Absolute property measurement with air calibration

Abstract

An instrument and method for providing accurate and reproducible measurement of absolute properties of a material under test without using conductivity or crack calibration standards. The instrument has a sensor designed to minimize unmodeled parasitic effects. To accomplish this, the sensor has one or more of the following features: dummy secondary elements located at the ends of a primary winding meandering, setting back of the sensing element from a connecting portion of the primary winding, or various grouping of secondary elements. The sensing elements of the sensor can be connected individually or in differential mode to gather absolute or differential sensitivity measurements. In addition, the instrumentation is configured such that a significant portion of the instrumentation electronics is placed as close to the sensor head to provide independently controllable amplification of the measurement signals therein reducing noise and other non-modeled effects.

Description

RELATED APPLICATIONS[0001]This application claims the benefit of provisional application 60 / 063,534, filed Oct. 29, 1997, provisional application 60 / 069,604 filed Dec. 15, 1997, and provisional application 60 / 104,526 filed Oct. 16, 1998, the entire teachings of are incorporated herein by reference.BACKGROUND OF THE INVENTION[0002]The technical field of this invention is magnetometry and, in particular, the nondestructive electromagnetic interrogation of materials of interest to deduce their physical properties and to measure kinematic properties such as proximity. The disclosed invention applies to both conducting and magnetic media.[0003]Conventional application of magnetometers, specifically eddy current sensors, involves the excitation of a conducting winding, the primary, with an electric current source of prescribed temporal frequency. This produces a time-varying magnetic field at the same frequency. The primary winding is located in close proximity to the material under test ...

AI Technical Summary

Benefits of technology

[0014]It is recognized that there is a need for measurement methods that provide estimates of the actual physical properties of the MUT Current techniques often measure “effective” properties that are only indirectly related to the actual physical properties (e.g., permeability and conductivity at a specified excitation frequency). These “effective” property measurements often provide insufficient characterization of the MUT For example, multiple temporal excitation frequencies are often used to obtain estimates of conductivity or permeability. This is not acceptable if these physical properties vary with temporal excitation frequency. In applications such as monitoring of aging and fatigue in ferrous and nonferrous metal alloys, it may be necessary to completely characterize the dispersive properties of the MUT, including the variations of conductivity and permeability with temporal excitation frequency. U.S. Pat. Nos. 5,015,951; 5,453,689; and 5,629,621 describe methods for such dispersive property measurement. However, the robustness of these earlier improvements is limited by the presence of unmodeled sensor and material behavior. There is a need for methods and sensors that can provide accurate and reproducible measurement of absolute properties without using conductivity or crack calibration standards. This will reduce errors caused by variations in sensor placement (e.g., lift-off) during calibration, variations in calibration standard properties that are uncontrolled, and human error.

[0015]Another enhancement that would extend the measurement performance capability of magnetometers is the ability to calibrate in air. This calibration accounts for instrument drift and unmodeled sensor behavior, which includes cable capacitance variations and manufacture or service created probe-to-probe variations. Often, variability in the manufacture of a given probe design is significant enough to require calibration on standards that have material properties and shape similar to the material under test. The ability to calibrate in air eliminates the inherent limitations of these standards. Other advantages include a reduced opportunity for human error in the selection of the property standards, self-consistently accounting for temperature variation since the calibration is not dependent upon any temperature variations in the standards, and self-consistently removing frequency-to-frequency variations without corrupting the calibration through the use of non-uniform reference standards. These advantages of an air calibration capability can lead to improved robustness and reproducibility of the measurements, reduced costs with the elimination of logistics issues for standards, and the capability for robust, self-consistent component-to-component comparisons with trend analysis.

[0016]It is desired to have magnetometers that can robustly provide absolute measurements of the material properties with minimal calibrations. In particular it is desired to have a sensor that does not require an extensive set of training or reference parts for calibration, that may also be required to have the same shape as the component to be tested. This can be accomplished with a sensor that is designed to minimize unmodeled parasitic effect so that only the response of the sensor to an insulating nonmagnetic material such as air can provide the necessary calibration information. While previous sensor designs did support “air calibration,” this invention introduces several new improvements. Design modifications to the sensor that minimize the unmodeled effects include altering :he layout for the primary and secondary windings, utilizing an equivalent circuit model to account for the parasitic effects on the sensor response, and constructing electrical instrumentation that can extend the dynamic range of the sensor.

Problems solved by technology

These “effective” property measurements often provide insufficient characterization of the MUT For example, multiple temporal excitation frequencies are often used to obtain estimates of conductivity or permeability.

This is not acceptable if these physical properties vary with temporal excitation frequency.

However, the robustness of these earlier improvements is limited by the presence of unmodeled sensor and material behavior.

Often, variability in the manufacture of a given probe design is significant enough to require calibration on standards that have material properties and shape similar to the material under test.

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