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

Tactile sensors are essential for robotic systems to interact safely and effectively with the external world, they also play a vital role in some smart healthcare systems. Despite advances in areas including materials/composites, electronics and fabrication techniques, it remains challenging to develop low cost, high performance, durable, robust, soft tactile sensors for real-world applications. This paper presents the first Soft Inductive Tactile Sensor (SITS) which exploits an inductance-transducer mechanism based on the eddy-current effect. SITSs measure the inductance variation caused by changes in AC magnetic field coupling between coils and conductive films. Design methodologies for SITSs are discussed by drawing on the underlying physics and computational models, which are used to develop a range of SITS prototypes. An exemplar prototype achieves a state-of-the-art resolution of 0.82 mN with a measurement range over 15 N. Further tests demonstrate that SITSs have low hysteresis, good repeatability, wide bandwidth, and an ability to operate in harsh environments. Moreover, they can be readily fabricated in a durable form and their design is inherently extensible as highlighted by a 4 × 4 SITS array prototype. These outcomes show the potential of SITS systems to further advance tactile sensing solutions for integration into demanding real-world applications.

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Robust and high-performance soft inductive tactile sensors based on the Eddy-current effect

This paper presents an eddy-current sensor (ECS) interface intended for sub-nanometer (sub-nm) displacement sensing in hi-tech applications. The interface employs a 126-MHz excitation frequency to mitigate the skin effect, and achieve high resolution and stability. An efficient on-chip sensor offset compensation scheme is introduced which removes sensor-offset proportional to the standoff distance. To assist in the ratiometric suppression of noise and drift of the excitation oscillator, the ECS interface consists of a highly linear amplitude demodulation scheme that employs passive capacitors for voltage-to-current (V2I) conversion. Using a printed circuit board-based pseudo-differential ECS, stability tests were performed which demonstrated a thermal drift of $\sqrt {\mathrm{ Hz}}$ which corresponds to a displacement resolution of 0.6 nm in a 2-kHz noise bandwidth. The ECS interface is fabricated in TSMC 0.18- $\mu \text{m}$ CMOS technology and dissipates only 19.8 mW from a 1.8-V supply.

/pdf/a-19-8-mw-eddy-current-displacement-sensor-interface-with-3epyfwtg9h.pdf

A 19.8-mW Eddy-Current Displacement Sensor Interface With Sub-Nanometer Resolution

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.

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

The miniaturization requirements have been put forward to the sensors for the development of the modern industry However, the traditional inductive displacement sensor applied to active magnetic bearings (AMBs) has complicated detection device and measuring circuit with excessive discrete components which cannot meet the requirements of the miniaturization To remedy the infliction, an optimized differential self-inductance displacement sensor was designed for AMBs Concretely, the operating principle of the self-inductance displacement sensor was analyzed in detail Then the optimized structure was proposed and the measuring circuit was designed After the designing, the performance of the sensor was analyzed Based on the analysis and designing, a series of experiments were carried out to test the performance The results demonstrated that the designed sensor can realize higher linearity, higher sensitivity, and wider dynamic response range than the traditional inductive displacement sensor And it is suitable to the AMBs system, with higher rotation speed and small volume

Optimized Differential Self-Inductance Displacement Sensor for Magnetic Bearings: Design, Analysis and Experiment

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.

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

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

Displacement sensing with sub-nanometer resolution is required in advanced metrology and high-tech industry, e.g., to measure the lens position in wafer scanners. Linear encoders and interferometers are often used for this purpose, but they are bulky and costly. Capacitive sensors [1], though compact, are sensitive to environment and require electrical access to the target. Eddy-current sensors (ECSs) do not have these disadvantages, but their resolution and stability are limited by the skin-effect [2–5]. For sub-nm measurements, this can be alleviated by using excitation frequencies >100MHz. This calls for stable flat sensing coils (to minimize parasitics) in close proximity to the ECS interface, whose power dissipation must then be low enough to avoid self-heating and displacement errors due to thermal expansion [2,6].

Vikram Chaturvedi

Papers

A 19.8-mW Eddy-Current Displacement Sensor Interface With Sub-Nanometer Resolution

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

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

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

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