Johan G. Vogel

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

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.

A 9.1 mW inductive displacement-to-digital converter with 1.85 nm resolution

A displacement-to-digital converter (DDC) based on inductive (eddy-current) sensor is presented. The sensor is embedded in a self-oscillating front-end, whose 145MHz output is then digitized by a ratiometric ΔΣ ADC. Over a 10μm range, the DDC achieves 1.85nm resolution (1.02 pH), in a 2kHz bandwidth. It draws 9.1mW from a 1.8 V supply making it the most energy-efficient ECS interface ever reported.

Eddy-Current Sensing Principle in Inertial Sensors

The eddy-current displacement sensing principle is, to the best of our knowledge, not yet used in inertial sensors. The main reasons for this are the important performance limitations of the existing eddy-current sensor solutions, such as low sensitivity, poor stability, high-power consumption, and bulkiness. Our novel high-frequency eddy-current displacement sensor (ECDS), however, has significantly improved performance with respect to these limitations and allows the use of planar, stable coils, making it a viable candidate for the use in inertial sensors. An implementation example of an ECDS-based inertial sensor with a bandwidth of 370 Hz and a noise floor of

Modelling the inductance of a novel eddy-current position sensor for high-precision applications

13\,\mu {\text{g}/ \surd \text{Hz}}

Study of the self-resonance frequency of a flat coil for an eddy-current position sensor

is proposed. Although not yet competitive with state-of-the-art inertial sensors, it performs better than other types of inductive accelerometers and offers the inherent advantages of ECDSs, such as insensitivity to the environment.

Self-Aligning and Self-Calibrating Capacitive Sensor System for Displacement Measurement in Inaccessible Industrial Environments

Due to its immunity to ambient conditions, the eddy-current position sensor is considered a good candidate for use in high-precision applications. Currently, the eddy-current sensor is not often used in such applications, due to its poor resolution as compared to, for example, the capacitive position sensor. A novel eddy-current sensor is proposed that uses small standoff distances in combination with a high excitation frequency to decrease the skin depth, thus aiming at an improved resolution. This paper studies how well some analytic models describe the sensors inductance at small standoff distances and high excitation frequencies. It is shown that the analytic methods can be used in the design process of the sensor but that some care must be taken, as the analytic methods do not fully model skin effect.

9.9 A 0.6nm resolution 19.8mW eddy-current displacement sensor interface with 126MHz excitation

At present, a novel eddy-current position sensor is being developed with an aim to attain sub-nanometre resolution. The sensor contains a flat Archimedean coil that should have a high enough Self-Resonance Frequency (SRF). This paper studies the SRF of coils with various geometries. The SRFs increase in the vicinity of a conductive target, but are relatively constant at small standoff distances. For accurate results, a 3D finite element model must be used instead of a 2D model. Based on the finite element results, a relatively simple expression has been obtained that predicts the SRF for standoffs between 10 pm and 100 pm with a RMS deviation below 6 %.

Humidity Sensitivity and Coil Design of a High-Precision Eddy-Current Displacement Sensor

High-precision positioning often requires high speed and high resolution displacement measurements in order to compensate for the small vibrations of critical components. The displacement sensor must be precise and stable over a long period of time to avoid expensive recalibration. This requires tight mounting tolerances, which are especially difficult to meet in inaccessible environments. The proposed sensor system is based on a capacitive sensor and consists of three subsystems: 1) a mechanical “zoom-in” system that performs self-alignment of the capacitive sensor electrode in order to reduce the mounting tolerances of the sensor; 2) a real-time capacitance-to-digital converter that employs an internal reference and electrical zoom-in technique to effectively reduce the dynamic range of the measured capacitance, thus improving the power efficiency; and 3) a self-calibration circuit that periodically calibrates the internal references to eliminate their drift. In previous publications, the three subsystems have been introduced. This paper shows how the different subsystems can be integrated to achieve optimal performance and presents new repeatability and stability measurement results. The overall system demonstrates a displacement measurement resolution of 65 pm (in terms of capacitance 65 aF) for a measurement time of 20

Vacuum packed particles as flexible endoscope guides with controllable rigidity

\mu \text{s}

Suppression Efficiency of the Correlated-noise and Drift of Self-oscillating Pseudo-differential Eddy Current Displacement Sensor

. Furthermore, the thermal drift of the sensor is within 6 ppm/K, owing to the self-calibration circuit. In measurement mode, the system consumes less than 16 mW.