, the width of the transmit pulse used to obtain the reflection amplitude, and the diameter of the surface area of the MUT, here shown as cylinder5, enables calculation of the arrival times of energy from each of the different paths10,11, and12for different dielectric spacer2thicknesses and dielectrics.

In use, a response from MUT3being measured with GPR antenna1of apparatus10arrives after a certain time delay, due to the presence of dielectric spacer2and the slow propagation of waves through the dielectric spacer. A difference in a peak of the measurement wave is more noticeable between a straight path and an angled path, the most angled path hitting an outer edge of the surface area whose dielectric is being measured. The diffraction at the edge, and reflection, cannot be separated from the rest of the dielectric measurements due to the more separate waveform produced in the response.

The dielectric spacer2can have a measured dielectric which is calibrated and accounted for (removed in the calculations of the received response) after measuring a medium there-through. This allows pucks, or thin cylindrical parts of asphalt or other MUTs to be measured, which pucks have a smaller diameter than previously able to be measured with this level of accuracy. A core sample, which has, for example, a 6 inch diameter, flat upper and lower side, and a round edge there-between of a thickness of perhaps ½ inch, can be measured using ground penetrating radar in embodiments of the disclosed technology.

FIG. 5is a graph demonstrating different reflection and diffraction arrival times calculated and used to determine the dielectric and thickness of the dielectric spacer placed between the antennas and the MUT according to an embodiment of the disclosed technology.

As seen inFIG. 5, for a cylinder having a diameter of 15 cm, a separation distance of 6 cm between the transmitting and receiving antennas, and a pulse width of 0.36 ns, a dielectric spacer with a dielectric of 3.0 will provide the required time isolation for the surface reflection arrival time if the spacer thickness is in the range of 4 cm to 5.5 cm. This is due to the fact that in this range of thicknesses, or antenna heights, the surface reflection arrival time slightly precedes the edge diffraction arrival start time.

FIG. 6is a graph demonstrating exemplary surface reflections obtained when the dielectric spacer2shown inFIG. 1is replaced by Styrofoam. As seen,FIG. 6includes isolated surface reflections obtained from two different MUT's of approximately the same dielectric but having differing surface areas, where the reflection from a MUT having a surface area of 24″×24″ is indicated with a solid line, and the reflection from a MUT having a surface area of 6″×6″ is indicated with a dashed line. The Styrofoam served to create an approximate air gap between the antenna and the surface.

As seen, the positive peak amplitude13of the isolated reflection from the 24″×24″ surface is noticeably different, and greater, than the positive peak amplitude14of the isolated surface reflection from the 6″×6″ surface area. This is due to interference of diffracted energy arriving from the edges of the 6″×6″ MUT.

FIG. 7shows a zoomed in view of the isolated surface reflection amplitudes from the same MUTs as inFIG. 6. As seen inFIG. 7, the difference in the maximum peak amplitudes13and14taken from the larger and smaller MUT surface areas, respectively, is in the order of several percent.

FIG. 8is a graph demonstrating exemplary surface reflections obtained using the apparatus ofFIG. 1, from a MUT with a surface area of 24″×24″ and a MUT of the approximate same dielectric with a surface area of 6″×6″, similar to the MUTs used for the graph ofFIG. 6, and using a dielectric spacer2between the antenna1and the MUT surface. As seen inFIG. 8, the maximum peak amplitude obtained from both MUTs is substantially identical. This is evidenced also by the graph ofFIG. 9, which is a zoomed in view of the waveform peaks shown inFIG. 8, and which shows that the positive peak amplitudes13and14obtained from the 24″×24″ and 6″×6″ MUT surfaces, respectively, are approximately the same. This is due to the fact that the dielectric spacer2, disposed between the MUT surfaces and the antenna, delays the arrival of diffracted energy from the edges of the 6″×6″ MUT sufficiently such that the arrival of the diffracted energy does not interfere with the arrival time of positive peak14. As such, comparison ofFIGS. 8 and 6, and ofFIGS. 9 and 7, demonstrates the advantage of the disclosed technology over prior art methods, and the accuracy of dielectric measurement from a small surface area when using the device10of the disclosed technology.

FIG. 10shows a high-level block diagram of a device that may be used to carry out the disclosed technology. Device500comprises a processor550that controls the overall operation of the computer by executing the measurement device's program instructions which define such operation. The measurement device's program instructions may be stored in a storage device520(e.g., magnetic disk, flash disk, database) and loaded into memory530when execution of the measurement device's program instructions is desired. Thus, the measurement device's operation will be defined by the measurement device's program instructions stored in memory530and/or storage520, and the measurement device will be controlled by processor550executing the measurement device's program instructions. A device500also includes one or a plurality of input network interfaces for communicating with other devices via a network (e.g., the internet). A device500also includes one or more output network interfaces510for communicating with other devices. Device500also includes input/output540representing devices which allow for user interaction with the computer500(e.g., display, keyboard, mouse, speakers, buttons, etc.). One skilled in the art will recognize that an implementation of actual devices will contain other components as well, and thatFIG. 10is a high level representation of some of the components of such a measurement device for illustrative purposes. It should also be understood by one skilled in the art that the method and devices depicted inFIGS. 1 through 9may be implemented on a device such as is shown inFIG. 10.

For purposes of this disclosure, the term “substantially” is defined as “at least 95% of” the term which it modifies. Any device or aspect of the technology can “comprise” or “consist of” the item it modifies, whether explicitly written as such or otherwise.

When the term “or” is used, it creates a group which has within either term being connected by the conjunction as well as both terms being connected by the conjunction.