Ruimin Yang

A time/resistor-referenced capacitive sensor interface for displacement measurement in the sub-nanometer range

This work presents a precision capacitive-sensor interface (CSI) designed for displacement measurement in the (sub)nanometer range. It digitizes a capacitance in the 8–12 pF range, corresponding to 4μm displacement range, with 12-bit resolution (150 pm) and excellent stability: thermal drift below 10ppm/oC in the temperature range from 20°C to 70°C. The conversion time is 10ms while the power consumption is only 660μW. The CSI is particularly suited for applications in which periodic calibration is not possible. For instance, position sensing in high precision industrial systems, such as lithographic machines, where the position of critical mechanical components needs to be measured with sub-nanometer precision.

Capacitive sensor interface with precision references

This paper presents an investigation on high-precision capacitance measurement techniques which are aimed for capacitive-sensor-based displacement measurement in advanced industrial applications. The paper analyzes the interface structure and the references used in a capacitive sensor interface (CSI), which define the precision of the capacitance measurement. The trade-offs of using different types of references are discussed. Finally, to validate the analysis, a prototype IC is presented and qualified. This prototype achieves a resolution of 17.5 bit for 10 ms conversion time, while consuming 230 μA from a 3.3 V supply. The measured thermal drift of the interface is ~6 ppm/°C.

Electronic system for control of a thermally actuated alignment device

This article presents the design of an electronic system that is used to control a thermally actuated alignment device. This device is ideal for aligning critical parts in complex, high-precision systems, like, for example, optical elements or the electrodes of capacitive sensors. Accurate temperature control of the thermal elements improves the performance of the alignment in terms of positioning accuracy and operating speed. Furthermore, the article presents the realized test setup and the initial measurement results. The measurement results show the benefits of the new controller.

Noise analysis and characterization of a charge-balancing-based capacitive sensor interface with a resistive reference

This paper presents a detailed noise analysis of a charge-balancing capacitive sensor interface (CSI) based on a resistor-capacitor (R-C) comparison. The CSI was proposed in [1] and [2] and shows superior precision and stability, due to the precision off-chip resistive reference used. In this work, an in-depth noise analysis is presented which provides optimal criteria for the CSI design. The result is applicable for all CSIs with a similar structure. Based on this analysis a prototype CSI is designed and tested, which delivers 17.5-bit resolution in a conversion time of 10 ms. The measurement results show good match with the theoretical analysis.

Autonomous Self-aligning and Self-calibrating Capacitive Sensor System

An autonomous capacitive sensor system for high accuracy and stability position measurement, such as required in high-precision industrial equipment, is presented. The system incorporates a self- alignment function based on a thermal stepping motor and a built-in capacitive reference, to guarantee that the relative position between the sensor electrodes is set to 10±0.1 μm. This is needed to achieve the performance specifications with the capacitive readout. In addition, an electronic zoom-in method is used to reach the 10 pm resolution with minimum power dissipation. Finally, periodic self-calibration of the electronic capacitance readout is realized using a very accurate and stable built-in resistive reference. The performance is evaluated experimentally and with simulations.

Performance optimization of a self-alignment system for capacitive sensors

Capacitive sensors are a good choice for sub-nanometer-resolution position measurements, but their performance is dependent on the correct alignment. Therefore, a capacitive sensor with an integrated self-alignment system has been developed which enables the use of capacitive sensors without alignment problems due to mounting and fabrication tolerance.

Qualification of a stable capacitive sensor interface based on capacitance-resistance comparison

This paper presents the principle of operation and the qualification results of a thermally stable high-precision capacitive sensor interface (CSI), which is based on a capacitance-resistance (C-R) comparison. An excellent performance of the CSI is demonstrated due to the high-accuracy and high-stability resistive reference implemented. A dedicated measurement set-up is built and a systematic measurement strategy is proposed to qualify the interface. The measurement results show a thermal stability better than 2.5 ppm/°C in all measurement ranges (5 pF, 10 pF, 20 pF, 50 pF, and 100 pF). Applying offset cancellation improves the thermal stability even further to 1.8 ppm/°C. The measured maximum deviation of the transfer characteristic from a linear one is 0.0023% for all measurement ranges. The dynamic range for 200 µs conversion time is extended up to 19 bits with the help of the “zoom-in” technique.

Optimized low-power thermal stepper system for harsh and inaccessible environments

This article presents an optimized thermal stepper system which is used to align the critical parts of highperformance smart sensors, such as capacitive sensors. In order to apply the system in harsh and inaccessible industrial environments, improvements have been made, such as reduced power consumption (1mW during measurement mode), increased alignment accuracy (within 0.1um), increased interconnect reliability, and better immunity to external interferences. Experimental results are shown which demonstrate the improved performance of the thermal stepper system.

High-performance eddy current sensor interface for small displacement measurements

This paper presents an improved interface for eddy current displacement sensor for accurate displacement measurement of thin targets in harsh environment. Two recently proposed state-of-the-art interfaces are analyzed and experimentally verified. The experimental results confirm high level of performance: displacement measurement of a 100 um thin target; displacement range ±1.5 mm; 15 bits dynamic range; 1 kHz signal bandwidth; 18-mW power consumption. The most important application constraints of the two interfaces are discussed, which facilitates a correct choice when targeting a specific set of application requirements. A new interface, which demonstrates the above mentioned performance, and at the same time allows continuous time displacement measurement useful in control loops and servo systems, is presented and evaluated.

Error analysis of a charge-balancing capacitive sensor interface with resistive reference

This paper presents a detailed error analysis of a charge-balancing capacitive sensor interface (CSI), which is based on resistor-capacitor (R-C) comparison. Precision resistive and time references can be utilized in the CSI to achieve high-precision capacitance-to-digital conversion. However, having precision references is not sufficient for the CSI to perform a precision measurement. The comparison of the measured capacitance with the reference introduces additional errors and instability. As a result, the errors of the comparison circuit degrade the performance of the CSI. For this reason, an in-depth error analysis is performed to identify the error sources and their impacts on the final measurement results. Based on that, error budgets can be properly allocated according to specific requirements. Besides, several circuit techniques are proposed to reduce the corresponding errors. A design example is presented, which achieves a thermal drift as low as 6ppm/°C.