An inductance measurement device and a method of measuring inductance

Abstract

The abstract pertains to an inductance measurement device that includes a circulator and a resonant tank circuit. The circulator is designed to receive a signal from a signal generator and direct it to the resonant tank circuit, which is configured to reflect the signal. The resonant tank circuit includes an inductor comprising a magnetostrictive material. The device also includes a processing device that receives the reflected signal from the circulator and processes it to determine a reflection coefficient.

Description

[0001] An inductance measurement device and a method of measuring inductance

[0002] Field

[0003] The technology pertains to the field of electrical engineering, specifically focusing on inductance measurement devices and methods.

[0004] Background In the field of bolt stress measurement, traditional methods involve strain gauges, optical interferometry or ultrasonic detection. While these methods exist, alternative approaches for stress measurement, particularly those leveraging the magnetostrictive effect, face distinct technical challenges, as described below.

[0005] Firstly, the detection of very small changes in inductance is required, which is a complex task. The sensitivity of the inductance measurement equipment is crucial in this regard.

[0006] Any lack of sensitivity can result in inaccurate readings, which in turn affects the reliability of stress and strain measurements in the materials.

[0007] Secondly, the high conductivity of the material, such as a bolt, necessitates low measurement frequencies to limit the generation of eddy currents, which not only restrict magnetic flux penetration but also produce opposing magnetic fields that adversely impact the accuracy of inductance measurements, particularly at higher frequencies.

[0008] To overcome these challenges, some approaches focus on analyzing the characteristics of a response signal across a range of frequencies. For example, document DE102022122172 describes a device for determining the mechanical load state of a mag- netostrictive fastening element by generating an excitation magnetic field and evaluating the frequency spectrum of the resulting response signal.

[0009] In summary, the prior art methods based on inductive principles for material stress measurement face significant challenges in terms of detecting small changes in inductance, maintaining low measurement frequencies, and ensuring magnetic flux penetration in con- ductive materials. These challenges directly impact the precision and reliability of stress and strain measurements in bolts. Summary

[0010] According to a first aspect of the disclosure, an inductance measurement device comprises a circulator configured to receive a signal from a signal generator and to direct the signal to a resonant tank circuit configured to reflect the signal. The resonant tank circuit comprises an inductor with rod core comprising a magnetostrictive material, a capacitor and a processing device configured to receive a reflected signal from the circulator and to process said reflected signal to determine a reflection coefficient. This configuration allows for precise measurement of inductance changes in the inductor, which corresponds to permeability changes in the magnetostrictive materials in response to mechanical strain, providing a highly sensitive and accurate measurement system.

[0011] Optionally in some examples, the signal generator is configured to be connected to a first port of the circulator. This ensures that the input signal is efficiently directed into the circulator, optimizing the signal flow and minimizing losses, which enhances the accuracy and reliability of the inductance measurement. Optionally in some examples, the resonant tank circuit is configured to be connected to a second port of the circulator. This setup allows the resonant tank circuit to effectively receive the input signal from the circulator and reflect it back, ensuring that the signal path is well-defined and that the reflected signal can be accurately captured and analyzed.

[0012] Optionally in some examples, the circulator is further configured to direct the reflected signal to a matched load. This feature ensures that the reflected signal is properly terminated, reducing signal reflections that could affect the accuracy of the measurement. By directing the reflected signal to a matched load, the system can achieve more precise and reliable measurements.

[0013] Optionally in some examples, the matched load is provided by the input impedance of the processing device. This integrated approach enhances measurement accuracy by improving the signal-to-noise ratio and minimizing parasitic impedance effects.

[0014] Optionally in some examples, the circulator is connected to the signal generator and the resonant tank circuit. This direct connection ensures that the signal flow between the components is seamless and efficient, reducing potential signal losses and interferences, which contributes to the overall accuracy and performance of the measurement device.

[0015] Optionally in some examples, the signal generator is configured to generate an AC signal. The use of an AC signal allows for the measurement of inductance over a range of frequencies, providing more detailed information about the inductive properties of the magnetostrictive material.

[0016] Optionally in some examples, the resonant tank circuit, the capacitor and the inductor are configured in a parallel resonant connection with the capacitor. This parallel configuration forms a resonant circuit that can enhance the sensitivity of the inductance measurement by creating a sharp resonance peak at a specific frequency, making it easier to detect changes in inductance.

[0017] Optionally in some examples, the resonant tank circuit the capacitor and the inductor are configured in a series resonant connection with the capacitor. This series configuration provides a pronounced resonance dip in impedance at a specific frequency, similarly allowing sensitive detection of changes in inductance from the magnetostrictive material. In some applications, the series configuration may also facilitate circuit integration or simplify impedance matching for particular measurement setups.

[0018] Optionally in some examples, the processing device comprises an analog-to-digital converter (ADC) configured to digitize the reflected signal. Digitizing the reflected signal allows for more precise and flexible signal processing, enabling analysis techniques to be applied. This can significantly improve the accuracy and reliability of the inductance measurement.

[0019] Optionally in some examples, the analog-to-digital converter (ADC) is a delta-sigma (AZ) ADC. This approach offers advantages such as high resolution and inherent anti-aliasing filtering, simplifying the analog front-end design and enabling precise digitization of even small changes in the reflected signal.

[0020] Optionally in some examples, the processing device comprises a window function configured to process the digitized signal. The window function helps to reduce spectral leakage and improve the frequency resolution of the digitized signal, which is for accurately deter- mining the reflection coefficient. This processing step enhances the quality of the signal analysis, leading to more precise inductance measurements.

[0021] Optionally in some examples, the processing device comprises a discrete Fourier transform (DFT) module configured to analyze the processed signal to determine a reflection coefficient. The DFT module allows for the transformation of the time-domain signal into the frequency domain, where the reflection coefficient can be more easily and accurately determined. This provides a robust method for determining inductance changes.

[0022] Optionally in some examples, the magnetostrictive material is a bolt. This application is particularly useful in structural health monitoring and other engineering applications where precise measurement of mechanical strain is relevant.

[0023] Optionally in some examples, the circulator is configured to provide isolation between at least two ports, such as between a signal generator port and a processing device port, to reduce sensitivity to imperfect impedance matching and maintain measurement accuracy. Optionally in some examples, the processing device is configured to perform calibration by recording a baseline reflection coefficient under substantially zero applied stress, and generating a calibration curve based on reflection coefficients obtained at a plurality of known stress levels applied to the magnetostrictive material. This step allows for establishing a baseline reference point for all subsequent stress measurements, enabling the creation of a lookup table or mathematical model for relating reflection coefficient to stress, which is crucial for the precision and reliability of the measurements.

[0024] According to a further aspect of the disclosure, a system for inductance measurement comprises an inductance measurement device according to the disclosure, and a signal generator. Optionally in some examples, the system further comprises a wireless communication module configured to transmit measurement data to a remote receiver. This integration enables the device to transmit data without physical cabling from the measurement location, providing a comprehensive solution for remote, real-time data acquisition and analysis, particularly beneficial for continuous structural health monitoring. Optionally in some examples, the system further comprises a temperature sensor operatively coupled to the processing device. This allows the processing device to use the temperature data to apply compensation algorithms, thereby effectively correcting for temperature-induced permeability changes and isolating the stress-induced changes, which improves the reliability and stability of the measurements in fluctuating temperature environments.

[0025] According to a further aspect of the disclosure, a method for determining a reflection coefficient comprises generating an input signal by means of a signal generator, directing the input signal to a resonant tank circuit by means of a circulator, wherein the resonant tank circuit comprises an inductor with rod-core comprising a magnetostrictive material, receiving a reflected signal from the resonant tank circuit by means of the circulator, and processing the reflected signal by means of a processing device to determine a reflection coefficient. This method provides a systematic approach to measuring inductance, through impedance, ensuring that each step is optimized for accuracy and reliability. Optionally in some examples, the reflected signal is digitized by means of an analog-to- digital converter (ADC). Digitizing the reflected signal allows for advanced digital signal processing techniques to be applied, improving the precision and flexibility of the measurement process. This step is for achieving high-resolution and accurate inductance measurements.

[0026] Optionally in some examples, the digitized signal is processed by means of a window function.

[0027] Optionally in some examples, the processed signal is analyzed by means of a discrete Fourier transform (DFT) module to determine the reflection coefficient. Optionally in some examples, a plurality of input signals are generated by the signal generator. The plurality of input signals may comprise signals at different frequencies applied sequentially or as a compound signal, and / or signals at different amplitudes to characterize potential non-linear effects. Generating multiple input signals allows for a more comprehensive analysis of the properties of the magnetostrictive material over a range of frequencies. This approach can provide more detailed information about the inductor's impedance behavior as the material's permeability changes and improve the accuracy of the inductance measurement.

[0028] Optionally in some examples, a plurality of strain levels are applied to the magnetostrictive material. Applying different strain levels to the magnetostrictive material allows for the measurement of how the inductance changes with mechanical strain. This information is valuable for applications such as structural health monitoring, where understanding the relationship between strain and inductance can provide insights into the integrity and performance of the structure being monitored.

[0029] Brief Description of the Drawings Examples are described in more detail below with reference to the appended drawings.

[0030] Figure 1 depicts an inductance measurement device according to various example embodiments of the present invention.

[0031] Figure 2 depicts an equivalent circuit model of the resonant tank circuit according to various example embodiments of the present invention. Figure 3a depicts a Smith chart representation of the reflection coefficient according to various example embodiments of the present invention, when AL=O, corresponding to the unstressed state of the magnetostrictive material.

[0032] Figure 3b depicts a Smith chart representation of the reflection coefficient according to various example embodiments of the present invention when AL£O, corresponding to a stressed state of the magnetostrictive material.

[0033] Figure 4 depicts a detailed view of the inductor according to various example embodiments of the present invention.

[0034] Detailed Description

[0035] The detailed description set forth below provides information and examples of the dis- closed technology with sufficient detail to enable those skilled in the art to practice the disclosure.

[0036] Figure 1 shows an inductance measurement Device 1, which comprises a Circulator 2, a Resonant Tank Circuit 4 and a Processing Device 5. A Signal Generator 3 generates an AC signal and is connected to port 21 of the Circulator 2. The Circulator 2 directs the signal from the Signal Generator 3 to the Resonant Tank Circuit 4. The Resonant Tank

[0037] Circuit 4 includes an Inductor 40, and a Capacitor 41 and equivalent series resistances ESR-L and ESR-C. The Inductor 40 is in parallel with the capacitor 41 . The Processing Device 5 is connected to port 23 of the circulator 2. The processing device 5 comprises an Analog-to-Digital Converter (ADC) 50, a Window Function 51, and a Discrete Fourier Transform (DFT) Module 52. Figure 1 depicts a parallel configuration. Equivalent series configurations are also within the invention and not separately illustrated for brevity.

[0038] Figure 2 shows the equivalent circuit models for the key components of the resonant tank circuit 4. The inductor 40, which comprises a coil 101 magnetically coupled to a magnetostrictive core 100, is represented by a model consisting of an ideal inductance (L3) in series with its equivalent series resistance (ESR_L). Separately, the capacitor 41 is represented by a model consisting of an ideal capacitance (C3) in series with its equivalent series resistance (ESR_C). These component models are used to analyze the behavior of the resonant tank circuit 4 described herein.

[0039] Figure 3a shows a Smith chart representing the response of the resonant tank circuit 4, with the reflection coefficient shown at multiple frequency points when AL=0, corresponding to the unstressed state of the magnetostrictive material 100. Figure 3b shows a Smith chart representing the response of the resonant tank circuit 4, with the reflection coefficient shown at multiple frequency points when AL£O, corresponding to a stressed state of the magnetostrictive material 100. The difference between the traces in Figures 3a and 3b indicates the change in reflection coefficient due to the stress- induced inductance change, which is used to determine the stress level.

[0040] Figure 4 shows a detailed view of the inductor 40. The inductor 40 may comprise a magnetostrictive material 100, here depicted as a bolt, which may be attached to an object 102. The inductor 40 may comprise a coil 101 enveloping the magnetostrictive material 100. Definitions of Technical Terms:

[0041] As used herein, the term 'magnetostrictive effect' refers to the phenomenon where ferromagnetic materials change their dimensions and magnetic properties when subjected to mechanical stress or magnetic fields. The inverse effect (Villari effect) refers specifically to permeability changes under mechanical stress. As used herein, the term 'reflection coefficient', or T refers to a complex number representing the ratio of reflected wave amplitude to incident wave amplitude at an impedance discontinuity. Its magnitude indicates how much signal is reflected (0 = perfect match, 1 = total reflection), and its phase indicates the nature of the impedance mismatch.

[0042] As used herein, the term 'equivalent series resistance' or ESR refers to the resistive losses inherent in reactive components (inductor 40 and capacitor 41), modeled as a resistance in series with the ideal component.

[0043] The signal digitization process is governed by the Nyquist-Shannon sampling theorem. This theorem dictates that to prevent aliasing and accurately represent an analog signal in digital form, the sampling frequency must be greater than twice the highest frequency present in the input signal, in accordance with the Nyquist-Shannon sampling theorem.

[0044] Therefore, the system is designed such that the analog signal is filtered to remove frequency components above half the sampling frequency before being digitized. As used herein, the term 'Nyquist frequency' refers to this operational limit, defined as half the sampling frequency, which represents the maximum signal frequency the system can ac- curately digitize.

[0045] As used herein, the term 'window function 51' refers to a mathematical function applied to sampled data to reduce spectral leakage in frequency analysis by smoothly tapering the signal at the boundaries. As used herein, the term 'capacitor 41 ' or 'capacitance' when referring to the resonant tank circuit 4 capacitive element, is to be understood broadly. It may encompass a discrete capacitor 41 component, or an inherent or parasitic capacitive property of the circuit configuration itself, which contributes to the resonant behavior. As used herein, the term 'amplitude' refers to the magnitude or strength of an oscillating signal, such as an AC voltage or current, representing its maximum displacement or intensity from a central value. The term 'frequency' refers to the rate at which an oscillating signal or phenomenon repeats a cycle, typically measured in Hertz (Hz). It represents the number of cycles per unit of time. The term 'phase' refers to the position of a point in time (or angle) on a waveform cycle. It describes the relative timing difference between two waveforms of the same frequency, or the state of a single waveform at a particular instant, often expressed in degrees or radians. The phase of the reflection coefficient, as used herein, refers to the argument (or angle) of the complex number r, typically expressed in degrees or radians, representing the phase shift introduced to the reflected signal relative to the incident signal.

[0046] As used herein, the term 'Impedance' refers to the total opposition that a circuit presents to the flow of alternating current, measured in ohms (Q). It is a complex quantity comprising both a resistive component (resistance) and a reactive component (reactance, which can be inductive or capacitive). As used herein, the term “magnetostrictive material 100” refers to a material that, under the operating conditions present in the device (i.e. , expected frequency, temperature, mechanical stress range), exhibits a change in magnetic permeability in response to mechanical stress (the Villari effect) that results in measurable variations in the inductance in the resonant tank circuit 4. Suitable materials are those that allow changes in mechanical stress or strain to be detected as corresponding, quantifiable changes in circuit inductance using the methods and apparatus described herein.

[0047] Non-limiting examples of suitable magnetostrictive materials include iron, certain steels, nickel, cobalt, Terfenol-D, and other alloys or materials known to those skilled in the art for exhibiting a measurable Villari effect under operating conditions. This section details the components of the inductance measurement device 1 and their respective functions within the system. The device 1 measures impedance, from which the change in inductance due to magnetostrictive effects is derived. The technology pertains to the field of electrical engineering, specifically focusing on inductance measurement device and methods. The principle of the disclosed inductance measurement device 1 and method for determining a reflection coefficient relies on the magnetostrictive effect to measure mechanical stress in materials 100, such as bolts. The components work together to measure the inductance of an inductor 40 comprising a magnetostrictive material 100, such as a bolt, by analyzing the reflected signal from a resonant tank circuit 4. In some im- piementations the inductance measurement device 1 may be used to measure impedance of an inductor 40 containing the magnetostrictive material 100.

[0048] The inductance measurement device 1 utilizes the principle of resonant coupling between an inductor 40 and a capacitor 41, forming a tank circuit 4 with a pronounced resonance at a characteristic frequency. This resonance condition, whether implemented in parallel or series configuration, amplifies the electrical signature of changes in the magnetostrictive element 100. The system achieves heightened sensitivity to mechanical stress or strain through detection of shifts in the resonance condition.

[0049] In a parallel resonant configuration, the circuit exhibits a high-impedance resonance peak, which enhances sensitivity to small inductance changes by sharpening the frequency re- sponse. Conversely, a series resonant configuration yields a low-impedance resonance at a specific frequency, also facilitating precise detection of inductance variations. Both configurations support the core measurement principle and may be selected according to the requirements of the application.

[0050] The measurement principle utilizes the magnetostrictive effect, which, when referring to permeability changes under stress, is also known as the Villari effect. This effect describes how mechanical stress applied to a magnetostrictive material 100 causes a change in its magnetic permeability (p). This permeability change directly affects the inductance (L) of an inductor 40 in which the material 100 serves as the core. As mechanical stress (a) is applied to the magnetostrictive material 100, its magnetic permeability (p) changes, directly influencing the inductance (L) of the coil 101.

[0051] The device operates by receiving an input signal, typically an AC signal , which is directed to a resonant tank circuit 4 containing an inductor 40 with the magnetostrictive material 100 and a capacitor 41. A portion of this signal is reflected back, carrying information about the impedance of the resonant tank circuit 4. As used throughout this application, the term “resonant tank circuit” encompasses both parallel and series resonant topologies, unless expressly stated otherwise. While the examples and certain preferred embodiments described herein employ a parallel configuration, the invention is equally applicable to implementations utilizing a series resonant connection of the inductor 40 and capacitor 41. The inventive concept concerns the use of resonant coupling for precise inductance measurement rather than any particular circuit topology, and series and parallel resonance may both be employed depending on application requirements.

[0052] The inductor 40 impedance can be expressed as Z_L = ESR_L+ jcoL, and the capacitor 41 impedance can be expressed as Z_C = ESR_C - jZ(coC), where ESR_L and ESR_C represent the equivalent series resistances of the inductor 40 and capacitor 41 respectively, co is the angular frequency in radians per second (rad / s), L is the inductance in Henry (H), C is the capacitance in Farad (F), and j is the imaginary unit. The configuration of the tank circuit 4 determines how these impedances combine:

[0053] For a parallel resonant tank circuit 4, the total impedance corresponds to the parallel combination of Z_L and Z_C (and their associated resistances).

[0054] For a series resonant tank circuit 4, the total impedance is the sum of Z_L and Z_C (and their associated resistances). In both configurations, the overall impedance of the resonant tank circuit 4 (Z_tank) is derived from the reflection coefficient (r) and the characteristic impedance (Zo) of the system using the following relationship:

[0055] Z_tank = Z0(l + T) / (l - r)

[0056] This calculation allows for the precise determination of the resonant tank circuit 4 impedance. The processing device 5 receives this reflected signal and processes it to determine the reflection coefficient. This processing may include digitization, filtering, and analysis using techniques like a discrete Fourier transform (DFT) module 52.

[0057] By analyzing changes in the inductor 40 impedance, which encompasses both its inductive and resistive components, the system can ascertain the stress state of the magnetostric- tive material 100. While the reflection coefficient directly yields the impedance, further analysis of the impedance's reactive component (inductive reactance) allows for the isolation and quantification of the inductance (L). Changes in this inductance (AL) are then directly correlated with the mechanical strain and / or stress applied to the magnetostrictive material 100. This provides a highly sensitive and accurate measurement system for applications such as structural health monitoring.

[0058] The inductance measurement device 1 comprises several components: a circulator 2, a resonant tank circuit 4, and processing device 5. These components are interconnected to facilitate the measurement process. The inductance measurement device 1 may further include a signal generator 3, that produces an AC signal which is directed by the circulator 2 to the resonant tank circuit 4. The resonant tank circuit 4 includes an inductor 40 with rod core comprising a magnetostrictive material 100, such as a bolt, and a capacitor 41, and exhibits equivalent series resistances in its model representation. The reflected signal from the resonant tank circuit 4 is routed by the circulator 2 to the processing device 5 for analysis. The processing device 5 analyzes the reflected signal to determine the reflection coefficient, which is then used to calculate the inductance of the inductor 40, which varies with the permeability of the magnetostrictive material 100. The reflection coefficient provides information about the impedance of the resonant tank circuit 4, including both its inductive reactance and resistive components.

[0059] In some implementations, a circulator 2 is a three-port device configured to direct signals from a first port 21 connected to the signal generator 3 to a second port 22 connected to the resonant tank circuit 4, and to direct reflected signals from the second port 22 to a third port 23 connected to the processing device 5. This ensures unidirectional signal flow and isolation between the signal generator 3 and processing device 5 while allowing both to interface with the resonant tank circuit 4.

[0060] This device offers a precise and efficient method for measuring inductance, particularly beneficial for materials 100 like magnetostrictive bolts where the magnetic permeability changes under stress. This enables real-time monitoring of the bolt's condition, contributing to improved structural health monitoring and predictive maintenance in various applications.

[0061] The inductance measurement device 1 includes a circulator 2 that directs the signal from the signal generator 3 to the resonant tank circuit 4 and the reflected signal to the process- ing device 5. In some configurations, the circulator 2 may be a three-port device connected to the signal generator 3, resonant tank circuit 4, and processing device 5. In some implementations, the input impedance of the processing device 5 serves as the matched load for the reflected signal. This matched load consists of the measuring electronics, usually the ADC 50. The matching can be achieved by selecting the circulator's characteristic impedance to match the ADC's input impedance through component selection. Alternatively, a matching network may be used to match the ADC's impedance to the circulator 2. In further embodiments, the matched load may be a discrete termination resistor, selected to provide impedance matching within the measurement system. The specific method of implementing the matched load (eg. by resistor selection, integration into the processing electronics, or other conventional arrangements) can be chosen according to routine prac- tices in the field. Calibration steps, such as reference measurements with known loads, may be performed as part of standard manufacturing and setup procedures.

[0062] In some embodiments, for enhanced measurement sensitivity, the characteristic impedance (Zo) of the circulator 2 is selected to be within the predetermined range of to the impedance of the resonant tank circuit 4 at its resonance frequency. This impedance matching condition maximizes the change in the phase of the reflection coefficient (I") in response to a change in inductance, thereby providing the highest sensitivity for phasebased measurements. However, as this condition of maximum phase sensitivity also corresponds to a minimum in the reflected signal's amplitude, which can make the measure- ment vulnerable to noise, a practical implementation may involve a compromise. Specifically, the characteristic impedance (Zo) may be selected to be close to, but not perfectly matched with, the impedance of the resonant tank circuit 4, so as to achieve a desired balance between phase response sensitivity and a sufficient signal-to-noise ratio for the processing device 5. As used herein, 'within a predetermined range ' indicates that the characteristic impedance of the circulator 2 is selected to be close to the impedance of the resonant tank circuit 4 at resonance, rather than requiring a perfect match. This predetermined range is chosen to achieve a desired balance between two competing factors: maximizing the phase sensitivity of the measurement, which occurs at a perfect impedance match, and maintaining a sufficient reflected signal amplitude for a robust signal-to-noise ratio at the processing device 5, which requires some degree of mismatch. In some embodiments, the predetermined range may correspond to the characteristic impedance being within approximately ±10% to ±30% of the impedance of the resonant tank circuit 4 at its resonance frequency. In some embodiments, the characteristic impedance is selected to achieve a desired balance between phase response sensitivity and a sufficient reflected signal amplitude for the processing device 5. In some embodiments, the characteristic impedance is selected to maintain a sufficient reflected signal amplitude for a robust signal-to-noise ratio at the processing device 5. In some embodiments, the characteristic impedance is selected to be proximate to the impedance of the resonant tank circuit 4 at its resonance frequency to achieve heightened phase sensitivity, but does not correspond to a perfect impedance match.

[0063] In some embodiments, the design of the circulator 2 provides isolation between port 21 (connected to the signal generator 3) and port 23 (connected to the processing device 5). This isolation advantageously improves measurement accuracy by reducing the sensitivity of results to imperfect impedance matching between the processing device 5 and port 23 of the circulator 2. Accordingly, accurate inductance measurements are enabled even where the input impedance of the measurement circuit is not perfectly matched to the characteristic impedance of the circulator 2.

[0064] The circulator 2 provides isolation between selected ports based on the assignment of system functions (signal generation, measurement, and load) to the circulator’s ports. By selecting which device is connected to each port, the measurement system can be configured so that isolation is achieved between the signal generator 3 and the processing device 5, or between other pairs, as required for optimal measurement performance in a given setup.

[0065] The achievable isolation depends on the manner in which the respective components are connected to the circulator 2 ports, as will be readily understood by those skilled in the art. In some embodiments, the measurement system may be arranged such that the circulator 2 provides isolation between, for example, the signal generator input (port 21) and the processing device (port 23), by connecting each device to the appropriate circulator 2 ports for the desired measurement flow. Such isolation improves measurement fidelity or reduce susceptibility to impedance mismatch. In some embodiments, the circulator 2 may be configured to provide isolation between at least two ports of the circulator 2.

[0066] The device 1 also may include a signal generator 3 that generates the input signal for the system. The input signal may be an AC signal. In some configurations, the signal generator 3 may be connected to port 21 of the circulator 2.

[0067] The inductance measurement device 1 includes a resonant tank circuit 4. In some config- urations, the resonant tank circuit 4 may be connected to a second port 22 of the circulator 2. The resonant tank circuit 4 may include an inductor 40. The inductor 40 and the capacitor 41 may be configured in a parallel resonant connection, where the inductor's equivalent series resistance is in series with its inductance, and this series combination is in parallel with the capacitor 41 and its equivalent series resistance. In further embodiments, the inductor 40 and capacitor 41 of the resonant tank circuit 4 may be configured in a series resonant connection. This provides flexibility in implementation and broadens the compatibility with various measurement and stress monitoring scenarios. In some instances, the inductor 40 may comprise a coil 101 arranged to a rod core comprising a magnetostrictive material 100, wherein the coil 101 comprises an inductance to be measured. The resonant tank circuit 4 may also include a capacitor 41. The capacitor forms part of the resonant tank circuit 4. In some configurations, the capacitor 41 may be in parallel with the inductor 40. In some configurations, the capacitor 41 may be in series with the inductor 40.

[0068] The inductance measurement device 1 includes a processing device 5. The processing device 5 may include an analog-to-digital converter (ADC) 50. The ADC 50 digitizes the reflected signal. The processing device 5 may also include a window function 51. The window function 51 processes the digitized signal. In some instances, the processing device 5 may include a discrete fourier transform (DFT) module 52. The DFT module 52 analyzes the processed signal to determine the reflection coefficient. The DFT module 52 may be implemented using various algorithms including Fast Fourier Transform (FFT).

[0069] In some embodiments, the DFT module 52 may implement the Goertzel algorithm for efficient single-frequency analysis at the excitation frequency.

[0070] The magnetostrictive effect responds differently to various types of mechanical stress.

[0071] Axial stress (tension or compression along the bolt axis) produces the primary inductance change used for measurement. Shear stress and torsional loads also affect the magnetic permeability but with different sensitivity coefficients. For a bolt under combined loading, the total permeability change can be expressed as:

[0072] Ap / p = X_a o_a + X_s i_s + X_t i_t where A_a, A_s, and X_t are the magnetostrictive coefficients for axial, shear, and torsional stress respectively, o_a is the axial stress, and i_s and i_t are the shear and torsional stresses. In typical bolt applications, the axial component dominates, but the system can detect combined stress states through careful calibration.

[0073] The matched load referenced in the system is implemented by the input impedance of the processing device 5, which is designed to match the characteristic impedance of the system to minimize unwanted reflections.

[0074] In some embodiments, temperature compensation may be implemented to account for temperature-dependent permeability changes in the magnetic permeability of the magnetostrictive material 100. Temperature compensation may be achieved for example through direct temperature measurement and correction algorithms, or through differential mea- surement using a reference inductor at the same temperature, typically positioned to experience the same temperature as the measurement inductor 40 but not subject to mechanical stress, optionally in combination with correction algorithms.

[0075] The method involves generating an input signal using a signal generator 3, which may involve generating a plurality of input signals. In some examples the plurality of input signals may comprise signals at different frequencies generated sequentially. In some examples, signals at multiple frequencies may be generated simultaneously. In some examples, measurement can be performed using a single frequency, which may be sufficient for certain applications where the frequency response characterization is not required. The input signal is directed to a resonant tank circuit 4 by means of a circulator 2. The resonant tank circuit 4 may comprise an inductor 40 formed by a coil 101 arranged to a rod core comprising the magnetostrictive material 100. In other words, the coil 101 may comprise an inductance which varies depending on the strain and / or stress level of the magnetostrictive material 100. The magnetostrictive material 100 may in itself have a magnetic permeabil- ity which varies depending the strain and / or stress levels applied to the magnetostrictive material 100. This setup may provide a plurality of strain levels to the magnetostrictive material 100. In some embodiments, the signal generator 3 is configured to generate a plurality of sinusoidal signals, either sequentially or simultaneously, at distinct frequencies to allow for frequency response characterization of the magnetostrictive material 100. A reflected signal is received from the resonant tank circuit 4 by means of the circulator 2. The reflected signal is processed by a processing device 5. This processing may involve digitizing the reflected signal using an analog-to-digital converter (ADC) 50, processing the digitized signal using a window function 51, and analyzing the processed signal using a discrete fourier transform (DFT) module 52 to determine the reflection coefficient. Finally, the impedance of the inductor 40 comprising a magnetostrictive material 100 is calculated based on the reflection coefficient. The impedance can be calculated from the reflection coefficient using Z_tank = Z0(l + T) / (l - T).

[0076] The method of measuring inductance of an inductor 40 comprising a magnetostrictive material 100 involves a series of steps that utilize the components of the inductance measure- ment device 1. The process begins with the generation of an input signal and culminates in the calculation of the inductor's 40 impedance, which reflects the permeability state of the magnetostrictive material 100.

[0077] The generated signal is directed to the resonant tank circuit 4 containing the magnetostrictive material 100. The reflected signal from this circuit 4 is then received and processed to determine the reflection coefficient. Finally, this coefficient is used to calculate the impedance of the inductor 40, providing a measure of its inductance.

[0078] Variations in the method may include generating multiple input signals at different frequencies or applying varying strain levels to the magnetostrictive material 100. These variations allow for an understanding of the inductor 40 impedance behavior under different conditions.

[0079] The generation of the input signal is the first step in the inductance measurement process. This signal, typically an AC signal, is generated by the signal generator 3.

[0080] The characteristics of the input signal, such as its frequency, amplitude, and phase can be adjusted to suit the specific requirements of the measurement. For instance, multiple frequency measurements can be used to analyze the frequency response of the magnetostrictive material 100.

[0081] The signal generator 3 is a component in the inductance measurement process. It is responsible for generating the input signal that probes the resonant tank circuit 4. The signal generator 3 is configured to produce an AC signal with precise and controllable characteristics. In various embodiments, this AC signal is a sinusoidal signal, or a combination of multiple sinusoidal signals at distinct frequencies. Such sinusoidal signals enable straightforward frequency-domain analysis using techniques like Discrete Fourier Transform. Alternatively, the AC signal could comprise other waveforms, such as pulsed or swept-frequency signals, tailored to specific measurement requirements or to excite particular modes of the magnetostrictive material 100 response.

[0082] The signal generator's ability to produce various types of signals, such as continuous wave or multiple discrete frequency signals, allows for flexibility in the measurement process. This adaptability enables the characterization of the inductor's 40 impedance as the mag- netostrictive material's 100 permeability varies under different conditions.

[0083] Following generation, the input signal is directed to the resonant tank circuit 4. This direction is controlled by the circulator 2, a device designed to route signals in a specific manner.

[0084] The circulator 2 ensures that the input signal reaches the resonant tank circuit 4 and that the reflected signal is subsequently directed to the appropriate components for processing.

[0085] The circulator 2 acts as a traffic controller for the signals within the inductance measurement device 1. It ensures that the input signal from the signal generator 3 is directed to the resonant tank circuit 4 and that the reflected signal from the resonant tank circuit 4 is routed to the processing device 5. This controlled signal routing allows for accurate and efficient measurement. By preventing unwanted signal paths, the circulator 2 maintains the integrity of the measurement process.

[0086] After interacting with the resonant tank circuit 4, a portion of the input signal is reflected back. This reflected signal carries information about the impedance of the inductance, which varies with the permeability of the magnetostrictive material 100.

[0087] The circulator 2 directs this reflected signal to the processing device 5 for analysis. This ensures that the reflected signal is properly routed for processing. In addition to directing the input signal, the circulator 2 receives the reflected signal from the resonant tank circuit 4. It ensures that this reflected signal, is properly routed to the processing device 5.

[0088] This directed routing of the reflected signal enables accurate analysis. By ensuring the signal reaches the processing device 5 without interference, the circulator 2 contributes to the overall precision of the inductance measurement.

[0089] The processing device 5 receives the reflected signal and performs a series of operations to extract the reflection coefficient. This typically involves digitization, filtering, and analysis of the signal. The processing device 5 is configured to analyze the reflected signal from the resonant tank circuit 4 via the circulator 2. It performs several functions to determine the reflection coefficient.

[0090] First, the processing device 5 digitizes the reflected analog signal using an analog-to- digital converter (ADC) 50. This conversion transforms the continuous analog signal into a discrete digital representation suitable for further processing by digital components.

[0091] In some embodiments, a delta-sigma ADC 50 may be employed, which offers advantages such as high resolution and inherent anti-aliasing filtering, simplifying the analog front-end design. Delta-sigma ADCs 50 achieve high resolution through oversampling and noise shaping, pushing quantization noise to higher frequencies where it can be more easily filtered. This allows for precise digitization of even small changes in the reflected signal, contributing to the overall accuracy and robust signal-to-noise ratio of the inductance measurement. A delta-sigma (AZ) analog-to-digital converter is a high-resolution ADC architecture that uses oversampling and noise-shaping to enable accurate conversion of small analog signal changes, particularly suitable for precision measurement systems. Next, the digitized signal is processed using a window function 51. This function applies a weighting to the sampled data to minimize spectral leakage and improve the accuracy of the subsequent frequency analysis. Different window function(s) 51, such as Hanning or Hamming windows, can be employed depending on the specific requirements of the measurement. Following windowing, the processed signal is analyzed using a discrete fourier transform (DFT) module 52. The DFT 52 may be implemented using various algorithms including Fast Fourier Transform (FFT). The DFT 52 decomposes the digitized signal into its constituent frequency components at the frequency (or frequencies) of interest, which are used for determining the reflection coefficient. This process allows for the precise determination of the reflection coefficient at each specific frequency point. For implementations utilizing a single excitation frequency (e.g., employing the Goertzel algorithm), the reflection coefficient is determined at that sole frequency point, enabling efficient single- frequency analysis. In implementations generating a plurality of input signals at different frequencies (e.g., sequentially or simultaneously), the DFT 52 is applied to each frequency of interest to determine the reflection coefficient at each respective frequency point. The DFT 52 provides both the magnitude and accurate phase information of the frequency components, which is critical for calculating the complex reflection coefficient and sub- sequently the impedance. Furthermore, the DFT 52 acts as a frequency-selective filter, effectively isolating the signal components at the excitation frequency(ies) while rejecting noise and interference at other frequencies, thereby enhancing the signal-to-noise ratio and measurement precision.

[0092] The DFT module 52 analyzes the processed signal to calculate the reflection coefficient, a complex quantity that represents the ratio of the reflected signal to the incident signal.

[0093] To determine this ratio, various methods can be employed. In some implementations, the system may measure both the incident signal (generated by the signal generator 3) and the reflected signal. Alternatively, in instances where the input signal is generated by the system itself and its characteristics can be precisely known or monitored, the re- flection coefficient can be determined by measuring only the reflected signal and comparing it against the known incident signal. When determining the reflection coefficient by comparing the received reflected signal with a known characteristic of the input signal, 'known characteristic' refers to parameters of the signal that are precisely controlled and recorded by the signal generator 3 itself, or by an internal monitoring circuit within the device 1. These characteristics can include, but are not limited to, the signal's amplitude, frequency, and phase at the point of generation or before it interacts with the resonant tank circuit 4. Since the input signal is generated by the system, its properties are inherently defined and can be stored, pre-calibrated, or dynamically monitored by the processing device 5. This internal knowledge of the input signal simplifies the measurement process by eliminating the need for a separate incident signal measurement. The magnitude and phase of the reflection coefficient carry information about the impedance of the resonant tank circuit 4.

[0094] In some implementations of the device 1 or the method, the processing device 5 is configured to determine the reflection coefficient by comparing the reflected signal with a known characteristic of the input signal generated by the signal generator 3. In some implementations, the processing device 5 is configured to determine the reflection coefficient by comparing the reflected signal with a known characteristic of the input signal generated by the signal generator 3, wherein the known characteristic of the input signal generated by the signal generator 3 is selected from the group consisting of: signal's amplitude, signal's frequency, signal's phase and combinations thereof. In some implementations of the device 1 or the method, the processing device 5 is configured to determine the reflection coefficient by measuring the input signal and comparing the measured input signal with the reflected signal.

[0095] The process commences with the generation of an AC signal by the signal generator 3. This signal acts as the stimulus for the impedance measurement. The signal's character- istics, such as frequency, amplitude, and phase are carefully controlled to ensure accurate and reliable measurements. The generated signal is then directed to the resonant tank circuit 4 via the circulator 2. The circulator 2 ensures that the signal reaches the resonant tank circuit 4 without interference, maintaining the integrity of the measurement process.

[0096] The signal generation process begins with setting the desired parameters of the AC sig- nal, such as frequency, amplitude, and waveform. The signal generator 3 then produces the signal according to these specifications. The generated signal is then routed to the circulator's input port. The circulator 2, acting as a directional device, guides the signal to the port connected to the resonant tank circuit 4. This ensures that the signal reaches the resonant tank circuit 4 with minimal loss or distortion. The specific type of circulator 2 used may vary depending on the frequency range and power level of the signal.

[0097] Upon reaching the resonant tank circuit 4, the signal interacts with the magnetostrictive material 100, causing a portion of the signal to be reflected. This reflected signal carries information about the inductor 40 impedance, which depends on the permeability of the magnetostrictive material 100. The circulator 2 directs the reflected signal to the process- ing device, which performs a series of operations to extract the reflection coefficient. These operations typically involve digitization, filtering, and analysis of the signal. The reflection coefficient, derived from the processed signal, is then used to calculate the impedance of the inductor 40, providing a measure of its inductance.

[0098] The reflected signal from the resonant tank circuit 4 is directed by the circulator 2 to the processing device 5. The processing device 5 first digitizes the analog signal using an analog-to-digital converter (ADC) 50. The digitized signal is then processed using a window function 51 to minimize spectral leakage and improve the accuracy of subsequent frequency analysis. A discrete Fourier transform (DFT) 52 is then applied to the windowed signal to extract the frequency components at the frequency (or frequencies) of interest and determine the reflection coefficient. This coefficient, a complex quantity, is then used to calculate the impedance of the inductor 40, from which the stress state of the magnetostrictive material 100 can be inferred.

[0099] The signal generator 3 is configured to generate a multiple frequency AC signal, allowing for analysis of the frequency response of the magnetostrictive material 100. The circulator 2 is a three-port device, ensuring unidirectional signal flow. The resonant tank circuit

[0100] 4 preferably includes a high-Q capacitor 41 to enhance sensitivity. A 'high-Q capacitor' refers to a capacitor 41 with a high quality factor (Q), meaning it has very low energy loss (low equivalent series resistance) relative to its reactance. Such capacitors 41 are desirable in resonant circuits as they lead to sharper resonance peaks or dips, thereby increasing the circuit's sensitivity to small changes in inductance. However, capacitors 41 with other Q factors may also be used, depending on application-specific requirements, cost, or availability. The processing device 5 utilizes a high-resolution ADC 50 for precise digitization and a carefully selected window function to minimize spectral leakage. The DFT Module 52 is optimized for fast and accurate analysis of the processed signal. This example describes a specific method for determining the reflection coefficient, a parameter for calculating the inductance of the inductor 40 comprising the magnetostrictive material 100.

[0101] The method begins with the signal generator 3 generating an AC signal. The circulator 2 directs this signal to the resonant tank circuit 4. The reflected signal from the resonant tank circuit 4 is then routed by the circulator 2 to the processing device 5. The processing device

[0102] 5 digitizes the reflected signal using an analog-to-digital converter (ADC) 50, processes it with a window function 51, and analyzes it using a discrete Fourier transform (DFT) module 52 to determine the reflection coefficient.

[0103] This subsection provides a step-by-step explanation of the reflection coefficient determ i- nation method. It details the signal processing steps performed by the processing device 5.

[0104] The reflected signal, received by the processing device 5, is first amplified and filtered to remove noise and frequency components at or above the Nyquist frequency. The ADC 50 then digitizes the filtered signal, converting it into a digital representation suitable for further processing. The window function 51 is applied to the digitized signal to minimize spectral leakage, a phenomenon that can distort the frequency spectrum . The DFT Module 52 then performs a frequency analysis of the windowed signal, determining the magnitude and phase of the reflected signal at different frequencies of interest. The reflection coefficient is calculated as the ratio of the reflected signal to the incident signal when the incident signal characteristics are known or predetermined. This detailed description clarifies the signal processing steps involved in determining the reflection coefficient, emphasizing the importance of each step in ensuring accurate measurements.

[0105] In some implementations, the device 1 may require calibration to establish the relation- ship between measured reflection coefficient and applied stress. This may involve the processing device 5 measuring a first reflection coefficient with the magnetostrictive material 100 under 'substantially zero applied stress' to establish a baseline, applying known stress levels and recording the corresponding reflection coefficients to create a calibration curve, and if temperature compensation is implemented, performing measurements at different temperatures to characterize temperature effects. The term 'substantially zero applied stress' refers to a condition where the mechanical stress on the magnetostrictive material 100 is minimal, typically within the elastic limits of the material where permanent deformation is negligible, and is generally below the sensitivity threshold of the measurement system. This allows for establishing an unstressed reference point. The calibration data may be used by the processing device 5 to create a lookup table or mathematical model relating reflection coefficient to stress, accounting for the specific magnetostrictive material 100 properties and geometry.

[0106] To streamline the calibration process, the device 1 may be configured for baseline calibration. This involves an initial measurement performed by the processing device 5 under 'substantially zero applied stress' conditions after device 1 installation, which serves as a reference point for all subsequent stress measurements. This baseline ensures consistency and reduces manual setup time. The system can store this baseline data internally, typically under the control of the processing device 5, or transmit it to a remote monitoring system for reference during continuous operation. In some embodiments, the baseline calibration may be an automatic baseline calibration, performed by the processing device 5 without user intervention. In other embodiments, calibration may be manual or semi-automatic, where the user initiates or assists with the calibration procedure, such as applying baseline (zero-stress) conditions or known stress levels, confirming conditions, and triggering or verifying measurement and recording of calibration data. The system may also provide prompts or guidance to support operator-driven calibration. Calibration routines can be executed at installation, during routine maintenance, or at scheduled intervals as appropriate for the application.

[0107] The inductance measurement device 1 offers a range of potential applications across various fields, capitalizing on its ability to accurately and efficiently measure the inductance of inductors 40 containing magnetostrictive materials 100. These applications span from material characterization and structural health monitoring to industrial quality control and construction monitoring.

[0108] The non-destructive nature of the measurement technique makes it particularly attractive for preserving the integrity of the material. This characteristic allows for repeated mea- surements without compromising the material's properties, enabling continuous monitoring and assessment over time. Furthermore, the device's 1 ability to measure inductance changes under varying strain levels opens up possibilities for real-time monitoring of structural components, providing information for predictive maintenance and preventing catastrophic failures. The inductance measurement device 1 finds utility in material testing, particularly for characterizing the properties of magnetostrictive materials 100. It provides a precise and efficient method for determining the impedance behavior of inductor 40 containing these materials under various conditions, such as different temperatures, stress levels, and magnetic fields. This information allows for understanding the material's 100 permeability behavior and optimizing its performance in specific applications.

[0109] Traditional methods for measuring inductance often involve complex setups and timeconsuming procedures. The inductance measurement device 1 simplifies this process, offering a more streamlined and efficient approach. Its ability to measure inductance changes in real-time allows for dynamic characterization of the inductor's 40 response to external stimuli, providing valuable insights into its magnetostrictive properties.

[0110] In material testing, the inductance measurement device 1 can be used to characterize the magnetostrictive properties of materials 100 under a wide range of conditions. By applying varying strain levels to the material 100 and measuring the corresponding changes in inductor's 40 impedance, researchers can gain understanding of the material's 100 per- meability behavior. This information allows for developing new magnetostrictive materials 100 with tailored properties for specific applications.

[0111] The device's 1 ability to measure inductance changes in real-time allows for dynamic testing, providing insights into the material's 100 permeability response to varying stress, temperature, and magnetic fields. This capability enables researchers to optimize ma- terial 100 composition and processing techniques to achieve desired performance characteristics. The non-destructive nature of the measurement ensures that the material 100 remains intact throughout the testing process, preserving its integrity for further analysis or use.

[0112] In industrial quality control, the inductance measurement device 1 offers a valuable tool for ensuring the consistency and reliability of magnetostrictive components. By accurately measuring the impedance of inductor 40 containing these materials, manufacturers can identify defects, variations in material 100 properties, and potential performance issues. This capability enables early detection of problems, reducing waste, improving product quality, and enhancing overall efficiency.

[0113] The device's 1 non-destructive nature is particularly advantageous in quality control applications, allowing for repeated measurements without damaging the component. This feature enables testing and ensures that only components meeting the required specifications are used in final products. The device's 1 ability to measure inductance changes under stress also allows for assessment of the component's structural integrity and its ability to withstand operational loads.

[0114] In industrial quality control, the inductance measurement device 1 can be integrated into automated testing systems for high-volume inspection of magnetostrictive components. The device's 1 speed and accuracy allow for rapid and reliable assessment of component quality, ensuring that only components meeting the required specifications proceed to the next stage of production.

[0115] The device's 1 ability to detect subtle changes in inductance enables identification of even minor defects or variations in material properties. This capability allows for maintaining consistent product quality and preventing potential failures in the field. The non-destructive nature of the measurement ensures that tested components remain undamaged and can be used in final products without compromise.

[0116] In a construction monitoring, the inductance measurement device 1 offers a valuable tool for measuring and monitoring stress level and change in stress in magnetostrictive parts of constructions such as bolts and beams. By accurately measuring the impedance of inductor 40 integrated with these components constructions can be monitored for early defect detection or sudden changes in stress level of bolts and beams.

[0117] In some embodiments, the device 1 can characterize the magnetic properties of a material at zero applied stress, enabling identification of whether a bolt or structural element is made of magnetostrictive material 100 before stress monitoring begins. This preliminary characterization is not possible with optical or strain-gauge methods.

[0118] The device's 1 non-destructive nature is particularly advantageous in quality control applications, allowing for repeated measurements without damaging the component.

[0119] The inductance measurement device 1 is particularly well-suited for monitoring compo- nents under dynamic loading conditions, where stress and strain levels may change over time. Its real-time measurement capabilities allow for continuous monitoring of these changes, providing critical information for detecting early defects or sudden shifts in stress levels in magnetostrictive parts such as bolts and beams, thereby enabling advanced structural health monitoring and predictive maintenance.

[0120] In various implementations, the inductance measurement device 1 may be configured for enhanced connectivity and data management. This can include the integration of wireless communication capabilities, enabling the device 1, under the control of the processing device 5, to transmit data without physical cabling from the measurement location. Further- more, the device 1 may incorporate functionalities for cloud-based monitoring, , wherein the processing device 5 enables measured data to be transmitted to and processed by remote servers or cloud platforms. The remote server or cloud platform may store, process, analyze, or display the received data, optionally providing real-time visualization, alerting, or maintenance recommendations. In some embodiments, two-way communication may be implemented, allowing updates, calibration parameters, or user commands to be sent from the cloud platform to the device 1. It is contemplated that these functionalities can be implemented independently, where the device 1 offers either wireless communication or cloud-based monitoring. Alternatively, in a combined approach, the device 1 may leverage wireless communication to facilitate cloud-based monitoring, providing a comprehensive solution for remote, real-time data acquisition, analysis, and storage, particularly beneficial for applications like continuous structural health monitoring and predictive maintenance.

[0121] Furthermore, to account for manufacturing variations in components or slight environmental shifts, the device 1 may incorporate carrier frequency adjustment capabilities. Before or during initial deployment, a diagnostic routine may be performed by the processing de- vice 5 to sweep a narrow range of frequencies around the nominal operating frequency to identify the exact resonant frequency of the installed resonant tank circuit 4. This optimal frequency is then used as the carrier frequency for subsequent measurements, ensuring that the system always operates at the most sensitive point of the resonant curve, thereby maximizing measurement accuracy despite component tolerances. For enhanced accuracy and stability, particularly in environments with fluctuating temperatures, the device 1 may integrate one or more temperature sensors, for example, in proximity to the magnetostrictive material 100. These sensors continuously monitor the ambient or material temperature. The processing device 5 then uses this temperature data to apply compensation algorithms or refer to pre-calibrated temperature-stress lookup ta- bles, effectively correcting for temperature-induced permeability changes and isolating the stress-induced changes, thereby improving the reliability of the measurements.

[0122] The terminology used herein is for the purpose of describing particular aspects only and is not intended to be limiting of the disclosure. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. As used herein, the term "and / or" includes any and all combinations of one or more of the associated listed items. It will be further understood that the terms "comprises," "comprising," "includes," and / or "including" when used herein specify the presence of stated features, integers, actions, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, actions, steps, operations, elements, components, and / or groups thereof.

[0123] It will be understood that, although the terms first, second, etc., may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element without departing from the scope of the present disclosure.

[0124] Relative terms such as "below" or "above" or "upper" or "lower" or "horizontal" or "vertical" may be used herein to describe a relationship of one element to another element as illustrated in the Figures. It will be understood that these terms and those discussed above are intended to encompass different orientations of the device in addition to the orientation depicted in the Figures. It will be understood that when an element is referred to as being "connected" or "coupled" to another element, it can be directly connected or coupled to the other element, or intervening elements may be present. In contrast, when an element is referred to as being "directly connected" or "directly coupled" to another element, there are no intervening elements present. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. It will be further understood that terms used herein should be interpreted as having a meaning consistent with their meaning in the context of this specification and the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0125] It is to be understood that the present disclosure is not limited to the aspects described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the present disclosure and appended claims. In the drawings and specification, there have been disclosed aspects for purposes of illustration only and not for purposes of limitation, the scope of the disclosure being set forth in the following claims.

Claims

Claims1. An inductance measurement device (1) comprising: a circulator (2); a resonant tank circuit (4), the resonant tank circuit (4) comprising an inductor5 (40) comprising a magnetostrictive material (100), and a capacitor (41); and a processing device (5), wherein the circulator (2) is configured to receive a signal from a signal generator (3) and to direct the signal to the resonant tank circuit (4), wherein the resonant tank circuit (4) has a load impedance that causes a portion10 of the signal to be reflected back to the circulator (2) as a reflected signal, and wherein the processing device (5) is configured to receive a reflected signal from the circulator (2) and to process said reflected signal to determine a reflection coefficient.

2. The inductance measurement device (1) according to claim 1, wherein the circulars tor (2) is further configured to provide an electrical isolation between the processing device (5) and the signal generator (3).

3. The inductance measurement device (1) according to any one of the preceding claims, wherein the circulator (2) is further configured to direct the reflected signal to a matched load.20 4. The inductance measurement device (1) according to claim 3, wherein the matched load is provided by the input impedance of the processing device (5).

5. The inductance measurement device (1) according to claim 1, wherein the inductor (40) is configured in a parallel resonant connection with the capacitor (41).

6. The inductance measurement device (1) according to claim 1, wherein the inductor25 (40) is configured in a series resonant connection with the capacitor (41).

7. The inductance measurement device (1) according to any one of the preceding claims, wherein the processing device (5) comprises an analog-to-digital converter (ADC) (50) configured to digitize the reflected signal.

8. The inductance measurement device (1) according to claim 7, wherein the analog-to- 30 digital converter (ADC) (50) is a delta-sigma (AZ) ADC.

9. The inductance measurement device (1) according to any of claims 7 to 8 wherein the processing device (5) comprises a window function (51) configured to process thedigitized signal.

10. The inductance measurement device (1) according to claim 9, wherein the processing device (5) comprises a discrete fourier transform (DFT) module (52) configured to analyze the processed signal to determine a reflection coefficient.

11. The inductance measurement device (1) according to any one of the preceding claims, wherein the magnetostrictive material (100) is a bolt.

12. The inductance measurement device (1) according to any one of the preceding claims, wherein the circulator (2) has a characteristic impedance (Zo), wherein the characteristic impedance is selected to be within a predetermined range of the impedance of the resonant tank circuit (4) at its resonance frequency.

13. The inductance measurement device (1) according to any one of the preceding claims, wherein the processing device (5) is configured to perform calibration by recording a baseline reflection coefficient under substantially zero applied stress, and generating a calibration curve based on reflection coefficients obtained at a plurality of known stress levels applied to the magnetostrictive material (100).

14. A system for inductance measurement comprising: an inductance measurement device (1) according to any of claims 1 to 13, and a signal generator (3).

15. The system according to claim 14, wherein the circulator (2) is connected to the signal generator (3) and the resonant tank circuit (4).

16. The system according to any of claims 14 to 15, wherein the signal generator (3) is configured to be connected to a first port (21) of the circulator (2).

17. The system according to any of claims 14 to 16, wherein the resonant tank circuit (4) is configured to be connected to a second port (22) of the circulator.

18. The system according to any of claims 14 to 17, wherein the signal generator (3) is configured to generate an AC signal.

19. The system according to any of claims 14 to 18 , wherein the system further comprises a wireless communication module configured to transmit measurement data to a remote receiver.

20. The system according to any of claims 14 to 19, further comprising a temperature sensor operatively coupled to the processing device (5).

21. The system according to claim 20, wherein the processing device (5) is configured toreceive and process temperature measurement data from the temperature sensor to facilitate temperature compensation.

22. A method for determining a reflection coefficient, the method comprising: generating an input signal by means of a signal generator (3), directing the input signal to a resonant tank circuit (4) by means of a circulator(2), wherein the resonant tank circuit (4) comprises an inductor (40) comprising a magnetostrictive material (100), and a capacitor (41), receiving a reflected signal from the resonant tank circuit (4) by means of the circulator (2), processing the reflected signal by means of a processing device (5) to determine a reflection coefficient.

23. The method according to claim 22, wherein the reflected signal is digitized by means of an analog-to-digital converter (ADC) (50).

24. The method according to claim 23, wherein the digitized signal is processed by means of a window function (51).

25. The method according to claim 24, wherein the processed signal is analyzed by means of a discrete Fourier transform (DFT) module (52) to determine the reflection coefficient.

26. The method according to any of claims 22 to 25, wherein a plurality of input signals are generated by the signal generator (3).

27. The method according to any of claims 22 to 26, wherein a plurality of strain levels are applied to the magnetostrictive material (100).

28. The method according to any of claims 22 to 27, further comprising transmitting measurement data from the processing device (5) to a remote receiver by wireless com- munication.

29. The method according to any of claims 22 to 28, further comprising performing temperature compensation by receiving temperature measurement data from a temperature sensor by means of the processing device (5), and processing the temperature measurement data to facilitate temperature compensation.

30. The method according to any of claims 22 to 29, further comprising performing calibration by recording a baseline reflection coefficient under substantially zero applied stress, and generating a calibration curve based on reflection coefficients obtained ata plurality of known stress levels to the magnetostrictive material (100).

31. The method according to any of claims 22 to 30, further comprising characterizing the magnetic properties of the magnetostrictive material (100) under substantially zero applied stress, prior to performing stress or strain monitoring.

32. The method according to any of claims 22 to 31, further comprising calibrating the system by setting the carrier frequency of the signal generator (3) to match a measured resonance frequency of the resonant tank circuit (4) prior to measurement.

33. The method according to any one of claims 22 to 32, wherein the method is performed using the system according to any one of claims 14 to 21.

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