Mohammad Reza Nabavi

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

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.

Eddy-Current Sensor Interface for Advanced Industrial Applications

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.

A Novel Interface for Eddy Current Displacement Sensors

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.

An interface for eddy current displacement sensors with 15-bit resolution and 20 MHz excitation

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 high-performance interface for eddy current displacement sensors, intended for use in IC fabrication equipment. The interface consists of a low-power front-end oscillator and a synchronous demodulator. With an excitation frequency of 20 MHz, it enables the use of targets with only a few tens of µm thickness. A ratio-metric measurement approach is applied to suppress the oscillators noise contribution. The demodulator is auto-zeroed to suppress its 1/f noise. The interface has been realized in a 0.35µm BiCMOS technology and consumes about 18 mW. Measurement results demonstrate a linearity of 0.4%, and a resolution of 15.3 bits (75 nm on a 3mm range) within a 1 kHz signal bandwidth.

A Low-Power Interface for Eddy Current Displacement Sensors in Sub-Micron Applications

In this paper we present a low-power interface for eddy current displacement sensors with digital output. A measurement method and a new front-end circuit are proposed. The front-end circuit demonstrates an excellent thermal stability, high-resolution and low power consumption. The demodulation principle, as well as the interface implementation, is addressed. This interface is introduced for measuring sub-micron displacements in medium to high-resolution applications. The system will consume less than 12 mW, and will have an extremely low thermal drift. The interface circuits 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 bits resolution (less than 500 nm). The signal conditioning circuit utilizes a standard 0.35 mum CMOS technology. The discussions are based on simulations and preliminary experimental results, promising a proper operation of the sensor.

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.

Stability considerations in a new interface circuit for inductive position sensors

A stable electronic interface circuit for sensing small inductances is discussed in this paper. The main application is for interfacing eddy current displacement sensors. The proposed circuit has been designed and implemented in a 0.35 µm BiCMOS process. The frequency of the excitation signal is above 22 MHz, which eliminates the skin effect on the measurement result when using very thin targets. Oscillation stability issues along with an offset cancellation approach for the interface circuit are particularly taken into account. In this design the full-scale displacement range is 1.5 mm. Primary experimental results show that the signal to noise ratio is about 75 dB, with 240 µs measurement time and only 9.5 mW power dissipation.

An eddy-current displacement-to-digital converter based on a ratio-metric delta-sigma ADC

This paper describes a smart Eddy-current displacement sensor for use in precision industrial applications. A novel readout scheme based on ratio-metric delta-sigma analog-to-digital conversion is proposed. The system employs two sensing coils incorporated in a low-power front-end oscillator. This produces two anti-phase outputs whose amplitudes are proportional to the inductances of the coils, and are thus a differential function of displacement. After synchronous down-conversion, these signals are fed into a second-order continuous-time delta-sigma modulator that directly produces a digital output that is a ratio-metric function of the coil inductances. This approach eliminates the need for a stable voltage reference, suppresses the oscillators multiplicative noise contributions, and effectively filters the ripple associated with the down-conversion. The sensors are excited at 15 MHz, which reduces the eddy-current penetration depth to only a few tens of μm. The interface has been realized in a 0.35 μm BiCMOS technology and consumes 18 mW from a 3.3 V supply. In a measurement time of 1 ms, it digitizes the inductance ratio with a resolution of 15 bits, and thus achieves a displacement resolution of 135 nm on a range of 3 mm.

Low-power front-end of eddy current sensor interfaces for industrial applications

This paper presents a high-performance electronic interface for eddy current displacement sensors (ECSs). The interface includes a low-power front-end oscillator specifically designed for high-stability performance. The demodulation principle used to detect the input signal (i.e., displacement information) is explained. A switched-channel technique is utilized to suppress the effect of the offset. Post-layout simulations along with primary experimental results confirm the effectiveness of the design approach as well as the validity of the theoritical analyses. The interface system dissipates 9.5 mW and demonstrates a dynamic range of about 75 dB with a 240 μs measurement time and with less than 20 ppm/°C thermal drift over the range of 0−85 °C.