This paper presents a comprehensive study of the design aspects of eddy-current displacement sensor (ECS) systems. In accordance with the sensor analysis presented in this paper, design strategies to compensate for important sensor imperfections are recommended. To this end, the challenges that are associated with ECS interfaces are identified, with focus on advanced industrial applications. This paper also provides a technical overview of the design advances of ECS interfaces proposed in the last decade and evaluates their pros and cons. Recently reported interface solutions for demanding industrial applications with respect to high resolution, stability, bandwidth, and low power consumption, at a sufficiently high excitation frequency, are addressed in more detail.

Design Strategies for Eddy-Current Displacement Sensor Systems: Review and Recommendations

This paper proposes a new method to reduce the thermal drift of eddy-current sensors (ECSs) by two orders of magnitude. Theoretical analysis shows that a well-designed bridge will help to decouple two vectors related to the resistance and inductance variations of the sensing coil of ECSs. Experiments show resistance variation has a considerably larger coefficient with temperature change compared to that of inductance variation. Other than being neglected, resistance variation compensates for the influence of temperature on inductance variation, which is used to derive true displacement information. A prototype ECS with high-resolution of sub-nanometer and ultrahigh thermal stability is manufactured and tested. Results show that the thermal drift of the prototype ECS is approximately 2.6 nm/°C, equivalent to 9.7 ppm/°C of the coil's inductance change. This self-temperature compensation method for ECS is simple, low cost, universal, very effective, and has competitive advantages in most applications.

Ultrastable and highly sensitive eddy current displacement sensor using self-temperature compensation

A simple method for measuring the thickness of metal films based on eddy-current sensors (ECSs) immune to distance variation is proposed. The slope of the lift-off curve (LOC) in the RL impedance plane is a good feature for characterizing target thickness independent of lift-off distance variation. A simple equivalent model was built to deal with the ECS problem, and the essential relationship between the slope of LOC (SLOC) and target properties was obtained. Full finite element analysis was conducted to analyze the relationship between SLOC feature and target thickness, and the results matched the modeling results very well. A sensor coil probe was then manufactured and used to measure the thickness of copper films with high performance, and the capability of this technique for online noncontact thickness measurement was verified. The basic characteristics and performances of this thickness measurement technique were tested and discussed. The SLOC feature for thickness measurement had significant advantages, such as simplicity, reliability, immunity to the lift-off effect (most important), high speed, simple signal processing, and negligible design limitation of the sensor probe. The results of this paper revealed that online thickness measurement systems could be developed for various advanced industrial applications.

Noncontact Thickness Measurement of Metal Films Using Eddy-Current Sensors Immune to Distance Variation

This paper presents a review of the latest advances in the field of capacitive, inductive (eddy current), and magnetic sensors, for measurement of absolute displacement. The need for accurate displacement and position measurement in the micrometer, nanometer, and subnanometer scales has increased significantly over the last few years. Application examples can be found in high-tech industries, metrology, and space equipment. Besides measuring displacement as a primary quantity, absolute displacement sensors are also used when physical quantities such as pressure, acceleration, vibration, inertia, etc., have to be measured. A better understanding of the commonalities between capacitive, inductive, and magnetic displacement sensors, as well as the main performance differences and limitations, will help one make the best choice for a specific application. This review is based on both theoretical analysis and experimental results. The main performance criteria used are: sensitivity, resolution, compactness, long-term stability, thermal drift, and power efficiency.

Advances in Capacitive, Eddy Current, and Magnetic Displacement Sensors and Corresponding Interfaces

In this paper, an inverse method was developed which can, in principle, reconstruct arbitrary permeability, conductivity, thickness, and lift-off with a multifrequency electromagnetic sensor from inductance spectroscopic measurements. Both the finite-element method and the Dodd and Deeds formulation are used to solve the forward problem during the inversion process. For the inverse solution, a modified Newton–Raphson method was used to adjust each set of parameters (permeability, conductivity, thickness, and lift-off) to fit inductances (measured or simulated) in a least-squared sense because of its known convergence properties. The approximate Jacobian matrix (sensitivity matrix) for each set of the parameter is obtained by the perturbation method. Results from an industrial-scale multifrequency sensor are presented including the effects of noise. The results are verified with measurements and simulations of selected cases. The findings are significant because they show for the first time that the inductance spectra can be inverted in practice to determine the key values (permeability, conductivity, thickness, and lift-off) with a relative error of less than 5% during the thermal processing of metallic plates.

Determination of the Magnetic Permeability, Electrical Conductivity, and Thickness of Ferrite Metallic Plates Using a Multifrequency Electromagnetic Sensing System

In this paper, a novel integrated eddy-current sensor interface is presented. The main targeted application is displacement measurement in an industrial environment, with resolution in the submicrometer range. A high excitation frequency of about 22 MHz is applied to minimize the skin effect of the generated eddy currents, when thin targets are used. The price to be paid is to process high-frequency signals. This is very challenging when high performance has to be achieved with respect to resolution and stability at minimum power consumption. To ensure high immunity of the interface to electromagnetic interferences, a second-order oscillator with a steep bandpass resonator is utilized as a front-end stage. The noise performance of the front-end stage is analyzed. To reduce the effect of this noise source on the resolution, a ratiometric measurement principle is proposed. In order to extract the displacement information, a novel amplitude-demodulation approach, including an offset cancellation technique, is introduced. The proposed circuit has been designed and implemented in a 0.35-μm BiCMOS process. In this design, the full-scale displacement range is 1.5 mm. The noise level allows a dynamic range of 75 dB with a measurement signal bandwidth of 1 kHz and only 9.5-mW power dissipation. A comparison with state-of-the-art eddy-current sensor interfaces shows an improved figure of merit, which confirms the high performance of the proposed interface.

Eddy-Current Sensor Interface for Advanced Industrial Applications

In this paper, we propose a novel interface concept for eddy current displacement sensors. A measurement method and a new front-end circuit are also proposed. The front-end circuit demonstrates excellent thermal stability, high resolution, and low-power consumption. The proposed idea is analytically investigated. The demodulation principle, as well as the interface implementation, is also addressed. This interface is being introduced for measuring submicrometer displacements in medium- to high-resolution applications. The interface system consumes less than 12 mW and has an extremely low thermal drift. The interface circuit will be implemented as a system-in-a-package (SIP). The full-scale range of displacement is 1 mm with 50-kHz signal bandwidth and 11-bit resolution (less than 500 nm). The signal conditioning circuit utilizes a standard 0.35- mum complementary metal-oxide semiconductor (CMOS) technology. Simulation results, which were achieved on the basis of experimental results of testing a prototype coil, also confirm the high performance of the interface system, as expected from analytical results. Compared with previous reports, this low-power interface system demonstrates a much lower temperature drift.

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A Novel Interface for Eddy Current Displacement Sensors

This paper presents an integrated interface for eddy-current sensors (ECSs) for displacement measurement. The employed architecture helps bridging the performance gap between the requirements of demanding and precision industrial applications and the performance of existing ECS interfaces. The interface operates with a sensor excitation frequency of 20 MHz, which is more than one order of magnitude higher than typical values. This high excitation frequency limits the eddy-current penetration depth in the target down to a few tens of micrometers, thus enabling the use of thin targets required in precision applications. The proposed interface consists of a low-power front-end oscillator that incorporates the sensor, and a two-channel offset-compensated synchronous demodulator. A ratio-metric measurement approach along with offset and 1/f noise reduction techniques is applied to improve the system stability. The interface has been realized in a 0.35-μm 3.3 V BiCMOS technology and consumes 18 mW. Measurement results obtained using two flat sensing coils show a full-range non-linearity of the sensor interface of only 0.4%, and a resolution of 15.5 bits (65 nm on a 3 mm measurement range), with 1 kHz signal bandwidth. This translates into 1.5 pico-Henry inductance-measurement resolution, which is comparable with the performance of the most advanced LCR meters. Using the proposed solution, a long-term instability below 20 ppm (for 17 hours) and a thermal drift of 30 ppm/°C are obtained without any temperature compensation. Compared to the state-of-the-art, the proposed interface achieves a considerably better trade-off between power consumption, resolution, bandwidth, and excitation frequency.

An Interface for Eddy-Current Displacement Sensors With 15-bit Resolution and 20 MHz Excitation

This paper presents a comprehensive study of demodulation techniques for high-frequency self-oscillating eddy-current displacement sensor (ECDS) interfaces. Increasing the excitation frequency is essential for lowering the skin depth in many demanding industrial applications, that require better resolution. However, a high excitation frequency poses design challenges in the readout electronics, and particularly in the demodulation functional block. We analyze noise, linearity, and stability design considerations in amplitude demodulators for nanometer and sub-nanometer ECDSs. A number of state-of-the-art amplitude demodulation techniques employed in high-frequency ECDSs are reviewed, and their pros and cons are evaluated.

M.R. Nabavi

Papers

Design Strategies for Eddy-Current Displacement Sensor Systems: Review and Recommendations

Eddy-Current Sensor Interface for Advanced Industrial Applications

A Novel Interface for Eddy Current Displacement Sensors

An Interface for Eddy-Current Displacement Sensors With 15-bit Resolution and 20 MHz Excitation

Demodulation Techniques for Self-Oscillating Eddy-Current Displacement Sensor Interfaces: A Review